DEPOSITION APPARATUS

Abstract: In order to deposit a film, such as a polysilicon film, on a substrate (7) by a low-pressure chemical vapour deposition (LPCVD) process, reduction of the pressure in the deposition chamber (1) to around 10-30 Pascals is necessary. This requires the use of a high-speed pump (5), and the mere substitution of a high-speed pump in place of a normal pump connected to the chamber results in poor uniformity of the deposited film. Good uniformity is achieved, in the present invention, by ensuring that the dimensions of the chamber (1) are such that the combined conductance of the chamber, the substrates (7) and a support (6) holding the substrates is at least ten times the combined conductance of the pump and a conduit (4) connecting the pump to the chamber.

Description: Deposition Apparatus This invention relates to deposition apparatus, and particularly to apparatus for the deposition of films by a low- pressure chemical vapour deposition (LPCVD) process. It is known, for example, to deposit polysilicon films on substrates by supporting the substrates within a deposition chamber, heating the chamber to a temperature of, say 630 * C, and maintaining the pressure in the chamber at a very low level, say 10 to 30 Pascals, while passing silane gas through the chamber. The need to achieve such a very low pressure level presents a problem. A high-capacity pump, such as a turbo-molecular pump, is required, but merely to substitute a high-speed pump in place of a normal pump connected to a LPCVD deposition chamber is found to produce poor uniformity of the deposited films across the substrate. It is an object of the present invention to provide an LPCVD apparatus in which the quality of deposition at very low pressures is improved. According to the invention there is provided apparatus for effecting deposition of a film on at least one substrate by a low pressure chemical vapour deposition process, comprising a deposition chamber; high-speed pump means connected to one end of the chamber; a gas inlet adjacent the opposite end of the chamber; and means to support the or each substrate within the chamber; wherein the dimensions of the chamber are selected such that the combined conductance of the chamber, the substrate support means and any substrate supported thereon is at least ten times .the conductance of the pump means. Apparatus according to the invention is found to give better deposition than conventional LPCVD apparatus for the following reasons. In a typical conventional LPCVD apparatus, the conductance of the chamber is much larger than the combined conductance of the pump- and the system for connecting the pump to the chamber. As a result, the pressure is substantially constant throughout the chamber, i.e. the chamber is isobaπ ' c. Now, if a pump of much higher pumping speed is connected to the chamber in place of the normal pump, via a high conductance connecting system to maximise the available pumping speed, the conductance of the high-speed pump and connecting system and the conductance of the conventional chamber will be of comparable values. When gas is admitted at one end of the chamber and is pumped out at the other end, it is found that a pressure gradient is set. up in the chamber due to a variation in the effective pumping speed along the length of the chamber, so that the growth rate on the wafers or substrates varies. By ensuring that the conductance of the chanter and its contents is at least ten times the combined conductance of the pump and the connecting system, this pressure gradient is avoided, and uniform deposition can therefore be achieved. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic cross -sect onal view of a deposition apparatus. Referring to the drawing, a deposition apparatus comprises a chamber 1 having an open base 2 which fits over and seals to-an inlet -flange 3 of a large-bore inlet duct 4 leading to a very high speed pump 5. The chamber may comprise a bell-jar, such as is commonly used for evaporation and deposition processes. A skeletal jig 6 located within the chamber is provided to hold a number of substrates 7 on which films are to be deposited. A gas injector pipe 8 passes into the chamber 1 through the flange 3 and terminates at an outlet 9 above the substrate mounting position. The jig 6 holds the. substrates in such a manner that their major surfaces lie substantially parallel to the direction of flow of gas from the outlet 9 to the duct 4, as indicated by an arrow 10, in order to minimise resistance to the gas flow. Furthermore, the jig 6 is so constructed as to present minimum resistance to the gas flow, while still adequately supporting the substrates. The jig is mounted on at least one support 11. In order to achieve a substantially constant pressure throughout the chamber 1, the dimensions of the chamber are selected such that the combined conductance of the chamber 1, the jig 6 and the substrates 7, from the gas outlet 9 to the flange 3, is at least ten times the combined conductance of the pump 5 and the duct 4. Any suitable means (not shown) may be provided for heating the substrates. Such means may heat the substrates 7 directly, or may heat the chamber 1 and/or the incoming gas. In use of the apparatus, substrates 7 are mounted in the jig 6, the bell-jar 1 is placed in position, and the pump 5 is operated to reduce the pressure in the bell-jar to a level in the range 10 ~6 Torτ to 0.55Torr, but preferably in the range 0.5-50 x 10 "3 Torτ, while the required gas or gases for film deposition are fed into the bell-jar. Such gas may be silane if polysilicon films are to be deposited on the substrates. In that case, the substrates are heated to 560-630 * C. Although the substrates are described above as mounted substantially parallel to the direction of gas flow, the invention is also applicable to the case where the substrates are mounted at other angles to the flow, including perpendicular to the flow. The criterion must still be met that the combined conductance of the bell-jar, the substrates and the jig is at least ten times that of the pump and the duct, and this will require the use of a larger bell -jar than in the case where the substrates are parallel to the flow. It will be apparent that whereas a bell -jar is a convenient and readily-available device for use as the chamber 1, any other suitable enclosure may be used, provided that its conductance conforms with the above-mentioned criterion. The gas injector pipe 8 may then enter the chamber at the opposite end to the pump. * Although a single pump 5 is mentioned above, pump means comprising more than one pump may be provided.

Claims: CLAIMS 1. Apparatus for effecting deposition of a film on at least one substrate (7) by a low pressure chemical vapour deposition process, comprising a deposition chamber (1); high-speed pump means (4, 5) connected to one end (2) of the chamber; a gas inlet (9) adjacent the opposite en of the chamber; and means (6) to support the or each substrate within the chamber; wherein the dimensions of the chamber are selected such that the combined conductance of the chamber, the substrate support means and any substrate supported thereon is at least ten times the conductance of the pump means. 2. Apparatus as claimed in Claim 1, wherein the substrate support means (6) is arranged to support the or each substrate (7) so that its major surfaces lie substantially parallel to the direction (10) of gas flow from the inlet (9) to the pump means (4, 5). 3. Apparatus as claimed in Claim 1 or Claim 2, wherein the chamber comprises an enclosure (1) having one open end (2) to which the pump means (4, 5) is connected; and wherein the gas inlet (9) comprises a conduit (8) which extends into the enclosure through the open end and terminates upstream of the substrate support means (6). 4. Apparatus as claimed in Claim 3, wherein the enclosure (1) comprises a bell-jar. 5. Apparatus as claimed in any preceding claim, wherein the pump means includes a duct (4) having a flange (3) against which said one end (2) of the chamber seals.

Inventors: MEAKIN DOUGLAS BOYD, MEAKIN, DOUGLAS, BOYD
Applicants: GEN ELECTRIC CO PLC, THE GENERAL ELECTRIC COMPANY, PLC
Application year: 1988
Main classification: C23C16
IPC codes: C23C16