Image-based surface deformation measurement

Abstract

A method for detecting localized tilted regions on a sample using imaging reflectometer measurements includes obtaining a first image without blocking any light reflected from the sample and obtaining a second image while blocking some light reflected from the sample at an aperture plane. The localized tilted regions are detected by comparing a first reflection intensity value of a pixel in the first image with a second reflection intensity value of a corresponding pixel in the second image.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Patent No. 16 / 877,866, filed May 19, 2020. The disclosure of this patent is incorporated herein by reference in its entirety for all purposes. Technical Field

[0003] Embodiments described herein generally relate to using image-based metrology measurements to map sample shape and / or detect localized tilted regions on a sample. Background Art

[0004] Depositing thin films on samples such as semiconductor wafers often results in the generation of localized stresses. Without proper processing, the resulting stress distribution can distort the sample shape, often in the form of a two-dimensional warp sometimes referred to as "potato chipping." While the sample can be clamped during some subsequent processing steps, which can flatten the sample and reduce warping, a certain amount of uncorrected residual displacement may still exist. This can lead to unacceptable errors in processes such as photolithography.

[0005] The map of the sample shape can be used to tune subsequent processes to reduce or correct for deformation. Preferably, this mapping technology is integrated into the processing tool. Therefore, it is desirable that the metrology solution is robust, reliable and relatively inexpensive.

[0006] Current methods for providing sample shape information include full-field sample interferometry, multi-point local slope metrology, and shearing interferometry of the entire sample using large gratings. Multi-point methods lack continuity in mapping, and full-field interferometry systems are complex and expensive. Furthermore, such systems are typically designed to perform only one function: detecting surface deformation, without providing any accompanying information, such as an image or any pattern features. In fact, many existing methods for determining sample shape or topography often have difficulty operating on patterned samples.

[0007] Therefore, there is a need for improved techniques for mapping changes in sample shape and topography that result in localized tilt or warping. Summary of the Invention

[0008] The embodiments described herein provide improved methods for mapping sample shapes and detecting local tilt or warped areas on a sample. Various embodiments utilize image-based reflectometry measurements, but it should be understood that other imaging techniques can also be used to map sample shapes and detect local tilted areas in a similar manner. As an example of an embodiment only, reflectometry measurements based on multiple images are used to detect changes in reflected intensity. One image can be obtained without blocking any light at the aperture plane of the detector, and another image can be obtained when some of the light at the aperture plane is blocked. The blocked light may be that portion reflected from uneven portions of the sample. These portions of the sample are distorted or have local tilt. The light from these portions of the sample is offset at the aperture plane, allowing portions of the light to be blocked so that corresponding points in the image have reduced intensity. Therefore, comparing the reflected intensities between images provides a means for detecting warping or local tilt on a sample. Other embodiments and techniques are described herein.

[0009] According to certain embodiments, for example, a method for detecting a localized tilt region on a sample using an imaging reflectometer includes illuminating a measurement region on the sample with an input beam and performing a first imaging reflectometry measurement. Performing the first imaging reflectometry measurement includes receiving a first imaging beam reflected from the sample at an imaging sensor. An aperture (iris) positioned at an aperture plane of the imaging reflectometer is configured such that the first imaging beam passes through a pupil of the aperture and does not block any portion of the first beam reflected from the sample. A first image of the measurement region is obtained using the first imaging beam reflected from the sample and received at the imaging sensor. The method also includes performing a second imaging reflectometry measurement by receiving a second imaging beam reflected from the sample at the imaging sensor. The aperture (iris) positioned at the aperture plane of the imaging reflectometer is configured such that the second imaging beam passes through a pupil of the aperture and blocks a portion of the second beam reflected from the sample. A second image of the measurement region is obtained using the second imaging beam reflected from the sample and received at the imaging sensor. The method also includes detecting a localized tilt region within the measurement region by comparing a first reflection intensity value of a pixel in the first image with a second reflection intensity value of a corresponding pixel in the second image.

[0010] According to another embodiment, a method for detecting a local tilted area on a sample using an imaging reflectometer configured for dark-field measurement includes: illuminating a measurement area on the sample using an input beam, a portion of the input beam passing through an illumination pupil; receiving, at an imaging sensor, a portion of the imaging beam reflected from the local tilted area on the sample, the portion of the imaging beam reflected from the local tilted area passing through an imaging pupil, wherein the imaging pupil is configured to block a portion of the imaging beam reflected from a flat area on the sample; obtaining a first image of the measurement area using the portion of the imaging beam reflected from the local tilted area on the sample and received at the imaging sensor; and detecting the local tilted area within the measurement area based on a position of a bright spot within the first image.

[0011] According to another embodiment, a method for detecting a localized tilt region on a sample using an imaging reflectometer includes illuminating a measurement region on the sample with an input beam and performing a first imaging reflectometry measurement. Performing the first imaging reflectometry measurement includes receiving a first imaging beam reflected from the sample at an imaging sensor. A pupil of an aperture, positioned at an aperture plane of the imaging reflectometer, is laterally offset relative to the first beam reflected from the sample, such that a portion of the first imaging beam passes through the aperture pupil and a portion of the first imaging beam is blocked at the aperture. A first image of the measurement region is obtained using the portion of the first imaging beam that passes through the aperture pupil and is received at the imaging sensor. The method also includes performing a second imaging reflectometry measurement by receiving a second imaging beam reflected from the sample at the imaging sensor. A pupil of an aperture, positioned at the aperture plane of the imaging reflectometer, is laterally offset relative to the second beam reflected from the sample, such that a portion of the second imaging beam passes through the aperture pupil and a portion of the second imaging beam is blocked at the aperture. A second image of the measurement region is obtained using the portion of the second imaging beam that passes through the aperture pupil and is received at the imaging sensor. The method further includes detecting a local tilt region within the measurement area by comparing a first reflection intensity value of a pixel in the first image with a second reflection intensity value of a corresponding pixel in the second image.

[0012] According to yet another embodiment, a method for detecting a localized tilt region on a sample using an imaging reflectometer includes illuminating a measurement region on the sample with an input beam; receiving an imaging beam reflected from the sample at an imaging sensor, the imaging beam reflected from the sample passing through an imaging pupil; and obtaining a first image of the measurement region using the imaging beam reflected from the sample and received at the imaging sensor. The method also includes illuminating the measurement region on the sample with the input beam, a portion of the input beam being blocked at the illumination pupil; receiving the imaging beam reflected from the sample at the imaging sensor, the imaging beam reflected from the sample passing through the imaging pupil; and obtaining a second image of the measurement region using the imaging beam reflected from the sample and received at the imaging sensor. The method also includes detecting a localized tilt region within the measurement region based on an amount of change in brightness of a pixel in the second image compared to the brightness of a corresponding pixel in the first image, and determining a slope of the localized tilt region.

[0013] In an embodiment, approximately half of the input beam is blocked at the illumination pupil.

[0014] In another embodiment, local slope areas with positive and negative slopes within the measurement area of ​​the second image appear as brighter or brighter areas compared to the first image, or vice versa.

[0015] Other aspects, advantages, and features will be apparent from the claims, the description, and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The various embodiments described herein, both as to organization and method of operation, and their features and advantages, may be best understood by reference to the following detailed description and accompanying drawings, in which:

[0017] Figure 1 is a simplified cross-sectional view of an imaging reflectometer.

[0018] Figure 2 is a simplified cross-sectional view of a multi-wavelength light source.

[0019] Figure 3 is a flow chart outlining a method for measuring the reflectance of a sample.

[0020] Figures 4A to 4C is a simplified cross-sectional diagram of light passing through an aperture at the aperture plane of an imaging reflectometer, according to some embodiments.

[0021] Figure 5 is a simplified graph showing the reflected intensity measured at different points around a localized tilted area on a sample according to an embodiment.

[0022] Figure 6 is a simplified diagram showing the reflected intensity measured at different points around a localized tilted area on a sample according to another embodiment.

[0023] 7A to 7B is a simplified diagram showing how a knife edge arrangement may be used to determine local tilt in a given direction according to an embodiment.

[0024] Figure 8 is a simplified diagram illustrating a lighting-based implementation according to an embodiment.

[0025] Figures 9A to 9B is a simplified diagram of a dark field implementation according to an embodiment.

[0026] FIG. 10A to FIG. 10B is a simplified diagram of a dark field implementation according to another embodiment.

[0027] Figures 11 to 12 is a flow chart illustrating a method of detecting localized tilted regions on a sample using an imaging reflectometer, according to some embodiments.

[0028] It should be understood that for simplicity and clarity of illustration, the elements shown in the figures are not necessarily drawn to scale. For example, the dimensions of some elements may be exaggerated relative to other elements for clarity. In addition, where deemed appropriate, reference numerals may be repeated in the various figures to indicate corresponding or similar elements. DETAILED DESCRIPTION

[0029] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it should be understood that the various embodiments may be practiced without these specific details. In other cases, well-known methods, processes, and components have not been described in detail in order to avoid obscuring the features being described.

[0030] Reference will now be made in detail to various embodiments, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation and not limitation. Furthermore, features illustrated or described as part of one embodiment may be used on or in conjunction with other embodiments to produce further embodiments. The description is intended to encompass such modifications and variations.

[0031] References herein to "specimen," "sample," or "wafer" include, but are not limited to, semiconductor wafers, semiconductor workpieces, photolithography masks, and other workpieces such as memory disks, etc. According to some embodiments, which may be combined with other embodiments described herein, systems and methods are configured for or applied to surface metrology applications such as reflectometry.

[0032] Embodiments described herein generally relate to improved methods for mapping wafer shape and / or detecting localized tilt areas on a wafer. In some embodiments, one image is acquired without blocking any light at the aperture plane of a detector, and another image is acquired while blocking some light at the aperture plane. Variations in reflected intensity between the images can be used to identify warpage and / or localized tilt on the wafer.

[0033] Figure 1 is a simplified cross-sectional view of an imaging reflectometer 100 according to an embodiment. The imaging reflectometer 100 shown in this example can be used to implement the methods described herein. However, the imaging reflectometer 100 is shown merely as an example, and other imaging reflectometers and / or other imaging technologies can perform the described methods. By way of example only, the methods can be performed using an imaging reflectometer configured to obtain an image of a portion of a sample rather than the entire sample and / or an imaging reflectometer using a single wavelength or multi-wavelength light source.

[0034] exist Figure 1 In the example of FIG, light from source module 102 is relayed to homogenizer 108 via light guide 106. Light 116 from homogenizer 108 passes through illumination pupil 114 and is directed to beam splitter 120. A portion 138 of light 116 is reflected by beam splitter 120 toward reference sensor 134, and a portion 122 of light passes through beam splitter 120 and continues along an optical path toward sample 130.

[0035] The portion 122 of light 116 that passes through beam splitter 120 is imaged by lens 126 onto sample 130. Light reflected from sample 130 is directed through at least a portion of lens 126 and reflected by beam splitter 120 toward imaging sensor 158.

[0036] The imaging reflectometer 100 may include a plurality of other lenses (e.g., 110, 112, 118, 136) that shape and / or direct light along an optical path to illuminate the sample 130, illuminate the reference sensor 134, direct the light to other lenses (e.g., 120, 140, 144), and direct the light reflected from the sample to the imaging sensor 158. In some embodiments, for example, light may pass through one or more polarizers (e.g., polarizers 110, 154). One or more polarizers may be inserted in the illumination and / or imaging path to provide enhanced sensitivity to different characteristics of the sample 130. Wave plates may also be inserted to change the phase of the polarized light. The wave plates and / or polarizers may be at a fixed angle to provide polarized reflectometry measurements, or may be rotated to provide ellipsometry measurements. It should be understood that an imaging reflectometer according to the embodiments described herein may not include Figure 1All optical elements shown in the examples may be included and / or other optical elements not included in this example may be included.

[0037] The source module 102 in this example provides a multi-wavelength light source that can sequentially generate different light beams, each light beam having a narrow wavelength range. In some embodiments, the multi-wavelength light source is provided by multiple light sources that can be activated individually. Each light source generates a light beam, and at least some of the light beams have different nominal wavelengths.

[0038] In other embodiments, a multi-wavelength light source is provided by adjusting the source power to the source module 102 to generate beams of light having different nominal wavelengths. The power of each wavelength can be independently controlled to optimize the dynamic range of the reflectivity measured at each wavelength.

[0039] In yet other embodiments, the multi-wavelength light source is provided by a broadband light source and a set of bandpass filters.The broadband light source can be used with the bandpass filters to produce a light beam of a selected nominal wavelength.

[0040] In yet other embodiments, the source module 102 may include multiple light sources, a broadband light source, and a set of bandpass filters.

[0041] In an embodiment, lens 126 has a measurement field size (or illumination area) that is slightly larger than the size of sample 130, allowing imaging sensor 158 to acquire a full sample image without scanning the light or moving stage 132. For example, lens 126 may have a measurement field size of 300 mm or greater for measuring a semiconductor wafer having a diameter of 300 mm. Lens 126 may be a telecentric lens, such that light rays traveling from lens 126 to sample 130 are approximately parallel to an optical axis that is substantially perpendicular to the surface of sample 130. This provides substantially perpendicular illumination across sample 130, or the entire measurement area. This can reduce measurement errors because the illumination angle is substantially the same. Telecentric imaging allows light reflected at substantially the same angle across the entire field of view to reach the imaging sensor. For example, in one embodiment, the light illuminating sample 130 has a telecentricity error of less than 0.3 degrees within the wavelength range of approximately 350 nm to approximately 1200 nm, and in some embodiments, has a telecentricity error of less than 1% within the wavelength range of approximately 350 nm to approximately 1100 nm. As used herein, telecentricity error is a measure of the angular deviation of light rays incident on and reflected from the wafer surface from the normal (or from the optical axis).

[0042] In some embodiments, lens 126 can be a lens with a field size smaller than sample 130. In this case, an area (or measurement area) is imaged, and the optics and / or stage 132 can be moved and / or the optical module can be scanned to image an adjacent field. Depending on the application, the measurement area can be approximately the same size as the die or stepper field. Adjacent images can be stitched together using known techniques to provide a multi-field or full sample image.

[0043] The imaging sensor 158 may be an area imaging sensor comprising one or more digital cameras for capturing light 142 reflected from the sample 130 and passing through the pupil at the aperture plane 150. The imaging sensor 158 provides an image of the sample 130 based on the received light 142. In some embodiments, the imaging sensor 158 may comprise a single camera configured to image the entire surface of the sample 130. In other embodiments, the imaging sensor 158 may comprise a single camera configured to image a portion of the sample 130. In yet other embodiments, the imaging sensor 158 may comprise multiple cameras, each of which images adjacent or slightly overlapping fields (or measurement areas) on the sample 130. Adjacent images may be stitched together using known techniques. Image resolution may be increased by using a higher resolution imaging sensor or by using multiple imaging sensors that each image a smaller field.

[0044] Imaging reflectometer 100 includes an illumination path that provides light to sample 130 and an imaging path that provides reflected light to imaging sensor 158. This allows for independent control of the illumination numerical aperture (NA) and the imaging NA. By way of example only, if imaging sensor 158 has an array size of 5120 pixels by 5120 pixels and an imaging NA of approximately 0.004, then for a 300 mm wafer, the pixel size on sample 130 is approximately 60 μm, and imaging sensor 158 has a Rayleigh resolution of approximately 55 μm at a wavelength of 365 nm and a Rayleigh resolution of approximately 153 μm at a wavelength of 1 μm. Typically, the illumination NA is greater than the imaging NA to correct for residual chromatic telecentricity errors and provide tolerance to tilt and curvature of sample 130. In some embodiments, the illumination NA may be in the range of approximately 0.005 to approximately 0.5, and the imaging NA may be in the range of approximately 0.003 to approximately 0.2.

[0045] The reference sensor 134 may include one or more digital cameras for capturing the light 138 reflected from the beam splitter 120. The reference sensor 134 may have a lower resolution than the imaging sensor 158. The reference sensor 134 may be used to monitor the uniformity and stability of the light 138 and provide real-time calibration of the reflection measurements made by the imaging sensor 158. The measurements at the reference sensor 134 may be used to adjust the characteristics of the light source (e.g., output power) to provide spatial and temporal corrections.

[0046] Figure 2 is a simplified cross-sectional view of a multi-wavelength light source according to an embodiment. The multi-wavelength light source can be used as, for example Figure 1 The multi-wavelength light source comprises a plurality of light sources 202 and a plurality of optical fibers 206. The light sources 202 may each comprise one or more light emitting diodes (LEDs) and / or laser diodes (LDs). The light sources 202 are each optically coupled to a homogenizer 208 via one of the optical fibers 206. Each of the light sources 202 generates a beam, and at least some of the beams may have different nominal wavelengths. The light from the homogenizer 208 may be directed to a lens and used to image a sample, as shown in FIG. Figure 1 As stated.

[0047] In an embodiment, the multi-wavelength light source generates different ones of the input beams sequentially and / or generates a combination of multiple input beams sequentially. The beams may be emitted in a manner typically associated with an imaging sensor (e.g., Figure 1 The image sensors 158 shown in FIG. 1 are sequentially generated at the same switching rate as the frame rate of the imaging sensor 158 shown in FIG. 1 to achieve one image per wavelength of the same field on the sample. In some embodiments, the frame rate of the sensor can be faster than the wavelength switching rate. A faster switching rate enables averaging of multiple images at each wavelength, thereby achieving a higher signal-to-noise ratio. The output power and / or integration time of each of the light sources 202 can be independently controlled and adjusted so that the sensor signal approaches saturation at each wavelength to maximize the signal-to-noise ratio. Each of the light sources 202 can have sufficient output power to achieve high-speed measurements (or measurements performed at or near the readout speed of the imaging sensor).

[0048] In some embodiments, the light throughput can be increased by inserting a diffuser between the optical fiber 206 and the homogenizer 208. Multiple light sources 202 can be combined by other means such as a dichroic beam splitter, and the light sources 202 can be coupled to the homogenizer 208 by other means such as a free-space optical relay.

[0049] In some embodiments, a bandpass filter can be inserted between each of the light sources 202 and its corresponding optical fiber 206 to narrow the bandwidth of each wavelength. Narrower bandwidths can provide better sensitivity for measuring thick film stacks or dense patterns on the sample surface. Bandpass filters can precisely define the measurement wavelength by eliminating wavelength drift of the LEDs, thereby improving measurement accuracy.

[0050] Imaging sensors (e.g. Figure 1 The imaging sensor 158 shown can have a high readout speed (e.g., 50 to 1000 frames per second (FPS) or higher, and up to 100 million pixels per frame or higher). As an example, at a readout speed of 100 FPS, the imaging sensor may be able to perform 6000 reflectance measurements per minute. The measurements can be taken at the same or different wavelengths. Obtaining multiple measurements at the same wavelength can improve the signal-to-noise ratio and increase measurement sensitivity.

[0051] Figure 3 The present invention is a flow chart outlining a method for measuring reflectivity of a sample using an imaging reflectometer. The method includes sequentially generating a plurality of input beams at a first switching rate (302). In some embodiments, each input beam in the plurality of input beams is generated by a different light source, and at least some of the input beams in the plurality of input beams may have a different nominal wavelength and / or integration time than other input beams in the plurality of input beams. In other embodiments, at least some of the input beams in the plurality of input beams are generated by a broadband light source, and a set of bandpass filters is used to limit the wavelength of each input beam in the plurality of input beams.

[0052] Each of the plurality of input beams is directed through an illumination pupil having a first NA (304). The illumination pupil can be arranged along a first optical path. In some embodiments, each of the plurality of input beams can be split, and a first portion of each of the plurality of input beams can be directed along the first optical path toward a reference sensor, and a second portion of each of the plurality of input beams can continue along the first optical path.

[0053] At least a portion of each of the plurality of input beams is provided as substantially telecentric illumination (306) on a sample imaged using the lens. A portion of each of the plurality of input beams may also be provided to a reference sensor for monitoring uniformity and stability of the input beams. In some embodiments, the measurement area of ​​the lens may be larger than the sample being imaged to provide full sample measurement.

[0054] A reflected portion of the substantially telecentric illumination reflected from the sample is received at a lens and directed through an imaging pupil having a second NA (308) lower than the first NA of the illumination pupil. A beam splitter may be used to direct the reflected portion through the imaging pupil.

[0055] The reflected portion is received and corresponding image information is generated at the imaging sensor module, wherein the image information is generated at a frame rate equal to or faster than the first switching rate (310). The image information can be calibrated or normalized based on information from a reference sensor.

[0056] In some embodiments, the image can be processed to identify process drift. The image can be processed according to known drift identification techniques. For example, the reflectivity measured at multiple wavelengths can be compared to the modeled reflectivity or a known good sample. Patterns measured at different locations on the sample can also be compared to identify variations and / or outliers. The variation in the measurement can be quantified by calculating the root mean square (RMS) difference at multiple wavelengths. The measurement sensitivity can be enhanced by selecting one or more wavelengths with the highest sensitivity based on the measurement data. The multi-wavelength reflectivity can be processed by nonlinear regression to a theoretical model to derive the film thickness and / or CD of the pattern.

[0057] It should be understood that the imaging reflectometers described herein can be configured as stand-alone metrology tools or integrated with other metrology or processing tools. As an example, an imaging reflectometer as described herein can be integrated with a processing tool and arranged outside a window that separates the imaging reflectometer from the processing chamber. In some embodiments, a lens arranged outside the window provides illumination for a sample arranged inside the processing chamber. The lens can be configured to provide illumination to all or part of the sample (e.g., the measurement area can be approximately the same size as the die or stepper field, or can be another size, such as a few centimeters on the side). This allows reflectometry measurements to be performed immediately during and / or after processing while the sample is in the vacuum chamber. This can shorten control loops, improve process control, and avoid material damage caused by air in the environment.

[0058] The imaging reflectometer can use a spectral imaging configuration in which fields of different sizes are sequentially imaged at different illumination wavelengths. The integration time for each field can be varied depending on the sample reflectivity at that wavelength. The images obtained at each wavelength can be stored at the imaging reflectometer or in remote storage.

[0059] The imaging path includes a pupil or aperture at an aperture plane. The aperture plane is located along the imaging path at a region through which substantially all light reflected from the sample passes. Figure 4A4, where reflected light 442 from a point on a sample 432 within a measurement region is focused by a lens 440 and passes through an aperture 452 of an aperture plane 450. After passing through the aperture 452, the light 442 is directed by a lens 444 toward an imaging sensor 458.

[0060] During normal operation of the imaging reflectometer, substantially all of the reflected light 442 passes through the aperture 452. Normal operation is where the surface of the sample 432 is substantially normal to the optical axis. However, when the sample includes a locally tilted region, the position of the light returning from the tilt point is offset. Figure 4B , which will be better understood. As shown in this example, sample 432 includes a tilted region 456, which causes a shift in reflected light 442a from this region. Reflected light 442 from other regions on sample 432 is not shifted. As shown in this example, reflected light 442a from tilted region 456 is shifted at the location where the aperture plane would be located. Figure 4A The aperture plane shown is Figure 4B 442a from the inclined region 456 to more clearly illustrate the offset of the reflected light 442a.

[0061] Normally, imaging systems are designed to tolerate this type of offset. Figure 4A As can be seen in , assuming the light field at aperture plane 450 is unobstructed, reflected light 442 from sample 432 passes unimpeded through aperture 452 to the intended image point on imaging sensor 458. Figure 4B In the scene, the only effect of the inclined area 456 is that the reflected light 442a reaches the corresponding image point at a certain angle. Since the angle of the inclined area 456 is usually very small, the angle is usually difficult to detect.

[0062] In some embodiments, aperture 452 is a variable aperture or iris. Some imaging reflectometry measurements can be performed with aperture 452 open so that none of reflected light 442 is blocked at aperture plane 450. Other imaging reflectometry measurements can be performed with aperture 452 reduced in size so as to include only the physical size of the reflected light under normal conditions. In such a configuration, any shift in reflected light 442 will have a commensurate change in the amount of energy that passes through aperture 452 toward imaging sensor 458. This can be referenced to Figure 4C 4. The example of the embodiment of the present invention is shown in FIG4 , where a portion of the deflected reflected light 442a is blocked at the aperture plane 450 (or aperture plate). In this simplified example, the deflected reflected light 442a is shown as passing through the aperture plane 450, but in actual operation, the aperture plane 450 blocks a portion of the reflected light 442a, so that less light reaches the imaging sensor 458. This will cause the signal level detected from the tilted area 456 on the sample 432 to change.

[0063] The amplitude of the signal change can be estimated.If the sample 432 is in the nominal focal plane of the objective, then a tilt of the sample θ results in a lateral shift of Fθ, where F is the focal length of the objective.

[0064] The position of reflected light 442a from tilted region 456 is shifted at aperture plane 450, regardless of the position of tilted region 456 on sample 432. Reflected light from all tilted regions within the field of view is shifted. Aperture 452 of the same size at aperture plane 450, which produces intensity variations due to a tilted region at one point on sample 432, produces intensity variations for all tilted regions within the measurement area (or field of view). That is, in addition to providing reflection intensity values, the image can also provide information about the local topography of the measurement area.

[0065] For any given point on sample 432, and for any given wavelength of light, the image of the measurement area on sample 432 can be expressed as K(λ)×M(λ)×I(λ)×(λ), where I is the illumination intensity and R is the reflectivity, both at a given wavelength (λ). Here, M represents the percentage of light that passes through aperture 452, and K is a proportionality constant. This representation is reasonable for a properly designed telecentric lens because the reflected light from every point on the sample will essentially uniformly fill aperture 452, and thus a shift in the light distribution position at aperture plane 450 appears as a multiplier of the amount of energy.

[0066] According to some embodiments, both reflectivity and tilt information can be obtained from imaging reflectometry measurements. Reflectivity information is obtained by performing measurements with the aperture or iris pupil wide open so that the aperture does not block light reflected from the sample. Tilt information is obtained by (i) performing measurements with the aperture or iris pupil wide open, and (ii) reducing the size of the aperture or pupil and performing measurements. The size of the aperture or pupil is reduced so that light displaced by the tilt region is blocked at the aperture (or aperture plane). All measurements can be performed at a single wavelength or at multiple wavelengths to obtain wavelength sensitivity information.

[0067] The image obtained in step (i) provides reflection intensity values.By normalizing the image obtained in step (ii) on a point-by-point basis with respect to the image obtained in step (i), the parameter M corresponding to the tilt can be extracted on a point-by-point basis.

[0068] The dependence of the detected signal on tilt is not a direct measure of the local topography, but rather a derivative of the topography (i.e., the local slope) along a particular direction of the sample 432. For example, local tilt along the x-direction on the sample 432 causes a shift in the x-direction of the reflected light 442a at the aperture plane 452. Similarly, local tilt along the y-direction on the sample 432 causes a shift in the y-direction of the reflected light 442a at the aperture plane 452. Tilt along other directions similarly shifts the reflected light 442a in the corresponding directions at the aperture plane 452.

[0069] Figure 5 is a simplified diagram showing the reflected intensity measured at different points around a localized tilted area on a sample, according to an embodiment. The description of the effect of tilt on the image has so far assumed an arrangement of a circular light distribution interrupted by a circular aperture at the aperture plane. For the case where the sample is symmetrically orthogonal to the illumination light and the light distribution is positioned symmetrically with respect to the pupil of the aperture, a given tilt will result in light passing through the pupil of the aperture having reduced intensity compared to light from adjacent flat areas on the sample. This reduction in the reflected signal applies regardless of whether the tilt has a positive or negative slope. Thus, the image of a symmetrical protrusion or depression on the sample is a dipole with a negative-going peak, as shown. That is, an arrangement in which the light distribution is positioned symmetrically with respect to the pupil of the aperture allows for the detection of features, but does not allow for the determination of whether the feature is a protrusion or a depression.

[0070] This is Figure 5 , where reflected light from flat areas on the sample, including from the peaks of the protrusions, essentially fills the pupil of the aperture at the aperture plane without being blocked at the aperture plane. The reflected intensity measured at these points is a nominal value. Reflected light from inclined areas on the sample, particularly from inclined areas on the sides of the protrusions, is offset at the aperture plane. Portions of the beam outside the pupil of the aperture are blocked at the aperture plane, thereby reducing the reflected intensity measured at these points. The right axis of the graph represents surface topography, and the left axis represents reflected intensity. The circles above the graph represent the aperture at the aperture plane for the corresponding points on the sample, and the circular shaded areas represent the light at the aperture plane. In reality, all light passes through a single aperture at the aperture plane, but the aperture and the light from each point are shown separately in this figure to illustrate the offset of reflected light from inclined areas. Similar reflected intensity values ​​would be obtained from depressions on the sample with inwardly sloping sidewalls of similar magnitude.

[0071] In some applications, it may be advantageous to determine the characteristics of a feature. For example, it may be advantageous to determine whether the slope across a localized sloped region is positive or negative. This allows the feature to be identified as a protrusion or a depression. According to an embodiment, the position of the pupil of the aperture is offset (or shifted) relative to the light distribution at the aperture plane along a direction of interest (e.g., along the x-direction or y-direction). This allows the characteristics of the feature to be determined. Such an arrangement may be Figure 6 As shown, the aperture pupil is offset horizontally, so that some light reflected from flat areas on the sample is blocked at the aperture plane (including light reflected from the peaks of the protrusions). This is illustrated by the circles and circular shading above the graph. Because some of the light is blocked, the reflected intensity measured from these flat areas is less than the maximum value. Furthermore, in the case of patterned samples, this arrangement slightly reduces the range of spatial frequencies detected emanating from the sample due to scattering. In some embodiments, the offset of the aperture pupil can be small, minimizing the impact on image brightness.

[0072] like Figure 6 As shown, the light reflected from the inclined area (or the side of the protrusion) is offset compared to the light reflected from the flat area. For the inclined area with a positive slope (on the left side of the protrusion), the reflected light is offset so that more light is blocked at the aperture plane than the flat area. For the inclined area with a negative slope (on the right side of the protrusion), the reflected light is offset so that less light is blocked at the aperture plane than the flat area. Therefore, in this example, the reflection intensity values ​​increase and decrease differently for positive and negative slopes. Using this arrangement, the image of the feature is a dipole, whose polarity (i.e., negative to positive, or positive to negative) determines the nature of the feature (i.e., protrusion or depression). In some embodiments, the offset of the pupil of the aperture is determined by the expected maximum tilt on the sample, so that the polarity of all expected features on the sample is maintained.

[0073] Figure 6 The example shows an arrangement where there is a linear offset between the pupil center of the aperture and the light distribution along a particular direction (e.g., along the x-direction), with maximum sensitivity to tilt also along that direction. The arrangement is less sensitive to tilt in the orthogonal direction (e.g., along the y-direction), although there may still be some variation in intensity due to tilt in the orthogonal direction.

[0074] In some applications, it may be advantageous to determine the local tilt in a given direction (e.g., the x-direction) while further minimizing the effects of local tilt in an orthogonal direction (e.g., the y-direction). 7A to 7B The bayonet joint arrangement shown is provided. Figure 7AIn the first example shown, a one-dimensional barrier or bayonet is arranged to block some of the reflected light at the aperture plane. The two circles and double arrows represent the offset of the reflected light at the aperture plane from a feature having a tilt along the x-axis. Depending on the offset of the reflected light, the bayonet blocks different amounts of the reflected light. Thus, Figure 7A The arrangement of is sensitive to features having tilt along the x-axis.

[0075] exist Figure 7B In the middle, the knife-type masonry is used with Figure 7A The same arrangement is made relative to the aperture plane. Figure 7B The two circles and double arrow in the figure represent the offset of the reflected light at the aperture plane from a feature with a tilt along the y-axis. In this case, the bayonet cutout blocks the same amount of reflected light regardless of the offset of the reflected light. Therefore, Figure 7B The arrangement is insensitive to features having a tilt along the y-axis. Regardless of the offset of the reflected light, the measured intensity will be the same.

[0076] The bayonet can be arranged in a similar manner in any direction to detect features that are tilted in that direction. As an example, the bayonet can be arranged horizontally (rather than horizontally as in FIG. 7A to 7B The y-axis is arranged to detect local tilt areas along the y-direction.

[0077] Figure 8 is a simplified diagram illustrating an illumination-based implementation according to an embodiment. In this arrangement, the illumination pupil can be partially blocked, so that only a portion of the light from the pupil illuminates the sample. In a suitably designed imaging system, the illumination pupil is imaged onto the imaging pupil. Therefore, if the sample does not have any localized tilted regions, the image of the illumination pupil is a partially blocked image. However, if the sample does have localized tilted regions, the image is shifted in the lateral direction, resulting in an increase or decrease in the amount of light passing through the imaging pupil. Thus, the system is able to distinguish between positive and negative slopes.

[0078] Figures 9A to 9B A darkfield implementation according to an embodiment is shown. In this arrangement, a portion of the illumination NA is blocked at the imaging pupil (black portion is blocked). For flat areas on the sample, all or nearly all of the reflected light is blocked at the imaging pupil (or at the aperture plane). For tilted areas on the sample, the reflected light will be offset and thus at least a portion will not be blocked at the imaging pupil, so that in this arrangement, the tilted areas will have an increased measured reflection intensity.

[0079] FIG. 10A to FIG. 10B The measurement of a two-dimensional surface profile with a one-dimensional pupil pattern according to an embodiment is shown. The patterns used in these figures are examples only, and other patterns can also be used in other embodiments. FIG. 10A to FIG. 10B As shown in , for a dark field implementation, the pattern at the illumination pupil can be the opposite of the pattern at the imaging pupil. The pattern can provide sensitivity to local tilt extending along a specific direction. For example, FIG. 10A to FIG. 10B The pattern used in is sensitive to local tilt extending along the x-axis, but insensitive to local tilt extending along the y-axis (no light shifted along the y-axis is detected in the case of a dark field implementation). According to an embodiment, the pattern is rotatable so that local tilt areas extending in different directions can be detected separately.

[0080] In some embodiments, patterns can also be achieved using an LED array for illumination and / or an LCD array for detection. Patterns for illumination and imaging can be generated by electronic control and / or mechanical patterns.

[0081] Figure 11 is a flow chart illustrating a method for detecting a local tilted area on a sample using an imaging reflectometer according to an embodiment. The method includes illuminating a measurement area on a sample with an input beam (1002).

[0082] The method also includes performing a first imaging reflectometry measurement (1004). Performing the first imaging reflectometry measurement may include receiving a first imaging beam reflected from the sample at an imaging sensor. An aperture positioned at an aperture plane of the imaging reflectometer may be configured such that the first imaging beam passes through a pupil of the aperture and the aperture does not block any portion of the first beam reflected from the sample. A first image of the measurement area may be obtained using the first imaging beam reflected from the sample and received at the imaging sensor. In some embodiments, the pupil size of the aperture may be variable.

[0083] The method also includes performing a second imaging reflectometry measurement (1006). Performing the second imaging reflectometry measurement may include receiving a second imaging beam reflected from the sample at an imaging sensor. An aperture positioned at an aperture plane of the imaging reflectometer may be configured such that the second imaging beam passes through a pupil of the aperture and the aperture blocks a portion of the second beam reflected from the sample. A second image of the measurement area may be obtained using the second imaging beam reflected from the sample and received at the imaging sensor. In some embodiments, a pupil diameter of the aperture during the second imaging reflectometry measurement may be smaller than a pupil diameter of the aperture during the first imaging reflectometry measurement.

[0084] The method also includes detecting a localized tilted region within the measurement area (1008). Detecting the localized tilted region may include comparing a first reflection intensity value of a pixel in the first image with a second reflection intensity value of a corresponding pixel in the second image. In some embodiments, detecting the localized tilted region may include identifying a pixel in the first image where the first reflection intensity value differs from the second reflection intensity value of the corresponding pixel in the second image by more than a predetermined amount. In other embodiments, detecting the localized tilted region may include comparing, for each pixel in the first plurality of pixels and each pixel in the second plurality of pixels, the first reflection intensity value of the pixel in the first image with the second reflection intensity value of the corresponding pixel in the second image.

[0085] Comparing the first reflection intensity value of a pixel in the first image to the second reflection intensity value of the corresponding pixel in the second image may include dividing the second reflection intensity value of the pixel in the second image by the first reflection intensity value of the corresponding pixel in the first image.

[0086] The height difference between the local inclined area and the surrounding flat area on the sample can be determined based on the difference in reflection intensity values ​​between the pixel in the first image and the corresponding pixel in the second image.

[0087] Figure 11 The method may further include arranging a bayonet to block a first portion of the first imaging beam reflected from the sample and a second portion of the second imaging beam reflected from the sample to determine a tilt direction associated with the local tilt region. Based on a change in reflection intensity values ​​between the first image and the second image, an area within the local tilt region having a tilt direction orthogonal to the bayonet can be identified.

[0088] According to some embodiments, a method for detecting a localized slope region on a sample using an imaging reflectometer includes performing a first imaging reflectometry measurement and a second imaging reflectometry measurement using an arrangement in which the pupil of an aperture is laterally offset relative to a first beam reflected from the sample. Because the pupil is offset, a portion of the imaging beam passes through the pupil of the aperture, and a portion of the imaging beam is blocked at the aperture. Using the offset pupil allows for the identification of positive and negative slope regions within the localized slope region.

[0089] Figure 12 is a flow chart illustrating a method for detecting local tilted areas on a sample using an imaging reflectometer configured for dark field measurement according to an embodiment.The method includes illuminating a measurement area on the sample with an input beam (1102).

[0090] The method also includes receiving, at an imaging sensor, a portion of an imaging beam reflected from a localized tilted region on the sample (1104). The portion of the imaging beam reflected from the localized tilted region can pass through an imaging pupil configured to block the portion of the imaging beam reflected from a flat region on the sample. In some embodiments, the portion of the input beam can pass through an illumination pupil having a first NA. The imaging pupil can have a second NA greater than the first NA.

[0091] The method also includes obtaining a first image of the measurement area using a portion of the imaging beam reflected from the local tilted area on the sample and received at the imaging sensor (1106).

[0092] The method also includes detecting a local tilt region within the measurement area based on a location of the bright spot within the first image (1108).

[0093] In some embodiments, a one-dimensional pattern may be provided at the illumination pupil, and a one-dimensional pattern may be provided at the imaging pupil. The one-dimensional pattern at the illumination pupil may be the inverse of the one-dimensional pattern at the imaging pupil. The orientation of the one-dimensional pattern at the illumination pupil may be aligned with the orientation of the one-dimensional pattern at the imaging pupil. The tilt direction associated with the local tilt region may be determined by rotating the one-dimensional pattern at the illumination pupil and the one-dimensional pattern at the imaging pupil by approximately 90°; obtaining a second image of the measurement region using a portion of the imaging beam reflected from the local tilt region on the sample and received at the imaging sensor; and determining the tilt direction associated with the local tilt region based on a change in reflection intensity values ​​between the first image and the second image. In some embodiments, the one-dimensional pattern at the illumination pupil and / or the one-dimensional pattern at the imaging pupil may be generated by electronic control, and the rotation of the one-dimensional pattern at the illumination pupil and / or the one-dimensional pattern at the imaging pupil may be provided by electronically generating a new one-dimensional pattern rather than by rotating a mechanical pattern generator.

[0094] It should be understood that Figures 11 to 12 The specific steps shown in FIG. 10 provide a specific method for measuring reflectivity according to some embodiments. According to alternative embodiments, other sequences of steps may also be performed. For example, alternative embodiments may perform the above steps in a different order. Figure 11 The various steps shown may include multiple sub-steps that may be performed in various orders. In addition, additional steps may be added or deleted depending on the specific application.

[0095] While the foregoing is directed to particular embodiments, other and further embodiments may be devised without departing from the basic scope of the invention, and the scope of the invention is determined by the claims that follow.

Claims

1. A method for detecting a local tilted area on a sample using an imaging reflectometer, the method comprising: illuminating a measurement area on the sample with an input beam; performing a first imaging reflectometry measurement, wherein performing the first imaging reflectometry measurement comprises: receiving, at an imaging sensor, a first imaging beam reflected from the sample, wherein an aperture positioned at an aperture plane of the imaging reflectometer is configured such that the first imaging beam passes through a pupil of the aperture and the aperture does not block any portion of the first imaging beam reflected from the sample; and obtaining a first image of the measurement region using the first imaging beam reflected from the sample and received at the imaging sensor, the first image comprising a first plurality of pixels; performing a second imaging reflectometry measurement, wherein performing the second imaging reflectometry measurement comprises: receiving, at the imaging sensor, a second imaging beam reflected from the sample, wherein the aperture positioned at the aperture plane of the imaging reflectometer is configured such that the second imaging beam passes through the pupil of the aperture and the aperture blocks a portion of the second imaging beam reflected from the sample; and obtaining a second image of the measurement region using the second imaging beam reflected from the sample and received at the imaging sensor, the second image comprising a second plurality of pixels; The local tilted region within the measurement region is detected, wherein detecting the local tilted region comprises comparing a first reflection intensity value of a pixel in the first image with a second reflection intensity value of a corresponding pixel in the second image. 2 . The method of claim 1 , wherein detecting the localized tilt region further comprises identifying pixels in the first image where the first reflection intensity value differs from the second reflection intensity value of a corresponding pixel in the second image by more than a predetermined amount. 3 . The method of claim 1 , wherein the height difference between the local inclined area and the surrounding flat area on the sample is determined based on the difference in reflection intensity values ​​between pixels in the first image and corresponding pixels in the second image. 4 . The method of claim 1 , wherein a diameter of the pupil of the aperture during the second imaging reflectometry measurement is smaller than a diameter of the pupil of the aperture during the first imaging reflectometry measurement. The method of claim 1 , wherein the size of the pupil of the aperture is variable.

6. The method of claim 1 , wherein comparing the first reflection intensity value of the pixel in the first image with the second reflection intensity value of the corresponding pixel in the second image comprises: The second reflection intensity value of the pixel in the second image is divided by the first reflection intensity value of the corresponding pixel in the first image.

7. The method according to claim 1, wherein detecting the local tilt region comprises: For each pixel in the first plurality of pixels and each pixel in the second plurality of pixels, a first reflected intensity value of the pixel in the first image is compared with a second reflected intensity value of the corresponding pixel in the second image.

8. The method of claim 1 , further comprising determining a tilt direction associated with the local tilt region, wherein determining the tilt direction comprises: arranging a knife edge to block a first portion of the first imaging beam reflected from the sample and a second portion of the second imaging beam reflected from the sample; as well as Based on the change in the reflection intensity value between the first image and the second image, a region in the local tilted region where the tilt direction is orthogonal to the bayonet joint is identified.

9. A method for detecting local tilted regions on a sample using an imaging reflectometer configured for dark field measurement, the method comprising: illuminating a measurement area on the sample with an input beam, a portion of the input beam passing through an illumination pupil; receiving, at an imaging sensor, a portion of the imaging beam reflected from the local tilted area on the sample, the portion of the imaging beam reflected from the local tilted area passing through an imaging pupil, wherein the imaging pupil is configured to block a portion of the imaging beam reflected from a flat area on the sample; obtaining a first image of the measurement area using the portion of the imaging beam reflected from the localized tilted area on the sample and received at the imaging sensor; as well as The local tilt region within the measurement region is detected based on the position of a bright spot within the first image.

10. The method of claim 9, wherein the illumination pupil has a first numerical aperture (NA) and the imaging pupil has a second NA that is greater than the first NA.

11. The method of claim 9 , wherein a one-dimensional pattern is provided at the illumination pupil and a one-dimensional pattern is provided at the imaging pupil, the one-dimensional pattern at the illumination pupil being the inverse of the one-dimensional pattern at the imaging pupil, and an orientation of the one-dimensional pattern at the illumination pupil being aligned with an orientation of the one-dimensional pattern at the imaging pupil, the method further comprising determining a tilt direction associated with the local tilt region, wherein determining the tilt direction comprises: rotating the one-dimensional pattern at the illumination pupil and the one-dimensional pattern at the imaging pupil by approximately 90°; obtaining a second image of the measurement area using the portion of the imaging beam reflected from the localized tilted area on the sample and received at the imaging sensor; as well as The tilt direction associated with the local tilt area is determined based on a change in reflection intensity values ​​between the first image and the second image.

12. The method of claim 11 , wherein the one-dimensional pattern at the illumination pupil and / or the one-dimensional pattern at the imaging pupil are generated by electronic control, and rotation of the one-dimensional pattern at the illumination pupil and / or the one-dimensional pattern at the imaging pupil is provided by electronically generating a new one-dimensional pattern rather than by rotating a mechanical pattern generator.

13. A method for detecting a local tilted area on a sample using an imaging reflectometer, the method comprising: illuminating a measurement area on the sample with an input beam; performing a first imaging reflectometry measurement, wherein performing the first imaging reflectometry measurement comprises: receiving, at an imaging sensor, a first imaging beam reflected from the sample, wherein a pupil of an aperture positioned at an aperture plane of the imaging reflectometer is laterally offset relative to the first imaging beam reflected from the sample such that a portion of the first imaging beam passes through the pupil of the aperture and a portion of the first imaging beam is blocked at the aperture; and obtaining a first image of the measurement area using the portion of the first imaging beam that passes through the pupil of the aperture and is received at the imaging sensor; performing a second imaging reflectometry measurement, wherein performing the second imaging reflectometry measurement comprises: receiving, at the imaging sensor, a second imaging beam reflected from the sample, wherein the pupil of the aperture positioned at the aperture plane of the imaging reflectometer is laterally offset relative to the second imaging beam reflected from the sample such that a portion of the second imaging beam passes through the pupil of the aperture and a portion of the second imaging beam is blocked at the aperture; and obtaining a second image of the measurement area using the portion of the second imaging beam that passes through the pupil of the aperture and is received at the imaging sensor; The local tilted region within the measurement region is detected, wherein detecting the local tilted region comprises comparing a first reflection intensity value of a pixel in the first image with a second reflection intensity value of a corresponding pixel in the second image.

14. The method of claim 13, further comprising identifying positive slope regions and negative slope regions within the local slope region based on the comparison of the first reflection intensity value of the pixel in the first image with the second reflection intensity value of the corresponding pixel in the second image.

15. The method of claim 13, wherein detecting the localized tilt region further comprises identifying pixels in the first image where the first reflection intensity value differs from the second reflection intensity value of a corresponding pixel in the second image by more than a predetermined amount.

16. The method of claim 13, wherein comparing the first reflection intensity value of the pixel in the first image with the second reflection intensity value of the corresponding pixel in the second image comprises: The first reflection intensity value of the pixel in the first image is divided by the second reflection intensity value of the corresponding pixel in the second image.

17. The method of claim 13, wherein detecting the local tilt region comprises: For each pixel of a first plurality of pixels in the first image and each pixel of a second plurality of pixels in the second image, a first reflected intensity value of the pixel in the first image is compared with a second reflected intensity value of the corresponding pixel in the second image.

18. A method for detecting a localized tilted area on a sample using an imaging reflectometer, the method comprising: illuminating a measurement area on the sample with an input beam; receiving, at an imaging sensor, an imaging beam reflected from the sample, the imaging beam reflected from the sample passing through an imaging pupil; obtaining a first image of the measurement region using the imaging beam reflected from the sample and received at the imaging sensor; illuminating the measurement area on the sample with the input beam, a portion of the input beam being blocked at an illumination pupil; receiving, at an imaging sensor, an imaging beam reflected from the sample, the imaging beam reflected from the sample passing through an imaging pupil; obtaining a second image of the measurement region using the imaging beam reflected from the sample and received at the imaging sensor; The local tilted region within the measurement area is detected and a slope of the local tilted region is determined based on an amount of change in brightness of pixels within the second image compared to brightness of corresponding pixels in the first image.

19. The method of claim 18, wherein approximately half of the input beam is blocked at the illumination pupil.

20. The method of claim 18, wherein the local slope regions having positive and negative slopes within the measurement area of ​​the second image appear as brighter or darker regions compared to the first image, or vice versa.

Images