GB2030180A

GB2030180A - Vapour deposition of metal in plasma discharge

Info

Publication number: GB2030180A
Application number: GB7925486A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1978-01-26
Filing date: 1979-07-20
Publication date: 1980-04-02
Also published as: GB2030180B

Abstract

A method for ion plating an electrically conducting substrate with a metal comprises generating a plasma discharge in an inert gas, e.g. argon or helium using DC, RF or DC biased RF, introducing the vapour of a volatile compound of the plating metal and decomposing the compound in the discharge using pyrolysis, microwaves, photolysis or chemical reduction. Suitable compounds include the halides, carbonyls, nitrosyls, alkyls, aryls or hydrides of W, Ta, MO, Nb or Cr. The discharge current required is proportional to the area of substrate to be coated, but also depends on the plating gas composition and flow rate with a minimum value of 0.5mA cm<2>. By reducing the input of the metal compound at intervals, the deposited coating may be sputter cleaned during the plating process. The method is used for plating the inner surface of a tube.

Description

SPECIFICATION Improvements in or relating to ion plating This invention relates to methods for effecting surface deposits onto conducting substrates with a view to modifying the physical or physico-chemical properties of their surface by means of a plasma chemical technique.

The technique of ion plating has recently been shown to be capable of producing exceptionally adherent coatings. In this process a metal vapour is introduced into a low pressure plasma discharge, which results in the formation of metal ions. These ions are then accelerated by a high electrical potential and subsequently discharged on the substrate to form a coherent coating. The metal vapour may be produced by metal evaporation, or by sputtering.

Both methods require a large input of electrical energy, and are slow, thus resulting in a low coating rate.

It is desirable to provide a means whereby a metal coating may be deposited on a surface at a lower energy cost than is normally achieved by using metal evaporation or sputtering and this will also facilitate high deposition rates.

According to the present invention, a process for ion plating a metal onto an electrically conducting substrate comprises enclosing the substrate and a refractory electrode in an airtight vessel fitted with means for continuously evacuating the vessel and means for introducing gases at a controlled pressure, establishing a plasma discharge within said vessel, introducing into the vessel at least one stream of gas comprosing a carrier gas on a compound of the metal to be plated and decomposing this compound to give ions of the metal which then migrate to the substrate surface and form the required coating.

Conveniently the means for evacuation of the vessel is at the opposite end of the vessel to the means for introducing gases so that the vessel may be flushed withe the gases.

The plasma discharge may be established by a DC voltage between an anode and the substrate, by RF methods, or by DC biased RF. The geometry of the electrodes used is variable, the shape and size being adjusted to achieve an optimum coating rate. In a particular embodiment, the process may be used to coat the interior of a conducting tube by inserting into the ends of the tube insulating plugs bearing a centrally located electrode and gas supply and evacuation means. The central electrode may be constructed of a refractory metal for example W, Ta, Mo, Nb, Cr or of another electrically conducting refractory material such as carbon but should preferably be made of the metal to be plated, and may be in the form of a rod or wire, or alternatively a hollow tube with openings into the vessel may be used and thus form one of the gas supply or extraction means.

When R.F. is used, the supply may be connected either to the substrate or to the counter-electrode, with D.C. the substrate is the cathode and the counter-electrode and anode. In certain circumstances, it is desirable to use both D.C. and R.F. in order to obtain a highly ionized plasma, the substrate again being the cathode.

It is necessary to clean the substrate surface prior to the plating process. This may advantageously be carried out in situ by a sputter cleaning technique.

For example the vessel may be evacuated and a cleaning gas introduced to maintain a dynamic pressure of about 5 torr. An electrical potential of approximately 1 kV cam~1 is then applied between the anode and the substrate. The resulting plasma discharge serves to sputter clean the substrate surface by bombardment with ions. The cleaning gas used may be an inert gas especially argon, but mixtures of an inert gas especially argon, with a reactive gas such as oxygen, chlorine or hydrogen in volume ratios of between 0.1 and 1.5 parts of reactive gas to 1 part of inert gas may be used.

Frequently an oxidising gas mixture is first used to remove carbon containing compounds, followed by a reducing gas mixture to remove oxides and finally an inert gas to remove absorbed reactive gases.

After this surface cleaning process, the plating process may be carried out in a generally similar manner except that the cleaning gas is replaced by a plating gas comprising vapour of a volatile compound containing the metal to be deposited normally mixed with an inert gas. Suitable volatile compounds include for example, the halides, volatile carbonyls, nitrosyls, alkyls, aryls and hydrides of W, Ta, Mo, Nb, Cr or other metals. The concentration of metal compound vapour in the mixture may be varied between 1 to 100% by volume and the dynamic pressure in the vessel may be varied typically between 10-3 and 10 torr to produce a suitable coating rate, depending upon the geometry of the electrodes, the metal compound used and the type of coating required.A suitable vapour pressure of the volatile metal compound in the plating gas may usually be obtained by passing the inert gas stream through a temperature controlled bath containing the metal compound at a temperature below its boiling or subliming point. The plating gas may be adjusted to an appropriate composition in the vessel, or may alternatively be adjusted in the gas input system before introduction to the vessel.

The volatile metal compound is continuously decomposed in the vessel and metal ions are produced in the plasma discharge by any one or more of a number of techniques including pyrolytic decomposition, chemical reduction, photolytic decomposition or by microwaves, as outlined below.

For pyrolytic decomposition, the plasma must be supplied with sufficient energy to provide the enthalpy of decomposition of the compound. In certain cases, sufficient thermal energy may be made available in the plasma by the supporting D.C. or R.F.

discharge and because of its relatively high ionization potential, the presence of helium may be employed to enable the plasma achieve a suffiently high enthalpy. In other circumstances it is necessary to feed energy into the plasma by photolysis or microwaves especially where high deposition rates are required. For photolytic decomposition of the metal compound, a laser of an appropriate wavelength may be shone through a window into the vessel so that the incident radiation is absorbed by the plasma. Alternatively, a suitable high energy flash tube may be shone through a window into the vessel, or may be positioned inside the vessel.

For microwave decomposition, microwaves of a suitable wavelength may be beamed into the vessel, or may be conveyed into the vessel by a surface wave propagation waveguide.

For chemical reduction a reductive material such as hydrogen or ammonia may be introduced into the vessel in a quantity preferably in excess of the stoichiometric ratio required for reduction of the metal compound. In this case the reductive material may partially or totally replace the inert gas.

During the plating process described above, some cleaning of the substrate surface will occur simultaneously with metal deposition as the surface is bombarded with ions. In addition to this, the input of metal compound vapour may be occasionally reduced or even stopped during the plating process, so that simultaneously the surface may be more thoroughly sputter cleaned. This cleaning process assists in the building up a metal coating of suitable tenacity by the removal of loosely bound metal atoms.

Typical apparatus for carrying out the process of the invention is shown by way of example in the accompanying drawings in which: Figure 1, illustrates an embodiment wherein the metal compound vapour may be pyrolysed or reduced by a D.C. discharge generated plasma and Figure 2, illustrates a further embodiment wherein the metal compound vapour is photolytically decomposed by a laser.

With reference to Figure 1, the substrate is in the form of a hollow conducting tube 1, of which the inside is to be plated, and which is fitted with a connection 2, enabling it to be made a cathode by connection to the negative pole of a suitable D C voltage supply (not shown). The ends are closed by insulating plugs 3 and 4to form an airtight vessel.

The plugs also carry a pipe 5 enabling evacuation of the tube via a pump 6, and a connection 7 to a vacuum gauge 8, and a tube 9 for introducing gases at a rate controlled by a metering device (not shown). A central concentric solid anode 10 made of tungsten is provided with a lead 11 enabling connection to the positive pole of the DC voltage supply.

Figure2 illustrates a modification of the apparatus which differs from that of Figure 1 in that the centrl solid anode 10 is replaced by a hollow anode 12 in the form of a tube which is used for introducing gases through a plurality of holes 13 in its walls. One of the plugs 3 is also fitted with a transparent window 14 enabling light from a suitable laser 15 to be shone into the tube 1 to photolytically decompose a metal compound vapour introduced via the tubes 9 or 12. In a modification of the apparatus shown in Figure 2, the laser 15 is replaced by a flash tube which serves to decompose the metal compound vapour by high intensity flashes of light of suitable wavelength. Alternatively the laser 15 and window 14 may be replaced by a flash tube within the conducting tube 1.In another modification of the apparatus shown in Figure 2, the laser 15 and window 14 are replaced by a microwave source and waveguide.

For an alternative mode of operation the tube 1 of the apparatus illustrated in both Figure 1 and Figure 2 may be connected to the live side of an R.F.

generator via the lead 2, and the central electrode 10 in Figure 1 and 12 in Figure 2 may be connected to the earth connection via the lead 11. In this embodimentthe plasma is generated by the R.F. Modifications for the use of a flash tube or microwaves may be made to the apparatus as described above.

Processes using the apparatus described above will now be indicated by way of example for the preparation of a coating of tungsten on the inside of a tubular metal substrate.

With reference to the apparatus of Figure 1, the interior of the tube 1 is evacuated to a pressure of at least 10~6tory as measured by the gauge 8 using the pump 6. A mixture of argon and oxygen containing 50% by volume of oxygen is then introduced into the tube 1 through tube 9 while pumping so as to maintain a dynamic pressure of about 50mtorr. The tube 1 and central anode 10 are then connected via the leads 2 and 11 to the negative and positive poles respectively of a suitable D C voltage supply so as to maintain a potential of the order of 1000 volts per cm. between the tube and the anode and thus generate a plasma discharge within the tube 1. This discharge is maintained for one hour to sputter clean the inside surface of the tube.This process is then repeated using a mixture of argon gas and hydrogen, containing 10% by volume of hydrogen for one hour and finally using pure argon alone for a similar period.

At the end of this period a mixture of argon, hydrogen and tungsten hexafluoride (hereafter called the plating gas) containing 5 to 40% by volume tungsten hexafluoride, 30 to 90% by volume of hydrogen and the balance made up of argon is introduced through tube 9 at such a rate as to maintain a dynamic pressure in the range 1.0 to 100 mtorrwhile pumping. Reactive hydrogen atoms generated within the plasma discharge reduce the tungsten hexafluoride and tungsten ions ar produced and deposited on the inside of the tube to form a dense coating. The deposition rates of tungsten obtained varies with the tube diameter.

The discharge current required is proportional to the internal tube area to be coated, but is also dependent on the plating gas composition and flow rate with a typical minimum value of approx 0.5 mA cm-2. For a 2.5 cm internal diameter tube tungsten deposition rates of the order of 2um/minute may readily be obtained.

In order to obtain good coating adherence, it has been found beneficial to reduce the proportion of tungsten hexafluoride to 5% within the plating gas, for periods of 10 minutes every 30 minutes during the plating process, and to finish off the process with a 10 minute sputter treatment cleaning period in pure inert gas (argon or helium). This treatment consolidates the coating and removes all traces of the halide vapour before the discharge is finally switched off and air re-admitted.

Claims

1. A process for ion plating a metal onto an electrically conducting substrate comprises enclosing the substrate and a refractory electrode in an airtight vessel fitted with means for continuously evacuating the vessel and means for introducing gases at a controlled pressure, establishing a plasma discharge within said vessel, introducing into the airtight vessel at least one stream of gas comprising a carrier gas and a compound of the metal to be plated and decomposing this compound to give ions of the metal which then migrate to the substrate and form the required coating.

2. A process according to Claim 1 in which the plasma discharge is established using a DC voltage between an anode and the substrate, or by RF methods or by DC biased RF.

3. A process according to Claim 1 or Claim 2 in which the airtight vessel is formed by plugging the open end or ends of a hollow tube, the interior surface of which comprises the substrate to be plated.

4. A process according to either Claim 1 or Claim 2 in which the refractory electrode is constructed of a refractory metal.

5. A process according to any preceding Claim in which the refractory electrode is made of the metal to be plated.

6. A process according to any preceding Claim in which the refractory metal is tungsten, tantalum, molybdenum, niobium or chromium.

7. A process according to any preceding Claim in which the refractory electrode is constructed of carbon.

8. A process according to any preceding Claim in which the refractory electrode is in the form of a hollow tube with openings into the vessel which may be used for introducing gases or evacuating the vessel.

9. A process according to any preceding Claim in which the substrate is cleaned prior to plating by a sputter cleaning process in an atmoshere of a reactive or an inert cleaning gas.

10. A process according to any Claim 7 in which the cleaning gas is a mixture of an inert gas and a reactive gas in volume ratios of between 0.1 and 1.5 parts of reactive gas to 1 part of inert gas.

11. A process according to either Claim 7 or Claim 8 in which the inert gas is argon or helium or a mixture thereof.

12. A process according to Claim 8 in which the reactive gas is oxygen, chlorine or hydrogen.

13. A process according to Claims 8 to 10 in which the substrate is sputter cleaned with an oxidising cleaning gas followed buy a reducing cleaning gas and finally by an inert cleaning gas.

14. A process according to any preceding claim in which the metal to be plated is tungsten, tantalum, molybdenum, niobium or chromium.

15. A process according to any preceding Claim in which the metal compound is a halide, carbonyl, nitrosyl, alkyl, aryl or hydride of the metal.

16. A process according to any preceding Claim in which the metal compound constitutes between 1 and 100% by volume of the gas stream introduced into the vessel, the balance being made up by the carrier gas.

17. A process according to Claim 14 in which the carrier gas is argon, helium or a mixture thereof.

18. A process according to any preceding Claim in which the dynamic pressure in the vessel is maintained between 10-3 and 10 torr.

19. A process according to Claim 16 in which the dynamic pressure in the vessel is maintained between 10-3 and 10-1 torr.

20. A process according to any preceding Claim in which the metal compound is decomposed py olytically.

21. A process according to any of Claims 1 to 17 in which the metal compound is decomposed by microwaves.

22. a process according to any of Claims 1 to 17 in which the metal compound is decomposed photo lytical ly.

23. A process according to any of Claims 1 to 17 in which the metal compound is reduced using a chemical reducing agent.

24. A process according to Claim 21 in which the chemical reducing agent is hydrogen or ammonia.

25. A process according to either Claim 22 or Claim 23 in which the chemical reducing agent is introduced in a quantity in excess of the stoichiometric ratio required for reduction of the metal compound.

26. A process according to Claims 23 or 24 in which the gas introduced into the vessel comprises 5 to 40% by volume of tungstem hexafluoride, 30 to 90% by volume of hydrogen and the balance made up with argon or helium.

27. A process according to any preceding claim in which the input of the gaseous metal compound is reduced either partially or completely at intervals during the ion plating operation.

28. A process for ion plating a metal onto an electrically conducting substrate substantially as herein described with reference to the accompanying drawings.

29. An electrically conducting substrate ion plated with a metal by a process according to any of the preceding Claims.