Color Imaging for CMP Monitoring

Abstract

A polishing system includes a polishing station including a platen for supporting a polishing pad; a support for holding a substrate; an inline metrology station for measuring the substrate before and after polishing the surface of the substrate in the polishing station; and a controller. The inline metrology station includes a color line scan camera; a white light source; a frame for supporting the light source and the camera; and a motor for causing relative motion between the camera and the support along a second axis perpendicular to the first axis to cause the light source and the camera to scan across the substrate. The controller is configured to receive color data from the camera, generate a two-dimensional color image from the color data, and control polishing at the polishing station based on the color image.

Description

[0001] This application is a divisional application of the Chinese patent application entitled “Color Imaging for CMP Monitoring” (PCT application number PCT / US2016 / 059501), filed on October 28, 2016, with application number 201680064804.5. Technical Field

[0002] The present disclosure relates to optical metrology, for example, to detect the thickness of a layer on a substrate. Background Art

[0003] Integrated circuits are typically formed on substrates by sequentially depositing conductive, semiconductive, or insulating layers on a silicon wafer. One manufacturing step involves depositing a filler layer on a non-planar surface and planarizing the filler layer. For some applications, the filler layer is planarized until the top surface of the patterned layer is exposed. For example, a conductive filler layer can be deposited on a patterned insulating layer to fill grooves or holes in the insulating layer. After planarization, the portion of the metal layer remaining between the raised patterns of the insulating layer forms vias, plugs, and lines that provide conductive paths between the thin-film circuits on the substrate. For other applications (such as oxide polishing), the filler layer is planarized until a predetermined thickness is left on the non-planar surface. In addition, for photolithography, planarization of the substrate surface is typically required.

[0004] Chemical mechanical polishing (CMP) is an acceptable planarization method. This planarization method typically requires mounting the substrate on a carrier or polishing head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate, pushing it against the polishing pad. An abrasive polishing slurry is typically applied to the surface of the polishing pad.

[0005] Variations in slurry distribution, polishing pad conditions, the relative speed between the polishing pad and substrate, and the load on the substrate can result in variations in the material removal rate. These variations, along with variations in the initial thickness of the substrate layer, result in variations in the time required to reach the polishing endpoint. Therefore, determining the polishing endpoint solely as a function of polishing time can result in over-polishing or under-polishing of the substrate.

[0006] Various optical metrology systems (e.g., spectroscopic or ellipsometry) can be used to measure the thickness of the substrate layer before and after polishing (e.g., at an inline or standalone metrology station). In addition, various in-situ monitoring techniques (such as optical or eddy current monitoring) can be used to detect the polishing endpoint. Summary of the Invention

[0007] In one aspect, a polishing system includes a polishing station including a platen for supporting a polishing pad; a support for holding a substrate; an in-line metrology station for measuring a substrate before or after polishing a surface of the substrate in the polishing station; and a controller. The in-line metrology station includes a color line scan camera having a detector element arranged along a first axis parallel to the surface of the substrate during scanning of the substrate; an elongated white light source having a longitudinal axis parallel to the first axis and configured to direct light toward the substrate at a non-zero angle of incidence during scanning of the substrate; a frame supporting the light source and the camera; and a motor for causing relative motion between the frame and the support along a second axis perpendicular to the first axis to cause the light source and camera to scan across the substrate. The controller is configured to receive color data from the camera, generate a two-dimensional color image from the color data, and control polishing at the polishing station based on the color image.

[0008] Implementations may include one or more of the following features: A diffuser may be located in the light path between the light source and the substrate. The non-zero angle is between 5° and 85°. A circular polarizer may be located in the light path between the substrate and the camera.

[0009] The motor may be coupled to the frame, and the controller may be configured to cause the substrate to be held stationary while the motor moves the frame and light source to cause the line scan camera to scan across the substrate. The frame may be fixed, the motor may be coupled to the support, and the controller may be configured to cause the motor to move the support while the light source and camera remain stationary to scan across the substrate. The support may include a carrier head for holding the substrate against the polishing pad, and the inline metrology station may be positioned between the polishing station and one of another polishing station or a transfer station. The support may include a transfer robot, and the inline metrology station may be positioned in a cassette interface unit.

[0010] In another aspect, a polishing system includes a polishing station including a platen for supporting a polishing pad; a support for holding a substrate; an inline metrology station for measuring the substrate before or after polishing the surface of the substrate in the polishing station; and a controller. The inline metrology station includes a color line scan camera having detector elements arranged along a first axis parallel to the surface of the substrate during scanning of the substrate; a white light source; and a motor for causing relative motion between the frame and the support along a second axis perpendicular to the first axis to cause the light source and the line scan camera to scan across the substrate. The controller is configured to receive color data from the camera, generate a two-dimensional color image from the color data, convert the color image into a hue-saturation-luminance color space, and compare hue and saturation values from the color image in the hue-saturation-luminance color space to corresponding hue and saturation thresholds to generate a thresholded image.

[0011] Implementations may include one or more of the following features. The controller may be configured to not compare brightness values from the color image in the hue-saturation-brightness color space to a threshold value when determining whether a pixel in the thresholded image is on or off. The controller may be configured to analyze the color image to locate wafer alignment features and convert the color image to a standard coordinate system. The controller may be configured to normalize the color image. The controller may be configured to apply a high-pass spatial filter to the image. The controller may be configured to generate a smoothed image by averaging pixel values within a threshold distance and to generate a filtered image by dividing the color image by the smoothed image. Spatial filtering may be performed only along the image axis corresponding to the first axis.

[0012] The controller may be configured to apply an image mask to the color image. The controller may be configured to count the number of pixels in the color image that do not meet a threshold and compare the number of pixels to the threshold. The controller may be configured to count the number of pixels for each die on the substrate in the color image. The controller may be configured to count the number of pixels for the entire substrate in the color image.

[0013] In another aspect, a computer program product includes a non-transitory computer-readable medium encoded with instructions for causing a processor to display a graph in a hue-saturation space of an image of a substrate on a display; receive user input selecting a portion of the graph by clicking and dragging from a first position to a second position in the graph; and generate an upper hue limit, a lower hue limit, an upper saturation limit, and a lower saturation limit based on the first position and the second position.

[0014] Embodiments may include one or more of the following potential advantages. Across-substrate variations in the thickness of a substrate layer can be determined, and unacceptable variations can be detected. This information can be used for feedforward or feedback purposes to control polishing parameters, thereby providing improved thickness uniformity. The algorithm used to determine the variations can be simple and have a low computational load. The user interface used to identify acceptable color thresholds is intuitive and allows the technology to be applied to many types of substrates, such as those with different patterns.

[0015] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A diagram showing an example of an in-line optical measurement system.

[0017] Figure 2 is a flow chart of a method for detecting acceptable changes.

[0018] Figure 3 Example images of a substrate shown in red, green, and blue channels.

[0019] Figure 4 Example image of a substrate displayed in the Hue, Saturation, and Lightness color channels.

[0020] Figure 5 Example graphs showing intensity as a function of pixel location for the unsmoothed image and the filtered image.

[0021] Figure 6 is a schematic top view of the substrate.

[0022] Figure 7 is a schematic diagram of a mask.

[0023] Figure 8 A graphical user interface is displayed that allows the user to select a region in hue-saturation space.

[0024] Like reference numerals in the various drawings indicate like components. DETAILED DESCRIPTION

[0025] The thickness of layers on substrates can be measured optically, before or after polishing, for example, at an inline or standalone metrology station. However, some optical techniques, such as spectrometry, require expensive spectrometers and computationally intensive manipulation of spectral data. Even with the computational overhead, in some cases, the algorithmic results do not meet the increasing accuracy demands of users. However, another metrology technique is to capture a color image of the substrate and analyze the image in color space to identify areas with acceptable thickness.

[0026] refer to Figure 1 The polishing apparatus 100 includes an in-line (also referred to as sequential) optical metrology system 160, such as a color imaging system.

[0027] The polishing apparatus 100 includes one or more carrier heads 126, each configured to carry a substrate 10; one or more polishing stations 106; and a transfer station for loading and unloading substrates onto and from the carrier heads. Each polishing station 106 includes a polishing pad 130 supported on a platen 120. The polishing pad 130 may be a two-layer polishing pad having an outer polishing layer and a softer backing layer.

[0028] The carrier head 126 can be suspended from a support 128 and can be moved between polishing stations. In some embodiments, the support 128 is an overhead track, and the carrier head 126 is coupled to the carriages 108 mounted to the track. The overhead track 128 allows each carriage 108 to be selectively positioned above the polishing station 106 and the transfer station. Alternatively, in some embodiments, the support 128 is a rotatable turntable, and the rotation of the turntable simultaneously moves the carrier head 126 along a circular path.

[0029] Each polishing station 106 of the polishing apparatus 100 may include a port (e.g., at the end of an arm 134) to dispense a polishing liquid 136 (such as an abrasive slurry) onto the polishing pad 130. Each polishing station 106 of the polishing apparatus 100 may also include a pad conditioning device to abrade the polishing pad 130 to maintain the polishing pad 130 in a consistent abrasive state.

[0030] Each carrier head 126 is operable to hold the substrate 10 against the polishing pad 130. Each carrier head 126 can have independent control over polishing parameters (e.g., pressure) associated with each respective substrate. In particular, each carrier head 126 can include a retaining ring 142 to hold the substrate 10 beneath a flexible membrane 144. Each carrier head 126 also includes a plurality of independently controllable pressurizable chambers (e.g., three chambers 146a-146c) defined by the membrane that can apply independently controllable pressures to associated areas on the flexible membrane 144, and thus to the substrate 10. Although for ease of illustration, Figure 1 Only three chambers are shown in the figure, but there may also be one or two chambers, or four or more chambers, such as five chambers.

[0031] Each carrier head 126 is suspended from a support 128 and connected to a carrier head rotation motor 156 via a drive shaft 154, enabling the carrier head to rotate about an axis 127. Optionally, each carrier head 126 may be laterally oscillated (e.g., by driving the carriage 108 on rails 128, or by rotational oscillation of the turntable itself). In operation, the platen rotates about its central axis 121, and each carrier head rotates about its central axis 127 and translates laterally across the top surface of the polishing pad. The lateral sweep is in a direction parallel to the polishing surface 212. The lateral sweep may be a linear or arcuate motion.

[0032] A controller 190 (such as a programmable computer) is connected to each motor to independently control the rotation rate of the platen 120 and the carrier head 126. For example, each motor may include an encoder that measures the angular position or rotation rate of the associated drive shaft. Similarly, the controller 190 is connected to the actuators in each carriage 108 and / or the rotary motor for the turntable to independently control the lateral motion of each carrier head 126. For example, each actuator may include a linear encoder that measures the position of the carriage 108 along the track 128.

[0033] Controller 190 may include a central processing unit (CPU) 192, memory 194, and support circuits 196 (e.g., input / output circuits), a power supply, clock circuits, cache memory, etc. The memory is connected to CPU 192. The memory is a non-transitory computer-readable medium and may be one or more readily available memories such as random access memory (RAM), read-only memory (ROM), floppy disk, hard disk, or other forms of digital storage. In addition, although shown as a single computer, controller 190 may be a distributed system, e.g., including multiple independently operating processors and memories.

[0034] The inline optical metrology system 160 is located within the polishing apparatus 100 but does not perform measurements during polishing operations; rather, measurements are collected between polishing operations, such as when a substrate is moved from one polishing station to another, or from a transfer station to a polishing station, or vice versa.

[0035] The inline optical metrology system 160 includes a sensor assembly 161 supported at a location between two of the polishing stations 106, such as between the two platens 120. In particular, the sensor assembly 161 is located so that the carrier head 126 supported by the support 128 can position the substrate 10 above the sensor assembly 161.

[0036] In an embodiment where the polishing apparatus 100 includes three polishing stations and the substrate is carried sequentially from a first polishing station to a second polishing station to a third polishing station, one or more sensor assemblies 161 may be positioned between the transfer station and the first polishing station, between the first and second polishing stations, between the second and third polishing stations, and / or between the third polishing station and the transfer station.

[0037] The sensor assembly 161 may include a light source 162 , a light detector 164 , and circuitry 166 for sending and receiving signals between the controller 190 and the light source 162 and light detector 164 .

[0038] Light source 162 is operable to emit white light. In one embodiment, the emitted white light comprises light having a wavelength of 200-800 nanometers. Suitable light sources are an array of white light emitting diodes (LEDs), or a xenon lamp or a xenon-mercury lamp. Light source 162 is oriented to direct light 168 onto the exposed surface of substrate 10 at a non-zero angle of incidence α. The angle of incidence α can be, for example, about 30° to 75°, such as 50°.

[0039] The light source can illuminate a substantially linear, elongated area across the width of substrate 10. Light source can 162 can include optical components, such as a beam expander, to diffuse light from the light source into the elongated area. Alternatively or additionally, light source 162 can include a linear array of light sources. Light source 162 itself (and the area illuminated on the substrate) can be elongated and have a longitudinal axis parallel to the substrate surface.

[0040] Diffuser 170 may be placed in the path of light 168 , or light source 162 may include a diffuser, to diffuse the light before it reaches substrate 10 .

[0041] Detector 164 is a color camera sensitive to light from light source 162. The camera includes an array of detector elements. For example, the camera may include a CCD array. In some embodiments, the array is a single row of detector elements. For example, the camera may be a line scan camera. The row of detector elements may extend parallel to the longitudinal axis of the elongated area illuminated by light source 162. In the case where light source 162 includes a row of light-emitting elements, the row of detector elements may extend along a first axis parallel to the longitudinal axis of light source 162. A row of detector elements may include 1024 or more elements.

[0042] The camera 164 is configured with appropriate focusing optics 172 to project a field of view of the substrate onto an array of detector elements 178. The field of view can be long enough to observe the entire width of the substrate 10, e.g., 150 to 300 mm long. The camera 164 (including associated optics 172) can be configured so that individual pixels correspond to regions having a length equal to or less than about 0.5 mm. For example, assuming the field of view is approximately 200 mm long and the detector 164 includes 1024 elements, the image generated by the line scan camera can have pixels having a length of approximately 0.5 mm. To determine the length resolution of the image, the length of the field of view (FOV) can be divided by the number of pixels onto which the FOV is imaged to yield the length resolution.

[0043] The camera 164 can also be configured so that the pixel width is comparable to the pixel length. For example, an advantage of a line scan camera is its very fast frame rate. The frame rate can be at least 5 kHz. The frame rate can be set to a frequency such that when the imaging area is scanned across the substrate 10, the pixel width is comparable to the pixel length (e.g., equal to or less than about 0.3 mm).

[0044] The light source 162 and the light detector 164 can be supported on a platform 180. In the case where the light detector 164 is a line scan camera, the light source 162 and the camera 164 can be moved relative to the substrate 10 so that the imaging area can be scanned across the length of the substrate. In particular, the relative motion can be in a direction parallel to the surface of the substrate 10 and perpendicular to the row of detector elements of the line scan camera 164.

[0045] In some embodiments, the platform 180 is stationary and the carrier head 126 is moved, for example, by movement of the carriage 108 or by rotational oscillation of the disk. In some embodiments, the platform 180 is movable while the carrier head remains stationary for image acquisition. For example, the platform 180 can be movable along a rail 184 via a linear actuator 182. In either case, this allows the light source 162 and camera 164 to remain in a fixed position relative to each other while the area being scanned moves across the substrate 10.

[0046] A potential advantage of moving the line scan camera and light source together across the substrate is that, for example, the relative angle between the light source and the camera remains constant for different locations across the wafer, as compared to conventional 2D cameras. Thus, artifacts caused by changes in viewing angle can be reduced or eliminated. Furthermore, line scan cameras can eliminate perspective distortion, whereas conventional 2D cameras exhibit inherent perspective distortion, which then needs to be corrected through image transformation.

[0047] The sensor assembly 161 may include a mechanism to adjust the vertical distance between the substrate 10 and the light source 162 and the detector 164. For example, the sensor assembly 161 may have an actuator to adjust the vertical position of the platform 180.

[0048] Optionally, a polarizing filter 174 may be positioned in the path of the light, e.g., between the substrate 10 and the detector 164. The polarizing filter 174 may be a circular polarizer (CPL). A typical CPL is a combination of a linear polarizer and a quarter-wave plate. Proper orientation of the polarization axis of the polarizing filter 174 can reduce haze in the image and sharpen or enhance desired visual features.

[0049] Assuming that the outermost layer on the substrate is a semi-transparent layer, such as a dielectric layer, the color of the light detected at detector 164 depends on (for example) the composition of the substrate surface, the smoothness of the substrate surface and / or the amount of interference between light reflected from different interfaces of one or more layers (e.g., dielectric layers) on the substrate.

[0050] As described above, the light source 162 and light detector 164 may be connected to a computing device, such as a controller 190 , that is operable to control their operation and receive their signals.

[0051] refer to Figure 2 , the controller assembles the individual image lines from the light detector 164 into a two-dimensional color image (step 200). The camera 164 may include separate detector elements for each of the red, blue, and green colors. The two-dimensional color image may include monochrome images 204, 206, 208 for each of the red, blue, and green channels (see Figure 3 ).

[0052] The controller may apply an offset and / or gain adjustment to the intensity values of the image in each color channel, step 210. Each color channel may have a different offset and / or gain.

[0053] Optionally, the image can be normalized (step 215). For example, the difference between the measured image and a standard predefined image can be calculated. For example, the controller can store a background image for each of the red, green, and blue channels and can subtract the background image from the measured image for each color channel. Alternatively, the measured image can be divided by the standard predefined image.

[0054] The image may be converted from the red, green, and blue (RGB) color space to the hue, saturation, and lightness (HSL) color space (step 220). This conversion may occur after gain adjustment in the RGB color space. An exemplary conversion from an 8-bit (0 to 255) RGB color space to an HSL color space is as follows:

[0055] RGB to HSL conversion formula

[0056] Divide the R, G, B values by 255 to change the range from 0..255 to 0..1:

[0057] R′=R / 255

[0058] G′=G / 255

[0059] B′=B / 255

[0060] Cmax=max(R′,G′,B′)

[0061] Cmin=min(R′,G′,B′)

[0062] Δ=Cmax-Cmin

[0063] Hue calculation:

[0064]

[0065] Saturation calculation:

[0066]

[0067] Brightness calculation:

[0068] L=(Cmax+Cmin) / 2

[0069] This provides scalar images 224, 226, 228 for each of the hue, saturation, and brightness channels (see Figure 4 ).

[0070] The image may be filtered to remove low-frequency spatial variations (step 230). In some embodiments, only the luminance channel is filtered to remove low-frequency spatial variations, i.e., the hue and saturation channels are not filtered. In some embodiments, the luminance channel is used to generate a filter that is then applied to the red, green, and blue images, and the resulting image is then converted back to the HSL color space.

[0071] In some embodiments of filtering, a smoothing filter is applied to the image in the luminance channel, e.g., averaged over a span of 20 pixels, to produce a smoothed luminance image. In some embodiments, the image in the luminance channel is divided by the smoothed luminance image to produce a filtered image; the hue and saturation channels are not modified as part of the spatial filtering. In some other embodiments, the image in each color channel (i.e., each of the red, green, and blue channels) is divided by the smoothed luminance image. The resulting filtered image with the filtered red, green, and blue images is then converted back to the HSL color space to produce luminance, hue, and saturation images. This can produce an image that removes low frequencies due to illumination variations, but maintains high-frequency spatial variations in the image data. For example, Figure 5 25 is an exemplary graph of intensity as a function of pixel position along a first axis in the luminance channel for both the unsmoothed image shown by line 252 and the filtered image shown by line 254 .

[0072] In some embodiments, smoothing is performed only along the first axis. For example, the brightness values of pixels along the direction of travel 186 can be averaged together to provide an average brightness value as a function of position along the first axis only. Each row of image pixels can then be divided by a corresponding portion of the average brightness value as a function of position along the first axis.

[0073] The controller may analyze the image using image processing techniques to locate wafer orientation features 16 (eg, wafer notches or wafer flats) on the substrate 10 (see Figure 6 ) (Step 240). Image processing techniques can also be used to locate the center 18 of the substrate 10 (see Figure 6 ).

[0074] Based on this data, the image is converted (e.g., scaled and / or rotated and / or translated) to a standard image coordinate frame (step 250). For example, the image can be translated so that the center of the wafer is at the center point of the image, and / or the image can be scaled so that the edge of the substrate is at the edge of the image, and / or the image can be rotated so that there is a 0° angle between the x-axis of the image and a radial segment connecting the wafer center and the wafer orientation feature.

[0075] Optionally, an image mask may be applied to mask out portions of the image data (step 260). Figure 6 , a typical substrate 10 includes a plurality of grains 12. Scribe lines 14 may separate the grains 12. For some applications, it may be useful to process only the image data corresponding to the grains. In this case, reference Figure 7 , an image mask may be stored by the controller in which unmasked areas 22 correspond in spatial location to the die 12, while masked areas 24 correspond to the scribe lines 14. Image data corresponding to masked areas 24 is not processed or used during the thresholding step. Alternatively, masked areas 24 may correspond to the die, such that the unmasked areas correspond to the scribe lines, or the unmasked areas may be only a portion of each die, with the remainder of each die masked.

[0076] The hue and saturation data at this stage can be used in feed-forward or feedback algorithms to control polishing parameters, thereby providing improved thickness uniformity. For example, the hue and saturation of each pixel can be compared to a target hue and saturation to generate an error signal image, and this error signal image can be used for feed-forward control or feedback control.

[0077] Next, the image is subjected to thresholding (step 270). In particular, thresholding can be performed on both the hue and saturation color channels. In some embodiments, thresholding is not performed on the luminance channel.

[0078] For example, the controller 190 may store an upper hue limit UHL and a lower hue limit LHL for the hue channel, and an upper saturation limit USL and a lower saturation limit LSL for the saturation channel. For each pixel, the hue value H of the pixel is compared with the upper hue limit UHL and the lower hue limit LHL, and the saturation value S of the pixel is compared with the upper saturation limit USL and the lower saturation limit LSL. If both the hue value and the saturation value fall within the limits, the pixel is marked as a "pass" threshold, e.g., assigned a value of 1. Conversely, if either the hue value or the saturation value falls outside the limits, the pixel is marked as a "fail" threshold, e.g., assigned a value of 0. In short, the threshold image T( x,y ) can be calculated as

[0079] If LHL≤H( x,y )≤UHL and LSL≤S( x,y )≤USL, then T( x,y )=1, otherwise T( x,y )=0

[0080] In some embodiments, the brightness value of a pixel is not compared to a threshold value to determine the threshold image.

[0081] An advantage of performing thresholding in hue-saturation color space is that spatial variations in hue-saturation color space are typically due to variations in the stack characteristics of the substrate, such as thickness variations in one or more layers in the stack, rather than variations in the light source or other environmental factors. Therefore, thresholding in this space (rather than using luminance or RGB color space) can provide increased reliability in detecting that a substrate meets thickness specifications.

[0082] exist Figure 8 3. A user interface for setting thresholds in hue-saturation space is shown in the user interface 300. The user interface 300 displays a histogram 302 of a test substrate in hue-saturation color space. For example, the system 100 can be used to generate a color image of a test substrate that is determined using a conventional metrology system (e.g., a profilometer) to meet a desired thickness specification. The number of pixels in the image having a given combination of hue and saturation is counted to generate a histogram in the hue-saturation color space. When displayed, each coordinate in the histogram 302 may be displayed with an intensity proportional to the number of pixels counted. As a result, the histogram 302 may have regions of different intensities, e.g., region 306 having a greater intensity than region 304 indicates that more pixels in the image have a particular combination of hue and saturation in region 306 than in the given combination of hue and saturation in region 304.

[0083] The user interface 300 may also display a cursor 310 overlaid on the histogram 302. The position of the cursor 310 may be adjusted using an input device (e.g., a mouse or a touch screen). By clicking and dragging from a first position 312 to a second position 314 (as indicated by arrow 316) in the histogram 302, a selection box 320 may be defined. Thus, the boundaries of the selection box 320 define the upper and lower limits of the threshold value.

[0084] For example, for Figure 8 For example, consider a histogram with hue on the y-axis and saturation on the x-axis. The upper hue limit (UHL) is the larger y value of the first and second positions; the lower hue limit (LHL) is the smaller y value of the first and second positions; the upper saturation limit (USL) is the larger x value of the first and second positions; and the lower saturation limit (LSL) is the smaller x value of the first and second positions. Of course, if hue were displayed on the x-axis and saturation on the y-axis, the x and y values would be reversed.

[0085] Next, a uniformity analysis can be performed on each region of the substrate (e.g., each die) or on the entire image (step 280). For example, for each die, the total number of "failed" pixels within the die can be calculated. This total can be compared to a threshold to determine whether the die is acceptable. For example, if the total is less than the threshold, the die is marked as acceptable. This provides a pass / fail indication for each die.

[0086] As another example, the total number of "failed" pixels within the unmasked area of the substrate can be calculated. This total can be compared to a threshold to determine whether the substrate is acceptable. For example, if the total is less than the threshold, the substrate is marked as acceptable. The threshold can be set by the user. This provides a pass / fail indication for the substrate.

[0087] If a die or wafer is determined to be "failed," the controller 190 may generate an alarm or cause the polishing system 100 to take corrective action. For example, an audible or visual alarm may be generated, or a data file may be generated indicating that a particular die is unusable. As another example, the substrate may be sent back for reprocessing.

[0088] In contrast to spectral processing, where pixels are typically represented by 1024 or more intensity values, in color images, pixels can be represented by only three intensity values (for hue, saturation, and brightness). As a result, the computational load for processing color images is significantly reduced.

[0089] Generally, the data can be used to control one or more operating parameters of the CMP apparatus. These operating parameters include, for example, platen rotation speed, substrate rotation speed, substrate polishing path, substrate speed across the platen, pressure applied to the substrate, slurry composition, slurry flow rate, and temperature at the substrate surface. These operating parameters can be controlled in real time and adjusted automatically without requiring further human intervention.

[0090] As used in this specification, the term substrate may include, for example, a production substrate (e.g., a production substrate including multiple memory or processor dies), a test substrate, a bare substrate, and a gate substrate. A substrate may be at various stages of integrated circuit fabrication; for example, a substrate may be a bare wafer, or it may include one or more deposited and / or patterned layers. The term substrate may include both circular disks and rectangular plates.

[0091] However, the color image processing techniques described above can be particularly useful in the context of 3D vertical NAND (VNAND) flash memory. In particular, the layer stacks used in VNAND manufacturing are so complex that current metrology methods (e.g., Nova spectral analysis) may not be able to perform with sufficient reliability to detect areas of inappropriate thickness. In contrast, color image processing techniques can have excellent reliability in this application.

[0092] The embodiments of the present invention and all functional operations described in this specification can be implemented in digital electronic circuits, or in computer software, firmware or hardware, including the structural means disclosed in this specification and their structural equivalent components, or a combination thereof. The embodiments of the present invention can be implemented as one or more computer program products, that is, one or more computer programs tangibly embodied in a non-transitory machine-readable storage medium for execution by a data processing device (e.g., a programmable processor, a computer, or multiple processors or computers) or for controlling the operation of a data processing device.

[0093] The term relatively positioned is used to refer to the positioning of the components of the system relative to each other, not necessarily relative to gravity; it is understood that the polishing surface and substrate can be maintained in a vertical orientation or some other orientation.

[0094] A number of embodiments have been described. However, it will be appreciated that various modifications may be made. For example

[0095] A camera that images the entire substrate can be used instead of a line scan camera. In this case, no movement of the camera relative to the substrate is required.

[0096] The camera may cover less than the entire width of the substrate. In this case, the camera will need to undergo motion in two perpendicular directions, such as being supported on an XY stage, to scan the entire substrate.

[0097] The light source can illuminate the entire substrate. In this case, the light source does not need to be moved relative to the substrate.

[0098] • The light detector can be a spectrometer instead of a color camera; the spectral data can then be reduced to the HSL color space.

[0099] The sensor assembly does not need to be located in an inline system between polishing stations or between a polishing station and a transfer station. For example, the sensor assembly can be located within a transfer station, located in a cassette interface unit, or be a standalone system.

[0100] • The uniformity analysis step is optional. For example, the image generated by applying the threshold transformation can be fed into a feed-forward process to adjust subsequent processing steps for the substrate, or fed into a feedback process to adjust processing steps for subsequent substrates.

[0101] Accordingly, other implementations are within the scope of the following claims.

Claims

1. A computer program product comprising a non-transitory computer-readable medium encoded with instructions for causing one or more processors to: storing data indicating a boundary of a region in a two-dimensional color space having a pair of color channels as axes of the color space, the pair of color channels including a first color channel and a second color channel; receiving color data of the substrate from the camera; generating a color image of the substrate from the color data; performing, for each pixel of a plurality of pixels of the color image, a comparison of a pair of color values of the pair of color channels of the pixel with the boundary of the region in the two-dimensional color space to determine whether the pair of color values satisfies a threshold provided by the boundary; counting the number of pixels in the color image that do not meet the threshold, and comparing the number of pixels to a second threshold; as well as A pass / fail indication for the substrate is generated based on a result of the comparison of the plurality of pixels.

2. The computer program product of claim 1, comprising instructions to store an image mask identifying one or more regions on the substrate, wherein the plurality of pixels are pixels within the regions identified by the image mask.

3. The computer program product of claim 2, comprising instructions to count a summed number of pixels in all regions of the one or more regions identified in the image mask and to compare the summed number to the second threshold.

4. The computer program product of claim 2, wherein the one or more regions correspond to one or more dies on the substrate.

5. The computer program product of claim 4, comprising instructions to count a summed number of pixels in each of the one or more regions identified in the image mask and to compare the summed number for each region to the second threshold.

6. The computer program product of claim 5, comprising instructions to store a data file indicating that a die is unusable if the summed number of pixels of an area corresponding to the die exceeds the second threshold.

7. The computer program product of claim 1, wherein the pair of color channels does not include a luminance channel.

8. A computer program product comprising a non-transitory computer-readable medium encoded with instructions to cause a processor to: displaying a graph of a two-dimensional color space on a display, the two-dimensional color space having a pair of color channels as axes of the color space, the pair of color channels including a first color channel and a second color channel, the graph providing a histogram of an image of the substrate for the two-dimensional color space; receiving a user input, the user input selecting a portion of the graphic, the user input comprising clicking and dragging from a first location to a second location within the graphic; generating data indicating a boundary of a region in the two-dimensional color space based on the user input, the generating utilizing pixels of the image within the region having a pair of color values indicating that a corresponding location on the substrate meets a thickness requirement; receiving color data of the second substrate from the camera; generating a color image of the second substrate from the color data; performing, for each pixel of a plurality of pixels of the color image, a comparison of a pair of color values of the pair of color channels of the pixel with the boundary of the region in the two-dimensional color space to determine whether the pair of color values satisfies a threshold provided by the boundary; counting the number of pixels in the color image that do not meet the threshold, and comparing the number of pixels to a second threshold; as well as A pass / fail indication for the second substrate is generated based on a result of the comparison of the plurality of pixels.

9. The computer program product of claim 8, wherein the pair of color channels does not include a luminance channel.

10. A computer program product comprising a non-transitory computer-readable medium encoded with instructions to cause one or more processors to: receiving color data of a substrate from a line scan camera that scans across the substrate along a first axis; generating a color image of the substrate from the color data; averaging pixel values of the color image only along an image axis corresponding to the first axis to generate a smoothed color image; generating a signal to an operator based on the smoothed color image; storing data indicating a boundary of a region in a two-dimensional color space having a pair of color channels as axes of the color space, the pair of color channels including a first color channel and a second color channel; performing, for each pixel of a plurality of pixels of the smoothed color image, a comparison of a pair of color values of the pair of color channels of the pixel with the boundary of the region in the two-dimensional color space to determine whether the pair of color values satisfies a threshold provided by the boundary; counting the number of pixels in the color image that do not meet the threshold, and comparing the number of pixels to a second threshold; as well as A pass / fail indication for the substrate is generated based on a result of the comparison of the plurality of pixels.

11. The computer program product of claim 10, wherein the instructions for averaging pixel values comprise instructions for averaging pixel values within a threshold distance.

12. The computer program product of claim 11, comprising instructions to generate a filtered image by dividing the color image by the smoothed image.

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