Abstract:
A method and apparatus for bonding on a silicon substrate are disclosed. An apparatus includes a membrane having a lower membrane surface and an upper membrane surface, a transducer having a transducer surface substantially parallel to the upper membrane surface, and an adhesive connecting the membrane to the transducer surface. In some implementations, the lower membrane surface is substantially contiguous and the upper membrane surface protrudes therefrom. In some other implementations, the upper membrane surface is substantially contiguous and the lower membrane surface is recessed therein.

Description:
BACKGROUND 
       [0001]    The following disclosure relates to bonding on a substrate, such as a silicon die. 
         [0002]    In some implementations of a fluid ejection device, fluid droplets are ejected from one or more nozzles onto a medium. The nozzles are fluidly connected to a fluid path that includes a fluid pumping chamber. The fluid pumping chamber is actuated by a transducer, and when actuated, the fluid pumping chamber causes ejection of a fluid droplet. The medium can be moved relative to the fluid ejection device. The ejection of a fluid droplet from a particular nozzle is timed with the movement of the medium to place a fluid droplet at a desired location on the substrate. In these fluid ejection devices, it is usually desirable to eject fluid droplets of uniform size and speed and in the same direction in order to provide uniform deposition of fluid droplets on the medium. 
       SUMMARY 
       [0003]    In one aspect, the systems, apparatus, and methods described herein include a membrane having a lower membrane surface and an upper membrane surface. A transducer can have a transducer surface that is substantially parallel to the upper membrane surface. An adhesive can connect the membrane to the transducer surface. 
         [0004]    In another aspect, the systems, apparatus, and methods described herein include arranging a transducer surface of a transducer proximate a membrane. The membrane can have an upper membrane surface and a lower membrane surface. The transducer surface can be facing the upper membrane surface and can be substantially parallel thereto. Adhesive can be applied to the transducer surface or the upper membrane surface or both. The transducer can be pressed against the membrane surface. At least some of the adhesive can be allowed to flow toward or along the lower membrane surface. 
         [0005]    Implementations can include one or more of the following features. In some implementations, the lower membrane surface can be generally contiguous and the upper membrane surface can protrude from the lower membrane surface, such as by between about 2.0 microns and about 5.0 microns. The apparatus can include multiple upper membrane surfaces. The multiple upper membrane surfaces can be formed at about equal height with respect to one another, can be substantially circular, and can be located near a critical bond area. In some other implementations, the upper membrane surface can be generally contiguous and the lower membrane surface can be recessed into the upper membrane surface, such as by between about 2.0 microns and about 5.0 microns. The apparatus can include multiple lower membrane surfaces recessed into the upper membrane surface. The multiple lower membrane surfaces can be formed at about equal depth with respect to one another, can be substantially circular, and can be located near a critical bond area. The adhesive can include benzocyclobutene. The transducer can include a piezoelectric material, such as lead zirconium titanate. 
         [0006]    In some embodiments, one or more of the following advantages may be provided. Flow of adhesive into recesses or grooves can reduce the thickness of adhesive between the transducer and the membrane. Reducing the thickness of adhesive can reduce the energy required to actuate a transducer to change the volume of a fluid pumping chamber so as to cause fluid droplet ejection. As a further advantage of recesses, providing space for adhesive to flow can mitigate or prevent a build-up of adhesive, which could press the membrane into the pumping chamber and thereby alter the effectiveness of the transducer when actuated. Because such build-up can be non-uniform across multiple actuators and fluid pumping chambers, the recesses can improve uniformity of fluid droplet ejection size and speed, as well as the accuracy of placement of fluid droplets on a medium. 
         [0007]    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 
         [0008]      FIG. 1  is a cross-sectional view of an apparatus for fluid droplet ejection. 
           [0009]      FIG. 2  is a flow diagram of a process of bonding a layer. 
           [0010]      FIGS. 3-7  are cross-sectional views of stages of forming an apparatus for fluid droplet ejection. 
           [0011]      FIG. 8A  is a cross-sectional view of a portion of an apparatus for fluid droplet ejection. 
           [0012]      FIG. 8B  is a cross-sectional view along line  8 - 8  in  FIG. 8A . 
           [0013]      FIG. 9  is a cross-sectional view of a portion of an apparatus for fluid droplet ejection. 
           [0014]      FIG. 10  is a plan view of a portion of the apparatus of  FIG. 9 . 
           [0015]      FIG. 11  is a flow diagram of a process of bonding a layer. 
           [0016]      FIGS. 12-17  are cross-sectional views of stages of forming an apparatus for fluid droplet ejection. 
       
    
    
       [0017]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0018]    An apparatus for fluid droplet ejection can have a fluid ejection module, e.g., a rectangular plate-shaped printhead module, which can be a die fabricated using semiconductor processing techniques. The fluid ejector can also include a housing to support the printhead module, along with other components such as a flex circuit to receive data from an external processor and provide drive signals to the printhead module. 
         [0019]    The printhead module includes a substrate in which a plurality of fluid flow paths are formed. The printhead module also includes a plurality of actuators, e.g., transducers, to cause fluid to be selectively ejected from the flow paths. Thus, each flow path with its associated actuator provides an individually controllable MEMS fluid ejector unit. 
         [0020]    The substrate can include a flow-path body, a nozzle layer, and a membrane layer. The flow-path body, nozzle layer, and membrane layer can each be silicon, e.g., single crystal silicon. The fluid flow path can include a fluid inlet, an ascender, a pumping chamber adjacent the membrane layer, and a descender that terminates in a nozzle formed through the nozzle layer. Activation of the actuator causes the membrane to deflect into the pumping chamber, forcing fluid out of the nozzle. 
         [0021]    The membrane can have recesses formed therein. An adhesive can bond or connect a transducer to the membrane, and the adhesive can at least partially occupy the recesses. The recesses can be arranged to define protrusions, such as posts, on the membrane. Alternatively, the recesses can be formed as grooves in the membrane. 
         [0022]      FIG. 1  is a cross-sectional view of a portion of a fluid droplet ejection apparatus. An inlet  100  fluidically connects a fluid supply (not shown) to a die  10  that includes a substrate  17  and a transducer  30 . The substrate  17  includes a flow-path body  11 . The inlet  100  is fluidly connected to an inlet passage  104  formed in the flow-path body  11  through a passage (not shown). The inlet passage  104  is fluidically connected to a pumping chamber  18 , such as through an ascender  106 . The pumping chamber  18  is fluidically connected to a descender  110 , at the end of which is a nozzle  112 . The nozzle  112  can be defined by a nozzle layer, such as a nozzle plate  12 , that is attached to the flow-path body  11 . The nozzle  112  includes an outlet  114  defined by an outer surface of the nozzle plate  12 . In some implementations, a recirculation passage  116  can be provided to fluidically connect the descender  110  to a recirculation channel  118 . A membrane  14  is formed on top of the flow-path body  11  in close proximity to and covering the pumping chamber  18 , e.g. a lower surface of the membrane  14  can define an upper boundary of the pumping chamber  18 . A transducer  30  is disposed on top of the membrane  14 , and a layer of adhesive  26  with a thickness T is between the transducer  30  and the membrane  14  to bond the two to one another. In some implementations, each pumping chamber  18  has a corresponding electrically isolated transducer  30  that can be actuated independently. The transducer  30  includes electrodes  84 ,  88  ( FIG. 9 ) to allow for actuation of the transducer  30  by a circuit (not shown). 
         [0023]    A top surface of the membrane  14 , i.e., the surface closer to the transducer, includes recesses  22  that are at least partially filled with the adhesive  26 , as discussed below. The recesses  22  extend partly, but not entirely, through the membrane  14 . The recesses  22  can be located only in regions of the membrane that are not directly over the pumping chambers  18 . However, in some implementations some recesses  22  can be located over the pumping chambers  18 . 
         [0024]    The membrane  14  can be formed of silicon (e.g., single crystalline silicon), some other semiconductor material, oxide, glass, aluminum nitride, silicon carbide, other ceramics or metals, silicon-on-insulator, or any depth-profilable layer. Depth profiling methods can include etching, sand blasting, machining, electrical-discharge machining (EDM), micro-molding, or spin-on of particles. For example, the membrane  14  can be composed of an inert material and have compliance such that actuation of the transducer  30  causes flexure of the membrane  14  sufficient to pressurize fluid in the pumping chamber  18  to eject fluid drops from the nozzle  112 . U.S. Patent Publication No. 2005/0099467, published May 12, 2005, the entire contents of which is hereby incorporated by reference, describes examples of a printhead module and fabrication techniques. In some implementations, the membrane  14  can be formed unitary with the flow-path body  11 . 
         [0025]    In operation, fluid flows through the inlet channel  100  into the flow-path body  11  and through the inlet passage  104 . Fluid flows up the ascender  106  and into the pumping chamber  18 . When a transducer  30  above a pumping chamber  18  is actuated, the transducer  30  deflects the membrane  14  into the pumping chamber  18 . The resulting change in volume of the pumping chamber  18  forces fluid out of the pumping chamber  18  and into the descender  110 . Fluid then passes through the nozzle  112  and out of the outlet  114 , provided that the transducer  30  has applied sufficient pressure to force a droplet of fluid through the nozzle  112 . That is, the transducer  30  pressurizes the fluid pumping chamber  18 , and a resulting pressure pulse, which can be referred to as a firing pulse, effects ejection of a droplet of fluid through the nozzle  112 . The droplet of fluid can then be deposited on a medium. 
         [0026]      FIG. 2  is a flow chart of a process for bonding the transducer  30  to the membrane  14  on a flow-path body  11 .  FIGS. 3-9  are cross-sectional diagrams of steps in the fabrication of an apparatus for fluid droplet ejection. As shown in  FIG. 3 , a photoresist layer  15  is formed on top of the membrane  14  (step  215 ). In some implementations, the nozzle plate  12  has multiple layers, some of which can be used for holding the apparatus during fabrication and can be removed during later fabrication steps. As shown in  FIG. 4 , the photoresist layer  15  is patterned using conventional photolithography techniques so that portions of the photoresist layer  15  are removed, and apertures  21  are thereby formed in the photoresist layer  15  (step  225 ). Referring to  FIG. 5 , the membrane  14  is etched through the apertures  21  in the photoresist layer  15  to form recesses  22  in the membrane  14  (step  235 ). As shown in  FIG. 6 , the photoresist layer  15  is then removed (step  245 ). 
         [0027]    In the implementation shown, the recesses  22  do not extend entirely through the membrane  14 . The depth of etching into the membrane  14  can be controlled, for example, by etching for a predetermined amount of time, stopping the etching process when a desired recess depth D r  of the recesses  22  has been achieved as detected by an in-situ monitoring system, or by including an etch-stop layer in the membrane  14  at depth D r . In some implementations, the recess depth D r  is between about 0.5 microns and about 10 microns, such as between about 2.0 microns and about 5.0 microns, and each of the recesses  22  are of substantially equal recess depth D r . The area between the recesses  22  defines posts  25 , which can also be referred to as protrusions. The posts  25  have a height equal to the recess depth D r . In alternative implementations, the recesses  22  can extend entirely through the membrane  14  so long as remaining membrane material adjacent the recesses  22  is adequately supported, such as by the flow-path body  11 . 
         [0028]    Referring to  FIG. 7 , adhesive  26  is applied to, or formed on, a surface of the transducer  30  facing the membrane  14  (step  255 ), and the transducer  30  with adhesive  26  is placed on the membrane  14  (step  265 ). Alternatively, adhesive  26  is applied to the membrane  14  instead of, or in addition to, adhesive  26  being applied to the transducer  30 . Pressure can be applied to press the substrate  17  and the transducer  30  toward each other, and adhesive  26  is allowed to at least partially flow into the recesses  22  (step  275 ). 
         [0029]    The membrane  14  can have a thickness of between about 1.0 micron and about 150 microns, such as between about 8.0 microns and about 20 microns. This thickness can be selected based in part on a desired recess depth D r . The depth selected for the recesses  22 , and thus the height of the posts  25 , can depend on the viscosity of the adhesive  26  during the curing state and the thickness of the adhesive  26  applied to either the membrane  14  or the transducer  30 . Temperature can affect the viscosity of the adhesive during the curing cycle, such as by making the adhesive  26  more viscous. A highly viscous adhesive  26  may flow slowly and need more space to flow sufficiently before curing. For example, relatively tall posts  25  may be needed to allow a highly viscous adhesive  26  to flow. Similarly, the greater the thickness of adhesive  26  between the membrane  14  and the transducer  30 , the more space may be needed to hold excess adhesive  26 . In some implementations, when a layer of adhesive  26  applied to the transducer  30  has a thickness of about 1.0 micron, the height of the posts  25  is between about 2.0 microns and about 5.0 microns. Alternatively, rather than defining posts  25 , the recesses  22  can define grooves, as described in U.S. Patent Application No. 61/098,187 filed concurrently herewith, the entire contents of which is incorporated herein by reference. 
         [0030]      FIG. 7  shows the transducer  30  and adhesive  26  on top of the membrane  14 . The adhesive  26  is between the transducer  30  and the membrane  14  and can partially or entirely fill the recesses  22 . The transducer  30  and the membrane  14  are not in direct contact because a layer of adhesive  26  is between them. As the thickness T of the layer of adhesive  26  increases, more energy (e.g., greater voltage) must be applied to the transducer  30  to cause sufficient deformation to effect fluid droplet ejection. Reducing the thickness of the layer of adhesive  26  is therefore desirable to minimize the energy requirements of the transducer  30 . 
         [0031]    In some implementations, the adhesive  26  must be present in a minimum thickness because of the material properties of the adhesive  26  or other limitations such as the process for applying the adhesive. For example, in the absence of the recesses  22 , with some types of adhesives, the minimum thickness of the adhesive  26  can be between about 1000 nanometers and about 1200 nanometers. The recesses  22  can reduce the minimum achievable thickness of adhesive  26  by allowing some adhesive to flow into the recesses  22  when the transducer  30  and adhesive  26  are pressed toward the membrane  14 . In contrast, where recesses  22  are present, the minimum achievable thickness of the adhesive  26  can be about 200 nanometers or less, such as about 100 nanometers or less. 
         [0032]    To attempt to achieve a minimum thickness of the adhesive  26 , the transducer  30  and the membrane  14  can be pressed together to squeeze out excess adhesive  26 . A flow resistance of the adhesive  26  increases linearly with an increase in a distance that the adhesive  26  travels before exiting from between the transducer  30  and the membrane  14 . For example, without the recesses  22 , adhesive  26  near a center of the transducer  30  and the membrane  14  travels about 75 millimeters before being squeezed out. As a contrasting example, where the membrane  14  has recesses  22  formed therein, adhesive  26  near the center only travels about 150 microns to flow into the recesses  22 . Since the flow resistance is proportional to the distance traveled, adhesive  26  flowing into the recesses  22  has a flow resistance that is about 500 times less than without the recesses  22 . Thus, more excess adhesive  26  can be squeezed out before curing, which can result in a relatively thinner layer of adhesive  26 . For example, if a 1.0 micron layer of adhesive  26  is applied between the two parts, the minimum thickness without recesses  22  might be between about 1000-1200 nanometers. With recesses  22 , by contrast, a minimum thickness of adhesive  26  may be about 200 nanometers or less. The flow resistance of the adhesive between the transducer  30  and membrane  14  can be described by the formula R=kμL/t 3 , where R is flow resistance, k is a constant, μ is a viscosity of the fluid, L is a length, and t is a thickness of the adhesive  26 . 
         [0033]    As noted, the adhesive  26  can be applied to the transducer  30  before bonding. In other implementations, the adhesive  26  is applied to the membrane  14  instead of, or in addition to, adhesive  26  being applied to the transducer  30 . The amount of adhesive  26  applied in the recesses  22  can be minimized to maximize the percentage of applied adhesive  26  that flows into the recesses  22 . The adhesive  26  can be an organic material, such as benzocyclobutene (BCB), or other suitable material. 
         [0034]      FIGS. 8A and 8B  show an implementation of the membrane  14 , adhesive  26 , and transducer  30 . The distance between the membrane  14  and the transducer  30  has been exaggerated for illustrative purposes. In this implementation, the posts  25  are substantially circular and adhesive  26  is positioned between the posts  25  and between the membrane  14  and the transducer  30 . Alternatively, the posts  25  can be square, rectangular, oval, some other closed shaped, or some other suitable shape. In addition, in this implementation, the recess  22  surrounds multiple posts  25 , e.g., the recess is a generally continuous single area on the substrate. 
         [0035]      FIG. 9  shows a cross-section of an implementation of transducers  30  on the membrane  14  above pumping chambers  18 . Multiple pumping chambers  18  are shown, and in this implementation, the membrane  14  includes recesses  22  in portions of the membrane  14  near, but not directly over, the pumping chambers  18 . The transducer  30  includes a top electrode  84 , a piezoelectric layer  80 , and a bottom electrode  88 . The top electrode  84  and the bottom electrode  88  are arranged on the top and bottom surface, respectively, of the piezoelectric layer  80 . The adhesive  26  bonds the transducer  80  to the membrane  14 . A circuit (not shown) can be electrically connected to the top electrode  84  and to the bottom electrode  88 . The circuit can apply a voltage between the electrodes  84 ,  88 . The applied voltage can actuate the transducer  30 , causing the piezoelectric material to deform. This deformation can deflect the membrane  14  into the pumping chamber  18 , thereby forcing fluid out of the pumping chamber  18 . 
         [0036]      FIG. 10  is a plan diagram of the implementation shown in  FIG. 9 , and two rows of transducers  30  are shown. These two rows of transducers  30  correspond to two rows of pumping chambers  18 , which can correspond to two rows of nozzles  112  beneath the pumping chamber  18 . 
         [0037]    In some implementations, the recesses  22  can be arranged in a continuous or interconnected manner to define protrusions on the surface of the membrane  14 , such as circular protrusions. In other implementations, the recesses  22  can be disconnected, and can themselves be circular. Alternatively, the recesses  22  can form grooves along a length of the membrane  14 . In some implementations, the recesses  22  can be formed in portions of the membrane  14  above a pumping chamber  18 . In other implementations, the recesses  22  can be formed in portions of the membrane  14  not above a pumping chamber  18 . In some implementations, the recesses  22  can be formed near critical bond areas. Critical bond areas can include portions of the membrane  14  near the edges of a pumping chamber  18 . 
         [0038]      FIG. 11  is a flow chart showing an alternative method of forming the recesses  22  in the membrane  14 .  FIGS. 12-17  are cross-sectional diagrams of steps in the fabrication of an apparatus for fluid droplet ejection. As shown in  FIG. 12 , a texture mask  13  is formed on top of the membrane  14  (step  905 ). The texture mask  13  can be made from an oxide, such as silicon oxide. Use of a texture mask  13  can be desirable where, for example, the texture mask  13  has a higher selectivity than a photoresist. That is, a relatively smaller thickness of texture mask  13  can be used to etch the membrane  14  to a relatively larger depth. A photoresist layer  15  is formed on top of the texture mask  13  (step  915 ). Referring to  FIG. 13 , the photoresist layer  15  is patterned using conventional photolithography techniques so that portions of the photoresist layer  15  are removed, and apertures  20  are thereby formed in the photoresist layer  15  (step  925 ). Referring to  FIG. 14 , the texture mask  13  is etched through the apertures  20  in the photoresist layer  15  to form apertures  21 ′ in the texture mask  13  (step  935 ). Referring to  FIG. 15 , the photoresist layer  15  is then removed (step  945 ). Referring to  FIG. 16 , the membrane  14  is then etched through the apertures  21 ′ in the texture mask  13  to form membrane recesses  22  (step  955 ). In some implementations, the membrane recesses  22  do not extend entirely through the membrane  14 , as described above. Referring to  FIG. 17 , the texture mask  13  is then removed, such as by grinding, by bathing in hydrofluoric acid, or some other suitable mechanical or chemical mechanism (step  965 ). Adhesive  26  is applied to, or formed on, a surface of the transducer  30  facing the membrane  14  (step  975 ), and the transducer  30  with adhesive  26  is placed on the membrane  14  (step  985 ), as shown in  FIG. 7 . Alternatively, adhesive  26  is applied to the membrane  14  instead of, or in addition to, adhesive  26  being applied to the transducer  30 . Pressure is applied, and adhesive  26  is allowed to at least partially flow into the membrane recesses  22  (step  995 ). 
         [0039]    The above-described implementations can provide none, some, or all of the following advantages. Flow of the adhesive into recesses or grooves can minimize the thickness of the adhesive between the transducer and the membrane. Reducing the thickness of adhesive can reduce the energy required to actuate a transducer and change the volume of a fluid pumping chamber so as to cause fluid droplet ejection. Further, where the thickness of applied adhesive is non-uniform, providing space for adhesive to flow can mitigate or prevent a build-up of adhesive, which might otherwise press the membrane into the pumping chamber and thereby influence the effectiveness of the transducer when actuated. Particularly where multiple pumping chambers and nozzles are used, varying degrees of deflection of the membrane into the pumping chambers can result in varying degrees of effectiveness among the multiple pumping chambers. Variations in the effectiveness across multiple pumping chambers can cause variation of fluid droplet ejection size or speed among the multiple nozzles, which may cause incorrect fluid droplet size or placement on a medium. By mitigating or preventing deflection of the membrane by adhesive, the recesses described above can improve uniformity of fluid droplet ejection size or speed. Uniformity among actuators on a die is thereby improved, which decreases the likelihood of incorrect fluid droplet placement. 
         [0040]    The use of terminology such as “front,” “back,” “top,” and “bottom” throughout the specification and claims is for illustrative purposes only, to distinguish between various components of the fluid droplet ejection apparatus and other elements described herein. The use of “front,” “back,” “top,” and “bottom” does not imply a particular orientation of the fluid droplet ejection apparatus, the substrate, the die, or any other component described herein. 
         [0041]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, recesses in the membrane could be any shape or profile that provides space for adhesive to flow or reside. Accordingly, other embodiments are within the scope of the following claims.