Method for forming amorphous silicon film

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to a method for forming a semiconductor thin film used in a thin film semiconductor device such as a thin film transistor for an active matrix type liquid crystal display device and a photodiode for an image reading device, and a method for forming a semiconductor thin film in which particles are less generated particularly at high speed formation of the semiconductor thin film. It is about.

【0002】

[Conventional technology]

A semiconductor thin film such as amorphous Si is formed by a continuous discharge plasma CVD method using time-continuous high-frequency power as shown in FIG.

【0003】

As an extension of this continuous discharge plasma CVD method, Japanese Patent Application Laid-Open No. 58-157600 has an example of forming a semiconductor thin film using unsteady high-frequency power. As shown in FIG. 4, this guides the generated high-frequency power to the forming device when the gate pulse is on, discharges and decomposes the raw material gas, while stopping the generation of high-frequency power when the gate pulse is off. , A method of forming a thin film that temporarily blocks discharge. The method of forming a thin film that turns the discharge on and off with a simple repeating pulse of the same period and the same duty as such a gate pulse is called a simple gate pulse modulation intermittent discharge plasma CVD method.

【0004】

It is known that the generation of particles is very small as a feature when forming amorphous Si by using this simple gate pulse modulation intermittent discharge CVD method. For example, as shown in Applied Physics Letters, Vol. 57, No. 16, pp. 1616-1618 (APPLIED PHYSICS LETTERS, VOL.57, NO.16, OCTOBER, 1990, pp.1616-1618). When forming amorphous Si, no particles were observed even at a high film formation rate of 360 A / min.

【0005】

[Problems to be Solved by the Invention]

In the mass production process of thin film semiconductor devices, the throughput of plasma CVD equipment is poor compared to other manufacturing equipment such as etching equipment. Therefore, line balance is ensured by installing a large number of plasma CVD equipment. From the viewpoint of investment cost of manufacturing equipment, it is important to improve the throughput of plasma CVD equipment, and for that purpose, there is an urgent need to develop a technology for significantly increasing the film formation speed.

【0006】

However, in the conventional simple gate pulse modulation intermittent discharge CVD method, the increase of particles is not observed up to about 400 A / min, but the generation of particles is observed when the duty ratio and high frequency power are increased aiming at higher speed film formation. Therefore, it is strongly desired to develop a thin film forming method that does not generate particles at a film forming rate of 400 A / min or more.

【0007】

[Means for solving problems]

In order to solve the above-mentioned problems, in the thin film forming method of the present invention, high frequency power is output when the gate pulse is on to cause discharge, and when the gate pulse is off, the high frequency power is cut off and the discharge is stopped. In the method of forming an amorphous silicon film in which an amorphous silicon film is deposited by intermittent discharge that repeats an off state, a pulse train having a high duty ratio and a pulse train having a low duty ratio or a pause pulse that is off for the entire period are alternately alternated as the gate pulse. It has the characteristic of repeating.

【0008】

[Action]

The cause of particle generation in the plasma CVD method is that the polymerization reaction of Si-based active species occurs rapidly in the gas phase, and this partition directly adheres to the sample or the inner wall of the growth chamber. It is considered that the particles adhering to the sample are peeled off from the wall and reattached to the sample.

【0009】

In the conventional simple gate pulse modulation intermittent discharge plasma CVD method, the generation of particles is prevented by killing active growth species that cause a polymerization reaction within the discharge suspension period.

【0010】

However, if the high-frequency power is further increased and the duty ratio is increased in order to form the film at high speed, the active growth species cannot be completely killed within the discharge suspension period as described above, and as a result, particles are regenerated.

【0011】

Therefore, as shown in Fig. 1, high-frequency power with a high duty ratio is applied for a certain period of time to form a high-speed film, and when particles increase in the gas phase, high-frequency power with a low duty ratio or a pause pulse that is off for the entire period By using a gate pulse such as inserting a gate pulse, the active growth species are killed by the long discharge pause time during the period of the low duty ratio, although the duty ratio is high on average, and low particle deposition is achieved. It will be realized. The rest pulse in the off state for the entire period means a waveform having a duty ratio of 0 that keeps the pulse train with a high duty ratio off for the entire period and does not cause high-frequency discharge. The forming method of the present invention will be referred to as a composite gate pulse modulation intermittent discharge plasma CVD method.

【0012】

[Example]

Next, the first embodiment of the present invention will be described. The amorphous silicon film forming apparatus used in this embodiment uses a high-frequency power supply with a pulse mode that can turn on / off the output of high-frequency power by a gate pulse and a matching box having a high-speed matching circuit with a response time of 100 μsec or less. There is. The size of the high frequency electrode is 600 mm square.

【0013】

The film forming conditions used in this example are a silane flow rate of 200 SCCM, a hydrogen flow rate of 600 SCCM, a vacuum degree of 200 Pa, and a substrate temperature of 300 ° C. Figure 2 shows the waveform of the gate pulse that controls high-frequency power. The nine pulses with a high duty ratio have an on-time of 800 microseconds, an off-time of 200 microseconds, and a duty ratio of 80%. On the other hand, one pulse with a duty ratio of 0 (that is, a pulse with all periods off) was inserted as a pulse with a low duty ratio. Therefore, a gate pulse was formed by repeating a pulse in which nine pulses having a duty ratio of 80% and one pulse having a duty ratio of 0% were connected.

【0014】

To evaluate the particles, the number of particles adhering to the substrate is increased by measuring the number of particles adhering to the substrate before and after film formation using a dust inspection device that utilizes the scattering phenomenon of laser light. The number of particles adhering in the plasma CVD apparatus was evaluated by the amount.

【0015】

When 400 W was applied as high frequency power in the ON state, the film formation speed was 810 A / min. For comparison, all other film formation conditions were the same, and an amorphous silicon film was formed by a simple gate pulse modulation intermittent discharge plasma CVD method using a gate pulse with a duty ratio of 80%. The film formation rate was 900 A / min. was gotten. On the other hand, when the number of particles is formed by the simple gate pulse modulation intermittent discharge plasma CVD method, the number of particles having a size of 2 μm or more in the range of 240 mm × 290 mm of the substrate is very large at 2400, whereas in this embodiment, the number of particles is very large. In the composite synthetic gate pulse modulation intermittent discharge plasma CVD method, the number was only 56, demonstrating the effect of the present invention.

【0016】

Next, a second embodiment of the present invention will be described. In this embodiment, the device for forming the amorphous silicon film is the same as that in Example 1. The conditions for forming the amorphous silicon film are a silane flow rate of 200 SCCM, a vacuum degree of 140 Pa, and a substrate temperature of 300 ° C as a raw material gas. As a gate pulse to control high frequency power, the repetition frequency was set to 500Hz, and 500W was introduced as high frequency power when on. Here, as the modulation of the duty ratio with respect to the gate pulse, which is a feature of the present invention, the duty ratio of the nine gate pulses is set to 70%, and the duty ratio of the tenth gate pulse is changed from 70% to 0%. .. The amount of particles generated at this time was measured. When the duty ratio of the 10th gate pulse was 70%, the forming rate of the amorphous silicon film was 950 A / min. When the duty ratio was 0%, it was 855 A / min. The results are shown in Figure 3. The vertical axis is the number of particles with a size of 2 μm or more in the range of 240 mm × 290 mm of the substrate. From Fig. 3, it can be seen that the particles decrease sharply when the due diligence ratio of the 10th gate pulse becomes 50% or less. This is the effect of low duty ratio pulse insertion. The effectiveness of the present invention has been demonstrated by the fact that the duty ratio is 10% or less and the number of particles is reduced by about an order of magnitude as compared with the simple gate pulse modulation intermittent discharge plasma CVD method with a duty ratio of 70%.

【0017】

[Effect of the invention]

As described above, according to the method for forming an amorphous silicon film using the composite synthetic gate pulse modulation intermittent discharge plasma CVD method according to the present invention, the amount of particles generated during high-speed film formation of a semiconductor thin film is higher than that of the conventional method. It is about 1/40, which improves the throughput of the plasma CVD process of the thin film transistor element array for liquid crystal display and the photodiode array for secret image sensor, and has a great effect on improving the yield.

[Simple explanation of drawings]

[Figure 1]

Waveform diagram showing the oscillation state of the gate pulse and high frequency power of the present invention [Figure 2]

Waveform diagram showing the gate pulse used in the first embodiment of the present invention [Fig. 3]

Characteristic diagram showing the effect of the second embodiment of the present invention [Fig. 4]

Waveform diagram showing the oscillation state of gate pulse and high frequency power in the conventional simple pulse modulation intermittent discharge plasma CVD method [Fig. 5]

Waveform diagram showing the oscillation state of high-frequency power in the conventional continuous discharge film forming method [Explanation of symbols]

1 High duty ratio pulse train 2 Low duty ratio pulse train 3 Discharge period 4 Discharge pause time 5 long pause pulse