JPH0697078A - Manufacture of amorphous silicon thin film - Google Patents

Abstract

PURPOSE:To provide a manufacture of an amorphous silicon thin film used in solar battery or the like at a high quality and a low cost. CONSTITUTION:In a manufacture of an amorphous silicon thin film by a plasma CVD method using a high-frequency discharge, a silicon compound gas such as SiH4 is used as a main raw gas, and a frequency f (MHz) of a high-frequency power source 405 is 30MHz or higher, and a negative voltage is applied to a substrate side electrode 401 in order to control an ion irradiation to the surface of a substrate 402. Also, in the manufacture of the amorphous silicon thin film, a relation of the distance d (cm) between the electrodes 401 and 403 to the frequency f (MHz) of the high-frequency power source 405 is satisfied with f/d <30.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an amorphous semiconductor thin film, and in particular, a thin film device such as a thin film transistor (hereinafter abbreviated as TFT), a thin film transistor type photosensor, a solar cell, an electrophotographic photoreceptor and the like. Amorphous silicon thin film (hereinafter referred to as a-Si thin film)
Manufacturing method.

[0002]

2. Description of the Related Art Conventionally, various CVD methods such as a vacuum vapor deposition method, a sputtering method, a plasma CVD method and a photo CVD method have been proposed as a method for depositing an a-Si thin film. Among them, a semiconductor gas such as SiH ₄ gas is diluted with such as H ₂ gas as required, RF plasma CVD method for forming a film on a substrate by glow discharge decomposition of a frequency 13.56MH _Z is good Since the a-Si thin film can be deposited, it has become widely used. This method is described in C.
It was started by Chittic et al. E. Spear
This is the first amorphous semiconductor to enable pn control of electrical conduction by impurities by this method, and it has received a great deal of attention since then. Typical applications include solar cells, photosensors, and electrophotographic photoreceptors. . This R
It is considered that the a-Si thin film formed by the F plasma CVD method contains several 10% of hydrogen, and this hydrogen makes it possible to control the photoconductivity of the a-Si thin film and the electric conductivity by impurities. .

On the other hand, a hydrogen-free a-Si film formed by a vacuum evaporation method or a sputtering method has a spin density of about 10 ²⁰ cm ^-3 and a very high dangling bond density.

From the above, it is considered that in the hydrogenated a-Si film formed by the RF plasma CVD method, hydrogen plays an important role in improving the film quality. Then, the spin density decreases to about 10 ¹⁶ cm ⁻³ , and pn control by impurities becomes possible. It was confirmed that a-Si by such RF plasma CVD method contained about 10% of hydrogen, and it was estimated that hydrogen terminated the dangling bond in the a-Si film and improved the film quality. If hydrogen has such a function, even if it is a vacuum deposition method or a sputtering method that does not use hydrogen-containing reaction gas such as SiH ₄ in the RF plasma CVD method, if hydrogen is supplied at the time of film formation, it becomes dangling. This fact was confirmed with the expectation that ring bonds could be reduced, and the role of hydrogen as a dangling bond terminator in the a-Si film became widely accepted.

Further, at present, from various experimental results, the film growth process by the RF plasma CVD method using SiH ₄ gas as a raw material is generally considered to be classified into the following processes.

(1) Radical generation process A process in which electrons repeatedly generate inelastic collisions with SiH ₄ molecules to generate various radicals, ions, and atoms in plasma. The main reaction precursors for film growth are SiH ₂ and SiH ₃
The possibility of radicals is high.

(2) Radical Transport Process A process in which neutral radicals generated in plasma are transported to the substrate surface by diffusion while causing various secondary chemical reactions mainly with SiH ₄ molecules. Considering the generation rate of radicals in plasma and the reaction lifetime in the transport process, it is presumed that most of the particles that reach the surface of the substrate are SiH ₃ radicals. However, when the arrival density of radicals such as Si, SiH, and SiH ₂ increases, the quality of the film deteriorates due to the difference in the reaction mode on the surface.

(3) Surface reaction process A process in which radicals reaching the film growth surface are adsorbed on the surface and diffused on the surface to form stable sites and chemical bonds to form an amorphous network. If the surface is covered with hydrogen at a high substrate temperature, SiH ₃ radicals are sufficiently diffused on the surface, and chemically bond with stable sites to obtain a high quality film.

According to the above-mentioned film forming mechanism, a-Si
By selectively diffusing SiH ₃ radicals, which are reaction precursors of deposition, to the substrate surface and diffusing the substrate surface,
A high quality a-Si film is formed. At this time, it is considered that the substrate surface is covered with hydrogen, which is important for promoting the surface mobility of radicals. Furthermore, the surface reaction SiH ₃ radicals abstraction surface hydrogen in the hydrogen-coated surface, the site with other SiH ₃ radicals is thought to carry out the chemical reactions.

Based on such an idea, in order to improve the quality of the a-Si film, attempts have generally been made to improve the manufacturing conditions such as the film forming temperature, the raw material gas flow rate, the pressure, and the input electric power. It is only confirmed that the film forming temperature has an optimum value, and that the film quality deteriorates when the film forming speed is increased. That is, as shown in FIG. 1, the film formation temperature has a substrate temperature at which the spin density is minimized.

Further, if the film formation speed is increased, increase in hydrogen in the film, increase in dangling bond density, etc., which are presumed to deteriorate the characteristics of the a-Si thin film, occur. For example, as shown in FIG. 2, the photoconductivity σp (S / cm), which is a basic characteristic of the thin film, decreases as the film formation rate increases.

After all, according to this method of manufacturing an a-Si thin film,
A film forming condition capable of maintaining device characteristics is a temperature of 250.
The film forming rate is around 0.2 to 2 Å / sec at around ℃. The raw material gas flow rate, pressure, input electric power, etc. are used. As the a-Si film quality, the spin density is about 10 ¹⁶ to 10 ¹⁷ , and the hydrogen concentration is It is around 10%. Of course, these physical property values are not considered to be the best values. For example, if hydrogen is necessary to terminate the dangling bond, it is considered that the hydrogen concentration of 1% is sufficient.

Therefore, various methods have been proposed for further improving the quality of the a-Si film by using such an RF plasma CVD method as a base.

As an example of a film forming technique which has recently received special attention, there is a chemical annealing method reported by Shimizu et al. Of Tokyo Institute of Technology in Applied Physics Volume 59 (1990) P1618. It is considered that this is because the photodegradation of the a-Si film is caused by the nonuniformity of the Si network structure, and attempts to stabilize the structure by densifying the Si network structure. The method aims at controlling the structure formation process involving hydrogen desorption on the film growth surface, and is achieved by supplying atomic hydrogen having a strong chemical interaction with Si. A large amount of atomic hydrogen is generated by microwave plasma and is transported to the deposition part. Si
H ₄ is decomposed by a normal RF glow discharge and deposited on the substrate, but the SiH ₄ supply time (T ₁ second) is controlled, and this film deposition and atomic hydrogen treatment on the surface of the deposited film (T ₂ seconds). Is repeated. By repeating such deposition surface treatment, the amount of hydrogen in the a-Si film is reduced to about 1%. As a result, improvements in carrier mobility and photodegradation have been confirmed.

However, the inventors of the present invention have invented a method of forming a high quality film based on a different idea, without using the complicated apparatus structure and the complicated film forming conditions as described above. Specifically, the present inventors have previously proposed VHF.
The most suitable range among the film forming methods is disclosed in Japanese Patent Application No. 4-10021.
The application was filed in No. 9, and this time it has been improved.

Various investigation examples relating to the VHF film formation method have been reported until now.

First, the 90th Fall and the 1991 Spring Joint Lectures on Applied Physics (28p-MF-14, 28p-S-4)
Then, Oda et al. Of Tokyo Institute of Technology discharges at a high frequency of 144 MHz to form and evaluate an a-Si film.

Further, as an effect of frequency, by increasing the frequency, it is possible to create a spatially uniform discharge over the film formation surface, and to realize a uniform film formation rate. JP-A-3-64466 It is disclosed in the official gazette.

Further, Japanese Patent Laid-Open No. 2-225674 describes a frequency of 1 KHz to 100 MHz.

Further, in US Pat. No. 4,933,203, V
A thin film is investigated using a high frequency in the HF band, and the relationship between the frequency and the distance between electrodes is optimized.

Although there are many reports relating to VHF as described above,
In many cases, the experiment is simply performed by raising the frequency to the VHF band.

[0022]

With the recent technological development, TFT type liquid crystal televisions, optical sensors, solar cells,
A- used in thin film devices such as electrophotographic photosensitive drums
High quality Si thin films are required, but as in the past,
The a-Si thin film using 13.56 MHz RF discharge still has the following problems.

(1) Problems concerning basic characteristics of thin film a. In the conventional a-Si film having a small photoconductivity by the RF glow discharge, the dangling bond density is about 10 ¹⁵ cm ⁻³ , and it is difficult to reduce the density below this value. This improvement can be expected to improve the photoconductivity.

B. Photodegradation is large The problem of photodegradation of photoconductivity, which has been a problem in conventional a-Si, has not been completely clarified.
It is believed to be due to a weak bond by bonding to. Therefore, there is a possibility that photo-deterioration can be improved by reducing hydrogen. The conventional GD method contains about several tens of percent of hydrogen.

C. Carrier mobility is low By decreasing the hydrogen concentration, carrier mobility may increase. It is possible to speed up the device, which is a drawback of the conventional a-Si film.

(2) Problems with Price High-quality films that can be used in thin-film devices have a low film-forming rate, and their production capacity does not increase, making it difficult to reduce the cost.

In other words, improving the basic characteristics of the a-Si thin film and at the same time realizing low cost, that is, high-speed film formation are still major problems to be solved.

[0028]

According to the present invention, in the plasma CVD method, the frequency f of the high frequency power source is specified to be 30 MHz or higher, preferably 50 MHz or higher and 100 MHz or lower, using a silicon compound as a main raw material gas.
By applying a negative voltage to the substrate-side electrode so that the irradiation of ions on the substrate surface is controlled, the thin film characteristics are improved and high-speed film formation becomes possible.

According to the conventional concept, as described above, if the film quality is improved by simply terminating the dangling bonds in the a-Si film by the surface coating hydrogen, the low substrate temperature is used. Since hydrogen is sufficiently adsorbed, the dangling bond density should be low. However, the dangling bond density actually increases unexpectedly.

The present inventors considered the cause as follows.
That is, the role of the surface-covering hydrogen is not to directly terminate the dangling bond, but it is merely the reaction precursor S.
It plays a role of enhancing the surface mobility of iH ₃ radicals, and since the surface mobility is promoted, it is considered that the dangling bonds are reduced as a result.
The reason why the dangling bond density becomes high at the low substrate temperature described above is that the surface mobility of the reaction precursor is low even though the substrate surface is covered with hydrogen because the substrate temperature is low. Can be estimated.

According to the above-mentioned idea, the present inventors have recognized that it is important to enhance the surface mobility of SiH ₃ radicals in order to produce a high quality a-Si thin film. I got the following guidelines.

[0032] First, to terminate the dangling bonds to increase the surface mobility of SiH ₃ radicals, further to the second, and the SiH ₃ radicals that reach adsorbed on the substrate it is necessary to increase Thought. The grounds for reaching the above conclusion will be specifically described below.

It is considered that hydrogen is always adsorbed and desorbed on the surface of the substrate. It is considered that the dangling bonds in the a-Si film are generated when surface hydrogen is desorbed and remains without being terminated by hydrogen or SiH ₃ radicals.

1) Surface state σ-SiH → σ-Si + (1/2) H ₂ (1) σ-SiH ← σ-Si + (1/2) H ₂ (2) where σ is the substrate surface Is the Si active point. When the substrate temperature is low, the equation 2) predominantly progresses, and when the substrate temperature is high, the equation 1) predominantly progresses. Roughly speaking, at a low substrate temperature of 300 ° C. or lower, the film forming surface is a hydrogen-covered surface, and at a high temperature of 300 ° C. or more, the film forming surface is a surface with dangling bonds. As an example that represents this state indirectly but well, there is a relationship between the film formation rate and the substrate temperature. That is, when the substrate temperature is 300 ° C. or higher, the film formation rate increases as the substrate temperature rises. On the contrary, at 300 ° C. or lower, the film formation rate becomes constant regardless of the substrate temperature. It can be estimated that the film-forming surface is a constant surface when the substrate temperature is 300 ° C. or lower. Generally, the surface is covered with hydrogen, and the SiH ₃ radical is a bimolecular reaction in which hydrogen is once extracted and then reacted with another SiH ₃ radical. On the other hand, when the temperature is 300 ° C. or higher, the state of the film formation surface changes with the rise of the substrate temperature. That is, considering that hydrogen on the surface is peeled off and SiH ₃ radicals react there, the film formation rate is increased by more peeled hydrogen.

Next, it is considered that the SiH ₃ radicals generated in the plasma diffuse on the surface of the substrate and react with the active points.

2) Surface reaction σ-SiH + SiH _{3 (act)} → σ-SiH ₃ + (1/2) H ₂ (3) σ-Si + SiH _{3 (act)} → σ-SiH ₃ (4) where 3 ), 4) SiH _{3 (act)} is a SiH ₃ radical that is in a state capable of reacting with an active point on the substrate surface, that is, is adsorbed on the surface and has a high reactivity that can be transferred to the active point by diffusion. Think of it as a radical. Where 2)
It can be seen that it is necessary to increase SiH _{3 (act)} in order to terminate the dangling bond represented by the formula by the reaction of the formula 4). That is, a large amount of SiH ₃ radicals are diffused into the substrate, and further the SiH ₃ radicals adsorbed on the substrate surface are activated. The equation (2) simply increases the hydrogen in the film.

According to the present invention, in the plasma CVD method, the frequency f of the high frequency power source is set to 30 MH in order to increase SiH ₃ radicals using a silicon compound as a main raw material gas.
z or more, preferably 50 MHz or more and 100 MHz or less, and the substrate-side electrode is controlled so that the ion irradiation to the substrate surface for activating SiH ₃ radicals on the film-forming surface and sufficiently performing surface diffusion is controlled. By applying a negative voltage, thin film characteristics are improved and high-speed film formation becomes possible.

[0038]

【Example】

[Example 1] The present inventors have focused on the power supply frequency in plasma CVD as a method of increasing SiH ₃ radicals with high efficiency. As an actual monitoring method, SiH ₃
The amount of radicals was indirectly determined by the film formation rate and the amount of light emission by plasma emission analysis. That, Si ^*, which can be confirmed by the plasma emission spectrometry, SiH ^*, H ₂ ^*, in H ^* light emitting line, Si ^*, SiH ^* emission intensity (hereinafter, respectively [Si ^*], denoted as [SiH ^*] ) And the film formation rate (DR) are correlated with each other, and since SiH ₃ is the main radical contributing to film formation, the amount of SiH ₃ radicals was indirectly confirmed. Of course, since it is necessary to increase only SiH ₃ radicals, it is premised that the film quality of the a-Si film at this time is not extremely deteriorated as compared with the conventional case. This is because it is presumed that the deterioration of the film quality is caused by the generation of Si and SiH ₂ radicals.

Further, in the present invention, among these, SiH ^* (4
14 nm) and the H ^* (656 nm) emission intensity relationship,
That is, it became clear that the magnitude of the strength is deeply related to the quality of the a-Si thin film. In particular, the emission intensity ratio [H ^* ] /
It is desirable to adopt a film forming condition such that [SiH ^* ] becomes the minimum value, but if [H ^* ] ≦ [SiH ^* ], a substantially satisfactory a-Si thin film can be obtained. FIG. 3 shows the relationship between the emission intensity ratio [H ^* ] / [SiH ^* ] and the hydrogen bonding mode in the thin film as the a-Si film quality.

[0040] In general, the absorption peak appearing in 2000cm ^-1 ~2100cm ^-1 in infrared absorption analysis in a-Si thin film, Si-H stretching vibration bond (2000c
m ^-1 ) and stretching vibration of Si-H ₂ bond (2100 cm
^-1 ), 2000 cm ^{-1 to} 2100 cm
The median value Rm (cm ^-1 ) of the absorption peak appearing at ^-1 can be considered to represent the ratio of SiH bond and SiH ₂ bond contained in the thin film. Here, for example, the median value Rm is 2
If it has changed from 000 cm ^-1 to 2100 cm ^-1 , S
This is an increase in iH ₂ bonds, and chain bonds or cyclic bonds of Si are contained in the film to generate defects, thus deteriorating the film quality.

That is, as shown in FIG. 3, the emission intensity ratio [H
^* ] / [SiH ^* ] increases, the median value Rm is also 21
In other words, the shift to the 00 cm ⁻¹ side means that if the emission intensity ratio [H ^* ] / [SiH ^* ] increases, then a−S
It can be said that the i film quality is deteriorated. In order to obtain a satisfactory a-Si film from the view of the present inventors, at least [H ^* ] ≦
We believe that it must be [SiH ^* ].

Next, the distance between electrodes, which is a component of the present invention, will be described. As shown in FIG. 4, the inter-electrode distance d
When the discharge frequency f (MHz) is large, the film thickness distribution T (%) in the substrate may become large depending on (cm). From the view of the present inventors, it was confirmed that the film thickness distribution mainly depends on the distance between the electrodes, and the film thickness distribution can be suppressed to the minimum by increasing the distance between the electrodes. When the relationship in which the distribution T (%) within the substrate is within 10% was obtained under various film forming conditions of the present invention, the inter-electrode distance d = 2 in the figure.
(Cm) has a remarkably large distribution and cannot be used, and f / d <30 (MHz /
It was found that a good distribution can be obtained with d satisfying cm).

FIG. 5 shows the relationship between the interelectrode distance and the defect level density in the film under certain film forming conditions. Distance between electrodes d ≧ 4c
If m, the defect density decreases, but the distance between electrodes d <4c
It can be seen that the defect density increases with m. It can be seen that the distance between the electrodes is preferably 4 cm or more. Therefore, in the examples of the present invention, the distance between the electrodes was set to 5 cm.

First, the manufacturing apparatus used in this embodiment is shown in FIG. As a basic structure, a conventional parallel plate type plasma C
It is similar to the VD device. As shown in the figure, 400 is a vacuum chamber, 401 is an anode electrode, 402 is a substrate, 4
Reference numeral 03 is a cathode electrode. 420 is a power supply terminal,
A bias voltage can be applied to the substrate as needed. Also,
A heater for heating the substrate (not shown) is built in the anode electrode 401. 404 is a matching unit, 405 is a high frequency power source, 407 is a gate valve, 408 is a turbo molecular pump, and 409 is a rotary pump. 410, 41
Mass flow controllers 1 and 412 are supplied from a gas cylinder (not shown). The electrode area is about 30
At 0 cm ² , the distance between the electrodes is 5 cm.

In this embodiment, SiH ₄ is used as a source gas,
Ar was introduced into the vacuum chamber 400 as a diluent gas, and was discharged at a frequency f (MHz) by the high frequency power source 405. The substrate temperature is 250 ° C by the substrate overheater
Here, the substrate bias is set to 0 v in order to simplify the description of the present invention. At this time, in order to obtain a satisfactory a-Si thin film, the relation of the emission intensity is [H ^* ] as described above.
≦ [SiH ^* ], preferably [H ^* ] / [SiH
^* ] Is the minimum pressure P (Torr) and input power Pw
Select (W / cm ² ) to form a film. Pressure is approximately 0.25To
rr or more and 2.5 Torr or less, and the input power is approximately 0.
It was 3 W / cm ² or less.

FIG. 7 shows the relationship between the power supply frequency f and the film formation rate DR. Further, FIG. 8 shows the relationship between the SiH ^* emission intensity and the film formation rate DR at this time. It can be estimated that the SiH ₃ radicals increase by increasing the discharge frequency, and it can be expected that there is a correlation between the SiH ₃ radicals and the film formation rate. Here, the film formation rate DR decreases in the region where the discharge frequency f exceeds 120 MHz, but it is estimated that this is a problem peculiar to the apparatus of the present invention and the effective input power is decreased. .

Next, as a relation with the film quality, the discharge frequency f
FIG. 9 shows the relationship between and the median value Rm in infrared absorption analysis.
FIG. 10 shows the relationship between the discharge frequency f and the spin density Ns.
Shown in. According to this result, it is understood that the film quality is improved when the discharge frequency is 30 MHz or more and 120 MHz or less. The reason for this is that in a region where the frequency f is less than 30 MHz, ion damage (details described later) incident on the substrate surface is large, so that the film contains a large amount of SiH ₂ bonds and has many defects. Further, the discharge frequency f is 120
Similarly, in the region over MHz, the content of SiH ₂ bonds in the film increases and the film quality deteriorates. It is SiH
_{This is because the four} molecules are highly decomposed and Si, SiH, and SiH ₂ radicals are also increased at the same time. It is also the point of the present invention described later, but since the ion energy incident on the substrate surface is small, the surface mobility of SiH ₃ is It is unclear at present, probably because it has decreased.

FIG. 11 shows the frequency dependence of the S / N ratio, which is the ratio of the photoconductivity σp (S / cm) and the dark conductivity σd (S / cm). Discharge frequency is 50MHz or more 100
If it is less than or equal to MHz, improvement of photoelectric characteristics can be expected.

From these results, the substrate bias Vb =
If the frequency in the case of 0 v is in the range of 30 MHz or more and 120 MHz or less, the film formation rate can be increased without degrading the film quality of the a-Si film.

As described above, in order to realize a higher quality a-Si film, it is necessary to increase SiH ₃ radicals and further increase the mobility on the substrate surface. Therefore, the inventors of the present invention set the discharge frequency to 30 MHz or more in order to increase the SiH ₃ radicals necessary for a high quality film, and further apply a negative bias to the substrate, and a large amount of reactive radical SiH 3 adsorbed on the surface. _It was possible to activate ₃ to increase the mobility on the substrate surface and realize a high quality a-Si film. In other words, a negative bias is applied to the substrate, ions are positively made to collide with the film formation surface, and the SiH ₃ radicals are surface-diffused by the kinetic energy.

FIG. 12 shows the invention of 30 MHz to 120M.
The relationship between each discharge frequency of Hz and the substrate bias Vb at 13.56 MHz and the median value Rm of the infrared absorption analysis is shown. Similarly, FIG. 13 shows the relationship between the substrate bias Vb and the spin density Ns. The substrate bias Vb is the distance between electrodes d (c
m) and the applied voltage V (v), Vb = V / d (v / c
m). What is important here is that the substrate bias has an optimum value in the frequency range of the present invention, and improvement of the film quality can be expected by applying a negative bias, but if the negative substrate bias is made too large, the reverse effect occurs. Cause deterioration of film quality.

Here, as the optimum substrate bias potential Vc, an applied potential which has the lowest spin density Ns and which does not increase the median value Rm of the infrared absorption analysis is shown in FIG. The relationship between f and the spin density Ns is shown.

On the other hand, at 13.56 MHz in the related art, the film quality is improved by applying a positive substrate bias rather than making the substrate bias negative. This is because of the 13.
The RF plasma CVD method at 56 MHz is generally known as a method for improving film quality. Based on the same idea, a third electrode is provided between the substrate and the cathode, and a positive bias is applied to this electrode. There is also a method.

Why is the frequency band of the present invention different from the conventional optimum substrate bias at 13.56 MHz, such as positive and negative? Before describing the reason, the ion damage to the substrate due to the discharge frequency will be described.

A mass spectrometer was installed at the substrate position, and the distribution of the incident energy of ions and the distribution of the incident amount were obtained. here,
In order to simplify the analysis, only Ar gas was introduced and the distribution of the incident energy and the incident amount of Ar ⁺ ions incident on the substrate was obtained. FIG. 15 shows a conventional 13.56.
The case is shown at MHz and 80 MHz according to the present invention. f
= 80MHz, the average incident energy becomes low,
Moreover, the energy distribution is sharp. Conversely, 13.
At 56 MHz, there are many high energy ions and the distribution is bad. According to the estimation by the present inventors, at 13.56 MHz in the related art, the ions are still oscillating according to the frequency, and it is estimated that the ions may be completely stopped by the increase of the discharge frequency.

That is, the difference between the substrate biases depending on the discharge frequencies in FIGS. 12 and 13 is that unnecessary high energy ions are incident when a negative bias is applied to the substrate at a low frequency such as 13.56 MHz. Rather, it is better to make the substrate bias positive to prevent high energy ions from entering. Ion damage to the film formation surface, which has been conventionally considered to be the greatest drawback of the RF plasma CVD method, can be expected to be the above-mentioned high-energy ions.

On the contrary, at a high frequency of 30 MHz or higher, the ion energy distribution is uniform, so that it is easy to control, and ions are positively made to collide with the substrate by setting a negative substrate bias, so that they are reactive radicals on the film growth surface. SiH ₃
This is because it is considered that there is an effect that energy is applied to the substrate to enhance the mobility of SiH ₃ radicals on the substrate surface. Therefore, if the negative substrate bias is too small, the surface mobility of SiH ₃ radicals cannot be increased, and if the negative substrate bias is too large, ion damage is caused. Therefore, it can be understood that there exists an optimum bias Vc in which the spin density has a minimum value depending on the discharge frequency. It is desirable that the substrate irradiation ions are irradiated with ions having the same or higher atomic weight as Si atoms than light elements such as hydrogen and helium that easily diffuse into the film. Actually, it is unclear how the substrate surface state is changed by the ion irradiation and how the SiH ₃ radicals are activated, but it is presumed by the present inventors that the ion irradiation causes the substrate surface of the substrate surface to be changed. The extreme surface, that is, several atomic layers on the surface is in a high temperature state, that is, in an activated state. Therefore, hydrogen is desorbed from the surface, and SiH ₃ radicals that reach the surface are adsorbed to obtain energy and diffuse. However, since the surface is reactive, the diffusion distance of radicals is short. However, since the number of radicals is much larger than in the past, the active sites on the surface are evenly distributed over SiH.
It is estimated that ₃ radicals can be diffused.

In the discharge frequency range of 30 MHz or higher which can increase the SiH ₃ radicals of the present invention with high efficiency, as shown in FIG. 16, the optimum bias voltage Vc is proportional to the increase of the discharge frequency f. It increases to the negative side. At the same time, since it also depends on the inter-electrode distance, when the voltage applied to the substrate-side electrode is Vb (v) and the inter-electrode distance d (cm) is considered, 0> Vb / d ≧ approximately −12
If it is (v / cm), improvement in film quality can be expected.

Further, as a feature of the present invention, the in-film hydrogen amount C _{H with} respect to each discharge frequency f under the optimum bias Vc does not change largely, but becomes approximately several percent, and the conventional substrate bias Vb = 0v, f = 13. The hydrogen concentration C _H was greatly improved compared to several 10% of the hydrogen concentration C _H by glow discharge at 0.56 MHz.
Further, as shown in FIG. 17, the substrate temperature Ts at the hydrogen concentration C _H is
It was also confirmed that the dependence was small. In the figure, as an example of the present invention, the discharge frequency f = 80 MHz, the substrate bias Vc
= -40V and the discharge frequency f = 1 as a conventional example
The one with 3.56 MHz and the substrate bias Vb = 0 v is shown. 18 shows the substrate temperature Ts and the spin density Ns.
The relationship with Similarly, it can be seen that the dependence on the substrate temperature Ts is reduced. The view here is that, even if the substrate temperature Ts is lower than that of the conventional case,
It is possible to obtain a sufficient a-Si film quality by using SiH ₃
It was further confirmed that the substrate bias Vb is an effective means for increasing the surface mobility of radicals.

Next, a comparison was made regarding photodegradation, specifically, after irradiating with white light of 10 mW / cm ² at V _D = 10 (v) for a certain period of time, changes in dark conductivity were measured and compared.
Here, the change is shown as a ratio to the initial dark conductivity. As shown in FIG. 19, it can be seen that the a-Si film of the present invention is less deteriorated than the conventional a-Si film as expected. This indicates that the film has a low defect such as a low hydrogen concentration and a low spin density. Example 2 Next, an example of a field effect transistor using an a-Si film manufactured by such a film forming method will be described.

FIG. 20 is a sectional view of an inverted stagger type TFT. The gate electrode 12 is formed on the insulating substrate 11, and the insulating layer 13 and the semiconductor layer 14 are further stacked thereon. Source / drain electrodes 16 are formed on the semiconductor layer 14 via an ohmic contact layer 15. Then, it is covered with the protective film 17. Next, a method for manufacturing this TFT will be described with reference to FIGS.

First, as shown in FIG. 21A, a Cr thin film (about 1000 Å) is formed on a Corning 7059 glass substrate 21 by a sputtering device and patterned to form a gate electrode 22.

After that, as the gate insulating layer 23, silicon nitride, SiNx (about 30
00 Å) A thin film is formed, and then, as the semiconductor layer 24,
Non-doped amorphous silicon, i-type a-Si (about 6000
Å) Thin film, as ohmic contact layer 25, phosphorus-doped microcrystalline silicon, n ⁺ -type μc-Si (about 1000
Å) Thin films are sequentially formed using the same equipment.

Second, as shown in FIG. 21B, an Al thin film (about 1 μm) is formed by a sputtering apparatus and patterned to form source / drain electrodes 26. The channel width w and the channel length L were set to W / L = 100.

Third, as shown in FIG. 21 (c), unnecessary n ⁺ type μc-S is formed by reactive ion etching.
The i layer is etched to form the gap portion 28.

Fourth, as shown in FIG. 21D, an unnecessary SiNx / i-type a-Si / n ⁺ -type μc-Si layer is further isolated, and then a protective film 27 is deposited. The thin film transistor is manufactured.

Here, a-Si which is the point of the present invention
The thin film manufacturing method will be described in detail.

As described in the first embodiment, the SiNx / i type a-Si / n ⁺ type μc-Si layer is formed by the load lock type plasma CVD apparatus as shown in FIG. In the figure, the detailed mechanism for continuously forming the SiNx / i-type a-Si / n ⁺ -type μc-Si layer is not shown. Descriptions are omitted except for the i-type a-Si layer deposition chamber of the present invention. In the figure, 300 is a vacuum chamber, 301 is a substrate, 302 is an anode electrode, 303 is a cathode electrode, 304 is a heater for heating a substrate, 305 is a bias applying terminal, 306 is a matching box, 307.
Is a high frequency power supply, 308 is an exhaust pump, 309 and 310
Is a sluice valve for the front and rear chambers. 320 is a raw material gas inlet,
321, 322, 323, 324 are valves, 325, 3
26 is a mass flow controller.

As described above, the substrate is loaded from the load chamber,
After the film is formed in the SiNx film forming chamber in the front chamber, it is carried in, and the chamber is evacuated to 1 * 10 ^-6 Torr or less.
Next, the source gas SiH ₄ and Ar are supplied by the mass flow controllers 325 and 326, and the pressure is 0.5T.
Orr and maintain the substrate heater 304,
After holding until the substrate temperature reaches 300 ° C, the high frequency power supply 307 sets the frequency to 80 MHz and the power to 0.04 W / c.
m ² and a substrate bias of −40 v are applied to the i-type a-Si
A film is formed at 6000Å. After the film formation is completed, the chamber is similarly evacuated to 1 * 10 ⁻⁶ Torr or less. Next, the film is moved to the n ⁺ -type μc-Si film forming chamber in the next chamber, a film is formed, and taken out from the unloading chamber in the next chamber.

A thin film transistor can be manufactured in this manner. As an example of the present invention, a high frequency discharge of 80 MHz was taken as an example. Of course, when the frequency f is changed, the input power Pw (W / cm ² ) and the pressure P (Tor.
r) is changed, and the emission intensity ratio [H ^* ] is obtained by emission spectroscopy.
/ [SiH ^* ] is set to a minimum value, and an optimum substrate bias Vc is applied. Conventional f = 13.56 MH
The electric field mobility μ of the a-Si thin film at z and the substrate bias Vb = 0 v is 0.5 cm ² / Vs, but f = 8 of the present invention.
With 0 MHz and substrate bias Vb = −8 v / cm applied,
It became 1.3 cm ² / Vs.

Further, as a result of the higher quality of the a-Si thin film, there is a difference in photodegradation shown in FIG. In the figure, as an example of the present invention, the above-mentioned f = 80 MHz and Vb =
Confirmation was performed using a TFT prepared at −8 v / cm. The change of the photoconductivity with respect to the initial stage under the irradiation of 500 lx of the gap 550 nm of this TFT at a source-drain voltage of 10 v and a gate voltage of 0 v is expressed as a function of irradiation time. It can be understood that the photodegradation has been remarkably improved. [Embodiment 3] As another embodiment of the present invention, by providing a third electrode between the anode electrode and the cathode electrode and applying a negative bias to this electrode, the same effect as described above can be obtained. it can. At this time, the distance between the electrodes, which is a component of the present invention, is the distance between the third electrode and the cathode electrode.

[0072]

As described above, according to the present invention, the frequency of the high frequency power source is 30 MHz in plasma CVD.
By setting z or more and applying a negative substrate bias,
The a-Si thin film can be manufactured inexpensively and with high quality. In particular, in thin film transistors, thin film transistor type optical sensors, solar cells, etc., improvements in electric field mobility, photoelectric characteristics, reliability, etc. can be achieved.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a film forming temperature, a hydrogen concentration, and a spin density.

FIG. 2 is a diagram showing a relationship between a film formation rate and photoconductivity.

FIG. 3 is a diagram showing a relationship between an emission intensity ratio and a hydrogen bond state.

FIG. 4 is a diagram showing a relationship between frequency and film thickness distribution.

FIG. 5 is a diagram showing a relationship between a distance between electrodes and a defect level density.

FIG. 6 is a schematic configuration diagram of a film forming apparatus.

FIG. 7 is a diagram showing a relationship between frequency and film formation rate.

FIG. 8 is a diagram showing a relationship between emission intensity and film formation rate.

FIG. 9 is a diagram showing a relationship between frequency and hydrogen bonding state.

FIG. 10 is a diagram showing a relationship between frequency and spin density.

FIG. 11 is a diagram showing a relationship between frequency and S / N ratio.

FIG. 12 is a diagram showing a relationship between a substrate bias and a hydrogen bond state.

FIG. 13 is a diagram showing the relationship between substrate bias and spin density.

FIG. 14 is a diagram showing a relationship between frequency and spin density.

FIG. 15 is a diagram showing substrate incident energy due to a difference in frequency.

FIG. 16 is a diagram showing the relationship between frequency and optimum substrate bias.

FIG. 17 is a graph showing the relationship between substrate temperature and hydrogen concentration according to the present invention.

FIG. 18 is a graph showing the relationship between substrate temperature and spin density according to the present invention.

FIG. 19 is a diagram showing light deterioration of the present invention.

FIG. 20 is a sectional view of a TFT.

FIG. 21 is a diagram showing a method of manufacturing a TFT.

FIG. 22 is a view showing a continuous film forming apparatus.

FIG. 23 is a diagram showing light deterioration of the present invention.

[Explanation of sign]

11, 21 glass substrate 12, 22 gate electrode 13, 23 gate insulating layer 14, 24 i-type semiconductor layer 15, 25 n ⁺ type semiconductor layer 16, 26 source / drain electrode 17, 27 protective film 300 vacuum chamber 301 substrate 302 anode Electrode 303 Cathode electrode 304 Substrate heating heater 305 Substrate bias application terminal 306 Matching box 307 High frequency power source 308 Exhaust pumps 309, 310 Gate valves 325, 326 Mass flow controller

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hidemasa Mizutani 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (3)

[Claims] 1. A method for producing an amorphous silicon thin film by a plasma CVD method using a high frequency discharge, wherein a frequency f (MHz) of a high frequency power source is 30 MHz or more, with a silicon compound gas as a main raw material gas, and a substrate surface. A method for producing an amorphous silicon thin film, characterized in that a voltage at which the substrate surface has a negative potential is applied to the substrate-side electrode so that ion irradiation to the substrate is controlled. 2. In the plasma CVD method, an inter-electrode distance d (cm) and a frequency f (MH) of the high frequency power source.
The method for producing an amorphous silicon thin film according to claim 1, wherein the relationship with z) satisfies f / d <30. 3. A distance d (cm) between electrodes and a voltage Vb applied to the substrate-side electrode in the plasma CVD method.
The method for producing an amorphous silicon thin film according to claim 1, wherein the relationship with (v) is Vb / d ≧ −12 (v / cm).