Abstract:
A flexible liquid crystal display (LCD) substrate support structure and a method of supporting a flexible LCD substrate during fabrication have been provided. The method forms channels or trenches in-between a rigid support substrate and the flexible LCD substrate. A vacuum is created in the channels or trenches to pull adhesive in. The adhesive formed in this manner contains no air or water bubbles whose expansion in subsequent LCD fabrication processes can destroy the integrity of thin film transistor films formed on the flexible LCD substrate.

Description:
RELATED APPLICATIONS 
   This application is a Divisional Application of a patent application entitled, STRUCTURE AND METHOD FOR SUPPORTING A FLEXIBLE SUBSTRATE, invented by Hirohiko Nishiki, Ser. No. 09/929,708, filed Aug. 13, 2001 now U.S. Pat. No. 6,934,001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to liquid crystal display (LCD) fabrication and, more particularly, to a structure and method for supporting a flexible LCD substrate during manufacturing. 
   2. Description of the Related Art 
   Thin film transistor (TFT) LCD displays are widely used with notebook type personal computers (PCs) due to their light weight and thin silhouette. More recently, TFT LCDs have been adapted for with personal digital assistants (PDAs), cellular telephones, and handheld game machines. These new applications demand less costly displays, while desktop PC applications demand higher image quality. TFT LCD displays are needed with enough diversification to satisfy both these contradictory requirements. For instance, poly-Si (polycrystalline silicon) TFT LCDs are suitable for the monitor of a notebook PC, because of their high image quality. Amorphous Si TFT LCDs are more suitable for use as a flat panel TV monitor, due to their lower cost. For the mobile devices, a flexible TFT LCD would be best type of display, as it can be made lighter and thinner, and less susceptible to breakable. 
   A number of researchers are attempting to realize more practical TFT LCDs on a plastic substrate. However, there are a number of problems that prevent the realization of a practical plastic TFT LCD display. Due to its flexibility and thinness, it is hard to securely hold a plastic substrate during the fabrication process. To overcome this issue, glue, or an adhesive have been used to attach the plastic substrate to glass. This method is suitable to fabricate plastic TFT LCD displays using conventional TFT LCD factory equipment and processes designed for use with a glass substrate LCD. To some extent, this method has been able to suppress the expansion of plastic substrate in response to heat or water absorption. However, if any air or water remains between the plastic and glass substrate, the air expands in the vacuum process and subsequently deposited films are deposited on the plastic in its expanded state. After the substrate is returned to normal atmospheric pressure, the plastic contracts and the overlying film can become cracked. To improve the plastic substrate LCD process, all the air and water between the plastic substrate and the glass support member must be removed when the plastic is attached to the glass. 
   As noted in U.S. Pat. No. 6,214,460 (Bluem et al.), screen printing of adhesives is known in the art and is used advantageously to apply adhesives to selected areas on a substrate. The adhesive printed or coated areas can subsequently be used to adhere to a second substrate. Typical screen-printable adhesives are pressure-sensitive adhesives which are tacky at room temperature, or heat-activatable adhesives, which are not tacky at room temperature, but become tacky when heated. Examples of screen-printable adhesives include (meth)acrylic polymers and copolymers dispersed in an organic solvent or water. 
   Acrylic adhesives, both pressure-sensitive and heat-activatable types, are widely used in industry because they are stable over time, and they can be formulated to adhere to a wide variety of different surfaces. With the advent of more stringent environmental controls, the technology in adhesives in general has evolved from solvent-based materials to water-based materials, and to a degree, solvent-free materials. Solvent-free acrylate adhesives are known and fall in various categories of processing such as heat-activatable coating and radiation curing which includes E-beam curing, ultraviolet light processing, and gamma radiation processing. Solvent-free crosslinked compositions are known in the art, but they provide little utility for adhesively bonding to other substrates since they are highly crosslinked and do not flow or become tacky on heating. Ultraviolet light processed adhesives are also used. While known adhesives processed by ultraviolet light have their own utility and advantages, they do not screen print well because they tend to become stringy during screen printing. 
   As noted in U.S. Pat. No. 5,699,139 (Aastuen et al.), thermal or barometric variations can affect LCD performance. The LC material in the display must fill the region between the two substrates perfectly, and the variation in the spacing of the two layers must be tightly controlled. As the LCD heats up due to either absorption of light energy or by ambient conditions, the pressure within the cell begins to build. Alternatively, the internal pressure may change due to ambient barometric pressure variations which must also be accounted for. The thermal expansion of the LC material and the thermal expansion of the substrates enclosing this material may not match, creating an internal pressure increase with rising temperature. If the substrate material is plastic (and therefore somewhat flexible), the portions of the substrate between the separation spacers can bow, changing the separation between the substrates. If the pressure variation becomes too great, the bonding of the separation spacers or the edge sealing can be compromised, and the LC cell can delaminate. Conversely, if the temperature or barometric pressure is lowered, a partial vacuum can be created in this region, creating bubbles within the LC material that may interfere with the display of information or otherwise damage the display. If cracks are formed in the adhesive during fabrication of LCD, this pressure problem is further accentuated. 
     FIGS. 1   a  through  1   e  are partial cross-sectional views of a flexible LCD substrate  10  during stages of fabrication (prior art). A layer of adhesive  12  binds a support substrate  14  to a flexible substrate  16 . Reference designator  18  is an area in the adhesive that contains a bubble of water, air, or some other gas or liquid. 
   In  FIG. 1   b  a vacuum has been created as a result of some LCD fabrication process, and the bubble  18  has expanded. The flexible substrate  16  does not lie flat. 
   In  FIG. 1   c  an integrated circuit (IC) film  20  has been deposited overlying the flexible substrate  16 . The IC film  20  can be a base coat layer of silicon dioxide, for example, of a thin film of silicon. Since the underlying flexible substrate  16  is not flat, the IC film  20  does not lie flat of the support substrate  14 . 
   In  FIG. 1   d  the LCD substrate  10  is returned to normal atmosphere. There is air pressure acting on the bubble region  18 , that has expanded in the vacuum of the previous fabrication process. 
   In  FIG. 1   e  the IC film has cracked as a result of the air pressure acting on the bubble region. If the IC film  20  had been a base coat, for example, the cracks in the film will permit impurities from the support substrate to migrate into overlying areas, such as into the active regions of transistors. It should be understood that the cracks may form as a result of several vacuum or annealing process cycles. It should also be understood that cracks may likewise form in IC films several layers above (not shown) the support substrate  14 . 
   It would be advantageous if a flexible plastic LCD substrate could be held completely rigid during fabrication to improve the mechanical and electrical characteristics of the final product. 
   It would be advantageous if the adhesive used to hold a flexible LCD substrate during fabrication could be more evenly distributed across the glass substrate. 
   It would be advantageous if the integrated circuit (IC) films overlying the flexible LCD substrate could be formed with a more uniform thickness, without cracks or weak areas. It would likewise be advantageous if the flexible LCD substrate could be adhered to remain flat during the LCD fabrication procedures, to promote the formation of more uniformly thick overlying IC films. 
   SUMMARY OF THE INVENTION 
   The present invention prevents cracking in the IC films overlying a flexible LCD substrate during fabrication, by insuring that no air or water remains between the flexible and glass substrates. This is accomplished by injecting adhesive between plastic (flexible) and glass (rigid) substrates using a vacuum. 
   Accordingly, a method is provided for mounting a flexible substrate during the fabrication of an LCD. The method comprises: forming a rigid support substrate with trenches, typically of glass; forming a flexible substrate overlying the support substrate, typically of plastic or metal films; injecting adhesive into support substrate trenches; curing the adhesive to attach the flexible substrate to the support substrate; depositing a plurality of patterned integrated circuit films overlying the flexible substrate and forming an LCD; and, detaching the support substrate and adhesive from the flexible substrate LCD. 
   More specifically, the trenches are formed with at least one trench mouth, and the adhesive is injected into support substrate trenches in a vacuum environment. Injecting adhesive into the support substrate trenches further includes: creating a vacuum environment in the support substrate trenches; supplying adhesive to the at least one mouth of the support substrate trenches; and, in response to returning the support substrate to ambient pressure, pulling the adhesive into the support substrate trenches vacuum environment through the at least one mouth. 
   Additional details of the above-described method, an alternate method for mounting a flexible substrate, and structures to support a flexible substrate LCD during fabrication are presented below. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIGS. 1   a  through  1   e  are partial cross-sectional views of a flexible LCD substrate during stages of fabrication (prior art). 
       FIG. 2  is a partial cross-sectional view of the present invention structure to support a flexible substrate LCD during fabrication. 
       FIG. 3  is a partial cross-sectional view of the LCD support structure of  FIG. 2  with IC films, formed into TFTs, overlying the first flexible substrate. 
       FIG. 4  is a partial cross-sectional view of the LCD support structure of  FIG. 3  with an additional flexible substrate. 
       FIG. 5  is a partial cross-sectional view of the completed LCD, following the removal of the LCD temporary rigid support structures. 
       FIG. 6  is a partial cross-sectional view of an alternate structure to support a flexible substrate LCD during fabrication. 
       FIG. 7  is a partial cross-sectional view of the structure of  FIG. 6  with integrated circuit films overlying the first flexible substrate. 
       FIG. 8  is a partial cross-sectional view of the LCD support structure of  FIG. 7  with an additional flexible substrate. 
       FIGS. 9   a  and  9   b  are schematic block diagrams illustrating the present invention flexible substrate support structure in an environmental IC process chamber. 
       FIG. 10  is a flowchart illustrating the present invention method for mounting a flexible substrate during the fabrication of a LCD. 
       FIG. 11  is an alternate method for mounting a flexible substrate in the fabrication of a LCD. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  is a partial cross-sectional view of the present invention structure to support a flexible substrate LCD during fabrication. The structure  100  comprises a first rigid temporary support substrate  102  with trenches  104 . Typically, the rigid temporary support structure  102  is a glass material, but other materials such as plastic can be used. A first flexible substrate  106  overlies the first temporary support substructure. As is well known in the art, flexible LCD substrates are made from several types of plastic and metal films. Vacuum injected adhesive  108 , represented by the cross-hatched areas, in the first temporary support substrate trenches  104  attaches the first temporary rigid support substrate  102  to the first flexible substrate  106 . 
   In order to prevent incorporating any air or water between the flexible  106  and temporary support  102  substrates, the adhesive  108  is injected in a vacuum ambience. In the beginning of the process, many trenches  104  are made on the temporary support substrate  102 . Then, the flexible substrate  106  is attached. After that, the adhesive is injected in the trenches and cured. 
   After attaching the flexible substrate  106  on the temporary support substrate  102 , TFTs can be fabricated by conventional semiconductor manufacturing tools using optimum process conditions. Because there is no air or water bubbles between the flexible substrate  106  and the temporary support substrate  102 , no cracks will form on any overlying films deposited in vacuum. 
     FIG. 3  is a partial cross-sectional view of the LCD support structure  100  of  FIG. 2  with IC films, formed into TFTs, overlying the first flexible substrate. As shown, a TFT is formed having a gate busline  300 , a gate insulator  302 , an amorphous silicon layer  304 , highly doped silicon regions  306 , interlevel dielectric  308 , source busline  310 , and drain busline  312 . The TFT is presented only as an example. The IC films could be other active or passive electrical devices, or the IC films could be stacks of TFTs. The present invention is not limited to any particular arrangement of IC films or the formation of any particular LCD active device. 
   Overlying the IC films (the TFT as shown) is a liquid crystal (LC) layer  320 . The LC layer can be formed from a variety of materials and through a variety of processes, as are well known in the art. The present invention is not limited to any particular kind of LC layer. A color film (CF)  322  is shown overlying the LC layer. Again, the color film  322  is not critical to the invention, but shown as a typical film layer that would be used in the fabrication of a flexible substrate LCD. 
     FIG. 4  is a partial cross-sectional view of the LCD support structure  100  of  FIG. 3  with an additional flexible substrate. A second flexible substrate  400  overlies the color filter  322 . A second rigid temporary support substrate  402  with trenches  404  overlies the second flexible substrate  400 . Vacuum injected adhesive  406 , shown as cross-hatched, in the second temporary support substrate trenches  402  attaches the second temporary rigid support structure  402  to the second flexible support structure  400 . 
     FIG. 5  is a partial cross-sectional view of the completed LCD  500 , following the removal of the LCD temporary rigid support structures  102  and  402 . At the final stage of the process, the temporary support substrates must be removed from the flexible substrate. In conventional processes this can be difficult because the adhesive is relatively thin and is formed uniformly between the flexible and temporary support substrates. However, with the present invention structure, the adhesive exists only in the trench, so the solvent can easily spread through the trench and remove the adhesive quickly and completely. 
     FIG. 6  is a partial cross-sectional view of an alternate structure to support a flexible substrate LCD during fabrication. The structure  600  comprises a first rigid temporary support substrate  602 . As above, the first rigid temporary support substrate is typically glass or plastic. A first temporary pattern of spacers  604 , with spacer channels  606  between the spacers  604 , overlies the first temporary support structure  602 . The spacers  604  can be a plastic or glass material, but other materials are acceptable. A first flexible substrate  608 , typically a plastic or metal film, overlies the first temporary pattern of spacers  604  (and spacer channels). Vacuum injected adhesive  610  shown in the cross-hatched spacer channels  606  attaches the first temporary support substrate  602  to the first flexible substrate  608 . As above, the first temporary support substrate  602  is glass and the first flexible substrate  608  is a plastic or metal film. 
     FIG. 7  is a partial cross-sectional view of the structure of  FIG. 6  with integrated circuit films  700  and  702  overlying the first flexible substrate  608 . For simplicity, two unpatterned film layers are shown. However, these film layers, with additional layers could be part of a TFT or other active device (see  FIG. 3 ). A liquid crystal (LC) layer  704  overlies the TFTs, or whatever the IC films  700  and  702  form. A color filter  706  overlies the LC layer  704 . 
     FIG. 8  is a partial cross-sectional view of the LCD support structure  600  of  FIG. 7  with an additional flexible substrate. A second flexible substrate  800  overlies the color filter  706 . A second temporary pattern of spacers  802 , with spacer channels  804  between the spacers  802 , overlies the second flexible substrate  800 . A second rigid temporary support substrate  808  overlies the second temporary pattern of spacers  802  (and openings  806 ). Vacuum injected adhesive  810  in the cross-hatched spacer channels  806  attaches the second temporary support substrate  808  to the second flexible substrate  800 . 
   At the finish of the fabrication processes the rigid support substrates are removed and the resulting LCD structure resembles the LCD of  FIG. 5 , described above. The structure  600  depicted in  FIGS. 6–8  has the advantage that special glass substrates with trenching are not required. 
     FIGS. 9   a  and  9   b  are schematic block diagrams illustrating the present invention flexible substrate support structure in an environmental IC process chamber. As shown, the chamber  900  has the input port  902  blocked and a pump (not shown) is engaged at the exhaust port  904  to create a chamber vacuum. The flexible substrate support structure  100  of  FIG. 4  is shown in a cross-sectional (section A-A 1 , see  FIG. 4 ) top plan view to expose the trenches  104  and trench mouths  906 . The support structure  100  is positioned over tray  908  containing the adhesive  108 . A vacuum is created in the trenches  104 , as well as in the chamber  900  in general. 
   In  FIG. 9   b  the mouth  906  of each rigid support substrate trench  104  has been immersed in the adhesive  108 , while the chamber  900  is returned to higher pressure atmosphere, typically ambient (approximately 1 atmosphere). Since the vacuum, or negative atmosphere, exists in the trenches  104 , the adhesive  108  is pulled through the mouths  906  to completely fill the trenches  104 . The adhesive filled trenches are represented by the cross-hatched areas. This vacuum process does not permit the formation of air or water bubbles in the adhesive-filled trenches  104 . In the event that the trenches are not completely filled with adhesive, the adhesive at the trench mouths  906  at least prevent the trench spaces from being filled with a gas or a liquid that will later expand in fabrication processes. 
     FIG. 10  is a flowchart illustrating the present invention method for mounting a flexible substrate during the fabrication of a LCD. Although this method, and the method of  FIG. 11  below, is depicted as a sequence of numbered steps for clarity, no order should be inferred from the numbering unless explicitly stated. The method starts at Step  1000 . Step  1002  forms a first rigid support substrate, typically of glass or plastic, with trenches. Step  1004  forms a first flexible substrate overlying the first support substrate. In some aspects of the invention the substrates mentioned in Steps  1002  and  1004  are formed previously, and these steps merely involve the introduction of these pre-formed substrates. Step  1006  injects adhesive into the first rigid support substrate trenches. Step  1008  cures the adhesive to attach the first flexible substrate to the first support substrate. Step  1010  deposits a plurality of patterned integrated circuit films overlying the first flexible substrate, and forms thin film transistors (TFTs). Step  1012  forms a liquid crystal (LC) layer overlying the TFTs. Step  1014  forms a color filter layer over the LC layer. Step  1016  forms a second flexible substrate overlying the color filter. Step  1018  forms a second rigid support substrate with trenches overlying the second flexible substrate. Step  1020  injects adhesive into the second rigid support substrate trenches. Step  1022  cures the adhesive to attach the second flexible substrate to the second support substrate. Step  1024 , subsequent to the additional LCD fabrication processes of Steps  1012  and  1014 , detaches the first support substrate and adhesive from the first flexible substrate. Typically, the second support structure is detached at the same time. 
   Forming a first rigid support substrate with trenches in Step  1002  includes forming trenches with at least one trench mouth, the same applies to Step  1018 . Injecting adhesive into the first rigid support substrate trenches in Step  1006  (as well as Step  1020 ) includes injecting the adhesive in a vacuum environment. Step  1006 , of injecting adhesive into the first support substrate trenches includes substeps. Step  1006   a  creates a vacuum environment in the first rigid support substrate trenches. Step  1006   b  supplies adhesive to the at least one mouth of the first rigid support substrate trenches. Step  1006   c,  in response to returning the first rigid support substrate to ambient pressure, pulls the adhesive into the first rigid support substrate trenches vacuum environment through the at least one mouth. Returning the first rigid support substrate to ambient pressure in Step  1006   c  includes supplying an N 2  atmosphere at ambient pressure. 
   Forming the first flexible substrate overlying the first rigid support substrate in Step  1004  includes forming a flexible substrate from a material selected from the group including plastic and metal films. 
   Forming the first rigid support substrate with trenches in Step  1002  includes substeps (not shown). Step  1002   a  forms a rigid support substrate with a top surface. Step  1002   b  forms a photoresist pattern with openings exposing the underlying support substrate top surface. Step  1002   c  etches the exposed support substrate top surface to form the trenches in the support substrate. Step  1002   d  removes the photoresist. 
     FIG. 11  is an alternate method for mounting a flexible substrate in the fabrication of a LCD. The method starts at Step  1100 . Step  1102  forms a first rigid support substrate, typically of glass or plastic. Step  1104  distributes a first pattern of spacers, with spacer channels between the spacers, overlying the first support substrate. Step  1106  forms a first flexible substrate overlying the first pattern of spacers, typically of a plastic of metal film material. Typically, Steps  1102 ,  1104 , and  1106  involve the introduction of pre-formed substrates and spacers. Step  1108  injects adhesive into the spacer channels. Step  1110  cures the adhesive to attach the first flexible substrate to the first support substrate. 
   Step  1112  deposits a plurality of patterned integrated circuit films overlying the first flexible substrate, forming TFTs. Step  1114  forms a liquid crystal (LC) layer overlying the TFTs. Step  1116  forms a color filter layer over the LC layer. Step  1118  forms a second flexible substrate overlying the color filter. Step  1120  distributes a second pattern of spacers, with spacer channels between the spacers, overlying the second flexible substrate. Step  1122  forms a second rigid support substrate overlying the second pattern of spacers. Step  1124  injects adhesive into the spacer channels. Step  1126  cures the adhesive to attach the second flexible substrate to the second support substrate. Step  1128 , subsequent to additional LCD fabrication processes of Steps  1112  through  1116 , detaches the first support substrate, spacers, and adhesive from the first flexible substrate. Typically, the second rigid support structure is removed in the same step. 
   Distributing a pattern of spacers, with spacer channels between the spacers in Step  1104  includes forming spacer channels with at least one mouth. Injecting adhesive into the spacer channels in Step  1108  includes injecting the adhesive in a vacuum environment. Injecting adhesive into spacer channels in Step  1108  includes substeps. Step  1108   a  creates a vacuum environment in the spacer channels. Step  1108   b  supplies adhesive to the at least one spacer channel mouth. Step  1108   c  returns the first rigid support substrate to ambient pressure. Step  1108   d,  in response to returning the first rigid support substrate to ambient pressure, pulls the adhesive into the spacer channels vacuum environment through the at least one mouth. In some aspects, returning the first rigid support substrate to ambient pressure in Step  1108   d  includes supplying an N 2  atmosphere at ambient pressure. 
   A structure and method have been providing for supporting a flexible LCD substrate in the fabrication process. Examples have been provided for injecting an adhesive between the flexible substrate and the rigid support substrate, using a vacuum to prevent the formation of air or water bubbles. However, other methods of using a vacuum to aid in the injection of adhesive will occur to those skilled in the art. Further, although the invention specifically describes supporting an LCD flexible substrate, the invention is applicable to the support of any kind of flexible substrate.