Patent Publication Number: US-2004056930-A1

Title: Piezoelectric ink jet print head and fabrication method for a vibrating layer thereof

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates to a vibrating layer of a piezoelectric ink jet print head, and more particularly to a fabrication method with SOI (silicon-on-insulator), grinding and etching for a vibrating layer and an ink chamber of a piezoelectric ink jet print head.  
       [0003] 2. Description of the Related Art  
       [0004] An ink jet print head is classified as a thermal bubble type or a piezoelectric type according to its primary working principle. The thermal bubble type employs a heater to vaporize ink droplets, and uses high-pressure bubbles to drive the ink droplets through the nozzle orifices, but has the problem of choosing suitable fluids for high-temperature gasification, causing limitations in application fields. The piezoelectric type employs a forced voltage to deform a piezoelectric ceramic body, and uses flexure displacement of the piezoelectric ceramic body to change the volume of a pressure-generating chamber, thus the chamber expels an ink droplet. The piezoelectric type has advantages as follows in comparison with the thermal bubble type. First, the piezoelectric ink jet print head has superior durability because the high-temperature gasification is omitted to avoid chemical variations. Second, the piezoelectric ink jet print head has a high-speed print performance because the piezoelectric ceramic body has quick response without the restriction of thermal conductivity. Third, the piezoelectric ink jet print head offers superior print quality because droplet volume is easily controlled.  
       [0005] The piezoelectric ink jet print head has been commercialized into a bend mode and a push mode according to the deformation mechanism of the piezoelectric body. Generally, the bend mode uses a face-shooter piezoelectric deformation, and the push mode uses an edge-shooter piezoelectric deformation.  
       [0006]FIG. 1 is a cross-section illustrating a conventional bend mode of the piezoelectric ink jet print head. The piezoelectric ink jet print head  10  comprises an actuator unit  12  and an ink path unit  14 . The actuator unit  12  is a stack structure consisting of a multi-layered piezoelectric ceramic body  16 , a vibrating plate  18 , and a substrate  20 , in which the substrate  20  has a plurality of pressure chambers  19  spaced apart from each other. The ink path unit  14  is a stack structure consisting of a first substrate  22 , a second substrate  24  and a nozzle plate  26 . The first substrate  22  has an ink slot  21  and an inlet/outlet hole  23 , the second substrate  24  has an outlet path  25 , and the nozzle plate  26  has a plurality of nozzle orifices  27 . When a voltage is exerted by control circuits, the piezoelectric ceramic body  16  is deformed and impeded by the vibrating plate  18  to bend laterally, thus extruding the ink in the ink chamber  19 . As a voltage difference arises between the internal space and the external circumference, the ink adjacent to the nozzle orifice  27  is accelerated and expelled as an ink droplet  28 .  
       [0007]FIG. 2 is a cross-section illustrating a conventional push mode of the piezoelectric ink jet print head. The piezoelectric ink jet print head  30  comprises a single-layer piezoelectric ceramic plate  32 , a transducer foot  34 , a vibrating plate  36 , a substrate  42  and a nozzle plate  44 . The substrate  42  comprises an ink chamber  37 , an ink slot  38 , an inlet path  39  and an outlet path  40 . The nozzle plate  44  comprises a plurality of nozzle orifices  43 . Also, an electrode layer is formed on the sidewall of the ink chamber  37  by an electroless nickel plating method, and two electrodes are connected between three ink chambers  37 . When the opposite potential of the applied voltage between the two electrodes is continuously increased, the ceramic sidewall of the ink chamber  37  bends outward to introduce ink. When the applied voltage is rapidly changed, the piezoelectric ceramic plate  32  is deformed to cause a greater bending motion, thus the ink in the ink chamber  37  is extruded by a right-hand thrust and expelled from the nozzle orifice  43  to form an ink droplet  46 .  
       [0008] Conventionally, the vibrating plate and the ink chamber are formed by a laminated ceramic co-fired method which includes steps of synthesizing raw materials (such as PZT, ZrO 2 , PbO, TiO 2  and other additives), mixing, drying, calcining, smashing, granulation, squeezing, shaping, sintering, and polarization. The complicated and difficult procedure of the laminated ceramic co-fired method, however, has disadvantages of low yield and high cost and is unfavorable to mass production. Accordingly, a modified method for forming the vibrating plate of the piezoelectric ink jet print head and increasing process reliability thereof is called for.  
       SUMMARY OF THE INVENTION  
       [0009] Accordingly, an object of the present invention is to provide a fabrication method with SOI (silicon-on-insulator), grinding and etching for a vibrating layer and an ink chamber of a piezoelectric ink jet print head to solve the problems caused by the conventional method.  
       [0010] According to the object of the invention, a piezoelectric ink jet print head comprises a first silicon wafer serving as a vibrating layer; a second silicon wafer having a plurality of ink chambers spaced apart from each other, in which the top of the second silicon wafer adheres to the bottom of the first silicon wafer; an adhesion layer formed between the first silicon wafer and the second silicon wafer for bonding the second silicon wafer and the first silicon wafer, and serving as an etching stop layer for the ink chambers; a piezoelectric material layer formed on the top of the first silicon wafer; and a hard mask layer formed on the bottom of the second silicon wafer for defining the pattern of the ink chambers.  
       [0011] According to the object of the invention, a fabrication method for a vibrating layer of a piezoelectric ink jet print head, comprising steps of: providing a first silicon wafer and a second silicon wafer; providing an adhesion layer on the bottom of the first silicon wafer and on the top of the second silicon wafer; bonding the bottom of the first silicon wafer to the top of the second silicon wafer, in which the adhesion layer is formed between the first silicon wafer and the second silicon wafer; grinding the top of the first silicon wafer until the thickness of the first silicon wafer reaches 5˜20 μm, which serves as a vibrating layer; forming a hard mask layer on the bottom of the second silicon wafer, in which the hard mask layer comprises a plurality of openings for defining a pattern of ink chambers; and etching the second silicon wafer and using the adhesion layer as an etching stop layer, in which the second silicon wafer exposed within the opening are removed to form a plurality of ink chambers spaced apart from each other. 
     
    
    
     DESCRIPTION OF THE DRAWINGS  
     [0012] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention.  
     [0013]FIG. 1 is a cross-section illustrating a conventional bend mode of the piezoelectric ink jet print head.  
     [0014]FIG. 2 is a cross-section illustrating a conventional push mode of the piezoelectric ink jet print head.  
     [0015]FIGS. 3A to  3 F are cross-sections illustrating a method of forming a vibrating layer of a piezoelectric ink jet print head according to the first embodiment of the present invention.  
     [0016]FIGS. 4A to  4 F are cross-sections illustrating a method of forming a vibrating layer of a piezoelectric ink jet print head according to the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0017] The present invention provides a vibrating layer of a piezoelectric ink jet print head and a fabrication method thereof. A SOI (silicon-on-insulator) technique and a grinding method are employed to form the main body of the piezoelectric ink jet print head, and then an etching process is employed to simultaneously complete a vibrating layer and an ink chamber. Preferably, the vibrating layer is composed of a silicon layer and a silicon oxide layer on a silicon wafer. The vibrating layer and the ink chamber are applied to a bend-mode piezoelectric ink jet print head or a push-mode piezoelectric ink jet print head.  
     First Embodiment  
     [0018]FIGS. 3A to  3 F are cross-sections illustrating a method of forming a vibrating layer of a piezoelectric ink jet print head according to the first embodiment of the present invention.  
     [0019] In FIG. 3A, a first silicon wafer  52  and a second silicon wafer  54  are provided. Then, using an oxidation process, a first silicon oxide layer  51   a  is formed on a predetermined adhesion surface of the first silicon wafer  52 , and a second silicon oxide layer  51   b  is formed on a predetermined adhesion surface of the second silicon wafer  54 .  
     [0020] In FIG. 3B, using a SOI (silicon-on-insulator) technique, the bottom (the predetermined adhesion surface) of the first silicon wafer  52  is compactly adhered to the top (the predetermined adhesion surface) of the second silicon wafer  54 . Preferably, using spin-on coating or spraying, a solution containing a hydrogen bond (such as acetone or alcohol) is formed on the predetermined adhesion surfaces of the first silicon wafer  52  and the second silicon wafer  54 . Thus, the first silicon oxide layer  51   a  temporarily adheres to the second silicon oxide layer  51   b  to become a silicon oxide adhesion layer  53 , which also serves as a stop layer  53  for a subsequent ink chamber process. Next, using a wafer alignment method and a wafer press method, the bottom of the first silicon wafer  52  is pressed downward to the top of the second silicon wafer  54 .  
     [0021] In FIG. 3C, using a grinding process, such as a chemical mechanical polishing (CMP) method, the top of the first silicon wafer  52  is polished until its thickness reaches 5˜20 μm. Thus, the remaining portion of the first silicon wafer  52  serves as a vibrating layer  52 A. Preferably, the silicon oxide layer adjacent to the bottom of the remaining portion of the first silicon wafer  52  also serves as a part of the vibrating layer  52 A. At the same time, using a grinding process, such as a chemical mechanical polishing (CMP) method, the bottom of the second silicon wafer  54  is polished until its thickness reaches a predetermined depth for the subsequent ink chamber process.  
     [0022] In FIG. 3D, a piezoelectric material layer  56  is formed on the vibrating layer  52 A, and then a sintering process is employed to complete the piezoelectric material layer  56  as a piezoelectric element. Alternatively, this sintering process can be performed after completing the subsequent ink chamber process. Next, a hard mask layer  58  is formed on the bottom of the second silicon wafer  54 . Preferably, the hard mask layer  58  with a thickness of 5˜20 μm is a SiO 2  layer or a Si 3 N 4  layer.  
     [0023] In FIG. 3E, using photolithography and etching, a plurality of openings  59  is formed in the hard mask layer  58  for defining a predetermined pattern of the ink chamber.  
     [0024] In FIG. 3F, using a dry etching process, a wet etching process or other enforceable methods from the bottom of the second silicon wafer  54 , in which the hard mask layer  58  serves as a mask and the stop layer  53  serves as an etching stop layer for controlling the etching depth, the second silicon wafer  54  exposed within the openings  59  is removed to form a plurality of ink chambers  60  spaced apart from each other. Thereafter, processes for an ink slot, nozzle orifices and a nozzle plate will be performed under the ink chamber  60 , which are omitted herein.  
     [0025] Compared with the conventional method, the present invention uses SOI, grinding, oxidation, photolithography and etching to form the remaining portion of the first silicon wafer  52  as the vibrating layer  52 A, and uses etching from the bottom of the second silicon wafer  58  to form the ink chamber  60 . Thus, the fabrication method of the vibrating layer  52 A and the ink chamber  60  in the present invention can simplify procedures, reduce process difficulties, and increase process reliability, resulting in high yield, low cost and greater production.  
     Second Embodiment  
     [0026] The fabrication method for a vibrating layer of a piezoelectric ink jet print head in the second embodiment is substantially similar to that of the first embodiment, with the similar portions omitted herein. The different portion is the wafer bonding method, in which an adhesion agent is used to replace the SOI technique so as to further simplify process steps and reduce process costs.  
     [0027]FIGS. 4A to  4 F are cross-sections illustrating a method of forming a vibrating layer of a piezoelectric ink jet print head according to the second embodiment of the present invention.  
     [0028] In FIG. 4A, a first silicon wafer  52  and a second silicon wafer  54  are provided. Then, a first adhesion agent layer  51   c  is formed on a predetermined adhesion surface of the first silicon wafer  52 , and a second adhesion agent layer  51   d  is formed on a predetermined adhesion surface of the second silicon wafer  54 . Preferably, the first adhesion agent layer  51   c  and the second adhesion agent layer  51   d  are made of resin, PSG (phosphosilicate glass), SOG (spin on glass) or a dry film.  
     [0029] In FIG. 4B, the first adhesion agent layer  51   c  is compactly adhered to the second adhesion agent layer  51   d  to become an adhesion layer  55 , thus the bottom (the predetermined adhesion surface) of the first silicon wafer  52  is temporarily adhered to the top (the predetermined adhesion surface) of the second silicon wafer  54 . The adhesion layer  55  serves as a stop layer  55  for a subsequent ink chamber process. Next, using a wafer alignment method and a wafer press method, the bottom of the first silicon wafer  52  is pressed downward to the top of the second silicon wafer  54 .  
     [0030] In FIG. 4C, using a grinding process, such as a chemical mechanical polishing (CMP) method, the top of the first silicon wafer  52  is polished until its thickness reaches 5˜20 μm. Thus, the remaining portion of the first silicon wafer  52  serves as a vibrating layer  52 A. Preferably, the silicon oxide layer adjacent to the bottom of the remaining portion of the first silicon wafer  52  also serves as a part of the vibrating layer  52 A. At the same time, using another grinding process, such as a chemical mechanical polishing (CMP) method, the bottom of the second silicon wafer  54  is polished until its thickness reaches a predetermined depth for the subsequent ink chamber process.  
     [0031] In FIG. 4D, a piezoelectric material layer  56  is formed on the vibrating layer  52 A, and then a sintering process is employed to complete the piezoelectric material layer  56  as a piezoelectric element. Alternatively, this sintering process can be performed after completing the subsequent ink chamber process. Next, a hard mask layer  58  is formed on the bottom of the second silicon wafer  54 . Preferably, the hard mask layer  58  with a thickness of 5˜20 μm is a SiO 2  layer or a Si 3 N 4  layer.  
     [0032] In FIG. 4E, using photolithography and etching, a plurality of openings  59  is formed in the hard mask layer  58  for defining a predetermined pattern of the ink chamber.  
     [0033] In FIG. 4F, using a dry etching process, a wet etching process or other enforceable methods from the bottom of the second silicon wafer  54 , in which the hard mask layer  58  serves as a mask and the stop layer  55  serves an etching stop layer for controlling the etching depth, the second silicon wafer  54  exposed within the openings  59  is removed to form a plurality of ink chambers  60  spaced apart from each other. Thereafter, processes for an ink slot, nozzle orifices and a nozzle plate will be performed under the ink chamber  60 , which are omitted herein.  
     [0034] Compared with the conventional method, the present invention uses adhesion agent, grinding, oxidation, photolithography and etching to form the remaining portion of the first silicon wafer  52  as the vibrating layer  52 A, and uses etching from the bottom of the second silicon wafer  58  to form the ink chambers  60 . Thus, the fabrication method for the vibrating layer  52 A and the ink chamber  60  in the present invention can simplify procedures, reduce process difficulties, and increase process reliability, resulting in high yield, low cost and greater production.  
     [0035] While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.