Abstract:
Electrostatic printing methods are used to allow the precise placement of small, discrete components on a substrate. The components are configured as liquid toners by coating one or more surfaces with a charge control agent which reacts with a charge director in a diluent to create a charge on the coated components allowing them to be manipulated and placed using electrical fields.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority of PCT application Ser. No. PCT/US02/06279 filed on Mar. 4, 2002 and U.S. Provisional Patent Application Ser. No. 60/272,595 filed on Mar. 2, 2001, the entire contents of both of which are hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention describes a manufacturing process for producing large physical area arrays of discrete components, more particularly to electro-photographic methods which can produce arrays of small components such as transistor circuits useful, for example, for flat panel displays. 
   2. Description of the Related Art 
   The most common flat panel display is the Active Matrix Liquid Crystal Display, typically used in lap top computers. In an active matrix each “pixel” location has a functioning transistor that switches “On” and “Off” the voltage to that pixel. The transistor modulates the voltage to achieve levels of brightness for continuous “tones” in the picture. The manufacture of the active array of transistors is a time-consuming, expensive task. Beginning with an X-Y grid of metallized conductors and transparent electrodes, the transistors are formed in a layered pattern using sputtering and photolithography. The lithographic steps consist of:
         A). spin coating an etch resist   B). soft bake to harden resist   C). photo expose   D). develop: i.e., wash away unexposed resist   E). cure remaining resist by baking   F). etch material underneath   G). strip away the resist.
 
Between steps B and C the substrate must be cooled back to room temperature otherwise thermal expansion will result in mechanical alignment errors.
       

   The formation of the simple transistor, usually a Field Effect Transistor (FET) requires the following layers, each of which is photolithographically patterned:
         1. Amorphous silicon layer   2. Silicon nitride dielectric layer   3. Adhesion layer   4. Metallization layer of Aluminum,
 
Two of the layers (1 and 3) are formed in an expensive vacuum sputtering chamber, while layer 4 is formed in a somewhat less expensive vacuum thermal evaporation process. Each of the layers is patterned by the 7 step photolithographic process previously described for a total of 35 steps.
       

   There is a desire for flat panel displays to be very large, perhaps one meter in diagonal. The above process is costly for very small panels, to scale it to “meter” dimensions has proved difficult despite $10&#39;s of billions and two decades of research worldwide. 
   Part of the difficulty of the above process is that the quality of the amorphous silicon is marginal, at best. It has very low “minority carrier mobility” (carrier velocity per unit of electric field) compared to the single crystal silicon (from wafers) that is used to make all other integrated circuits. To this end, an improvement could be effected if the amorphous silicon could be crystallized resulting in substantial improvements in performance. If the amorphous silicon can be crystallized and transformed to “Poly Si”, then other circuits can be incorporated into the display beyond the transistor at each “pixel” element (e.g., the “Housekeeping” circuits like row and column drivers, data shift registers, etc.). This would also substantially reduce the interconnects required and lower the cost. 
   Substantial research has been executed attempting to produce such “poly” silicon. High powered lasers “Zap” the amorphous silicon, raising its temperature thereby crystallizing it. Solid phase crystallization through the agent of a catalyst like palladium or other metal with heating to 600° C. for 10 minutes has been disclosed (see Fonash, U.S. Pat. No. 5,275,851). But none of the “poly Si” processed have been successfully commercialized. 
   Many people have proposed mounting “real silicon” die, cut from a silicon wafer. These parts may be as small as 100×100 microns in dimension. They would be very inexpensive themselves; however, the problem would be in applying them at each fixed location. In a very high resolution display there may be as many as 3 million pixels, each of 3 colors, for a total of 9 million “pixels” meaning 9 million transistors that need to be properly and precisely located. 
   One approach to this problem is that of Alien Technology of Hayward, Calif. Alien describes die that are fabricated with beveled rear faces and are “fluidically” located in a preformed back plate, to “micron tolerances”. This back plate is part of the final product and is shipped “out the door” to the customer. Furthermore there are no “imaging forces” that move the “die” into the preformed cavities. The only force available is that of gravity. 
   There remains the need to assemble discrete active silicon parts; cheaply and massively that this invention is presented. 
   The basic technology of “electrophotography” or images made by electrical forces is approximately 50 years old and generates approximately $100 billion in revenue, worldwide. Its legacy includes the copy machine and the laser printer. In virtually all manifestations the machines put a colored resin toner on a sheet of paper for a human being to read. In recent years the Electrox Corp. of Denville, N.J. has pioneered the use of the technology as a manufacturing tool; printing functional materials configured as liquid toners, on metal, glass and even paper surfaces. Toners include Metals(Ag, Al); Catalysts (Pb, Sn); Dielectrics (Glass, Phosphors); and Resins (Etch resists, Color Filter Materials, Organic LED Materials). 
   SUMMARY OF THE INVENTION 
   The invention extends electrostatic printing technology to precisely place a plurality of components on a substrate. This method allows the placement of components ranging in size from sub-micron up to 0.1 inches in size. The components are coated on one or more surfaces with a charge control agent and are then dispersed in an aliphatic hydrocarbon diluent, already containing a suitable charge director material. The interaction of the charge control agent, bonded to the components and the charge director in the diluent, produce acid/base reactions that impart an ionic charge to the component/toner particle and produces oppositely charged inverse “micelles” in the diluent This yields a liquid electrophoretic toner. The components now possess a stable electric charge and can be moved, imaged and transferred from one surface to another using electric fields. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of a silicon “die” toner particle with a net negative charge surrounded by positively charged inverse micelles in a diluent liquid. 
       FIG. 2  is an illustration of a master template electrostatic printing plate onto which silicon die toner particles are being attracted. 
       FIG. 3  is an illustration of the production of an RFID device. 
       FIG. 4  is an illustration of the silicon die toner particles being transferred from the master template to the receiving plate or flexible film, and the end product. 
       FIG. 5  is an illustration of a configuration in which the toner die are imaged directly to a final substrate. 
       FIG. 6  is an illustration of an image mask as part of the final substrate. 
   

   DESCRIPTION OF THE INVENTION 
   The invention extends electrostatic “toner technology” to the printing of physical components on a variety of final substrates. The components are configured as liquid toner materials which enables their use in electrostatic printing processes. This method allows the precise placement of components ranging in size from sub-micron up to 0.1 inches in size. 
   In one embodiment,  FIG. 1 , the components are “active matrix” transistors which are small silicon die on the order of 100 micron by 100 micron. These silicon die are temporarily coated with a charge control agent and then dispersed in an aliphatic hydrocarbon diluent, already containing a suitable charge director material. The interaction of the charge control agent, bonded to silicon die and the charge director in the diluent, produce acid/base reactions that impart an ionic charge to the toner particle and forming oppositely charged inverse “micelles” in the diluent. This yields a liquid electrophoretic toner. The silicon die now possess a stable electric charge and can be moved, imaged and transferred by electric fields from one surface to another. Silicon die  40  shown in  FIG. 1  are shaped like pyramids with the top chopped off. They are coated with a thin layer of charge control agent, which interact with the charge director material to form inverse micelles  44 , positive hydrogen ions in the hydrocarbon diluent and hydroxyl ions  42  (negative charged) attached to the charge control agent. The toner particle (the “die”) now has a negative charge. As such it can be “imaged” by electric fields and transferred from one surface to another with electric fields. 
   The charge control agent is a material that coats the silicon die toner particle and forms acid/base reactions with the charge director material dissolved in the Isopar diluent. These charge control materials can be photo-lithographically applied to the front or back side of the die (the non-active region) or even an overall coating. Since some coatings can be as thin as molecular mono-layers, and selective (on the passivation layer of the die, but not on the input/output metal contacts); they can remain in place and never need to be removed. These materials can include photo-imagable materials like: epoxy cationic, photosensitized polyimide and acrylic free radical, etc. or the broad overall coating agents like organo-silanes and organo-titinate or aluminate coupling agents. Examples of suitable charge control agents. include, but are not limited to, Cyclotene® (Dow Chem. Midland, Mich.), AZ9260 (Clariant Chemical), SU8 (MCM Corp., Newtown, Mass.) and Dynamask® #5030 Solder Mask (Dynachem, Calif.). 
   Compounds which can be used as charge directors are polar or non-polar and are compatible with the diluent used and provide an acid/base moiety which can react with the complimentary acid/base moiety on the charge control agent. Examples of such compounds include, but are not limited to: basic barium petronate (available from e.g., Cromptom Corp, Connecticut), Zirconium Hexchem (commercially available from (NSG America, Cleveland) and #1276 (from Hunt Imaging of Varin, Ohio). 
   Suitable diluents/solvents include, but are not limited to, Norpar® (Exxon), Isopar® (Exxon), silicon oils and ShellSol® (Shell Oil). 
   In  FIG. 2 , image plate  50  has inverse pyramidal cavities molded or ground in to it at the exact locations, for precise component placement Plate  50  is coated on top and bottom with electrically conducting layers  54  and  52  respectively. A voltage (of the opposite polarity of the charge on the charged components)  56  is applied between  54  and  52 . Electric fields from this plate draw toner particles  58  into their proper locations in the image plate  50 . In a preferred embodiment, diluent  60  is lightly pumped from below to push away components not properly oriented into the cavities of plate  50 . Properly oriented components are strongly pulled into the cavities in the plate  50  and are securely held there. 
     FIG. 3  shows another embodiment of the invention where it is used to manufacture inexpensive radio frequency identification devices. Antenna substrate  160  consists of film or paper substrate  162  with an etched metal pattern  164 . It is desired to place the silicon die toner particles on the two electrodes of the antenna,  170 . Antenna  160  is placed on insulating plate  166  which contains print electrode  168 . A positive voltage  172  is applied to the antenna metal  164  (by means not shown). An appropriate negative voltage  174  is applied to the print electrode  168  in an insulating platen  176  (by means not shown). The fringing electric fields from these voltages cause a negatively charged silicon die to be attracted to the electrodes  168  only. 
     FIG. 4  illustrates the transfer step. Image plate  50  can be expensive and preferably is reused. It can be quite large, for example a meter by meter, quite thin less than 5 mm (preferably about 1 mm) and has perhaps one to two million cavities ground into it. The image plate is used as an assembly fixture to transfer the components to a receiving surface, such as glass or polymeric film. 
   In  FIG. 4  plate  50 , imaged with a complete array of silicon die  40 , is spaced away from receiving surface  80  by a mechanical gap  86 . Means for this spacing are not shown but are known in the art. Mechanical gap  86  is filled with diluent fluid  88  to allow the toner die to be transferred using methods known in the art, e.g., in the disclosure of Bujese (U.S. Pat. No. 4,786,576). Imaging voltage  56  is removed and the transfer voltage  84  connection to back electrode  82  is activated. “Toner” die  40  now precisely transfer across gap  86  to receiving surface  80  by action of the electric field produced by voltage  84  connected to plate  82 . 
   The receiving surface  80  and image plate  50  are now mechanically separated with voltage  84  retained to hold the toner die  40  in place. 
   During pre-transfer assembly of receiving surface  80  and image plate  50  voltage  84  might be adjusted to hold the toner die  40  rigidly in place. Similarly during separation of the plate, image voltage  56  might be adjusted to perform the same function of holding the transferred die in place on the receiving substrate. 
     FIG. 5  shows an alternate embodiment of the invention wherein the silicon die toner particles are placed on the receiving object in one step (without the transfer step) with the aid of an image defining or placement mask  90 . This placement mask is an etched metal mask that serves as a mechanical support much like that of a solder paste stencil used in the printed wiring board industry. It is preferably etched from the top only to produce the tapered entry ways into the holes. After etching stencil  90  is coated by photopolymer  92  (for example, either dry film or liquid photopolymer, or the like) which is photo-lithographically imaged in an identical pattern as the stencil  90 . Finally cured photopolymer  92  is coated with a thin metal layer  94  by sputtering, vacuum evaporation or any other means. 
   Now receiving sheet  96  is placed on chuck  100  with imaging mask ( 90 ,  92 ,  94 ) placed on top. Layer  94  is grounded and mask  90  is raised to a suitable positive potential, —Vo. The manifold structure is flooded with liquid toner diluent containing a dissolved charge director (both not shown) and also containing toner die  40  coated with charge control agent  98 , on the active side only of the silicon die (where the transistors, diodes, passive components and interconnects are located). The charge control agent  98  forms acid/base couples with charge director (not shown) imparting a charge to the die, in this case a positive charge. 
   After all holes are filled with die, voltage Vo is reduced to zero and holding voltage −Vh is applied while the stencil structure ( 90 - 92 - 94 ) is removed leaving all die in place. 
     FIG. 6  shows another alternate embodiment of the invention wherein the patterning or positioning mask of the invention remains with the final product. The substrate of the final product  140  contains surface circuit patterns, not shown, and an anti-static layer  141 . The circuitry on the substrate is preferably low voltage and relatively high current (microamperes) and is compatible with anti-static layers which are of the order of 10+8 ohms per square and conducts currents of the order of pico amperes during the charging step. 
   Substrate  140  with anti-stat layer  142  is coated with photopolymer  144  which is photo-lithographically patterned as shown, yielding cavities in the desired locations. Photopolymer  144  is now charged by a corona unit (not shown) resulting in a latent image of charges  148  as illustrated in  FIG. 6 . 
   Liquid toner containing die  146  floods the plate and depositing die  146  in the cavities indicated. These die are subsequently fixed in place by a method known in the art, such as by a heat of pressure bonding step of ordinary means. 
   The following examples will further describe the invention. These examples are intended only to be illustrative. Other variations and modifications may be made in form and detail described herein without departing from or limiting the scope of the invention which is determined by the attached claims. 
   EXAMPLE I 
   A silicon wafer is spin coated with an approximate 5 micron thick layer of DuPont Pyralin™ photo imageable polyimide resin. The Pyralin is processed by normal means of prebake, exposure, image development and post bake cure. The cured Pyralin serves as a tough moisture/sodium barrier thereby protecting the silicon from future degradation. 
   The wafer is singulated into individual die that are dispersed in a diluent (Isopar™ G solvent, Exxon). 4 gm of silicon die are dispersed in 50 ml of diluent. A charge director material, Indigo Imaging agent (Indigo of America, Inc., Woburn, Mass.) is added to the mix. 0.1 ml charge director per 50 ml of diluent/die mix. The mix is now gently agitated for 30 minutes with a magnetic stir bar. The toner mix is allowed to stabilize for 12 hrs. after stirring. 
   During subsequent imaging tests the silicon die toner particles are found to possess a substantial negative “zeta” potential charge. 
   While the invention has been described with reference to the preferred embodiments thereof, it will be appreciated that various modifications can be made to the parts and methods that comprise the invention, without departing from the spirit and scope thereof.