GB2261226A

GB2261226A - Deposition of non-conductive material using D.C. biased, thermionically enhanced, plasma assisted PVD

Info

Publication number: GB2261226A
Application number: GB9123798A
Authority: GB
Inventors: Allan Matthews; Adrian Stanley James; Kevin Steven Fancey; Adrian Leyland
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1991-11-08
Filing date: 1991-11-08
Publication date: 1993-05-12
Anticipated expiration: 2011-11-08
Also published as: GB2261226B; GB9123798D0

Abstract

A layer of zirconia, or other ceramic oxide material, is deposited on a number of conductive substrates 9 using a d.c. biased, thermionically enhanced, plasma assisted physical vapour deposition technique. The layer may be deposited oxygen deficient, and subsequently oxidised by heating in an oxygen environment. A preferred material deposited is a zirconia based material. In the specific embodiment, as illustrated in the Figure, there is provided a thermionic emitter, 11, which may be a tungsten or titanium filament, the support platform 8 for the substrate being connected to a d.c. power supply 10 arranged to maintain the platform 8 at a negative potential. In use a glow discharge may be initiated by applying a potential of several kV to the support platform so as to initiate a d.c. diode mode glow discharge. Alternatively the filament heater 11 itself may be activated so as to produce a filament omission current with a negative potential applied to the filament and a negative bias being applied to the support platform 8 and initiate a d.c. triode mode glow discharge. A positively biased electrode may be introduced into the plasma generated to raise the plasma potential and cause the surfaces of the workpieces to be bombarded with ions and neutral species as well. <IMAGE>

Description

DEPOSITION OF NON-CONDUCTIVE MATERIAL This invention relates to methods and apparatus for depositing non-conductive materials. The invention has particular, although not exclusive, relevance to the deposition of metallic oxides on to a conductive substrate, in particular to the deposition of zirconia based materials on to a conductive substrate.

zirconia based materials are used in a wide range of applications, for example, optical coatings, thermal barrier coatings and oxygen sensors. It is also expected that zirconia based materials will be used in wear-resistant coatings. Where the coating is to be used in optical applications, the coatings are usually deposited by rf sputtering. Where the coating is to be used as a thermal barrier, the coating will generally be deposited by electron-beam physical vapour deposition, or by plasma spraying. It has also been proposed in an article in "Surface and Coatings Technology" 1990, Vol.

43/44, pages 436-445, by A.S. James and A Matthews, that rf-biased plasma assisted physical vapour deposition may be used to deposit zirconia based materials on to a conductive substrate.

It is an object of the present invention to provide an alternative method and apparatus suitable for depositing zirconia based materials on to a conductive substrate, in which better coating structure control is possible than in the techniques previously used.

According to a first aspect of the present invention, there is provided a method of depositing non-conductive material on to a conductive substrate using a d.c.

based, thermionically enhanced, plasma assisted physical vapour deposition technique.

The method suitably includes introducing a positive electrode into the plasma It has been observed that the structure of the coatings produced by such a technique are considerably denser than those produced by rf diode deposition. This is contrary to accepted theory which suggests that it should not be possible to produce coatings of insulating materials using a d.c. technique.

Where the material is an oxide, the method suitably includes the steps of depositing a layer of oxygen deficient oxide material on to the conductive substrate, and subsequently oxidising the oxygen deficient material.

Preferably, the temperature of the substrate is arranged to be increased for the deposition process. This heating may be produced by sputter cleaning the substrate prior to the deposition, or alternatively or additionally by the use of substrate heater means.

The non-conductive material may be a metal oxide, such as a zirconia based material.

According to a second aspect of the present invention there is provided an apparatus for depositing nonconductive material upon a conductive substrate, the apparatus comprising a thermionically enhanced, d.c.

biased, plasma assisted physical vapour deposition apparatus.

The apparatus suitably includes a positive electrode within the plasma.

Where the material is an oxide, the apparatus suitably includes means for depositing a layer of oxygen deficient material on the conductive substrate, and means for subsequently oxidising the layer of material.

One method of depositing a coating of a zirconia based material in accordance with an embodiment of the invention, together with an apparatus in accordance with an embodiment of the invention for performing the method, will now be described, by way of example only, with reference to the sole figure which is a schematic diagram of the apparatus.

Referring to the figure, the apparatus comprises a vacuum chamber 1 having a pumping port 2 which in use of the apparatus is connected to a vacuum pumping system (not shown), and a gas inlet 3. Within the chamber 1 there is provided a crucible 4 containing the source material 5 for evaporation, for example zirconia or another generally non-conductive material, adjacent to an electron beam arrangement 6 connected to a power supply 7. Also arranged within the chamber 1 is a support platform 8 which carries the conductive substrates 9 to be coated. The support platform 8 is connected to a d.c. power supply 10 arranged to maintain the platform 8 at a negative potential Between the melt 4 and the support platform 8 there is provided a thermionic emitter 11 provided with a filament heater power supply 12 and a bias supply 13.

The thermionic emitter 11 may be of any of the usual forms which will be evident to a person skilled in the art of the deposition of materials, for example a tungsten or tantalum filament.

In use of the apparatus the vacuum chamber 1 is pumped via the pumping port 2 to an ultimate pressure of less than 10 5 mTorr, so as to remove any background impurities from the chamber 1. If this pressure level is not reached then the chamber 1 may be purged with an inert gas such as argon, which may be introduced through the gas inlet 3, the chamber 1 being additionally, or alternatively, baked out by heating the chamber 1, this heating being conveniently performed by the filament 11, using the heater power supply 12.

If it is considered necessary to sputter clean or condition the conductive substrates 9, this may be achieved by letting an inert gas such as argon or hydrogen into the chamber 1 through the gas inlet 3, the pumping rate through the pumping port 2 being reduced until the pressure within the chamber 1 reaches a level of typically between 7 mTorr and 20 mTorr. A glow discharge can then be initiated by applying a potential of several kV to the support platform 8 so as to initiate a d.c. diode mode glow discharge.

Alternatively, the filament heater 11 may be activated so as to produce a filament emission current of several amps with a negative potential of typically between 50 and 200 V being applied to the filament by the bias supply 13, and a negative bias of less than 1 kV, and typically less than 200 V, being applied to the support platform 8 by the power supply 10 so as to initiate a d.c. triode mode glow discharge.

A third alternative is to introduce a positively biased electrode (not shown) into the chamber 1 so as to bombard the surfaces of the workpieces 9 with ions and/or neutrals (even if they are earthed), thereby sputtering impurities from the surface of the conductive substrates 9 Typically, the substrates 9 will be sputter cleaned for 30 minutes.

After sputter cleaning the workpieces 9, deposition of the zirconia coating on to the substrate 9 may take place.

Typically the support platform 8 and consequently the conductive substrates 9, will be negatively biased to between 50 and 200 V, with the bias supply 13 biasing the filament 11 to between 50 and 100 V and the filament 11 being arranged to produce an emission current of typically between 1 and 10 amps. Argon is introduced into the chamber 1 via the gas inlet 3, the pumping being controlled to maintain the pressure within the chamber at approximately 7 mTorr. The electron beam arrangement 6 is initiated, the electron trajectory being indicated in the figure, and the zirconia evaporated into the argon plasma. Optionally a stream of oxygen may be bled into the chamber 1 via the gas inlet 3 in order to increase the stoichiometry of the coatings on the substrates 9. The deposition process will be continued until the desired coating thickness is achieved on the substrates 9.

The temperatures of the substrates 9 during the deposition process will typically be between 400 and 8000C. If required this temperature can be increased by the introduction of additional substrate heaters (not shown), or by performing the sputter cleaning operation at greater powers and for greater time periods so as to pre-heat the substrates 9.

A deliberately oxygen deficient source material 8 as the evaporant may be used in the crucible 4. Thus, the coatings will require to be oxidised after deposition, for example, by heating the coated substrates 9 in an oxygen environment, this being conveniently achieved in the deposition chamber 1 after the deposition process.

It may be advantageous to include a positively biased electrode within the deposition chamber 1. This could be achieved by applying a suitable positive bias to an additional electrode (not shown) introduced into the chamber 1. A further alternative is to apply a positive bias to the crucible 4. The addition of such a positively biased electrode will have a number of effects, the net result being an increase in the amount of ionisation occuring in the deposition chamber 1, this being effective to enhance the properties of the coating on the substrates 9.

Alternatively, or additionally0 a positive bias applied to the crucible 4 may improve the properties of the coatings deposited on the substrates 9 If the crucible 4 is so biased it may, in some cases, be possible to dispense with the negative bias applied to the holder 8 and thus the substrates 9.

Whilst the example described herebefore relates to the deposition of zirconia on to the substrates 9, the technique is also applicable to other non-conductive materials. Examples of such alternative materials include yttria and alumina. If it is required to produce complex materials, for example, zirconia based material including other phases such as yttria, magnesia, calcia, ceria etc., the coatings may be produced by reactive deposition by evaporating the appropriate metallic species from the crucible 4, for example, zirconium and yttrium in appropriate concentrations to produce a zirconia based material including yttria.

Claims

1. A method of depositing non-conductive material on to a conductive substrate using a d.c. biased, thermionically enhanced, plasma assisted physical vapour deposition technique.

2. A method according to claim 1 including introducing a positive electrode into the plasma to raise the plasma potential.

3. A method according to either of the preceding claims in which the material is an oxide, the method including the steps of depositing a layer of oxygen-deficient oxide material on to the conductive substrate0 and subsequently oxidising the oxygen deficient material.

4. A method according to any one of the preceding claims including arranging for the temperature of the substrate to be increased for the deposition process.

5. A method according to claim 4 in which the increase in temperature of the substrate is produced by sputter cleaning the substrate prior to the deposition.

6. A method according to claim 4 or claim 5 using other substrate heating means to effectively increase the temperature of the substrate.

7. A method according to any one of the preceding claims in which the non-conductive material is a metal oxide.

8. A method according to claim 7 in which the metal oxide is a zirconia based material.

9. A method of depositing non-conductive material on to a conductive substrate substantially as hereinbefore described with reference to the accompanying figure.

10. An apparatus for depositing non-conductive material upon a conductive substrate, comprising a thermionically enhanced, d.c. biased, plasma assisted physical vapour deposition apparatus.

11. An apparatus according to claim 10, incorporating a positive electrode within the region containing the plasma.

12. An apparatus according to claim 11 in which the electrode is the crucible holding the evaporant material.

13. An apparatus according to any one of claims 10 to 12 in which the material is an oxide, the apparatus including means for depositing a layer of oxygen-deficient oxide material on to the conductive substrate, and means for subsequently oxidising the oxygen-deficient material.

14. An apparatus according to claims 10 to 13 including means for increasing the temperature of the substrate for the deposition process.

15. An apparatus according to claim 14 including means for sputter cleaning the substrates prior to the deposition process so as to increase the temperature of the substrate.

16. An apparatus according to claim 14 including other substrate heating means to effectively increase the temperature of the substrate.

17. An apparatus according to any of claims 10 to 16 in which the non-conductive material is an oxide.

18. An apparatus according to claim 17 in which the oxide is a zirconia based material.

19. An apparatus for depositing a non-conductive material upon a conductive substrate, substantially as hereinbefore described with reference to the accompanying figure.