WO2011018601A1

WO2011018601A1 - Method of forming an electrical circuit using fullerene derivatives

Info

Publication number: WO2011018601A1
Application number: PCT/GB2010/001409
Authority: WO
Inventors: Francis Gibbons; Alex Robinson; Richard Palmer; Jon Andrew Preece
Original assignee: The University Of Birmingham
Priority date: 2009-08-10
Filing date: 2010-07-26
Publication date: 2011-02-17
Also published as: GB0913919D0

Abstract

The present invention resides in a method of forming an electrical circuit, comprising forming on a substrate a non-conducting film having as its major constituent one or more fullerene derivatives, followed by exposing a selected region of said film to actinic radiation whereby to cause said selected region to become conductive. At least one electrical contact is provided (at any stage of the process) in physical contact with said selected region. The invention also reides in an electrical circuit producible by the above method.

Description

Method of Forming an Electrical Circuit Using Fullerene Derivatives

The present invention relates to the formation of electrical circuits using fullerene derivatives whose conductivity can be selectively controlled by exposure to actinic radiation.

US 2008/0118874 describes the use of fullerene derived materials as photoresist layers that are capable of fine patterning on the substrate material surface of an electronic or semiconductor device. The patterned layer is utilised as a passive mask on the surface layer for a subsequent sequence of complex treatments such as etching, metal deposition, lift off etc. The fullerene materials have demonstrated that they are capable of recording patterns with a resolution in excess of 20 nm using actinic radiation, such as electron beam or EUV light. It has been found by Onoe et al ("The Electron Beam Transport Properties of Photo- and Electron-Beam-Irradiated C₆₀ Films"; Journal of Physics and Chemistry of Solids (2004) vol. 65, pp 343-348) that exposure of a fullerene film to electrons or photons results in polymerisation of the fullerene and a reduction in the resistivity of the fullerene film. The whole film was exposed for long periods (400 hours) and the increase in conductivity was poor. Resistivity values in the order of 1-10 Ωcm were recorded which are in the range of undoped semiconductor materials.

It would be an advantage to deposit a selectively activatable electrical material that could be applied by industry standard coating technologies and converted into a highly conductive layer (i.e. resistivity < 0.1 Ωcm) through exposure to actinic rays, such as electron beam or EUV. It would be advantageous if the unexposed material remained non-conductive so that it did not require removal after patterning. It would be doubly advantageous if the unexposed material could also be removed if required without affecting the exposed areas to leave a fine resolution network of electrically conductive connections without the need for the additional processing stages normally associated with known lithography techniques for manufacturing electrical contacts.

According to the present invention there is provided a method of forming an electrical circuit, comprising

(i) forming on a substrate a non-conducting film having as its major constituent one or more fullerene derivatives, (ii) exposing a selected region of said film to actinic radiation whereby to cause said selected region to become conductive, and

(iii) providing at least one electrical contact in physical contact with said selected region.

As used herein "fullerene derivative" means any closed cage-like fused-ring polycyclic system consisting of carbon having at least one addend extending from the cage system. The minimum theoretical number of carbon atoms in the cage is 20. Certain embodiments are based on C₆₀ but other materials (e.g. C₅₀ or C₇₀) may be used.

The nature and number of the addends is not particularly limited. In certain embodiments the fullerene derivative is a methanofullerene, diazafulleroid, diels-alder fullerene or a fullerenol. Specific reference is also made to WO2006/030234 and WO2006/030240 which disclose suitable fullerene derivatives and methods for their synthesis.

Mixtures of fullerene derivatives may be used to form the non-conducting film, but in certain embodiments only a single fullerene derivative will be employed. Optionally, after exposure, the unexposed (non-conducting) region of the film may be removed.

The at least one electrical contact may be applied directly to the substrate prior to film formation, onto the film prior to actinic radiation exposure, or before or after removal of the unexposed region of the film where such removal is carried out.

It will be understood that for those applications where current is required to flow through the circuit, at least two electrical contacts will be required. Film forming may be achieved by application of the fullerene derivative in solution, followed by removal of the solvent. Solvent may be removed by heating, a process which is commonly referred to as a soft pre-bake (which is also known as a post application bake) or by evaporation at ambient temperature. Convenient coating techniques include spin coating, dipping and roller coating.

In certain embodiments film forming is by spin coating. When spin coating, it will be appreciated that the solids content of the coating solution can be adjusted to provide a desired film thickness based upon the specific spinning equipment utilized, and parameters such as the viscosity of the solution, the speed of the spinner and the amount of time allowed for spinning. The film thickness is not particularly limited and may be 10 nm or less to several hundred (e.g. 500) nanometers.

The nature of the substrate is not particularly limited, but in the preparation of electronic components, the substrate will normally be a silicon or silicon dioxide wafer for the production of microprocessors and other integrated circuit components. Aluminium- aluminium oxide, titanium dioxide, gallium arsenide, sapphire and silicon nitride wafers can also serve as substrate.

Solvents which may be used for film forming include chloroform, chlorobenzene, dichlorobenzene, anisole, ethyl-3-ethoxy propionate and acetone. In certain embodiments the concentration of methanofullerene derivative is from about 0.5 to 20 mg/ml.

The exposure may be conducted using electron beam energy although excimer laser beams, EUV light and X-rays may also be used. The exposure may be achieved by scanning the actinic radiation source according to a required pattern (so called "direct write"), or by projecting the actinic radiation through a mask formed to the required pattern over the substrate.

Suitable direct write technologies include Electron Beam Lithography, Ion Beam Lithography, Focus Electron Beam Ion Deposition, Maskless Photolithography and SPM Lithography.

The skilled reader will understand that exposure dose and penetration depth can be controlled by varying parameters such as the energy of the actinic radiation, current (in the case of electron beams) and time of exposure. In certain embodiments the minimum exposure dose is 0.01 Can^'2. In certain embodiments the maximum exposure dose is 1 Ccm^"2, 0.5 Ccm ² or 0.1 Ccm^"2

It will be appreciated that although reference is made to exposing "a selected region", exposure may be according to any pattern. Thus in certain embodiments, the selected region may consist of a plurality of unconnected sub-regions. The method may include an additional step of a post exposure bake, in which the substrate is heated to an elevated temperature (e.g. 50 to 150 ⁰C, such as about 100⁰C), for a predetermined period (e.g. 0.1 to 20 minutes, such as about 1-2 minutes). In those embodiments in which the unexposed (non-conducting) region of the film is to be removed, this may conveniently be achieved by selectively dissolving the

unexposed region of the film in a solvent (e.g. by immersion or spraying). Solvents which may be used for this purpose include chloroform, chlorobenzene,

dichlorobenzene, anisole, ethyl-3-ethoxy propionate, acetone and isopropyl

alcohol.water.

The electrical contact may be in the form of a bond pad of a conductive material (e.g. gold) or a conductive wire attached directly to the conductive material. The invention also resides in an electrical circuit produced by the method of the first aspect.

According to a second aspect of the present invention, there is provided an electrical circuit having

a conductive path consisting predominantly of glassy or amorphous carbon with minor quantities of residues derived from the fragmentation of one or more fullerene derivatives, and

at least one electrical contact in physical contact with said conductive path. Wherein said minor quantities are less than 10wt%, less than 5wt% or less than 1wt% relative to the glassy or amorphous carbon.

The invention will be further described by way of example only with reference to the accompanying drawings in which:-

Figure 1 is a series of line profiles taken from atomic force micrographs of fullerene derivative microstructures at a range of electron beam radiation doses,

Figure 2 is a scanning electron micrograph of a set of four point probes fabricated over a carbon microstructure,

Figure 3 shows a plot of resistivity as a function of electron beam radiation dose, and

Figures 4 to 9 are schematics representing the stepwise formation of electrical circuits in accordance with the present invention. Synthesis of a Fullerene Derivative

The synthesis of the diazafulleroid MF03-02 was achieved by treatment of a concentrated solution of fullerene with an equimolar amount of diethyleneglycol monomethyl ether, to yield the diazafulleroid MF03-02 by the reaction route shown in scheme 1 below:

Scheme 1 : Synthesis of Compound 1

1. SOCl₂

2, NaN₃

MeO MeO' ^- -^ ^{^}N₃

Compound 1

Qo

1 -chloronaphthalene

60°c

Compound 2 Compound 3 MF03-02

R = CH₂CH₂OCH₂CH₂OMe

To a solution of diethyleneglycol monomethyl ether (16.4 g, 0.10 mol) in dry toluene (100 mL) was added dropwise a solution of thionyl chloride (18.0 g, 0.15 mol) in dry toluene (20 mL). The reaction mixture was then heated at reflux under nitrogen for 2 hours, after which the mixture was cooled and evaporated to dryness. Dry toluene (50 mL) was added, the mixture filtered, and the filtrate evaporated to dryness to remove residual traces of thionyl chloride. To the crude chloro derivative (18.0 g, 0.10 mol) dissolved in dry dimethylsulfoxide (100 mL) was added sodium azide (13.0 g, 0.20 mol) and the mixture heated at 6O⁰C for 16 hours. After it was cooled, the mixture was added to water (500 mL) and extracted with dichloromethane (1 * 50 mL). The combined extracts were dried (MgSO₄) and evaporated to dryness. The crude azide was purified by distillation to afford 15.3 g (81%) of compound 1 as clear oil. A solution of Ceo (100 mg, 0.14 mmol) and compound 1 (20 mg, 0.14 mmol) in 1- chloronaphthalene (15 mL) was stirred overnight at 6 ⁰C. The reaction mixture was then separated by column chromatography eluting with dichloromethane to afford compound 2 (50 mg, 42 % ) and traces of compound 3 and MF03-02 (12 mg, 9% and 5 mg, 4%respectively) as a purple solid: High Performance Liquid Chromatography (HPLC) run in 50 % MeCN and 50 % H₂O showed:

• compound 2 was 99% pure;

• compound 3 was 98% pure; and

• MF03-02 was 99% pure. Any unreacted C₆₀ was removed by silica gel plug filtration and the product of the reaction purified by flash chromatography on silica gel. The expected structure was confirmed by NMR and mass spectrometry. Reactions were carried out under a nitrogen atmosphere. Reactant chemicals were purchased from Sigma-Aldrich Company Limited of Gillingham, Dorset, England and used as received

Yields refer to chromatographically pure products Thin-layer Chromatography (TLC) was carried out on aluminium sheets coated with silica gel 60 (Merck 5554 mesh). Column chromatography was performed on silica gel 60 (Merck 230-400). High-performance liquid chromatography (HPLC) results were recorded on a Dionex Summit System with Chromeleon Software, with a Summit UV 170s UV/visible multichannel detector with analytical flow cell. Analytical HPLC runs were performed on a Luna (Phenomenex), C18, 250 mm * 4.6 mm ID, with 10 μm pore size column using a gradient of MeCN/H₂O 50/50 over 60 min. FABMS was performed on Micromass ProSpec and Micromass ZabSpec instruments using perfluorokerosene (PFK) as calibrant for the El and Cesium Iodide or Polyethylene glycol (PEG). Example 1

Fabrication of M ic restructures Figure 1 shows line profiles taken from atomic force micrographs of microstructures for fullerene derivatives of the invention at electron beam doses of (a) 0.001 , (b) 0.01 , (c) 0.1 and (d) 1 Ccm^'2 prepared by the following method.

Silicon wafers with a 500 nm thermal oxide layer were diced into 4 cm² samples. Cleaning was performed using semiconductor-grade Puranal chemicals from Riedel- de-Haen, and other solvents were from Aldrich. The samples were washed ultrasonically in IPA for 15 min and then for 1 minute in flowing deionized (Dl) water (Purite Neptune 18.2 MΩcm) before cleaning with fresh piranha solution (H₂SO₄IH₂O₂ [1 :1]) to remove organic contaminants, and a further 1 minute wash in Dl water. The samples were then dried with nitrogen and used immediately. The MF03-02 was dissolved in chloroform with a concentration in the range 0.6 to 20 gl^~1 and deposited onto the substrate via spin coating at speeds of between 600 and 3000 RPM for 60 to 120 seconds. No post application bake was applied. Films of 15 to 120 nm thickness, dependent on the spin speed and solution concentration, and measured with a surface profiler (Veeco Dektak 3st), were prepared. All results presented herein are for a film thickness of ~24 nm prior to irradiation. Thinner films resulted in discontinuous microstructures due to the roughness of the substrate. A scanning electron microscope (FEI XL30SFEG), with a pattern generator (Raith Elphy Quantum) for electron beam lithography, was used to irradiate the films. A beam energy of 5 keV was used to irradiate the MF03-02 film. After exposure, un-irradiated material was removed by dissolution in monochlorobenzene, to ensure that only exposed areas contributed to the measured resistivity. The exposed material was not soluble in monochlorobenzene and the patterned areas were thus retained on the substrate. The microstructures obtained are long thin wires. They are about 400 nm wide, 30 nm tall and several microns long. An interesting feature of the microstructures is the small trough running down the centre along the length of the wires (particularly pronounced in panel B of figure 1). Although not wishing to be bound by theory it is postulated that this feature may arise through desorption of the fullerene additive addends at the centre of the electron beam where temperature is at its highest. Fabrication of Overlaid Four Point Probe Electrodes

Electrical contacts to the microstructure were prepared via lift-off to enable resistivity measurements to be made. In this case the carbon microstructure was created first. The substrate was spin coated with 100 nm of poly(methyl methacrylate) resist (Micro- Chem 950K A1) and baked for 10 minutes at 12O⁰C to drive off the resist solvent. The probe pattern was written into the resist with electron beam lithography using a dose of 0.0002 Ccm^"2. Development was in methyl isobutyl ketone : isopropyl alcohol [1:3] for 90 seconds, before rinsing with isopropyl alcohol for 30 seconds and drying with nitrogen. A chromium adhesion layer of approximately 20 nm thickness was applied by sputter coating (Edwards Auto 306) using an RF power of 300 W for 20 seconds at a pressure of 2.8 * 10^"3 mbar. A gold layer of ~50 nm thickness was then deposited at an RF power of 100 W, with a deposition time of 60 seconds and pressure as before. The sample was sonicated for 10 minutes in acetone to remove the unexposed areas of PMMA together with the metal on top of the resist. The final assembly was examined using a scanning electron microscope and the resulting micrograph is shown in Figure 2. In Figure 2, the conductive nanowire 2 is clearly visible as are the four gold electrodes 4. Electrical Measurements

Electrical measurements were taken using a Source Measure Unit (Keithley 238) using tinned copper wire leads (not shown), attached to the macroscopic bond pads of the four point probe electrodes with silver dag. A good mechanical contact between the copper wire and the bond pad was achieved before application of the dag. Measurements were taken at room temperature, in the dark and under ambient atmospheric conditions, using a current source and voltage measurement. Currents in the microamp range were used to limit thermal effects. The measurements of resistivity are plotted against electron beam dose in Figure 3.

From Figure 3 it can be seen that 0.001 CCcm^'2 is insufficient to reduce resistivity and the microstructure produced is non-conducting. A dose of 0.01 CCm^'2 is sufficient to reduce resistivity to <0.1 Ωcm. Although higher exposure doses do result in

conductivity, there is significant feature broadening (see panels c and d of figure 1). This may not be important in embodiments where there is no need to remove the unexposed film region, but is likely to be problematic where film removal is required. Further Examples of electrical circuits Electrical Connection via Bond Pads A circuit may be formed by creating a conductive pathway in the fullerene derivative material that is contacted by bond pads, themselves of a conductive material. The bond pads may then be wired to the outside world, using a number of methods including wires attached with conductive glue, or with solder, or by wires in mechanical contact with the bond pads, or by wires pressure bonded to the bond pads, or by other techniques. The bond pads may be created on top of, within, or below the film. The bond pads may be contacted to the external wire in any orientation, including where necessary, by pushing the wires through the fullerene derivative film to reach the bond pads (for instance in the case where bottom contacts are used and wires from above would be beneficial.

Figure 4 is a schematic showing the stepwise production of an electrical circuit using bond pads as top contacts (panel A) and bottom contacts (panel B). In each case the figure on the left is a cross section and on the right a top view. Step 1 : For the bottom contact method, the electrical contact 10 is prepared on the substrate 12 first. The electrical contact 10 may be a metal or other conducting material. It may be formed by evaporation, sputter coating, electroplating, spin coating, or other suitable deposition method. It may be a single contact or multiple contacts and may be of arbitrary shapes created through the use of a patterning method such as but not limited to etching, lithographic liftoff, moulding, pressing, rolling, printing, etc. For this particular geometry a non-conductive substrate would be required (however, that is not an overall requirement as demonstrated in later examples). For top contacts step one is not required. Step 2: The fullerene derivative material 14 is deposited on the substrate.

Step 3: The required electrical circuit is created in the fullerene derivative using actinic irradiation, such as electron beam lithography. Arbitrary patterns may be constructed as required. In figure 4, a simple wire 16 is shown.

Step 4: For the top contact method, the electrical contact 10 is prepared on top of the film after exposure to irradiation, although it will be understood that step 4 could come before step 3 (not shown). The electrical contact may be a metal or other conducting material. It may be formed by evaporation, sputter coating, electroplating, spin coating, or other suitable deposition method. It may be a single contact or multiple contacts and may be off arbitrary shapes created through the use of a patterning method such as but not limited to etching, lithographic liftoff, moulding, pressing, rolling, printing, etc.

Step 5: The unexposed fullerene derivative material is removed after patterning the electrical circuit leaving only the exposed, conductive areas of the film 16. Step 5 is not required for the basic function of the material, but may be a requirement of a particular device. In a variation of the above method, step 5 is carried out prior to step 4.

Electrical Connection with integral bond pads

A circuit may be formed by creating a conductive pathway in the fullerene derivative material, with "pseudo" bond pads that are themselves created in the fullerene derivative material. These integrally formed bond pads may then be wired to the outside world, using a number of methods including wires attached with conductive glue, or with solder, or by wires in mechanical contact with the bond pads, or by wires pressure bonded to the bond pads, or by other techniques.

Figure 5 is a schematic showing the stepwise production of an electrical circuit in which the equivalent of bond pads are created in the fullerene film layer itself. In each case the figure on the left is a cross section and on the right a top view. Step 1 : The fullerene derivative material 14 is deposited on the substrate 12.

Step 2: The required electrical circuit is created in the fullerene derivative 14 using actinic irradiation, such as electron beam lithography. Arbitrary patterns may be constructed as required. Here a simple wire 16 is shown. Where contact to the outside world is required a bond pad 16' is defined (by exposure to actinic radiation) in the material whose dimensions are large enough to accommodate the wiring to the outside world.

Step 3: Electrical connection to the outside world is created by attaching external wires 18 directly to the integrally formed bond pads 16'. Electrical connectivity may be achieved using conductive glue, by mechanical contact, or by other appropriate methods. Step 4: The unexposed fullerene derivative material is removed after patterning the electrical circuit leaving only the exposed, conductive areas of the film 16, 16'. This is not required for the basic function of the material, but may be a requirement of a particular device. Where step 4 is applied, step 3 may come after step 4.

Partial Thickness Exposure

A circuit may be formed by creating a conductive pathway in the fullerene derivative material without exposing the full thickness of the fullerene film 14. For instance in electron beam lithography it is possible to limit the penetration depth of the electrons by reducing the beam energy of the exposure tool. In this way it is possible to create a conductive pathway 16,16' that does not extend through the full thickness of the film. In this case it is not important as to whether the substrate 20 is conductive or not as the unexposed material 14' at the bottom of the film forms an insulating layer between the conductive pathway 16,16' and the substrate 20.

Referring to Figure 6, in which the figure on the left is a cross section and on the right a top view:

Step 1 : The fullerene derivative material 14 is deposited on the substrate 20.

Step 2: The required electrical circuit is created in the fullerene derivative using actinic irradiation, such as electron beam lithography. Arbitrary patterns may be constructed as required. Here a simple wire 16 is shown. A low beam energy is used to prevent full thickness exposure, thereby leaving a layer of unexposed material 14' between the substrate 12 and the exposed regions 16, 16'.

Step 3: Electrical connection to the outside world is created by attaching external wires 18 directly to the exposed fullerene derivative material 16'. Electrical connectivity may be achieved using conductive glue, by mechanical contact, or by other appropriate methods.

Alternative Partial Thickness Exposure

A circuit may be formed by creating a conductive pathway in the fullerene derivative material where the thickness of the conductive pathway varies as required, so that in some areas the conductive pathway is in contact with the substrate and in other areas it is not. This can be done by controlling the penetration depth of the actinic radiation, increasing it in some areas and decreasing in other areas. For instance in electron beam lithography it is possible to limit the penetration depth of the electrons by reducing the beam energy of the exposure tool.

Referring to Figure 7, in which the figure on the left is a cross section and on the right a top view: Step 1 : The fullerene derivative material 14 is deposited on a conductive substrate 30.

Step 2: The required electrical circuit is created in the fullerene derivative using actinic irradiation, such as electron beam lithography. Arbitrary patterns may be constructed as required. Here a simple wire 16 is shown, together with an integral bond pad 16'. A low beam energy is used to prevent full thickness exposure.

Step 3: A higher beam energy is used to exposed a full film thickness bond pad 16" at one end of the wire. In this way a contact to the substrate 30 is created. In a variation not shown, step 3 may come before step 2.

Step 4: Electrical connection to the outside world is created by attaching a first wire 18 directly to the integral bond pad 16' and a second wire 18' to the substrate 30.

Electrical connectivity may be achieved using conductive glue, by mechanical contact, or by other appropriate methods. There exists an electrical connection between the wires 18,18', via the substrate 30 and then via the conductive pathway 16,16' in the exposed fullerene derivative material.

Formation of an electronic device Using the capability of partial thickness exposure an electronic device might quickly be fabricated using the technique described here. The carbon based nature of the conductive pathways created from the fullerene derivative may be particularly beneficial in the field of organic electronic devices. Referring to Figure 8, in which the figure on the left is a cross section and on the right a top view: Step 1: A semiconducting material 40 is deposited on a substrate 42.

Step 2: The fullerene derivative material 14 is deposited on the semiconductor 40. Step 3: The required electrical circuit is created in the fullerene derivative using actinic irradiation, such as electron beam lithography. Arbitrary patterns may be constructed as required. Here the wiring 44 for a simple transistor (when combined with the underlying semiconductor 40) is shown. A low beam energy is used to prevent full thickness exposure, in some areas, whilst where contact to the semiconductor is required a high penetration depth irradiation is used to achieved full thickness exposure. It will be noted from Figure 8 that the transistor wiring 44 is formed by the exposure of three unconnected sub-regions of the fullerene derivative film.

Step 4: Electrical connection to the outside world is created by attaching external wires 18 directly to the exposed fullerene derivative material 44. Electrical connectivity may be achieved using conductive glue, by mechanical contact, or by other appropriate methods.

Formation of "Lab on Chip"

The glassy carbon nature of the exposed fullerene derivative material makes it ideal for use as electrodes in chemically harsh environments. Glassy carbon is often used as electrodes in environments where metal electrodes would rapidly corrode, or otherwise be damaged. This makes the fullerene derivative material ideal for forming electrodes on microfabrication chemical handling systems, such as lab on a chip.

Figure 9 is a schematic showing the stepwise production of a Lab on Chip having an open structure (panel A) and an embedded structure (panel B), In each case the figure on the left is a cross section and on the right a top view.

Step 1 : A lab on a chip device is fabricated. In the embedded version, additional standoff features 50 are used so that when the unexposed fullerene derivative is removed later in the process the top will remain structurally viable. Step 2: The fullerene derivative material 14 is deposited on the substrate.

Step 3: In the embedded device a top cover 52 is added over the fullerene film. Step 4 The required electrical circuit is created in the fullerene derivative using actinic irradiation, such as electron beam lithography Arbitrary patterns may be constructed as required Here a pair of electrodes 54 is shown Provided that the top cover 52 in the embedded system is sufficiently transparent to the actinic radiation used to expose the fullerene derivative material 14, the conductive pathways can be created by patterning through the cover 52

Step 5: The unexposed material is removed using a solvent

Step 6: Optionally the structure in panel A could then be covered after patterning with the conductive features

Claims

1. A method of forming an electrical circuit, comprising

(i) forming on a substrate a non-conducting film having as its major constituent one or more fullerene derivatives,

(ii) exposing a selected region of said film to actinic radiation whereby to cause said selected region to become conductive, and

2. The method of claim 1 , wherein the fullerene derivative is one or more methanofullerene, diazafulleroid, diels-alder fullerene orfullerenol, or a mixture thereof.

3. The method of claim 1 or 2, wherein, after exposure, the unexposed (non- conducting) region of the film is removed.

4. The method of claim 3 wherein the unexposed (non-conducting) region of the film is removed by selectively dissolving the unexposed region of the film in a solvent.

5. The method of any preceding claim, wherein the at least one electrical contact is applied after actinic radiation exposure.

6. The method of any preceding claim, wherein at least two electrical contacts are provided in physical contact with said selected region, so as to permit current to flow in said electrical circuit.

7. The method of any preceding claim, wherein film forming is achieved by application of the fullerene derivative in solution, followed by removal of the solvent.

8. The method of claim 7, wherein the solvent used for film forming is chloroform, chlorobenzene, dichlorobenzene, anisole, ethyl-3-ethoxy propionate, acetone or a mixture thereof.

9. The method of any preceding claim, wherein the substrate is a wafer of silicon, silicon dioxide, aluminium-aluminium oxide, titanium dioxide, gallium arsenide, sapphire or silicon nitride.

10. The method of any preceding claim, wherein the exposure is conducted using electron beam energy, excimer laser beams, EUV light or X-rays.

11. The method of any preceding claim, wherein the exposure is achieved by scanning the actinic radiation source according to a required pattern or by projecting the actinic radiation through a mask formed to the required pattern over the substrate.

12. The method of any preceding claim, wherein the exposure dose is from 0.01 Ccm^'2 to 1 Ccm^"2.

13. The method of any preceding claim, comprising an additional step of a post exposure bake, in which the substrate is heated to an elevated temperature for a predetermined period.

14. An electrical circuit producible by the method of any one of claims 1 to 13.

15. An electrical circuit having

at least one electrical contact in physical contact with said conductive path.