JP7440580B2 - Apparatus and method for inspecting a reticle - Google Patents

Description

TECHNICAL FIELD This invention relates generally to reticle inspection. More particularly, the present invention relates to pattern evaluation.

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Patent Application No. 15/803,628 filed on November 3, 2017 by Rui-fang Shi et al. This application claims priority to U.S. Provisional Application No. 62/508,369, filed May 18, 2017. These applications and patents are incorporated herein by reference in their entirety for all purposes.

Generally, the semiconductor manufacturing industry involves highly complex techniques for manufacturing integrated circuits using patterned semiconductor materials layered onto a substrate such as silicon. As circuit integration increases and the size of semiconductor devices decreases, manufactured devices are becoming increasingly sensitive to defects. That is, the defects that cause device failure are becoming smaller and smaller. The device shall be fault-free before being shipped to the end user or customer.

Typically, integrated circuits are manufactured from multiple reticles. Initially, a circuit designer provides a reticle production system or reticle writer with circuit pattern data that describes a particular integrated circuit (IC) design. Typically, circuit pattern data is in the form of a representation layout of the physical layers of the IC device being manufactured. This representation layout includes a representation layer for each physical layer of the IC device (e.g., gate oxide, polysilicon, metallization, etc.), with each representation layer containing multiple polygons that define the patterning of the layers of a particular IC device. Composed of squares. Reticle writers use circuit pattern data to write multiple reticles that are later used to manufacture a particular IC design (e.g., typically an electron beam writer or laser scanner is used to write reticles). pattern).

Some reticles or photomasks are in the form of optical elements that include at least transparent and opaque regions, translucent and phase-shifting regions, or absorber and reflective regions, both of which can be used for electronic devices such as integrated circuits. Define a pattern of coplanar features in . Reticles are used during photolithography to define specific areas of a semiconductor wafer for etching processes, ion implantation processes, or other manufacturing processes.

US Patent Application Publication No. 2016/0012579

After manufacturing each reticle or group of reticles, each new reticle is typically evaluated for use in wafer manufacturing. For example, the reticle pattern must be free of transferable defects. Furthermore, any wafer produced with the reticle must be free of defects. Accordingly, there is a continuing need for improved reticle and wafer inspection and evaluation techniques.

The following presents a brief summary of the disclosure in order to provide a basic understanding of some embodiments of the invention. This summary is not an extensive overview of the disclosure and does not identify key/essential elements of the invention or delineate the scope of the invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

In one embodiment, a method of evaluating a photolithographic reticle is disclosed. The imaging tool is used to acquire multiple images with different illumination configurations and/or different imaging configurations from each of the multiple pattern areas of the test reticle. The near field of the reticle is reconstructed for each pattern area of the test reticle based on images acquired from each pattern area of the test reticle. The reconstructed reticle near field is then used to determine whether the test reticle or another reticle is likely to result in an unstable wafer pattern or a defective wafer.

In one implementation, the near field of the reticle is directly analyzed to determine whether the test reticle or another reticle is likely to result in an unstable wafer pattern or a defective wafer. In another aspect, the reconstructed reticle near field is used to detect defects in a test reticle or in a simulated wafer image that is simulated from the reconstructed reticle near field. Yes, defect detection is performed using intensity and/or defect detection for the same die at different times, for adjacent dies, for a die and its corresponding golden die, or for a corresponding die from a reticle copy with the same design as the die and test reticle. or involving comparing phases.

In one aspect, images are acquired in the field plane or pupil plane. In certain embodiments, the near field of the reticle is reproduced without the use of the design database used to manufacture the reticle. In another aspect, the acquired images include at least three reflection/transmission images acquired with different imaging conditions selected to be in the near field of the same reticle. In this aspect, the different imaging conditions include different focus settings and different pupil shapes, and the different illumination conditions include different source intensity distributions and/or polarization settings.

In an alternative implementation, the method includes the steps of: (i) applying a lithography model to a reticle near field for a test reticle to simulate a plurality of test wafer images; analyzing the image to determine if the test reticle is likely to result in an unstable or defective wafer. In this aspect, the lithography model is configured to simulate a photolithography process. In a further aspect, the lithography model simulates an illumination source having a different shape than an illumination shape of an inspection tool for acquiring an image of the test reticle or another reticle or wafer. In another aspect, the lithography model is comprised of images drawn from a design database for a calibration reticle. In another example, a lithography model is calibrated using images obtained from a calibration reticle. In yet a further aspect, the lithography model is applied to a reticle near field recreated for a test reticle under a plurality of different lithography process conditions, and the step of analyzing the simulated test wafer image comprises The method includes determining whether the test reticle is likely to become an unstable wafer under different lithography process conditions by comparing portions of the simulated test image that are associated with the reticle area.

In an alternative embodiment, the invention relates to an inspection system for evaluating photolithographic reticles. The system includes a light source that generates an input beam and an illumination optics module that directs the input beam onto a reticle. The system further includes a focusing optics module that directs the output beam from each pattern area of the reticle to at least one sensor, and at least one sensor that detects the output beam and generates an image or signal based on the output beam. Be prepared. The system further comprises a controller configured to perform operations similar to one or more of the operations of the above method.

These and other aspects of the invention are explained below with reference to the figures.

3 is a flowchart illustrating a procedure for reproducing a near field of a mask according to an embodiment of the present invention. 3 is a flowchart illustrating a model calibration process according to certain embodiments of the invention. 2 is a flow diagram depicting a reticle evaluation process according to one embodiment of the invention. 2 is a flowchart illustrating a process for determining stability of a reticle pattern according to one application of the present invention. 5 is a flowchart illustrating a defect inspection procedure according to another embodiment of the invention. 5 is a flowchart illustrating a reticle evaluation process applied to a reconstructed near-field image or result of a mask according to an alternative embodiment of the invention. 1 is a schematic diagram of an example inspection system capable of implementing the techniques of the present invention; FIG. 1 is a simplified schematic diagram of a lithography system for transferring a mask pattern from a photomask to a wafer according to some embodiments; FIG. 1 is a schematic diagram of a photomask inspection apparatus according to some embodiments. FIG.

In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. The invention may be practiced without some or all of these specific details. In other instances, well-known process operations or apparatus components have not been described in detail so as not to unnecessarily obscure the present invention. Although the invention will be described in conjunction with particular embodiments, it will be understood that they are not intended to limit the invention to these embodiments.

Prior to shipping a mask to a manufacturing facility, prior to using such mask to fabricate wafers for manufacturing, and/or after such mask has been used for a period of time in a manufacturing process. For periodic re-evaluation, it is beneficial to detect defects in each mask and otherwise characterize various aspects of the mask (eg, pattern stability, CD, CD uniformity).

One embodiment of the invention includes a technique for reconstructing a near-field image of a reticle based on images of the reticle obtained from an inspection tool under a plurality of different imaging parameters. This near-field image of the reticle may then be used for several reticle evaluation applications. In one example, a near-field image of a reticle is input into a lithography model to predict features of the wafer image or various wafer patterns related to how the resulting pattern will be printed on the wafer. can do. The predicted wafer image and/or various wafer features can then be analyzed for defect detection, reticle evaluation or re-evaluation, and/or any other suitable metrology or inspection applications. The near-field image of the reticle can also be analyzed itself for various purposes, as further described herein.

The terms "reticle,""mask," and "photomask" are used interchangeably herein, and each generally includes a material such as glass, borosilicate glass, quartz, or fused silica. A transparent substrate having a layer of opaque material formed thereon may be included. Opaque (or substantially opaque) materials can include any suitable material that completely or partially blocks photolithography light (eg, deep or extreme UV light). Examples of materials include chromium, molybdenum silicide (MoSi), tantalum silicide, tungsten silicide, opaque MoSi on glass (OMOG), and the like. A polysilicon film may be added between the opaque layer and the transparent substrate to improve adhesion. A low reflection film such as molybdenum oxide (MoO ₂ ), tungsten oxide (WO ₂ ), titanium oxide (TiO ₂ ), or chromium oxide (CrO ₂ ) may be formed over the opaque material. In certain examples, EUV reticles have low absorption properties (such as molybdenum (Mo) and silicon (Si)) and absorber materials (such as tantalum boron nitride films covered with a thin anti-reflective oxide). Multiple layers may be provided with alternating layers of different refractive index.

The term reticle includes, but is not limited to, clear field reticle, dark field reticle, binary reticle, phase shift mask (PSM), alternating PSM, attenuated or halftone PSM, ternary attenuated PSM, Refers to various types of reticles, including chromeless phase lithography PSM, and chromeless phase lithography (CPL). A clear field reticle has a transparent field or background area, and a dark field reticle has an opaque field or background area. A binary reticle is a reticle that has patterned areas that are either transparent or opaque. For example, a photomask made from a transparent fused silica blank with a pattern defined by a film that adsorbs chromium metal can be used. Binary reticles are different from phase shift masks (PSM), one type of which can include a membrane that only partially transmits light, and these reticles are generally halftone phase shift masks such as ArF and KrF masks. Alternatively, it is sometimes called an embedded phase-shift mask (EPSM). If the phase-shifting material is placed on alternating clear spaces of the reticle, the reticle is called an alternating PSM, ALT PSM, or Levenson-type PSM. One type of phase-shifting material applied to any layout pattern is called attenuated PSM or halftone PSM, which involves replacing an opaque material with a partially transparent, or "halftone" film. It can be manufactured by A ternary damped PSM is a damped PSM that also includes fully opaque features.

The next generation of lithography has ushered in the use of extreme ultraviolet (EUV, wavelength 13.5 nm), which is absorbed by normal air and glass. To this end, the lithographic EUV process is performed in vacuum and an optical reflective lens/mirror is used for focusing onto the EUV photomask, and instead of translucent and opaque patterns, reflective pattern and an absorbent pattern.

FIG. 1 is a flowchart illustrating a mask near-field reconstruction procedure 100 according to one embodiment of the invention. The following mask reproduction process 100 may be applied to a particular reticle or set of reticles at any suitable time within the reticle's commercial life, as further described below in various use cases for the near field of the reproduced mask. It can be done about. As an example, the near field of a mask may be recreated before manufacturing any wafers using such a reticle, before beginning production of large quantities of wafers, or during re-evaluation of such a reticle.

In operation 102, at least three images of a mask are first acquired with different imaging configurations using a mask inspection tool. Alternatively, two images may be used, but the use of three images has been found to work well. Acquisitions using different imaging configurations may be simultaneous or sequential. The acquired image does not have to be in the plane of the field. As an example, two or more images may be acquired at a pupil plane with direct access to the diffraction intensity.

Various suitable combinations of illumination configurations and/or collection configurations may be utilized to obtain two or more images. Generally, various imaging configurations are selected to provide images from which the near field of the mask can be calculated. Any suitable imaging or optical configuration may be selected such that the near field of the mask remains the same under various operating conditions. Examples include different focus settings, different illumination shapes (e.g., different directions or patterns), different polarizations for the entire illumination pupil or for various parts of the illumination pupil, different appointments to blur different parts of the collection beam, etc. Contains daization settings. In one embodiment, different focus settings by focusing and defocusing (such as 0 focus, ±800, or ±1600 defocus) may be used to obtain different images. In another example, different quadrants of the illumination pupil can have different polarization settings. In another example, the imaging configuration may include high resolution images such as transmission images with different pupil shapes and/or different focus conditions (eg, for an ArF mask). In another embodiment, more than two reflection images can be obtained with different pupil shapes and/or different focus conditions (eg, in the case of an EUV mask).

The reticle can be imaged with "substantially low resolution" using a relatively low NA (eg, less than 0.5). In contrast, a "substantially high-resolution image" generally means that the features printed on the reticle (within the optical limits of the reticle inspection system used to generate the image) are Refers to the image of a reticle that substantially appears as being formed. A "substantially high-resolution image" of a reticle means imaging the physical reticle at the reticle plane using a substantially high-resolution reticle inspection system (e.g., numerical aperture (NA) greater than 0.8). This is an image generated by The "substantially low NA" used to generate the reticle image may be approximately the same as the reticle-side NA used by the exposure/lithography system to project the reticle image onto the wafer. This transfers the features on the reticle onto the wafer. In substantially low NA images (also LNI), reticle features may have a significantly different appearance than actual reticle features. For example, a reticle feature may appear to have rounder corners in the feature's LNI than the actual feature being formed on the reticle.

In general, any suitable imaging tool may be used for the mask near-field reconstruction process. In some embodiments described herein, the results of the initial reproduction process determine pattern stability or defects with respect to the same reticle or other reticles based on further reticle images from a particular inspection tool. May be used later for evaluation of detection. For consistency in these use cases, the image of the reticle for near-field reproduction of the mask may be imaged with the detector of the reticle inspection system used for subsequent inspection of the same reticle or other reticles, or It may be acquired using a similarly configured detector of a similarly configured reticle inspection system (eg, a different reticle inspection system of the same make and model as the reticle inspection system used for inspection). In other words, images that can be used for mask reproduction can be acquired under the same optical conditions used during subsequent mask inspection and evaluation processes. In this way, the interaction of the electromagnetic waves for illuminating the reticle and the inspection system can be measured as directly as possible.

In alternative embodiments, the tool used to reproduce the near field of the mask may be different from the reticle inspection system. For example, the imaging tool may utilize the same wavelength as the lithography system in which the reticle is used to fabricate the wafer (eg, a wavelength of 193.3 nm for DUV or 13.5 nm for EUV). In fact, any suitable electromagnetic wavelength may be used to reproduce the near field of the mask.

Returning to the illustrated example, in operation 104, three or more images may be registered with each other, or each image may be registered against a database of post-OPCs. For example, the acquired images can be registered by spatial domain or frequency domain methods. Registration adjustments may depend on the particular geometry of the inspection system used. If different images are obtained using different collection paths, partial image adjustments may be made to compensate for the differences in optical paths.

In imaging tools, reticles with various patterns are illuminated by electromagnetic (EM) waves incident from many directions. This incident light is diffracted from various points on the mask pattern with different electromagnetic field phases that interfere with each other. The reticle's near field is an electromagnetic field that is several wavelengths away from the reticle.

Collection optics typically direct a diffraction-limited portion of light from the reticle toward a detector (or wafer) to form an image. The detector detects the intensity, but not the phase, which is the result of the near-field interference of the mask.

Although the far field strength is obtained in the detected signal, it is desirable to reproduce the near field of the mask, including amplitude and phase. In an exemplary embodiment, the near field of the mask is reconstructed and stored based on such acquired mask images, as shown in operation 106. Multiple images (or signals) are generally used to reconstruct the near field of the mask, including both phase and amplitude components. Near field data can be determined by regression techniques based on images obtained from the reticle. For example, the near field of a selected portion of the reticle can be reconstructed (regressed) using quasi-Newton or conjugate gradient methods from its acquired optical image or intensity of the image recorded at the detector plane. . Additionally, any other suitable regression method and/or algorithm may be used to determine near-field data from one or more actual images.

Reconstruction of the near-field of the mask is achieved by solving an optimization problem that attempts to minimize the difference between the observed intensity image and the resulting image of the assumed optical field of the mask. can do. In particular, reconstructing the near field of the reticle from an image of its intensity is an inverse or regression problem. The near field can be iteratively recreated by minimizing a cost function (eg, energy or penalty function). The penalty to be minimized may be the sum of the squared differences between the acquired image and the image of the intensity at the detector calculated from the near field of the mask. In other words, images of the intensity can be computed from the final mask near field for various sets of optical system properties, and these computed images are calculated when the mask near field is found. Most closely fits the acquired image. Embodiments of various mask near-field reconstruction techniques and systems are further described in U.S. Pat. , which patent is incorporated herein by reference in its entirety for all purposes.

If multiple images are acquired under different optical conditions, the reconstructed near-field mask m, which carries phase and amplitude information, can be determined by the following equation:

In Equation 1 above, I _α is the image measured for imaging condition α,
is a set of eigenvectors describing the examination imaging system,
is a set of corresponding eigenvalues for the imaging system, and c _α is a non-negative weighting coefficient between 0 and 1. The above equations can be solved iteratively by methods such as quasi-Newtonian or conjugate gradients, for example.

Another example is the Gerchberg-Saxton algorithm, where a combination of field plane images and pupil plane diffraction orders can be utilized to solve for both the amplitude and phase of the object.

In one embodiment, the near field of the mask can be determined based on the acquired image by a Hopkins approximation. In another embodiment, the regression does not include a thin-mask approximation. For example, the near field of a reticle is the electromagnetic field that is calculated to exist near the surface of the reticle when the reticle is illuminated by a normally incident plane wave. In lithography and inspection, reticles are illuminated by plane waves incident from many directions. When the direction of incidence changes, according to the Hopkins approximation, the directions of the diffraction orders change, but their amplitude and phase remain approximately unchanged. The embodiments described herein may use the Hopkins phase approximation, but do not perform the so-called thin mask or Kirchhoff approximations.

The formulation of reproduction is

can also be varied with a different norm or the addition of a normalization term R that penalizes the vibrations in the near field as Previous information about the field or predictions can be incorporated. Additionally, the norm used for image difference may be a norm of 1 and may be adjusted based on the specific needs of the optimization function.

An interesting note to note is that vector interference of the mask's electromagnetic field as a result of higher NA is more pronounced due to the wider range of light incidence angles and associated interfering electric field components for higher NA (than for lower NA inspection systems). )growing.

The actual mask may vary from the intended design pattern due to the mask writing process. Obtaining a near-field mask from an image of the mask means that such a near-field mask is obtained from the actual physical mask rather than from a design database. That is, the near field of the mask can be reproduced without using a design database.

The near-field results of the mask can then be used for various applications. In one embodiment, the near-field results of the mask can be used to predict the wafer pattern using one or more models. That is, the reconstructed near field of the mask can be used to simulate lithography images. Any suitable technique can be used to simulate the lithography image based on the near field image of the mask. One embodiment is a partial coherence model, i.e.

Including calculation of lithographic images by.

However, λ _i represents the eigenvalue (transfer cross coefficient) of the lithography TCC,
represents the eigenvector (kernel) of the TCC, s is the wafer stack containing the film index, f is the focal point, and z is the vertical position of the lithography plane in the resist material. The transfer section coefficient (TCC) in Equation 2 may include the vector propagation of a field through a lithography projector including a stack of films on a wafer.

Before using a model to predict wafer results, the model may be calibrated to produce as accurate results as possible. The model can be calibrated using any suitable technique. Some embodiments of the invention provide techniques for calibrating a lithography model based on mask near-field results that are reproduced from a calibration mask. In an alternative embodiment, a design database is used to calibrate the model. For example, an image of a calibration reticle may be drawn from a design database.

Typically, the calibration reticle is designed to have characteristics that are substantially similar to the reticle being inspected for defect detection or measured for metrology purposes. For example, the calibration reticle and test reticle are preferably manufactured from substantially the same material having substantially the same thickness and composition. Additionally, the two reticles may be formed using the same process. The two reticles do not necessarily have the same pattern printed on them, as long as the pattern on the reticles can be divided into segments that are approximately the same (eg, lines have similar widths, etc.). Additionally, the reticle being inspected and the reticle used to capture the image can be one and the same reticle.

FIG. 2 is a flow diagram illustrating a model calibration process 200 according to certain embodiments of the invention. As shown, the photolithography process and photoresist are modeled when applied to a near-field image (201) of a mask reconstructed from a calibration reticle using a first set of model parameters in operation 208. obtain. Alternatively, the calibration process 200 may use a simulated calibration reticle image (202) that is simulated from a design database. Reticle images may be drawn from the database by simulating the reticle manufacturing and imaging process on the design database. Any suitable model may be used to generate optical images of the features in the design database. As an example, such simulations can include using Sum Of Coherent Systems (SOCS), or Abbe methodologies, as described herein. Several software packages exist that can simulate intensity images of optical systems from known design databases. An example is the Dr. This is LiTHO. When simulating images from design database 202, the near field may first be simulated using the software packages mentioned above, as well as Prolith by KLA-Tencor and HyperLith by Panoramic Technologies, among others. It can be done by several other packages including:

A model that generates a wafer image based on a near-field image of the reticle, the model can include only the effects of a photolithography scanner, and the model can include only the effects of resist, etch, CMP, or any other wafer process. It can also include influences. One example of a model tool for simulating processes is Prolith, available from KLA-Tencor Corp., Milpitas, California. Resist and etch processes can be modeled closely or approximately. In certain embodiments, the model can be in the form of a model of a compact resist that includes 3D acid diffusion within a particular resist material and configuration, with imposed boundary conditions, and a single threshold. , applied to form a latent image.

Note that the modeled lithography tool may have a different illumination shape or source than the reticle inspection tool to obtain the actual image of the reticle. In some embodiments, the modeled lithography tool can have the same or similar sources as the reticle inspection tool.

Other simulation techniques such as SOCS or Abbe may also be used. An algorithm commonly known as summation of coherent systems (SOCS) attempts to transform an imaging system into a family of linear systems whose outputs are squared, scaled, and summed. The SOCS method is based on Nicolas Cobb's doctoral thesis, "Fast Optical and Process Proximity Correction Algorithms for Integrated Circuit Manufacturing." , University of California, Berkeley, Spring 1998. The Abbe algorithm involves computing an image of the object for each point source one by one, then adding the intensity images together and taking into account the relative intensities of each source point.

The model and its modeling parameter inputs include a set of process conditions to be applied to the reconstructed near-field mask. That is, the model is configured to simulate different sets of process conditions for the reconstructed near-field mask (or simulated mask image). Each set of process conditions generally corresponds to a set of wafer fabrication process parameters that characterize or partially characterize a wafer process for forming a wafer pattern from a mask. For example, specific settings for focus and exposure can be entered into the model. Other adjustable model parameters may include one or more of the following parameters: That is, the wavefront parameters of the projection lens, apodization parameters, chromatic aberration focus error parameters, vibration parameters, resist profile index, resist film metric, top loss metric, etc. The use of such models with various sets of process conditions can result in a set of simulated wafer or resist pattern images formed by reconstructed near-field masks under different process conditions. Together, these simulated wafer images can be used for pattern stability and defect detection evaluation as further described herein.

A calibration reticle may also be used to fabricate a calibration wafer from which the actual image is obtained in operation 216. In one example, the actual image is acquired using a critical dimension (CD) scanning electron microscope (SEM). Although other imaging tools may be utilized, high resolution tools are preferred.

In general, a calibration wafer includes any number of known structures, which can vary widely. This structure may be in the form of a lattice, typically periodic. Each grating can be periodic in one direction (X or Y), e.g. as a grating in line space, or each grating can be periodic in two directions (X and Y), e.g. as a grating in grid space. could be. An example of a grid space lattice may include an array of lines in the Y direction, with each line segmented in the X direction. Another example of a grid space is an array of dot structures. That is, each structure can take the form of a line space lattice, a grid space lattice, a checkerboard pattern structure, and the like. Structure design features include line width (width at a particular height), line space width, line length, shape, sidewall angle, height, pitch, grid orientation, top profile (top radius or degree of topping), lower profile (footing), etc. The calibration wafer can include structures having various combinations of these feature properties. As will probably be appreciated, the characteristics of different structures (different widths, spacings, shapes, pitches, etc.) exhibit different responses to focusing, so preferably the calibration mask includes different structures with different characteristics.

In certain embodiments, the calibration wafer may take the form of a "Design of Experiments" (DOE) wafer having different measurement locations subjected to different processing conditions. In a more general embodiment, variations in process parameters are organized into a pattern on the surface of a semiconductor wafer (referred to as a DOE wafer). In this way, the measurement locations correspond to different locations on the wafer surface having different associated process parameter values. In one example, the DOE pattern is a Focus/Exposure Matrix (FEM) pattern. Typically, a DOE wafer exhibiting a FEM pattern includes a grid pattern of measurement locations. In one grid direction (eg, the x direction), the exposure dose is varied while the depth of focus is held constant. In the orthogonal grid direction (eg, the y direction), the depth of focus is varied while the exposure dose is held constant. In this way, the measurement data collected from the FEM wafer includes data related to known variations in process parameters of focus and dose.

FEM measurement locations are generally provided throughout a focus exposure matrix wafer. In practice, there may generally be one or more measurement points per field. Each field can be created using different combinations of focus and exposure (or just focus or exposure). For example, a first field can be generated using a first combination, and a second field can be generated using a second combination different from the first combination. Multiple combinations can be generated using varying focus and varying exposure, varying focus and constant exposure, constant focus and varying exposure, and so on.

The number of measurement points may also be different. The number of locations per field is generally smaller on a product wafer because the real estate on the product wafer is very valuable. Also, due to time constraints during production, fewer measurements are performed on product wafers than on FEM wafers. In one embodiment, a single location is measured per location. In another embodiment, multiple locations are measured per location.

In most FEM cases, the measurement site structure is formed from similarly designed patterns using different processing parameters. However, it should be noted that matrices of different focal exposures may have different structures. For example, a first matrix can be implemented with a first lattice type and a second matrix can be implemented with a second lattice type that is different than the first lattice type.

In an alternative embodiment, a simulated calibration image (202) drawn from a design database for a calibration reticle may be used as input to the model. That is, the model can be calibrated without recreating the near field from a physical calibration reticle. Instead, lithography images are simulated by simulating (not reproducing) the near field from a design database and applying a model of lithography imaging to the simulated near field to obtain the actual image from the wafer. The lithography results are arrived at which are compared with the results (216).

Generally, optical signal data associated with known variations of any set of process parameters, structural parameters, or both are contemplated. Regardless of the form, the calibration wafer structures can be printed on a variety of different wafer layers. In particular, the printed structures are generally printed onto a layer of photoresist using standard lithography processes (e.g., by projecting a circuit image through a reticle onto a silicon wafer coated with photoresist). . The wafer may be a calibration wafer having layers of material that correspond to materials typically present on production wafers at that step in the testing process. Printed structures can be printed on top of other structures in underlying layers. A calibration wafer may be a production wafer that has the potential to become a working device. A calibration wafer can be a simple wafer used only to calibrate the model. The calibration wafer may be the same wafer used to calibrate the OPC design model. More than one calibration wafer may be used to calibrate the lithography model. When using multiple calibration wafers, the same calibration reticle or different calibration reticles may be used. Different calibration reticles can have patterns with different dimensions to produce a wider range of image data.

The process parameters used to form the calibration structure are generally set to keep the characteristics of the pattern within desired specifications. For example, the calibration structure can be printed on a calibration wafer as part of a calibration procedure, or the calibration structure can be printed on a product wafer during production. During production, calibration structures are typically printed when inscribing lines between device areas (eg, dies that define an IC) disposed on a product wafer. The measurement location may be a dedicated calibration structure disposed around the device structure, or the measurement location may be part of the device structure (eg, a periodic section). As perhaps appreciated, using part of the device structure may be more difficult, but since it is part of the device structure, it tends to be more accurate. In another embodiment, the calibration structure can be printed across the calibration wafer.

Returning to FIG. 2, in operation 210, corresponding modeled results and calibration results (eg, images) may be compared. It can then be determined whether the model parameters should be adjusted in operation 212. If the model parameters are to be adjusted, the model parameters are adjusted in operation 214, and procedure 200 repeats operation 208 to model the lithography process (and resist) using the adjusted parameters. The model parameters can be adjusted until a minimum value is reached at which the quantification of the difference between the model and calibration images is also less than a predetermined threshold. The quantity minimized may be the sum of the squared differences between the acquired calibration image and the simulated image. The output of this process 200 is a lithography/resist model and its final model parameters. This set of model parameters overcomes the technical hurdles associated with mask process modeling and mask 3D diffraction calculations by the nature of using the near field of the mask.

The simulated wafer pattern based on the reproduced mask near-field results may be used for several mask inspection, metrology, and/or evaluation purposes. In one embodiment, reticle evaluation is performed by estimating how the reproduced mask near field is likely to result in defects in the wafer's pattern under a range of simulated wafer manufacturing conditions. . For defect detection, the transferability of reticle defects on the wafer is important, and the transferability of reticle defects is directly dependent on the reticle near field and the lithography system.

After the final calibrated lithography/resist/etch model for a particular process is obtained, regardless of how such model is obtained, such model can be It can be used to generate a precise wafer plane resist image from a mask (eg, after development or etching) before manufacturing a wafer or for re-evaluation of such a mask. These resist images allow one to assess the wafer image for any inspection pattern with high fidelity and with different focus and exposure settings or other lithography parameters. Since this evaluation process can be performed prior to wafer fabrication, the evaluation and defect detection cycle can be significantly shortened. The simulated wafer images can also enable isolation of the root cause of problems with different patterning by comparing the simulated wafer images after lithography, after resist model application, and after etching. .

FIG. 3 shows a flow diagram representing a reticle evaluation process 300 according to one embodiment of the invention. In operation 302, a near-field image of the mask is reconstructed, eg, for a particular reticle, based on an image acquired from such particular reticle. This operation may include an operation of near-field reproduction of the mask of FIG. After the near-field of the mask is obtained, in operation 303 the lithography process (and resist) can be modeled using the final model parameters associated with the calculated near-field mask field. For example, the final model is used to simulate a wafer image using a near-field image of the mask.

The simulated wafer pattern may then be assessed in operation 322 to determine pattern stability and/or locate defects. It can be generally determined whether the corresponding reticle is likely to result in an unstable wafer or resulting wafer pattern. In one embodiment, the model is applied to near-field images or results of a mask with multiple different process conditions, such as focus and dose, to assess the design stability of the reticle under various process conditions. Ru.

FIG. 4A is a flow diagram illustrating a process 400 for determining stability of a wafer pattern according to one application of the present invention. In operation 402, each test image may first be aligned with its corresponding reference image that was also produced by the model under a different set of process conditions. Different test and reference images are calculated by the model under different processing conditions/parameters.

At operation 404, each pair of registered images may be compared to each other to obtain one or more wafer pattern differences. A threshold may then be associated with each wafer pattern difference in operation 406. Wafer pattern differences and their associated thresholds can be used to jointly characterize pattern stability. That is, the stability of a pattern is jointly characterized by the amount of deviation of a particular pattern under different simulated process conditions (pattern difference) and whether such deviation crosses an associated threshold. The process window of a manufacturing process specifies an expected or defined amount of process deviation, such that the resulting patterns remain stable or within certain tolerances (e.g. , thresholds).

Different thresholds for assessing pattern stability can be assigned to different areas of the reticle and thus the corresponding wafer pattern. The thresholds may all be the same or may vary depending on the design of the pattern, the MEEF (or Mask Error Enhancement Factor as further described below) level of the pattern, or the device for changes in the wafer pattern. May vary based on various factors such as performance sensitivity, etc. For example, one may choose a tighter threshold for patterns in dense regions of the reticle compared to semi-dense regions.

A first set of hot spots or areas of pattern weakness may be identified in both the reference mask pattern and the test mask pattern, as appropriate. For example, a designer may prepare a list of coordinates of design hotspots that are important to the functionality of the device. For example, areas defined as hotspots may be assigned one detection threshold, while non-hotspot areas may be assigned a higher threshold (for defect detection). This differentiation can be used to optimize testing resources.

This evaluation of pattern stability can be used to aid reticle evaluation, thereby overcoming many challenges in this field. As the density and complexity of integrated circuits (ICs) continue to increase, inspection of photolithographic mask patterns becomes increasingly more difficult. Every new generation of ICs has denser and more complex patterns that reach and exceed the optical limits of current lithography systems. To overcome these optical limitations, various resolution enhancement techniques (RET) have been introduced, such as optical proximity correction (OPC). For example, OPC helps overcome some diffraction limitations by improving the photomask pattern so that the resulting pattern corresponds to the original desired pattern. Such improvements may include size and edge disturbances of the main IC features, ie, transferable features. Other improvements involve the addition of serifs to the corners of the pattern and/or the provision of sub-resolution assist features (SRAFs) near the printed features. features that are not expected to occur and are therefore referred to as non-printable features. These non-printable features are expected to cancel pattern perturbations that would otherwise occur during the printing process. However, OPC makes the mask pattern much more complex and usually makes the resulting wafer images very dissimilar. Furthermore, OPC defects often do not convert into transferable defects. The increasing complexity of photomask patterns, and the fact that not all pattern elements are expected to directly impact the printed pattern, has made the task of inspecting photomasks for meaningful pattern defects difficult. Make it much more difficult. As the semiconductor industry moves to ever smaller features, leading edge manufacturers are beginning to use newer OPC technologies, such as inverse lithography technology (ILT), which allows the pattern on the mask to become much smaller. It has become complicated. Therefore, it is highly desirable to know the mask writing fidelity and print quality of the wafer before physically fabricating the wafer.

One measure of the importance of a defect is its MEEF or mask error enhancement factor. This factor relates the size of a defect in the mask plane to the magnitude of the impact it has on the printed image. High MEEF defects have a large impact on the printed pattern, and low MEEF defects have little or no impact on the printed pattern. Small main pattern features in dense fine line portions of the pattern are an example of defects with high MEEF, and small mask plane sizing errors can cause complete collapse of the printed pattern. A small isolated pinhole is an example of a defect with low MEEF, where the defect itself is too small to print and is far enough away from the nearest main pattern edge to not affect how that edge is printed. There is a distance. As these examples demonstrate, the MEEF of a defect is a somewhat complex function of the defect type and the context of the pattern in which the defect is located.

In addition to higher MEEF mask defects causing significantly larger wafer defects, some design patterns, and corresponding mask patterns, may be more robust to process variations than other designs and mask patterns. When the manufacturing process begins to drift from optimal process conditions, certain mask patterns can result in much larger wafer pattern perturbations and defects.

FIG. 4B is a flow diagram illustrating a defect inspection procedure 450 according to another embodiment of the invention. At act 452, each modeled test wafer image may be aligned with its corresponding reference image. In one embodiment, die-to-die or cell-to-cell alignment may be achieved. In another embodiment, the modeled test wafer image is aligned with a reference image drawn from a corresponding post-OPC design. For example, a post-OPC design is processed to simulate the reticle manufacturing process for such a design. For example, corners are rounded. Generally, the reference image may originate from the same die as the test image at an earlier time, originate from the same adjacent die, or be drawn from a design database. In a particular example, the reference image is obtained from a "golden" die that is proven to be free of defects (e.g., immediately after the reticle is manufactured and evaluated). The image of the golden reticle obtained from the reticle when it is known to be free of defects is stored and later, when needed, the near-field image of the golden reticle and the wafer image are generated on request. can be used to calculate. Alternatively, the near field image of the golden reticle can be stored for quick access without the need to recalculate the near field in future inspections.

In operation 454, each pair of registered test and reference images is compared based on an associated threshold to locate defects in the reticle. As further discussed above, any suitable mechanism may be used to associate a threshold with a particular reticle area. Any suitable metric of the test image and reference image may be compared. For example, test and reference wafer image contours may be compared as a metric for edge placement error (EPE).

Then, in operation 456, for each reticle defect, the area of the corresponding simulated wafer defect may be compared to the area of its corresponding reference pre-OPC. That is, the simulated wafer pattern is assessed to determine whether a defect in the reticle results in a defect in the wafer that deviates from the intended design.

Returning to FIG. 3, it may then be determined in operation 324 whether the design is defective based on the simulated reticle image. In one embodiment, it is determined whether the design pattern results in unacceptable wafer pattern variation under a particular range of process conditions (or process window). Process variability will determine whether significant differences exist. If the difference between differently processed wafer patterns is greater than a corresponding threshold, such wafer patterns may be considered defective. These systematic defects are known as hot spots. It may also be determined that any difference between the simulated wafer pattern from the reticle and its corresponding pre-OPC pattern exceeds a predetermined threshold. If the design is determined to be flawed, the design may be improved in operation 332.

Once the reticle design is verified, the reticle may still contain hot spots that should be monitored. The following operations are described as being performed on a mask in which at least some identified hotspots are present. Of course, if the mask does not include any identified hotspot operations, the following operations in Figure 3 may be skipped, and the mask may be used without hotspot monitoring occurring during manufacturing and inspection. may be done.

In the illustrated example, if the design is not deemed flawed, then in operation 326 it may be determined whether the hotspot can be monitored. If it is determined that the hot spot can be monitored, then, for example, in operation 334, the hot spot can be monitored during wafer processing. For example, hot spot patterns can be monitored during wafer fabrication to determine if the process has gone out of specification and caused the corresponding wafer pattern to have a critical parameter change to an unacceptable value. Ru. One implementation may include setting relatively high MEEF levels for inspection of the reticle and/or wafer pattern of corresponding hot spots. As conditions move further away from the nominal process conditions, the CD or EPE can become larger and can compromise the integrity of the wafer fabrication process.

The hotspot pattern changes when the test mask pattern changes by a predetermined amount, regardless of how such change compares to the original intended design (e.g., pre-OPC data). can be simply identified. In other words, significant changes in the physical mask pattern under different process conditions may indicate a problem with the intended design pattern. Differences between corresponding modeled image portions represent differences in the effects of process conditions on the designed pattern and manufactured mask. Differences associated with specific design patterns are commonly referred to as "design hotspots," or simply "hot spots," and perhaps also identify weaknesses in the design with respect to the specific process conditions tested on the manufactured mask. represent. Examples of the types of differences that may be found between images modeled for different process conditions are critical dimension (CD) or edge placement error (EPE).

In another embodiment, when the model is applied to a post-OPC design database, the resulting wafer pattern may correspond to the pattern intended to be printed on the wafer by the designer. Optionally, the results obtained from applying the model to the database of post-OPCs can be used, and the modeled images improve hotspot detection. For example, the model of the post-OPC database takes into account only design effects, and thus can be used to separate the effects of wafer processing on the design and the effects of wafer processing on the manufactured mask. The modeled pattern from the near field of the mask can be compared to the modeled wafer image from the corresponding post-OPC pattern. For example, when a set of modeled wafer patterns for different process variations is matched to a corresponding modeled posterior OPC wafer pattern for the same process variation, the wafer pattern due to the process variation (or It can be determined that changes in the resist pattern (resist pattern) result from the design pattern rather than from defects in the mask pattern, which can be redesigned or monitored. However, if the changes on the wafer due to process variations from the post-OPC database are different from the changes on the wafer due to the same process variations from the reconstructed mask (or near field of the mask), then these hot spots are assumed to result from hot spots from the actual mask, which can be repaired or monitored.

Differences in simulated wafer images can also be analyzed to determine wafer CD uniformity (CDU) metrics across the die or over time as reticle changes occur during exposure in the manufacturing process. . For example, CD can be measured for each target in each image by analyzing and measuring the distance between the edges of the targets if the resolution is high enough. Alternatively, U.S. Patent Application No. 14/664,565 filed March 20, 2015 by Carl E. Hess et al. As further described in U.S. patent application Ser. These applications are incorporated herein by reference in their entirety for all purposes.

At operation 328, it may also be determined whether the reticle is to be repaired. Expected wafer pattern variations may be determined to be out of specification for the process window expected to be used during the lithography process. In some cases, in operation 336, the reticle may include a defect to be repaired. The reticle may then be evaluated. Otherwise, the reticle may be discarded in operation 330 if it is not repairable. A new reticle can then be manufactured and evaluated.

In addition to or as an alternative to using the reconstructed mask near-field image to simulate the wafer image during the evaluation process, the mask near-field image or results may be used during the reticle evaluation process. Direct assessment is also possible. FIG. 5 is a flow diagram illustrating a reticle evaluation process 500 applied to a reconstructed mask near-field image or result according to an alternative embodiment of the invention. In operation 502, the near field results of the mask are first reconstructed from the reticle. A near-field image of this mask can be reconstructed for a particular reticle based on images acquired from that particular reticle. This operation may be performed similar to the near-field reconstruction operation of the mask in FIG. Additionally, some of the operations in FIG. 5 may be performed similar to the operations in FIG. Contains ingredients.

As shown, in operation 522, the near-field results of the mask may then be assessed to characterize and/or locate the defect. It can generally be determined whether the corresponding reticle is likely to be defective or have hot spots that require monitoring. More specifically, some of the techniques described herein that assess simulated wafer images may be performed on near-field images of the mask. In the process of defect detection, any suitable metrics of near-field images of test and reference masks may be compared. For example, intensity and/or phase may be compared. Different defect types have different effects on intensity and/or phase values. Are these differences actual defects (as opposed to nuisance defects that have no impact) that could result in defective wafers that can be repaired or monitored or identify patterns or areas of hot spots? can be determined.

For example, it may then be determined in operation 524 whether the design is flawed. If the design is determined to be flawed, the design may be modified in operation 532. For example, it may be determined whether any difference between a near-field image of a reticle and its corresponding posterior OPC-based near-field exceeds a predetermined threshold for detecting defects. Procedure 500 can continue to monitor hot spots on the wafer and determine whether to repair or redesign the reticle, as described above. If the design is not deemed flawed, then in operation 526 it may be determined whether any hotspots can be monitored. For example, it may be determined that any intensity and/or phase difference between the near-field image of the test reticle and the near-field image of the reference reticle is close to an associated threshold.

If it is determined that the hot spot can be monitored, the hot spot can then be monitored during wafer processing, for example, in operation 534. For example, hot spot patterns can be monitored during wafer fabrication to determine if the process has gone out of specification and caused the corresponding wafer pattern to have a critical parameter change to an unacceptable value. be done. One implementation may include setting a relatively high level of sensitivity for inspection of the reticle and/or wafer pattern of corresponding hot spots. As conditions move further away from nominal process conditions, CD errors or EPE can become larger and can compromise the integrity of the wafer fabrication process.

At operation 528, a determination may also be made as to whether the reticle is to be repaired. In some cases, in operation 536, the reticle may include a defect to be repaired. The reticle may then be re-evaluated. Otherwise, the reticle may be discarded in operation 530 if it is not repairable. A new reticle can then be manufactured and re-evaluated.

Some techniques of the present invention provide mask pattern evaluation and early detection of weak patterns or hot spots on the physical mask prior to wafer fabrication. In addition to performing near-field reconstruction of the reticle based on the reticle image, the full range of wafer process effects (many settings for focus and exposure, as well as wafer resist, etch, CMP, and any other wafer (including process effects) and how they adversely affect the wafer pattern. No prior knowledge of the mask is required since the near field of the mask is reconstructed using only the reticle image without using reticle design data. Since the mask pattern is typically four times larger than the wafer pattern, a more accurate position of the pattern relative to the design database can be determined. The above techniques can also be extended to any suitable type of mask, such as pattern evaluation for EUV masks.

The techniques of the present invention may be implemented in any suitable combination of hardware and/or software. FIG. 6 is a schematic diagram of an example inspection system 600 that can implement the techniques of the present invention. Inspection system 600 may receive input 602 from a low NA inspector (not shown) that mimics a high NA inspection tool or scanner. The inspection system includes a data distribution system (e.g., 604a and 604b) that delivers received input 602 and an intensity signal (or patch) processing system (e.g., patch) for mask near-field and wafer reproduction, process modeling, etc. a processor and reticle evaluation system (e.g., 612)), a network (e.g., a switching network 608) to enable communication between inspection system components, optional mass storage 616, and a near-field image of the mask. One or more inspection control and/or review stations (e.g., 610). Each processor of test system 600 typically includes one or more microprocessor integrated circuits, may also include an interface and/or memory integrated circuit, and has one or more shared and and/or may be further coupled to a global memory device.

The inspector or data acquisition system (not shown) generating input data 602 may take the form of any suitable equipment (e.g., as further described herein) for obtaining an intensity signal or image of a reticle. You can take it. For example, a low NA inspector may construct an optical image based on a portion of the detected light that is reflected, transmitted, or otherwise directed to one or more optical sensors, or a reticle. It is possible to generate an intensity value for a portion of . The lower NA inspector can then output intensity values or images.

The low NA inspection tool may be operable to detect and collect reflected and/or transmitted light as the incident optical beam scans across each patch of the reticle. As mentioned above, the incident optical beam can be scanned across a swath of the reticle, each consisting of a plurality of patches. Light is collected from multiple points or subareas of each patch in response to this incident beam.

A low NA inspection tool may generally be operable to convert such detected light into a detected signal corresponding to an intensity value. The detected signal may be in the form of an electromagnetic waveform having amplitude values corresponding to different intensity values at different locations on the reticle. The detected signal may also take the form of a simple list of intensity values and associated reticle point coordinates. The detected signal may also take the form of an image with different intensity values corresponding to different locations or scan points on the reticle. Two or more images of the reticle may be generated after all positions of the reticle are scanned and converted into detected signals, or two or more image portions may be generated after each reticle portion is scanned and converted into detected signals. After scanning the entire reticle, a final two or more images of the reticle can be generated as it is completed.

The detected signal may also take the form of an aerial image. That is, spatial imaging techniques can be used to simulate the optical effects of a photolithography system to generate an aerial image of a photoresist pattern exposed on a wafer. Generally, the optics of a photolithography tool are emulated to generate an aerial image based on detected signals from a reticle. The aerial image corresponds to the pattern generated from the light passed through the photolithography optics and reticle onto the photoresist layer of the wafer. Additionally, photoresist exposure processes for specific types of photoresist materials can also be emulated.

The incident or detected light may pass through any suitable spatial aperture to produce any incident or detected light profile at any suitable angle of incidence. As an example, programmable illumination or detection apertures can be utilized to generate specific beam profiles such as dipoles, quadrupole, quasar, annulus, etc. In particular examples, Source Mask Optimization (SMO) or any pixelated illumination technique may be implemented. The incident light may also pass through a linear polarizer to linearly polarize all or part of the illumination pupil in one or more polarizations. The detected light may be passed through an apodization component to block specific areas of the collection beam.

Intensity or image data 602 may be received by a data distribution system via network 608. The data distribution system may be associated with one or more memory devices, such as a RAM buffer, to retain at least a portion of the received data 602. Preferably, the total memory is large enough to hold the entire sample of data. For example, 1 gigabyte of memory works well for a sample that is 1 million by 1000 pixels or points.

The data distribution system (eg, 604a and 604b) can also control the distribution of a portion of the received input data 602 to the processors (eg, 606a and 606b). For example, the data distribution system may route data for a first patch to first patch processor 606a and may route data for a second patch to patch processor 606b. Also, multiple datasets for multiple patches may be routed to each patch processor.

A patch processor can receive intensity values or images corresponding to at least a portion of the reticle or patch. Each patch processor may also be coupled to or integrated with one or more memory devices (not shown), such as DRAM devices, which provide local memory functions such as retaining received data portions. can. Preferably, the memory is large enough to hold data corresponding to patches on the reticle. For example, 8 megabytes of memory works well for intensity values or images corresponding to patches that are 512 x 1024 pixels. Alternatively, patch processors may share memory.

Each set of input data 602 can correspond to a swath of a reticle. One or more data sets may be stored in memory of the data distribution system. This memory may be controlled by one or more processors within the data distribution system, and the memory may be divided into multiple partitions. For example, the data distribution system may accept data corresponding to a portion of a swath into a first memory partition (not shown), and the data distribution system may accept other data corresponding to another swath into a second memory partition (not shown). memory partition (not shown). Preferably, each memory partition of the data distribution system only holds a portion of the data that is routed to the processor associated with such memory partition. For example, a first memory partition of the data distribution system may hold and route first data to patch processor 606a, and a second memory partition may hold and route second data to patch processor 606b. can do.

The data distribution system can define and distribute each dataset of data based on any suitable parameters of the data. For example, data can be defined and distributed based on the corresponding location of the patch on the reticle. In one embodiment, each band is associated with a range of column positions that corresponds to the horizontal position of pixels within the band. For example, columns 0 through 256 of the band may correspond to a first patch, and the pixels within these columns are routed to one or more patch processors in a first image or a first set. contains the intensity value. Similarly, swatch columns 257 to 512 may correspond to a second patch, and the pixels in these columns represent a second image or a second set of intensity values that are routed to a different patch processor. include.

The inspection apparatus may be suitable for inspecting semiconductor devices or wafers, as well as optical reticles and EUV reticles or masks. An example of a suitable inspection tool is the Teron™, operating at 193 nm, available from KLA-Tencor of Milpitas, California, or the TeraScan™ DUV reticle inspection tool. Other types of specimens that can be inspected or imaged using the inspection apparatus of the present invention include any surface such as a flat panel display.

As further described herein, the inspection tool includes at least one light source that generates an incident beam of light, illumination optics that directs the incident beam to the sample, and at least one light source that directs the incident beam to the sample. Collection optics for directing the output beam, sensors for detecting the output beam and generating images or signals about the output beam, and techniques for controlling the components of the inspection tool and generating and analyzing the near field of the mask. and a controller/processor to assist in the process.

In the following exemplary inspection systems, the input beam may be in any suitable form of coherent light. Additionally, any suitable lens configuration can be used to direct the input beam to the sample and the output beam resulting from the sample to the detector. The output beam may be reflected or scattered from the sample, or transmitted through the sample. For EUV reticle inspection, the output beam is typically reflected from the sample. Similarly, based on the characteristics (e.g., intensity) of the received output beam, any suitable detector type or number of detection elements may be used to receive the output beam and provide an image or signal. can.

First, describing generalized photolithography tools, EUV photolithography tools typically have only reflective type optics. FIG. 7A is a simplified schematic diagram of an exemplary lithography system 700 that can be used to transfer a mask pattern from a photomask M onto a wafer W according to some embodiments. Examples of such systems include scanners and steppers, and more specifically the TWINSCAN NXT:1970Ci Step-and-Scan system available from ASML, Veldhoven, The Netherlands. Generally, illumination source 703 directs a light beam through illumination optics 707 (eg, lens 705) onto photomask M located at mask plane 702. Illumination lens 705 has a numerical aperture 701 in plane 702. The value of numerical aperture 701 influences which defects on the photomask are lithographically significant defects and which are not. The portion of the beam that passes through photomask M forms a patterned optical signal that is directed onto wafer W through imaging optics 713 to initiate pattern transfer. In a reflective system (not shown), the illumination beam is reflected from some parts of the mask M (and absorbed by other parts of such mask M) by reflective imaging optics on the wafer W. forming a directed patterned signal.

The inspection tool may utilize similar components or may be similarly configured as the photolithography tool described above, eg, with LNI capability. However, alternatively or in addition, the inspection tool may be configurable to generate high resolution images. FIG. 7B shows a schematic diagram of an example inspection system 750 that includes an imaging lens system with illumination optics 751a and a relatively large numerical aperture 751b at the reticle plane 752, according to some embodiments. For example, the numerical aperture 751b at the reticle plane 752 of the inspection system can be significantly larger than the numerical aperture 701 at the reticle plane 702 of the lithography system 700, thereby reducing the difference between the test inspection image and the actual printed image. become.

The inspection techniques described herein can be implemented in a variety of specially configured inspection systems, such as the system shown schematically in FIG. 7B. The illustrated system 750 includes an illumination source 760 that generates a light beam that is directed onto a photomask M in a reticle plane 752 through illumination optics 751a. Examples of light sources include coherent laser sources (eg, deep UV or gas laser generators), filtered lamps, LED light sources, and the like. In some embodiments, the light source can generally provide high pulse repetition rates, low noise, high power, stability, reliability, and scalability. Note that while EUV scanners operate at a wavelength of 13.5 nm, inspection tools for EUV reticles can, but do not have to operate at the same wavelength. In one example, the light source is a 193 nm laser.

Illumination optics 751a can include beam steering devices to precisely direct the beam and beam conditioning devices that can be used to provide light level control, speckle noise reduction, and high beam uniformity. The beam steering and/or beam conditioning device may be a separate physical device from the laser, for example. The illumination optical system 751a can also include an optical system that controls polarization, focus, magnification, distribution of illumination intensity, and the like.

As mentioned above, inspection system 750 has a numerical aperture 751b in reticle plane 752 that can be equal to or greater than the numerical aperture of the reticle plane of a corresponding lithography system (e.g., element 701 in FIG. 7A). You can also do that. The photomask M to be inspected is placed on the mask stage at the reticle plane 752 and exposed to a light source.

The illustrated inspection system 750 comprises detection optics 753a and 753b and may also include magnification optics of a microscope designed to provide magnification of, for example, 60 to 200 times or more, for enhanced inspection. can. Collection optics 753a and 753b may include any suitable optics that condition the output light/beam. For example, condensing optical systems 753a and 753b can include optical systems that control focus, pupil shape, polarization analyzer settings, and the like.

In transmission mode, the patterned image from mask M may be directed through the condenser of optical element 753a, which projects the patterned image onto sensor 754a. In reflection mode, the focusing elements (eg, beam splitter 776 and detection lens 778) direct and capture reflected light from mask M onto sensor 754b. Although two sensors are shown, a single sensor can be used to detect reflected and transmitted light during different scans of the same reticle area. Suitable sensors include charge coupled devices (CCDs), CCD arrays, time delay integral (TDI) sensors, TDI sensor arrays, photomultiplier tubes (PMTs), and other sensors.

The column of illumination optics can be moved relative to the mask stage and/or the stage can be moved relative to the detector or camera by any suitable mechanism to scan the patch of the reticle. . For example, a motor mechanism can be used to move the stage. The motor mechanism can be constituted, by way of example, by a screw drive and a stepper motor, a linear drive with feedback position, or a band actuator and a stepper motor. System 700 may utilize one or more monitoring mechanisms to move any of the system components relative to the illumination or collection optical path.

The signals captured by each sensor (e.g. 754a and/or 754b) can be processed by computer system 773, or more generally by one or more signal processing devices; Each of these includes an analog-to-digital converter configured to convert the analog signal from each sensor to a digital signal for processing. Typically, computer system 773 has one or more processors coupled to input/output ports and one or more memories via a suitable bus or other communication mechanism.

Computer system 773 may also include one or more input devices (eg, keyboard, mouse, joystick) for providing user input, such as changing focus and other examination procedure parameters. Additionally, computer system 773 can be connected to a stage, for example, to control sample position (e.g., focusing and scanning), as well as other inspection parameters and configuration of such other inspection system components. It can also be connected to other inspection system components for control.

Computer system 773 provides a user interface (e.g., computer screen) that displays the near-field intensity and phase (value, image, or difference) of the mask, reticle/wafer image, identified hotspot CD, CDU map, It can be configured (eg, by programming instructions) to provide process parameters, etc. Computer system 773 is configured to analyze intensity, phase, and/or other characteristics of reflected and/or transmitted detected and/or simulated signals or images, reproduced reticle near-field results, etc. may be configured. Computer system 773 is configured (e.g., by programming instructions) to provide a user interface (e.g., on a computer screen) for displaying the resulting intensity and/or phase values, images, and other inspection characteristics. can do. In some embodiments, computer system 773 is configured to perform the inspection techniques detailed above.

Such information and program instructions may be implemented on a specially configured computer system so that such system can perform the various operations described herein that may be stored on a computer-readable medium. It comprises program instructions/computer code for execution. Examples of machine-readable media include, but are not limited to, hard disks, floppy disks, magnetic media such as magnetic tape, optical media such as CD-ROM disks, and optical media such as optical disks. Included are magnetic media and hardware devices specifically configured to store and execute program instructions, such as read-only memory devices (ROM) and random access memory (RAM). Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that can be executed by a computer using an interpreter.

FIG. 7B shows an example in which the illumination light beam is directed onto the sample surface approximately perpendicular to the inspection surface. In other embodiments, the illumination light beam can be directed at an oblique angle, thereby allowing separation of the illumination and reflected beams. In these embodiments, an attenuator may be placed in the path of the reflected light beam to attenuate the zero order component of the reflected light beam before reaching the detector. Additionally, an imaging hole may be placed on the path of the reflected light beam to phase shift the zero-order component of the reflected light beam.

Note that the above description and drawings are not to be construed as limitations with respect to particular components of the system, and that the system may be embodied in many other forms. For example, the inspection or metrology tool may detect any suitable feature from any number of known imaging or metrology tools arranged to detect defects and/or resolve significant aspects of the reticle or wafer feature. It is considered that it may be included. By way of example, the inspection or measurement tool may be applied to bright field imaging microscopy, dark field imaging microscopy, full sky imaging microscopy, phase contrast microscopy, polarized light contrast microscopy, and coherence probe microscopy. It is also contemplated that single and multiple image methods can be used to capture images of the target. These methods include, for example, single grab, double grab, single grab coherence probe microscopy (CPM), and double grab CPM. Non-imaging optical methods such as scatterometry scatterometry can also be considered as forming part of the inspection or metrology device.

Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be obvious that changes and modifications may be practiced within the scope of the appended claims. It should be understood that there are many alternative ways of implementing the processes, systems, and apparatus of the present invention. Accordingly, the present embodiments are to be regarded as illustrative, not limiting, and the invention is not limited to the details set forth herein.

Claims (10)

A method for evaluating a photolithography reticle, the method comprising:
using an imaging tool to obtain a plurality of images with different illumination configurations or different imaging configurations from each of the plurality of pattern areas of the test reticle;
A reticle near field regression technique for each pattern area of the test reticle based on the acquired images from each pattern area of the test reticle without using the design database used to manufacture the test reticle. a step of calculating and reproducing by
Analyzing the intensity and/or phase of the reconstructed reticle near-field image to determine the intensity and/or phase of the test and reference reticle near-field images without simulating a wafer image. determining whether the test reticle is likely to produce a defective wafer depending on whether the difference is within a certain threshold. The method according to claim 1,
The method characterized in that the plurality of images are acquired in a pupil plane. The method according to claim 1,
The near field of the reconstructed reticle is analyzed to detect defects in the test reticle, and the defect detection is performed for the same die at different times, for adjacent dies, for a die and its corresponding golden A method comprising comparing the near field strength and/or phase for a die or for a die and a corresponding die from a reticle copy having the same design as the test reticle. The method according to claim 1,
The acquired images include at least three reflection images acquired with different imaging configurations selected to be in the near field of the same reticle, the different imaging configurations having different focus settings, different pupil shapes . , wherein the different illumination configurations include different source intensity distributions . The method according to claim 1,
The acquired images include at least three transmission images acquired with different imaging configurations selected to be in the near field of the same reticle, the different imaging configurations having different focus settings, different pupil shapes . , wherein the different illumination configurations include different source intensity distributions . An imaging system for evaluating a photolithography reticle, the imaging system comprising:
a light source that generates an incident beam;
an illumination optics module that directs the incident beam onto a reticle;
a focusing optics module that directs an output beam from each pattern area of the reticle to at least one sensor;
at least one sensor that detects the output beam and generates an image or signal based on the output beam;
The following actions, i.e.
acquiring multiple images with different illumination configurations or different imaging configurations from each of the multiple pattern areas of the test reticle;
A reticle near field regression technique for each pattern area of the test reticle based on the acquired images from each pattern area of the test reticle without using the design database used to manufacture the test reticle. Calculate and reproduce by
Analyzing the intensity and/or phase of the reconstructed reticle near-field image to determine the intensity and/or phase of the test and reference reticle near-field images without simulating a wafer image. and a controller configured to perform determining whether the test reticle is likely to produce a defective wafer depending on whether the difference is within a certain threshold. Imaging system. 7. The system according to claim 6,
A system characterized in that the plurality of images are acquired in a pupil plane. 7. The system according to claim 6,
The near field of the reconstructed reticle is analyzed to detect defects in the test reticle, and the defect detection is performed for the same die at different times, for adjacent dies, for a die and its corresponding golden A system comprising comparing the near field strength and/or phase for a die or for a die and a corresponding die from a reticle copy having the same design as the test reticle. 7. The system according to claim 6,
The acquired images include at least three reflection images acquired with different imaging configurations selected to be in the near field of the same reticle, the different imaging configurations having different focus settings, different pupil shapes . , wherein the different illumination configurations include different source intensity distributions . 7. The system according to claim 6,
The acquired images include at least three transmission images acquired with different imaging configurations selected to be in the near field of the same reticle, the different imaging configurations having different focus settings, different pupil shapes . , wherein the different illumination configurations include different source intensity distributions .

Images