Abstract:
A high-speed, high-resolution inkjet printhead. At least two ink-supply paths used to supply ink to the ink chamber are arranged on the substrate in a two-dimensional array. The present invention overcomes the disadvantages of conventional inkjet printheads, i.e., low degree of integration arising from nozzles aligned in a line around a single ink-supply path. Thus, according to the present invention, a large number of nozzles can be integrated on the substrate, thus resulting in high-speed, high-resolution printing.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a printhead and, more particularly, to an inkjet printhead capable of high-resolution, high-speed printing and a method of fabricating such printhead with a process employing the same degree of integration as in conventional processes.  
           [0003]    2. Description of the Prior Art  
           [0004]    Methods of ejecting ink in conventional inkjet printheads are classified into two types. The first type, as disclosed in U.S. Pat. No. 4,338,611 to Eida et al., is the side-shooter type in which nozzles are formed on the side face of a printhead substrate and ink is ejected in the side direction. The second type, as disclosed in U.S. Pat. No. 4,931,813 to Pan et al., is the roof-shooter type in which nozzles are formed on the top face of the printhead substrate and ink is ejected to the upper direction.  
           [0005]    Among the above two types, the side-shooter type has a drawback that nozzles can only be arranged in a single line, because nozzles are arranged on the side face of the printhead substrate. Further, the roof-shooter type also has a drawback that nozzles can only be arranged in a single line or double lines, even though nozzles are arranged on the top face of the printhead substrate. This is because, as shown in FIG. 1., the main ink-supply path  15  used to supply ink to an ink chamber is formed as a single orifice on the silicon substrate  10 , and the ink channels  14  and the nozzles  12  are arranged around the main ink-supply path  15 . When the main ink-supply path  15  is formed in the center of the silicon substrate  10  and the nozzles  12  are arranged in a single line, as shown in FIG. 1, the distance between the printed lines cannot be less than 40 μm, if the technology used to form the printhead does not allow the distance (μ) between the nozzles  12  to be less than 40 μm.  
           [0006]    As an improvement to the roof-shooter type, U.S. Pat. No. 5,648,806 to Steinfield et al. discloses simplifying the formation of the main ink-supply path by utilizing the side face of the substrate as the main ink-supply path. However, this still has a drawback that no more than two rows of nozzles can be formed on the side face of the substrate. Accordingly, the degree of integration in the arrangement of nozzles becomes lower and the number of nozzles integrated on the printhead becomes fewer, thus lowering the ink ejection speed. In order to double the resolution in high-resolution inkjet printheads while printing the same area in the same amount of time, the ink ejection speed needs to be four times faster than that of conventional printheads, because the size of the droplets of ink is small. Therefore, even if high-resolution nozzles are made with conventional arrangement of nozzles, slow printing speed always becomes a problem and thus high-resolution printing cannot be achieved practically.  
           [0007]    In addition, U.S. Pat. No. 4,558,333 to Sugitani et al. discloses dividing an ink chamber to many small chambers in order to improve the degree of integration in the arrangement of nozzles. However, this arrangement still has only a single ink-supply path and the ink-supply speed is different for each ink chamber. Thus, it has a drawback that ink cannot be supplied fast and smoothly and that the fluid dynamics interference between ink chambers is very strong so that it is impractical for actual use.  
         SUMMARY OF THE INVENTION  
         [0008]    Therefore, it is an object of the present invention to provide an inkjet printhead capable of high-resolution, high-speed printing and a method of fabricating such inkjet printhead with a process having the same degree of integration as in conventional processes.  
           [0009]    To this end, an inkjet printhead is provided, the inkjet printhead comprising a substrate having at least four ink-supply path orifices arranged in a two-dimensional array, nozzles connected to each of the ink-supply path orifices, driving means for driving the nozzles, and an electrical device for decoding an electric signal provided from outside the inkjet printhead and transmitting the decoded electric signal to the driving means in order to selectively drive the nozzles.  
           [0010]    It is preferable that the two-dimensional array of the ink-supply orifices is an n×n array or a 1×n array, wherein n is a natural number greater than 1.  
           [0011]    It is also possible to make the size of the nozzles different in each area of the two-dimensional array, thereby implementing a variety of resolutions and enhancing the printing speed.  
           [0012]    The driving means ejects ink from the nozzles by heat ejection or piezoelectric ejection, and the electrical device is a switching device such as a diode or a metal-oxide-silicon (MOS) transistor and is preferably integrated on the substrate.  
           [0013]    The nozzles are formed in the nozzle plate that covers the ink-supply path orifices, and it is preferable for the nozzle plate to include a conductive layer that can function as a power supply line or a ground line for driving the inkjet printhead.  
           [0014]    It is also preferable that the two-dimensional array of ink-supply path orifices and nozzles comprises rows forming an angle with respect to a printing-movement direction of the printhead.  
           [0015]    In order to achieve the above technical objects of the present invention, the method of fabricating an inkjet printhead of the present invention comprises monolithic processes consistent with a high-resolution printhead. That is, the method includes forming nozzles directly on a silicon substrate and forming at least two ink-supply paths in a two-dimensional array on the silicon substrate by an electro-chemical etching process or by deep reactive ion etching (DRIE).  
         BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]    [0016]FIG. 1 is a diagram illustrating the arrangement of nozzles in a conventional inkjet printhead;  
           [0017]    [0017]FIG. 2 is a diagram illustrating the two-dimensional arrangement of ink-supply paths and nozzles in an inkjet printhead according to an embodiment of the present invention;  
           [0018]    [0018]FIG. 3 is a perspective view of an inkjet printhead integrated on a silicon substrate according to an embodiment of the present invention.  
           [0019]    [0019]FIG. 4 is a diagram illustrating the relation between the arrangement of nozzles and printing of the inkjet printhead according to an embodiment of the present invention;  
           [0020]    [0020]FIGS. 5 a  and  5   b  are diagrams illustrating the arrangement of nozzles in order to enhance printing speed;  
           [0021]    [0021]FIG. 6 is a diagram illustrating the arrangement of nozzle blocks and pads according to an embodiment of the present invention;  
           [0022]    [0022]FIG. 7 is a layout diagram of a printhead according to an embodiment of the present invention;  
           [0023]    [0023]FIGS. 8 a  through  8   k  are process cross-sectional views illustrating the method of fabricating an inkjet printhead according to a first embodiment of the present invention;  
           [0024]    [0024]FIGS. 9 a  through  9   c  are process cross-sectional views illustrating the method of fabricating an inkjet printhead according to a second embodiment of the present invention;  
           [0025]    [0025]FIGS. 10 a  through  10   c  are perspective views illustrating the use of a single photolithography process to form a three-dimensional nozzle mold to be used for plating; and  
           [0026]    FIGS.  11  is a cross-sectional view of an etching apparatus used in an electro-chemical etching process to implement the method according to the first embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    The preferred embodiments of the present invention will be described hereinafter with reference to the attached drawings.  
         [0028]    [0028]FIG. 2 is a diagram illustrating the two-dimensional arrangement of ink-supply paths and nozzles in an inkjet printhead according to an embodiment of the present invention. With reference to FIG. 2, ink-supply paths  15 ′ are formed on the silicon substrate  10  in a two-dimensional array, and nozzles  12 ′ are formed on the respective ink-supply paths  15 ′. With this arrangement, it is possible to print lines separated from each other by 10 μm even if the nozzles are separated from each other by 40 μm.  
         [0029]    [0029]FIG. 3 is a perspective view of an inkjet printhead integrated on a silicon substrate according to an embodiment of the present invention. Part of the silicon substrate is shown as cut off for clear illustration of the present invention. With reference to FIG. 3, the inkjet printhead shown therein is a roof-shooter type printhead. A number of ink-supply paths  15 ′ are formed on the silicon substrate in a two-dimensional array, and each ink-supply path  15 ′ is covered with a nozzle plate  11  in which an ink channel  14 ′, ink chamber  13 , and a nozzle  12 ′ are formed. In order to show the interior structure of the nozzle plate  11  clearly, the nozzle plates  11  are shown as removed or cut off horizontally or partially. Switching devices  17  are arranged near each nozzle  12 ′ and used to decode the signal applied from outside the printhead and to transmit the decoded signal in the form of electrical energy to the driver of each nozzle in order to selectively drive each nozzle  12 ′. The electrical energy heats up the heater resistor  16 . Ink supplied via the ink-supply paths  15 ′ expands by this heat so that ink droplets d are ejected via the nozzle  12 ′. In addition, Vcc and GND pads  18  and  19 , switching devices  17 , heater resistors  16 , and heat pads  20  are formed on the substrate  10 .  
         [0030]    [0030]FIG. 4 is a diagram illustrating the relation between the arrangement of nozzles and printing of the inkjet printhead according to an embodiment of the present invention. With reference to FIG. 4, hypothetical pixels  3  of c×c mm size are formed on the printing paper  2 . The resolution is 25.4/c. Nozzles  12 ′ on the printhead  1  are sequentially selected from the first row to the last row. At the moment when a row is selected, all image data given to all the columns on that row is printed. Nozzles  12 ′ are located on the printhead  1  with coordinates given by the following formula: 
           Nij =(− L   x   ·j−T   row   ·V·i, L   y   ·i+c·j )  Formula 1 
         [0031]    wherein L x  is the horizontal distance and L y  is the vertical distance between adjacent nozzles, T row  is the amount of time during which each row is selected, V is the relative velocity of the printhead  1  with respect to the printing paper  2 . For example, if the resolution of the printer is 2400×2400 dpi (dots per inch), then c, L x , L y , and T row  can be set as 10 μm, 210 μm, 200 μm, and 3.3 μs, respectively. As shown in FIG. 4, nozzles N 00  and N 01  are skewed by distance c in the vertical direction in order to print pixels separated from adjacent pixels by distance c in the vertical direction.  
         [0032]    [0032]FIGS. 5 a  and  5   b  are diagrams illustrating the arrangement of nozzles in order to improve printing speed. As illustrated in FIG. 5 a , many two-dimensional nozzle blocks (A, B, and C) are repeatedly arranged and each nozzle block only carries out the printing of a designated area. In this manner, the use of three nozzle blocks results in enhancement of printing speed by threefold. Generally, the use of n nozzle blocks results in enhancing printing speed by n times.  
         [0033]    On the other hand, it is also possible to use large nozzles in an area of the printhead along with small nozzles as shown in FIG. 5 b . When high-resolution printing is required, only small nozzles are used. When high-speed, low-resolution printing is required, it is possible to selectively drive both the large and small nozzles using electrical signals controlled by a software program to enhance printing speed.  
         [0034]    [0034]FIG. 6 is a diagram illustrating the arrangement of nozzle blocks and pads according to an embodiment of the present invention. The term “nozzle block” is used to indicate an area of the printhead in which nozzles of the same size are arranged equidistant from adjacent nozzles. As shown in FIG. 6, two-dimensional nozzle blocks (A, B, C and A′, B′, C′) are repeatedly arranged in order to enhance printing speed by use of more nozzles. Column pads  4  and  5  and row pads  6  and  7  are located around the nozzle blocks (A, B, C and A′, B′, C′) to supply electrical energy to the switching devices and heater resistors.  
         [0035]    [0035]FIG. 7 is a layout diagram of a printhead according to an embodiment of the present invention. FIG. 7 shows nozzles arranged in a 2×2 array and devices and wiring for driving the nozzles. The nozzle plate  11  covers the ink-supply path  15 ′, and ink channels  14 ′ and nozzles  12 ′ are formed in the nozzle plate  11 . The nozzle plates  15 ′ are shown with a see-through view in order to clearly illustrate the elements covered by the nozzle plate  15 ′. Heater resistors used to eject ink from the nozzles  12 ′ are not shown in FIG. 7, because it is located under the nozzles  12 ′. Vcc wiring  22  is connected to the heater resistors in order to supply electrical energy to the heater resistors. Also, switching transistors, comprising polysilicon gate electrodes  21  and gate oxide under the gate electrodes  21 , are used to apply electrical signals for driving the heater resistors. FIG. 7 also shows ground (GND) wiring  23 .  
         [0036]    The method of fabricating an inkjet printhead according to embodiments of the present invention will be illustrated below.  
       Fabrication Method According to the First Embodiment  
       [0037]    [0037]FIGS. 8 a  through  8   k  are process cross-sectional views illustrating the method of fabricating an inkjet printhead according to a first embodiment of the present invention.  
         [0038]    Referring to FIG. 8 a , a silicon oxide layer  31  and a silicon nitride layer  30  are  30  formed on the boron-doped p-type silicon substrate  10  to a thickness of 500 Å and 1500 Å, respectively, for a LOCOS (local oxidation of silicon) process for separation of devices.  
         [0039]    Subsequently, as shown in FIG. 8b, in order to prevent over-erosion of the silicon substrate  10  by electrolytic polishing process, the silicon oxide layer  31  and the silicon nitride layer  30  are etched away using the first mask except in the switching device area  25  and the main ink-supply path area  24 , and then phosphorous doping is carried out to form a phosphorous-doped layer  32 . Subsequently, a thermal oxide layer  33  for prevention of heat transfer during ejection of ink is formed to a thickness of 1.2 μm by wet oxidation in a high-temperature furnace at 1100° C. for 200 minutes.  
         [0040]    With reference to FIG. 8 c , the oxide layer over the main ink-supply path area  24  and the switching device separation area  26  is removed using the second mask, and the silicon nitride layer over the main ink-supply path area  24  and the switching device separation area  26  is etched away by use of phosphoric acid. Subsequently, boron is doped at 900° C. for 20 minutes to form a boron diffusion area  34  for separation of devices so that, in a subsequent electrolytic polishing process, contact resistance between the electrode and silicon is enhanced and leakage current in transistors is reduced. Then, a heat treatment process in nitrogen environment at 1150° C. for 60 minutes, an oxidation process in vapor environment at 1100° C. for 70 minutes, and a heat treatment process in nitrogen environment at 1100° C. for 20 minutes is carried out sequentially, in order to reduce the boron concentration in the boron diffusion area  34 , form a device-separation silicon oxide layer  35 , and increase the thickness of the thermal oxide layer  33  for prevention of heat transfer. Thereafter, all the silicon nitride layers are removed using phosphoric acid, and the thin silicon oxide layer under the silicon oxide layer is etched away using BOE (buffered oxide etchant) solution for 1 minute. Additionally, in order to remove the white strip formed during the LOCOS process, a sacrificial oxide layer is formed by an oxidation process in an oxygen environment at 1000° C. for 65 minutes and etching is carried out in BOE solution for 1 minute.  
         [0041]    Referring to FIG. 8 d , the gate oxide layer of the transistor is formed to a thickness of 300 Å by an oxidation process in oxygen environment at 1000° C. for 20 minutes. Thereafter, a heat treatment process for the gate oxide layer is carried out in nitrogen environment at 1000° C. for 20 minutes in order to improve the electrical characteristics of the gate oxide layer. In order to form the gate electrode of the transistor, a polysilicon layer is deposited to a thickness of 4500 Å and then etched away using the third mask to form the gate  21  of the transistor. The gate oxide over the areas for the source-drain of the transistor is removed, and the source-drain area  36  is formed by doping phosphorous at 970° C. for 30 minutes. In order to compensate for the etching of the side face of the gate oxide layer while etching the gate oxide layer over the source-drain area, an additional oxidation process is carried out in oxygen environment at 1000° C. for 20 minutes. Also, prior to depositing the metal electrode, an oxide layer  37  is deposited to a thickness of 5000 Å for insulation.  
         [0042]    Referring to FIG. 8 e,  the oxide layer over the ink-supply path area is removed using the fourth mask. In addition, boron doping is carried out in this ink-supply path area at 915° C. for 30 minutes to form a boron diffusion layer  38 , so that the contact resistance between the ink-supply path area and metal wiring is reduced.  
         [0043]    Subsequently, as shown in FIG. 8f, the oxide layer over the source-drain area is removed, and the thin layer for metal wiring and for the heater resistor is deposited and etched using the sixth and seventh mask to form the metal wiring  39  and the heater resistor  40 .  
         [0044]    Thereafter, as shown in FIG. 8 g,  in order to protect the transistor, heater resistor, and the wiring from ink, first and second passivation layers  41  and  42  are sequentially deposited. The second passivation layer  42  is etched away using the eighth mask except for the area around the heater resistor. Also, the first passivation layer  41  over the pad-wiring contact window area  27  and the ink-supply path area is etched away using the ninth mask.  
         [0045]    Referring to FIG. 8 h,  the base metal layer  43  for plating of the nozzle plate is deposited, and the plating mold  44  for plating of the nozzle plate is formed by patterning a photoresistor layer. In this embodiment, the base metal layer  43  is a titanium-gold composite layer (Ti/Au). As shown in FIGS. 10 a  through  10   c,  the thick photoresistor layer for the plating mold  44  is exposed to ultraviolet light using sequentially the tenth mask corresponding to the ink channel—ink chamber mask  63  and the eleventh mask corresponding to the nozzle mask  64 . At this time, if the exposure period for the tenth mask is long (FIG. 10 a ) whereas the exposure time for the eleventh mask is short (FIG. 10 b ), then as shown in FIG. 10 c  a three-dimensional photoresistor mold comprising a nozzle mold  60 , an ink chamber mold  61 , and an ink channel mold  62  can be formed by a single photolithography process.  
         [0046]    Subsequently, as shown in FIG. 8 i,  the nozzle plate  45  is formed by a plating process using the plating mold  44 . The thickness of plating should be less than that of the photoresistor layer.  
         [0047]    With reference to FIG. 8 j,  the ink-supply path  15 ′ is formed in the silicon substrate  10  by electro-chemical etching. Electro-chemical etching is carried out in the etching apparatus as shown in FIG. 11. Referring to FIG. 11, the closed space carrying the electro-chemical etching solution  72  is formed by the Teflon bath  70 , the bottom surface of the silicon substrate  10 , and the O-ring  75 . The electrochemical etching solution  72  is typically a mixture of nitric acid, fluoric acid, and water or acetic acid. One end of the direct current device  74  is connected to the platinum electrode  73  inserted in the electro-chemical etching solution  72 , and the other end is connected to the copper electrode  71  that is in contact with the silicon substrate  10  and the contact window  76  of the thin metal layer. Thus, the current from the direct current apparatus  74  flows to the silicon substrate  10  via the contact window  76  to form the ink-supply path  15 ′ of the shape of the contact window  76  in the silicon substrate  10  as shown in FIG. 8 j.    
         [0048]    Subsequently, as shown in FIG. 8 k , boiled acetic acid is used to remove the photoresistor layer covering the ink channel  14 ′, ink chamber  13 , and the nozzle  12 ′. Finally, the entire process is completed by removing the base metal layer (Ti/Au) using BOE and metal-etching solution.  
       Fabrication Method According to the Second Embodiment  
       [0049]    In the method of fabricating an inkjet printhead according to the second embodiment of the present invention, the ink-supply path is formed using a DRIE process. The first half of the process is identical to the process as illustrated in FIGS. 8 a  through  8   h.  That is, after completing the plating process of the nozzle, then as shown in FIG. 9 a , a photoresistor layer  46  is deposited on the bottom face of the silicon substrate  10 . Then, the photoresistor layer in the ink-supply path area is removed using a two-sided aligned exposure apparatus.  
         [0050]    Subsequently, as shown in FIG. 9 b,  the silicon substrate  10  is etched from the bottom surface thereof using a DRIE process. At this time, the base metal  43  for plating or the photoresistor layer  44  used as the plating mold functions as the etch stop layer.  
         [0051]    Thereafter, as shown in FIG. 9 c , boiled acetic acid is used to remove the photoresistor layer  46  used in the DRIE process and the photoresistor layer  44  covering the ink channel  14 ′, ink chamber  13 , and the nozzle  12 ′. Finally, the entire process is completed by removing the base metal layer  43  (Ti/Au) for plating using BOE and metal-etching solution.  
         [0052]    In order to achieve high-resolution printing at the same level as photographs as demanded by customers, an inkjet printhead that is capable of high-resolution printing at the level of 2400-3600 dpi is required. However, conventional methods of fabricating inkjet printheads merely produced printheads of 600 dpi resolution considering the nozzle size and the nozzle arrangement pitch. The method of the present invention is capable of realizing an inkjet printhead of 2400×2400 dpi resolution. In addition, the printing speed is not deteriorated at all in the inkjet printhead of 2400×2400 dpi resolution according to the present invention. Therefore, use of inkjet printhead of the present invention can result in prints of the same resolution as in photographs, and the market for such inkjet printhead will be enormous.  
         [0053]    Although the present invention has been illustrated with reference to embodiments of the present invention, various modifications are possible within the scope of the present invention. Therefore, the scope of the present invention should be defined not by the illustrated embodiments but by the attached claims.