Pupil filter with spatially varying transmission

Abstract

An inspection system includes a pupil filter positioned in an imaging pupil of the inspection system. The pupil filter is configured to provide spatially varying intensity transmission. The pupil filter is a glass sheet having a patterned layer disposed on a surface thereof. The inspection system uses the pupil filter to introduce spatially varying intensity transmission.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to inspection of masks used in semiconductor manufacturing, such as measuring extreme ultraviolet (EUV) marks to detect defects. BACKGROUND

[0002] Evolution of the semiconductor manufacturing industry places increasingly stringent demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions continue to shrink while there is a demand for reducing the time for achieving high yield, high value production. Minimizing the total time from detecting a yield problem to fixing the problem maximizes the return on investment for semiconductor manufacturers.

[0003] Fabricating semiconductor devices typically includes processing a workpiece, e.g., a semiconductor wafer, using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a photomask to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. The arrangement of multiple semiconductor devices fabricated on a single semiconductor wafer can be separated into individual semiconductor devices.

[0004] Inspection processes are used at various steps during semiconductor manufacturing to detect defects on the workpiece in order to facilitate higher yield and, thus, higher profits in the manufacturing process. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits (ICs). However, as semiconductor devices shrink, inspection becomes even more important to successfully fabricate acceptable semiconductor devices because smaller defects can easily cause devices to fail. For example, as the size of semiconductor devices shrink, it becomes necessary to detect defects that are shrinking in size because even relatively small defects can cause unwanted aberrations in the semiconductor devices.

[0005] However, as design rules shrink, semiconductor manufacturing processes can operate closer to the limits of the performance capabilities of the processes. In addition, as design rules shrink, smaller defects can affect electrical parameters of the devices, which drives more sensitive inspection. As design rules shrink, the population of potential yield-related defects detected by inspection grows dramatically, and the population of nuisance defects detected by inspection also grows dramatically. Thus, more defects can be detected on a workpiece, and correcting the process to eliminate all defects can be difficult and expensive. Determining which defects actually affect electrical parameters and yield of the devices can enable process control methods to focus on those defects and largely ignore other defects. Furthermore, at smaller design rules, process-induced failures tend to be systematic in some cases. That is, process-induced failures tend to occur at predetermined design patterns that are often repeated multiple times within a design. Eliminating spatially systematic electrical-related defects can affect yield.

[0006] Inspection of extreme ultraviolet (EUV) reticles for EUV lithography can be difficult. Current inspection systems are unable to detect certain defects. From a process yield management perspective, control of the defectivity of EUV reticles that define patterns printed on semiconductor wafers or other workpieces is critical. However, due to the lack of a reliable optical EUV reticle inspector that inspects photomasks at the same wavelength as used by EUV scanners (e.g., 13.5 nm), defect detection has been identified as one of the high-risk areas of EUV lithography development. Electron beam inspection tools that can provide good sensitivity typically have orders of magnitude slower inspection throughput than desired and are not a practical solution for inspection. Currently and for the foreseeable future, inspection of patterned EUV reticles can rely on more readily available, higher throughput inspection tools that operate in the deep UV (DUV) wavelength range (1.90 nm to 260 nm).

[0007] This large difference in wavelength between inspection systems and lithography systems can affect the performance of DUV inspection systems when applied to EUV reticle defect detection. For example, DUV inspection systems have lower optical resolution than EUV lithography scanners, which results in lower image contrast. In addition, different materials that make up EUV reticles (e.g., multilayer (ML) background material versus absorber material patterns) have different optical properties between EUV wavelengths and DUV wavelengths, which affects the amplitude and phase of light reflected from EUV reticles. In a more particular example, defect sensitivity can be compromised because light scattered from defects tends to be out of phase with light reflected from background patterns. This effect has been a limiting factor in determining the detection sensitivity that can be achieved by DUV inspection systems for EUV mask detection.

[0008] Improved systems and techniques are needed. SUMMARY

[0009] In a first embodiment, an inspection system is provided. The inspection system includes a light source generating a light beam, illumination optics configured to direct the light beam onto an EUV reticle, a pupil filter positioned in an imaging pupil of the inspection system, a detector receiving an output beam from the pupil filter and configured to generate an image for the output beam, and collection optics for directing the output beam reflected and scattered from the EUV reticle in response to the light beam. The pupil filter is configured to provide a spatially varying intensity transmission. The pupil filter is a glass plate having a patterned layer disposed on a surface thereof. The output beam is directed through the pupil filter toward the detector.

[0010] The patterned layer can include at least two different shapes and / or sizes across the surface.

[0011] The patterned layer can have a varying density across the surface.

[0012] The patterned layer can be rotationally symmetric or non-rotationally symmetric across the surface.

[0013] The patterned layer can be chromium, another metal, or a dielectric.

[0014] The pupil filter can be configured to provide a phase contrast in the output beam. In an example, the glass plate has an etched portion having a depth corresponding to a phase variation introduced into a portion of the output beam transmitted through the pupil filter. The patterned layer can be disposed on an opposite side of the glass plate from the etched portion.

[0015] The patterned layer can have rings with a width from 100 nm to 5000 nm.

[0016] The patterned layer can have rings with a thickness from 10 nm to 250 nm.

[0017] The patterned layer can have a pitch from 100 nm to 5000 nm between rings.

[0018] A method of inspecting an extreme ultraviolet (EUV) photomask is provided in a second embodiment. The method includes obtaining a test image from an output beam using an inspection system, the output beam being reflected and scattered from a test portion of an EUV test photomask. The inspection system is configured to provide a spatially varying intensity transmission. The inspection system introduces the spatially varying intensity transmission using a pupil filter, the pupil filter being a glass plate having a patterned layer disposed on a surface thereof. A reference image of a reference photomask portion is obtained, the reference photomask portion being designed to be identical to the test photomask portion. The test image is compared to the reference image using a processor. Whether the test photomask portion has a candidate defect is determined based on the comparison using the processor.

[0019] The method can include repeating the using the inspection system, the obtaining a reference image, the comparing, and the determining for each of a plurality of test photomask portions of the photomask. A defect report can be generated based on the candidate defects that have been determined to be present.

[0020] The patterned layer can include at least two different shapes and / or sizes across the surface.

[0021] The patterned layer can have a varying density across the surface.

[0022] The patterned layer can be rotationally symmetric or non-rotationally symmetric across the surface.

[0023] The pupil filter can be configured to provide a phase contrast in the output beam. In an example, the glass plate has an etched portion having a depth corresponding to a phase variation introduced into a portion of the output beam transmitted through the pupil filter. BRIEF DESCRIPTION OF DRAWINGS

[0024] For a more complete understanding of the nature and objects of this disclosure, reference should be made to the following detailed description taken in connection with the accompanying drawings in which:

[0025] Figure 1 is a pictorial representation of an inspection system configured for imaging according to an embodiment of the present disclosure;

[0026] Figure 2 is a cross-sectional pictorial representation of a side view of a pupil filter according to an embodiment of the present disclosure;

[0027] Figure 3 is a cross-sectional pictorial representation of a side view of a pupil filter according to another embodiment of the present disclosure;

[0028] Figure 4 exemplary spatially varying transmission characteristics of a rotationally symmetric transmission of a pupil filter are shown;

[0029] Figure 5 An exemplary pattern of a patterned layer of a pupil filter is shown;

[0030] Figure 6 is a cross-sectional graphical representation of a side view of a pupil filter according to another embodiment of the disclosure, where the patterned layer has different shapes and / or sizes;

[0031] Figure 7 is a cross-sectional graphical representation of a side view of a pupil filter according to another embodiment of the disclosure, where the patterned layer has varying density;

[0032] Figure 8 is a cross-sectional graphical representation of a side view of a pupil filter according to another embodiment of the disclosure, where the patterned layer is asymmetric; and

[0033] Figure 9 is a flowchart showing an embodiment of a method according to the disclosure. DETAILED DESCRIPTION

[0034] Although the claimed subject matter will be described in terms of certain embodiments, other embodiments comprising not providing all of the benefits and features set forth herein are within the scope of the present disclosure. Various structural, logical, process steps, and electronic changes can be made without departing from the scope of the present disclosure. Accordingly, the scope of the present disclosure is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

[0035] To enhance defect signatures in an image (and increase the chance of detecting a defect), a pupil filter (PF) is used in an inspection system. Embodiments of the pupil filter disclosed herein provide improved results during testing. The pupil filter can have a spatially varying intensity transmission that enhances a defect signal. The shape of the spatially varying transmission profile can enhance the defect signal.

[0036] Any suitable combination of hardware and software can be used to implement a spatially varying intensity transmission for reticle inspection. Figure 1 is a graphical representation of an inspection system 100 configured for spatially varying intensity transmission imaging. This inspection system 100 generally includes a light source 102 suitable for inspecting EUV reticles. One example of a light source is a quasi-continuous wave laser. In particular embodiments, the light source can generally provide high pulse repetition rate, low noise, high power, stability, reliability, and scalability. While EUV scanners operate at 13.5 nm wavelength, the inspection system for EUV reticles does not have to operate at the same wavelength. For example, the TERON system from KLA Corporation operating at 193 nm can be used to inspect EUV reticles.

[0037] The inspection system 100 can generally be set up with a set of operational parameters or "recipe" settings. Recipe settings can include one or more of the following settings: pupil filter configuration, zoom setting, one or more defect detection thresholds, focus setting, illumination or detection aperture setting, incident beam angle and wavelength settings, detector settings, setting of amount of reflected or transmitted light, aerial imaging parameters, etc. Certain embodiments of the present invention utilize an inspection system that is in a reflection mode and has a set polarization (e.g., S, P, circular, etc.).

[0038] The inspection system 100 includes a series of optical elements for focusing an illumination beam onto the surface under inspection 112. For example, the inspection system 100 can include beam steering devices for precise beam positioning and beam conditioning devices that can be used to provide photometric control, speckle reduction, and high beam uniformity. The beam steering and / or beam conditioning devices can be separate physical devices from, for example, the laser. For the sake of brevity, Figure 1 Only a condenser lens 104, a beam splitter 106, and an objective lens 108 for the illumination optics are illustrated. However, one skilled in the art will appreciate that the inspection system can include other optical or electronic elements for achieving particular inspection functions. The objective lens 108 can be relatively large in order to meet certain low aberration requirements. The objective lens can be adjusted for different pixel sizes, for example, less than about 100 nm or, more particularly, less than about 75 nm or even less than 60 nm per pixel.

[0039] The sample 110 can also be placed on a stage 117 of the inspection system 100, and the inspection system 100 can also include positioning mechanisms for moving the stage 117 (and the sample 110) relative to the incident beam. By way of example, one or more motor mechanisms can each be formed from a screw drive and stepper motor, a linear drive with feedback position, or a belt actuator and stepper motor. The sample 110 can be an EUV photomask or reticle or another type of workpiece (e.g., a semiconductor wafer or flat panel).

[0040] After the incident beam(s) are illuminated on the sample 110, the light can then be reflected and diffracted / scattered from the sample 110 in the form of "output light" or "output beam." The inspection system 100 also includes any suitable lens arrangement for directing the output light toward one or more detectors. As shown, the output beam can be received by a detector or imaging lens 113 that directs the output beam toward a detector 114. In one embodiment, the detector 114 is a time delay integration (TDI) detector. A typical TDI detector accumulates multiple exposures of the same area of the surface under inspection, effectively increasing the integration time that can be used to collect incident light. In general, the detector 114 can include a transducer, a concentrator, a charge-coupled device (CCD), or other type of radiation sensor.

[0041] When imaging a reticle as a sample 110, light from both the ML and the absorber material can be analyzed. For example, the Kirchhoff complex reflectance of both the ML and the absorber at 193 nm wavelength can be calculated. This calculation shows that the absorber has a reflection amplitude that is about half of the reflection amplitude of the ML, with a -90° phase angle difference between them, which indicates that the EUV mask is a strong phase object. It should be noted that the EUV absorber stack film thickness and type can actually vary. The phase properties of the mask generally persist, but there is a certain variability in the exact phase angle.

[0042] In Figure 1 , the inspection system 100 includes a pupil filter 107 to provide spatially varying intensity transmission of the output light. The pupil filter 107 can be an apodizer. The pupil filter 107 can be made by etching into a thin glass plate 120, such as fused silica. Figure 2 and 3 are graphical representations of side views of the pupil filter 107. As shown in Figure 3 , the etch depth or height h determines the phase of the filter relative to the unetched region. For example, at 19.3 nm, an etch depth of about 85 nm will result in a phase angle close to 90°.

[0043] The geometry of the pupil filter 107 is configured to match the shape of the illumination aperture. That is, the pupil filter 107 is configured to provide spatially varying intensity transmission with or without phase change within the illumination area, which also corresponds to the reflected portion of the output light, but not outside of this illumination area, which mostly corresponds to the scattered light. In Figure 3 , the width (w) of the etched portion can be sized to match the aperture diameter, such as a σ 0.5 aperture. This selection of σ 0.5 illumination and circularly polarized light is used as an example to illustrate the effect of phase contrast imaging. Phase contrast imaging can enhance the signal strength of phase objects compared to non-phase or differently phased objects in the test reticle portion. Other illumination options for EUV photomask inspection are possible.

[0044] Thus, the pupil filter 107 can be configurable to different phase change values. For example, different pupil filters 107 with different etched depths and resulting phase change values can be selectively inserted into the imaging pupil plane along direction 115, as well as all pupil filters removed from the pupil plane for inspection without phase contrast, such as for pinhole defect detection. While optional, phase contrast imaging can improve defect classification, which relies on information contained in the defect residual image. For example, for EUV contact hole patterns, over-sized detection will tend to have a bright tone, while under-sized or encroachment defects tend to have a dark tone. The under-sized and encroachment defect intensity tone can be incorrect without phase contrast, but correct with phase contrast at best focus. Thus, phase contrast imaging can provide more accurate defect classification results for certain applications, as the intensity tone is correct for all defect types.

[0045] While Figure 3 The embodiments in are shown with etched portions in the glass plate 120, but the patterned layer 121 can be used with a glass plate 120 that is not etched and does not provide phase contrast. The example in Figure 2 shows an example without phase contrast.

[0046] An anti-reflective coating (e.g., MgF2, etc.) can be deposited on one or both sides of the glass plate 120 to reduce stray light. The transmission of the pupil filter 107 is generally left unchanged, but it can also be controlled by placing the patterned layer 121 on the flat side of the glass plate 120, made of a material that is compatible with DUV light (e.g., chrome, aluminum, nickel, another metal, or a dielectric). Figure 2 and 3 A side view of the pupil filter 107 with the patterned layer 121 for controlling transmission is shown. The patterned layer 121 can vary in size and pitch in order to provide a spatially varying intensity transmission to improve defect detection sensitivity for certain defect types or mask pattern types.

[0047] Figure 4 Exemplary spatially varying transmission characteristics of a rotationally symmetric transmission of the pupil filter 107 are shown. The lines on the graph show a gradual transmission. The intensity transmission line shows the total transmission through the pupil filter (i.e., the glass plate 121 with the patterned layer 121).

[0048] Figure 5 An exemplary pattern of the patterned layer 121 of the pupil filter 107 is shown. On the left, two sections show 100% transmission (1.0) and 45% transmission (0.45). This is a step function. Figure 5 Different patterns in can be used to enhance different mask defect types. As Figure 5As seen in FIG. 1, the patterned layer 121 can be a ring (e.g., a concentric ring), although other shapes are possible. Although Figure 5 The ring in FIG. 1 is shown as solid and unbroken, but the ring can also be segmented.

[0049] Figure 5 The left middle instance in FIG. 1 shows another step function. The progression is 100% transmission, 0% transmission, 45% transmission, 0% transmission, and 100% transmission from inside to outside.

[0050] Figure 5 The right middle instance in FIG. 1 shows 0% transmission in the center and 100% transmission at the edges.

[0051] Figure 5 The right instance in FIG. 1 shows an asymmetric transmission pattern. The asymmetric transmission pattern can be used to correct errors in the inspection system 100.

[0052] In one embodiment, the patterned layer 121 includes at least two different shapes and / or sizes across the surface of the glass sheet 120. This is shown in Figure 6 The ring 121a and the ring 121b have different widths. Different widths can block different amounts of light and thus create a spatially varying transmission. Figure 5 Other instances with different ring thicknesses are shown in FIG. 1. The width of the rings in the patterned layer 121 can be from 100 nm to 5000 nm. The rings in the patterned layer 121 or other features of the patterned layer 121 can have a thickness extending from the surface of the glass sheet 120 from 10 nm to 250 nm. These widths and thicknesses can enhance the defect signatures in the image. Although disclosed with respect to rings or circles, these dimensions can also apply to other shapes. The dimensions between the features of the patterned layer 121 can vary. Thus, the patterned layer 121 can not have a uniform width or thickness.

[0053] In one embodiment, the patterned layer 121 includes varying density across the surface of the glass sheet 120. This is shown in Figure 7 The spacing of the ring 121a and the ring 121c is different than the spacing of the ring 121c and the circle 121d. For example, a higher density will result in a lower transmission value. Other instances with different spacing are shown in Figure 5 Figure 7 The varying density of FIG. 1 can be combined with the different shapes and / or sizes of FIG. 1. The spacing between the rings of the patterned layer 121 can be from 100 nm to 5000 nm. This spacing can enhance the defect signatures in the image. Although disclosed with respect to rings or circles, this spacing can also apply to other shapes. The spacing between the features of the patterned layer 121 can vary. Thus, the patterned layer 121 can not have a uniform spacing. Figure 6

[0054] ​​In one embodiment, the patterned layer 121 is rotationally symmetric across the surface of the glass plate 120. For example, this is shown in Figure 2 , 3 , 6, and 7.

[0055] In one embodiment, the patterned layer 121 is not rotationally symmetric across the surface of the glass plate 120. For example, this is shown in the rightmost example in Figure 5 or Figure 8 . In Figure 8 , the patterned layer 121 includes a circle 121e that exists only on a portion of the surface.

[0056] In another example, the patterned layer 121 is a single monolithic layer having different thicknesses at different points on the surface of the glass plate 120. The single monolithic layer can be symmetric or asymmetric. Apertures can be formed in the single monolithic layer.

[0057] Multiple pupil filters 107 can be used in a given inspection system. Which particular pupil filter is selected for use in a given inspection pass can depend on the details of the EUV mask stack and the type of defects to be detected. Multiple inspection passes each utilizing a different pupil filter can be performed initially in order to achieve the optimal overall inspection sensitivity for all key defect types. A set of pupil filters 107 can be disposed in the imaging path. During inspection, while a particular swath N is being processed, the processor can analyze the database corresponding to swath N+1 and recommend the best pupil filter for that swath. The filter used to optically scan swath N+1 can be selected. That is, the filter used for the next swath can be selected based on the results of using one or more (or multiple) pupil filters on the current or previous swath or other reticle area.

[0058] A defect report for the candidate defects can be generated and stored. The defect report can be in any suitable format. In one implementation, the defect report can contain a reference to an image and location of each candidate defect. The defect report can contain the difference between the test image and the reference image for each candidate defect. The image and location of each candidate defect can also be stored with the defect report for later re-inspection. In another example, the detection report is in the form of an image composed of intensity differences defined or marked as potential defects. The report can be in the form of a defect map with varying colors corresponding to varying intensity or average intensity differences of the candidate defects, as described further below.

[0059] Referring back to the inspection system of Figure 1 , the illumination beam can be directed toward the sample surface 112 at a substantially normal angle with respect to the surface being inspected. In other embodiments, the illumination beam can be directed at an oblique angle, which enables the illumination beam to be separated from the reflected beam.

[0060] The detector 114 is also typically coupled with a processor 116 in an image processing system, which can include an analog-to-digital converter configured to convert analog signals from the detector 114 into digital signals or images for processing. The processor 116 can be configured to analyze the intensity, phase, and / or other characteristics of one or more reflected and scattered light beams. The processor 116 can be configured (e.g., with programming instructions) to provide a user interface (e.g., a computer screen) for displaying generated test images and other inspection characteristics. The processor 116 can also include one or more input devices (e.g., a keyboard, mouse, joystick) for providing input. The processor 116 can also be coupled with a stage for controlling, for example, sample position (e.g., focusing and scanning), pupil filter configuration, zoom settings, and other inspection parameters and configurations of the inspection system elements. In certain embodiments, the processor 116 is configured to perform the inspection techniques detailed above. The processor 116 typically has one or more processors coupled to input / output ports and one or more memory stores via an appropriate bus or other communication mechanism.

[0061] Because such information and program instructions can be employed to implement the systems / methods described herein, the present systems include program instructions / computer code that can be stored in a computer readable medium such as a hard disk drive, a floppy disk, a CD-ROM, a DVD, a Blu-ray Disc, a magnetic tape, a silicon memory, or any other appropriate medium. The program instructions / computer code can be written in any of a number of suitable programming languages and / or programming or scripting tools and then compiled as necessary. Alternatively, the program instructions / computer code can be provided without being compiled, such as by being provided in an interpreted version. The program instructions / computer code can be downloaded to the user's computer from an appropriate computer readable medium or to an appropriate server or datacenter for distribution.

[0062] It is noted that the above description and drawings are not to be taken in a limiting sense as the specific components of the inspection system 100 are not to be viewed as limiting, and the inspection system 100 can be embodied in a number of other forms. For example, it is contemplated that the inspection or measurement tool can have any suitable features from any number of known imaging or metrology tools arranged for detecting defects and / or resolving features of a reticle or workpiece (e.g., a semiconductor wafer). By way of example, the inspection or measurement tool can be adapted for bright field imaging microscopy, dark field imaging microscopy, full sky imaging microscopy, phase contrast microscopy, polarized contrast microscopy, and coherent probe microscopy. It is also contemplated that single image and multi-image methods can be used to capture images of the target. These methods include, for example, single acquisition, dual acquisition, single acquisition coherent probe microscopy (CPM), and dual acquisition CPM methods. It is contemplated that non-imaging optical methods such as scatterometry can also form part of the inspection or metrology apparatus.

[0063] In other inspection applications, the incident or detected light can pass through any suitable spatial light apertures, in turn producing any incident or detected light profile at any suitable angle of incidence. By way of example, programmable illumination or detection apertures can be utilized to produce specific beam profiles, such as dipole, quadrupole, starburst, annulus, etc. In a particular example, pixelated illumination techniques can be implemented. In addition to any of the phase contrast techniques described above, programmable illumination and spatial apertures can also be used for the purpose of enhancing feature contrast of specific patterns on the reticle.

[0064] The inspection system 100 can be suitable for inspecting semiconductor devices or wafers and optical reticles as well as EUV reticles or masks. Other types of samples that can be inspected or imaged using the inspection apparatus and techniques of the present disclosure include any surface such as flat panel displays.

[0065] Figure 9 is a flowchart showing a method 200 for inspecting an EUV reticle. At 201, a test image is obtained from an output beam using an inspection system, the output beam being reflected and scattered from a test portion of an EUV test reticle. The inspection system can be an embodiment of the inspection system 100. The inspection system can be configured to provide spatially varying intensity transmission to enhance signals from any defects compared to signals from noise on the EUV reticle. The inspection system can introduce spatially varying intensity transmission (and optionally phase contrast) using a pupil filter, which is a glass plate having a patterned layer disposed on a surface thereof. The pupil filter, implemented via a patterned layer or an unpatterned layer having varying thickness, enhances the relative amount of light in image spatial frequencies that magnify signatures of particular mask defects. When embodiments of the disclosed pupil filter are incorporated into the inspection system, mask defects in the image appear more prominently above background image acquisition noise and stand out more clearly. Where the defect signatures are enhanced by the pupil filter relative to image noise, such defects can be detected and reported in the image.

[0066] At 202, a reference image of a reference reticle portion is obtained, the reference reticle portion being designed to be the same as the test reticle portion.

[0067] At 203, the test image is compared to the reference image. At 204, it is determined whether the test reticle portion has a candidate defect based on the comparison.

[0068] For multiple test reticle portions of a reticle, the use of the inspection system at 201, the obtaining of the reference image at 202, the comparison at 203, and the determination at 204 can be repeated. A defect report can be generated based on candidate defects that have been determined to be present.

[0069] Accordingly, the operation can be repeated for each reticle area, such that the entire reticle is inspected. After inspecting the reticle, it can then be determined whether the reticle passed inspection. Each image difference or intensity value difference above a predefined threshold can then be re-detected more carefully to determine whether the reticle is defective and cannot be used again. For example, a scanning electron microscope (SEM) can be used to re-detect each detection candidate to determine whether the critical dimension (CD) is out of specification. This re-detection process can be performed on any or all reported candidate defects.

[0070] Without regard to the inspection method performed, if the reticle fails re-detection, the corresponding reticle can be repaired or discarded and the inspection ends. For example, the particular defect can be removed from the reticle.

[0071] If the reticle is qualified, the re-detection process can end without discarding or repairing the reticle. The qualified reticle can be used to make workpieces (e.g., semiconductor wafers). After the reticle is used again (either repaired or qualified), the reticle can be inspected again.

[0072] In an alternative embodiment, if the reticle passes inspection, all candidate defects can be considered "acceptable differences" and can be stored and later reused to quickly re-certify the reticle after it is used. In this example, "acceptable differences" serve as a set of baseline events. If such baseline events are present on a used reticle, such baseline events can be considered acceptable and not reported as candidate defects. Only differences that occur after a baseline event is detected are determined to be candidate defects and subjected to defect re-detection.

[0073] While the present disclosure has been described with respect to one or more particular embodiments thereof, it is not intended to limit the present disclosure to these particular embodiments and / or examples. To the contrary, it is intended to cover all modifications, equivalents, and alternatives falling within the scope of the present disclosure. The scope of the present disclosure should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with their full scope of equivalents.

Claims

1. An inspection system, comprising: a light source that generates a light beam; illumination optics configured to direct the light beam onto an extreme ultraviolet (EUV) reticle; a pupil filter positioned in an imaging pupil of the inspection system, wherein the pupil filter is configured to provide spatially varying intensity transmission, and wherein the pupil filter is a glass plate having a patterned layer disposed on a surface thereof; a detector that receives an output light beam from the pupil filter and is configured to generate an image for the output light beam; and collection optics for directing the output light beam that is reflected and scattered from the EUV reticle in response to the light beam, wherein the output light beam is directed through the pupil filter toward the detector.

2. The inspection system of claim 1, wherein the patterned layer comprises at least two different shapes and / or sizes across the surface.

3. The inspection system of claim 1, wherein the patterned layer has a varying density across the surface.

4. The inspection system of claim 1, wherein the patterned layer is rotationally symmetric across the surface.

5. The inspection system of claim 1, wherein the patterned layer is non-rotationally symmetric across the surface.

6. The inspection system of claim 1, wherein the patterned layer is chromium, another metal, or a dielectric.

7. The inspection system of claim 1, wherein the pupil filter is further configured to provide phase contrast in the output light beam.

8. The inspection system of claim 7, wherein the glass plate has an etched portion having a depth corresponding to a phase variation that is introduced into a portion of the output light beam that is transmitted through the pupil filter.

9. The inspection system of claim 8, wherein the patterned layer is disposed on an opposite side of the glass plate from the etched portion.

10. The inspection system of claim 1, wherein the patterned layer has a ring with a width from 100 nm to 5000 nm.

11. The inspection system of claim 1, wherein the patterned layer has a ring with a thickness from 10 nm to 250 nm.

12. The inspection system of claim 1, wherein the patterned layer has a pitch from 100 nm to 5000 nm between rings.

13. A method of inspecting an extreme ultraviolet (EUV) reticle, the method comprising: obtaining a test image from an output light beam that is reflected and scattered from a test portion of an EUV test reticle using an inspection system, wherein the inspection system is configured to provide spatially varying intensity transmission, wherein the inspection system uses a pupil filter to introduce the spatially varying intensity transmission, the pupil filter being a glass plate having a patterned layer disposed on a surface thereof; obtaining a reference image of a reference reticle portion that is designed to be the same as the test reticle portion; comparing, using a processor, the test image to the reference image; and determining, using the processor, whether the test reticle portion has a candidate defect based on the comparison.

14. The method of claim 13, further comprising: repeating the using the inspection system, the obtaining the reference image, the comparing, and the determining for each of a plurality of test reticle portions of the reticle; and generating a defect report based on the candidate defects that have been determined to be present.

15. The method of claim 13, wherein the patterned layer comprises at least two different shapes and / or sizes across the surface.

16. The method of claim 13, wherein the patterned layer has a varying density across the surface.

17. The method of claim 13, wherein the patterned layer is rotationally symmetric across the surface.

18. The method of claim 13, wherein the patterned layer is non-rotationally symmetric across the surface.

19. The method of claim 13, wherein the pupil filter is further configured to provide phase contrast in the output beam.

20. The method of claim 19, wherein the glass sheet has an etched portion having a depth corresponding to a phase variation introduced into portions of the output beam transmitted through the pupil filter.

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