JPH06283435A - Method of forming amorphous silicon film by plasma cvd - Google Patents

Abstract

PURPOSE:To form amorphous silicon film of a small optical gap by plasma CVD in which the source gas for amorphous silicon is heated before it is introduced into a plasma region. CONSTITUTION:Wafers 20 are placed on a holder 14 in a chamber 12, and the temperature of the wafers is controlled by a heater 16. Above the holder 14, a heater 22 is provided, and heated source gas is introduced through an entrance 22a to a plasma region 26. The source gas is thermally energized to vibrate and rotate so that the reaction at the surface of the wafer is accelerated during the formation of amorphous silicon. Therefore, amorphous silicon film I of a low optical gap can be formed at a relatively low substrate temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming amorphous silicon by plasma CVD, and more particularly to a method for forming an I layer in a PIN junction of a semiconductor layer of a solar cell.

[0002]

2. Description of the Related Art For example, in a solar cell or the like, an amorphous silicon film having a PIN junction is formed by a plasma CVD apparatus. An example of such a conventional plasma CVD apparatus is shown in FIG. Figure 6
The conventional plasma CVD apparatus 1 shown in FIG. 1 includes a chamber 2, a substrate holder 3 is arranged in the chamber 2, and a heater 4 is built in the substrate 3. The temperature of the heater 4 is detected and controlled by the thermocouple 5 arranged close to it. The substrate 6 is placed on the substrate holder 3. Therefore, the temperature Ts of the substrate 6 is controlled by the heater 5. R is provided above the substrate holder 3 in the chamber 2.
An F electrode 7 is arranged, and this RF electrode 7 is a shield electrode 8
Shielded by. An RF output is applied to the RF electrode 7 from a high frequency power source (not shown). Also, the RF electrode 7
Includes a pipe-shaped portion 7a, and from this pipe-shaped portion 7a, a material gas that constitutes amorphous silicon, such as silane,
Disilane or higher order silane is introduced. A large number of small holes 7b are formed on the front surface of the RF electrode 7, and the material gas is drawn by a pump (not shown) connected to the lower portion of the chamber 2 to form a shower shape from the small holes 7b. Are introduced into the plasma region 9.

In such a conventional plasma CVD apparatus 1, in order to change the optical gap (Eopt),
The RF output, gas flow rate or gas pressure has been changed.

[0004]

However, if the optical gap is narrowed by changing such factors, other film characteristics may be adversely affected. For example, in order to narrow the optical gap, the substrate temperature Ts may be raised, but when the substrate temperature is raised, in the case of the PIN structure, the P layer formed prior to the I layer may be damaged, Therefore, there is a problem that the characteristics of the solar cell are changed.

Therefore, a main object of the present invention is to provide a method for forming an amorphous silicon film by plasma CVD which can narrow the optical gap without adversely affecting the other.

[0006]

Briefly, the present invention is a method for depositing amorphous silicon by plasma CVD in which a material gas for forming amorphous silicon is introduced into a plasma region, and the material gas is introduced into the plasma region. It is a film forming method characterized by heating before.

[0007]

By heating the material gas before it is introduced into the plasma region, sufficient heat energy is provided to the gas. Therefore, vibration / rotational motion is excited in the gas, and the surface reaction is promoted when the amorphous silicon grows. Therefore, the I layer having a narrow optical gap can be formed without increasing the substrate temperature.

[0008]

According to the present invention, since it is not necessary to raise the substrate temperature in order to narrow the optical gap, the film previously formed is not damaged or otherwise adversely affected. The above-mentioned objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of the embodiments with reference to the drawings.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A plasma CVD apparatus 10 shown in FIG. 1 includes a chamber 12 in which a substrate holder 1 is placed.
4 are arranged. A heater 16 for heating the substrate and a thermocouple 18 for controlling the temperature of the heater 16 are provided in the substrate holder 14. The substrate 20 is placed on the substrate holder 14, and thus the heater 16 controls the temperature Ts of the substrate 20. A gas heater 22 is arranged above the substrate holder 14 in the chamber 12. The gas heater 22 has an inlet 2 from its upper surface.
The material gas is introduced through 2a, and the heated material gas is discharged from the outlet 22b formed in the lower surface substantially in the center. The gas heater 22 is illustrated in FIGS. 2 and 3. The gas heater 22 is formed in a disk shape as a whole, and an inlet 22a is formed on the outer periphery of the upper surface thereof, and an outlet 22b is formed substantially at the center of the lower surface thereof. A wall 22d forming a spiral gas passage 22c from the inlet 22a to the outlet 22b is formed in the gas heater 22, and a heater 22e is provided on the inner surfaces of the upper plate and the lower plate. Therefore, the material gas introduced from the inlet 22a is discharged from the outlet 22b through the spiral gas passage 22c communicating with the outlet 22b. At this time,
The gas is heated by the heater 22e.

It should be noted that the time during which the material gas flows in the gas heater 22 while reaching the heating set temperature (for example, 100 ° C.) is outside the chamber 12 after the material gas is introduced into the plasma region 26 described later. The length of the gas passage 22c is designed so as to be sufficiently longer than the time until it is discharged. This is to raise the temperature of the gas sufficiently.

Referring again to FIG. 1, a shower electrode 24 is arranged on the outlet 22b side of the gas heater 22, and a large number of small holes 24a are formed in the shower electrode 24. An RF output is applied to this shower electrode 24 from a high frequency power supply (not shown). Therefore, the material gas discharged from the outlet 22b of the gas heater 22 and being heated is showered through the small holes 24a of the shower power supply 24 in the plasma region 26.
Will be introduced in.

As described above, the characteristic feature of the present invention is that the material gas itself is heated before being introduced into the plasma region 26. By imparting heat energy to the material gas in this way, vibration / rotation motion can be achieved. Since it is excited, the substrate 20
The surface reaction at the time of forming the amorphous silicon film can be promoted. Thereby, an amorphous silicon film having a narrow optical gap can be formed without raising the substrate temperature Ts so much.

In the experiment, the material gas is about 100 to 380 ° C.
It was heated once, but as shown in the graphs of Figs.
Good results have been obtained. FIG. 4 shows the gas addition as shown in FIG.
When the material gas is heated by using the heater 22,
This figure shows the relationship between the cap and the deposition rate.
4 shows that even if the substrate temperature Ts is not increased, the optical gap
It can be seen that can be made sufficiently narrow. Well
The graph in Fig. 5 shows the characteristics of the film formed by this method.
Indicates whether or not has changed compared to the past.
The vertical axis represents the silicon atoms and water in the amorphous silicon.
Bonding ratio with elementary atoms SiH₂/ SiH is taken, and the horizontal axis is optical
It shows a gap. This ratio SiH₂/ SiH is small
The film characteristics are better, but the material gas is heated according to the example.
If the optical gap is narrow, the SiH ₂
It can be seen that / SiH can be reduced. Furthermore, FIG.
It is a graph showing the relationship between the optical gap and the conductivity,
The side shows photoconductivity and the lower side shows dark conductivity. This Figure 6
From the graph of, the optical
Even if the gap is narrowed, the conductivity is worse than before.
It turns out that there is no change. That is, subordinate
In the past, in order to narrow the optical gap, it was necessary to increase the substrate temperature.
If I had to raise it to about 180 ° C,
However, according to the present invention, the substrate temperature is, for example, about 140 ° C.
The optical gap can be narrowed by degrees, and
Therefore, the conductivity does not change.

According to the present invention, in order to heat the material gas before introducing it into the plasma region, any other method other than the gas heater as shown in FIGS. 1 to 3 may be used. Of course good things.

[Brief description of drawings]

FIG. 1 is an illustrative view showing an example of a plasma CVD apparatus to which the present invention is applied.

FIG. 2 is a cross sectional view showing a gas heater according to the embodiment of FIG.

FIG. 3 is a top view showing the gas heater of FIG. 1 embodiment.

FIG. 4 is a graph showing a relationship between a film forming rate and an optical gap with a substrate temperature as a parameter.

FIG. 5 is a graph showing the relationship between the optical gap and the bond ratio between silicon atoms and hydrogen atoms.

FIG. 6 is a graph showing the relationship between optical gap and conductivity.

FIG. 7 is an illustrative view showing an example of a conventional plasma CVD apparatus.

[Explanation of symbols]

 10 ... Plasma CVD apparatus 14 ... Substrate holder 20 ... Substrate 22 ... Gas heating heater 22a ... Inlet 22b ... Outlet 22c ... Gas passage 22e ... Heater 24 ... Shower electrode 26 ... Plasma region

Claims (1)

[Claims] 1. A method for depositing amorphous silicon by plasma CVD in which a material gas for forming amorphous silicon is introduced into a plasma region, wherein the material gas is heated before being introduced into the plasma region. Membrane method.