Abstract:
A method of forming an array of piezoelectric actuators on a membrane ( 18 ) which includes the steps of preparing a piezoelectric comb-like structure having an array of islands that are integrally connected by a continuous top portion and that form piezoelectric layers of the actuators, the islands having an electrode at a bottom side, attaching the comb-like structure with its bottom electrode to a surface of the membrane, removing the continuous top portion of the comb-like structure to thereby separate the actuators from one another, and forming top electrodes on the top surfaces of the piezoelectric layers of the actuators.

Description:
[0001]    This non-provisional application claims priority under 35 U.S.C. §119(a) on European Patent Application No. 07109196.1 filed in the European Patent Office on May 30, 2007, which is herein incorporated by reference 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method of forming an array of piezoelectric actuators on a membrane. More particularly, the invention relates to a method of forming an array of piezoelectric actuators that forms part of an ink jet printing device. 
         [0003]    An ink jet device is used in an ink jet printer for expelling an ink droplet in response to an electric signal energizing the piezoelectric actuator. The actuator, when energized, causes the membrane to flex into a pressure chamber, so that the pressure of liquid ink contained in that chamber is increased and an ink droplet is ejected from a nozzle that communicates with the pressure chamber. 
         [0004]    In a typical ink jet printer, the ink jet device takes the form of an array of a large number of nozzles and actuator units, and the nozzles are arranged with a very small pitch so as to achieve a high resolution of the printer. As a result, a manufacturing process is required which permits a high nozzle density of the ink jet device. Further, since the membrane and the actuator are subject to mechanical strains that vary with high frequency, the membrane must firmly and reliably be connected with the actuator. In many conventional ink jet printers, the actuators operate in a longitudinal expansion mode of the piezoelectric material. The array of actuators is formed by a comb-like structure with a continuous top region of piezoelectric material that is formed integrally with a number of piezoelectric fingers that project towards the membrane and form the individual actuators. Electrodes are provided on the top surface of the continuous layer and on the tip ends of the fingers. These electrodes of the fingers are attached to the membrane. Such a structure may be produced from a solid block of piezoelectric material by cutting slots into the block, so as to form the individual fingers. Then, the comb structure is attached to the surface of the membrane. 
         [0005]    In contrast, the present invention is concerned with a method of manufacturing an array of piezoelectric actuators that operate in a flexural mode of the piezoelectric material. In this case, an individual actuator is formed by a flat, relatively thin layer of piezoelectric material that is sandwiched between top and bottom electrodes and is attached to the membrane with its bottom electrode. When a voltage is applied to the electrodes, the piezoelectric layer experiences a bending deformation which causes the membrane to flex. 
         [0006]    US 2005/0046678 A1 discloses an ink jet device of this latter type, and a manufacturing process wherein electrode layers and piezoelectric layers forming the individual actuators are successively formed and patterned on the membrane. 
         [0007]    US 2006/008257 discloses a method according to the preamble of claim  1 , wherein the actuators are formed on a body that includes a plurality of ink chambers and their respective membranes. 
         [0008]    A similar method is known from US 2004/0066524 where the ink chambers are formed by etching away the corresponding parts of the body so as to leave only the membranes and the walls separating the ink chambers. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the invention to provide a more reliable and efficient manufacturing process. 
         [0010]    In order to achieve this object, the manufacturing process of forming an array of piezoelectric actuators on a common membrane comprises the steps of preparing a piezoelectric comb-like structure having an array of islands that are integrally connected by a continuous top portion and that are to form piezoelectric layers of the actuators, said islands having an electrode at a bottom side, attaching the comb-like structure with its bottom electrode to a surface of the membrane, removing the continuous top portion of the comb-like structure, thereby separating the actuators from one another, and forming top electrodes on the top surfaces of the piezoelectric layers of the actuators, wherein the membrane is initially configured as a thick carrier plate and, after the comb-like structure has been attached, is brought to a desired uniform thickness by removing material from the side opposite to the actuators so as to obtain a continuous flat bottom surface of the membrane extending over the a plurality of actuators. 
         [0011]    The present method has the advantage that the piezoelectric actuators can be prepared in advance, before they are attached to the membrane. That is, before attachment, the piezoelectric material is already formed and poled, electrical conducting layers are deposited and the islands are defined. 
         [0012]    The piezoelectric comb-like structure may be prepared by first preparing a piezoelectric slab with an electrode on a bottom side, and subsequently cutting grooves into the bottom surface of the slab thereby forming the array of islands that are integrally connected by a continuous top portion. 
         [0013]    Further, although the piezoelectric layers of the individual actuators are relatively thin and are extended in a plane parallel to the plane of the membrane, it is possible, according to the present invention, to prepare a plurality of actuators, preferably the entire array or even a plurality of arrays on separate chips, from a single slab of piezoelectric material. 
         [0014]    By cutting grooves into the bottom surface of the slab, the electrode that is provided on that surface is divided into individual bottom electrodes of the actuators. The cutting process precisely defines the shape and arrangement of the actuators in the array. When the slab is bonded to the membrane, the array of actuators formed on the bottom of the slab still form an integral structure, so that the task of precisely aligning the actuators relative to one another and relative to a member forming the pressure chambers below the membrane is greatly facilitated. 
         [0015]    Thanks to the stability of the integrated piezoelectric slab, the attachment step can be performed reliably and without any risk of damage. Moreover, the attachment step that may be carried out by means of an adhesive is relatively robust and does not require a high surface quality of the surfaces of the electrodes and the membrane. When the actuators have been reliably attached to the membrane in this manner, the actuators can easily be separated from one another by simply grinding away the continuous top portion of the piezoelectric slab. Then, the actuators may be completed by forming the top electrodes on the separated piezoelectric islands. 
         [0016]    The step of bonding the actuators to the membrane is performed in a state in which the membrane is still a relatively thick carrier plate with a sufficient strength to withstand any mechanical strains that may occur during the bonding step. Then, the membrane part of the carrier plate may be securely attached to a rigid substrate, and the membrane is brought to the desired thickness by removing material of the carrier plate on the side opposite to the actuators. 
         [0017]    Preferably, the material of the carrier plate is removed by etching, grinding or a combination thereof. In a particularly preferred embodiment, the carrier plate is formed by an SOI wafer (Silicon On Insulator) with a relatively thin silicon layer forming the desired membrane, an insulating layer, e.g., silicon dioxide, serving as an etch stop, and another silicon layer forming the bulk of the carrier plate that will later be removed. Alternatively, the membrane part of a carrier plate may be attached to the bulk of a carrier plate by means of a temporary wafer bond. In such a case, the membrane may be separated, for example, by means of a thermal treatment. 
         [0018]    Preferably, the slab is attached to the membrane by means of thermocompression bonding. In this thermocompression bonding step, the bottom electrode formed on the actuators may automatically be contacted with an electrical conductive structure on the membrane, so that the bonding step assures not only a high mechanical stability but also a good and reliable electrical contact. 
         [0019]    The continuous top region of the slab from which the actuators are formed may be provided with a top electrode and, during the thermocompression bonding step, the top and bottom electrodes may be short-circuited in order to avoid any possibility of electrical damage that might otherwise be caused by the pyroelectric properties of the piezoelectric material. 
         [0020]    Electronic components, e.g., for controlling the actuators, detecting malfunctions, or measuring temperature may be formed directly in the silicon layer that will later form the membrane. Electrical leads and electrodes for contacting the electronic components and the actuators may be formed on the surface of that layer that has been covered by a dielectric layer. 
         [0021]    The rigid substrate may be formed by another silicon wafer which is suitably etched to form chambers accommodating the individual actuators, ink supply channels, feedthroughs and the like. 
         [0022]    Electrical leads for contacting the top electrodes of the actuators may be formed directly on the surface of the membrane. Preferably, before forming the top electrodes, the peripheral portions of the piezoelectric actuators are covered by a ring of insulating material for insulating the side faces of the piezoelectric layer and for reliably separating and insulating the top and bottom electrodes of the actuators from one another. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Preferred embodiments of the invention will now be described in conjunction with the drawings, wherein: 
           [0024]      FIG. 1  is a cross-sectional view of an individual ink jet device manufactured by the method according to the present invention; 
           [0025]      FIG. 2  is an enlarged detail of the device shown in  FIG. 1 ; 
           [0026]      FIG. 3  is a partial sectional view of components of an ink jet device forming an array of a plurality of nozzle and actuator units; 
           [0027]      FIG. 4  is a partial plan view of arrays of the type shown in  FIG. 3 , as manufactured from a wafer; 
           [0028]      FIGS. 5-8  illustrate several steps of a method for preparing and mounting piezoelectric actuators on a membrane; 
           [0029]      FIGS. 9-11  illustrate several steps of a method for completing the actuators on the membrane; 
           [0030]      FIG. 12  illustrates a step of attaching the membrane to a rigid substrate; 
           [0031]      FIG. 13  illustrates a step of releasing the membrane; and 
           [0032]      FIGS. 14-16  illustrate steps analogous to  FIGS. 9-11  for a modified embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    As is shown in  FIG. 1 , an ink jet device according to the invention has a layered structure comprising, from the bottom to the top in  FIG. 1 , a nozzle plate  10  with a nozzle  12  formed therein, a chamber plate  14  defining a pressure chamber  16  that communicates with the nozzle  12 , a flexible membrane  18  carrying a piezoelectric actuator  20 , a distribution plate  22  for supplying liquid ink to the pressure chamber  16 , and an optional cover plate  24 . 
         [0034]    The chamber plate  14 , the membrane  18  and the distribution plate  22  are preferably made of silicon, so that etching and photolithographic techniques known from the art of semiconductor processing can be utilised for reliably and efficiently forming minute structures of these components, preferably from silicon wafers. While  FIG. 1  shows only a single nozzle and actuator unit, it is possible and preferable that an entire chip comprising a plurality of nozzle and actuator units, or a plurality of such chips, are formed in parallel by wafer processing. The use of identical, respectively similar materials for the above components has the further advantage that problems resulting from differential thermal expansion of the components can be avoided or effectively minimized. 
         [0035]    The flexible membrane  18  is securely bonded to the chamber plate  14  by means of an adhesive layer  26  so as to cover the pressure chamber  16  and to define a top wall thereof. An electrically conductive structure  28  is formed on the top surface of the membrane and may be led out on at least one side, so that it may be in electrical contact with a wire bond  30 , for example. 
         [0036]    The piezoelectric actuator  20  comprises a bottom electrode  32  held in intimate large-area contact with the electrically conductive structure  28 , a top electrode  34 , and a piezoelectric layer  36  sandwiched therebetween. The piezoelectric layer  36  may be made of a piezoelectric ceramic such as PZT (Lead Zirconate Titanate) and may optionally contain additional internal electrodes. 
         [0037]    The peripheral edge of the top surface of the piezoelectric layer  36  as well as the lateral surfaces of that layer are covered by an insulating layer  38 . The peripheral portion of the top electrode  34  is superimposed on the insulating layer  38  and is led out to one side on the surface of the membrane  18 , so that it may be in electrical contact with a wire bond  40 . 
         [0038]    At the locations where the electrical contacts, such as wirebonds  30  and  40 , are made, the electrical leads are secured to the distribution plate  22  by means of another adhesive layer  42  that is also used to securely attach the top surface of the membrane  18  to the distribution plate. 
         [0039]    It is observed that the bottom electrode  32  and preferably also the top electrode  34  of the actuator cover the entire surface of the piezoelectric layer  36 , including the edge portions thereof, which contributes to an increase in power gain and volume displacement of the actuator. The insulating layer  38  reliably prevents the top and bottom electrodes from becoming short-circuited and also assures that the electrodes are separated everywhere by a sufficient distance, so that, when a voltage is applied to the electrodes, the strength of the electric field established therebetween will reliably be limited to a value that is not harmful to the piezoelectric material. 
         [0040]    The distribution plate  22  is securely bonded to the top surface of the membrane  18  by means of adhesive layer  42  and defines a chamber  44  that accommodates the actuator  20  with sufficient play so as not to obstruct the piezoelectric deformation of the actuator. The actuator  20  will thus be shielded not only from the ink in the pressure chamber  16  and in the supply system but also from ambient air, so that a degradation of the actuator due to ageing of the piezoelectric material is minimized. 
         [0041]    The chamber  44  may be filled with a gas such as nitrogen or argon that does not react with the piezoelectric material, or may be evacuated or held under a slight sub-atmospheric pressure. If, in another embodiment, the chamber  44  contains air at atmospheric pressure, it preferably communicates with the environment through a restricted vent hole, so that the pressure in the chamber may be balanced with the atmospheric pressure, but the exchange of air is restricted so as to avoid ageing of the piezo. 
         [0042]    Above the actuator chamber  44  and separated therefrom, the distribution plate  22  defines a wide ink supply channel  46  that is connected, at at least one end thereof, to an ink reservoir (not shown). Optionally, the ink reservoir may be provided directly on top of the ink channel  46  in place of the cover plate  24 . 
         [0043]    In a position laterally offset from the actuator chamber  44 , the distribution plate  22  defines a feedthrough  48  that connects the ink supply channel  46  to the pressure chamber  16  via a filter passage  50  formed by small perforations in the membrane  18 . The filter passage  50  prevents impurities that may be contained in the ink from entering into the pressure chamber  16  and at the same time restricts the communication between the ink supply channel  46  and the pressure chamber  16  to such an extent that a pressure may be built up in the pressure chamber  16  by means of the actuator  20 . To that end, the piezoelectric layer  36  of the actuator deforms in a flexural mode when a voltage is applied to the electrodes  32 ,  34 . 
         [0044]    When an ink droplet is to be expelled from the nozzle  12 , the actuator is preferably energized with a first voltage having such a polarity that the piezoelectric layer  36  bulges away from the pressure chamber  16  and thus deflects the membrane  18  so as to increase the volume of the pressure chamber. As a result, ink will be sucked in through the filter passage  50 . Then, the voltage is turned off, or a voltage pulse with opposite polarity is applied, so that the volume of the pressure chamber  16  is reduced again and a pressure wave is generated in the liquid ink contained in the pressure chamber. This pressure wave propagates to the nozzle  12  and causes the ejection of the ink droplet. 
         [0045]    The above-described construction of the ink jet device, with the ink supply channel  46  being formed on top of the pressure chamber  16  (and on top of the actuator  20 ) has the advantage that it permits a compact configuration of a single nozzle and actuator unit and, consequently, permits a high integration density of a chip formed by a plurality of such units. As a result, a high nozzle density can be achieved for high resolution and high speed printing. Nevertheless, the device may be produced in a simple and efficient manufacturing process that is particularly suited for mass production. In particular, the electrical connections and, optionally, electrical components  52  can easily be formed at one side of the membrane  18  before the same is assembled with the distribution plate  22 . 
         [0046]    It will be understood that the metal layer forming the ground electrode  32  (or, alternatively, an electrode for energising the actuator) is led out in a position offset from the filter passage  50  in the direction normal to the plane of the drawing in  FIG. 1  or is formed around that filter passage. 
         [0047]      FIG. 2  is an enlarged view of a detail that has been marked by a circle X in  FIG. 1 . In the example shown, part of an electronic component  52 , e.g., a sensor or a switching transistor or driving circuit for controlling the actuator  20 , has been embedded in the top surface of the membrane  18  by suitably doping the silicon material. Further, in that example, an extension or tab of the electrode  32  forms a reliable connection with the electronic component  52  through an opening  54  in the dielectric layer  51  on the surface of the membrane. 
         [0048]      FIG. 3  illustrates a chip  56  comprising a plurality of nozzle and actuator units that are constructed in accordance with the principles that have been described in conjunction with  FIG. 1 . Here, the main components of the chip, i.e., the chamber plate  14 , the membrane  18  with the actuators  20 , and the distribution plate  22 , have been shown separated from one another for reasons of clarity. 
         [0049]    In this example, the pressure chambers  16  are altematingly arranged and rotation-symmetrically disposed, so that pairs of these chambers may be supplied with ink from a common channel  46  and a common feedthrough  48 . The filter passages  50  for each pressure chamber  16  are arranged above an end portion of the respective pressure chamber  16  opposite to the end portion that is connected to the nozzle  12 . This has the advantage that the pressure chambers may be flushed with ink so as to remove any air bubbles that might be contained therein and would be detrimental to the droplet generation process. 
         [0050]    The chip  56  shown in  FIG. 3  forms a two-dimensional array of nozzle and actuator units with a plurality of such units being aligned in the direction normal to the plane of the drawing in  FIG. 3 . In the example shown, each actuator  20  is accommodated in an individual chamber  44  that is separated from adjacent chambers by transverse walls  58  formed integrally with the distribution plate  22 . As mentioned above, these chambers may communicate via restricted vent holes  60 . As an alternative, the transverse walls  58  may be dispensed with, so that the actuators  20  aligned in a same column are accommodated in a common, continuous chamber  44 . 
         [0051]    Each of the membrane  18 , the distribution plate  22 , and, optionally, the chamber plate  14  may be formed by processing a respective wafer  62 , as has been indicated in  FIG. 4 . The components of a plurality of chips  56  may be formed of a single wafer. What has been illustrated for the chip  56  shown on the right side in  FIG. 4 , is a top plan view of the distribution plate  22  with the ink supply channels  46  and feedthroughs  48 . The chip on the left side in  FIG. 4  has been shown partly broken away, so that the layer structure of the chip is visible. 
         [0052]    A layer  64  directly underneath the distribution plate  22  shows five rows of actuators. The first two rows show top plan views of the top electrodes  34  with their projected leads. In this embodiment, the entire surface of the membrane  18 , except the areas of the electrodes  34  and the areas coinciding with the feedthroughs  48 , is covered by the insulating layer  38 , as will later be explained in detail in conjunction with  FIGS. 14 to 16 . The first row in  FIG. 4  shows also electrical tracks  66  connected to the leads and provided on the surface of the insulating layer  38 . The last three rows in the layer  64  show the piezoelectric layers  36  without top electrodes. 
         [0053]    In the next layer  68 , the insulating layer  38  has been removed so that the membrane  18  with the filter passages  50  becomes visible. In the second row of this layer, the piezoelectric layers  36  have also been removed so as to illustrate the bottom electrodes  32 . 
         [0054]    The lowermost three rows of the chip show a top plan view of the chamber plate  14  with the pressure chambers  16  and the nozzles  12 . In this example, the filter passages communicate with the pressure chambers  16  via labyrinths  70 . These labyrinths serve to provide for a sufficient flow restriction. As shown, the pressure chambers  16  have an approximately square shape, and the labyrinth opens into the corner of the chamber that is diagonally opposite to the nozzle  12 . 
         [0055]    Preferred embodiments of the present method for producing the ink jet device and the chip  56 , respectively, will now be described. 
         [0056]      FIGS. 5 to 13  illustrate a method of forming the membrane  18  with the actuators  20 . 
         [0057]    First, as is shown in  FIG. 5 , a slab  72  of piezoelectric material is prepared and is provided with the bottom electrode  32  and another electrode  74  on the top surface. These electrodes may be used for polarising the piezoelectric material. The slab  72  should preferably have at least the size of an entire chip  56  which. If available, a slab of wafer size could be used, or a plurality of slabs may be attached with their electrodes  74  to a wafer-size carrier plate. The thickness of the slab  72  may, for example, be in the range from 200 to 500 μm. 
         [0058]    As is shown in  FIG. 6 , grooves  76  are cut into the bottom side of the slab  72  to a depth slightly larger than the intended thickness of the piezoelectric layer  36  of the actuator. Although not shown in the drawings, the grooves  76  extend cross-wise, thus leaving projecting platforms that will later form the piezoelectric layers  36  covered by the bottom electrodes  32 . The pattern of these platforms corresponds to the intended array of actuators on the chip  56 . 
         [0059]    As is shown in  FIG. 7 , the bottom side of the bottom electrode  32  is covered with an adhesive layer  78 , e.g., by tampon printing, roller coating, spray coating or the like. Further, a wafer-size carrier plate  80  is prepared, and the electrically conductive structure  28  is formed with a suitable pattern on the top surface thereof. The carrier plate  18  is preferably formed by an SOI wafer having a top silicon layer which will later form the membrane  18 , a bottom silicon layer  82  that will later be etched away, and a silicon dioxide layer  84  separating the two silicon layers and serving as an etch stop. 
         [0060]    In a practical embodiment, the top silicon layer and hence the membrane  18  may have a thickness between 1 μm and 25 μm, or about 10 μm, the etch stop has a thickness of 0.1 to 2 μm and the bottom silicon layer  82  may have a thickness between 150 and 1000 μm, so that a high mechanical stability is assured. 
         [0061]    The slab  72  is then pressed against the top surface of the carrier plate  80 , and the bottom electrodes  32  of the intended actuators are firmly bonded to the conductive structures  28  by thermocompression bonding. In this process, as has been shown in  FIG. 8 , the adhesive layer  78  will be squeezed out and will form a meniscus around the periphery of each piezoelectric layer  36 , while the conductive structures  28  and electrodes  32  are brought into electrical contact with one another. 
         [0062]    Since the piezoelectric material of the slab  72  will typically have pyroelectric properties, it is convenient to short-circuit the electrodes  32  and  74  during the thermocompression bonding process in order to avoid electrical damage. Alternatively instead of thermocompression bonding ultrasonic bonding may be used where instead of an adhesive layer a gold layer or gold bumps are provided on the bottom electrodes of the intended actuators and/or on the ground electrodes. 
         [0063]    As is shown in  FIG. 8 , the electrode  74  and the continuous top portion of the slab  72  are removed, e.g., by grinding, so that only the desired array of piezoelectric layers  36  of the actuators is left on the carrier plate  80 . 
         [0064]    As is shown in  FIG. 9 , the next step is to form the insulating layer  38 . This layer is formed, e.g., by spin coating, spray coating, sputtering PVD, CVD or the like, at least on the entire surface of the piezoelectric layer  36 , on the side walls thereof and on the meniscus formed by the adhesive layer  78 , respectively. The insulating layer  38  is preferably formed by a photo-curable epoxy resin such as SU 8  or BCB. The portions of the layer  38  that are to be retained are exposed with light so as to cure the resin, and the non-exposed portions are removed. 
         [0065]    As is shown in  FIG. 10 , the layer  38  is removed at least from the central portion of the insulating layer  36  where the top electrode  34  is to be applied. 
         [0066]    As is shown in  FIG. 11 , the top electrode  34  is formed on the exposed top surface of the piezoelectric layer  36 , e.g., by sputtering or any other suitable process. In order to be able to electrically contact the top electrode, this electrode is extended on at least one side over the insulating layer  38  and onto the top surface of the carrier plate  80 , as is shown on the right side in  FIG. 11 . The insulating layer  38  assures that the metal of the top electrode  34  is reliably kept away by a sufficient distance from the bottom electrode  32  and the conductive structures  28 , so as to avoid short circuits and to limit the strength of the electric field developed between the electrodes. 
         [0067]    The step shown in  FIG. 11  completes the formation of the piezoelectric actuators  20 . 
         [0068]    In the next step, shown in  FIG. 12 , the distribution plate  22  is bonded to the top surface of the carrier plate  80 . The distribution plate  22  will be prepared separartely by etching a suitable silicon wafer. For example, the relatively coarse structures of the supply channels  46  may be formed in a cost-efficient anisotropic wet etching process, whereas the minute structures of the actuator chambers  44  and feedthroughs  48  may be formed by dry etching from below. 
         [0069]    The distribution plate  22  then serves as a rigid substrate that can be used as a handle for manipulating the assembly. The joint wafers forming the distribution plate  22  and the carrier plate  80  are transferred to an etching stage where the lower silicon layer  82  of the carrier plate  80  is etched away up to the etch stop formed by the silicon oxide layer  84 . The silicon oxide layer is subsequently removed, which leaves only the thin, flexible membrane  18  with the actuators  20  mounted thereon and firmly secured to the rigid distribution plate  22 . 
         [0070]    The filter passages  50  may be formed in the same or in a separate etching step or by another process such as laser cutting. The result is shown in  FIG. 13 . Since the flexible membrane  18  is backed by the distribution plate  22 , it may safely be handled in the further processing steps which include bonding the membrane  18  to the chamber plate  14 . If, in this stage, the assembly of the membrane  18  and the distribution plate  22  on the one side and the chamber plate  14  on the other side have wafer size, the actuators  20  and filter passages  50  may accurately be aligned with the pressure chambers  16  for all the chips on the wafers in a single alignment step. Finally, the joint wafers will be diced to form the individual chips  56 . 
         [0071]    As an alternative, it is of course possible to dice only the joint wafers forming the membrane  18  and the distribution plate  22  and to assemble them with the separate chamber plates  14 . 
         [0072]    In the example shown in  FIGS. 9-13 , the insulating layer  38  has a relatively small thickness on the top side of the piezoelectric layer  36  and a larger thickness on the surface of the membrane and the electrically conductive structures  28 , respectively. For comparison,  FIG. 1  illustrates an embodiment where the insulating layer  38  has a uniform thickness. 
         [0073]      FIG. 14  illustrates yet another embodiment, wherein the step of  FIG. 9  is modified in that the insulating layer  38  is formed on the entire surface of the carrier plate  80  with a flat, continuous top surface, i.e., the piezoelectric layers  36 , the bottom electrodes  32 , and the electrically conductive structures  28  are entirely buried in the insulating layer  38 . This embodiment corresponds to the example shown in  FIG. 4 . 
         [0074]    Again, as is shown in  FIG. 15 , the photo-curable insulating layer  38  is exposed, and the resin is removed at least in the portions covering the piezoelectric layers  36  and portions  86  coinciding with the feedthroughs  48 . 
         [0075]    Finally, as is shown in  FIG. 16 , the top electrodes  34  of the actuators are applied and extended on the flat top surface of the insulating layer  38 . Depending on the procedures employed for electrically contacting the actuators, this may facilitate the formation of the electrical contacts. The rest of the procedure corresponds to that explained in conjunction with  FIGS. 9 to 12 . 
         [0076]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.