Abstract:
A method and apparatus for laminating a TFT panel with a glass support plate without the need to touch an active area of the TFT panel. To accomplish this result, a touchless vacuum lamination chuck secures the TFT panel by its outer margins that do not carry TFTs. To facilitate lamination of the TFT panel, a pressure chamber is formed within the laminating chuck to provide support to the center region of the TFT panel as it is brought into contact with the glass support plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of commonly owned, co-pending provisional application Ser. No. 60/525,481, filed Nov. 25, 2003 and entitled “TOUCHLESS TFT PANEL LAMINATION FIXTURE AND PROCESS”, the entire contents of which are incorporated by reference herein. 
    
    
     FIELD 
     This patent specification relates to laminating a TFT panel with a glass support plate. More particularly, this patent specification pertains to a method and apparatus for laminating a TFT panel with a glass support plate while refraining from making contact with any portion of the TFT panel&#39;s front face other than on peripheral strips that are free of TFTs. 
     BACKGROUND 
     Thin film transistor (TFT) panels are frequently fabricated on very thin substrates such as 0.7 mm thick glass panels. The back faces of the TFT panels, i.e. the sides opposite the front faces carrying the TFTs, are subsequently laminated onto glass support plates, commonly using a UV-curable resin, so that subsequent process steps can be performed without damaging or distorting the fragile TFT panels. This approach is well known in the art and is described in U.S. Pat. No. 5,827,757 issued Oct. 27, 1998 to Robinson, et al., which is hereby incorporated by reference. During this process, solid objects should not make contact with the front face of the TFT panel because burrs or dust particles found on solid objects can crush films on the TFT panel. 
     A previous approach for laminating a TFT panel onto a glass support plate involves holding the TFT panel using a vacuum chuck with a piece of lint-free paper forming a cushion between the chuck and the front face of the TFT panel to avoid having the metal chuck make direct contact with the front face of the TFT panel. Lint-free paper can be used as a cushion because it is both porous and cushioning. Panel laminated according to this approach are offered commercially in this country by the assignee hereof, and further information therein is available at its website, HOLOGIC.COM. 
     Artifacts can sometimes be found on images captured using TFT panels that have been manufactured using lint-free paper. These artifacts are a result of uneven curing of UV resin used in the TFT panels due to reflection of UV light from the lint-free free paper to the TFT panels. Lint-free paper can also leave small paper particle on the TFT panel that must subsequently be removed using a spin cleaning process. Spin cleaning can add cost and complexity to the TFT panel manufacturing process and can occasionally promote crazing of the TFT panel thereby lowering the manufacturing yield. The vacuum chuck used in this process does not shield the TFT panel from potentially harmful vapors that emanate from the UV-curable resin used to laminate the TFT panel to the glass support plate. 
     SUMMARY 
     An object of the disclosed system and method is to solve problems discussed above relating to laminating TFT panels onto glass support plates. 
     Specifically, an object is to provide a system and method for laminating TFT panels onto glass support plates without making contact with the front face of the TFT panel (the face carrying the TFTs) other than on peripheral strips that are free of TFTs. It is also an object to provide a system and method for laminating TFT arrays onto sturdy glass plates without the use of lint-free paper. It is also an object to reduce artifacts found on images captured using TFT panels relative to images captured from TFT panels manufactured using a previous approach involving lint-free paper. It is also an object to provide a system and method for laminating TFT panels onto glass support plates that may avoid the need for spin cleaning, shield the TFT panel from potentially harmful vapors that emanate from UV-curable resins used to laminate the TFT panel to the glass support plate, and reduce instances of crazing over TFT panels that have been laminated pursuant to the previous approach. 
     The disclosed system and method laminate TFT panels onto glass support plates without making contact with the front face of the TFT panel other than on peripheral strips that are free of TFTs, while reducing artifacts found on images captured using TFT panels, avoiding the need for spin cleaning, shielding the TFT panel from potentially harmful vapors that emanate from UV-curable resins, and reducing instances of crazing. 
     The disclosed system comprises a lamination chuck which contacts the TFT panel only on the peripheral strips. The TFT panel is oriented horizontally with the face to be laminated (the back face) facing up and the sensitive front face facing down. The TFT panel is supported by a cushion of gas, preferably dry nitrogen (N 2 ), to prevent the thin TFT panel from sagging under its own weight and to cause the TFT panel to bow up at the start of the lamination process. The gas is prevented from escaping by sealing the perimeter of the lamination chuck with sealing tape. The pressure within the cushion of gas is monitored by a manometer and regulated by a bleed valve, a gas inlet, and a pressure regulator. The peripheral strips are held to the chuck by vacuum channels that are built into the rim of the chuck. While the TFT panel is held firmly in place and bowed up, a UV-curable resin is applied to the TFT panel. The glass support plate is gradually brought into contact with the TFT panel as the pressure within the cushion of gas is reduced. The resin spreads out as the area of contact between the TFT panel and the glass support plate increases. After the TFT panel is flat against the glass support plate, the resin is cured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  and  1   b  illustrate a laminating chuck in accordance with a preferred embodiment. 
         FIG. 2  is a plan view of the laminating chuck shown in  FIG. 1  detailing the chuck&#39;s gas connections and instrumentation. 
         FIGS. 3   a  and  3   b  illustrate a sealing cover in accordance with a preferred embodiment. 
         FIGS. 4   a  and  4   b  illustrate another view of the sealing cover shown in  FIG. 4 . 
         FIG. 5  illustrates a TFT panel being laminated in accordance with a preferred embodiment. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1   a  and  1   b  illustrate a lamination chuck  10  machined out of a cast aluminum plate to maintain its flatness and dimensional stability. The top surface of the chuck  10  is machined into a 3 mm deep tray or pressure chamber cavity  11 , which is slightly larger in length and width than the active area of a TFT panel, thereby forming a ridge  12  on a second edge  17  of the chuck  10 , third edge  18  of the chuck  10 , and fourth edge  19  of the chuck  10 , leaving the first edge  16  of the chuck  10  without a ridge. No ridge is formed on the first edge  16  of the chuck  10  because the TFT panel  100  (see  FIG. 5 ) preferably has active elements extending all the way to the first edge of the panel (the chest wall edge). Sealing tape (not shown) is affixed to the ridge  12  to help maintain desired pressure. The side walls  13  of the chuck  10  are recessed to form a shoulder  14  on all four sides, onto which an adhesive tape drip skirt (not shown) is attached. The drip skirt is used to help catch excess UV-curable resin that escapes from the sides of the chuck  10  during lamination. Vacuum channels  15  are machined into all three sides of the ridge  12  so that the TFT panel can be held firmly in place by three of its edges. 
       FIG. 2  illustrates the lamination chuck  10  in plan view. An input port  20  leads to a recess  11  that serves as a pressure chamber during the lamination process. N 2  gas enters the input port  20  through a pressure hose  26  and pressure regulator  26   a.  Two bleed holes  21  allow excess gas to escape through a needle bleed valve  22 , and two pressure sensing holes  23  allow the pressure inside the pressure chamber  11  to be monitored by a manometer  24 . A second pressure hose  27  connects the manometer  24  to the pressure sensing holes  23 . Suction is applied to the vacuum channels  15  by a vacuum connection hose  25  leading to a vacuum pump (not shown), which may be of the type used for vacuum chucks using lint-free paper. 
     Preferably, the holes  21 ,  23  are positioned as close to the ridge  12  as practical to avoid uneven reflection of UV radiation that is used for curing the UV-curable resin. Preferably, the pressure chamber  11  is sandblasted and the entire chuck  10  is anodized black to minimize reflection of the UV radiation that may cause exposure intensity variations. Preferably, all four sides of the chuck  10  are covered with a Teflon tape  101  ( FIG. 6 ) to avoid scratching the TFT panel, and in particular, bonding pads and pad routing lines on the TFT panel&#39;s peripheral strips. 
       FIGS. 3   a  and  3   b  illustrate a vacuum sealing cover  40  that is used to prevent the TFT panel  100  from sagging while on the chuck  10 . Two handles  41  are mounted on the top side of the sealing cover  40 . A sealing cover vacuum channel  42  is formed at the underside of the sealing cover  40 . This sealing cover vacuum channel  42  is connected to a vacuum port  43  positioned at the top side of the sealing cover  40 , and the vacuum port  43  is connected to a vacuum pump (not shown). Suction may then be applied through the sealing cover vacuum channel  42  to allow the sealing cover  40  to hold the TFT panel  100  straight and prevent it from sagging down. A bleeder valve port  44  located on the underside of the sealing cover  40  is connected to a bleeder valve knob  45  on the top side of the sealing cover  40  to regulate suction within the sealing cover vacuum channel  42 . Sealing tape  46  is affixed around the vacuum channel  42  and around the edges of the underside of the sealing cover  40  to help maintain the desired suction.  FIGS. 4   a  and  4   b  also illustrate the sealing cover  42  of  FIG. 3 . 
     The chuck  10 , the glass support plate and a UV-curable resin are preheated to 50° C. to reduce the viscosity of the resin. The chuck  10  is preferably heated with a resistance element (chuck heater)(not shown) and a thermocouple (not shown) is used to monitor the temperature of the chuck  10 . The glass support plate and the resin are preferably heated in a convection oven. 
     As illustrated in  FIG. 5 , the TFT panel  100  is positioned on the chuck  10  with the back face (the face to be laminated) up and the front face (the face carrying the TFTs) down. Locating pins (not shown) are used to precisely align the TFT panel  100  on the chuck  10 . The vacuum hoses  25  ( FIG. 2 ) are connected and the vacuum pump is turned on. The drip skirt (not shown) is applied to the second edge  17 , third edge  18 , and fourth edge  19  of the chuck  10  ( FIG. 1 ). The sealing cover  40  is placed on top of the TFT panel  100  with the vacuum channel  42  near the chest wall edge of the TFT panel  100 . Vacuum is applied to the channel  42  to straighten the TFT panel  100  so the drip skirt can be applied to the panel while the panel is straight. The pressure is regulated, preferably to a pressure of 1.2 inch water column (WC). The bleed valve  22  is partly opened to stabilize the pressure and to allow for pressure reduction later. At a pressure of 1.2 inch WC the TFT panel  100  bows up and out to form a dome that is approximately 1 mm high in the center. The resin is poured onto the glass support plate  102  or on the bottom face of the TFT panel  100 , preferably in a dog-bone shape or an oval shape puddle, with the long axis of the puddle parallel to the longer dimension of the glass support plate  102 . The glass support plate  102  is then guided by locating pins (not shown) onto the TFT panel  100  where it is let stand for a length of time, preferably five minutes. In this position, initially only the center of the TFT panel  100  makes contact with the glass support plate  102  causing the excess resin to be squeezed away from the center. Over another length of time, also preferably five minutes, the pressure in the pressure chamber  11  is reduced linearly from 1.2 inch WC to 0.3 inch WC. As the pressure is reduced, the dome flattens out and the excess resin is squeezed out into the drip skirt (not shown). At 0.3 inch WC, the TFT panel  100  is planar. This technique helps to minimize the occurrence of air-bubbles in the resin. Over a settling time, preferably five minutes, potential variations in resin thickness will smooth out. The resin is UV cured with light from a UV source (not shown), preferably for 240 seconds, a shorter length of time than that which is required for the previous method of manufacture.