Wafer bonding apparatus and methods to reduce post-bond wafer distortion

Abstract

This document describes numerous embodiments of wafer bonding apparatus and methods for reducing post-bonding wafer pair deformation. More specifically, this disclosure provides embodiments of wafer bonding apparatus and methods to reduce post-bonding wafer deformation primarily occurring within the center and / or edges of the post-bonded wafer pair. In this disclosure, post-bonding wafer deformation is reduced by correcting for differences in the pre-bonding wafer shape. These differences are corrected or compensated for by hardware adjustments to the wafer chuck. Such adjustments may include, but are not limited to, adjusting the surface height and / or temperature of the wafer chuck. Although this document discloses hardware adjustments to reduce post-bonding wafer deformation near the center and / or edges of the post-bonded wafer pair, similar adjustments can be made to reduce post-bonding wafer deformation in other areas or ranges of the post-bonded wafer pair.

Description

[Technical Field]

[0001] This disclosure relates to substrate processing. Specifically, this disclosure provides an improved chuck design and wafer bonding process to reduce post-bonding wafer warping. [Cross-reference to related applications]

[0002] This application claims priority to U.S. non-provisional application No. 17 / 236,785, filed on April 21, 2021, the entire contents of which are incorporated herein by reference. [Previous Technology]

[0003] Wafer bonding equipment (or "wafer bonding machine") physically joins two wafers (upper wafer and lower wafer) together through interface attachment. Lateral misalignment along the bonding interface is called post-bonding deformation error. Post-bonding deformation error is a function of the wafer shape before bonding and the wafer bonding process (e.g., tooling formulation), and is often used to judge the performance of wafer bonding equipment.

[0004] There are two main types of wafers used in wafer bonding processes: device / device pairs and device / carrier pairs. Device wafers are patterned wafers with nanoscale electronic circuitry formed on their surface. Carrier wafers are typically blank wafers without any pattern on their surface. For device / device pairs, the absolute lateral misalignment between the two wafers at any point along the bonding interface is crucial, and for state-of-the-art technologies, it is typically around 100 nm. In contrast, relative lateral misalignment is more important in device / carrier wafer pairs. Because there is no pattern on the carrier wafer, the absolute lateral misalignment between the two bonded wafers is affected by the ability of lithography tools to pattern the carrier wafer in subsequent (multiple) processing steps. Such misalignment can be at least partially corrected by lithography tools. Therefore, the relative lateral misalignment of a device / carrier wafer pair is the absolute misalignment minus the amount of correction that can be applied in subsequent (multiple) processing steps. Depending on the corrective capabilities of subsequent (complex) steps, the relative lateral misalignment between component / carrier wafer pairs can be less than 10 nm in state-of-the-art technology. In the following paragraphs, absolute and relative lateral misalignment will be referred to as “deformation”.

[0005] Figures 1A-1C (Prior Art) illustrate a conventional wafer bonding apparatus 10 and a process that can be used to bond two wafers together. The wafer bonding apparatus 10 includes an upper chuck 12 and a lower chuck 14, which hold an upper wafer 16 and a lower wafer 18 in place, respectively. In some cases, vacuum pressure can be used to hold the wafers on the chuck. As shown in Figure 1A, the two wafers are initially separated by a small gap (g). Then, the centers of the upper and lower wafers are pushed to contact, sometimes assisted by a mechanical force applied to the center of the upper wafer 16. As shown in Figure 1B, for example, a striker (or bonding pin) 19 can be used to apply a mechanical force to the center of the upper wafer 16, which causes the upper wafer to warp or bend until the center of the upper wafer 16 contacts the center of the lower wafer 18. Such contact triggers a bonding leading edge that spontaneously propagates radially outward from the centers of the upper and lower wafers. As the contact surface area increases, the upper and lower wafers elastically deform to close the initial gap (g) between the wafers. Vacuum pressure on the upper chuck 12 is then closed. When the contact surface area is sufficiently large, the vacuum seal between the upper chuck 12 and the upper wafer 16 is broken due to wafer deformation or a predetermined formulation setting. The bonding front continues to propagate outward until the two wafers are fully bonded. Once bonding is complete, the pin 19 retracts, and vacuum pressure on the lower chuck 14 is closed, as shown in Figure 1C. The bonded wafer pair 16 / 18 can then be released from the wafer bonding apparatus 10 and transferred to a metrology tool for deformation characterization, and / or proceed to the next processing step.

[0006] In some cases, the conventional wafer bonding apparatus 10 and process shown in Figures 1A-1C may cause undesirable post-bonded wafer deformation near the center and / or edge (e.g., the outermost 15 mm) of the bonded wafer pair 16 / 18. Specifically, Figure 1C illustrates an upward bend of the bonded wafer pair 16 / 18 near the edge or periphery of the bonded wafer pair. The upward bend shown in Figure 1C is an example of post-bonded wafer deformation (e.g., outward deformation), which may occur after the two wafers are bonded together using a conventional wafer bonding process. When using a conventional wafer bonding process, deformation may also occur in other areas of the bonded wafer pair (e.g., near the center). Regardless of location, post-bonded wafer deformation in the deformed area results in a lateral offset (or lateral misalignment) between the two wafers.

[0007] The wafer deformation after bonding shown in Figure 1C can be due to a number of reasons. First, pre-bonding wafers often have different specifications near the edges. For example, some wafers may have tapered edges or a smaller thickness at the edges or periphery of the wafer (e.g., the outermost 3 mm). This results in differences in the mechanical properties of the wafer at / near the edges (compared to the rest of the wafer), which can lead to undesirable deformation of the wafer pair after bonding. Furthermore, pre-bonding wafers can be processed differently in other processing steps (e.g., different patterning settings) to improve device yield. Process variations often result in different pre-bonding wafer shapes near the edges or periphery. Differences in pre-bonding wafer shape are another reason for post-bonding wafer deformation. Moreover, the wafer bonding process can be considered a two-stage process—before and after the chuck vacuum seal is broken. Once the vacuum seal between the chuck and the wafer is broken, deformation control of the wafer bonding equipment is reduced, which can lead to undesirable deformation at the edges.

[0008] Figure 2 is a graph illustrating the deformation (i.e., post-bonding wafer deformation) of a wafer pair after bonding when two wafers are bonded together using conventional wafer bonding equipment and processes. This deformation is a function of radius (r), as discussed above with reference to Figures 1A-1C. In the graph shown in Figure 2, the deformation 20 increases significantly near the edge of the bonded wafer pair. In some wafer bonding processes, the deformation 20 near the edge of the bonded wafer pair can be as high as, for example, 20 nm worse than the deformation 20 in other parts of the wafer pair. It is estimated that improving the wafer edge deformation can improve the post-bonding deformation error, for example, by 15% or more. [Summary of the Invention]

[0009] This document describes numerous embodiments of wafer bonding apparatus and methods for reducing post-bonding wafer pair deformation. In many cases, as shown in FIG2 and above, post-bonding wafer deformation near the center and / or edges of the post-bonding wafer pair may be undesirable. Therefore, this disclosure provides embodiments of wafer bonding apparatus and methods to reduce post-bonding wafer deformation primarily occurring within the center and / or edges of the post-bonding wafer pair. In this disclosure, post-bonding wafer deformation is reduced by correcting for differences in the pre-bonding wafer shape. Differences in the pre-bonding wafer shape are corrected or compensated for by hardware adjustments to the wafer chuck. Such adjustments may include, but are not limited to, adjusting the surface height and / or temperature of the wafer chuck. Although this document discloses hardware adjustments to reduce post-bonding wafer deformation near the center and / or edges of the post-bonding wafer pair, similar adjustments may be made to reduce post-bonding wafer deformation in other areas or ranges of the post-bonding wafer pair.

[0010] According to one embodiment, a wafer bonding apparatus may generally include a first chuck including a first mounting surface configured to hold a first wafer; and a second chuck including a second mounting surface configured to hold a second wafer. In this embodiment, the second chuck may further include a central region, a peripheral region extending radially outward from the edge of the central region to an outer edge of the second chuck, and a plurality of radial ribs extending vertically from the upper surface of the second chuck to provide the second mounting surface. The second mounting surface is flat in the central region and linearly upward or downward in the peripheral region, such that the height of the second chuck in the peripheral region differs from the height of the second chuck in the central region.

[0011] In some embodiments, the second mounting surface may be linearly downward sloped in the peripheral region, such that the height of the second chuck in the peripheral region is less than the height of the second chuck in the central region. In some embodiments, for example, the height difference between the edge of the central region and the outer edge of the second chuck may be selected from a range including -1 μm to -5 μm. In some embodiments, one or more radial ribs of the plurality of radial ribs may be located in the peripheral region. In some embodiments, the height difference may be provided by reducing the height of one or more radial ribs in the peripheral region.

[0012] In some embodiments, the second mounting surface may be linearly upwardly sloped in the peripheral region, such that the height of the second chuck in the peripheral region is greater than the height of the second chuck in the central region. In some embodiments, for example, the height difference between the edge of the central region and the outer edge of the second chuck may be selected from a range including +1 μm to +5 μm. In some embodiments, one or more radial ribs of the plurality of radial ribs may be located in the peripheral region. In some embodiments, the height difference may be provided by increasing the height of one or more radial ribs in the peripheral region.

[0013] According to another embodiment, the wafer bonding apparatus may generally include a first chuck including a first mounting surface configured to hold a first wafer; and a second chuck including a second mounting surface configured to hold a second wafer. Similar to the foregoing embodiments, the second chuck may also include a central region and a peripheral region extending radially outward from the edge of the central region to the outer edge of the second chuck. However, in this embodiment, the second chuck may also include a plurality of radial regions extending radially outward from the center of the second chuck to the outer edge of the second chuck. A first subset of the plurality of radial regions may be located within the central region of the second chuck. A second subset of the plurality of radial regions may be located within the peripheral region of the second chuck. The height of the first chuck in one or more radial regions may differ from the height of the second chuck in other radial regions.

[0014] In some embodiments, the second chuck may further include a plurality of radial ribs, each radial rib located at the boundary between two radial regions and extending perpendicularly from the upper surface of the second chuck to provide a second mounting surface. A first subset of the plurality of radial ribs may be located within a first subset of the plurality of radial regions. A second subset of the plurality of radial ribs may be located within a second subset of the plurality of radial regions. The height of the second subset of the plurality of radial ribs may differ from the height of the first subset of the plurality of radial ribs.

[0015] In some embodiments, the second mounting surface is flat or planar within a first subset of the plurality of radial ribs and linearly downsloping or linearly upsloping within a second subset of the plurality of radial ribs.

[0016] In some embodiments, for example, the second mounting surface is linearly downsloping or has a downward angle within a second subset of the plurality of radial ribs, such that the height of the second chuck at its outer edge is less than the height of the second chuck in the central region. In such an implementation, the reduced height in the peripheral region of the second chuck can cause the peripheral portion of the second wafer to bend downwards before bonding. When bonding between the first and second wafers is completed, the peripheral portion of the second wafer bends upwards to provide inward deformation, which reduces lateral misalignment between the first and second wafers.

[0017] In another embodiment, the second mounting surface is linearly upwardly inclined or has an upward tilt angle within a second subset of the plurality of radial ribs, such that the height of the second chuck at its outer edge is greater than the height of the second chuck in the central region. In such an implementation, the increased height in the peripheral region of the second chuck can cause the peripheral portion of the second wafer to bend upwards before bonding. When bonding between the first and second wafers is completed, the peripheral portion of the second wafer bends downwards to provide outward deformation that reduces lateral misalignment between the first and second wafers.

[0018] In some embodiments, at least one of the first chuck and the second chuck may include one or more temperature regulating elements embedded within one or more radial regions of a plurality of radial regions. In some embodiments, the one or more temperature regulating elements may be configured to transfer heat to regions of the first wafer and / or the second wafer located above one or more radial regions, causing the regions of the first wafer and / or the second wafer to laterally expand and deform outward, the outward deformation reducing lateral misalignment between the first wafer and the second wafer. In other embodiments, the one or more temperature regulating elements may be configured to transfer heat from regions of the first wafer and / or the second wafer located above one or more radial regions, causing the regions of the first wafer and / or the second wafer to laterally contract and deform inward, the inward deformation reducing lateral misalignment between the first wafer and the second wafer.

[0019] According to another embodiment, a method is provided to reduce deformation in a post-bonded wafer pair. The method may generally include receiving an upper wafer on the mounting surface of an upper chuck of a wafer bonding apparatus and supplying vacuum pressure to the upper chuck to hold the upper wafer on the mounting surface of the upper chuck. The method may also include receiving a lower wafer on the mounting surface of a lower chuck of the wafer bonding apparatus and supplying vacuum pressure to the lower chuck to hold the lower wafer on the mounting surface of the lower chuck. The method may further include bonding the upper wafer to the lower wafer to form a post-bonded wafer pair.

[0020] In some embodiments of the disclosed method, the lower chuck may include a central region and a peripheral region extending radially outward from the edge of the central region to the outer edge of the lower chuck. The mounting surface of the lower chuck may be flat in the central region and linearly upward or downward in the peripheral region, such that the height of the lower chuck in the peripheral region differs from its height in the central region. When a vacuum pressure is supplied to the lower chuck, the vacuum pressure causes the lower wafer to adhere to the mounting surface of the lower chuck, such that the lower wafer is pre-deformed according to the shape of the mounting surface of the lower chuck before the upper wafer is bonded to the lower wafer. This pre-deformation of the lower wafer before bonding advantageously reduces deformation during wafer alignment after bonding and minimizes lateral misalignment between the upper and lower wafers.

[0021] In some embodiments of the disclosed method, the upper chuck includes a central region and a peripheral region extending radially outward from the edge of the central region to the outer edge of the lower chuck. The mounting surface of the upper chuck may be flat in the central region and linearly upward or downward in the peripheral region, such that the height of the upper chuck in the peripheral region differs from its height in the central region. When a vacuum pressure is supplied to the upper chuck, the vacuum pressure causes the upper wafer to adhere to the mounting surface of the upper chuck, such that the lower wafer is pre-deformed according to the shape of the mounting surface of the upper chuck before being bonded to the upper wafer. This pre-deformation of the upper wafer before bonding advantageously reduces deformation during wafer alignment after bonding and minimizes lateral misalignment between the upper and lower wafers.

[0022] In some embodiments, the bonding step may include applying mechanical force to the center of the upper wafer to initiate bonding between the upper and lower wafers, removing the vacuum pressure supplied to the upper chuck before the bonding of the upper and lower wafers is completed, and removing the vacuum pressure supplied to the lower chuck after the bonding of the upper and lower wafers is completed, to form a bonded wafer pair. In such an embodiment, the bonded wafer pair may remain on the lower chuck until the vacuum pressure supplied to the lower chuck is removed.

[0023] In other embodiments, the bonding step may include applying a mechanical force to the center of the upper wafer to initiate bonding between the upper and lower wafers, removing the vacuum pressure supplied to the lower chuck before bonding between the upper and lower wafers is completed, and removing the vacuum pressure supplied to the upper chuck after bonding between the upper and lower wafers is completed to form a bonded wafer pair, wherein the bonded wafer pair remains on the upper chuck until the vacuum pressure supplied to the upper chuck is removed. In such an embodiment, the bonding force between the upper and lower wafers before bonding is completed and / or before the vacuum pressure supplied to the upper chuck is removed causes the vacuum seal between the lower wafer and the lower chuck to break. In such an embodiment, the bonded wafer pair may remain on the upper chuck until the vacuum pressure supplied to the upper chuck is removed.

[0024] Different or additional features, variations and embodiments may also be implemented, and related systems and methods may also be utilized.

Implementation Method

[0040] Silicon wafers are typically processed in batches, such as up to 25 wafers. Due to processing conditions, inherent wafer deformation, and other less noticeable factors, wafers often vary from batch to batch. Such differences can be localized, meaning that the average surface pattern of one batch differs from that of another; in logic devices, the standard deviation of shape differences can range from 1 μm to 5 μm. Differences can also be global, meaning that peak-to-valence differences between batches can be as low as 10 μm. Batch-to-batch wafer variation is a significant cause of wafer deformation after bonding. In this disclosure, batch-to-batch wafer variation is minimized by correcting for differences in wafer shape before bonding.

[0041] Differences in wafer shape before bonding are directly related to in-plane deformation or lateral misalignment between the two wafers in the post-bonding wafer pair. In one example, a 1 μm difference in wafer shape before bonding can lead to approximately 5 nm of deformation. The amount of deformation in the post-bonding wafer pair (i.e., post-bonding deformation error) is a key indicator, often used to assess the performance of wafer bonding equipment and processes. Therefore, minimizing the amount of deformation in the post-bonding wafer pair is generally desirable.

[0042] This disclosure provides numerous embodiments of wafer bonding apparatus and methods for reducing deformation in post-bonded wafer pairs. In the disclosed embodiments, post-bonded wafer deformation is reduced by correcting for differences in wafer shape prior to bonding. Numerous hardware improvements are used herein to correct for these differences. The hardware design is driven by a physical modeling of a specific wafer bonding process. The physical model provides guidance for unique hardware designs at specific locations (e.g., near the edge and / or center of the wafer) to counteract unwanted deformation areas in the post-bonded wafer pair.

[0043] In many cases, post-bonding wafer deformation may be more severe near the wafer center (e.g., due to the design of the snap-fit ​​connector) and / or near the wafer edge, as shown and described in Figure 2 above. Therefore, this disclosure provides numerous embodiments of wafer bonding apparatus and methods to reduce post-bonding wafer deformation primarily occurring in the center and / or edges of the wafer. Such embodiments may include, but are not limited to, adjusting the surface height and / or temperature of the wafer chuck. Although this document discloses hardware adjustments for reducing post-bonding wafer deformation near the center and / or edges of a bonded wafer pair, those skilled in the art will recognize that similar adjustments can be made to reduce post-bonding wafer deformation in other areas or ranges of the bonded wafer pair.

[0044] FIG3 illustrates an example of a wafer bonding apparatus 100 in which the techniques described herein may be used. In some embodiments, the wafer bonding apparatus 100 shown in FIG3 may be a wafer bonding module included in a wafer bonding system or another substrate processing system. In other embodiments, the wafer bonding apparatus 100 shown in FIG3 may be a stand-alone module configured to bond two wafers together. It should be appreciated that although the wafer bonding apparatus 100 is shown as an example for illustrative purposes, the techniques described herein are not limited to the specific configuration shown in FIG3.

[0045] The wafer bonding apparatus 100 is used to bond two semiconductor substrates together, such as a first wafer (W1) and a second wafer (W2). In some embodiments, the first wafer W1 and the second wafer W2 may have substantially the same diameter (e.g., 300 mm in one embodiment). In some embodiments, the first wafer W1 may be a semiconductor substrate, such as a silicon wafer or a compound semiconductor wafer (i.e., a device wafer) on which multiple electronic circuits are formed, and the second substrate W2 may be a bare wafer on which no electronic circuits are formed (i.e., a carrier wafer). In other embodiments, the first wafer W1 and the second wafer W2 may each be a device wafer. In the following description, the first wafer W1 may be referred to as the "upper wafer," and the second wafer W2 may be referred to as the "lower wafer," or vice versa. When the first wafer W1 and the second wafer W2 are bonded, the combined substrates may be referred to as a "bonded wafer pair."

[0046] As shown in FIG3, the wafer bonding apparatus 100 includes a processing chamber 105 for bonding a first wafer W1 to a second wafer W2. In some embodiments, the first and second wafers may be transferred to the processing chamber 105 by a processing arm 110, which may be configured to transfer the wafers in and out of the processing chamber 105 through an opening 115 provided in the sidewall of the processing chamber. In order to transfer the wafers in and out of the processing chamber 105, the processing arm 110 may be configured to move in the vertical direction (Z direction) and the horizontal direction (X direction and Y direction). In some cases, the processing arm 110 may also pivot about a vertical axis to transfer the first wafer W1 and the second wafer W2 into the processing chamber 105, and to transfer the bonded wafer pair out of the processing chamber 105.

[0047] The processing chamber 105 typically includes an upper chuck 120 for receiving and holding a first wafer (or upper wafer) W1 and a lower chuck 125 for receiving and holding a second wafer (or lower wafer) W2. The upper chuck 120 is supported by a support member 130, which can be coupled to the top of the processing chamber 105 by a plurality of support posts 132, as shown in FIG3. As described in more detail below with reference to FIG4A-4C, a hole 134 is formed in the support member 130 and the upper chuck 120, and extends vertically through the support member 130 and the upper chuck 120. The hole 134 is positioned to correspond to the central region or range of the upper wafer W1 held by the upper chuck 120. The engagement pin of the striker 140 is inserted into the hole 134 and is used to provide mechanical force to the upper wafer W1 during the wafer bonding process.

[0048] In the wafer bonding apparatus 100 shown in FIG. 3, the lower chuck 125 is supported by a first lower chuck moving member 135 and a second lower chuck moving member 150 disposed below the lower chuck 125, which are used to translate or move the lower chuck 125 in the horizontal direction. For example, the bottom surface of the first lower chuck moving member 135 is coupled to a first pair of guide rails 137, which are mounted to the upper surface of the second lower chuck moving member 150 to extend in a first horizontal direction (e.g., the X-axis direction). Similarly, the bottom surface of the second lower chuck moving member 150 is coupled to a second pair of rails 152, which are mounted on a placement stage 155 to extend in a second horizontal direction (e.g., the Y-axis direction). In the embodiment shown in FIG. 3, the placement stage 155 is coupled to the bottom of the processing chamber 105.

[0049] When configured as shown in FIG3, the first lower chuck moving member 135 is configured to move along the first pair of tracks 137 to translate the lower chuck 125 (and the lower wafer W2) in the X direction. Similarly, the second lower chuck moving member 150 is configured to move along the second pair of tracks 152 to translate the lower chuck 125 (and the lower wafer W2) in the Y direction. After the upper wafer W1 and the lower wafer W2 are positioned on the upper and lower chucks, the first lower chuck moving member 135 and the second lower chuck moving member 150 can be used to translate the lower chuck 125 (and the lower wafer W2) in the X and / or Y directions until the lower wafer W2 is aligned with the upper wafer W1. In some embodiments, the first lower chuck moving member 135 and the second lower chuck moving member 150 can also be used to move the lower chuck 125 in the Z direction.

[0050] In some embodiments, the upper chuck 120 and lower chuck 125 may be vacuum chucks. Once the upper wafer W1 and lower wafer W2 are transferred into the processing chamber 105, the vacuum pump 160 may be turned on to provide sufficient vacuum pressure (i.e., negative pressure) to hold the upper wafer W1 on the upper chuck 120 and the lower wafer W2 on the lower chuck 125. As described in more detail below with reference to Figures 4A-4C, the upper chuck 120 and lower chuck 125 may each have a mounting surface including a plurality of holes through which a vacuum pressure is generated between the mounting surface of the chuck and the back side of the wafer. When the wafer is placed on the vacuum chuck and the vacuum pump 160 is turned on, the vacuum pressure provided by the vacuum pump 160 forms a vacuum seal between the back side of the wafer and the mounting surface of the chuck to hold the wafer on the vacuum chuck. Although a vacuum chuck is described, those skilled in the art will recognize that other types of chucks (e.g., electrostatic chucks, magnetic chucks, etc.) can also be used to implement the upper chuck 120 and lower chuck 125 shown in FIG3, while utilizing the techniques described herein.

[0051] Figures 4A-4C are cross-sectional views through the upper and lower chuck assemblies shown in Figure 3. As shown in Figure 4A, the striker 140 disposed within the upper chuck assembly (including, for example, the upper chuck 120 and the support member 130) is equipped with an engagement pin 144, an actuator unit 142, and a linear movement mechanism 143. The engagement pin 144 is a columnar member supported by the actuator unit 142 and extending vertically through a hole 134 formed in the upper chuck assembly.

[0052] The actuator unit 142 is supported by a linear movement mechanism 143. The linear movement mechanism 143 moves the actuator unit 142 and the engagement pin 144 vertically by means of a drive unit (not shown), which may include, for example, a motor. The actuator unit 142 is configured to generate a constant pressure in a vertically downward direction. In one example, the actuator unit 142 may generate a constant pressure, for example, from air supplied from an electro-pneumatic regulator (not shown). In this way, when the engagement pin 144 is brought into contact with the center portion of the upper wafer W1, the actuator unit 142 is able to control the mechanical force applied to the center portion of the upper wafer W1. When configured as described above, the impact member 140 controls the movement of the actuator unit 142 by means of the linear movement mechanism 143, and controls the mechanical force applied to the upper wafer W1 from the engagement pin 144 by means of the actuator unit 142.

[0053] Figures 4A-4C illustrate various wafer bonding process steps that can be performed by the wafer bonding apparatus 100. As shown in Figure 4A, the upper wafer W1 and the lower wafer W2 are held by the upper chuck 120 and the lower chuck 125, respectively. In the embodiments shown in Figures 3 and 4A, the back side of the upper wafer W1 is held on the mounting surface 122 of the upper chuck 120 by a vacuum pressure 165 provided by the vacuum pump 160. Similarly, the back side of the lower wafer W2 is held on the mounting surface 127 of the lower chuck 125 by a vacuum pressure 165 provided by the vacuum pump 160. The two wafers are initially separated by a small gap (g).

[0054] As shown in Figure 4B, the impactor 140 applies a mechanical force to the center of the upper wafer W1, causing the upper wafer W1 to warp or bend until the center of the upper wafer W1 contacts the center of the lower wafer W2. This contact initiates a bonding front that spontaneously propagates radially outward from the centers of the upper and lower wafers. As the contact surface area increases, the upper and lower wafers elastically deform to close the initial gap (g) between the two wafers.

[0055] In some embodiments, the vacuum pressure 165 applied to the upper chuck 120 may be turned off before the vacuum pressure 165 of the lower chuck 125 is removed, as shown in FIG4C. When the vacuum pressure 165 on the upper chuck 120 is turned off first and the contact surface area is large enough, the vacuum seal between the upper chuck 120 and the upper wafer W1 is broken, and the peripheral portion of the upper wafer W1 falls downward onto the peripheral portion of the lower wafer W2, and the bonding front continues to propagate outward until the two wafers are fully bonded. Once bonding is complete, the impactor 140 retracts, the vacuum pressure 165 on the lower chuck 125 is turned off, and the bonded wafer pair W1 / W2 is released from the wafer bonding apparatus 100.

[0056] In other embodiments (not shown in FIG. 4C), the vacuum pressure 165 supplied to the lower chuck 125 may be shut off before the vacuum pressure 165 of the upper chuck 120 is removed. When the vacuum pressure 165 to the lower chuck 125 is shut off first and the contact surface area is large enough, the vacuum seal between the lower chuck 125 and the lower wafer W2 is broken, the peripheral portion of the lower wafer W2 is lifted upward toward the peripheral portion of the wafer W1, and the bonding front continues to propagate outward until the two wafers are fully bonded. In this embodiment, the bonding force between the upper wafer W1 and the lower wafer W2 can resist gravity to bond the lower wafer W2 to the upper wafer W1 held on the upper chuck 120. Once bonding is complete, the impactor 140 retracts, the vacuum pressure 165 to the upper chuck 120 is shut off, and the bonded wafer pair W1 / W2 is released from the wafer bonding apparatus 100.

[0057] As described above, when two wafers are bonded together using conventional wafer bonding systems and methods, the bonded wafer pair typically exhibits significant deformation and lateral misalignment. In many cases, the post-bonded wafer deformation generated in such systems and methods may be more severe near the center and / or edges of the bonded wafer pair, as shown and described in Figure 2 above. Therefore, this disclosure provides numerous embodiments of wafer bonding systems and methods to reduce post-bonded wafer deformation primarily occurring within the center and / or edges of the bonded wafer pair. More specifically, this disclosure provides numerous hardware improvements that reduce post-bonded wafer deformation by correcting for differences in the pre-bonded wafer shape. Although hardware adjustments are disclosed herein to reduce post-bonded wafer deformation near the center and / or edges of the bonded wafer pair, those skilled in the art will recognize that similar adjustments can be made to reduce post-bonded wafer deformation in other areas or ranges of the bonded wafer pair.

[0058] In some embodiments, post-deposition wafer deformation is reduced by hardware adjustments to the lower chuck 125. Such adjustments may include, but are not limited to, adjusting the surface height and / or temperature of the lower chuck 125. However, the techniques described herein are not strictly limited to the lower chuck 125. In some embodiments, similar hardware adjustments may be made to the upper chuck 120 as an additional or alternative measure.

[0059] Figures 5-7 illustrate various embodiments of wafer chucks in which the techniques described herein can be utilized. In particular, Figure 5 illustrates a top view of a wafer chuck 200 comprising a plurality of radial regions (Z1-Z5) extending radially outward from the center (C) of the wafer chuck to the outer edge (E) of the wafer chuck. In some embodiments, radial ribs (R1-R5) may be provided at the boundaries of each radial region (Z1-Z5), as shown in Figure 5. When included in the design of the wafer chuck 200, the radial ribs (R1-R5) extend vertically from the upper surface of the wafer chuck to provide a mounting surface for the wafer (W). Although shown and described in various embodiments (see, for example, Figures 6B, 7B and 10), it should be noted that the radial ribs (R1-R5) are not strictly necessary for the techniques described herein and may be omitted in other embodiments (see, for example, Figures 6A and 7A). When the radial ribs (R1~R5) are omitted, the wafer (W) can be directly mounted onto the upper surface of the wafer chuck 200, as shown in Figures 4A~4C.

[0060] In some embodiments, the surface height of the wafer chuck 200 shown in FIG. 5 can be configured to reduce post-bonding wafer deformation. In particular, the surface height of the wafer chuck 200 can vary within the range of the wafer chuck, such that the wafer chuck height in one or more radial regions (Z1~Z5) differs from the wafer chuck height in other radial regions. A wafer chuck with a variable surface height may also be referred to herein as a "variable height wafer chuck". FIG. 6A~6B provide side views of variable height wafer chucks 210 / 220, which can be used to reduce post-bonding wafer deformation near the edges of the wafer pair after bonding. FIG. 7A~7B provide side views of variable height wafer chucks 230 / 240, which can be used to reduce post-bonding wafer deformation near the center of the wafer pair after bonding.

[0061] FIG. 6A illustrates a variable height wafer chuck 210 according to a first embodiment of the present disclosure. As shown in FIG. 6A, the variable height wafer chuck 210 includes a mounting surface 212 for holding a wafer. Furthermore, the variable height wafer chuck 210 includes a central region 214 and a peripheral region 216 extending radially outward from the edge of the central region 214 to the outer edge of the chuck. In some embodiments, the peripheral region 216 may extend a distance (d) from the outer edge of the chuck. When the variable height wafer chuck 210 is configured to support a 300mm wafer, the distance (d) is typically less than or equal to 50mm. In one example, the distance (d) shown in FIG. 6A may range between about 0mm and about 20mm. In some embodiments, the central region 214 and the peripheral region 216 may each include one or more radial regions (Z1~Z5) as shown in FIG. 5. In one example, the central region 214 may include a first subset of the radial regions (e.g., Z1-Z3), and the peripheral region 216 may include a second subset of the radial regions (e.g., Z4-Z5). However, it should be noted that the central region 214 and the peripheral region 216 are not limited to the exemplary radial regions described above, and may include different radial regions in other embodiments.

[0062] In the embodiment shown in FIG. 6A, the mounting surface 212 of the variable height wafer chuck 210 is flat (or planar) in the central region 214 and linearly sloping (or having a downward tilt angle) in the peripheral region 216, such that the height of the variable height wafer chuck 210 in the peripheral region 216 is less than the height of the variable height wafer chuck 210 in the central region 214. In some embodiments, the height (h) of the variable height wafer chuck 210 may decrease linearly between the edge of the central region 214 and the edge (i.e., the outer edge) of the peripheral region 216, as shown in FIG. 6A. In some embodiments, the height difference (Δh) or the range of height reduction between the edge of the central region 214 and the edge (i.e., the outer edge) of the peripheral region 216 of the variable height wafer chuck 210 may be between -1 μm (or less) and -5 μm (or more).

[0063] FIG. 6B illustrates a variable height wafer chuck 220 according to a second embodiment of the present disclosure. Similar to the previous embodiment shown in FIG. 6A, the variable height wafer chuck 220 shown in FIG. 6B includes a central region 224 and a peripheral region 226 extending radially outward from the edge of the central region 224 to the outer edge of the chuck. The variable height wafer chuck 220 shown in FIG. 6B differs from the variable height wafer chuck 210 shown in FIG. 6A in that it includes a plurality of radial ribs (R1-R5) extending vertically from the upper surface of the variable height wafer chuck 220 to provide a mounting surface 222 for the wafer. A first subset of the radial ribs (e.g., R1-R3) is included within the central region 224 of the variable height wafer chuck 220, and a second subset of the radial ribs (e.g., R4-R5) is included within the peripheral region 226.

[0064] In the embodiment shown in FIG. 6B, the mounting surface 222 of the variable height wafer chuck 220 is flat (or planar) in the central region 224 and linearly downsloping (or having a downward tilt angle) in the peripheral region 226, such that the height of the variable height wafer chuck 220 in the peripheral region 226 is less than the height of the variable height wafer chuck 220 in the central region 224. Unlike the embodiment shown in FIG. 6A, in FIG. 6B, the height difference (Δh) in the peripheral region 226 is provided by reducing the height of a second subset of radial ribs (e.g., radial ribs R4 and R5) located in the peripheral region 226. In some embodiments, the rib height in the peripheral region 226 may be reduced from -1 μm (or less) to -5 μm (or more) to provide a stepwise height reduction, as shown in FIG. 6B. In one exemplary embodiment, one or more radial ribs located in the peripheral region 226 may be reduced by about -3 μm.

[0065] The embodiments shown in Figures 6A and 6B illustrate a variable-height wafer chuck 210 / 220, which can be used to reduce post-bonding wafer deformation near the edges of the wafer pair after bonding. When a pre-bonding wafer (e.g., the lower wafer) is mounted onto the variable-height wafer chuck 210 / 220 during the wafer bonding process, the pre-bonding wafer adheres to and takes the shape of the mounting surface 212 / 222. In other words, the pre-bonding wafer is pre-deformed according to the shape of the mounting surface 212 / 222 of the variable-height wafer chuck 210 / 220.

[0066] In the embodiments shown in Figures 6A and 6B, the height reduction (Δh) within the peripheral regions 216 / 226 of the variable height wafer chuck 210 / 220 causes the peripheral portion of the pre-joint wafer (e.g., the lower wafer) to flex or bend in a downward direction (see, for example, Figure 8A). This causes the joining interface between the upper and lower wafers to extend radially outward. When the upper wafer joins to the lower wafer, the deformation along the joining interface is frictionally "locked," which prevents further lateral displacement along the interface. When joining is complete and the joined wafer pair is released from the lower chuck (see, for example, Figure 8E), the peripheral portion of the lower wafer can bend in an upward direction to rebalance the joined wafer pair. Thus, the peripheral portion of the lower wafer is pushed inward along the joining interface, thereby providing inward deformation to compensate for the outward deformation shown in Figures 1C and 2. In this way, the inward deformation generated within the peripheral portion of the lower wafer can be used to reduce lateral misalignment between the upper and lower wafers.

[0067] The variable height wafer chucks 210 / 220 shown in Figures 6A and 6B can be used in the wafer bonding apparatus 100 shown in Figures 3-4 and the wafer bonding process shown in Figures 8A-8E to reduce post-bonding wafer deformation near the edges or periphery of the wafer pair. In some embodiments, for example, the variable height wafer chucks 210 / 220 shown in Figures 6A or 6B can be used as the lower chuck 125 in the wafer bonding apparatus 100 and the wafer bonding process shown in Figures 8A-8E. However, the variable height wafer chucks 210 / 220 are not strictly limited to the lower chuck 125. In some embodiments, the variable height wafer chucks 210 / 220 shown in Figures 6A and 6B can be additionally used in the wafer bonding apparatus 100 to implement the upper chuck 120. In such an embodiment, the upper chuck 120 may be implemented with some degree of elastic fit, allowing it to elastically deform and act like a striker to push the center of the upper wafer (W1) downwards to initiate engagement with the lower wafer (W2). In other embodiments, in alternative wafer bonding apparatuses, the variable height wafer chucks 210 / 220 shown in Figures 6A and 6B may be used instead to implement the upper chuck, wherein a striker is provided within the lower chuck to push the lower wafer upwards to initiate engagement with the upper wafer. When used to implement the upper chuck 120, the variable height wafer chucks 210 / 220 shown in Figures 6A and 6B can provide the same effect as the lower chuck 125 described above.

[0068] Note that the techniques described herein are not strictly limited to reducing post-joint wafer deformation near the edges or periphery of the joined wafer pair. In other embodiments, variable-height wafer chucks may be configured to reduce post-joint wafer deformation in other areas or ranges of the joined wafer pair. In some cases, for example, the impactor 140 used to initiate the joining between the upper wafer (W1) and lower wafer (W2) shown in FIG. 4B may produce undesirable deformation near the center of the joined wafer pair W1 / W2 (e.g., at r <= 10 mm). To avoid this problem, FIG. 7A-7B provides various embodiments of variable-height wafer chucks 230 / 240, which can be used in the wafer bonding apparatus 100 shown in FIG. 3-4 and the wafer bonding process shown in FIG. 8A-8E to reduce post-joint wafer deformation near the center of the joined wafer pair.

[0069] FIG. 7A illustrates a variable height wafer chuck 230 according to a third embodiment of the present disclosure. Similar to the embodiment shown in FIG. 6A, the variable height wafer chuck 230 shown in FIG. 7A includes a mounting surface 232 for holding a wafer, a central region 234, and a peripheral region 236 that extends radially outward from the edge of the central region 234 to the outer edge of the chuck. In FIG. 7A, when the variable height wafer chuck 210 is configured to support a 300 mm wafer, the distance (d) is typically greater than or equal to 130 mm. In the embodiment shown in FIG. 7A, the distance (d) may be in the range of about 135 mm and about 145 mm. In some embodiments, the central region 234 and the peripheral region 236 may each include one or more radial regions (Z1-Z5) as shown in FIG. 5. In one example, the central region 234 may include a first subset of the radial regions (e.g., Z1-Z2), and the peripheral region 236 may include a second subset of the regions (e.g., Z3-Z5). However, it should be noted that the central region 234 and the peripheral region 236 are not limited to the exemplary regions described above, and may include different radial regions in other embodiments.

[0070] In the embodiment shown in FIG. 7A, the mounting surface 232 of the variable height wafer chuck 230 is flat (or planar) in the central region 214 and linearly upward (or has an upward tilt angle) in the peripheral region 236, such that the height of the variable height wafer chuck 230 in the peripheral region 236 is greater than the height of the variable height wafer chuck 230 in the central region 234. In such an embodiment, the height (h) of the variable height wafer chuck 230 can increase linearly between the edge of the central region 234 and the edge (i.e., the outer edge) of the peripheral region 236, as shown in FIG. 7A. In one example, the height difference (Δh) or the range of height increase between the edge of the central region 234 and the edge (i.e., the outer edge) of the peripheral region 236 of the variable height wafer chuck 230 can be between +1 μm (or less) and +5 μm (or more). In other embodiments (not shown), the mounting surface 232 of the variable height wafer chuck 230 may be flat (or planar) within the central region 214, linearly upsloping (or having an upward tilt angle) inside the peripheral region 236, and flat (or planar) outside the peripheral region 236. The outer perimeter of the peripheral region 236 may typically be smaller than the inner perimeter of the peripheral region 236, such that the inner perimeter of the peripheral region 236 provides a sufficient linear upsloping area to offset wafer deformation near the center after bonding.

[0071] FIG. 7B illustrates a variable height wafer chuck 240 according to a fourth embodiment of the present disclosure. Similar to the aforementioned embodiment shown in FIG. 7A, the variable height wafer chuck 240 shown in FIG. 7B includes a central region 244 and a peripheral region 246, the peripheral region 246 extending radially outward from the edge of the central region 244 to the outer edge of the chuck. Unlike the variable height wafer chuck 230 shown in FIG. 7A, the variable height wafer chuck 240 shown in FIG. 7B includes a plurality of radial ribs (R1~R5) extending vertically from the upper surface of the variable height wafer chuck 240 to provide a mounting surface 242 for the wafer. A first subset of the radial ribs (e.g., R1~R2) is included within the central region 244, and a second subset of the radial ribs (e.g., R3~R5) is included within the peripheral region 246 of the variable height wafer chuck 240.

[0072] In the embodiment shown in FIG. 7B, the mounting surface 242 of the variable height wafer chuck 240 is flat (or planar) in the central region 244 and linearly upsloping (or having an upward tilt angle) in the peripheral region 246, such that the height of the variable height wafer chuck 240 in the peripheral region 246 is greater than the height of the variable height wafer chuck 240 in the central region 244. Unlike the embodiment shown in FIG. 7A, in FIG. 7B, the height difference (Δh) in the peripheral region 246 is provided by increasing the height of a second subset of radial ribs located in the peripheral region 236 (e.g., ribs R3, R4, and R5). In some embodiments, the rib height in the peripheral region 246 may be increased from +1 μm (or less) to +5 μm (or more) to provide a stepwise height increase, as shown in FIG. 7B. Alternatively, the rib height within the peripheral region 246 may remain constant, while the rib height within the central region 244 may be reduced by -1 μm (or less) to -5 μm (or more) to provide the desired height difference (Δh). In yet another alternative embodiment, the height of one or more radial ribs (e.g., ribs R3 and R4) located within the peripheral region 236 may be increased by +1 μm (or less) to +5 μm (or more) to provide a progressive height increase within the peripheral region 236, while the height of one or more radial ribs (e.g., rib R5) located outside the peripheral region 236 remains constant or equal to the rib height within the central region 244.

[0073] The embodiments shown in Figures 7A and 7B illustrate a variable-height wafer chuck 230 / 240, which can be used to reduce post-bonding wafer deformation near the center of the bonded wafer pair. When a pre-bonding wafer (e.g., the lower wafer) is mounted onto the variable-height wafer chuck 230 / 240 during the wafer bonding process, the pre-bonding wafer adheres to and conforms to the shape of the mounting surfaces 232 / 242. In other words, the pre-bonding wafer is pre-deformed according to the shape of the mounting surfaces 232 / 242 of the variable-height wafer chuck 230 / 240.

[0074] In the embodiments shown in Figures 7A and 7B, the increase in height (Δh) within the peripheral regions 236 / 246 of the variable height wafer chuck 230 / 240 (or the decrease in height within the central region 234 / 244) causes the peripheral portion of the pre-joint wafer (e.g., the lower wafer) to flex or bend in the upward direction. This causes the joining interface of the pre-joint wafer to deform radially inward. When the upper wafer joins the lower wafer, the deformation along the joining interface is frictionally "locked," which prevents any further lateral displacement along the interface. When joining is complete and the joined wafer pair is released from the lower chuck, the peripheral portion of the lower wafer can be bent in the downward direction to rebalance the joined wafer pair. This causes the peripheral portion of the lower wafer to be pushed outward along the joining interface, thus providing outward deformation to compensate for the inward deformation generated in the pre-joint wafer. In this way, the outward deformation generated within the peripheral portion of the lower wafer can be used to reduce the lateral misalignment between the upper and lower wafers caused by the impactor 140.

[0075] The variable height wafer chucks 230 / 240 shown in Figures 7A and 7B can be used in the wafer bonding apparatus 100 shown in Figures 3-4 and the wafer bonding process shown in Figures 8A-8E to reduce post-bonding wafer deformation near the center of the bonded wafer pair. In some embodiments, for example, the variable height wafer chucks 230 / 240 shown in Figures 7A or 7B can be used as the lower chuck 125 in the wafer bonding apparatus 100 to reduce post-bonding wafer deformation near the center of the bonded wafer pair. However, the variable height wafer chucks 230 / 240 are not strictly limited to the lower chuck 125. In some embodiments, in the wafer bonding apparatus 100, the variable height wafer chucks 230 / 240 shown in Figures 7A and 7B can be additionally used to implement the upper chuck 120. In such an embodiment, the upper chuck 120 may be implemented with some degree of elastic fit, allowing it to elastically deform and act like a striker to push the center of the upper wafer (W1) downwards to initiate engagement with the lower wafer (W2). In other embodiments, in alternative wafer bonding apparatuses, the variable height wafer chucks 230 / 240 shown in Figures 7A and 7B may be used instead to implement the upper chuck, wherein a striker is provided within the lower chuck to push the lower wafer upwards to initiate engagement with the upper wafer. When used to implement the upper chuck 120, the variable height wafer chucks 230 / 240 shown in Figures 7A and 7B can provide the same effect as the lower chuck 125 described above.

[0076] In some embodiments, one or more embodiments shown in Figures 6-7 may be combined to reduce post-bonded wafer deformation near the edges and center of the bonded wafer pair. It should be noted that the embodiments of the variable height wafer chuck shown in Figures 6-7 are illustrative, and the techniques described herein can be used to reduce post-bonded wafer deformation in other regions or ranges of the bonded wafer pair. It should also be noted that the height difference (Δh) and distance (d) described herein are illustrative. Specific values ​​of Δh and d are typically selected based on a physical model of a particular wafer bonding process and the pre-bonding wafer shapes of the upper and lower wafers. For example, the height difference (Δh) and / or distance (d) may be empirically optimized from experimental measurements of the bonded wafers.

[0077] Figures 8A-8E illustrate an improved wafer bonding process that uses a variable-height wafer chuck 210 / 220 as shown in Figure 6A or 6B to bond two wafers (e.g., an upper wafer and a lower wafer). In the illustrated embodiment, a variable-height wafer chuck is used to implement the lower chuck. In other embodiments, a variable-height wafer chuck can be used to implement the upper chuck, or both the lower and upper chucks. Although the wafer bonding process shown in Figures 8A-8E uses a variable-height wafer chuck configured to reduce post-bonded wafer deformation near the edges of the bonded wafer pair, those skilled in the art will understand how the illustrated process can utilize other variable-height wafer chucks to reduce post-bonded wafer deformation in other areas or ranges of the bonded wafer pair. For example, the variable-height wafer chuck 230 / 240 shown in Figures 7A or 7B can be used in a similar wafer bonding process to reduce post-bonded wafer deformation near the center of the bonded wafer pair.

[0078] In some embodiments, the improved wafer bonding process may begin at step 300 of FIG. 8A, receiving an upper wafer 302 on the mounting surface of an upper chuck 304 and a lower wafer 306 on the mounting surface of a lower chuck 308. When the upper chuck 304 and lower chuck 308 are implemented as vacuum chucks, the upper wafer 302 and lower wafer 306 may be held on their respective chucks by, for example, a vacuum pressure provided by a vacuum pump. The upper and lower wafers may initially be separated by a small gap (g), as shown in FIG. 8A.

[0079] As shown in FIG8B, in step 310, a mechanical force can be applied to the center of the upper wafer 302 using the impact member 309 to initiate bonding. The mechanical force provided by the impact member 309 causes the center of the upper wafer 302 to warp or bend in a downward direction until the center of the upper wafer 302 contacts the center of the lower wafer 306. Such contact triggers a bonding front that spontaneously propagates radially outward from the centers of the upper and lower wafers. As the contact surface area increases, the upper and lower wafers elastically deform to close the initial gap (g) between the two wafers.

[0080] As shown in Figure 8C, the vacuum pressure on the upper chuck 304 can be turned off in step 320 before the bonding between the upper and lower wafers is completed. When the contact area is large enough, the vacuum seal between the upper chuck 304 and the upper wafer 302 is broken, and the bonding front continues to propagate outward until the two wafers are fully bonded, as shown in Figure 8D. Once the bonding between the upper and lower wafers is completed, in step 330 of Figure 8D, the impactor 309 retracts, and in step 340 of Figure 8E, the vacuum pressure on the lower chuck 308 is turned off to release the bonded wafer pair 302 / 306 from the lower chuck 308.

[0081] In an alternative wafer bonding process (not shown), the impactor 309 is retractable and the vacuum pressure on the lower chuck 308 can be closed before the vacuum pressure on the upper chuck 304 is closed in FIG. 8C. In such a process, the bonding force between the upper wafer 302 and the lower wafer 306 before bonding is completed and / or before the vacuum pressure on the upper chuck 304 is closed can cause the vacuum seal between the lower wafer 306 and the lower chuck 308 to break. When this occurs, the bonding force between the upper wafer 302 and the lower wafer 306 can resist gravity to bond the lower wafer 306 to the upper wafer 302 held on the upper chuck 304. Once bonding is complete, the vacuum pressure on the upper chuck 304 is closed to release the bonded wafer pair 302 / 306 from the upper chuck 304.

[0082] The improved wafer bonding process shown in Figures 8A-8E reduces post-bonded wafer deformation near the edges of the bonded wafer pair 302 / 306 by using a variable-height wafer chuck. This variable-height wafer chuck pre-deforms the pre-bonded wafer (e.g., the lower wafer) by bending the peripheral portion of at least one of the pre-bonded wafers downward (see, for example, Figure 8A). Such pre-deformation causes the bonding interface between the upper and lower wafers to be stretched or deformed radially outward. When the upper wafer 302 is bonded to the lower wafer 306, the deformation along the bonding interface is frictionally "locked," preventing further lateral displacement along the interface. When bonding is complete (see, for example, Figure 8D) and the vacuum pressure on the lower chuck 308 is turned off to release the bonded wafer pair from the lower chuck 308 (see, for example, Figure 8E), the peripheral portion of the lower wafer 306 can be bent upward to rebalance the bonded wafer pair 302 / 306. Therefore, the peripheral portion of the lower wafer 306 is pushed inward along the bonding interface, thus providing inward deformation to compensate for the outward deformation shown in Figures 1C and 2.

[0083] Figure 9 is a graph showing the deformation of the wafer pair after bonding as a function of radius (r), where the improved wafer bonding process shown in Figures 8A-8E is used to bond the two wafers together. As shown in Figure 9, the improved wafer bonding process shown in Figures 8A-8E produces much smaller deformation compared to the deformation produced by conventional wafer bonding processes. Reducing deformation near the edges of the wafer pair after bonding reduces post-bonding deformation errors caused by the wafer bonding process and minimizes lateral offset (or lateral misalignment) between the two wafers.

[0084] The improved wafer bonding process shown in Figures 8A-8E and the variable height wafer chucks 210 / 220 shown in Figures 6A and 6B are particularly suitable for reducing the deformation 90 at the edges of the bonded wafer pair, because the deformation at the edges is very similar for all wafers. In conventional wafer bonding processes, the deformation at the bonding interface at the edges is usually greater (i.e., positive, outward displacement) than at the center of the bonded wafer pair, which has a steep slope toward the edges. This significant characteristic can be corrected by using the variable height wafer chuck shown in Figure 6A or 6B. Conversely, if the bonded wafer pair exhibits the opposite deformation behavior, with less deformation at the edges (i.e., negative, inward displacement) and more deformation at the center, then the wafer bonding process using the variable height wafer chuck shown in Figure 7A or 7B can be used to reduce the deformation of the bonded wafer at the center.

[0085] The embodiments shown in Figures 6-8 reduce post-bonding wafer deformation by adjusting the surface height of at least one of the wafer chucks. Other hardware adjustments can also be used to reduce post-bonding wafer deformation. For example, a ring (multiple) of temperature regulating elements can be disposed in one or more areas or ranges of the wafer chuck to improve deformation in these areas or ranges. In one exemplary embodiment, a ring of Peltier chips can be embedded in the lower chuck and / or upper chuck to regulate the temperature of the chuck surface. Prior to the bonding process, the ring (multiple) of temperature regulating elements can be activated to transfer heat to or from the wafer area above and / or adjacent to the ring (multiple) of temperature regulating elements. Such heat transfer causes localized temperature changes in the wafer area, resulting in lateral expansion or contraction of that area. This creates a lateral displacement mismatch between wafer pairs, which becomes permanent after the bonding step.

[0086] FIG10 is a side view of a wafer chuck 400 according to a fifth embodiment of the present disclosure. The wafer chuck 400 shown in FIG10 includes one or more temperature regulating elements 410 embedded in one or more regions or ranges of the wafer chuck. Thus, the wafer chuck 400 shown in FIG10 may be otherwise referred to herein as a "temperature-regulated wafer chuck".

[0087] The temperature-regulating wafer chuck 400 shown in FIG. 10 includes a central region 414 and a peripheral region 416 extending radially outward from the edge of the central region 414 to the outer edge of the chuck. In some embodiments, the peripheral region 416 may extend a distance (d) from the outer edge of the chuck. When the temperature-regulating wafer chuck 400 is configured to support a 300 mm wafer, the distance (d) is typically less than or equal to 50 mm. In one example, the distance (d) shown in FIG. 10 may range between about 5 mm and about 10 mm. In some embodiments, the central region 414 and the peripheral region 416 may each include one or more radial regions (Z1-Z5) shown in FIG. 5. In one example, the central region 414 may include a first subset of the radial regions (e.g., Z1-Z3), and the peripheral region 416 may include a second subset of the radial regions (e.g., Z4-Z5). However, it should be noted that the central region 414 and the peripheral region 416 are not limited to the exemplary regions described above, and may include different radial regions in other embodiments.

[0088] The temperature-regulated wafer chuck 400 shown in FIG. 10 further includes a plurality of radial ribs (R1~R5) extending vertically from the upper surface of the temperature-regulated wafer chuck 400 to provide a mounting surface 418 for the wafer. Alternatively, the ribs shown in FIG. 10 may be omitted, and the wafer may be directly mounted on the upper surface of the temperature-regulated wafer chuck 400. In the embodiment shown in FIG. 10, the mounting surface 418 of the temperature-regulated wafer chuck 400 is flat over the entire wafer chuck area (i.e., within the central region 414 and the peripheral region 416). Alternatively, the mounting surface of the temperature-regulated chuck may be flat in the central region 414 and, as described above, may slope downward in the peripheral region 416 (as shown in FIG. 6A~6B) or upward in the peripheral region 416 (as shown in FIG. 7A~7B).

[0089] The temperature-regulated wafer chuck 400 shown in FIG. 10 further includes one or more temperature-regulating elements 410 embedded in one or more regions or areas of the wafer chuck. In the embodiment shown in FIG. 10, the (plural) temperature-regulating elements 410 are embedded in the peripheral region 416 of the temperature-regulated wafer chuck 400 to transfer heat to the edge of the wafer mounted on the mounting surface 418 (or upper surface) of the temperature-regulated wafer chuck 400, or to transfer heat from the edge. However, it should be noted that the (plural) temperature-regulating elements 410 are not limited to the exemplary arrangement shown in FIG. 10, and may be additionally or alternatively arranged in other regions or areas of the temperature-regulated wafer chuck 400 to transfer heat to or from other regions of the wafer. In one embodiment, the (plural) temperature-regulating elements 410 may be (plural) Peltier elements.

[0090] Figure 11 is a graph illustrating wafer temperature as a function of radius (r), wherein the temperature-regulated wafer chuck 400 shown in Figure 10 is used to bond two wafers together in the wafer bonding process shown in Figures 8A-8E. As shown in Figure 11, (multiple) temperature-regulating elements 410 embedded in the peripheral region 416 of the temperature-regulated wafer chuck 400 can be activated before the bonding process to heat the wafer chuck so that heat can be transferred to the wafer during the bonding process. Transferring heat to the wafer edge causes the wafer temperature 500 to increase near the wafer edge, as shown in Figure 11, and causes the region near the wafer edge to expand laterally. The lateral expansion of the wafer edge produces outward deformation, which creates a lateral displacement mismatch between the upper and lower wafers, which becomes permanent after the bonding step. In some embodiments, for every 0.1°C increase in wafer temperature, the (multiple) temperature-regulating elements 410 can provide an outward deformation of approximately 5 nm at the wafer edge.

[0091] In other embodiments, the (plural) temperature regulating elements 410 may instead be used to cool one or more regions or areas of the wafer, or to absorb heat from one or more regions or areas of the wafer. Cooling a region causes that region to shrink laterally and produce inward deformation, resulting in a lateral displacement mismatch between the upper and lower wafers. In some embodiments, the (plural) temperature regulating elements 410 may provide an inward deformation of about 5 nm at the wafer edge for every 0.1°C decrease in wafer temperature.

[0092] In some embodiments, the temperature-regulating wafer chuck 400 shown in FIG. 10 can be used in the wafer bonding apparatus 100 shown in FIG. 3-4 and the wafer bonding process shown in FIG. 8A-8E to reduce post-bonded wafer deformation in one or more regions or ranges of the bonded wafer pair. The temperature-regulating wafer chuck 400 allows for flexibility in deformation adjustment based on incoming wafer metering. The amount and direction of deformation can be adjusted by regulating the temperature of (multiple) temperature regulating elements 410 relative to other parts of the wafer. In some embodiments, the temperature regulating element 410 shown in FIG. 10 may be included in one of the variable height chucks shown in FIG. 6-7.

[0093] Figure 12 is a flowchart illustrating an embodiment of method 600 to reduce deformation in the post-bonded wafer pair. Generally, method 600 may include (in step 610) receiving an upper wafer on the mounting surface of an upper chuck of a wafer bonding apparatus, and (in step 620) supplying vacuum pressure to the upper chuck to hold the upper wafer on the mounting surface of the upper chuck. Method 600 may also include (in step 630) receiving a lower wafer on the mounting surface of a lower chuck of a wafer bonding apparatus, and (in step 640) supplying vacuum pressure to the lower chuck to hold the lower wafer on the mounting surface of the lower chuck. Method 600 may also include (in step 650) bonding the upper wafer to the lower wafer to form a post-bonded wafer pair.

[0094] In some embodiments of the method 600 described herein, the lower chuck includes a central region and a peripheral region extending radially outward from the edge of the central region to the outer edge of the lower chuck. The mounting surface of the lower chuck is flat in the central region and linearly upward or downward in the peripheral region, such that the height of the lower chuck in the peripheral region differs from its height in the central region. When (in step 640) a vacuum pressure is supplied to the lower chuck, the vacuum pressure causes the lower wafer to adhere to the mounting surface of the lower chuck, such that (in step 650) before the upper wafer is bonded to the lower wafer, the lower wafer is pre-deformed according to the shape of the mounting surface of the lower chuck. This pre-deformation of the lower wafer before bonding advantageously reduces deformation during wafer alignment after bonding and minimizes lateral misalignment between the upper and lower wafers.

[0095] In some embodiments of the method 600 described herein, the upper chuck includes a central region and a peripheral region extending radially outward from the edge of the central region to the outer edge of the upper chuck. The mounting surface of the upper chuck is flat in the central region and linearly upward or downward in the peripheral region, such that the height of the upper chuck in the peripheral region differs from its height in the central region. When (in step 640) a vacuum pressure is supplied to the upper chuck, the vacuum pressure causes the upper wafer to adhere to the mounting surface of the upper chuck, such that (in step 650) before the lower wafer is bonded to the upper wafer, the upper wafer is pre-deformed according to the shape of the upper chuck mounting surface. This pre-deformation of the upper wafer before bonding advantageously reduces deformation during wafer alignment after bonding and minimizes lateral misalignment between the upper and lower wafers.

[0096] In some embodiments, the bonding step (i.e., step 650) may include: applying mechanical force to the center of the upper wafer to initiate bonding between the upper and lower wafers; removing the vacuum pressure supplied to the upper chuck before completing bonding between the upper and lower wafers; and removing the vacuum pressure supplied to the lower chuck after completing bonding between the upper and lower wafers to form a bonded wafer pair. In such an embodiment, the bonded wafer pair may remain on the lower chuck until the vacuum pressure supplied to the lower chuck is removed.

[0097] In other embodiments, the bonding step (i.e., step 650) may include: applying a mechanical force to the center of the upper wafer to initiate bonding between the upper and lower wafers; removing the vacuum pressure supplied to the lower chuck before completing bonding between the upper and lower wafers; and removing the vacuum pressure supplied to the upper chuck after completing bonding between the upper and lower wafers to form a bonded wafer pair. In such an embodiment, the bonding force between the upper and lower wafers may cause the vacuum seal between the lower wafer and the lower chuck to break before the vacuum pressure supplied to the lower chuck is removed. When this occurs, the bonding force between the upper and lower wafers may resist gravity to bond the lower wafer to the upper wafer, and the upper wafer may be held on the upper chuck. In such an embodiment, the bonded wafer pair may remain on the upper chuck until the vacuum pressure supplied to the upper chuck is removed.

[0098] It should be noted that the reference to "an embodiment" or "one embodiment" throughout this specification means that a particular feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention, but does not mean that it exists in every embodiment. Therefore, the phrases "in one embodiment" or "in one embodiment" appearing in various places throughout the specification do not necessarily refer to the same embodiment of the invention. Furthermore, a particular feature, structure, material, or characteristic may be combined in any suitable manner in one or more embodiments. In other embodiments, numerous additional coatings and / or structures may be included, and / or the described features may be omitted.

[0099] As used herein, the term "substrate" means and includes the base material or structure on which materials are formed. It will be appreciated that a substrate may include a single material, multiple layers of different materials, one or more layers having different material regions or different structures, etc. These materials may include semiconductors, insulators, conductors, or combinations thereof. For example, a substrate may be a semiconductor substrate, a base semiconductor layer on a support structure, a metal electrode, or a semiconductor substrate on which one or more cladding layers, structures, or regions are formed. The substrate may be a conventional silicon substrate or a host substrate comprising layers of semiconductor material. As used herein, the term "host substrate" means and includes not only silicon wafers, but also silicon-insulator ("SOI") substrates (e.g., silicon-sapphire ("SOS") substrates and silicon-glass ("SOG") substrates), silicon epitaxial layers on a base semiconductor substrate, and other semiconductor or optoelectronic materials, such as silicon germanium, germanium, gallium arsenide, gallium nitride, and indium phosphide. The substrate may be doped or undoped.

[0100] Systems and methods for processing substrates are described in numerous embodiments. A substrate may include any material portion or structure of an element, particularly a semiconductor or other electronic element, and may be, for example, a base substrate structure, such as a semiconductor substrate, or a cladding or overlay layer, such as a thin film, on a base substrate structure. Therefore, the term "substrate" is not intended to be limited to any particular base structure, underlay or overlay layer, patterned or unpatterned, but is intended to include any such cladding or base structure, and any combination of cladding and / or base structures. In some cases, the term "substrate" may be used to describe, for example, patterned or unpatterned element wafers or carrier wafers.

[0101] Those skilled in the art will recognize that various embodiments may be implemented without one or more specific details, or by using other alternatives and / or additional methods, materials, or components. In other instances, well-known structures, materials, or operations have not been shown or described in detail so as not to obscure the various embodiments of the invention. Similarly, specific figures, materials, and configurations are set forth for illustrative purposes to provide a thorough understanding of the invention. However, the invention may be implemented without specific details. Furthermore, it should be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

[0102] In view of this description, further modifications and alternative embodiments of the systems and methods described will be apparent to those skilled in the art. Therefore, it will be appreciated that the systems and methods described are not limited to these exemplary embodiments. It should be understood that the forms of the systems and methods shown and described herein are to be considered exemplary embodiments. Numerous changes may be made in implementation. Therefore, although the invention has been described herein with reference to specific embodiments, numerous modifications and changes may be made without departing from the scope of the invention. Therefore, the specification and drawings are to be considered illustrative rather than restrictive, and such modifications are intended to be included within the scope of the invention. Furthermore, any benefits, advantages, or solutions to problems described herein with respect to specific embodiments are not intended to be construed as critical, essential, or fundamental features or elements of any or all claims. [Simplified Explanation of the Diagram]

[0025] A more complete understanding of the invention and its advantages can be obtained by referring to the following description, taken in conjunction with the accompanying drawings, wherein similar reference numerals indicate similar features. However, it should be noted that the drawings illustrate only exemplary embodiments of the disclosed concepts and should therefore not be considered as limiting the scope, as the disclosed concepts may allow for other equally effective embodiments.

[0026] Figures 1A-1C (Prior Art) illustrate a conventional wafer bonding process for bonding two wafers together.

[0027] Figure 2 (Prior Art) illustrates the deformation of the wafer pair after bonding as a function of radius (r) when two wafers are bonded together using conventional wafer bonding equipment and processes.

[0028] Figure 3 is a side view of an exemplary wafer bonding apparatus in which the techniques described herein can be used.

[0029] Figures 4A-4C are cross-sectional views through the upper and lower chuck assemblies shown in Figure 3, illustrating a number of wafer bonding process steps that can be performed by the exemplary wafer bonding equipment.

[0030] Figure 5 is a top view of a wafer chuck, in which the techniques described herein can be used.

[0031] FIG6A is a side view of a variable height wafer chuck according to a first embodiment of the present disclosure.

[0032] FIG6B is a side view of a variable height wafer chuck according to a second embodiment of the present disclosure.

[0033] FIG7A is a side view of a variable height wafer chuck according to a third embodiment of the present disclosure.

[0034] FIG7B is a side view of a variable height wafer chuck according to a fourth embodiment of the present disclosure.

[0035] Figures 8A-8E illustrate an improved wafer bonding process, which uses a variable height wafer chuck as shown in 6A or 6B to bond two wafers together.

[0036] Figure 9 illustrates the deformation of the wafer pair after bonding as a function of radius (r) when two wafers are bonded together using the improved wafer bonding process shown in Figures 8A-8E.

[0037] FIG10 is a side view of a wafer chuck according to a fifth embodiment of the present disclosure, the wafer chuck having temperature regulation settings in one or more areas of the wafer chuck.

[0038] Figure 11 illustrates the wafer temperature as a function of radius (r) when two wafers are joined together using the wafer chuck shown in Figure 10 in the wafer bonding process shown in Figures 8A-8E.

[0039] Figure 12 is a flowchart illustrating an embodiment of a method for reducing wafer alignment deformation after bonding.

Claims

1. A wafer bonding apparatus, comprising: A first chuck, the first chuck including a first mounting surface configured to hold a first wafer; And a second chuck, the second chuck including a second mounting surface configured to hold a second wafer, wherein the second chuck further includes a central region, a peripheral region extending radially outward from an edge of the central region to an outer edge of the second chuck, and a plurality of radial ribs extending vertically from a planar upper surface of the second chuck to provide the second mounting surface; wherein the second mounting surface is flat in the central region and linearly upward or downward in the peripheral region, such that the height of the second chuck in the peripheral region is different from the height of the second chuck in the central region.

2. The wafer bonding equipment as described in claim 1, wherein, The second mounting surface slopes linearly downward in the peripheral region, and the height of the second chuck in the peripheral region is less than the height of the second chuck in the central region.

3. The wafer bonding equipment as described in claim 2, wherein, The height difference between the edge of the central region and the outer edge of the second chuck is selected from a range including -1 μm to -5 μm.

4. The wafer bonding equipment as described in claim 3, wherein, The plurality of radial ribs includes: a first subset of radial ribs located in the central region; and a second subset of radial ribs located in the peripheral region, wherein the height difference is provided by configuring the plurality of radial ribs such that the height of the second subset of radial ribs is lower than the height of the first subset of radial ribs.

5. The wafer bonding apparatus as claimed in claim 1, wherein, The second mounting surface is linearly upward in the peripheral region, and the height of the second chuck in the peripheral region is greater than the height of the second chuck in the central region.

6. The wafer bonding apparatus as claimed in claim 5, wherein, The height difference between the edge of the central region and the outer edge of the second chuck is selected from a range including +1μm to +5μm.

7. The wafer bonding apparatus as claimed in claim 6, wherein, The plurality of radial ribs includes: a first subset of radial ribs located in the central region; and a second subset of radial ribs located in the peripheral region, wherein the height difference is provided by configuring the plurality of radial ribs such that the height of the second subset of radial ribs is greater than the height of the first subset of radial ribs.

8. A wafer bonding apparatus, comprising: A first chuck, the first chuck including a first mounting surface configured to hold a first wafer; The second chuck includes a second mounting surface configured to hold a second wafer, wherein the second chuck further includes: a planar upper surface comprising a plurality of radial regions extending radially outward from the center of the second chuck to an outer edge of the second chuck, wherein a first subset of the plurality of radial regions is located within the central region of the second chuck, and wherein a second subset of the plurality of radial regions is located within a peripheral region of the second chuck, the peripheral region extending radially outward from an edge of the central region to an outer edge of the second chuck; and a plurality of radial ribs, each radial rib being located at a boundary between two of the plurality of radial regions and extending perpendicularly from the planar upper surface of the second chuck to provide the second mounting surface, wherein a first subset of the plurality of radial ribs is located within the first subset of the plurality of radial regions; a second subset of the plurality of radial ribs is located within the second subset of the plurality of radial regions; and wherein the height of the second subset of the plurality of radial ribs is different from the height of the first subset of the plurality of radial ribs.

9. The wafer bonding apparatus as claimed in claim 8, wherein, The second mounting surface is flat or planar within the first subset of the plurality of radial ribs, and wherein the second mounting surface is linearly upward or linearly downward within the second subset of the plurality of radial ribs.

10. The wafer bonding apparatus as claimed in claim 9, wherein, The second mounting surface is linearly downward sloped or has a downward angle within the second subset of the plurality of radial ribs, such that the height of the second chuck at the outer edge of the second chuck is less than the height of the second chuck in the central region.

11. The wafer bonding apparatus as claimed in claim 10, wherein, A height reduction in the peripheral region of the second chuck causes the peripheral portion of the second wafer to bend in a downward direction before bonding, and wherein the peripheral portion of the second wafer bends in an upward direction when bonding between the first wafer and the second wafer is completed to provide an inward deformation that reduces lateral misalignment between the first wafer and the second wafer.

12. The wafer bonding apparatus as claimed in claim 9, wherein, The second mounting surface is linearly inclined or has an upward angle within the second subset of the plurality of radial ribs, such that the height of the second chuck at the outer edge of the second chuck is greater than the height of the second chuck in the central region.

13. The wafer bonding apparatus as claimed in claim 12, wherein, An increase in height in the peripheral region of the second chuck causes the peripheral portion of the second wafer to bend in an upward direction before bonding, and wherein, when bonding between the first wafer and the second wafer is completed, the peripheral portion of the second wafer bends in a downward direction to provide an outward deformation that reduces lateral misalignment between the first wafer and the second wafer.

14. The wafer bonding apparatus as claimed in claim 8, wherein, At least one of the first chuck and the second chuck includes one or more temperature regulating elements embedded in one or more radial regions of the plurality of radial regions.

15. The wafer bonding apparatus as claimed in claim 14, wherein, The one or more temperature regulating elements are configured to transfer heat to a region of the first wafer and / or the second wafer, the region being located above the one or more radial regions, which causes the region of the first wafer and / or the second wafer to expand laterally and produce an outward deformation that reduces lateral misalignment between the first wafer and the second wafer.

16. The wafer bonding apparatus as claimed in claim 14, wherein, The one or more temperature regulating elements are configured to transfer heat from a region of the first wafer and / or the second wafer, the region being located above the one or more radial regions, which causes the region of the first wafer and / or the second wafer to laterally contract and produce an inward deformation, the inward deformation reducing the lateral misalignment between the first wafer and the second wafer.

17. A method for reducing wafer deformation after bonding, comprising: An upper wafer is received on a mounting surface of an upper chuck of a wafer bonding apparatus; a vacuum pressure is supplied to the upper chuck to hold the upper wafer on the mounting surface of the upper chuck; a lower wafer is received on a mounting surface of a lower chuck of the wafer bonding apparatus, wherein the lower chuck includes: a planar upper surface; a central region; a peripheral region extending radially outward from an edge of the central region to an outer edge of the lower chuck; and a plurality of radial ribs extending perpendicularly from the planar upper surface of the lower chuck to provide the mounting surface of the lower chuck, wherein the plurality of radial ribs includes: a first subset of radial ribs located in the central region; and a second subset of radial ribs located in the peripheral region, and wherein by configuring the plurality of radial ribs, the mounting surface of the lower chuck is flat in the central region and linearly upward or downward in the peripheral region, such that the height of the second subset of radial ribs in the peripheral region is different from the height of the first subset of radial ribs in the central region; Vacuum pressure is supplied to the lower chuck to hold the lower wafer on the mounting surface of the lower chuck, wherein the vacuum pressure supplied to the lower chuck causes the lower wafer to adhere to the mounting surface of the lower chuck, such that the lower wafer is pre-deformed according to the shape of the mounting surface of the lower chuck before the upper wafer is bonded to the lower wafer; and the upper wafer is bonded to the lower wafer to form a bonded wafer pair, wherein the pre-deformation of the lower wafer before bonding reduces deformation in the bonded wafer pair and minimizes lateral misalignment between the upper wafer and the lower wafer.

18. The method for reducing wafer deformation after bonding, as described in claim 17, wherein, The bonding process includes: applying a mechanical force to the center of the upper wafer to initiate bonding between the upper wafer and the lower wafer; after initiating bonding, but before bonding between the upper and lower wafers is complete, removing the vacuum pressure supplied to the upper chuck; and after bonding between the upper and lower wafers is complete, removing the vacuum pressure supplied to the lower chuck to form the bonded wafer pair, wherein the bonded wafer pair remains on the lower chuck until the vacuum pressure supplied to the lower chuck is removed.

19. The method for reducing wafer deformation after bonding, as described in claim 17, wherein, The bonding process includes: applying a mechanical force to the center of the upper wafer to initiate bonding between the upper wafer and the lower wafer; removing the vacuum pressure supplied to the lower chuck after initiating bonding but before the bonding of the upper wafer and the lower wafer is completed; and removing the vacuum pressure supplied to the upper chuck after the bonding of the upper wafer and the lower wafer is completed to form the bonded wafer pair, wherein the bonded wafer pair remains on the upper chuck until the vacuum pressure supplied to the upper chuck is removed.

20. The method for reducing wafer deformation after bonding, as described in claim 19, wherein, Before the bonding is completed and / or before the vacuum pressure supplied to the upper chuck is removed, the bonding force between the upper wafer and the lower wafer causes the vacuum seal between the lower wafer and the lower chuck to break.

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