Ink-jet print head and method thereof

[0002] 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.

[0003] 1. Field of the Invention

[0004] The present invention relates to an ink-jet print head, and more particularly, to an ink-jet 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.

[0005] 2. Description of the Related Art

[0006] 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 electro-mechanical transducer having a piezoelectric device varying a volume of the ink caused by deformation of the piezoelectric device to eject the ink.

[0007] 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.

[0008] 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 Silverbrook issued Feb. 1, 2000.

[0009] FIG. 1 is a cross-sectional view of a conventional ink-jet print head.

[0010] A chamber 1ahaving a hemispheric shape is formed in a substrate 1, which is formed of silicon, etc., and an ink inlet 1bconnected to an ink supply source (not shown) is formed in a lower portion of the chamber 1a.A nozzle plate 2 is formed on the substrate 1 and above the chamber 1a,a nozzle 3 is formed in the nozzle plate 2, and an ink droplet 15ais ejected from the nozzle 3.

[0011] The nozzle plate 2 includes a thermal insulation layer 2aand a chemical vapor deposition (CVD) overcoat 2bformed on the thermal insulation layer 2a.The insulation layer 2aand the CVD overcoat 2bcorrespond to a portion of the substrate 1. The insulation layer 2ahas a first surface facing the substrate 1 and a second surface contacting the heat element 8.

[0012] 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 2aand the overcoat 2b,and a thermal shunt 9 transferring heat from the heat element 8 to ink 15 in the chamber 1aand transferring redundant heat to the substrate 1 through the insulation layer 2ais formed above an upper side of the heat element 8.

[0013] 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 2a. 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 1a,and thus the droplet 15aof 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 1aalong an ink channel or passage direction 5, and thus the chamber 1ais refilled with the additional ink 15.

[0014] 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 2aand the overcoat 2b,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 2aand the overcoat 2b.

[0015] 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 1a,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 has not been transferred to the ink 15 in the chamber 1a,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.

[0016] 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.

[0017] To solve the above problems, it is an object of the present invention to provide an ink-jet print head, which is capable of more effectively preventing excessive thermal accumulation on a nozzle plate.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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 ink-jet 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.

[0024] 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.

[0025] 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:

[0026] FIG. 1 illustrates a conventional ink-jet print head;

[0027] FIG. 2 is a schematic plan view of an ink-jet print head according to an embodiment of the present invention;

[0028] FIG. 3 is a schematic cross-sectional view of the ink-jet print head taken along line A—A of FIG. 2

[0029] FIG. 4 illustrates an arrangement of a nozzle, a heat element, and a thermal shunt in the ink-jet print head of FIG. 3;

[0030] FIG. 5 illustrates an arrangement of the nozzle, the heat element, and the thermal shunt in the ink-jet pint head according to another embodiment of the present invention;

[0031] FIG. 6 is a cross-sectional view of the ink-jet print head taken along line B—B of FIG. 5;

[0032] 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

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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 10aare arranged in a line at predetermined intervals along long opposite sides of the print head 10. The pads 10aare 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 10aand 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, 11aand 12cdenote an ink channel or passage having the same axis as the nozzle 13, a chamber and a thermal conduction layer, respectively.

[0037] 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 11afilled 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 12a′ of an underlying insulation layer 12a,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.

[0038] In addition, it is necessary that the thermal shunt 19 is sufficiently spaced-apart from the chamber 11asuch 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 11a.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 11ato be increased if the thermal shunt 19 is disposed too close the chamber 11a.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.

[0039] The thermal conduction layer 12cmade of diamond like carbon (DIC) or silicon carbide (SiC) is formed on the thermal shunt 19. The thermal conduction layer 12cis electrically non-conductive and is made of a material having a very low heat resistance. The thermal conduction layer 12cis 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 12ccovers the nozzles 13 and the chamber 11aand may be a single layer or divided into a plurality of layers or a plurality of regions. The thermal conduction layer 12cis formed on an intermediate insulation layer 12bto be spaced-apart from the heat element 18 by a predetermined second vertical distance in the vertical direction.

[0040] The intermediate insulation layer 12bis 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 12dhaving a hydrophobic property is formed on an upper surface of the thermal conduction layer 12c.Since the DLC or SiC forming the thermal conduction layer 12chas large residual-stress and generates high compression stress, there is a limitation in increasing a thickness of the thermal conduction layer 12c,and the thickness of the thermal conduction layer 12cis about 0.3-0.5 μm. Thus, the passivation layer 12dis 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 12d,and a hydrophobic material, such as the DLC or fluoro-carbon (FC), may be coated on the passivation layer 12dfor hydrophobic processing in a case where the passivation layer 12ddoes not have the hydrophobic property.

[0041] In the above structure, the thermal conduction layer 12cis formed over the heat element 18, absorbs the heat generated from the heat element 18 and passed through the intermediate insulation layer 12b,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.

[0042] As described above, the thermal conduction layer 12ccovers the heat element 18 and maintains the predetermined second vertical distance from the heat element 18. When the thermal conduction layer 12cis spaced-apart the predetermined second distance from the heat element 18, the thermal conduction layer 12cis 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 12cis 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 12cis 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 12cis 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.

[0043] 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 12cto 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.

[0044] 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.

[0045] 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 12cand the substrate 11, and thus provide a path where thermal energy from the thermal conduction layer 12cis 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.

[0046] Referring to FIG. 6, the nozzle plate 12 is formed on a top of the substrate 11 in which the chamber 11ahaving a hemispheric shape is formed. The nozzle 13 having the central axis passing through the center of the chamber 11ais 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.

[0047] The underlying insulation layer 12aof 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 12a,and the intermediate insulation layer 12bis formed on the heat element 18. The intermediate insulation layer 12bincludes a first intermediate insulation layer 121band a second intermediate insulation layer 122b,and the first electrode 181 and a first metal layer 181aare formed between the first and second intermediate insulation layers 121band 122b.The first electrode 181 and the first metal layer 181aare simultaneously formed of the same material such as aluminum. A second electrode 182 and a second metal layer 182aare formed on the second intermediate insulation layer 122b.The second electrode 182 and the second metal layer 182aare 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 122b′ formed on the second intermediate insulation layer 122b.The second metal layer 182ais also physically in contact with the first metal layer 181athrough the first through hole 12a′.

[0048] The first metal layer 181aand the second metal layer 182ain 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.

[0049] The thermal conduction layer 12chaving 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 182a.The thermal conduction layer 12cmay be formed through the PE-CVD method, etc. The thermal conduction layer 12cis formed to cover all of the first and second thermal shunts 191, 192 and intermediate insulation layers 121a,122b,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.

[0050] The passivation layer 12dis formed on the thermal conduction layer 12c,and a hydrophobic layer (not shown) may be formed on an outer surface of the passivation layer 12din a case where the passivation layer 12ddoes not have the hydrophobic property.

[0051] 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 122b′ of the second intermediate insulation layer 122band are separated from the substrate 11 by the underlying insulation layer 12a.In FIG. 7, as shown in a left upper side of the chamber 11a,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.

[0052] 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 122b′ of the second intermediate insulation layer 122b,and the first electrode 181 is directly in contact with the substrate 11 through the first through hole 12a′of the underlying insulation layer 12a,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 12cis directly transferred to the substrate 11.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.