JPS6147226B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for growing an amorphous silicon film that is effectively used as a material for solar cells, etc.
In particular, the present invention relates to a growth method capable of controlling the amount of bonded hydrogen in such an amorphous silicon film.

A common method for growing amorphous silicon films is the glow discharge method, which is preferred as a relatively simple method. It is known that when an amorphous silicon film is deposited from a raw material gas, hydrogen atoms are contained in the amorphous silicon film in a state bonded to silicon atoms.

However, it would be even more advantageous if the amount of bonded hydrogen in the amorphous silicon film could be controlled without deteriorating the photoelectric conversion characteristics when depositing the amorphous silicon film as described above using this type of method. This is because, even though the amount of bound hydrogen does not have a large effect on photoelectric conversion characteristics, there are various other factors that affect the amount of bonded hydrogen, such as the use of the amorphous silicon film created in this way as a solar cell material. This is because it has a great effect on the mechanical stability, thermal stability, etc. of the final product, such as the mechanical stability and thermal stability.

However, conventionally, attempts have been made to control the amount of bonded hydrogen in the amorphous silicon film in order to improve external factors, while satisfying the condition that the photoelectric conversion characteristics are not impaired, if not improved. I couldn't see the idea of doing so.

FIG. 1 shows a schematic basic configuration of a conventional amorphous silicon film growth apparatus using this type of glow discharge method. Briefly, a plasma electrode 11 and a heater electrode 12 holding a substrate S on which an amorphous silicon film is to be grown are placed in a reduced pressure tank 10 which is adjusted to a pressure suitable for film growth by an appropriate exhaust device 20. An AC power source 1 for generating glow discharge is connected between both electrodes, while a raw material gas containing monosilane, disilane, etc. is supplied from a gas supply device 30 into the reduced pressure tank. Conventionally, in such an amorphous silicon film growth apparatus,
The most common frequency of AC power is
A relatively high frequency of 13.56 MHz is employed, at which the amount of bound hydrogen in the deposited amorphous film is typically on the order of 15 atomic percent under conditions of good film photoconductivity.

However, such a relatively high frequency, which greatly exceeds the conventional high cutoff frequency of ionized ions, may cause major problems in terms of equipment. For one thing, if the above-mentioned 13.56MHz is selected as the power supply frequency, the quarter wavelength of the frequency is approximately 5.53m, but as the size of the growth equipment approaches this size, the internal Standing waves are generated, and the power distribution within the device becomes non-uniform.As a result, the thickness distribution of the growing amorphous silicon film may become significantly non-uniform. Another reason is that the matching circuit 40 shown in FIG. 1 is required in the power supply circuit. Of course, it is preferable not to have such a circuit.

If the power supply frequency is set to a low frequency, the above-mentioned equipment disadvantages will be resolved, but if the equipment is kept in the conventional configuration and the frequency is simply lowered to below the high cutoff frequency, the deposited The amount of bonded hydrogen in the amorphous amorphous film often exceeded 20 atomic percent. It is true that a photoelectric conversion element using an amorphous silicon film requires a certain amount of bonded hydrogen to reduce the gap level and improve the photoelectric conversion characteristics, but this order of magnitude is The value is clearly too high. In particular, concerns arise about thermal stability, etc., and in some cases, it is conceivable that the photoelectric conversion characteristics themselves, which should be improved, may deteriorate.

The present invention has been made in view of the above-mentioned circumstances, and is capable of reducing the amount of bonded hydrogen in an amorphous silicon film without at least deteriorating the photoelectric conversion characteristics and without causing any inconvenience when configuring the device. This provides a method for controlling the

To first outline the present invention configured in accordance with the above objectives, in the growth apparatus described above, the frequency of the AC power source is lower than the high cutoff frequency of ionized ions (generally up to about 100 KHz). At the same time, by applying a DC bias to at least one of the two opposing electrodes in the decompression tank relative to the other, a DC potential difference is created between the two electrodes, and by adjusting the DC bias value, the amorphous silicon This is an attempt to control the amount of bound hydrogen in the film. The reason why the AC power frequency is set to be lower than the high cutoff frequency of ionized ions has been briefly mentioned above, but will be described in more detail later in connection with the embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, through the description of embodiments of the present invention in accordance with the accompanying drawings, it will be made clear that the configuration of the present invention certainly has the desired effects.

FIG. 2 shows an amorphous silicon film growth apparatus suitable for use in embodiments of the present invention. The main structural parts of the device, excluding the power supply system, are
No modification is required to the illustrated device structure. Therefore, such parts that do not particularly require modification are given the same reference numerals as in FIG. 1, and their explanations may be omitted.

In the device shown in the second diagram, the AC power supply 1 has a frequency below the high cutoff frequency of ionized ions,
For example, it is 100 KHz or less, and there is no need to go through the matching circuit 40 shown in FIG. (hereinafter simply referred to as bypass capacitor 3)
Connected to 2.

For this closed loop of the AC power supply system, a DC power supply 2, preferably with a variable voltage, is provided in parallel with the bypass capacitor 3. It should be noted that in the case of the present invention, a DC bias is reliably applied between the heater electrode 12 and the plasma electrode 11.
This is because, although the purpose is slightly different, there was a previous report that no significant effect was found even when applying a DC bias to this type of device, but with the device configuration used in this report, This is because, in reality, no DC bias was applied between the two electrodes, and the floating DC potential of the plasma electrode was simply measured.

The drawings from FIG. 3 onward show that the required amount of bound hydrogen can be controlled by selecting the AC power frequency and the DC bias configuration according to the present invention.

Figure 3 shows that the AC power frequency is lower than the high cutoff frequency of ionized ions, as will be seen later.
30KHz, and when a positive bias is applied to the heater electrode 12 with the configuration shown in the second figure, that is, when a relatively negative bias is applied to the plasma electrode 11,
It shows how the amount of bonded hydrogen in the silicon film changes with changes in the bias potential. Of course, this data was first collected by the inventor. In any case, according to what this data shows, with the configuration shown in Figure 2, when the heater electrode is biased positively, that is, when the plasma electrode is biased negatively, the amount of bound hydrogen is lower than when it is biased to the opposite polarity. It can be seen that this can be reduced to about 70%.

However, as mentioned at the beginning, even if the amount of bound hydrogen is controlled to an appropriate value, the photoelectric conversion characteristics will not deteriorate. However, according to the method of the present invention, even if the amount of bonded hydrogen in the amorphous silicon film is controlled by controlling the DC bias voltage according to the curve shown in FIG. FIG. 4 proves that the deterioration does not cause a problem. In other words, Figure 4 shows the relationship between the photoconductivity, which is an index of the photoelectric conversion characteristics of the fabricated device, and the applied DC bias voltage value.As is clear from this curve, the photoconductivity It remains almost constant regardless of the DC bias voltage value.

Having described the effects of the present invention, further practical considerations will be made.

As mentioned above, in view of the problem of standing waves and the need to eliminate the need for a matching circuit, the lower the frequency of the AC power supply, the better. However, compared to an amorphous film deposited using a conventional high-frequency device, if the amount of bound hydrogen is too large as mentioned above, there is still a problem, and even at low frequencies, it is possible to obtain a bonded hydrogen amount of an appropriate amount. There is a need for assurance that it will be possible.

As mentioned above, we searched for the cause of why the amount of bound hydrogen increases simply by lowering the power frequency below the high cutoff frequency of ionized ions, and as a result of our research, we found that the A mechanism in which when the AC power frequency is lowered to below the high cutoff frequency of ionized ions, the ionized ions hit the surface of the sample or substrate, and hydrogen recombines with the broken bonds on the membrane surface. It was found that this occurs.

Conversely, the present invention is based on this knowledge, and as a result, the amount of bound hydrogen can be reduced by applying a polar DC bias that moves ionized ions away from the substrate, as shown in Figure 3. The result was that the film could be controlled with good controllability at or below the level of conventional amorphous amorphous films deposited by high frequency.

Here, let us consider what the high cutoff frequency of ionized ions actually is.

The high cutoff frequency c of an ion running at a speed v over a length d can be expressed by the following equation.

c=v/2πd...(1) In the case of CVD amorphous silicon film growth by the glow discharge method using monosilane as the raw material gas, the length of the dark part of the cathode is generally about 1 cm in the normal equipment shown in Figure 1. , the voltage between the electrodes is 600V ~
Since the voltage is 800V, the electric field E in the part where the Si:H ⁺ ions are accelerated is E=6 to 8×10 ² /cm. Therefore, the ratio of electric field E to gas pressure Po is E/Po
is E/Po=6 to 8×10 ² /cm·mmHg at a gas pressure of 1 Torr, which is common in this type of device.

In addition, for such a value, based on already published data, the velocity of the ion is (E/
Po) It is known that it is proportional to ^1/2 . Furthermore, based on existing data, the velocity v of monosilane ions at these values is considered to be v2×10 ⁵ cm/sec.

Therefore, under each of the above values, if the cutoff frequency c is determined according to the above-mentioned equation (1), it becomes c(1/3)×10 ⁵ Hz. However, in normal cases, the gas pressure is 0.1 Torr ~
Since it is selected between several Torr, the cutoff frequency c generally becomes a value up to about 100 KHz.

This is a highly desirable result. At such a low frequency, there is sufficient space for increasing the size of the equipment, and it is clear that there is no need to consider countermeasures against standing waves, and there is no need for matching circuits, etc. It can be seen that there is a certain effect.

In the above, the gist and effects of the present invention have been confirmed, but below, consideration will be given to making the device more suitable for mass production in terms of the device for carrying out the present invention.

In the second illustrated device, the heater electrode has a DC potential with respect to ground. Therefore, in the current configuration, it is necessary to insulate the heater electrode, insulate the substrate S, and then apply a DC bias to the tray that conveys the substrate. However, this is of course often inconvenient in practice. To solve this problem, the heater electrode or the substrate should be set at the reference potential or ground potential of the entire device. Conventionally, the plasma electrode 11 was generally designed to have a DC floating potential, but the examples proposed by the applicant for the above purpose are shown in FIGS. The results are as shown in Figure 6.

In the case shown in FIG. 5, the heater electrode to be biased is connected to a common line, that is, grounded, and the AC power source 1 is inserted in series with the DC power source 2. In this embodiment, the potential of the DC power source 2 is shown as a switching circuit so that either positive or negative potential can be selected. In the case of this circuit, it is of course equivalently the same as the configuration shown in the second figure, but the bypass capacitor 3 inserted in parallel with the DC power supply may not be provided in principle. However, considering heat generation loss in the DC power source of AC power and interference with the DC potential control system, it is practically desirable to provide it.

In the case shown in Figure 6, the DC power supply 2 is inserted between the plasma electrode 11 side terminal of the AC power supply 1 and the ground via the low-pass filter circuit 6. 12
can be placed in a predetermined DC bias relationship.

In this way, if the actual device configuration as shown in FIGS. 5 and 6 is adopted, the present invention can be applied without any special measures while keeping the substrate transfer tray, as well as the work area and the worker, at ground potential. be able to.

As described in detail above, according to the present invention, it is possible to convert an amorphous silicon film without sacrificing photoelectric conversion characteristics by simply connecting a DC bias power source without requiring any major modification to conventional equipment. It is possible to control the amount of bound hydrogen, simplify the circuit configuration, and eliminate the need to take measures against standing waves. Therefore, in the future, the use of amorphous silicon thin films in solar cells, thin film FETs, etc. It is expected that it will make a major contribution to technology.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a conventional amorphous silicon film growth apparatus, FIG. 2 is a schematic configuration diagram of an example of an apparatus used for carrying out the present invention, and FIGS.
The figure is a characteristic diagram showing an example of the effects of the present invention, and FIGS. 5 and 6 are schematic configuration diagrams of other examples of the device configuration used in the present invention. In the figure, 1 is an AC power source for glow discharge, 2 is a DC power source for DC bias, 10 is a pressure reducing tank, 11 is a plasma electrode, 12 is a heater electrode, 30 is a source gas supply device, and S is an amorphous silicon film. This is the substrate on which to grow.

Claims (1)

[Claims] 1. A glow discharge is caused by an AC power source between two electrodes placed oppositely in a reduced pressure tank to which raw material gas is supplied, and a glow discharge is caused by an AC power source, A method for growing an amorphous silicon film on a substrate, comprising: setting the frequency of the AC power source to be lower than the high cutoff frequency of ionized ions caused by the glow discharge;
A method for growing an amorphous silicon film, characterized in that a DC bias is applied to one of the two electrodes with respect to the other, and the amount of bonded hydrogen in the amorphous silicon film is controlled by controlling the DC bias value.