Device and method for plasma etching a substrate

Abstract

This application relates to an apparatus and a method. In at least some of its embodiments, this application provides an apparatus adapted to determine an etch endpoint by providing a strong endpoint signal even when the fraction of the open area of the mask is low (e.g., less than about 1%). In at least some embodiments, this application further provides an endpoint signal representative of a wide area across the substrate. In at least some embodiments, this application can further provide a strong endpoint signal in the case of features exposed at the bottom of high aspect ratio features or trenches.

Description

Technical Field

[0001] The present invention relates to an apparatus for plasma etching a substrate, and more particularly to etching features. The present invention also relates to an apparatus for determining an endpoint of a plasma etching process. The present invention also relates to an associated method for plasma etching a substrate; and to an associated method for determining an endpoint of a plasma etching process. Background Art

[0002] Plasma etching can be used to etch features in a wafer such as a silicon wafer. A mask resistant to the etching conditions can be provided on the wafer. The surface area of the wafer not covered by the mask (also referred to as the "open area") is exposed to the etching conditions and can be selectively etched.

[0003] An endpoint signal can be used to determine the termination of a plasma etching process step. The endpoint signal can be, for example, a measured change in an optical property of the wafer or a change in an optical and / or chemical property of the plasma. Known methods for determining an endpoint include laser-based reflectometry, interferometry, and optical emission spectroscopy (OES). However, these techniques are not always well-suited to provide a strong endpoint signal, particularly when the percentage of the open area in the mask is low (e.g., less than about 1%).

[0004] Reflectometry and interferometry techniques rely on changes in the optical properties of the wafer. Changes in the optical properties of the wafer can be due to reaching an etch stop layer. Laser-based techniques typically provide measurements only over a localized area determined by the spot size of the laser. Thus, the measurement represents only a small area (usually covering about one or a few features) and may not truly represent the entire wafer. To obtain a signal, the laser must be accurately positioned and aligned in the area where the feature is to be measured, which can be challenging, particularly when the percentage of the open area in the mask is low or when the size of the etched feature is less than or similar to the spot size of the laser. If the laser is not properly aligned, the feature may not be observed and the endpoint signal may not be detected. Some methods utilize moving components to enable the entire surface of the wafer to be scanned by the laser. However, these systems increase the cost and complexity of the manufacture and operation of the apparatus.

[0005] OES depends on changes in the optical properties of the plasma indicating an endpoint. However, the OES sensitivity decreases as the percentage of the "open area" in the mask decreases because the amount of material etched from the wafer is correspondingly low. Thus, when the percentage of the "open area" of the mask is low, OES may only provide a weak endpoint signal.

[0006] The present invention, in at least some of its embodiments, seeks to address some of the above problems, desires, and needs. The present invention, in at least some of its embodiments, provides an apparatus adapted to determine an etch endpoint by providing a strong endpoint signal even when the fraction of the open area of the mask is low (e.g., less than about 1%). In at least some embodiments, the present invention further provides an endpoint signal representative of a wide area across the substrate. In at least some embodiments, the present invention can further provide a strong endpoint signal in the case of features exposed at the bottom of high aspect ratio features or trenches. SUMMARY OF THE INVENTION

[0007] According to a first aspect of the present invention, there is provided an apparatus for plasma etching a substrate, the apparatus comprising:

[0008] a chamber;

[0009] a substrate support disposed within the chamber to support the substrate to be plasma etched;

[0010] a white light illumination source for illuminating a region of the substrate during a plasma etching process, wherein the illumination source is mounted to illuminate the region of the substrate with an incident beam having an incident angle of less than about 10° relative to the normal;

[0011] a camera arranged to capture successive images of the region illuminated by the illumination source; and

[0012] a processor configured to perform image processing techniques on the images to identify the location of at least one feature on the substrate and measure a reflectivity signal from the feature at the location;

[0013] wherein the processor is configured to modify the plasma etching process in response to the measured reflectivity signal at the location.

[0014] The processor may be configured to modify the plasma etching process in response to a change in the measured reflectivity signal at the location.

[0015] The illumination source may be mounted to illuminate the region of the substrate with an incident beam having an incident angle of less than about 5° relative to the normal. Optionally, the incident angle of the incident beam relative to the normal is less than about 2°, optionally less than about 1°, or optionally about 0°. A low incident angle allows the bottom of high aspect ratio features to reflect light to the camera (i.e., be imaged by the camera). Thus, due to the collimation of light in the features, it is beneficial to position the camera and the light source at approximately 90° to the wafer surface. This helps to avoid the exposed features being hidden out of the field of view and thus can increase the intensity of the reflectivity signal.

[0016] As is known in the art, the white light illumination source provides light across a wide optical bandwidth (typically about 100 nm or greater). The white light source can be used to illuminate a wide area of the substrate to be etched. This enables the detection of reflectivity signals across a wide area of the substrate.

[0017] The apparatus may further include an optical arrangement that is arranged to focus the light emitted by the illumination source onto the substrate. The optical arrangement may also focus light into the camera. The optical arrangement may include one or more lenses and / or optical filters.

[0018] The apparatus may further include an optical filter that is positioned to filter light of selected wavelengths entering the camera. The optical filter can be used to prevent light of certain wavelengths (e.g., certain wavelengths emitted by the plasma) from being detected by the camera. The optical filter can be a band-pass filter.

[0019] The camera can be fixed in place such that the field of view of the camera can image a fixed area on the substrate support. In use, the processed substrate and the camera can be fastened relative to each other in a fixed position. Fixing the position of the camera will allow corresponding pixels on each successive image to correspond to the same position on the substrate. Thus, it is possible to monitor the progress of the etching process by analyzing these pixels (rather than analyzing data across an integrated area of the substrate). Thus, even if the proportion of the open area in the mask is low, a strong endpoint signal can still be achieved due to the measurement. The camera can have a complementary metal oxide semiconductor (CMOS) detector.

[0020] According to a second aspect of the present invention, there is provided a method of plasma etching a substrate, the method comprising the steps of:

[0021] (a) using a white light illumination source to illuminate a region of the substrate to be plasma-etched with an incident beam having an incident angle of less than 10° with respect to the normal;

[0022] (b) using a camera to take successive images of the illuminated region during the plasma etching process;

[0023] (c) applying image processing techniques to the images in order to identify the position of at least one feature on the substrate and measure the reflectivity signal at that position; and

[0024] (d) modifying the plasma etching process in response to the measured reflectivity signal at that position.

[0025] The images taken by the camera have a resolution that can resolve discrete features in the illuminated region. That is, the pixel size is typically smaller than the size of the features imaged by the camera.

[0026] Step (c) may include applying an image processing technique to the image to identify one or more pixels in the image, wherein the one or more pixels correspond to the location of a feature on the substrate. The reflectivity signal of the one or more pixels may be measured. The reflectivity signals of these pixels are generally associated with the reflectivity signal from the location of the feature. These reflectivity signals may be analyzed to measure the value or change of the reflectivity signal during the plasma etching process. The plasma etching process may be modified in response to the amplitude of the reflectivity signal of one or more pixels corresponding to the location of the at least one feature being higher or lower than a predetermined value. The plasma etching process may be modified in response to a change in the reflectivity signal of one or more pixels corresponding to the location of the at least one feature.

[0027] The plasma etching process may be modified in response to the amplitude of the reflectivity signal at the location being higher or lower than a predetermined value.

[0028] Step (d) may include modifying the plasma etching process in response to a change in the measured reflectivity signal at the location.

[0029] Analyzing a specific location on the substrate rather than using an integrated signal across the illuminated area enables the detection of localized changes in reflectivity. This has a particular application when the open area of the mask layer is less than about 1% of the substrate surface to be etched.

[0030] The illuminated area may have an area in the range of 0.75 mm 2 to 100 mm 2 , optionally 1 mm 2 to 50 mm 2 , optionally 2 mm 2 to 25 mm 2 or optionally 3 mm 2 to 5 mm 2 The illuminated area may be large enough to illuminate the location of at least one feature while allowing for tolerances in substrate positioning and alignment. The illumination source may illuminate an area of the substrate that includes multiple features.

[0031] The illuminated area may be illuminated with an incident beam having an incident angle of less than 5°, optionally less than 2° or optionally less than 1°. The incident beam may be substantially perpendicular to the substrate. A low incident angle allows the bottom of high aspect ratio features to reflect light into the camera (i.e., to be imaged by the camera). Thus, due to the collimation of light in the features, it is beneficial to position the camera and the light source at approximately 90° to the wafer surface. This helps to avoid the exposed features being hidden out of the field of view and thus may increase the intensity of the reflectivity signal.

[0032] The camera can detect light reflected from the substrate at the detection wavelength. The light emitted by the illumination source can have a higher intensity at the detection wavelength compared to the light emitted by the plasma of the plasma etching process at the detection wavelength. The user can select the wavelength to which the camera is sensitive. Preferably, a detection wavelength different from the peak wavelength of the light emitted by the plasma is used so that the signal from the plasma does not saturate the captured image.

[0033] The image processing technique can include image pattern recognition and / or image pattern matching. Image pattern matching can be used to cancel vibrations in the plasma etching equipment and any relative movement between the camera and the substrate. Image pattern recognition can define and match the open areas of the mask layer before etching. Image pattern recognition can include image correlation from an existing (e.g., reference) image. Image pattern recognition can include looking for reference marks on the image, such as the edge of a die adjacent to a cut or device design, and calculating the goodness of fit. Other image pattern recognition methods can be envisioned, such as using artificial intelligence and machine learning.

[0034] Before the plasma etching process, the substrate can include a front surface that includes at least two regions having different reflectivity properties.

[0035] The substrate can include a mask layer that partially covers the material to be etched. The mask layer can have different reflectivity properties from the material to be etched. The mask layer can cover at least 90%, optionally at least 95%, or optionally at least about 99% of the material to be etched. That is, the proportion of the open area on the surface of the substrate can be about 10% or less, 5% or less, 2% or less, or about 1% or less. Even when the proportion of the open area on the substrate is very low, the method can enable a strong endpoint signal to be provided.

[0036] Step (c) can include applying an image processing technique to a reference image (e.g., a first image) taken by the camera to identify the location of the at least one feature. The image processing technique can include, for example, measuring a reference reflectivity signal at the location of the at least one feature in the reference image. The image processing technique can include comparing the reference reflectivity signal with the reflectivity signal measured at the location of the at least one feature on a successive image. Step (c) can include measuring the change in the reflectivity signal at the location of the at least one feature by comparison with the reference image. For example, the reflectivity signal from the location in the reference image can be compared with the reflectivity signal from a location in a subsequent image. As another example, the reflectivity signal from the location of a feature in one (successive) image can be compared with a reference value, such as the reflectivity signal from a location not associated with the feature (e.g., a portion of the mask layer).

[0037] Identifying the location of the at least one feature may include locating a reference pattern. The location of the at least one feature may be identified relative to the location of the reference pattern.

[0038] The reflectivity signal may be the luminance signal, intensity, and / or color of the reflected light. The change in the reflectivity signal may be a change in the luminance signal, intensity change, and / or color change. For example, the change in the reflectivity signal may be a change in the light intensity at the detection wavelength. Step (d) may include terminating the plasma etching process in response to the change in the reflectivity signal at the location. The change in the reflectivity signal at the location may be related to an etch stop signal.

[0039] The at least one feature (e.g., an etched via) typically has different reflectivity properties compared to another region of the substrate that is not associated with the feature (e.g., the mask layer). For example, in the case where the feature is an open area of the mask layer, the open area may have different reflectivity properties compared to the mask layer. This allows the location of the feature to be identified using only reflectivity. The at least one feature may be a via. The at least one feature may be a trench, such as a trench opening in a mask layer. The feature may be a copper plug embedded in a substrate such as a silicon substrate.

[0040] The substrate may include an etch stop layer, such as a GaAs layer. The etch stop layer typically has different reflectivity properties compared to the etched material (e.g., the feature) and / or another region of the substrate that is not associated with the feature (e.g., the mask layer). The change in the reflectivity signal at the location may be the result of approaching or exposing a portion of the etch stop layer. This change may indicate an etch stop signal.

[0041] The substrate may be a wafer, a wafer on a carrier structure, or a wafer attached to a frame by a tape. The substrate may be a semiconductor wafer, such as a silicon or silicon carbide wafer. That is, the material to be etched may be a semiconductor material, such as silicon or silicon carbide.

[0042] Although the present invention has been described above, the present invention extends to any inventive combination of the features set forth in the above or following description, drawings, or claims. For example, any feature disclosed with respect to a first aspect of the present invention may be combined with any feature of a third aspect of the present invention. Description of the Drawings

[0043] Embodiments of a substrate and method according to the present invention will now be described with reference to the drawings, in which:

[0044] Figure 1 is a schematic diagram of an apparatus according to an embodiment of the present invention;

[0045] Figure 2A is a cross-sectional view of a feature in a substrate before plasma etching;

[0046] Figure 2B is a cross-sectional view of a feature in a substrate after plasma etching;

[0047] Figure 3 is a plot showing an image of a region of the substrate superimposed with a laser reflectometry trace for comparison;

[0048] Figure 4A is a cross-sectional view of a buried feature in a substrate before plasma etching;

[0049] Figure 4B is a cross-sectional view of an exposed feature in a substrate after plasma etching; and

[0050] Figures 5 to 8 is a flowchart of a process according to an embodiment of the present invention. Detailed Description

[0051] Figure 1 Shows a schematic view of a plasma etching apparatus generally designated 10. For simplicity, some well-known features of the plasma etching apparatus 10, such as plasma generating components and gas inlets / outlets, have been omitted from the figure. The operation of generating plasma in such a plasma etching chamber is well known in the art and will not be described herein except where necessary to understand the present invention.

[0052] The apparatus 10 includes a substrate support 12 disposed within a chamber 14. The chamber 14 is adapted to contain a plasma 32. The substrate support 12 is adapted to hold the substrate 16 in a substantially horizontal orientation with the front side of the substrate 16 to be etched facing upward. The substrate 16 can be, for example, a wafer, a wafer on a carrier structure, or a wafer attached to a frame by a tape. The substrate 16 can, for example, include a plurality of die separated by scribe lines. A portion of the front side of the substrate 16 can be patterned with a mask layer resistant to plasma processing conditions. The portion of the substrate covered by the mask is not etched. On the other hand, the portion of the substrate 16 that remains exposed to the plasma processing conditions (also referred to as the "open area") will be selectively etched during the plasma etching process.

[0053] The chamber 14 includes a top wall 18 (or lid) that is above and faces the substrate support. A window (or viewing port) 20 is disposed in the top wall 18 above the substrate support 12 such that at least a portion of the substrate support 12 can be viewed in plan view through the window 20.

[0054] A white light illumination source 22 is arranged to illuminate an area of the substrate 16 with an incident beam 24 through the window 20. The white light illumination source 22 provides light across a broad optical bandwidth, typically about 100 nm or greater, as is known in the art. The area of the substrate 16 illuminated by the illumination source 22 is determined by the spot size of the incident beam 24. The spot size may have a diameter of about 2 mm or about 5 mm. The spot size is larger than the size of the individual features etched on the substrate. The spot size is large enough to illuminate an area of the substrate 16 that includes a plurality of features. The features typically have different reflectivity properties compared to another area of the substrate that is not associated with the features to be etched. By way of example only, the features may be openings in a mask that are used to form trenches or vias in the etched material. The mask and the openings have different reflectivity properties, which results in the openings exhibiting different reflectivity signals compared to the mask. Thus, the open areas are visible prior to the plasma etching process. Buried features, such as copper plugs, are embedded in the substrate and may not be visible at the start of the etching process. However, these features will become visible as the silicon is etched back to expose the buried features, such as copper plugs.

[0055] When etching the open areas, the open areas will typically be etched to stop at an etch stop layer. Alternatively, the etching may be timed, however, this may result in less uniform etch depth control. Exposing the etch stop layer at the bottom of the features may be an indication that the etching step is complete, e.g., a trench or via feature is fully formed. For example, the etch stop layer may have different reflectivity properties compared to positions on the etched material and / or substrate that are not associated with the features, such as a mask layer or bulk substrate material. In another example, a reference pattern may be used to identify a known location on the substrate, and the location of the features may be determined relative to the location of the reference pattern. The features typically have dimensions less than about 100 μm.

[0056] An optical device 26 is used to focus the incident beam and control the spot size. The optical device 26 may include one or more lenses for focusing the beam. The optical device 26 may further include an optical filter that only allows certain wavelengths of light to pass through.

[0057] The incident beam 24 is reflected from the front surface of the substrate 16 to provide a reflected beam 28. The incident beam is directed to illuminate the bottom of an etched feature, such as a trench, such that the reflected beam 28 includes a reflectivity signal from the location of the feature. Preferably, the incident beam is substantially perpendicular (i.e., about 90°) to the substrate surface. This helps to resolve and view high aspect ratio features, such as high aspect ratio trenches, because a higher proportion of the incident light can be reflected from the bottom of the high aspect ratio features. However, a larger angle of incidence may be used for lower aspect ratio features.

[0058] The camera 30 is positioned to detect the reflected beam 28. The camera 30 is mounted in a fixed position relative to the substrate support 12. The reflected beam 28 can pass through the optical device 26 and / or through a filter (not shown) before entering the lens of the camera 30. The filter can determine which wavelengths of light are detected by the camera 30. The camera 30 can be a CMOS camera. However, other cameras, such as CCD cameras, can also be used.

[0059] The camera 30 has a field of view that can substantially correspond to the illuminated area on the substrate. Optionally, the field of view can be smaller or larger than the illuminated area of the substrate. The camera 30 is operable to capture the reflected light from the illuminated area and take successive images of the illuminated area of the substrate during the plasma etching process. The illumination source illuminates a relatively large area of the substrate and is large enough to encompass a plurality of locations where features are present. The illuminated area is also relatively large compared to the size of the etched features. Generally, the image size (and thus, the illuminated area) is large enough to encompass at least one feature to be etched while allowing tolerances for substrate position and alignment. The camera 30 can detect the reflected light 28 from the etched features.

[0060] The camera 30 is focused such that each pixel in the captured image corresponds to a dimension smaller than the dimension of the etched feature. Individual features can be resolved in the image captured by the camera. For example, the etched feature can have a dimension of about 25 μm, while each pixel in the first image can correspond to approximately 10 μm × 10 μm. Thus, the position of the etched feature can span one or more pixels in the captured image. Each pixel contains information about the reflectivity signal at a particular location. For example, the pixel can contain information about the intensity and / or color of the reflected beam at a particular location on the substrate.

[0061] The images taken by the camera 30 are transmitted to a processor for image processing. Image processing techniques such as image pattern matching and image pattern recognition are applied to the images. Commercially available software such as Cognex VisionPro(RTM) sold by Cognex Corporation (Natick, Massachusetts, USA) can be used to apply the image processing techniques. The pixels corresponding to the positions of the features in the image are identified. The reflectivity signals of these pixels can be analyzed. If the change in the reflectivity signal at the position of the feature exceeds a threshold, then the processor can send a signal to modify the plasma etching process. For example, the plasma etching process can be stopped, or the next stage of the plasma process can be initiated.

[0062] Each successive image captured by the camera is focused on the same area of the substrate. Thus, corresponding pixels in the successive images contain reflectivity information about the same location on the substrate. Therefore, once the location of a feature has been determined, there is no need to use reflectivity signals from other locations (e.g., using other pixels) to analyze the progress of the etching process. Instead, the changes occurring in the pixels corresponding to the location of the feature can be analyzed to indicate the progress of the etching process. Thus, when a feature is exposed (e.g., the etch stop layer is revealed), a strong endpoint signal can be determined, regardless of the proportion of the open area in the mask layer.

[0063] Example 1

[0064] Merely by way of example, Figure 2A A cross-sectional schematic view of an exemplary substrate 34 before plasma etching is shown. Substrate 34 includes a mask layer 36 that partially covers a bulk silicon carbide (SiC) layer 38. SiC layer 38 has a thickness of approximately 100 μm. The mask layer is made of copper having a thickness of about 5 μm. The proportion of the open area 40 in the mask layer is about 0.2%. Substrate 34 further includes a gallium arsenide (GaAs) layer 42 formed on a gold layer 44, and gold layer 44 serves as an etch stop layer. At certain wavelengths of light, the reflectivity properties of the SiC layer are different from those of the GaAs stop layer. At certain wavelengths, the reflectivity properties of the SiC layer are different from those of the mask layer. At certain wavelengths, the reflectivity properties of the GaAs stop layer are different from those of the mask layer.

[0065] A white light illumination source illuminates the area of the substrate. A CMOS camera is used to capture an image of the illuminated area. The image covers an area on the substrate having a size of approximately 2 mm × 2 mm. Each pixel corresponds to an area on the substrate having a size of approximately 10 μm × 10 μm. A green filter having a 10 nm bandwidth is used to filter the light entering the camera.

[0066] The etching endpoint is determined using the apparatus and method according to the above embodiments of the present invention. Figure 2B Is a cross-sectional schematic view of substrate 34 at the etching endpoint. The GaAs etch stop layer 42 is exposed, and the via hole 46 is completely formed.

[0067] As a comparative example, the etching endpoint is also determined using a known laser reflectometry and interferometry system having a 25 μm laser spot size. The laser beam has a size similar to that of the etched feature. Aligning a laser beam having a size similar to that of the etched feature can be challenging because a small movement in the substrate can cause misalignment of the laser, which can result in poor measurements or no measurements at all. This would require realignment of the substrate or the laser, which is time-consuming.

[0068] Figure 3Shows the laser reflection method traces of comparative examples, i.e., the intensity of the reflected (laser) signal varies with time during the etching process. Figure 3 Also includes four camera images obtained using embodiments of the present invention during plasma etching. Figure 3 Includes four representative images taken by the camera. Although not shown, a much larger number of images were taken by the camera during the etching process. The frequency and number of images taken by the camera during the etching process can be selected depending on the available processing power.

[0069] The plasma etching process is carried out in three steps. The first step is the slag removal step 48. The slag removal step prepares the open area 40 of the mask for subsequent etching processes, such as by removing unwanted materials from the open area 40 before bulk etching. Image 148 is a camera image captured 300 s after the start of the etching process. Since the exposed SiC layer 36 has different reflectivity properties from the mask layer 34, the location of the feature to be etched is visible and can be identified as an array of lighter color pixels 149 in this case.

[0070] The second step is the main bulk etching step 50. This involves selectively etching the main part of the SiC layer at the location of the open area 40. The bulk etching can use any process conditions known in the art. Image 152 is a camera image captured 1800 s from the start of the etching process and was taken during the main etching step 50. The reflectivity signal at the location of feature 149 deteriorates, and the open area in the mask is no longer visible. Without wishing to be bound by any theory or speculation, it is believed that during the bulk etching step, the surface of the substrate becomes rough or curved, causing the reflectivity signal from the feature to deteriorate temporarily.

[0071] The third step is the soft landing step 52, where the etching rate of the landing step is less than the etching rate of the bulk etching step 50. The soft landing step 52 exhibits improved etching selectivity between the SiC layer 38 and the GaAs etch stop layer 42. This helps to ensure the formation of each etched feature (e.g., via hole) across the entire substrate 34 is completed (i.e., there is no residual SiC material in the via hole), and thus helps to improve the etching uniformity across the entire substrate 34. Image 152 is a camera image captured approximately 4500 s after the start of the etching process and was taken during the soft landing step 52. As the SiC layer thins, the reflectivity properties of this layer change. Therefore, as the etching front approaches the etch stop layer 42, there is a change in the reflectivity signal detected by the camera at discrete locations where the features reside, and the discrete locations of the features become observable again.

[0072] A change in the reflectivity signal (e.g., a change in brightness, intensity, and / or color) at the location of the feature exceeding a threshold indicates that the etch stop layer (or via) is exposed. That is, it indicates that the plasma etching process is complete. Image 154 is a camera image captured approximately 4800 s after the start of the etching process, where the buried feature 155 is visible and the etching process is complete.

[0073] Embodiments of the present invention provide measurements that closely match comparative reflectometry traces. However, this embodiment has several advantages. First, it is simpler to align and focus the incident beam to ensure illumination of the feature to be etched, especially while allowing for the tolerances of the wafer positioning equipment. This can help avoid time-consuming substrate realignment steps. Second, since reflectivity signals are obtained and analyzed from discrete features (rather than integrating the signal over the entire illuminated area), it is possible to detect strong endpoint signals regardless of the proportion of the open area in the mask. For example, embodiments of the present invention have particular applications in detecting endpoint signals in substrates with low open areas (e.g., less than about 1%). Further, due to the wide field of view of the camera, the processed data provides a better representation of the progress of plasma etching over a wider area of the substrate.

[0074] Example 2

[0075] Figure 4A A cross-sectional schematic view of an exemplary silicon substrate 56 before plasma etching is shown. The substrate 56 does not include a mask layer. However, the substrate 56 includes two buried features 58 in trenches 60. The buried features 58 are copper-silicon through-silicon via (TSV) plugs typically encapsulated in a thin dielectric layer such as SiO2.

[0076] A region of the substrate is illuminated by an illumination source, and the camera takes consecutive images of the illuminated area during the plasma etching process. The illumination source is positioned to direct the incident beam substantially perpendicular to the surface of the substrate 54, as shown by the Figure 4A arrow in. A very low angle of incidence (i.e., relative to the normal) is preferred to ensure that the incident beam can be reflected substantially from the entire area of the bottom surface of the trench. If the angle of incidence is higher, some portions of the bottom of the trench may be hidden from view and thus not provide any reflectivity information to the camera.

[0077] At the start of the etching process, the buried feature is not visible, and its location within the substrate 56 may not be determinable. The camera takes consecutive images of the illuminated area during the plasma etching process. Processing techniques scan the images to look for changes in the reflectivity signal across the wafer. If the change in the reflectivity signal at a certain location exceeds a threshold, the processor can initiate a change to the etching process in response. For example, the etching process can be stopped in response to a change in the reflectivity signal (e.g., a change in brightness, intensity, and / or color) at a specific location.

[0078] Example 3

[0079] Figure 5 is a flow chart of a substrate verification method. The substrate to be etched is positioned on a substrate support and clamped in place. The substrate to be etched includes a visible reference pattern, such as a pattern of open areas in a mask layer. The area of the substrate including the reference pattern is illuminated by an illumination source, and a camera captures an image of the illuminated area. A processor uses image pattern recognition and / or image pattern matching techniques to identify the reference pattern. If the reference pattern is present, then the correct substrate has been verified and the next stage of substrate processing can proceed. If the reference pattern is absent, then an incorrect substrate has been loaded into the apparatus and the process can be aborted.

[0080] Example 4

[0081] Figure 6 is a flow chart of a substrate verification method. This method can also be used to determine an etch endpoint. The substrate to be etched is positioned and clamped in the same manner as in Example 3. The substrate to be etched includes a buried reference pattern, such as a buried feature, such that the pattern appears as etching proceeds. The area of the substrate is illuminated by an illumination source, and a camera captures successive images of the illuminated area. A plasma is generated in the chamber, and a plasma etching process is started. As the reference pattern is revealed during the process, the processor uses image pattern recognition or image pattern matching to identify the reference pattern (such as the buried feature). When the change in the reflectivity signal at the location of the reference pattern exceeds a threshold, the processor modifies the plasma etching process. This change indicates an endpoint signal. For example, in response to the change in the reflectivity signal at the location of the reference pattern, the processor can initiate an over-etch process for an absolute over-etch time or a percentage of the time required to reach the endpoint signal. Then, the etching process can be stopped or proceed to the next step in the etching process.

[0082] Example 5

[0083] Figure 7 is a flow chart showing a method for determining an endpoint. The substrate to be etched (including a mask layer) is positioned and clamped in the same manner as in Example 3. The substrate to be etched includes a visible reference pattern, such as the edge of a die adjacent to a scribe lane. The visible reference pattern can be a pattern of open areas in a mask layer. The area of the substrate is illuminated by an illumination source, and a camera captures successive images of the illuminated area. The processor uses image pattern recognition or image pattern matching techniques to identify the reference pattern. If the reference pattern is absent, then the processor will detect that an incorrect substrate has been loaded into the apparatus and the process can be aborted.

[0084] The reference pattern is located in a known position on the substrate. The positions of the features to be etched can be identified relative to the position of the reference pattern. When identifying the positions of the features to be etched, these positions are analyzed to determine when the reflectivity signal at these discrete positions changes to be above a threshold. The change in the reflectivity signal at the positions of the features can be measured relative to a first reference image. When the reflectivity signal changes by more than the threshold, an endpoint signal is detected.

[0085] The processor can modify the plasma etching process in response to a change in the reflectivity signal detected at the positions of the features. For example, the processor can initiate an over-etch process in the same manner as described for Example 4 before terminating or changing the etching process. Alternatively, the etching process can be terminated immediately or proceed to the next stage of the process.

[0086] Example 6

[0087] Figure 8 is a flowchart showing a method for determining an endpoint. The substrate to be etched is positioned and clamped in the same manner as in Example 3. The substrate with the mask is subjected to plasma etching until the etch stop layer is reached. During the plasma etching process, the area of the substrate is illuminated by an illumination source, and the camera captures consecutive images of the illuminated area.

[0088] As the reference pattern is revealed during the plasma etching process, the processor uses image pattern recognition or image pattern matching to identify the position of the reference pattern. The positions of the features to be etched can be identified relative to the position of the reference pattern. When identifying the positions of the features to be etched, these positions are analyzed to determine when the reflectivity signal at these positions changes to indicate an endpoint signal. In this example, the reflectivity signal is the luminance level.

[0089] The processor can modify the plasma etching process in the same manner as in Example 5 in response to a change in the reflectivity signal detected at the discrete positions of the features.

Claims

1. An apparatus for plasma etching a substrate, the apparatus comprising: A chamber; A substrate support disposed within the chamber, the substrate support having a support surface for supporting the substrate to be plasma etched; A white light illumination source for illuminating a region of the substrate during a plasma etching process, wherein the illumination source is mounted to illuminate the region of the substrate with an incident light beam having an incident angle of less than 10° with respect to the normal of the support surface of the substrate support; A camera arranged to capture successive images of the region illuminated by the illumination source, wherein the white light illumination source and the camera are disposed in the chamber opposite the support surface so as to face the support surface, and wherein all of the light received by the camera is illuminated on the region of the substrate at the incident angle; And A processor configured to perform image processing techniques on the images to identify one or more pixels in the images corresponding to the location of at least one feature on the substrate and to measure a reflectance signal from the one or more pixels; Wherein the processor is configured to modify the plasma etching process in response to the measured reflectance signal at the location, and wherein the one or more pixels have an area smaller than the size of the feature on the substrate.

2. The apparatus according to claim 1, wherein the processor is configured to modify the plasma etching process in response to a change in the measured reflectance signal at the location.

3. The apparatus according to claim 1, wherein the illumination source is mounted to illuminate the region of the substrate with an incident light beam having an incident angle of less than 5° with respect to the normal of the support surface of the substrate support.

4. The apparatus according to claim 1, wherein the illumination source is mounted to illuminate the region of the substrate with an incident light beam having an incident angle of less than 2° with respect to the normal of the support surface of the substrate support.

5. The apparatus according to claim 1, wherein the illumination source is mounted to illuminate the region of the substrate with an incident light beam having an incident angle of approximately 0° with respect to the normal of the support surface of the substrate support.

6. The apparatus according to any one of claims 1 to 5, further comprising an optical arrangement arranged to focus the light emitted by the illumination source onto the substrate and / or into the camera.

7. The apparatus according to any one of claims 1 to 5, further comprising an optical filter positioned to filter light of a selected wavelength entering the camera.

8. The apparatus according to any one of claims 1 to 5, wherein the image processing techniques include image pattern recognition and / or image pattern matching.

9. A method for plasma etching a substrate, the method comprising the steps of: (a) Use a white light illumination source to illuminate a region of the substrate to be plasma-etched with an incident light beam, wherein all the light used to illuminate the region of the substrate has an incident angle of less than 10° with respect to the normal of the front surface of the substrate; (b) Use a camera to capture successive images of the illuminated region during the plasma etching process; (c) Apply image processing techniques to the images in order to identify one or more pixels in the images corresponding to the location of at least one feature on the substrate and measure the reflectivity signal at the one or more pixels, wherein the one or more pixels have an area smaller than the size of the feature on the substrate; and (d) Modify the plasma etching process in response to the measured reflectivity signal at the location.

10. The method according to claim 9, wherein step (d) includes modifying the plasma etching process in response to a change in the measured reflectivity signal at the location.

11. The method according to claim 9, wherein the illuminated area has an area in the range of 0.75 mm 2 to 100 mm 2 in area.

12. The method according to claim 9, wherein the illuminated area has an area in the range of 1 mm 2 to 50 mm 2 .

13. The method according to claim 9, wherein the illuminated area has an area in the range of 2 mm 2 to 25 mm 2 in area.

14. The method according to claim 9, wherein the illuminated area has an area in the range of 3 mm 2 to 5 mm 2 .

15. The method according to any one of claims 9 to 14, wherein the illuminated region is illuminated with an incident light beam having an incident angle of less than 5° with respect to the normal of the front surface of the substrate.

16. The method according to any one of claims 9 to 14, wherein the illuminated region is illuminated with an incident light beam having an incident angle of less than 2° with respect to the normal of the front surface of the substrate.

17. The method according to any one of claims 9 to 14, wherein the illuminated region is illuminated with an incident light beam having an incident angle of less than 1° with respect to the normal of the front surface of the substrate.

18. The method according to any one of claims 9 to 14, wherein the incident light beam is substantially perpendicular to the front surface of the substrate.

19. The method according to any one of claims 9 to 14, wherein the camera detects light reflected from the substrate at a detection wavelength, and the light emitted by the illumination source has a higher intensity at the detection wavelength compared to the light emitted by the plasma of the plasma etching process at the detection wavelength.

20. The method according to any one of claims 9 to 14, wherein the image processing techniques include image pattern recognition and / or image pattern matching.

21. The method according to any one of claims 9 to 14, wherein prior to the plasma etching process, the substrate includes the front surface, and the front surface includes at least two regions having different reflectivity properties.

22. The method according to any one of claims 9 to 14, wherein the substrate includes a mask layer partially covering the material to be etched, and the mask layer has a different reflectivity property from the material to be etched.

23. The method according to claim 22, wherein the mask layer covers at least 90% of the material to be etched.

24. The method according to claim 22, wherein the mask layer covers at least 95% of the material to be etched.

25. The method according to claim 22, wherein the mask layer covers at least 99% of the material to be etched.

26. The method according to any one of claims 9 to 14, wherein step (c) includes applying image processing techniques to a reference image captured by the camera to identify the position of the at least one feature; measuring a reference reflectance signal at the position of the at least one feature in the reference image; and comparing the reference reflectance signal with the reflectance signal measured at the position of the at least one feature on a successive image.

27. The method according to any one of claims 9 to 14, wherein identifying the position of the at least one feature includes locating a reference pattern and determining the position of the at least one feature relative to the position of the reference pattern.

28. The method according to any one of claims 9 to 14, wherein the reflectance signal is the luminance signal, intensity, and / or color of the reflected light.

29. The method according to any one of claims 9 to 14, wherein step (d) includes terminating the plasma etching process in response to a change in the reflectance signal at the position.

30. The method according to any one of claims 9 to 14, wherein the at least one feature is a via hole; a copper plug embedded in a silicon substrate; or a trench.

31. The method according to any one of claims 9 to 14, wherein the substrate is a wafer.

32. The method according to claim 31, wherein the wafer is a semiconductor wafer, a wafer on a carrier structure, or a wafer attached to a frame by a tape.

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