Patent Application
20230408911
The present invention relates to the field of optimizing a defect correction for an optical element used in a lithographic process.
As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks and/or templates of the nanoimprint lithography have to project smaller and smaller structures onto a photosensitive layer, i.e. a photoresist dispensed on a wafer. To fulfil this demand, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the deep ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers.
As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In near future, photolithographic masks will increasingly use significantly smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum (approximately 10 nm to 15 nm).
Therefore, photolithographic masks, photomasks or simply masks have to fulfil highest demands with respect to, for example, transmission homogeneity, planarity, pureness, and temperature stability. In order to fabricate photolithographic masks with a reasonable yield, defects or errors of masks cannot be excluded during the manufacturing process. These defects have to be corrected at the end of the manufacturing process. Various types of defects may occur. U.S. Pat. Nos. 8,539,394 and 10,061,192 describe methods for correcting overlay errors of various masks of a mask set. U.S. Pat. No. 9,658,527 describes a mathematical physical model which can be used for determining arrangements of pixels which can correct various types of errors, as for example, registration errors, overlay errors, and/or optical transmission errors. U.S. Pat. No. 9,753,366 describes a method for determining tool specific parameters of a tool which is used to write a pixel arrangement into an optical element calculated with the deformation model explained in U.S. Pat. No. 9,658,527. U.S. Pat. No. 8,735,030 describes a method for modifying a substrate surface by locally generating colour centers and/or writing arrangement(s) of pixels close to a distorted substrate surface. U.S. Pat. No. 9,606,444 describes methods for locally deforming an optical element used for a photolithography process by designing specific pixels. U.S. Pat. No. 10,353,295 describes a method for generating a predetermined three-dimensional contour of an optical element and/or a wafer. These patents of the applicant are hereby incorporated herein in their entirety by reference.
A relatively new type of lithography is the nanoimprint technique in which pattern elements are transferred via a polymer layer to an underlying wafer. Since the reproduction scale of pattern elements in a template for the nanoimprint lithography is 1:1, high demands are made to a template with respect to admissible errors. Thus, defective templates have also to be corrected whenever possible.
Femtosecond light pulses of a laser source can be used to correct errors of an optical element which can for example be a photolithographic mask, a template for the nanoimprint lithography and/or a wafer during its manufacturing process. For this purpose, the laser source applies a huge local energy density on the optical element, for example, a substrate of a photolithographic mask, a template or a wafer which leads to a local nonlinear ionization process resulting in a local plasma generation in the optical element. This local plasma generation induces a local variation of the density of, for example, the substrate or of the template material. A local density variation induced by an ultra-short light pulse is in the following also called a pixel. The introduction of a local density variation by locally applying the laser beam on the optical element is in the following denoted as the writing of pixels into the optical element. A cluster of pixels to be written or which has already been written into an optical element is called a pixel arrangement.
The writing of pixels into an optical element, for example to correct registration defects, locally changes the material density of the optical element. This results in a deformation of the optical element in three dimensions. The deformation in the x- and y-direction is mainly used for correcting registration defects. If pixels are written in a middle level of an optical element, for example a photomask, the global flatness of the photomask is not changed. However, pixels are often written in a plane which is close to the patterned surface of a mask to increase the spatial resolution of the pixel writing process. This modification of the process results in a global flatness change of the photomask, or generally of an optical element. This effect can be compensated by writing a second arrangement of pixels close to the rear surface of the mask. In this approach, errors in the writing depth of the pixel arrangement essentially impact the resulting flatness of the optical element.
This means the writing of pixels into an optical element, for example to correct registration defects, typically has two effects on a surface of the optical element: (a) It shifts pattern elements of the optical element towards their predetermined positions. This is a desired tangential surface deformation of the optical element. (b) But a pixel writing process also causes a second surface deformation which is normal to the local surface of the optical element. Typically, the normal surface deformation is an unwanted effect when correcting registration defects and/or overlay defects.
For modern lithography systems and in particular for future systems using the extreme ultraviolet (EUV) portion of the electromagnetic spectrum, the surface profile of the optical elements becomes critical. For example, half of a wavelength is only 6.75 nm for an exposure wavelength of 13.5 nm in the EUV wavelength range. The overall flatness budget for the individual optical elements used in a lithographic process is very tight. For example, registration and/or overlay error correction processes writing pixels in an optical element for the deep ultraviolet (DUV) wavelength range may not induce surface deformations larger than 30 nm.
This means that it is necessary to know the effect of the deformation a single pixel writing process induces in an optical element. But presently no model is available which is precisely enough to forecast the deformation of a pixel writing process based on the parameters of the laser system used for writing the pixel and the material parameters characterizing the optical element. Therefore, it is necessary to perform systematic calibration measurements to determine the changes the pixel causes in the optical element.
Calibration measurements can be done by direct registration change measurements. But registration measurements consume expensive patterned masks and require expensive metrology tools as well as complex logistics. For photomasks operating in the DUV wavelength range, birefringence measurements of optically transparent fused silica mask blanks can be used for calibration measurements. This approach is fast and cost-effective but does not allow direct deformation measurements. Furthermore, it requires some normalizations. Moreover, birefringence measurements cannot be used for calibrating EUV masks or EUV mask blanks.
It is therefore one general aspect of the present invention to provide methods and apparatuses for optimizing a defect correction of an optical element which at least partially avoid the drawbacks discussed above.
According to a first aspect, a method and an apparatus for determining a writing depth of at least one pixel writing map written into an optical element used in a lithographic process from a surface contour deformation of the optical element are provided for solving the above problem at least partly. The method for determining a writing depth of at least one pixel writing map written into an optical element used in a lithographic process from a surface contour deformation of the optical element comprises: (a) determining a surface contour deformation induced by the at least one pixel writing map written into the optical element; (b) simulating at least two surface contour deformations of the at least one pixel writing map for at least two different writing depths; and (c) determining the writing depth of the at least one pixel writing map by comparing the determined surface contour deformation with the at least two simulated surface contour deformations. The apparatus for determining a writing depth of at least one pixel writing map written into an optical element used in a lithographic process from a surface contour deformation of the optical element comprises: (a) means for determining a surface contour deformation induced by the at least one pixel writing map written into the optical element; (b) means for simulating at least two surface contour deformations of the pixel writing map for at least two different writing depths; and (c) means for determining the writing depth of the at least one pixel writing map by comparing the determined surface contour deformation with the at least two simulated surface contour deformations.
In a second aspect, a method and an apparatus are provided for optimizing a defect correction of an optical element used in a lithographic process. The method for optimizing a defect correction of an optical element used in a lithographic process comprises: (a) determining whether the optical element has at least one defect; (b) determining whether the optical element has at least one surface contour deformation; and (c) determining at least one pixel arrangement for writing into the optical element based on whether at least one defect has been determined and on whether at least one surface contour deformation has been determined, the at least one pixel arrangement correcting the at least one defect, the at least one surface contour deformation or both. The apparatus for optimizing a defect correction of an optical element used in a lithographic process comprises: (a) means for determining whether the optical element has at least one defect; (b) means for determining whether the optical element has at least one surface contour deformation; and (c) means for determining at least one pixel arrangement for writing into the optical element based on whether at least one defect has been determined and on whether at least one surface contour deformation has been determined, the at least one pixel arrangement correcting the at least one defect, the at least one surface contour deformation or both.
In a first embodiment, a method for determining a writing depth of at least one pixel writing map written into an optical element used in a lithographic process from a surface contour deformation of the optical element comprises the steps: (a) determining a surface contour deformation induced by the at least one pixel writing map written into the optical element; (b) simulating at least two surface contour deformations of the at least one pixel writing map for at least two different writing depths, and (c) determining the writing depth of the at least one pixel writing map by comparing the determined surface contour deformation with the at least two simulated surface contour deformations.
A portion of the above-discussed drawbacks can be avoided if a relation can be established which links a surface contour deformation induced by a pixel writing process to the depth of an optical element into which of the pixels are generated. Such a relationship allows to calibrate the impact a pixel has on an optical element. Thus, establishing a relation between a pixel writing depth and a surface contour deformation complements calibration processes based on birefringence measurements and based on registration measurements for optical elements operating in the DUV wavelength range. As outlined above, for optical elements used in the EUV wavelength range, birefringence measurements of EUV mask blanks are virtually impossible. Further, registration measurements are involved, time consuming and thus cost intensive. Moreover, these measurements cannot be performed by the pixel writing tool. Rather, a time-consuming mask transport and an adjustment process has be performed on a respective metrology tool. Additionally, this calibration process consumes one or more wafers, and thus is a cost intensive approach.
Therefore, the knowledge of a relationship connecting an induced surface contour deformation with the pixel writing depth which can be used to reliably determine a writing depth, for example of an EUV substrate, from a determined surface contour deformation allows to partially avoid an experimental calibration of the pixel writing depth. Further, it enables verifying registration processes. Moreover, knowing the relation between the writing depth and the surface contour deformation allows a simultaneous tuning of registration problems, overlay problems and flatness problems. Thus, it allows to avoid the drawbacks outlined above to a large extent. What is more, such surface contour deformations may be determined with much less efforts than registration change and/or overlay change measurements.
A writing map or a pixel writing map is an arrangement of pixels or a pixel arrangement which is specifically designed for generating a surface contour deformation which can precisely be detected, and thus be utilized for establishing a reliable relationship between an induced surface contour deformation and the writing depth of the designed pixel writing map.
Determining the surface contour deformation may comprise:
A surface contour may be a flat surface of, for example, a photomask, a template used in nanolithography, a mirror, and/or a wafer. The flat surface may be a rear surface of a photomask, a template, a mirror, and/or a wafer. The surface contour of an optical element may be a curved surface of, for example, a lens or a curved surface of a mirror. The terms surface and surface contour are used interchangeably in this application.
A surface contour deformation of an optical element may be determined or measured by determining or measuring an initial surface contour, i.e. a surface prior to performing a pixel writing process or introducing a writing map into an optical element, and determining or measuring the surface contour resulting as a consequence of the generated writing map. The surface contour deformation may be determined by comparing the initial and the final surface contour, i.e. prior to and after the pixel writing.
An optical element may be a test optical element. For example, the test optical element may be a test photomask. The test mask may have a structured or patterned front surface which can be used to determine a tangential component of a surface deformation induced by a pixel writing process. A structured front surface of a test mask may have pattern elements of a photolithographic mask and/or marks for performing overlay adjustments in a photolithographic process.
The at least one pixel writing map may be adapted to an expected writing depth of the pixel writing map.
As already discussed above, the effect of a pixel depends on the depth it is written into an optical element. On the one hand, this correlation can be used for establishing a robust relation between a surface contour deformation and the writing depth of the pixel writing map. This relation can be used for correcting a determined or an identified surface contour deformation. Further, this connection may also be used for generating a predetermined surface contour starting from a determined or measured surface contour. Thus, knowing this relationship, allows designing optical elements having predetermined optical properties. This means, the typically unwanted effect of a surface contour deformation when correcting registration and/or overlay defects can be exploited for designing optical elements having specific optical characteristics.
On the other hand, the correlation between the writing depth and the surface contour deformation may also be utilized for designing a specific arrangement of pixels which corrects a defect of the optical element.
A high frequency content of a spatial spectral density of the at least one pixel writing map may decrease with an increasing expected writing depth of the at least one pixel writing map into the optical element.
The spatial frequency content transmitted by an arrangement of pixels to a surface of the optical element may be understood as a function of the pixel writing depth. This relationship can be exploited when designing pixel writing maps for determining the implication of a pixel writing process on the induced surface contour deformation.
The at least one pixel writing map may not be written in a writing depth within a range around a depth center of the optical element of ±20 μm, preferably ±50 μm, more preferably, ±100 μm, and most preferably ±200 μm. For example, if the optical element has a depth of 10 mm, its depth center is 5 mm.
Writing pixels into the center depth which is half the depth of the optical element has a minimum effect on the surface contour of the surface through which the pixel writing map in introduced and also on the surface opposite to the surface used for the pixel writing process. When it is the objective to set-up a relation between the writing depth and the surface contour deformation, it is beneficial to exclude this range from measurement, since it is difficult to interpret the respective measurements.
The at least one pixel writing map may have a distance from a front surface and a rear surface of the optical element which is >20 μm, preferably >30 μm, more preferred >50 μm, and most preferred >80 μm.
Excluding this range from pixel writing may secure that the respective surface of the optical element is not damaged by the huge optical intensities typically used in a pixel writing process.
If the optical element comprises a photolithographic mask, a pixel writing map has a distance to the patterned front surface of >100 μm, preferably >150 μm, more preferred 200 μm, and most preferred >250 μm, and/or the pixel writing may have a distance to the rear surface of >20 μm, preferably >30 μm, more preferred >50 μm, and most preferred >80 μm. In case the optical element is an EUV mask having a rear side coating, a safety distance to the coated surface of 500 μm or more is typically used to avoid damaging the rear side coating.
Determining the surface contour deformation may comprise defining a grid across a surface of the optical element the at least one pixel writing map is introduced into and/or the surface of which the surface contour deformation is to be determined and determining the surface contour at the grid nodes.
Determining the surface contour deformation may comprise determining a difference between the initial and the final surface contour at the grid nodes, squaring the differences of the grid nodes, and adding the squared differences of the grid nodes. It is also possible to add the absolute values of the of the measured differences of the various grid nodes.
Simulating the surface contour deformation may comprise using a three-dimensional deformation model of the optical element.
A three-dimensional (3D) deformation model interconnects an induced tangential modification which, for example, shifts or displaces pattern elements on the surface of a photomask with an induced normal modification which typically results in a surface contour modification.
The three-dimensional deformation model may use material parameters of the optical element, parameters specific for a tool writing the at least one pixel writing map, and basic physical laws. An exemplary 3D deformation model is explained in detail in the U.S. Pat. No. 9,658,527.
Step c. of claim 1 may further comprise determining tool specific parameters of a three-dimensional deformation model and the writing depth of the at least one pixel writing map in a common fitting process.
This approach minimizes the expense for setting up a relation between determined surface contour deformations and the writing depth of the writing map.
Determining tool specific parameters of the three-dimensional deformation model may comprise using at least one of a registration measurement, an overlay measurement, an optical transmission measurement, and a birefringence measurement of the optical element.
In an alternative approach, the tool specific parameters may be obtained by performing a separate measurement. For example, optical transmission measurements and/or birefringence measurements of the optical element may be performed by the pixel writing tool or by a metrology tool which is close by the pixel writing tool. Registration measurements and/or overlay measurements typically require one or more wafer exposures and the detection of the wafer exposure with a separate metrology tool. Hence, the determination of the tool specific parameters is a rather complex and cost intensive process when the exposure of wafer(s) and/or test structures is involved.
The tool specific parameters may have at least two specific manifestations, namely as an expansion signature (ES) or as a mode signature (MS). The expansion signature is a three-dimensional (3D) deformation caused by one pixel of a specific pixel type written into a predetermined depth of an optical element. The mode signature specifies the reduction of the optical intensity transmitted through an optical element having a specific pixel type in a predetermined depth referred to one pixel of the specific pixel type. The parameter MS is relevant for transmissive optical elements, as the variation of the optical transmission is important. The ES is normalized to the pixel density, i.e. number of pixels per square millimeter. The ES is the more relevant parameter for EUV optical elements where the optical property of the pixel writing process is of less importance. Both, the ES and the MS are conventionally determined by writing specific arrangements of pixels into a test optical element, determining or measuring the resulting effect and normalizing the obtained result to one pixel.
The determined writing depth is the depth into which the at least one pixel writing map is introduced in the optical element which minimizes a difference between the determined surface contour deformation and one of the at least two simulated surface contour deformations.
Simulating the surface contour deformation may comprise simulating the surface contour deformation at three or more writing depths of the at least one pixel writing map, and/or may comprise determining the writing depth by interpolating between the simulated surface deformations having the two lowest differences to the determined surface contour deformation.
Determining the writing depth may comprise fitting the three or more simulated surface contour deformations with a polynomial for determining the writing depth which minimizes a mismatch between the determined surface contour deformation and the simulated surface contour deformations.
This additional simulation step may increase the accuracy with which the writing depth can be obtained.
As already discussed above, at least a portion of the optical element is optically transmissive for the tool writing the at least one pixel writing map in the optical element. For example, if the optical element is a photomask, the substrate of the photomask is at least partially optically transparent to the wavelength of the laser writing the pixels. The writing tool for the at least one pixel writing map may comprise a laser system generating ultra-short light pulses. The wavelength of the pixel writing tool may be adapted to a material of the optical element.
A computer program may have instructions for causing a computer system to perform any of the method steps of any of the aspects described above when the computer system executes the computer program.
In a second embodiment, a method for optimizing a defect correction of an optical element used in a lithographic process comprises the steps: (a) determining whether the optical element has at least one defect; (b) determining whether the optical element has at least one surface contour deformation; and (c) determining at least one pixel arrangement for writing into the optical element based on whether at least one defect has been determined and on whether at least one surface contour deformation has been determined, the at least one pixel arrangement correcting the at least one defect, the at least one surface contour deformation or both.
In a conventional defect correction procedure, an identified defect is corrected. Then it is detected whether the executed correction process has induced an inadmissible change of a quantity of the optical element. If this is the case, the quantity of the optical element which is out of specification is corrected in a second correction step.
The method of the second embodiment allows to perform a defect correction process in such a way that it does essentially not induce any detrimental effect on the optical element. This beneficial effect is achieved by designing a defect correction process that simultaneously corrects a defect and a surface contour deformation. In this way, the method may optimize the defect correction of an optical element. A first special or limiting case is defined above in the context of the first embodiment. The method comprises a second special case which is correcting a defect of the optical element without inducing any surface contour deformations. The general case is between these two special cases and allows a simultaneous correction of one or more defects and one or more surface contour deformations.
The method may further comprise the step of writing the at least one pixel arrangement into the optical element. A pixel writing tool may write the at least one pixel arrangement into the optical element.
Writing the at least one pixel arrangement may comprise at least one of
The at least one defect may comprise at least one of: at least one placement defect of at least one pattern element of a photolithographic mask, at least one overlay defect of a photolithographic mask, at least one phase shifting defect of a photolithographic mask, at least one optical transmission defect of a transmissive photolithographic mask, at least one absorber defect and/or at least one phase defect of a reflective photolithographic mask, at least one placement defect of a template used in a nanolithography process, and at least one placement defect and/or at least one optical transmission defect of a wafer during a manufacturing process.
The pattern element may comprise at least one of: an absorbing pattern element, a phase shifting pattern element, and/or an absorbing and phase shifting pattern element.
The optical element may comprise at least one of: a photolithographic mask, a template for the nanoimprint lithography, a mirror and/or a lens used in a photolithographic exposure system, a mirror and/or a lens used in a photolithographic exposure system, and a wafer. The photolithographic mask may be a transmissive mask, a reflective mask, or a mask used in double or multi patterning processes. Further, the photolithographic mask may be of any patterning type, for example, having absorbing pattern elements, phase shifting pattern elements, and/or absorbing and phase shifting pattern elements.
The template may be a template used in any type of nanoimprint lithography, as for example, thermoplastic nanoimprint lithography, photo nanoimprint lithography or resist-free direct thermal nanoimprint lithography.
A wafer may be of any semiconductor material, as for example silicon or germanium, or any compound semiconductor material, as for example, gallium arsenide (GaAs), indium phosphide (InP) or gallium nitride (GaN) to just name a few.
A surface contour deformation may comprise a deviation between a determined surface contour and a predetermined surface contour. A determined surface contour may be a measured surface contour. A predetermined surface contour may be surface contour predetermined by the design of the optical element.
The method may further comprise:
The method may be executed in a stand-alone computing unit specifically designed for determining at least one pixel arrangement based on data of at least one defect and/or at least one surface contour deformation. It is also possible to integrate the computing unit into a metrology tool used for determining whether the optical element has at least one defect, to integrate the computing unit in a metrology tool used for determining whether the optical element has at least one surface contour deformation, or to integrate the computing unit in a tool used for writing the at least one pixel arrangement into the optical element.
Obtaining data of the at least one defect and/or obtaining data for determining the at least one surface contour deformation may comprise at least one of: loading the data from a memory, and receiving the data from at least one metrology tool.
Determining the at least one surface contour deformation may comprise using at least one of: one or more confocal microscopes, one or more probes of a scanning probe microscope, one or more capacitive distance gauges, an electron beam of a scanning electron microscope, one or more photomask metrology tools, one or more contact profilometers, one or more pseudo-contact profilometers, one or more non-contact profilometers, one or more interferometers, and one or more white light interferometers.
Determining the at least one defect may comprise using at least one of: a registration measuring tool, an overlay measuring tool, an optical transmission measuring tool, an optical reflection measuring tool, and a phase measuring tool. The registration measuring tool and/or the overlay measuring tool may comprise a scanning particle beam microscope and/or a scanning probe microscope.
Determining the at least one pixel arrangement may comprise adapting the spatial frequency content of the at least one pixel arrangement to the determined writing depth of the at least one pixel arrangement.
Determining the at least one pixel arrangement may comprise determining a writing depth of the at least one pixel arrangement.
This approach allows to adapt a spatial frequency content of the at least one pixel arrangement to the depth into which the at least one pixel arrangement is generated. The simultaneous determination of the writing depth and the pixel arrangement enables an optimal adaption of both for the task of simultaneously correcting one or more defects and one or more surface contour deformations of an optical element.
Determining the at least one pixel arrangement may comprise using a three-dimensional deformation model parameterized for the optical element.
Determining the at least one pixel arrangement may comprise determining tool-specific parameters of a three-dimensional deformation model for the tool used for writing the at least one pixel arrangement into the optical element.
Determining tool specific parameters of the three-dimensional deformation model may comprise determining the writing depth of the at least one pixel arrangement and the tool specific parameters in a common fitting process.
Applying a common fitting process for determining the writing depth as well as the tool specific parameters minimizes the effort for the determination of the at least one pixel arrangement. For example, U.S. Pat. No. 9,753,366 discusses a method for determining tool specific parameters of a tool used for introducing a pixel arrangement calculated by means of a 3D deformation model into an optical element.
Determining tool specific parameters of the 3D deformation model may comprise performing at least one of a registration measurement of the optical element, a birefringence measurement of the optical element, an optical transmission measurement, an overlay measurement, and a measurement for determining a surface contour deformation as a function of the writing depth of a pixel writing map. The last item is discussed when describing the first embodiment.
The at least one pixel arrangement may comprise a two-dimensional layer of pixels which is written parallel to the surface of the optical element through which the at least one pixel arrangement is written into the optical element.
The surface through which the at least one pixel arrangement is introduced into an optical element is in the following also called rear side or back side of the optical element. For example, if the optical element is an EUV mask, the rear side is the substrate side typically having an electrically conductive coating. The front or front side is the side of a mask which has the mask pattern.
Determining the at least one pixel arrangement may comprise determining a writing depth for the at least one pixel arrangement. A 3D deformation model enables to calculate both the tangential and the normal surface changes caused by the generation of one pixel. Thus, the 3D deformation model is the basis for the determination or calculation of one or more pixel arrangements to be introduced into the optical element.
Determining the at least one pixel arrangement may comprise determining at least one pixel type used in the at least one pixel arrangement. A pixel type is characterized by the parameters of the pixel writing tool. The pixel writing tool is typically a laser system which can generate ultra-short laser pulses. The pixel writing tool may generate a pixel focusing a single ultrashort light pulse to a specific position within the optical element. It is also possible to generate a pixel by subsequentially applying two or more light pulses onto the same position of the optical element. A pixel writing tool may comprise a laser system. A relation connecting the writing depth of the at least one pixel arrangement with the surface contour deformation induced by the at least one pixel arrangement—as discussed above—may be used to facilitate the determination e the at least one pixel arrangement.
The wavelength of the pixel writing tool may be adapted to the band gap of the optical element. Typically, a writing wavelength is in the infrared (IR), the visible or the ultraviolet (UV) range of the electromagnetic spectrum. A pixel type may be characterized by a pulse energy, a pulse length, a repetition rate, a numerical aperture (NA) of the objective focusing the light pulses into the optical element, a beam polarization, wavefront determining parameters of the beam, as an astigmatism, a defocus, a trefoil and higher order distortions, a beam intensity distribution, and a beam spectrum (wavelength).
Determining the at least one pixel arrangement may comprise determining a two-dimensional (2D) pixel density used in the at least one pixel arrangement. The 2D pixel density is a lateral distance (i.e. normal or perpendicular to the beam direction) between two pixels.
Determining the at least one pixel arrangement may comprise determining a varying two-dimensional pixel density within the at least one pixel arrangement. The pixel density may vary in one direction or in two directions. Further, the pixel density may vary differently in two directions.
The at least one pixel arrangement may comprise at least two pixel layers written into different depths. The difference in the depth between the at least two pixel layers of the at least one pixel arrangement may be larger than a dimension of the pixels within a layer. This means the pixels in the various layers of the pixel arrangement do not overlap or merge. The at least two pixel layers within the at least one pixel arrangement may be parallel. The pixel type and the pixel density within the at least two different layers may be different. Further, the pixel types may be different within the at least two different layers of the at least one pixel arrangement.
The method may further comprise the step of determining the surface contour deformation of the optical element after writing the first pixel layer of the at least one pixel arrangement.
Moreover, the method may further comprise the step of adapting the at least one second pixel layer of the at least one pixel arrangement to the determined surface contour deformation.
In the general case, if the at least one pixel arrangement has N pixel layers, at least the (i+1)th pixel layer of the at least one pixel arrangement of the remaining (N−1) pixel layers to be written into the optical element is adapted to the surface contour deformation determined or measured after writing the ith pixel layer of the at least one pixel arrangement.
It is a beneficial effect of the present application that a pixel writing process may be interrupted after the introduction of a first pixel layer (generally an ith pixel layer) and the surface contour deformation induced by the writing process of the first (generally first to ith) pixel layer can be determined. Then one, several or all of the (N−1) pixel layers of the at least one pixel arrangement remaining to be written into the optical element can be adapted to the currently measured surface contour deformation or may be newly calculated. This means that the method may be performed as an iterative process. The correction of a defect, of a determined surface contour deformation or of both can be controlled during the correction process. If the correction process runs out of a specified range, one or several of the pixel layers remaining to be written can be corrected so that the correction process is brought back on track. The knowledge of the relationship between a surface contour deformation induced by a pixel layer and its writing depth can be used when adapting the pixel layers remaining to be written into the optical element of the pixel arrangement to a detected defect rest and/or a determined surface contour deformation.
It is an optimizing effect of the method described in this application that a closed feedback loop can be set up for a defect correction process of an optical element that allows a constant adaption to the progress of the defect correction. This enables to perform a defect correction of an optical element in a single process and simultaneously secures that no other characteristics of the optical element runs out of a predetermined specification.
The at least one pixel arrangement may comprise 1 to 50, preferably 2 to 25, more preferred 3 to 14, and most preferred 4 to 8 pixel layers written into different depths of the optical element.
Adapting the at least one pixel arrangement to the determined surface contour deformation may comprise adapting at least one of a pixel type, a two-dimensional pixel density, the writing depth of the at least one second layer of the pixel arrangement, introducing and/or adapting a sub-structure of the at least one second pixel layer, and a size of the least one second pixel layer of the at least one pixel arrangement.
Determining the at least one pixel arrangement may comprise determining a minimum distance of the at least one pixel arrangement from a front surface and/or a rear surface of the optical element.
The determination of the minimum distance of the at least one pixel arrangement to the front and/or rear surface of the optical element can be performed by using the 3D deformation model. By not generating pixels within the minimum distance secures that the front surface and/or the rear surface of the optical element may not be damaged.
In contrast to the first embodiment, it may be beneficial to write the at least one pixel arrangement or at least a portion thereof close to the center depth of the optical element. By writing pixels in the center depth of the optical element, the effect of the normal component of the induced surface modification is minimized. Thus, the ratio of the induced tangential and normal surface modifications is maximized. The relationship between the induced surface contour deformation and the writing depth set up in the first embodiment can be used to determine the optimum writing depth.
The at least one pixel arrangement may have a distance to a front surface and/or a rear surface of the optical element >20 μm, preferably >30 μm, more preferred >40 nm, and most preferred >50 μm.
In case the optical element comprises a photolithographic mask, a pixel writing map has a distance to the patterned front surface of >100 μm, preferably >150 μm, more preferred 200 μm, and most preferred >250 μm, and/or the pixel writing may have a distance to the rear surface of >20 μm. preferably >30 μm, more preferred >50 μm, and most preferred >50 μm. In case the optical element is an EUV mask having a rear side coating, a safety distance to the coated surface of 500 μm or more is typically used to avoid damaging the rear side coating.
A distance between the at least two layers of the at least one pixel arrangement may be larger than a pixel dimension parallel to a writing direction of the pixels in the at least two layers of the at least one pixel arrangement. This means the pixels between two of the two or more pixel layers do not overlap or merge. Therefore, the effect of the two or more generated pixel layers is linear and additive. The writing direction may be perpendicular to the rear surface and/or the front surface of the optical element.
An optical element may be manufactured according to a method of any one of the aspects of the second embodiment.
In a first embodiment, an apparatus for determining a writing depth of at least one pixel writing map written into an optical element used in a lithographic process from a surface contour deformation of the optical element comprises: (a) means for determining a surface contour deformation induced by the at least one pixel writing map written into the optical element; (b) means for simulating at least two surface contour deformations of the pixel writing map for at least two different writing depths, and (c) means for determining the writing depth of the at least one pixel writing map by comparing the determined surface contour deformation with the at least two simulated surface contour deformations.
The means of simulating at least two surface contour deformations may comprise a computing unit adapted to use a 3D deformation model for simulating surface contour deformations induced by a pixel writing map introduced in at least two different depths of the optical element.
The apparatus may further comprise means for writing the pixel writing map into the optical element.
In a second embodiment, an apparatus for optimizing a defect correction of an optical element used in a lithographic process comprises: (a) means for determining whether the optical element has at least one defect; (b) means for determining whether the optical element has at least one surface contour deformation; and (c) means for determining at least one pixel arrangement for writing into the optical element based on whether at least one defect has been determined and on whether at least one surface contour has been determined, the at least one pixel arrangement correcting the at least one defect, the at least one surface contour deformation or both.
The apparatus may comprise means for writing the at least one determined pixel arrangement into the optical element. The means for determining the at least one pixel arrangement may comprise a computing unit.
A computer program may have instructions for causing a computer system to perform any of the method steps of any of the aspects described above with respect to the second embodiment when the computer system executes the computer program.
In order to better understand the present invention and to appreciate its practical applications, the following figures are provided and referenced hereafter. It should be noted that the figures are given as examples only and in no way limit the scope of the invention.
In the following, the present invention will be more fully described hereinafter with reference to the accompanying figures, in which exemplary embodiments of the invention are illustrated. However, the present invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and will convey the scope of the invention to persons skilled in the art.
In the following, the present invention is described by taking photolithographic masks as examples. However, the present invention can also be applied to optical elements, in particular to optical elements used in various kinds of lithographic systems, as for example, photolithographic exposure systems and nanolithography systems. Apart from photomasks and templates, optical elements in form of, for example, mirrors, lenses, and wafers can also be corrected with the methods described in this application to just mention a few examples. Further, the operation wavelength range of the optical elements is not restricted to the wavelengths used in lithography systems. Rather, the optical elements processed with the methods can be used across the entire optical range. Moreover, the present invention is not restricted to semiconductor wafers, as for example silicon or germanium wafers. Rather, they can be applied to all kind of wafers used for fabricating any kind of integrated circuits.
Finally, the methods defined above can be applied to all optical elements whose materials are at least partially optically transparent for a specific wavelength range of the electromagnetic spectrum. If a material of an optical element fulfils this requirement, pixels can be introduced into the material of the optical element. It is presently preferred that the transparent wavelength range of a material of an optical element is in the ultra-violet (UV), the visible and/or the infrared (IR) part of the electromagnetic spectrum as numerous laser sources are available for these spectral ranges.
In the following, the operational principle of a confocal sensor 100 or a confocal microscope 100 is outlined. A confocal microscope 100 uses a point illumination of a pinhole in an optically conjugate plane in front of a detector to eliminate out-of-focus signal components. In the example of
For example, the light source 120 may be a laser source. If an optical fiber 125 is used for guiding the electromagnetic radiation from the light source 120 to the pinhole 130, the pinhole 130 may be omitted as the fiber 125 has a similar effect as the pinhole 130. The detector 175 may for example be a photodiode, an avalanche photodiode, or a photomultiplier.
A photomask 110 or generally an optical element 110 may be arranged on a sample stage 105. The sample stage 105 may be movable and is moved by a scan unit 180 in a plane which is perpendicular to the incident light beam (x- and/or y-direction). For example, micromanipulators or servomotors can be used for moving the sample stage 105 (not shown in
A control unit 170 controls the light source 120, the detector 175 as well as the scan unit 180. It is also possible that the control unit 170 controls the movement of the objective 140 (not shown in
The resolution of the confocal microscope 100 in lateral direction is limited by diffraction. This means that the lateral resolution depends on the diameter or opening of the pinholes 130 and 165, the numerical aperture (NA) of the objective 140 as well as on the wavelength of the light source 120. In lateral direction, the resolution is in the range of the wavelength of the light source 120. The resolution in the beam direction is not diffraction limited but limited by the determination of the position of the maximum intensity. The resolution in beam direction reaches the one-digit nanometer range.
The apparatus 200 comprises a chuck 220 which may be movable in three dimensions. The optical element 110, for example a mask 110, may be fixed to the chuck 220 by using various techniques as for example clamping. The photolithographic mask 110 may be a transmissive mask mounted upside down, so that its rear substrate surface is directed towards the objective 240.
The apparatus 200 includes a pulse laser source 230 which produces a beam or a light beam 235 of pulses or light pulses. The laser source 230 generates light pulses of variable duration. The pulse duration may be as low as 10 fs but may also be continuously increased up to 100 ps. The pulse energy of the light pulses generated by the pulsed laser source 230 can also be adjusted across a huge range reaching from 0.001 μJ per pulse up to 10 mJ per pulse. Further, the repetition rate of the light pulses comprises the range extending from 1 Hz to 100 MHz. In an example, the light pulses may be generated by a Ti:sapphire laser operating at a wavelength of about 800 nm. However, the methods described in the following are not limited to this laser type, principally all laser types may be used having a photon energy which is smaller than the band gap of the optical element 110 and which are able to generate ultra-short pulses with durations in the femtosecond range. This criterion also holds for wafers. Generally, the emission wavelength of the apparatus 200 has to be adapted to the band gap of the optical element 110.
Therefore, for example, YAG laser systems doped with at least one of the elements neodymium (Nd), thulium (Tm), holmium (Ho), and erbium (Er) can also be used. As a further example, dye laser systems may also be applied. The apparatus 200 may also comprise more than one pulse laser source 230 of the same type or of different types (not shown in
The requirement that the band gap of the optical element 110 is larger than the emission wavelength of the laser source used to apply ultra-short laser pulses is typically equivalent to the requirement that the optical element 110 is optically transparent in the near-infrared, the visible, and/or the near-ultraviolet wavelength range. In these wavelength ranges, laser sources are available which can generate ultra-short pulses having a large energy density in their focus.
The following table represents an overview of an example of laser beam parameters of a frequency-doubled Nd-YAG laser system which is used to write a pixel writing map and/or a pixel arrangement into an optical element 110.
The pixel density refers to a 2D density in a plane perpendicular to the beam direction (lateral plane) or parallel to a local rear surface and/or a local front surface of the optical element.
As indicated in table 1, the parameter ranges of laser pulses used for the generation of pixels are large. Hence, the properties of pixels may also be quite different. Typically, a laser pulse does not generate a pixel in form of a sphere but creates a pixel having a form similar to an ellipsoid or a spheroid. The length of the spheroid is in a range of 1 μm to 50 μm and its width or thickness extends, for example, from 0.5 μm to 10 μm. Characteristic lengths to thickness ratios are in the range of 2 to 10. As indicated in table 1, pixel densities in the lateral plane are 103 to 107 pixels per square mm. One-dimensional pixel densities in beam or depth direction cover a range of 1 to 100 pixel(s) or 2D pixel layers per mm.
The steering mirror 290 directs the pulsed laser beam 225 into the focusing objective 240. The objective 240 focuses the pulsed laser beam 235 through the rear substrate surface into the substrate of the photolithographic mask 110. The NA (numerical aperture) of the applied objective depends on the predetermined spot size of the focal point and the position of the focal point within the substrate of the transmissive mask 110 relative to the rear substrate surface. As indicated in table 1, the NA of the objective 240 may be up to 0.9 which results in a focal point spot diameter of essentially 1 μm and a maximum intensity in focus of essentially 1020 W/cm2.
The apparatus 200 also includes a controller 280 and a computer 260 which manage the translations of the two-axis positioning stage of the sample stage 220 in the plane perpendicular to the laser beam (x- and y-direction). The controller 280 and the computer 260 also control the translation of the objective 240 perpendicular to the plane of the chuck 220 (z-direction) via the one-axis positioning stage 250 to which the objective 240 is fixed. It should be noted that in other implementations of the apparatus 200 the chuck 220 may be equipped with a three-axis positioning system in order to move the optical element 110 or the mask 110 to the target location and the objective 240 may be fixed, or the chuck 220 may be fixed and the objective 240 may be moveable in three dimensions. It is also conceivable to equip both the objective 240 and the chuck 220 with three-axis positioning systems. It should be noted that manual positioning stages can also be used for the movement of the optical element 110, as for example a transmissive or a reflective mask, to the target location of the pulsed laser beam 235 in x-, y- and z-directions and/or the objective 240 may have manual positioning stages for a movement in three dimensions.
Further, the apparatus 200 may also provide a viewing system including a detector 265, as for example a CCD (charge-coupled device) camera, which receives light from an illumination source arranged to the chuck 220 (not shown in
The computer 260 may be a microprocessor, a general-purpose processor, a special purpose processor, a CPU (central processing unit), a GPU (graphic processing unit), or the like. It may be arranged in the controller 280, or may be a separate unit such as a PC (personal computer), a workstation, a mainframe, etc. The computer 260 may further comprise I/O (input/output) units like a keyboard, a touchpad, a mouse, a video/graphic display, a printer, etc. Moreover, the computer 260 may comprise one or several connection ports through which it may send a request for experimental data to, for example, the control unit 170 of the confocal microscope 100 of
A non-volatile storage of the computer 260 may store a three-dimensional (3D) surface contour of an optical element 110. The computer 260 may receive experimental raw data from the control unit 170 of the confocal microscope 100. Alternatively, it may receive processed data from the control unit 170. The computing unit of the computer 260 may comprise a 3D deformation model. The 3D deformation model may use a surface contour or a surface contour deformation and a pixel writing map as input data to determine or calculate a writing depth for the pixel writing map. It is also possible that the 3D deformation model determines one or more pixel arrangements which correct a defect of the optical element 110 without inducing any significant changes of the surface contour of the optical element 110. Further, the 3D deformation model may also use defect data of an optical element 110 and a surface contour deformation of the optical element 110 as input data to calculate one or more pixel arrangements which correct both, the determined surface contour deformation and the defect of the optical element 110.
The one or more pixel arrangement determined by the computer 260 based on a 3D deformation model may comprise two or more pixel layers to be written into various depths of the optical element 110. The pixel writing process by the apparatus 200 may be interrupted after the generation of the first or generally a ith pixel layer and the confocal microscope 100 may be used in order to measure or determine the surface contour deformation induced by the introduction of the first (or the ith) pixel layer. It is further possible to analyze a remaining defect rest. If the defect correction process is not executed as planned, the computing unit of the computer 260 may use the 3D deformation model to calculate a modified second (or the (i+1)th), several of the pixel layers which remain to be written, or all pixel layers of the pixel arrangement which remain to be written so that the corrected surface contour agrees with a predetermined surface contour and/or a remaining defect rest is below a predetermined threshold at the end of the correction process.
It is also possible that the supplier of an optical element provides data with respect to a deviation of an existing surface contour from a predetermined surface contour.
The mask substrate 510 comprises a rear surface 515 and a front surface 545. The rear surface 515 has a surface contour 520 which deviates from a predetermined flat surface contour 540. This means the rear substrate surface 515 has a surface contour deformation 530.
The substrate 510 of an EUV mask 500 typically has lateral dimensions of 152 mm×152 mm and a thickness or height of essentially 6.35 mm. The rear surface or backside 515 of the substrate 510 normally has a thin metallic coating. For example, the coating may comprise at least one metal of the group of nickel (Ni), chromium (Cr), aluminum (Al), gold (Au), silver (Ag), copper (Cu), tantalum (Ta), titanium (Ti), wolfram (W), indium (In), platinum (Pt), molybdenum (Mo), rhodium (Rh), and/or zinc (Zn) and/or mixtures of at least two of these metals. The thickness of the metal coating layer is typically in a range of 50 nm or smaller. The metallic coating is used for fixing the mask 500 to an EUV scanner by using an electrostatic chuck. Other materials as for example boron (B) or chromium nitride (CrN) as also be used for generating thin backside coating for an EUV mask 500.
In order to have access to the mask substrate 510 for the pixel writing tool 200, the metallic coating can be replaced by an optically transparent and electrically conductive coating 550. Such a transparent conductive coating 550 may for example comprise indium tin oxide (ITO). Alternative materials for optically transparent and electrically conductive coatings are, for example, fluorine tin oxide (FTO) and/or aluminium zinc oxide (AZO) and/or antimony tin oxide (ATO). These materials can easily be applied to the rear substrate surface 515 of a fused silica substrate 510 and have an electrical conductivity which is high enough for electrostatically chucking a photomask to an EUV scanner. The optically transparent and electrically conductive coating 550 enables to irradiate an EUV mask 500 with light pulses of the pixel writing tool 200 through the rear substrate surface 515.
A multilayer (ML) mirror system 560 is deposited on a front surface 555 of the substrate 510. The multilayer mirror system 560 typically comprises 20 to 80 pairs of alternating molybdenum (Mo) and silicon (Si) layers. The normal thickness of each Mo layer is 4.15 nm and that of the Si layer amounts to 2.80 nm. To protect the ML structure 560, a capping layer of silicon with a native oxide of 7 nm depth is typically arranged on top of the ML 560 (not shown in
When fixing the EUV mask 500 with an electrostatic chuck, typically its rear surface 515 is flattened during the operation of the EUV mask 500. This in turn will lead to aberrations of the multilayer structure 560 arranged on the substrate 510 compared with an essentially flat front surface 555. The aberrations caused by the rear surface contour deformation 530 can neither be removed by varying the position of the wafer nor by changing the reproduction scale. Thus, the rear surface contour deformation 530 has to be corrected.
An absorbing layer is generated on a buffer layer (not presented in
At first, the quality of the 3D deformation model is tested and the tool specific parameters in form of a 3D expansion signature (ES) are determined. For this purpose, a test mask for the deep ultraviolet (DUV) wavelength range is selected. The test mask should have a respective density of structure elements either in form of pattern elements and/or in form of overlay marks.
In a first step, the positions of the structure elements of the test mask are measured by using a registration metrology tool. In the second step, a pixel writing map is generated or written in the test mask.
In the next step, a shift of the structure elements is measured by using the registration metrology tool which have been used to determine the positions of the original structure elements. The diagram 700 of
Diagram 800 of
Diagram 900 of
The 3σ numerical value of the remaining or residual mismatch of the registration fit is 0.44 nm. This value is significantly lower than the noise of a registration metrology tool which is around 0.7 nm. This result has been reached by multiple sampling the induced registration displacements of
In the following, ES parameters derived from registration measurements are also referred to ESR (expansion signature registration).
The 3D deformation model connects induced tangential surface changes of an optical element, as for example registration displacements of the test mask 600, with normal surface changes of the optical element, as for example a surface contour deformation of the test mask 600. This means that in principle a detected surface contour deformation may be used to deduce the tool specific ES parameters. For determining the ES parameters from a mask bending or generally from a surface contour deformation of an optical element, the same approach as for the determination of ESR is used. The ES parameters deduced from a determined surface contour deformation are in the following also called ESF (F for flatness).
In a first step, parallel to the initial positions of the structure elements of the test mask 600, the initial rear surface contour of the test mask 600 is measured, for example by using the confocal microscope 100 of
Similar to
It is also possible to determine the tool specific ES parameters from birefringence measurements of the optical element 110, 600 having the pixel writing map 610. Table 2 of
Table 2 reveals that both registration data and surface contour deformation data can be the basis for determining tool specific ES parameters. Like the other approaches for determining ES and/or MS parameters, there are also some limitations. As already discussed above, when writing pixels close to the depth center, ES parameters can only be determined having a large error interval. Further, not all parameters of a 3D ES (ES3) may be deduced from a surface contour deformation, because the effects of a surface contour deformation are similar to the effects of an induced registration change.
A full set of ES parameters describes all possible deformations of a cubic or more generally of a cuboid of the optical element induced by writing of the pixel writing map 610 into the test mask 600. A cubic or a cuboid has eight corners each having three degrees of freedom (x-, y- and z-direction). In total this results in 24 degrees of freedom. When removing three directions of shift and three directions of rotation of the overall optical element 18 degrees of freedom are left. But not all these parameters essentially contribute to a deformation of the investigated optical element. For example, if the optical element is a photomask, as illustrated in
In a second item, the writing depth of a pixel writing map or a pixel arrangement is now deduced from a determined or measured surface contour deformation. At first, the writing depth is derived by matching a measured surface contour with a simulated surface contour deformation wherein the ES parameters are determined from induced registration changes by writing the pixel writing map 610 into the test mask 600 as discussed above.
The dots 1310 of diagram 1300 of
As already mentioned above, the ES parameters have been determined based on measured registration changes induced by a pixel writing map 610. Further, as summarized in table 1, the ES parameters can also be deduced from one or more surface contour deformations. In the next step, it is the objective to determine both, the ES parameters and the writing depth 650 from induced surface contour deformations. The criteria for simultaneously establishing the ES parameters and the writing depth is again the difference between the determined or measured surface contour deformation and the simulated surface contour deformations for various writing depths 650 of the pixel writing map 600.
Diagrams 1400 and 1450 of
At first glance, the results of diagrams 1400 and 1450 are disappointing. The residual mismatch does not show a defined minimum at all. There are two large plateaus 1410 and 1420 and the curves of
For measuring and simulating the surface contour deformations which are the basis of the curves in diagrams 1400 and 1450 of
Similar to
The residual mismatch of
Therefore, a third pixel writing map has been designed which has a sub-structure adapted to the writing depth 650 of 5080 μm. The third pixel writing map 1710 still further increases the ratio of circumference and area ratio compared to the second pixel writing map 1510 of
Diagrams 1800 and 1850 of
The spatial frequency content of a pixel writing map 610, 1510, 1710 that is transmitted to the front surface 655 of the test mask depends on the depth 650 the pixel writing map 610, 1510, 1710 is generated in the test mask 600. In order to distinguish between the contribution of the ES parameters, i.e. the pixel size, and the contribution of the writing depth to the induced surface contour deformation, a pixel writing map must contain high spatial frequencies. However, the high spatial frequencies do not propagate far in the test mask 600, or generally in an optical element. This means that the spatial frequency content of the pixel writing map 1710 or its ratio of circumference and area should be adapted to the depth 650 into which the pixel writing map 610, 1510, 1710 is to be introduced. This secures an optimum accuracy at the determination of both the ES parameters and the writing depth from measured and simulated surface contour deformations.
Diagrams 2000 and 2050 of
In the following, it is discussed how an induced surface contour deformation can be used for monitoring the correction of registration defects, as for example registration defect 590 of the EUV mask 500. In the example described below, it is assumed that the one or more registration defects are on the rear surface 615 of the test mask 600 and the surface contour deformation is also detected at rear surface 615. This is not a restriction of the described method. Rather, front surface registration defects may also be corrected, and the surface contour deformation induced at the rear surface of the mask or optical element may be used to monitor the registration defect correction or vice versa.
For correcting registration defects, a pixel arrangement is determined by using the 3D deformation model. The pixel arrangement contains pixels which predominantly induce a tangential surface modification at the rear surface 615 of the test mask 600 for correcting one or more registration defects. This means that the pixels of the determined writing map have essentially a 2D expanding expansion signature (ES) perpendicular to the beam direction of the pixel writing tool 200. It is assumed that the determination of the ES parameters has a mistake of 10%. This leads to an error of 10% when writing the determined pixel arrangement into a mask 110, 500, 600, or generally into an optical element. In most cases, an error of 10% of the ES parameters is not critical for correcting one or more registration defects. However, this error may result in a significant uncertainty of the induced surface contour deformation.
For example, if pixel arrangement 2110 does not completely remove the registration defect 590, a second pixel arrangement can be designed based on the detected defect rest which forces the residual defect rest below a prescribed threshold.
Often a pixel arrangement 2110 designed for correcting one or more registration defects and/or overlay defects comprises several layers or pixel layers. The defect correction process may be interrupted when the determined surface contour deformation signalizes that the correction process is no longer on track. The pixel layers of the pixel arrangement which remain to be written into the mask may be newly calculated so that they correct the identified deviation of the defect correction process. The determination of a relation which connects an induced surface contour deformation with the writing depth of a pixel arrangement, or a pixel layer—as discussed above—may be used for calculating new pixel layer(s) for the pixel arrangement.
It is also possible not to modify the pixel layers of a designed or calculated pixel arrangement but to calculate one or more additional layers which compensate the deviation of the performed defect correction process.
It might occur that the mistake of the pixel writing process does not or at least not predominantly result from a mistake at the determination of the ES parameters but is caused by one or more other factors. If this is the case, the difference between the measured and simulated surface contour deformation has a high residual mismatch. Such a result requires a more thorough analysis of the measured surface contour deformation. For example, induced registration and/or overlay measurements can be performed to obtain more details of the problem. Further, the ES parameters may newly be determined based on a measured registration change and/or a measured overlay change. Then one of the options discussed above can be applied to correct any defect rest.
Finally, a simultaneous correction of a defect and a surface contour deformation is described in the following. As already discussed above, the tangential surface change and the normal surface change of an optical element induced by a pixel or a pixel arrangement are coupled. Therefore, the correction of registration defects and/or overlay defects potentially impairs a surface contour of an optical element. If the optical element is a photomask their flatness may be degraded when one or both above defect types are corrected. The influence of the defect correction process on the surface contour of an optical element gets more important if the correction process requires a high spatial resolution which in turn needs to design pixel arrangements which have a significant content of high spatial frequencies. Typically, this requirement forces to generate the pixel arrangement close to the patterned mask surface.
A lithographic process typically uses a configuration having a high numerical aperture (NA) to maximize the optical resolution. But lithographic exposure systems having a high NA are very sensitive to the flatness of the patterned surface of a photomask. The allowed overall flatness budget is very tight and thus leads to serious constraints for the contribution resulting from registration and/or overlay correction processes.
However, the knowledge of the actual mask flatness, or generally of the surface contour deformation of an optical element, enables formulating the registration and/or overlay correction process in a different way. Instead of limiting the induced mask bending or surface contour deformation as in the conventional approach, it is possible to target a registration and/or an overlay correction process so that it simultaneously also corrects or at least significantly improves an existing surface contour deformation or a mask surface flatness.
To evaluate the potential of a correction process which corrects both, a registration and/or an overlay defect and a surface contour deformation, a test mask is designed which has both defect types.
Diagram 2350 of
The conventional approach of solving these problems is illustrated in
When performing the registration correction simulation, the surface contour deformation has not been considered. Diagram 2450 of
Diagram 2490 of
The new approach presented in this application simultaneously corrects the registration problem of the defined mask 2300 presented in
Diagram 2550 of
Diagram 2590 of
The constraints of additionally correcting a surface contour deformation together with a registration defect slightly reduces the registration improvement from 89.2% to 84%. This reduction may be ignored, as it typically disappears in the metrology noise of the tool used to measure the registration defect.
However, the registration correction process having the additional constraints has an essential effect on the residual surface contour deformation. This is very positive, because even a mask having a surface contour deformation of more than 60 nm can be corrected in a single correction step. As a consequence, by monitoring the induced surface contour deformation, the error budget for an allowed surface contour deformation may be relaxed significantly.
The present application demonstrates that the determination of surface contour deformations may complement or substitute birefringence measurements for the determination of tool specific ES parameters. Further, the determination of surface contour deformations may provide an effective method of process control. Moreover, the determination of surface contour deformations enables establishing of a reliable process feedback.
At step 2630, at least two surface contour deformations of the at least one pixel writing map 610, 1510, 1710, 1910 are simulated for at least two different writing depths 650, 2150. This step may be performed by the computer 170 of the confocal microscope 100, the computer 260 of the pixel writing tool 200 and/or the computing unit 360 of the apparatus 300.
At step 2640, the writing depth 650, 2150 of the at least one pixel writing map 610, 1510, 1710, 1910 is determined by comparing the determined surface contour deformation with the at least two simulated surface contour deformations. This step may also be performed by the computer 170 of the confocal microscope 100, the computer 260 of the pixel writing tool 200 and/or the computing unit 360 of the apparatus 300. Then the method ends at 2650.
Finally,
At step 2730, it is determined whether the optical element 110, 500, 600, 2100 has at least one surface contour deformation 530. For example, the confocal microscope 100 may be used to determine whether the optical element 110, 500, 600, 2100 has a surface contour deformation 530.
At step 2740, at least one pixel arrangement 2110 is determined for writing into the optical element 110, 500, 600, 2100 based on whether at least one defect 590 has been determined and whether at least one surface contour deformation 530 has been determined, wherein the at least on pixel arrangement 2110 corrects the at least one defect 590, the at least one surface contour deformation 530 or both. This step may also be performed by the computer 170 of the confocal microscope 100, the computer 260 of the pixel writing tool 200 and/or the computing unit 360 of the apparatus 300. Then the method ends at 2750.
This application is a continuation of and claims priority under 35 U.S.C. § 120 from PCT application No. PCT/IL2021/050223, filed on Mar. 1, 2021. The entire content of this priority application is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/IL2021/050223 | Mar 2021 | US |
| Child | 18238730 | US |
