Patent Publication Number: US-7712888-B2

Title: Inkjet printing system for manufacturing thin film transistor array

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2005-0123136 filed in the Korean Intellectual Property Office on Dec. 14, 2005, the contents of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to an inkjet printing system for manufacturing a thin film transistor array for a flat panel display. 
   DESCRIPTION OF THE RELATED ART 
   Generally, a flat panel display, such as a liquid crystal display (LCD), an organic light emitting diode (OLED) display, and an electrophoretic display, includes a plurality of pairs of field generating electrodes and an electro-optical activation layer disposed therebetween. The liquid crystal display includes a liquid crystal layer as the electro-optical activation layer, and the organic light emitting diode display includes an organic emission layer as the electro-optical activation layer. 
   One of the pair of field generating electrodes is generally connected to a switching element so as to receive an electrical signal which the electro-optical activation layer converts to an image. 
   In the flat panel display, a thin film transistor (TFT), which is a three terminal element, is used as the switching element. On the display panel a gate line transmits a scanning signal to control the turning on and off of the thin film transistor to connect the image signal from data line to the pixel electrode. 
   Because an organic thin film transistor, constructed mainly of a crystalline material that has enhanced crystallinity and molecular ordering, can be manufactured by a solution process at a low temperature, particularly by an inkjet printing method, its applicability to a wide area flat panel display is limited only by the deposition process employed. However, organic semiconductors formed by inkjet printing may have poor crystal growth resulting in inferior transistor characteristics. 
   SUMMARY OF THE INVENTION 
   The present invention provides an inkjet printing system that produces an organic semiconductor having improved the crystallinity. An exemplary embodiment of the present invention provides an inkjet printing system including an inkjet printing chamber depositing ink on a substrate, and a drying chamber spaced from inkjet printing chamber for drying the ink by regulating the vapor pressure of a solvent deposited on substrate. 
   The method for manufacturing a thin film transistor array panel according to an exemplary embodiment of the present invention includes forming an organic semiconductor by depositing a solvent containing ink on a substrate within an inkjet printing chamber, taking the substrate out of the inkjet printing chamber and then placing substrate in a drying chamber, and drying the organic semiconductor by regulating the vapor pressure of the ink solvent using a vapor pressure regulating device. Advantageously, the organic solvent may be one of mesitylen and tetralin. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The forgoing and other objects and features of the present invention may become more apparent from a reading of the ensuing description together with the drawing, in which: 
       FIG. 1  is a perspective view of an inkjet printing system according to an exemplary embodiment of the present invention. 
       FIG. 2  is a bottom plan view of a head unit of an inkjet printing system according to an exemplary embodiment of the present invention. 
       FIG. 3  is a drawing schematically showing a method for forming an organic semiconductor using an inkjet head of an inkjet printing system according to an exemplary embodiment of the present invention. 
       FIG. 4  is a layout view showing the first step of a method for manufacturing an organic thin film transistor array panel according to an exemplary embodiment of the present invention. 
       FIG. 5  is a cross-sectional view of the organic thin film transistor array panel taken along the line V-V of  FIG. 4 . 
       FIG. 6  is a layout view showing a subsequent step to that shown in  FIG. 4 . 
       FIG. 7  is a cross-sectional view of the organic thin film transistor array panel taken along the line VII-VII of  FIG. 6 . 
       FIG. 8  is a layout view showing a subsequent step to that shown in  FIG. 6 . 
       FIG. 9  is a cross-sectional view of the organic thin film transistor array panel taken along the line IX-IX of  FIG. 8 . 
       FIG. 10  is a layout view showing a subsequent step to that shown in  FIG. 8 . 
       FIG. 11  is a cross-sectional view of the organic thin film transistor array panel taken along the line XI-XI of  FIG. 10 . 
       FIG. 12  is a layout view showing a subsequent step to that shown in  FIG. 10 . 
       FIG. 13  is a cross-sectional view of the organic thin film transistor array panel taken along the line XIII-XIII of  FIG. 12 . 
       FIG. 14  is a layout view showing a subsequent step to that shown in  FIG. 12 . 
       FIG. 15  is a cross-sectional view of the organic thin film transistor array panel taken along the line XV-XV of  FIG. 14 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element, such as a layer, film, region, or substrate, is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
   An inkjet printing system according to an exemplary embodiment of the present invention will be explained in detail with reference to  FIG. 1  to  FIG. 3  hereinafter.  FIG. 1  is a perspective view of an inkjet printing system according to an exemplary embodiment of the present invention,  FIG. 2  is a bottom plan view of a head unit of inkjet printing system according to the exemplary embodiment of the present invention, and  FIG. 3  is a drawing schematically showing a method for forming an organic semiconductor using inkjet head of inkjet printing system according to the exemplary embodiment of the present invention. 
   As shown in  FIG. 1  to  FIG. 3 , an inkjet printing system includes an inkjet printing chamber  51  in which an inkjet printing process is performed and a drying chamber  52  spaced from inkjet printing chamber  51  that regulates the ink solvent&#39;s vapor pressure. 
   Within inkjet printing chamber  51  are installed a stage  510  to which substrate  110  is mounted, a head unit  700  positioned over stage  510  and a transfer device  300  for moving head unit  700 . 
   Head unit  700  includes an inkjet head  400  and sensor  600  for positioning inkjet head  400 . Inkjet head  400  has the shape of a long bar and includes a plurality of nozzles  410  disposed on a bottom surface thereof. Ink  5  for forming an organic semiconductor  154  is deposited on substrate  110  through the nozzles  410 . The ink solvent may be mesitylen, tetralin, cyclohexanone, etc. In addition, other suitable ink solvents can be used. 
   Inkjet head  400  is slanted with respect to the Y direction by a predetermined angle θ. Nozzle pitch D is the distance between the nozzles  410  formed in inkjet head  400 . The distance P is identified as the distance between the organic semiconductors  154 , which will be printed. As distances D and P are different from each other, the distance P between the neighboring organic semiconductors  154  is accommodated by rotating inkjet head  400  through a predetermined angle θ. Although inkjet head unit  700  is shown as a unitary part, it may be constructed of a plurality of parts. 
   Transfer device  300  includes a Y direction transfer member  310  positioning head unit  700  above substrate  110  for moving head unit  700  in the Y direction, an X direction transfer member  330  moving head unit  700  in an X direction, and a lifter  340  for raising and lowering head unit  700 . 
   Within drying chamber  52 A, stage  520 , vapor pressure regulating device  800 , and a vapor pressure detector  900  are installed. Vapor pressure regulating device  800  heats the solvent for ink  5  to a vapor  801  and then sprays the vapor  801  into an inner space of the drying chamber  52 . Inside drying chamber  52  vapor pressure detector  900  detects the vapor pressure of the solvent. 
   Operations for forming an organic semiconductor on substrate  110  using inkjet printing system having the above-mentioned structure will now be explained. 
   First, head unit  700  is positioned above the corresponding substrate  110  in inkjet printing chamber  51  by the operations of the X or Y direction transfer member  330  or  310  and the lifter  340 . 
   Subsequently, by driving the X direction transfer member  330  of transfer device  300  and the nozzle  410  of inkjet head  400 , the ink  5  is deposited while head unit  700  is moved in the X direction, thereby forming an organic semiconductor  154  on the respective pixels. 
   Subsequently, substrate  110  is taken out of inkjet printing chamber  51  and is then placed in the drying chamber  52 . Before substrate  110  is placed in the drying chamber  52 , the size of crystal  154   a  of the organic semiconductor  154  formed on substrate  110  is very small. 
   Subsequently, the drying speed of the ink  5  is regulated by regulating the vapor pressure of the solvent inside the drying chamber  52  using the vapor pressure regulating device  800  so that the crystallinity of the organic semiconductor  154  is improved. As the vapor pressure of the solvent increases, the crystal growth of the organic semiconductor  154  is expedited, so that the size of the crystal  154   b  increases thereby improving its crystallinity. It is preferable that the vapor pressure of the solvent is increased to a value at which the organic semiconductor  154  does not dissolve again. At this time, by checking the vapor pressure of the solvent inside the chamber  52  using the vapor pressure detector  900 , the vapor pressure of the solvent is regulated so as not to be excessive. 
   As such, by providing the separate drying chamber  52 , the crystallinity of the organic semiconductor  154  can be improved, and a plurality of substrates can be simultaneously treated. 
   A method for manufacturing an organic thin film transistor array panel using inkjet printing system shown in  FIG. 1  to  FIG. 3  will be explained in detail with reference to  FIG. 4  to  FIG. 15 . 
     FIG. 4  is a layout view showing the first step of the method for manufacturing an organic thin film transistor array panel according to an exemplary embodiment of the present invention;  FIG. 5  is a cross-sectional view of the organic thin film transistor array panel taken along the line V-V of  FIG. 4 ;  FIG. 6  is a layout view showing a subsequent step to that shown in  FIG. 4 ; and  FIG. 7  is a cross-sectional view of the organic thin film transistor array panel taken along the line VII-VII of  FIG. 6 .  FIG. 8  is a layout view showing a subsequent step to that shown in  FIG. 6 ,  FIG. 9  is a cross-sectional view of the organic thin film transistor array panel taken along the line IX-IX of  FIG. 8 ,  FIG. 10  is a layout view showing a subsequent step to that shown in  FIG. 8 , and  FIG. 11  is a cross-sectional view of the organic thin film transistor array panel taken along the line XI-XI of  FIG. 10 .  FIG. 12  is a layout view showing a subsequent step to that shown in  FIG. 10 ,  FIG. 13  is a cross-sectional view of the organic thin film transistor array panel taken along the line XIII-XIII of  FIG. 12 ,  FIG. 14  is a layout view showing a subsequent step to that shown in  FIG. 12 , and  FIG. 15  is a cross-sectional view of the organic thin film transistor array panel taken along the line XV-XV of  FIG. 14 . 
   First, by depositing a metal layer on substrate  110  by, for example, a sputtering method, and etching the same by photolithography, data lines  171 , each including a plurality of protrusions  173  and an end portion  179 , and storage electrode lines  131 , each including a plurality of storage electrodes  137 , as shown in  FIG. 4  and  FIG. 5  are formed. 
   Subsequently, a lower interlayer insulating layer  160  having contact holes  163  and  162  is formed by performing a chemical vapor deposition (CVD) with an inorganic material or a spin coating with an organic material. The contact holes  163  and  162  can be formed by photolithography using a photosensitive film in the case of an inorganic material or only by lithography in the case of an organic material. 
   Referring to  FIG. 6  and  FIG. 7 , by depositing a metal layer on the lower interlayer insulating layer  160 , and etching the same by photolithography, gate lines  121 , each including a plurality of gate electrodes  124  and an end portion  129 , and storage capacitor conductors  127  are formed. 
   Subsequently, referring to  FIG. 8  and  FIG. 9 , by performing a spin coating with, for example, a photosensitive organic material, and patterning the same, an upper interlayer insulating layer  140 , having upper side walls of an opening  144  and contact holes  141 ,  143 , and  147 , is formed. At this time, the end portions  179  of data lines  171  are formed such that all the organic material is removed. 
   Subsequently, a gate insulator  146  is formed in opening  144  of the upper interlayer insulating layer  140  by, for example, an inkjet printing method. To form gate insulator  146  by inkjet printing, a solution is deposited in opening  144  and is then dried. However, the present invention is not limited to this, and gate insulator  146  can be formed by various solution processes, such as a spin coating and a slit coating. 
   Referring to  FIG. 10  and  FIG. 11 , by sputtering, for example, an amorphous ITO and then performing photolithography, pixel electrodes  191  including a drain electrode  195 , source electrodes  193 , and contact assistants  81  and  82  are formed. It is preferable that a temperature be a low temperature of 25° C. to 130° C., such as room temperature, and it is preferable that the amorphous ITO be etched using a weak basic etchant. By forming the ITO at a low temperature and etching the same with a weak basic etchant, gate insulator  146  and upper interlayer insulating layer  140 , which are made of an organic material, can be prevented from being damaged by heat or the chemical solution. 
   Subsequently, as shown in  FIG. 12  and  FIG. 13 , by depositing a photosensitive organic layer and developing the same, a bank  180  having an opening  184  is formed. Then, ink  5  from nozzle  410  of inkjet head  400  is deposited in opening  184  to form organic semiconductor  154 . 
   Subsequently, substrate  110  is transferred to the drying chamber  52  where vapor pressure regulating device  800  controls the drying speed of the ink solvent&#39;s vapor pressure so as to improve the crystallinity of the deposited organic semiconductor  154 . 
   Subsequently, as shown in  FIG. 14  and  FIG. 15 , a light blocking member  186  is formed on the organic semiconductor  154  thereby completing the organic thin film transistor array panel. 
   An organic thin film transistor array panel manufactured by the manufacturing method of an organic thin film transistor array panel according to the above exemplary embodiment of the present invention will be explained in detail hereinafter. 
   A plurality of data lines  171  and a plurality of storage electrode lines  131  are formed on an insulation substrate  110  that is made of transparent glass, silicone, plastic, etc. 
   Data lines  171  transmit data signals and generally extend in a vertical direction. Each of data lines  171  includes a plurality of protrusions  173  protruding laterally and an end portion  179  that is enlarged to have a wide area for connection with another layer or an external driving circuit. A data driving circuit (not shown) generating data signals may be mounted on a flexible printed circuit film (not shown) attached on substrate  110 , directly mounted on substrate  110 , or integrated with substrate  110 . In the case that data driving circuit is integrated with substrate  110 , data lines  171  may extend so as to be directly connected to the same. 
   Storage electrode lines  131  receive a predetermined voltage and extend substantially in parallel with data lines  171 . Each of storage electrode lines  131  is disposed between two data lines  171  and is closer to the left one of the two data lines  171 . Storage electrode lines  131  include storage electrodes  137  extending laterally. However, the shape and the disposition of storage electrode lines  131  can be variously modified. 
   Data lines  171  and storage electrode lines  131  may be made of an aluminum-based metal such as aluminum Al or an aluminum alloy, a silver-based metal such as silver Ag or a silver alloy, a gold-based metal such as gold Au or a gold alloy, a copper-based metal such as copper Cu or a copper alloy, a molybdenum-based metal such as molybdenum Mo or a molybdenum alloy, chromium Cr, tantalum Ta, titanium Ti, etc. They can have a multilayer structure including two conductive layers (not shown) having different physical properties. One of the conductive layers is made of a metal with a low resistivity, for example, an aluminum-based metal, a silver-based metal, and a copper-based metal, so as to decrease a signal delay or a voltage drop. In contrast, the other conductive layer is made of a material having an excellent adhesive property to substrate or a material having excellent physical, chemical, and electrical contact characteristics with other materials, particularly with indium tin oxide (ITO) and indium zinc oxide (IZO), for example, a molybdenum-based metal, chromium, titanium, and tantalum. As examples of such a combination, there may be a chromium lower layer and an aluminum (alloy) upper layer or an aluminum (alloy) lower layer and a molybdenum (alloy) upper layer. However, data lines  171  and storage electrode lines  131  can be made of various metals or conductors. 
   It is preferable that the sides of data lines  171  and storage electrode lines  131  are slanted by 30 to 80 degrees with respect to the surface of substrate  110 . 
   On data lines  171  and storage electrode lines  131 , a lower interlayer insulating layer  160  is formed. The lower interlayer insulating layer  160  can be made of an inorganic insulator or an organic insulator. As an example of the inorganic insulator, silicon nitride SiNx or silicon oxide SiO2 may be used. A thickness of the lower interlayer insulating layer  160  may be about 2,000 Å to 4 μm. 
   Lower interlayer insulating layer  160  may have a plurality of contact holes  163  and  162  respectively exposing the protrusions  173  and the end portions  179  of data lines  171 . 
   On the lower interlayer insulating layer  160 , a plurality of gate lines  121  and a plurality of storage capacitor conductors  127  are formed. 
   Gate lines  121  transmit a gate signal and generally extend in a horizontal direction so as to cross data lines  171  and storage electrode lines  131 . Each of gate lines  121  includes a plurality of gate electrodes  124  upwardly protruding and an end portion  129  that is enlarged so as to have a wide area for connection to another layer or an external driving circuit. A gate driving circuit (not shown) generating gate signals may be mounted on a flexible printed circuit film (not shown) attached to substrate  110 , directly mounted on substrate  110 , or integrated with substrate  110 . In the case that gate driving circuit is integrated with substrate  110 , gate lines  121  may extend to be directly connected to gate driving circuit. 
   Storage capacitor conductors  127  are separated from gate lines  121  and overlap storage electrodes  137 . 
   Gate lines  121  and storage capacitor conductors  127  can be made of the same material as data lines  171  and storage electrode lines  131 . The sides of gate lines  121  and storage capacitor conductors  127  are slanted with respect to the surface of substrate  110 , and the slanted angle may preferably be between about 30° to about 80°. 
   An upper interlayer insulating layer  140  is formed on gate lines  121  and storage capacitor conductors  127 . The upper interlayer insulating layer  140  is made of an organic material or an inorganic material having a relatively low dielectric constant of about 2.5 to 4.0. As examples of an organic material, a polyacryl-based compound, a polystyrene-based compound, and a soluble high molecular compound such as benzocyclobutene (BCB) may be used, and as examples of an inorganic material, silicon nitride and silicon oxide may be used. A thickness of the upper interlayer insulating layer  140  may be about 5,000 Å to 4 μm. 
   By using an upper interlayer insulating layer  140  having a low dielectric constant, parasitic capacitance between data lines  171  and gate lines  121  and the upper conductive layer is reduced. 
   Upper interlayer insulating layer  140  is not present near end portions  179  of data lines  171 . The reason for this is not only to prevent too much separation between lower interlayer insulating layer  160  and interlayer insulating layer  140  formed on the end portions  179  of data lines  171 , but also to decrease the thickness of the interlayer insulating layer such that the end portions  179  of data lines  171  and the external circuit can be effectively connected to each other. 
   A plurality of openings  144  exposing gate electrodes  124 , a plurality of contact holes  141  exposing the end portions  129  of gate lines  121 , a plurality of contact holes  143  exposing the protrusions  173  of data lines  171 , and a plurality of contact holes  147  exposing storage capacitor conductors  127  are formed in the upper interlayer insulating layer  140 . 
   Gate insulators  146  are formed within openings  144  of upper interlayer insulating layer  140 . Gate insulators  146  cover gate electrodes  124 , and the thickness thereof is about 1,000 to 10,000 Å. The side walls of the openings  144  are higher than gate insulators  146  so that the upper interlayer insulating layer  140  serves as a bank, and the openings  144  have sufficient size as the surface of gate insulator  146  becomes planar. 
   Gate insulators  146  are made of an organic material or an inorganic material having a relatively high dielectric constant of about 3.5 to 10. As examples of an organic material, a soluble high molecular compound such as a polyimide-based compound, a polyvinyl alcohol-based compound, a polyfluorane-based compound, and parylene may be used, and as an example of an inorganic material, silicon oxide surface-treated by octadecyl trichloro silane (OTS) may be used. Particularly, it is preferable that the dielectric constant of gate insulators  146  is higher than that of the upper interlayer insulating layer  140 . 
   By disposing gate insulators  146  with a high dielectric constant, the threshold voltage of the organic thin film transistor can be decreased and the amount of ion current thereof can be increased, thereby enhancing the efficiency of the organic thin film transistor. 
   A plurality of source electrodes  193 , a plurality of pixel electrodes  191 , and a plurality of contact assistants  81  and  82  may be formed on the upper interlayer insulating layer  140  and gate insulator  146 . They can be made of a transparent conductive material such as IZO and ITO, and a thickness thereof may be about 300 Å to about 800 Å. 
   Source electrodes  193  are connected to data lines  171  through contact holes  143  and extend over gate electrodes  124 . 
   Pixel electrodes  191  includes portions  195  (hereinafter referred to as drain electrodes) facing the source electrodes  193  centering on gate electrodes  124 , and are connected to storage capacitor conductors  127  through the contact holes  147 . Respective facing sides of the drain electrodes  195  and the source electrodes  193  are parallel with each other and snake windingly. The pixel electrodes  191  overlap gate lines  121  and data lines  171  so as to enhance an aperture ratio. 
   Contact assistants  81  and  82  are respectively connected to the end portions  129  of gate lines  121  and the end portions of data lines  171  through the contact holes  141  and  162 . The contact assistants  81  and  82  complement the adhesive property of the end portions  129  of gate lines  121  and the end portions  179  of data lines  171  to an external device, and also protect these members. 
   A plurality of banks  180  are formed on the source electrodes  193 , the pixel electrodes  191 , and the upper interlayer insulating layer  140 . 
   A plurality of openings  184  are formed in the banks  180 . The openings  184  are positioned on gate electrodes  124  and the openings  144  of the upper interlayer insulating layer  140 , and expose portions of the source electrodes  193  and the drain electrodes  195  and gate insulators  146  therebetween. 
   Banks  180  are made of a photosensitive organic material having a thickness of about 5,000 Å to 4 μm to which a solution process may be applied. Openings  184  of banks  180  are smaller than the openings  144  of the upper interlayer insulating layer  140 . Accordingly, the banks  180  firmly fix gate insulators  146  formed below so that lifting of gate insulators  146  can be prevented and permeation of a chemical solution in the subsequent process can be reduced. 
   A plurality of organic semiconductor islands  154  are formed within openings  184  of banks  180 . Organic semiconductors  154  contact source electrodes  193  and drain electrodes  195  above gate electrodes  124 , and the height thereof is lower than that of the banks  180  so that the organic semiconductors  154  are completely surrounded by the banks  180 . Since the organic semiconductors  154  are completely surrounded by banks  180  so that sides thereof are not exposed, permeation of a chemical solution into the sides of the semiconductors  154  in the subsequent process can be prevented. 
   Organic semiconductors  154  may include a high molecular compound or a low molecular compound that is dissolved in an organic solvent, and can be formed by an inkjet printing method. 
   Organic semiconductors  154  may include a derivative having a substituent of tetracene or pentacene. Organic semiconductors  154  may include oligothiophene having four to eight thiophene connected to positions 2 or 5 of the thiophene ring. 
   Organic semiconductors  154  may include polythienylenevinylene, poly 3-hexylthiophene, polythiophene, phthalocyanine, metalized phthalocyanine, or halogenated derivatives thereof. The organic semiconductors  154  may include perylenetetracarboxylic dianhydride (PTCDA), naphthalenetetracarboxylic dianhydride (NTCDA), or imide derivatives thereof. The organic semiconductor  154  may include perylene or coronene and derivatives including substituents thereof. The thickness of organic semiconductor may be about 300 Å to 3,000 Å. 
   One gate electrode  124 , one source electrode  193 , and one drain electrode  195  form one thin film transistor (TFT) Q together with one organic semiconductor  154 . The channel of thin film transistor Q is formed on the organic semiconductor  154  between source electrode  193  and drain electrode  195 . 
   The thin film transistors Q apply a data voltage to pixel electrodes  191  so as to generate an electric field together with the common voltage applied to the common electrode (not shown) of the display panel (not shown), thereby determining the the direction of liquid crystal molecules of the liquid crystal layer (not shown) between the two electrodes. Pixel electrodes  191  and the common electrode form a capacitor (hereinafter referred to as a liquid crystal capacitor) thereby maintaining the applied voltage after the thin film transistor is turned off. 
   Light blocking members  186  are formed on the organic semiconductors  154 . The blocking members  186  are made of a fluorine-based hydrocarbon compound, a polyvinyl alcohol-based compound, etc., and protect the organic semiconductors  154  from outer heat, plasma, and chemical substances. 
   A passivation layer (not shown) for enhancing the protection of the organic semiconductors  154  may be formed on the blocking members  186 . 
   In inkjet printing system and the manufacturing method of an organic thin film transistor array panel according to an embodiment of the present invention, the drying chamber is separately provided, and the vapor pressure of a solvent within the drying chamber is regulated so that the drying speed of the ink is regulated so as to improve the crystallinity of the organic semiconductor. 
   While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements that will be apparent to those skilled in the art without, however, departing from the spirit and scope of the invention.