JPH0816040B2 - Molecular beam growth method for silicon - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a molecular beam growth method for silicon using a gas source as a raw material, and more particularly to selective growth.

(Prior Art) In recent years, research and development relating to the production of silicon-based bipolar transistors characterized by having a thin base layer for the purpose of application to high-speed bipolar devices, microwave devices, etc. have been actively conducted. When a thin base layer having an impurity concentration that is designed in advance is formed, a silicon molecular beam growth technique that is good in film thickness controllability and that can form an epitaxial film having a steep impurity profile is effective. However, in the conventional molecular beam growth method of heating a solid silicon source with an electron gun, a silicon film is epitaxially grown on a silicon substrate, but polysilicon is also grown on a SiO ₂ film. Therefore, when actually manufacturing a bipolar transistor device, a process for removing the excess polysilicon attached on SiO ₂ is required.

In addition, recently, the inventors have applied Applied Physics Letter 51.
Volume (1987) page 2213, a gas source silicon molecular beam growth method using silane gas was developed. When this method was used, a poly silicon film on a silicon substrate partially having an oxide film pattern was formed on the oxide film. It is reported that silicon can be selectively epitaxially grown only on the exposed silicon portion without depositing silicon.

(Problems to be solved by the invention) In the conventional silicon molecular beam technology using a solid source, when a silicon film is grown on a substrate having a partial SiO ₂ film pattern on a silicon wafer, the silicon is exposed on the exposed silicon part. Not only the film grows epitaxially, but also the polysilicon film grows on the SiO ₂ film. It is necessary to remove this polysilicon when manufacturing a device such as a transistor. This complicates the process. In addition, since patterning is required to remove excess polysilicon, it is necessary to allow for alignment accuracy during mask alignment at this time, and it is necessary to reduce the area of the junction between the base and emitter. This is disadvantageous in the manufacturing process of high speed bipolar transistors and the like. These points are very serious problems in using the ordinary molecular beam growth method using a solid source in a semiconductor manufacturing process.

Further, in the gas source silicon molecular beam growth method using silane, since the silane gas is chemically stable, a high substrate temperature is required for epitaxial growth on a silicon substrate, and the growth rate is extremely slow. Therefore, when considering application to the fabrication of semiconductor devices such as bipolar transistors, the high substrate temperature and long growth time obscure the dopant profile created in the crystal due to thermal diffusion, resulting in high-speed operation characteristics of the device. Is deteriorated.

(Means for Solving Problems) Problems in the conventional molecular beam growth method described above, that is, when a solid source is used, a silicon film is epitaxially grown only on the silicon surface and nothing is formed on SiO _2. It solves the problems of non-deposition selective growth not possible and the problem of doping profile blurring due to high substrate temperature during growth and slow growth rate in the gas source silicon molecular beam growth method using silane gas. Therefore, gas source molecular beam growth of silicon using disilane gas is used as a molecular beam source of silicon.

(Function) Disilane gas molecules cause a dissociative adsorption reaction on the substrate heated by the substrate heating mechanism to supply silicon atoms necessary for epitaxial growth to the substrate. Since disilane molecules are chemically more active than silane molecules, the substrate temperature required for growth on a silicon substrate can be low. In addition, the growth rate is about one digit higher than that of silane. When disilane gas molecules are used as a molecular beam source, in order to contribute to the growth of a silicon film, dissociative adsorption occurs on the surface and silicon atoms must be generated on the surface. On the other hand, it was found from the experiments by the inventors that the dissociative adsorption of disilane molecules is promoted by the chemically active dangling bonds on the substrate surface. This dangling bond exists on the silicon crystal surface but does not exist on SiO ₂ . Therefore, dissociative adsorption of disilane gas molecules
It is predominantly promoted on the silicon surface as compared to the SiO ₂ surface.
For this reason, it is considered that selective growth is possible with the molecular beam growth method using disilane gas molecules.

Experiments conducted by the present inventors have revealed that selective growth largely depends on growth conditions. It has been found that it is necessary to appropriately select the substrate temperature during growth and the flow rate of disilane gas supplied to the substrate during growth in order to perform selective growth. It is as if subsequently occur nucleation occurs frequently in this dissociation and also of disilane on hardly SiO ₂ that the dissociative adsorption when and gas flow rates the substrate temperature is too high too much, from the top Accordingly SiO ₂ This is considered to be because it cannot be ignored in comparison with the rate of thermal desorption of, and a polysilicon film is also formed on SiO ₂ . That is, at high temperature, the rates of desorption and dissociation of disilane both increase, but since disilane is active, the dissociation increases more, and as a result, it is considered that selectivity cannot be obtained at high temperature. In fact, in the disilane gas source molecular beam growth method, Si partially covered with a 1 μm thick SiO ₂ film
FIG. 1 shows the results of investigating the conditions under which the growth is possible on the (100) substrate and the selective growth is possible by changing the substrate temperature and the flow rate of disilane gas. In FIG. 1, a circle indicates that selective growth is possible, and a cross indicates that selective growth does not occur. As is clear from the figure, perfect selective growth is achieved when a disilane gas of about 0.5 sccm or less is supplied at a substrate temperature of about 700 ° C or less. As is clear from these results, in the disilane gas source molecular beam growth method, the substrate temperature and the disilane gas flow rate were lowered to obtain SiO ₂
Selective growth is possible under the condition that growth of the upper polysilicon film is suppressed.

(Example) Hereinafter, a detailed description will be given with reference to the drawings. FIG. 2 is a schematic view of an apparatus for explaining an embodiment of the present invention. This is an example of a case where a SiO ₂ film pattern is partially formed on a silicon substrate and gas source molecular beam growth of a silicon thin film is selectively performed only on the exposed silicon portion. An outline is shown when disilane is used as the source gas. In this case, the molecular beam source of silicon is a gas introduction valve from a disilane gas cylinder,
It is a disilane gas molecular beam guided into the growth chamber through a gas cell.

As described in detail in FIG. 1 of the action section, in the disilane gas source molecular beam growth method, complete selective growth is realized at a substrate temperature of about 700 ° C. or less and a disilane gas flow rate of about 0.5 sccm or less. In this embodiment, as an example of the conditions for the selective growth, a case where the growth is performed under the following conditions will be described. 0.5 sccm of disilane gas in the growth chamber
Introduce. At this time, the vacuum degree in the growth chamber is 1 × 10 ^-5 Torr, and the mean free path of the gas molecules is sufficiently longer than the size in the growth chamber, so the gas molecules move linearly in the growth chamber and the dissociation reaction of the molecules causes the substrate surface Therefore, the above-mentioned selective molecular beam growth occurs. The substrate temperature during growth was 650 ° C. As a substrate treatment before growth, Branson cleaning was performed after chemical etching to form a thin oxide film on the surface, which was then introduced into the growth chamber and heated in a high vacuum at 900 ° C or higher for 10 minutes to clean the surface. did. I used silicon (10
0) Substrate. The growth was performed for 2 hours under the above conditions. FIG. 2 shows the structure of the silicon wafer used as the substrate in the experiment for confirming the selective growth in the gas source molecular beam growth method in this example. As shown in FIG. 2, there is an exposed silicon portion in the web and a 1 μm thick SiO ₂ coating portion partially formed on the exposed silicon portion. FIG. 3 shows the result after the growth in this example is performed on this. FIG. 3 is a schematic diagram of the result of observing the cross section of the substrate by cutting it through a scanning electron microscope after performing gas source molecular beam growth on the substrate shown in FIG. As is clear from FIG. 3, in the gas source molecular beam growth method using disilane, the exposed silicon portion does not grow at all in the SiO ₂ coating portion where the silicon film is epitaxially grown. That is, it can be seen that complete selective growth is performed. It was confirmed that the degree of vacuum in the growth chamber at the time of growth was in the range of 3 × 10 ⁻⁶ to 1 × 10 ⁻⁵ torr, and the growth was possible.

In this example, the selective growth on the silicon substrate having the SiO ₂ film pattern was shown, but the present invention is not limited to Si ₃ N.
Selective growth is possible even if the Si substrate is covered with another coating film such as ₆ films.

(Effect of the Invention) As described in detail above, if the gas source silicon molecular beam epitaxial growth method using disilane gas is used,
It is possible to selectively epitaxially grow a silicon film only on the exposed silicon portion without depositing silicon on the SiO ₂ coating portion.

Furthermore, since selectivity is obtained when the substrate temperature is low,
It can be applied to the fabrication of devices that require low-temperature processes.

[Brief description of drawings]

FIG. 1 is a diagram showing conditions under which selective growth is possible in a silicon molecular beam growth method using a disilane gas as a gas source for explaining the function of the present invention, and FIG. 2 is for explaining an embodiment of the present invention. 3 is a schematic view of an apparatus for a silicon molecular beam growth method using disilane gas as a gas source, FIG. 3 is a schematic view of the structure of a silicon substrate used to show the effect of the present invention, (a) is the surface of the substrate, (B) shows a cross section of the substrate, and FIG. 4 shows a cross section of the substrate actually grown by the gas source molecular beam growth method using disilane gas on the substrate shown in FIG. 3 to show the effect of the present invention. It is a schematic diagram of the result observed with the scanning electron microscope. In the figure, 1 is a growth chamber, 2, 7 and 12 are silicon (10
0) substrate, 3 gas cell, 4 substrate heating mechanism, 5 gas inlet tube valve, 6 disilane cylinder, 8 and 13 silicon exposed portion, 9 and 11 SiO ₂ coated portion, and 10 silicon epitaxial growth film .

Claims (1)

[Claims] 1. A silicon substrate on which a coating film has been partially formed in the silicon molecular beam growth method, and is exposed without being covered with the coating film by a gas source silicon molecular beam growth method using disilane as a source gas A method for growing a molecular beam of silicon, which comprises selectively epitaxially growing a silicon film on a silicon substrate portion being formed.