Abstract:
A method of bonding two substrates includes placing a separating member between a first substrate and a second substrate, applying pressure to the first substrate to initiate a bond wave between the first substrate and the second substrates with the separating member between the first substrate and the second substrate, and controlling movement of the bond wave by translating the separating member away from a center of the first substrate or the second substrate.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to silicon substrate processing. 
       BACKGROUND 
       [0002]    A microelectromechanical system (MEMS) typically has mechanical structures formed in a semiconductor substrate using conventional semiconductor processing techniques. A MEMS can include a single structure or multiple structures. The electromechanical aspect of MEMS is that an electrical signal activates each or is produced by actuation of each structure in the MEMS. 
         [0003]    Various processing techniques are used to form MEMS. These processing techniques can include layer formation, such as deposition and bonding, and layer modification, such as laser ablation, etching, punching and cutting. The techniques that are used are selected based on a desired pathway, recess and hole geometry to be formed in a body along with the material of the body. 
         [0004]    One implementation of a MEMS includes a body having chambers formed therein and a piezoelectric actuator formed on an exterior surface of the body. The piezoelectric actuator includes a layer of piezoelectric material, such as a ceramic, and conductive elements, such as electrodes, on opposite sides of the piezoelectric material. The electrodes of the piezoelectric actuator can either apply a voltage across the piezoelectric material to cause it to deform, or deformation of the piezoelectric material can generate a voltage difference between the electrodes. 
         [0005]    One type of MEMS with piezoelectric actuators is micro-fluidic ejection devices. An actuator can include piezoelectric material that can be actuated by electrodes, causing the piezoelectric material to deform. This deformed actuator pressurizes a chamber, causing fluid in the chamber to exit, for example, through a nozzle. The structure components, including the actuator, the chamber and the nozzle, can affect how much fluid is ejected. In a MEMS with multiple structures, forming uniformly sized components for each structure across the MEMS can improve the uniformity of performance of the MEMS, such as the uniformity of fluid quantities that are ejected. Forming structures with uniformity of size of a few microns can be challenging. 
       SUMMARY 
       [0006]    In general, in one aspect, a method of bonding two substrates includes placing a separating member between a first substrate and a second substrate, applying pressure to the first substrate to initiate a bond wave between the first substrate and the second substrates with the separating member between the first substrate and the second substrate, and controlling movement of the bond wave by translating the separating member away from a center of the first substrate or the second substrate. 
         [0007]    This and other embodiments can optionally include one or more of the following features. The method can further include monitoring the bond wave as the bond wave moves between the first substrate and the second substrate. The method can further include removing the separating member from between the first substrate and the second substrate after translating the separating member. The separating member can include a tapered portion and a non-tapered portion, and removing the separating member can include removing the tapered portion after the non-tapered portion. The method can further include determining a stopping point of the bond wave, and controlling movement of the bond wave can begin after the stopping point has been determined. 
         [0008]    The separating member can be translated at a rate that is less than a maximum rate above which voids and bubbles can be trapped between the first and second substrates. The separating member can be translated at a rate of between about 50 mm/s to 70 mm/s. Pressure can be applied at between about 0.5 psi and 5 psi, such as about 1 psi. 
         [0009]    The first substrate or the second substrate can include a patterned region including at least one die. The method can further include positioning the substrate having the patterned region such that a length of the at least one die is positioned along an axis that is at an angle of less than 30° from an axis extending along a length of the separating member. The angle can be about 17°. 
         [0010]    Placing the separating member between the first substrate and the second substrate can cause there to be a gap of between about 0.5 mm and 5 mm at least one point between the first substrate and the second substrate. The gap can be about 1 mm. 
         [0011]    The separating member can be placed approximately along a radial axis of the first substrate or the second substrate, and the separating member can extend along the radial axis by an amount that is less than a radial distance of the first substrate or the second substrate. The separating member can extend about 0.5 mm to 50 mm along the radial axis. The separating member can extend about 3 mm along the radial axis. 
         [0012]    The pressure can be applied with a manual mechanism. The pressure can be applied by air from an automated air cylinder. The bond can be further initiated by sliding a pressure mechanism across a surface of the first substrate or the second substrate. The pressure mechanism can include a compliant material. The compliant material can be rubber. The pressure can be applied at a single pressure point on the first or second substrate. 
         [0013]    The separating member can be the only separating member between the first and second substrates. 
         [0014]    In general, in one aspect, an apparatus for bonding two substrates includes a substrate holding member configured to hold a first substrate, a separating member configured to separate the first substrate and a second substrate, a pressure inducer configured to apply pressure to the first or second substrate and initiate a bond wave between the first substrate and the second substrate, a monitoring device configured to generate images of a bond wave between the first and second substrates, and a mechanism connected to the separating member. The mechanism is configured to translate the separating member away from a center of the first or second substrate to control movement of the bond wave. 
         [0015]    This and other embodiments can optionally include one or more of the following features. The monitoring device can be an infrared camera. The separating member can include a tapered portion. The separating member can have a length that is less than a radial distance of the first substrate or the second substrate. The separating member can be configured to align about along a line that bisects a center of the first or second substrate and a point where pressure is applied to the first substrate or the second substrate. The apparatus can further include a handle configured to move the separating member away from the substrate holding member when not in use. The mechanism can include a pocket configured to hold the separating member when not in use. The pressure inducer can be capable of exerting a pressure on the first substrate or the second substrate at an angle other than parallel to a main surface of the first substrate. The pressure inducer can be configured to apply a pressure at an angle between 90 degrees and 45 degrees to the main surface. The pressure inducer can have a tip that is less than 5 mm in diameter. The pressure inducer can be actuatable. 
         [0016]    By placing a separating member between two substrates and translating the separating member away from the center of the substrates, the bond wave between two substrates can be precisely controlled. Controlling the bond wave can avoid the formation of voids and bubbles between substrates. Avoiding bubbles and voids when bonding substrates can result in fewer defects in substrates, which can increase product yield. Moreover, controlling the bond wave to ensure that the bond is not defective can reduce the number of defects that need to be tested for in the completed device. 
         [0017]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a top perspective view of a mechanical device for bonding substrates. 
           [0019]      FIG. 2  is a bottom view of a mechanical device for bonding substrates. 
           [0020]      FIG. 3A  is a schematic of a separator unit having an extended separating member. 
           [0021]      FIG. 3B  is a schematic of a separator unit having a separating member stored in a pocket of the separator unit. 
           [0022]      FIG. 4  is a side view of a mechanical device for bonding substrates. 
           [0023]      FIG. 4A  is close-up view of a portion of  FIG. 4 . 
           [0024]      FIGS. 5A-5F , viewed from the top as if the upper substrate is transparent, show movement of an exemplary bond wave between substrates. 
       
    
    
       [0025]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0026]    When two substrates are bonded together, e.g., with room temperature fusion bonding, the bond typically begins at an initial bonding region and propagates outward in a bond wave. If at least one of the substrates includes patterned or etched features, the movement of the resulting bond wave is affected by the patterned regions of the substrate. As a result, the bond wave will move faster over some areas of the substrate than other areas. Such uneven movement of the bond wave can cause voids and air bubbles to be trapped between the substrates, reducing the strength of the bond and creating defects in unbonded areas. By placing a separating member between the substrates, monitoring the bond wave as it moves between the substrates, and translating the separating member away from the center of the substrates, the bond wave can be controlled to move uniformly across the substrates and avoid the formation of voids and bubbles between the substrates. In some devices, there are features, such as recesses or apertures that are formed in one or both of the substrates. The voids and bubbles that are avoided using the techniques and devices described herein are other than desired recesses and/or apertures that are purposely formed in a substrate and required for proper device construction. In some implementations, the voids and bubbles that are created by improper bonding of two substrates are greater than 2 millimeters in diameter. 
         [0027]    Referring to  FIGS. 1 and 2 , a mechanical device  100  can hold a lower substrate  240  and an upper substrate  200 . The upper substrate  200  can sit on the lower substrate  240  at one edge and be angled apart from the lower substrate  240  at the opposite edge The device can include a substrate support  610  that can be actuated up and down. The substrate support  610  can include substrate holders  612 , such as between three and six substrates holders, attached to the support  610 . The substrate holders  612  can be configured to project inwardly from the support and to touch only a small portion of the lower substrate  240 , such as a perimeter portion or edge of the substrate, thereby helping to ensure that the lower substrate  240  is kept both flat and clean. In some implementations, the substrate holders  612  are spaced to hold a 300 mm substrate. The substrate holders can be sized and positioned to accommodate other substrate sizes, such as 200 mm substrates or smaller or larger substrates. 
         [0028]    A separator unit  630  can be used to prevent portions of the substrates  200 ,  240  from contacting. The separator unit  630  can include a separating member  620 . The separating member  620  can project from the separator unit  630  and can be positioned to project between the main surfaces of the upper substrate  200  and the lower substrate  240 , e.g., generally parallel to the surface of the lower substrate. 
         [0029]    As shown in  FIG. 3A , the separating member  620  can include a tapered portion  622 . The tapered portion is tapered to be progressively thinner along at least one axis, e.g., its longitudinal axis, and the taper can be uniform along the length of the tapered portion  622 . For example, the separating member  620  can be in the shape of a pin or a wedge. In operation, the separator unit  630  can be held such that the separating member  620  is generally parallel to the surface of the lower substrate, and the width of the tapered portion  622 , as measured perpendicular to the surface of the substrates, progressively decreases toward the center of the substrates. As such, the tapered portion  622  can ensure that when the separating member  620  is removed from between the substrates  200 ,  240 , the substrates  200 ,  240  gradually come together in a controlled manner, e.g., without an abrupt drop. The cross-section of the separating member  620  normal to its longitudinal axis can be circular so that the separating member  620  does not have to be precisely aligned with the main surfaces of the substrates  200 ,  240 . The width of the separating member  620  at its thickest point can be between about 1 mm and 12 mm, such as 6 mm. The separating member  620  can have a length that is less than a radial distance of either of the substrates  200  or  240 . Further, the separating member  620  can have a maximum width of less than 3 mm, for example less than 1 mm. The separating member can be made of a material that does not scratch the surfaces of the substrates  200 ,  240 , such as plastic, ceramic, or metal, e.g., stainless steel. 
         [0030]    Each separator unit  630  can include a holding member  632 , e.g., a clamp, for securing the separating member  620 . A motor  650  (see  FIG. 4 ), such as a stepper motor, can be configured to actuate the separator member  620  in an outward and inward direction with respect to a central axis perpendicular to the surface of the substrate support  610 , as discussed further below. Thus, when the substrates  200 ,  240  are properly supported in the device  100  and the separator unit  630  is in operation, the separating member  620  can move inward or outward along an axis parallel to the surfaces of the substrate. The motor  650  can either be part of the separator unit  630  or a separate unit. 
         [0031]    In some implementations, the separating member  620  can be mounted on the clamp  632  such that it can pivot freely in the vertical direction, i.e. rather than being mounted rigidly in the clamp  632 . Mounting the separating member  620  to pivot freely in the vertical direction can both facilitate alignment of the separating member  620  and facilitate loading of substrates  200 ,  240 . For example, if the separating member is mounted to pivot freely in the vertical direction, then the separating member can be pivoted such that both substrates  200 ,  240  follow the taper as the separating member is removed to ensure that the substrates  200 ,  240  will come together smoothly. 
         [0032]    The separator unit  630  can further include a handle  634  to move the separating member  620  from an extended state as shown in  FIG. 3A  to a retracted state as shown in  FIG. 3B . In the retracted state, the separating member  620  can be located in a pocket  636  of the separator unit  630  away from the substrate support  610 . Placing the separating member  620  in the pocket  636  can avoid damage to the separating member  620 , e.g., the tapered portion  622  or sharp point of the separating member, when not in use. The handle  634  can further be used to move the separating member  620  out of the way before loading the lower substrate  240  and then to lower the separating member  620  before loading the upper substrate  200 . Optionally, the handle  634  can be automated, for example using an air cylinder. The automated process can cause the separating member  620  to automatically retract after the substrates  200 ,  240  have been bonded together. 
         [0033]    As shown in  FIG. 4 , the mechanical device can also include a monitoring device  400 , such as an infrared camera, to generate images of a bond wave between the substrates  200 ,  240 . The monitoring device  400  and/or the motor  650  can be connected to a controller  660 . 
         [0034]    In operation, a lower substrate  240  is placed on the substrate holders  612  of the substrate support  610 , the separating member  620  is lowered, and then the upper substrate  200  is placed on top of the supported lower substrate  240  at one edge and on the separating member  620  on the opposite edge. The substrates can be, for example, silicon or piezoelectric (e.g. PZT) substrates. The interface between the two substrates  200 ,  240  can be, for example, silicon-to-silicon, silicon-to-oxide, oxide-to-oxide, or BCB-to-silicon. One substrate can be, for example, a sacrificial substrate. 
         [0035]    At least one of the substrates can have an etched or patterned portion  202 , as shown in  FIG. 1 . The surface having the patterned portion  202  can have recesses on the surface at the interface between the two substrates that extend only partially through the substrate, or, as shown in  FIG. 1 , the patterned portion of the substrate can have apertures that extend all the way through the substrate. In some implementations, the patterned portion  202  includes inlet channels or pumping chambers for use in an ink jet printer. In some implementations, the patterned portion  202  has features, i.e., recesses or apertures, that are grouped into dies  204 . At some point during the process, the dies can be removed from the substrates. However, after the bonding step, multiple dies can remain part of an integral substrate. In some implementations, the dies have a length in one direction that is greater than a width in a perpendicular direction. 
         [0036]    The substrates and separating member  620  can be positioned so that an axis through a center of the length of the separating member  620  can be at an angle to an axis that runs along a length of ones or more of the dies  204 . The angle can be less than 30°, such as about 17° or about 0° (i.e., be parallel). Further, the separating member can be aligned approximately along an axis that intersects the center of the substrates  200 ,  240 , i.e. is aligned along a radial axis of the substrates  200 ,  240 . 
         [0037]    Referring back to  FIG. 4 , the separating member  620  can be moved in toward the center of the substrates  200 ,  240  along an axis  422  parallel to the plane defined by the substrate holders  612  using the motor  650 . The distance at which the separating member  620  is placed along the radial axis of the substrates can be determined based upon the ability of the substrates  200 ,  240  to bond. For example, if the separating member  620  is placed too far in between the substrates  200 ,  240 , then the substrates will not be able to bond together due to the amount of space between them. Therefore, the separating member  620  can be extended between the substrates  200 ,  240  by less than a radial distance, such as 0.5 mm to 50 mm, for example 3 mm. In some implementations, the separating member  620  can be permanently mounted in the desired alignment so that additional alignment is not necessary. 
         [0038]    Referring to  FIGS. 4 and 4A , the separating member  620  can cause the substrates  200 ,  240  to separate and form a gap  408  between the substrates at the edge of the substrates. The maximum gap length L at the edge of the substrates can be about 0.5 mm to 5 mm. 
         [0039]    After the separating member  620  has been placed between the substrates  200 ,  240 , a pressure can be applied to the substrates  200 ,  240 , such as by pressing on upper substrate  200 . The pressure can be applied at a point  414  that is about 180° from the separating member  620 , i.e., the pressure point can be applied on the opposite side of the substrates  200 ,  240  as the separating member  620 . In some implementations, the pressure point is close to the substrates&#39; edge. The pressure can be applied with a pressure inducer  412 , which can be manually actuatable. Alternatively, the pressure inducer  412  can be an automated pressure inducer that actuates on a signal from the controller  660 . The pressure inducer  412  can be made, for example, of a resin, such as polypropylene, for example, if it is a manual pressure inducer. The pressure inducer  412  can also be made, for example, of a compliant material, such as rubber, for example if it is an automated pressure inducer, so that when the inducer contacts the surface, it can flex and slide slightly across the surface of the substrate to initiate the bond between the two substrates  200 ,  240 . The pressure inducer can have a tip that is less than 5 mm in diameter. Alternatively, the pressure inducer  412  can be an air cylinder, which ejects air onto the substrates to put pressure between the two substrates. The pressure inducer  412  is capable of exerting a pressure on the upper substrate  200  that is at an angle other than parallel to the main surface  680  of the lower substrate  240 , for example at an angle of between 45° and 90° with the surface  680 . A pressure of between about 0.5 psi and 5 psi, such as about 1 psi can be applied with the pressure inducer  412  at the pressure point  414 . 
         [0040]    Referring to  FIGS. 5A-5C , the pressure can initiate room temperature fusion bonding between the substrates  200 ,  240  of a substrate assembly (upper substrate  200  is treated as transparent to show the bond wave). Fusion bonding, which creates Van der Waals bonds between the two surfaces, occurs when two flat, highly polished, clean surfaces are brought together with no intermediate adhesive layer between the surfaces. Referring to  FIG. 5B , the initial pressure application at pressure point  414  will start a bond between the substrates  200 ,  240 . The edge  502  of the bond (i.e., the edge that divides the bonded portion  510  from the unbonded portion  512 ) can be called the bond front. Starting from the regions closest to the bond front  502 , the remaining portions of the substrates will then be attracted to one another due to Van der Waals forces. As a result, shown in  FIGS. 5A-5C , the bond front  502  propagates across the substrates. This traveling of the bond front can be called a “bond wave.” 
         [0041]    As the substrates  200 ,  240  are bonded together, the bond wave can be monitored using the monitoring device  400 . The monitoring device can reveal the position of the bond front  502  between the substrates  200 ,  240 . At a certain point, for example when the monitoring device  400  shows that the bond wave has stopped due to the separating member  420  pulling the substrates  200 ,  240  too far apart to bond, or when a sensor detects a particular position of the bond wave, the separating member  620  can be translated radially away from a center of the substrates  200 ,  240  along the axis  422 , for example using the motor  650 . As shown in  FIG. 4 , the lower substrate  240  has a primary face  680  and a thin side  670 . The separating member  620  moves in a direction perpendicular to the thin side  670  of the substrates and parallel to the primary face  680 . The separating member  620  can be translated at a rate that is less than a maximum rate above which voids and bubbles can be trapped between the substrates  200 ,  240 . For example, the separating member  620  can be translated at between about 50 mm/s and 75 mm/s. The rate at which the separating member  620  moves can be controlled, for example, using the controller  660 . 
         [0042]    The rate at which the separating member  620  is translated can relate to the rate at which the bond front  502  propagates across the substrates. Further, the rate at which the bond front  502  propagates can relate to the activation level of the substrates  200 ,  240 . For example, a silicon-to-silicon bond is considered highly active and bonds quickly, causing the bond front to move quickly across the substrates. As a result, the rate of translation of the separating member can be faster. If, however, one of the substrate surfaces is contaminated, then the substrates will be less active, and the bond front will move slower. As a result, the rate of translation of the separating member may be slower. Similarly, a silicon-to-oxide bond or oxide-to-oxide is less active than a silicon-to-silicon bond, so the bond front moves slower across the substrates, and the rate of the separating member may therefore also be slower than with the silicon-to-silicon bond. 
         [0043]    Referring to  FIGS. 5D-5F , the movement of the separating member  620  can be controlled to ensure that the bond wave moves evenly across the substrates  200 ,  240 . That is, as the separating member  620  is translated, portions of the substrates  200 ,  240  that are unbonded due to the gap between them are brought close enough that Van der Waals forces can create a bond. A velocity profile can be created based on the speed of the bond wave at different points between the substrates and to determine the resulting rate of translation of the separating member  620  and the time at which the translation should begin. For example, if the bond wave speeds up near the end, then the separator can slow down near the end to slow down the bond wave, and vice versa. Because the translation of the separating member  620  can be controlled, the speed of the bond wave can be controlled to ensure that the bond wave moves evenly across the substrate. In some implementations, the bond front  502  is controlled to remain about straight or linear as it moves between the substrates  200 ,  240 . The process continues until the separating member  620  has been removed completely from between the substrates  200 ,  240  and the substrates  200 ,  240  are fully bonded together. 
         [0044]    When fusion bonding is used to bond two substrates together without using a separating member as described herein, the movement of the bond front can be uneven. For example, the bond wave can move slower across patterned areas than nonpatterned areas. Likewise, the bond wave can move slower across patterned areas with deep etchings than patterned areas with shallow etchings. In some cases, the bond wave can move around a circular area between the two substrates, creating an area of trapped air that prevents the substrates on either side of the air bubble from coming in close enough contact to form the requisite Van der Waals bonding. Thus, uneven movement can cause voids and air bubbles to be trapped between the substrates, which can reduce the effectiveness of the bond or even form defectively bonded dies or devices. By translating the separating member away from the center of the substrates, the bond wave between the substrates  200 ,  240  can be precisely controlled so that the bond front moves straight across the substrates. That is, the bond front does not move such that two portions of the front move faster across the substrates than a third portion between the two portions and meet one another, trapping the third portion of the front as an edge of an air bubble. As a result of using the separator described herein, the bond wave can be forced to move across all portions of the substrates, e.g. portions that are deeply etched, shallowly etched, or not etched, at about the same rate, thereby significantly reducing or avoiding the generation of voids or air bubbles between the substrates. 
         [0045]    A number of embodiments of the invention have been described. Other embodiments are within the scope of the following claims.