Method and apparatus for mask feature analysis

Abstract

A method and apparatus for mask feature analysis includes measuring an interference signal of a reflective or transmissive mask used for lithography and determining a quality indicator of the reflective or transmissive mask based on the interference signal. An apparatus for mask feature analysis includes a light source, an optical grating, and an optical detector array. The light source is configured to illuminate the reflective or transmissive mask with light, thereby generating mask reflection or mask transmission light. The optical grating is configured to convert the mask reflection or mask transmission light into an interference pattern. The optical detector array is configured to generate the interference signal by measuring the interference pattern.

Description

Technical Field

[0001] This disclosure relates to methods and apparatus for mask feature analysis. Background Technology

[0002] The following content concerns extreme ultraviolet (EUV) lithography, online monitoring of lithography masking processes, and related technologies. Summary of the Invention

[0003] According to one embodiment of this disclosure, a method for mask feature analysis includes measuring an interference signal of a reflective or transmissive mask used for semiconductor lithography, and determining a quality index of the reflective or transmissive mask based on the interference signal.

[0004] According to another embodiment of this disclosure, a method for mask feature analysis includes reflecting light from the mask or passing light through the mask to generate reflected or transmitted light, reflecting the reflected or transmitted light from an optical grating or passing the reflected or transmitted light through an optical grating to generate an interference pattern, and determining a quality index of the mask based on the interference pattern.

[0005] According to another embodiment of this disclosure, an apparatus for mask feature analysis includes a light source, an optical grating, and an array of optical detectors. The light source is configured to illuminate a reflective or transmissive mask, thereby generating mask-reflected or mask-transmitted light. The optical grating is configured to convert the mask-reflected or mask-transmitted light into an interference pattern. The array of optical detectors is configured to generate an interference signal by measuring the interference pattern. Attached Figure Description

[0006] Please read the following embodiments in conjunction with the accompanying drawings for a clear understanding of the viewpoints of this disclosure. It should be noted that, according to industry standard practice, the various features are not drawn to scale. In fact, for the sake of clear discussion, the dimensions of the various features may be arbitrarily enlarged or reduced.

[0007] Figure 1 A device for mask feature analysis is schematically illustrated according to some embodiments;

[0008] Figure 2 A device for mask feature analysis is schematically illustrated according to some embodiments;

[0009] Figure 3 A device for mask feature analysis is schematically illustrated according to some embodiments;

[0010] Figure 4 The interference pattern measured by the optical detector array of the device for mask feature analysis is schematically illustrated according to some embodiments.

[0011] Figure 5The reference and target interference signals measured by the device for mask feature analysis are schematically illustrated according to some embodiments;

[0012] Figure 6 A method for mask feature analysis is schematically illustrated according to some embodiments;

[0013] Figure 7 A method for mask feature analysis is schematically illustrated according to some embodiments;

[0014] Figure 8 A method for determining the quality index of a mask based on measured reference and target interference signals is illustrated schematically according to some embodiments;

[0015] Figure 9 A method for determining the quality index of a mask based on measured reference and target interference signals is illustrated schematically according to some embodiments;

[0016] Figure 10 Some embodiments illustrate methods for using machine learning to determine one or more quality metrics of a mask.

[0017] [Symbol Explanation]

[0018] 10: Reflection Mask

[0019] 10': Transmission shield

[0020] 10 R :Reflection mask / Reference mask

[0021] 10' R Transmission mask / reference mask

[0022] 10 T Reflection mask / Target mask

[0023] 10' T Transmission mask / Target mask

[0024] 11: Installation components

[0025] 11': Installation component

[0026] 12: Light source

[0027] 14, 15: Optical gratings

[0028] 16: Optical Detector Array

[0029] 18: Electronic Processor

[0030] 20: Reflective multilayer stacking

[0031] 21: Light transmission substrate

[0032] 22: Absorption layer

[0033] 30, 32, 34, 36, 40, 50, 60, 62, 70, 72, 82: Operation

[0034] 42: Film thickness measured by AFM

[0035] 52: Computer

[0036] 64: Quality Indicators

[0037] 74: Quality Indicators

[0038] 80: Marked reference interference signal combination

[0039] 84: Mask Quality Classifier

[0040] 86: Quality Indicators

[0041] A: Area

[0042] A D Defect area

[0043] F I Interference fringes

[0044] L1: Light

[0045] L2: Light reflected from the mask / EUV light reflected from the mask

[0046] L2': Light transmitted through the mask / EUV light transmitted through the mask

[0047] P: Interference pattern

[0048] R C Central area

[0049] R P :Outer area

[0050] S: Interference signal

[0051] S R Reference interferometer signal

[0052] ST: Target Interference Signal Detailed Implementation

[0053] The following disclosure provides many different embodiments or examples to demonstrate the different features of this disclosure. Specific examples of the elements and their arrangements will be disclosed below to simplify the description of this disclosure. Of course, these specific examples are not intended to limit this disclosure. For example, if the following description of forming a first structure on or above a second structure indicates that it includes embodiments where the first and second structures are in direct contact, as well as embodiments where additional structures can be formed between the first and second structures, in which case the first and second structures are not in direct contact. Furthermore, the various examples in this disclosure may use repeated reference numerals and / or words. These repeated numerals or words are for simplification and clarity and are not intended to limit the various embodiments and / or the relationship between the described appearance structures.

[0054] Furthermore, to facilitate the description of the relationship between one element or feature element(s) and another element or feature element(s) in the accompanying drawings, spatially related terms such as "below," "under," "lower," "above," "upper," and similar terms may be used. In addition to the orientations shown in the accompanying drawings, spatially related terms also cover different orientations of the device in use or operation. When the device is turned to a different orientation (e.g., rotated 90 degrees or otherwise), the spatially related adjectives used therein will also be interpreted according to the orientation after the turn.

[0055] The thin-film optical properties of a lithography mask affect the lithography performed using the mask. Typically, the thin-film optical properties on an EUV lithography blank mask are measured using a synchrotron EUV source before mask fabrication. However, this method is difficult to apply to in-line monitoring during the mask fabrication process. Atomic force microscopy (AFM) can be used to characterize the mask during various stages of fabrication or at the end of fabrication; however, while AFM provides information on surface structure features, it does not provide sufficient information on the optical properties of the processed thin film. Furthermore, AFM is a slow technique.

[0056] refer to Figure 1The apparatus for feature analysis of a reflective mask 10 is schematically illustrated according to some embodiments. The reflective mask 10 is sometimes also referred to as a magnifying mask; these terms are considered interchangeable herein. A light source 12 is configured to illuminate the reflective mask 10, which is mounted on a mounting 11 (e.g., an electrostatic or electronic chuck suitable for use in a vacuum environment), with light L1. The light L1 is reflected by the reflective mask 10 to produce EUV light L2 reflected by the mask. In some embodiments, the light source 12 is an extreme ultraviolet (EUV) light source, wherein the EUV spectral range is between 10 nanometers (nm) and 124 nm (10 electron volts (eV) to 124 eV). In some embodiments, the light L1 emitted by the light source 12 has a maximum spectral peak at a wavelength of 100 nm or less. It is contemplated that a maximum spectral peak at a wavelength of 100 nm or less may be shorter than 10 nm; for example, a maximum spectral peak at a wavelength of 100 nm or less may be 1 nm. Light L1 with a maximum spectral peak at a wavelength of 100 nm or less can be X-ray radiation. In some further embodiments suitable for EUV lithography processes operating at 13.4 nm to 13.6 nm, light L1 output from light source 12 can have a maximum spectral peak in the spectral range of 13.4 nm to 13.6 nm. These are merely non-limiting illustrative examples, and light L1 can alternatively have a maximum spectral peak in another wavelength range (e.g., in the ultraviolet range at higher wavelengths (e.g., in the range of 10 nm to 400 nm), the visible spectral range (e.g., in the range of 400 nm to 700 nm, i.e., light L1 can be visible light), or the infrared spectral range).

[0057] Figure 1 The apparatus for analyzing the mask features further includes an optical grating 14 configured to convert the light L2 reflected from the mask (if light L1 is EUV light, then the light L2 reflected from the mask is EUV light) into an interference pattern P. An optical detector array 16 is configured to generate an interference signal S by measuring the interference pattern P. An electronic processor 18 is programmed to determine, at least, a quality index of the reflective mask 10 based on the interference signal S. In various non-limiting illustrative embodiments, the electronic processor 18 may include a computer or other microprocessor-based digital data processing device, a microcontroller, etc., and may include suitable auxiliary elements, such as one or more analog-to-digital converters to convert the interference signal S into a digital representation, a display, a speaker, and / or one or more means for generating a human-perceptible representation of the quality index, etc.

[0058] Generally, light source 12 may include a string of optical elements (not shown) comprising one or more reflective and / or refractive optical elements configured to shape and guide light L1 onto reflective shield 10. In embodiments where light source 12 outputs light L1 as EUV light, the light source can be implemented in various ways. For example, light source 12 may include a free electron laser (FEL) source or a high harmonic generation (HHG) EUV source or a laser-produced plasma (LPP) EUV source, such as a pulsed tin plasma EUV source incorporating spatial filtering or a synchrotron source. In embodiments where light source 12 outputs light L1 as EUV light, reflective optics are typically used to construct the string of optical elements. In some non-limiting embodiments, light source 12 may include a synchrotron EUV source that does not include a synchrotron source.

[0059] Optical grating 14 is designed to be used with light L1 output from light source 12. As a non-limiting illustrative example of EUV light, the light L1 output from light source 12 may be a reflective EUV grating. Optical detector array 16 is also designed to be used with light L1 output from light source 12. As a non-limiting illustrative example of EUV light, the light L1 output from light source 12 may be a charge-coupled device (CCD) array, a CMOS detector array, or other EUV-sensitive detector array.

[0060] Figure 1 The device for analyzing the masking features employs a reflective optical grating 14. (Brief Reference) Figure 2 The apparatus for mask feature analysis is illustrated schematically according to some different embodiments. Figure 2 The apparatus for analyzing the mask features again includes a light source 12, which is configured to illuminate a reflective mask 10 mounted on a mounting 11 with light L1. The light L1 is reflected by the reflective mask 10 to produce EUV light L2 reflected by the mask. However, in Figure 2 In this embodiment, the optical grating 15 is a transmission optical grating, and light passing through the transmission optical grating 15 converts the light L2 reflected by the shield into an interference pattern P. For example, in... Figure 1 In this embodiment, the optical detector array 16 is configured to measure the interference pattern P to generate an interference signal S, and the electronic processor 18 is programmed to determine (at least) the quality index of the reflective shield 10 based on the interference signal S. The transmission optical grating 15 is again designed to be suitable for the light L1 output by the light source 12. As a non-limiting illustrative example of EUV light, the light L1 output by the light source 12 can be a transmission EUV grating.

[0061] Figure 1 and Figure 2 The apparatus for mask feature analysis is used to perform feature analysis on the reflective mask 10. (Brief Reference) Figure 3 The apparatus for mask feature analysis is schematically illustrated according to several different embodiments. Except that the reflective mask 10 to be feature-analyzed is replaced with a transmissive mask 10', Figure 3 The device for analyzing mask features is similar to Figure 2 The apparatus for analyzing the characteristics of a transmission mask 10' is mounted on a mounting member 11', the mounting member 11' having a central opening or similar to allow light L1 emitted from a light source 12 to pass through the transmission mask 10' to produce EUV light L2' transmitted through the mask. For example, transmission masks are commonly used in visible light lithography. Figure 1 and Figure 2 In this embodiment, the optical detector array 16 is configured to measure the interference pattern P to generate an interference signal S, and the electronic processor 18 is programmed to determine (at least) the quality index of the transmission shield 10' (in this embodiment) based on the interference signal S. The transmission optical grating 15 is again designed to be suitable for the light L1 output by the light source 12.

[0062] Although not explicitly stated, it should be understood that in yet another variation of the embodiment, for Figure 3 The equipment for feature analysis of the transmission mask 10' can be adopted. Figure 1 The reflective optical grating 14 replaces the transmission optical grating 15.

[0063] Re-reference Figure 1 and Figure 2 The illustration schematically depicts two illustrative reflective masks 10, more specifically referred to herein as reference masks 10. R and target masking 10 T Explanatory masking 10 R and 10 TFor use as a reflective lithography mask in lithography, the material comprises a reflective multilayer stack 20 disposed on a substrate (not shown) and an absorption layer 22 disposed on the reflective multilayer stack 20. More specifically, the reflective multilayer stack 20 reflects light L1, while the absorption layer 22 absorbs light L1. In an illustrative example where light L1 is EUV light, the EUV reflective multilayer stack 20 may, for example, comprise alternating layers of silicon (Si) and molybdenum (Mo). In some more specific embodiments, sometimes used for 13.5nm EUV lithography, the reflective multilayer stack 20 may comprise 40 pairs of Si / Mo. For the case of a reflective EUV mask, the absorption layer 22 may comprise, for example, a layer of chromium (Cr), chromium nitride (CrN), chromium oxynitride (CrON), chromium carbonitride (CrCON), etc., having a sufficient thickness to absorb the EUV light L1 irradiating the absorption layer 22. The reflective shield 10 used in EUV embodiments may include other features not shown in the illustrations, such as a substrate of low thermal expansion material (LTEM) (e.g., titanium dioxide (TiO2) or doped silicon dioxide (SiO2)) for supporting the reflective multilayer stack 20 and the absorption layer 22, and / or a back-side multilayer stack (optionally used) to help secure the shield 10 with a mounting member 11 such as an electrostatic chuck. The reflective shield 10 may also include structural supports and / or protective films, depending on the situation. Furthermore, it should be understood that these are merely non-limiting illustrative examples of suitable shields for EUV lithography, and the structure of the reflective shield 10 is generally designed to suit the type of lithography used. For example, for visible light lithography, the reflective multilayer stack may be replaced by a single metal layer with high reflectivity to visible light, and similarly, the absorption layer may be a material with strong absorption to visible light.

[0064] Furthermore, features of the reflective shield 10 are formed in the surface of the reflective shield 10, and these features cause the surface of the reflective shield 10 to have different reflectivities for light L1. For example, in the case of an EUV shield, the illustrative reference shield 10 R and target masking 10 T Region A is included, where the absorption layer 22 is removed, by methods such as electron beam writing and subsequent etching. To maximize the reflection of light L1 in region A, the absorption layer 22 is completely removed in region A to expose the surface of the underlying EUV reflective multilayer stack 20. However, as shown, the target mask 10... T Includes defect area A D In defect area A D The absorption layer 22 was not completely removed. Therefore, defect region A... D When absorbing EUV light L1, or when compared to region A, at least defective region A DIt has low reflectivity to EUV light L1. Illustrative defect area A D It's possible that the mask wasn't fully etched during manufacturing. More general defect areas (e.g., defect area A) D It may be defective for various reasons (in the sense of having a reflectivity for light L1 that differs from the reflectivity based on the design), such as (but not limited to) incomplete etching (as illustrated), one or more particulate contaminations, oxides or other coatings formed after manufacturing, and various combinations thereof.

[0065] refer to Figure 3 The illustration schematically shows two illustrative transmission masks 10', which are more specifically referred to herein as reference transmission masks 10'. R and target transmission shield 10' T Explanatory transmission shield 10' R and 10' T For lithography, a transmissive lithography mask is used, and does not contain... Figure 1 and Figure 2 Reflective mask 10 of the embodiment R and 10 T The reflective multilayer stack 20. More precisely, the illustrative transmission mask 10' R and 10' T An absorption layer 22 is disposed on a light-transmitting substrate 21. More specifically, the light-transmitting substrate 21 is transparent to light L1, while the absorption layer 22 absorbs light L1. Depending on the wavelength of light L1, the light-transmitting substrate 21 may include, for example, quartz, titanium dioxide (TiO2), doped silicon dioxide (SiO2), etc.

[0066] refer to Figure 4 and Figure 5 The interference pattern P (and the corresponding interference signal S measured by the optical detector array 16) typically has interference fringes F. I At least in the central region R of the interference pattern P C In the case of strong interference patterns, the interference fringes may extend significantly into the peripheral region R of the interference pattern P. P middle. Figure 4 An example of an interference pattern P is illustrated schematically, while Figure 5 The diagram illustrates the equipment for mask feature analysis for reference mask 10. R or 10' R The acquired reference interferometric signal S R It also explains that the device for analyzing mask features targets mask 10. T or 10' T The acquired target interference signal S T Interference fringes F IThe reflection area A and the defect area A of the reflection shield 10 D When region A has uniform reflectivity, interference fringes F are generated. I Often the strongest, and the interference fringes F I The intensity of the interference fringes can also be enhanced by the regularity of the lateral spacing of region A. On the other hand, the interference fringes F I This may be due to defective regions (e.g., defective region A that reduces the uniformity of reflectivity). D The interference fringes F shift and / or weaken, and the interference fringes F I It may shift and / or weaken due to a decrease in the regularity of the lateral spacing of the reflective area.

[0067] More specifically, the central region R of the interference pattern P C The detailed shape and symmetry (or asymmetry) of the corresponding interference signal S strongly depend on the reflection region A and the defect region A of the reflective shield 10. D Detailed settings, geometry (e.g., size and shape), and reflectivity. Interference fringes F I The number of interference fringes, the spacing of interference fringes FI, and the interference fringes F I Intensity (weakness) and interference fringe F I The general location depends on the detailed layout and reflectivity of the reflective area A and the defective area AD of the mask 10. Therefore, as Figure 5 As seen in the image, refer to mask 10. R or 10' R Reference interference signal S R The interference fringes F are strong and regular. I In contrast, compared to the reference interferometric signal S R In comparison, the target mask is 10. T or 10' T Target interference signal S T The interference fringes F are weak and irregular. I Reference interference signal S R Interference signal S with target T The difference comes from the target mask 10. T or 10' T Defect area A D .

[0068] Interference fringes F measured by a given reflective shield 10 I The intensity also depends on the spatial coherence of light L1. Generally speaking, and for a given mask 10, the interference fringe F I The interference fringes will be strongest when the light L1 is highly spatially coherent. In contrast, when the light L1 is not coherent, the interference fringes F... IThe light L1 will be the weakest or even non-existent. Therefore, light L1 can include spatially coherent light. For example, the spatial coherence of light L1 is such that interference pattern P has interference fringes F at least in the central region of interference pattern P. I This is effective. To provide sufficient spatial coherence for light L1, light source 12 may selectively include a spatial filter to filter out specific spatial frequency components of the light, thereby increasing the spatial coherence of light L1 (albeit at the cost of reduced photon flux). Therefore, for example, for characterization of EUV masks, it is conceivable to implement light source 12 as an LPP-EUV light source, such as a pulsed tin plasma EUV light source. The output of an LPP-EUV light source typically has relatively low coherence, but when combined with spatial filtering to improve coherence, it can provide sufficiently high spatial coherence for light L1 to produce interference fringes F. I The interference pattern P is at least in the central region of the interference pattern P. In another non-limiting illustrative embodiment of the feature analysis of the EUV mask, the light source 12 can be a FEL EUV light source or an HHG EUV light source, which can output highly spatially coherent light without using spatial filtering.

[0069] Figure 4 Explanatory interference pattern P and Figure 5 Reference interference signal S R and target interference signal S T The interference pattern is one-dimensional. A one-dimensional or linear optical grating is used as optical grating 14 or 15 to obtain this one-dimensional or linear interference pattern. In these embodiments, the optical detector array 16 is also a one-dimensional or linear detector array, for example, in the case of a CCD detector array, having one-dimensionally single-line arranged CCD elements. In other embodiments, optical grating 14 or 15 is a two-dimensional optical grating, and the resulting interference pattern P is a two-dimensional interference pattern. In these embodiments, the optical detector array 16 may be a two-dimensional detector array, for example, in the case of a CCD detector array, having a two-dimensional CCD element array to measure the two-dimensional interference pattern.

[0070] Generally speaking, the interference signal S measured by a specific mask 10 can be considered a characteristic of that mask. Various methods can be used to form a mask with the optical properties of the interference signal S.

[0071] refer to Figure 6 In one method, the target reflection mask 10 T (or equivalently, a 10' transmission shield) T The quality index is determined by comparing the target interference signal S. T With reference mask 10 R (or equivalently, for reference transmission shield 10') R The measured reference interference signal S RTo determine this. In this method, refer to mask 10. R As a standard (i.e., reference) mask, its reflection area A is the target mask 10. T (or 10') T The intended design basis. For example, reference mask 10 R or 10' R and target masking 10 T or 10' T Both may have been manufactured in the same factory, to the same specifications, and in the same batch. Refer to mask 10, depending on the situation. R or 10' R Verification can be performed using one or more standard mask feature analysis methods, such as atomic force microscopy (AFM) and / or optical feature analysis using a synchrotron EUV light source at the mask manufacturing facility. In operation 30, for example using... Figure 1 or Figure 2 (or for transmission shields) Figure 3 The device is used for mask feature analysis to measure the reference mask 10. R (As shown in the figure, or 10') R If we perform feature analysis on the transmission shield, the interference signal S R .

[0072] On the other hand, because the target mask is 10 T or 10' T Used in lithography processes, therefore, the target may be masked 10 times before, for example, shipping to a customer or before deployment in a semiconductor lithography system. T or 10' T Perform the test. In operation 34, for example, using... Figure 1 or Figure 2 (or for transmission shields) Figure 3 The device is used to analyze the masking features of the target to measure the reflective mask. T (or equivalently, target transmission shield 10') T The interference signal S T To maximize the two interference signals S R With S T The comparability between them is demonstrated in some embodiments by using the same masking feature analysis equipment to measure the interference signal S. R and S T Both, although this is not necessary. In operation 36, by comparing the reference interference signal S R Interference signal S with target T To evaluate target masking. For example, measuring the reference interferometric signal S. R Interference signal S with target TThe similarity between them is used to determine quality indicators. In one method for performing operation 36, since a reference mask 10 has been determined... R (or equivalently, refer to transmission shield 10') R ) is acceptable, therefore when the target interference signal S T The closer to the reference interference signal S R The higher the quality index value, the better. If a target mask of 10 is obtained... T (or equivalently, a 10' transmission shield) T Other information, such as the film thickness measured by AFM in operation 40 and the film thickness 42 measured by AFM, may also be considered in operation 36 to evaluate the quality of the target mask.

[0073] Figure 6 The advantage of this method is that it can mask the target by 10. T or 10' T Online feature analysis is performed. For example, the reference interference signals S for the reference and target mask respectively. R and target interference signal S T Measurements can be taken at any stage of the masking process, and the two interference signals can then be compared to generate a quality index, which can provide a reference interference signal S at a certain stage of the masking process. R Interference signal S with target T The similarity between them is measured. Similarly, the individual reference interference signals S of the reference and target masking targets can be measured at the end of the masking process. R and target interference signal S T The two interference signals are then compared to generate a quality index, which can be used as a reference interference signal S at the end of the masking process. R Interference signal S with target T The measurement of similarity between them.

[0074] In a typical implementation for online monitoring of the masking process, a reference masking measurement operation 30 is performed at the beginning of the operation to generate a reference interference signal S. R The reference interference signal S is then stored. R Furthermore, the reference interferometric signal S can be retrieved at any subsequent time point. R An example of performing target mask evaluation operation 36. Therefore, operation 30 can be performed once, and subsequently, target mask 10 is generated in the masking process. T And use online mask feature analysis equipment to mask the target 10 T At any time during the evaluation, the reference interferometric signal S from operation 30 is reused. R Even a single stored reference interference signal S RIt can be retrieved and used to perform target masking evaluation operation 36 to evaluate the expected and reference masks 10. R Equivalent to any target mask (e.g., in relation to reference mask 10) R Any mask manufactured to the same specifications in the same OEM factory.

[0075] Another advantage of the revealed mask feature analysis is that it is fast because it measures the interference signal across the entire mask. In contrast, AFM mask scanning is slower.

[0076] Another advantage of the disclosed mask feature analysis is that, compared to existing mask feature analysis hardware (such as synchrotron EUV light sources), the mask feature analysis disclosed herein uses relatively inexpensive components.

[0077] refer to Figure 7 In another method, a reference interferometric signal S can be generated using computer simulation. R . Figure 7 The methods include those previously Figure 6 The described operations 32, 34, and 36, and operations 40 and 42 (optional, depending on the situation) (and again, can be used) Figure 3 Equipment for measuring transmission target shield 10' T Instead of the target reflection mask 10 described T However, in Figure 7 The method does not use a solid reference mask 10 R (or 10') R ),and Figure 6 Reference mask measurement operation 30 in Figure 7 The method is performed by computer 52 (which can be connected with...) Figures 1 to 3 The computer simulation operation 50, which may be the same as or different from the electronic processor 18, is used instead. The computer simulation operation 50 can simulate the reference interference signal S using ray tracing, Monte Carlo simulation, or another form of optical system simulation. R By inputting specific information about the design layout of the reference reflection (or transmission) mask (e.g., the geometry of region A, the optical properties of the material constituting the absorption layer 22 and the reflectivity of the surface of the reflective multilayer stack 20 (or the optical transmittance of the substrate 21 in the case of a transmission mask)), the specifications of the optical grating 14 or 15 (e.g., lines / mm at the design reference wavelength) and the incident light L1 (e.g., optical coherence and center wavelength and spectral characteristics for multicolor light sources, such as full width at half maximum (FWHM), or the spectrum of light L1).

[0078] refer to Figure 8 This describes a non-limiting illustrative embodiment of the target masking evaluation operation 36. In this embodiment, the reference interference signal S R and target interference signal ST The difference between the values ​​is calculated in operation 60, and a threshold value is compared with this difference in operation 62. If the difference calculated in operation 60 is less than the threshold value compared in operation 62, quality indicator 64 is set to indicate that the target mask is acceptable; however, if the difference calculated in operation 60 is greater than the threshold value compared in operation 62, quality indicator 64 is set to indicate that the target mask is unacceptable. In the latter case, the completed mask or the mask under manufacturing may be discarded along with all masks in the same manufacturing batch.

[0079] Operation 60 can calculate the reference interference signal S in various ways. R Interference signal S with target T The difference between them. In one method, the difference is calculated as the mean squared error (MSE) as follows:

[0080]

[0081] Where N is the reference interference signal S R and target interference signal S T The number of data points in the signal (assuming both signals have the same number of data points), s T,i Indicates the target interference signal S T The i-th data point, and s R,i Represents the reference interference signal S R The i-th data point. Depending on the situation, refer to the interference signal S. R and target interference signal S T Preprocessing can be performed before calculating the difference, such as removing low-frequency (e.g., DC) components by applying a high-pass filter and / or normalizing the filtered signal. Furthermore, in the case of two-dimensional interference signals acquired using a two-dimensional grating and a two-dimensional optical detector array, the sum of equation (1) is the calculation of data points in both dimensions, and can be written as follows:

[0082]

[0083] The subscripts i and j operate in the corresponding dimensions of the two-dimensional interference signal.

[0084] refer to Figure 9 Another non-limiting illustrative embodiment of target masking evaluation operation 36 is described. In this embodiment, in operation 70, the difference index Δμ = |μ(S) is calculated. T )-μ(S R )|, where μ(S) T ) is the target interference signal S T The mean of μ(S)R S is the reference interference signal. R The mean value is calculated. In operation 72, the critical limit value is compared with the difference index Δμ. If the difference index Δμ calculated in operation 70 is less than the critical limit value compared in operation 72, then quality index 74 is set to indicate that the target mask is acceptable; however, if the difference index Δμ calculated in operation 70 is greater than the critical limit value compared in operation 72, then quality index 74 is set to indicate that the target mask is unacceptable. Again, in the latter case, various remedial measures can be taken.

[0085] Explanatory difference indicator MSE (reference) Figure 8 (Description) and Δμ (Reference) Figure 9 The description is a non-restrictive descriptive difference indicator, and any of the many other difference indicators may be applied in operation 36.

[0086] refer to Figure 10 Another non-limiting illustrative embodiment of the target mask evaluation operation 36 is described. In this embodiment, for example, by measuring the reference interference signal S of one or more reference masks. R A labeled reference interferometric signal combination 80 is obtained. During measurement, each reference interferometric signal in the labeled reference interferometric signal combination 80 is labeled with one or more characteristics of the reference mask, such as whether the characteristic is suitable for lithography, and / or information such as the thickness of the absorption layer 22, the reflectivity of the reflective multilayer stack 20, etc. The labeled items are assigned based on the information measured by AFM, the optical properties measured by the synchrotron EUV light source, etc. In some embodiments, the labeled reference interferometric signal combination 80 includes both acceptable and unacceptable reference masks.

[0087] In another embodiment, as previously referenced Figure 7 As described, a computer simulation operation 50, performed by computer 52, generates a marked reference interference signal combination 80. Simulations can be performed for various simulated reference mask configurations that explicitly describe known variations in the design configuration of the input reference reflection (or transmission) mask in the geometry of region A, the optical properties of the material constituting the absorption layer 22, and the reflectivity of the surface of the reflective multilayer stack 20 (or, in the case of a transmission mask, the optical transmittance of the light-transmitting substrate 21), etc. These known variations can encompass both acceptable and unacceptable simulated reference mask configurations to generate the marked reference interference signal combination 80.

[0088] In operation 82, one or more mask quality classifiers are trained using a labeled reference interferometric signal combination 80 to output one or more mask quality indices. During training, the parameters of the machine learning (ML) model (or multiple models) can be adjusted so that the classifier outputs mask quality indices, where the mask quality indices exhibit optimal consistency with the labeled reference interferometric signal combination 80. For example, the ML model can be: a support vector machine (SVM) classifier with model parameters to be optimized, including the normal vector w and the offset b; or an artificial neural network (ANN) with model parameters to be optimized, including neuron weights and activation functions, etc. Training operation 82 outputs the trained mask quality classifier (or multiple classifiers) 84. After training operation 82, the target interferometric signal S is... T The input is fed into a trained mask quality classifier (or multiple classifiers) 84, which evaluates the target interference signal S measured from the target mask. T Then, the trained mask quality classifier (or multiple classifiers) 84 outputs the quality index (or multiple indexes) of the target mask 86.

[0089] As a non-limiting illustrative example, if in training operation 82 the labeled reference interference signal combination 80 represents acceptable / unacceptable quality and absorption layer thickness, then two classifiers can be trained: a binary classifier that outputs acceptable or unacceptable values ​​and a continuous value classifier that outputs the absorption layer thickness. Therefore, when these trained mask quality classifiers 84 are used in mask evaluation operation 36, two quality indices 86 are output: an acceptable / unacceptable index and an absorption layer thickness index. Again, this is merely a non-limiting illustrative example, and in general, based on the training data points available for the labeled reference interference signal combination, the data classifier can predict various quality indices after training.

[0090] The following discloses some further non-limiting illustrative embodiments.

[0091] In one non-limiting illustrative embodiment, the method for mask feature analysis includes measuring the interference signal of a reflective or transmissive mask used for semiconductor lithography, and determining a quality index of the reflective or transmissive mask based on the interference signal.

[0092] In some embodiments, measuring the interference signal includes reflecting light through a self-reflecting or transmitting shield or passing light through a reflective or transmitting shield to generate reflected or transmitted light, generating an interference pattern from the reflected or transmitted light using an optical grating, and measuring the interference pattern using an array of optical detectors. In some embodiments, the light includes spatially coherent light. In some embodiments, the light has spatial coherence, wherein spatial coherence is effective for the interference pattern to have at least a number of interference fringes in the central region of the interference pattern. In some embodiments, the light includes extreme ultraviolet light having a maximum spectral peak at a wavelength of 100 nanometers or less. In some embodiments, the quality index is determined by comparing the interference signal with a reference interference signal. In some embodiments, the method of shield feature analysis further includes measuring a reference interference signal by reflecting light through a self-reference reflective or transmitting shield or passing light through a reference reflective or transmitting shield to generate reference reflected or transmitted light, generating a reference interference pattern from the reference reflected or transmitted light using an optical grating, and measuring the reference interference pattern using an array of optical detectors. In some embodiments, the method for mask feature analysis further includes one of the following: (i) measuring a reference interference signal of a reference reflective mask, and (ii) calculating the reference interference signal using a computer. A quality index is determined by comparing the interference signal with the reference interference signal. In some embodiments, the reflective or transmissive mask is a reflective mask and the light includes extreme ultraviolet (EUV) light having a maximum spectral peak at a wavelength of 100 nm or less, and the reflective mask includes an EUV reflective multilayer stack and an EUV absorbing layer disposed on the EUV reflective multilayer stack. In some embodiments, the method for mask feature analysis further includes acquiring an atomic force microscope (AFM) image of the reflective or transmissive mask using an atomic force microscope, wherein a quality index of the reflective or transmissive mask is further determined based on the AFM image of the reflective or transmissive mask.

[0093] In another non-limiting illustrative embodiment, the method for mask feature analysis includes reflecting light from the mask or passing light through the mask to produce reflected or transmitted light, reflecting the reflected or transmitted light from an optical grating or passing the reflected or transmitted light through an optical grating to produce an interference pattern, and determining a quality index of the mask based on the interference pattern.

[0094] In some embodiments, the mask feature analysis method further includes reflecting light from a reference mask or passing light through a reference mask to generate reference reflected or transmitted light, and reflecting reference reflected or transmitted light from an optical grating or passing reference reflected or transmitted light through an optical grating to generate a reference interference pattern. The quality index of the mask is determined by comparing the interference pattern with the reference interference pattern. In some embodiments, the light has spatial coherence, which is effective for the interference pattern to have at least a few interference fringes in the central region of the interference pattern. In some embodiments, the light has a maximum spectral peak at a wavelength of 100 nanometers or less.

[0095] In another non-limiting illustrative embodiment, the apparatus for mask feature analysis includes a light source, an optical grating, and an array of optical detectors. The light source is configured to illuminate a reflective or transmissive mask, thereby generating mask-reflected or mask-transmitted light. The optical grating is configured to convert the mask-reflected or mask-transmitted light into an interference pattern. The array of optical detectors is configured to generate an interference signal by measuring the interference pattern.

[0096] In some embodiments, the apparatus for mask feature analysis further includes an electronic processor programmed to determine a quality index of the reflective or transmissive mask based on the interference signal. In some embodiments, the electronic processor is programmed to determine the quality index of the reflective or transmissive mask by comparing the interference signal with a reference interference signal of a reference reflective or transmissive mask, wherein the reference interference signal of the reference reflective or transmissive mask is obtained using a light source, an optical grating, and an array of optical detectors. In some embodiments, the light source includes a coherent light source having spatial coherence, which is effective for the interference pattern to have at least a few interference fringes in the central region of the interference pattern. In some embodiments, the reflective or transmissive mask is a reflective mask. The light source includes an extreme ultraviolet (EUV) light source configured to illuminate the reflective mask with EUV light, thereby generating EUV light reflected by the mask. The optical grating is configured to convert the EUV light reflected by the mask into an interference pattern. The optical detectors are EUV-sensitive optical detector arrays configured to measure the interference pattern to generate the interference signal. In some embodiments, the EUV light source does not include a synchrotron.

[0097] The foregoing provides a brief overview of the features of several embodiments of this disclosure, enabling those skilled in the art to more readily understand it. Anyone skilled in the art should understand that this disclosure can easily serve as a basis for changes or designs to other structures or processes to achieve the same purpose and / or obtain the same advantages as the embodiments of this disclosure. Anyone skilled in the art will also understand that equivalent structures as described above do not depart from the spirit and scope of this disclosure, and that modifications, substitutions, and alterations can be made without departing from the spirit and scope of this disclosure.

Claims

1. A method for mask feature analysis, characterized in that, include: A light is reflected from or transmitted through a reflective or transmissive shield to produce a reflected or transmitted light, wherein the reflective or transmissive shield is used for semiconductor lithography. An interference pattern is produced by using an optical grating to reflect or transmit the light or to allow the light to pass through the optical grating. An optical detector array is configured to generate an interference signal by measuring the interference pattern; Using an electronic processor, employing ray tracing or Monte Carlo simulation, generate a reference interference signal of a reference interference pattern with a reference reflection or transmission mask; and Determine a quality index for the reflection or transmission shield based on a comparison of the intensity and regularity of several interference fringes of the measured interference pattern with the intensity and regularity of several interference fringes of a reference interference pattern, including: Calculate the mean of the interference signal. ; Calculate the mean of the reference interference signal. ; and Calculate a difference index .

2. The method for mask feature analysis according to claim 1, characterized in that, The measured interference fringes of the interference pattern are at least in a central region of the interference pattern.

3. The method for mask feature analysis according to claim 2, characterized in that, The light has spatial homology.

4. The method for mask feature analysis according to claim 2, characterized in that, The light has a spatial coherence that is effective for the interference pattern to have interference fringes at least in the central region of the interference pattern.

5. The method for mask feature analysis according to claim 2, characterized in that, The light includes extreme ultraviolet light, which has a maximum spectral peak at a wavelength of 100 nanometers or less.

6. The method for mask feature analysis according to claim 2, characterized in that, The reflective or transmissive shield includes a defective area.

7. The method for mask feature analysis according to claim 1, characterized in that, The electronic processor uses ray tracing to generate the reference interference pattern.

8. The method for mask feature analysis according to claim 1, characterized in that, The electronic processor uses Monte Carlo simulation to generate the reference interference pattern.

9. The method for mask feature analysis according to claim 2, characterized in that, The reflective or transmissive shield is a reflective shield and the light includes extreme ultraviolet light having a maximum spectral peak at a wavelength of 100 nanometers or less, and the reflective shield includes an extreme ultraviolet reflective multilayer stack and an extreme ultraviolet absorption layer disposed on the extreme ultraviolet reflective multilayer stack.

10. The method for mask feature analysis according to claim 1, characterized in that, Further includes: Use an atomic force microscope to acquire an atomic force microscope image of the reflective or transmissive shield; The quality index of the reflective or transmissive shield is further determined based on the atomic force microscope image of the reflective or transmissive shield.

11. A method for mask feature analysis, characterized in that, include: A light is reflected from or transmitted through a reflective or transmissive shield to produce a reflected or transmitted light. Reflecting or transmitting light from an optical grating or passing the reflected or transmitted light through the optical grating to produce an interference pattern; An optical detector array is configured to generate an interference signal by measuring the interference pattern; and A quality metric for the mask is determined using an electronic processor, including: Calculate the mean of the interference signal. ; Calculate the mean of a reference interference signal ; and Calculate a difference index .

12. The method for mask feature analysis according to claim 11, characterized in that, Further includes: The light is reflected from a reference shield or the light is passed through the reference shield to produce a reference reflected or transmitted light; and The optical grating reflects the reference reflected or transmitted light or allows the reference reflected or transmitted light to pass through the optical grating to generate a reference interference pattern; The quality index of the mask is determined by comparing the mean value of the interference signal of the interference pattern with the mean value of the reference interference signal of the reference interference pattern.

13. The method for mask feature analysis according to claim 11, characterized in that, The light has a spatial coherence that is effective for the interference pattern to have at least a few interference fringes in a central region of the interference pattern.

14. The method for mask feature analysis according to claim 11, characterized in that, The light has a maximum spectral peak at a wavelength of 100 nanometers or less.

15. A device for mask feature analysis, characterized in that, include: A light source configured to illuminate a reflective or transmissive shield with light, thereby producing shield-reflected or shield-transmissive light. An optical grating configured to convert light reflected or transmitted by the mask into an interference pattern; An array of optical detectors configured to generate an interference signal by measuring the interference pattern; and An electronic processor, employing ray tracing or Monte Carlo simulation, generates a reference interference signal from a reference interference pattern of a reference reflection or transmission mask. The electronic processor is programmed to determine the quality indicators of the reflection or transmission mask by comparing the intensity and regularity of several interference fringes of the interference signal with the intensity and regularity of several interference fringes of the reference interference pattern, including: Calculate the mean of the interference signal. ; Calculate the mean of the reference interference signal. ; and Calculate a difference index .

16. The apparatus for mask feature analysis according to claim 15, characterized in that, The light has a maximum spectral peak at a wavelength of 100 nanometers or less.

17. The apparatus for mask feature analysis according to claim 15, characterized in that, The reference interferometer signal was obtained using Monte Carlo simulation.

18. The apparatus for mask feature analysis according to claim 15, characterized in that, The light source includes a co-coherent light source having spatial coherence, which is effective for the interference pattern at least in a central region of the interference pattern for the interference fringes.

19. The apparatus for mask feature analysis according to claim 15, characterized in that, in: The reflective or transmissive shield is a reflective shield; The light source includes an extreme ultraviolet light source configured to irradiate the reflective shield with extreme ultraviolet light, thereby generating extreme ultraviolet light reflected by the shield. The optical grating is configured to convert the extreme ultraviolet light reflected by the shield into the interference pattern; and The optical detector is an extreme ultraviolet-sensitive optical detector array configured to measure the interference pattern and generate the interference signal.

20. The apparatus for mask feature analysis according to claim 19, characterized in that, This extreme ultraviolet light source does not include a synchrotron.

Images