Patent Publication Number: US-2013235125-A1

Title: Inkjet head and methods for forming same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-055149, filed Mar. 12, 2012; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate to an inkjet head wherein an electrode is covered by a parylene film, and methods for forming same. 
     BACKGROUND 
     An inkjet head that ejects ink from plural nozzles has plural slots for feeding the ink, and it has electrodes formed in each slot from the bottom surface to the side surface. If the electrodes are exposed to the ink, the electrodes may be dissolved so that wire breakage takes place, due to the characteristics of the ink. Consequently, for a related inkjet head, the electrodes are covered by a parylene film to protect the electrodes from the ink. 
     For the parylene film, the probability of generation of pinholes is lower than films made from various types of oxides and nitrides. Consequently, even when various types of water soluble or electroconductive inks are utilized, it is still impossible to guarantee electrical insulation between the ink and the electrode. For example, a parylene film composed mostly of polyparaxylene may deteriorate in insulation characteristics when such film reacts with oxygen. Thus, when the parylene film is exposed to oxygen in the atmosphere while the parylene film is deposited on an electrode, it is difficult for the parylene film to maintain its original insulation characteristics. 
     As a result, the electrode is not sufficiently protected, and durability of the inkjet head is reduced. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an oblique view of the inkjet head according to an embodiment. 
         FIG. 2  is a plane view of the inkjet head according to the embodiment. 
         FIG. 3  is a cross-sectional view illustrating the inkjet head, taken across F 3 -F 3  in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the inkjet head, taken across F 4 -F 4  in  FIG. 3 . 
         FIG. 5  is an enlarged cross-sectional view illustrating the F 5  portion of  FIG. 4 . 
         FIG. 6  is a characteristics diagram illustrating the UV absorption spectrum of the parylene film, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to an embodiment of the present disclosure, an inkjet head has a pressure chamber configured to pressurize ink, a nozzle connected to the pressure chamber configured to eject the ink, and an electrode arranged in the pressure chamber. The electrode is configured to receive a driving voltage to deform the pressure chamber in order to apply pressure on the ink. A parylene film covers the electrode. The parylene film is configured to protect the electrode from the ink. A barrier film is deposited on the parylene film. The barrier film is configured to isolate the parylene film from oxygen. 
     According to an embodiment of the present disclosure, an inkjet head has a pressure chamber configured to pressurize ink, and an electrode arranged in the pressure chamber. The electrode is configured to receive a driving voltage applied to it to deform the pressure chamber in order to apply pressure on the ink. A parylene film covers the electrode in the pressure chamber. A barrier film is deposited on the parylene film. The barrier film contains no oxygen and is exposed in the pressure chamber. 
     According to another embodiment of the present disclosure, an inkjet head is formed. A slot is formed in a body. A nozzle plate having a nozzle is bonded to the body to cover the slot and to form a pressure chamber. An electrode is formed in the slot, the electrode being configured to receive a driving voltage to deform the pressure chamber in order to eject ink from the nozzle. A parylene film is formed on the electrode, the parylene film being configured to protect the electrode from the ink. A barrier film is formed on the parylene film, the barrier film configured to isolate the parylene film from oxygen. 
       FIGS. 1 to 3  show an on-demand type inkjet head  1  carried on a carriage of a printer, according to an embodiment. The inkjet head  1  has an ink vessel  2 , a substrate  3 , a spacer  4  and a nozzle plate  5 . 
     The ink vessel  2  is connected to an ink cartridge (not shown) via an ink feeding pipe  6  and the ink returning pipe  7 . 
     The substrate  3  has a rectangular shape. The substrate  3  has a slender assembling surface  3   a  and is superposed on the ink vessel  2  to cover the open end of the ink vessel  2 . The substrate  3  may be made of alumina (Al 2 O 3 ), silicon nitride (Si 3 N 4 ), silicon carbide (SiC), aluminum nitride (AlN), lead zirconate titanate (PZT: Pb(Zr, Ti)O 3 ), etc. 
     As shown in  FIGS. 2 and 3 , plural ink feeding ports  8  and plural ink exhausting ports  9  are arranged on the substrate  3 . The ink feeding ports  8  and the ink exhausting ports  9  have openings on the assembling surface  3   a  of the substrate  3 . 
     The ink feeding ports  8  are arranged so that the respective openings on a first side of the substrate  3  and on a second side of the substrate Sin the lateral direction of the substrate  3  are set as a row. The openings of the ink feeding ports on the first side of the substrate  3  have a uniform spacing between each other in the longitudinal direction of the substrate  3 . Likewise, the openings of the ink feeding ports on the second side of the substrate  3  have a uniform spacing between each other in the longitudinal direction of the substrate  3   
     The ink exhausting ports  9  are arranged so that the openings form a column with uniform spacing between each other in the longitudinal direction of the substrate  3 . 
     The spacer  4  has a quadrangle frame shape. The spacer  4  is bonded on the assembling surface  3   a  of the substrate  3 , and it surrounds the ink feeding ports  8  and ink exhausting ports  9 . 
     The nozzle plate  5  is bonded on the spacer  4  and facing the assembling surface  3   a  of the substrate  3 . The nozzle plate  5  may be made of a 50-μm-thick polyimide film. 
     As shown in  FIG. 3 , the substrate  3 , the spacer  4  and the nozzle plate  5  together form an ink flowing chamber  11 . The ink flowing chamber  11  is connected to the ink vessel  2  via the ink feeding ports  8  and the ink exhausting ports  9 . The ink feeding ports  8  feed the ink from the ink vessel  2  to the ink flowing chamber  11 . The excessive quantity of the ink fed to the ink flowing chamber  11  is recycled to the ink vessel  2  through the ink exhausting ports  9 . 
     As shown in  FIGS. 1 and 2 , a pair of nozzle rows  12   a,    12   b  are formed on the nozzle plate  5 . The nozzle rows  12   a,    12   b  are formed extending in the longitudinal direction of the nozzle plate  5  with uniform spacing between individual nozzles  13 , discussed further below. The nozzle rows  12   a,    12   b  are arranged parallel with each other in the lateral direction of the nozzle plate  5 . 
     The nozzle rows  12   a,    12   b  each have plural nozzles  13 . The nozzles  13  are minute holes through the nozzle plate  5  in the thickness direction. For example, a diameter of a nozzle  13  may be measured in microns. The nozzles  13  are arranged side-by-side with a uniform spacing between them. The nozzles include a first end opened to the ink flowing chamber  11 . The nozzles also include a second end arranged facing a recording media for printing. 
     As shown in  FIG. 4 , the nozzles  13  are each formed with a tapered shape having a diameter increasing gradually towards the ink flowing chamber  11 . For the nozzles  13  in the present embodiment, for example, when the ink ejecting rate is 3 pL, the diameter of the second end (opening towards the ink flowing chamber  11 ) is 40 μm, and the diameter of the first end (opening towards the recording media) is 20 μm. The dimensions of the nozzles  13  are of preset values corresponding to the ejecting rate of the ink. 
     As shown in  FIGS. 2 and 3 , a pair of actuators  15   a,    15   b  are accommodated in the ink flowing chamber  11 . One actuator  15   a  is bonded on the assembling surface  3   a  of the substrate  3  located right below one nozzle row  12   a.  The other actuator  15   b  is bonded on the assembling surface  3   a  of the substrate  3  located right below the other nozzle row  12   b.    
     The actuators  15   a,    15   b  are a common structure. Consequently, in the present embodiment, only one actuator  15   a  will be explained as an example. The same keys as those in the above are adopted for the other actuator  15   b,  and they will not be explained in detail again. 
     The actuator  15   a  has a slender main body  16  extending along the nozzle row  12   a.  As shown in  FIG. 4 , the main body  16  is composed of two piezoelectric members  17   a,    17   b.  The piezoelectric members  17   a,    17   b  are superposed and bonded with each other in the thickness direction. Further, the polarization directions of the piezoelectric members  17   a,    17   b  are opposite to each other in the thickness direction. 
     According to some embodiments, the piezoelectric members  17   a,    17   b  maybe made of lead zirconate titanate (PZT), lithium niobate (LiNbO 3 ), lithium titanate (LiTaO 3 ), etc. In one embodiment, PZT with a high piezoelectric constant is adopted. 
     The main body  16  has an outer surface  18 , a back surface  19  and side surfaces  20   a,    20   b.  The outer surface  18  faces the nozzle plate  5 . The back surface  19  is located on the side opposite to the outer surface  18 , and it faces the assembling surface  3   a  of the substrate  3 . 
     One end of side surface  20   a  extends along the lateral direction of the outer surface  18  and another end extends along the lateral direction of the back surface  19 . Likewise, with respect to the other side surface  20   b,  one end extends along the lateral direction of the outer surface  18  and another end extends along the lateral direction of the back surface  19 . In addition, the side surfaces  20   a,    20   b  are inclined so that they become farther from each other from the outer surface  18  towards the back surface  19 . 
     As shown in  FIGS. 2 and 4 , plural slots  22  are formed on the main body  16 . The slots  22  are formed side-by-side as a sequence with a uniform spacing between them in the longitudinal direction of the main body  16 . The slots  22  are connected to and opened on the outer surface  18  and side surfaces  20   a,    20   b  of the main body  16 . Along the main body, the portion located between the slots  22  of the main body  16  works as a wall  23  that separates the adjacent slots  22 . 
     As shown in  FIG. 4 , the slots  22  are each defined by a bottom surface  24  and a pair of side surfaces  25   a,    25   b.  The side surfaces  25   a,    25   b  are arranged facing each other with a uniform spacing in the lateral direction of the slots  22 . In addition, the slots  22  go through the piezoelectric member  17   a  on the upper side to reach halfway along the thickness direction of the piezoelectric member  17   b  on the lower side. Consequently, the bottom surface  24  of the slots  22  is made of the piezoelectric member  17   b  on the lower side. 
     The depth of the slots  22  is selected to be larger than the width of the slots  22 . The aspect ratio defined to be the ratio of depth to width (depth/width) of the slots  22  is greater than 1.0, i.e., the slots  22  are deeper than they are wide. The aspect ratio and the spacing of the slots  22  are selected at prescribed values corresponding to a resolution and an ink ejecting rate desired for the inkjet head  1 . 
     According to the present embodiment, the slots  22  are formed by cutting operation using, e.g., a diamond blade on the main body  16 . As shown in  FIG. 4 , on the bottom surfaces  24  and side surfaces  25   a,    25   b  of the slots  22  and the inner surfaces of slots  22 , plural protrusions/recessions  28  are formed. The plural protrusions/recessions  28  have a width measured in microns. In addition, in the process of cutting the main body  16 , the inner surfaces of the slots  22  are partially lost because the main body  16  made of PZT is brittle. As a result, the inner surfaces of the slots  22  after cutting become rough surfaces without smoothness. 
     As shown in  FIG. 4 , the nozzle plate  5  is bonded on the outer surface  18  of the main body  16  where the slots  22  are opened via an adhesive  30 . The spaces defined by the slots  22  and the nozzle plate  5  form plural pressure chambers  31 . The pressure chambers  31  are connected to the ink flowing chamber  11  so that the fed ink flows into the ink flowing chamber  11 . In addition, the nozzles  13  of the nozzle plate  5  are each open in the central portion of the respective individual pressure chambers  31 . 
     Electrodes  32  are arranged on the inner side of the pressure chambers  31 , respectively. The electrodes  32  cover the entire bottom surfaces  24  of slots  22  and the entire side surfaces  25   a,    25   b  of the slots  22 . The electrodes  32  of the adjacent slots  22  are cut from each other so that they are electrically independent from one another. 
     As shown in  FIG. 5 , the electrodes  32  have a double-layer structure including, e.g., a nickel plating layer  33  and a gold plating layer  34 . Here, the nickel plating layer  33  is formed by performing electroless nickel plating on the main body  16  and the slots  22 . The thickness of the nickel plating layer  33  is, for example, 0.8 μm. 
     The gold plating layer  34  is formed by performing electrolytic gold plating on the nickel plating layer  33 . The gold plating layer  34  is laminated on the nickel plating layer  33 , so that the nickel plating layer  33  is coated. The thickness of the gold plating layer  34  is, for example, 0.1 μm. 
     The electrodes  32  are electrically connected to the plural conductor patterns  35  formed on the assembling surface  3   a  of the substrate  3 . The tips of the conductor patterns  35  extend outside the spacer  4 , and, at the same time, they are connected to plural tape carrier patterns  36 . The tape carrier patterns  36  carry driver signals that drives the inkjet head  1 . 
     The driver signals apply driving pulses (driving voltage) to the electrodes  32  of the inkjet head  1 . As a result, a potential difference takes place between the adjacent electrodes  32  with the pressure chamber  31  sandwiched between them. Also, electric fields are generated on the side surfaces  25   a,    25   b  of the slots  22  corresponding to the electrodes  32 . As a result, the side surfaces  25   a,    25   b  are deformed in the shear mode, and the ink in the pressure chambers  31  is pressurized. A portion of the pressurized ink forms plural ink droplets that are ejected from the nozzles  13  towards the recording media. 
     As shown in  FIGS. 4 and 5 , the electrodes  32  of the slots  22  are each covered with a parylene film  38 . The parylene film is formed from an organic material mainly made of polyparaxylene. The parylene film  38  is laminated on the gold plating layer  34 , which is the outer layer of the electrodes  32 . The parylene film  38  protects the electrodes  32  from the ink fed into the pressure chambers  31 . 
     The bottom surface  24  and side surfaces  25   a,    25   b  of the slots  22  as the base of the electrodes  32  are rough surfaces with plural minute bumps/dips  28 . Consequently, the electrodes  32  are affected by the surface roughness of the bottom surfaces  24  and side surfaces  25   a,    25   b  of the slots  22 . In other words, when the bottom surfaces  24  and side surfaces  25   a,    25   b  of the slots  22  are rougher, the bumps/dips  28  are not absorbed when they are covered by the electrodes  32 . Consequently, the outer surfaces of the electrodes  32  covered by the parylene film  38  also become rough surfaces, and the probability of generation of pinholes on the parylene film  38  increases. In order to prevent generation of pinholes on the parylene film  38 , it is preferred that the film thickness of the parylene film  38  be 3 μm or thicker. 
     As shown in  FIG. 5 , a barrier film  39  is deposited on the parylene film  38 . According to an embodiment, the barrier film  39  is formed from an organic material, such as silicon nitride film. The silicon nitride film has the barrier function of suppressing penetration of moisture and impurities, and, at the same time, possesses excellent electrical insulating properties. 
     According to the present embodiment, the barrier film  39  is formed on the parylene film  38  using, e.g., the PE-CVD method (plasma-enhanced chemical vapor deposition). The silicon nitride film obtained using the PE-CVD method does not contain oxygen in its composition. In addition, the silicon nitride film is deposited on the parylene film  38  in a vacuum atmosphere. Thus, it is possible to exclude oxygen from between the silicon nitride film and the parylene film  38 . Consequently, the silicon nitride film deposited on the parylene film  38  in the vacuum atmosphere is a preferable barrier film  39  possessing oxygen barrier characteristics. 
     The barrier film  39  covers the entire surface of the parylene film  38  inside the pressure chambers  31  of the inkjet head  1 . Consequently, the barrier film  39  is exposed inside the pressure chambers  31 , and it isolates the parylene film  38  from oxygen. According to an embodiment, the film thickness of the barrier film  39  is preferably 1 μm or thicker. 
     The inkjet head  1  of the present embodiment has the parylene film  38  for protecting the electrodes  32  covered by the barrier film  39 . As an example, the barrier film  39  is made of silicon nitride film, wherein no oxygen is contained in the composition, and it is deposited on the parylene film  38  in vacuum. As a result, there is no oxygen between the barrier film  39  and the parylene film  38 . 
     Consequently, the parylene film  38  is isolated from oxygen, and degradation of the parylene film  38  by oxygen can be prevented. Therefore, it is possible to maintain good electrical insulation between the ink fed to the pressure chambers  31  and the electrodes  32  over a long period of time. Even when the ink is electroconductive, it is still possible to avoid corrosion of the electrodes  32  and electrolysis of the ink caused by the flowing of current in the ink. Consequently, it is possible to obtain an inkjet head  1  with excellent printing quality and durability. 
     In addition, according to the present embodiment, the outer surface of the parylene film  38  covering the electrodes  32  becomes a rough surface as influenced by the bumps/dips  28  on the bottom surfaces  24  and the side surfaces  25   a,    25   b  of the slots  22 . As a result, the barrier film  39  assumes the shape of the fine bumps/dips  28  present on the outer surface of the parylene film  38 . Consequently, the close contact property of the barrier film  39  on the parylene film  38  is increased. Therefore, it is possible to form the barrier film  39 , such as a silicon nitride film, having the desired oxygen barrier property reliably on the parylene film  38 . 
     In the following, the electrical insulating property of the parylene film  38  will be explained with reference to the mode wherein the electrical insulating property degrades by oxygen. 
     As shown in chemical formula 1, the parylene film  38  has a structure wherein plural benzene rings are bonded via plural methylene groups (CH 2 ). 
     
       
         
         
             
             
         
       
     
     When the parylene film  38  receives UV light, heat or other external energy in an oxygen atmosphere, as shown in chemical formula 2, the portion of the methylene group (CH 2 ) is oxidized to a ketone group (CO). That is, as the bonds between the atoms in oxygen are broken, the electrical insulating property of the parylene film  38  degrades. 
     
       
         
         
             
             
         
       
     
       FIG. 6  is a diagram illustrating the UV absorption spectrum of the parylene film  38  deposited on the electrodes. In  FIG. 6 , peak A indicates the main peak of ketone observed after irradiation by UV light on the parylene film  38  in an oxygen atmosphere. In the step before irradiation by UV light on the parylene film  38  in an oxygen atmosphere, absorption of the ketone groups is not observed. However, after irradiation by UV light on the parylene film  38 , absorption by ketone groups is observed as shown in  FIG. 6 . According to the present embodiment, the absorptivity of the ketone groups observed here is 1700 cm −1 . 
     Consequently, it can be seen that when external energy, such as UV light, is irradiated on the parylene film  38  in an oxygen atmosphere, the electrical insulating property of the parylene film  38  degrades. For the parylene film, the crystal structures of other types are different. However, the parylene film possessing methylene groups (CH 2 ) in its structure has degraded electrical insulating property due to this principle. 
     The parylene film  38  is covered by the barrier film  39 , which is made of, for example silicon nitride film containing no oxygen. Moreover, oxygen is excluded from between the parylene film  38  and the barrier film  39 , so that it is possible to prevent degradation of the parylene film  38  caused by oxygen. 
     The barrier film is not limited to the silicon nitride film. One may also adopt, e.g., titanium nitride film in place of the silicon nitride film. 
     Additionally, the method for depositing the barrier film on the parylene film  38  is not limited to the PE-CVD method. Any method may be adopted as long as it is possible to deposit an organic feed material containing no oxygen on the parylene film in a vacuum. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.