GB1599369A

GB1599369A - Deposition techniques

Info

Publication number: GB1599369A
Application number: GB2263578A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-05-25
Filing date: 1978-05-25
Publication date: 1981-09-30

Description

(54) DEPOSITION TECHNIQUES (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Com- pany, of 190 Strand, London, WC2R lDU, England do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the vapour deposition of metal films for use as ohmic contacts on various electronic components. In particular it relates to the thermally-induced deposition of aluminium for the above purpose from volatile aluminium compounds.

Many processes require the deposition of metal films, e.g. to provide such ohmic contacts. Generally such metal films are provided by vacuum evaporation of the metal, but this process needs careful control to ensure that a hard vacuum is maintained and that traces of impurities are excluded.

Further vacuum processing needs expensive airlocks and involves considerable process time. To overcome these difficulties atmospheric pressure processes have been developed in which aluminium is thermally deposited by decomposition of a volatile aluminium compound, generally tri-isobutyl aluminium (TIBA). Such a process, however, has so far proved difficult to control and does not, in general, give reproducible results.

In our Patent Specification No. 22632/78 Serial No. 1,595,659 there is described a process for the pyrolytic deposition of aluminium from tri-isobutyl aluminium (TIBA) the process including passing a stream of dry, oxygen free, argon or nitrogen through a quantity of liquid TIBA maintained at a temperature below 900 C, so as to entrain a proportion of TIBA vapour, feeding the argon or nitrogen and entrained TIBA to a reaction chamber containing workpieces to be coated in the aluminium, selectively pulsing dry, oxygen free, argon or nitrogen into the reaction chamber so as to provide a substantially uniform TIBA concentration within the vessel, and maintaining the workpieces at a temperature in the range 250--290"C thereby causing pyrolytic deposition of aluminium thereon.

A process for vapour deposition of aluminium films has been described in our Patent Application No. 22633/78 Serial No.

1,594,399 and an object of this invention is to enable such a process to be applied to the specific purpose referred to above.

According to the invention there is provided a process for depositing aluminium on an electrical or electronic device for the production thereupon of one or more ohmic contacts, which process includes the thermal decomposition of tri-isobutyl aluminium (TIBA) supplied in vapour form to a reaction chamber maintained at a temperature in the range 250-2900C, in which the TIBA, prior to entering the reaction chamber, is maintained at a temperature of 900C or below, and in which each said device is provided with masking means so formed that deposition can only take place on the areas at which ohmic contacts are to be produced.

According to the invention there is also provided a process for the pyrolytic deposition of aluminium on an electrical or electronic device for the production thereupon of one or more ohmic contacts, which process includes passing a stream of dry, oxygen free, argon or nitrogen through a quantity of liquid tri-isobutyl aluminium (TIBA) maintained at a temperature of 900C or below so as to entrain a proportion of TIBA vapour, feeding the argon or nitrogen and entrained TIBA - to a reaction chamber containing electrical or electronic devices on which ohmic contacts are to be produced, each said device being provided with masking means so formed that deposition can only take place on the areas at which ohmic contacts are to be produced, selectively pulsing dry, oxygen free, argon or nitrogen into the reaction vessel so as to provide a substantially uniform TIBA concentration within the chamber and maintaining the devices at a temperature in the range 250-290 C thereby causing pyrolytic deposition of aluminium thereon.

Embodiments of the invention will now be described with reference to the accompanying drawing; which is a schematic view of an apparatus for the thermal deposition of aluminium.

Referring to the drawing, the apparatus includes a reaction vessel 11 in which pyrolytic deposition of aluminium is effected and which is maintained at a temperature in the range 25e2700C by a furnace (not shown). TIBA vapour is carried into the reaction vessel 11 in a steady stream of dry, oxygen free, argon or nitrogen which is fed through a bubbler 12 containing liquid TIBA and which is maintained at a temperature of 80--90"C by an oven 13.

Supplies of gases to the apparatus are provided via inlet control valves 14, flow- meters 15 and valves 16. A solenoid-operable change-over valve 17 couples the various gas supplies selectively via a tube 18 to the reactor 11. Pipe 19 supplies argon to the bubbler 12 via an expansion chamber 20. To reduce the rate of conversion of TIBA into diisobutyl aluminium hydride (DIBAH), which is probably catalysed by the surfaces of the vapour feed lines of the apparatus, it is necessary to maintain the TIBA vapour temperature below 900C and preferably within the range 84 to 86 C. The amount of TIBA fed into the reaction vessel 11 is determined both by the temperature and the flow rate of argon through the bubbler.In the arrangement described an argon flow rate of 7 litres/ min at a TIBA temperature of 85 0C was found to be suitable.

Pyrolytic deposition of aluminium on work pieces is effected in the reaction vessel 11, which vessel is maintained at a temperature preferably between 250 and 2700C. We have found that at temperatures below 2500C little deposition takes place, whilst at temperatures above 270"C the film quality becomes poor.

While not in operation the apparatus is purged with purified nitrogen fed via the tube 18 to the reaction vessel 11. Further during the periods when the reactor 11 is cold or at a reduced temperature, isobutylene is passed through the bubbler at a rate of about one bubble per second to convert any DIBAH contamination in the gas lines back into TIBA.

We have found that this technique of purging the apparatus with isobutylene results in a superior quality deposited product. Note that when the apparatus is in use the isobutylene purge is turned off.

For deposition the work pieces are loaded through a door 19a at one end of the reactor 11. The door is dosed against gasket 20 and the reactor 11 is purged with nitrogen.

After the work pieces have had sufficient time to reach the temperature of the reactor the nitrogen purge is switched off and argon is supplied to the bubbler so as to introduce TIBA to the reactor 11. Uniform distribution of the gas within the reactor is ensured by supplying periodic pulses of dry, oxygen free argon or nitrogen via the pipe 18 and the associated solenoid valve. The valve may be controlled advantageously by a timer, a suitable pulse rate being 1 second on in every 20 seconds. Typical operation conditions are as follows: (a) Oven temperature 850C (b) Reactor temperature 250--2700C (c) Argon flow rate 7 litres/minute (d) Argon pulse rate 1 second in 20 seconds.

This results in a deposition rate of the order of 0.1 microns/minute.

The deposition process is terminated by switching off the carrier gas and purging the reactor with purified nitrogen for a further two minutes.

In some applications the quality of the deposited aluminium may be further improved by pretreatment of the work pieces in a hydrogen glow discharge in the presence of certain metals such as gold or nickel. It is observed that the work piece surface is activated by transport of the metal via the plasma.

The thus activated surfaces of the work pieces are then ready for aluminium deposition.

The activated surface is stable, and the work piece may if necessary be left in air for several days with little degradation.

One of the types of devices to which ohmic contacts are applied by the process referred to above is barium titanate positive temperature coefficient (PTC) thermistors. To do this the thermistors are placed in the reactor 11 with each thermistor masked, the masking being so effected that deposition can only occur on the area or areas on which ohmic contacts are required. As a follow-up step to this deposition, where the contacts require to be plated, the devices are removed from the reactor, whereafter nickel is electrolessly deposited on the area or areas on which aluminium has already been deposited. This nickel coating provides a solderable surface.

This has been found not to need any catalysts, and also does not need heat.

The above process is easier to "automate" than many known processes, and also does not need centre-less grinding.

The next devices to be considered are barium titanate capacitors. Here the technique used is substantially the same as that used for the thermistors. In this case we have certain advantages specific for capacitors: (a) the tan 8 is lower than with known methods.

(b) the capacitors can be electrically cleaned.

(c) it eliminates expensive and difficult silver screen printing processes.

Other capactitors on which the aluminium may be deposited in this way are these using titanium dioxide or zirconium oxide mixed with barium titanate, or calcium titanate or calcium zirconate.

Another device onto which ohmic contacts can be deposited by the above method is a ceramic heating element. In this case the aluminium is deposited on the top and bottom areas of a honeycomb-like structure. This is then followed by the deposition on the aluminium of an easily weldable surface.

The temperature range referred to. above is not the absolute maximum: thus if temperature is increased to 2900C, deposition is accelerated. This is especially suitable where deposition is on a rough surface.

WHAT WE CLAIM IS: 1. A process for depositing aluminium on an electrical or electronic device for the production thereupon of one or more ohmic contacts, which process includes the thermal decomposition of tri-isobutyl aluminium (TIBA) supplied in vapour form to a reaction chamber maintained at a temperature in the range 250--290"C, in which the TIBA, prior to entering the reaction chamber, is maintained at a temperature of 900C or below, and in which each said device is provided with masking means so formed that deposition can only take place on the areas at which ohmic contacts are to be produced.

2. A process for the pyrolytic deposition of aluminium on an electrical or electronic device for the production thereupon of one or more ohmic contacts, which process includes passing a stream of dry, oxygen free, argon or nitrogen through a quantity of liquid tri-isobutyl aluminium (TIBA) maintained at a temperature of 900C or below so as to entrain a proportion of TIBA vapour, feeding the argon or nitrogen and entrained TIBA to a reaction chamber containing electrical or electronic devices on which ohmic contacts are to be produced, each said device being provided with masking means so formed that deposition can only take place on the areas at which ohmic contacts are to be produced, selectively pulsing dry, oxygen free, argon or nitrogen into the reaction vessel so as to provide a substantially uniform TIBA concentration within the chamber and maintaining the devices at a temperature in the range 250-2900C thereby causing pyrolytic deposition of aluminium thereon.

3. A process as claimed in claim 2, and which includes purging the reaction chamber with isobutylene prior to and after aluminium deposition.

4. A process for the pyrolytic deposition of aluminium on an electrical or electronic device for the production thereupon of one or more ohmic contacts, which process includes passing a stream of dry, oxygen free, argon or nitrogen through a quantity of liquid tri-isobutyl aluminium (TIBA) maintained at a temperature in the range of 8486 C so as to entrain a proportion of TIBA vapour, feeding the argon or nitrogen and entrained TIBA to a reaction chamber containing electrical or electronic devices on which ohmic contacts are to be produced, each said device being provided with masking means so formed that deposition can only take place on the areas at which ohmic contacts are to be produced, selectively pulsing dry, oxygen free, argon or nitrogen into the reaction vessel so as to provide a substantially uniform TIBA concentration within the chamber and maintaining the devices at a temperature in the range of 250-2700C thereby causing pyrolytic deposition of aluminium onto the unmasked areas of the devices, the process also involving purging the reaction chamber with isobutylene prior to and after the aluminium deposition.

5. A process as claimed in claim 1, 2, 3, or 4, and in which the devices on which ohmic contacts are to be produced are barium titanate positive temperature coefficient thermistors.

6. A process as claimed in claim 1, 2, 3, or 4, and in which the devices on which ohmic contacts are to be produced are capacitors.

7. A process as claimed in claim 6, and in which the capacitors are barium titanate capacitors.

8. A process as claimed in claim 1, 2, 3, or 4, and in which after the aluminium deposition has been effected, nickel is electrolessly deposited on the aluminium areas.

9. A process as claimed in claim 1, 2, 3, or 4, and in which the devices on which ohmic contacts are to be produced are ceramic heating elements, the aluminium deposition being followed by the deposition of a readily weldable surface on to the aluminium.

10. A process for the deposition of aluminium ohmic contacts onto electrical devices, as claimed in claim 1, and substantially as described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. may be deposited in this way are these using titanium dioxide or zirconium oxide mixed with barium titanate, or calcium titanate or calcium zirconate. Another device onto which ohmic contacts can be deposited by the above method is a ceramic heating element. In this case the aluminium is deposited on the top and bottom areas of a honeycomb-like structure. This is then followed by the deposition on the aluminium of an easily weldable surface. The temperature range referred to. above is not the absolute maximum: thus if temperature is increased to 2900C, deposition is accelerated. This is especially suitable where deposition is on a rough surface. WHAT WE CLAIM IS:

1. A process for depositing aluminium on an electrical or electronic device for the production thereupon of one or more ohmic contacts, which process includes the thermal decomposition of tri-isobutyl aluminium (TIBA) supplied in vapour form to a reaction chamber maintained at a temperature in the range 250--290"C, in which the TIBA, prior to entering the reaction chamber, is maintained at a temperature of 900C or below, and in which each said device is provided with masking means so formed that deposition can only take place on the areas at which ohmic contacts are to be produced.