Patent Publication Number: US-7914128-B2

Title: Liquid ejecting head, piezoelectric element, and liquid ejecting apparatus

Description:
This application claims priority to Japanese Patent Application No. 2008-088176 filed on Mar. 28, 2008 and Japanese Patent Application No. 2009-002958, filed on Jan. 8, 2009, the entire disclosures of which are expressly incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a liquid ejecting head, a piezoelectric element, and a liquid ejecting apparatus. 
     2. Related Art 
     As an example of a liquid ejecting head, an ink ejecting recording head in which a piezoelectric element consisting of an upper electrode, a piezoelectric layer, and a lower electrode is used as a pressure generating unit is known (reference should be made to, for example, claim  2  and  FIG. 6  of JP-A-2000-246888). In such an ink ejecting recording head, an insulation layer for preventing moisture degradation of the piezoelectric layer is formed in a state of covering the piezoelectric layer and the upper electrode. 
     According to the above-mentioned configuration, although it is possible to obtain a sufficiently large displacement amount in response to driving of the piezoelectric element and excellent ink ejection characteristics, there is a pending need to further increase the displacement amount. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides a piezoelectric element capable of providing a large displacement amount and a liquid ejecting head using the piezoelectric element. Another advantage of some aspects of the invention is that it provides a liquid ejecting apparatus equipped with the liquid ejecting head, capable of exhibiting excellent printing quality. 
     According to an aspect of the invention, there is provided a liquid ejecting head including: a piezoelectric element in which a lower electrode, a piezoelectric layer, and an upper electrode are laminated in this order; and a flow path forming substrate in which the piezoelectric element is formed above one surface thereof and a pressure generating chamber being communicated with a nozzle opening is provided, in which the upper electrode is formed so as to extend over an upper surface of the piezoelectric layer and an upper portion of side faces of the piezoelectric layer, and a protective film is formed above portions of the piezoelectric layer, which are not covered by the upper electrode. 
     Since the upper electrode of the piezoelectric element is formed so as to extend over the upper surface of the piezoelectric layer and the upper portion of the side faces of the piezoelectric layer, it is possible to increase the area where electric field is produced during driving of the piezoelectric element. Moreover, since the protective film is not configured to cover an entire of the piezoelectric element but is configured to cover only the portions of the piezoelectric layer, which are not covered by the upper electrode, the possibility that the movement of the piezoelectric layer is inhibited by the protective film is low. Therefore, according to the configuration, it is possible to further increase the displacement amount in response to driving of the piezoelectric element. In this case, since the piezoelectric layer is covered by the upper electrode and the protective film, it is possible to prevent degradation thereof due to moisture. 
     In the above aspect of the liquid ejecting head of the invention, a width of the piezoelectric layer may increase toward the lower electrode, a thickness of a portion of the upper electrode formed above the side faces may gradually decrease toward the lower electrode, and the protective film may be provided so as to overlap with the gradually decreasing thickness portion of the upper electrode. If the side faces of the piezoelectric layer are sloped surfaces, electric field can be produced over a wide range of areas when the upper electrode is configured to cover the upper surface and the side faces of the piezoelectric layer. Therefore, it is possible to further increase the displacement amount in response to driving of the piezoelectric element. Moreover, the portion of the upper electrode formed above the side faces has a width thereof which gradually decreases toward the lower portions of the side faces, and the protective film is provided so as to overlap with the gradually decreasing thickness portion of the upper electrode. Therefore, the protective film is easily adhered onto the piezoelectric layer, and thus the degradation of the piezoelectric element can be prevented. 
     In the above aspect of the liquid ejecting head of the invention, the upper electrode may be configured to cover an upper half portion of each of the side faces of the piezoelectric layer. Owing to such a configuration, it is not only possible to prevent the upper electrode and the lower electrode from making contact with each other to be shorted but also to further increase the displacement amount. 
     According to another aspect of the invention, there is provided a piezoelectric element including: a lower electrode formed above a substrate; a piezoelectric layer formed above the lower electrode; and an upper electrode that covers an upper surface of the piezoelectric layer and an upper portion of side faces of the piezoelectric layer, in which a protective film is formed above portions of the piezoelectric layer, which are not covered by the upper electrode. In such a piezoelectric element, it is possible to further increase the displacement amount. 
     According to a further aspect of the invention, there is provided a liquid ejecting apparatus including the liquid ejecting head according to the above aspect of the invention. Since the liquid ejecting apparatus is equipped with the liquid ejecting head having good ink ejection characteristics, the printing characteristics of the liquid ejecting apparatus can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is an exploded perspective view of a recording head according to an embodiment of the invention. 
         FIGS. 2A and 2B  are a top plan view and a sectional view of the recording head according to the embodiment of the invention, respectively. 
         FIG. 3  is a sectional view of the recording head according to the embodiment of the invention. 
         FIG. 4  is a schematic view showing an example of a recording apparatus according to the embodiment of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments will be described herein below with reference to the accompanying drawings.  FIG. 1  is an exploded perspective view showing a simplified structure of an ink ejecting recording head which is an example of a liquid ejecting head according to the present embodiment.  FIG. 2A  is a top plan view of  FIG. 1 , and  FIG. 2B  is a sectional view taken along the line IIA-IIA′ in  FIG. 2A . For better understanding of the drawing, a later-described protective film  310  is not depicted in  FIG. 2A . 
     A flow path forming substrate  10  is formed of a single crystal silicon substrate which has a plane (110) of the plane orientation in the present embodiment. An elastic film  50  which is preliminarily formed of silicon dioxide by thermal oxidation is formed on one surface of the flow path forming substrate  10 . An insulation film  55  formed of zirconium oxide (ZrO 2 ) or the like is formed on the elastic film  50 . On the other surface of the flow path forming substrate  10 , pressure generating chambers  12  which are partitioned by a plurality of partition walls  11  are arranged in a width direction thereof (short-axis direction) by anisotropically etching from the other surface. A communicating portion  13  is formed in an outer region in the longitudinal direction of the pressure generating chambers  12  of each row, and the communicating portion  13  and each of the pressure generating chambers  12  are communicated with each other via an ink supply path  14  and a communicating path  15  which are provided for each of the pressure generating chambers  12 . That is, in the flow path forming substrate  10 , the pressure generating chamber  12 , the communicating portion  13 , the ink supply path  14 , and the communicating path  15  are formed as a liquid flow path. 
     The communicating portion  13  is communicated with a reservoir portion  31  of a later-described protective substrate  30 , thereby constituting a portion of a reservoir  100  which serves as a common ink chamber for the rows of the pressure generating chambers  12 . The ink supply path  14  is formed with a width narrower than that of the pressure generating chambers  12 , and is configured to keep constant flow path resistance of ink flowing from the communicating portion  13  into the pressure generating chambers  12 . In the present embodiment, although the ink supply path  14  is formed by narrowing the width of one of the pressure generating chamber  12  and the communicating path  15 , the invention is not particularly limited to this. For example, the ink supply path  14  may be formed by narrowing the width of both the pressure generating chamber  12  and the communicating path  15 , and the ink supply path  14  may be formed by narrowing the size in a thickness direction thereof. 
     Onto the opening surface (the other surface) of the flow path forming substrate  10  where an opening is formed, a nozzle plate  20  having nozzle openings  21  bored therein which are communicated with a zone near the end portions of the pressure generating chambers  12  on the side opposite to the liquid supply paths  14  is fixedly secured by an adhesive or a heat welding film. 
     On the insulation film  55 , a lower electrode  60 , a piezoelectric layer  70  formed of lead zirconate titanate (PZT), which is an example of a piezoelectric film, and an upper electrode  80  are formed in a laminated state, thereby constituting a piezoelectric element  300 . The piezoelectric element  300  refers to a portion including the lower electrode  60 , the piezoelectric layer  70 , and the upper electrode  80 . The piezoelectric element  300  functions as a pressure generating element that causes a pressure change to the ink (liquid) in the pressure generating chamber  12 . 
     In the present embodiment, the lower electrode  60  of the piezoelectric element  300  is used as a common electrode, the upper electrode  80  is used as an individual electrode of the piezoelectric element  300 , so that the piezoelectric layer  70  is driven by electric voltage applied between the upper electrode  80  and the lower electrode  60 . The piezoelectric layer  70  is formed of a piezoelectric material which is formed on the lower electrode  60  and exhibits electromechanical conversion action, and among the piezoelectric materials, a ferroelectric material having the Perovskite structure. 
     A structure of the piezoelectric element  300  will be described in detail with reference to  FIG. 3 .  FIG. 3  is a main part sectional view for explaining the structure of the piezoelectric element  300  and is a sectional view taken along the line IIB-IIB′ in  FIG. 2B . The piezoelectric layer  70  on the lower electrode  60  which is the common electrode is patterned by anisotropic etching so that a width thereof increases toward the lower electrode  60 . That is, the piezoelectric layer  70  has sloped side faces  71 . An upper side of both side faces  71  and an upper surface  72  of the piezoelectric layer  70  are covered by the upper electrode  80 . In this manner, since the upper electrode  80  covers not only the upper surface  72  of the piezoelectric layer  70  but also the upper portion of the side faces  71 , when electric voltage is applied between the upper electrode  80  and the lower electrode  60 , it is possible to produce electric field over a wider range of areas of the piezoelectric layer  70  than that in the case where the upper electrode  80  is formed in only the upper surface  72 . Therefore, it is possible to further increase the displacement amount of the piezoelectric element  300 . 
     In order to protect the portion of the side faces  71  of the piezoelectric layer  70  which is not covered by the upper electrode  80 , that is, the lower portion of the side faces  71 , from moisture, a protective film  310  functioning as a moisture-resistant protective film is provided. The protective film  310  is formed, for example, of an oxide film such as aluminum oxide or an organic film such as polyimide. In this manner, in the present embodiment, since the side faces  71  of the piezoelectric layer  70  are covered by the upper electrode  80  and the protective film  310 , it is possible to prevent degradation of the piezoelectric layer  70  due to moisture. In this case, it is preferable that the protective film  310  is provided to extend a wide range of areas of the upper surface of the insulation film  55  as shown in  FIG. 1  so that moisture cannot make contact with the piezoelectric layer  70 . Moreover, since the protective film  310  is not configured to cover an entire of the side faces  71  among the surface of the piezoelectric layer  70  but is configured to cover only the lower portion of the side faces  71 , which are not covered by the upper electrode  80 , the movement thereof during the driving of the piezoelectric element  300  is not inhibited, and thus, it is possible to further increase the displacement amount of the piezoelectric layer  70 . 
     Moreover, it is preferable that the portion of the upper electrode  80  covering the side faces  71  has a thickness thereof which gradually decreases toward the lower portion of the side faces  71 , and the gradually decreasing thickness portion overlaps with the protective film  310 . Owing to such a configuration, since the protective film  310  is easily adhered onto the piezoelectric element  300 , it is possible to prevent the piezoelectric element  300  from making contact with the atmosphere to be degraded due to moisture, which results from the poor adhesion of the protective film  310 . The upper electrode  80  is preferably configured to cover the upper half portion of the side faces  71 . Although it is preferable that the upper electrode  80  covers a wide range of areas of the side faces  71  as much as possible in order to increase the displacement amount, there is a fear that the upper electrode  80  is shorted to the lower electrode  60  when they are brought into contact with each other. Therefore, when the upper electrode  80  is configured to cover the upper half portion of the side faces  71 , it is possible to obtain a sufficient displacement amount with no fear of short-circuits. 
     Such a piezoelectric element  300  is constructed in such a manner that the lower electrode  60  and the piezoelectric layer  70  are formed in this order by film forming and patterning processes and thereafter, the upper electrode  80  is formed thereon by film forming and patterning processes. Thereafter, a later-described lead electrode  90  is formed, and the protective film  301  is provided at a predetermined position, whereby the piezoelectric element according to the present embodiment is provided. 
     The upper electrodes  80  which are the individual electrodes of the piezoelectric element  300  are connected to the lead electrodes  90  which are formed, for example, of gold (Au) and are led out from the vicinity of the end portions close to the ink supply path  14  to be extended to be positioned on the insulation film  55 . In the above-described example, although the elastic film  50  and the insulation film  55  function as the vibration plate, either one of the elastic film  50  or the insulation film  55  may be provided as the vibration plate. 
     On the flow path forming substrate  10  where the piezoelectric elements  300  are formed, that is, on the lower electrode  60 , the insulation film  55 , and the lead electrode  90 , a protective substrate  30  having a reservoir portion  31  constituting at least a portion of the reservoir  100  is bonded via an adhesive  35 . In the present embodiment, the reservoir portion  31  is provided along the width direction of the pressure generating chambers  12  so as to penetrate through the protective substrate  30  in a thickness direction thereof. The reservoir portion  31  is communicated with the communicating portion  13  of the flow path forming substrate  10 , thereby constituting the reservoir  100  which serves as a common ink chamber for the respective pressure generating chambers  12 . Moreover, the communicating portion  13  of the flow path forming substrate  10  may be divided into a plurality of parts which correspond to the pressure generating chambers  12 , so that the reservoir is constituted by only the reservoir portion  31 . Furthermore, only the pressure generating chambers  12  may be provided in the flow path forming substrate  10 , and the ink supply path  14  may be provided to the member (for example, the elastic film  50 , the insulation film  55 , and the like) disposed between the flow path forming substrate  10  and the protective substrate  30  so as to be communicated with the reservoir and the respective pressure generating chambers  12 . 
     The protective substrate  30  has a piezoelectric element holding portion  32  which is defined in a region of the protective substrate  30  opposed to the piezoelectric element  300  and has such a space that the movement of the piezoelectric element  300  is not inhibited. As long as the space of the piezoelectric element holding portion  32  does not inhibit the movement of the piezoelectric element  300 , the space may be, or may not be, hermetically sealed. 
     In the protective substrate  30 , a through-hole  33  is bored so as to penetrate through the protective substrate  30  in the thickness direction thereof. The lead electrodes  90  which are led out from the respective piezoelectric elements  300  have the distal ends thereof being exposed to the inside of the through-hole  33 . 
     On the protective substrate  30 , a driving circuit  120  for driving the piezoelectric elements  300  is fixedly secured. As the driving circuit  120 , a circuit board or a semiconductor integrated circuit (IC), for example, can be used. The driving circuit  120  and the lead electrode  90  are electrically connected to each other via a connection wiring  121  which is inserted through the through-hole  33  and is configured by a conductive wire such as a bonding wire. 
     Preferably, the protective substrate  30  is formed of a material having approximately the same thermal expansion coefficient as that of the flow path forming substrate  10 , such as, glass or a ceramic material. In the present embodiment, the protective substrate  30  is formed using a single crystal silicon substrate which has a plane (110) of the plane orientation and is formed of the same material as that of the flow path forming substrate  10 . 
     Furthermore, a compliance plate  40 , which consists of a sealing film  41  and a fixing plate  42 , is bonded onto the protective substrate  30 . The sealing film  41  is formed of a material having a low rigidity and flexibility (for example, a polyphenylene sulfide (PPS) film), and the sealing film  41  seals one surface of the reservoir portion  31 . The fixing plate  42  is formed of a hard material such as a metal (for example, stainless steel (SUS)). A region of the fixing plate  42  opposed to the reservoir  100  defines an opening portion  43  which is completely deprived of the plate in the thickness direction. Thus, one surface of the reservoir  100  is sealed only with the sealing film  41  having flexibility. 
     In the ink ejecting recording head of the present embodiment, ink is taken in from a non-illustrated external ink supply unit, and the interior of the head ranging from the reservoir  100  to the nozzle openings  21  is filled with the ink. Then, according to recording signals from the drive IC  210 , voltage is applied between the lower electrode film  60  and the upper electrode film  80  corresponding to each of the pressure generating chambers  12  to warp and deform the elastic film  50 , the insulation film  55 , the lower electrode  60 , and the piezoelectric layer  70 . As a result, the pressure in each of the pressure generating chambers  12  rises, and thus ink is ejected from the nozzle openings  21 . 
     In this case, in the ink ejecting recording head according to the present embodiment, the upper electrode  80  is formed so as to cover the upper surface  72  of the piezoelectric layer  70  and the upper portion of the side faces  71  of the piezoelectric layer  70 , and the protective film  310  is provided so as to cover only the lower portion of the side faces  71  of the piezoelectric layer  70 . When electric voltage is applied between the lower electrode  60  and the upper electrode  80 , since it is possible to further increase the displacement amount of the piezoelectric element  300 , a desired amount of ink droplets can be ejected. 
     The above-described ink ejecting recording head constitutes a portion of the recording head unit provided with an ink flow path being communicated with an ink cartridge or the like and is mounted on the ink ejecting recording apparatus.  FIG. 4  is a schematic view showing an example of the ink ejecting recording apparatus. As illustrated in the drawing, recording head units  1 A and  1 B, which have ink ejecting recording heads, respectively, are provided so as to be respectively detachably attached to cartridges  2 A and  2 B which form an ink supply unit, and a carriage  3  mounting thereon the recording head units  1 A and  1 B is axially movably provided to a carriage shaft  5  which is attached to an apparatus body  4 . The recording head units  1 A and  1 B are configured to eject, for example, black ink composition and color ink composition, respectively. 
     When a driving force of a driving motor  6  is transferred to the carriage  3  via a plurality of non-illustrated gears and a timing belt  7 , the carriage  3  mounting thereon the recording head units  1 A and  1 B is moved along the carriage shaft  5 . On the other hand, a platen  8  is provided to the apparatus body  4  along the carriage shaft  5  so that a recording sheet S which is a recording medium such as paper fed by a non-illustrated feed roller or the like is transported on the platen  8 . 
     In the ink ejecting recording apparatus according to the present embodiment, since the above-described ink ejecting recording head is used in the recording head units  1 A and  1 B, the ink ejection characteristics thereof are excellent, and thus excellent printing characteristics can be provided. 
     While an exemplary embodiment of the invention has been described, the invention is not limited to the above-described embodiment. For example, although a single crystal silicon substrate has bee illustrated as the flow path forming substrate  10 , the invention is not particularly limited to this. For example, a SOI substrate, a glass substrate, a MgO substrate, and the like can be effectively used in the invention. 
     Furthermore, in the above-described embodiments, the ink ejecting recording head is taken for illustration as an example of the liquid ejecting head. However, the invention is aimed to broadly cover the overall liquid ejecting head and, needless to say, can be applied to liquid ejecting heads for ejecting liquid other than ink. Examples of other liquid ejecting heads include a variety of types of recording heads for use in an image recording apparatus such as a printer, a coloring-material ejecting head for use in manufacture of a color filter of a liquid crystal display or the like, an electrode-material ejecting head for use in forming an electrode of an organic EL display, an FED (field emission display) or the like, a bioorganic-material ejecting head for use in manufacture of a biochip, and the like. 
     Moreover, in the above-described ink ejecting recording apparatus, although the head units  1 A and  1 B are illustrated as being mounted on the carriage  3  to be moved in the main scanning direction, the invention is not particularly limited to this. For example, the invention may be applied to a so-called line type recording apparatus in which the ink ejecting recording head (or the head unit) may be fixedly secured, and only the recording sheet S such as paper is moved in the sub-scanning direction, whereby printing is performed thereon. Furthermore, although the ink ejecting recording apparatus has been described as an example of the liquid ejecting apparatus, the invention can be similarly applied to a liquid ejecting apparatus using the above-mentioned other liquid ejecting heads.