Wafer bonding apparatus and method for reducing post-bonding wafer distortion - Patents.com

Abstract

Various embodiments of wafer bonding apparatus and methods for reducing distortion of a bonded wafer pair are described herein. More specifically, the present disclosure provides embodiments of wafer bonding apparatus and methods for reducing post-bond wafer distortion that occurs primarily within the center and / or edge of a bonded wafer pair. In the present disclosure, post-bond wafer distortion is reduced by correcting variations in pre-bond wafer shape. The variations in pre-bond wafer shape are corrected or compensated for by applying hardware modifications to the wafer chuck. Such modifications may include, but are not limited to, modifying the surface height and / or temperature of the wafer chuck. Although hardware modifications are disclosed herein for reducing post-bond wafer distortion near the center and / or edge of a bonded wafer pair, similar modifications can be applied to reduce post-bond wafer distortion in other areas or zones of a bonded wafer pair.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Non-provisional Patent Application No. 17 / 236,785, filed April 21, 2021, the entirety of which is incorporated by reference herein. [Background technology]

[0002] The present disclosure relates to substrate processing. In particular, it provides an improved chuck design and wafer bonding process that reduces post-bonding wafer distortion.

[0003] A wafer bonding machine (or "wafer bonder") physically bonds two wafers (a top wafer and a bottom wafer) together through an interfacial bond. The lateral misalignment along the bond interface is called the post-bond distortion error. The post-bond distortion error is a function of the pre-bond wafer shape and the wafer bonding process (e.g., tool recipe) and is often used to judge the performance of a wafer bonding machine.

[0004] There are two main types of wafers used in the wafer bonding process: device / device pairs and device / carrier pairs. Device wafers are patterned wafers with nanoscale electrical circuits created on their surface. Carrier wafers are usually blank wafers without any patterns on their surface. For device / device wafer pairs, the absolute lateral misalignment between the two wafers anywhere along the bond interface is important, which can typically be around 100 nm in state-of-the-art technology. In contrast, for device / carrier wafer pairs, the relative lateral misalignment is more important. Since the carrier wafer has no patterns, the absolute lateral misalignment between the two bonded wafers is influenced by the ability of the lithography tool to pattern the carrier wafer in subsequent processing steps. This misalignment can be at least partially corrected by the lithography tool. Therefore, the relative lateral misalignment of a device / carrier wafer pair is the absolute misalignment minus the correction that can be applied in subsequent processing steps. Depending on the correction capabilities of subsequent processes, the relative lateral misalignment between device / carrier wafer pairs can be less than 10 nm in the state of the art. The absolute and relative lateral misalignments are referred to as "distortion" in the following paragraphs.

[0005] 1A-1C (PRIOR ART) illustrate a conventional wafer bonding apparatus 10 and process that may be used to bond two wafers together. The wafer bonding apparatus 10 includes an upper chuck 12 and a lower chuck 14 that hold an upper wafer 16 and a lower wafer 18 in place, respectively. In some cases, vacuum pressure may be used to hold the wafers on the chucks. As shown in FIG. 1A, the two wafers are initially separated by a small gap (g). The centers of the top and bottom wafers are then forced into contact, possibly assisted by a mechanical force applied to the center of the top wafer 16. For example, as shown in FIG. 1B, a striker pin (or bonding pin) 19 may be used to apply a mechanical force to the center of the top wafer 16, which causes the top wafer to bend or flex until the center of the top wafer 16 contacts the center of the bottom wafer 18. This contact initiates a bond front that spontaneously propagates radially outward from the centers of the top and bottom wafers. As the contact surface area increases, the top and bottom wafers elastically deform to close the initial gap (g) between the wafers. The vacuum pressure to the upper chuck 12 is turned off. When the contact surface area becomes large enough, either due to wafer deformation or due to a predefined recipe setting, the vacuum seal between the upper chuck 12 and the upper wafer 16 breaks. The bond front continues to propagate outward until the two wafers are fully bonded. Once bonding is complete, the striker pins 19 retract and the vacuum pressure to the lower chuck 14 is turned off, as shown in FIG. 1C. The bonded wafer pair 16 / 18 can then be released from the wafer bonding apparatus 10 and sent to a metrology tool for distortion characterization and / or continued to the next processing step.

[0006] In some cases, the conventional wafer bonding apparatus 10 and process shown in Figures 1A-1C can result in poor post-bond wafer distortion near the center and / or edge (e.g., within the outermost 15 mm) of the bonded wafer pair 16 / 18. Specifically, Figure 1C illustrates an upward bending of the bonded wafer pair 16 / 18 near the edge or periphery of the bonded wafer pair. The upward bending shown in Figure 1C is an example of a post-bond wafer distortion (e.g., outward distortion) that can occur after bonding two wafers together using a conventional wafer bonding process. Distortion can also occur in other areas of the bonded wafer pair (e.g., near the center) when using a conventional wafer bonding process. Regardless of the location, the post-bond wafer distortion causes a lateral shift (or lateral misalignment) between the two wafers at the area of ​​the distortion.

[0007] The post-bond wafer distortion shown in FIG. 1C can be attributed to many reasons. First, pre-bond wafers usually have different specifications near the edge. For example, some wafers may have tapered edges or have a thinner thickness near the edge or periphery of the wafer (e.g., the outermost 3 mm). This can lead to differences in the mechanical performance of the wafer near the edge / edge (compared to the rest of the wafer), which can lead to poor distortion of the post-bond wafer pair. In addition, pre-bond wafers may be processed differently in other processing steps (e.g., different patterning arrangements) to improve device yield. Process differences often lead to different pre-bond wafer shapes near the edge or periphery. Variations in pre-bond wafer shape are another source of post-bond wafer distortion. Furthermore, the wafer bonding process can be considered as a two-step process before and after the chuck vacuum seal is broken. If the vacuum seal between the chuck and the wafer is broken, there is less distortion control from the wafer bonding apparatus, which can result in distortion defects at the edges.

[0008] FIG. 2 is a graph illustrating the distortion of a bonded wafer pair (i.e., post-bond wafer distortion) as a function of radius (r) when two wafers are bonded together using conventional wafer bonding equipment and processes, e.g., as discussed above with reference to FIGS. 1A-1C. In the graph shown in FIG. 2, the distortion 20 increases significantly near the edge of the bonded wafer pair. In some wafer bonding processes, the distortion 20 near the edge of the bonded wafer pair can be up to, e.g., 20 nm, worse than the distortion 20 across the remainder of the wafer pair. It is estimated that when the distortion at the wafer edge is improved, the post-bond distortion error can improve by, e.g., 15% or more. Summary of the Invention [Means for solving the problem]

[0009] Various embodiments of wafer bonding apparatus and methods are described herein for reducing the distortion of a bonded wafer pair. Often, as shown in FIG. 2 and discussed above, the post-bond wafer distortion may be worse near the center and / or near the edge of the bonded wafer pair. For this reason, the present disclosure provides embodiments of wafer bonding apparatus and methods for reducing the post-bond wafer distortion that occurs primarily within the center and / or edge of the bonded wafer pair. In the present disclosure, the post-bond wafer distortion is reduced by correcting the variations in the pre-bond wafer shape. The variations in the pre-bond wafer shape are corrected or compensated for by applying hardware modifications to the wafer chuck. Such modifications may include, but are not limited to, modifying the surface height and / or temperature of the wafer chuck. Although hardware modifications are disclosed herein for reducing the post-bond wafer distortion near the center and / or edge of the bonded wafer pair, similar modifications can be applied to reduce the post-bond wafer distortion in other areas or zones of the bonded wafer pair.

[0010] According to one embodiment, a wafer bonding apparatus may generally include a first chuck with a first mounting surface configured to hold a first wafer and a second chuck with a second mounting surface configured to hold a second wafer. In this embodiment, the second chuck may include 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 an upper surface of the second chuck to provide the second mounting surface. The second mounting surface is flat in the central region and linearly slopes or linearly slopes 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.

[0011] In some embodiments, the second mounting surface may be linearly downwardly sloping in the peripheral region, such that the height of the second chuck in the peripheral region is lower than the height of the second chuck in the central region. In some embodiments, 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, for example, -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 is provided by reducing the height of one or more radial ribs located in the peripheral region.

[0012] In some embodiments, the second mounting surface may be linearly tapered in the peripheral region, such that the height of the second chuck in the peripheral region is higher than the height of the second chuck in the central region. In some embodiments, 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, for example, +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 is provided by increasing the height of one or more radial ribs located in the peripheral region.

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

[0014] In some embodiments, the second chuck may further include a plurality of radial ribs, each located at a boundary between two of the radial zones and extending perpendicularly from a top 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 zones. A second subset of the plurality of radial ribs may be located within a second subset of the plurality of radial zones. A height of the second subset of the plurality of radial ribs may differ from a height of the first subset of the plurality of radial ribs.

[0015] In some embodiments, the second mounting surface may be flat or planar across a first subset of the plurality of radial ribs and linearly sloped or linearly sloping across a second subset of the plurality of radial ribs.

[0016] In one embodiment, for example, the second mounting surface may be linearly sloped or angled downward across 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. In such an embodiment, the reduced height in the peripheral region of the second chuck may cause the peripheral portion of the second wafer to bend downward prior to bonding. When bonding between the first and second wafers is completed, the peripheral portion of the second wafer may bend upward to provide an inward deflection, thereby reducing lateral misalignment between the first and second wafers.

[0017] In another embodiment, the second mounting surface may be linearly inclined or angled upward across 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. In such an embodiment, the increased height in the peripheral region of the second chuck may cause the peripheral portion of the second wafer to bend upward prior to bonding. When bonding between the first and second wafers is completed, the peripheral portion of the second wafer may bend downward to provide an outward deflection, thereby reducing 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 adjustment elements embedded within one or more of the radial zones. In some embodiments, the one or more temperature adjustment elements may be configured to transfer heat to an area of ​​the first and / or second wafer overlapping the one or more radial zones, causing the area of ​​the first and / or second wafer to expand laterally and generate an outward distortion that reduces the lateral misalignment between the first and second wafers. In other embodiments, the one or more temperature adjustment elements may be configured to transfer heat from an area of ​​the first and / or second wafer overlapping the one or more radial zones, causing the area of ​​the first and / or second wafer to contract laterally and generate an inward distortion that reduces the lateral misalignment between the first and second wafers.

[0019] According to another embodiment, a method for reducing distortion of a bonded wafer pair is provided. The method may generally include receiving an upper wafer on a mounting surface of an upper chuck of a wafer bonding apparatus and applying a vacuum pressure to the upper chuck to hold the upper wafer on the mounting surface of the upper chuck. The method may further include receiving a lower wafer on a mounting surface of a lower chuck of the wafer bonding apparatus and applying a 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 bonded wafer pair.

[0020] In some embodiments of the disclosed method, the lower chuck may comprise a central region and a peripheral region extending radially outward from an edge of the central region to an outer edge of the lower chuck. The mounting surface of the lower chuck may be flat in the central region and linearly sloped or linearly down-sloped in the peripheral region, such that the height of the lower chuck in the peripheral region is different from the height of the lower chuck in the central region. When vacuum pressure is applied to the lower chuck, the vacuum pressure applied to the lower chuck causes the lower wafer to adhere to the mounting surface of the lower chuck, and thus the shape of the mounting surface of the lower chuck pre-distorts the lower wafer before the upper wafer is bonded to the lower wafer. Pre-distorting the lower wafer before bonding advantageously reduces distortion of the bonded wafer pair and minimizes lateral misalignment between the upper and lower wafers.

[0021] In some embodiments of the disclosed method, the upper chuck may comprise a central region and a peripheral region extending radially outward from an edge of the central region to an outer edge of the upper chuck. The mounting surface of the upper chuck may be flat in the central region and linearly sloped or linearly down-sloped in the peripheral region, such that the height of the upper chuck in the peripheral region is different from the height of the upper chuck in the central region. When a vacuum pressure is applied to the upper chuck, the vacuum pressure applied to the upper chuck causes the upper wafer to adhere to the mounting surface of the upper chuck, and thus the shape of the mounting surface of the upper chuck pre-distorts the lower wafer before the lower wafer is bonded to the upper wafer. Pre-distorting the upper wafer before bonding advantageously reduces distortion of the bonded wafer pair and minimizes lateral misalignment between the upper and lower wafers.

[0022] In some embodiments, the bonding process may include applying a mechanical force to a center of the upper wafer to initiate bonding between the upper and lower wafers, removing the vacuum pressure supplied to the upper chuck before bonding between the upper and lower wafers is completed, and removing the vacuum pressure supplied to the lower chuck after bonding between the upper and lower wafers is completed to form a bonded wafer pair. In such embodiments, 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 process may include applying a mechanical force to a 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. In such embodiments, the bonding force between the upper and lower wafers breaks the vacuum seal between the lower wafer and the lower chuck before bonding is completed and / or before the vacuum pressure supplied to the upper chuck is removed. In such embodiments, 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 the related systems and methods may be utilized as well.

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

[0026] [Figure 1A] 1 (PRIOR ART) illustrates a conventional wafer bonding process for bonding two wafers together. [Figure 1B] 1 (PRIOR ART) illustrates a conventional wafer bonding process for bonding two wafers together. [Figure 1C] 1 (PRIOR ART) illustrates a conventional wafer bonding process for bonding two wafers together. [Diagram 2] 1 (PRIOR ART) is a graph illustrating distortion of a post-bonded wafer pair as a function of radius (r) when two wafers are bonded together using conventional wafer bonding equipment and processes. [Diagram 3] FIG. 1 is a side view of an exemplary wafer bonding apparatus that can utilize the techniques described herein. [Figure 4A-4C] 4A-4D are cross-sectional views through the upper chuck and lower chuck subassemblies shown in FIG. 3 illustrating various wafer bonding process steps that may be performed by an exemplary wafer bonding apparatus. [Diagram 5] FIG. 1 is a plan view of a wafer chuck that can utilize the techniques described herein. [Figure 6A] FIG. 2 is a side view of a variable height wafer chuck according to a first embodiment of the present disclosure. [Figure 6B] FIG. 13 is a side view of a variable height wafer chuck according to a second embodiment of the present disclosure. [Figure 7A] FIG. 13 is a side view of a variable height wafer chuck according to a third embodiment of the present disclosure. [Figure 7B] FIG. 13 is a side view of a variable height wafer chuck according to a fourth embodiment of the present disclosure. [Figure 8A] 6A or 6B illustrate an improved wafer bonding process for bonding two wafers together using a variable height wafer chuck as shown in FIG. 6A or 6B. [Figure 8B] 6A or 6B illustrate an improved wafer bonding process for bonding two wafers together using a variable height wafer chuck as shown in FIG. 6A or 6B. [Figure 8C] 6A or 6B illustrate an improved wafer bonding process for bonding two wafers together using a variable height wafer chuck as shown in FIG. 6A or 6B. [Figure 8D] 6A or 6B illustrate an improved wafer bonding process for bonding two wafers together using a variable height wafer chuck as shown in FIG. 6A or 6B. [Figure 8E] 6A or 6B illustrate an improved wafer bonding process for bonding two wafers together using a variable height wafer chuck as shown in FIG. 6A or 6B. [Figure 9] 8A-8E are graphs illustrating distortion of a bonded wafer pair as a function of radius (r) when the two wafers are bonded together using the improved wafer bonding process shown in FIGS. 8A-8E. [Figure 10] FIG. 13 is a side view of a wafer chuck having a temperature adjustment means within one or more zones of the wafer chuck in accordance with a fifth embodiment of the present disclosure. [Figure 11] 11 is a graph illustrating wafer temperature as a function of radius (r) when the wafer chuck shown in FIG. 10 is used in the wafer bonding process shown in FIGS. 8A-8E to bond two wafers together. [Figure 12] FIG. 1 is a flow chart diagram illustrating one embodiment of a method for reducing distortion in a bonded wafer pair. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Silicon wafers are typically processed in lots containing, for example, up to 25 wafers. Wafers typically vary from lot to lot due to processing conditions, inherent wafer deformations, and other less obvious factors. This variation can be local, meaning that one standard deviation of shape variation ranges from 1 μm up to 5 μm for logic devices, meaning that the average surface map of one lot differs from another lot. The variation can also be global, meaning that peak-to-valley variations can be on the order of 10 μm from lot to lot. Lot-to-lot wafer variation is a major contributor to post-bond wafer distortion. In the present disclosure, lot-to-lot wafer variation is minimized by correcting for pre-bond wafer shape variations.

[0028] The variation in pre-bond wafer shape directly correlates to the in-plane distortion or lateral misalignment created between the two wafers of the bonded wafer pair. In one example, a pre-bond wafer shape variation of 1 μm can result in a distortion of about 5 nm. The amount of distortion of the bonded wafer pair (i.e., post-bond distortion error) is an important metric that is often used to judge the performance of wafer bonding equipment and processes. Therefore, it is generally desirable to reduce the amount of distortion of the bonded wafer pair.

[0029] The present disclosure provides various embodiments of wafer bonding apparatus and methods for reducing distortion of bonded wafer pairs. In the disclosed embodiments, post-bond wafer distortion is reduced by compensating for variations in pre-bond wafer shape. Various hardware improvements are used herein to compensate for variations in pre-bond wafer shape. The design of the hardware is driven by physical modeling of the specific wafer bonding process. The physical model provides guidelines for unique hardware design at specific locations (e.g., near the edge and / or center of the wafers) to counteract poorly distorted areas in the bonded wafer pairs.

[0030] In many cases, as shown in FIG. 2 and discussed above, post-bond wafer distortion may be worse near the center of the wafer (e.g., due to striker bond pin design) and / or near the edge of the wafer. For this reason, the present disclosure provides various embodiments of wafer bonding apparatus and methods to reduce post-bond wafer distortion that occurs primarily within the center and / or edge of the wafer. Such embodiments may include, but are not limited to, modifying the surface height and / or temperature of the wafer chuck. Although hardware modifications are disclosed herein to reduce post-bond wafer distortion near the center and / or edge of the bonded wafer pair, one of ordinary skill in the art will recognize how similar modifications can be made to reduce post-bond wafer distortion in other areas or zones of the bonded wafer pair.

[0031] 3 illustrates an example of a wafer bonding apparatus 100 that may utilize the techniques described herein. In some embodiments, the wafer bonding apparatus 100 shown in FIG. 3 may be a wafer bonding module included within a wafer bonding system or another substrate processing system. In other embodiments, the wafer bonding apparatus 100 shown in FIG. 3 may be a stand-alone module configured to bond two wafers together. While the exemplary wafer bonding apparatus 100 is shown for illustrative purposes, it will be appreciated that the techniques described herein are not limited to the particular configuration shown in FIG. 3.

[0032] 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 (i.e., a device wafer), such as a silicon wafer or a compound semiconductor wafer, on which a plurality of electronic circuits are formed, and the second substrate W2 may be a bare wafer (i.e., a carrier wafer) on which no electronic circuits are formed. 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 alternatively referred to as the "upper wafer" and the second wafer W2 may be alternatively referred to as the "lower wafer". When the first wafer W1 is bonded to the second wafer W2, the bonded substrates may be referred to as a "bonded wafer pair".

[0033] As shown in FIG. 3, the wafer bonding apparatus 100 includes a process chamber 105 for bonding a first wafer W1 to a second wafer W2. In some embodiments, the first and second wafers can be delivered to the process chamber 105 by a process arm 110, which can be configured to transfer the wafers to / from the process chamber 105 through an opening 115 in a sidewall of the process chamber. To transfer the wafers to / from the process chamber 105, the process arm 110 can be configured to move vertically (Z direction) and horizontally (X direction and Y direction). In some cases, the process arm 110 can also be pivotable about a vertical axis to transfer the first wafer W1 and the second wafer W2 to the process chamber 105 and transfer the bonded wafer pair out of the process chamber 105.

[0034] The processing chamber 105 generally 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 may be coupled to the ceiling of the processing chamber 105 by a number of posts 132, as shown in FIG. 3. As described in more detail below with reference to FIGS. 4A-4C, holes 134 are formed in and extend vertically through the support member 130 and the upper chuck 120. The holes 134 are positioned to correspond to a central region or zone of the upper wafer W1 held by the upper chuck 120. Bonding pins of a striker 140 are inserted through the holes 134 and are used to apply mechanical force to the upper wafer W1 during the wafer bonding process.

[0035] In the wafer bonding apparatus 100 shown in FIG. 3, the lower chuck 125 is supported by a first lower chuck mover 135 and a second lower chuck mover 150 provided below the lower chuck 125 to translate or move the lower chuck 125 in a horizontal direction. For example, the bottom surface of the first lower chuck mover 135 is coupled to a first pair of rails 137 attached to an upper surface of the second lower chuck mover 150 so as to extend in a first horizontal direction (e.g., the X-axis direction). Similarly, the bottom surface of the second lower chuck mover 150 is coupled to a second pair of rails 152 attached to a mounting table 155 so as to extend in a second horizontal direction (e.g., the Y-axis direction). In the embodiment shown in FIG. 3, the mounting table 155 is coupled to the bottom of the processing chamber 105.

[0036] 3, the first lower chuck mover 135 is configured to move along a first pair of rails 137 to translate the lower chuck 125 (and lower wafer W2) in the X-direction. Similarly, the second lower chuck mover 150 is configured to move along a second pair of rails 152 to translate the lower chuck 125 (and lower wafer W2) in the Y-direction. After the upper and lower wafers W1 and W2 are provided to the upper and lower chucks, the first lower chuck mover 135 and the second lower chuck mover 150 can be used to translate the lower chuck 125 (and lower wafer W2) in the X-direction and / or Y-direction until the lower wafer W2 is aligned with the upper wafer W1. In some embodiments, the first lower chuck mover 135 and the second lower chuck mover 150 may also be used to translate the lower chuck 125 in the Z direction.

[0037] In some embodiments, the upper chuck 120 and the lower chuck 125 may be vacuum chucks. Once the upper wafer W1 and the lower wafer W2 are transferred to the process 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 FIGS. 4A-4C, the upper chuck 120 and the lower chuck 125 may each have a mounting surface with a number of holes through which vacuum pressure is created between the mounting surface of the chuck and the backside of the wafer. When a wafer is placed on the vacuum chuck and the vacuum pump 160 is turned on, the vacuum pressure provided by the vacuum pump 160 creates a vacuum seal between the backside of the wafer and the mounting surface of the chuck to hold the wafer on the chuck. Although described in the context of a vacuum chuck, 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 FIG. 3 while utilizing the techniques described herein.

[0038] 4A to 4C are cross-sectional views through the subassemblies of the upper chuck and the lower chuck shown in Fig. 3. As shown in Fig. 4A, a striker 140 provided in the upper chuck subassembly (e.g., including the upper chuck 120 and the support member 130) is equipped with a joint pin 144, an actuator unit 142, and a linear movement mechanism 143. The joint pin 144 is a columnar member supported by the actuator unit 142 and extending vertically through a hole 134 formed in the upper chuck subassembly.

[0039] The actuator unit 142 is supported by a linear movement mechanism 143. The linear movement mechanism 143 moves the actuator unit 142 and the bonding pin 144 up and down in a vertical direction by a drive unit (not shown) that may include, for example, a motor. The actuator unit 142 is configured to generate a constant pressure in a vertical downward direction. In one example, the actuator unit 142 can generate the constant pressure using, for example, air supplied from an electro-pneumatic regulator (not shown). In this manner, the actuator unit 142 can control the mechanical force applied to the central portion of the upper wafer W1 when the bonding pin 144 contacts the central portion of the upper wafer W1. When configured as described above, the striker 140 controls the movement of the actuator unit 142 by the linear movement mechanism 143 and the mechanical force applied by the actuator unit 142 from the bonding pin 144 to the upper wafer W1.

[0040] 4A-4C illustrate various wafer bonding process steps that may be performed by the wafer bonding apparatus 100. As shown in FIG. 4A, an upper wafer W1 and a lower wafer W2 are held by an upper chuck 120 and a lower chuck 125, respectively. In the embodiment shown in FIG. 3 and FIG. 4A, the backside of the upper wafer W1 is held to the mounting surface 122 of the upper chuck 120 via a vacuum pressure 165 provided by a vacuum pump 160. Similarly, the backside of the lower wafer W2 is held to the mounting surface 127 of the lower chuck 125 via a vacuum pressure 165 provided by the vacuum pump 160. The two wafers are initially separated by a small gap (g).

[0041] As shown in Figure 4B, the striker 140 applies a mechanical force to the center of the upper wafer W1, causing the upper wafer W1 to bend or flex 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 top and bottom wafers. As the contact surface area increases, the top and bottom wafers elastically deform to close the initial gap (g) between the two wafers.

[0042] In some embodiments, the vacuum pressure 165 supplied to the upper chuck 120 can be turned off before the vacuum pressure 165 to the lower chuck 125 is removed, as shown in FIG. 4C. When the vacuum pressure 165 to 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, the peripheral portion of the upper wafer W1 falls towards the peripheral portion of the lower wafer W2, and the bond front continues to propagate outward until the two wafers are completely bonded. Once bonding is completed, the striker 140 retracts, the vacuum pressure 165 to the lower chuck 125 is turned off, and the bonded wafer pair W1 / W2 is released from the wafer bonding apparatus 100.

[0043] In another embodiment (not shown in FIG. 4C ), the vacuum pressure 165 supplied to the lower chuck 125 can be turned off before the vacuum pressure 165 to the upper chuck 120 is removed. When the vacuum pressure 165 to the lower chuck 125 is turned 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 lifts towards the peripheral portion of the upper wafer W1, and the bond front continues to propagate outward until the two wafers are completely bonded. In this embodiment, the bonding force between the upper wafer W1 and the lower wafer W2 can act against gravity to bond the lower wafer W2 to the upper wafer W1 held on the upper chuck 120. Once bonding is completed, the striker 140 retracts, the vacuum pressure 165 to the upper chuck 120 is turned off, and the bonded wafer pair W1 / W2 is released from the wafer bonding apparatus 100.

[0044] As noted above, when two wafers are bonded together using conventional wafer bonding systems and methods, the bonded wafer pair often exhibits significant distortion and lateral misalignment. Often, as shown in FIG. 2 and discussed above, the post-bond wafer distortions generated by such systems and methods can be worse near the center and / or near the edge of the bonded wafer pair. For this reason, the present disclosure provides various embodiments of wafer bonding systems and methods for reducing post-bond wafer distortions that occur primarily within the center and / or edge of the bonded wafer pair. More specifically, the present disclosure provides various hardware improvements that reduce post-bond wafer distortions by correcting for variations in pre-bond wafer shape. Although hardware modifications are disclosed herein for reducing post-bond wafer distortions near the center and / or edge of the bonded wafer pair, one skilled in the art would recognize how similar modifications can be made to reduce post-bond wafer distortions in other areas or zones of the bonded wafer pair.

[0045] In some embodiments, post-bond wafer distortion is reduced by applying hardware modifications to the lower chuck 125. Such modifications may include, but are not limited to, modifying 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 modifications may additionally or alternatively be applied to the upper chuck 120.

[0046] 5-7 illustrate various embodiments of a wafer chuck that may utilize the techniques described herein. In particular, FIG. 5 illustrates a top view of a wafer chuck 200 that includes multiple radial zones (Z1-Z5) that extend radially outward from the center (C) of the wafer chuck to the outer edge (E) of the wafer chuck. In some embodiments, as shown in FIG. 5, radial ribs (R1-R5) may be provided at the boundaries of each radial zone (Z1-Z5). When included within the wafer chuck 200 design, the radial ribs (R1-R5) extend vertically from the top surface of the wafer chuck to provide a mounting surface for the wafer (W). Although shown and described in various embodiments (see, e.g., FIGS. 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, e.g., FIGS. 6A and 7A). When the radial ribs (R1-R5) are omitted, the wafer (W) can be mounted directly on the upper surface of the wafer chuck 200, for example as shown in FIGS. 4A-4C.

[0047] In some embodiments, the surface height of the wafer chuck 200 shown in FIG. 5 may be configured to reduce post-bond wafer distortion. In particular, the surface height of the wafer chuck 200 may vary across the wafer chuck such that the height of the wafer chuck in one or more of the radial zones (Z1-Z5) is different from the height of the wafer chuck in other radial zones. A wafer chuck with a variable surface height may be alternatively referred to herein as a "variable height wafer chuck." FIGS. 6A-6B provide side views of a variable height wafer chuck 210 / 220 that may be used to reduce post-bond wafer distortion near the edge of a bonded wafer pair. FIGS. 7A-7B provide side views of a variable height wafer chuck 230 / 240 that may be used to reduce post-bond wafer distortion near the center of a bonded wafer pair.

[0048] 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. In addition, the variable height wafer chuck 210 includes a central region 214 and a peripheral region 216 extending radially outward from an edge of the central region 214 to an outer edge of the chuck. In some embodiments, the peripheral region 216 can extend a distance (d) from the outer edge of the chuck. The distance (d) can generally be 50 mm or less when the variable height wafer chuck 210 is configured to support a 300 mm wafer. In one example, the distance (d) shown in FIG. 6A can range between about 0 mm and about 20 mm. In some embodiments, the central region 214 and the peripheral region 216 can each include one or more of the radial zones (Z1-Z5) shown in FIG. 5. In one example, the central region 214 may include a first subset of radial zones (e.g., Z1-Z3) and the peripheral region 216 may include a second subset of radial zones (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 zones described above and may include different radial zones in other embodiments.

[0049] 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 slopes down (or angled downward) 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, as shown in FIG. 6A, 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., outer edge) of the peripheral region 216 of the variable height wafer chuck 210. In some embodiments, the height difference (Δh), or height reduction, between the edge of the central region 214 and the edge (i.e., outer edge) of the peripheral region 216 of the variable height wafer chuck 210 may range between −1 μm (or less) to −5 μm (or more).

[0050] 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 comprises a central region 224 and a peripheral region 226 that extends radially outward a distance (d) from an edge of the central region 224 to an 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 by including a plurality of radial ribs (R1-R5) that extend vertically from an upper surface of the variable height wafer chuck 220 to provide a mounting surface 222 for a wafer. A first subset of the radial ribs (e.g., R1-R3) are included within the central region 224 and a second subset of the radial ribs (e.g., R4-R5) are included within the peripheral region 226 of the variable height wafer chuck 220.

[0051] 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 slopes down (or angled downward) in the peripheral region 226, such that the height of the variable height wafer chuck 220 in the peripheral region 226 is lower than the height of the variable height wafer chuck 220 in the central region 224. Unlike the embodiment shown in FIG. 6A, the height difference (Δh) in the peripheral region 226 is provided in FIG. 6B 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 can be reduced by −1 μm (or less) to −5 μm (or more) to provide a step-down height reduction, as shown in FIG. 6B. In one exemplary embodiment, one or more of the radial ribs located in the peripheral region 226 can be reduced by about −3 μm.

[0052] 6A and 6B illustrate a variable height wafer chuck 210 / 220 that may be used to reduce post-bond wafer distortion near the edges of a bonded wafer pair. During a wafer bonding process, when a pre-bond wafer (e.g., a bottom wafer) is attached to the variable height wafer chuck 210 / 220, the pre-bond wafer adheres to and assumes the shape of the mounting surface 212 / 222. In other words, the pre-bond wafer is pre-distorted by the shape of the mounting surface 212 / 222 of the variable height wafer chuck 210 / 220.

[0053] In the embodiment shown in Figures 6A and 6B, the height reduction (Δh) in the peripheral region 216 / 226 of the variable height wafer chuck 210 / 220 causes the peripheral portion of the pre-bonded wafer (e.g., the lower wafer) to curve or bend downward (see, e.g., Figure 8A). This causes the bond interface between the upper and lower wafers to expand radially outward. When the upper wafer is bonded onto the lower wafer, the strain along the bond interface is "locked" by friction that prevents further lateral shift along the interface. When bonding is completed and the bonded wafer pair is released from the lower chuck (see, e.g., Figure 8E), the peripheral portion of the lower wafer can bend upward to return the bonded wafer pair to an equilibrium state. As a result, the peripheral portion of the lower wafer is pushed inward along the bond interface, resulting in an inward strain that compensates for the outward strain shown, for example, in Figures 1C and 2. In this manner, the inward distortion created within the peripheral portion of the lower wafer can be used to reduce lateral misalignment between the upper and lower wafers.

[0054] The variable height wafer chuck 210 / 220 shown in Figures 6A and 6B may 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 distortion near the edge or periphery of the bonded wafer pair. In some embodiments, the variable height wafer chuck 210 / 220 as shown in Figures 6A or 6B, for example, may 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 chuck 210 / 220 is not strictly limited to the lower chuck 125. In some embodiments, the variable height wafer chuck 210 / 220 shown in Figures 6A and 6B may additionally be used to implement the upper chuck 120 in the wafer bonding apparatus 100. In such an embodiment, the upper chuck 120 may be implemented with some elastic compliance, allowing the upper chuck to elastically deform and act like a striker that pushes down on the center of the upper wafer (W1) to initiate bonding with the lower wafer (W2). In other embodiments, the variable height wafer chuck 210 / 220 shown in Figures 6A and 6B can alternatively be used to implement the upper chuck in an alternative wafer bonding apparatus, in which a striker is provided in the lower chuck to push the lower wafer upward to initiate bonding with the upper wafer. When used to implement the upper chuck 120, the variable height wafer chuck 210 / 220 shown in Figures 6A and 6B can provide the same benefits as noted above for the lower chuck 125.

[0055] It should be noted that the techniques described herein are not strictly limited to reducing post-bond wafer distortion near the edge or perimeter of the bonded wafer pair. In other embodiments, the variable height wafer chuck may be configured to reduce post-bond wafer distortion in other areas or zones of the bonded wafer pair. In some cases, for example, the striker 140 used to initiate bonding between the top wafer (W1) and the bottom wafer (W2) shown in FIG. 4B may generate poor distortion near the center (e.g., r≦10 mm) of the bonded wafer pair W1 / W2. To avoid this problem, FIGS. 7A-7B provide various embodiments of variable height wafer chucks 230 / 240 that may be used in the wafer bonding apparatus 100 shown in FIGS. 3-4 and the wafer bonding process shown in FIGS. 8A-8E to reduce post-bond wafer distortion near the center of the bonded wafer pair.

[0056] 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 extending radially outward a distance (d) from an edge of the central region 234 to an outer edge of the chuck. In FIG. 7A, the distance (d) may generally be 130 mm or greater when the variable height wafer chuck 210 is configured to support a 300 mm wafer. In the embodiment shown in FIG. 7A, the distance (d) may range between 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 of the radial zones (Z1-Z5) shown in FIG. 5. In one example, the central region 234 may include a first subset of radial zones (e.g., Z1-Z2) and the peripheral region 236 may include a second subset of zones (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 zones described above and may include different radial zones in other embodiments.

[0057] 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 sloped (or angled upward) 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, as shown in FIG. 7A, the height (h) of the variable height wafer chuck 230 may increase linearly between the edge of the central region 234 and the edge (i.e., outer edge) of the peripheral region 236 of the variable height wafer chuck 230. In one example, the height difference (Δh), or height increase, between the edge of the central region 234 and the edge (i.e., outer edge) of the peripheral region 236 of the variable height wafer chuck 230 may range between +1 μm (or less) to +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) in the central region 214, linearly sloped (or angled upwardly) in an inner portion of the peripheral region 236, and flat (or planar) in an outer portion of the peripheral region 236. The outer portion of the peripheral region 236 may generally be smaller than the inner portion of the peripheral region 236, such that the inner portion of the peripheral region 236 provides a linearly sloped area sufficient to counteract distortion near the center of the bonded wafer pair.

[0058] FIG. 7B illustrates a variable height wafer chuck 240 according to a fourth embodiment of the present disclosure. Similar to the previous embodiment shown in FIG. 7A, the variable height wafer chuck 240 shown in FIG. 7B comprises a central region 244 and a peripheral region 246 that extends radially outward a distance (d) from an edge of the central region 244 to an outer edge of the chuck. The variable height wafer chuck 240 shown in FIG. 7B differs from the variable height wafer chuck 230 shown in FIG. 7A by including a plurality of radial ribs (R1-R5) that extend perpendicularly from a top surface of the variable height wafer chuck 240 to provide a mounting surface 242 for a wafer. A first subset of the radial ribs (e.g., R1-R2) are included within the central region 244, and a second subset of the radial ribs (e.g., R3-R5) are included within the peripheral region 246 of the variable height wafer chuck 240.

[0059] 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 sloped (or angled upward) in the peripheral region 246, such that the height of the variable height wafer chuck 240 in the peripheral region 246 is higher than the height of the variable height wafer chuck 240 in the central region 244. Unlike the embodiment shown in FIG. 7A, the height difference (Δh) in the peripheral region 246 is provided in FIG. 7B by increasing the height of a second subset of radial ribs (e.g., ribs R3, R4, and R5) located in the peripheral region 236. In some embodiments, the rib height in the peripheral region 246 can be increased by +1 μm (or less) to +5 μm (or more) to provide a step-up height increase, as shown in FIG. 7B. Alternatively, the rib height in the peripheral region 246 can remain unchanged, while the rib height in the central region 244 can be reduced by −1 μm (or less) to −5 μm (or more) to provide a desired height difference (Δh). In yet another alternative embodiment, the height of one or more of the radial ribs located in the inner portion of the peripheral region 236 (e.g., ribs R3 and R4) can be increased by +1 μm (or less) to +5 μm (or more) to provide a step-up height increase in the inner portion of the peripheral region 236, while the height of one or more radial ribs located in the outer portion of the peripheral region 236 (e.g., rib R5) remains unchanged or equal to the rib height in the central region 244.

[0060] 7A and 7B illustrate a variable height wafer chuck 230 / 240 that may be used to reduce post-bond wafer distortion near the center of a bonded wafer pair. During the wafer bonding process, when a pre-bond wafer (e.g., a bottom wafer) is attached to the variable height wafer chuck 230 / 240, the pre-bond wafer adheres to and assumes the shape of the mounting surface 232 / 242. In other words, the pre-bond wafer is pre-distorted by the shape of the mounting surface 232 / 242 of the variable height wafer chuck 230 / 240.

[0061] In the embodiment shown in FIGS. 7A and 7B, the height increase (Δh) in the peripheral region 236 / 246 of the variable height wafer chuck 230 / 240 (or alternatively, the height decrease in the central region 234 / 244) causes the peripheral portion of the pre-bonded wafer (e.g., the lower wafer) to curve or bend upward. This distorts the bond interface on the pre-bonded wafer radially inward. When the upper wafer is bonded onto the lower wafer, the strain along the bond interface is "locked" by friction that prevents further lateral shift along the interface. When bonding is completed and the bonded wafer pair is released from the lower chuck, the peripheral portion of the lower wafer can bend downward to return the bonded wafer pair to an equilibrium state. This allows the peripheral portion of the lower wafer to be pushed outward along the bond interface, thereby providing an outward strain that compensates for the inward strain created in the pre-bonded wafer. In this manner, the outward deflection created within the peripheral portion of the lower wafer can be used to reduce the lateral misalignment between the upper and lower wafers produced by the striker 140 .

[0062] The variable height wafer chuck 230 / 240 shown in Figures 7A and 7B may 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 distortion near the center of the bonded wafer pair. In some embodiments, the variable height wafer chuck 230 / 240 as shown in Figures 7A or 7B, for example, may be used as the lower chuck 125 in the wafer bonding apparatus 100 to reduce post-bonding wafer distortion near the center of the bonded wafer pair. However, the variable height wafer chuck 230 / 240 is not strictly limited to the lower chuck 125. In some embodiments, the variable height wafer chuck 230 / 240 shown in Figures 7A and 7B may additionally be used to implement the upper chuck 120 in the wafer bonding apparatus 100. In such an embodiment, the upper chuck 120 may be implemented with some elastic compliance, allowing the upper chuck to elastically deform and act like a striker that pushes down on the center of the upper wafer (W1) to initiate bonding with the lower wafer (W2). In other embodiments, the variable height wafer chuck 230 / 240 shown in Figures 7A and 7B can alternatively be used to implement the upper chuck in an alternative wafer bonding apparatus, in which a striker is provided in the lower chuck to push the lower wafer upward to initiate bonding with the upper wafer. When used to implement the upper chuck 120, the variable height wafer chuck 230 / 240 shown in Figures 7A and 7B can provide the same benefits as noted above for the lower chuck 125.

[0063] In some embodiments, one or more of the embodiments shown in FIGS. 6-7 may be combined to reduce post-bond wafer distortion near the edge and near the center of the bonded wafer pair. It is noted that the variable height wafer chuck embodiments shown in FIGS. 6-7 are exemplary, and the techniques described herein may be used to reduce post-bond wafer distortion in other areas or zones of the bonded wafer pair. It is further noted that the height difference (Δh) and distance (d) described herein are exemplary. The particular values ​​of Δh and d may generally be selected based on physical modeling of a particular wafer bonding process and the pre-bond wafer geometries of the top and bottom wafers. For example, the height difference (Δh) and / or distance (d) may be empirically derived and optimized from experimental measurements of bonded wafers.

[0064] 8A-8E show an improved wafer bonding process for bonding two wafers together (e.g., an upper wafer and a lower wafer) using the variable height wafer chuck 210 / 220 shown in FIG. 6A or FIG. 6B. In the illustrated embodiment, a variable height wafer chuck is used to mount the lower chuck. In other embodiments, a variable wafer chuck may be used to mount the upper chuck, or the lower chuck and the upper chuck. Although the wafer bonding process shown in FIG. 8A-8E uses a variable height wafer chuck configured to reduce post-bonding wafer distortion near the edge of the bonded wafer pair, one skilled in the art will understand how the illustrated process may utilize other variable height wafer chucks to reduce post-bonding wafer distortion in other areas or zones of the bonded wafer pair. For example, the variable height wafer chuck 230 / 240 shown in FIG. 7A or FIG. 7B may be used in a similar wafer bonding process to reduce post-bonding wafer distortion near the center of the bonded wafer pair.

[0065] In some embodiments, the improved wafer bonding process may begin at step 300 of FIG. 8A with receipt of an upper wafer 302 on a mounting surface of an upper chuck 304 and a lower wafer 306 on a mounting surface of a lower chuck 308. When the upper chuck 304 and the lower chuck 308 are implemented as vacuum chucks, the upper wafer 302 and the lower wafer 306 may be held on their respective chucks by vacuum pressure provided, for example, by a vacuum pump. The upper and lower wafers may initially be separated by a small gap (g), as shown in FIG. 8A.

[0066] As shown in FIG. 8B, to initiate bonding in step 310, a mechanical force can be applied to the center of the top wafer 302 using a striker 309. The mechanical force applied by the striker 309 causes the center of the top wafer 302 to bend or flex downward until it contacts the center of the bottom wafer 306. This contact initiates a bonding front that spontaneously propagates radially outward from the centers of the top and bottom wafers. As the contact surface area increases, the top and bottom wafers elastically deform to close the initial gap (g) between the two wafers.

[0067] Before the bonding between the top and bottom wafers is completed, the vacuum pressure to the top chuck 304 can be turned off in step 320, as shown in Figure 8C. When the contact surface area is large enough, the vacuum seal between the top chuck 304 and the top wafer 302 is broken and the bond front continues to propagate outward until the two wafers are fully bonded, as shown in Figure 8D. Once the bonding between the top and bottom wafers is completed, the striker 309 retracts in step 330, Figure 8D, and the vacuum pressure to the bottom chuck 308 is turned off in step 340, Figure 8E, to release the bonded wafer pair 302 / 306 from the bottom chuck 308.

[0068] In an alternative wafer bonding process (not shown), the striker 309 may be retracted and the vacuum pressure to the lower chuck 308 may be turned off before the vacuum pressure to the upper chuck 304 is turned off in FIG. 8C. In such a process, the bonding forces between the upper wafer 302 and the lower wafer 306 may cause the vacuum seal between the lower wafer 306 and the lower chuck 308 to break before bonding is completed and / or before the vacuum pressure to the upper chuck 304 is turned off. When this occurs, the bonding forces between the upper wafer 302 and the lower wafer 306 may act against gravity to bond the lower wafer 306 to the upper wafer 302 held on the upper chuck 304. Once bonding is completed, the vacuum pressure to the upper chuck 304 is turned off and the post-bond wafer pair 302 / 306 is released from the upper chuck 304.

[0069] The improved wafer bonding process shown in Figures 8A-8E reduces post-bond wafer distortion near the edges of the bonded wafer pair 302 / 306 by utilizing a variable height wafer chuck that pre-strains at least one of the pre-bond wafers (e.g., the bottom wafer) by bending a peripheral portion of the pre-bond wafer downward (see, e.g., Figure 8A). This pre-strain causes the bond interface between the top and bottom wafers to spread or distort radially outward. Once the top wafer 302 is bonded onto the bottom wafer 306, the distortion along the bond interface is "pinned" by friction that prevents further lateral shift along the interface. When bonding is completed (see, e.g., FIG. 8D) and the vacuum pressure to the lower chuck 308 is turned off and the bonded wafer pair is released from the lower chuck 308 (see, e.g., FIG. 8E), the peripheral portion of the lower wafer 306 may bend upwardly to return the bonded wafer pair 302 / 306 to an equilibrium state. As a result, the peripheral portion of the lower wafer 306 is pressed inward along the bond interface, thereby providing an inward distortion that compensates for the outward distortion shown, for example, in FIGS. 1C and 2.

[0070] FIG 9 is a graph illustrating the distortion of a bonded wafer pair as a function of radius (r) when two wafers are bonded together using the improved wafer bonding process shown in FIG 8A-8E. As shown in FIG 9, the improved wafer bonding process shown in FIG 8A-8E creates much less distortion 90 near the edges of the bonded wafer pair than the distortion 20 created by the conventional wafer bonding process. By reducing the distortion 90 near the edges of the bonded wafer pair, the post-bond distortion error caused by the wafer bonding process is reduced and the lateral shift (or lateral misalignment) between the two wafers is minimized.

[0071] The improved wafer bonding process shown in Figures 8A-8E and the variable height wafer chuck 210 / 220 shown in Figures 6A and 6B are particularly well suited to reduce the distortion 90 at the edge of the bonded wafer pair, since the distortion performance at the edge is very similar for all wafers. In conventional wafer bonding processes, the distortion 20 at the bonded interface is generally higher at the edge (i.e., positive outward displacement) than at the center of the bonded wafer pair, with a steeper gradient toward the edge. This distinct characteristic can be corrected by using a variable height wafer chuck as shown in Figures 6A or 6B. Conversely, if the bonded wafer pair exhibits an opposite distortion behavior, with distortion lower at the edge (i.e., negative inward displacement) and higher at the center, a wafer bonding process using a variable height wafer chuck as shown in Figures 7A or 7B can also be used to reduce the post-bonded wafer distortion at the center.

[0072] The embodiment shown in Figures 6-8 reduces post-bond wafer distortion by modifying at least one surface height of the wafer chuck. Other hardware modifications can also be used to reduce post-bond wafer distortion. For example, a ring of temperature adjustment elements can be placed in one or more areas or zones of the wafer chuck to ameliorate distortion in those areas or zones. In one exemplary implementation, a ring of Peltier chips can be embedded in the lower chuck and / or upper chuck to adjust the temperature of the chuck surface. Prior to the bonding process, the temperature adjustment elements can be activated to transfer heat to (or from) areas of the wafer that overlap and / or are adjacent to the temperature adjustment elements. This heat transfer locally changes the temperature in areas of the wafer, which causes the areas to expand or contract laterally. This creates a lateral displacement mismatch between the wafer pair that becomes permanent after the bonding process.

[0073] Figure 10 is a side view of a wafer chuck 400 in accordance with a fifth embodiment of the present disclosure. The wafer chuck 400 shown in Figure 10 includes one or more temperature adjustment elements 410 embedded within one or more areas or zones of the wafer chuck. As such, the wafer chuck 400 shown in Figure 10 may be alternatively referred to herein as a "temperature conditioned wafer chuck."

[0074] The temperature-conditioned wafer chuck 400 shown in FIG. 10 includes a central region 414 and a peripheral region 416 that extends radially outward from an edge of the central region 414 to an outer edge of the chuck. In some embodiments, the peripheral region 416 can extend a distance (d) from the outer edge of the chuck. The distance (d) can generally be 50 mm or less when the temperature-conditioned wafer chuck 400 is configured to support a 300 mm wafer. In one example, the distance (d) shown in FIG. 10 can range between about 5 mm and about 10 mm. In some embodiments, the central region 414 and the peripheral region 416 can each include one or more of the radial zones (Z1-Z5) shown in FIG. 5. In one example, the central region 414 can include a first subset of radial zones (e.g., Z1-Z3) and the peripheral region 416 can include a second subset of radial zones (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 zones discussed above, and may include different radial zones in other embodiments.

[0075] The temperature-conditioned wafer chuck 400 shown in FIG. 10 further includes a plurality of radial ribs (R1-R5) extending vertically from the top surface of the temperature-conditioned wafer chuck 400 to provide a mounting surface 418 for a wafer. Alternatively, the ribs shown in FIG. 10 may be omitted and the wafer may be mounted directly on the top surface of the temperature-conditioned wafer chuck 400. In the embodiment shown in FIG. 10, the mounting surface 418 of the temperature-conditioned wafer chuck 400 is flat across the wafer chuck (i.e., across the central region 414 and the peripheral region 416). Alternatively, the mounting surface of the temperature-conditioned chuck may be flat in the central region 414 and slope downwardly in the peripheral region 416 (as shown in FIGS. 6A-6B) or sloped in the peripheral region 416 (as shown in FIGS. 7A-7B) as described above.

[0076] The temperature conditioned wafer chuck 400 shown in FIG. 10 further comprises one or more temperature adjustment elements 410 embedded within one or more areas or zones of the wafer chuck. In the embodiment shown in FIG. 10, the temperature adjustment elements 410 are embedded within a peripheral region 416 of the temperature conditioned wafer chuck 400 to transfer heat to (or from) the edge of a wafer mounted on a mounting surface 418 (or top surface) of the temperature conditioned wafer chuck 400. However, it should be noted that the temperature adjustment elements 410 are not limited to the exemplary placement shown in FIG. 10 and may additionally or alternatively be positioned within other areas or zones of the temperature conditioned wafer chuck 400 to transfer heat to (or from) other areas of the wafer. In one embodiment, the temperature adjustment elements 410 may be Peltier elements.

[0077] FIG. 11 is a graph illustrating wafer temperature as a function of radius (r) when the temperature conditioned wafer chuck 400 shown in FIG. 10 is used in the wafer bonding process shown in FIGS. 8A-8E to bond two wafers together. As shown in FIG. 11, the temperature adjustment element 410 embedded in the peripheral region 416 of the temperature conditioned wafer chuck 400 can be activated to heat the wafer chuck prior to the bonding process so that heat can be transferred to the wafer during the bonding process. By transferring heat to the wafer edge, the wafer temperature 500 increases near the edge of the wafer and the region near the wafer edge expands laterally as shown in FIG. 11. The lateral expansion of the wafer edge generates outward distortion, which creates a lateral displacement mismatch between the top and bottom wafers that becomes permanent after the bonding process. In some embodiments, the temperature adjustment element 410 can provide about 5 nm of outward distortion at the wafer edge for every 0.1° C. increase in wafer temperature.

[0078] In other embodiments, the temperature adjustment element 410 can alternatively be used to cool or extract heat from one or more regions or zones of the wafer. Cooling the region causes the region to contract laterally, generating inward distortion, which creates a lateral displacement mismatch between the top and bottom wafers. In some embodiments, the temperature adjustment element 410 can provide about 5 nm of inward distortion at the wafer edge for every 0.1° C. decrease in wafer temperature.

[0079] In some embodiments, the temperature conditioned wafer chuck 400 shown in FIG. 10 may be used in the wafer bonding apparatus 100 shown in FIGS. 3-4 and the wafer bonding process shown in FIGS. 8A-8E to reduce post-bond wafer distortion in one or more areas or zones of the bonded wafer pair. The temperature conditioned wafer chuck 400 allows for flexible adjustment of distortion based on incoming wafer measurements. The amount and direction of distortion can be adjusted by adjusting the temperature of the temperature conditioned element 410 relative to the remainder of the wafer. In some embodiments, the temperature conditioned element 410 shown in FIG. 10 may be included in one of the variable height chuck designs shown in FIGS. 6-7.

[0080] 12 is a flow chart diagram illustrating one embodiment of a method 600 for reducing distortion of a bonded wafer pair. In general, the method 600 may include receiving an upper wafer on a mounting surface of an upper chuck of a wafer bonding apparatus (step 610) and applying a vacuum pressure to the upper chuck to hold the upper wafer on the mounting surface of the upper chuck (step 620). The method 600 may further include receiving a lower wafer on a mounting surface of a lower chuck of the wafer bonding apparatus (step 630) and applying a vacuum pressure to the lower chuck to hold the lower wafer on the mounting surface of the lower chuck (step 640). The method 600 may further include bonding the upper wafer to the lower wafer to form a bonded wafer pair (step 650).

[0081] In some embodiments of the method 600 described herein, the lower chuck comprises a central region and a peripheral region extending radially outward from an edge of the central region to an outer edge of the lower chuck. The mounting surface of the lower chuck is flat in the central region and linearly slopes or linearly slopes down in the peripheral region, such that the height of the lower chuck in the peripheral region is different from the height of the lower chuck in the central region. When vacuum pressure is applied to the lower chuck (step 640), the vacuum pressure applied to the lower chuck causes the lower wafer to adhere to the mounting surface of the lower chuck, and thus the lower wafer is pre-distorted by the shape of the mounting surface of the lower chuck before the upper wafer is bonded to the lower wafer (step 650). Pre-distorting the lower wafer before bonding advantageously reduces distortion of the bonded wafer pair and minimizes lateral misalignment between the upper and lower wafers.

[0082] In some embodiments of the method 600 described herein, the upper chuck comprises a central region and a peripheral region extending radially outward from an edge of the central region to an outer edge of the upper chuck. The mounting surface of the upper chuck is flat in the central region and linearly slopes or linearly slopes down in the peripheral region, such that the height of the upper chuck in the peripheral region is different from the height of the upper chuck in the central region. When vacuum pressure is applied to the upper chuck (step 640), the vacuum pressure applied to the upper chuck causes the upper wafer to adhere to the mounting surface of the upper chuck, and thus the shape of the mounting surface of the upper chuck pre-distorts the upper wafer before the lower wafer is bonded to the upper wafer (step 650). Pre-distorting the upper wafer before bonding advantageously reduces distortion of the bonded wafer pair and minimizes lateral misalignment between the upper and lower wafers.

[0083] In some embodiments, the bonding process (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 upper chuck before bonding between the upper and lower wafers is completed, and removing the vacuum pressure supplied to the lower chuck after bonding between the upper and lower wafers is completed to form a bonded wafer pair. In such embodiments, the bonded wafer pair may remain on the lower chuck until the vacuum pressure supplied to the lower chuck is removed.

[0084] In other embodiments, the bonding process (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 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. In such embodiments, the bonding force between the upper and lower wafers may break the vacuum seal between the lower wafer and the lower chuck before the vacuum pressure supplied to the lower chuck is removed. When this occurs, the bonding force between the upper and lower wafers may act against gravity to bond the lower wafer to the upper wafer while the upper wafer is held on the upper chuck. In such embodiments, the bonded wafer pair may remain on the upper chuck until the vacuum pressure supplied to the upper chuck is removed.

[0085] It should be noted that throughout this specification, reference to "one embodiment" or "an embodiment" 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 indicate that they are present in all embodiments. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification does not necessarily refer to the same embodiment of the invention. Furthermore, particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. In other embodiments, various additional layers and / or structures may be included and / or described features may be omitted.

[0086] The term "substrate" as used herein means and includes a base material or structure on which a material is formed. It will be understood that a substrate may include a single material, multiple layers of different materials, a layer or layers having regions of different materials or structures therein, and the like. These materials may include semiconductors, insulators, conductors, or combinations thereof. For example, the substrate may be a semiconductor substrate, a base semiconductor layer on a supporting structure, a metal electrode, or a semiconductor substrate with one or more layers, structures, or regions formed thereon. The substrate may be a conventional silicon substrate or other bulk substrate including a layer of semiconducting material. As used herein, the term "bulk substrate" means and includes not only silicon wafers, but also silicon-on-insulator ("SOI") substrates such as silicon-on-sapphire ("SOS") substrates and silicon-on-glass ("SOG") substrates, epitaxial layers of silicon 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.

[0087] Systems and methods for processing a substrate are described in various embodiments. The substrate may include any material portion or structure of a device, particularly a semiconductor or other electronic device, and may be, for example, a base substrate structure, such as a semiconductor substrate, or a layer on or overlying the base substrate structure, such as a thin film. Thus, the term "substrate" is not intended to be limited to any particular base structure, lower layer, or upper layer, patterned or unpatterned, but rather is intended to include any such layer or base structure and any combination of layers and / or base structures. In some cases, the term "substrate" may be used to describe a patterned or unpatterned wafer, such as a device wafer or a carrier wafer.

[0088] Those skilled in the relevant art will recognize that the various embodiments may be practiced without one or more of the specific details, or with other alternative and / or additional methods, materials, or components. In other instances, well-known structures, materials, or operations have not been shown or described in detail to avoid obscuring aspects of the various embodiments of the invention. Similarly, for purposes of explanation, specific numbers, materials, and configurations are set forth to provide a thorough understanding of the invention. However, the invention may be practiced without the 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.

[0089] Further modifications and alternative embodiments of the described system and method should be apparent to those skilled in the art upon review of this specification. It will therefore be appreciated that the described system and method are not limited to these exemplary configurations. It should be understood that the forms of the system and method shown and described herein should be construed as exemplary embodiments. Various modifications can be made to the implementation. Thus, although the present invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention. Accordingly, the specification and drawings should be regarded in an illustrative rather than a restrictive sense, and such modifications are intended to be included within the scope of the present invention. Furthermore, any benefits, advantages, or solutions to problems described herein with respect to specific embodiments are not intended to be construed as any or all of the essential, necessary, or essential features or elements of the claims.

Claims

1. A wafer bonding apparatus comprising: a first chuck having a first mounting surface configured to hold a first wafer; a second chuck having a second mounting surface configured to hold a second wafer, the second chuck further having 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 perpendicularly from a flat upper surface of the second chuck to provide the second mounting surface; having 13. A wafer bonding apparatus comprising: a wafer bonding surface that is flat in the central region and that slopes linearly upward or downward in the peripheral region, the second mounting surface being flat in the central region and that slopes linearly upward or downward in the peripheral region, the height of the second chuck in the peripheral region being different from the height of the second chuck in the central region.

2. 2. The wafer bonding apparatus of 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. 3. The wafer bonding apparatus of claim 2, wherein a 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 plurality of radial ribs having a first subset of radial ribs disposed within the central region and a second subset of radial ribs disposed within the peripheral region; 4. The wafer bonding apparatus of claim 3, wherein the height difference is provided by configuring the plurality of radial ribs such that a height of a second subset of the radial ribs is less than a height of a first subset of the radial ribs.

5. 2. The wafer bonding apparatus of claim 1, wherein the second mounting surface slopes 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. 6. The wafer bonding apparatus of claim 5, wherein a 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.

7. the plurality of radial ribs having a first subset of radial ribs disposed within the central region and a second subset of radial ribs disposed within the peripheral region; 7. The wafer bonding apparatus of claim 6, wherein the height difference is provided by configuring the plurality of radial ribs such that a height of a second subset of the radial ribs is greater than a height of a first subset of the radial ribs.

8. A wafer bonding apparatus comprising: a first chuck having a first mounting surface configured to hold a first wafer; a second chuck having a second mounting surface configured to hold a second wafer; a planar upper surface having a plurality of radial zones extending radially outward from a center of the second chuck to an outer edge of the second chuck, a first subset of the plurality of radial zones being disposed in a central region of the second chuck and a second subset of the plurality of radial zones being disposed in a peripheral region of the second chuck extending radially outward from an edge of the central region to the outer edge of the second chuck; a plurality of radial ribs, each disposed at a boundary between two of the radial zones and extending perpendicularly from the flat upper surface of the second chuck, providing the second mounting surface; a second chuck having having a first subset of the plurality of radial ribs disposed within the first subset of the plurality of radial zones; a second subset of the plurality of radial ribs disposed within the second subset of the plurality of radial zones; a height of the second subset of the plurality of radial ribs is different from a height of the first subset of the plurality of radial ribs.

9. the second mounting surface is flat or planar across the first subset of the plurality of radial ribs; 9. The wafer bonding apparatus of claim 8, wherein the second mounting surface slopes linearly downward or linearly upward across the second subset of the plurality of radial ribs.

10. 10. The wafer bonding apparatus of claim 9, wherein the second mounting surface slopes or angles linearly downward across the second subset of the plurality of radial ribs, and 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. a reduction in height of the second chuck in the peripheral region causes a peripheral portion of the second wafer prior to bonding to bow downward; 11. The wafer bonding apparatus of claim 10, wherein when bonding between the first and second wafers is completed, the peripheral portion of the second wafer bends upward to provide an inward distortion that reduces lateral misalignment between the first and second wafers.

12. 10. The wafer bonding apparatus of claim 9, wherein the second mounting surface slopes or angles upwardly linearly across the second subset of the plurality of radial ribs, and 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. an increase in height of the second chuck in the peripheral region causes a peripheral portion of the second wafer prior to bonding to bend upward; 13. The wafer bonding apparatus of claim 12, wherein when bonding between the first and second wafers is completed, the peripheral portion of the second wafer bends downward to provide an outward distortion that reduces lateral misalignment between the first and second wafers.

14. 10. The wafer bonding apparatus of claim 8, wherein at least one of the first chuck and the second chuck has one or more temperature adjustment elements embedded within one or more of the plurality of radial zones.

15. the one or more temperature adjustment elements are configured to transfer heat to areas of the first wafer and / or the second wafer that overlap with the one or more radial zones; 15. The wafer bonding apparatus of claim 14, wherein the transfer of heat causes the regions of the first wafer and / or the second wafer to expand laterally, creating an outward distortion and reducing lateral misalignment between the first wafer and the second wafer.

16. the one or more temperature adjustment elements are configured to transfer heat from regions of the first wafer and / or the second wafer that overlap the one or more radial zones; 15. The wafer bonding apparatus of claim 14, wherein the transfer of heat causes the regions of the first wafer and / or the second wafer to contract laterally, creating an inward distortion and reducing lateral misalignment between the first wafer and the second wafer.

17. receiving an upper wafer on a mounting surface of an upper chuck of a wafer bonding apparatus; applying a vacuum pressure to the upper chuck to hold the upper wafer against the mounting surface of the upper chuck; receiving a lower wafer on a mounting surface of a lower chuck of the wafer bonding apparatus, the lower chuck having a flat 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 flat upper surface of the lower chuck and providing the mounting surface of the lower chuck, the plurality of radial ribs including a first subset of radial ribs disposed in the central region and a second subset of radial ribs disposed in the peripheral region, the mounting surface of the lower chuck being flat in the central region and linearly sloping upward or downward in the peripheral region by configuring the plurality of radial ribs such that a height of the second subset of radial ribs in the peripheral region is different from a height of the first set of radial ribs in the central region; applying a vacuum pressure to the lower chuck to hold the lower wafer against the mounting surface of the lower chuck, the vacuum pressure applied to the lower chuck causing the lower wafer to adhere to the mounting surface of the lower chuck and causing the lower wafer to be pre-strained by a shape of the mounting surface of the lower chuck before the upper wafer is bonded to the lower wafer; bonding the upper wafer to the lower wafer to form a bonded wafer pair, where pre-distortion of the lower wafer prior to bonding reduces distortion in the bonded wafer pair and minimizes lateral misalignment between the upper and lower wafers; The method comprising:

18. The joining step includes: applying a mechanical force to a center of the upper wafer to initiate bonding between the upper and lower wafers; removing the vacuum pressure supplied to the upper chuck after the initiating bonding step and before bonding between the upper wafer and the lower wafer is completed; removing the vacuum pressure supplied to the lower chuck after bonding between the upper wafer and the lower wafer is completed and the bonded wafer pair is formed, the bonded wafer pair remaining on the lower chuck until the vacuum pressure supplied to the lower chuck is removed; 20. The method of claim 17, comprising:

19. The joining step includes: applying a mechanical force to a center of the upper wafer to initiate bonding between the upper and lower wafers; removing the vacuum pressure supplied to the lower chuck after the initiating bonding step and before bonding between the upper wafer and the lower wafer is completed; removing the vacuum pressure supplied to the upper chuck after the bonding between the upper wafer and the lower wafer is completed and the bonded wafer pair is formed, the bonded wafer pair remaining on the upper chuck until the vacuum pressure supplied to the upper chuck is removed; 20. The method of claim 17, comprising:

20. 20. The method of claim 19, wherein a bonding force between the upper wafer and the lower wafer causes a vacuum seal between the lower wafer and the lower chuck to be broken before the bonding step is completed and / or before the vacuum pressure supplied to the upper chuck is removed.

Images