WO2014037736A1

WO2014037736A1 - Method of coating and etching

Info

Publication number: WO2014037736A1
Application number: PCT/GB2013/052339
Authority: WO
Inventors: David Sheel; John Hodgkinson
Original assignee: University of Salford
Current assignee: University of Salford
Priority date: 2012-09-07
Filing date: 2013-09-06
Publication date: 2014-03-13
Anticipated expiration: 2015-03-07
Also published as: GB201215996D0; GB2505685A; GB2505685B

Description

Method of coating and etching

FIELD OF THE INVENTION

The invention relates to a method of addition or removal of a material to or from a substrate, particularly but not necessarily exclusively chemical vapour deposition or discharge etch, and equipment for performing said method.

BACKGROUND OF THE INVENTION

Chemical vapour deposition (CVD) is a chemical process used to produce solid materials and is often used to create thin films. Thin films produced using CVD are generally hard and have desirable surface properties (e.g. desirable crystallinity) for a given application. Films may also be grown rapidly when produced using CVD. Examples of thin films or coatings formed using CVD include coatings or films used for energy efficiency, hardening, semiconductor devices, wear resistance, or photovoltaic applications.

CVD can be particularly beneficial when performed at atmospheric pressure (atmospheric pressure chemical vapour deposition (APCVD)). Performing CVD at atmospheric pressure alleviates the need for pressure control, which in turn reduces the capital cost for equipment and also permits continuous manufacture. However, to conduct CVD at atmospheric pressure requires the substrate to be heated to a high temperature, which can cause damage to the substrate and also limits the choice of substrate available for use.

Lower temperature CVD processes are known, for example vacuum plasma CVD, but problematically vacuum plasma CVD must be carried out at low-pressure and therefore is often incompatible with the need for low capital costs and continuous manufacture. There are also further size/volume constraints that restrict the application and exploitation of vacuum plasma CVD.

Attempts have been made to conduct APCVD at lower temperatures using plasma enhanced activation. In such systems dielectric plates are used to create a shield between the electrodes and plasma, and is known as a dielectric barrier discharge system. The dielectric layers act to rapidly extinguish discharge current to a few mA.cm^"2. This reduces the current density and means there is less chance of thermal runaway in the plasma, which improves the stability of the system, reducing the risk of transition to filamentary conditions which can result in substrate damage. However, the significant reduction in current (hence lower electron density) results in a reduced density of chemically reactive species, and therefore reduced film formation.

A similar plasma enhanced dielectric set up can also be used for etching a surface.

SUMMARY OF THE INVENTION The invention relates to using a short pulsed plasma in a plasma enhanced chemical vapour deposition and/or discharge etch process. Using a short pulsed plasma instead of a conventional sinusoidal plasma can result in improved surface properties and increased control of surface properties.

Accordingly, a first aspect of the present invention provides a method of chemical vapour deposition of a material on a substrate, or etch of material from a substrate. The method comprises forming a plasma using a short pulsed power signal so that the plasma is a short pulsed plasma. The short pulsed plasma being used to enhance the deposition of material or the etch of material to or from the substrate.

Advantageously, the use of a short pulsed plasma means that for the same energy input, the peak current is higher. The increase in peak current means that there is a higher activation in the gas phase, which means that an improved coating can be applied to the surface of the substrate.

Short pulsed plasmas have been researched in other fields, but never in the field of chemical vapour deposition or etching. The inventors of the present invention have found that using a short pulsed plasma in plasma enhanced chemical vapour deposition and barrier discharge etch surprisingly gave improved surface properties and improved control of the surface properties of a coating or an etched surface.

For example, in the case of chemical vapour deposition, the short pulsed plasma permits a coating of greater crystallinity or greater controlled crystallinity to be deposited on a surface. In the case of barrier discharge etch, better control of the surface topography can be achieved, as well as a surface having asperities with more rounded peaks and troughs, which is desirable particularly in applications involving subsequent deposition steps. Short pulsed plasma is a term clearly defined in other technical fields. The pulsed power supply used to form the short pulsed plasma typically has a sub-microsecond "on phase" pulse width. The sub microsecond pulse widths and the breakdown voltage applied to form the plasma are such that the capacitive charge and discharge currents at each end of the "on phase" pulse have a duration of less than 100 ns, more particularly are in the order of 10 ns. A sample short pulsed voltage signal suitable for forming a short pulsed plasma when helium is the process gas is shown in Figure 10, and the resulting capacitive charge and discharge currents are shown in Figure 11. In Figure 10 the axis labelled y3 is "Voltage (V)" and the axis labelled x3 is "Time (s)", and in Figure 11 the axis labelled y4 is "Current (A)" and the axis labelled x4 is "Time (s)". In this example the peak current is ± 1.5 to 2 Amps.

As is known to a person skilled in the art, in the case of chemical vapour deposition, the method may comprise the step of providing a reaction gas, for example providing a flow of reaction gas towards or over the substrate. The reaction gas being of the chemical composition required to form a desired material addition on a substrate. Suitable gases to use in such a process are well known in the art, and include for example TiCLt and for example 0₂ if depositing a titania film on a substrate.

The method may comprise the step of positioning a substrate in a region near a pair of opposing electrodes. The short pulsed plasma may be formed between the opposing electrodes. A shield may be provided for controlling a discharge current from an electrode. A shield may be positioned adjacent each electrode. The shield may be a plate. Two plates may be provided, and each plate may shield one of the pair of electrodes. The two plates may be positioned in opposition in a region between the pair of electrodes. The shield may be a dielectric shield.

The power signal may be a square wave or a trapezoidal wave, or a near square or near trapezoidal wave. A square wave or near square wave is advantageous because of a near instant rise time, for example, 0V to the maximum output voltage. The short rise time permits substantially all the gases to breakdown at substantially the same time. This is advantageous over conventional sinusoidal activation of gases because the steady increase of voltage in a sinusoidal activation system means that the gases breakdown at different times, leading to less control of the deposition or etch process. The power signal may have an asymmetric duty cycle. For example the time between each "on phase" pulse may be longer than the length of each "off phase" pulse. The "on phase" being a period of increased voltage output.

The pulsed power supply may be a short pulsed power supply. For example, the pulse width may be less than or equal to about 1 μβ. For example, the pulse width may be less than or equal to about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or any value therebetween.

The frequency of the power signal may range from 0.5 kHz to 100 kHz. The frequency of the pulsed power supply can be varied so as to vary the properties of the coating or etched surface.

The power signal may range from 2.5 kV to 10 kV. For example, 3.5 kV, 4.5 kV, 5 kV, 6 kV or any voltage therebetween. The output of the power signal may be selected based on factors such as the reactor design and the distance over which the plasma is formed.

The plasma may be formed using helium, argon or nitrogen, or any other suitable inert gas. A sinusoidal activation system of the prior art optimally works using helium, which is an expensive process gas. However, the pulsed power supply permits nitrogen to be used which is a much less expensive gas, and as such reduces the cost of a deposition or etch process.

The method may be performed at atmospheric or near atmospheric pressure. As discussed previously, the short pulsed plasma means that for the same energy input the peak current is higher, which results in increased activation of the desired chemical reactions. This means that to achieve a similar level of coating or etch as a sinusoidal process the chemical vapour deposition or etch process does not need to be performed at low pressure, i.e. the process can be performed at atmospheric pressure. Performing the process at atmospheric pressure means that the process can be continuous and the high capital costs may be reduced. The method may be performed at sub-atmospheric pressure. Performing the method as sub-atmospheric pressure may further improve the properties of a coating or an etch, and also may permit coatings to be deposited using materials not previously possible. The substrate may be heated to a temperature ranging from 50°C to 300°C, for example below 200°C, below 150°C or below 100°C. The heating of the substrate may be physical heating, for example using a heated block or pre-heating in a furnace/oven. The high peak current possible with the pulsed power supply means that the substrate can be heated to a reduced temperature compared to the chemical vapour deposition and etch processes of the prior art. This is desirable because it permits substrates such as plastics to be used, which may not have previously been possible. It also means that the properties of the substrate are less likely to be changed, for example it may be undesirable to re-heat a metal substrate which has previously been heat treated. However, the coating properties and capabilities (e.g. materials that may be deposited) can be further improved if the substrate is heated to a high temperature. For example, the substrate may be heated to a temperature equal to or below 500°C, for example below 400°C.

A second aspect of the invention provides a method of chemical vapour deposition. The method comprises the step of forming a short pulsed plasma in a plasma enhanced chemical vapour deposition reactor.

The second aspect may have any of the features of the first aspect.

A third aspect of the invention provides a method for etching a surface of a substrate. The method comprises the step of forming a short pulsed plasma in a discharge etch reactor. The third aspect may have any of the features of the first aspect. The discharge etch reactor may be a barrier discharge etch reactor, for example a dielectric barrier discharge etch reactor.

A fourth aspect provides a method of chemical vapour deposition. The method comprises the step of providing a substrate. A reaction gas is provided in a region near the substrate. A short pulsed plasma is formed for activating chemical reaction of the reaction gas to form a coating on the substrate.

The reaction gas may be provided as a flow to or near the substrate.

The fourth aspect may have any of the features of the first aspect. A fifth aspect of the invention provides a method of discharge etch. The method comprises the step of providing a substrate having a surface layer for at least partial removal. A short pulsed plasma is formed in a region near the substrate to activate removal of material from the surface layer of the substrate.

The fifth aspect may have any of the features of the first or third aspect. A sixth aspect of the invention provides a method of addition or removal of material to or from a substrate. The method being performed at atmospheric or near atmospheric pressure. The method comprises the step of forming a plasma using a short pulsed power signal so that the plasma is a short pulsed plasma. The short pulsed plasma is used to enhance the addition or removal of material to or from the substrate.

A seventh aspect of the invention provides equipment for the chemical vapour deposition of a material on a substrate or etch of a material from a substrate. The equipment comprises a holder for a substrate. The equipment also comprises apparatus for forming a short pulsed plasma. The equipment further comprises a power supply configured to provide a pulsed power signal to form the short pulsed plasma. The short pulsed plasma enhances the deposition of material on the substrate or etch of material from the substrate.

The equipment may be used to perform the method of any one of the first, second, third, fourth, fifth or sixth aspects. The power supply may be configured to output a square wave, a trapezoidal wave or a near square or near trapezoidal power signal.

The apparatus may comprise a gas source. The gas source may provide a process gas for forming a short pulsed plasma, or any other suitable inert gas. The process gas may be substantially helium, argon or nitrogen, or any suitable inert gas. In the case of addition of a material, the gas source may further provide a reaction gas for forming a film (or coating) of material on a substrate.

The equipment may comprise two opposing electrodes connected to the power supply for forming an electric field to activate the short pulsed plasma. The equipment may comprise a shield for shielding an electrode to control the current output. The shield may be a plate. Two plates may be provided each shielding one electrode, and the plates may be positioned in opposition and/or adjacent the electrodes. The shield may be a dielectric shield. The holder may be configured to translate during operation of the equipment.

The equipment may comprise a housing for the apparatus, and the housing may be held at atmospheric or near atmospheric pressure. Alternatively, the equipment may comprise a housing for the apparatus, and the housing may be held at a sub- atmospheric pressure. For example, the housing may be a pressure chamber. The power supply may be configured to supply a power signal having a frequency of between about 0.5 kHz and about 100 kHz. The power signal may have an asymmetric duty cycle (i.e. the "on phase" pulse time may be shorter than the "off phase" pulse time). The power supply may be configured to supply a power signal ranging from 2.5 kV to 10 kV. The power supply may be configured to supply a power signal with a pulse width less than or equal to about 1 μβ.

The apparatus may be a chemical vapour deposition reactor or a barrier discharge etch reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows schematically an arrangement for plasma enhanced chemical vapour deposition; Figure 2 shows photographs of titania films produced using the equipment of Figure 1 at varying repetition rates;

Figure 3 shows X-ray diffraction patterns for a selection of the films shown in Figure

2; Figure 4 shows AFM images showing the surface topography of a selection of the films shown in Figure 2;

Figure 5 shows cross sectional SEM images of a selection of the films shown in Figure 2;

Figure 6 shows schematically an arrangement for plasma enhanced barrier discharge etch;

Figure 7 shows AFM images showing the surface topography of surfaces etched using the equipment of Figure 6;

Figure 8 shows an angular distribution of the etched surfaces of the AFM images of Figure 7; Figure 9 shows a height distribution of the etched surfaces of the AFM images of Figure 7;

Figure 10 shows an example waveform of a short pulsed power signal for forming a short pulsed plasma; and

Figure 11 shows an example current resulting from the waveform shown in Figure 10. DETAILED DESCRIPTION OF EMBODIMENT(S)

Referring to Figure 1 an arrangement for atmospheric pressure plasma enhanced chemical vapour deposition (AP PECVD) is indicated generally at 10. The arrangement has an electrode 12 opposing a further electrode, which in this embodiment is a steel block 14. Optionally, the steel block 14 may be heated to warm the substrate. A dielectric plate 16, 18 is positioned adjacent each electrode 12, 14 on a side of the electrode 12, 14 near the opposing electrode 14, 12 and positioned such that the dielectric plates 16, 18 are in opposition. Each of the dielectric plates 16, 18 has a length greater than the length of each of the electrodes 12, 14, so as to better shield the electrodes.

The steel block 14 is mounted to an insulated support 22, which is earthed.

A process gas inlet 20 is positioned adjacent one end of each of the dielectric plates 16, 18, such that in use, process gas can be directed to flow through a region created between the two dielectric plates to an opposite end of the dielectric plates. The gas inlet 20 supplies a process gas for forming a plasma in a region 36 between the opposing electrodes. In this embodiment the process gas is helium, but in alternative embodiments the process gas may be nitrogen or argon, or any suitable inert gas. The region 36 between the electrodes forms part of a reaction region, i.e. a region for chemical reactions during use of the reactor.

The gas inlet 20 may also supply a reaction gas to form reactants for coating a surface of a substrate 32. In this embodiment the reactants are TiCLt and 0₂ to form a titania coating, but any suitable reactants may be used to form a desired coating. The electrodes 12, 14, dielectric plates 16, 18, insulated support 22 and gas inlet 20 are positioned within a housing 24. An exhaust 26 is positioned at an end of the housing opposite the process gas inlet 20 for the process gas to exit the housing (as indicated by arrow 28). The housing 24 has a funnelled portion 30 adjacent the exhaust to guide the process gas to the exhaust 26. A gas ring 34 for supplying a purging gas, for example nitrogen, is positioned at an end of the housing spaced from the gas inlet 20 in a direction away from the exhaust and the electrodes and dielectric plates.

During use, i.e. to deposit a film on the substrate 32, the substrate 32 is positioned on the dielectric plate 16 that is adjacent the metal block 14. The positioning of the substrate is such that one surface rests on the dielectric plate 16 and the opposing surface is exposed to a reaction region between the dielectric plates. The substrate may be placed in a holder, or as in the present embodiment the dielectric plate acts a holder for the substrate. In this embodiment the substrate is glass, but as known in the art any suitable substrate may be used. Further substrates such as plastics or heat- treated metals not previously suitable for use in chemical vapour deposition may be used. In alternative embodiments the substrate may be positioned in a region near an end of the electrodes rather than in a region between the electrodes.

The plasma is activated using a power supply 38 that is configured to output a short pulsed signal. In this embodiment, the power signal is substantially a square wave. A square wave is advantageous because of a near instant rise time from, for example, 0V to the maximum output voltage. It is suspected that a short rise time permits substantially all gases to breakdown at the same time. This is advantageous over the sinusoidal activation of gases of the prior art because the steady increase of voltage in a sinusoidal activation system means that the gases breakdown at different times, leading to less control of the deposition or etch process.

The maximum voltage output of the power supply is selected depending on the size of the reaction chamber, for example, the larger the chamber the greater the maximum output voltage. In this embodiment, the power supply provides a 3.5 kV power signal.

The length of the pulse is less than 1 μβ, and the repetition rate (or frequency) can be varied to optimise the desired surface characteristics, as will be shown in the later described examples.

Figure 10 shows a sample waveform used in the present embodiment, and Figure 11 shows the resulting current. In Figure 10 the axis labelled y3 is "Voltage (V)" and the axis labelled x3 is "Time (s)", and in Figure 11 the axis labelled y4 is "Current (A)" and the axis labelled x4 is "Time (s)". In this example the peak current is ± 1.5 to 2 Amps.

Other parameters of the method such as gas flow rate are similar to conventional CVD methods and are clear to the person skilled in the art and as such are not described in more detail here. The following describes tests performed using the above described method at varying frequencies.

Titania films were deposited on a glass substrate using a short pulsed plasma. The power supply signal was a 3.5 kV with a 500 ns square wave, and the repetition rate was from 10 to 100 kHz in 10 kHz increments. The tests were all performed at atmospheric pressure with the substrate heated to a surface temperature of 275 °C.

For comparison, a titania film was deposited on a glass substrate heated to a surface temperature of 275°C using TiCLj and 0₂, and grown thermally with no addition of plasma activation, under atmospheric pressure. The titania film thermally grown had a slow growth rate and analysis via X-ray diffraction (XRD) showed the film to be amorphous.

Referring to Figure 2, photographs of titania films produced during these tests are shown, the repetition rate increases from the top left along to the top right and then from the bottom left to the bottom right: the film formed by a repetition rate of 10 kHz is indicated at 40, 20 kHz at 42, 30 kHz at 42, 40 kHz at 46, 50 kHz at 48, 60 kHz at 50, 70 kHz at 52, 80 kHz at 54, 90 kHz at 56, and 100 kHz at 58.

The tests showed that a thicker film was achieved at a higher pulse repetition rate. The higher repetition rates also showed an increase in adhered particulates. It is believed that using a pulsed power supply permits a greater activation current to be achieved, which results in a more energetic gas phase which at higher repetition rates may increase homogeneous nucleation. In a conventional thermal atmospheric pressure CVD process the surface energy associated with the substrate and the new solid material favours heterogeneous reaction resulting in nucleation on the substrate forming a film. If the gas phase is heated beyond a critical level, there is sufficient energy to allow competitive homogeneous reaction to occur in the gas phase resulting in gas phase nucleation of solid material and powder formation.

Analysis of the films using XRD showed that the films were anatase. Referring to Figure 3, the output of the XRD at 10 kHz is indicated at 60, at 80 kHz at 62, at 90 kHz at 64 and at 100 kHz at 66. It can be seen from Figure 3 that the degree of structure and relative intensity of the peaks varies between a repetition rate of 10 kHz, 80 kHz, 90 kHz and 100 kHz, i.e. the proportion of crystalline material increases with repetition rate. Figure 3 also shows the relative intensity of the peaks change with a higher repetition rate, indicating a transition in growth mechanism, which results in a change of preferred orientation. It is predicted that if the described procedure is conducted at elevated temperatures it will be possible to use the repetition rate to control the transition between anatase and rutile morphologies.

The samples were then analysed using atomic force microscopy (AFM). The resulting AFM images are shown in Figure 4, and indicated at 68 for a repetition rate of 20 kHz, 70 for a repetition rate of 90 kHz and 72 for a repetition rate of 100 kHz. These images show changes in topography related to increase in crystallinity, change in orientation and a decrease in amorphous material at a higher repetition rate.

In Figure 4, the view angle is indicated by a symbol 0 and the light angle is indicated by a symbol . The scan size is 5 μπι, scan rate is 0.5003 Hz, the number of samples is 512. The image 68 has an x-axis of 1 μιη/div and z-axis of 100 nm/div. The image 70 has an x-axis of 1 μιη/div and z-axis of 1000 nm/div. The image 72 has an x-axis of 1 μιη/div and z-axis of 150 nm/div.

The samples were then analysed under a scanning electron microscope (SEM), and the results are shown in Figure 5; the SEM image for a repetition rate of 10 kHz is indicated at 74 and 76, for 50 kHz at 78 and 100 kHz at 80. Two images are shown for a repetition rate of 10 kHz, one image 74 is an in lens detector image to highlight the film thickness, and the second image 76 is an off-set detector image to give increased depth of field to show the film structure. The scale "a" of image 74 indicates a distance of 200 nm, the scale "b" of image 76 indicates a distance of 200 nm, the scale "c" of image 78 indicates a distance of 100 nm, and the scale "d" of image 80 indicates a distance of 2 μτη. The SEM image for a repetition rate of 10 kHz show a continuous film approximately 200 nm thick and a crystalline structure with individual crystallites in the order of 100 nm. Increasing the plasma pulse repetition rate results in increased growth rate, for example at 80 kHz the film was 600 nm thick. Further it can be seen that at 100 kHz the crystalline structure is different to the crystalline structure at the lower repetition rates.

These tests show that the use of a short pulsed power supply in a plasma enhanced CVD process permits increased control of the structure and growth rate of a film formed. Further, the pulsed drive permits films to be formed at atmospheric pressure and low temperature that are comparable to films formed using high temperature APCVD methods of the prior art. The short pulsed drive is further advantageous because of its stability compared to sinusoidal systems of the prior art.

In the present embodiment the process is conducted at atmospheric pressure, but in alternative embodiments the process may be performed at low pressure. In alternative embodiments, the temperature the substrate is heated to may be higher or lower than 275°C. Increased temperature and/or reduced pressure can achieve improved surface coating and can give a different crystalline structure compared to conventional sinusoidal activation of the plasma.

Referring to Figure 6 an arrangement for a barrier discharge etch process is indicated generally at 110. In this embodiment a substrate 132 is provided in a region at one end of two opposing electrodes 112, 114. The substrate 132 is positioned in a translating holder that can move the substrate in a direction transverse, and in this embodiment substantially perpendicular to a direction parallel to the axial alignment of the electrodes. At an opposite end of the electrodes 112, 114 to the position of the substrate 132 is a process gas inlet and a process gas exhaust. The arrow 128 indicates the flow of gas during operation of the equipment 10.

A dielectric plate 116, 118 is positioned adjacent each electrode 1 12, 114 on a side of the electrode 112, 114 near the opposing electrode 114, 112 and surrounding each end side of the respective electrode. The two dielectric plates 116, 118 are in opposition. Each of the dielectric plates 116, 118 has a length greater than the length of each of the electrodes 112, 114, so as to better shield the electrodes. An end of each dielectric plate near the substrate 132 is rounded so as to direct process gas flow. A further dielectric plate 117, 119 is positioned spaced from but substantially parallel to the remaining exposed side of each electrode.

During use, i.e. to etch a surface of a substrate 132, the substrate is positioned at an end of the electrodes 112, 114 opposite the process gas supply. The gas flow (indicated by arrow 128) flows from the gas supply inlet towards the substrate, between a reaction region formed between the two electrodes. In a region between the two electrodes a plasma 137 is formed due to the flow of process gas and an electric field formed by the two opposing electrodes. The process gas then flows around an end of the dielectric plates 116, 118 near the substrate 132, and back towards the process gas exhaust between the exposed face of the electrode 112, 114 or the dielectric plate 116, 118 and the further dielectric plate 117, 119 spaced from each respective electrode 112, 114. The current supplied to the electrodes 112, 114 to form the electric field to create the plasma is supplied using a power supply 138 that is configured for a pulsed power output. In this embodiment, the power supply output is substantially a square wave, a sample pulse from the waveform used is shown in Figure 10. A square wave is advantageous for similar reasons to those described for the deposition arrangement. The maximum voltage output of the power supply is selected depending on the size of the reaction chamber, for example, the larger the electrode separation the greater the maximum output voltage. In this embodiment, the power supply supplies a 4.5 kV power signal.

The length of the pulse is less than or equal to 1 μβ, and the repetition rate (or frequency) can be varied to optimise the desired surface characteristics, as will be shown in the later described examples.

Again, parameters such as the flow rate of process gases are similar to those of conventional barrier discharge etch processes and are therefore known to the person skilled in the art. The advantages of the above method are illustrated in the following examples. In this example, the samples were formed by depositing a zinc oxide (ZnO) film on a glass substrate using conventional APCVD.

The power supply signal output was 4.5 kV with a pulse width of 0.5 and a repetition rate of 10 kHz, 20 kHz, 30 kHz, 40 kHz, and 50 kHz. Each sample was exposed to 100 passes with a speed of 2.3 x 10^"2 ms^"1.

An AFM of the final surfaces is shown in Figure 7. The image 182 in the top left corner is of a non-etched sample piece, i.e. a glass substrate with a ZnO film. The AFM image 184 is a surface produced using a repetition rate of 10 kHz, 186 was produced at 20 kHz, 188 at 30kHz, 190 at 40 kHz, and 192 at 50 kHz. In Figure 7, the view angle is indicated by a symbol 0 and the light angle is indicated by a symbol . The data for each image is shown in the table below.

The AFM images of Figure 7 show that the short pulsed method described provides better control over the surface topography of an etched surface, and can also provide a more desirable surface topography.

For example, it can be seen that the peaks are rounded and the valleys are broad. In applications where it is desired to apply a further layer to the etched layer, rounded peaks and broad valleys (i.e. a low aspect ratio) means that it is easier to grow a further layer on the surface by CVD. This is particularly beneficial for photovoltaic applications to facilitate conformal growth of subsequent layers and minimise faults.

Figure 8 shows an angular distribution of each of the surfaces of the samples. In Figures 8 the axis labelled yl is "Number" and the axis labelled xl is the "Angle (Degrees)". It can be seen from Figure 8, that etching tends to increase the number of large angles to the surface resulting in larger, more rounded features.

Figure 9 shows the height distribution of the asperities of the etched surface In Figure 9 the axis labelled y2 is "Number" and the axis labelled x2 is the "Cantilever Deflection (nm)". It can be seen from Figure 9 that most of the asperities of the etched surface have a low negative height and a low positive height. The low negative and positive heights show that the valleys and peaks are wide and rounded compared to barrier discharge etched surfaces of atmospheric pressure etching of the prior art. However, it will be noted that at a different repetition rate the peaks are sharper, which illustrates the controllability offered by a pulsed power supply operation.

As illustrated, the pulsed power supply CVD operation provides the advantage of improved surface properties and increased control of the surface topography. This method is particularly advantageous because it can be performed at atmospheric pressure and at atmospheric temperature.

In alternative embodiments the process may be performed at elevated temperature and low pressure, which can further improve the control of the surface properties. For example, it is expected that increased temperature will enable alternative structures and morphologies, and also enable alternative materials to be etched.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. For example, although the described embodiments deposit a titania coating and etch a zinc oxide coating, and suitable material may be deposited including but not limited to any one or more of a variety of metals and oxides, and any suitable surface may be etched, including but not limited to, any one or more of a variety of metals and oxides.

Claims

1. A method of chemical vapour deposition of a material on a substrate, or etch of material from a substrate, the method comprising:

forming a plasma using a short pulsed power signal so that the plasma is a short pulsed plasma for enhancing the deposition of material to the substrate or the etch of material from the substrate.

2. The method according to claim 1, wherein the power signal is a square or trapezoidal wave, or a near square or near trapezoidal wave.

3. The method according to claim 1 or 2, wherein the plasma is formed using helium, argon or nitrogen.

4. The method according to any one of the previous claims, wherein the method is performed at atmospheric or near atmospheric pressure.

5. The method according to any one of claims 1 to 3, wherein the method is performed at sub-atmospheric pressure.

6. The method according to any one of the previous claims, wherein the substrate is heated to a temperature ranging from 50°C to 300°C.

7. The method according to any one of the previous claims, wherein the frequency of the pulsed power signal ranges from 0.5 kHz to 100 kHz.

8. The method according to any one of the previous claims, wherein the power supply signal ranges from 2.5 kV to 10 kV.

9. The method according to any one of the previous claims, wherein the "on phase" pulse width of the short pulsed power signal is less than or equal to 1 μβ.

10. Equipment for chemical vapour deposition of material on a substrate or etch of a material from a substrate, the equipment comprising:

a holder for a substrate;

apparatus for forming a short pulsed plasma near the holder; and

a power supply configured to provide a pulsed power signal to form the short pulsed plasma to enhance the deposition of material on to the substrate or etch of material from the substrate.

11. The equipment according to claim 10, wherein the power supply is configured to output a square wave or near square wave power signal, or a trapezoidal or near trapezoidal power signal.

12. The equipment according to claim 10 or 11, wherein a process gas provided by the gas source for forming a plasma is helium, argon or nitrogen.

13. The equipment according to any one of claims 10 to 12, wherein the equipment comprises a housing for the apparatus, and the housing is held at atmospheric pressure.

14. The equipment according to any one of claims 10 to 12, wherein the equipment comprises a housing for the apparatus, and the housing is held at a sub- atmospheric pressure.

15. The equipment according to any one of claims 10 to 14, wherein the power supply is configured to supply pulsed power at a frequency ranging from 10 kHz to

100 kHz.

16. The equipment according to any one of claims 10 to 15, wherein the power supply is configured to supply pulsed power signal ranging from 2.5 kV to 10 kV.

17. The equipment according to any one of claims 10 to 16, wherein the power supply is configured to supply pulsed power signal with an "on phase" pulse width less than or equal to 1 μβ.

18. A method and/or equipment substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.