Background to the Invention Standard photolithographic techniques are commonly used to pattern thin films of substrates such as tin-doped indium oxide (referred to hereinafter as ITO) on glass. A typical process involves the following steps: (i) a thin film of photoresist is spin-coated on the substrate; (ii) the photoresist is exposed to UV light through a photomask and subsequently developed; (iii) the areas that are not covered by the remainder of the photoresist are then etched away using a variety of etch techniques (wet and dry) dependent upon the nature of the photoresist material and the substrate on which it has been spun-coated; and (iv) the photoresist on the non-etched areas is then finally removed by rinsing with a solvent.

This process involves many steps, wastes large percentages of the expensive photoresist material and requires the use of organic solvents, many of which are toxic and/or expensive and/or have an adverse effect on the earth's atmospheric chemistry. It is therefore desirable to try to find an alternative process for the patterning of substrates.

Self-assembled monolayers (referred to hereinafter as SAMs) have attracted much attention in areas such as device engineering because of the versatility they provide for surface modification. SAMs are highly ordered molecular assemblies that form spontaneously by chemisorption of functionalised molecules on a variety of substrates such as metals, indium tin oxide, silicon, and glass. These molecules organise themselves laterally, most commonly via van der Waals interactions between long aliphatic chains. The principles and practice of deposition of monolayers are described in detail in a publication by A Ulman entitled"Introduction to Thin Organic Films : From Langmuir-Blodgett to Self-Assembly", published by Boston Academic Press, 1991. SAMs have found widespread research interest because of potential applications related to control over wettability, biocompatibility and corrosion resistance of surfaces.

For many electronic, optical and electro-optical devices for example, the ability to modify the properties of surface areas of the devices makes SAMs attractive for many applications, such as modification of surface hydrophobicity, packaging and electrical insulation. Furthermore, as SAMs exhibit interesting barrier properties, they are considered very appropriate for use as protective coatings on metal surfaces because they form thin highly crystalline barrier films. A large number of SAMs on a wide range of substrates has been reported. The most widely studied systems are thiols on gold or silver and silanes on glass or silicon wafers.

Gold has found widespread application and, for example, is used extensively in the electronics industry in integrated circuit technology. Also, as a relatively inert metal it has been used as a protective layer in certain chemical environments, such as a liner material for the ink chambers in ink jet print heads. However, gold will dissolve under appropriate chemical or electrochemical conditions, so the ability of SAMs to provide a very thin protective layer to such metal layers in harsh chemical environments where metal layer corrosion is known to occur is also considered extremely attractive. Thiols on gold can form very densely-packed monolayers, completely covering the surface. The molecules are bound via a gold thiolate bond that has part covalent character.

In the case of silanes on glass or silicon wafers, a two-dimensional network is formed that is covalently bound to the surface. In principle, silane SAMs are less dense but more stable than thiol SAMs.

Both types of SAMs (and others such as alkanephosphonates on silicon wafers and ITO) have been studied for their potential use as etch resists. The principle is that after a SAM is formed on the surface of the target substrate, the SAM can then be patterned to form the desired etch resist through use of various conventional techniques including microcontact printing, short wavelength UV lithography, and e-beam or focused ion beam lithography.

Although some encouraging results have been obtained (see, for example, Breen et al. , Languir, 2002, 18, 194), the general consensus in the field is that these approximately 2 nm thick films are not stable enough to act as commercially attractive etch resists, thus severely limiting their commercial application in industrial processes.

To date, the SAM material is deposited by dissolving the material in an appropriate organic solvent and, as such, the monolayer formation over the required flat surface areas, which usually include surface discontinuities arising from design features dictated by the practical application of devices, is difficult to control. As the layers are self-aligning, they often exhibit molecular sized defects or holes in the layer. These defects can limit their use as etch resists because the barrier properties provided by the densely packed molecules of the SAM material can be breached through the molecular sized defects.

Furthermore, although SAMs are typically in the order of only about 2 nm thickness, they are relatively slow to deposit. Typical deposition times range from several hours to a few days with the normal solvents used for the compounds. SAMs have also been fabricated on silicon substrates using semi-fluorinated silane derivatives. However, SAMs of these compounds are usually deposited by way of a vapour deposition process which is very time consuming.

Metal-doped indium oxides such as ITO are widely used as a transparent electrode in a wide range of devices such as light emitting devices and photovoltaic devices. However, it is particularly difficult to make a SAM on substrates such as ITO because the surface of ITO is rough and it contains very few reactive groups that can be used to form covalently-bound SAMs.

Supercritical carbon dioxide has been used for polymer synthesis and polymer processing. This has been extensively reviewed in the past and the state of the art is summarised in an article by Cooper [A. I. Cooper, J. Mater. Chem., 2000,10, 207].

Compressed carbon dioxide is also used as a solvent for the preparation of organic molecules and this has been summarised in a special issue of Chemical Reviews. [see Special Issue: Chem. Rev. 1999,99, #2]. Unlike conventional liquid solvents, carbon dioxide is highly compressible and the density (and therefore solvent properties) can be tuned over a wide range by varying the pressure [see M. McHugh et al."Supercritical Fluid Extraction"Boston, Butterworth-Heinemann, 1994]. Compressed carbon dioxide is a superior solvent medium for heavily fluorinated compounds. A supercritical fluid may be defined as a substance for which both temperature and pressure are above the critical values for the substance and which has a density close to or higher than the critical density.

The use of supercritical fluids for the production of particles has increased enormously within the past few years due to their readily adjustable densities and particularly their pressure-dependent solvent power. In the RESS-process (rapid expansion of supercritical solutions) the expansion of a supercritical solution leads to a decrease in solvent power and hence to the precipitation of the organic or inorganic solute [J. W. Tom and P. G. Debenedetti, J. Aerosol Sci., 1991,22, 555]. As the decompression is fast, high supersaturation is reached and fine powders can be obtained. The most common solvent in this process is carbon dioxide because of its mild critical temperature (31. 1°C) and its relatively low critical pressure (73.8 bar). As a result of the mild temperature, the RESS-process offers the opportunity to micronise heat-sensitive organics. These are otherwise difficult to comminute as they are thermally degraded. Another advantage of carbon dioxide is that it is a gas under atmospheric conditions so that the end-reaction mixture is solvent-free.

In co-pending GB application no. 0029535.2, the contents of which are incorporated herein by reference thereto, a method is disclosed for the fabrication of a SAM of a substance such as an organic silane on a substrate such as ITO comprising depositing the substance on the substrate using compressed carbon dioxide as the solvent for said substance. Cao et al, Langmvif ; 2001, 17, 757 discloses a method for the deposition of organosilane SAMs on silica surfaces using compressed carbon dioxide as the solvent for the substance. The deposition times are much shorter compared to organic solvents and the hysteresis between the contact angles is very small which indicates the formation of a very tightly-packed monolayer, approaching the quality of comparable SAMs on the surface of silicon wafers.

Summary of the Invention It is an object of the present invention to provide an improved method for the patterning of an organosilane on the surface of a substrate such as ITO so that the resulting patterned organosilane is suitable for use industrially as, for example, an etch resist. We have now discovered that using compressed carbon dioxide as a solvent for an organosilane it is possible to form densely-packed, strongly bound organosilane SAMs on the surface of a target substrate such as ITO which can then be patterned using a conventional, non-solvent technique such as short wavelength UV lithography or focused ion beam lithography. It is a further object of the present invention to utilise the resulting patterned substrate by reacting it with an etchant to etch the surface of said substrate that is not protected by the patterned organosilane SAM, said patterned organosilane SAM acting as an etch resist.

Thus, in a first aspect of the present invention there is provided a method for the deposition and patterning of an organosilane on a substrate, said method comprising the steps of : (i) forming a self-assembled monolayer of said organosilane on said substrate by depositing said organosilane on said substrate using compressed carbon dioxide as the solvent for said organosilane; and (ii) patterning the organosilane self-assembled monolayer thus formed using a patterning technique that does not use a solvent to give the desired pattern.

By compressed carbon dioxide, we mean carbon dioxide which has been compressed under pressure to produce liquid carbon dioxide or supercritical carbon dioxide. The use of compressed carbon dioxide is advantageous because it is inexpensive, non-toxic and non- flammable. Furthermore, compressed carbon dioxide is highly compressible and the density (and therefore solvent properties) can be tuned over a wide range by varying the pressure.

Organosilanes that are to be deposited to form SAMs are generally highly soluble in compressed carbon dioxide. The improved solubility facilitates the interaction of these organosilane molecules with the substrate and thus facilitates the formation of the organosilane SAMs. The resulting organosilane SAMs have greatly improved surface integrity, not displaying the molecular sized defects or holes in the organosilane SAMs produced using conventional organic solvents. As a result, when the organosilane SAMs thus produced in the first step are then patterned using a technique that does not use a solvent, the resulting patterned organosilane SAMs do not have the sort of defects that are observed in the prior art, thus making them particularly suitable for use as etch resists. Furthermore, the organosilane SAMs can be fabricated much more rapidly if they are deposited from carbon dioxide so this gives clear process advantages, reducing the time that it takes to form a patterned substrate from several hours or days using conventional techniques to just a few hours.

The substrate to be patterned can be any substrate that might be patterned by an organosilane. Typical examples of the substrate include transparent conducting materials such as metal-doped indium oxides, examples of which include ITO, zinc-doped indium oxide (IZO), zirconium-doped indium oxide and aluminium-doped indium oxide (which can be used as the anode material in devices such as LCDs and organic light emitting diodes), tin oxide, Si/Si02, AVA1203 and Ti/Ti02. Of these, we prefer ITO and IZO and we particularly prefer ITO.

The substance that is to be deposited to form an SAM on the surface of the substrate is an organosilane. The organo groups can be any suitable group, but typically the organosilanes have at least one long-chain aliphatic group attached to the silicon atom thereof.

Specific examples are semi-fluorinated silane derivatives of the following formula (I): wherein: Z comprises a group of formula Si (Rl) 3 wherein each Rl is the same or different and is selected from the group consisting of halogen atoms, alkyl groups (preferably Cl 6 alkyl groups) and alkoxy groups (preferably Cl 6 alkoxy groups) [examples of such groups include but are not limited to SiCl3, Si (OCH3) 3, Si (OCH2CH2CH3) 3, Si (OCH3) 2Cl, and Si (CH2CH3) 2Cl], preferably Z is group of formula Si (Rl) 3 wherein each Rl is the same and is either a halogen atom or an alkoxy group, and most preferably Z is SiC13 ; m and n are the same or different and each is an integer of from 1 to 20 (preferably, m is an integer of from 5 to 10 and n is an integer of from 1 to 5, and most preferably m = 7 and n= 1) ; each X is the same or different and is selected from the group consisting of hydrogen, deuterium and fluorine atoms, and preferably fluorine; and Y represents a group of formula-C (R2) 3 wherein each R is the same or different and is selected from the group consisting of hydrogen, deuterium and fluorine atoms and functional groups that, after formation of the SAM, can be reacted with other reagents to give further functionalisation or can react with other functional groups on other organosilane chains of formula (1) to give cross-linking, preferred examples of such functional groups being selected from the group consisting of vinyl, styryl, acryloyl, methacryloyl and alkenyl groups (preferably C2 6 alkenyl groups), of which Y representing a trifluoromethyl group is most preferred.

Of the above organosilanes of formula (I), those wherein Z is a trialkoxysilyl group or a trihalosilyl group, m is an integer of from 5 to 10, n is an integer of from 1 to 5, each X is the same or different and is a hydrogen or fluorine atom, and Y is a group of formula-C (R) 3 wherein each R2 is the same or different and is a hydrogen or fluorine atom are preferred; and those wherein Z is a trimethoxysilyl group or a trichlorosilyl group, m is 7, n is 1, each X is a fluorine atom, and Y is a trifluoromethyl group are particularly preferred.

The addition of co-solvents to the carbon dioxide such as H2O, CH30H, CF30H, CF3CH20H, CFsCFzOH, (CF3) CHOH, CH4, C2H4, C2F6, CHF3 CCIF3, C2H6, SF6, propylene, propane, NH3, Pentane, i-PrOH, MeOH, EtOH, i-BuOH, benzene, and pyridine may also be adopted to provide further improvements in the monolayer formation in step (i).

Where the substrate to be patterned is a metal-doped indium oxide such as ITO, the formation of well-controlled and reproducible SAMs in the first step is not simple, mostly because of the high degree of roughness of the surface and the low number of hydroxy groups on the surface. Preferably, the latter problem can be addressed by pre-treating the surface of the metal-doped indium oxide such as ITO with, for example, an acid followed by a base, or with piranha solution [a 7: 3 (v/v) mixture of 98% sulphuric acid and 30% aqueous hydrogen peroxide solution; typically, treatment is for 10 minutes, after which the metal-doped indium oxide is rinsed in deionised water and then dried on a hotplate], or with a 5: 1: 1 (v/v) mixture of water, 30% aqueous hydrogen peroxide solution and 30% aqueous ammonia (typically, treatment is for 1 hour at 70 °C, followed by rinsing with water and finally drying in the oven at 100 °C), or with UV and ozone.

Where the substrate to be patterned is a metal-doped indium oxide such as ITO on a flexible polymer substrate such as PTE, the use of compressed carbon dioxide is particularly advantageous as it does not attack the polymer substrate in the way that many of the conventional organic solvents do.

Any suitable temperature can be used for the organosilane SAM formation in step (i), but preferred temperatures for the formation of an organosilane SAM on a metal-doped indium oxide such as ITO are in the region of 20 °C to 80 °C, more preferred is 30 °C to 50 °C and most preferred is 40 °C.

Any suitable time can be used for the oganosilane SAM formation in step (i), but preferred times for the formation of an organosilane SAM on a metal-doped indium oxide such as ITO are in the region of 1 to 15 hours.

After the formation of an organosilane SAM on the substrate, said organosilane SAM is then patterned using a patterning technique that does not use a solvent to give the desired pattern. Suitable examples of the patterning technique include microcontact printing, short wavelength UV lithography [e. g. see M. Ishida, M. Kasuga, T. Kaneko and T. Shimoda, Jpn.

J. Appl. Phys, 2000, (39) L227-L229], and e-beam and focused ion beam lithography [e. g see P. M. St. John and H. G. Craighead, J. Vac. Sci. Technol. B. , 1996, (14), 69-74]. Of these techniques, short wavelength UV lithography and e-beam and focused ion beam lithography are particularly preferred.

The patterning of the organosilane SAMs produced on the substrates in the process of the present invention can be used to act as an etch resist in a process for etching of the substrate. Thus, in a further aspect of the present invention there is provided a process for the patterned etching of a substrate comprising treating a substrate patterned with an organosilane self-assembled monolayer produced according to the process of the first aspect of the invention described above with an etchant. The exact nature of the etchant will vary depending on the identity of the substrate to be etched and the organosilane substance that has been patterned on the surface to form the etch resist. Where the substrate is a metal-doped indium oxide such as ITO and the patterned monolayer is an organosilane, a mildly acidic etchant such as oxalic acid or zinc powder/aqueous HC1 can be used. Where the substrate is Si/Si02 and the patterned monolayer is an organosilane, a strongly basic etchant such as aqueous potassium hydroxide, aqueous ammonium hydroxide or aqueous tetramethylammonium hydroxide, e. g. a mixture of 30% potassium hydroxide solution and 1% isopropanol can be used. Alternatively, an etchant containing fluoride ions can be used, e, g, buffered hydrofluoric acid solution (6: 1 ammonium fluoride: aqueous HF), aqueous HF or a 60: 3: 2 mixture of water, HF and HNO3. Where the substrate is Al/A1203 and the patterned monolayer is an organosilane, a strongly acidic etchant such as Aluminium Etchant <BR> <BR> "A" (available from Transene Co. , USA), which is a pre-mixed 16: 2: 1: 1 mixture of phosphoric acid, deionised water, acetic acid and nitric acid, can be used.

Optionally, after the etching of the substrate above the resulting product can be treated with plasma (e. g. with oxygen plasma or under ozone in the presence of UV irradiation) so as to remove the remainder of the organosilane monolayer or to make the surface hydrophilic.

This makes the resulting product suitable for the fabrication of devices such as light emitting devices and photovoltaic devices by spin-coating, spray-coating or spray-coating using the technique disclosed in co-pending PCT application no. PCT/GB2001/05402.

The smallest patterns that can be generated using the processes of the present invention are 10 nm while the largest are 1 mm.

The products of the present invention can be incorporated into electronic, optical and electro-optical devices. In a further aspect of the present invention there is provided a light emitting device or a photovoltaic device comprising a layer of an etched substrate produced according to the process of the present invention described above.

Typical examples of organic light-emitting devices that can be formed incorporating, for example, etched ITO produced according to the process of the present invention are described in, for example, WO-A-90/13148, US 5,512, 654 and WO-A- 95/06400.

Typical examples of photovoltaic devices that can incorporate etched substrates produced according to the process of the present invention are organic field-effect transistors (FETs) which have recently become of interest for applications in cheap, logic circuits integrated on plastic substrates [C. Drury, et al. , APL 73,108 (1998) ] and optoelectronic integrated devices and pixel transistor switches in high-resolution active-matrix displays [H.

Sirringhaus, et al., Science 280, 1741 (1998), A. Dodabalapur, et al. Appl. Phys. Lett. 73,142 (1998)]. Typical architecture of polymer FETs is shown in Horowitz, Advanced Materials, 10,365, (1998).

Another example of the use of the etched substrates such as patterned ITO lines obtainable using the process of the present invention is in the manufacture of polarizers.

Brief Description of the Drawings The present invention may be further understood by consideration of the following embodiments of the present invention, with reference to the following drawings in which: Figure l (a) shows cyclic voltammograms recorded at a scan rate of 150 mV/s of bare ITO and ITO patterned with a perfluorinated silane produced in accordance with the present invention; Figure 1 (b) shows a plot of the fractional surface coverage of ITO with a perfluorinated silane against time during the production of a SAM in accordance with the present invention; Figure 2 shows a comparison of ITO produced after a perfluorinated silane SAM has been deposited thereon in accordance with the present invention and bare ITO, and of said treated and bare ITO after treatment with oxalic acid; and Figure 3 shows etched ITO produced by etching ITO patterned with a perfluorinated silane SAM produced according to the present invention.

Example 1 Formation of a Self-Assembled Monolayer of a Silane on ITO ITO plates (obtained from IVC Technologies) having a surface conductivity of 40 Q su-1 were cleaned by sonication with acetone (10 minutes), dichloromethane (10 minutes) and finally water (5 x 2 minutes). Each slide was immersed into a 1: 1: 5 by volume mixture of 30% aqueous hydrogen peroxide/30% aqueous ammonia/water for a period of 1 hour at 70 °C with occasional stirring. At the end of this time, the slides were washed with large amounts of water and then dried in an oven at 1000 °C for 4 hours. The ITO slides thus-produced were placed in a 10 mL stainless steel high-pressure vessel. 1H, 1H, 2H, 2H-perfluorodecyl- trichlorosilane (2 mL) was flushed into the vessel with liquid carbon dioxide via an injection loop. The vessel was filled with liquid carbon dioxide (23 °C, 1000 psi) and heated to 40 °C.

At the end of the desired reaction time (various times were tried; 15 hours gave the best results), the vessel was cooled, and the substrate in the cell was rinsed by filling the cell five more times with liquid carbon dioxide.

Cyclic voltammograms were measured with a Solartron 1287 Potentiostat. The cyclic voltammograms of an ITO electrode in 1mM Ru (NH3) 63+ in 0. 1M Na2SO4 were measured before and after the deposition with the perfluoroalkylsilane as described above and are presented in Figure 1 (a). The voltage was applied versus an Ag/AgCl reference electrode with varying scan rates (25 mV/s, 50 mV/s, 100 mV/s and 150m V/s). Figure 1 (a) shows the voltammograms obtained at 150 mV/s with ITO treated with the perfluoroalkylsilane for 0 hr, 0.5 hr, 1 hr, 2 hr, 5 hr, 10 hr and 15 hr. The bare ITO (0 hr) shows a large peak typical of diffusion-limited electron transfer process. After treatment with the perfluoroalkylsilane, the peak quickly reduces showing that there is a monolayer forming, preventing ions from reaching the ITO surface.

Figure l (b) shows the surface coverage with time. The surface coverage can be estimated using rate constants which can be obtained from the method of Nicholson and Shain using the voltammograms obtained above [ref. R. S. Nicholson and I. Shain, Anal. <BR> <BR> <P>Chem. , 1964, (36), 706-723]. The surface coverage increases with reaction time reaching a maximum coverage of 0.96 after 10 hr. This is a very high surface coverage which can only be obtained after a week using conventional organic solvents, showing one of the significant advantages of the process of the present invention.

Figure 2 shows a comparison of the ITO covered with an organosilane SAM produced by the above process with bare ITO and of said ITO covered with an organosilane SAM and bare ITO after etching with oxalic acid. Optical microscope images were taken using a Nikon Eclipse ME 600 under *500 magnification. For bare ITO (with no SAM), treatment with oxalic acid quickly produced holes in the ITO and after 5 minutes all of the ITO was stripped off such that it was no longer conductive. For the ITO covered with an organosilane SAM, however, treatment with oxalic acid resulted in no change to the monolayer. The ITO plate remained conducting showing that the even after more than 10 hours, the SAM and ITO are still present, showing that the organosilane SAM is indeed an excellent etch resist.

Example 2 Patterning a SAM on ITO and then Etching the ITO 2 (a) Patterning a Silane SAM on ITO The ITO covered with a self-assembled monolayer of 1H, 1H, 2H, 2H- perfluorodecyltrichlorosilane produced in Example 1 above was subjected to focused ion beam lithography (milling time 0.1 seconds) using an FEI200 workstation to produce a pattern consisting of lines ranging from 300 nm to 500 nm wide with a periodicity of from 400 nm to 1000 nm.

2 (b) Etching ITO Patterned with a Silane SAM The patterned ITO produced in step 2 (a) above was etched with a 0.05 M solution of oxalic acid at room temperature with mild agitation for 5 minutes. The resulting etched ITO is shown in the Figure 3. The figure shows images of patterned ITO after FIB treatment and etching with oxalic acid. Optical microscope images were taken using a Nikon Eclipse ME 600 under *500 magnification. The darker lines represent where the ITO has been etched away and lighter areas are where the SAM and ITO are still present.