This invention relates to reaction vessels for vapour deposition apparatus and the like.

In, for example, metal organic chemical vapour deposition (MOCVD) apparatus, a quartz reaction vessel is usually employed with an inlet orifice communicating with the necessary gas handling equipment to direct into the reaction vessel a carrier gas stream containing the species to be deposited. Care is taken in the design of this gas handling equipment to ensure that a laminar flow is injected into the vessel and, in the usual case where different species are being deposited sequentially to build up a number of layers, equal care is taken to enable rapid switching within the gas carrier stream from one species to another. This, as will be understood, is necessary to permit the deposition of uniform layers with clearly defined boundaries between layers.

The reaction vessel includes a reaction chamber which is of sufficient size to hold the semiconductor wafer or other substrate upon which deposition is to take place. Normally, the size constraint will require an expansion of the reaction vessel from the gas stream inlet to the reaction chamber, at least in one plane. If this expansion took place in a step-wise fashion, it had previously been recognised that there would be a probability of a breakdown in laminar flow. Low pressure

regions caused by breakaway of the flow from the walls would tend to cause re-circulation of gas within the reaction vessel. Such re-circulation would encourage inter-diffusion of species at the point in the carrier gas stream at which switching occurs, tending in turn to the creation of poorly defined interfaces in the deposited structure.

To overcome this problem, it is usual to employ a throat portion between the orifice and the reaction chamber with the walls in this throat portion expanding from the inlet orifice to the reaction chamber at an angle which is less than the critical angle at which breakaway occurs. For a three dimensional expansion, the critical angle is typically seven degrees; with a two dimensional arrangement the critical angle is typically twelve degrees. The requirement for such shallow angles leads to relatively long reaction vessels, particularly where - as is increasingly the case - the substrate is relatively large. This increases the difficulties and cost of manufacturing the quartz vessel and of course increases the overall dimensions of the vapour deposition apparatus. A longer flow time from the inlet orifice to the reaction chamber is felt to be undesirable for the further reason that it increases the problem of inter-diffusion of species which occurs even in laminar flow.

It is one object of this invention to provide an improved reaction vessel for vapour deposition apparatus and the like in which turbulance can be avoided whilst maintaining a relatively short flow path.

050

Accordingly, the present invention consists in a reaction vessel for vapour deposition apparatus or the like, the vessel having walls extending from a gas stream inlet, in the direction of a gas stream axis, through a throat portion to a reaction chamber having in a defined plane a dimension transverse to said axis which is substantially greater than the transverse dimension in said plane of the gas stream inlet, characterised in that the mutual spacing of the walls of the throat portion in said plane varies along said axis in such a manner as to provide substantially constant rate of change of gas velocity along said axis.

Preferably, in the case where the height of the throat portion normal to said plane is constant, said mutual spacing of the walls (W) varies with the dimension (L) along said axis substantially according to = k(L+C) where k and C are constants.

Alternatively, in the case where the mutual spacing of the walls (W) expands equally in all directions normal to said axis, W = k'(L+C) /_ where k' and C are constants.

This invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a schematic view partly in section of vapour deposition apparatus incorporating a reaction vessel according to this invention;

- _ -

Figure 2 is a front sectional view of the reaction vessel shown in Figure 1; and

Figure 3 is a plot of linear gas velocity against distance for the gas stream in the apparatus shown in Figure 1.

Referring to Figure 1, there is illustrated vapour deposition apparatus utilising a fused silica reaction vessel according to this invention. Apart from the form of the reaction vessel, the apparatus is of generally conventional form and will not require detailed description. In the apparatus, an inlet switching manifold 12 receives a carrier gas stream at carrier inlet 1. and a plurality of source gases (typically up to sixteen) through inlets 16, only one of which appears in the drawing. The carrier gas stream carrying the source gases (for instance metal organics) enters the reaction vessel 18 at orifice 20. Over a throat portion 22, the walls of the reaction vessel expand in one plane from the relatively narrow inlet orifice 20 to a width which may be five or more typically,ten or more times greater. The expansion of the walls is not constant, as has hitherto been the case, and is governed by a relationship will be described later.

The throat portion 22 extends to a reaction chamber 24 having generally parallel walls. Within the reaction chamber 2k the lower wall of the reaction vessel is stepped at 26 to provide a well 28. This well accommodates a graphite block 30 providing a support surface 32 for the substrate. It will be noted that the substrate lies in the plane of the paper of Figure 1, that is to say the plane in which the dimension of the reaction vessel is - in the throat portion - expanded. The support

surface 3 of the graphite block 30 is generally coplanar with the lower wall in the throat portion 22 so that in the direction out of the paper of Figure 1, that is to say the thickness direction of the substrate, the effective height of the reaction vessel is generally constant between the inlet orifice 20 and the reaction chamber 2.. This is shown more clearly in Figure 2.

Downstream of the block 30, the reaction vessel is provided with a relatively short expansion chamber 34 closed by a water cooled end housing 36. This communicates with an exhaust 38. The end housing 36 provides access to the reaction chamber through a suitable door arrangement (not shown) . An induction heating coil 40 is provided helically around the reaction chamber. Heating of the substrate, the operation of manifold 12 and other such matters as the application of a potential to the substrate carrier, are controlled through a suitable controller.

The shape of the throat portion 22, and more particularly the dependence upon the axial dimension (L) of the throat portion thickness (W) is selected so that there is a substantially constant rate of change of gas

- velocity (V) . If there is assumed a constant gas flow (F m /sec) and a constant height (H m) of the throat portion, the gas velocity is given by:-

V = _F_

WxH (1)

According to this invention, it is required that:-

=. CaΛSf ^β ^r , _ >

^• a^Λ

- fe-

Now:

Therefore the requirement of (2) can be expressed from (3) and (4) as:- dW _£ , W ² (5) dL

Therefore

A solution of (6) is w = k (L+C) ^"1 (7) where k and C are constants.

By the use of the described formation of throat portion, the necessary expansion in one dimension can be achieved over axial lengths which are significantly shorter than would be possible using conventional designs of reaction vessel. It is found, surprisingly, that once the necessary degree of expansion has been obtained, the discontinuity in curvature between the throat portion and the reaction chamber itself has no deleterious effect upon the flow.

In one example, the expansion in dimension is from an inlet diameter of 13mm to a reaction chamber width of 74mm. An overall length of 260mm from the inlet to the step 26, is formed of parallel walled first section A of length 50mm, a parabolic section B of length 150mm and a cylindrical section C again of parallel walls and of length 60mm.

There is as shown in Figure 1 a sharp discontinuity between section B and C. The profile of section B is given in the following table where L is the length in mm from the begining of section B and W is the dimension for the axis to one wall.

L W L W

0 6.500 80.625 11.671

5.625 6.707 86.250 12.357

11.250 6.928 91.875 13.129

16.875 7.164 97.500 14.003

22.500 7.417 103.125 15.002

28.125 7.688 108.750 16.154

33.750 7.980 114.375 17.494

39-375 8.295 120.000 19.087

.5.000 8.636 125.625 20.993

52.500 9.136 131.250 23.321

58.125 9.550 136.875 26.230

67.750 10.005 142.500 29.969

69-375 10.505 146.250 33.115

75-000 11.057 150.000 37.000

Referring by way of example to Figure 3. curve I illustrates the constant rate of change of linear gas flow velocity that is achieved with a reaction vessel according to the present invention. By way of contrast, Curve II illustrates the linear gas flow velocity of a normal

conical expansion section where - unless an. extended length is employed the slope in a region adjacent the orifice will exceed the critical value which leads to breakaway. It will be recognised that use of the present invention leads to a reaction vessel which is of reduced length as compared to a conventional reaction vessel providing the same breadth reaction chamber. The reaction vessel according to this invention is thus more robust and - having regard to high material costs - less expensive to produce. As outlined above, the decrease in actual length has the functional advantage of reducing the period of time over which inter-diffusion of species takes place.

It is expected that in most cases expansion of the throat portion in the plane of the substrate will be sufficient. If this is not the case, or if deposition is required otherwise then on a planar substrate, the invention also includes an expansion in two planes. In the case where the reaction, vessel is symmetrical about the vessel axis, the gas velocity (V) is given by:-

V = F

TTR ² (8)

where R is the dimension of the throat portion radially of the axis.

According to the invention, it is required that

^~ Constant (9)

formation of a well to receive a graphite or the support block is felt to have advantages but is not essential. The construction of the vessel integrally from fused silica is preferred but alternative fabrications or materials may in certain cases be appropriate.