Abstract:
Microcontact printing to pattern a self-assembled monolayer (SAM) of an alkanephosphonic acid on a film of indium zinc oxide (IZO). The SAM is robust enough to protect the undelying IZO from wet chemical etching, and thus defines a pattern of IZO on the substrate. In the microcontact printing process, a patterned, elastomeric stamp is inked with a solution of octadecylphosphonic acid and brought into conformal contact with the IZO surface. A SAM of alkanesulfonic acid forms where the stamp and the surface make contact; the rest remains underivatized. The stamp is then removed from the surface. Etching the sample in aqueous oxalic acid removes the unprotected areas, while the areas protected by the SAM remain in place.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to derivatization and patterning of surfaces, and more particularly to the formation of self-assembled molecular monolayers on metal oxide surfaces using microcontact printing and derivative articles produced thereby. 
     BACKGROUND 
     In the field of microelectronic devices, sensors, optical elements and electronic displays, the development of devices that are conveniently and relatively inexpensively produced with a relatively low failure rate is important. 
     A well-known method of production of such devices is photolithography. According to this technique, a negative or positive resist (photoresist) is coated onto the exposed surface of a substrate. The resist is then irradiated in a predetermined pattern, and irradiated (positive resist) or nonirradiated (negative resist) portions of the resist are washed from the surface to produce a predetermined pattern of resist on the surface. This is followed by one or more procedures. For example, the resist may serve as a mask in an etching process in which areas of the material not covered by resist are chemically removed, followed by removal of resist to expose a predetermined pattern of the conducting, insulating, or semiconducting material on the substrate. 
     According to another example, the patterned surface is exposed to a plating medium or to metal deposition (for example, under a vacuum), followed by removal of the resist, resulting in a predetermined plated pattern on the surface of the material. In addition to photolithography, x-ray and electron-beam lithography have found analogous use. 
     While the above-described irradiative lithographic methods may be advantageous in many circumstances, all require relatively sophisticated and expensive apparatus to reproduce a particular pattern on a plurality of substrates. Additionally, they generally consume more reactants and produce more by-products in collateral fabrication steps than is optimal. Further, they are relatively time-consuming. 
     A need exists in the art for a convenient, inexpensive, and reproducible method of etching a surface according to a predetermined pattern. The method would ideally result in patterns having features in the micron and submicron domain, and would provide for convenient reproduction of existing patterns. 
     The study of self-assembled monolayers (SAMs) is an area of significant scientific research. Such monolayers are typically formed of molecules each having a functional group that selectively attaches to a particular surface, the remainder of each molecule interacting with neighboring molecules in the monolayer to form a relatively ordered array. Such SAMs have been formed on a variety of substrates including metals, silicon dioxide, gallium arsenide, and others. SAMs have been applied to surfaces in predetermined patterns in a variety of ways including simple flooding of a surface along with more sophisticated methods such as irradiative patterning and microcontact printing. 
     An example of a SAM that has been extensively studied and has been the subject of several patents is composed of a molecular species having a long-chain alkyl group and a thiol (—SH) group at one terminus. These SAMs are formed on metallic surfaces like gold, silver, copper, etc. For example, U.S. Pat. No. 5,512,131 to Kumar et al. describes patterned transfer of alkyl thiols to metallic surfaces such as gold. Other molecular species capable of forming SAMs on oxide surfaces usually contain, at one terminus, trichloro or trialkoxy silane groups that form covalent bonds with hydroxylated surfaces of metal oxides. The drawback of using silane-containing compounds is that these materials are very reactive, forming crosslinked structures in solution or on the surface of a stamp used for microcontact printing. Trichlorosilanes must be handled under an inert atmosphere to prevent their decomposition. A general description of patterning is found in U.S. Pat. No. 5,900,160 to Whitesides, et al. relating to methods of etching articles via microcontact printing.. 
     Accordingly, a general purpose of the present invention is to solve problems associated with expense, complicated apparatus, and other complications associated with patterning indium zinc oxide (IZO) and indium tin oxide (ITO) surfaces for optical and electronic devices. 
     One object is to provide a method of conveniently and reproducibly producing a variety of SAM patterns on IZO and ITO surfaces, the pattern having resolution in the micron domain, and being amenable to etching. 
     Another general purpose of the invention is to provide electronic and optical elements and devices that are conveniently and inexpensively manufactured, and that are adaptable to a variety of systems. 
     SUMMARY 
     The method of patterning a metal oxide surface comprising the present invention, involves the steps of: 
     a) providing a solid substrate which may be a silicon wafer, glass, quartz, or polymeric materials such as polyimide, polyacrylate, and polyester coated with a thin metal oxide, preferably IZO and ITO, wherein the thickness of said metal oxide layer is from about 50 to about 1000 nm; 
     b) providing a stamp having a substantially continuous surface with at least one indentation, said indentation being formed by indenting means; 
     c) coating said stamp surface with a molecular species terminating in a functional group which is capable of reacting with said metal oxide surface to form a bond between said functional group and said metal oxide surface; 
     d) positioning said coated stamp on the surface of said metal oxide such that said functional group contacts and reacts with at least a portion of said metal oxide surface to form a bond therebetween; 
     e) removing said stamp from the surface of said metal oxide to obtain a self-assembled monolayer comprising the reaction product of said functional group of said molecular species and said metal oxide. 
     The non-contacted portion of said metal oxide surface which is not reacted with said molecular species, is removed by contact with an etchant. 
     The molecular species used in according with the present invention is an organic compound consisting of an alkyl group terminating with a phosphonic acid group and has the formula [CH 3 (CH 2 ) n PO 3 H 2 ], wherein in the alkyl portion, n=5-21. 
     Alternatively, the molecular species comprises an organic compound consisting of an alkyl group which is fully or partially fluorinated terminating with a phosphonic acid group and has the formula [CF 3 (CF 2 ) m (CH 2 ) n —PO 3 H 2 ], where n=0-20 and n+m=5-20. 
     The present invention provides a method of forming a patterned film of either IZO or ITO by microcontact printing and etching. The method involves contacting the surface of the IZO or ITO film with a stamp to transfer to it a self-assembled monolayer of a molecular species in a pattern. The self-assembled monolayer is contiguous with an exposed portion of the surface of the IZO or ITO film in a second pattern. The IZO or ITO film is removed from the underlying substrate, according to the second pattern, by contacting the exposed portion of the IZO or ITO film with an etchant that reacts chemically with the IZO or ITO and is inert with respect to the self-assembled monolayer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a-f  illustrate microcontact printing of a self-assembled molecular monolayer on the surface of a film of IZO or ITO on Si/SiO 2 . 
     FIGS. 2 a-b  illustrate IZO surfaces derivatized using the technique illustrated in FIGS. 1 a-f.    
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the drawings and descriptive matter which are illustrated and described in the preferred embodiments of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1 a-f , a method for forming a patterned film of IZO or ITO that uses microcontact printing is illustrated schematically. At FIG. 1 a , a stamp  20  is illustrated having a surface  21  including a plurality of indentations  22  formed therein that form an indentation pattern. The indentations are contiguous with a stamping surface  23  that defines a stamping pattern. 
     Prior to microprinting, stamping surface  23 , typically the entire surface  21  of the stamp, is coated with a molecular species  24 . Molecular species  24  terminates in a phosphonic acid functional group, selected to bind to the IZO or ITO surface and to form an ordered self-assembled monolayer thereupon. Referring to FIGS. 1 a-f , the phosphonic acid functional group at the terminus of species  24  is selected to bind to surface  25  of material  26  (IZO or ITO) provided on substrate  27 , and to form a self-assembled monolayer on surface  25 . 
     Referring to FIG. 1 b , stamp  20  is placed, in a predetermined orientation, adjacent to substrate  27  such that stamping surface  23  contacts first portions  28  of article surface  25 . Stamping surface  23  is held against portions of surface  25  of material  26  to hold molecular species  24  against the material surface to allow the phosphonic acid group to bind thereto. 
     In FIG. 1 c , stamp  20  has been removed from surface  25  to provide SAM  29  of the molecular species  24  on surface  25  according to the stamping pattern of stamping surface  23 . Second portions  30  of surface  25 , contiguous with first portions  28 , remain free of molecular species  24 . FIG. 1 d  is a scanning electron micrograph of a silicon article  27 , coated with a thin layer of material  26  (IZO), upon the surface of which species  24  has been microcontact printed in accordance with the method illustrated in FIGS. 1 a-c.    
     After removal of stamp  20 , an etchant is applied to the surface. Species  24  is undisturbed by the etchant, and therefore the etchant does not attack the area of surface  25  that is covered by species  24 . The etchant contacts surface  25  of material  26  on substrate  27  via gap  30 , and dissolves material  26  at gap  30 . The pattern of stamping surface  23  is thus transferred to the film of material  26 . The result is illustrated in FIG. 1 e . A scanning electron micrograph of material  26  (IZO) on silicon substrate  27  that has been patterned according to the embodiment illustrated is shown in FIG. 1 f.    
     Molecular species  24  may be coated onto stamping surface  23  by any convenient method, for example, molecular species  24  may be sprayed onto stamping surface  23 . Alternatively, a cotton swab or the like may be used to transfer molecular species  24  to stamping surface  23 , or molecular species  24  may be absorbed into a piece of paper, and stamping surface  23  may then be pressed against the piece of paper. 
     Generally, molecular species  24  is dissolved in a solvent for transfer to stamping surface  23 . The concentration of molecular species  24  in such a solvent for transfer should be selected to be low enough that a well-defined SAM may be transferred to surface  25  without blurring, and high enough that the SAM protects underlying surface  25  from the etchant. Typically, species  24  will be transferred to stamping surface  23  in a solvent at a concentration of less than 100 mM, preferably from about 0.5 to about 20.0 mM, and more preferably from about 1.0 to about 10.0 mM. Any organic solvent within which molecular species  24  dissolves is suitable. When molecular species  24  is transferred to stamping surface  23 , either near or in a solvent, the stamping surface should be dried before the stamping process is carried out. If stamping surface  23  is not dry when the SAM is stamped onto the material surface, that is, if gross liquid remains on the stamping surface, blurring of the SAM will result from dispersion of the liquid from under the stamping surface. Stamping surface  23  may be air dried, blow dried, or dried in any other convenient manner. The drying manner should simply be selected so as not to degrade the SAM-forming molecular species. 
     The film of material  26  is deposited, by any convenient method such as sputtering, on any convenient substrate, for example, silicon, silicon dioxide etc. The thickness of the film of material  26  is typically less than 500 nm, preferably from about 25 to about 100 nm, and more preferably from about 50 to about 75 nm. 
     According to another embodiment of the invention, SAM  29  is formed over the entirety of surface  25 . For example, SAM  29  may be formed by immersing surface  25  in a solution of molecular species  24 . Alternatively, a stamp in which stamp surface  21  does not include any indentations may be used in a microcontact printing process as in FIG.  1 . 
     The function and advantage of these and other embodiments of the present invention will be more fully understood from the examples below. The following examples are intended to illustrate the benefits of the present invention, but do not exemplify the full scope of the invention. 
     EXAMPLE 1 
     Fabrication of a Microcontact Printing Stamp 
     A template consisting of an exposed and developed photoresist pattern was fabricated by photolithography. A 2-mm-high border was formed by fixing strips of 2-mm-thick adhesive-backed foam around the perimeter of the surface of the template. A 10:1 (w:w or v:v) mixture of PDMS-Sylgard Silicone Elastomer 184 and Sylgard Curing Agent 184 (Dow Corning Corp., Midland, Mich.) was degassed under vacuum for about 10 minutes, then the mixture was poured over the template to approximately the height of the foam border. The PDMS cured at 65° C. for 60 minutes. After cooling to room temperature, the PDMS-stamp was carefully peeled from the template. 
     EXAMPLE 2 
     Patterning IZO Films by Microcontact Printing and Wet Etching 
     A stamp fabricated in accordance with Example 1 was made. The stamp was fabricated so as to have an array of pixels surrounded by indented regions. That is, the stamp had a pattern of indentations separating stamping surface pixels. The surface was coated with octadecylphosphonic acid in ethanol by applying a small volume of solution to cover the surface of the stamp. After allowing the solution to contact the surface of the stamp for 30 s, the stamp surface was dried under a stream of dry dinitrogen. The stamp was applied to an IZO surface and removed, whereby a pattern of alkanephosphonic acid is formed. The remaining unprotected IZO was removed from the surface of the silicon substrate using an aqueous oxalic acid solution (10% w:v), thus transferring the pattern of the stamp to the IZO film. An optical micrograph of the resultant pattern is shown in FIG. 2 a  and a scanning electron micrograph in FIG. 2 b.    
     EXAMPLE 3 
     Patterning ITO Films by Microcontact Printing and Wet Etching 
     A stamp fabricated in accordance with Example 1 was made. The procedure of Example 2 above was followed and a similar result obtained. The pattern of the stamp was transferred to the ITO film. An optical micrograph of the resultant pattern was similar to that shown in FIG. 2 a  and a scanning electron micrograph obtained was similar to that shown in FIG. 2 b.    
     Thus while there have been shown and described and pointed out fundamental novel features of the invention as applied to currently preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the method and apparatus illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. In addition, it is to be understood that the drawings are not necessarily drawn to scale but that they are merely conceptual in nature. It is the inention, therefore, to be limited only as indicated by the scope of the claims.