Process and method for in-situ dry cleaning of thin film deposition reactors and thin film layers

Abstract

The invention relates to the use of thionyl chloride and related materials for dry etching of internal surfaces of metalorganic vapor phase epitaxy (MOVPE) reactors to remove deposits. The method is also useful for the dry etching of process substrates within such reactors for cleaning and processing of those substrates. The invention may be particularly adaptable to chemical vapor deposition reactors used in the manufacture of high brightness LED's based on III-V semiconductors such as GaN and related materials. Features of the process include thermal, UV, and plasma activated dry cleaning, and the use of etchant gases such as COCl.sub.2, COBr.sub.2, COl.sub.2, SOl2, SOCl.sub.2, SOBr.sub.2, SO2Cl.sub.2, SO.sub.2Br.sub.2, NOCI, NOBr, NOl, S.sub.2Cl.sub.2, S.sub.2Br.sub.2, SCI.sub.2, SBr.sub.2, SOClBr, SOClF and SOFBr, either formed from neat materials or combinations of constituent gases such as CO, SO, SO.sub.2 or NO with halogens, to achieve the desired effect.

Claims

1. A method for cleaning reaction product deposits from interior surfaces of a chemical reactor chamber or from a substrate within said chemical reactor chamber, comprising the steps of: heating said reactor chamber to an elevated temperature of at least 100 C.; introducing an etchant gas into said reactor chamber, said etchant gas being of the general formula AO.sub.mX.sub.n, wherein: A is selected from the group consisting of C, N, and S; O is oxygen; X is a halogen; and the subscripts .sub.m and .sub.n are greater than zero; activating said etchant gas either before or after said introduction; allowing an etching reaction to proceed between said activated etchant gas and said reaction product deposits to remove said reaction product deposits without substantial re-deposition of etching reaction products within said reactor chamber; and evacuating said activated etchant gas together with substantially all products of said etching reaction; wherein pressure within said reactor chamber is between 20 mBar and 1000 mBar during said etching reaction.

2. The method of claim 1 comprising the further step of generating said etchant gas prior to said introduction into said chamber.

3. The method of claim 1 comprising the further step of bubbling a carrier gas through a liquid chemical constituent to volatilize said liquid chemical constituent into said etchant, prior to said introduction of said etchant gas into said chamber.

4. The method of claim 2 wherein said etchant gas is generated by bubbling a carrier gas through a plurality of liquid chemical constituents and then combining the resulting gases.

5. The method of claim 2 wherein said etchant gas is generated by mixing two or more chemical constituent gases.

6. The method of claim 1 wherein said etchant gas is activated by exposing it to an activation mechanism in a gas activation chamber prior to introduction into said reactor chamber; said gas activation mechanism being selected from the group consisting of heat, ultraviolet light and plasma discharge.

7. The method of claim 1 wherein said etchant gas is activated by exposing it to a thermal activation mechanism after introduction into said reactor chamber; said thermal activation mechanism being selected from the group consisting of: the overall temperature within said reactor chamber and a local heat source within the reactor chamber.

8. The method of claim 1 wherein said etchant gas further contains an additive of the general formula RX, wherein: R is selected from the group consisting of H and Me; and X is selected from the group consisting of F, Cl, Br and I.

9. The method of claim 1 wherein said etchant gas also contains a halogen gas additive.

10. The method of claim 1 wherein said pressure is between 500 mBar and 1000 mBar.

11. The method of claim 1 wherein said elevated temperature is between 100 C. and 400 C.

12. The method of claim 1 wherein said elevated temperature is between 250 C. and 400 C.

13. The method of claim 1 wherein said temperature is substantially constant during said etching reaction.

14. The method of claim 1 wherein said pressure is substantially constant during said etching reaction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The preferred embodiment of the invention will be described by reference to the drawings in which:

(2) FIG. 1 is a diagrammatical sectional view of a typical MOCVD reactor;

(3) FIG. 2 is a gas activation chamber used to activate etchant gas flowing through chamber by exposure to heat, UV light, or plasma discharge;

(4) FIG. 3 is an experimental apparatus used to demonstrate thermally activated dry etching on substrates coated with materials typical of deposits in MOVPE reactors;

(5) FIG. 4 is an experimental apparatus used to demonstrate thermally activated dry etching with multiple etchant components on substrates coated with materials typical of deposits in MOVPE reactors;

(6) FIG. 5 is an experimental apparatus used to demonstrate UV light activated dry etching on substrates coated with materials typical of deposits in MOVPE reactors; and

(7) FIG. 6 is an experimental apparatus used to demonstrate thermally activated dry etching using a mixture of reagent gasses according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(8) Referring to FIG. 1, a typical reactor comprises a reactor chamber 1, in which a susceptor plate 2 is capable of supporting one or more substrates 4. One or more heating elements 3 may be used to control the temperature of the susceptor 2, holding it as necessary in the proper range for effective thin film deposition. Reactive process gas enters the chamber 1 through one or more feed lines 9, and is distributed through the chamber 1 via a dispersal means such as showerhead 5, which comprises a lower plate 6 having a plurality of orifices 7 to spread the gas feed evenly throughout the chamber 1. After a deposition cycle has taken place, the chamber may be purged by feeding purge gas through a feed line 8. The chamber is evacuated by removing gas through gas outlet ring 10, comprising one or more orifices 11, into a gas evacuation line 12, which may be connected to a vacuum pump and gas disposal means or recycling means, depending on the system.

(9) In use, the dry etching process consists of heating the entire reactor chamber 1 to an elevated temperature, typically in the range between 100 C. and 400 C., and providing an etchant gas, which is introduced into reactor chamber 1 via one or more process gas feed lines 9. The gas may be introduced through a gas distribution mechanism, such as shower head 5 or via gas feed line 8. The etchant gas flows through the reactor chamber 1, before being evacuated together with any produced reaction products through gas evacuation ring 10 and pumped out through gas evacuation line 12. The gas pressure in the reactor chamber 1 is typically maintained at between 20 mBar and 1000 mBar (atmospheric pressure) during the dry etching cycle.

(10) The etchant gas is preferably activated to enhance the generation of free radicals, and thereby to enhance the etching process. This may be achieved by thermal activation, ultraviolet (UV) excitation, or plasma discharge, as shown in FIG. 2. The etchant gas 14 enters the activation chamber through inlet 13. Within the activation chamber, a volume of gas 15 is exposed to an activation energy source 16, such as a heater (thermal activation), a UV lamp (UV activation) or an ionizing RF field (plasma activation). Thermal activation may be effected by heating of the MOVPE reactor vessel, or the etchant gas may be pre-heated in a heating chamber prior to injection into the reactor vessel. In the case of UV or plasma discharge activation the etchant gas is activated in an activation chamber by exposure to UV light or radio frequency plasma discharge prior to injection into the reactor vessel. The activated gas exits the activation chamber through outlet 17, and passes into the MOCVD reactor chamber 1 (not shown).

(11) The etchant gas comprises a carbonyl, thionyl, or nitrosyl group (CO, SO, or NO) combined with a halogen: chlorine, bromine, or iodine (Cl, Br, or I). COCl.sub.2, COBr.sub.2, COI.sub.2, SOI.sub.2, SOCl.sub.2, SOBr.sub.2, SO.sub.2Cl.sub.2, SO.sub.2Br.sub.2, NOCl, NOBr, NOI, S.sub.2Cl.sub.2, S.sub.2Br.sub.2, SCl.sub.2, SBr.sub.2, SOClBr, SOClF and SOFBr are examples of suitable etchant gases. The etchant gas may be derived from the neat material, or may alternatively be generated by a combination of separate components, such as CO, SO, SO.sub.2, or NO mixed with Cl.sub.2, Br.sub.2, or I.sub.2. The etchant gas may be mixed with a carrier gas such as argon, nitrogen, or hydrogen. The etchant gas or its components may be supplied directly from one or more gas cylinders, in the case of etchant constituents that are available in the gaseous state. In the case of etchant gas constituents that are normally in a liquid state, the required gaseous state is preferably achieved by bubbling the carrier gas through a container with the liquid etchant component, volatilizing the liquid constituents into the etchant, thereby producing a mixture of carrier gas and etchant vapour. Alternatively, the liquid chemical constituents may be heated until they vaporize, at which point the vapor is combined with a carrier gas, if necessary, and introduced into the reactor chamber 1. The etchant gas may contain additional amounts of halogen to enhance etching. The etchant gas may contain additional amounts of methyl halide, hydrogen halide or other halogen compounds to enhance etching.

(12) Within the reaction chamber the etchant gas reacts with metal containing deposits to form volatile metal halides which are removed with the purging of the etchant gas. A typical reaction involves the reaction of the metallic oxide with the purge gas to form metallic halide with the residual oxygen combining with the carbonyl/thionyl/nitrosyl group. For example:
Ga.sub.2O.sub.3+3SOBr.sub.2.fwdarw.2GaBr.sub.3+3SO.sub.2
2Ga.sub.2O.sub.3+6SOBrCl.fwdarw.GaClBr.sub.2+GaBrCl.sub.2+GaCl.sub.3+GaBr.sub.3+6SO.sub.2
Ga.sub.2O.sub.3+3NOBr.fwdarw.GaBr.sub.3+3NO.sub.2
In.sub.2O.sub.3+3COCl.sub.2.fwdarw.2InCl.sub.3+3CO.sub.2

(13) Other metal containing deposits also react to form metal halides; the oxide is given as an example of the most difficult deposit to remove due to the strong affinity of the metal for oxygen.

(14) Once the metallic halide is formed, it must be removed from the reaction chamber. One way to do so is to reduce the pressure in the chamber to mobilize the halides for removal from the reactor chamber. Another option, which may be used alone or in combination with a reduced chamber pressure, is to heat the chamber to a temperature sufficient to evaporate or sublimate the halide. Table 1 lists some typical reaction products and their boiling points.

(15) TABLE-US-00001 TABLE 1 Reaction products and their boiling point temperatures. Material B.P ( C.) Material B.P ( C.) GaCl.sub.3 201 AlBr.sub.3 263 GaBr.sub.3 279 AlI.sub.3 191 GaI.sub.3 345 (sub) NCl.sub.3 <71 InCl.sub.3 600 NBr.sub.3 Explodes upon melting InBr.sub.3 >600 NI.sub.3 Explodes InI.sub.3 210 NH.sub.3 33 AlCl.sub.3 183 Me.sub.3N 3
Experimental Results

(16) Referring generally to FIG. 3, a first embodiment of the dry etching process of the invention is carried out in an experimental reaction chamber 27 arranged as follows. The reaction chamber 27, being a quartz tube having an input end flange 26 and an output end flange 33, contains a first substrate holder 28, which is able to support a test substrate 32. A second substrate holder 30 is also provided, but for the purposes of the experiment does not hold a substrate. Each substrate is heated by a suitable heater 31, which may be for example, an infrared heater. A pre-heating zone is created around substrate holder 30, wherein the etchant gas flow is pre-heated before it reaches the test substrate 32 on substrate holder 28. The temperature of the substrate holder 28 may be monitored as needed, such as by thermocouple wires 29 which pass through 35 an end of the reactor chamber, and produce a readout on recorder 36. The temperature within the reactor chamber is preferably controlled such that a cooled section 37 is maintained near the output end flange 33. An output valve 34 connected to a vacuum pump (not shown) regulates the pressure within the reactor chamber.

(17) Gas input line 20 feeds a carrier gas, such as argon, nitrogen, or hydrogen, into the system. The carrier gas feed may pass through direct input valve 21 directly into the reaction chamber, as may be required during a purge cycle, or it may pass through bubbler valve 22. The carrier gas entering the bubbler 23 passes through liquid reagent etching component 24, where it picks up etchant vapour and transports it to the reactor in the gas phase, controlled by valve 25. Once operational temperature of the substrate holders is reached, the flow of carrier gas is directed through the reagent 24 by closing valve 21 and opening valves 22 and 25. The resultant etchant gas flows across test substrate 32 and dry etches the reactor deposits on the substrate 32. Volatile non-gaseous reaction products are condensed onto the cooled section 37 of chamber 27.

(18) For the purpose of these experiments, test substrate 32 comprises stainless steel, a material typical of MOVPE reactor structural material. The substrates each bore a typical MOPVE reactor deposit resulting from III-V fabrication processes, namely a coating formed during the deposition of GaN using trimethyl gallium Ga(CH.sub.3).sub.3 and ammonia NH.sub.3 at high temperatures (>700 C.) using MOCVD.

Example 1SOCl2 200 C.

(19) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 200 C. under a continuous carrier gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. Once the substrate reached a temperature of 200 C., the flow of the carrier gas was diverted through bubbler valve 22 into bubbler 23 containing reagent 24 (SOCl.sub.2), such that SOCl.sub.2 was picked up in the carrier gas, forming an etchant gas. The etchant gas entered the reactor chamber 27 through valve 25, passed over the substrate 32 and etched away the deposit on the stainless steel substrate 32. The reaction was very rapid over the first five minutes, and then proceeded more slowly, such that the etching process took 10-13 minutes to clean the substrate to a shiny stainless condition. The by-products, which were mainly GaCl.sub.3 and SO.sub.2, were collected in the cooled area 37 of chamber 27.

Example 2SOBr2 200 C.

(20) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 200 C. under a continuous carrier gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. Once the substrate reached 200 C., the flow of the carrier gas was diverted through bubbler valve 22 into bubbler 23 containing reagent 24 (SOBr.sub.2), such that SOBr.sub.2 was picked up in the carrier gas, producing an etchant gas. The etchant gas entered the reactor chamber 27 through valve 25, passed over the substrate 32 and etched away the deposit on the stainless steel substrate 32. The reaction was very quick and efficient, cleaning the substrate completely within 7-9 minutes. The by-products, which were mainly GaBr.sub.3 and SO.sub.2, were collected in the cooled area 37 of chamber 27.

Example 3SOBr2 200 Preheat 300 C.

(21) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 200 C. under a continuous carrier gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. The second substrate holder 30 was heated to 300 C. with the IR heater 31. Once the appropriate temperatures were reached, the flow of the carrier gas was diverted through bubbler valve 22 into bubbler 23 containing reagent 24 (SOBr.sub.2), such that SOBr.sub.2 was picked up in the carrier gas, and an etchant gas was produced. The etchant gas entered the reactor chamber 27 through valve 25, was thermally activated as it passed over the substrate holder 30, passed over substrate 32 and etched away the deposit on the stainless steel substrate 32. The reaction was very quick and efficient, cleaning the substrate completely within 3-5 minutes. The by-products, which were mainly GaBr.sub.3 and SO.sub.2, were collected in the cooled area 37 of chamber 27.

Example 4SOClBr 200 C.

(22) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 200 C. under a continuous carrier gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. Once the substrate reached 200 C., the flow of the carrier gas was diverted through bubbler valve 22 into bubbler 23 containing reagent 24 (SOClBr), such that SOClBr was picked up in the carrier gas, producing an etchant gas. The etchant gas entered the reactor chamber 27 through valve 25, passed over the substrate 32 and etched away the deposit on the stainless steel substrate 32. The reaction was quick and efficient, cleaning the substrate completely within 3-5 minutes. The by-products, which were mainly GaClBr.sub.2, GaBrCl.sub.2, GaBr.sub.3, GaCl.sub.3 and SO.sub.2, were collected in the cooled area 37 of chamber 27.

Example 5SOBr2 with 5% Br2 at 250 C.

(23) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 250 C. under a continuous carrier gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. The second substrate holder 30 was heated to 300 C. with the IR heater 31. Once the appropriate temperatures were reached, the flow of the carrier gas was diverted through bubbler valve 22 into bubbler 23 containing reagent 24 (SOBr.sub.2 and 5% Br.sub.2 by volume), such that SOBr.sub.2/Br.sub.2 was picked up in the carrier gas, and an etchant gas was produced. The etchant gas entered the reactor chamber 27 through valve 25, was thermally activated as it passed over the substrate holder 30, passed over substrate 32 and etched away the deposit on the stainless steel substrate 32. The reaction was very quick and efficient, cleaning the substrate completely within 5-6 minutes. The by-products, which were mainly GaBr.sub.3 and SO.sub.2, were collected in the cooled area 37 of chamber 27.

(24) Referring now to FIG. 4, an experimental apparatus for carrying out a second embodiment of the invention comprises a similar arrangement of reaction chamber 27 bounded by an input end flange 26 and an output end flange 33. First and second substrate holders 28, 30 are provided, each heated by a heater 31, which may, for example be an infrared heater or any suitable heater. As in the first arrangement, only substrate holder 28 supports a test substrate 32, while second substrate holder 30 is used to create a pre-heating zone wherein the etchant gas flow is heated before it reaches the test substrate. Recorder 36 and thermocouple wires 29 monitor the substrate temperature as needed. A cooled section 37 is maintained near the output end flange 33. An output valve 34 connected to a vacuum pump (not shown) regulates the pressure within the reactor chamber.

(25) In this embodiment, the input gas is treated differently before it enters the reaction chamber 27. As before, the carrier gas feed may pass through direct input valve 21 directly into the reaction chamber, as may be required during a purge cycle, but this embodiment contains two bubbler valves, 38, 42, each accessing a separate bubbler 39, 43, which contains a different liquid reagent etching component, 40, 44. The carrier gas entering the bubblers 39, 43 can thus pick up traces of two different etchant vapours, and transport them to the reactor in the gas phase, controlled by valves 41, 45. Once operational temperature of the substrate holders is reached, the flow of carrier gas is directed through the reagents 40, 44 by closing valve 21 and opening valves 38 and 41 for the first reagent, and valves 42 and 45 for the second reagent, creating two different etchant gases. The two etchant gases mix before entering the reactor chamber 27 and the flow of the mixed etchant gas dry etches the reactor deposits on test substrate 32. Volatile non-gaseous reaction products are condensed onto the cooled section 37 of chamber 27.

Example 6SOBr2+Br2 200 C.

(26) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 200 C. under a continuous carrier gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. Once the appropriate temperature was reached, the flow of the gas was diverted through bubbler valve 38 into bubbler 39 containing reagent 40 (SOBr.sub.2), and through bubbler valve 42 into bubbler 43 containing reagent 44 (Br.sub.2) such that SOBr.sub.2 and Br.sub.2 were picked up in the carrier gas flows. The gases containing SOBr.sub.2 and Br.sub.2 exited through valves 41 and 45 respectively and combined to form an etchant gas before entering the reactor chamber 27. The etchant gas passed over substrate 32 and etched away the deposit on the stainless steel substrate 32. Although the etching process removed most of the coating from the substrate, a persistent film of greyish orange streaks remained on the substrate after 15 minutes of etching. The by-products, which were mainly GaBr.sub.3 and SO.sub.2, were collected in the cooled area 37 of chamber 27.

Example 7SOBr2+MeBr 200 C.

(27) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 200 C. under a continuous carrier gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. Once the appropriate temperature was reached, the flow of the gas was diverted through bubbler valve 38 into bubbler 39 containing reagent 40 (SOBr.sub.2), and through bubbler valve 42 into bubbler 43 containing reagent 44 (MeBr) such that SOBr.sub.2 and MeBr (about 4-6% as compared to SOBr.sub.2) were picked up in the carrier gas flows. The gases containing SOBr.sub.2 and MeBr exited through valves 41 and 45 respectively and combined to form an etchant gas before entering the reactor chamber 27. The etchant gas passed over substrate 32 and etched away the deposit on the stainless steel substrate 32 completely in approximately 6 minutes. The by-products, which were mainly GaBr.sub.3, MeGaBr.sub.2, Me.sub.2GaBr and SO.sub.2, were collected in the cooled area 37 of chamber 27.

(28) Referring now to FIG. 5, an experimental apparatus for carrying out a third embodiment of the invention comprises a similar arrangement of reaction chamber 27 bounded by an input end flange 26 and an output end flange 33. Only one substrate holder 28 is provided, heated by suitable heater 31, and supporting a test substrate 32. Recorder 36 and thermocouple wires 29 monitor the substrate temperature as needed. A cooled section 37 is maintained near the output end flange 33. An output valve 34 connected to a vacuum pump (not shown) regulates the pressure within the reactor chamber 27.

(29) In this apparatus, the input gas is treated in the same manner as in the first embodiment before it enters the reaction chamber 27. Gas input line 20 feeds a carrier gas into the system. The carrier gas feed may pass through direct input valve 21 directly into the reaction chamber or through bubbler valve 22. The carrier gas entering the bubbler 23 passes through liquid reagent etching component 24, where it picks up etchant vapour and transports it to the reactor in the gas phase, controlled by valve 25. Once operational temperature of the substrate holders is reached, the flow of carrier gas is directed through the reagent 24 by closing valve 21 and opening valves 22 and 25, producing an etchant gas. The etchant gas is then passed through an activation chamber, in this case a UV activation chamber 46, in which the gas flow is irradiated to activate the etchant gas. The activated gas flows across the test substrate 32 and dry etches the reactor deposits on the substrate 32. Volatile non-gaseous reaction products are condensed onto the cooled section 37 of chamber 27.

Example 8SOCl2 150 C.UV Pre-Excitation

(30) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 150 C. under a continuous carrier gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. Once the substrate reached 150 C., the flow of the gas was diverted through bubbler valve 22 into bubbler 23 containing reagent 24 (SOCl.sub.2), such that SOCl.sub.2 was picked up in the gas. The SOCl.sub.2 exited the bubbler through valve 25, passed through activation chamber 46, then entered the reaction chamber 27, where it passed over the substrate 32 and etched away the deposit on the stainless steel substrate 32. The reaction was very quick and efficient, cleaning the substrate completely within 3-5 minutes. The by-products, which were mainly GaCl.sub.3 and SO.sub.2, were collected in the cooled area 37 of the chamber 27.

(31) FIG. 6 shows an experimental apparatus for carrying out a fourth embodiment of the invention. The reactor chamber 27 and its contents are all the same as the first embodiment, but in this case the etchant gas is supplied directly from one or more cylinders 47, 49 of compressed gas, regulated by valves 48, 50. Gas input line 20 feeds a carrier gas, such as argon, into the system. The carrier gas feed may pass through direct input valve 21 directly into the reaction chamber, as may be required during a purge cycle. Once operational temperature of the substrate holders is reached, the flow of carrier gas is stopped by closing valve 21 and the flow of etchant gas components from compressed gas cylinders 47 and 49 is initiated by opening valves 48 and 50. The gases combine before entering the reaction chamber 27, and the resultant combined etchant flows across test substrate 32 to dry etch the reactor deposits on test substrate 32. Volatile non-gaseous reaction products are condensed onto the cooled section 37 of the chamber 27.

Example 9No Carrier Gas, CO+Br2 200 C.; Preheat to 300 C.

(32) A coated substrate 32 was placed on the first substrate holder 28. The substrate 32 was heated with IR heater 31 to 200 C. under a continuous gas (Ar) flow through the reactor chamber 27 from the gas inlet 20 and direct input valve 21. The second substrate holder 30 was heated to 300 C. with the IR heater 31. Once the appropriate temperatures were reached, the flow of the carrier gas was stopped by closing valve 21, while valves 48 and 50 were opened sequentially, allowing the etchant gases (Br.sub.2 and CO; about 5% Br.sub.2 compared to CO) to combine and enter the reactor chamber 27. The etchant gas combination was thermally activated as it passed through the heated area over the substrate holder 30. When the activated gas passed over substrate 32, it etched away a substantial part of the coating on the substrate 32, though a very faint film remained on the surface. The by-products, which were mainly GaBr.sub.3 and CO.sub.2, were collected in the cooled area 37 of the chamber 27.

(33) Most of these experiments were conducted using the SOX.sub.2 system, as the thionyl halide, being a liquid at room temperature, is simpler to handle. It was also shown to be effective. Because the chemistry (i.e. the reactivity) of other similar systems such as carbonyl (COX.sub.2) and nitrosyl (NOX) is very similar, it is anticipated that those chemicals will also work for the applications outlined herein. In all cases, the process for the etching of the metal oxides is driven by generation of SO.sub.2, CO.sub.2 and NO.sub.2 gases from the etchant gas, as those gases are more stable than the starting material and also very volatile. The formation of metal halides GaCl.sub.3, GaBr.sub.3, which are again stable yet volatile at the process temperature, also drives the reaction.

(34) Further, each of the materials can normally be prepared by reaction of the individual gases. For example, NO and Cl.sub.2 will react in the gas phase to generate NOCl. It is therefore anticipated that when individual gases are passed into the chamber with heat activation, active species will be generated, which will then react with the surface oxides, etching away the coating on the substrate.

(35) It will be appreciated by those skilled in the art that other variations to the preferred embodiment described herein may be practiced without departing from the scope of the invention, such scope being properly defined by reference to the specification and to the following claims.