Patent Application
20060012781
The present invention relates to the field of the use of programmable spatial filters based on the use of MEMS devices, especially for use as a Fourier plane filter in the imaging system of a wafer inspection system.
Wafer inspection systems are used in the semiconductor industry for the detection of small defects and anomalies occurring within the chips on the wafers, generally arising during the fabrication process. The geometry on a semiconductor wafer generally consists of a large-scale multiply repetitive pattern that defines the dies of the wafer. Within each die, there are often areas in which there appears an array of a repetitive pattern with a cycle of a few microns or less. This occurs especially in memory chips or in the memory area in a logic chip. The inspection system should be capable of detecting even defects occurring within these repetitive regions.
When coherent or partially coherent illumination is incident in a dark field configuration on such a repetitive array, the array serves as a diffraction grating that reflects the light at angles corresponding to the defined diffraction orders. The reflected light produces a diffraction pattern of spots in the back focal plane of the objective lens of the imaging system. This plane is also referred to as the Fourier plane of the lens, since the image obtained in this plane is a two-dimensional Fourier transform of the object. The smaller the cycle in the object plane, the larger the distance between the spots in the Fourier plane. The size of these spots depends on the optical quality of the objective lens, but even more on the geometrical nature of the incident light. When the input light is a collimated beam, the spot size is very small. Furthermore, certain known features of the wafer, even if non-repetitive, may scatter the incident light beam in known directions, which can be observed as known areas of the Fourier plane.
The system for the detection of wafer defects operates by looking for the very small anomalies resulting in the optical image information from such defects. These small anomalies usually appear as non-periodic, small signals, that override the medley of information that exists on the wafer. The light scattered from the repetitive structures on the wafer can be filtered in the Fourier plane, since it is concentrated only in certain specific areas, while the light from the defect can be spread over the entire Fourier plane. Similarly, the light scattered at certain selected angles, arising from known, not necessarily repetitive features on the wafer, can also be filtered in the Fourier plane. This task is facilitated by the use of a programmable Fourier plane filter.
In U.S. Pat. No. 5,970,168 to Montesanto et al., for “Fourier Filtering Mechanism for Inspecting Wafers” there is described the use of a spring array as a Fourier plane filter, with a built-in damping mechanism to prevent interference from mechanical vibrations. However, this prior art always relates to use of a laser as the light source, which is a collimated coherent light source.
In co-pending U.S. patent application Ser. No. 10/345,097, for “System for Detection of Wafer Defects”, commonly assigned with the present application, and herein incorporated by reference in its entirety, there are described Fourier plane filters using a mechanical array of small bars that can be physically moved by means of thin wires to change the cycle and phase of the mask in the Fourier plane. In that application, and elsewhere, the use of Spatial Light Modulators (SLM) using pixelated Liquid Crystal Displays (LCD) has been proposed for use as Fourier plane filters in wafer inspection systems. Such LCD SLM's are particularly useful as they may be programmed electronically to the Fourier plane pattern desired. However, many LCD materials do not stand up well to the UV illumination used in wafer inspection systems. In U.S. Patent Application Publication No. US-2003/0184739 to D. E. Wilk et al, for “UV Compatible programmable Spatial Filter”, and assigned to KLA-Tencor Technologies Corporation, there is described such an LCD programmable spatial filter using materials specially selected for use with ultra-violet illumination sources.
However, the use of any LCD array, regardless of the materials used, generally results in a limited transmission level in the regions which are switched to the “open” or transparent state, and a limited blocking level in the regions which are switched to the “closed” or opaque state. Additionally, changes in the polarization of the parts of the illuminating beam diffracted or scattered from the object may cause changes in the transmission and blocking properties of the LCD array, thus reducing its efficiency. Furthermore, even the most carefully selected materials, such as described in the above-mentioned Publication No. U.S. 2003/0184739, may eventually show deterioration in time under constant UV illumination.
There therefore exists a need for a new programmable spatial filter for use as a Fourier plane filter in wafer inspection systems, which will overcome some of the disadvantages of prior art filters.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are herein incorporated by reference, each in its entirety.
The present invention seeks to provide, according to a first preferred embodiment, a dark field wafer inspection system, utilizing a programmable spatial Fourier plane filter based on the use of MEMS (Micro-Electro-Mechanical Systems) devices. In comparison with LCD prior art devices, MEMS devices have a number of potential advantages. Such advantages include:
A Fourier plane addressable filter according to the present invention, can be constructed using a number of different MEMS device geometry's for providing the required shutter action. One such type of MEMS geometry for this purpose is the flipping shutter type of MEMS, as available from Flixel Ltd., of Tel Aviv, Israel. Such a flipping pixel array is made up of an array of addressable hinged shutters which can open to an out-of-plane angle of 90° or more, though 90° is optimal for most optical transmission devices. Such devices provide a fill factor of up to 90%.
A further type of MEMS geometry suitable for use in the MEMS Fourier plane SLM of the present invention, is that developed at the NASA Goddard Space Flight Center, for use in the NIR Spectrometer of the Next Generation Space Telescope (NGST), and as described in the article entitled “Programmable 2-Dimensional Microshutter Arrays” by S. H. Moseley et al, published in the ASP Conference Series, Vol. XXX, 2000. An array using this geometry uses a double layer of shutters, each layer of shutters being hinged at opposite ends, and in which the opening mechanism is actuated by selection of the appropriate shutter or shutters by means of micro-motion of an actuation element to latch the edge of the shutter(s) to be opened, followed by macro-motion of the entire array to open the preselected shutter(s).
Another suitable type of MEMS geometry for this use is what is known in the art as the electrostatically operated artificial eyelid device, such as described in U.S. Pat. No. 6,456,420 for “Micromechanical elevating structures” to S. Goodwin-Johansson, and as supplied by MCNC Research and Development Institute of Research Triangle Park, N.C. 27709, or as described in U.S. Pat. No. 5,784,189 for “Spatial light modulator” to C. Bozler et al., as developed at the MIT Lincoln Laboratory, of Lexington, Mass. The artificial eyelid generally has one flexible electrode in the form of a curled flexible film, the curled nature of the film generally being created by an inbuilt stress, and a second flat electrode fixed in the substrate. The curled lid is attached to a window in the substrate at one of the edges of the window, and insulating films preferably cover at least the flat electrode to prevent direct contact between the two electrodes when the lid is rolled out. When a voltage is applied between the two electrodes, electrostatic attraction is established between the rolled up eyelid electrode and the flat substrate electrode. As the electrostatic force overcomes the material rigidity, the flexible film begins to unroll until the entire flap is rolled out against the substrate. Upon the removal of the applied voltage, the inbuilt stress in the flexible film curls it back into its original shape. Operation is achieved at readily attainable operating voltages, with low power consumption and at high speed. Arrays of such actuators can be readily fabricated by standard microelectronic fabrication techniques, and the elements of such an array can either be activated together or they can be individually addressed. Individually addressable artificial eyelids arrays can be used as a programmable Fourier plane filter in wafer inspection systems with visible and/or ultraviolet dark field illumination. Each of the eyelids can be set to one of two states: an “open” or transparent state, in which the flap is rolled up, and a “closed” or opaque state, in which the flap is rolled out. The individual eyelids are preferably controlled from the wafer inspection system controller in order to provide the desired pattern to block the Fourier diffraction spots arising from the repetitive features on the wafer.
A magnetically actuated artificial eyelid MEMS device has been described in U.S. Pat. No. 6,226,116 for “Magnetic micro-shutters” to D. R. Dowe et al., and assigned to the Eastman Kodak Company of Rochester, N.Y.
Spatial light modulator arrays generally have dead areas between the individual pixels, where the transmission of the light does not follow the transmission being selected for the adjacent pixel. The effect of such dead areas is to reduce what is known as the fill-factor of the array. In the case of a pivoting MEMS device, such as the Flixel shutters or the NASA NGST shutters, this dead area is the region occupied by the frame in which the MEMS is installed, and particularly, the pivoting or actuating mechanism by which the MEMS shutter is operated. In the case of the artificial eyelid MEMS device, this dead area arises from the area covered by the rolled up eyelid flap when the MEMS is open. In the above-described MEMS devices, the dead area blocks transmission of light even when the adjacent pixel is switched to be open.
Even LCD SLM arrays have dead areas between the pixels of the array. In such LCD arrays, there are dead areas, generally on one side of each pixel, to contain the on-board transistors for switching the pixels, and often also the conduction leads for the electrodes. There are also dead areas formed in the regions where the actuating electrodes over the LCD layer are absent in order to divide the LCD layer up into its separate pixels, but these are generally very narrow. In the case of LCD arrays, the dead area is not necessarily a completely opaque area, but can be an area with a different and unswitchable transmission from the active area of the pixel. The dead area for an LCD array is thus properly described as an area which does not behave in tandem with the operation of its adjacent pixel.
According to a further preferred embodiment of the present invention, there is provided a novel, double layer SLM, in which the fill factor is increased in comparison to a single layer SLM using the same devices. The SLM arrays of this double SLM array are essentially identical, and are arranged one on top of the other and in close proximity, such that the light to be spatially modulated has to pass serially through both of the individual arrays. The double SLM array relies for its operation on the asymmetric placement of the dead area within each pixel. Two conditions are necessary for the correct operation of the double layered SLM embodiment of the present invention. Firstly, the individual arrays are laterally positioned such that their dead areas are collinearly located in relation to the light transmission through the array. Secondly, the direction of symmetry of the pixel devices in one SLM array is opposite to that of the other array, such that the pixels of the two arrays open in opposite directions. Thus, if for example, in one of the arrays, the dead areas are on the left hand sides of the pixels relative to the direction of propagation of the light beam passing therethrough, then the other array is rotated such that the equivalent dead areas are on the right hand sides of the pixel. Each layer is thus arranged to open in the opposite direction to the other, with the result that the co-positioned overlapping dead areas are common to both layers, thus increasing the overall fill factor. The blocked dead area associated with a single pixel in a single SLM array, thus suffices, at least to a first order approximation, for two pixels in the double SLM array of the present invention.
For optimum fill factor, the pixels in each array are preferably spaced apart by a distance equal to twice the spacing that would be required on an equivalent single SLM array using identical pixel devices. The area thus covered by adjacent pixels is maximized relative to the size of each pixel and each dead area. If the pixels are closer, then there is a superfluous overlap between the active switched areas of the double array. If the pixels are spaced further apart, then there will be an unswitched open gap between the active switched areas of the double array.
There is thus provided in accordance with a preferred embodiment of the present invention, an optical inspection system for inspecting a sample, comprising a light source for directing an incident light beam onto the sample, an objective element having a back focal plane and operative to form an image of the sample from light collected from the sample, and a programmable spatial filter positioned at the back focal plane, the programmable spatial filter comprising an array of Micro-Electro-Mechanical System (MEMS) devices, at least some of the MEMS devices having switched configurations which are alternately generally optically transmissive and optically blocking. The above described optical inspection system preferably also comprises an image analyzer module for analyzing the image and for switching devices of the MEMS array accordingly, such that at least light collected from the sample at selected angles of scattering is blocked. This light collected from the sample at selected angles of scattering generally arises from selected features of the sample, and the selected angles of scattering are preferably predetermined diffraction orders. In this preferred embodiment of the optical inspection system, these predetermined diffraction orders are such as arise in general from repetitive features of the sample.
In accordance with yet other preferred embodiments of the present invention, the light source of the system may be a visible light source, or an ultra-violet light source.
There is further provided in accordance with yet more preferred embodiments of the present invention, an optical inspection system for inspecting a sample, as described above, in which at least one of the MEMS devices is an artificial eyelid device, or a hinged flap device, or a double shutter flap device.
In accordance with still another preferred embodiment of the present invention, there is provided a method of optically inspecting a sample, comprising the steps of, illuminating the sample with a beam of incident light, forming an image of the sample by means of an objective element, the objective element having a back focal plane, positioning at the back focal plane, a programmable spatial filter comprising an array of Micro-Electro-Mechanical System (MEMS) devices, at least some of which have switched alternate configurations which are generally optically transmissive and optically blocking, and adjusting the programmable spatial filter to a pattern such that information related to selected features of the sample is blocked. The pattern is preferably obtained by analysis of an image of the light distribution at the back focal plane to determine light arising from the selected features of the sample and scattered at specific angles. In this case, the specific angles preferably correspond to predetermined diffraction orders, and the selected features of the sample are preferably repetitive features of the sample.
In accordance with yet other preferred embodiments of the present invention, the light may be in the ultra violet spectral range, or in the visible spectral range.
There is further provided in accordance with further preferred embodiments of the present invention, a method of optically inspecting a sample, as described above, in which at least one of the MEMS devices is an artificial eyelid device, or a hinged flap device, or a double shutter flap device.
In accordance with a further preferred embodiment of the present invention, there is also provided a filter for controlling the spatial transmission of a light beam, comprising at least a first optical shutter comprising a section switchable between optically transmissive and optically blocking states, and an unswitchable dead area, and at least a second optical shutter comprising a section switchable between optically transmissive and optically blocking states, and an unswitchable dead area, wherein the at least second optical shutter is disposed in the path of the light beam serially to the at least first optical shutter and is aligned such that in the path of the light beam, the dead area of the at least second optical shutter overlaps the dead area of the at least first optical shutter, and the at least first and at least second optical shutters are mutually aligned such that in a plane perpendicular to the light beam, the switchable section of the at least first optical shutter and the switchable section of the at least second optical shutter face opposite directions relative to the overlapping dead areas.
In accordance with a further preferred embodiment of the present invention, there is also provided a filter for controlling the spatial transmission of a light beam, comprising at least a first optical shutter comprising a section switchable between optically transmissive and optically blocking states, and an unswitchable dead area, and at least a second optical shutter comprising a section switchable between optically transmissive and optically blocking states, and an unswitchable dead area, wherein the at least second optical shutter is disposed in the path of the light beam serially to the at least first optical shutter and is aligned such that in the path of the light beam, the dead area of the at least second optical shutter overlaps the dead area of the at least first optical shutter, and the at least first and at least second optical shutters are mutually aligned with their planes generally parallel, and rotated in the planes at essentially 180° to each other.
In either of the above-described filters, the at least first optical shutter may preferably be part of a first array of optical shutters, and the at least second optical shutter may preferably be part of a second array of optical shutters. The optical shutters are preferably arranged in rows in the arrays. In such a case, the optical shutters are preferably linearly disposed in the rows of the arrays such that the dead areas are spaced apart a distance equal to approximately twice the length of the switchable sections.
In accordance with other preferred embodiments of the present invention, at least some of the optical shutters may be MEMS devices. In this case, the MEMS devices may be flap devices which open generally at right angles to the planes of the devices. The flap devices of the first array and the flap devices of the second array flip then preferably open in opposite directions.
Alternatively and preferably, the MEMS devices may be artificial eyelid devices which open generally along the planes of the arrays. The artificial eyelid devices of the first array and the artificial eyelid devices of the second array then preferably roll open in opposite directions.
In accordance with another preferred embodiments of the present invention, at least some of the optical shutters of the filter may be LCD devices.
Furthermore, in any of the above-described filter devices for controlling the spatial transmission of a light beam, the light beam may preferably be a visible light beam or an ultra-violet light beam.
The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:
Reference is now made to
Reference is now made to
Reference is now made to
Reference is now made to
To obtain high transmission and to avoid interference effects arising from the illumination incident on the grating array formed when all the pixels are “open”, a high fill factor is required. Since each pixel has a certain minimum “dead area” due to the minimal curvature diameter into which it is possible to roll up the flap, however small, this implies that in order to increase the fill factor, large pixels are required. However, pixels that are too large limit the resolution of the device, and as a result a larger area than desired will be blocked in the Fourier plane, leading to a decrease in the amount of light gathered from potentially detectable defects. Therefore, in order to increase the fill factor of the SLM without reducing the resolution, pixels with smaller dead areas are desired.
The extent of the dead areas in the preferred examples of the eyelid pixels shown in FIGS. 2 to 4 are such that the fill factors are only of the order of 60% to 65%. In order to increase this fill factor to achieve more advantageous SLM characteristics for the Fourier plane array, reference is made now to
In the preferred embodiment of
Reference is now made to
According to this preferred embodiment of the present invention, transmission of light through the open half 76 of the length between two pixels is shuttered by means of closure of the flaps or the active area of the second array of the pair, such that the serial combination of the two arrays ensures that the illumination is completely blocked along the whole of the array.
Reference is now made to
When the pixels used are other flap-type MEMS, or LCD pixels, then an equivalent explanation applies with the dead area of the pixels in the top and the bottom arrays being arranged collinearly and in mutually opposite directions.
Since according to the above-described preferred embodiment of the present invention, the dead spaces of the arrays are arranged one on top of the other, the total dead space taken in each double array assembly is reduced to half of that of a single array, since the position taken by the dead space of one array is in the same position serially in the light illuminating beam path as that of the other array. This preferred double array embodiment thus reduces the dead space by approximately half, with a commensurate increase in fill factor. Thus, for instance, if the fill factor for a specific design of single eyelid MEMS array is 60%, then for the double array embodiment of the present invention, it may be increased to close to 80%.
According to further preferred embodiments of the present invention, various methods are provided whereby programmable spatial filters can be utilized in wafer inspection systems for dynamically blocking diffraction orders or other known angular portions of the scattered light, relating to repetitive features or other specific features which it is desired to eliminate from the images of the wafer under inspection. In the above-mentioned co-pending U.S. patent application Ser. No. 10/345,097, there is described a method and apparatus for initially viewing the image obtained in the Fourier plane, in order to learn the Fourier plane topography of preselected regions of the wafer under inspection, and then to actively adapt a spatial Fourier filter design to a specific layer or region or feature of the wafer under inspection in accordance, with the Fourier plane image obtained in the learning stage.
According to these preferred embodiments of the present invention, the various required layouts of the programmable filter, each layout in accordance with the region or set of features which it is desired to eliminate from the image, are stored in advance as part of the inspection protocol or “inspection recipe” for each specific wafer design. Then, during the inspection procedure itself, the programmable SLM is activated to generate each required pattern layout in synchronization to the inspection path being followed by the system. According to this preferred embodiment of the present invention, the programmable filter layout becomes part of the inspection protocol, and each time a wafer having a specific recipe is inspected, the required layout of the filter that was obtained during the pre-inspection learning stage, is activated. This method is applicable using any of the systems and programmable spatial light modulators of the present invention, or of prior art systems.
Reference is now made to
According to the preferred procedure illustrated in
In step 91, the wafer is then moved to the next known layer, region or feature which it is desired to eliminate from the inspection image, and a second Fourier plane image recorded and correlated to this second known position. According to step 92, this procedure is repeated over the whole wafer, and through all of the required layers thereof, until the complete wafer is “learned”. The resulting Fourier plane images are stored in the control system in step 93, as a series of spatial filter patterns, one for each layer/region/feature of the wafer which it is desired to filter out of the inspection image. This series of spatial filter patterns are thus made part of the inspection protocol or “inspection recipe” for each wafer to be inspected.
Finally, as shown in step 94, each of these spatial filter patters is converted into the correct drive signal information to generate a corresponding spatial filter in the programmable spatial light filter, such as those described in the various preferred embodiments of the present invention. As the inspection path of the wafer is followed, at each known inspection step, the spatial light filter is activated with the corresponding spatial filter pattern so as to filter out the layer/region/feature which it is desired to eliminate from the inspection image, as defined in the predetermined inspection protocol.
It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.
