JP2833044B2 - Thin film formation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a method for forming a semiconductor film and an insulator film by a microwave plasma CVD method.

[Prior art]

A CVD method is often used as a thin film forming method. In this method, a source gas is excited to cause a gas phase reaction to form a thin film on a substrate. Heat is the most commonly used excitation energy. However, depending on the substrate, the heating temperature cannot be raised sufficiently, and is not universal. Some use light as an excitation source. This has no good light source and is not efficient. Those that use plasma have a relatively low substrate heating temperature,
Widely used. In particular, plasma CVD is mainly used for an amorphous silicon thin film. At present, 13.56 MHz radio frequency (RF) glow discharge is generally used in the plasma CVD method. However, in this method, the deposition rate of the film is low. At most several tens of km / s. For this reason, large-scale substrates such as solar cells and electrophotographic photoreceptors,
Application to the substrate is not progressing. On the other hand, there is a plasma excitation source that uses microwaves instead of radio frequency (RF). For example, the raw material gas is excited by a microwave of 2.45 GHz to generate plasma. The microwave plasma CVD method can obtain a higher density plasma than the high frequency glow discharge. Therefore, it is used as a vapor phase synthesis method for diamond and BN thin films. Also, a higher deposition rate can be obtained in the production of an a-Si thin film. In this method, a source gas is passed through a vacuum chamber, and microwaves are introduced through a window having a small dielectric constant. Microwaves exist in a mode that follows the vacuum chamber geometry.

[Problems to be solved by the invention]

In microwave plasma CVD, the mode is determined by the geometric shape of the vacuum chamber. Since the microwave is introduced in the longitudinal direction, a standing wave is generated by interference between the incident wave and the reflected wave. FIG. 2 shows the average longitudinal electric field strength in the vacuum chamber as a function of the distance from the termination plane. λ _g
Is the guide wavelength. Since the terminal surface of the vacuum chamber is a conductor, the electric field is zero. A standing wave is generated, and the guide wavelength λ _g
An electric field minimum (node) point is formed at an integral multiple of 1/2 of. A point (antinode) where the electric field is maximum occurs at a position shifted by 1/4 wavelength. Since the density of the plasma excited by the microwave depends on the electric field strength of the microwave, the spatial density of the plasma becomes extremely non-uniform. Since the gas phase reaction of the raw material gas also depends on the plasma density, the deposition rate becomes uneven in the longitudinal direction. In the case of a flat substrate, the non-uniformity in the longitudinal direction is not a problem, but the non-uniformity in the planar direction has a problem in uniform deposition of a large-area substrate. Also, cylinders, cylinders, prisms, prisms, etc.
This is a serious problem when forming a thin film on a substrate having a three-dimensional spread. In order to reduce the effects of standing waves, a variable short-circuit plate is provided,
A method of moving this has been proposed. But this has not been effective enough. In microwave plasma CVD, the present invention is to provide a method for eliminating non-uniform microwaves in the longitudinal direction and forming a uniform thin film at a high speed on a long, large substrate, a cylindrical substrate, or a large-area substrate. It is an object of the invention.

[Means for Solving the Problems]

The present invention is directed to a microwave plasma CVD apparatus that excites and decomposes a source gas in a vacuum chamber by microwave energy to form a thin film on a substrate placed in the vacuum chamber. It is characterized by changing the oscillation frequency. FIG. 1 is a schematic configuration diagram showing an example of a microwave plasma CVD apparatus to which the present invention can be applied. It comprises a vacuum chamber 1, a source gas inlet 3, an exhaust pipe 4, a substrate heater 5, a microwave introduction window 6, a waveguide 7, a microwave oscillator 8, a power supply 9, and the like. Since the base 2 is cylindrical, the cylindrical vacuum chamber 1
Is installed in the middle of the vertical direction in accordance with the axis of the chamber. The substrate 2 may be stationary, but if it is rotated, the uniformity is further improved. The substrate heater 5 heats the substrate to an appropriate temperature. The source gas is a gas containing a constituent material of the thin film. SiH ₄ ,
CH ₄ , NH ₃ , H ₂ , B ₂ H _{6 and the} like are optional. The pressure is the same as normal microwave plasma CVD,
It is about 01-100 Torr. The microwave introduction window 6 must maintain a vacuum and pass microwaves. A dielectric such as ceramic is used. It should be noted that there is little microwave loss upon entering the vacuum chamber 1. The example of FIG. 1 is suitable for forming a thin film on a substrate on a cylinder. FIG. 4 shows an apparatus according to another example to which the present invention is applied. The apparatus shown in FIG. 4 is an apparatus used in the vapor phase synthesis of diamond or BN thin films. The raw material gas flows from top to bottom in the vertically long cylindrical vacuum chamber 1. A flat substrate 2 is supported by a susceptor 10 in the middle of the vacuum chamber 1. A source gas inlet 3 is provided at an upper end of the vacuum chamber 1, and an exhaust pipe 4 is provided at a lower end of the vacuum chamber 1. A cavity 11 is provided surrounding the vacuum chamber 1. The waveguide 11 is connected to the cavity 11, and a microwave is introduced from the microwave oscillator 8 to the waveguide 7. The power supply 9 supplies power for driving the microwave oscillator 9. The microwave propagates horizontally in the cavity, but since there are conductors at both ends, it is repeatedly reflected to produce a standing wave. If the substrate 2 is long in the horizontal direction, the intensity of microwaves on the substrate becomes uneven. Strong at the antinode of the standing wave,
Weak at the section. Therefore, in the present invention, the oscillation frequency f of the microwave oscillator 8 is changed. This is the most important point of the present invention. The reason for changing the oscillation frequency is to cancel the effect of the standing wave. Even if a standing wave is temporarily generated, the oscillation frequency changes immediately, so that the wavelength of the standing wave changes and the electric field intensity is averaged.

[Operation]

In the apparatus shown in FIGS. 1 and 4, a substrate 2 is attached to a vacuum chamber 1. Close the vacuum chamber 1 and apply a vacuum. The substrate 2 is heated by the substrate heater 5. Feed the source gas. Microwaves are introduced from the microwave oscillator 8 into the vacuum chamber 1. The microwave oscillation frequency is changed with time. The energy of the standing wave changes in a cycle of half the wavelength, but the oscillation frequency changes, so that the energy of the standing wave is spatially averaged. This will be described in more detail. A wavelength in free space of the microwave and lambda, the cutoff wavelength is lambda _c. Guide wavelength λ _g is Becomes It is λ that is controlled, but by changing this, the guide wavelength λ _g can also be changed. The cutoff wavelength has a fixed value for each mode. If there are multiple modes, the cutoff wavelength is not a single value, but this is no problem. This is because the fact that there are a plurality of guide wavelengths λ _g for one free space wavelength works to cancel the standing wave. By appropriately determining the dimensions of the cavity and the vacuum chamber, only the lowest mode can be present for a certain range of wavelengths. Now, since the free space wavelength λ is changed, the guide wavelength λ _g
Changes. largest field strength, the minimum position changes according to the change of the lambda _g. Since they cancel each other, the electric field intensity distribution is spatially averaged, and a uniform plasma density can be obtained. When a microwave having an oscillation frequency f is frequency-modulated by qsin ωt, the average electric field strength is as follows. Here, Kg (2π / λ _g ) is the wave number in the tube, and Q is the amplitude of the wave number change in the tube due to frequency modulation. x is the distance from the terminal surface. ω is a modulation frequency, which should be appropriately determined depending on the film formation time, but is generally preferably 2πrad / s or more. Actually, film deposition was performed by changing Q, that is, q. x
FIG. 3 shows the film thickness distribution in the range of = 0 to 3λ _g . The horizontal axis is the distance x from the terminal surface. The vertical axis is the film thickness. When the frequency is not changed (Q / Kg = 0), the fluctuation of the film thickness distribution reaches ± 50%. This is smaller than the fluctuation of the average electric field strength shown in FIG. The plasma generation is almost proportional to the electric field, but is diffused by thermal motion, so that the film thickness distribution is less than the electric field strength. Still, 50% fluctuation is a serious problem. When the oscillation frequency is changed, the fluctuation of the film thickness distribution is reduced. For example, when Q / Kg = 0.05, the fluctuation of the film thickness distribution is considerably reduced as compared with the case where the frequency modulation is not performed. In particular, the fluctuation decreases as the distance from the end surface increases. When Q / Kg = 0.1, the fluctuation of the film thickness distribution can be suppressed to ± 12% in the range of x ≧ λ _g / 2. If x is greater than this, the fluctuation will be less. This is because the standing wave is generated by the reflection of the terminal surface. Microwaves having different frequencies are superimposed one on the other as the distance from the end increases, so that fluctuations are reduced. This can be seen from the above integral. The vacuum chamber is long because the substrate is long. Most of the substrate is farther away than near the end, and the plasma distribution is almost uniform around here. For this reason, the film thickness distribution becomes uniform. From this result, it can be seen that it is better to perform frequency modulation so that Q / Kg becomes 0.1 or more, and to separate the substrate from the terminal surface by λ _g / 2 or more.

[Example I]

An amorphous silicon (a-Si) thin film was formed on a cylindrical substrate using the apparatus shown in FIG. 1 to produce a photoconductor for electrophotography. Table 1 shows the film forming conditions. Substrate temperature 300 ° C. substrate Al drum (40Φ, l = 300mm) installed from the end surface at the position of the lambda _g / 2 microwave 2.45GHz + 0.3sin 120t GHz 1.2kW (ω = 120rad / sec) a-Si in such conditions A thin film was formed. The film thickness distribution on the substrate drum was 30 μm ± 3 μm (± 10%).
It can be seen that the uniformity is good. The photoreceptor thus produced was subjected to corona charging, and the electrophotographic characteristics were measured. Good characteristics could be obtained with both positive and negative polarities.

[Example II]

Using the microwave plasma CVD apparatus shown in FIG.
A thin film of BN and diamond was grown on a flat substrate. Table 2 shows the film forming conditions. The frequency of the microwave was changed to 2.45 GHz + 0.3 sin 120 t GHz as in Example I. The thin films of BN and diamond grown on the silicon substrate were analyzed by Raman and FTIR. As a result, the BN and diamond thin films were formed over the entire flat substrate, and the inner surface film thickness distribution was ± 10%, which was an extremely good thin film. On the other hand, when the film was formed under the same conditions as in Table 2 while keeping the microwave oscillation frequency constant, the film could only be formed to a diameter of about 100 mm near the center of the substrate.

【The invention's effect】

In the microwave plasma CVD method, the microwave oscillation frequency is changed to prevent generation of a fixed standing wave, so that a uniform plasma distribution in the longitudinal direction can be obtained. A thin film can be formed quickly and uniformly on a large-area substrate having a three-dimensional shape or a two-dimensional shape such as a columnar substrate. There is an excellent effect that a high-quality electrophotographic photosensitive member can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a thin film forming apparatus of the present invention. FIG. 2 is an average electric field intensity distribution diagram by microwaves in a vacuum chamber of a conventional thin film forming apparatus. FIG. 3 is a diagram showing a film thickness distribution of a thin film by the apparatus of the present invention when the microwave oscillation frequency is periodically changed. FIG. 4 is a schematic structural view of another microwave plasma thin film growth apparatus to which the present invention is applied. DESCRIPTION OF SYMBOLS 1 ... Vacuum chamber 2 ... Substrate 3 ... Source gas inlet 4 ... Exhaust pipe 5 ... Substrate heater 6 ... Microwave introduction window 7 ... Waveguide 8 ... Microwave oscillator 9 ... Power supply 10 …… Susceptor 11 …… Cabiday

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takeshi Oishi 1-3-1 Shimaya, Konohana-ku, Osaka-shi, Osaka Sumitomo Electric Industries, Ltd. Osaka Works (72) Inventor Yoshio Tsuchizaki 1, Shimaya, Konohana-ku, Osaka-shi, Osaka Chome 1-3, Sumitomo Electric Industries, Ltd., Osaka Works (58) Field surveyed (Int. Cl. ⁶ , DB name) C23C 16/00-16/56 C23F 1/00-4/04 H01L 21/205, 21 / 31,21 / 365 H01L 21 / 469,21 / 86 H01L 21 / 302,21 / 3065,21 / 461

Claims (1)

(57) [Claims] 1. A substrate is placed in a vacuum chamber and heated, a source gas is passed through the vacuum chamber, microwaves are introduced to excite the source gas, and a plasma state is formed to form a thin film on the substrate. A method for forming a thin film, comprising: changing a microwave oscillation frequency.