Abstract:
A method of patterning soluble materials on a substrate is described. In the method, a stamp is applied to a liquid carrier solution. The raised areas of the stamp removes mainly a liquid carrier leaving behind a precipitate while the non-raised areas of the stamp lifts both the liquid carrier and the precipitate from the substrate. The result is a precipitate pattern residue that matches the raised area of the stamp. One use of the method is for patterning large areas of polymers used in large area electronics such as displays and sensors.

Description:
REFERENCE TO GOVERNMENT CONTRACT 
     This invention was made with Government support by NIST, under NIST contract number NIST 70NANB0H3033, and the Government has certain rights in this invention. 
    
    
     BACKGROUND 
     Large-area electronics based on polymeric semiconductors, for applications such as display systems, often require the deposition and patterning of solution processable polymeric materials over large areas. Various printing techniques have been used to achieve the deposition and patterning. However, each of these printing techniques suffers from a number of problems. 
     One polymeric material deposition method uses ink jet printing to deposit droplets of polymeric material. However, ink jet printing is a slow sequential process. Using multiple ink jet nozzles to print in parallel speeds up the process but also dramatically increases complexity and expense. 
     A second patterning method uses liquid embossing. A publication by Bulthaup et al., Applied Physics Letters 79 (10) 1525, (2001), describes depositing an “ink” on a substrate and patterning the ink in a liquid embossing process. In order to pattern the ink, a stamp displaces the “ink” and creates a reverse or negative image on the substrate relative to the pattern in the stamp. In addition, after removal of the stamp, the “ink” is still liquid and is cured before handling. The curing process reduces the robustness and throughput of the process. Furthermore, heating the substrate to cure the “ink” can also degrade the electrical properties of the embossed polymer. 
     S. Y. Chou in U.S. Pat. No. 5,772,905 describes using conventional embossing and nanoprint lithography to flow a thin film under a stamp to create a pattern. An anisotropic etching step, such as reactive ion etching (RIE), finishes the pattern definition. Conventional nanoprint lithography often involves exposing the patterned polymer to high temperatures, UV exposure and etching processes. These processes result in a harsh environment that potentially degrades the electrical properties of the polymeric semiconductor. 
     Still other techniques use a surface-energy pattern on a substrate to pattern a polymer. C. R. Kagan et al. in Appl. Phys. Lett. 79 (21) 3536 (2001) describes patterning self-assembled monolayers using such a surface-energy pattern. Such patterns are typically generated using surface energy modulation. However, use of such a system in electronic device fabrication is restricted to surfaces on which a self-assembled monolayer can be deposited (typically the noble metals such as gold or palladium). An additional coating step, typically accomplished through dip-coating the surface-energy pattern of the substrate over the entire substrate area is complex and slow, lowering throughput and yield. 
     Thus an improved method of patterning a polymer is needed. 
     BRIEF SUMMARY 
     A method of forming an using a stamping procedure to pattern a surface is described. In the method, a liquid carrier solution including a liquid carrier and a precipitate is deposited on a substrate. A relief pattern on a stamp is brought into contact with the liquid carrier solution such that raised portions of the relief pattern absorbs the liquid carrier leaving a thin precipitate layer between the raised portions of the relief pattern and the substrate. Both liquid carrier and precipitate are removed from substrate regions between the substrate and non-raised portions of the relief pattern. When, the stamp is removed, a precipitate pattern or residue pattern that matches the raised portions of the relief pattern on the stamp remains on the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1–4  shows the operations used in forming a relief pattern on a stamp. 
         FIG. 5  shows the reaction of water to a treated area of the stamp and the reaction of water to an untreated area of the stamp. 
         FIG. 6–8  shows the sequence of operations in using the stamp to form a polymer pattern on a substrate. 
         FIG. 9  shows the interactions that occur between stamp and polymer solution to produce the desired pattern. 
         FIGS. 10–13  show one sequence of operations to treat a stamp to adjust hydrophobicity across various stamp surfaces. 
         FIG. 14  shows the use of the stamped surface in a display device. 
     
    
    
     DETAILED DESCRIPTION 
     A novel stamping procedure to pattern a polymer is described. In the procedure a relief pattern that includes raised portions and non-raised portions is formed on a stamp. The relief pattern is brought into contact with a polymer. When the relief pattern is removed, the remaining polymer pattern matches the raised portions of the relief pattern. 
       FIGS. 1–4  show one method of forming a relief pattern on a stamp. In  FIG. 1 , a material used to form a stamp  104  is deposited over a relief master  108 . The stamp material should be a material that is capable of making a conformal contact between stamp  104  and a polymer to be stamped. In one embodiment, the stamp material is polydimethylsiloxane (PDMS). 
     A surface  112  of relief master  108  includes a negative of the relief pattern to be formed on stamp  104 . A number of well known techniques, including but not limited to, photolithography and wax printing patterning, may be used to form surface  112  of relief master  108 . In one example, height  116  of raised portions  120  on surface  112  of relief master  108  exceeds the width  124  of the raised portion resulting in a height to width ratio in excess of approximately 0.1. In one embodiment, non-raised or “recessed” portions  128  of stamps formed from such relief masters will have a width to depth ratio of less than approximately 10. Actual dimensions of the stamp and the ratios of width to depth may vary considerably as will be later discussed in connection with  FIG. 9 . 
       FIG. 2  shows PDMS stamp  104  after removal of relief master  108 . Typically, untreated stamp  104  is strongly hydrophobic. In order to reduce stamp hydrophobicity, the stamp is treated.  FIGS. 3 and 4  show one treatment that reduces stamp hyrdophobicity by exposing stamp  104  to an oxygen plasma followed by exposure to a reacting agent. 
       FIG. 3  shows applying an oxygen plasma  304  to the surface of stamp  104 . Oxygen plasma  304  oxidizes stamp  104  surface thereby preparing the surface for a reacting agent. In one embodiment, the oxygen prepares the stamp surface to allow covalent bonds to from between a reacting agent and the stamp surface. 
     After oxidation, stamp  104  is exposed to a reacting solution  404 . In one embodiment, a compound in reacting solution  104  forms covalent bonds to the stamp. An example of a compound that forms such covalent bonds are chlorosilane compounds. A hexadecane solution of benzyltrichlorosilane (BTS) is one example of a suitable chlorosilane compound. The reaction with reacting solution  104  reduces stamp hydrophobicity. 
       FIG. 5  shows the result of reduced hydrophobicity. A water droplet  504  placed on an untreated portion  508  of a test surface  510  of the stamp will have a contact angle  512  that is larger than the contact angle  516  of a second water droplet  520  deposited on a treated portion  524  of the test surface  510 . In one example, the difference in contact angle is exceeds 30 degrees. An example range of contact angles  512  before treatment is between 90 and 110 degrees. An example range of contact angles after treatment range is between 0 and 60 degrees. 
       FIG. 6–8  illustrates using a stamp to pattern a polymer deposited on a substrate. A typical polymer may be a polymer solution including a semiconductor material dissolved in a liquid carrier. As used herein a liquid carrier solution is broadly defined to be a liquid that includes a liquid carrier and a material (a precipitate) that remains as a solid when the liquid carrier is removed. In one example, the liquid is a solvent. The precipitate may be a material that is dissolved as a solute in the solvent, such as salt (serving as the precipitate material) in water (the solvent material). Alternately, the precipitate may be a particle in suspension in the liquid carrier, such as a colloidal semiconductor or a nonparticle suspended in a fluid. As used herein, precipitate is broadly defined to be the material carried by the liquid carrier, whether or not it is mixed, dissolved or otherwise combined with the liquid carrier or has been separated out as a solid. 
     For simplicity, the discussion that follows, the material that precipitates will be described as a polymer and the liquid carrier will be a solvent, although other materials may be used and the claims should not be limited to solvents and liquid polymers. 
     When forming an electronic device, the precipitate carried by the liquid carrier is a semiconductor material that often has electrical properties suitable for forming an electronic device. The solvent solution keeps the semiconductor in a liquid state. One example of a typical polymer is poly -9.9′, dioctyl-fluorene-cobithiophene (F8T2).  FIG. 6  shows a thin polymer solution  604 , typically having a depth between 1 micrometer and 100 micrometers, deposited on a substrate  608 . Possible methods for depositing polymer solution  604  include spin coating for a period of a few seconds at low speed, doctor-blading or dip-coating. 
     After polymer deposition,  FIG. 7  shows bringing stamp  104  in contact with polymer solution  604 . Contact is usually maintained for a few seconds, sufficient time for capillary action or another absorption mechanism to draw the solvent into the stamp. As will be described in  FIG. 9 , in one embodiment, the walls of the relief pattern on stamp  104  absorbs the solvent drawing semiconductor or polymer residue onto the walls of the relief pattern of stamp  104 . The stamp thus removes material directly beneath recessed portions of the stamp  104  relief pattern. 
       FIG. 8  shows a cross section of material remaining on substrate  608  after stamp  104  is removed. The dry semiconductor and/or polymer material  804  surrounds opening  808 . Opening  808  corresponds to the non-raised portions or recessed portions of stamp  104 . Thus dry polymer material  804  reproduces a pattern that corresponds to the raised features of stamp  104 . The stamping process is a process very similar to relief printing with openings in dry polymer material  804  corresponding to non-raised features of stamp  104 . 
     The remaining precipitate, or dry semiconductor and/or polymer material  804  may have distinctive characteristics. One example characteristic is a very uniform deposition of polymer material  804  across substrate  608 . In other deposition techniques that rely on evaporation, a “coffee ring” effect may occur in that uneven evaporation of solute causes uneven distribution of precipitate, more particularly, a very slight increase in precipitate height occurs toward the center of the deposited layer. By using a stamp that uniformly absorbs solvent across the surface of polymer material  804 , the described stamping technique can be adjusted to avoid such uneven effects. 
       FIG. 9  is a cross sectional view illustrating the interaction between stamp  904  and a polymer solution  908 . Capillary forces and solvent absorption in stamp  904  permits removal of material, including semiconductor material from regions underlying recessed areas of stamp  904 . 
     Stamp  904  is pressed into polymer solution  908  forming an airtight or conformal contact. A force, typically from a pressure differential, forces liquid in polymer solution  908  into recesses  912 ,  916 . The pressure differential may be caused by inducing a gas flow along channels coupling the recessed areas of the stamp to lower the pressure in the recessed areas of the stamp. Alternately, the reduced hydrophobic nature of the stamp surface causes capillary action that draws the liquid polymer solution into the recess such that a concave meniscus  918  forms in the recess. The edge of concave meniscus  918  wets the walls of the stamp recess. 
     Arrows  934  in  FIG. 9  illustrate absorption of the solvent along recess walls  920 ,  924 . The absorption leaves a thin polymer film on recess walls  920 ,  924 . The solvent absorption prevents the complete filling of recess  912 ,  916  with polymer solution and the creation of back-pressure. In one embodiment capillary action distributes solution to recess walls  920 ,  924 . When capillary action is used, the spacing  928  between adjacent recess walls should not exceed the distance by which capillary action may take place. For capillary action, an example maximum spacing between adjacent recess walls in a F8T2 structure is 250 micrometers. Other elements such as surface energies, viscosity of the fluid, and drying rates may also affect the maximum width of the recess. The ratio of recess depth  932  to width (spacing  290 ) should not fall below minimum values to prevent solution from completely filling the recess. A second factor in computing a minimum ratio is what ratio prevents solution drawn into the recess from exerting back-pressure. An example minimum ratio of recess depth  932  to recess spacing  290  might be 2. 
     Arrows  938  of  FIG. 9  shows absorption of the solvent in the raised portions  929 ,  930 ,  931  of stamp  904 . The absorption supersaturates the polymer solution  908  in regions between raised portions  929 ,  930 ,  931  and substrate  900 . The polymer precipitates out of the supersaturated solution and deposits on substrate  900 . 
     After removal of stamp  904 , the stamp recess walls, such as recesses  912 ,  916  are coated with a thin polymer film while the stamp raised portions  929 ,  930 ,  931  remain largely uncoated. In order to minimize distortion in the pattern from stamp swelling, stamp  904  is preferably significantly larger than the volume of solvent absorbed. 
       FIGS. 10–14  show the procedures used to create a stamp with variable hydrophobicity characteristics. In particular,  FIGS. 10–14  show a process that allows for fabrication of a stamp that has low surface energy (higher hydrophobicity) in raised regions and a higher surface energy (lower hydrophobicity) in the recessed regions. Such variable hydrophobicity stamps reduce the probability of polymer adhesion to raised portions of the stamp and increase the probability of polymer adhesion to recessed portions of the stamp. 
     In  FIG. 10 , a relief surface  1008  of stamp  1000  is exposed to an oxygen plasma  1004 . Oxygen plasma  1004  oxidizes relief surface  1008 . Oxidation facilitates covalent bond formation between relief surface  1008  and chlorosilane compounds. After oxidation, relief surface  1008  is placed in conformal contact with a flat surface  1104  as shown in  FIG. 11 . A silicon wafer may be used as a typical flat surface. A first chlorolsilane vapor, such as benzyltrichlorosilane (BTS) gas, introduced into chamber  1108  surrounds stamp  1000 . The first chlorosilane vapor wicks into relief surface channels contacting recessed regions  1112 . The walls of the recessed regions thus form covalent bonds with the first chlorosilane compound. Raised portions of the relief pattern in contact with flat surface  1104  avoid contact with the first chlorosilane vapor. 
     After flat surface  1104  is removed, the relief surface  1008  is exposed to a second chlorosilane compound  1204 , such as tridecafluoro-1,1,2,2,-tetrahydrooctyl trichlorosilane (FTS) as shown in  FIG. 12 . The second chlorosilane compound differs from the first cholosilane compound in that the second chlorosilane compound reduces the surface energy of the treated surface. Second chlorosilane compound deposition may occur either by vapor deposition or solution deposition. The entire relief surface  1008  may contact the second chlorosilane compound, however, only relief pattern areas unexposed to the first chlorosilane reacts with the second chlorosilane. More particularly, only raised regions  1208  of relief surface  1008  that have not already formed covalent bonds with the first chlorosilane compound reacts by forming covalent bonds with the second chlorosilane compound. 
       FIG. 13  shows resulting stamp  1000 . Stamp  1000  includes a recessed region bonded to the first chlorosilane compound. The high surface energy of the recessed region facilitates solution wicking into the channel. Stamp  1000  also includes raised regions  1208  bonded to the second chlorosilane compound. The low surface energy of the raised regions reduce the probability that patterned polymer film will adhere to raised regions  1208 . The varying surface energy shown in  FIG. 13  improves the output of the stamping system of  FIG. 9 . 
       FIG. 14  shows the using the polymer patterns formed by the stamping procedure in a display device. By coupling contacts  1410  and  1420  to polymer pixels  1404  formed by stamping, a pixel of the display device is formed. Each polymer pixel  1404  ranges in width from approximately 50 microns to 500 microns (dimensions). A similar structure also may be used to form a sensor array. 
     In the preceding description, a number of details have been provided. For example, polymer compounds and particular treatments of adjusting the stamp surface have been described. However, such details are included to assist the reader in understanding various ways in which the invention may be used and should not be interpreted to limit the scope of the invention. The invention itself should only be limited by the following claims.