Abstract:
A micromachined fluid ejector includes an ejector body having a fluid cavity for holding fluid to be ejected and a piezoelectric actuator for ejecting the fluid. A nozzle plate is placed in operable association with the ejector body. The configuration of the nozzle plate is selected to adjust a volume of the fluid cavity to obtain a desired mechanical impedance matching between the fluid and the actuator.

Description:
BACKGROUND  
       [0001]     The present application is directed to fluid ejectors, and more particularly, to fluid ejectors using piezoelectric actuation, and methods to make the same. Micromachined fluid ejectors, such as ink jet printheads, using either electrostatic or piezoelectric actuation have been discussed. When electrostatic actuation is employed, the fluid ejectors are fabricated using standard silicon micromachining processes. Because the energy density of electrostatic actuators is very small, the required driving voltage is quite high (e.g., commonly 50V or more). Use of electrostatic actuation also makes the ejectors vulnerable to damage caused by the snap-down operation of the active diaphragm.  
         [0002]     Fluid ejectors employing piezoelectric actuators have also been considered. Several advantages exist in the use of piezoelectric actuation, including lower driving voltages and elimination of device failure occurring due to snap-down of an active diaphragm. Bulk piezoelectric actuation systems commonly require larger driving voltages than ejectors which employ piezoelectric thin films since, for example, the distance between the electrodes is larger in the bulk piezoelectric actuators. In either case, either type of piezoelectric actuator based fluid ejector requires lower driving voltages than electrostatic based ejectors. While lower driving voltages are expected for thin film piezoelectric actuators, there are several challenges in making operable piezoelectric thin film based fluid ejectors, especially for micromachined fluid ejectors. Particularly, sufficient energy must be developed by the piezoelectric material, and that energy must be effectively transferred to the fluid for consistent controllable drop ejection.  
       BRIEF DESCRIPTION  
       [0003]     A micromachined fluid ejector includes an ejector body having a fluid cavity for holding fluid to be ejected and a piezoelectric actuator for ejecting the fluid. A nozzle plate is placed in operable association with the ejector body. The configuration of the nozzle plate is selected to adjust a volume of the fluid cavity to obtain a desired mechanical impedance matching between the fluid and the actuator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  illustrates a schematic of a micromachined fluid ejector in accordance with the present application;  
         [0005]      FIGS. 2   a - 2   i  depict a process flow for manufacturing the fluid ejector of  FIG. 1 ;  
         [0006]      FIGS. 3   a - 3   c  depict a first embodiment for forming a recessed nozzle plate used with the fluid ejector of  FIG. 1 ;  
         [0007]      FIGS. 4   a - 4   c  depict a second embodiment for formation of a recessed fluid plate used with the fluid ejector of  FIG. 1 ;  
         [0008]      FIG. 5  shows a modified version for a fluid ejector according to the present application;  
         [0009]      FIGS. 6   a - 6   c  depict top view sketches shown conceptual fluid cavity structures;  
         [0010]      FIG. 7  shows a second embodiment for a structure of a micromachined fluid ejector according to the present application;  
         [0011]      FIGS. 8   a - 8   i  depict a process flow for manufacturing a fluid ejector such as shown in  FIG. 7 ; and  
         [0012]      FIG. 9  shows a third embodiment for a structure of a micromachined fluid injector in accordance with the present application; and.  
         [0013]      FIGS. 10   a - 10   f  depict a process flow for manufacturing the fluid injector of  FIG. 9 . 
     
    
     DETAILED DESCRIPTION  
       [0014]     The following description sets forth improved design and manufacturing processes of micromachined, fluid ejectors such as piezoelectric actuated fluid ejectors. While fluid ejectors employing thin film piezoelectric actuation will theoretically require lower driving voltages than other actuation arrangements, several challenges exist to the manufacture of actual usable thin film piezoelectric actuation based fluid ejectors. Initially, when thin film piezoelectric actuators are used, it has been determined by the inventors that they have to have a sufficiently small sized fluid cavity to mechanically match the impedance between the actuator and the fluid being ejected. This makes it difficult to directly use a conventional silicon wafer to build the fluid cavity since the thickness of the conventional silicon wafer is too large, usually between 300 μm to 500 μm thick: Thus, constructing an efficient fluid structure becomes very complicated. Further, the compatibility of depositing piezoelectric thin films with integrated CMOS silicon microelectronics is an issue, as the process for depositing the piezoelectric thin film will tend to destroy the integrated CMOS circuit on the silicon substrate. The present application makes it possible to use conventionally sized silicon wafers in the construction of fluid ejectors, without the need of more polishing, grinding or otherwise making the entire silicon wafer thinner than the conventional thickness.  
         [0015]     In a first approach a recess structure formed in the nozzle plate is employed. Thus when the nozzle plate is bonded to the silicon wafer substrate, the formed recessed portion part fits into an open area in the body of the silicon wafer substrate, selectively reducing the volume of the fluid cavity formed on the substrate. In a second approach, a multi-layer structure including a diaphragm thin film piezoelectric and reduced fluid cavity is fabricated onto one side of the silicon wafer substrate. These two approaches allow the fluid cavity to be small enough to achieve mechanical impedance matching between the fluid cavity and the thin film piezoelectric actuator which is less than approximately 10 μm thick. This impedance matching allows for the use of driving voltages as low as a few volts (e.g., 4 volts). In addition, a laser liftoff transfer method is used to transfer the thin film piezoelectric from a fabrication substrate (e.g., sapphire) to a silicon substrate having integrated driving electronics. Use of the laser liftoff procedure avoids contamination and damage problems due to the piezoelectric deposition procedures.  
         [0016]     Turning to  FIG. 1 , illustrated is a fluid ejector  10 , including a bulk silicon wafer  12  which has integrated drive electronics  14 , and which is micromachined to form an open area  16  with sidewalls  16   a ,  16   b . Deposited on a surface of silicon wafer  12  is a thin structure layer (or membrane)  18 , preferably with a thickness of a few micrometers (e.g., 1 μm to 10 μm, and more preferably 1 μm to 3 μm thick). Thin structure layer  18  can be a silicon based material such as polysilicon, silicon nitride or oxide, a metal or other appropriate material. In one embodiment thin structure layer  18  is a patterned metal layer, which is also used as a bottom electric connection for the piezoelectric thin film layer  20 , which is preferably 1 μm to 10 μm thick, and more preferably 1 μm to 5 μm thick. In another embodiment thin structure layer  18  is a patterned silicon nitride or oxide, and on which is a very thin metal layer (not shown in the figure) deposited and patterned to connect the piezoelectric actuator to the drive electronics  14 , as is well known in the art. Piezoelectric layer  20  is bonded to thin structure layer  18  via bonding layer  22 , and forms a bending mode diaphragm actuator for pushing fluid. A fluid channel  24  is formed by micromachined or laser drilled opening  24   a  and micromachined channel  24   b . Additional fluid channels may be formed as needed.  
         [0017]     A separately fabricated nozzle plate  26  having vertical walls  26   a ,  26   b , a recessed nozzle structure  28 , and an aperture  30 , is bonded and sealed to a second side of silicon wafer  12 . Silicon sidewalls  16   a ,  16   b , thin structure layer  18  and recessed portion  28  of nozzle plate  26  define a reduced volume fluid cavity  32  within the silicon wafer  12 . The recessed portion  28  of nozzle plate  26  is fitted into open area  16  of silicon wafer  12  to form a top portion of fluid cavity  32 . The depth of recess  28  acts to define the height (or depth) of fluid cavity  32 , where the height (or depth) of fluid cavity  32  is less than the thickness of silicon wafer  12 . In one embodiment, recess  28  is selected so the height (or depth) of fluid cavity  32  is about 200 μm or less. Nozzle plate  26  can be made from metal such as nickel or other appropriate material.  
         [0018]     While a single fluid ejector is shown, arrays of fluid ejectors, having the same or similar structure as shown in  FIG. 1 , can be made on a silicon wafer.  
         [0019]     Turning to  FIGS. 2   a - 2   i ,  3   a - 3   c  and  4   a - 4   c , illustrated are the major steps used to make fluid ejector  10  of  FIG. 1 , including forming the recessed nozzle plate.  
         [0020]     As depicted in  FIG. 2   a , starting with silicon wafer  12 , which has integrated drive electronics  14  on a first side of the silicon wafer, a thin and relatively long well or channel  24   b  (which will be part of fluid inlet  24 ) is etched and then filled with sacrificial material  34 , such as PSG glass (phosphosilicate glass) or other etchable or removable material. Several wells will be made if several channels are to be used.  
         [0021]     In  FIG. 2   b , thin structure layer  18 , preferably with the thickness of a few micrometers (μm), is deposited onto a surface of silicon wafer  12  to cover sacrificial material  34 . The material of thin structure layer  18  can be a silicon based material such as polysilicon, silicon nitride or oxide, or other material such as metal, so that selective etching can be undertaken between the bulk silicon wafer  12  and thin structure layer  18 . In one embodiment, thin structure layer  18  is deposited as a thin metal layer by use of a shadow mask. This patterned thin metal layer can also then be used as a bottom connection for piezoelectric thin film  20 . In another embodiment, thin structure layer  18  is deposited as a thin silicon oxide or nitride which can be patterned using a dry or wet etching method. In this case a very thin metal layer (not shown in the figure) will be deposited on the thin silicon oxide or nitride layer with a shadow mask, or patterned using dry or wet chemical etching methods after deposition. The very thin metal layer is used to connect to the piezoelectric thin film  20 .  
         [0022]     Turning to  FIG. 2   c , piezoelectric thin film  20  is fabricated on a separate transparent substrate  36 . This includes but is not limited to depositing piezoelectric thin film  20  on transparent substrate  36 , with a transparent electrode such as ITO (Indium-Tin oxide) on a coated sapphire substrate using a deposition method such as sol-gel, depositing a top surface electrode (not shown), patterning the film and electrode, and then poling the piezoelectric thin film  20 . In one embodiment, the piezoelectric thin film is PZT (lead zirconate titanate) material made by sol-gel, sputtering, CVD (chemical vapor deposition), PLD (pulsed laser deposition), or other suitable deposition methods.  
         [0023]     Next, bonding of piezoelectric thin film  20  to thin structure layer  18  via bonding layer  22  is depicted in  FIG. 2   d , using a bonding technique such as but not limited to a thin film metal transient liquid phase bonding.  
         [0024]     In  FIG. 2   e , transparent.(e.g., sapphire) substrate  36  is removed, such as by a laser liftoff process method, and an ion mill operation is used to remove any laser induced surface damage, then an electrode (not shown) is deposited on the piezoelectric surface, and the piezoelectric thin film is connected to the drive electronics  14  by well-known connection techniques (not shown). More details of the formation of the piezoelectric and the laser liftoff procedure are discussed for example as in U.S. Pat. No. 6,964,201, issued Nov. 15, 2005, entitled “Large Dimension, Flexible Piezoelectric Ceramic Tapes,” by Baomin Xu et al.; U.S. Pat. No. 6,895,645, issued May 24, 2005, entitled “Methods to Make Bimorph MEMS,” by Baomin Xu et al.; and U.S. patent application Ser. No. 10/376,544, filed Feb. 25, 2003, entitled “Methods to Make Piezoelectric Ceramic Thick Film Array and Single Elements and Devices,” by Baomin Xu, et al., each hereby incorporated herein by reference in their entirety.  
         [0025]     Next, as shown in  FIG. 2   f , hole  24   a  is etched or drilled in the thin structure layer  18 . Then, sacrificial material  34  is etched away by use of hole  24   a , to form ink inlet channel  24 . As illustrated in  FIG. 2   g  (where the described structure has been rotated top-to-bottom from its presentation in  FIG. 2   f ), on the other or second side of silicon wafer  12 , micromachining of the silicon wafer is undertaken to selectively remove silicon and form an opening area  16  having sidewalls  16   a ,  16   b . Fluid cavity  32  is to be defined within open area  16 .  
         [0026]      FIG. 2   h  shows, nozzle plate  26  produced according to the required structure, i.e., including recessed portion  28  and aperture  30 . Details on the manufacture of nozzle plate  26  will be provided in connection with  FIGS. 3   a - 3   c  and  4   a - 4   c.    
         [0027]     Finally, as depicted in  FIG. 2   i , nozzle plate  26  is bonded to silicon wafer  12  to form fluid ejector  10  with selectably sizable fluid cavity  32 . The nozzle plate  26  may be bonded with adhesive or solder which will fill in gaps to avoid air bubbles and seal the ink cavity.  
         [0028]     Turning now to  FIGS. 3   a - 3   c  and  4   a - 4   c , two methods to make a nozzle plate in accordance with the present concepts are set forth. The first embodiment uses a mechanical stamping process. The second embodiment uses an electroplating method.  
         [0029]     In  FIG. 3   a , the process employs a metal foil  40  and a lower metal mold portion  42   a , which has an opening with similar dimensions as open area  16  of silicon wafer  12  but with a different depth. Attention is directed to dotted line  43 . This dotted line is intended to show an alternative representation of the lower metal mold portion  42   a . In particular, dotted line  43  is provided to emphasize that nozzle plates, such as nozzle plate  26  of  FIG. 1  can have selectively alterable configurations. In this specific example, dotted line  43  emphasizes that the depth of the recessed portion of the nozzle plate, such as recessed portion  28  of  FIG. 1 , is controllable during the manufacturing process. More particularly, a manufacturer or user of the present concepts would provide a specific depth in the recessed portion such that a high level of impedance matching will exist between the fluid within the fluid cavity and the actuator of a particular fluid ejector device. It is to be understood that dotted line  43  is simply provided as showing the adjustable or selective features of the nozzle plate according to the present application, and other depths and/or configurations of the nozzle plate to improve the mechanical impedance are within the realm of the present application.  
         [0030]     Next, as depicted in  FIG. 3   b , metal foil  40  is pressed into lower mold portion  42   a , by use of an upper mold stamp portion  42   b . While maintaining pressure, mold  42  is heated by heater  44  to a temperature sufficient to induce permanent deformation of metal foil  40 .  
         [0031]     Lastly, in  FIG. 3   c  mold portions  42   a ,  42   b  are removed and aperture  30  is etched or laser drilled in deformed metal foil  40 , to form nozzle plate  26  with recess  28 . Aperture  30  can also be formed by etching or laser drilling before stamping the metal foil  40 .  
         [0032]     Turning to a second embodiment, in  FIG. 4   a , the process starts with a metal or silicon mold  46 . The mold has an opening with similar dimensions as of silicon wafer  12  but a different depth. A sacrificial layer  48 , and then a thin metal film  50  are deposited onto mold  46 .  
         [0033]     Next, as shown in  FIG. 4   b , a relatively thick metal layer  52  is deposited on thin metal film  50 , with a thickness about several micrometers (μm) (e.g, 1 μm to 10 μm) by using a manufacturing procedure such as an electroplating method. This deposited metal layer  52  could be either the same or different metal as the thin metal film  50 . Following the deposition, an aperture  30  and holes  54 ,  56  are laser drilled or etched through layers  52  and  50  to reach sacrificial layer  48 . Holes  54 ,  56  are provided if needed to etch away the sacrificial layer  48 . Alternatively, holes  54 ,  56  might not be provided, and etching of sacrificial layer  48  may be undertaken through aperture  30  alone.  
         [0034]     Then, as shown in  FIG. 4   c , sacrificial layer  48  as shown in  FIG. 4   a  is etched away, and the metal or silicon mold  46  is removed, providing fabricated nozzle plate  58 , which may be used in the fluid ejector of  FIG. 1 .  
         [0035]     Turning to  FIG. 5 , a modified structure of the micromachined fluid ejector of  FIG. 1  is depicted. As will be understood from a review of  FIG. 5 , fluid ejector  60  is constructed substantially similar to ejector  10  of  FIG. 1 . However, in this design nozzle plate  62  has sloping sidewalls  62   a ,  62   b  as opposed to the substantially vertical sidewalls  26   a ,  26   b  of  FIG. 1 . By this construction, additional material is provided in the nozzle plate for increased strength of the nozzle plate. A nozzle plate of this design can be configured by use of, for example, an electroplating method.  
         [0036]     Turning to  FIGS. 6   a - 6   c , top views of alternative fluid cavity shapes are provided. The fluid cavity can be formed as a square shape  64 , a thin and long rectangular shape  66 , or a curved shape  68 , among others. While fluid apertures  64   a ,  66   a ,  68   a  shown in  FIGS. 6   a - 6   c  are made close to the center of the nozzle plate, this is not necessary for many applications. Several inlets  64   b - 64   e ,  66   b - 66   c , and  68   b - 68   c  are shown as being provided to the fluid cavity, which are intended to be placed strategically to help minimize the undesirable generation of air bubbles which may form during the initial fluid filling of the cavities. While four inlets are shown for  FIG. 6   a  and two inlets for  FIGS. 6   b  and  6   c , this is not necessary, and different numbers of inlets could be used for different designs or applications. Each of  FIGS. 6   a ,  6   b ,  6   c  also show piezoelectric thin films  64   f ,  66   d  and  68   d , and fluid cavities  64   g ,  66   e ,  68   e . The curved design of  FIG. 6   c  is intended to incorporate features such as inlet impedance within the ink chamber. The curved design can be arranged in a staggered arrangement when an array of fluid ejectors is formed.  
         [0037]     It is to be appreciated, the processes for manufacturing the nozzle plates as shown in  FIGS. 3   a - 3   c , and  4   a - 4   c  may include molds and machining processes which result in the manufacture of nozzle plates having profiles similar to the fluid cavity to which it is to be associated. For example, the processes of  FIGS. 3   a - 3   c  and  4   a - 4   c  can be modified to form nozzles having square shapes, thin and long rectangular shapes or curved shapes, among others, as for example as discussed in connection with  FIGS. 6   a - 6   c.    
         [0038]     Turning to  FIG. 7 , depicted is a second design for a fluid ejector  70 . Instead of using the silicon wafer to form the fluid cavity, a structure with several layers on one side of the silicon wafer is built. The fluid cavity, fluid inlet and ejector aperture are constructed within this multi-layer structure. The height or depth of the ink cavity being preferably controlled to be 200 μm or less, and more preferably in a range of about 100 μm to 200 μm.  
         [0039]     With more particular attention to fluid ejector  70  of  FIG. 7 , in this structure, silicon wafer  72  has a monolithic structure  74  built on one side. The structure includes a first structure layer  76 , a sacrificial (e.g., polysilicon) layer  78  sandwiched between the first structure layer  76  and a second structure layer  80 . The second structure layer includes a horizontal portion  80   a  and filled trenches or vertical sidewalls  80   b  and  80   c . The first structure layer  76 , horizontal portion  80   a  and filled trenches/vertical sidewalls  80   b  and  80   c  of the second structure layer define a fluid cavity  82 . Holes or openings  84   a  and  84   b  are formed within the second structure layer  80  to act as fluid inlets, and aperture  88  is formed in the first structure layer  76  to emit fluid. The silicon wafer  72  has been etched through a second surface to create an open area  90  exposing portions of the first structure layer  76  whereby aperture  88  is open to free space. A piezoelectric thin film  92  is bonded to the horizontal portion of the second structure layer  80  via a bonding layer  94 .  
         [0040]     With particular attention to  FIG. 8   a , the process for fabricating a fluid ejector as shown in  FIG. 7  begins with obtaining a silicon substrate  72 , and then as shown in  FIG. 8   b , depositing a first structure layer  76  thereto, where structure layer  76  may be a metal conductive layer, or silicon oxide or nitride layer deposited by any of known depositing methods, such as CVD, PVD, electroplating or other depositing procedure.  
         [0041]     Next, as shown in  FIG. 8   c , a sacrificial layer  78  is deposited on top of the first structure layer  76 . Sacrificial layer  78  can be a polysilicon or other material having characteristics which permit its selective etching or otherwise removal during the formation of the fluid ejector. The depth or height of sacrificial layer  78  is particularly controlled, as it will define the height of the fluid cavity.  
         [0042]     In  FIG. 8   d , portions of sacrificial layer  78  are etched or otherwise removed to form closed trenches with parts of which shown as  79   a  and  79   b . As can be seen in this FIGURE, trenches  79   a  and  79   b  are made within sacrificial layer  78 , such that a surface of first structure layer  76  is exposed. The formation of closed trenches  79   a  and  79   b  cause the sacrificial layer  78  to be divided into two sections, including a center section  78   a , and an outer section  78   b . Thereafter, and as depicted in  FIGS. 9   e  and  9   f , a second structure layer  80  is deposited, which in some embodiments is a metal layer or a thin oxide or nitride layer. Second structure layer  80  includes a horizontal layer portion  80   a  and portions which fill in the closed trenches in the sacrificial layer and which are formed as closed, filled trenches or vertical sidewall structures. Parts of the closed, filled trenches or vertical sidewalls are shown in the FIGURE as  80   b  and  80   c . By this design, end surfaces of filled trenches  80   b  and  80   c  come into contact with a surface of the first structure layer  76 .  FIG. 9   f  shows that holes  84   a  and  84   b  are formed in the second structure layer  80 , where holes  84   a  and  84   b  are created such that sections of the surface for center sacrificial portion  78   a  are exposed. Holes  84   a  and  84   b  are positioned to act as fluid inlets in the formed fluid ejector.  
         [0043]     Next, in  FIG. 8   g  a piezoelectric thin film  92  is shown bonded to a surface of the second structure layer  80  via bonding layer  94 .  
         [0044]     Turning to  FIG. 8   h , the side of the device with the piezoelectric is protected through the application of resist material and/or tape  96 . It is desirable to protect the piezoelectric side of the device, as the next step in the process includes etching, drilling or otherwise removing portions of silicon wafer  72  to create opening  90 .  
         [0045]     Opening  90  exposes a surface portion of the first structure layer  76 , corresponding to at least a portion of the center sacrificial layer portion  78   a . Thereafter, and as illustrated in  FIG. 8   i , aperture  88  is formed in first structure layer  76  by a laser drilling or etching step. Aperture  88  also works as an opening into the center sacrificial layer portion  78   a , whereby etching for removal of the sacrificial material is undertaken. By this process, fluid cavity  82  is formed. Once these processes are complete, the protective layer  96  is removed. By removal of layer  96 , holes or inlets  84   a  and  84   b  provide passages for fluid cavity  82 , wherein fluid within fluid cavity  82  is ejected via aperture  88  from fluid ejector  70 .  
         [0046]     It is pointed out that in  FIGS. 1 and 5  drive electronics are shown integrated with the silicon wafer. A similar arrangement may be provided in connection with the described fluid ejector  70  of  FIG. 7 . However, considering the cost issue providing integrated electronics may not be necessary for all cases. For example, if the nozzle density is very low, surface mounting the drive electronics (which are manufactured separately) may be more cost effective. When it is necessary to have integrated drive electronics a laser liftoff process can be used to transfer the piezoelectric elements. The laser transfer method may also be used to avoid the contamination problem. On the other hand, if the drive electronics are fabricated separately, the piezoelectric thin film can be directly deposited on the silicon wafer.  
         [0047]     Turning to  FIG. 9 , illustrated is a fluid ejector  100 , including a bulk silicon wafer  102  which has surface mounted drive electronics  104 . The bulk silicon wafer is micromachined to form an open area  106  having sidewalls  106   a ,  106   b . Deposited on a surface of silicon wafer  102  is a thin structure layer (or membrane)  108 , preferably with a thickness of a few micrometers (e.g., 1 μm to 10 μm, and more preferably 1 μm to 3 μm thick). Thin structure layer  108  can be a silicon based material such as polysilicon, silicon nitride or oxide. In  FIG. 9  thin structure layer  108  is a patterned silicon nitride or oxide, on which is a very thin metal layer  110  which acts as a bottom electrode of deposited and patterned piezoelectric  112 . Bottom electrode  110  is also used to connect piezoelectric  112  to surface mounted drive electronics  104 . A top electrode  114  is deposited on a second side of piezoelectric  112 . The top electrode  114  can be connected to the drive electronics  104  by any well-known connection method, such as but not limited to, wire bonding (not shown in the FIGURE). Piezoelectric  112  and thin structure layer  108  forming a bending mode diaphragm actuator for pushing fluid. A fluid channel  116  is formed by micromachined or laser drilled opening  116   a  and micromachined channel  116   b . Additional fluid channels may be formed as needed.  
         [0048]     A separately fabricated nozzle plate  118  having vertical walls  118   a ,  118   b , a recessed nozzle structure  120 , and an aperture  122 , is bonded and sealed to a second side of silicon wafer  102 . Silicon sidewalls  106   a ,  106   b , thin structure layer  108  and recessed portion  120  of nozzle plate  118  define a reduced volume fluid cavity  124  within the silicon wafer  102 . The recessed portion  120  of nozzle plate  118  is fitted into open area  106  of silicon wafer  102  to form a top portion of fluid cavity  124 . The depth of recess  120  acts to define the height (or depth) of fluid cavity  124 , where the height (or depth) of fluid cavity  124  is less than the thickness of silicon wafer  102 . In one embodiment, recess  120  is selected so the height (or depth) of fluid cavity  124  is about 200 μm or less (and more preferably in a range of 100 μm to 200 μm). Nozzle plate  118  can be made from metal such as nickel or other appropriate material.  
         [0049]     While a single fluid ejector is shown, arrays of fluid ejectors, having the same or similar structure as shown in  FIG. 9 , can be made on a silicon wafer.  
         [0050]     Turning to  FIGS. 10   a - 10   f , illustrated are the major steps used to make fluid ejector  100  of  FIG. 9 .  
         [0051]     As depicted in  FIG. 10   a , starting with silicon wafer  102  having a first side and a second side, a thin and relatively long well or channel  116   b  (which will be part of fluid inlet  116 ) is etched on the first side and then filled with sacrificial material  126 , such as PSG glass (phosphosilicate glass) or other etchable or removable material. Several wells will be made if several channels are to be used.  
         [0052]     In  FIG. 10   b , thin structure layer  108 , with a thickness of a few micrometers (e.g., 1 μm to 10 μm, and preferably 1 μm to 3 μm thick), is deposited onto a surface of silicon wafer  102  to covering sacrificial material  126 . The material of thin structure layer  108  can be a silicon based material such as polysilicon, silicon nitride or oxide, so that selective etching can be made between the bulk silicon wafer and this membrane layer. Next, the bottom electrode  110  is deposited on a surface of structure layer  108 . The bottom electrode  110  also works as a buffer layer to prevent a reaction between the piezoelectric film  110  and the silicon thin layer structure, and therefore an inert/noble metal material is preferred. A specific material which may be used is platinum (Pt). In order to enhance the adhesion between the bottom electrode and the silicon thin layer structure, commonly another thin metal layer, such as titanium (Ti), may be deposited between the silicon thin layer structure and the platinum (Pt) bottom electrode layer.  
         [0053]     Turning to  FIG. 10   c , piezoelectric thin film  112  is shown deposited on bottom electrode  110 . This depositing step includes but is not limited to using a deposition method such as sol-gel, sputtering, CVD (chemical vapor deposition), PLD (pulsed laser deposition), or other suitable deposition method. Next, top electrode  114  is deposited, and the piezoelectric thin film  112  is poled to generate the piezoelectric property.  
         [0054]     As shown in  FIG. 10   d , top electrode  114 , piezoelectric  112  and bottom electrode  110  are patterned. Then hole  116   a  is etched or drilled in the thin structure layer  108 , and sacrificial material  126  is etched away by use of hole  116   a , in order to form ink inlet channel  116 . Then, as illustrated in  FIG. 10   e  (where the described structure has been rotated top-to-bottom from its presentation in  FIG. 10   d ), the drive electronics  104  has been surface mounted to the first side of the silicon wafer and connected to the piezoelectric thin film  11 . After that, on the second side of silicon wafer  102 , micromachining of the silicon wafer is undertaken to selectively remove silicon and form opening area  106  having sidewalls  106   a ,  106   b . Fluid cavity  124  is to be defined within open area  106 .  
         [0055]      FIG. 10   f  shows nozzle plate  118  produced according to the required structure, i.e., including recessed portion  120  and aperture  122 . Details on the manufacture of nozzle plate  118  have previously been provided in connection with  FIGS. 3   a - 3   c  and  4   a - 4   c.    
         [0056]     As depicted in  FIG. 10   f , nozzle plate  118  is bonded to silicon wafer  102  to form fluid ejector  100  with selectably sizable fluid cavity  124 . The nozzle plate  118  may be bonded with adhesive or solder which will fill in gaps to avoid air bubbles and seal the ink cavity.  
         [0057]     In each of the foregoing embodiments, the manufacturing process may provide an appropriate thickness ratio between the piezoelectric layer and the structure layer (i.e., structure layer  18  of  FIG. 1 , and structure layer portion  80   a  of  FIG. 7 ) to optimize the actuation performance.  
         [0058]     Through controlling the variable features of (i) the thickness and materials of structural layer  18  (of  FIG. 1 ), or center horizontal layer portion  80   a  (of  FIG. 7 ), (ii) the piezoelectric thickness ( 20  of  FIG. 1  and  92  of  FIG. 7 ), and (iii) the depth of the fluid cavity ( 32  of  FIG. 1, 82  of  FIG. 7 ) appropriate impedance matching may be selected to optimize the transfer of energy into the fluid cavity for fluid ejection.  
         [0059]     It has been further considered by the inventors that a range of a piezoelectric layer of 1 μm to 10 μm (and more preferably in a range of 1 μm to 5 μm), in combination with a structure layer ( 18  in  FIG. 1  and  80  or  80   a  in  FIG. 7 ) of 1 μm to 10 μm (and more preferably 1 μm to 3 μm) with a cavity depth of 200 μm or less (and more preferably 100 μm to 200 μm), will also provide desirable results.  
         [0060]     The disclosures related to  FIGS. 1 and 5 , illustrate that a fluid ejector employing piezoelectric actuation can have the depth of the fluid cavity  32  adjusted to obtain a desirable mechanical impedance matching. More specifically, when the thickness of the piezoelectric and/or silicon layers are varied, the depth of the recess  28  may also be varied, either increasing or decreasing the depth of the fluid cavity to permit an optimized mechanical impedance matching for optimized transfer of energy from the piezoelectric actuator into the fluid cavity. Thus, it is to be understood the processes shown in  FIGS. 3   a - 3   c  and  4   a - 4   c  are adjustable in order to provide nozzle plates having different recessed portions. As mentioned above, while a single fluid ejector for each of the embodiments in  FIGS. 1, 5  and  7  have been depicted and discussed, a multitude or array of each of these fluid ejectors may be manufactured on a single piece of silicon wafer. In these embodiments, it is therefore possible to have in a single array fluid ejector cavities having different depths. For example, in the embodiment of  FIG. 1 , a depth of recess  28  for nozzle plate  26  may be adjusted during the manufacturing processes of  FIGS. 3   a - 3   c  and/or  4   a - 4   c , whereby the depth or height of the fluid cavity can be changed. Similarly, in the process according to  FIG. 7 , the depth or height of layer  78  may be made to provide distinct heights or depths in the corresponding fluid cavity.  
         [0061]     Also, while the nozzle plate with the recessed portion has been described to be used with the piezoelectric actuation system, it is to be understood benefits may be obtained when a nozzle plate having a recessed profile as shown in the foregoing discussion is applied to other fluid ejectors such as those using electrostatic actuation. More particularly, even with the non-piezoelectric based actuation systems, impedance matching between actuators of whatever type, and the depth of the fluid cavity, may improve or optimize the mechanical impedance matching of a fluid ejector.  
         [0062]     In consideration of the lower driving voltages needed for piezoelectric thin film actuation, the following discussion is provided. The inventors have studied an electrostatic membrane driving structure which has a polysilicon membrane that is about 1000 μm×120 μm×2 μm and the membrane air gap (the distance between the lower surface of the polysilicon membrane and the bottom electrode) is about 1 μm. It has been found that with about 100V driving voltage, the center point displacement of the membrane is about 0.25 μm. The membrane moves only along one direction, a downward movement.  
         [0063]     The inventors have also calculated the center point displacement of a piezoelectric diaphragm actuator which has similar lateral dimensions as the electrostatic membrane actuator described above but the diaphragm or membrane is composed of 1 μm thick polysilicon and 2 μm thick sol-gel piezoelectric (e.g., PZT, lead zirconate titanate) thin film. The mechanical stiffness of 1 μm thick polysilicon and 2 μm thick sol-gel piezoelectric (e.g., PZT) thin film is about the same as that of 2 μm thick polysilicon, which means this arrangement can generate the same force if the same displacement is achieved. It has been calculated by the inventors that only 4V applied voltage can generate 0.173 μm center point displacement for the piezoelectric diaphragm actuator. Considering that a piezoelectric actuator can move in two directions (up and down), by applying ±4V it is possible to generate a 0.346 μm center point displacement. Thus it can be seen that to generate a similar displacement and force, the driving voltage can be significantly reduced by using piezoelectric actuation instead of electrostatic actuation.  
         [0064]     The present disclosure thus describes a manner to easily change the fluid cavity size to realize the mechanical impedance matching between the fluid in the fluid cavity and the actuator. When using a thin film piezoelectric actuator or even an electrostatic membrane actuator, the fluid cavity needs to be relatively small, especially for the cavity height, which needs to be about 200 μm or less. As a conventional silicon wafer is about 300 μm thick or more, this makes it difficult to form a small ink cavity using the entire thickness of the silicon wafer body. However, by using a recessed nozzle plate to fit into the opening area made on the silicon wafer body, the fluid cavity height can easily be reduced to about 200 μm or less, without reducing the thickness of the silicon wafer. For the embodiment of  FIGS. 7, 8   a - 8   i , the fluid cavity height can be easily controlled during the manufacturing process.  
         [0065]     Thus, the present application specifically shows a fluid ejector which permits the use of a nozzle plate which may change its shape, and in particular, the amount of recess in the nozzle plate, in order to adjust the fluid cavity volume. This adjustment is made in order to improve the performance of the ejector through improving the impedance matching between the fluid and the actuator.  
         [0066]     The foregoing discussion sets forth the major processing steps for manufacturing various embodiments of the described fluid ejectors. Various minor processing steps, such as depositing electrodes and making certain electrical attachments, have not been specifically recited. These processing steps are well known in the art, and have not been specifically set forth, in some instances, simply to focus the application and to provide clarity in the drawings and discussion.  
         [0067]     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.