Deposition apparatus for growing a material with reduced hazard

Abstract: An apparatus for growing a material comprises a reaction vessel (1) of a metal extending generally in a vertical direction, the reaction vessel forming a reaction chamber (1a) therein, a cooling system (8) provided on said reaction vessel to maintain the reaction vessel at a temperature that the metal forming the reaction vessel does not cause contamination, an inlet (4) provided on the reaction vessel for introducing a reactant gas, an outlet (5) provided on the reaction vessel for exhausting a product gas that is formed in the reaction chamber, a support mechanism (3) at the bottom part of the reaction chamber to seal the reaction chamber, wherein the support mechanism has a rod member (3) extending vertically in said reaction chamber to hold thereon a susceptor (2) that is adapted to hold a wafer (10) on which the material is to be grown. Further there is provided a sleeve member (9) of a stable material particularly at a temperature that is used for growing the material, such that the sleeve member is provided in the reaction chamber along the inner surface.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to deposition apparatuses for growing material layers, and more particularly to an improvement of an MOCVD apparatus for growing a compound semiconductor material epitaxially on a substrate. Particularly, the present invention relates to an MOCVD apparatus having a barrel-type reaction chamber that uses arsine for the source of the group V elements. The barrel type MOCVD apparatuses generally use a reaction chamber of a quartz glass. The quartz glass is stable even when heated to the temperature above 1000 °C and produces little contamination. On the other hand, the quartz glass is brittle and tends to cause a difficulty in the handling particularly when the size of the reaction chamber has increased above 300 mm. Because of this problem of mechanical stability, recent MOCVD apparatuses having a reaction chamber of large size tend to use stainless steel for the reaction chamber. As the stainless steel produces unwanted contaminating gases at the high temperature that is employed in the CVD processes, such a MOCVD apparatus generally uses a cooling system to maintain the reaction chamber to a temperature in which the release of the contaminating gases does not occur. Such a cooling is necessary also from the view point of minimizing the contaminant gases from being released. FIG.1 shows a typical example of the MOCVD apparatus that uses a stainless steel reaction chamber 1. Referring to FIG.1, a carbon susceptor 2 is placed in the reaction chamber 1 for supporting semiconductor wafers 10, and the susceptor 2 is held on a quartz supporting rod 3 that is movable up and down between the position 3 and a position 3′. Under the reaction chamber 1, there is provided a chamber 11 for accommodating the wafers 10 held on the susceptor 2, and the wafers are lowered to the position 3′ as a result of lowering of the support rod 3. It should be noted that the support rod 3 has a flange member 3a that establishes a seal of the reaction chamber when moved to the position shown by the continuous line, by making a contact with an upper part of the chamber 11. The mechanism for raising and lowering the rod 3 is not illustrated for the sake of simplicity of the drawing. The reaction chamber 1 is provided with a source gas inlet 4 for introducing a source gas such as arsine into the chamber 1 and an exhaust gas outlet 5 for exhausting a product gas that is formed in the chamber 1 as a result of the reaction. In order to maintain the temperature of the inner wall of the reaction chamber 1 low, it will be seen that a cooling system 8 is provided on the inner wall for circulating a cooling medium that is introduced at an inlet 6 and exhausted at an outlet 7. Without such a cooling, the stainless steel reaction chamber 1 releases impurity elements upon the heating and causes a contamination in the crystal that was grown by the apparatus. The chamber 11 is used for mounting and dismounting the wafer 10 on and from the susceptor 2. During this process, a nitrogen gas N₂ is introduced into the chamber 11 via an inlet 11₁ and the chamber 11 is flushed with N₂ while evacuating via an outlet 11₃. For this purpose, a blower 13 is connected to the outlet 11₃. The gas thus evacuated from the chamber 11 is sent to a processing facility 14 for neutralizing the toxicity. In order to monitor the progress of the flushing in the chamber 11, a sensor 12 is connected at a port 11₂. The sensor 12 may be the model HD2P of Japan Oxygen Company, Ltd., and is used to detect the concentration of the arsine in the chamber 11. When arsine is no longer detected, the chamber 11 is opened and the wafer 10 is replaced. In this conventional MOCVD apparatus that has the metal reaction chamber 1, it was found that a considerable time is needed after the experiment, until the replacement of the wafer 10 becomes possible. More particularly, arsine of 20 - 50 ppb was detected when the support rod 3 lowered to the position 3' and the chamber 11 opened for the apparatus having the chamber diameter of 200 mm . A similar detection of arsine was made when an air is introduced into the reaction chamber 1. Such a detection of arsine indicates that the opening of the deposition apparatus for replacing the wafer has to be prohibited for a considerable time, and the operational efficiency of the MOCVD apparatus is inevitably reduced. This problem is particularly serious in the apparatuses that are used in the production line. Although the source of arsine is not completely explored, it is thought that a grayish-white or brownish material deposited on the cooled inner wall of the reaction chamber is responsible for this undesirable phenomenon. It was shown that the material is a compound of gallium and arsenic. However, the exact composition or crystal structure is not identified yet. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful deposition apparatus, wherein the foregoing problems are eliminated. Another object of the present invention is to provide a deposition apparatus for growing compound semiconductor materials, wherein the release of toxic arsine gas at the time of replacement of the wafer is eliminated. The present invention provides an apparatus as defined in claim 1. According to the present invention, the deposition of toxic materials that release arsine on the cooled inner wall of the metal reaction chamber is eliminated by the shield. More particularly, the deposition of the material occurs not on the cooled inner wall of the reaction chamber but on the shield of which temperature is substantially higher than the temperature of the inner wall of the reaction chamber. Thereby, it is believed that the material takes a stable crystal form of gallium arsenide or a modification thereof and the release of toxic arsine does not occur anymore. It is believed that the materials that are deposited directly on the cooled inner wall of the reaction chamber take a complex crystal form that is close to the amorphous state and easily releases arsine or other toxic compounds when contacted with the air. In the present invention, the problem of hazard in the operation of the apparatus is substantially reduced by preventing the direct deposition on the cooled inner wall of the reaction chamber. Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a diagram showing a conventional MOCVD apparatus that uses a metal reaction chamber; FIG.2 is a diagram showing a first embodiment of the MOCVD apparatus according to the present invention; FIG.3 is a diagram showing the growth of the compound semiconductor layer achieved by the apparatus of FIG.2; FIG.4 is a diagram showing a second embodiment of the MOCVD apparatus according to the present invention; and FIG.5 is a diagram showing the growth of the compound semiconductor layer achieved by the apparatus of FIG.4. DETAILED DESCRIPTION FIG.2 shows the deposition apparatus according to a first embodiment of the present invention. In FIG.2, the parts that correspond to those shown in FIG.1 are designated by the same reference numerals and the description thereof will be omitted. In FIG.2, it will be seen that there is provided a sleeve 9 along the inner wall of the metal reaction chamber 1. Typically, a stainless steel is used for the reaction chamber 1, and the chamber 1 has an inner diameter of about 200 mm and a height of about 300 mm. It will be noted that the susceptor 2 is made of carbon while the support rod 3 is made of a quartz glass tube. In the apparatus of FIG.2, it will be seen that a sleeve 9 of quartz glass is provided along the inner wall of the chamber 1 with a thickness of about 2mm to surround the susceptor 2 including the wafer 10 held thereon as well as the most part of the support rod 3. The sleeve 9 may be provided in contact with the inner wall of the reaction chamber 1, particularly when a heat insulating material is used for the sleeve 9. However, it is more preferable to provide the sleeve 9 in separation from the inner wall by a minute distance that is set to prohibit the reactant gases from entering into the space between the sleeve 9 and the inner wall. Typically, the sleeve 9 is separated from the inner wall of the reaction chamber 1 by a distance of 1 - 2 mm. In operation, the wafer 10 is set on the susceptor 2 and placed inside the reaction chamber 1 as shown in FIG. 2. After evacuating the reaction chamber 1 to a pressure of about 6666 Pa (50 Torr) via the exhaust outlet 5, the susceptor 2 is subjected to a heating process by activating an infrared lamp in the reaction chamber (not illustrated) while maintaining the chamber pressure. In a typical experiment, trimethyl gallium and arsine are introduced from the inlet 4 while maintaining the temperature of the wafer 10 at 650 °C. Thereby, a GaAs crystal is grown on the wafer 10. After the growth for one hour, the supply of the source gas is interrupted and replaced with an inert gas such as nitrogen. Further, the susceptor 2 is lowered together with the support rod 3 to the chamber 11 shown in FIG.1, and the chamber 11 is flushed by nitrogen. In the experiment conducted on the apparatus having the diameter of 200 mm for the reaction chamber 1, no arsine was detected at the time of replacement of the wafers 10. It should be noted that the model HD2P sensor 12 has a detection limit of about 2 ppb. On the other hand, in the experiment for the apparatus having the diameter of 600 mm for the reaction chamber 1, it took about 5 minutes until the concentration of arsine is reduced below the detection limit. It should be noted that a time of about 30 minutes or more has been needed conventionally by the apparatus that does not use the sleeve 9. A similar effect was observed when other refractory materials such as pyrolytic boron nitride and carbon are used for the sleeve 9. Generally, any chemically stable material at the elevated temperatures such as pyrolytic boron nitride (PBN), carbon or even molybdenum may be used for the sleeve 9. FIG.3 shows the result of the experiment undertaken in the apparatus of FIG.2. In the apparatus of FIG.2, it was found that the deposition rate changes depending on the axial position of the wafer 10, wherein "U" represents the wafer 10 at the upper part of the reaction chamber 1, "M" represents the wafer at the central part of the reaction chamber 1, and "B" represents the wafer at the bottom part of the reaction chamber 1. In FIG.3, the vertical axis represents the deposition rate and the horizontal axis represents the vertical position of the wafer 10 in the reaction chamber 1. The gas flows in the reaction chamber 1 from the top to the bottom in correspondence to the introduction via the inlet 4 at the top of the chamber 1 and the evacuation via the outlet 5 at the bottom of the chamber 1. It will be noted that the line designated by "C" in FIG.3 changes little with the vertical position of the wafer in the reaction chamber. This line corresponds to the case where there is no sleeve 9 provided in the apparatus. In this case, therefore, the deposition rate becomes uniform for all the vertical positions of the wafer 10 in the reaction chamber 1. When the sleeve 9 is provided with a quartz glass tube, the deposition rate changes slightly in the top part, in the medium part and in the bottom part of the reaction chamber 1 as shown by the line A. Further, when the sleeve 9 is made of a PBN, the deposition rate changes as shown by a line B in FIG.3. Summarizing the lines A - C, it seems that there is a tendency that the deposition rate increases in the bottom part of the reaction chamber 1. FIGS.4 shows an improvement of the deposition apparatus of FIG.2. In FIG.4, the parts that corresponds to those described previously are designated by the same reference numerals and the description will be omitted. In the present embodiment, the diameter of the reaction chamber 1 is increased a little at the bottom part thereof. For example, the bottom diameter of the chamber 1 may be set to about 230 mm that is 30 mm larger than the top diameter. According to the present construction, the flow of the gases in the chamber 1 is changed slightly and the deposition rate becomes uniform through the vertical positions when there is a sleeve as shown in FIG.4. Referring to FIG.5, it will be seen that the line A corresponding to the use of the sleeve 9 of quartz glass shows a generally uniform deposition rate as compared the line C where no sleeve is provided. Similar to the diagram of FIG.3, the line B represents the use of PBN for the sleeve 9. Even in this case, the uniformity in the deposition rate is significantly improved.

Claims: An apparatus for growing a material comprising: a reaction vessel (1) of a metal, said reaction vessel having outer and inner surfaces and extending generally in a vertical direction, said inner surface forming a reaction chamber (1a) having a bottom part and a closed top part; a cooling system (8) provided on said reaction vessel to maintain the inner surface of the vessel at a low temperature so that the metal forming the reaction vessel does not cause a contamination of the apparatus; an inlet (4) provided on said reaction vessel for introducing an arsine reactant gas; an outlet (5) provided on said reaction vessel for exhausting a product gas that is formed in the reaction chamber; and a support mechanism (3) provided at the bottom part of the reaction chamber to seal the reaction chamber, said support mechanism having a rod member (3) extending vertically in said reaction chamber to hold thereon a susceptor (2) that is adapted to hold a wafer (10) on which the material is to be grown; characterized in that the apparatus further comprises a sleeve member (9) of a non-volatile solid material selected from a group of quartz glass, pyrolytic boron nitride and carbon, said solid material being chemically inert against the reactant gas and further against the product gas particularly at a temperature that is used for growing the material, said sleeve member being provided in said reaction chamber along the inner surface for protecting the inner surface of the reaction chamber from being deposited by a material. An apparatus as claimed in claim 1 characterized in that said metal forming the reaction vessel (1) comprises a stainless steel. An apparatus as claimed in claim 1 characterized in that said sleeve member (9) is provided to surround the rod member (3) in the reaction chamber (1a). An apparatus as claimed in claim 1 characterized in that said apparatus further comprises a base member (11) to support the reaction vessel thereon, said base member having therein a space in continuation of the bottom part of the reaction chamber (10a), said support mechanism being provided movable between a first position in which said reaction chamber is sealed and a second, lowered position in which said rod member is accommodated in said space, said base member having an inlet (11₁) for introducing an inert gas into said space for flushing the space, and an outlet (11₃) for exhausting the inert gas. An apparatus as claimed in claim 1 characterized in that said reaction vessel (1) includes a generally cylindrical part extending vertically in correspondence to the reaction chamber (1a), wherein said cylindrical part has a greater diameter at a bottom part thereof as compared with a top part thereof.

Inventors: ITO HIROMI C O FUJITSU LIMITED, OHORI TATSUYA C O FUJITSU LIMI, SHIINA KAZUSHIGE C O FUJITSU L, TANAKA HITOSHI C O FUJITSU LIM, TOMESAKAI NOBUAKI C O FUJITSU, ITO, HIROMI, C/O FUJITSU LIMITED, OHORI, TATSUYA, C/O FUJITSU LIMITED, SHIINA, KAZUSHIGE, C/O FUJITSU LIMITED, TANAKA, HITOSHI, C/O FUJITSU LIMITED, TOMESAKAI, NOBUAKI, C/O FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: C23C16
IPC codes: C23C16, C30B25, H01L21