JP2663518B2 - Silicon substrate cleaning method - Google Patents

Description

The present invention relates to a method for growing a crystal or the like on a silicon substrate, and more particularly, to a method for cleaning a substrate before growth.

(Prior Art) In recent years, research and development on silicon-based bipolar transistors characterized by having a thin base layer for the purpose of application to high-speed bipolar devices, microwave devices, and the like have been actively conducted. In order to form a thin base layer having an impurity concentration as designed in advance, a silicon molecular beam growth technique capable of forming an epitaxial film having good film thickness controllability and a steep impurity profile is effective. In the molecular beam growth method of silicon, it is important to clean the substrate prior to the growth. If the initial cleaning of the substrate is not performed well, a good epitaxial film cannot be obtained. Furthermore, when the silicon molecular beam growth method is applied to device fabrication, the substrate must be initially cleaned and epitaxially grown at a low temperature so that the impurity profile created in advance does not collapse due to thermal diffusion. Recently, Hirayama et al. Reported at the 35th Joint Lecture Meeting on Applied Physics that the epitaxial growth temperature can be reduced to about 630 ° C by gas source molecular beam growth using disilane. -Q-17). However, the substrate cleaning temperature is higher than the epitaxial temperature, and it is important to lower this temperature. In the current silicon molecular beam epitaxial growth, the substrate is usually cleaned by the following method. The substrate is treated with a mixed solution of ammonia: hydrogen peroxide: water = 1: 6: 20, and 10
A protective oxide film of about A is formed, and in this state, it is loaded into a molecular growth apparatus. Amorphous silicon of about 10A is deposited on this in the growth chamber, then the substrate temperature is raised to 800 ° C, the oxide film on the surface is changed to volatile SiO, and removed to form a protective oxide film on the substrate surface. Removes existing impurities and performs cleaning. Tatsumi et al., Japanese Journal of Applied Physics, Vol. 24, pp. L227-L229 (Japan J. Appl. Phys., 24 (1985) pp. L227-L229)
In order to obtain a good film with a surface defect density of 10 ² cm ^-2 or less, the substrate grown at a temperature of 780 ° C. or higher is required to clean the epitaxially grown film after cleaning by this method. Required, which is higher than the epitaxial temperature.

(Problems to be solved by the present invention) A conventional method for cleaning a substrate in a silicon molecular beam growth method, that is, a chemical treatment using ammonia: hydrogen peroxide solution: water = 1: 6: 20, and a thin protective oxidation on the surface. The silicon substrate with the film is loaded into the growth chamber, an amorphous silicon film of about 10A is deposited on it, and then the substrate temperature is increased to clean the substrate. is required. Therefore, when considering the application of silicon molecular beam epitaxy to the fabrication of semiconductor devices, the problem is that the impurity profile created in the crystal due to the high substrate temperature is distorted by thermal diffusion and the device characteristics are degraded. There is a point.

An object of the present invention is to solve the problem of the substrate cleaning method in the conventional molecular beam growth method described above that the cleaning temperature is high.

(Means for solving the problem) First, the substrate is treated with ammonia: hydrogen peroxide solution: water = 1: 6: 20 as usual, and a thin protective oxide film is formed on the substrate surface. Thereafter, the substrate is loaded into the growth chamber.
In the present invention, the cleaning of the substrate is performed by applying electron cyclotron resonance (Electron Cyctron) to the surface of the substrate loaded in the growth chamber.
This is achieved by irradiating after disilane gas cracking by a cracking cell utilizing lotron resonance (ECR).

(Action) When the disilane is cracked by using electron cyclotron resonance, the disilane molecule is dissociated to generate mainly SiH ₂ and H. Substrate cleaning is achieved by clean removal of the protective oxide on the substrate covered with a thin protective oxide, but the SiH ₂ generated by cracking using the electron cyclotron resonance of disilane acts as a source of Si, Reacts with the SiO ₂ oxide film to form volatile SiO, thereby promoting the desorption of the protective oxide film. H atoms are SiO ₂
It has a reducing effect on oxygen atoms in the film and exhibits an etching effect on the SiO ₂ film. For this reason, when disilane cracked by electron cyclotron resonance is supplied, the oxide film can be etched efficiently, and it is lower in temperature than the method of loading amorphous silicon of about 10 A on a protective oxide film and removing it to SiO by heating the substrate. And a good clean surface can be obtained. In addition, on the exposed Si surface, SiH ₂ also acts as a source of Si and reacts with dangling bonds of Si atoms on the surface and contributes to epitaxial growth of Si. It doesn't even become.

In a growth apparatus using ordinary electron cyclotron resonance, the distance between the substrate and the electron cyclotron resonance portion is short,
Surface damage is severe because the substrate is directly exposed to the plasma. In this method, the substrate is separated from the electron cyclotron resonance portion by limiting the electron cyclotron resonance portion to the inside of the cracking cell, and damage due to direct exposure of the substrate to plasma can be prevented.

(Example) Hereinafter, a detailed description will be given with reference to the drawings. FIG. 1 is a schematic view of an apparatus for explaining an embodiment of the present invention. A silicon substrate with a thin protective oxide film formed on the surface by cleaning with a mixed solution of ammonia: hydrogen peroxide water: water = 1: 6: 20 is cleaned with a gas source silicon molecular beam epitaxy apparatus equipped with an electron cyclotron resonance cell. This is an example of the conversion. A Si (111) substrate covered with a protective oxide film is set in a growth chamber of the apparatus. In contrast, 100% disilane gas is supplied toward the substrate through an electron cyclotron resonance cracking cell. At this time, the state in which the protective oxide film of the substrate is removed is observed by reflected electron beam diffraction. When covered with a protective oxide film, a halo reflected electron beam diffraction pattern due to the oxide film is observed. When the protective oxide film is removed and a clean surface appears, a sharp 7 × 7 surface superstructure reflected electron beam diffraction pattern unique to a clean Si (111) surface is observed. FIG. 2 shows the results of measurement of the substrate temperature required for cleaning the substrate surface and the time required for cleaning using this data as disilane irradiation (with ECR) 16. In the figure, for the purpose of comparison, only the substrate is heated and the disilane molecular beam is supplied without the cracking by electron cyclotron resonance. The results in the case where they are performed are also shown. The supply amount of disilane molecular beam is 1scc
m, and the degree of vacuum in the growth chamber at this time is 5 × 10 ⁻⁵ to 2 × 10 ^5
-4 Torr. When the substrate is cleaned only by heat without supplying disilane, the protective oxide film on the surface reacts with underlying Si to become SiO and is desorbed. When disilane is supplied, some of the disilane molecules dissociate on the substrate,
It is thought that this reaction is further accelerated because it becomes a source of Si. When cracked disilane is supplied using electron cyclotron resonance, predissociated SiH ₂ and H are supplied to the substrate surface, which further promotes substrate cleaning according to the mechanism described in the section of action. Is done. In fact, when the cracked disilane is supplied to the substrate by using electron cyclotron resonance, the cleaning temperature can be lowered to 600 ° C., and the required cleaning time is 1 hour.
It was as short as 5 minutes and practical. Si (111) which was initially cleaned at 600 ℃ using cracked disilane
When an epitaxial film is grown on a substrate by disilane gas source molecular beam growth, the surface defect density of the obtained film is
It was 10 ² cm ^-2 or less. The fact that such a high-quality epitaxial film was obtained indicates that the initial cleaning of the substrate was sufficiently achieved by this method.

In the above embodiment, the case where the epitaxial growth of Si was performed on the Si substrate by the molecular beam growth method was described. However, the method of cleaning the substrate of the present invention is not limited to the epitaxial growth of Si, but also for the growth of other semiconductors and It is also used as a pre-treatment of the substrate when forming a metal or the like, and the growth method is not limited to molecular beam growth, but may be another vapor phase growth method.

(Effects of the Invention) As described in detail above, when a substrate is cleaned using disilane cracked using an electron cyclotron resonance cracking cell, the substrate can be sufficiently cleaned even at a low temperature of 600 ° C. You can do it.

[Brief description of the drawings]

FIG. 1 is a schematic view of a disilane gas source molecular beam growing apparatus used in an embodiment of the present invention. FIG. 2 is a diagram showing the substrate temperature and time required for cleaning using the present invention and a conventional cleaning method. In the figure, reference numeral 1 denotes a growth chamber of a silicon molecular beam growth apparatus;
Is a silicon substrate heating device, 3 is a protective oxide film covered
Inch Si (111) substrate, 4 is a substrate shutter, 5 is a diverging magnetic field of an electron cyclotron resonance cell, 6 is a cracking cell using electron cyclotron resonance, 7 is a waveguide that transmits microwaves to an electron cyclotron resonance device, and 8 is a waveguide. Klystrons for generating microwaves, 9 and 10 denote valves of a disilane gas introduction pipe, 11 denotes a disilane gas cylinder, 12 denotes an electron gun for reflection electron beam diffraction, and 13 denotes a fluorescent screen for reflection electron beam diffraction.

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Claims (1)

(57) [Claims] In the cleaning of a substrate before growing a material to be grown on the silicon substrate, a disilane gas is cracked by a cracking cell utilizing electron cyclotron resonance and then supplied to the substrate. Method.