WO1988005836A1

WO1988005836A1 - Method of improving the sag resistance of a component constructed from fused quartz

Info

Publication number: WO1988005836A1
Application number: PCT/GB1988/000072
Authority: WO
Inventors: Rene Frederick Ivan Joseph Docherty; John Alexander Winterburn; Joseph Ignatius Henry Allen
Original assignee: Tsl Group Plc
Priority date: 1987-02-09
Filing date: 1988-02-08
Publication date: 1988-08-11
Also published as: GB8702834D0

Abstract

A high temperature jig is made from fused quartz tubing (1, 2: 6) reinforced internally by a sag-resistant tubular skeleton (5). To prevent reaction between the tube (6) and skeleton (5) at high temperature (i.e. above 900°C) a refractory barrier layer (7) is interposed between the tube and skeleton. A woven sock of high silica glass or aluminosilicate fibres makes a suitable barrier layer.

Description

Method of improving the sag resistance of a component constructed from fused σuartz

Technical Field

This invention relates to a method of improving the sag resistance of a component constructed from fused quartz.

Fused quartz, which is the massive vitreous form of silicon dioxide, is widely used in the semi-conductor industry for the construction of components used in the processing of silicon and other semi-conductor materials particularly for use at temperature above 900°C (e.g. in the range 1100 to 1600°C). Because of its vitreous nature, fused quartz exhibits plastic flow at high temperatures and this is a known disadvantage in its use. This disadvantage is however, accepted because of the other desirable properties of the material such as chemical purity and stability, excellent thermal shock resistance and the relative ease of fabrication of components from it.

One group of fused quartz components which is particularly susceptible to plastic deformation in use is that including the jigs and boats used as carriers for silicon wafers during high temperature oxidation, deposi¬ tion and diffusion operations. Such components normally have a closely specified slotting arrangement for the location of the silicon wafers and the long term stability of the spacing of the slots is particularly important when the carriers are used in the dump transfer method of wafer processing.

Other materials of greater refractor ness than quartz can be used for the construction of such carriers but all suffer from some disadvantage. For example silicon is expensive and difficult to fabricate; alumina is also difficult to fabricate, has a relatively poor resistance to thermal shock and can be a source of sodium impurity. Discussion of Prior Art

It is known from GB-A-936,129 that refractory silica bodies can be reinforced against high temperature deforma¬ tion by means of insertions of fused silica rods, fibres or tubes and from GB-A-1 ,549,819 that fused quartz bodies can be reinforced with a fired particulate mass of reinforcing material, having greater strength at the temperature of use than the fused quartz.

Using a two component structure with the reinforcing material embedded in the body of fused quartz provides a significant improvement in the high temperature sag resistance of a fused quartz component. Such a composite has the chemical inertness, stability and thermal shock resistance of fused quartz, and the high temperature sag resistance provided by the reinforcing material (e.g. an alumina, rod or tube).

Additionally, providing the reinforcing material is fully enclosed within fused quartz, then the enveloping fused quartz acts as a barrier against the diffusion of traces of impurity (e.g. sodium) evolved at high tempera¬ tures from the reinforcing material.

Unfortunately, such two component structures suffer from random breakage in use which it has been found relates to the manner in which the inner surface of the fused quartz envelope . devitrifies, that is reverts from the vitreous to the crystalline state. Thus if devitrification is slow to occur, the fused quartz can stick to an alumina reinforcement at high temperatures and then when the fused quartz component is cooled, breakage occurs due to the differential contraction between the fused quartz and the alumina re nforcement; at the other extreme, devitrifica¬ tion of the inner surface of the fused quartz can be excessive due to a higher than normal level of sodium impurity in the alumina and this may also cause breakage after use at high temperatures due to the differential contraction between the fused quartz and the product of devitrification, Cristobal ite. Achieving the desirable degree of devitrification of the inner layer of fused quartz so that, on the one hand, it is just sufficient to prevent its sticking to the alumina without, on the other hand, being sufficient to seriously weaken the envelope, is difficult and can result in variable results in a produc¬ tion situation.

Summary of the Invention It has now been found that by interposing a suitable barrier between the reinforcing material and the fused quartz, sticking and/or excessive devitrification can be avoided.

Thus according to one aspect of the invention there is provided a method of fabricating an article of vitreous silica having superior resistance to deformation and breakage when heated to a temperature above 900°C by

, incorporating a skeleton of material having greater refractoriness than vitreous silica, and separated from the vitreous silica by an intermediate barrier layer chosen so as to at least suppress reaction and/or sticking between the reinforcing skeleton and the vitreous silica.

According to a further aspect of the invention a fused quartz component for use at high temperatures comprises an envelope of fused quartz and a skeleton of a refractory reinforcing material surrounded by the envelope and is characterised in that intermediate the envelope and the skeleton of reinforcing material there is provided a barrier layer which prevents the skeleton adhering to the envelope under the conditions of use of the component.

One example of such a barrier layer is a sock, which covers the skeleton, made from a woven high silica glass such as that marketed under the trade name Rafrasil. In use, the sock readily devitrifies, preventing sticking between the fused quartz envelope and the skeleton. It is possible that the product of devitrification also acts in such a manner as to absorb traces of sodium emitted from (for example)an alumina reinforcing skeleton.

The skeleton can be fabricated from, for example, aluminium oxide, silicon carbide, silicon nitride, silicon, aluminous porcelain, zirconium silicate and stabilised zirconium oxide. Other high temperature refractory materials can be used,, provided that are without sig- nif cant action on silica at the relevant use temperature.

The barrier layer can be in the form of fibres (e.g. as a thin mat or woven cloth), or as a thin layer of refractory powder. The material forming the barrier layer - should have no or only limited reaction with both silica and the material of the skeleton at the relevant use temperature. Examples of alternate barrier materials include zircon a fibres or alumina fibres, both in the form of a mat. The aluminosilicate fibre marketed under the trade name Fiberfrax is also suitable.

Desirably the materials forming the skeleton and the barrier layer show negligible outgassing at the temperature of use. If outgassing could occur these materials should be thoroughly outgassed prior to enclosure within the vitreous silica envelope since a pressure-rise in the enclosed volume could be itself a cause of distortion of the component in question.

The barrier layer may also be formed by coating either the reinforcing member and/or the inner surface of the fuzed quartz with a suitable barrier material. The method of coating can comprise for example, sputtering, evapora¬ tion/condensation, deposition from the vapour phase, painting, dipping or spraying with a sol/gel or suspension and the material for the barrier layer can consist of the previously mentioned refractory materials for example silicon, alumina, zircon or zirconia.

Brief Description of the Drawings

The invention will be further described, by way of example, with reference to the accompanying drawing, in which:

Figure 1 shows a reinforced fused quartz semi¬ conductor jig, and

Figure 2 an enlarged sectional detail of part of the jig of Figure 1 taken on the line II-II.

Description of Preferred Embodiment

This jig consists essentially of two longitudinal members 1 and 2 made from fused quartz tubing. To these, solid fused quartz cross-members 3 are welded at regular intervals. The cross-members are provided with regularly spaced slots 4 to locate and support separately silicon wafers (not shown) during known treatments to form semi¬ conductor components. The cross-section of a longitudinal member shown in Figure 2 illustrates the manner in which that member is reinforced. A central alumina reinforcing tube 5 defines a reinforcing skeleton and is separated from the outer fused quartz tube 6 by a woven sock of "Refrasil" fibre material forming an intermediate barrier layer 7.

The jig described above has been cycled from room temperature to a temperature of around 1280°C and has been shown to have a useful life which is up to ten times greater than a comparable jig constructed from pure fused quartz without any reinforcement.

Although the specific example has been directed to a jig for silicon wafers the invention also finds application in a wide range of components which, in use, are regularly cycled to very high temperatures. In place of a woven sock forming the barrier layer 7, an intermediate layer can be deposited from the vapour phase onto at least one of the inner surface of the vitreous silica article and the outer surface of the skeleton. Alternatively the deposited barrier layer is formed by first depositing a precursor coating of the final layer and then subjecting the coating to chemical treatment to form the layer .in the chemical form required.

Claims

CLAIMS 1. A method of fabricating an article of vitreous silica having superior resistance to deformation and breakage when heated to a temperature above 900°C by incorporating a skeleton of material having greater refractoriness than vitreous silica, and separated from the vitreous silica by an intermediate barrier layer chosen so as to at least suppress reaction and/or sticking between the reinforcing skeleton and the vitreous silica.

2. A method according to claim 1, characterised in that the reinforcing skeleton is fabricated from one of the group of materials consisting of aluminium oxide, silicon carbide, silicon nitride, silicon, aluminous porcelain, zirconium silicate and stabilised zirconium oxide.

3. A method according to claim 1, characterised in that the intermediate barrier layer is a fibrous material selected from the group consisting . of zirconium oxide, aluminium oxide, high silica glass, aluminosilicate and vitreous silica.

4. A method according to claim 3, characterised in that the fibrous material is woven to form the intermediate barrier layer.

5. A method according to claim 1, characterised in that the intermediate barrier layer is a powder selected from the group consisting of aluminium oxide, silicon carbide, silicon nitride, vitreous silica, quartz, Cristobal ite, zircon and zirconium oxide.

6. A method according to claim 5, characterised in that the intermediate barrier layer is deposited from the vapour phase onto at least one of the inner surface of the vitreous silica article and the outer surface of the skeleton.

7. A method as claimed in claim 6, characterised in that the deposited barrier layer is formed by first depositing a precursor coating of the final layer and then subjecting the coating to chemical treatment to form the layer in the chemical form required.

8. A method according to claim 1 , characterised in that the intermediate barrier layer is produced by coating at least one of the inner surface of the vitreous silica and the outer surface of the reinforcing skeleton with a slip incorporating at least one of aluminium oxide, silica, zirconium oxide, silicon carbide and silicon nitride powder.

9. A method according to claim 1, characterised in that the intermediate barrier layer is produced by coating at least one of the inner surface of the vitreous silica and the outer surface of the reinforcing skeleton with a sol incorporating at- least one of oxide and hydroxide of one of aluminium, silicon and zirconium.

10. A fused quartz component for use at high tempera¬ tures comprising an envelope of fused quartz and a skeleton of a refractory reinforcing material surrounded by the envelope characterised in that intermediate the envelope and the skeleton of reinforcing material there is provided a barrier layer which prevents the skeleton adhering to the envelope under tfre conditions of use of the component.

11. A component according to claim 10, designed to be used as a support for silicon wafers during semi-conductor fabrication processing.