Patent Publication Number: US-2011057991-A1

Title: Inkjet head and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of Korean Patent Application No. 10-2009-0084208 filed on Sep. 7, 2009, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet head and a manufacturing method thereof, and more particularly, to an inkjet head and a manufacturing method thereof allowing for improved manufacturing yield due to the densification and facilitation of bonding between substrates by using anodic bonding between a silicon substrate and a ceramic substrate. 
     2. Description of the Related Art 
     An inkjet head converts electric signals into physical impulses so that ink droplets are ejected through small nozzles. In the inkjet head, several structures may be formed to perform various functions. A piezoelectric material (PZT) may be used for an actuator allowing the inkjet head to be driven. Also, materials such as stainless steel, ceramic and silicon may be used for the inkjet head structures. 
     With recent developments in semiconductor technology accompanied by developments in silicon wafer processing technology, it is now possible to manufacture an inkjet head without a separate adhesive layer, by processing each layer of the inkjet head to be a silicon wafer and bonding the layers together by silicon direct bonding. In the case of stainless steel or ceramic, a polymer adhesive layer may be needed for bonding each layer. In the case of silicon, however, such an adhesive layer is not required. Accordingly, such an inkjet head not requiring an adhesive layer may eject a variety of functional ink, as compared to the inkjet head having the adhesive layer. Also, the inkjet head formed of stainless steel or ceramic may require molds for manufacturing the structures and may not readily allow for changes in design, whereas the structures of the inkjet head formed of silicon may readily be modified by employing a photolithography method. Accordingly, the inkjet head, manufactured by fabricating the structures formed of silicon having a low chemical reaction rate and bonding them together by silicon direct bonding, may be appropriate in an industrial inkjet market. However, silicon direct bonding has disadvantages such as being a difficult process, having low yield, and being a time-consuming process. 
     A method of manufacturing an inkjet head using single-crystal silicon wafers according to the related art may include fabricating structures having respective functions from two or three wafers and bonding them together. 
     In order to manufacture an inkjet head using silicon wafers, several structures such as a chamber and a membrane may need to be formed, and then a bonding process may be required for integrating the structures. The bonding process may be performed by aligning each silicon wafer, preliminarily bonding the silicon wafers, and then applying thermal treatment at a high temperature of about 1000° C. 
     In silicon direct bonding technology, however, the preliminarily bonding process is performed using only intermolecular attraction, so even fine impurities on the surface of a wafer may lead to poor bonding quality. Accordingly, it is significantly difficult to bond several layers of silicon wafers through the use of a silicon direct bonding technology sensitive to external environmental conditions, and thus it is difficult to expect high yield in bonding. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides an inkjet head and a manufacturing method thereof allowing for improved manufacturing yield due to the densification and facilitation of bonding between substrates by using anodic bonding between a silicon substrate and a ceramic substrate. 
     According to an aspect of the present invention, there is provided an inkjet head, the inkjet head including: an upper substrate formed of a silicon material and having an ink chamber storing ink provided therein; an intermediate substrate bonded to the upper substrate, formed of a low temperature co-fired ceramic material, and having a connection path and a restrictor provided therein while the connection path and the restrictor are connected to the ink chamber; and a lower substrate bonded to the intermediate substrate, formed of a silicon material, and having a nozzle connected to the connection path provided therein. 
     The intermediate substrate may have a difference in thermal expansion coefficient by 2 ppm/C or less in comparison with the upper or lower substrate. 
     The restrictor may have a diameter of 100 μm or less. 
     The restrictor may have a smaller diameter than the connection path. 
     The connection path may include a plurality of filter holes. 
     According to another aspect of the present invention, there is provided a method of manufacturing an inkjet head, the method including: providing an upper substrate formed of a silicon material and having an ink chamber formed therein; providing an intermediate substrate formed of a low temperature co-fired ceramic material and having a connection path and a restrictor formed therein while the connection path and the restrictor are connected to the ink chamber; providing a lower substrate formed of a silicon material and having a nozzle connected to the connection path formed therein; and bonding the intermediate substrate to the upper substrate, the lower substrate, or the upper and lower substrates. 
     The intermediate substrate may have a difference in thermal expansion coefficient by 2 ppm/C or less in comparison with the upper or lower substrate. 
     The restrictor may have a diameter of 100 μm or less. 
     The restrictor may have a smaller diameter than the connection path. 
     The connection path may include a plurality of filter holes. 
     The bonding of the intermediate substrate to each of the upper and lower substrate may include an anodic bonding. 
     The anodic bonding may be performed by applied voltage in a range of 800 V to 1000 V at a temperature of 400° C. to 650° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view illustrating an inkjet head according to an exemplary embodiment of the present invention; and 
         FIGS. 2A through 2C  are schematic cross-sectional views illustrating a method of manufacturing an inkjet head according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the shapes and dimensions may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like components. 
       FIG. 1  is a schematic cross-sectional view illustrating an inkjet head according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , an inkjet head  1  includes an upper substrate  10  having an ink chamber  15  for storing ink formed therein; an intermediate substrate  20  bonded to the upper substrate  10 , formed of a ceramic material, and having a connection path  27  and a restrictor  23  formed therein while the connection path  27  and the restrictor  23  are connected to the ink chamber  15 ; and a lower substrate  30  bonded to the intermediate substrate  20  and having a nozzle  35  connected to the connection path  27  formed therein. 
     Here, the upper and lower substrates  10  and  30  may be formed by processing a silicon substrate having good workability. In the upper substrate  10 , the ink chamber  15  is formed to accommodate and pressurize ink. In the lower substrate  30 , the nozzle  35  is formed to eject the ink in the form of droplets. Since the upper and lower substrates  10  and  30  have structures required to ensure a certain degree of precision among the structures of the inkjet head  1 , in this embodiment they are manufactured by processing a silicon substrate having good workability. 
     In contrast to the upper and lower substrates  10  and  30  formed by processing the silicon substrate, the intermediate substrate  20  may be formed by processing a ceramic substrate, especially, a low temperature co-fired ceramic (LTCC) substrate. It is very important that the LTCC substrate has firing behaviors similar to those of the silicon substrates constituting the upper and lower substrates  10  and  30 , and thus to maintain the precision of dimensions of the structures in the inkjet head  1  even after firing. Accordingly, in the present embodiment, the intermediate substrate  20  is formed by using an LTCC substrate having a difference in thermal expansion coefficient by 2 ppm/C or less, as compared to that of the upper or lower substrate  10  or  30 . This is because using such an LTCC substrate for the intermediate substrate  20 , in which the LTCC substrate has little difference in thermal expansion coefficient in comparison with the upper or lower substrate  10  or  30 , may allow for stable bonding at a bonding interface between individual structures of the inkjet head without distortion or looseness even after firing. 
     The upper substrate  10  may further include an ink inlet  13 . The intermediate substrate  20  may further include an ink path  29  connected to the connection path  27  and a reservoir  25  connected to the restrictor  23 . The lower substrate  30  may further include a damper  33  formed between the ink path  29  and the nozzle  35 . Also, the upper substrate  10  may further include a piezoelectric actuator  40  allowing ink to be moved by pressurizing the ink chamber  15 . Here, the restrictor  23  may have a diameter d smaller than the diameter D of the connection path  27  to efficiently adjust the ink ejection amount. Also, the connection path  27  may further include a plurality of filter holes constituting an ink filter F. 
     In the case that a plurality of substrates are fabricated by processing silicon substrates and they are bonded together to thereby manufacture an inkjet head, bonding between the silicon substrates may not be facilitated. Also, in the case that a bond contains a defect, the defect may cause general bonding failure. 
     According to the present embodiment, the intermediate substrate  20 , disposed between the upper and lower substrates  10  and  30  formed by processing the silicon substrates, is formed by processing the LTCC substrate having firing behaviors similar to those of the silicon substrates, thereby forming a structure having an essential function of the inkjet head  1  and improving bonding strength between each substrates  10 ,  20  and  30  of the inkjet head  1 . That is, by taking advantage of the fact that bonding strength between a ceramic substrate and a silicon substrate is greater than bonding strength between silicon substrates, the upper and lower substrates  10  and  30  are manufactured by using the silicon substrates and the intermediate substrate  20  is manufactured by using the LTCC substrate. In this manner, a bonding structure of silicon substrate-LTCC substrate-silicon substrate is formed. 
     The LTCC substrate has good workability, and also it has superior hardness in comparison to the silicon substrate. Accordingly, the connection path  27  and the restrictor  23  requiring for precise processes are formed in the intermediate substrate  20  formed by processing the LTCC substrate. 
     Here, the restrictor  23  may commonly have a diameter d of 100 μm or less. Its diameter is designed to show an optimal ejection behavior in comparison with the diameter d′ of the nozzle  35 . 
     The restrictor  23  is a path transferring ink from the reservoir  25  to the ink chamber  15 . The ink introduced from the ink inlet  13  is stored in the reservoir  25  and the stored ink is transferred through the restrictor  23 . The ink is transferred to the ink chamber  15  according to the driving force of the piezoelectric actuator  40  allowing the ink to be moved by pressurizing the ink chamber  15 . Then, the ink is accommodated in the damper  33  through the connection path  27 . After that, the ink is ejected to a printing medium in the form of droplets through the nozzle  35 . Accordingly, the ink ejection amount may be adjusted according to the diameter d of the restrictor  23  formed at the boundary between the reservoir  25  and the ink chamber  15  and the diameter d′ of the nozzle  35 . 
     The connection path  27  serves to adjust the amount of ink transferred from the ink chamber  15  to the nozzle  35 , by being formed to be narrowed as compared to that of an existing inkjet head. The damper  33  allows the ink ejected by the piezoelectric actuator  40  from the ink chamber  15  to be transferred to the nozzle  35 . Here, the damper may be variably formed by changing its shapes, thereby adjusting the amount of ink received from the ink chamber  15  and the amount of ink transferred to the nozzle  35 . The damper  33  is optional, so the formation of the damper  33  may be omitted. 
     Hereinafter, a method of manufacturing an inkjet head according to an exemplary embodiment of the invention will be described with reference to  FIGS. 2A through 2C . 
       FIGS. 2A through 2C  are schematic cross-sectional views illustrating a method of manufacturing an inkjet head according to an exemplary embodiment of the present invention. 
     First of all, referring to  FIGS. 2A through 2C , each of upper and lower substrates  10   a  and  30   a  formed of a silicon material is disposed to have an intermediate substrate  20   a  formed of an LTCC material interposed therebetween. 
     Next, the upper substrate  10   a  formed of the silicon material is processed to manufacture an upper substrate  10   b  including the ink chamber  15  and the ink inlet  13 , and the lower substrate  30   a  formed of the silicon material is processed to manufacture a lower substrate  30   b  including the damper  33  and the nozzle  35 . The intermediate substrate  20   a  formed of the LTCC material is processed to manufacture an intermediate substrate  20   b  including the connection path  27  connected to the ink chamber  15 , the ink path  29 , the restrictor  23 , and the reservoir  25 . 
     It is very important that the LTCC substrate has firing behaviors similar to those of the silicon substrates constituting the upper and lower substrates  10  and  30 , and thus to maintain the precision of dimensions of the structures in the inkjet head  1  even after firing. Accordingly, in the present embodiment, the intermediate substrate  20  is formed by using an LTCC substrate having a difference in thermal expansion coefficient by 2 ppm/C or less, as compared to that of the upper or lower substrate  10  or  30 . This is because using such an LTCC substrate for the intermediate substrate  20 , in which the LTCC substrate has little difference in thermal expansion coefficient in comparison with the upper or lower substrate  10  or  30 , may allow for stable bonding at a bonding interface between individual structures of the inkjet head without distortion or looseness even after firing. 
     Here, the restrictor  23  may commonly have a diameter d of 100 μm or less. Its diameter d is designed to show an optimal ejection behavior, as compared to the diameter d′ of the nozzle  35 . 
     The restrictor  23  is a path which is designed to transfer ink from the reservoir  25  to the ink chamber  15 . The ink introduced from the ink inlet  13  is stored in the reservoir  25  and the stored ink is transferred through the restrictor  23 . The ink is transferred to the ink chamber  15  according to the driving force of the piezoelectric actuator  40  allowing the ink to be moved by pressurizing the ink chamber  15 . Then, the ink is accommodated in the damper  33  through the connection path  27 . After that, the ink is ejected to a printing medium in the form of droplets through the nozzle  35 . Accordingly, the ink ejection amount may be adjusted according to the diameter d of the restrictor  23  formed at the boundary between the reservoir  25  and the ink chamber  15  and the diameter d′ of the nozzle  35 . 
     Here, the connection path  27  is bonded after being formed to be narrowed as compared to that of an existing inkjet head, thereby adjusting the amount of ink transferred from the ink chamber  15  to the nozzle  35 . The connection path  27  may further include a plurality of filter holes constituting an ink filter F. 
     Here, the damper  33  allows the ink ejected by the piezoelectric actuator  40  from the ink chamber  15  to be transferred to the nozzle  35 . The damper may be variably formed by changing its shapes, thereby adjusting the amount of ink received from the ink chamber  15  and the amount of ink transferred to the nozzle  35 . The damper  33  is optional, so the formation of the damper  33  may be omitted. 
     Then, the process-finished upper and intermediate substrates  10   b  and  20   b  are bonded together and the process-finished intermediate and lower substrates  20   b  and  30   b  are bonded together, thereby forming the bonded upper, intermediate and lower substrates  10 ,  20  and  30 . 
     Here, the upper substrate  10   b  formed by processing the silicon substrate and the intermediate substrate  20   b  formed by processing the LTCC substrate are bonded by anodic bonding. Also, the intermediate substrate  20   b  formed by processing the LTCC substrate and the lower substrate  30   b  formed by processing the silicon substrate are bonded by anodic bonding. 
     Anodic bonding leads to ionic bonding between materials to thereby prevent ink leakage at a bonding interface and allow for physically and chemically stable bonding. The LTCC substrate and the silicon substrate are allowed to be bonded together due to the ion bonding therebetween without a separate adhesive layer, thereby preventing physical and chemical reactions of ink at the bonding interface and forming a strong head structure. By the use of the anodic bonding performed by applied voltage in the range of approximately 800 V to 1000 V at a temperature of approximately 400° C. to 650° C., the LTCC substrate and the silicon substrate may be bonded together after melting the interface therebetween. 
     This basic structure of the inkjet head fabricated in the above manner is combined with the piezoelectric actuator  40 , thereby completing the manufacturing of the inkjet head  1  as shown in  FIG. 1 . 
     The inkjet head and the manufacturing method thereof according to exemplary embodiments of the invention are allowed to secure precision and maintain hardness by bonding the silicon substrate and the ceramic substrate. 
     Also, the silicon substrate and the ceramic substrate are bonded by the anodic bonding, whereby the densification and facilitation of bonding between the substrates are achieved and the manufacturing yield of the inkjet head is enhanced. 
     As set forth above, according to exemplary embodiments of the invention, there is provided an inkjet head and a manufacturing method thereof capable of securing precision and maintaining hardness by bonding a silicon substrate and a ceramic substrate. 
     Also, according to exemplary embodiments of the invention, there is also provided an inkjet head and a manufacturing method thereof allowing for improved manufacturing yield due to the densification and facilitation of bonding between substrates by using anodic bonding between a silicon substrate and a ceramic substrate. 
     While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.