Patent Application
20240087838
The disclosure relates to multi-beam microscopes and methods of operating such microscopes. In some embodiments, the methods comprise using a multi-beam microscope using settings adjusted to an inspection site.
With the continuous development of ever smaller and ever more complex microstructures such as semiconductor components, there is often a desire to develop and optimize planar production techniques and inspection systems for producing and inspecting small dimensions of the microstructures. By way of example, the development and production of the semiconductor components typically involves monitoring of the design of wafers, and the planar production techniques typically involve process monitoring and process optimization for a reliable production with a high throughput. Moreover, there have been recent desires for an analysis of semiconductor wafers for reverse engineering and for a customer-specific, individual configuration of semiconductor components. Therefore, it can be desirable to provide a mechanism for inspection which can be used with a high throughput for examining the microstructures on wafers with great accuracy. Typical silicon wafers used in the production of semiconductor components have diameters of up to 300 mm. Each wafer is often subdivided into 30 to 60 or more repeating regions (“dies”) with a size of up to 800 mm2. A semiconductor apparatus comprises a plurality of semiconductor structures, which are produced in layers on a surface of the wafer by planar integration techniques. Semiconductor wafers typically have a plane surface on account of the production processes. The structure sizes of the integrated semiconductor structures in this case can extend from a few micrometres (μm) to the critical dimensions (CD) of 5 nanometres (nm), wherein the structure dimensions will become even smaller in the near future; in future, structure sizes or critical dimensions (CD) are expected to be less than 3 nm, for example 2 nm, or even under 1 nm. In the case of the small structure sizes, it is generally desirable to relatively quickly identify defects of the size of the critical dimensions in a relatively large area. For several applications, the desired properties for the accuracy of a measurement provided by an inspection device can be even higher, for example by a factor of two or one order of magnitude. By way of example, it is often desirable to measure a width of a semiconductor feature with an accuracy of below 1 nm, for example 0.3 nm or even less, and to determine a relative position of semiconductor structures with a superposition accuracy of below 1 nm, for example 0.3 nm or even less.
The MSEM, a multi-beam scanning electron microscope, is a relatively new development in the field of charged particle systems (charged particle microscopes, CPMs). By way of example, a multi-beam scanning electron microscope is disclosed in U.S. Pat. No. 7,244,949 B2 and in US 2019/0355544 A1. In the case of a multi-beam electron microscope or MSEM, a sample is irradiated simultaneously with a plurality of individual electron beams, which are arranged in a field or raster. By way of example, 4 to 10 000 individual electron beams can be provided as primary radiation, with each individual electron beam being separated from an adjacent individual electron beam by a pitch of 1 to 200 micrometres. By way of example, an MSEM has approximately J=100 separated individual electron beams (“beamlets”), which for example are arranged in a hexagonal raster, with the individual electron beams being separated by a pitch of approximately 10 μm. The plurality of J individual charged particle beams (primary beams) are focused on a surface of a sample to be examined by way of a common objective lens. By way of example, the sample can be a semiconductor wafer which is accommodated via a wafer chuck that is assembled on a movable stage. During the illumination of the wafer surface with the primary individual particle beams, interaction products, for example secondary electrons or backscattered electrons, emanate from the surface of the wafer. Their start points correspond to those locations on the sample on which the plurality of J primary individual particle beams are focused in each case. The amount and the energy of the interaction products depends on the material composition and the topography of the wafer surface. The interaction products form a plurality of secondary individual particle beams (secondary beams), which are collected by the common objective lens, and which are directed by a projection imaging system of the multi-beam inspection system at a detector arranged in a detection plane. The detector comprises a plurality of detection regions, each of which comprises a plurality of detection pixels, and the detector measures an intensity distribution for each of the J secondary individual particle beams. A digital image of an image field of for example 100 μm×100 μm is obtained in the process.
Certain known multi-beam electron microscopes comprise a sequence of electrostatic and magnetic elements. At least some of the electrostatic and magnetic elements are adjustable in order to adapt the focus position and the stigmation of the plurality of charged individual particle beams. Such a multi-beam system with charged particles moreover can comprise at least one crossover plane of the primary or the individual secondary charged particle beams. Moreover, the system can comprise detection systems to make the setting easier. The multi-beam particle microscope can comprise at least one beam deflector (“deflection scanner”) for collective scanning of a region of the sample surface via the plurality of individual primary particle beams in order to completely sweep over the image field of the sample surface with the plurality of the primary beams. Moreover, such a system from can comprise a beam splitter arrangement which is configured such that the bundle of primary beams is guided from the generation device of the bundle of primary beams to the objective lens, and the bundle of secondary beams is guided from the objective lens to the detection system. Further details regarding an example of a multi-beam electron microscope and a method for operating same are described in the PCT application PCT/EP2021/061216, filed on 29 Apr. 2021, the disclosure of which is incorporated in full here within by reference.
In the case of scanning electron microscopes for wafer inspection, it is often desirable to keep the imaging conditions stable such that the imaging can be carried out with great reliability, great imaging fidelity and high repeatability. The throughput generally depends on a plurality of parameters, for example the speed of the displacement stage and of the realignment at new measurement sites, and the area measured per unit of capture time. The latter is determined, inter alia, by the dwell time on the pixel, the pixel size and the number of individual particle beams. Additionally, time-consuming image postprocessing may be involved for a multi-beam electron microscope; by way of example, the signal generated from charged particles by the detection system of the multi-beam system are often digitally corrected before the image field from a plurality of image sub-fields or sub-fields is put together (“stitching”).
Previous methods may not longer be sufficient in all cases, such as in the case of inspecting semiconductors using a multi-beam microscope and the high demands on the measurement accuracy connected therewith. A number of special effects which can reduce the desired high measurement accuracy occur when semiconductors are inspected. The complex effects relate to the raster arrangement of the plurality of charged particle beams and a different shape or size of individual particle beams from the plurality of charged particle beams. By way of example, some of these effects do not occur in the case of biological samples or cannot be observed there. Other effects are sometimes so small that they might only play a role in the case of the semiconductor inspection with the increased measurement accuracy of better than 2 nm or better than 1 nm. The effects can reduce the resolution or the signal strengths during the wafer inspection. Moreover, the measurement accuracy of dimensions or distances of structures on wafer surfaces, for example, can be adversely affected when these effects occur. In principle, some of these effects can be at least partly compensated by a change in the settings of the multi-beam microscope. However, certain previous methods for determining and setting a best setting of the multi-beam microscope for the purposes of avoiding the unwanted effects can be too slow or too complicated for a wafer inspection task with a high throughput. By way of example, a previous method for determining and setting a best focal plane of the multiplicity of the primary beams can have a negative effect on the throughput. U.S. Pat. No. 10,388,487 describes a determination of object properties in a first measurement with first setting parameters and a derivation of second setting parameters resulting therefrom in order to measure the object therewith in a second measurement. Beam properties such as the focal position and stigmation, for example, are determined from the object properties. However, this method can reduce the throughput since a second, improved measurement is preceded by a first measurement with a high resolution. U.S. Pat. No. 10,535,494 describes a detection system used to determine the beam shape of secondary beams, but not the raster arrangement of the beams itself. DE 10 2018 124 044 B3 describes a detection system that it is believed can only correct relatively small effects of local sample charging by assigning the plurality of secondary electron beams to detector channels. For a measurement accuracy of better than 2 nm or better than 1 nm, only considering the detection system and the secondary particle path may no longer sufficient.
A multi-beam system can provide a higher throughput, and there can be the occurrence of complex effects that relate to the plurality of beams of charged particles, for example the raster arrangement of the plurality of charged particle beams or a different shape or size of individual particle beams. According to certain known multi-beam systems and methods for operating a multi-beam system, these complex effects can involve complicated analysis and adjustment of the multi-beam system, which can significantly reduce the throughput of the multi-beam system.
There are increasing demands on throughput/speed and on precise measurement of ever smaller structures, and it is believed that it would be desirable to improve the existing multi-beam systems and methods for operating a multi-beam system. This can apply, for example, to the inspection of polished wafer surfaces with HV structures. Thus—even under the not entirely realistic assumption of a lack of system drift and the like—it may no longer sufficient to set the multiple electron microscope at a predefined working point with an associated working distance using the methods from the prior art.
It is believed that the complex multi-beam effect of the plurality of primary beams cannot be determined directly without effort. An occurrence of a complex multi-beam effect of the primary beams, for example a distortion of the raster arrangement of the primary beams, a magnification difference of the raster arrangement of the primary beams or a deviation in the shape and size of focal points of the primary beams, can lead to defective imaging, for example to incorrect positioning of the images of surface structures of a wafer, or to an incorrect measurement of dimensions or areas of surface structures. If there is a significant occurrence of a complex multi-beam effect of the primary beams, this may furthermore lead to the expected drop in the signal strength of the secondary particles, right up to a complete loss of the signal strength of the secondary particles. Especially if the known methods for fast adjustment of the detection path are applied and thus compensate the complex multi-beam effect of the primary beams for the purposes of keeping the signal strength of the secondary particle highs, the effect of incorrect imaging of the object remains. However, especially in the case of wafer inspection, the positions and dimensions of surface structures are relevant and have to be determined with high precision of less than 2 nm, ideally less than 1 nm or even less.
The present disclosure seeks to provide an improved multi-beam system and an improved method for operating a multi-beam system, by which complex effects, such as those discussed above, are reduced or compensated without the throughput of a wafer inspection being reduced in the process.
The disclosure also seeks to provide a multiple particle beam system that operates with charged particles and an associated method for operating same with a high throughput, which facilitates a highly precise measurement of semiconductor features with a resolution of below 4 nm, below 3 nm or even below 2 nm.
The disclosure provides an improved multi-beam system and improved method for operating a multi-beam system, by which complex multi-beam effects that occur during the interaction of a plurality of beams of charged particles with a wafer surface are compensated. According to an embodiment of the disclosure, the complex multi-beam effects can be characterized by a combination of a distortion of the plurality of secondary beams and a change in the size and shape of the focal points on a detector. According to a further embodiment, a complex multi-beam effect occurs in particular in the vicinity of an edge of the wafer or in the vicinity of a preceding inspection position, or the size of the one complex multi-beam effect depends directly on a distance of an inspection site from an edge of the wafer or a preceding inspection position. For the compensation there is a characterization or classification of the complex multi-beam effects and a derivation of measures, for example an adjustment of the primary or illumination path and an adjustment of the secondary or detection path. According to this derivation of measures, parameters for setting or adjusting primary and secondary paths are derived, which are suitable for counteracting the complex effects. Moreover, a multi-beam system may comprise a mechanism in the direct vicinity of the wafer or the wafer surface which is suitable for minimizing or compensating a complex multi-beam effect. Using this mechanism, an electric field between the wafer surface and a last electrode of an objective lens of the multi-beam system is influenced, the field simultaneously acting on the primary particle beams and the secondary particle beams.
The multi-beam system and the methods for operating the multi-beam system of the present disclosure solve the problem of fast wafer inspection with a great imaging fidelity by virtue of a complex multi-beam effect of the primary beams and the secondary beams being determined at an inspection site and measures being carried out for compensating the complex multi-beam effect by way of changing parameters, both of components of the illumination system and of the detection system. In particular, this is facilitated during a routine inspection of wafers being always similar in principle, and similar multi-beam effects with similar causes are always occurring.
In an embodiment, the complex multi-beam effect of the primary beams is determined by a time-averaged measurement of the raster arrangement of the plurality of secondary beams and the shape or size of at least one focal point of a secondary beam. In this case, influence of the surface structuring of the semiconductor wafer on the measurement signal is reduced by time averaging. An analysis of the raster arrangement of the plurality of secondary beams and the shape or size of at least one focal point of a secondary beam is used to deduce the probable complex multi-beam effect of the primary beams and correction measures for the illumination path are introduced. In this case, the method of the measurement can be repeated. It is consequently possible within the scope of the wafer inspection to determine a complex multi-beam effect of the primary beams at an inspection position on the surface of a structured wafer purely from determining the accumulated complex multi-beam effect of the secondary beams without having to use a reference object. As a result of the time averaging of the measurement, the measurement can be carried out very quickly here, for example by way of very quickly scanning the object surface at the inspection position using the plurality of primary beams.
In a further embodiment, the complex multi-beam effect of the primary beams is determined from a priori information. Further, it was found that some complex multi-beam effects depend on the inspection position on a wafer surface, in particular on the distance of an inspection position from an edge of the wafer or a preceding inspection position. Since the inspection positions on a wafer are known in advance, this dependence can be used in accordance with one embodiment of the disclosure to compensate the complex multi-beam effects. By way of example, the parameters for driving the multi-beam system may depend on the inspection position known in advance. By way of example, a sequence of inspection tasks can be modified in order to reduce effects of preceding inspection positions. In a further embodiment, an edge effect caused by an inhomogeneous extraction field is reduced by additional electrodes in the periphery of a wafer reception area. During inspection, correction voltages are applied to the additional electrodes. In one example, an extraction field is set by way of a counter electrode, formed by a plurality of differently driveable electrode segments, so that a complex multi-beam effect is reduced.
In principle, the method of determining the complex multi-beam effect can also be carried out during an inspection task. Consequently, it is also possible to detect changeable complex multi-beam effects of the primary beams, or unexpected deviations of the primary beams.
According to a first embodiment of the disclosure, the improved multi-beam system comprises a spatially resolving detection device which is configured to detect focal points of a plurality of secondary beams during an inspection task, independently of a surface contrast of a wafer surface at an inspection position. Further, the improved multi-beam system comprises a control unit having a memory and a computing unit which is configured to determine a current raster arrangement of the focal points of the plurality of primary beams from the focal points of the plurality of secondary beams and to determine a deviation from a predefined raster arrangement using this current raster arrangement. According to an embodiment, the control unit is further configured to determine at least one current shape and size of a predetermined focal point of the plurality of secondary beams. By way of example, a differing shape or size of at least two focal points of the plurality of secondary beams is determined. The control unit is further configured to analyse the deviation of the current raster arrangement from a predefined raster arrangement and use this to deduce the occurrence of a certain complex multi-beam effect of the primary beams. According to an embodiment, the control unit determines a deviation of the current shape and size of the at least one predetermined focal point from a predetermined shape and size of the focal point. Further, the control unit is configured to determine a possible cause for the complex multi-beam effect of the primary beams. In an example, the control unit determines a plurality of possible causes for the complex multi-beam effect of the primary beams, sorted according to their occurrence probability. In an example, during the determination of the possible cause for the complex multi-beam effect, a machine learning algorithm is applied. The machine learning algorithm can be trained by a growing set of frequently occurring complex multi-beam effect with frequently occurring causes, for example a vicinity to a previous inspection position or a proximity to an edge of a wafer.
The control unit is further configured to determine measures for adjusting the illumination path and the detection path of the multi-beam system according to the most likely cause of the complex multi-beam effect at an inspection position. A plurality of control parameters are determined within the scope of this determination, the parameters being used to drive or set components within the illumination path and within the detection path of the multi-beam system. These parameters may also be changes in relation to already set parameter values at certain components of the illumination path or detection path of the multi-beam system. Possible components include a quasi-static deflector for the plurality of beams of charged particles, a dynamic deflector for the scanning deflection of the plurality of beams of charged particles, electrostatic or magnetic lenses with changeable focusing effects for the plurality of beams of charged particles, multi-pole elements and energy filters for the plurality of beams of charged particles, or else array components, by which each individual beam of the plurality of beams of charged particles can be influenced. In an example, the adjustment mechanism can comprise for example the parameters for setting a homogeneous extraction field between the object surface and an objective lens system of the multi-beam system.
In an embodiment, the control unit is connected to a unit for image evaluation and configured to provide a correction signal to the unit for image evaluation for the purposes of correcting at least a part of the complex multi-beam effect, for example by image processing. The unit for image evaluation is connected to a detection unit of the multi-beam system and is configured to carry out a correction of the image information of the detection unit using the correction signal. By way of example, a distortion known with great accuracy, a perspective distortion or a magnification aberration of the raster arrangement of the plurality of primary beams can be compensated by digital image processing in a downstream image evaluation. By way of example, a positional deviation of individual primary beams can be taken into account when putting together (“stitching”) the individual images.
The multi-beam system comprises a displacement stage with an object holder for the semiconductor wafer, the object holder being suitable for receiving and positioning the wafer below the objective lens of the multi-beam system. To this end, the object holder comprises a reception area or wafer chuck for receiving the substantially planar wafer with a thickness T and an external diameter D. The reception area for the wafer comprises electrical contacts to the control unit in order to apply a voltage difference between an electrode system of the multi-beam system and the wafer. The electrode system is situated below the objective lens or is part of the objective lens and comprises electrical contacts to the control unit. The control unit is configured to supply suitable voltages to the electrode system and the wafer surface during operation, in order to build up, between the wafer surface and the electrode system, an electric field profile of an extraction field perpendicular to the wafer surface with equipotential lines parallel to the wafer surface during operation. This field is referred to as the extraction field.
For an inspection task without complex multi-beam effects, it can be desirable, for example, for the extraction field to have a homogeneous form and to form a constant predefined electric field strength on the wafer surface over an inspection position. Attempts are therefore made to generate by way of the voltage difference an extraction field that is as homogeneous as possible. However, inhomogeneities of the extraction field occur, in particular in the vicinity of an edge of a wafer. This complex multi-beam effect is also referred to as edge effect or boundary effect. In a second embodiment, the object holder further comprises a ring-shaped correction electrode with a height DE above the reception area, the correction electrode being arranged in the periphery of the reception area and having an internal diameter DI>D such that when the wafer is received, a constant distance G is formed in each direction between the edge of the wafer and the ring-shaped electrode. The ring-shaped electrode is insulated with respect to the reception area and electrically connected to the control unit such that a voltage difference relative to the voltage of a wafer arranged on the reception area can be applied to the ring-shaped electrode during operation. The control unit of the multi-beam system is configured during operation to supply a first voltage to the reception area and the wafer arranged thereon and supply a second voltage to the ring-shaped electrode for reducing an edge effect, for the purposes of generating the homogeneous extraction field.
In a third embodiment, the aforementioned electrode system is formed by a plurality of for example two, four, eight or more electrodes which are insulated from one another and are each electrically connected to the control unit. The control unit is configured during operation to supply different voltages to the plurality of electrodes in order to generate a homogeneous extraction field at an inspection position during operation. Since measurements are only carried out at one inspection position at any one time, it is advantageous to vary the voltage of at least one segment of the ring electrode dependent on the inspection position.
Many effects related to imaging with a multi-beam microscope are linked very strongly to the topological conditions. By way of example, the edge of a wafer has a significant influence during the wafer inspection. Since the relative position of the inspection positions with respect to the edge of the wafer is known in advance in the context of the wafer inspection in particular, the improved adjustment of both the detection path and the illumination path dependent on a distance of an inspection position from an edge of the wafer can already be implemented when homing in on the inspection site. In the fourth embodiment, a multi-beam system and a method for operating the multi-beam system are provided, within the scope of which parameters of components of the illumination path and of the detection path of the multi-beam system are set dependent on a distance of an inspection position from an edge or boundary of the wafer. To this end, the multi-beam system comprises a control unit which determines the distance of an inspection position from an edge or boundary of the wafer. The control unit determines a complex multi-beam effect from the distance and the current working point of the multi-beam system. Further, the control unit is configured during the operation of the multi-beam system at the inspection position to drive components of the illumination path and of the detection path of the multi-beam system with parameters which are suitable for reducing or completely avoiding the complex multi-beam effect. One embodiment of the method contains the acquisition and storage of parameters of an improved adjustment of the detection path and illumination path for different inspection sites dependent on a distance from an edge of the wafer. Then, the optimal parameters of an improved adjustment of both the detection path and the illumination path are determined and set dependent on a next inspection site from the predetermined and stored parameters during a wafer inspection.
It was found that a number of further complex multi-beam effects depend on the inspection position on the surface of a wafer and are therefore, in principle, known in advance. In the fifth embodiment, a multi-beam system and a method for operating the multi-beam system are provided, within the scope of which parameters of components of the illumination path and of the detection path of the multi-beam system are set dependent on a priori information. In one example, the multi-beam system comprises a control unit to this end, which control unit determines the composition of the object, at least at an inspection position, prior to measurement or inspection at the inspection position. In this case, the determination of the composition of the object comprises a determination of the material composition of the object, for example from CAD information about the semiconductor structures formed in a wafer at the inspection position. On the basis of the composition there is a determination of expected complex multi-beam effects and a setting of parameters of the multi-beam system which are suitable for reducing or completely avoiding the complex multi-beam effect. In an alternative example, the a priori information consists of information from earlier inspections at similar inspection sites, for example on other wafers. In an example, the method contains storing a dynamic correction dependent on an identical inspection position during the sequential inspection of a plurality of wafers.
It was found that a number of further complex multi-beam effects depend on adjacent inspection position on the surface of a wafer and are therefore, in principle, known in advance. In the sixth embodiment, a multi-beam system and a method for operating the multi-beam system are provided, within the scope of which parameters of components of the illumination path and of the detection path of the multi-beam system are set dependent on adjacent inspection positions. In one example, the multi-beam system comprises a control unit to this end, which control unit determines for example a current charge distribution of the object at an inspection position caused by preceding inspections on the same object, prior to measurement or inspection at the inspection position. On the basis of the current charge distribution there is a determination of expected complex multi-beam effects and a setting of parameters of the multi-beam system which are suitable for reducing or completely avoiding the complex multi-beam effect. A special example is formed by a method of repeatedly homing in on the same inspection position on the same wafer.
The methods and the multi-beam microscope designed for the application of the methods according to the embodiments facilitate an improved adjustment of both the detection path and the illumination path for a specific inspection position on a surface of an object. According to the seventh embodiments, a method is based on the basis of acquiring and evaluating two fundamentally different information items about the multi-beam microscope and the interaction with an object. Firstly, the raster arrangement of the plurality of secondary beams is detected and evaluated. Secondly, the shape and size of the at least one focal point of the secondary beams is detected and evaluated. It is also possible to evaluate shapes and sizes of a plurality of focal points, for example of at least three focal points, of the secondary beams.
Both information items are acquired during scanning imaging of a portion of a surface of an object. In this case, a plurality of J focal points of J primary beams are moved in scanning fashion over the surface of the object and a plurality of J scan positions on the object surface are illuminated at the same time. To this end, a first deflection unit for scanning deflection of the plurality of J primary beams is situated in the primary path or illumination path. Each incidence location of the J focal points of the J primary beams forms a source location for secondary electrons, which are collected and imaged on a detector, during the short period of time of scanning irradiation using the J primary beams. The plurality of the J source locations of the secondary electrons move synchronously over the object surface in accordance with the scanning irradiation using J primary beams. Therefore, a second deflection unit for scanning deflection of the J secondary beams that emanate from the J source locations is situated in the imaging path of the secondary electrons, which is also referred to as detection path or secondary path, so that the focal point of the J secondary beams on the detector remain at J same detection locations. In this case, the second deflection unit in the secondary path is synchronized with the first deflection unit in the primary path.
A plurality of J time-sequential data streams which are converted into a plurality of J two-dimensional digital image information items are acquired as a result of the scanning illumination with the plurality of J primary beams and the acquisition of the signals of the plurality of J secondary beams synchronously with the scanning illumination. Each image information item represents the spatially resolved rate of generation of secondary electrons by the spatially resolved illumination of the object surface by a focal point of a primary beam. In this case, the rate of the generation of secondary electrons depends on the local surface condition, for example the local material composition of a structured wafer surface. The information about the shape and size of the focal points themselves and the raster arrangement of the focal points for adjusting both the detection path and the illumination path are acquired in time-averaged fashion such that the influence of the structuring on the object surface is reduced by averaging over a plurality of scan positions on the surface. However, the measurement can also be implemented on a completely unstructured wafer or an unstructured test object. Consequently, the method is rendered possible for a plurality of objects, and there is no need for special measurement or calibration objects. In particular, the method of adjusting both the detection path and the illumination path may also be implemented during an inspection task at an inspection position on an object surface.
By way of example, this therefore also allows the method of adjusting both the detection path and the illumination path to be used for a fast autofocus. In general, this allows the method of adjusting both the detection path and the illumination path to be used for a dynamic correction. In respect of a dynamic correction, reference is made to the PCT patent application PCT/EP2021/061216, filed on 29 Apr. 2021, which as a result is included in full in the disclosure by reference.
Determining the parameters of an improved adjustment of both the detection path and the illumination path dependent on an inspection site is implemented iteratively in an example. Initially, an image is recorded without corrections or changes in the parameters. The deviation of the raster arrangement of the plurality of secondary beams from a predefined or expected raster arrangement is detected and evaluated, and the deviation of the shape and size of at least one focal point from a predefined or expected shape and size of the focal point is detected and evaluated at the same time. As described above, the deviations are detected within the scope of time averaging during the scanning of the object surface with a plurality of primary beams in order to eliminate influences of the composition of the wafer. A probable cause for the deviation is determined from the deviations, and suitable parameters are determined for the adjustment of the illumination path and detection path. The detection of the various deviations, specifically the deviation of the raster arrangement of the plurality of secondary beams and of the shape and size of the focal points, allows more targeted conclusions to be drawn about the cause, for example whether there is a disruption or error in the illumination path and a deviation of the plurality of focal points of the primary beams is already present on the wafer surface or whether an edge or topography of the wafer is a cause for the deviation, whether global or local charging effects are present, or whether there is a disruption in the detection path.
A compensation of the complex multi-beam effect is determined in accordance with a model following the analysis of the raster arrangement and the complex multi-beam effect. In general, the success of the compensation can be verified at a sample site, for example of a reference sample, and a fine correction can be carried out. The model for calculating the compensation can be improved by the fine correction.
Further information, for example from additional detectors or a priori information, can be used to carry out the determination of the most probable cause for the deviations with even greater accuracy. Further detectors may comprise a distance sensor for determining the distance of the sample surface from a reference area. The use of such a distance sensor for example allows a better distinction to be made between global charging of the object and purely mechanical defocusing. Further examples comprise field sensors for measuring an electric or magnetic field strength in the vicinity of the object surface. A priori information may comprise CAD information about the inspection position, or stored information from earlier measurements on similar objects or at similar inspection sites. By way of example, possible inhomogeneous or local charging effects of the object may be determined from CAD information. By way of example, regions of a wafer may be conductively connected and scatter charging effects beyond an inspection site. By way of example, regions of a wafer may comprise capacitances which store charging effects over a relatively long period of time.
In general, the position of the inspection site in relation to the sample edge is also an information known in advance. Consequently, it is possible to take account of distortions of the raster arrangement by way of edge effects and distortions as a result of inhomogeneous charging. Preceding measurements form further a priori information. By way of example, charges may arise due to preceding measurements and may only dissipate slowly by way of leakage currents. The charging of already scanned, adjacent inspection sites lead to distortions in the raster arrangement and this a priori information may be considered when determining the cause for the deviations.
In principle, one may already compensate a compensation of a complex multi-beam effect in advance, for example when homing in on an inspection position. Once the probable cause of the deviations has been determined, it is possible to implement correction measures or an adjustment of the detection path and illumination path. Then, the determination of the deviations is repeated. Should the deviations lie within a predetermined tolerance range, the portion of the object surface at the inspection site is measured or imaged in a next step. Should the deviation still exceed a predetermined tolerance limit, determining the cause and determining new parameters for adjusting the detection path and illumination path is repeated. By way of example, a fine correction is determined and carried out in a second step.
The causes of the deviations of the raster arrangement and of the shape and size of beam focal points may be subject to a dynamic change. By way of example, global charging of a sample increases when the illumination by the plurality of primary beams increases and may lead to an increasing deviation of the raster arrangement while imaging. Such dynamic effects are determined in an eighth embodiment of the disclosure and for example the speed of a change or deviation of the raster arrangement and of the shape and size of beam focal points is taken into account. This allows the deviations of the raster arrangement and of the shape and size of beam focal points to be corrected dynamically and allows the parameters for adjusting the detection path and illumination path to be dynamically altered in predetermined fashion during the capture of an image portion of the object surface.
In a ninth embodiment a multi-beam system for inspecting wafers contains a first and a second electron detector and a beam deflector for deflecting the secondary electron beams from the first to the second electron detector. The first electron detector can detect the plurality of J secondary electron beams during an inspection task, the object contrast of a wafer with a high data rate and with little noise. The second electron detector can detect the raster arrangement and the shape or size of the focal points of the plurality of J secondary electron beams with a high spatial resolution, time-averaging of the signal over a plurality of scanning points on the surface of the wafer being implemented at the same time in order to suppress an object contrast. As a result, a complex multi-beam aberration can be determined very quickly during an inspection task and optimal parameters for a complex multi-beam system can be set. From the analysis and evaluation of the raster arrangement and the shape and size of the focal points of the secondary electrons, it is possible to deduce properties of the illumination system, the detection system or the inspection position on the wafer.
In general, a multi-beam system according to the disclosure can be configured in such a way that it is configured to quickly carry out an inspection task in relation to a surface of a wafer in a first setting, and configured to detect a complex multi-beam aberration in a second setting. In this case, a complex multi-beam aberration is given by a deviation of the raster arrangement of the plurality of particle beams and the deviation of the shape and size of at least one focal point of a particle beam. In an example, the complex multi-beam aberration is given by a deviation of the raster arrangement of the plurality of particle beams and the deviation of the shape and size of the focal points of at least three secondary beams on a detector. In a second setting, the complex multi-beam aberration is detected by way of time averaging over a plurality of raster points on the wafer surface, as a result of which an object contrast is averaged. Consequently, it is possible to quickly switch between an inspection task and the detection of the complex multi-beam aberration, and a high throughput is obtained. In some embodiments the multi-beam system according to the disclosure is configured such that a complex multi-beam aberration is detected while an inspection task in relation to a surface of a wafer is carried out quickly. The complex multi-beam aberration is detected by way of time averaging over a plurality of raster points on the wafer surface, as a result of which an object contrast is averaged. Consequently, an inspection task and the detection of the complex multi-beam aberration can be implemented simultaneously, and a high throughput is obtained.
A multi-beam system according to the disclosure has available a plurality of primary particle beams and a plurality of secondary particle beams, and comprises a spatially resolving detector, at least one deflection system for deflecting the plurality of primary and secondary particle beams for the purposes of collective scanning of a portion of a structured surface of a wafer, and a control device for driving the detector and the deflection system, the control device and the detector being designed to capture a time-averaged inspection image of a raster arrangement of the plurality of secondary particle beams and/or capture a digital image of the portion of the structured surface with a spatial resolution of 2 nm, 1 nm or less. The control device is configured, in a first mode of operation for capturing the time-averaged inspection image of the raster arrangement, to quickly scan a plurality of the primary particle beams over the portion of the structured surface of the wafer in a time T1 using the deflection system and, in a second mode of operation for recording the digital image of the portion of the structured surface, to slowly scan the plurality of primary particle beams over the portion of the structured surface of the wafer in a time T2 using the deflection system, where T1<T2, such as T1<T2/10, for example T1<T2/100. The detector can contain a first detector and a second detector and the multi-beam system can comprise a detection unit having a beam deflector which is driven by the control unit and configured to deflect the plurality of secondary particle beams either onto the first detector or on the second detector during operation. The beam deflector can additionally be configured to keep the plurality of secondary particle beams at a constant position either on the first detector or on the second detector during operation. In an alternative example, the detector can be designed for the simultaneous capture of the time-averaged inspection image of the raster arrangement of the plurality of secondary particle beams and of the digital image of the portion of the structured surface with a high spatial resolution with a pixel dimension of 2 nm, 1 nm or less. To this end, the detector may contain an electron conversion element which generates photons from electrons, and photons are detected simultaneously using a first, fast light detector for capturing a portion of the wafer surface and a second, slow light detector for capturing the inspection image of the raster arrangement.
In an example, the control device is further configured to determine a complex multi-beam effect consisting in a change in the incidence locations of the plurality of particle beams and a change in the shape and size of the focal points of the particle beams from the inspection image of the raster arrangement, and to derive and set changes in the setting parameters of the multi-beam system on the basis of the complex multi-beam effect. In an example, the control device is connected to a plurality of components of an illumination path and of a detection path, including components for setting a homogeneous extraction field, and is configured to suitably adjust parameters of the components of the illumination path and of the detection path, including components for setting a homogeneous extraction field, for the purposes of reducing the complex multi-beam effect.
A multi-beam system according to an embodiment contains the following components which are connected to the control device for driving purposes:
In an example, the control unit of the multi-beam system further comprises a unit for image evaluation. Then, the control unit is configured to drive the unit for image evaluation with a correction signal for the purposes of correcting at least one part of the complex multi-beam effect.
In an embodiment, the wafer inspection multi-beam system comprises a displacement stage for receiving a wafer, a spatially resolving detector, a first deflection system for deflecting the plurality of primary particle beams for the purposes of collective scanning of a portion of a structured surface of the wafer, and a second deflection system for deflecting the plurality of secondary particle beams in order to keep the focal points of the secondary particle beams on the detector constant. Further, the multi-beam system contains a control device, the control device being configured to acquire a list of inspection tasks at a plurality of inspection positions and to work through the list, and the control device further being configured to set the setting parameters of components of the illumination path and of the detection path, including components for setting the homogeneous extraction field, for the purposes of reducing a complex multi-beam effect at an inspection position. To this end, the control unit is configured to detect the distance of an inspection position from an edge of the wafer and to compensate a complex multi-beam effect caused by the wafer edge. The control unit can be further configured to determine the composition of the wafer at an inspection position from CAD data prior to a measurement or inspection at the inspection position, and to compensate a complex multi-beam effect caused by the composition. To this end, the control unit comprises a memory and can determine stored parameters from stored inspection tasks at similar inspection sites and can set these for the purposes of reducing a complex multi-beam effect at an inspection position. The control unit can determine parameters from preceding inspection tasks at adjacent inspection sites and can set these for the purposes of reducing a complex multi-beam effect at an actual or subsequent inspection position. The control unit may change a scanning program for driving the first and second deflection systems in order to at least partly compensate a complex multi-beam effect, the control unit further being configured to change a scanning program for driving the first and second deflection systems in order to at least partly compensate a complex multi-beam effect.
A method of wafer inspection using a multi-beam system includes homing in on an inspection position on a wafer and, on the basis of the inspection position, determining the setting parameters of the multi-beam microscope, determined in advance, for optimal imaging at the inspection position. The determined setting parameters are set and an image is taken of a portion of the surface of the wafer at the inspection position. The setting parameters of the multi-beam system may be determined from predefined setting parameters assigned to the inspection position or the setting parameters for optimal imaging at the inspection position can be determined from at least two setting parameters assigned two adjacent inspection positions. Additionally or alternatively, optimized setting parameters may be determined from a priori information about the inspection position. A priori information may contain the distance of the inspection position from an edge of the wafer or from preceding image recordings at preceding inspection positions, or CAD information about the material composition at the surface of the wafer at the inspection position. The setting parameters comprise voltage values for generating a homogeneous extraction field at the surface of the wafer, for example the voltage values being supplied to electrodes.
The disclosure is not restricted to the specific embodiments, but variations of the embodiments are also possible. Although in principle reference is made to a wafer as an object, the disclosure is also applicable to other objects as used in semiconductor manufacturing. By way of example, the object can also be a mask, for example a mask for EUV lithography, rather than a semiconductor wafer. In contrast to semiconductor wafers, such masks are generally rectangular and have a significantly greater thickness. By way of example, in this case the electrode around the object reception area does not have a ring-shaped embodiment but a rectangular embodiment. The disclosure is further described on the basis of a multi-beam system having a plurality of primary electron beams, but other charged particles, for example helium ions, may also be used.
The described embodiments of the disclosure can be combined with one another in full or in part, provided that no technical contradictions arise as a result. It is self-evident that a person skilled in the art considers obvious variations of the exemplary embodiments to be possible and not excluded in the description.
The disclosure will be understood even better with reference to the accompanying figures, in which:
Below, the same reference signs denote the same features, even if these are not explicitly mentioned in the text.
The plurality of beam focal points 5 of the primary beams form a regular raster arrangement of incidence locations, which are formed in the first plane 101. The number J of incidence locations may be five, twenty-five, or more. In practice, the number of beams J, and hence the number of incidence locations 5, can be chosen to be significantly greater, such as, for example, J=10×10, J=20×30 or J=100×100. Exemplary values of the pitch Pi between the incidence locations are 1 micrometre, 10 micrometres and 40 micrometres or more.
A diameter of the minimal beam spots or focal points 5 shaped in the first plane 101 can be small. Exemplary values of this diameter are below 4 nanometres, for example 3 nm or less. The focusing of the particle beams 3 for shaping the beam spots 5 is carried out by the objective lens system 102. In this case, the objective lens system 102 can comprise a magnetic immersion lens, for example. Further examples of focusing mechanisms are described in the German patent DE 102020125534 B3, filed on 30 Sep. 2020, the entire content of which is herewith incorporated in the disclosure.
The primary particles 3 striking the object 7 generate interaction products, e.g. secondary electrons, back-scattered electrons or primary particles that have experienced a reversal of movement for other reasons, which emanate from the surface of the object 7. The interaction products emanating from the surface 25 of the object 7 are shaped by the objective lens 102 to form secondary particle beams 9. The particle beam system 1 provides a detection beam path 11 for guiding the plurality of secondary particle beams 9 to a detection system 200. The detector system 200 comprises a particle optical unit with at least one projection objective 205 for directing the secondary particle beams 9 towards a spatially resolving particle detector 207. In this case, the imaging with the detection system is strongly magnifying such that both the raster pitch of the primary beams on the wafer surface and the size and shape of focal points of the primary beams are imaged in much magnified fashion. By way of example, a magnification is between 100× and 300× such that one nm on the wafer surface is imaged enlarged to between 100 nm and 300 nm. In the process, an image field of a multi-beam system with for example 100 μm diameter is enlarged to approximately 30 mm. In the case of a sufficient signal strength, a small change in the centroid of a focal point of a particle beam on the detector 207 can be determined with great precision. In the case of multi-beam systems with for example F beams along one direction, a scale error becomes visible additionally magnified by the factor F, in accordance with the larger image field. Consequently, a complex multi-beam effect of the particle beams can be determined with great accuracy, in particular a deviation from a specified raster arrangement, for example at opposing focal points which are situated furthest away from the optical axis 105.
The primary particle beams 3 are generated in a beam generation apparatus 300 comprising at least one particle source 301 (e.g. an electron source), at least one collimation lens 303.1 and 303.2, a multi-aperture arrangement 305 and a field lens 307, or a field lens system made of a plurality of field lenses. The particle source 301 generates at least one diverging particle beam 309, which is collimated or at least substantially collimated by the at least one collimation lens 303, and which illuminates the multi-aperture arrangement 305. The multi-aperture arrangement 305 comprises least one multi-aperture plate 306.1, which has a plurality of J openings formed therein in the raster arrangement. Particles of the illuminating particle beam pass through the J apertures or openings and form the plurality J of primary beams 3. Particles of the illuminating beam which strike the plate 306.1 are absorbed by the latter and do not contribute to the formation of the primary beams 3. The multi-aperture arrangement usually has at least a further multi-aperture plate 306.2, for example a lens array, a stigmator array or an array of deflection elements.
Together with the field lens 307 and a second field lens 308, the multi-aperture arrangement 305 focuses each of the primary beams 3 in such a way that beam focal points 311 are formed in an intermediate image plane 321. Alternatively, the beam foci 311 can be virtual. A diameter of the beam foci 311 can be, for example, 10 nanometres, 100 nanometres and 1 micrometre. A further multi-aperture plate 390, for example in the form of a deflector array, may be arranged in the intermediate image plane 321.
The field lens 103.1 and 103.2 and the objective lens 102 provide a first imaging particle optical unit for imaging the plane 321, in which the beam foci 311 are formed, onto the first plane 101 such that the raster arrangement of incidence locations or focal points 5 arises there. Should a surface 25 of the object 7 be arranged in the first plane 101, the focal points 5 are correspondingly formed on the object surface 25 (see also
The objective lens 102 and the projection lens arrangement 205 of the projection system 200 provide a second imaging particle optical unit for imaging the first plane 101 onto the detection plane. The objective lens 102 is thus a lens or a lens system that is part of both the first and the second particle optical unit, while the field lenses 103, 307 and 308 belong only to the first particle optical unit, or the illumination path 13, and the projection lens 205 belongs only to the second particle optical unit, or the detection path 11.
A beam divider 400 is arranged in the beam path of the first particle optical unit between the field lens 103 and the objective lens system 102. The beam divider 400 is also part of the second optical unit in the beam path between the objective lens system 102 and the projection objective 205. The beam divider 400 additionally has a correction element 420 available for the purposes of compensating aberrations of the beam divider 400, at least in the illumination beam path 13.
The detection system 200 comprises a plurality of further components, for example the electrostatic lens 206 and a plurality of further magnetic lenses 208, 209. Together with the projection lens 210, the lenses serve to focus the secondary beams on the spatially resolving detector and, in the process, compensate the imaging scale and the twist of the plurality of beams as a result of a magnetic lens such that the raster arrangement of the focal points 15 of the plurality of secondary beams 9 on the detector plane 207 remains constant. In this case, the first and second magnetic lenses 208 and 209 are designed reversed to one another and have oppositely directed magnetic fields. A Larmor rotation of the secondary electron beams can be compensated by suitably driving the magnetic lenses 208 and 209. Additionally, a further crossover point 212 of the secondary beams, at which an aperture stop 214 is arranged, is arranged in the projection objective 205. Moreover, the detection system 200 has available a second, collective beam deflector 222 which is located in the vicinity of a crossover point of the secondary beams 9, which is operated synchronously with the first beam deflector 110 and which compensates a beam deflection of the primary beams 3 such that the focal points 15 of the secondary beams 9 remain at constant position on the detection plane 207. The detection system 200 has further correction elements available, for example a multi-aperture plate 216 and a further, third deflection system 218.
Further information relating to such multi-beam particle beam systems and components used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005/024881, WO 2007/028595, WO 2007/028596, WO 2011/124352 and WO 2007/060017 and the German patent applications having the publication numbers DE 10 2013 016 113 A1 and DE 10 2013 014 976 A1, the disclosure of which in the full scope thereof is incorporated by reference in the present application.
The multiple particle beam system furthermore comprises a computer system or control system 800 configured both for controlling the individual particle optical components of the multiple particle beam system and for evaluating and analysing the signals obtained by the multi-detector 207. In this case, the control or controller system 800 can be constructed from a plurality of individual computers or components. By way of example, the control unit 800 has available a first control module 820 for the detection system 200 and a second control unit 830 for the illumination system 100.
Further, the control unit 800 has available a control module 503 for supplying a voltage to the sample 7, the voltage also being referred to as sample voltage below. During use, a field 113 is generated between the objective 102 and the surface 25 of the object 7, for example of the wafer. During use, field 113 decelerates the primary particles of the primary beams 3 before the sample surface 25 is reached and generates an additional focusing effect on the plurality of primary beams 3. At the same time, this field 113 serves during use to accelerate the secondary particles out of the surface 25 of the object 7. Therefore, the field 113 is also referred to as an extraction field 113 but express reference is made at this point to the fact that the extraction field 113 has two effects: firstly, decelerating and focusing the primary beams 3 and, secondly, aligning and accelerating the secondary electron beams 9. The extraction field 113, or the strength and homogeneity of the extraction field 113, therefore has a great influence on the raster arrangement of the primary particle beams 3, and on the shape and size of the focal points of the primary particle beams 3, and additionally also on the yield of the secondary particles and the shape and direction of the secondary particles 9. In the ideal case, the secondary particles are extracted at right angles, or perpendicular to the object surface 25. Inhomogeneous extraction fields 113 may lead to directional deviations of the secondary beams 9, for example, which together with further aberrations lead to a deviation of the raster arrangement of the secondary beams 9 in the detection path 13 or to a change in the size and shape of the focal points 15 of the secondary beams 9 on the detector 207. Inhomogeneous extraction fields 113 already lead to a deflection of the primary particles at an earlier stage, and hence to a deviation of the raster arrangement of the illumination beams 3 and to a change in shape and size of the focal points 5 of the primary beams 3 on the object surface 25. Consequently, the inhomogeneities of the extraction field 113 superimpose and amplify one another in twofold fashion. Global variations of the extraction field 113 may arise due to global effects, for example a tilt or a z-offset of the object 7, or uniform charging of the object 7. Local variations in the electric extraction field 113 may arise due to local effects, for example an object edge or edge of the wafer, general height differences of the object 7 caused by an object topography, or local charging. Variations in the electric extraction field 113 may be static or time varying. By way of example, static variations arise from an unchanging topography or edge of an object 7. Time-varying variations arise from time-varying charging effects. In particular, the raster arrangement reacts very sensitively to variations in the electric extraction field 113 at the object 7 or between the object surface 25 and an electrode within the objective 102 of the multi-beam microscope 1.
In addition to the spatially resolving detector 207, the detection system 200 of the multi-beam system 1 further comprises slow compensators 230 of the detection system 200 and fast compensators 232 of the detection system 200. By way of example, the magnetic lenses 208 and 209 and the magnetic lens 210 are slow compensators 230. By way of example, the beam deflector 214 or the electrostatic lens 206 are fast compensators 232. Further, the detection system 200 comprises the second, fast beam deflector 222 for fast, collective beam deflection of the secondary beams 9. The second beam deflector 222 is driven synchronously with the first beam deflector 110 by the scanning module 860. The secondary beams 9 pass through both the first beam deflector 110 and the second beam deflector 222. The second beam deflector 222 is designed to carry out the so-called anti-scan, which compensates the otherwise arising scanning movement of the secondary beams 9 upon incidence on the detection unit 207. The detection system 200 further has available further sensors 238 which are arranged in the periphery of an aperture stop 214, for example.
The semiconductor wafer 7 is positioned by a displacement stage 500 below the objective lens 102. The displacement stage 500 can be a 6-axis displacement stage which can position the surface 25 of the sample 7 in the object plane or first plane 101 in 6 degrees of freedom. In this case, the position accuracy in the z-direction is under 50 nm, for example better than 30 nm. The position of the displacement stage 500 is monitored and controlled by sensors 520 of the control unit 880 in this case. The sample voltage for a homogeneous extraction field is controlled by way of the control module 820 of the detection unit 820, together with the slow and fast compensators 230, 232 of the detection module 200. It is additionally possible to drive at least one further correction electrode, which is arranged in the periphery of the wafer 7 in accordance with the second exemplary embodiment.
The detection unit 207 may comprise at least one scintillator for converting the secondary electrons into light, and a plurality of light-optical detectors. Such detectors can be CMOS or CCD sensors or else be formed by a plurality of photodiodes, for example avalanche photodiodes. The sensors may be arranged directly behind the scintillator or an optical imaging system or light guides may be arranged between the scintillator and the sensors. It is also possible to use sensors that detect electrons directly and convert these into electrical signals. A special form of the detection unit is described in the German patent with the number DE 102018124044 B3, the disclosure of which is incorporated into the present application in full by reference. Therein, the detection unit 207 consists of a scintillator on which the plurality of focal points 15 of the secondary particle beams are formed. The light generated is imaged onto a fibre bundle by way of an imaging system, with each fibre being coupled to a photodiode. The imaging system further contains a beam divider which steers some of the generated light onto a CMOS sensor. Using this sensor, it is possible to monitor the raster arrangement and the shape and size of the individual focal points 15. Instead of output coupling some of the light using the beam divider, it is alternatively also possible to capture and evaluate the light emitted by the scintillator in the backward direction, that is to say the light emitted in the direction of the incident particle beams, by way of a CMOS camera.
The detection unit 207 is connected to an image data converter 280 which converts the temporal sequence of analogue electrical signals, for example voltages of the sensors, into a temporal sequence of digital signals. By way of example, the image data converter 280 of the plurality of J secondary beams has a parallel computer architecture available. In this case, the image data converter 280 contains a plurality of analogue-to-digital converters connected in parallel, which can be designed as ASICs connected in parallel, for example. The scanning frequency FS of the two deflection systems 110 and 222 approximately corresponds to the inverse of the dwell time of a primary beam 3 on a focal point 5 on the sample surface 25. This dwell time is typically 50 ns. However, dwell times of 10 ns, 20 ns or 100 ns are also possible. While an image is recorded, the read-out frequency or the frequency FC of the data conversion using the image data converter 280 corresponds to the scanning frequency FS, and so digital image data for a plurality of image pixels are acquired for a plurality of focal points with FS=FC. Usual clock rates of the deflection systems and the analogue-to-digital conversion using the image data converter 280 are between FS=FC=10 MHz to 100 MHz in this case, but higher clock rates of more than 100 MHz are also possible.
The control unit 800 has available a control module 830 for controlling the illumination device 100, a control module 880 for controlling the displacement stage 500, and a control module 820 for controlling the detection unit 200. A data acquisition device 810 is connected firstly to the image data converter 280 and secondly to an image data memory 814. Additionally, a digital image processing unit 812 is arranged between image data memory 814 and data acquisition device 810. A sensor data module 818 receives time-averaged data of the raster arrangement of the secondary beams 9 and other sensor signals, for example from the further sensors 238 of the detection module 200 or from the control module of the displacement stage 500 with the position sensors 520. The control unit 800 further has available a control processor 840, which evaluates the sensor data of the sensor data module 818 and determines corresponding control signals, for example parameters for setting components of the illumination system 100 and of the detection system 200.
In a multi-beam particle beam system 1 as per
A method of wafer inspection is described with reference to
The plurality J of primary beams 3 are then deflected together by the scan deflector 110 over in each case small sub-fields 31.11 to 31.MN and, in the process, each beam scans a different sub-field, for example sub-field 31.mm or sub-field 31.m(n+1). Exemplary scanning patterns or scanning paths 27.11 and 27.MN are schematically illustrated in the first sub-field 31.11 and in the last sub-field 31.MN. Further, in exemplary fashion, the focal points 5.11, . . . , 5.MN of the respective different primary beams are illustrated in each case at the upper left corner of an assigned sub-field. Further, sub-fields 31 each have a centre; the centre 29.mn of the sub-field 31.mn is labelled by a cross in exemplary fashion. Here, a plurality of sub-fields 31.11, . . . , 31.MN are in each case scanned in parallel by the plurality of the J primary beams with focal points 5.11 to 5.MN and a digital image data record is acquired for each of the J sub-fields 31.11 to 31.MN, each image data record being able to comprise 8000×8000 pixels, for example. In this case, the pixel size can be defined and be 2 nm×2 nm, for example. However, a different number of pixels between 4000×4000 and more than 10 000×10 000 pixels are also possible, and other pixel sizes of for example 3 nm, 1 nm or less can be set. Once the digital image data of the first image field 17.1 have been acquired, the image data of the individual sub-fields 31.1 to 31.MN of the first image field 17.1 are combined to form an image data record. Subsequently, the second inspection position 34 is positioned under the axis of the objective 102 and the digital image data of the second image field 17.2 are acquired. The procedure is continued, for example with the second inspection position 35 with the image field 17.k. Naturally, the raster arrangement 41 of the primary beams 3 is not restricted to rectangular raster arrangements, other raster arrangements include, for example, hexagonal rasters or an arrangement of the primary beams on concentric rings or one ring. In this case, the lateral resolution of the digital image data is determined substantially by the diameter of the focal points 5 of the primary beams 3 on the object surface 25.
During an inspection task, the raster arrangement 41 is displaced synchronously over an object surface 25, and image data of a surface 25 of a wafer 7 are acquired. The anti-scan using the deflection device 222 ensures that the raster arrangement 41 of the secondary beams 9 remains fixed in position or stationary on the detector 207. However, there may be changes of the raster arrangement 41 in the detection plane of the detector 207 before or during the inspection task, the changes significantly impairing the position and reproduction fidelity of the semiconductor structures on the object surface 25. In this case, disruptive changes of the raster arrangement 41 arise in the primary beam 11 and bring about a change in the raster arrangement 41 of the focal points 5 of the primary beams 3. The changes of the raster arrangement 41 of the primary beams bring about corresponding changes of the raster arrangement 41 of the secondary beams 9, the latter of course arising at the focal points 5 of the primary beams 3 on the object surface 25. The changes of the raster arrangement 41 of the secondary beams 9 may still be amplified in the secondary path and ultimately lead to, for example, the changed raster arrangements 41a to 41g of the focal points 15 of the secondary beams 9 in the plane of the detector 207. The changed raster arrangements of the focal points 15 of the secondary beams 9, together with the changed shapes and sizes of the focal points 15, are also referred to as complex multi-beam effect.
To acquire the raster arrangement 41 of the plurality of J secondary beams 9, the multi-beam system 1 according to the disclosure can be configured to carry out various methods. In a first method there is time-averaging of the signals by the image data converter 280 by temporal integration of the image signals. To this end, the image data converter 280 is operated at a significantly lower data conversion frequency FC<FS in relation to the scanning frequency FS, and so averaging of the image data is implemented over a plurality of focal points on the object surface 25. By way of example, the data conversion frequency FC can be 1/10 of the scanning frequency, FC<FS/10, or even less, for example FC<FS/100 or FC<FS/1000 or even significantly less. In one example, the image data acquisition is implemented with two detectors in parallel, and a first detector is operated for high-resolution imaging using a first image data converter at a first data conversion frequency FC1=FS, which equals the scanning frequency; a second detector is operated using a second image data converter at an image evaluation frequency, and so only a few images or only one image of the raster arrangement is determined during high-resolution imaging of an image portion at an inspection position using the first detector. In an example, the high-resolution image comprises 8000×8000 pixels; with a dwell time of 50 ns or a scanning frequency of FS=20 MHz, this then yields an image recording time T2 of approximately 3.2 s. By way of example, the second detector camera can be a CMOS sensor which for example has a frame rate of 10 to 100 frames per second with an image recording time T1 of 0.1 s to 10 ms, or an image frequency of 1 Hz to approximately 0.1 kHz. Consequently, approximately 30 to 300 inspection images of the raster arrangement can be generated while recording the high-resolution image.
As an alternative to reducing the data conversion frequency FC, it is also possible to increase the scanning frequency. By way of example, a scanning frequency for measuring the raster arrangement 41 can be increased by a factor of 10, from 50 MHz to 500 MHz. By way of example, the scanning frequency FS can be increased to FS=10× FC or FS>100×FC. In the first method there is time averaging of the signals by the sensor data module 818, which evaluates mean values for each secondary beam from spatially resolved digital image data and which detects a change in the raster arrangement. In the second method, the time averaging of the signals is implemented by fast scanning using the scanning deflector 110 and 222.
By way of the magnification of the detection path, the complex multi-beam effect is imaged in enlarged fashion on the detection camera. Additionally, the cause for the complex multi-beam effect (charging, edge, tilt on the wafer, etc.) also affects the secondary electrons. By way of example, the lower energy secondary electrons react more sensitively in this case than the higher energy primary electrons to variations in the extraction field, and for example a further distortion is added in addition to the deviations of the primary beam on account of sample influences.
Other deviations of the raster arrangement are also possible, for example a keystone distortion. Further, the deviations usually occur as a combination or superposition of individual deviations.
The multi-beam system 1 is set with predefined parameters for an inspection task. The control processor 840 is configured to determine the various predefined parameters for an inspection task during operation, and to use this to drive the components of the multi-beam system 1. Components driven by parameters include for example the slow and fast compensators 130, 132 of the illumination system 100, slow and fast compensators 330, 332 of the multi-beam generation device 300, the slow and fast compensators 230, 232 of the detection system 200, or the displacement stage 500. By way of example, the spacing or pitch ps of the individual beam focal points 5 on the surface 25 of the wafer 7 are set by way of these parameters and the focal points are set in an optimal focal plane in the plane 101. Further changeable parameters include the beam intensity, which may be set using the condenser lenses 303 for example. The noise performance can be set by way of the beam intensity and the dwell time. The parameter that determines the strength of the extraction field further influences the resolution and the kinetic energy of the secondary electrons. A certain twist of the raster arrangement on the object surface sets in by way of the focusing effect of the magnetic lens of the objective lens system 102. The scanning program is set using further parameters. Further components of the detection system are driven by parameters such that the plurality of focal points 15 of the secondary beams 9 are incident on the detector 207 at predefined positions and are kept constant there such that the image data can be acquired in a time-sequential sequence. In summary, a set of parameters is also referred to as working point. The control processor 840 of a multi-beam system 1 according to the first embodiment is designed to operate the multi-beam system 1 at a plurality of different, predefined working points.
By way of example, the sensor data module 818 (see
A few disruptive influences or causes for changes in the raster arrangement and a change in the shape and size of a focal point are listed below.
A mechanical defocus, for example as a result of a deviating thickness of a wafer, brings about a change in the magnification or the pitch of the focal points in the raster arrangement and brings about increased spot diameters in accordance with the raster arrangement 41a. Additionally, there can be a changed size of a focal point, as illustrated for example on the basis of the focal point 5.ir in
A local inclination of the sample surface, for example when a wafer bends, leads to a homogeneous gradient of the extraction field and to an offset of the raster arrangement according to the raster arrangements 41b. Additionally, a typical astigmatism, for example a constant astigmatism, may occur over the plurality of beams and may lead to a constant elliptical beam shape, like the beam shape of beam 5.ka in
Effects of an inhomogeneous extraction field 113 at the edge of a wafer lead to a migration or an offset of the raster arrangement 41b in combination with a constant astigmatism. A mechanism for correcting an edge effect is discussed below. A complex multi-beam effect at the edge of the wafer 7 and an effect resulting from tilting may be very similar. The compensation measures may be similar. However, aberrations that vary more significantly over the image field arise at the edge of a wafer, for example there is no constant offset and no uniform astigmatism over the image field but a slight distortion in the positions of the focal points or a more complex field dependency of the astigmatism.
Homogeneous charging of the sample surface 25 likewise causes a magnification change and enlarged spot diameters. At the same time, there is a lateral displacement of the raster arrangement 41b corresponding to
Local charging of a portion of the sample surface at the inspection position (33, 35) according to the image field 17 of the raster arrangement 41 of the primary beams 3 likewise leads to a change in magnification in conjunction with a lateral offset. However, there additionally is a change in the shape of the focal points 5 of the edge beams of the raster arrangement 41. Edge beams are those beams that no longer have an adjacent beam in one direction. The shape changing and size changing effect of the focal points 5 is particularly pronounced in the corners of the raster arrangement 41.
Inspection positions such as the inspection positions 33 and 35 are influenced by latent charging of preceding or adjacent inspection positions or adjacent image fields. This may occur, in particular, if like in the example of
Local charging only distorts individual spot positions or spot shapes, as illustrated in
The control processor 840 is configured to store the predetermined relationship of disruptive influences or causes for a complex multi-beam aberration. A complex multi-beam aberration is understood to mean a change in the raster arrangement 41 and a change in the shape and size of at least one focal point, for example of three focal points or all focal points. Further, the control processor 840 is configured to store predefined parameters for correcting or compensating the disruptive influences. The control processor is further configured to deduce disruptive influences or a cause from a currently determined complex multi-beam aberration. In this case, the control processor resorts to the stored relationship of disruptive influences or causes and the suitably altered parameters for correcting or compensating the disruptive influences or causes, and drives the multi-beam system 1 using the altered parameters.
Hence, the first embodiment of the multi-beam system comprises an improved method for operating the multi-beam system 1 for inspecting objects 7, such as a semiconductor wafer. The method is illustrated in
In this case, the deviation of the raster arrangement 41 comprises at least one of the following errors: a scale error 41a, an offset error 41b, a distortion 41c, a twist 41g, or a local deviation 41d of only individual beams of the raster arrangement 41.
The change in the shape and size of the focal points 15 comprises at least one of the following aberrations: a constant astigmatism, a linear astigmatism with a linear dependency of the astigmatism over the position in the raster arrangement 41, a constant focal aberration, a linear focal aberration with a linear dependency of the focal aberration over the position in the raster arrangement 41.
In a further step, a decomposition of the change of the raster arrangement 41 can be implemented according to global changes and local changes of the raster arrangement.
In this case, the determination is implemented for example via a stored table of parameter changes which are suitable for compensating individual, normalized effects of the complex multi-beam effect. Then, the optimized parameters are calculated for example by multiplying the amplitude of a change in the raster arrangement by the associated stored parameter changes. Specific examples of the first embodiment comprise a determination of a local inclination error of the planar object surface 25 from the combination of an offset error of the raster arrangement 41 and a beam shape deviation in the form of an ellipticity. In a further example, the first embodiment comprises determining a spacing error of the planar object surface 25 from the combination of a scale difference of the raster arrangement 41 and a beam shape deviation in the form of a diameter deviation of the at least one focal point. In a further example, the first embodiment comprises determining a global charging effect of the planar object surface from the combination of a scale error of the raster arrangement 41 and an offset of the raster arrangement 41 in the case of a virtually unchanging beam diameter. In a further example, the first embodiment comprises determining a distance from a topographic structure, for example the edge of the object 43, from an offset of the raster arrangement 41 and a distortion in the direction of the offset or a magnification difference between two non-parallel directions. In a further example, the first embodiment comprises determining a local charging effect from an irregular change in the raster arrangement, consisting of at least two different position deviations of at least two focal points from the predefined raster arrangement. In a further example, the first embodiment comprises determining a local charging effect from an irregular change in the raster arrangement, consisting of at least two deviations of at least two focal points comprising at least one beam shaping deviation and at least one position deviation from the predefined raster arrangement.
The optimized setting parameters contain parameters of components within the illumination path 13 and within the detection path 11 of the multi-beam system 1, and may also contain a realignment of the wafer 7 with the displacement stage 500 at the first inspection position (33, 35). Further, the extraction field 113 is arranged in the illumination path and detection path, and setting the optimized parameters of the multi-beam microscope contains driving correction electrodes for influencing an extraction field 113 at the inspection position (33, 35) on the surface 15 of the wafer 7. Compensators comprise a deflection apparatus 107 for compensating an offset of the raster arrangement 41 on the surface 25 of the wafer 7, and a change in the working point of the multi-beam system 1, for example for setting a scale of the raster arrangement 41, a change in the scanning program for the beam deflector 110, and a change in the digital image evaluation.
Between steps 8 and 9, there may optionally be a switchover from a second operating method for acquiring the complex multi-beam effect to the first mode of operation for a quick and high-resolution image capture of a portion of an object surface in a step STU.
Hence, a method for determining a complex multi-beam effect 41 comprises recording an image of a time-averaged inspection image of the raster arrangement 41 of the multiplicity of particle beams 9 using a detector camera 207 by scanning a portion of a structured surface 25 of a wafer 7 and averaging out an image contrast of the surface structure of the wafer 7, and analysing the inspection image for the purposes of determining at least one deviation of the raster arrangement 41 of the incidence locations of the plurality of particle beams from a predefined or ideal raster arrangement 45 and a change in the shape and size of the focal points 15 of the particle beams.
In an example, averaging out the image contrast is achieved by quickly scanning the portion of the surface 25 of the wafer 7 with an image recording time of T1<T2, such as less than T1<T2/10, for example T1<T/100, where T2 corresponds to the time for recording an image of the portion of the surface 25 with a high spatial resolution and a pixel dimension of 2 nm, 1 nm or less. T1 is typically less than 100 ms, such as less than ms. In an example, the averaging out of the image contrast of the surface structure of the wafer 7 is implemented by averaging the detection signal over time.
It was found that multi-beam effects as a result of an inhomogeneous extraction field occur at the edges of an object in particular. By way of example, electrons are deflected in the direction of the wafer edge.
The objective lens unit comprises a solenoid 149 for forming a focusing magnetic field for the purposes of focusing the primary beams on the wafer surface 25. A voltage V2 ranging from 1 kV to 4 kV, for example 2 kV, is supplied to the wafer 7 or wafer surface by way of the wafer reception area 505 in the displacement stage 500. An extraction field 113a forms between the counter electrode 151 and the wafer surface 25 by way of the voltage difference V2−V1. The extraction field 113 typically has a field strength of 1-5 kV per mm at the wafer surface 25, as a result of which the primary electrons 3 are decelerated. In this case, the wafer reception area 505 is insulated with respect to the wafer stage 500 and the wafer stage 500 is at earth potential or 0 kV. The extraction field 113a is illustrated schematically by way of equipotential surfaces. However, there is a height difference DW at the edge 43 of the wafer 7 and the equipotential surfaces no longer extend parallel to the surface 25 of the wafer 7 in the vicinity of the edge 43 of the wafer, and the primary beams 3a are deflected. As a consequence of the inhomogeneous edge field, the raster arrangement of the primary beams therefore experiences a distortion similar to the distortion depicted in
An embodiment of the disclosure therefore comprises a displacement stage 500 for a multi-beam microscope 1 and a multi-beam microscope 1 having the displacement stage 500 with a reception area 505 for receiving a wafer 7 with an edge 43 and a diameter D, by which a voltage V2 can be applied to the wafer 7 during operation. Further, a ring-shaped electrode 153 is arranged on the displacement stage 500 in the periphery of the reception area 505. The ring-shaped electrode 153 has an internal diameter DI>D such that, when the wafer 7 is received, a distance is formed between the edge 43 of the wafer 7 and the ring-shaped electrode 153. The ring-shaped electrode 153 is insulated from the reception area 505 so that a voltage V3 can be applied to the ring-shaped electrode 153 during operation. In an example, the ring-shaped electrode 153 is formed from a plurality of mutually insulated electrode segments, for example two, four, eight or more, to which at least one first voltage V3 can be applied.
Many effects related to imaging with a multi-beam microscope are linked very strongly to the topological conditions. As illustrated on the basis of
The second and third embodiment of the disclosure thus describe a displacement stage 500 with a reception area 505 for receiving a substantially planar object 7 with an edge 43, a thickness DW and an external diameter D, and a ring-shaped electrode 153 with a height DE over the reception area 505, the latter being arranged in the periphery of the reception area 505 and having an internal diameter DI>D such that, when an object 7 is received, a distance G is formed between the edge 43 and the ring-shaped electrode 153. In this case, the electrode 153 is insulated from the reception area 505 so that a different voltage difference can be applied to the ring-shaped electrode 153 during operation.
The second and third embodiment further describes a multi-beam system comprising the displacement stage 500 and a measuring device for determining an edge effect, and a control unit configured during operation to supply a first voltage V2 to a received object and a second voltage V3 at the ring-shaped electrode for reducing the edge effect, for the purposes of generating the voltage difference.
In the third embodiment, the electrode 153 has available a plurality of segments 153.1 to 153.8, to which a plurality of different voltages V3.1 to V3.8 can be applied. In a further embodiment, the objective lens system 102 of the multi-beam system 1 according to the third embodiment has available a plurality of counter electrodes 151.1 to 151.8, to which a plurality of different voltages V1.1 to V1.8 can be applied. The voltage supply of the electrodes is designed in this way in order to generate, together with the object voltage, a homogeneous extraction field 113 between the objective lens system 102 and the surface of the object.
It was found that a number of complex multi-beam effects depend on the inspection position on the surface 25 of a wafer and are therefore, in principle, known in advance. An improved method, based thereon, for operating a multi-beam system 1 is depicted in
A complex multi-beam effect for a next inspection task is predicted in a next step SE. The prediction of the complex multi-beam effect may be put together from a plurality of components and the complex multi-beam effect may be caused by a plurality of causes. Complex multi-beam effect denotes the effects illustrated in the context of
In step SER, a complex multi-beam effect VKR is predicted from a distance of an inspection position from an edge 43 of a wafer 7. In general, the position of the inspection site in relation to the sample edge 43 is information known in advance. Consequently, it is possible to take account of distortions of the raster arrangement 41 by way of edge effects and distortions as a result of inhomogeneous charging.
An example of the method according to the fourth embodiment contains the detection and storage of parameters for an improved adjustment of the detection path and illumination path and the setting of the voltages for a homogeneous extraction field for different inspection sites on the basis of a distance from an edge of an object. Then, the optimal parameters of an improved adjustment of both the detection path and the illumination path, including the setting of the voltages for a homogeneous extraction field, are determined and set dependent on a next inspection site from the predetermined and stored parameters during a wafer inspection.
In step SED, a complex multi-beam effect VKA is predicted from a priori information about the inspection position, for example from design information, CAD information or preceding measurements.
In the fifth embodiment, a multi-beam system and a method for operating the multi-beam system are provided, within the scope of which parameters of components of the illumination path and of the detection path and voltages for a homogeneous extraction field of the multi-beam system are set dependent on a priori information. In an example, the determination of the composition of the object at least at the next inspection position is implemented in step SED. In this case, the determination of the composition of the object comprises the determination of the material composition of the object, for example from CAD information about the semiconductor structures formed in a wafer at the inspection position. By way of example, possible inhomogeneous or local charging effects of the object 7 may be determined from CAD information. On the basis of the composition there is a determination of expected complex multi-beam effects and a setting of parameters of the multi-beam system which are suitable for reducing or completely avoiding the complex multi-beam effect. In an alternative example, the a priori information consists of information from earlier inspections at similar inspection sites, for example on other wafers.
Preceding measurements form further a priori information. By way of example, charges may arise due to preceding measurements and may only dissipate slowly by way of leakage currents. The charging of already scanned, adjacent inspection sites leads to distortions in the raster arrangement and this information may be considered when determining the cause for the deviations. In step SEH, a complex multi-beam effect VKS is predicted from information from preceding inspection tasks, for example a detection of a current state of the multi-beam system 1, or from expected charging effects of the wafer from preceding measurements at preceding inspection positions. In this case, for example the position and the distance of the next inspection position in relation to the preceding inspection positions can be determined and evaluated. Additionally, the time difference to previous inspection tasks can be evaluated in order to take account of discharging effects from preceding sample charging.
It was found that a number of further complex multi-beam effects depend on adjacent inspection position on the surface 25 of a wafer and are therefore, in principle, known in advance. In the sixth embodiment, a multi-beam system and a method for operating the multi-beam system are provided, within the scope of which parameters of components of the illumination path and of the detection path including the voltages for a homogeneous extraction field 113 of the multi-beam system are set dependent on adjacent inspection positions or preceding inspection tasks. In one example, the multi-beam system comprises a control unit to this end, which control unit determines for example a current charge distribution of the object at an inspection position caused by preceding inspections on the same object, prior to measurement or inspection at the inspection position. In an example, step SEH includes determining a current charge distribution of the object surface at the inspection position caused by preceding inspections on the same object. By way of example, regions of a wafer may be conductively connected and scatter charging effects beyond an inspection site. By way of example, regions of a wafer may comprise capacitances which store charging effects over a relatively long period of time. On the basis of the current charge distribution there is a determination of expected complex multi-beam effects and a setting of parameters of the multi-beam system which are suitable for reducing or completely avoiding the complex multi-beam effect. A special example is formed by a method of repeatedly homing in on the same inspection position on the same wafer.
In step SEC, the predictions VKR, VKA or VKS from distance of the inspection position from the edges, a priori information or information from preceding inspection tasks are combined, and a combined complex multi-beam effect VKK is predicted.
Optimized parameters for driving the multi-beam system 1 are determined in step PE. Proceeding from a working point AP of a multi-beam system 1, the multi-beam system is operated using a certain set of parameters. The parameters of the working point AP for example describe currents or voltages of electromagnetic or electrostatic components, for example for setting a beam current, a beam pitch or a predefined magnification, a scanning program, the size of an extraction field, or a focal position.
In step PEI, the standard parameters according to the working point AP are determined in accordance with the next inspection task at an idealized inspection position.
In step PEC, a change in at least one parameter value of a parameter is determined from the predicted complex multi-beam effect VKK. Examples of parameter changes that are suitable for minimizing a complex multi-beam effect are listed further above in conjunction with the first embodiment of the disclosure. By way of example, the determination is implemented on the basis of previously determined and stored optimal parameter values, from which the changes in the parameter values are determined by interpolation for example.
In step PC, the current parameters PA according to the working point AP and including the parameter changes in step PEC are transmitted in accordance with the next inspection task at the next inspection position, and the multi-beam microscope 1 is driven using the determined parameter values.
In step IN, the next inspection task at the next inspection position is implemented. To this end, the next inspection position of the wafer 7 is centred with respect to the optical axis 105 of the multi-beam system 1 and the surface 25 of the wafer 7 is aligned in the setting plane or focal plane 101 of the multi-beam system 1. The multi-beam system 1 is operated with the current parameter values PA and the inspection task is carried out. By way of example, an image portion of the wafer surface at the inspection position is captured with a high resolution and imaging fidelity of better than 5 nm, better than 2 nm or even better than 1 nm.
Simultaneously with step IN, the raster arrangement of the secondary particle beams and the shape or the size of at least one focal point of a secondary beam path is monitored in step M. Monitoring is implemented by way of acquiring a time-averaged signal of the spatially resolving detector 207 of the multi-particle system 1. As a result of time-averaging during a scanning of the object surface 25, areal averaging of the signal of the secondary particles over a plurality of object structures on the object surface 25 is achieved, and the current raster arrangement of the secondary particle beams and the shape or the size of at least one focal point of a secondary particle beam can be detected reliably with a high accuracy of less than 1 nm. The capture of the current raster arrangement of the secondary particle beams and the shape or the size of at least one focal point of a secondary particle beam can be implemented multiple times during an inspection task, for example ten times or one hundred times.
In step Q, a current complex multi-beam effect is determined from the monitoring result of step M. Should a current complex multi-beam effect during an inspection task exceed a predetermined threshold, there is a signal to step PE to continually change or update the setting parameters of the multi-beam system 1 and step PE is repeated during the inspection task of step IN. By way of example, this therefore also allows the method of driving both the detection path and the illumination path, including the voltages for a homogeneous extraction field, to be used for a fast autofocus. In general, this allows the method of adjusting both the detection path and the illumination path to be used for a dynamic correction. In respect of a dynamic correction, reference is made to the international patent application WO 2021239380 A1, which is hereby incorporated in the disclosure by reference.
In step ES, the result, for example the digital image of the portion of the wafer surface at the inspection position, is finally stored. In the example, the digital image information is stored together with the ongoing information from monitoring step M. The ongoing information about the raster arrangement of the secondary particle beams and the shape or the size of at least one focal point of a secondary particle beam is taken into account in a subsequent step DV of digital image processing and data evaluation.
An improved adjustment of both the detection path 11 and illumination path 13, including the settings of the voltages for a homogeneous extraction field 113, is facilitated for a specific inspection position of a surface 25 of an object 7 using the multi-beam system 1 according to the first embodiment and the methods for using the multi-beam system 1 according to
Both information items are acquired during scanning imaging of a portion of a surface 25 of an object 7. In this case, a plurality of J focal points 5 of J primary beams 3 are moved in scanning fashion over the surface 25 of the object 7 and a plurality of J scan positions on the object surface 25 are illuminated at the same time. To this end, a first deflection unit 110 for scanning deflection of the plurality of J primary beams 3 is situated in the primary path or illumination path 13. Each incidence location of the J focal points 5 of the J primary beams 3 forms a source location for secondary electrons, which are collected and imaged on a detector 207, during the short period of time of scanning irradiation using the J primary beams 3. The plurality of the J source locations of the secondary electrons move synchronously over the object surface in accordance with the scanning irradiation using J primary beams. Therefore, a second deflection unit 222 for scanning deflection of the J secondary beams 9 that emanate from the J source locations is situated in the imaging path of the secondary electrons, which is also referred to as detection path or secondary path 11, so that the focal point 15 of the J secondary beams on the detector remain at J same detection locations. In this case, the second deflection unit 222 in secondary path is synchronized with the first deflection unit 110 in the primary path.
A plurality of J time-sequential data streams which are converted into a plurality of J two-dimensional digital image information items are acquired as a result of the scanning illumination with the plurality of J primary beams 3 and the acquisition of the signals of the plurality of J secondary beams 9 synchronously with the scanning illumination. Each image information item represents the spatially resolved rate of generation of secondary electrons by the spatially resolved illumination of the object surface 25 by a focal point 5 of a primary beam 3. In this case, the rate of the generation of secondary electrons depends on the local surface condition, for example the local material composition of a structured wafer surface. The information about the shape and size of the focal points themselves and the raster arrangement 41 of the focal points for adjusting both the detection path and the illumination path are acquired in time-averaged fashion such that the influence of the structuring of the object surface is reduced by averaging over a plurality of scan positions on the surface. Consequently, the method is rendered possible for a plurality of objects, and there is no need for special measurement or calibration objects. In particular, the method of adjusting both the detection path and the illumination path may also be implemented during an inspection task at an inspection position on an object surface.
In an example of the method there is an assignment of selected setting parameters to assigned inspection positions, and a storage of the assignment. A wafer inspection method then comprises homing in on a next inspection position in step SI, followed by determining, on the basis of the next inspection position, setting parameters of the multi-beam microscope for optimal imaging at the inspection position in step SE, and the setting of the determined setting parameters in a step PE. In this case, step SE further includes loading predefined setting parameters of the multi-beam microscope, which are assigned to at least one inspection position. In an example, step SE comprises determining predefined setting parameters of the multi-beam microscope at the next inspection position. In an example, there is an interpolation of the setting parameters for optimal imaging at the next inspection position from at least two setting parameters which are assigned to two adjacent inspection positions. The method can therefore be suitable for repeatedly inspecting objects, in particular portions of surfaces 25 of wafers 7, at repeated or similar inspection positions.
In a further embodiment there is an optimization of inspection positions on the basis of predefined different setting parameters of the multi-beam microscope, which are each assigned to an inspection position. Consequently, it is possible to prevent frequent changes to setting parameters of the multi-beam microscope 1. By way of example, there is an optimization of the sequence of inspection positions on the basis of predefined different setting parameters of the multi-beam microscope, which are each assigned to a number of inspection positions. By way of example, there is an optimization of the sequence of inspection positions on the basis of local charging effects. In one case, successive inspection positions can be arranged adjacent in a targeted manner in order to have long-term compensation of pre-existing local charges. In another case, successive inspection positions could be arranged with maximum spacing in a targeted fashion, in order to allow local charges responsible for charging effects to be discharged for as long as possible by way of leakage currents.
A wafer inspection method using a multi-beam system 1 having a plurality of primary and secondary particle beams (3, 9) includes the following steps:
In an example, the step of determining setting parameters comprises homing in on the inspection position (33, 35) and recording an image of a time-averaged first inspection image of the raster arrangement 41 of the plurality of secondary particle beams 9 using a detector camera 207 by quickly scanning the inspection position with an image recording time of T1<T2, such as T1<T2/100 or T1<T2/1000. The first inspection image is analysed to determine a complex multi-beam effect and setting parameters are determined therefrom such that the complex multi-beam effect is at least partly compensated.
In an example, the step of determining setting parameters further comprises homing in on a reference position and recording an image of a time-averaged first reference image of the raster arrangement 41 of the plurality of secondary particle beams 9 using the detector camera 207 by quickly scanning the reference position with an image recording time of T1. The analysis of the first inspection image of the raster arrangement 41 contains a comparison with the reference image of the raster arrangement 41. The reference position may be a preceding inspection position or a reference position on a reference object which is additionally arranged on the displacement stage 500.
In an example, the method may include recording a further image of a time-averaged second inspection image of the raster arrangement 41 by quickly scanning the inspection position (33, 35) using the determined setting parameters. A success of the compensation can be determined from the analysis of the second inspection image. A residual complex multi-beam effect can be determined and there can be a renewed determination of improved setting parameters such that the residual complex multi-beam effect is at least partly compensated. The determined setting parameters can be assigned to an inspection position (33, 35) and can be stored such that there can be repeated inspection, for example of at least one second wafer, at the same inspection position (33, 35) using the setting parameters assigned to the inspection position (33, 35). In general, predefined or stored setting parameters of the multi-beam microscope 1 can be used for a wafer inspection task. In an example, the setting parameters can be determined by interpolating at least two predefined or stored setting parameters at at least two adjacent reference positions. Further, at least one information item known in advance can be considered when determining the setting parameters, the information known in advance containing CAD information about composition of the wafer 7 at an inspection position (33, 35) in addition to the information from preceding measurements at the inspection position (33, 35) or at adjacent inspection positions (33, 35). Additionally, the method may comprise determining a distance of the inspection position (33, 35) from an edge of the wafer 7.
Within the scope of optimizing the sequence of inspection tasks, a determination of a sequence of setting parameters of the multi-beam system 1 for optimal imaging for each of a sequence of inspection positions (33, 35) may be altered such that the number of changes in the setting parameters of the multi-beam system 1 is minimized.
Determining the parameters of an improved adjustment of both the detection path and the illumination path, including the setting of the voltages for a homogeneous extraction field, dependent on an inspection site is implemented iteratively in an example. The method is illustrated in
During an inspection step IN, the raster arrangement 41 is displaced synchronously over an object surface 25, and image data of an object surface 25, for example of a wafer, are acquired. The anti-scan using the deflection device 222 ensures that the raster arrangement 41 remains fixed in position or stationary on the detector 207. This parallel acquisition of the plurality of J image data points is implemented at a scanning frequency FS of for example 100 MHz; further examples of conventional scanning frequencies are specified above.
In an example, the scanning frequency is increased during the step M1. By way of example, a scanning frequency FS can be increased by a factor of 10, from for example 50 MHz to 500 MHz, or from 100 MHz to 1 GHz. An averaging of the data recording is implemented over larger focal regions on the object surface 15 as a result of the increased scanning frequency.
A probable cause for the deviation is determined in step Q1 from the deviations. In step PE, suitable parameters for adjusting illumination path and detection path, including the voltages for a homogeneous extraction field 113, are determined. The detection of the various deviations, specifically the deviation of the raster arrangement 41 of the plurality of secondary beams and of the shape and size of the focal points 15, allows more targeted conclusions to be drawn about the cause, for example whether there is a disruption in the illumination path 13 and a deviation of the plurality of focal points 15 of the primary beams 3 is already present on the object surface 25 or whether an edge 43 or topography of the object 7 is a cause for the deviation, whether global or local charging effects are present, or whether there is a disruption in the detection path 11.
In step ZS, further information, for example from additional detectors, or a priori information can be used to determine the probable cause of the deviations. Further detectors may comprise a distance sensor for determining the distance of the sample surface from a reference area. The use of such a distance sensor for example allows a better distinction to be made between global charging of the object 7 and purely mechanical defocusing. Further examples comprise field sensors for measuring an electric or magnetic field strength in the vicinity of the object surface 25. A priori information is described above in the context of
After the probable cause of the deviations has been determined, correction measures or an adjustment of detection path and illumination path, including the voltages for a homogeneous extraction field, are/is determined in step PE. Steps SM and PE may also be carried out multiple times in iterative fashion. By way of example, a fine correction is calculated in a second step. Finally, the multi-beam system is driven with the altered parameters in step IN and the inspection is carried out at the same inspection site.
Simultaneously with the inspection step IN, the determination of the deviations can be repeated in step M again. Should the deviation exceed a predetermined tolerance limit, the determination of the cause in step Q and the determination of the new parameters for adjusting the detection path and illumination path is repeated. Above-described steps ES and DV follow.
In a further example of the seventh embodiment, the method for setting a multi-beam microscope for inspecting objects comprises a variation of the steps listed above. In step SM, an image of a time-averaged first reference image of the raster arrangement of the plurality of primary beams is initially taken using a detector camera by quickly scanning a reference position of the object within a first period of time T1, which for example is shorter than a second period of time T2 by a factor of 10 or 100, 1000 or 10 000, the second period of time corresponding to the period of time for recording a high-resolution image of a portion of the surface of the object. By way of example, the first period of time T1 can be 1 ms to 100 ms. By way of example, the second period of time T2 can be approximately 0.8 s, is or more.
An image of a time-averaged, second inspection image of the raster arrangement of the plurality of primary beams is recorded using the detector camera by quickly scanning the inspection position within T1 of for example 1 ms to 100 ms. The second inspection image of the raster arrangement is compared to the first or reference image of the raster arrangement and the deviations or differences of the raster arrangement vis-a-vis the reference image are analysed in a step Q1 or step Q. In step PE, there is a determination of changes of selected setting parameters of the multi-beam microscope for adapting the multi-beam microscope to the inspection site. The changes of the selected setting parameters are carried out as correction measures. Following the implementation of the correction measures, the method may additionally include renewed recording of an image of a time-averaged second reference image of the raster arrangement of the plurality of primary beams using the detector camera by quickly scanning the inspection position within 1 ms to 100 ms, and a renewed analysis and determination of optimized setting parameters of the multi-beam system. In step IN, an inspection image of the surface 25 of the object 7 is recorded with a high spatial resolution by slowly scanning the inspection position within 100 ms to 2000 ms, for example. In this case, the selected setting parameters may comprise at least one of the following parameters:
Method of setting a multi-beam system 1 for inspecting a wafer 7 hence includes the following steps:
The causes of the deviations of the raster arrangement and of the shape and size of beam focal points may be subject to a dynamic change. By way of example, global charging of a sample increases when the illumination by the plurality of primary beams increases and may lead to an increasing deviation of the raster arrangement while imaging. Such dynamic effects are determined in an eighth embodiment of the disclosure and for example the speed of a change or deviation of the raster arrangement and of the shape and size of beam focal points is taken into account. This allows the deviations of the raster arrangement and of the shape and size of beam focal points to be corrected dynamically and allows the parameters for adjusting the detection path and illumination path to be dynamically altered in predetermined fashion during the capture of an image portion of the object surface. An example is illustrated in
To measure the raster arrangement of the plurality of secondary beams and at least one shape or size of a focal point of a secondary beam, the disclosure uses a time-averaged measurement signal which can be acquired during an inspection task. In this case, the measurement can be carried out by the same detector that is also used for high-resolution imaging, with the time averaging carried out as described above being set by way of the sampling rate of the analogue-to-digital converter or the scanning frequency or both. Alternatively, in the case of a detection system which initially converts the signals of the secondary electrons into light by way of a scintillator, a beam divider or deflector may be inserted in the light optical unit disposed downstream of the converter and steer at least a fraction of the generated light onto a CMOS camera. CMOS cameras typically have a lower refresh rate of for example 10-100 frames per second, and so averaging is obtained by the reduced refresh rate. Alternatively, the electron-optical path may be split in the projection optical unit 205 in a ninth embodiment, for example by way of a modified deflection system 224.
The disclosure can be described by following clauses. The disclosure is, however, not limited to the clauses:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 205 394.7 | May 2021 | DE | national |
The present application is a continuation of, and claims benefit under 35 USC 120 to, international application PCT/EP2022/061042, filed Apr. 26, 2022, which claims benefit under 35 USC 119 of German Application No 10 2021 205 394.7, filed May 27, 2021. The entire disclosure of each these applications is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2022/061042 | Apr 2022 | US |
| Child | 18501863 | US |
