Abstract:
Fluid ejection apparatuses and processes for making the same are disclosed. An apparatus for ejecting fluid droplets includes a substrate having a plurality of flow paths formed therein, each flow path including a respective pumping chamber and a respective nozzle, and the respective nozzle being configured to eject fluid droplets through a first surface of the substrate in response to actuation of the respective pumping chamber; and an actuation assembly including a drive electrode layer over a second surface of the substrate opposite to the first surface, a piezoelectric layer over the drive electrode layer, and a reference electrode layer over the piezoelectric layer, the drive electrode layer being patterned to define an individually controllable drive electrode over each of two or more pumping chambers in the substrate, and the reference electrode layer including a continuous reference electrode spanning the two or more pumping chambers in the substrate.

Description:
TECHNICAL FIELD 
     This specification relates to MEMS actuators. 
     BACKGROUND 
     An example fluid ejection module typically has a line or an array of nozzles with a corresponding array of flow paths. Each flow path includes a pumping chamber controllable by an associated actuator, such as a piezoelectric actuator. The piezoelectric actuator includes a layer of piezoelectric material that changes geometry (or actuates) in response to a voltage applied across the piezoelectric layer by a pair of opposing electrodes. The pair of opposing electrodes includes a drive electrode and a reference electrode, and the voltage applied across the piezoelectric layer can be controlled by maintaining the reference electrode at a fixed reference voltage or potential while varying a driving voltage or potential on the driving electrode. The piezoelectric actuator is attached to a membrane over the pumping chamber. The actuation of the piezoelectric layer in response to the voltages applied across the piezoelectric layer causes the membrane to flex. The flexing of the membrane pressurizes fluid in the pumping chamber along the flow path and eventually ejects a fluid droplet out of the nozzle. 
     SUMMARY 
     This specification describes technologies related to MEMS actuators. 
     In one aspect, an apparatus for ejecting fluid droplets includes: a substrate having a plurality of flow paths formed therein, each flow path including a respective pumping chamber and a respective nozzle, and the respective nozzle being configured to eject fluid droplets through a first surface of the substrate in response to actuation of the respective pumping chamber; and an actuation assembly including a drive electrode layer over a second surface of the substrate opposite to the first surface, a piezoelectric layer over the drive electrode layer, and a reference electrode layer over the piezoelectric layer, the drive electrode layer being patterned to define an individually controllable drive electrode over each of two or more pumping chambers in the substrate, and the reference electrode layer including a continuous reference electrode spanning the two or more pumping chambers in the substrate. 
     In some implementations, the apparatus further includes circuitry electrically connected to each drive electrode and to the continuous reference electrode, the circuitry being configured such that during fluid droplet ejection, the continuous reference electrode being maintained at a reference voltage at or near an earth ground voltage, and each drive electrode being configured to actuate the piezoelectric layer using driving voltages whose minimum is at or above the reference voltage. 
     In some implementations, the apparatus further includes an integrated circuit layer, the integrated circuit layer being attached to the actuation assembly on a side of the actuation assembly opposite to the substrate, the integrated circuit layer including one or more application-specific integrated circuits (ASICs) for individually controlling the drive electrodes in the actuation assembly. 
     In some implementations, the integrated circuit layer includes fluid passages for transporting fluid to the flow paths in the substrate. 
     In some implementations, the integrated circuit layer includes one or more active circuit components and an electrical ground, the one or more active circuit component being electrically connect to the drive electrodes in the actuation assembly, and the electrical ground being electrically connected to the reference electrode in the actuation assembly. 
     In some implementations, the one or more active circuit components comprise positive logic transistors having a low state at or above a voltage of the electrical ground. 
     In some implementations, the one or more active circuit components comprise NMOS or NDMOS transistors. 
     In some implementations, the electrical ground in the integrated circuit layer is maintained at a reference voltage at or near an earth ground voltage, and the one or more active circuit components have minimum output voltages at or above the reference voltage. 
     In some implementations, the piezoelectric layer is poled in a direction pointing from the drive electrode layer to the reference electrode layer. 
     In some implementations, the piezoelectric layer comprises a layer of Lead Zirconate Titanate (PZT). 
     In some implementations, the piezoelectric layer comprises a layer of sputtered Lead Zirconate Titanate (PZT). 
     In some implementations, the substrate has a nozzle density sufficient to generate a resolution of at least 100 dpi. 
     In some implementations, the pumping chambers in the substrate form a parallelogram-shaped array, and each drive electrode has at least one pair of edges along a direction substantially parallel to an edge of the parallelogram-shaped array and spans a cross-section of a single pumping chamber in the substrate. 
     In some implementations, the drive electrode layer is made of a first conductor and the reference electrode is made of a second, different conductor. 
     In some implementations, at least one of the drive electrode layer or the reference electrode layer has a thickness of 1000 to 3000 Angstroms. 
     In some implementations, the piezoelectric layer is patterned to define an individual piezoelectric element within an area over each drive electrode in the drive electrode layer. 
     In some implementations, the continuous reference electrode includes one or more apertures. 
     In some implementations, the apparatus further includes an integrated circuit layer attached to the actuation assembly on a side of the actuation assembly opposite to the substrate, and each of the one or more apertures encircling a respective electrical connection between a drive electrode in the actuation assembly and the integrated circuit layer. 
     In some implementations, the respective electrical connection between the drive electrode and the integrated circuit layer is a vertically-oriented conductive column going through the aperture, and wherein the vertically-oriented conductive column is electrically connected to the drive electrode and is electrically insulated from the reference electrode. 
     In some implementations, the apparatus further includes: an integrated circuit layer attached to the actuation assembly on a side of the actuation assembly opposite to the substrate; and one or more vertically-oriented conductive columns between the integrated circuit layer and the continuous reference electrode layer. 
     In some implementations, at least some of the one or more conductive columns electrically connect the reference electrode and the integrated circuit layer. 
     In some implementations, at least some of the one or more conductive columns are electrically connected to a voltage ground in the integrated circuit layer. 
     In another aspect, a method for forming a fluid ejection apparatus includes the actions of: attaching an actuation assembly to a first surface of a substrate, the actuation assembly including a drive electrode layer attached to the first surface the substrate, a piezoelectric layer over the drive electrode layer, and a reference electrode layer over the piezoelectric layer, the drive electrode layer being patterned to define an individually controllable drive electrode over each of two or more pumping chambers formed or to be formed in the substrate, and the reference electrode layer including a continuous reference electrode spanning the two or more pumping chambers in the substrate; and forming a plurality of flow paths in the substrate, each flow path including a respective pumping chamber and a respective nozzle, and the respective nozzle being configured to eject fluid droplets through a second surface of the substrate in response to actuation of the respective pumping chamber. 
     In some implementations, the action of attaching the actuation assembly to the first surface of the substrate further includes the actions of: depositing a first conductive layer over the first surface of the substrate, the first conductive layer forming the drive electrode layer; sputtering a layer of piezoelectric material over the first conductive layer, the layer of sputtered piezoelectric material forming the piezoelectric layer; patterning the layer of sputtered piezoelectric material to form an individual piezoelectric element over each of the two or more pumping chambers formed or to be formed in the substrate; after patterning the layer of sputtered piezoelectric material, patterning the first conductive layer to form the individually controlled drive electrode over each of the two or more pumping chambers formed or to be formed in the substrate; depositing an insulating layer over the individual piezoelectric elements and the individually controlled drive electrodes formed after the patterning of the piezoelectric layer and the drive electrode layer; and depositing a second conductive layer over the insulating layer, the second conductive layer forming the reference electrode layer. 
     In some implementations, the method further includes the action of: attaching an integrated circuit layer to the actuation assembly on a side of the actuation assembly opposite to the substrate, the integrated circuit layer including one or more application-specific integrated circuits (ASICs) for individually controlling the drive electrodes in the actuation assembly. 
     In some implementations, the integrated circuit layer includes vertically-oriented fluid passages for transporting fluid to the flow paths in the substrate. 
     In some implementations, the integrated circuit layer includes one or more active circuit components and an electrical ground, and the method further includes the actions of: electrically connecting the one or more active circuit component to the drive electrodes in the substrate; and electrically connecting the electrical ground to the reference electrode in the substrate. 
     In some implementations, the action of attaching the integrated circuit layer to the actuation assembly further includes the action of: electrically connecting one or more active components of the integrated circuit layer to the drive electrodes by eutectic bonding. 
     In another aspect, an apparatus for ejecting fluid droplets includes: a substrate having a plurality of flow paths formed therein, each flow path including a respective pumping chamber and a respective nozzle, and the respective nozzle being configured to eject fluid droplets through a first surface of the substrate in response to actuation of the respective pumping chamber; an actuation assembly, the actuation assembly including a drive electrode layer attached to a second surface the substrate opposite to the first surface, a piezoelectric layer over the drive electrode layer, and a reference electrode layer over the piezoelectric layer, the drive electrode layer including a respective drive electrode over each pumping chamber in the substrate, and the reference electrode layer including a continuous reference electrode; and an integrated circuit layer, the integrated circuit layer being attached to the actuation assembly on a side of the actuation assembly opposite to the substrate, the integrated circuit layer including circuitry electrically connected to each drive electrode and to the continuous reference electrode, the circuitry being configured such that during fluid droplet ejection, the reference electrode is maintained a reference voltage at or near an earth ground voltage, and each drive electrode being configured to actuate the piezoelectric layer using driving voltages whose minimum is at or above the reference voltage. 
     In some implementations, the integrated circuit layer includes vertically-oriented fluid passages for transporting fluid to or from the flow paths in the substrate. 
     In some implementations, the fluid transported through the fluid passages in the integrated circuit layer is at the reference voltage of the reference electrode. 
     In some implementations, the continuous reference electrode, an electric ground of the integrated circuit layer, and the fluid transported through the fluid passages in the integrated circuit layer at maintained at or near the earth ground voltage. 
     In some implementations, the continuous reference electrode includes one or more apertures, where a respective electrical connection is made between each drive electrode in the drive electrode layer and the circuitry in the integrated circuit layer, and where each respective electrical connection is conductive column going through a respective one of the one or more apertures in the continuous reference electrode. 
     Particular implementations of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. 
     In a design for an actuation assembly that has a piezoelectric layer with an as-deposited poling direction pointing from the substrate to the piezoelectric layer, the electrode configuration is that: a first conductive layer below the piezoelectric layer and between the piezoelectric layer and the substrate serves as the drive electrode layer and includes multiple drive electrodes, while a second conductive layer above the piezoelectric layer serves as the reference electrode layer and includes a continuous reference electrode spanning multiple drive electrodes. With such an electrode configuration, positive driving voltages between each drive electrode and the reference electrode can be used to cause fluid ejection from a corresponding pumping chamber in the substrate. 
     By enabling the use of positive driving voltages to cause fluid ejection, the above design of the actuation assembly allows an integrated circuit layer to be attached to the side of the actuation assembly opposite to the substrate, where the integrated circuit layer includes circuitry for individually controlling a large array of drive electrodes in the drive electrode layer. In addition, fluid passages can be included in the integrated circuit layer for channeling fluid (e.g., from a fluid supply located above the integrated circuit layer) through the integrated circuit layer into the flow paths in the substrate. The body of the integrated circuit layer can be kept at an electric potential at or near the earth ground potential, thus the fluid going through the fluid passages in the integrated circuit layer need not be insulated or can have reduced insulation from the body of the integrated circuit layer to avoid electrolysis-caused erosion in the integrated circuit layer and fluid decomposition. At the same time, the body of the integrated circuit layer can be kept at a potential below the lowest potential (e.g., the minimum of the driving voltages) that the drive electrodes can have during fluid ejection operation, to isolate the active circuit components (e.g., transistors) in the integrated circuit layer. This can permit the transistors of the integrated circuit layer to be fabricated as NMOS or NDMOS devices, which can have a lower fabrication cost than PMOS devices. 
     In addition, if insulation were needed to isolate fluid from the integrated circuit layer, compatibility between the fluid and the insulating materials would have to be considered to avoid possible chemical interactions between the fluid and the insulating materials. In contrast, when the fluid does not need to be insulated from the body of the integrated circuit layer (e.g., as enabled by the electrode configuration described herein), a wider range of fluid can be used in the fluid ejection module because the fluid selection is no longer constrained by any insulating materials used to isolate the fluid from the integrated circuit layer. Therefore, the fluid ejection module can have a wider applicability. 
     In addition, the integrated circuit layer attached to the actuation assembly on the side of the reference electrode can pack a large number of integrated circuit components into a small space over the substrate of the fluid ejection module. A respective electrical connection can be made from each individual drive electrode in the drive electrode layer to a corresponding control circuit in the integrated circuit layer by a vertically-oriented conductive column going through a respective aperture in the continuous reference electrode. Thus, in this design, the need for conductive traces running laterally in a plane of the drive electrode layer from the drive electrodes to their respective external control circuits can be avoided. Consequently, a higher density of individually controllable actuators can be packed into the space over the substrate, and a higher nozzle density and print resolution can be achieved in the fluid ejection module. 
     In addition, in one implementation of the design, the body of the integrated circuit layer, the fluid going through the fluid passages in the integrated circuit layer, and the reference electrode can all be kept at or near the earth ground potential, while the drive electrode carries the positive driving signals to create the necessary voltage differentials across the piezoelectric layer and to cause fluid ejection. By keeping the components of the fluid ejection module other than the drive electrodes at or near the earth ground potential, the fluid ejection module can be substantially inert relative to its environment, which leads to better handling safety and improved lifetime of the fluid ejection module. 
     The details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-section of an example fluid ejection module including an actuation assembly and an integrated circuit layer above the actuation assembly. 
         FIG. 2  is a perspective view of an example substrate of the fluid ejection module. 
         FIG. 3  is a perspective view of an example drive electrode layer of the actuation assembly. 
         FIG. 4  is a perspective view of a side of the integrated circuit layer facing the reference electrode layer of the actuation assembly. 
         FIGS. 5A-5J  illustrate an example process for making the example fluid ejection module shown in  FIG. 1 . 
     
    
    
     Many of the layers and features are exaggerated to better show the process steps and results. Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Fluid droplet ejection can be implemented with a fluid ejection module which includes a printhead die fabricated using semiconductor processing techniques. The printhead die includes a substrate in which a plurality of microfabricated fluid flow paths are formed, and a plurality of actuators to cause fluid to be selectively ejected from nozzles of the flow paths. Thus, each flow path with its associated actuator provides an individually controllable fluid ejector unit. 
     In a fluid ejection module, the plurality of actuators form an actuation assembly. The actuation assembly can be attached to the substrate on a side of the substrate opposite to the side that includes the nozzle openings. An integrated circuit layer (e.g., an ASIC layer) can be attached to the actuation assembly and used to control individual actuators in the actuation assembly. Fluid passages can be formed through the integrated circuit layer to channel fluid in and out of the flow paths in the substrate. 
     The fluid flowing through the fluid passages in the integrated circuit layer is typically in contact with elements (e.g., heater, fluid reservoir, etc.) of the fluid ejection module that are kept at the earth ground potential. In order to avoid electrolysis-caused erosion in the integrated circuit layer and fluid decomposition, the body of the integrated circuit layer should be kept at or near the earth ground potential, such that no or minimal electrolysis-causing current would be induced between the fluid and the inner walls of the fluid passages in the integrated circuit layer by a voltage difference between the fluid and the inner walls of the fluid passages. 
     For example, for a given fluid flowing through the fluid passages in the integrated layer, a threshold voltage difference between the fluid and the body of the integrated circuit layer is needed to cause electrolysis in the fluid flowing through the fluid channels. The threshold voltage difference is dependent on the composition of the fluid and the minimum energies (e.g., measured by the Gibb&#39;s free energy) required to oxidize or to reduce the substances in the fluid and the body of the inner walls of the fluid passages in contact with the fluid. Therefore, to avoid electrolysis-caused erosion or fluid decomposition, the body of the integrated circuit layer (and hence, the inner walls of the fluid passages in the integrated circuit layer) should be kept within a threshold voltage range around and include the earth ground potential. In various implementations, depending on the composition of the fluid, the absolute size of the threshold voltage range can be 0.1 volt, 0.6 volt, 2 volt, 4 volt, or 6 volt, for example. In various implementations, the earth ground potential may be located at the center of the threshold voltage range, at an upper end of the threshold voltage range, at a lower end of the voltage range, or any locations in the threshold voltage range, depending (at least in part) on the composition of the fluid and/or the body of the integrated circuit layer. 
     If the electrical potential of the body of the integrated circuit layer cannot be kept at or near earth ground potential (e.g., within the threshold voltage range), the fluid going through the fluid passages in the integrated circuit layer may need to be insulated from the body of the integrated circuit layer, which can add manufacturing cost and complexity to the fluid ejection module. 
     The integrated circuit layer includes active circuit components (e.g., NMOS transistors) that are connected to the drive electrodes in the actuation assembly and provides the driving voltage signals to the drive electrodes. In order to isolate the active circuit components in the integrated circuit layer, the body of the integrated circuit layer should be kept at an electric potential at or below the lowest potential that the drive electrode can have when the drive electrode is in operation (or in other words, the minimum of the driving voltages). This poses an upper bound on the electric potential of the body of the integrated circuit layer. If the body of the integrated circuit layer cannot be kept below this upper bound, special transistor components and designs may be needed to form the control circuits in the integrated circuit layer, which can add design complexity and manufacturing cost to the fluid ejection module. 
     Given the two constraints set forth above on the electrical potential of the body of the integrated circuit layer, it is advantageous to use a positive driving signal whose minimum is at or above the upper bound of the threshold voltage range for avoiding electrolysis-caused erosion and fluid decomposition, because that helps to eliminate the need to insulate the fluid passages from the fluid and the need to use special transistors and designs to isolate the active circuit elements in the integrated circuit layer. At the same time, it is disadvantageous to use a negative driving signal whose minimum is below the lower bound of the threshold voltage range for avoiding electrolysis-caused erosion and fluid decomposition, because that would limit the choices of the fluid that can be used in the fluid ejection system, require insulation of fluid passages, require use of more expensive transistors or special designs in the integrated circuit layer, all of which can add cost or limit the applications of the fluid ejection system. 
     In a piezoelectric fluid ejection system, a piezoelectric membrane disposed between a pair of opposing electrodes can flex in response to a driving voltage applied between the pair of opposing electrodes. When the driving voltage causes an electric field pointing in the same direction as the poling direction of the piezoelectric membrane, the piezoelectric membrane expands. When the driving voltage causes an electric field pointing in the opposite direction from the poling direction of the piezoelectric membrane, the piezoelectric membrane contracts. Although it is possible to have a fluid ejection cycle in which the piezoelectric membrane goes into both an expanded state and a contracted state, such a fluid ejection cycle would require a driving signal that has both a positive portion and a negative portion. The driving signal that has both a positive portion and a negative portion can be less desirable in some cases because the portion of the driving signal that causes the piezoelectric membrane to go into the contracted state can tend to de-pole the piezoelectric membrane over time, leading to shorter lifetime of the piezoelectric membrane. Therefore, it can be advantageous to use a driving signal that is either entirely positive or entirely negative during a fluid ejection cycle, where the driving signal only causes electric fields pointing in the same direction as the poling direction of the piezoelectric membrane, and where the electric fields cause the piezoelectric membrane to flex between a maximum expanded state and a minimum expanded state (or the resting state). 
     In some fluid ejection systems, when bulk piezoelectric materials are used between the drive electrode layer and the reference electrode layer of the actuation assembly, the piezoelectric layer formed from the bulk piezoelectric material can be oriented such that its poling direction points in a direction going from the piezoelectric layer to the substrate (e.g., pointing downwards toward the substrate). Based on such orientation of the piezoelectric layer, a conductive layer below the piezoelectric layer and between the piezoelectric layer and the substrate can serve as the reference electrode layer, while a conductive layer above the piezoelectric layer can serve as the drive electrode layer. With such a configuration, positive driving signals can be used to cause fluid ejection in these fluid ejection systems, and the above mentioned constraints on the electric potential of the integrated circuit layer can be satisfied. 
     In contrast to a piezoelectric layer formed from bulk piezoelectric materials, a piezoelectric layer formed by depositing (e.g., by sputtering) particles of piezoelectric materials on a substrate is not easily reoriented after its formation on the substrate. The piezoelectric layer deposited on the substrate has an as-deposited poling direction pointing in a direction going from the substrate to the piezoelectric layer (e.g., pointing up away from the substrate). Based on such orientation of the piezoelectric layer, if the conductive layer below the piezoelectric layer and between the piezoelectric layer and the substrate were to serve as the reference electrode layer, while the conductive layer above the piezoelectric layer were to serve as the drive electrode layer, negative driving voltages would have to be used to cause an electric field pointing in the same direction as the poling direction of the piezoelectric layer. As set forth above with respect to the constraints on the electric potential of the body of the integrated circuit layer, the use of negative driving voltages would require special designs to be made for the integrated circuit layer, adding complexity and manufacturing cost to the fluid ejection module. Thus, such electrode configuration (i.e., reference electrode is below the piezoelectric layer and closer to the substrate, and the drive electrode is above the piezoelectric layer and farther from the substrate) can be less desirable in some implementations. 
     In an alternative design for the actuation assembly that includes a piezoelectric layer having an as-deposited poling direction pointing from the substrate to the piezoelectric layer (e.g., pointing upward away from the substrate), the conductive layer below the piezoelectric layer and between the piezoelectric layer and the substrate serves as the drive electrode layer, while the conductive layer above the piezoelectric layer serves as the reference electrode layer. With such a configuration, positive driving signals can be used to cause electric fields pointing in the same direction as the poling direction of the piezoelectric layer, and to cause fluid ejection in the fluid ejection module. 
     Consequently, in the alternative design, the body of the integrated circuit layer can be maintained at a potential at or near the earth ground potential, the fluid going through the fluid passages in the integrated circuit layer need not be insulated from the body of the integrated circuit layer to avoid electrolysis-caused erosion in the integrated circuit layer and fluid decomposition. At the same time, the body of the integrated circuit layer can be kept at a potential below the lowest potential that the drive electrode can have during operation (e.g., the minimum of the driving voltages) to isolate the circuit components (e.g., transistors) in the integrated circuit layer. 
     In one implementation of this alternative design, the body of the integrated circuit layer, the fluid going through the fluid passages in the integrated circuit layer, and the reference electrode can all be kept at the earth ground potential (e.g.,  0  volts), while the drive electrode carries the positive driving signals (e.g., entirely positive driving voltages) to create the necessary voltage differentials across the piezoelectric layer and to cause fluid ejection. 
       FIG. 1  is a schematic cross-sectional view of a portion of an exemplary fluid ejection module  100 . 
     The fluid ejection module  100  includes a substrate  102  in which a plurality of passages are formed. The substrate  102  can be a semiconductor (e.g., silicon) body including multiple layers, such as a nozzle layer  104  and a pumping chamber layer  106 . Each of the nozzle layer  104  and the pumping chamber layer  106  may be a single layer (e.g., a silicon layer, a polymer layer, a polysilicon layer, etc.) or include multiple layers of different materials. Each passage through the substrate  102  defines a flow path for fluid (e.g. ink) to be ejected or re-circulated. 
     Each flow path passage includes a fluid inlet  108 , a pumping chamber  110 , and a fluid ejection nozzle  112 . Fluid enters the pumping chamber  110  through the fluid inlet  108 , and can be ejected through the fluid ejection nozzle  112  (as shown by the dashed lines leading from the fluid inlet  108  to the pumping chambers  110 ). Optionally, fluid not ejected out of the fluid ejection nozzle  112  can exit the pumping chamber  110  through a fluid outlet (not shown in  FIG. 1 ). The fluid inlet  108  can be fluidically connected to a fluid supply channel (not shown), and the fluid outlet can be fluidically connected to a fluid return channel (not shown). Each fluid supply channel and each fluid return channel can be shared by multiple flow paths in the substrate  102 . 
     As shown in  FIG. 1 , the substrate  102  includes a pumping chamber layer  106  attached to the top side of the nozzle layer  104 . Each pumping chamber  110  is a cavity formed in the pumping chamber layer  106 . The fluid ejection nozzle  112  is an aperture formed through the nozzle layer  104 . The fluid ejection nozzle  112  is fluidically connected to the pumping chamber cavity on one side, and has an opening through the bottom surface of nozzle layer  104  on the opposite side. The openings of the nozzles  112  form an array of apertures on the exposed surface of the nozzle layer  104  (or in other words, the nozzle face of the substrate  102 ). 
     The substrate  102  also optionally includes a membrane layer  114  attached to the top side of the pumping chamber layer  106 . The membrane layer  114  can be an oxide layer that seals the pumping chambers  110  from above. The portion of the membrane layer  114  over each pumping chamber cavity is flexible and capable of flexing under the actuation of a corresponding piezoelectric actuator. The flexing of the membrane expands and contracts the pumping chamber cavity and pumps the fluid along the flow path, and eventually ejects the fluid out through the nozzle opening. 
     Above the membrane layer  114  is an actuation assembly  116 . The actuation assembly  116  includes a plurality of piezoelectric actuator structures  118  disposed on the substrate  102 , with each actuator structure  118  positioned over an associated pumping chamber  110 . The piezoelectric actuator structures  118  can be supported on the top side of the membrane layer  114 . If the membrane layer  114  does not exist in a particular implementation, the actuation assembly  116  can be disposed directly on the top side of the pumping chamber layer  106 , and the bottom surface of the piezoelectric actuator structures  118  can seal the pumping chambers  110  from above. 
     The actuation assembly  116  includes a first conductive layer (e.g., a reference electrode layer  120 ), a second conductive layer (e.g., a drive electrode layer  122 ), and a piezoelectric layer  124  disposed between the first and the second conductive layers. The second conductive layer (e.g., a drive electrode layer  122 ) is on the side of the piezoelectric layer  124  closer to the pumping chamber  110 ; the drive electrode layer  122  can be formed directly on the membrane  114 . The first conductive layer (e.g., a reference electrode layer  120 ) is on the side of the piezoelectric layer  124  farther from the pumping chamber  110 ; the reference electrode layer  120  can be formed directly on the piezoelectric layer  124 . 
     In some implementations, the reference electrode layer  120  and the drive electrode layer  122  can be of the same conductive material. In some implementations, the reference electrode layer  120  and the drive electrode layer  122  can be made of different conductive materials. The reference electrode layer  120  and the drive electrode layer  122  can each include one or more layers of different conductive materials, such as to improve conductivity, chemical compatibility, and/or adhesion to the substrate  102  and/or piezoelectric layer  124 . The conductive materials for the reference electrode layer  120  and the drive electrode layer  122  can include one or more of various metals (e.g., Au, Pt, Ni, Ir, etc.), alloys (e.g., Au/Sn, Ir/TiW, Au/TiW, AuNi, etc.), metal oxides (e.g., IrO 2 , NiO 2 , PtO 2 , etc.), or combinations thereof, for example. The reference electrode layer  120  and the drive electrode layer  122  can each have a thickness of a few thousand angstroms, such as 1000-3000 angstroms. The thickness of the drive electrode layer  122  and the reference electrode layer  120  can be selected based on the current that is needed to drive the piezoelectric actuators  118  of the fluid ejection module  100 , and the resistivity of the conductive material(s) used for the drive electrode layer  122  and the reference electrode layer  120 , respectively. 
     The piezoelectric layer  124  can be formed over the second conductive layer by gradually depositing (e.g., sputtering) particles of piezoelectric materials (e.g., Lead Zirconate Titanate (PZT)) over the second conductive layer. Within the actuation assembly  116 , the piezoelectric electric layer  124  has an as-deposited polarization (or poling direction) pointing in a direction from the drive electrode layer  122  to the reference electrode layer  120 , where the drive electrode layer  122  is disposed closer to the substrate  102  than the reference electrode layer  120 . 
     Some environments that are used for sputtering the piezoelectric material include a direct current (DC) bias during sputtering. The DC field causes the piezoelectric material to be polarized (or “poled”) in the direction of the DC field. Without being limited to any particular theory, the as-deposited poling direction of the sputtered piezoelectric layer points away from the underlying substrate surface along the grain structure of the sputtered piezoelectric layer. Therefore, the as-deposited poling direction is substantially locally perpendicular to the surface of the sputtered piezoelectric layer. Such as-deposited poling direction of the sputtered piezoelectric layer can reduce the stress in the piezoelectric layer during actuation, which can result in extended useful life of the piezoelectric actuator. 
     The drive electrode layer  122  is patterned to form a respective drive electrode  126  above each pumping chamber  110  in the substrate  102 . The piezoelectric layer  124  can be patterned to form a respective piezoelectric element  128  above each drive electrode  126  in the drive electrode layer  122 . The respective drive electrode  126  above each pumping chamber  110  can be separated from the drive electrodes  126  above other pumping chambers  110 , and spans only a single pumping chamber. Similarly, the respective piezoelectric element  128  above each drive electrode  126  can be separated from the piezoelectric elements  128  above other drive electrodes  126 , and spans only a single drive electrode  126 . However, in some implementations, the piezoelectric elements  128  above multiple drive electrodes  126  can form a continuous sheet spanning the multiple drive electrodes. The reference electrode layer  120  can be a continuous conductive layer that spans the drive electrodes  126  over multiple pumping chambers  110 , and forms a common reference electrode  132  for multiple piezoelectric actuators  118  in the actuation assembly  116 . 
     An insulating layer  130  can be disposed over the patterned piezoelectric layer  124  and drive electrode layer  122  to insulate individual drive electrodes  126  from one another, and optionally, to insulate individual piezoelectric elements from one another. Thus, each individual drive electrode  126  can be independently drivable. The insulating layer  130  further insulates the reference electrode layer  120  from the piezoelectric elements  128  and the drive electrodes  126 , except in the regions over the pumping chamber cavities. In the regions over the pumping chamber cavities, the reference electrode layer  120  is in direct contact with the piezoelectric elements  128 , and the piezoelectric elements  128  separate the reference electrode layer  120  from coming into contact with the drive electrodes  126  over the pumping chamber cavities. 
     The reference electrode layer  120  or the common reference electrode  132  shown in  FIG. 1  includes a first plurality of apertures that are connected to the fluid inlets  108  and fluid outlets (not shown in  FIG. 1 ) in the substrate  102 , respectively. The apertures are located over respective areas in the piezoelectric layer  124  and the drive electrode layer  122  where the piezoelectric materials and conductive materials of the layers have been removed. The same apertures can be formed in the insulating layer  130  directly below the reference electrode layer  120 , if needed. Fluid seals  134  can be formed around the apertures in the space above the reference electrode layer  120 . The fluid seals  134  prevent fluid passing through the apertures into and out of the substrate  102  from escaping into regions near the piezoelectric actuators  118  and coming in contact with the elements of the piezoelectric actuators  118 . In some implementations, the fluid seals  134  are metal seals formed using a eutectic bonding process, where the metal(s) used to form the seals are resistant to the attack of the fluid passing through the apertures. The first plurality of apertures in the fluid reference electrode layer  120  can form a first aperture array, and are distributed in proximity to respective pumping chambers in the substrate  102 . 
     As shown in  FIG. 1 , the drive electrode layer  122  of the actuation assembly  116  is disposed on the side of the piezoelectric layer  124  that is closer to the substrate  102 , while the reference electrode layer  120  is disposed on the side of the piezoelectric layer  124  that is farther away from the substrate  102 . Therefore, the common reference electrode  132  also includes a second plurality of apertures (e.g., in the areas  125 ) through each of which an electrical connection to a respective drive electrode  126  below the common reference electrode  132  can be made. The boundaries of the apertures are insulated from the electric connections (e.g., the electric connections below the conductive columns  146 ) and the drive electrodes  126  by the insulating layer  130 . The second plurality of apertures can form a second aperture array, and are distributed in proximity to respective pumping chambers in the substrate  102 . 
     The piezoelectric element  128  of the piezoelectric actuator structure  118  expands or contracts in response to a voltage applied across the piezoelectric element  128  between the respective drive electrode  126  of the piezoelectric actuator structure  118  and the common reference electrode  132 . The expansion and contraction of the piezoelectric element  128  causes the membrane over the pumping chamber  110  to change geometry, and in turn causes the pumping chamber  110  to expand or contract. The expansion of the pumping chamber  110  draws the fluid along the flow path into the pumping chamber  110 , and the contraction of the pumping chamber  110  forces a fluid droplet through and out of the fluid ejection nozzle  112 . 
     In some implementations, each of the drive electrode  126 , the piezoelectric element  128 , and the reference electrode  132  of an actuator structure  118  can be substantially planar in the region over the pumping chamber  110 . In some implementations, each of the drive electrode  126 , the piezoelectric element  128 , and the reference electrode  132  of an actuator structure  118  can be convex (e.g., curved away from the substrate  102 ) or concave (e.g., curved towards the substrate  102 ) in the region over the pumping chamber  110 . 
     Each pumping chamber  110  with its associated actuator structure  118  provides an individually controllable fluid ejection unit. The drive electrode  126  and the reference electrode  132  for each actuator structure  118  can be electrically coupled to a controller which supplies the voltage differential across the piezoelectric element  128  of the actuator  118  at appropriate times and for appropriate durations in a fluid ejection cycle. In the actuator structure  118 , the poling direction of the piezoelectric element  128  points from the drive electrode  126  to the reference electrode  132 . Consequently, when a positive voltage is applied between the drive electrode  126  and the reference electrode  132  (in other words, when the electric potential of the drive electrode  126  is raised above the electric potential of the reference electrode  132 ), an electric field that is parallel to the poling direction can be created in the piezoelectric element  128 , causing the piezoelectric element  128  to actuate in a manner that contracts the pumping chamber  110 . The contraction of the pumping chamber can cause fluid to be ejected from the pumping chamber. In some implementations, the drive electrode  126  can have a resting potential above the electric potential of the reference electrode  132 , and during each fluid ejection cycle, the driving signal can include an initial portion below the resting potential and a final portion above the resting potential, such that the pumping chamber  110  first expands to draw in fluid and then contracts to expel the fluid out of the nozzle  112 . During the entire fluid ejection cycle, the electric potential of the drive electrode  126  can be kept above the electric potential of the reference electrode  132 , such that the entire drive signal is a positive voltage signal. 
     As shown in  FIG. 1 , the drive signal for each piezoelectric actuator  118  can be individually controlled by one or more controllers. The controllers can be implemented at least in part in an integrated circuit layer  136 , such as an application-specific integrated circuit (ASIC) layer, disposed above the actuator assembly  116 . The integrated circuit layer  136  can include a semiconductor body  138  and a plurality of active circuit elements, such as transistors  140  (e.g., NMOS transistors or NDMOS transistors). The active circuit elements of the integrated circuit layer  136  provide the driving signals to the drive electrodes  126  in the drive electrode layer  122 . One advantage of using an integrated circuit layer  136  in the fluid ejection module  100  is that the integrated circuit layer  136  (e.g., an ASIC layer) can use a relative small number of inputs to provide a large number of output signals. A typical integrated circuit layer can have over 2000 outputs for about 200 inputs. By using the integrated circuit layer  136  that is of a similar lateral dimension as the substrate  102  (e.g., the printhead die), a high resolution (e.g., a resolution of 600 dpi or 1200 dpi, or at least 100 dpi) fluid ejection module  100  can be made very compact. 
     As shown in  FIG. 1 , the integrated circuit layer  136  can be supported above the actuation assembly  116  by a plurality of spacer bumps  142  to allow room for the flexing of the piezoelectric actuator structure  118 . The active circuit elements (e.g., transistors  140 ) inside the integrated circuit layer  136  (e.g., the ASIC layer) can be electrically connected to the drive electrodes  126 , while an electrical ground (e.g., a conductive bottom surface layer  137  of the semiconductor body  138  of the ASIC layer) in the integrated circuit layer  136  can be electrically connected to the common reference electrode  132 . The electrical connections between the common reference electrode  132  and the electrical ground of the integrated circuit layer  136  can be a plurality of vertically-oriented conductive columns  144  formed between the common reference electrode  132  and the semiconductor body of the integrated circuit layer  136 , for example. Similarly, the electrical connection between each drive electrode  126  and its corresponding active circuit elements (e.g., transistors  140 ) in the integrated circuit layer  136  can be a vertically-oriented conductive column  146  formed between the drive electrode  126  and the corresponding active circuit elements (e.g., transistors  140 ) in the integrated circuit layer  136 , for example. The vertically-oriented conductive column  146  formed between each drive electrode  126  and corresponding active circuit elements (e.g., transistors  140 ) in the integrated circuit layer  136  can go through a respective aperture in the continuous common reference electrode  132  and be insulated from the continuous common reference electrode  132 . In some implementations, one or more of the conductive columns  144  and  146  can also serve as spacer bumps to support the integrated circuit layer  136  above the actuation assembly  116 . 
     As shown in  FIG. 1 , the integrated circuit layer  136  includes a plurality of vertically oriented fluid passages  148 . Each fluid passage  148  in the integrated circuit layer  136  is fluidically connected to a corresponding fluid inlet  108  or fluid outlet (not shown in  FIG. 1 ) of a flow path in the substrate  102 . Fluid can enter a fluid passage  148  from a fluid supply (not shown) positioned above the integrated circuit layer  136 , pass through an opening of a respective fluid seal  134 , and enter a respective fluid inlet  108 . The fluid then flows through a respective flow path in the substrate  102 , and is either ejected from a respective nozzle  112  of the flow path or re-circulated to a respective fluid outlet (not shown) of the flow path. The re-circulated fluid can exit the fluid outlet, pass through an opening of a respective fluid seal (not shown in  FIG. 1 ) into another fluid passage (not shown in  FIG. 1 ) in the integrated circuit layer  136 , and enter a fluid return (not shown in  FIG. 1 ) positioned above the integrated circuit layer  136 . The plurality of vertically-oriented fluid passages  148  in the integrated circuit layer  136  form an array of fluid passages, each of which is positioned at a location in the integrated circuit layer  136  in proximity to a respective pumping chamber  110  in the substrate  102 . 
     In some implementations, the fluid going through the integrated circuit layer  136  may be in contact with other components (e.g., heater, chiller, or fluid reservoir) of the fluid ejection system that are kept at the earth ground potential. In order to avoid electrolysis-caused erosion to the body (e.g., the side walls of the fluid passages  148 ) of the integrated circuit layer  136  and fluid decomposition, the body of the integrated circuit layer  136  should be maintained at an electric potential at or near the earth ground potential (e.g., within +/−0.1 volts from the earth ground potential, within +/−0.5 volts from the earth ground potential, or within +/−1 volt from the earth ground potential). 
     In addition, in order to isolate the active circuit elements in the integrated circuit layer  136  which includes transistors  140 , the body  138  of the integrated circuit layer  136  should be kept at an electric potential below the minimum electric potential that the active circuit components (e.g., NMOS transistors) and the drive electrodes  126  can have during fluid ejection operation. In the electrode configuration shown in the example ejection module  100 , the drive electrode  126  is below the reference electrode  132 , and the poling direction of the piezoelectric layer  124  points in a direction from the drive electrode  126  to the reference electrode  132 . In this configuration, purely positive driving signals can be used to cause fluid ejection from the pumping chamber  110 . Therefore, the body  138  of the integrated circuit layer  136  (or in other words, the electrical ground of the integrated circuit layer  136 ) can be kept at an electric potential at or below the minimum potential that the drive electrode  126  can have during a fluid ejection cycle, for example, at the earth ground potential. 
     Various advantages of keeping the body of the integrated circuit layer  136  at the earth ground potential have been set forth in the summary section and other parts of this specification. 
       FIG. 2  is a perspective view of an example substrate  102  of the fluid ejection module  100 . The substrate  102  includes a pumping chamber layer  106  and a nozzle layer below the pumping chamber layer  106 . As shown in  FIG. 2 , the pumping chamber layer  106  includes a plurality of pumping chamber cavities  110 . Each pumping chamber cavity  110  is situated over a corresponding nozzle  112  in the nozzle layer. Each pumping chamber cavity  110  is further connected to a respective inlet feed  202  that leads to a respective neighboring nozzle inlet  108 , and a respective outlet feed  204  that leads to a respective neighboring nozzle outlet  206 . Also, as shown in  FIG. 2 , the nozzles  112  form a nozzle array (e.g., a parallelogram-shaped nozzle array with parallel columns of nozzles) in the nozzle face of the substrate  102 . Each fluid inlet  108  in the pumping chamber layer  106  is positioned in proximity to a corresponding pumping chamber  110 . Similarly, each fluid outlet  206  in the pumping chamber layer  106  is also positioned in proximity to a corresponding pumping chamber  110 . 
     Also as shown in  FIG. 2 , positions of the respective fluid seals  134  around the fluid inlets  108  and the respective fluid seals around the fluid outlets  206  are indicated around the fluid inlets  108  and the fluid outlets  206 , respectively; positions of the respective electrical connections  208  to the drive electrodes are indicated in proximity to respective pumping chamber cavities  110 ; and positions of respective electrical connections  210  to the reference electrode, and positions of the spacer bumps  212  are also indicated throughout on the top surface of the pumping chamber layer  106 . In some implementations, one or more electrical connections  210  to the reference electrode can also serve as the spacer bumps  212 , and vice versa. 
       FIG. 3  is a perspective view of an example drive electrode layer  122  of the actuation assembly  116 . As shown in  FIG. 3 , a respective drive electrode  126  is positioned over each pumping chamber  110 . Each of the drive electrodes  126  can be a solid conductive leaf of a selected shape, such as an oval shape, an elliptical shape, or a polygonal shape, etc. In some implementations, the nozzles in the substrate  102  form parallel nozzle columns, and the parallel nozzle columns can form a parallelogram-shaped nozzle array. Each of the drive electrodes  126  can be a polygonal shape that has at least a pair of edges along the same direction as that of an edge of the parallelogram-shaped nozzle array on the nozzle face of the substrate  102 , such that the nozzles (and the fluid ejection units) can be packed more closely on the substrate  102 , leading to higher printing resolution and more a compact fluid ejection module.  FIG. 3  also shows the fluid seals  134  around the fluid inlets  108  and the fluid outlets  206  deposited over the top surface of the pumping chamber layer  106 . 
       FIG. 4  is a perspective view of a side of the integrated circuit layer  136  facing the reference electrode layer  120 . A plurality of fluid passages for supplying fluid to the fluid inlets  108  and a plurality of fluid passages for collecting un-ejected fluid from the fluid outlets  206  are formed through the integrated circuit layer  136  to channel fluid from the fluid supply channels in a fluid distribution layer (not shown) to the fluid inlets  108  of the flow paths in the substrate  102 , and to channel fluid from the fluid outlets  206  to the fluid return channels in the fluid distribution layer. 
     In some implementations, the fluid inlets  108  are distributed along a first plurality of parallel columns in the top surface of the pumping chamber layer, while fluid outlets  206  are distributed along a second plurality of parallel columns in the pumping chamber layer. Therefore, the openings  402  of the fluid passages leading from the fluid supply channels are also distributed along a first plurality of parallel columns  404  that correspond to the first plurality of parallel columns in the top surface of the pumping chamber layer. Similarly, the openings  406  of the fluid passages leading to the fluid return channels are also distributed along a second plurality of parallel columns  408  that correspond to the second plurality of parallel columns in the top surface of the pumping chamber layer. 
     Also shown in  FIG. 4 , the positions of the fluid seals, electrical connections to the drive electrodes, and electrical connections to the reference electrodes, the spacer bumps are also indicated on the bottom surface of the integrated circuit layer  136 . 
       FIGS. 5A-5J  illustrate an example process for forming a fluid ejection module including an actuation assembly that has its drive electrode layer located closer to the substrate than the reference electrode layer. For illustrative purposes, the process is shown in  FIGS. 5A-5J  with respect to only a single fluid ejection unit of the fluid ejection module, in actual manufacturing, many fluid ejection units can be formed on the same wafer (or a portion thereof) in this process. 
     As shown in  FIG. 5A , an example substrate  500  can be prepared. A substrate can be an unprocessed single semiconductor body in which flow paths will be subsequently formed, or a product of multiple prior processing steps that have defined locations (or portions) of the pumping chambers on either the surface of the substrate or within the substrate. As shown in  FIG. 5A , the example substrate  500  has multiple layers, including a pumping chamber layer  502  on one side (e.g., the top side) and a handle layer  504  on an opposite side (e.g., the bottom side). On the exposed top surface of the pumping chamber layer  502 , respective locations of the pumping chambers have been defined. In this example, the actuators in the actuation assembly have curved surface structures (e.g., concave or convex structures), and curved surface areas  506  (e.g., dents or domes) have been defined on the top surface of the pumping chamber layer  502  above the locations of the pumping chambers (either already formed or to be formed subsequently). In this example, previous processing steps have been performed to produce the substrate  502 , and portions of the pumping chambers and the fluid inlets and outlets associated therewith have been formed in the pumping chamber layer  502 . In some implementations, if actuators that have planar surface structures are to be used, the exposed top surface of the pumping chamber layer  502  can be planar, and without the dents  506  currently shown. 
     Optionally, a layer of thermal oxide (e.g., 0.25 microns thick) can be formed on the exposed top surface of the pumping chamber layer  502 , the oxide layer  508  can serve as the etch stop used when pumping chambers are subsequently formed in the pumping chamber layer  502  by etching the pumping chamber layer  502  from the bottom side going upwards. During the etching process, the features defining the locations of the pumping chambers and the fluid inlets and fluid outlets can serve as the masks for the etching process. After the pumping chambers are subsequently formed, the remaining oxide layer  508  can also serve as the pumping chamber membrane that keep the actuator structure formed on top from coming in to direct contact with the fluid inside the pumping chambers. 
     A first layer of conductive material is then deposited over the substrate  502  (e.g., on the oxide layer  508 ). The first layer of conductive material will be used as the drive electrode layer  510  to define the individual drive electrodes for the actuation assembly. The conductive material used for the drive electrodes can be a metal, alloy, conductive metal oxide, or a combination thereof, such as Ir, Ir/TiW, IrO 2 , Pt, Ni, and so on. 
     Considerations for choosing the materials of the drive electrode layer  510  include the adhesion properties of the material relative to the substrate  502  (or any interface layers that is already deposited on the substrate  502 ), the conductivity of the materials, and the impendence of the materials as drive electrodes, the stability and durability of the materials, and the chemical compatibility with the piezoelectric materials that will be deposited over the drive electrode layer  510 , and so on. Various suitable methods of depositing a conductive layer may be used, such as sputtering, chemical vapor deposition, physical vapor deposition, atomic layer deposition, plasma-enhanced chemical vapor deposition, and so on. The thickness of the first conductive layer can be a few thousands of angstroms (e.g., 1000-3000 angstroms). 
     After the first layer of conductive materials (i.e., the drive electrode layer  510 ) is deposited, a layer of piezoelectric materials can be deposited on the first layer of conductive materials. The layer of piezoelectric materials can be used as the piezoelectric layer  512  of the actuation assembly. Because piezoelectric materials sometimes do not develop well over small pieces of metals or conductive materials, the drive electrode layer  510  is not patterned to define the individual drive electrodes before the piezoelectric layer  512  is deposited. 
     Suitable methods for depositing the piezoelectric material over drive electrode layer  510  include, for example, sputtering, chemical vapor deposition, physical vapor deposition, atomic layer deposition, plasma-enhanced chemical vapor deposition, and so on. Types of sputter deposition can include magnetron sputter deposition (e.g., RF sputtering), ion beam sputtering, reactive sputtering, ion assisted deposition, high target utilization sputtering, and high power impulse magnetron sputtering. The thickness of the piezoelectric layer is selected such that the piezoelectric element is sufficiently flexible to flex under the driving voltages, and yet still enough to push fluid along the flow paths and out of the fluid ejection nozzles. 
       FIG. 5A  shows the product formed after the drive electrode layer  510  and the piezoelectric layer  512  have been deposited over the substrate  502  (e.g., over the oxide layer  508  on top of the substrate  502 ). After the drive electrode layer  510  and the piezoelectric layer  512  have been deposited over the substrate  502 , the piezoelectric layer  512  is patterned to define an individual piezoelectric element  514  over the location of each pumping chamber in the substrate  502 . The individual piezoelectric element  514  can have a shape and size that resembles the lateral cross-section (e.g., an oval having a radius of 150 microns) of the pumping chamber, or extends slightly beyond (e.g., by 2-5 microns, or 5-15 microns) the lateral cross-section of the pumping chamber. The small extensions can help reduce operation stress in the piezoelectric element and extend lifetime of the actuator structure. 
     After the piezoelectric layer  512  has been patterned, portions of the drive electrode layer  510  become re-exposed in areas where the piezoelectric materials have been removed. Then, the exposed portions of the drive electrode layer  510  can be patterned (e.g., etched) to define individual drive electrodes  516 . When patterning the drive electrode layer  510 , the respective electrical connection pads  518  leading from the electrical connection bumps (to be formed) to the drive electrodes can be defined in the drive electrode layer  510  as well. 
     Each drive electrode  516  can have a shape and size that resembles the lateral cross-section of the pumping chamber below the drive electrode  516 , or extends slightly beyond the lateral cross-section of the pumping chamber. In some implementations, the shape of the drive electrode  516  does not have to resemble that of the lateral cross-section of the pumping chamber. For example, the pumping chamber can have an oval lateral cross-section, while the drive electrode can have hexagonal shape but be large enough to cover the entire lateral cross-section of the pumping chamber. In some implementations, at least one pair of opposing edges of each drive electrode are parallel to each other, and along the same direction as an edge of the nozzle array on the substrate or to an edge of the substrate  502 , such that a large number of drive electrodes (and pumping chambers) can be packed into the space over the surface of the substrate  502 . 
       FIG. 5B  shows the product formed after the piezoelectric layer  512  and the drive electrode layer  510  have been patterned to define the individual piezoelectric elements and the individual drive electrodes underneath the piezoelectric elements. At this stage, the piezoelectric element  514  and drive electrode  516  that belong to the same actuator structure have been isolated from the piezoelectric elements  514  and drive electrodes  516  that belong to other actuator structures in the actuation assembly. In addition, a respective electrical connection has been made from a respective electrical connection pad  518  to a corresponding drive electrode  516 . 
     After the drive electrode layer  510  is patterned and the drive electrodes are defined, a first layer of insulator material can be deposited over the substrate  502 . The first insulator layer  520  covers at least the exposed surface of the piezoelectric elements  514 , the exposed surface of the drive electrodes, and the exposed surface of the oxide layer  508  shown in  FIG. 5B . The material of the first insulator layer  520  can include, for example, silicon oxide or silicon nitride, or a combination thereof. 
     The first insulator layer  520  can then be patterned (e.g., etched) to expose central portions of the top surface of the piezoelectric elements  514  over the pumping chambers, and to expose the electrical connection pads  518  in the drive electrode layer  510 . The patterned insulator layer  520  also covers areas on the oxide layer  510  at which the fluid seals for the fluid inlets and the fluid outlets will be formed subsequently, at which the spacer bumps will be formed subsequently, and at which the electrical connections to the reference electrodes will be formed subsequently, respectively. 
     After the first insulator layer  520  is patterned, a second conductive layer  522  (e.g., a layer of Au over a Ti/W interface layer) can be deposited over the substrate  502  and patterned. The patterned second conductive layer  522  covers the exposed surface of the drive electrode layer  512  at locations  522  where electrical connections will be made to the drive electrodes (e.g., over the connection pads  518  in the drive electrode layer). The newly deposited conductive materials (e.g., Au/TiW) over the connection pads  518  in the drive electrode layer will form the raised connection pads for the vertically-oriented conductive columns (or the electrical connection bumps) between the drive electrodes and the integrated circuit layer (to be subsequently attached). The raised connection pads formed by the newly deposited conductive materials are insulated from the edge of the piezoelectric elements  514  and other elements on the substrate  502  other than their respective drive electrode connection pads  518  in the drive electrode layer  510 . 
     The patterned second conductive layer  522  also covers areas where the fluid seals, spacer bumps, and connections to the reference electrodes will be made. The patterned second conductive layer  522  can leave the previously exposed portions of the oxide layer  508 , previously exposed portions of the first insulator layer  520 , and the previously exposed portions of the piezoelectric elements  514 , respectively.  FIG. 5C  shows the product formed after the second conductive layer  520  has been patterned. 
     Then, a second insulator layer  524  (e.g., SiN, SiOx, or a combination thereof) can be deposited over the substrate  502  and patterned. The patterned second insulator layer  524  covers the peripheral portions of the piezoelectric element  514 , and further insulates the piezoelectric element  514  from the conductive materials deposited over the connection pads  518  to the drive electrodes  516 . In addition, the patterned second insulator layer  524  also leaves exposed the central portions of the piezoelectric element  514  directly above the pumping chamber. Other portions of the patterned second insulator layer  524  cover the areas where the fluid seals, and electric connection bumps, and spacer bumps will be formed on the substrate  502 , such that the height of these areas remain substantially identical after each processing step. 
     After the second insulator layer  524  is deposited and patterned, a third conductive layer  526  can be deposited over the piezoelectric elements  514 , and part of the patterned insulator layer  524  near the peripheral portions of the piezoelectric elements  514 . The patterned third conductive layer  526  can form an individual reference electrode on each piezoelectric element  514 . The individual reference electrode can be subsequently connected by a conductive sheet deposited over the individual reference electrodes to form a continuous common reference electrode. The third conductive layer  526  can be of a different material than the first conductive layer (i.e., the drive electrode layer  510 ), and of a different material than the conductive sheet that would be subsequently deposited. Since the third conductive layer  526  will be used as the reference electrodes, and kept inactive (e.g., at the earth ground potential) during operation, constraints on the choice of the conductive metal for the third conductive layer  526  can be fewer and/or different from those for the first conductive layer (i.e., the drive electrode layer  510 ). In addition, since the third conductive layer  526  is in direct contact with the piezoelectric elements  514 , the material choice for the third conductive layer  526  can take into account the compatibility with the piezoelectric material used for the piezoelectric element  514 . In some implementations, the third conductive layer  526  includes a layer of sputtered Iridium. Other conductive materials (e.g., metals, metal oxides, alloys, etc.) can be used for the third conductive layer  526 .  FIG. 5D  shows the product formed after the third conductive layer has been deposited and patterned. 
     After the individual reference electrodes (e.g., formed by the patterned third conductive layer  526 ) are defined over the piezoelectric elements  514 . A metal seed layer  528  (e.g., a layer of Au/Nb) can be deposited over the entire top surface of the substrate  502 , covering all of the individual reference electrodes (e.g., the Ir top electrodes) that have been formed on the piezoelectric elements  514 . The seed metal layer  528  can electrically connect all the individual reference electrodes that have been formed over the piezoelectric elements  514 , and forms a continuous common reference electrode that spans multiple individual drive electrodes. The seed metal layer  528  also covers the areas where the fluid seals, spacer bumps, and connection pads will be formed on the substrate  502 . 
     After the seed metal layer  528  has been deposited, one or more conductive plating materials (e.g., one or more metals) can be plated over areas of the fluid seals, the spacer bumps, connections pads to the drive electrodes, and connection pads on the continuous common reference electrode. The plated conductive materials  530  can form at least portions of the fluid seals (e.g.,  530   a ), spacer bumps (e.g.,  530   b ), electrical connection bumps or columns (e.g.,  530   c ) for the drive electrodes, and the electrical connection bumps or columns (e.g.,  530   d ) for the common reference electrode. 
     The plated conductive (e.g., Au) fluid seals can serve to prevent fluid from coming in contact with the actuation assembly as the fluid passes through the fluid channels into the substrate. The plated spacer bumps can serve to support the integrated circulate layer that is to be subsequently attached to the actuation assembly from above and create space for the actuation of the actuator structures. The plated electrical connection bumps or columns can serve as spacer bumps for the actuator assembly, and to provide the necessary electrical connections from the integrated circuit layer (to be attached) to the individual drive electrodes and the common reference electrode.  FIG. 5E  shows the product formed after the plating has been completed. 
     After the plating, the seed metal layer  528  can be patterned to isolate the drive electrodes, the reference electrode, the fluid seals, and the spacer bumps from one another. The remaining seed metal layer  528  can continue to connect multiple individual reference electrodes and form the continuous reference electrode. Apertures can be formed in the continuous reference electrode such that the plated metals for fluid seals, and the electrical connection bumps to the drive electrodes can go through the apertures to the integrated circuit layer (to be attached) while still be insulated from the continuous common reference electrode. For example, each aperture can encircle a respective fluid seal, or a respective electrical connection for a single pumping chamber. These apertures can form an array of apertures, each aperture located at a location in the seed metal layer  528  in proximity to a respective pumping chamber in the substrate. In some implementations, more of the metal seed layer can be removed (e.g., by etching), leaving just sufficient portions to form the continuous common reference electrode. For example, portions of the seed metal layer  528  can be removed from above the individual reference electrodes, such that the individual reference electrodes are electrically connected by the seed metal layer  528 , but do not have excessive thickness due to the materials in the seed metal layer  528 . 
     Then, portions of the oxide layer  508  that is within the central openings of the fluid seals can be removed, and the fluid inlets and fluid outlets can be opened up on the surface of the pumping chamber layer  502 , for example by etching.  FIG. 5F  shows the product formed after the inlet and outlet openings have been formed in the surface of the pumping chamber layer  502 . 
       FIGS. 5G-5I  shows example processing steps for forming the pumping chamber cavities and the nozzles in the substrate. These processing steps are merely illustrative, and the flow paths, including the pumping chamber cavities and the nozzles can be formed in the substrate at other times, and according to other process flows. 
     As shown in  FIG. 5G , a sacrificial wafer  532  can be attached to the substrate  502  above the actuation assembly. The sacrificial wafer  532  can be used as a handle layer, when the handle layer  504  is removed. The pumping chamber cavities  534  and fluid passages  536  (e.g., inlet feeds and outlet feeds) can then be formed in the pumping chamber layer  502 . And a nozzle layer  538  can be bonded to the pumping chamber layer  502 . The nozzle layer  538  can itself be attached to a handle layer, which can be used as the handle layer for the entire structure, when the sacrificial wafer  532  is subsequently removed.  FIG. 5H  shows the product formed after the nozzle layer  538  and its handle layer have been bonded to the pumping chamber layer  502 . Then, the sacrificial wafer  532  can be removed to re-expose the actuation assembly, such that an integrated circuit layer can be attached to the exposed side of the actuation assembly.  FIG. 5I  shows the product formed after the sacrificial wafer  532  have been removed and the actuation assembly has been exposed. 
     The integrated circuit layer can be included in a pre-formed integrated circuit wafer (e.g., an ASIC wafer  540 ). The ASIC wafer  540  includes active circuit elements  542  (e.g., transistors) that can provide individually controlled drive signals to the individual drive electrodes in the actuation assembly. The ASIC wafer  540  also includes an electrical ground  544  that is kept at a ground voltage that is at or below the lowest voltage that the active circuit elements will provide to the drive electrodes. In some implementations, the body of the ASIC wafer (e.g., the semiconductor body in which the integrated circuit elements (e.g., NMOS transistors) have been formed) is kept at the ground voltage, and serves as the electrical ground of the integrated circuit layer. 
     As set forth earlier in the specification, the integrated circuit layer includes fluid passages  546  (e.g., fluid passages vertically oriented relative to the plane of the integrated circuit layer) that go through the integrated circuit layer from one side (e.g., the top side) to the other side (e.g., the bottom side) of the integrated circuit layer (e.g., the body of the ASIC wafer  540 ). The openings of the fluid passages on the face of the ASIC wafer  540  facing the actuation assembly are illustrated in  FIG. 4 . 
     In addition to the openings of the fluid passages  546 , the face of the ASIC wafer  540  facing the actuation assembly can also include corresponding conductive fluid seals, spacer bumps, electrical connection bumps connected to the electrical ground of the ASIC wafer, and electrical connection bumps connected to the active circuit elements in the ASIC wafer, that can be bonded respectively to the fluid seals, spacer bumps, electrical connection bumps connected to the common reference electrode of the actuation assembly, and electrical connection bumps connected to the individual drive electrodes of the actuation assembly. 
     In some implementations, the bonding between the ASIC wafer  540  and the actuation assembly can be accomplished through a eutectic bonding processing that join the conductive fluid seals, spacer bumps, electrical connection bumps connected to the electrical ground of the ASIC wafer, and electrical connection bumps connected to the active circuit elements in the ASIC wafer, to their respective fluid seals, spacer bumps, electrical connection bumps connected to the common reference electrode of the actuation assembly, and electrical connection bumps connected to the individual drive electrodes of the actuation assembly. The joined electrical connection bumps can form the conductive columns  548  that electrical connect the common reference electrode in the actuation assembly to the electrical ground of the integrated circuit layer, and the conductive columns  550  that electrically connect the active circuit elements in the integrated circuit layer to their respective drive electrodes in the actuation assembly. 
     After the ASIC wafer  540  is bonded to the actuation assembly (e.g., by eutectic bonding), the ASIC wafer  540  can serve as the handle layer for subsequent processing, such as the removal of the handle layer below the nozzle layer  538  to open up the nozzles in the nozzle face.  FIG. 5J  shows a product formed after the nozzles  552  have been opened in the nozzle face. 
     The use of terminology such as “front,” “back,” “top,” “bottom,” “left,” “right,” “over,” “above,” and “below” throughout the specification and claims is for describing the relative positions of various components of the system(s) and relative positions of various parts of the various components described herein. Similarly, the use of any horizontal or vertical terms throughout the specification and claims is for describing the relative orientations of various components of the system(s) and the relative orientations of various parts of the various components described herein. Except where a relative orientation or position set forth below is explicitly stated in the description for a particular component, system, or device, the use of such terminology does not imply any particular positions or orientations of the system, device, component or part(s) thereof, relative to (1) the direction of the Earth&#39;s gravitational force, (2) the Earth ground surface or ground plane, (3) a direction that the system(s), device(s), or particular component(s) thereof may have in actual manufacturing, usage, or transportation; or (4) a surface that the system(s), device(s), or particular component(s) thereof may be disposed on during actual manufacturing, usage, or transportation. 
     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 inventions. For example, some processing steps may be carried out in a different order, modified, or omitted. The layout and configuration of the nozzles, pumping chambers, electrical connections, may be varied.