Abstract:
A method of forming a piezoelectric actuator of an inkjet head formed on a vibrating plate to provide a driving power for ejecting ink to each of pressure chambers is provided. The method includes forming a lower electrode on a vibrating plate, forming a piezoelectric layer on the lower electrode to be located above each of pressure chambers, forming a protecting layer covering the lower electrode and the piezoelectric layer, exposing an upper surface of the piezoelectric layer by decreasing a thickness of the protecting layer and the piezoelectric layer, forming an upper electrode on the upper surface of the piezoelectric layer, removing the protecting layer. According to the present invention, since the piezoelectric layer having a flat upper surface is formed in uniform figure, area and thickness of the upper electrode formed thereon is uniformly controlled.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2006-0012598, filed on Feb. 9, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present general inventive concept relates to an inkjet head, and more particularly, to a method of forming a piezoelectric actuator in a uniform shape, the piezoelectric actuator providing a driving force to eject ink from a piezoelectric inkjet head. 
     2. Description of the Related Art 
     Generally, inkjet heads are devices that can print a color image on a printing medium by ejecting droplets of ink onto a desired region of the printing medium. Depending on the ink ejecting method, the inkjet heads can be classified into two types: thermal inkjet heads and piezoelectric inkjet heads. The thermal inkjet head generates bubbles in the ink to be ejected by using heat and ejects the ink using expansion of the bubbles, and the piezoelectric inkjet head ejects ink using a pressure generated by deforming a piezoelectric material. 
       FIG. 1A  is a sectional view illustrating a general structure of a conventional piezoelectric inkjet head, and  FIG. 1B  is a sectional view along a line A-A′ of  FIG. 1A . 
     Referring to  FIG. 1A  and  FIG. 1B , a manifold  11 , a plurality of restrictors  12 , and a plurality of pressure chambers  13  are disposed in a flow channel plate  10  to form an ink flow channel. A vibrating plate  20 , which becomes deformed by driving a piezoelectric actuator  40 , is bonded to an upper surface of the flow channel plate  10 . A nozzle plate  30 , having a plurality of nozzles  31 , is bonded to a lower surface of the flow channel plate  10 . The flow channel plate  10  and the vibrating plate  20  may be integrally formed, and so may the flow channel plate  10  and the nozzle plate  30 . 
     The manifold  11  is a passage that supplies ink flowing from an ink storage (not illustrated) to each of the pressure chambers  13 , and the restrictor  12  is a passage through which ink flows from the manifold  11  into each of the pressure chambers  13 . The pressure chambers  13  are arranged along one side or both sides of the manifold  11  to store the ink to be ejected. The nozzles  31  are formed by penetrating the nozzle plate  30  and are each connected to a respective one of the pressure chambers  13 . The vibrating plate  20  is bonded to an upper surface of the flow channel plate  10  to cover the pressure chambers  13 . The vibrating plate  20  is deformed by the operation of the piezoelectric actuator  40  to supply the pressure variation, to eject ink, to each of the pressure chambers  13 . The piezoelectric actuator  40  includes a lower electrode  41 , a piezoelectric layer  42 , and an upper electrode  43 , which are successively stacked on the vibrating plate  20 . The lower electrode  41  is formed on a whole surface of the vibrating plate  20  to serve as a common electrode. The piezoelectric layer  42  is formed on the lower electrode  41  so as to be located above each of the pressure chambers  13 . The upper electrode  43  is formed on the piezoelectric layer  42  to serve as a driving electrode to apply a voltage to the piezoelectric layer  42 . 
     The piezoelectric actuator  40  of the conventional piezoelectric inkjet head is, generally, formed as described below. The lower electrode  41  is formed by depositing a predetermined metal material at a predetermined thickness on the vibrating plate  20  using a sputtering process. The piezoelectric layer  42  is formed by coating a ceramic material of a paste state having a piezoelectricity at a predetermined thickness on the lower electrode  41  using a screen-printing process, and sintering the same. The upper electrode  43  is formed by coating a conductive material on the piezoelectric layer  42  using a screen-printing process, and sintering the same. 
     However, since the conventional piezoelectric layer  42  formed by the screen-printing tends to spread laterally because of a property of the material of the paste state, it is difficult to form the conventional piezoelectric layer  42  in a uniform thickness. That is, a middle portion of the piezoelectric layer  42  is thick, while both edge portions of the piezoelectric layer  42  are thin, as illustrated in  FIG. 1B . The upper electrode  43 , which is formed on the piezoelectric layer  42  by a screen-printing process, also may not be uniform in shape, area, and thickness, due to a fluidity of the paste. Particularly, since a thickness of the piezoelectric layer  42  is not uniform, a distance between the upper electrode  43  and the lower electrode  41 , which are formed respectively on the upper surface and the lower surface of the piezoelectric layer  42 , is not uniform. Accordingly, an electric field formed between the upper electrode  43  and the lower electrode  41  is also not uniform. In addition, when the upper electrode  43  is formed on the thin edge portion of the piezoelectric layer  42 , an interval between the upper electrode  43  and the lower electrode  41  becomes a lot smaller, so that the upper electrode  43  and the lower electrode  41  may be shorted. Moreover, a paste may flow down along a curved surface of the piezoelectric layer  42  and directly contact the lower electrode  41  in the forming process of the upper electrode  43 , leading to a defective piezoelectric actuator  40 . 
     As described above, the conventional method of the piezoelectric actuator  40  cannot control formation of a uniform width, area, and thickness etc., of the upper electrode  43 . 
     SUMMARY OF THE INVENTION 
     The present general inventive concept provides a method of forming a piezoelectric actuator of an inkjet head that can uniformly control a formation of an upper electrode and can prevent a short-circuit between the upper electrode and a lower electrode. 
     Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
     The foregoing and/or other aspects and utilities of the present general inventive concept are achieved by providing a method of forming a piezoelectric actuator of an inkjet head formed on a vibrating plate to provide a driving force to eject an ink to each of a plurality of pressure chambers, the method including forming a lower electrode on the vibrating plate, forming a piezoelectric layer on the lower electrode to correspond to each of the plurality of pressure chambers; forming a protecting layer covering the lower electrode and the piezoelectric layer; exposing an upper surface of the piezoelectric layer by decreasing a thickness of the protecting layer and the piezoelectric layer; forming an upper electrode on the upper surface of the piezoelectric layer; and removing the protecting layer. 
     A silicon oxide layer or a silicon nitride layer may be formed as an insulating layer between the vibrating layer and the lower electrode. 
     The lower electrode may be formed by depositing a conductive metal material at a predetermined thickness. The lower electrode may be formed by sequentially depositing a Ti layer and a Pt layer using a sputtering process. 
     The piezoelectric layer may be formed by coating a piezoelectric material of a paste state using a screen-printing process. The forming of the piezoelectric layer may include drying and sintering the piezoelectric layer of a paste state. A cold isostatic press (CIP) process may be performed to densify a construction of the dried piezoelectric layer. 
     The protecting layer may be formed of an organic material selected from a group of a polydimethylsiloxane (PDMS), a polymethylmethacrylate (PMMA) and a photosensitive polymer. The protecting layer may be formed by coating the organic material using a spin coating process. 
     A thickness of the protecting layer and the piezoelectric layer may be decreased by a chemical-mechanical polishing (CMP) process or a lapping process. 
     The upper electrode may be formed by coating an electrode material of a paste state on the piezoelectric layer using a screen-printing process. The forming of the upper electrode may be performed by drying and sintering the upper electrode of a paste state. 
     The upper electrode may be formed by depositing a conductive material at a predetermined thickness on the piezoelectric layer by a sputtering process. 
     The protecting layer may be removed by an O 2  ashing or by using a sulphuric acid solution or an acetone. 
     The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a method of forming a piezoelectric actuator of an inkjet head formed on a vibrating plate, the method including forming a lower electrode on the vibrating plate; forming a piezoelectric layer in a predetermined pattern on the lower electrode to correspond with a plurality of pressure chambers to contain ink therein; forming a protecting layer covering the lower electrode and the piezoelectric layer pattern; etching the protecting layer and a portion of the piezoelectric layer pattern to a predetermined thickness to expose the piezoelectric layer pattern within a same plane with the protecting layer; and forming an upper electrode above the etched region to correspond with the exposed piezoelectric layer pattern. 
     The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a method of forming a piezoelectric actuator of an inkjet head formed on a vibrating plate, the method including forming a lower electrode on the vibrating plate; forming a piezoelectric layer in a predetermined pattern on the lower electrode to correspond with a plurality of pressure chambers to contain ink therein; etching the formed piezoelectric layer to a predetermined thickness; and forming an upper electrode on the etched piezoelectric layer pattern and corresponding with the predetermined pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1A  is a sectional view illustrating a general structure of a conventional piezoelectric inkjet head; 
         FIG. 1B  is a sectional view along a line A-A′ of  FIG. 1A ; 
         FIG. 2A  through  FIG. 2F  is a view sequentially illustrating a method of forming a piezoelectric actuator of an inkjet head according to an embodiment of the present general inventive concept; and 
         FIG. 3  is a view illustrating another embodiment of the forming operation of an upper electrode illustrated in  FIG. 2E . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. 
       FIG. 2A  through  FIG. 2F  are views sequentially illustrating a method of forming a piezoelectric actuator of an inkjet head according to an embodiment of the present general inventive concept. The drawings illustrate a part of the inkjet head, and generally, several tens or hundreds of pressure chambers and nozzles are arranged along one line or a plurality of lines in an inkjet head. 
     Referring to  FIG. 2A , a piezoelectric inkjet head may include an ink flow channel, which may be formed on plates, for example, a flow channel plate  110 , a vibrating plate  120 , and a nozzle plate  130 . A plurality of pressure chambers  113  are formed between the flow channel plates  110  of the inkjet head. The vibrating plate  120  is bonded to an upper surface of the flow channel plates  110  to cover the pressure chambers  113 , and the nozzle plate  130 , through which a plurality of nozzles  31  are formed, is bonded to a lower surface of the flow channel plates  110 . A manifold and a plurality of restrictors (not illustrated) may also be formed between the flow channel plates  110 . The flow channel plates  110  and the vibrating plate  120  may be integrally formed, and so may the flow channel plates  110  and the nozzle plate  130 . 
     A piezoelectric actuator  140  (see  FIG. 2F ) is formed on the vibrating plate  120  of the inkjet head by processes described below. The piezoelectric actuator  140  provides a driving force to eject ink to each of the pressure chambers  113  by deforming the vibrating plate  120 . 
     As illustrated in  FIG. 2A , a lower electrode  141  is formed on a whole surface of the vibrating plate  120  to serve as a common electrode. An insulating layer  121  to provide insulation between the lower electrode  141  and the vibrating plate  120  may be formed on a whole surface of the vibrating plate  120  before forming the lower electrode  141 . In this case, the lower electrode  141  is formed on a whole surface of the insulating layer  121 . When the vibrating plate  120  is formed of a silicon substrate, the insulating layer  121  may be formed of a silicon oxide layer or a silicon nitride layer. 
     The lower electrode  141  may be formed by depositing a conductive metal material at a predetermined thickness on a whole surface of the vibrating plate  120  or the insulating layer  121 . For example, the lower electrode  141  may be formed of one metal layer or two metal layers consisting of a Ti layer and a Pt layer. When the lower electrode  141  is formed of the two layers, the Ti layer may be formed approximately 400 Å thick by a sputtering process, and the Pt layer may be formed approximately 5000 Å thick also by a sputtering process. 
     Next, as illustrated in  FIG. 2B , a piezoelectric layer  142  is formed on the lower electrode  141  to be located above each of the pressure chambers  113 . The piezoelectric layer  142  may be formed by coating a piezoelectric material of a paste state, for example, a lead ziroconate titanate (PZT) ceramic material, to a predetermined thickness using a screen-printing process. A thickness T 1  of the piezoelectric layer  142  may be thicker than a final thickness T 2  in  FIG. 2D  of the piezoelectric layer  142 , for example, approximately 50 μm thick. Next, the piezoelectric layer  142  of a paste state is dried, and then sintered at approximately 900° C.˜1200° C. A cold isostatic press (CIP) process may be performed on the piezoelectric layer  142  of a paste state before the sintering. The CIP process is a process of densifying a construction by applying a same pressure to the piezoelectric layer  142  from all directions. 
     Next, as illustrated in  FIG. 2C , a protecting layer  150  is formed to cover the lower electrode  141  and the piezoelectric layer  142 . An organic material removable after being solidified from a liquid state, for example, a polydimethylsiloxane (PDMS), a polymethylmethacrylate (PMMA), or a photosensitive polymer such as photoresist, may be used as the protecting layer  150 . The protecting layer  150  may be formed by coating the removable material (such as the organic material) using a spin coating process. 
     Next, as illustrated in  FIG. 2D , thicknesses of the piezoelectric layer  142  and the protecting layer  150  are decreased to a desired thickness T 2 , for example, approximately 10-30 μm. A final thickness T 2  of the piezoelectric layer  142  may be varied depending on a size of the pressure chamber  113  and a thickness of the vibrating plate  120 . The decreasing of thicknesses of the piezoelectric layer  142  and the protecting layer  150  may be performed by a chemical-mechanical polishing (CMP) process or a lapping process. 
     After the above operations are completed, the piezoelectric layer  142  having the uniform thickness T 2  and a flat upper surface is completely formed on the vibrating plate  120 . When the piezoelectric layer  142  has the uniform thickness T 2 , a distance between an upper electrode  143  as illustrated in  FIG. 2E  and the lower electrode  141 , which are formed respectively above and below the piezoelectric layer  142 , is uniform, so that a uniform electric field is formed. 
     Referring to  FIG. 2E , the upper electrode  143  is formed on an exposed upper surface of the piezoelectric layer  142 , as illustrated in  FIG. 2D , to serve as a driving electrode. The upper electrode  143  may be formed by screen-printing an electrode material, for example, an Ag—Pd paste, on the piezoelectric layer  142 , and then drying the same and sintering the same at a temperature range of approximately 100-400° C. 
     As described above, according to an embodiment of the present general inventive concept, the upper electrode  143  is formed in a state where the upper surface of the piezoelectric layer  142  is exposed and the upper surface of the lower electrode  141  is covered with the protecting layer  150 . Therefore, the upper electrode  143  and the lower electrode  141  are prevented from being shorted as a fluidity of the paste of the upper electrode  143  is prevented. Also, since the upper surface of the piezoelectric layer  142  is flat, it is easy to form the upper electrode  143  to a uniform thickness. In addition, since only the upper surface of the piezoelectric layer  142  is exposed at the time of forming the upper electrode  143 , although the electrode material is coated on the protecting layer  150  out of the range of the upper surface of the piezoelectric layer  142 , the electrode material coated on the protecting layer  150  is removed along with the removal of the protecting layer  150 , thereby forming the upper electrode  143  having a uniform area and shape. 
     In another embodiment of the present general inventive concept, an upper electrode  143  may be formed by depositing the electrode material at a predetermined thickness on the piezoelectric layer  142  by using a sputtering process, which will be described below with reference to  FIG. 3 . 
     The protecting layer  150  remaining on the lower electrode  141  is removed, so that the piezoelectric actuator  140  including the lower electrode  141 , the piezoelectric layer  142  and the upper electrode  143 , sequentially stacked, is formed as illustrated in  FIG. 2F . The protecting layer  150  may be removed by various known methods, for example, by an O 2  ashing process or by using a sulphuric acid solution or an acetone, depending on the type of the material used to form the protecting layer  50 . 
       FIG. 3  is a view illustrating another embodiment of forming the upper electrode in  FIG. 2E . 
     Referring  FIG. 3 , the upper electrode  143  may be formed by depositing a metal material, for example, a conductive metal material, such as Au or Pt, at a predetermined thickness on the exposed upper surface of the piezoelectric layer  142  illustrated in  FIG. 2D  using a sputtering process. At this time, the upper electrode  143  is formed on the protecting layer  150  as well as the piezoelectric layer  142 . Subsequently, when the protecting layer  150  is removed as descried above, the upper electrode  143  deposited on the protecting layer  150  is lifted off and removed together with the protecting layer  150 , and only the upper electrode  143  deposited on the piezoelectric layer  142  remains, as illustrated in  FIG. 2F . 
     As described above, according to the method of forming the piezoelectric actuator of the inkjet head of the present general inventive concept, since the piezoelectric layer having a flat upper surface is formed to a uniform thickness, a shape, area, and thickness of the upper electrode formed thereon is uniformly controlled. Therefore, a distance between the upper electrode and the lower electrode is uniform, so that a uniform electric field is formed. Also, the upper electrode and the lower electrode are prevented from being shorted due to a fluidity of a paste. 
     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.