Patent Publication Number: US-6663226-B2

Title: Ink-jet print head and method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of application Ser. No. 10/121,723, filed Apr. 15, 2002, now pending. 
    
    
     This application claims the benefit of Korean Patent Application No. 2001-80902, filed Dec. 18, 2001, in the Korean Industrial 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 ink-jet print head, and more particularly, to an inkjet print head having a nozzle plate, a heat element formed on the nozzle plate, and a thermal shunt formed in the nozzle plate such that thermal accumulation on the nozzle plate can be effectively prevented 
     2. Description of the Related Art 
     Ink ejection mechanisms of ink-jet print heads include an electro-thermal transducer having a heat source generating bubbles to eject ink by using a bubble-jet method, and an electromechanical transducer having a piezoelectric device varying a volume of the ink caused by deformation of the piezoelectric device to eject the ink. 
     The bubble-jet method of the electro-thermal transducer is classified into a top-shooting method, a side-shooting method, and a back-shooting method according to a relationship between a growing direction of the bubbles and an ejecting direction of an ink droplet of the ink. In the top-shooting method, the growing direction of the bubbles is the same as the ejecting direction of the ink droplet, in the side-shooting method, the growing direction of the bubbles is perpendicular to the ejecting direction of the ink droplet, and in the back-shooting method, the growing direction of the bubbles is opposite to the ejecting direction of the ink droplet. 
     A basic principle of the back-shooting method and a structure of an ink-jet print head using the same are disclosed in U.S. Pat. No. 5,760,804 to Heinzl et al. issued Jun. 2, 1998. In addition, various structures used for the back-shooting method are disclosed in U.S. Pat. No. 4,847,630 to Bhaskar et al. issued Jul. 11, 1989 and U.S. Pat. No. 6,019,457 to Silberbrook issued Feb. 1, 2000. 
     FIG. 1 is a cross-sectional view of a conventional ink-jet print head. 
     A chamber  1   a  having a hemispheric shape is formed in a substrate  1 , which is formed of silicon, etc., and an ink inlet  1   b  connected to an ink supply source (not shown) is formed in a lower portion of the chamber  1   a . A nozzle plate  2  is formed on the substrate  1  and above the chamber  1   a , a nozzle  3  is formed in the nozzle plate  2 , and an ink droplet  15   a  is ejected from the nozzle  3 . 
     The nozzle plate  2  includes a thermal insulation layer  2   a  and a chemical vapor deposition (CVD) overcoat  2   b  formed on the thermal insulation layer  2   a . The insulation layer  2   a  and the CVD overcoat  2   b  correspond to a portion of the substrate  1 . The insulation layer  2   a  has a first surface facing the substrate  1  and a second surface contacting the heat element  8 . 
     A heat element  8  is disposed adjacent to the nozzle  3  to surround the nozzle  3 . The heat element  8  is disposed in an interface area between the thermal insulation layer  2   a  and the overcoat  2   b , and a thermal shunt  9  transferring heat from the heat element  8  to ink  15  in the chamber  1   a  and transferring redundant heat to the substrate  1  through the insulation layer  2   a  is formed above an upper side of the heat element  8 . 
     In the conventional ink-jet print head, if a current pulse is applied to the heat element  8 , the heat is generated from the heat element  8 , and bubbles  7  are formed from the first surface of the insulation layer  2   a . After that, while heat is continuously generated from the heat element  8 , the heat is continuously supplied to the bubbles  7 , and thus the bubbles  7  expand. Due to the expansion of the bubbles  7 , pressure is applied to the ink  15  disposed in the chamber  1   a , and thus the droplet  15   a  of the ink  15  in a vicinity of the nozzle  3  is ejected to an outside of the nozzle plate  2  through the nozzle  3 . After that, additional ink  15  is sucked into the chamber  1   a  along an ink channel or passage direction  5 , and thus the chamber  1   a  is refilled with the additional ink  15 . 
     In the conventional ink-jet print head using the back-shooting method, as described above, the heat element  8  arranged around the nozzle  3  of the nozzle plate  2  is formed between the insulation layer  2   a  and the overcoat  2   b , which constitute the nozzle plate  2 , and the heat element  8  is connected to an electric line (not shown) to receive current from a power source. The electric line is also formed between the insulation layer  2   a  and the overcoat  2   b.    
     If the current is supplied to the heat element  8 , heat generated from the heat element  8  is transferred to the ink  15  in the chamber  1   a , and thus the bubbles  7  are formed in the ink  15 . However, remaining redundant heat may be accumulated on the nozzle plate  2 , but the thermal accumulation of the remaining redundant heat is prevented by the thermal shunt  9 . In other words, the thermal shunt  9  prevents the thermal accumulation on the nozzle plate  2 . The temperature of the nozzle plate  2  raised by the remaining redundant heat, which is has not been transferred to the ink  15  in the chamber  1   a , is lowered when the remaining redundant heat is transmitted to the substrate  1 . If the temperature of the nozzle plate  2  is increased to more than a predetermined temperature, a lifetime of the ink-jet print head is shortened, and the performance of an ink-jet ejection operation is lowered. The problem with the thermal accumulation may not occur in a structure in which the heat element  8  is directly formed on the substrate  1  but occurs in another structure having the heat element  8  formed on a portion spaced-apart from the substrate  1 , for example, on the nozzle plate  2  having a membrane structure with a large heat transfer resistance as shown in FIG.  1 . 
     Likewise, in the ink-jet print head having the heat element  8  formed on the nozzle plate  2 , the thermal shunt  9  is used to improve the above thermal accumulation. However, with the thermal shunt  9  of the conventional ink-jet print head, it is very difficult to efficiently transfer or radiate the remaining redundant heat to the substrate  1 . In addition, the thermal shunt  9  is made of a conductor, such as aluminum, and is extended above the heat element  8  and between upper and lower material layers. Since the thermal shunt  9  is disposed very close to the heat element  8 , cracks are generated due to the thermal stress caused by a difference between thermal expansion coefficients of the thermal shunt  9  and the upper and lower material layers. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an object of the present invention to provide an inkjet print head, which is capable of more effectively preventing excessive thermal accumulation on a nozzle plate. 
     Additional objects and advantageous of the invention 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 invention. 
     Accordingly, to achieve the above and other objects, there is provided an ink-jet print head. The ink-jet print head includes a substrate, a channel formed on the substrate to supply ink in an ink passage direction, a nozzle plate connected to the substrate and including a nozzle corresponding to the channel, a heat element disposed in the nozzle plate to surround the nozzle, a thermal conduction layer formed on an upper side of the heat element, an intermediate insulation layer formed between the thermal conduction layer and the heat element, and a first thermal shunt spaced-apart from the heat element by a predetermined interval in a direction parallel to a major surface of the nozzle plate not to overlap the heat element and connecting the thermal conduction layer to the substrate. 
     The thermal conduction layer is made of diamond like carbon (DLC) or silicon carbide (SiC), and a passivation layer is formed on an upper surface of the thermal conduction layer, and a hydrophobic layer is formed on the passivation layer. 
     An electrode applying current to the heat element is formed on the nozzle plate, and the first thermal shunt is formed of the same material as that of the electrode. 
     The first thermal shunt includes first and second metal layers formed on the nozzle plate, an insulation layer is formed between the first and second metal layers, and a first through hole formed on the insulation layer to allow the first and second metal layers to contact each other. Here, the first through hole is spaced-apart from a wall defining the chamber so as not to thermally affect the ink in the chamber. The electrode includes a first electrode directly connected to the heat element and a second electrode formed on an upper layer formed on the first electrode, an insulation layer formed between the first electrode and the second electrode, and a second through hole formed on the insulation layer to allow the first electrode to be electrically connected to the second electrode. Thereby, a second thermal shunt including the first and second electrodes is provided. The first and second thermal shunts surround the heat element at a predetermined interval. 
     The above and other objects are achieved by providing a structure in which redundant heat generated from the heat element can be effectively transferred to a bulk silicon substrate in the ink-jet print head using a back-shooting method in which the heat element is spaced-apart from the substrate. That is, the inkjet print head includes a membrane. The chamber having a hemispheric shape is formed in the membrane, and the nozzle is formed above the chamber of the membrane. A thermal conduction layer is made of the DLC or the SiC to absorb the heat generated from the heat element and formed above the heat element with by the predetermined interval in the direction parallel to the major surface of the nozzle plate or parallel to a plane disposed between the nozzle plate and the substrate. A thermal shunt or bridge is formed between the thermal conduction layer and the substrate and spaced-apart from the heat element to rapidly transfer the heat from the thermal conduction layer to the substrate. An insulation layer having a predetermined thickness is made of a material having thermal conductivity lower than the DLC, such as an inter-metal dielectric (IMD) material, and disposed between the thermal conduction layer and the heat element, and thereby preventing the heat generated from the heat element from being excessively absorbed into the thermal conduction layer. Due to the excessive absorption and exhaustion of the heat, it is very difficult to effectively generate the bubbles. 
     The thermal conduction layer has an electrical insulation characteristic and is made of an inorganic material having a very high thermal conductivity and a low thermal expansion rate lower than a metal. As a result, the occurrence of the cracks caused by the thermal stress is prevented. The thermal shunt connecting the thermal conduction layer to the substrate is spaced-apart from the heat element by the predetermined second vertical distance and is simultaneously formed with the electrode constituting an electric circuit for the heat element. Thus, a design for the thermal shunt is applied to a mask forming the electrode in the nozzle plate when the electrode is formed, and thereby the thermal shunt is formed together when the electrode having one or two metal layers is formed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 illustrates a conventional ink-jet print head; 
     FIG. 2 is a schematic plan view of an ink-jet print head according to an embodiment of the present invention; 
     FIG. 3 is a schematic cross-sectional view of the ink-jet print head taken along line A—A of FIG. 2; 
     FIG. 4 illustrates an arrangement of a nozzle, a heat element, and a thermal shunt in the ink-jet print head of FIG. 3; 
     FIG. 5 illustrates an arrangement of the nozzle, the heat element, and the thermal shunt in the ink-jet print head according to another embodiment of the present invention; 
     FIG. 6 is a cross-sectional view of the ink-jet print head taken along line B—B of FIG. 5; 
     FIG. 7 schematically illustrates the ink-jet print head excluding the second thermal shunt from the nozzle plate of FIG. 6 according to another embodiment of the present invention; and 
     FIG. 8 schematically illustrates the ink-jet print head excluding the first thermal shunt from the nozzle plate of FIG. 6 according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described in order to explain the present invention by referring to the figures. 
     FIG. 2 is a schematic plan view of an ink-jet print head  10  according to an embodiment of the present invention, and FIG. 3 is a schematic cross-sectional view taken along line A—A of FIG. 2, illustrating the arrangement of a nozzle  13 , a heat element  18 , and a thermal shunt  19  of the ink-jet print head  10  of FIG.  2 . 
     As shown in FIG. 2, in the print head  10 , a plurality of nozzles  13  are arranged on a nozzle plate  12  in a plurality of lines, for example two lines in this embodiment. The nozzle plate  12  is a membrane formed on a substrate  11  to be described later. A plurality of pads  10   a  are arranged in a line at predetermined intervals along long opposite sides of the print head  10 . The pads  10   a  are terminals applying electric signals to corresponding heat elements  18 , and a switching device, such as an electric line and a transistor, controlling the electric signals may be arranged between the pads  10   a  and the corresponding heat elements  18 . Here, the switching device is positioned between the substrate  11  and the nozzle plate  12  and is formed through a generally known semiconductor manufacturing process on the substrate  11 . A position and a structure of the switching device in the nozzle plate  12  may be easily formed through general techniques of the generally known semiconductor manufacturing process. Reference numerals  5 ,  11   a  and  12   c  denote an ink channel or passage having the same axis as the nozzle  13 , a chamber and a thermal conduction layer, respectively. 
     As shown in FIGS. 2 through 4, the nozzle  13  is surrounded by the heat element  18  as a circular heating unit, and has a central axis passing through a center line of the chamber  11   a  filled with ink  15  supplied through an ink channel in an ink channel or passage direction  5  parallel to the central axis and the center line and perpendicular to a major surface of the nozzle plate  12 . As shown in FIGS. 3 and 4, a thermal shunt  19  surrounds the heat element  18  in a state where the thermal shunt  19  is spaced-apart from the heat element  18  by a predetermined horizontal distance ‘d’ in a horizontal direction parallel to the major surface of the nozzle plate  12 . One side of the thermal shunt  19  is directly in contact with a surface of the substrate  11  through a first through hole  12   a ′ of an underlying insulation layer  12   a , and thus absorbed heat is rapidly transferred from the thermal shunt  19  to the substrate  11  formed of silicon (Si). Here, the predetermined horizontal distance ‘d’ is in the range where the thermal shunt  19  does not overlap the heat element  18  in the horizontal direction such that another side of the thermal shunt  19  maintains the predetermined horizontal distance ‘d’ from the heat element  18 , and thereby preventing the thermal shunt  19  from being heated directly by heat generated from the heat element  18 . 
     In addition, it is necessary that the thermal shunt  19  is sufficiently spaced-apart from the chamber  11   a  such that parts or portion, such as a metal forming the thermal shunt  19  disposed along a heat transfer path, do not affect the temperature of the ink in the chamber  11   a . The heat always flows into the thermal shunt  19 , and thus this flowing of the heat may cause the temperature of the ink  5  in the chamber  11   a  to be increased if the thermal shunt  19  is disposed too close the chamber  11   a . When the temperature of the ink  15  increases, the viscosity of the ink  15  is lowered, and thus the lowered viscosity of the ink  15  may cause a bad influence on an ejection operation of the ink  5  and a printing performance of the ink-jet print head  10 . 
     The thermal conduction layer  12   c  made of diamond like carbon (DIC) or silicon carbide (SiC) is formed on the thermal shunt  19 . The thermal conduction layer  12   c  is electrically non-conductive and is made of a material having a very low heat resistance. The thermal conduction layer  12   c  is physically in contact with the thermal shunt  19  and is extended in the horizontal direction to cover the heat element  18 . As shown in FIG. 2, the thermal conduction layer  12   c  covers the nozzles  13  and the chamber  11   a  and may be a single layer or divided into a plurality of layers or a plurality of regions. The thermal conduction layer  12   c  is formed on an intermediate insulation layer  12   b  to be spaced-apart from the heat element  18  by a predetermined second vertical distance in the vertical direction. 
     The intermediate insulation layer  12   b  is an electrical insulation material, is obtained through a stack of one or more insulation materials and is preferably formed of inter-metal dielectric (IMD) material. A passivation layer  12   d  having a hydrophobic property is formed on an upper surface of the thermal conduction layer  12   c . Since the DLC or SiC forming the thermal conduction layer  12   c  has large residual-stress and generates high compression stress, there is a limitation in increasing a thickness of the thermal conduction layer  12   c , and the thickness of the thermal conduction layer  12   c  is about 0.3-0.5 μm. Thus, the passivation layer  12   d  is used to prevent an electrical short caused by the ink  15  penetrating the nozzle plate  12 . An oxide formed through a plasma enhanced-chemical vapor deposition (PE-CVD) method is used as the passivation layer  12   d , and a hydrophobic material, such as the DLC or fluorocarbon (FC), may be coated on the passivation layer  12   d  for hydrophobic processing in a case where the passivation layer  12   d  does not have the hydrophobic property. 
     In the above structure, the thermal conduction layer  12   c  is formed over the heat element  18 , absorbs the heat generated from the heat element  18  and passed through the intermediate insulation layer  12   b , and transfers the absorbed heat to the substrate  10  through the thermal shunt  19 . According to the heat transfer structure, thermal accumulation on the nozzle plate  12  is suppressed, and thereby a series of operations, such as heat/vaporization/ejection of the ink  15  is smoothly performed. 
     As described above, the thermal conduction layer  12   c  covers the heat element  18  and maintains the predetermined second vertical distance from the heat element  18 . When the thermal conduction layer  12   c  is spaced-apart the predetermined second distance from the heat element  18 , the thermal conduction layer  12   c  is prevented from excessively absorbing the heat and a minimum amount of the heat is absorbed to avoid the excessive thermal accumulation on the nozzle plate  12 . Since the thermal conduction layer  12   c  is formed of an inorganic matter such as the DLC or the SiC, the thermal stress caused by a difference in thermal expansion rates of materials stacked on upper and lower sides of the thermal conduction layer  12   c  is lowered, and thus the cracks due to thermal stress are prevented. The thermal shunt  19  made of a metallic material is spaced-apart from the heat element  18  by the predetermined horizontal distance not to overlap the heat element  18  in the horizontal direction and provides a path through which the heat from the thermal conduction layer  12   c  is passed. As a result, the thermal shunt  19  is not directly heated by the heat element  18  in the vertical direction, and the occurrence of the cracks is prevented. 
     The above embodiment illustrates an example of the ink-jet print head of the present invention and may be modified in various forms. according to the principles of the present invention, a different type of a thermal conduction structure connecting the thermal conduction layer  12   c  to the substrate  11  may be formed with a structural change of an electrode connected to the heat element  18  excluding the thermal shunt  19  as a separate element as described above. In the above structure, the thermal shunt  19  has a circular shape and completely surrounds the heat element  18  but may be partially formed around the heat element  18 . Also, the thermal shunt  19  may not overlap the heat element  18 . 
     FIG. 5 illustrates a structure having first and second thermal shunts  191  and  192  surrounding the heat element  18 , and FIG. 6 is a cross-sectional view of the ink-jet print head taken along line B—B of FIG.  5 . 
     As shown in FIG. 5, the first and second thermal shunts  191  and  192  are spaced-apart from the heat element  18  and disposed around the heat element  18  at a predetermined interval. As mentioned previously, the first and second thermal shunts  191  and  192  are physically in contact with the thermal conduction layer  12   c  and the substrate  11 , and thus provide a path where thermal energy from the thermal conduction layer  12   c  is transmitted to the substrate  11 . In such a case, the second thermal shunts  192  are also formed on first electrodes  181  formed on both ends the heat element  18  or may be formed on only one of the first electrodes  181  of the heat element  18  as a separate element. If the second thermal shunts  192  are formed on the first electrodes  181  at the both ends of the heat element  18 , each of the two second thermal shunts  192  must be electrically separated from each other. 
     Referring to FIG. 6, the nozzle plate  12  is formed on a top of the substrate  11  in which the chamber  11   a  having a hemispheric shape is formed. The nozzle  13  having the central axis passing through the center of the chamber  11   a  is formed on the nozzle plate  12 . The nozzle plate  12  is a membrane formed through a process of forming a thin film on the substrate  11 . 
     The underlying insulation layer  12   a  of the nozzle plate  12  directly contacts the substrate  11  and is a SiOx layer formed through the PE-CVD method. The heat element  18  surrounding the nozzles  13  is formed on the underlying insulation layer  12   a , and the intermediate insulation layer  12   b  is formed on the heat element  18 . The intermediate insulation layer  12   b  includes a first intermediate insulation layer  121   b  and a second intermediate insulation layer  122   b , and the first electrode  181  and a first metal layer  181   a  are formed between the first and second intermediate insulation layers  121   b  and  122   b . The first electrode  181  and the first metal layer  181   a  are simultaneously formed of the same material such as aluminum. A second electrode  182  and a second metal layer  182   a  are formed on the second intermediate insulation layer  122   b . The second electrode  182  and the second metal layer  182   a  are simultaneously formed of the same material as the aluminum. The second electrode  182  is physically and electrically connected to the first electrode  181  through a second through hole  122   b ′ formed on the second intermediate insulation layer  122   b . The second metal layer  182   a  is also physically in contact with the first metal layer  181   a  through the first through hole  12   a′.    
     The first metal layer  181   a  and the second metal layer  182   a  in the above structure are elements of the first thermal shunt  191  having the same function as above and act as only the path for transferring the heat to the substrate, and the first electrode  181  and the second electrode  182  act as elements of the second thermal shunts  192  for providing the path for transferring the heat to the substrate  11  and further act as an electrical connector connected to the heat element  18 . 
     The thermal conduction layer  12   c  having electrical insulation and high thermal conductivity such as the DLC or the SiC, is formed on the second electrode  182  and the second metal layer  182   a . The thermal conduction layer  12   c  may be formed through the PE-CVD method, etc. The thermal conduction layer  12   c  is formed to cover all of the first and second thermal shunts  191 ,  192  and intermediate insulation layers  121   a ,  122   b , absorbs redundant heat generated from the heat element  18  and exhausts the redundant heat to the substrate  11  through the first and second thermal shunts  191  and  192 . 
     The passivation layer  12   d  is formed on the thermal conduction layer  12   c , and a hydrophobic layer (not shown) may be formed on an outer surface of the passivation layer  12   d  in a case where the passivation layer  12   d  does not have the hydrophobic property. 
     According to a third embodiment of the present invention, as shown in FIG. 7, the second thermal shunt  192  is excluded from the nozzle plate  12  of FIG. 6, and only the first thermal shunt  191  is used. The first electrode  181  and the second electrode  182  are electrically in contact with each other through the second through hole  122   b ′ of the second intermediate insulation layer  122   b  and are separated from the substrate  11  by the underlying insulation layer  12   a . In FIG. 7, as shown in a left upper side of the chamber  11   a , the first thermal shunt  191  directly contacting the substrate  11  is arranged on a portion where the first and second electrodes  181 ,  182  are not formed. 
     According to a fourth embodiment of the present invention, as shown in FIG. 8, unlike the previous embodiment of FIG. 7, the first thermal shunt  191  is excluded from the nozzle plate  12  of FIG. 6, and only the second thermal shunt  192  is used. That is, the first electrode  181  and the second electrode  182 , which are included in the second thermal shunt  192 , are electrically in contact with each other through the second through hole  122   b ′ of the second intermediate insulation layer  122   b , and the first electrode  181  is directly in contact with the substrate  11  through the first through hole  12   a ′ of the underlying insulation layer  12   a , and the second electrode  182  is directly in contact with the thermal conduction layer  12  thereon, and thereby the path is provided where the heat absorbed into the thermal conduction layer  12   c  is directly transferred to the substrate  11 . 
     As with the embodiments of FIGS. 7 and 8, the selective use of the first and second thermal shunts  191 ,  192  depends on the amount of the redundant heat on the nozzle plate  12  and other design matters. Of course, as with the embodiments of FIGS. 5 and 6, all of the first and second thermal shunts may be used. 
     In the ink-jet print head according to the present invention, an active element required to drive the heat element, such as a power transistor or a CMOS for constituting a logic circuit, is formed on the substrate. The active element is formed before the above membrane is formed on the substrate. The active element forms an electric circuit, such as the heat element. 
     According to the present invention, redundant heat generated from a heat element is not accumulated on a membrane but is rapidly absorbed into an inorganic thermal conduction layer existing in the membrane and is transferred to a bulk silicon substrate through a metallic thermal bridge. The redundant heat is rapidly exhausted to prevent a shortened lifetime of an ink-jet print head, and an ink droplet is rapidly and successively ejected under a high pressure. Thus, the ink-jet print head according to the present invention can be maintained in a stable condition for a long life time of the ink-jet print head, and due to a very quick response speed, the ink-jet print head is suitable for a high speed printing apparatus. 
     Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and sprit of the invention, the scope of which is defined in the claims and their equivalents.