WAFER CHUCK COMPRISING VARIABLE STIFFNESS MATERIAL - Patent application

Abstract

The wafer bonding apparatus comprises: a first chuck in a processing chamber, the first chuck configured to hold a first wafer, the first chuck comprising a chuck body and a variable stiffness layer comprising a plurality of actuators, the plurality of actuators comprising a variable stiffness material, the variable stiffness layer being disposed below the chuck body; a controller configured to send control signals to one or more of the plurality of actuators; a vacuum line on the chuck body configured to apply vacuum pressure from a vacuum pump to the first chuck; and a second chuck in the processing chamber, the second chuck configured to hold a second wafer to be bonded to the first wafer, the stiffness of the plurality of actuators being configured to change based on a control signal from the controller.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 63 / 303,787, filed January 27, 2022, which is incorporated herein by reference.

[0002] The present invention relates generally to wafer chuck designs and, in particular embodiments, to wafer chucks that include variable stiffness materials. [Background technology]

[0003] In semiconductor manufacturing processes, micro-electrical devices are created on silicon wafer substrates through deposition, patterning, etching, and integration techniques. One process of bonding two silicon wafers together is known as wafer bonding. Wafer bonding is a packaging technique used in the manufacturing of Microelectromechanical Systems (MEMS), Nanoelectromechanical Systems (NEMS), microelectronics, and optoelectronics. In one wafer bonding process, one patterned (device) wafer may be bonded onto a blank (carrier) wafer. In another wafer bonding process, one device wafer may be bonded onto another device wafer. In both cases, the alignment between the wafers plays a key role in the overall device product performance. If there is a large misalignment (or distortion) between the two wafers during the bonding process, the electrical circuits on the devices may not connect properly. The success of minimizing wafer bonding distortion is directly related to the yield of devices on the wafer. The latest distortion requirement for wafer bonding process can be less than 100 nm in 3σ variation across the wafer. Due to process variations, the distortion can vary locally on the wafer. In addition to improving the global distortion, it is also important to mitigate the local distortion variation. Summary of the Invention

[0004] According to one embodiment of the present invention, a wafer bonding apparatus includes a first chuck in a processing chamber, the first chuck configured to hold a first wafer, the first chuck comprising a chuck body and a variable stiffness layer comprising a plurality of actuators, the plurality of actuators comprising a variable stiffness material, the variable stiffness layer being disposed below the chuck body; a controller configured to send control signals to one or more of the plurality of actuators; a vacuum line on the chuck body configured to apply vacuum pressure from a vacuum pump to the first wafer; and a second chuck in the processing chamber, the second chuck configured to hold a second wafer to be bonded to the first wafer, the stiffness of the plurality of actuators being configured to change based on a control signal from the controller.

[0005] According to one embodiment of the present invention, a wafer bonding method includes applying a vacuum pressure through a first chuck to a process chamber of a wafer bonding apparatus; placing a first wafer on the first chuck, where the first wafer is held to the first chuck by the vacuum pressure, the first chuck comprising a chuck body and a variable stiffness layer comprising a plurality of actuators, the plurality of actuators comprising a variable stiffness material, the variable stiffness layer being disposed below the chuck body; sending a control signal to one or more of the plurality of actuators; and actuating the one or more of the plurality of actuators based on the control signal to change the stiffness of the variable stiffness layer.

[0006] According to one embodiment of the present invention, a wafer bonding apparatus includes a first chuck in a processing chamber, the first chuck configured to hold a first wafer, the first chuck including a variable stiffness layer with a first actuator, the first actuator including a variable stiffness material; a controller configured to send a control signal to the first actuator; and a second chuck in the processing chamber, the second chuck configured to hold a second wafer to be bonded to the first wafer, the stiffness of the variable stiffness layer configured to change based on the control signal from the controller. [Brief description of the drawings]

[0007] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0008] [Figure 1] 1 illustrates a cross-sectional view of an exemplary wafer bonding apparatus including a variable stiffness material, according to various embodiments. [Diagram 2] 4 is a cross-sectional view of another exemplary wafer bonding apparatus including a variable stiffness material according to another embodiment. [Diagram 3] 1 is a cross-sectional view of an exemplary wafer chuck including a variable stiffness material in accordance with another embodiment. [Figure 4] FIG. 2 illustrates a top view of a chuck body of an exemplary wafer chuck in accordance with various embodiments. [Diagram 5] FIG. 2 illustrates a top view of a variable stiffness layer of an exemplary wafer chuck in accordance with various embodiments. [Figure 6] 1 is a cross-sectional view of an exemplary wafer chuck including a variable stiffness material when adjusting the stiffness of the variable stiffness material in accordance with various embodiments. [Figure 7] FIG. 13 is a cross-sectional view of another exemplary wafer chuck including a variable stiffness material when adjusting the stiffness of the variable stiffness material in accordance with another embodiment. [Figure 8] 1 is a cross-sectional view of an exemplary wafer chuck with a variable stiffness material and a sensor in accordance with various embodiments. [Figure 9] 1A-1C are cross-sectional views of an exemplary actuator including a variable stiffness material that induces vertical deformation, in accordance with various embodiments. [Figure 10] 1A-1C are cross-sectional views of an exemplary actuator including a variable stiffness material that induces shear deformation, in accordance with various embodiments. [Figure 11] FIG. 2 illustrates a top view of a variable stiffness layer in a shear-deformed state, according to one embodiment. [Figure 12] FIG. 13 is a top view of a variable stiffness layer in a shear-deformed state according to another embodiment. [Figure 13] FIG. 1 illustrates a process flow diagram of a wafer bonding process, according to various embodiments. [Figure 14] 13 illustrates simulated results of chuck stiffness versus vertical displacement. [Figure 15] 13 is a diagram showing simulated results of chuck stiffness and vacuum pressure versus lateral displacement. [Figure 16] 1 is a plot of simulated maximum lateral displacement as a function of chuck stiffness at a vacuum pressure differential of 10 kPa. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] This application relates to wafer chuck designs, and more particularly to wafer chucks that include variable stiffness materials. In various embodiments, the wafer chuck is part of a wafer bonding apparatus used in semiconductor device manufacturing. In a typical wafer bonding process, it is important to minimize local and global distortion of the wafers being bonded, since misalignment between the wafers can reduce the yield of devices on the wafers. For example, the local distortion may be small in the center region of the wafer, but the distortion at the edge of the wafer may be significant and out of specification. This local distortion may be due to inherent variations in the wafer, variations in the preceding process, or the wafer bonding process. One solution to the local distortion is to provide multiple independently controlled vacuum zones in the lower chuck. By applying different degrees of vacuum pressure to correct the local and global distortion, the wafer deforms differently in different zones. However, the number and size of the vacuum zones are limited by the number of pressure regulators available in the system. Furthermore, some of these knobs have limited ranges for adjusting distortion. Therefore, it may be desirable to design the wafer chuck using a different concept for distortion correction. The embodiments of the present application disclose a wafer chuck including a variable stiffness material that allows for local distortion correction by locally tailoring the stiffness of the wafer chuck. In various embodiments, wafer distortion of less than 100 nm with 3σ variation across the wafer can be achieved in the wafer bonding process. The wafer chuck described in the present disclosure may advantageously enable a system having only one vacuum zone instead of multiple vacuum zones while providing multiple points of distortion correction. Although the present disclosure mainly describes an embodiment of local distortion correction, the wafer chuck in various embodiments may also enable overall global distortion correction.

[0010] In the following, first, with reference to Figs. 1-3, designs of a wafer bonding apparatus and a wafer chuck of the wafer bonding apparatus according to various embodiments are described. Then, with reference to Figs. 4-5, designs of an exemplary chuck body and variable stiffness layer are described. Figs. 6-7 provide examples of a variable stiffness layer while locally varying its stiffness. Fig. 8 describes another exemplary wafer chuck with a sensor that reveals the characteristics of the wafer held by the wafer chuck. With reference to Figs. 9-12, vertical and lateral (e.g., shear) deformations of the variable stiffness layer are described. Fig. 13 presents a process flow diagram of a wafer bonding process according to various embodiments. With reference to Figs. 14-16, the effect of chuck stiffness on various displacements on the wafer chuck is simulated and described. All figures in this disclosure are drawn for illustrative purposes only and are not to scale, including the aspect ratios of features. The wafer bonding apparatus may include any other useful components for wafer bonding that are not shown in the figures. Although the following description in this disclosure is primarily directed to wafer bonding apparatus, the disclosed wafer chuck designs may be applied to other systems, such as metrology chambers.

[0011] FIG. 1 illustrates a cross-sectional view of an exemplary wafer bonding apparatus 10 including a variable stiffness material, according to various embodiments.

[0012] In FIG. 1 , the wafer bonding apparatus 10 includes a lower chuck 11 and an upper chuck 12 configured to hold a first wafer 100 and a second wafer 105, respectively. The two wafers are to be bonded using the wafer bonding apparatus 10. In a particular embodiment, the upper chuck 12 is configured to move vertically to reposition the second wafer 105 so that the first wafer 100 and the second wafer 105 can be brought into contact for bonding in the wafer bonding process. In some embodiments, the lower chuck 11 can be configured to move vertically to move the first wafer 100. The wafer bonding apparatus 10 can be disposed in a process chamber 13 that can appropriately control process parameters (e.g., gas environment, temperature, and pressure) for the wafer bonding process. The process chamber 13 can include a gas supply / exhaust system 14 that controls the gas environment of the process chamber 13. In various embodiments, the wafer bonding apparatus 10 can be configured to accommodate 200 mm or 300 mm wafers. The components of the wafer bonding apparatus 10 (eg, lower chuck 11, upper chuck 12, and processing chamber 13) are sized accordingly.

[0013] In various embodiments, the first wafer 100 and the second wafer 105 may contain a number of semiconductor devices to be fabricated. Accordingly, the first wafer 100 and the second wafer 105 may comprise layers of semiconductors useful for various microelectronics. In certain embodiments, the first wafer 100 and the second wafer 105 may be silicon wafers or silicon-on-insulator (SOI) wafers. In some embodiments, the first wafer 100 and the second wafer 105 may include silicon germanium wafers, silicon carbide wafers, gallium arsenide wafers, gallium nitride wafers, and other compound semiconductors. In other embodiments, the first wafer 100 and the second wafer 105 may include heterogeneous layers such as silicon germanium on silicon, gallium nitride on silicon, silicon carbon on silicon, as well as layers of silicon on silicon or SOI substrates.

[0014] In various embodiments, the lower chuck 11 comprises two layers: a chuck body 120 and a variable stiffness layer 130 disposed below the chuck body 120. In various embodiments, the chuck body 120 may comprise any suitable material having sufficient stiffness (e.g., 10 GPa / m or greater) to support the first wafer 100. In certain embodiments, the chuck body 120 may comprise aluminum. In one or more embodiments, the chuck body 120, the variable stiffness layer 130, or both may have a thickness between 1 cm and 15 cm. As shown, the surface of the chuck body 120 may have grooves 110. The grooves 110 may advantageously reduce the contact area between the first wafer 100 and the chuck body 120 such that any damage to the first wafer 100 due to friction may be avoided. In certain embodiments, the chuck body 120 may not have any grooves or may have a different number of grooves than in FIG. 1. The wafer bonding apparatus 10 further includes a first vacuum system 160 coupled to the chuck body 120 via a first vacuum line 150. The first vacuum line 150 is configured to apply a vacuum pressure from the first vacuum system 160 to the first wafer 100 so that the first wafer 100 can be held in a vacuum by the lower chuck 11. Although the wafer bonding apparatus 10 in FIG. 1 is shown as a vacuum chuck driven by a vacuum, in other embodiments, other forces may be used in the wafer bonding apparatus 10 instead of or in addition to a vacuum. For example, in certain embodiments, the wafer bonding apparatus 10 may include an electrostatic chuck (ESC). In one embodiment, the variable stiffness layer 130 may be disposed between the first wafer 100 and the ESC body.

[0015] Continuing to refer to FIG. 1, the variable stiffness layer 130 has actuators 140 that include a variable stiffness material. In FIG. 1, each of the actuators 140 is coupled to a controller 170 that sends a control signal to the actuators 140 to locally vary the stiffness of the actuators 140. In various embodiments, the local variation in stiffness in the variable stiffness layer 130 is utilized to correct distortions before or during the wafer bonding process, as described in more detail below. In certain embodiments, as shown, the controller 170 may also be coupled to a first vacuum system 160 to allow dynamic control of stiffness and vacuum. While each of the actuators 140 may be controlled individually, in other embodiments, some of the actuators 140 may be grouped together and controlled with a single control signal.

[0016] In various embodiments, the variable stiffness material is any material whose stiffness can be adjusted by a control signal. The control signal can include a thermal signal, a pressure signal, an electrical signal, or a magnetic signal. In certain embodiments, the variable stiffness material can be a polymeric material. Some examples of polymeric materials for the variable stiffness material include a shape memory polymer that responds to a thermal or magnetic signal, a granular material that responds to pressure, an electroactive polymer that responds to an electrical signal, or a magnetorheological elastomer that responds to a magnetic signal. In one embodiment, a thermal actuator can be used that responds to a signal that provides thermal energy. Such a signal can be heat or an electromagnetic wave that can be absorbed by the variable stiffness material used in the thermal actuator. In another embodiment, if a pressure actuator (e.g., a granular material) is used instead of a control signal, the pressure applied to the actuator can directly induce a change in stiffness. In some embodiments, the stiffness of the variable stiffness material can be adjusted by other mechanisms or any combination of the above signals. Other materials, such as ceramic materials, can also be used. In various embodiments, the variable stiffness material has a minimum stiffness that is comparable to the chuck body 120 (e.g., 10 GPa / m or more). In certain embodiments, the variable stiffness material is capable of varying its Young's modulus by at least 100 GPa in a layer thickness of 0.1 m.

[0017] In certain embodiments, the variable stiffness material may include a piezoelectric material that may deform in response to a control signal. When a piezoelectric material is used for the actuator 140 of the variable stiffness layer 130, a control signal (e.g., an electrical signal) may induce a deformation of the actuator 140. The direction and degree of deformation of the actuator 140 may be adjusted such that local distortions of the first wafer 100 may be locally corrected, as described in more detail below (FIGS. 9 and 10). In one embodiment, the actuator 140 may comprise a 2 mm cube-shaped piezoelectric chip capable of applying a force of 65 N and inducing a deformation of up to 2 μm.

[0018] In one or more embodiments, the wafer bonding apparatus may further include a temperature control system (e.g., a cooling system) for thermal management of the actuator 140. Thermal management may be especially important when controlling the actuator 140 because changes in stiffness generate significant heat that may affect the actuator 140 or surrounding components.

[0019] FIG. 2 illustrates a cross-sectional view of another exemplary wafer bonding apparatus 20 including a variable stiffness material, according to another embodiment.

[0020] In various embodiments, as shown in FIG. 1, the upper chuck 12 may include an upper chuck body 125, a second vacuum system 165, and a second vacuum line 155 (FIG. 1). In certain embodiments, as shown in FIG. 2, the upper chuck 12 may further include an upper variable stiffness layer 135 above the upper chuck body 125. The upper variable stiffness layer 135 includes actuators 145 that include a variable stiffness material. As with the lower chuck 11, each of the actuators 145 is coupled to an upper controller 175, which sends control signals to the actuators 145 to locally change the stiffness of the actuators 145. In certain embodiments, as shown, the upper controller 175 may also be coupled to a second vacuum system 165 that allows dynamic control of stiffness and vacuum. The design of the upper chuck 12 in FIG. 1 and FIG. 2 is only an example, and in other embodiments, any other chuck design may be used, whether the upper chuck 12 includes a variable stiffness material or not.

[0021] FIG. 3 illustrates a cross-sectional view of an exemplary lower wafer chuck 11 including a variable stiffness material, according to another embodiment.

[0022] In FIG. 3, unlike the previous embodiment having a two-layer structure in the lower chuck, the lower chuck 11 may comprise a variable stiffness layer 130 constituting the chuck body and configured to contact the first wafer 100. Such an embodiment may advantageously simplify the chuck design and may be beneficial in translating the stiffness change in the variable stiffness layer 130 to the first wafer 100 for local distortion correction. Other features of the lower chuck 11, such as the vacuum system 160, are the same as the previous embodiment shown in FIG. 1 and FIG. 2 and will not be repeated. In the following drawings (FIG. 6-FIG. 8) in which the wafer bonding apparatus is shown, only the lower chuck is shown for the sake of explanation, and some features of the upper chuck and the lower chuck (e.g., the controller 170) are not shown. However, in various embodiments, it is assumed that these features are present in the illustrated examples.

[0023] FIG. 4 illustrates a top view of a chuck body 120 of an exemplary wafer chuck, in accordance with various embodiments.

[0024] 4, seven grooves 110 are provided as circular grooves. The grooves 110 have a common center and are distributed on the surface of the chuck body 120. In other embodiments, the grooves 110 may have different shapes, such as square, oval, etc., and there may be more or fewer grooves.

[0025] 5 illustrates a top view of a variable stiffness layer 130 of an exemplary wafer chuck, in accordance with various embodiments. The chuck body 120 may be disposed on top of the variable stiffness layer 130, but is not shown in FIG. 5 for purposes of illustration.

[0026] In FIG. 5, the variable stiffness layer 130 is divided into a number of square sections by a lattice (e.g., more than 500 sections in FIG. 5), and each of the square sections can be made with an actuator 140 whose stiffness can be adjusted individually. The size and number of square sections shown in FIG. 5 are examples, and different sizes and numbers of the square sections can be selected. In various embodiments, the variable stiffness layer 130 can be provided with 700 to 18,000 square sections. In certain embodiments, the square sections can have a side of 2 mm to 10 mm. In other embodiments, the shape of the sections can also be different from a square, for example, triangular. In various embodiments, each of the sections can be used for an actuator 140, but in other embodiments, a portion of the sections can be selected for the actuator 140. By increasing the number of sections in the variable stiffness layer 130 and correspondingly increasing the number of actuators 140, local distortions of the first wafer 100 can be corrected by improving the vertical and lateral resolution. On the other hand, reducing the number of sections and the number of actuators 140 may be advantageous to simplify the design of the wafer chuck, thereby reducing the manufacturing costs of the wafer chuck. In certain embodiments, the actuators 140 may be spaced apart, advantageously providing space for lateral deformation correction.

[0027] FIG. 6 illustrates a cross-sectional view of an exemplary lower chuck 11 including a variable stiffness material when adjusting the stiffness of the variable stiffness material, according to various embodiments.

[0028] In FIG. 6, the lower chuck 11 may have the same features as those previously shown in FIGS. 1 and 2, and some of the actuators 140 are actuated to change their stiffness (shown as actuated actuators 142) to allow for local distortion correction of the first wafer 100. The initial state of the first wafer 100 is shown in dotted lines, and local distortions are present with some indentations. The local distortions of the first wafer 100 may be due, for example, to pressure differences in different locations. These local distortions may be corrected using the variable stiffness layer 130. The actuators 140, which include a variable stiffness material, may change their stiffness when actuated. In the illustrated example, the actuators 140 below the indented areas of the first wafer 100 may be actuated to increase their stiffness so that the distortion of the first wafer 100 may be locally counteracted. As a result of this local distortion correction, shown by the arrows in FIG. 6, the indentations of the first wafer 100 may be eliminated or minimized. The first wafer 100 locally modified by the actuated actuators 142 is shown in solid lines.

[0029] FIG. 7 illustrates a cross-sectional view of another exemplary lower chuck 11 including a variable stiffness material when adjusting the stiffness of the variable stiffness material according to another embodiment.

[0030] In FIG. 7, the first wafer 100 before local distortion correction has only one depression centered on the location of the vacuum line 150, as shown by the dotted line. This depression is larger and deeper than the depression in FIG. 6. As in FIG. 6, local distortion correction can be performed by activating some of the actuators 140. While actuators 140 in separate regions are activated in FIG. 6, in these embodiments of FIG. 7, only one region of actuators 140 is activated, but multiple discrete degrees of stiffness change are produced for the activated actuators 142 across the variable stiffness layer 130. In FIG. 7, the degree of stiffness change of the activated actuators 412 is qualitatively shown by the difference in contrast and the size of the arrows. The stiffness change of the variable stiffness layer can be locally adjusted to be larger near the first vacuum line 150, where the initial depression is largest, and smaller away from the vacuum line 150, as the initial depression becomes smaller.

[0031] As shown in Figures 6 and 7, the wafer bonding apparatus 10 may enable local distortion correction. In these examples, the first wafer 100 is locally warped with surface distortion. Thus, the change in stiffness of the variable stiffness layer is configured to compensate for the surface distortion at the surface of the first wafer 100 facing the lower chuck 11. Alternatively, information regarding the change in stiffness of the variable stiffness layer required for the expected local distortion correction can be stored in memory, and the stiffness change can be controlled accordingly, even before the first wafer 100 develops surface distortion (i.e., distortion).

[0032] Various embodiments may also be used in other conditions, such as plane stress or plane strain conditions, For example, in some embodiments, the change in stiffness of the variable stiffness layer is configured to compensate for residual surface stresses in the surface of the first wafer 100 that faces the lower chuck 11 in plane strain.

[0033] 6 and 7 are shown primarily to illustrate correcting local distortion based on the wafer shape on the lower chuck 11 before the wafer bonding process, the stiffness change may be further utilized for local distortion correction when the first wafer 100 is bonded to the second wafer 105. The wafer bonding process may also generate some distortion. The stiffness of the variable stiffness layer 130 may also be adjusted to offset the distortion from the bonding process by minimizing or eliminating the overall distortion. In certain embodiments, the stiffness change for local distortion correction may or may not result in a flat surface on the surface of the lower chuck 11.

[0034] Because the wafer bonding apparatus with variable stiffness material in various embodiments allows for local distortion correction based on stiffness changes, this design of the wafer bonding apparatus advantageously eliminates the need for multiple vacuum zones in conventional apparatus and replaces them with only one vacuum zone per chuck as shown in Figures 1-3 and 6-8. Replacing multiple vacuum zones with one vacuum zone can greatly simplify the design of the wafer chuck, reduce costs, and save space. The space required for actuators and control circuits can be significantly less than the vacuum lines required for multiple vacuum zones. Alternatively, the design of a wafer chuck with multiple vacuum zones can advantageously provide an additional control parameter that can supplement the local distortion correction by vacuum control by adding a variable stiffness layer.

[0035] In various embodiments, the actuators 140 may be individually controlled to provide ideal stiffness by sending control signals from the controller 170 to the actuators 140. To this end, prior to the skew correction and wafer bonding process, the incoming wafer may be characterized with respect to its shape so that the type and extent of the global and local skew can be determined. In certain embodiments, such measurements may be performed in a separate metrology system, while in other embodiments, they may be performed in the processing chamber for the wafer bonding process. Furthermore, the measurements may be performed at various stages, i.e., before, during, or after the wafer bonding process. An appropriate sensing system may be coupled to the processing chamber to enable real-time measurements of the first wafer 100 during the local skew correction. In various embodiments, the process of local skew correction may include determining a control signal based on measurements of the wafer shape, which may also include determining which one or more of the actuators 140 may be actuated according to the control signal.

[0036] In certain embodiments, the distortion correction can be performed in multiple modes based on multiple measurements of the wafer shape. For example, a local distortion correction can be performed based on a first measurement before bonding the two wafers. Then, after contacting the two wafers, a local or global distortion correction can be performed based on a second measurement. This can be particularly useful because when the two wafers are contacted, additional forces are applied to the first wafer 100, which can cause changes in the local and / or global distortion. Thus, the second measurement of the wafer shape can be used to update the control signal and fine-tune the actuator 140.

[0037] FIG. 8 illustrates a cross-sectional view of an exemplary lower chuck 11 with a variable stiffness material and sensor 800, according to various embodiments.

[0038] In FIG. 8, all the features of the lower chuck 11, except for the sensor 800, are the same as the previous embodiment shown in FIG. 1 and FIG. 2 and will not be repeated. Some sections of the variable stiffness layer 130 may be equipped with a sensor 800 that allows real-time measurement of the first wafer 100 during local distortion correction. For example, the sensor 800 may include a pressure sensor that locally measures the pressure on the first wafer 100 to identify areas where the vacuum pressure is relatively high. In a particular embodiment, pressure data may be collected before the distortion correction process. According to the data collected by the pressure sensor, a pressure map may be generated and used to determine which actuators 140 are actuated and the degree of stiffness change to be achieved. In a further embodiment, after some of the actuators 140 are actuated in the distortion correction, a new data set may be collected to determine whether the local distortion has been sufficiently corrected. In an embodiment, a threshold value of acceptable distortion may be predetermined, and if the new data set indicates that the corrected distortion is below the threshold value, the wafer bonding process may proceed to the next step. On the other hand, if the corrected distortion is above the threshold, the actuators 140 may be adjusted to further minimize the local distortion. In various embodiments, the wafer bonding apparatus 10 may include a microprocessor having a program that performs the process of local distortion correction, including, for example, the steps described above.

[0039] FIG. 9 illustrates a cross-sectional view of an exemplary actuated actuator 142 inducing vertical deformation, according to various embodiments.

[0040] In FIG. 9, two actuated actuators 142 of a variable stiffness layer including a piezoelectric material are shown. The dotted lines indicate the initial position of the top surface of the actuators. In response to a control signal, the piezoelectric material deforms. In the illustrated example, the actuators are positioned to cause deformation in the vertical direction. In certain embodiments, when actuated by a control signal, the piezoelectric material may stretch in the vertical direction and lift the top surface, as shown by the arrows in FIG. 9. By identifying which actuators may be actuated and selectively actuating those actuators, it is possible to locally induce a vertical deformation in the variable stiffness layer. Such a local deformation of the variable stiffness layer may be utilized to correct a local distortion of the first wafer 100. In one embodiment, the degree of deformation of the piezoelectric material at one location of the first wafer 100 may be adjusted in the vertical direction in a range from 10 nm to 5 μm. In certain embodiments, the first wafer 100 may be vacuum chucked before the actuators are actuated, while in other embodiments, the first wafer 100 may be vacuum chucked after the actuators are actuated.

[0041] FIG. 10 illustrates a cross-sectional view of an exemplary actuated actuator 142 for inducing shear deformation, according to various embodiments.

[0042] In addition to or instead of the vertical deformation, the piezoelectric material may also induce deformation of the actuator in the lateral direction (e.g., shear deformation induced by a piezoelectric shear actuator as shown in FIG. 10). The dotted line indicates the initial position of the actuator. In one embodiment, when actuated by a control signal, the piezoelectric material may induce shear deformation as shown by the arrow in FIG. 10. In various embodiments, the first wafer 100 may first be chucked with a vacuum. When sufficient vacuum pressure is applied to the first wafer 100, this shear deformation of the actuator may be translated to the first wafer 100 to allow for local distortion correction in the lateral direction. In certain embodiments, the shear deformation of the actuator may first be translated to the chuck body 120, and then the deformation of the chuck body 120 may be translated to the first wafer 100, which may advantageously smooth out the deformation occurring in the first wafer 100. In other embodiments, the variable stiffness layer 130 comprising the actuators can be in direct contact with the first wafer 100, so that the deformation of the actuators can be directly and efficiently translated to the first wafer 100. In various embodiments, the underside of the actuators can be fixed to a base so that the lateral deformation can be translated to the first wafer 100. In one embodiment, the degree of deformation of the piezoelectric material at one location of the first wafer 100 can be tuned in the lateral direction from 1 nm to 50 nm.

[0043] FIG. 11 illustrates a top view of the variable stiffness layer 130 in a shear-deformed state, according to one embodiment.

[0044] In FIG. 11, an example of local distortion correction is shown with a subset of actuators 140 (i.e., actuated actuator 142) actuated. The subset is comprised of, for example, ten separate regions of different sizes in the illustrated embodiment, although any configuration of actuators for the subset is possible in other embodiments. The arrows in FIG. 11 indicate the direction and extent of shear deformation at the top surface of the variable stiffness layer 130. Each of the separate regions induces shear deformation according to a control signal received from the controller 170, and as indicated by the arrows, the direction and extent of shear deformation can be determined individually for each of the separate regions. For example, each region can have actuators configured to actuate in the X and Y directions (i.e., two orthogonal directions on the wafer surface). By varying the amount of actuation, any effective actuation direction can be achieved.

[0045] In various embodiments, this local distortion correction may be performed on the first wafer 100 before contacting the second wafer 105 to the first wafer 100. In certain embodiments, the local distortion correction may be performed in the wafer bonding process while the second wafer 105 is in contact with the first wafer 100.

[0046] FIG. 12 shows a top view of a shear deformed variable stiffness layer 130 according to another embodiment.

[0047] In FIG. 12, an example of global distortion correction is shown with all actuators 140 (i.e., actuated actuators 142) actuated. The arrows in FIG. 12 indicate the direction of shear deformation at the top surface of the variable stiffness layer 130. If all actuators 140 are synchronously deformed in one direction to the same extent, local distortion correction may not result. However, the entire top surface of the variable stiffness layer 130 may be displaced to reduce misalignment between the two wafers. In various embodiments, this mode of controlling the actuators 140 may be utilized to fine-tune the position of the first wafer 100 relative to the second wafer 105 before or during the wafer bonding process. Alternatively, synchronously deforming all actuators 140 in one direction but to different extents may also enable local distortion correction.

[0048] FIG. 13 shows a process flow diagram of a wafer bonding process 1300, according to various embodiments.

[0049] In FIG. 13, the process flow of the wafer bonding process 1300 for the first and second wafers begins by measuring the wafer shape to characterize the degree of distortion of the first wafer (block 1310). This measurement to obtain information related to the degree of distortion may be performed in the processing chamber for the wafer bonding process using a sensor or in a separate metrology system before transferring the wafer to the processing chamber. Based on the data from the measurement, a control signal for the variable stiffness layer of the lower wafer chuck may be determined and sent to a controller coupled to the actuator of the variable stiffness layer (block 1320). In one embodiment, the data from the measurement may also be utilized to adjust the vacuum pressure applied to the first wafer. One or more of the actuators are then controlled according to the control signal received from the controller (block 1330). Once the process of local distortion correction is completed, the two wafers may be bonded (block 1340).

[0050] In certain embodiments, the steps of measuring the wafer shape, determining and sending the control signals, and controlling the actuators may be performed iteratively. In one or more embodiments, after one or more of the actuators have been controlled (block 1330) and before bonding (block 1340), the shape of the wafer on the chuck (i.e., the chucked wafer) may again be measured (block 1335) and further correction of distortion may be achieved by repeating the steps of determining and sending the control signals (blocks 1320 and 1330). Such iterative embodiments are also possible, including the bonding step (block 1340). For example, a threshold may be set regarding the degree of distortion that is acceptable for device fabrication, and if the degree of final distortion after wafer bonding (block 1340) is unacceptable (i.e., greater than the threshold), the bonded wafers may be separated and rebonded according to an updated process recipe. The process recipe may be updated based on the degree of final distortion from the preceding bonding process. In another embodiment, a similarly updated process recipe may be utilized for the next set of wafers to be bonded. In one or more embodiments, the wafer bonding process (block 1340) and the change in stiffness of the variable stiffness layer (block 1330) may overlap in time or proceed simultaneously. In other words, information related to the degree of distortion may be obtained before, during, and / or after the bonding step (block 1340) and utilized to update the control signals to minimize local / global distortion. Thus, the measurement of wafer shape (block 1310) may be for the first wafer or the bonded wafer.

[0051] In various embodiments, local distortion correction based on stiffness variation may provide a wider range of tunability for distortion correction. For example, while a maximum differential pressure in a vacuum zone alone is less than about 100 kPa, when stiffness variation is utilized for local distortion correction, a differential pressure of the equivalent pressure greater than 100 kPa may be possible.

[0052] In certain embodiments, the wafer bonding process uses a process model and may include a model optimization step. For example, the process model may include a distribution map of control signals (e.g., voltage values ​​for sending electrical signals to actuators) for a desired wafer shape, and an optimization step may be used to update the distribution map based on incoming wafer measurements.

[0053] In various embodiments, the two wafers may be bonded (block 1340) by conventional methods, such as fusion bonding (also commonly referred to as direct bonding). In fusion bonding embodiments, the two wafers begin to bond when the wafers are brought together and the surfaces of the wafers begin to touch, forming a bonded wafer. The bonded wafer is then annealed at high temperature to increase the bond strength between the two wafers and form a fused wafer. Examples of wafer bonding process conditions include, among others, the bonding temperature, the environmental conditions of the processing chamber, and the applied force.

[0054] FIG. 14 shows a graph of simulated results of chuck stiffness versus vertical displacement.

[0055] To investigate the effect of chuck stiffness on the displacement, finite element simulations were performed with the first wafer held under vacuum on the lower chuck. In each simulation, the chuck stiffness is set to a specified value. Stiffness is expressed per area. In Figure 14, the effect of chuck stiffness on the displacement in the vertical direction (z direction) of the lower wafer chuck is shown as a function of radial position. For a 300 mm wafer (i.e., radius 150 mm), the effect was simulated at four different chuck stiffness values ​​(15, 30, 45, 60 GPa / m). For all four chuck stiffness values, the vertical displacement shows three plateau regions: a first plateau region at about 25-60 mm, a second plateau region at about 75-100 mm, and a third plateau region at about 120-130 mm. For example, at 15 GPa / m, the vertical displacement is about -4 μm in the first plateau region, about -4.7 μm in the second plateau region, and about -3.2 to -3.5 μm in the third plateau region. There are transition regions between the plateau regions where the vertical displacement transitions from one level to another. These general trends across the wafer are common to all four chuck stiffness values, but as the chuck stiffness value increases, the vertical displacement decreases. For example, at 60 GPa / m, the vertical displacement is only about -1 μm in the first plateau region, about one-quarter of the vertical displacement at 15 GPa / m. This decrease in vertical displacement therefore indicates that an increase in chuck stiffness can be used to correct local distortions on the wafer in the vertical direction.

[0056] FIG. 15 shows the simulated results of chuck stiffness and vacuum pressure versus lateral displacement.

[0057] In FIG. 15, the effect of chuck stiffness and vacuum pressure on the lateral (r-direction) displacement of the lower wafer chuck is shown as a function of radial position. In addition to the same four chuck stiffness values ​​as in FIG. 12, three different vacuum pressure values ​​(-10, -50, and -70 kPa) are used as simulation variables. For all four chuck stiffness values, the lateral displacement shows four peaks, namely, a first peak at about 0-25 mm, a second peak at about 60-70 mm, a third peak at about 100-120 mm, and a fourth peak at about 140-150 mm. For example, at 15 GPa / m, the lateral displacement is about 95 nm for the first peak, about 20 nm for the second peak, about -40 nm for the third peak, and about -100 nm for the fourth peak. As the chuck stiffness value increases, the lateral displacement decreases. For example, at 60 GPa / m, the lateral displacement is only about 5 nm at the second peak, about one-quarter of the lateral displacement at 15 GPa / m. Similar to Figure 12, the simulated results show that increasing the chuck stiffness can also be used to correct local distortions on the wafer in the lateral direction, but the displacement can be several orders of magnitude smaller than in the vertical direction.

[0058] FIG. 16 shows the simulated maximum lateral displacement as a function of chuck stiffness at a vacuum pressure differential of 10 kPa.

[0059] In Figure 16, as the chuck stiffness increases, the maximum lateral displacement decreases, a trend consistent with that shown in Figure 15. The maximum lateral displacement is up to about 95 nm at 15 GPa and gradually decreases to about 30 nm at 60 GPa / m.

[0060] Exemplary embodiments are summarized below. Other embodiments can be understood from the entire specification and claims filed herewith. Reference numerals are provided below for illustrative purposes only, and various examples may be implemented differently and should not be construed as being limited to these examples only.

[0061] Example 1. A wafer bonding apparatus comprising: a first chuck in a processing chamber, the first chuck configured to hold a first wafer, the first chuck comprising a chuck body and a variable stiffness layer comprising a plurality of actuators, the plurality of actuators comprising a variable stiffness material, the variable stiffness layer being disposed below the chuck body; a controller configured to send control signals to one or more of the plurality of actuators; a vacuum line on the chuck body configured to apply vacuum pressure from a vacuum pump to the first chuck; and a second chuck in the processing chamber, the second chuck configured to hold a second wafer to be bonded to the first wafer, the stiffness of the plurality of actuators being configured to change based on a control signal from the controller.

[0062] Example 2. The wafer bonding apparatus of Example 1, wherein the change in stiffness of the plurality of actuators is configured to compensate for surface stress on a surface of the first wafer facing the first chuck.

[0063] Example 3. The wafer bonding apparatus of example 1 or 2, wherein the change in stiffness of the plurality of actuators is configured to compensate for surface distortions on a surface of the first wafer facing the first chuck.

[0064] Example 4. The wafer bonding apparatus of any one of Examples 1-3, wherein the plurality of actuators comprises thermal actuators, the variable stiffness material comprises a shape memory material, and the control signal is configured to supply thermal energy to the thermal actuators.

[0065] Example 5. The wafer bonding apparatus of any one of Examples 1-4, wherein the plurality of actuators includes a pressure actuator, the variable stiffness material includes a granular material, and the stiffness of the variable stiffness material varies based on pressure applied to the pressure actuator.

[0066] Example 6. The wafer bonding apparatus of any one of Examples 1-5, wherein the plurality of actuators comprises electrical actuators, the variable stiffness material comprises a pressure sensitive material, and the control signal comprises an electrical signal.

[0067] Example 7. The wafer bonding apparatus of any one of Examples 1-6, wherein the plurality of actuators comprises a magnetic actuator, the variable stiffness material comprises a magnetorheological elastomer, and the control signal comprises a magnetic signal.

[0068] Example 8. The wafer bonding apparatus of any one of Examples 1-7, wherein the controller determines the control signal according to information related to a degree of distortion of the first wafer on the first chuck.

[0069] Example 9. The wafer bonding apparatus of any one of Examples 1-8, further comprising a metrology system including a sensor configured to sense an extent of distortion of the first wafer on the first chuck.

[0070] Example 10. A method of wafer bonding comprising: applying vacuum pressure through a first chuck to a processing chamber of a wafer bonding apparatus; disposing a first wafer on the first chuck, where the first wafer is held to the first chuck by the vacuum pressure, the first chuck comprising a chuck body and a variable stiffness layer comprising a plurality of actuators, the plurality of actuators comprising a variable stiffness material, the variable stiffness layer being disposed below the chuck body; sending a control signal to one or more of the plurality of actuators; and actuating the one or more of the plurality of actuators based on the control signal to change the stiffness of the variable stiffness layer.

[0071] Example 11. The method of example 10, wherein varying the stiffness of the plurality of actuators compensates for surface stress at a surface of the first wafer facing the first chuck.

[0072] Example 12. The method of example 10 or 11, further comprising receiving information from the metrology system relating to a degree of distortion of the first wafer on the first chuck, and determining, in the controller, the control signal according to the information.

[0073] Example 13. The method of any one of Examples 10-12, wherein the metrology system is part of the processing chamber, and further comprising adjusting the vacuum pressure based on the information.

[0074] Example 14. The method of any one of Examples 10-13, further comprising: placing a second wafer on a second chuck within the processing chamber; and bonding the second wafer to the first wafer to form a bonded wafer.

[0075] Example 15. The method according to any one of Examples 10 to 14, further comprising: after placing the second wafer on the second chuck, sending another control signal to another plurality of actuators of the variable stiffness layer, the another plurality of actuators including a variable stiffness material; and activating the another plurality of actuators based on the another control signal to further change the stiffness of the variable stiffness layer, the another control signal indicating a stiffness setting of the variable stiffness layer different from the control signal.

[0076] Example 16. The method of any one of Examples 10-15, further comprising, after bonding the second wafer to the first wafer, receiving information related to a degree of distortion of the bonded wafers on the first chuck.

[0077] Example 17. The method according to any one of Examples 10 to 16, further comprising: determining whether the degree of distortion of the bonded wafer is below a threshold based on the information related to the degree of distortion of the bonded wafer; and if the degree of distortion of the bonded wafer is greater than the threshold, separating the bonded wafer into a first wafer and a second wafer, and rebonding the second wafer to the first wafer to perform a rebonding process.

[0078] Example 18. A wafer bonding apparatus comprising: a first chuck in a processing chamber, the first chuck configured to hold a first wafer, the first chuck comprising a variable stiffness layer comprising a first actuator, the first actuator comprising a variable stiffness material; a controller configured to send a control signal to the first actuator; and a second chuck in the processing chamber, the second chuck configured to hold a second wafer to be bonded to the first wafer, the stiffness of the variable stiffness layer configured to change based on a control signal from the controller.

[0079] Example 19. A wafer bonding apparatus as described in Example 18, further comprising a plurality of actuators coupled to the controller and configured to receive a further control signal from the controller, wherein the stiffness of the variable stiffness layer is configured to vary based on the further control signal.

[0080] Example 20. The wafer bonding apparatus of example 18 or 19, wherein the variable stiffness layer comprises a polymer material having a stiffness of 10 GPa / m or more.

[0081] Example 21. The wafer bonding apparatus of any one of Examples 18-20, wherein the first chuck further comprises a plurality of sensors configured to sense a degree of distortion of the first wafer on the first chuck and to transmit information related to the degree of distortion to the controller, and the controller determines the control signal according to the received information from the plurality of sensors.

[0082] Example 22. The wafer bonding apparatus of any one of Examples 18-21, wherein the plurality of sensors are further configured to sense an extent of distortion of the first wafer in situ after placing the first wafer on the first chuck.

[0083] Example 23. A wafer bonding apparatus comprising: a first chuck in a processing chamber, the first chuck configured to hold a first wafer, the first chuck comprising a chuck body and a variable stiffness layer comprising a plurality of piezoelectric shear actuators, the variable stiffness layer being disposed below the chuck body; a controller configured to send a signal to one of the plurality of piezoelectric shear actuators; a vacuum line on the chuck body configured to apply vacuum pressure from a vacuum pump to the first wafer; and a second chuck configured to hold a second wafer to be bonded to the first wafer, wherein each of the plurality of piezoelectric shear actuators is configured to laterally induce shear deformation of the variable stiffness layer based on a signal from the controller.

[0084] Example 24. A wafer bonding apparatus as described in example 23, wherein the shear deformation of the variable stiffness layer occurs simultaneously in one direction.

[0085] Example 25. A wafer bonding apparatus as described in Example 23 or 24, further comprising a plurality of sensors configured to sense the degree of deformation of the first wafer on the chuck in situ while the first chuck induces a shear deformation of the variable stiffness layer in the lateral direction and transmit information related to the degree of distortion to a controller, the controller updating the signals according to the received information from the plurality of sensors.

[0086] While the present invention has been described with reference to exemplary embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the exemplary embodiments, as well as other embodiments of the present invention, will be apparent to those skilled in the art upon reference to the above description. It is therefore intended that the appended claims cover any such modifications or embodiments.

Claims

1. a first chuck in a processing chamber, the first chuck configured to hold a first wafer, the first chuck comprising: A chuck body; a variable stiffness layer comprising a plurality of actuators, the plurality of actuators comprising a variable stiffness material, the variable stiffness layer being disposed below the chuck body; A first chuck comprising: a controller configured to send a control signal to one or more of the plurality of actuators; a vacuum line on the chuck body configured to apply a vacuum pressure to the first wafer; a second chuck in the processing chamber, the second chuck configured to hold a second wafer to be bonded to the first wafer; and A wafer bonding apparatus comprising: A wafer bonding apparatus configured such that stiffness of the plurality of actuators varies based on the control signal from the controller.

2. 10. The wafer bonding apparatus of claim 1, wherein the change in the stiffness of the plurality of actuators is configured to compensate for surface stresses on a surface of the first wafer facing the first chuck.

3. 10. The wafer bonding apparatus of claim 1, wherein the change in the stiffness of the plurality of actuators is configured to compensate for surface distortions on a surface of the first wafer facing the first chuck.

4. The wafer bonding apparatus of claim 1 , wherein the plurality of actuators comprises thermal actuators, the variable stiffness material comprises a shape memory material, and the control signal is configured to supply thermal energy to the thermal actuators.

5. 10. The wafer bonding apparatus of claim 1, wherein the plurality of actuators comprises pressure actuators, the variable stiffness material comprises a granular material, and a stiffness of the variable stiffness material varies based on pressure applied to the pressure actuators.

6. The wafer bonding apparatus of claim 1 , wherein the plurality of actuators comprises electrical actuators, the variable stiffness material comprises a pressure sensitive material, and the control signals comprise electrical signals.

7. The wafer bonding apparatus of claim 1 , wherein the plurality of actuators comprises magnetic actuators, the variable stiffness material comprises a magnetorheological elastomer, and the control signal comprises a magnetic signal.

8. 2. The wafer bonding apparatus of claim 1, wherein the controller determines the control signal according to received information related to a degree of distortion of the first wafer on the first chuck.

9. 10. The wafer bonding apparatus of claim 8, further comprising a metrology system comprising a sensor configured to sense the degree of distortion of the first wafer on the first chuck.

10. applying a vacuum pressure through the first chuck to a process chamber of a wafer bonding apparatus; placing a first wafer on the first chuck, the first wafer being held to the first chuck by the vacuum pressure, the first chuck comprising: A chuck body; a variable stiffness layer comprising a plurality of actuators, the plurality of actuators comprising a variable stiffness material, the variable stiffness layer being disposed below the chuck body; and sending a control signal to one or more of the plurality of actuators; actuating the one or more of the plurality of actuators based on the control signal to vary the stiffness of the variable stiffness layer; A method for wafer bonding comprising:

11. 11. The method of claim 10, wherein varying the stiffness of the variable stiffness layer compensates for surface stresses at a surface of the first wafer facing the first chuck.

12. receiving information from a metrology system relating to a degree of distortion of the first wafer on the first chuck; determining, in a controller, the control signal according to the information; The method of claim 10 further comprising:

13. The method of claim 12 , wherein the metrology system is part of the processing chamber, and further comprising adjusting the vacuum pressure based on the information.

14. placing a second wafer on a second chuck within the processing chamber; bonding the second wafer to the first wafer to form a bonded wafer; The method of claim 10 further comprising:

15. sending another control signal to another plurality of actuators of the variable stiffness layer after placing the second wafer on the second chuck, the another plurality of actuators including the variable stiffness material; and actuating the other plurality of actuators to further vary the stiffness of the variable stiffness layer based on the other control signal, the other control signal indicating a stiffness setting of the variable stiffness layer different from the control signal; and The method of claim 14 further comprising:

16. 15. The method of claim 14, further comprising receiving information related to a degree of distortion of the bonded wafers on the first chuck after bonding the second wafer to the first wafer.

17. determining whether the degree of distortion of the bonded wafer is below a threshold based on the information related to the degree of distortion of the bonded wafer; if the degree of distortion of the bonded wafer is greater than the threshold, separating the bonded wafer into the first wafer and the second wafer and rebonding the second wafer to the first wafer to perform a rebonding process; The method of claim 16 further comprising:

18. a first chuck in a processing chamber, the first chuck configured to hold a first wafer, the first chuck comprising a variable stiffness layer comprising a first actuator, the first actuator comprising a variable stiffness material; and a controller configured to send a control signal to the first actuator; a second chuck in the processing chamber, the second chuck configured to hold a second wafer to be bonded to the first wafer; and A wafer bonding apparatus comprising: A wafer bonding apparatus configured such that a stiffness of the variable stiffness layer is varied based on the control signal from the controller.

19. a plurality of actuators coupled to the controller and configured to receive a further control signal from the controller, the stiffness of the variable stiffness layer being changed based on the further control signal; The wafer bonding apparatus of claim 18 further comprising:

20. 20. The wafer bonding apparatus of claim 18, wherein the variable stiffness layer comprises a polymeric material having a stiffness of 10 GPa / m or greater.

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