WO2009125189A1

WO2009125189A1 - Biased plasma assisted processing

Info

Publication number: WO2009125189A1
Application number: PCT/GB2009/000934
Authority: WO
Inventors: Martynas Audronis; Allan Matthews
Original assignee: The University Of Sheffield
Priority date: 2008-04-09
Filing date: 2009-04-09
Publication date: 2009-10-15
Also published as: GB0806410D0; GB2459103A

Abstract

The present invention provides a method for use in a plasma-assisted physical vapour deposition and/or other plasma processing system, in which low or medium frequency power is applied to at least two electrodes at different individual frequencies and with a defined relationship between them. The invention also provides an enhanced process created by the abovementioned method, which is also simple, reliable and straightforward, to scale up. One embodiment includes a plasma-assisted physical vapour deposition system that employs a target and a substrate being processed. The low or medium frequency voltage or current waveforms are applied to the target and the substrate at different frequencies by power supplies (or a supply) that are (is) constructed and configured to output signals of different frequencies with a defined desirable relationship between them, desirable synchronisation and desirable adjustable mismatch between them.

Description

BIASED PLASMA ASSISTED PROCESSING

Field of the Invention

The present invention relates to plasma assisted surface processing, for example plasma assisted vapour deposition.

Background of the invention

Plasma assisted surface engineering methods (e.g. plasma-assisted physical vapour deposition) are applied in a number of industries for variety of purposes, such as etching and/or coating. Low and medium frequency (typically 2 - 350 kHz) power technologies used for sputtering, such as alternating current (AC) (US4046659) and pulsed direct current (pulsed-DC) technologies (US5576939, US5718813, US2005098430) , vacuum arc evaporation (US2007000772, Ramm et al. Surface and Coatings Technology 202 2007 876 and Buschecl et al. Surface and Coatings Technology 142-144 2001665) or plasma processing by high density plasma (WO9859087) , are also known.

The main objective in developing low and medium frequency sputtering technologies was to overcome arcing-related problems encountered when operating in the reactive sputtering mode (US5576939, US4046659,

US5718813) . Consequently arcing-free sputtering processes and coatings with significantly reduced number of defects have been achieved using these technologies. In the case of vacuum arc evaporation, medium frequency power applied to the target/cathode has been found to be useful in stabilising the process and improving the quality of coatings (Buschecl et al. Surface and Coatings Technology 142-144 2001665, Ramm et al.

Surface and Coatings Technology 202 2007 876, US2007000772) . Figure

1 shows schematically several examples of commonly used cathode voltage waveforms: AC (in Figure IA) ; symmetric bipolar pulsed-DC (in Figure IB and Figure 1C) ; unipolar pulsed-DC (in Figure ID) and asymmetric bipolar pulsed-DC (in Figure IE) .

Also, it is now well known that methods such as low and medium frequency AC and asymmetric bipolar pulsed-DC sputtering have a significant beneficial impact on the structure (and resultant properties) of both reactively and non-reactively produced coatings (see for example Scherer et al. Journal of Vacuum Science and Technology A 10 1992 1772 and Audronis et al. Surface and Coatings Technology 200 2006 4166) . Concurrently, studies of the pulsed-DC sputtering process have shown that application of pulsed-DC power to the sputter target provides plasma enhancement due to voltage transients and overshoots exhibited by pulsed cathode voltage waveforms as, for example, shown in Figure 2. Consequently, in the proximity of the substrate, deposition parameters such as energy distribution functions and the fluxes of bombarding charged and neutral species are affected (e.g. Bradley et al. Plasma Sources Science and Technology 11 2002 165) . Effects are stronger when higher frequencies are used. Also, the process can be further enhanced if, for example, a medium frequency wave is superimposed with a high frequency waveform at the cathode, as schematically shown in Figure 3, or if a low or medium frequency cathode waveform is frequency-, amplitude-, pulse width- or pulse position-modulated (as, for example, described in US2006278524) .

The technique of medium frequency biased deposition has started to gain popularity mainly because it has been realized that it provides improved control of substrate bombarding ion energy and flux (e.g. Barnat et al. Journal of Vacuum Science and Technology A 17 1999 3322) and can be utilised to bias substrates during the reactive deposition of insulating films (as, for example, described in US5789071 and Schneider et al. Surface and Coatings Technology 94-95 1997 179) . Pulsed-DC substrate biasing can be carried out in combination with pulsed-DC biasing of the target. In such a situation, the target and the substrate are usually pulsed at the same frequency in one of three configurations : i) synchronous (schematically shown in Figure 4A) , sometimes called 'master-slave' , (e.g. Schneider et al. Surface and Coatings Technology 94-95 1997 179) ; ii) 'dual cathode' (schematically shown in Figure 4B) in which the angle between target and substrate waveforms is 180 degrees and the duty cycle is limited to 50% for both, target and substrate (Kelly et al. Journal of Vacuum Science and Technology A 19 2001 2856) ; iii) asynchronous (schematically shown in Figure 4C) in which there is no defined relationship between target and substrate waveforms (e.g. Kelly et al. Journal of Vacuum Science and Technology A 19 2001 2856) . The asynchronous configuration is however rarely used in practice, as it can cause instabilities in the deposition process and reproducibility issues. The same considerations apply to biased AC processing.

Although low and medium frequency deposition processes can offer significant improvements in terms of aspects such as control of the flux and energy of ionised and neutral species delivered to surfaces of items treated, the desirable properties of products are not always achieved. For example, reactive physical vapour deposition of crystalline coatings of non-conducting oxides such as alumina and chromia is desirable, but often problematic. It is usually found that the required crystalline structure and stoichiometry of coatings cannot be easily achieved. Methods such as substrate heating (e.g. US5789071) , radio frequency (RF) plasma enhancement (e.g. US5789071) and the application of a negative substrate bias (e.g. US5789071, US6818103) or any combination of these methods, are often used to improve the structure and stoichiometry of such films. However, the necessity to heat the substrates can complicate the deposition process, and limit the range of substrates that can be coated; plasma enhancement by RF powered coils is difficult to scale up; AC substrate biasing technologies, as, for example, described in US6818103 are relatively more complex; RF substrate biasing is complicated because of the need to minimise the reflected power, whilst direct current biasing of non-conducting coatings generally causes damage by arcing, which limits applicability of this process.

Consequently the need exists for a simple technology which could beneficially enhance the process and address the problematic deposition or processing issues of certain materials, including, but not limited to, the problems mentioned above.

Summary of the Invention

The present invention provides a plasma assisted surface processing system comprising two electrodes and power supply means arranged to generate two cyclically varying signals of different respective frequencies for application to the two electrodes respectively. The power supply means may comprise control means arranged to control the ratio of the frequencies of the two signals, or to control the phase relationship between the two signals.

The signals may be frequency, amplitude, pulse width, pulse position, or pulse amplitude modulated.

The present invention further provides a method of surface processing an object comprising locating the object and a target in a chamber with a gas in the chamber, and applying respective cyclically varying signals to the object and the target. The signals may be of different frequencies. The ratio of the frequencies may be controlled or varied. The ratio of the frequencies may be limited to discrete values, so that the ratio can be varied by switching between the discrete values. The phase relationship between the signals may be controllable or variable. This control or variation may be provided by means of a user input, giving manual control, or may be in response to some sensing or control mechanism so as to provide automatic control or variation. For example the sensing means may be arranged to sense the temperature of the object or the species present in plasma in the vicinity of the object.

The present invention, therefore, may provide a method used in a deposition and/or processing system, in which low or medium frequency power is applied to at least two electrodes at different individual frequencies and with a defined relationship between them.

Some embodiments of the present invention can provide an enhanced process created by the abovementioned method, which is also simple, reliable and straightforward to scale up.

One embodiment includes a plasma-assisted physical vapour deposition system that employs a target and an item being processed. The low or medium frequency voltage or current waveforms are applied to the target and the substrate at different frequencies by power supplies (or a supply) that are (is) constructed and configured to output signals of different frequencies with a defined relationship between them. The signals may be voltages. The voltages may be relative to a reference voltage or potential. The system may further comprise an anode, and the reference voltage may be the voltage of the anode. The reference voltage may be earth, or it may be a fixed voltage or potential relative to earth.

Preferred embodiments of the present invention will now be described by way of example only with reference to the remainder of the accompanying drawings.

Brief Description of the Drawings Figure IA shows schematically an example of an AC voltage waveform.

Figure IB-C show schematically examples of symmetric bipolar pulsed-

DC voltage waveforms;

Figure ID shows schematically an example of a unipolar pulsed-DC voltage waveform.

Figure IE shows schematically an example of asymmetric bipolar pulsed- DC voltage waveform;

Figure 2 shows the measured pulsed-DC waveform;

Figure 3 shows waveforms, as produced by pulsed-DC (Output 1) and RF (Output 2) power supplies, and an RF-superimposed pulsed-DC waveform

(Cathode Voltage) ;

Figure 4A shows target and substrate waveforms in a conventional synchronous configuration;

Figure 4B shows target and substrate waveforms in a 'dual cathode' configuration;

Figure 4C shows target and substrate waveforms in an asynchronous configuration;

Figure 5A-D show schematically examples of waveforms of different frequencies applied to the target and the substrate, and relationships between them, achievable with embodiments of the present invention;

Figure 6 shows schematically an example of waveforms of different frequencies applied to the target and the substrate, arid the relationship between them, achievable with a further embodiment of the present invention; Figure 7 shows schematically an example of waveforms of different frequencies applied to the target and the substrate, and the relationship between them, achievable with another embodiment of the present invention;

Figures 8A-D show examples of power supply arrangements constructed and configured in accordance with embodiments of the present invention; Figure 9A shows an example of different conditions at the substrate created by one embodiment of the present invention, where pulsed-DC signals are supplied to the target and substrate during the reactive sputtering of metal in an atmosphere containing argon and oxygen; Figure 9B shows an example of different conditions at the substrate created by conventional single frequency synchronous biased sputtering, where pulsed-DC signals are supplied to the target and substrate during the reactive sputtering of metal in the atmosphere containing argon and oxygen; and Figures 10A-E show examples of deposition systems according to embodiments of the present invention.

Detailed description of the Preferred Embodiments

Figure 5A shows schematically an example of one embodiment of the present invention where, in a deposition (or other plasma processing) system, pulsed-DC signals of different frequencies are applied to both the target and the substrate. In each case the horizontal axis is marked as OV, indicating that the signal voltages are relative to earth. However, they may alternatively be relative to another fixed non-zero reference voltage. The relationship between the periods (T) , and, hence, frequencies (F) (as F= IIT), of target and substrate waveforms (T₁ and T, and F₁ and F_s, respectively) is defined as T₁ = n Ts, where n is preferably a rational number and preferably, but not necessarily, an integer number (e.g. 2, 3, 4, 5 etc.) . In this embodiment the target signal is a pulsed DC square wave and the substrate signal is also a pulsed DC square wave with twice the frequency of the target signal, i.e. /z = 2. The signals are applied by respective power supplies that are configured to output signals of the different periods/frequencies with only such a defined frequency/period relationship between them. The frequency ratios are limited to the discrete ratios defined above. The actual frequencies may be continuously variable, or variable in discrete steps, within the constraints of these fixed frequency ratios. The power supplies are further configured to synchronise the signals of these different frequencies, bound by the abovementioned relationship, so that period 7¹, at the target and a group of n periods T_s at the substrate begin and end at the same time. In a modification to this embodiment, a small mismatch may occur between the beginning of the periods of the two waveforms, which may occur due to the particular technology chosen to carry out synchronisation of the signals. In some cases this mismatch may be desirable, in which case a controlled delay may be provided between the beginning of the periods of the two waveforms, as described in more detail below.

Figure 5B shows schematically an example of another embodiment of the present invention where, in a deposition (or other plasma processing) system, pulsed-DC and AC signals of different frequencies are applied to the target and the substrate, respectively. In this embodiment the target signal is again a pulsed DC square wave, but the substrate signal is a sinusoidal AC wave, again of twice the frequency of the target signal. Again, as discussed for the Figure 5A, the signals are applied by the power supplies that are configured to output synchronised signals of the different periods/frequencies with a defined frequency/period relationship between them. In the same manner, Figures 5C and 5D show schematically examples of other embodiments of the present invention where, in a deposition (or other plasma processing) system, AC and pulsed-DC (Figure 5C) and AC and AC (Figure 5D) signals of different frequencies are applied to the target and the substrate, respectively. Again, the signals are applied by the power supplies that are configured to output synchronised signals of the different periods/frequencies with a defined frequency /period relationship between them. In the embodiment of Figure 5C the target signal is sinusoidal AC and the substrate signal is a pulsed DC square wave, and n = 4. In the embodiment of Figure 5D both signals are sinusoidal AC and n = 2. It is also understood that the relationship T_x = nT_s can also be written as mT. = nT_s or 'nF_x = mF, , where m is preferably, but not necessarily, an integer number. Figure 6 illustrates one situation where, in accordance to the principles of the present invention, signals of different frequencies are applied by power supplies to the sputter target and the substrate and the relationship between these signals is m 7¹, = n T_s where m = 2 and n - 5. The ratio of the two frequencies is therefore a rational number n/m where n and m are both integers. In a practical system, n and m are preferably in the range from 1 to 10 inclusive, but in some cases they may be anywhere in the range from 1 to 200. Similarly to what was discussed for Figures 5 A to 5D, the power supplies are configured in a way that the output signals are synchronised so that period m T_x at the target and period n T_s at the substrate begin and end at the same time or with a small mismatch, which may occur due to the particular technology chosen to carry out synchronisation of signals.

Figure 7 shows schematically an example of another embodiment of the present invention where, in a deposition (or other plasma processing) system, pulsed-DC signals of different frequencies are applied to the target and the substrate. The signals are applied by the power supplies which are configured to output synchronised signals of the different periods/frequencies with a defined frequency/period relationship between them and, in order to provide more control over processing parameters, with intentionally induced adjustable mismatch (T_d in Figure 7) between them. T_ά preferably has a value between 0 and T_s or 0 and 7¹,, whichever of them is the smaller.

Figure 8A illustrates one power supply arrangement constructed and configured in accordance with an embodiment of the present invention.

Such a power supply arrangement can, for example, include two pulsed- DC power supplies or two AC power supplies or one AC and one pulsed- DC power supply. The two power supplies are arranged to be synchronized so that the frequencies and phases of the two signals can be controlled. In order to achieve this, one of the power supplies is arranged to send a trigger signal to the other at a trigger point, such as the beginning of each period of its signal, so that the other power supply, on receiving the trigger signal, can synchronize its signal with that of the first to provide the required ratio of frequencies and the required phase offset. In Figure 8 A, as well as Figure 8B, outputs 1 and 2 are the outputs connected to the cathode (e.g. the target in the sputter/ vacuum arc gun or the item being processed) and the anode (e.g. the anode in the sputter/vacuum arc gun or the chamber) . In an alternative embodiment the power supply for both signals can be embodied in a single unit, as illustrated in Figure 8B. Such a power supply unit is capable of producing two waveforms (as, for example, shown in Figures 1A-1E and Figure 3) of desirable frequencies, with desirable relationship between them, desirable synchronisation and desirable adjustable phase offset or mismatch between them, as, for example, is shown in Figures 5A-D, 6 and 7.

Figures 8C and 8D show examples of other power supply arrangements constructed and configured in accordance with embodiments of the present invention. Such embodiments can, for example, be used in the deposition systems with multiple targets. Such examples of power supplies can include any number (from 1 to k, where k is any integer number, as illustrated in Figure 8C) of power supplies, in any combination of pulsed-DC, AC or other types of power supplies. Such power supply systems are capable of supplying signals (as, for example, shown in Figures 1A-1E and Figure 3) of desirable frequencies, to the substrate and to one or more sputter targets, with desirable relationship between those signals, desirable synchronisation and desirable adjustable mismatch between them, as for example, is shown in Figures 5A-D, 6 and 7. In each of these embodiments, the frequencies of the two signals, and in particular the ratio of their frequencies and the degree of offset of their phases, is variable. For example a user input can be provided on the power supply, with the controller of the power supply being arranged to respond to operation of the user input to control the parameters of the signals, so that a user can select and vary those parameters as required. Alternatively the controller of the power supply can be connected to one or more sensors arranged to sense one or more parameters of the system, such as the temperature of the object and the species present in the plasma in the vicinity of the object. In that case the controller is arranged to control the parameters of the signals in response to signals from the sensors. The system defines a number of discrete frequency ratios for the two (or more) voltage signals, and is arranged to switch between these discrete ratios in response to the inputs.

Furthermore, it is understood that power supplies constructed and configured in accordance with some embodiments of the present invention may include a number of extra features or devices, such as arc handling, high frequency power supplies, tuners, match boxes and any other circuitry, that are useful in aiding processes such as sputter deposition, plasma etching or arc evaporation.

Figure 9A shows an example of different conditions at the substrate (arbitrarily shown as zones 1 , 2 and 3) created by one implementation of the present invention, where pulsed-DC signals are supplied to the target and substrate during the reactive sputtering of metal (Me) in an atmosphere containing argon (Ar) and oxygen (O) . Different species that are expected to be preferentially accelerated through the sheath at the substrate and to bombard the surface of the substrate during different periods of the substrate bias pulsing cycle are discussed below. A comparison is also made to conventional technology where synchronised pulsed-DC signals of the same frequency (as shown schematically in Figure 4A) are supplied to both, the target and the substrate (Figure 9B) .

In Zone 1 (Figure 9 A and 9B) , the system is operating in a first state in which both the target and the substrate are at negative (or positive) potential relative to a common reference potential, which may be ground or the anode which may not be grounded as discussed above. In this case the target and substrate are at substantially the same potential. The plasma potential in such a situation is expected to be few Volts above the most positive electrode potential (e.g. ground) and positively charged species (gas and metal ions, such as Ar⁺ , O⁺ , Me⁺ , and molecules, such as O₂ ⁺ , MeO⁺) in the plasma, near the substrate, will be preferentially accelerated across the sheath towards the substrate. (This, of course, does not mean that other species, such as electrons, will not reach the substrate; the same applies to the following two paragraphs) . In 'conventional synchronous mode' Zone 1 (Figure 9B) is essentially the only possible condition.

In Zone 2 (Figure 9A) , the system is operating in a second state in which the target is in sputtering regime (pulse-on) at some negative potential relative to ground or the anode, the potential being high enough to generate some negative species at the surface of the target (there will be other sources of negative species also) , while the substrate is in pulse-off period at some positive potential relative to ground or the anode. This situation creates ideal conditions for negatively charged species (e.g. e\ 0", O₂ ", MeO") in the region of the object to be preferentially drawn across the substrate sheath towards the object. Such situation however is not possible when operating in a conventional synchronous mode (Figure 9B) . In Zone 3 (Figure 9A) , the system is operating in a third state in which the target is at the beginning of pulse-off period (when the plasma potential is raised, due to a large positive overshoot at the target [e.g. see Figures 2 and 3] , and higher energy positive ions are generated [e.g. see Bradley et al. Plasma Sources Science and Technology 11 2002 165 and Vetushka et al. Journal of Vacuum Science and Technology A 22 2004 2459]) and the substrate is at some negative potential. In these circumstances positive ions (some possessing significantly higher energies [Bradley et al. Plasma Sources Science and Technology 11 2002 165 and Vetushka et al. Journal of Vacuum Science and Technology A 22 2004 2459]) will be preferentially accelerated across the substrate sheath. Again, processes taking place in Zone 3 will not be characteristic of conventional synchronous configuration (Figure 9B) .

Processes carried out in accordance with some embodiments of the present invention therefore offer substantial process enhancements, for example in terms of control of energy and flux delivered to the surfaces of items being processed, and, hence, enhancement in resultant properties of products. With a power supply in which the frequencies and relative phases of the different signals can be controlled and varied, the process can be controlled in a variety of ways depending on what is required. For example the times for which the system is in each of the states corresponding to zones 1 to 3 can be controlled and varied to suit a number of different processing techniques or methods.

Figures 1OA to 1OE show several examples of embodiments in which the present invention can be implemented. Figure 1OA shows an example of a deposition (or other plasma processing) system which includes a target 1 and a stationary or moving substrate' (or substrate holder) 2, powered by power sources, located in a gas-filled chamber, exemplary embodiments of which (shown in Figures 8A-D) were discussed above. In Figure 1OA, the anode 6 is also shown, which in this case is earthed, and the signals applied to the target 1 and substrate 2 are varied with respect to the earthed anode. In modifications to this arrangement, the anode 6 may be held at a fixed non-zero voltage, or the voltage of the anode 6 may be pulsed or varied with the voltage of the substrate.

Figures 1OB and 1OC show examples of deposition (or other plasma processing) systems, each of which include two targets 1 and a stationary or moving substrate (or substrate holder) 2, powered by power sources, exemplary embodiments of which (shown in Figures 8A-D) were discussed above. The anode is omitted from Figures 1OB and 1OC for clarity.

Figure 1OD shows an example of a deposition (or other plasma processing) system which is otherwise similar to the system shown in

Figure 1OB, but, differently from that system, has additional electrodes/targets 3, to which power is supplied either by power sources, exemplary embodiments of which (shown in Figures 8A-D) were discussed above, or by any other kind of power supplies (such as DC or RF power supplies) with no defined relationship to the signal supplied to substrate 2.

Figure 1OE shows an example of a deposition (or other plasma processing) system which includes a sputter target 1 , a substrate transportation system 4, an electrode 5 through which the substrate is biased and a substrate 2, biased by power sources, exemplary embodiments of which (shown in Figures 8A-D) were discussed above. A power supply 10 provides the voltage signals to both the sample and the target and has a user input 12 which can be operated to select the ratio of frequencies of the two voltage signals and a second user input 14 which can be operated to control the phase offset between the two voltage signals. Also a temperature sensor 16 is arranged to sense the temperature of the substrate and output a temperature signal to the power supply 10 indicative of that temperature, and a species sensor 18 is arranged to sense the species present in the plasma near the substrate, and to output signals to the power supply 10 indicative of what those species are. Other sensors may be included to sense other parameters as appropriate. The power supply 10 includes control electronics which are arranged to control the frequencies and'phases of the two voltage signals as described above either in response to inputs from the user inputs 12, 14 or in response to inputs from the sensors 16, 18. While the power supply is shown as a single unit in this embodiment, separate power supplies which are arranged to coordinate their signals as described above can also be used. It will be appreciated that the systems of Figures 1OA to 1OD can also include power supplies and sensors in a similar manner.

In accordance with some embodiments of the present invention, in these (Figure 1 OA-E) and other possible embodiments, which are not shown here, but which are within the scope of the present invention, desirable different frequency signals, preferably, but not necessarily, in a range of 1 to 500 kHz, are applied to the substrate and one or more targets/electrodes, with desirable frequency relationship between those signals, desirable synchronisation and desirable adjustable phase offset or mismatch between them.

Claims

1. A plasma assisted surface processing system comprising two electrodes and power supply means arranged to generate two cyclically varying signals of different respective frequencies for application to the two electrodes respectively, wherein the power supply means comprises control means arranged to control the ratio of the frequencies of the two signals .

2. A system according to claim 1 wherein the control means is arranged to define a plurality of rational number ratios between the frequencies and to control the frequencies so that the ratio of their frequencies is one of the rational number ratios.

3. A system according to claim 2 wherein the rational number ratios include a plurality of integer ratios.

4. A system according to any foregoing claim wherein one of the electrodes is a target electrode for connection to a target and one of the electrodes is a substrate electrode for connection to a substrate, and the signal for application to the substrate electrode is of a higher frequency than the signal for application to the target.

5. A system according to any foregoing claim wherein the control means is arranged to control the phase relationship between the signals.

6. A system according to claim 5 wherein the control means is arranged to control the phase offset between the two signals .

7. A system according to claim 6 wherein the control means is arranged to define a range of phase offsets between the two signals, and to vary the phase offset within that range.

8. A system according to any foregoing claim further comprising user input means connected to the control means, wherein the control means is arranged to vary the relationship between the signals in response to operation of the user input means.

9. A system according to any foregoing claim further comprising sensing means arranged to produce sensor output indicative of at least one parameter of the system, wherein the control means is arranged to vary the relationship between the signals in response to the sensor output.

10. A system according to any foregoing claim wherein the signals are voltages measured with respect to a common reference voltage.

11. A system according to any foregoing claim further comprising an anode.

12. A system according to claim 11 when dependent on claim 10 wherein the reference voltage is the voltage of the anode.

13. A method of surface processing an object comprising locating the object and a target in a chamber with a gas in the chamber, and applying respective cyclically varying signals to the object and the target wherein the signals are of different frequencies, and controlling the ratio of the frequencies .

14. A method according to claim 13 further comprising controlling the phase relationship between the signals.

15. A method according to claim 13 or claim 14 wherein the signals are arranged so that during a part of the signal cycle, the target is at a negative potential relative to the object so that negative species are drawn towards the object.

16. A method according to any of claims 13 to 15 wherein during a part of the signal cycle, the target is switched to a more positive potential while the object potential is held substantially constant.

17. A method according to any of claims 13 to 16 wherein during a part of the signal cycle both the target and the object are at a negative potential relative to a reference potential.

18. A power supply for a plasma. assisted surface processing system comprising two electrodes, the power supply comprising signal generation means arranged to generate two cyclically varying signals of different respective frequencies for application to the two electrodes respectively, and control means arranged to control the signal generation means to control the ratio of the frequencies of the two signals.

19. An object which has been treated according to the method of any of claims 13 to 17.