PLASMA JET DEPOSITION PROCESS

Abstract

Processes and apparatus are described for atmospheric pressure plasma jet deposition onto a substrate. The process comprises feeding a solution comprising a dissolved metal precursor into a plasma jet. The dissolved metal precursor comprises a precursor metal selected from Groups 2 to 16, with the proviso that the precursor metal does not comprise Mn. The plasma jet is directed towards a surface of the substrate such that material from the plasma jet becomes deposited onto the surface of the substrate. The process provides a means to manufacture conductive, semiconducting or insulating deposits on a substrate in a material-efficient manner without the need for high-temperature post-treatment steps.

Claims

1. A process for preparing a deposit on a substrate using atmospheric pressure plasma jet deposition, comprising: feeding a solution comprising a dissolved metal precursor into a plasma jet, wherein the dissolved metal precursor comprises a precursor metal selected from Groups 2 to 16, with the proviso that the precursor metal does not comprise Mn; and directing the plasma jet towards a surface of the substrate such that material from the plasma jet becomes deposited onto the surface of the substrate.

2. A process according to claim 1, wherein the concentration of the precursor metal in the solution is at least 0.001 M.

3. A process according to claim 2, wherein the concentration of the precursor metal in the solution is at least 0.2 M, preferably at least 0.3 M.

4. A process according to claim 1, wherein the precursor metal is a metal selected from Groups 2 to 6 or Groups 8 to 16.

5. A process according to claim 1, wherein the plasma is a non-thermal plasma.

6. A process according to claim 1, wherein the solution is an aqueous solution.

7. A process according to claim 1, wherein the solution contains less than 0.5 wt % solid material.

8. A process according to claim 1, wherein the solution is at least 25% saturated.

9. A process according to claim 1, wherein the dissolved metal precursor contains a single species of precursor metal.

10. A process according to claim 1, wherein the dissolved metal precursor comprises a metal salt, a metal complex or a mixture thereof.

11. A process according to claim 10, wherein the dissolved metal precursor comprises a transition metal salt, transition metal complex or mixture thereof.

12. A process according to claim 11, wherein the transition metal salt or complex comprises one or more metals selected from copper (Cu), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), chromium (Cr), gold (Au) and mercury (Hg).

13. A process according to claim 11, wherein the transition metal salt or complex comprises one or more metals selected from Cu, Au and Ag.

14. A process according to claim 1, wherein the deposit on the substrate is a metallic deposit.

15. A process according to claim 1, wherein the deposit comprises a conductive, semiconducting or insulating track.

16. A process according to claim 1, wherein the deposit on the substrate provides one or more properties selected from antibacterial properties, magnetic properties, optical properties, sensor properties, catalytic properties and anti-corrosion properties.

17. A process according to claim 1, wherein the substrate is a planar substrate or wherein the substrate is a three-dimensional substrate.

18. A process according to claim 1, wherein the surface of the substrate comprises a material selected from plastic, metal, ceramic, biological material and glass.

19. A process according to claim 1, wherein the solution is nebulized to produce an aerosol which is fed into the plasma jet.

20. A process according to claim 19, further comprising providing a feed gas and mixing the feed gas with the aerosol to provide a mixture before feeding the mixture into the plasma jet.

21. A process according to claim 20, wherein the feed gas comprises an inert gas.

22. A process according to claim 21, wherein the feed gas further comprises a reducing gas.

23. A process according to claim 22, wherein the reducing gas is selected from hydrogen and methane.

24. A process according to claim 21, wherein the inert gas is selected from one or more of He and Ar.

25. A process according to claim 1, wherein the plasma jet is ignited and stabilized stabilised by an electrode connected to a radio frequency generator.

26. A process according to claim 1, wherein the plasma jet is pulsed, optionally providing a duty cycle of 10 to 40%.

27. A process according to claim 1, comprising depositing a first layer of a first material onto the surface of the substrate, followed by depositing a second layer of a second material onto the first layer, wherein the first and second materials are different.

28. A process according to claim 27, wherein the first material is a material which exhibits greater adhesion to the surface of the substrate than the second material.

29. A process according to claim 27, wherein the first material is selected from Ag and Au.

30. A process according to claim 1, further comprising sintering at least a portion of the deposit after deposition onto the substrate, wherein the sintering is performed by exposing the deposit to the plasma jet.

31. Apparatus for atmospheric pressure plasma jet deposition of a deposit on a substrate, the apparatus comprising: a feed solution comprising a dissolved metal precursor, wherein the dissolved metal precursor comprises a metal selected from Groups 2 to 16, with the proviso that the dissolved metal precursor does not comprise Mn; a plasma jet generator which generates a plasma jet directable towards the substrate; and means to direct the feed solution into the plasma jet.

32. A deposit on the surface of a substrate obtained or obtainable by a process according to claim 1.

33. A substrate comprising a deposit on a surface thereof, wherein the deposit is obtained or obtainable by a process according to claim 1.

Description

DESCRIPTION OF THE FIGURES

[0126] FIG. 1 shows a schematic drawing of the plasma jet deposition apparatus used in the present invention.

[0127] FIG. 2 shows a schematic drawing of the plasma jet nozzle used in the present invention.

[0128] FIG. 3 shows a schematic diagram of the plasma jet deposition apparatus including the aerosol and electrical supply used in the present invention

[0129] FIG. 4 shows the plasma driving waveforms used to generate plasma in the plasma jet deposition apparatus.

[0130] FIG. 5 shows a diagram of the concentric nebulizer aerosol generating apparatus.

[0131] FIG. 6 shows a diagram of the ultrasonic aerosol generating apparatus.

[0132] FIG. 7 shows a photograph of copper tracks being deposited on a glass surface using the plasma jet deposition apparatus.

[0133] FIG. 8 shows plots of the measured resistivity of copper tracks deposited from various concentrations of copper sulphate solution, as the number of passes of the plasma jet increases.

[0134] FIG. 9 shows scanning electron micrographs (SEM) of copper tracks deposited onto glass from copper sulphate precursor using the plasma deposition apparatus.

[0135] FIG. 10 shows a plot of calculated cross sectional areas of the plasma printed tracks deposited using precursor solutions of increasing concentration. The concentrations are given in the inset tables

[0136] FIG. 11 shows an example x-ray photoelectron spectrum (low resolution survey spectrum) collected from a sample film deposited at 25% duty cycle from 1.25 M CuSO.sub.4 solution for 120 s, with major peaks identified.

[0137] FIG. 12 shows the deconvolution of the Cu 2p XPS spectrum of copper tracks deposited with 1.25 M CuSO.sub.4 and at 25% duty cycle at 120 seconds of deposition duration. The associated relative peak areas are given in the inset tables.

[0138] FIG. 13 shows stacked Raman spectra taken from the copper track surface at increasing deposition durations. Normalized intensities are plotted.

[0139] FIG. 14 shows Raman spectra of cuprous deposits obtained with different concentrations of H.sub.2 in plasma gas.

[0140] FIG. 15 shows scanning electron micrographs of copper deposited on polyimide using copper formate precursor using the plasma deposition apparatus.

[0141] FIG. 16 shows scanning electron micrographs of silver deposited on acrylic using silver nitrate precursor using the plasma deposition apparatus.

[0142] FIG. 17 shows scanning electron micrograph of gold deposit on glass using hydrogen tetrachloroaurate precursor using the plasma deposition apparatus.

EXAMPLES

[0143] Aspects and embodiments of the present invention will now be discussed in the following examples. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

Experimental Setup

[0144] The plasma jet apparatus used in the following examples is shown in FIG. 1. The plasma jet apparatus includes a powered tungsten electrode 1 of 0.25 mm diameter placed centrally in a 40 mm long capillary tube 2 of 1.4 mm inner diameter. One end of the capillary tapers down to 1 mm, with a 0.5 mm diameter orifice present at the end to serve as the plasma nozzle 3, shown in more detail in FIG. 2. The capillary is mounted in a larger borosilicate glass tube 4 (20 mm O.D.) through a rubber septum 5 in contact with a screw-thread cap 6 with the nozzle end facing outwards. Working gas is introduced into the plasma jet apparatus via a gas inlet.

[0145] A schematic depiction of the deposition system is given in FIG. 3. Hydrogen, helium and argon gas sources were used. The hydrogen concentrations used for the experiments ranged from 0 to 5 vol % of the total amount of hydrogen, argon and helium. A typical feed gas composition was around 5 vol % hydrogen, 35 vol % helium and 60 vol % argon. The overall gas flow rate through the nozzle during the experiments varied from 50 mL/min up to 500 mL/min, with a typical overall gas flow rate being 200 mL/min. All gases were fed from high pressure cylinders 8, 9, 10 through computer-controlled digital mass flow controllers 11 (Brooks Instrument) to maintain the chosen ratio.

[0146] Two different types of nebulizer were used in the experiments to nebulize the metal precursor solutions, “Nebulizer A” and “Nebulizer B”.

[0147] Nebulizer A was a concentric nebulizer 12 (Meinhard Type K), shown in FIG. 5. The solution was fed into a capillary through the liquid inlet 13, the capillary was concentric with a glass tube connected to a pressurized gas inlet 14 through which argon was flown at a constant flow rate of 40 mL/min. The capillary and the outer tube were open on one end which serves as the aerosol nozzle 15. The fast-flowing gas around the nozzle causes the liquid inside the capillary to break into droplets that nominally range from 1 to 10 μm in size. The droplets were ejected from the nebulizer nozzle. The aerosol was injected directly into a large round bottom flask through a dip tube 16 that reaches nearly to the bottom. The larger droplets coalesce in the dip tube and collect at the bottom of the flask whereas the smaller droplets travel back up the neck and out of the flask through the aerosol outlet 17.

[0148] Nebulizer B was an ultrasonic nebulizer, shown in FIG. 6. A long necked (30 cm) round bottom flask 24 was positioned in a ultrasonic nebulizer bath. The nebulizer bath consisted of a 20 W, 1.6 MHz piezoelectric atomizer element 25 positioned under 3 cm depth of distilled water acting as a sonic transfer medium 26. The round bottom flask was positioned directly above the atomizer element at a distance of 1.2 cm. Upon energizing the piezoelectric element, atomized solution was generated within the flask.

[0149] A dip tube 27 was inserted into the round bottom flask through the flask neck, positioned approximately 1 cm above the liquid level to introduce the aerosol carrier gas. Carrier gas was introduced through the gas inlet 28 and then flown up through the neck of the flask and through an outlet 29 positioned at the top of the flask.

[0150] The argon flow containing aerosol was then sent through a flow divider made up of two needle valves 18, 19 and two rotameters 20, 21. 60% of that flow was diverted and bubbled through water to recover the copper sulfate feedstock (denoted “waste” in FIG. 5). The remaining 40% was mixed close to the flow divider exit in a wet mixing manifold with a dry mixture of hydrogen and helium which had been mixed in a dry mixing manifold. 2 metres of tubing 24 led from the wet mixing manifold to the plasma jet, to ensure good drying and mixing of the aerosol and gas mixture. The flow divider was used to adjust the amount of aerosol in the final plasma jet gas mixture. Gas was passed through the capillary where it was ignited and ejected from the nozzle onto the substrate 22 directly underneath. The jet used in this work produces dielectric barrier discharge plasma. The deposition substrate 22 serves as the dielectric barrier between the ground and the powered electrode.

[0151] FIG. 3 shows a schematic diagram of the electrical circuit used in the apparatus. The powered electrode is connected, via an L topology matching network, to a 13.56 MHz RF Generator (Coaxial Power UK). The generator supplies the power required for sustaining the plasma. The matching network is used to minimize the impedance mismatch between the plasma jet and the 500 generator output, ensuring efficient power transfer from the generator to the jet.

[0152] The RF generator output is modulated via a pulse generator using pulse width modulation. The pulse voltage and RF current waveforms that drive the apparatus are given in FIG. 4. The RF output is enabled on the rising edge of the pulse waveform, the RF output is disabled on the falling edge of the pulse waveform. This results in discrete pulses of high voltage radio frequency on the electrode, igniting the plasma. The period of the pulse waveform is defined as the time elapsed between two rising edges of the pulse generator waveform. The frequency is defined as the inverse of the period. The duty cycle is given by the ratio between the on time and the period of the pulse waveform.

[0153] Plasma printing was started by placing the glass substrate on the positioning stage 23 and turning on the pulse generator, RF generator and working gas flow.

Example 1—Plasma Printing of Conductive Copper Tracks on Glass Surfaces

[0154] Copper sulfate solution feedstock was aerosolized with the nebulizer system shown in FIG. 5, using argon feed gas. The copper sulfate solution was prepared by mixing an excess amount of copper sulfate in deionized water and mixing to achieve a saturated salt solution. This 100% saturated solution was then filtered to remove the excess undissolved copper sulfate. To prepare solutions of lower copper concentration, this solution was diluted with deionized water as necessary.

[0155] The plasma was ignited and the CuSO.sub.4 solution was nebulized into the gas stream using the “Nebulizer A” apparatus shown in FIG. 5. FIG. 7 shows a photograph of conductive copper traces being plasma-deposited with the system. Printing was performed by positioning the glass slide 0.5 to 0.75 mm beneath the powered jet nozzle and translating the stage back and forth at a rate of 1 mm/s with a computer controlled two-axis translation stage (Physik Instrumente). Static deposits were also produced in this manner by keeping the translation stage at a fixed position.

Analysis of Deposits

X-ray Photoelectron Spectroscopy (XPS)

[0156] A Thermo K-alpha photoelectron spectroscopy system equipped with a monochromatic Al—Ka (1486.6 eV) X-ray source was used for XPS spectrum acquisition. The system was operated at a spot size of 50 μm. For the deposited films, low resolution Survey spectra from 0 to 1200 eV were collected.

[0157] Depth profiling of the copper samples were done to probe further into the bulk of the material, this was done with argon ion bombardment at 1 kV acceleration voltage, 500 nA ion current and 1 mm raster size. A low energy electron flood gun was used during spectra collection to prevent charging, adventitious C 1 s peaks for all spectra were found at 284.8±0.1 eV binding energy, confirming that surface charging was minimal.

[0158] Samples for the XPS were produced with the plasma jet by depositing copper spots of roughly 1 mm diameter onto glass slides. The prepared samples were washed with deionized water prior to analysis. The samples were not cleaned via argon ion etching to remove surface contaminants unless reported otherwise.

[0159] XPS spectrum deconvolution was performed with the CasaXPS software package. The spectral peaks were first fit with a Shirley type background. If the identity of the samples were established, peaks were fit to the spectra using constraints on the line shape, FWHM and position. The parameters for the known peaks were all based on the work of Biesinger et al. (Advanced analysis of copper X-ray photoelectron spectra. Surface and Interface Analysis 2017, 49 (13), 1325-1334).

Micro Raman Spectroscopy

[0160] A Renishaw In-Via confocal Raman microscope with an unpolarized 532 nm argon ion laser excitation source was used for characterization of copper containing films. 50x objectives were used to focus the laser beam. Samples were prepared in an identical fashion to the XPS and AES experiments of Example 2. Raman spectra were taken from an area of approximately 10 μm diameter with a 100x objective. The spectra were recorded in the 140 cm.sup.−1 to 900 cm.sup.−1 Raman shift range at ambient temperature.

Stylus Profilometry

[0161] A Bruker DekTak XT stylus profilometer was used to measure the track profiles and thicknesses. The stylus radius was 5 μm and the linear scan resolution was set to 520 nm for all profilometry measurements. After data collection, the background slope due to the non-uniform glass substrate was subtracted from all results to provide a flat background.

Scanning Electron Microscopy (SEM)

[0162] Samples for SEM imaging were produced in the form of 1 cm long traces. Samples were deposited in 5 vol % hydrogen. In order to image the time-evolution of the film morphology, different numbers of passes from 1 to 200 were made over the substrate to obtain different film thicknesses.

[0163] Scanning electron micrographs were taken with a JEOL JSM 6701 F SEM at an accelerating voltage of 10 kV with samples placed perpendicularly to the electron gun. Samples were cleaned by washing in distilled water followed by acetone. The samples were sputter-coated with gold for 10 seconds prior to imaging to avoid charging artifacts in the produced images.

Resistivity Measurements

[0164] Resistivities of the deposits were measured with a Fluke 179 digital multimeter. Samples for these measurements were identical to the samples produced for the SEM experiments. The measurements were made with a two probe configuration, measuring the trace resistance from one end to the other.

Conductivity of Deposited Tracks

[0165] Measured conductivities for various films at different times and feedstock concentrations are shown in FIG. 8. After 60 passes at 1 mm/s with a 1.25 M concentration of copper salt solution that is aerosolized the resistivity drops to below 10 Ω cm.sup.−1. With a lower concentration of copper sulfate it takes a greater number of passes to achieve similar conductivity. Minimum resistivity was measured at 0.3 Ω cm.sup.−1 (0.62 M, 300 passes). Based on the trace length of 10 mm and approximate width of 1 mm, the sheet resistance is calculated to be 30 mΩ/sq, which is 60 times higher than pure copper (0.5 mΩ/sq).

[0166] Concentration of the CuSO.sub.4 feedstock was found have a strong effect on the time required for onset of conductivity. A sharp drop at around 80, 90 and 200 passes of the plasma jet was noted for 1.25, 0.94 and 0.62 M solutions respectively. Deposition performed with 0.31 M solutions did not become conductive during the timescale of the experiments here.

Morphology of Deposited Tracks

[0167] Scanning electron microscopy of the surface of the copper traces reveals the evolution of film morphology and surface coverage as deposition progresses. The micrographs shown in FIG. 9 belong to copper traces deposited at 25% duty cycle and 1.25 M CuSO.sub.4 concentration. FIGS. 9 (a)-(c) show the surface of the same deposited copper track moving from the centre of the track to the edge of the track.

[0168] As shown in FIG. 9, near the centre of the deposit (FIG. 9(a)), the copper is a contiguous layer of particles that are laid down and appear to grow. The formation of one interconnected mass in this way supports the low resistivity evidenced in FIG. 8. The edges of the deposit (FIG. 9(c)) show scattered individual particles 0.5-1 μm in diameter. Further in (FIG. 9(b)), the particles are seen to be sintered together with large voids remaining in between.

[0169] The onset of conductivity of the copper films at 70 passes seems to coincide with an observed complete surface connectivity of the copper layer, as shown in FIG. 9(a). The addition of further layers to the film does not change the morphology or conductivity to any significant extent after this point.

[0170] Cross-sectional areas of the tracks calculated from the profilometry experiments are shown in FIG. 11. The deposits show initial rapid growth with increased number of passes which appears to slow and reverse at around 130, 100, 90 and 80 passes for the 0.31 M, 0.62 M, 0.94 M and 1.25 M solutions respectively before continuing at a slower growth rate. These points, for the given samples, approximately correspond to the onset of electrical conductivity with the exception of the 0.31 M sample.

[0171] At decreasing concentrations of CuSO.sub.4 in the aerosol solution, the average particle sizes in the deposit are found to shrink from approximately 1 μm diameter at 1.25 M to 0.10 μm at 0.31 M.

Chemical Composition of the Deposit

[0172] An example XPS survey spectrum is given in FIG. 11. Elements present in all samples were determined to be mostly identical, differing only in relative quantities and chemical environments. There is no incorporation of the anion or products of the anion, in this case sulfate. The low resolution survey spectra displays photoelectron signals from silicon, copper, oxygen, adventitious carbon and the corresponding Auger electron signals. The major surface contaminant was carbon, likely due to contamination from the atmosphere (dust etc.) present on all samples at a level of 9-14 at. %

[0173] In order to quantify the change in the proportions of Cu(II) and Cu (I/0) contained in the sample, deconvolution of the high resolution Cu 2p X-ray photoelectron spectra was employed. FIG. 12 shows deconvolution of XPS spectra at into 2 major components. The two components were assigned to Cu(I/0) 2p3/2 and 2p1/2 signals. Based on the conductivity of the traces and the XPS spectra obtained from the surface, the traces are composed mainly of Cu (0) species.

[0174] Raman spectroscopy shows the presence of little copper oxide. The copper oxide observed by XPS is from a surface layer which is formed after the deposition process and is probably only a few 10 s of nanometers thick. Raman spectra collected from the samples (FIG. 13) at different stages of deposition show five major bands centred around 149, 215, 413, 510 and 645 cm.sup.−1, these bands can all be assigned to Cu.sub.2O (Debbichi et al., Vibrational Properties of CuO and Cu.sub.4O.sub.3 from First-Principles Calculations, and Raman and Infrared Spectroscopy. J. Phys. Chem. C 2012, 116 (18), 10232-10237) and band positions agree with reference spectra in literature (Sander et al., Correlation of intrinsic point defects and the Raman modes of cuprous oxide. Phys. Rev. B 2014, 90 (4), 8). The peak intensities attributed to Cu.sub.2O are seen to vanish with increased deposition time. Beyond 120 s, the sample is composed mainly of metallic copper which is Raman inactive.

[0175] When the H.sub.2 gas was not added to the plasma gas mixture, a thin layer of porous black deposit was obtained after 100 seconds of deposition time. The Raman spectra show that the films produced without H.sub.2 addition are composed of CuO (FIG. 14). Increasing the H.sub.2 gas concentration to 2% yields the lower oxide Cu.sub.2O and further increasing the H.sub.2 concentration above 5% yields metallic Cu as evidenced by XPS spectra.

[0176] Raman spectra show that initially, these deposited layers are mostly composed of Cu.sub.2O. This provides film adherence to the glass surface, presumably because of Cu—O—Si bonds that are formed between the glass and the oxide layer, similar to the situation in the well-known metal-to-glass bonding technique “Housekeeper's seal” (Hull et al., Glass-to-metal seals. Physics-J Gen. Appl. Phys. 1934, 5 (1), 384-405).

[0177] The adhesion to the surface of the glass is excellent due to this layer.

Example 2—Plasma Printing of Conductive Copper Tracks on Polyimide Surfaces

[0178] An aqueous solution of copper (II) formate was made to 1.1 M concentration by dissolving solid copper (II) formate (>97%) in water. 20 mL of this solution was poured into the ultrasonic nebulizer apparatus (“Nebulizer B”) shown in FIG. 6. For the deposition process, the aerosol carrier gas was chosen as helium, the carrier was flown at a rate of 60 ml/min through the nebulizer apparatus. The aerosol laden gas was then mixed with a dry mixture of 6% v/v hydrogen in helium, flown at a rate of 150 ml/min, in a mixing manifold. This final working gas mixture, comprised of helium, hydrogen, water vapor and copper formate aerosol was flown through 2 m of tubing for thorough mixing and drying of copper formate particles.

[0179] Polyimide sheet (Kapton H N, DuPont) of 0.075 mm thickness was used as the print substrate. The flat sheet was placed on a translation stage for track printing. Printing was performed by positioning the polyimide substrate 0.2 mm beneath the powered jet nozzle and translating the stage back and forth at a rate of 0.2 mm/s, for a distance of 11 mm with a computer controlled two-axis translation stage (Physik Instrumente). Printing was initiated by starting the gas flows and setting the duty cycle of the pulse generator at 55% at 11 kHz. The peak continuous power output of the generator was set at 25W. The RF generator was then turned on, achieving ignition and creating stable plasma. The duty cycle of the pulse generator was then brought down to 30% and the substrate was translated under the plasma jet, resulting in a well sintered conductive copper track.

Deposit Characterisation

[0180] After a single pass, the deposited copper tracks showed a resistivity of 1.2 kΩ cm.sup.−1, this value was found to decrease to 2.3 Ω cm.sup.−1 following a second deposition pass. Increasing the number of passes with the plasma jet did not decrease the resistivity further. This was likely due to the large contribution from probe resistivity leading to a minimum measurement of around 1 D. The track width was measured to be 150 μm under an optical microscope.

[0181] The produced tracks after two passes of the plasma jet were examined under scanning electron microscopy (SEM) for morphological characteristics, the produced micrographs are given in FIG. 15(a-b). The connectivity of the tracks was found to be excellent with no discernible cracks or large voids. The surface of the deposit was composed of sintered particles with mean diameter around 1 μm. Small voids on the surface with mean diameters comparable to the particles were also visible. The sintered particle connectivity was consistent across the length and width of the track with clearly defined edges present.

[0182] Raman spectroscopy was used to characterize the chemical composition of the deposited tracks. No signal associated with copper(I/II) was present in the collected spectra suggesting minimal oxide impurity presence on the deposit surface.

[0183] Example 3—Plasma Printing of Conductive Silver Tracks on Poly(methylmethacrylate)

[0184] An aqueous solution of silver nitrate was made to 0.35 M concentration by dissolving solid silver nitrate in water. 20 mL of this stock solution was used to generate silver laden aerosol in the ultrasonic nebulizer apparatus (“Nebulizer B”) shown in FIG. 6. PMMA (acrylic) sheet (RS Pro, UK) of 2 mm thickness was used as the print substrate. Printing was initiated by starting the gas flow and setting the duty cycle of the pulse generator at 40% at 20 kHz and turning on the RF Generator. The peak continuous power output of the generator was set at 20 W. Once ignition and stable plasma was created, the duty cycle was brought down to 20% and the substrate was translated under the plasma jet, resulting in a well sintered conductive silver track.

Deposit Characterisation

[0185] After a single pass, the deposited silver tracks showed a resistivity of 6.1 Ω cm.sup.−1, this value was found to decrease to 0.7 Ω cm.sup.−1 following a second deposition pass. Further deposition did not appreciably decrease the track resistivity. The track width was measured as 180 μm under an optical microscope.

[0186] The produced tracks after two passes of the plasma jet were examined under scanning electron microscopy (SEM) for morphological characteristics, the produced micrographs are given in FIG. 16(a-b). The central 50 μm of the track was found to be well sintered and devoid of large cracks or discontinuities. The peripheral sections of the tracks were found to be incompletely sintered with small cracks apparent. Under high magnification, the center of the tracks were found to be comprised of sintered silver particles around 100 nm in diameter.

Example 4—Plasma Printing of Conductive Gold Tracks on Soda Lime Glass

[0187] An aqueous solution of hydrogen tetrachloro aurate (>99%) was made to 0.08 M concentration by dissolving solid hydrogen tetrachloro aurate in distilled water. 20 mL of this stock solution was used to generate gold laden aerosol in the ultrasonic nebulizer apparatus (“Nebulizer B”) shown in FIG. 6. Glass microscope slide (VWR International, Belgium) of 1 mm thickness was used as the print substrate. Printing was initiated by starting the gas flow and setting the duty cycle of the pulse generator at 40% at 20 kHz and turning on the RF generator. The peak continuous power output of the generator was set at 20 W. Once ignition and stable plasma was created, the duty cycle was brought down to 20% and the substrate was translated under the plasma jet, resulting in a well sintered conductive gold track.

Deposit Characterisation

[0188] After a single pass, the deposited silver tracks showed a resistivity of 0.8 Ω cm.sup.−1, this value did not decrease with repeated passes. The track width was measured to be 250 μm under an optical microscope.

[0189] SEM was used to characterise the track surface, the produced high magnification micrograph is given in FIG. 17. The surface appeared exceptionally uniform and well sintered under low magnification, no voids or cracks were present. Under higher magnification, the surface was seen to be made up of deposited gold particles with an average diameter of 20 nm.

[0190] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0191] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0192] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0193] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0194] Throughout this specification, including the claims which follow, unless the context requires otherwise, the words “have”, “comprise”, and “include”, and variations such as “having”, “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0195] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means, for example, +/−10%.

[0196] The words “preferred” and “preferably” are used herein refer to embodiments of the invention that may provide certain benefits under some circumstances. It is to be appreciated, however, that other embodiments may also be preferred under the same or different circumstances. The recitation of one or more preferred embodiments therefore does not mean or imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the disclosure, or from the scope of the claims.