JPH01730A - Method of forming multilayer thin film - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for forming a multilayer thin film, which is suitably employed for forming an amorphous semiconductor superlattice thin film.

[Issues and problems to be solved by conventional techniques and inventions] Amorphous semiconductor thin films are being put to practical use in solar cells, thin film transistors, etc., but research results in terms of performance tend to be saturated. The main reason for this is that the mobility of carriers is low in amorphous semiconductors, which inherently have many defects. Amorphous semiconductor superlattices are attracting attention as a new material that can overcome this situation. .

An amorphous semiconductor superlattice is one in which two types of amorphous semiconductors (hereinafter referred to as AJI and B layers) with different compositions, band gaps, or doped impurities are stacked in several to several dozen large periods. Conventionally, methods for producing this amorphous semiconductor superlattice thin film include plasma or photo-excited CVD (chemical vapor deposition).
A method is adopted in which AWj and 119 are alternately layered by mechanically switching the introduction of raw material gas.

That is, a raw material gas for depositing Ai is introduced into a reaction chamber, the raw material gas is excited and decomposed by light or plasma, and 15
is deposited until the desired film thickness is reached.

Next, after stopping the supply of the raw material gas and evacuating the reaction chamber, the raw material gas for depositing the B layer was introduced.

Similarly, layer B is deposited until the desired thickness is achieved.

Next, one reaction chamber is evacuated again to prevent the layers A and B from contaminating each other at the boundary with impurities. A common method is to repeat this operation a predetermined number of times to produce a superlattice thin film. For example, when stacking a-8i (amorphous-5i) and a-SiC (amorphous-3iC), when depositing a-3i, the source gas containing silicon is decomposed, and when depositing a-8iC, the source gas containing silicon is decomposed. This method decomposes a mixed gas of a source gas containing silicon and a source gas containing carbon, and a superlattice thin film is produced by repeating the above operations.

However, the conventional superlattice thin film fabrication method using plasma or photo-excited CVD essentially requires mechanical switching of raw material gases, and each time the reaction to form a film of each layer is performed, The productivity of superlattice films was extremely inefficient because the process involved exhausting the raw material gas and creating a vacuum in the system.

Therefore, in the above method, it is necessary to switch the raw material gas, and the deposition of one layer is essentially one work step, so the work steps are enormous when producing a superlattice thin film, which is a multilayer film, and it is difficult to put it into practical use. There was a problem that was said to be a major hindrance.

In addition, the reaction chamber for forming Al and the reaction chamber for forming the B layer are adjacent to each other via a partition wall, and a rotating body (rotary) that alternately rotates between the two chambers at a predetermined speed is connected to the partition wall. There is also a method in which layers A and B are alternately laminated on the substrate by placing the substrate on this rotating body and rotating the substrate together with the rotating body, but this method is also very slow. ineffective.

The present invention has been made in order to improve the above-mentioned situation, and it is an object of the present invention to form a multilayer thin film such as an amorphous semiconductor superlattice thin film efficiently, easily, and reliably in one reaction chamber without repeating the introduction and exhaust of raw material gases. The purpose of the present invention is to provide a method for forming a multilayer thin film.

[Means and effects for solving the problems] In order to achieve the above object, the present invention provides a first raw material in which only one of the plasma-excited chemical reaction and the photo-excited chemical reaction is possible or preferentially occurs. A mixed gas of a gas and a second raw material gas that has significantly different reactivity to these chemical reactions than the above-mentioned raw materials, such as a raw material gas that can be re-reacted, is introduced into the reaction chamber, and plasma is excited in the reaction chamber. One of the chemical reaction and the photoexcitation chemical reaction is performed singly or preferentially and intermittently, and the other chemical reaction is performed intermittently or continuously in a phase different from that of the other chemical reaction, and the chemical reaction is performed on the substrate. A multilayer thin film is formed by sequentially and alternately laminating a thin film when one chemical reaction is carried out and a thin film when one chemical reaction is stopped.

That is, the present invention enables the supply of raw material gas to be carried out without interruption,
In order to continuously form a multilayer thin film such as a superlattice structure, it utilizes the difference in chemical reactivity of two or more raw material gases.
Two types of excitation sources are used simultaneously to excite the reactant gas. For example, a light source that generates excitation energy with a narrow distribution, such as a low-pressure mercury lamp or an excimer laser, and a discharge plasma that generates high-speed electrons with a wide energy distribution, such as high energy of 10 eV or more, are used.

Thus, the present invention discontinuously supplies either plasma excitation energy or optical excitation energy to a reaction chamber filled with a mixture of gases having mutually different optical reactivities, and supplies the other excitation energy to one of them. A method in which excitation energy is applied intermittently or continuously with a phase different from that of the excitation energy, and a thin film when one excitation energy is used and a thin film when one excitation energy is stopped are sequentially and alternately laminated on the substrate. For example, as raw material gases, we select one that can be decomposed by light or plasma, and another that can be decomposed by plasma but not by light, and constantly introduce a mixture of these raw material gases into the reaction chamber and remove it. By continuously irradiating light and pulsing the plasma by periodically turning the discharge on and off, it is possible to continuously switch between light-only CVD and CVD with combined excitation of light and plasma in the same reaction chamber. , this allows light-only CvD when the discharge is off.
When the discharge is on, a film with an alloy composition from both source gases is formed.

Therefore, according to the present invention, various desired multilayer thin films can be formed by selecting various mixed gases and reaction modes, and in this case, as described above, the mixed gas serving as the raw material can be continuously supplied, There is no need to repeat introduction and exhaust, there is only one reaction chamber, and there is no need to switch reaction chambers, so multilayer thin films can be formed efficiently, easily, and reliably, which eliminates the need for complicated equipment and high costs. This will not cause any inconvenience.

In addition, the superlattice film produced according to the present invention has, on average, superior semiconductor properties to a single layer film with the same composition, such as a conventional bulk film with a similar optical band gap. It has a very high photoelectric rate compared to the conventional monolayers, and compared to each of the monolayers that are laminated. Also,
Since plasma excitation and optical excitation occur repeatedly with different phases, the structure and properties of the resulting superlattice film are improved compared to those obtained by conventional gas exchange methods or single excitation methods. Therefore, according to the present invention, it is possible to obtain an amorphous semiconductor superlattice film that can be used in solar cells, optical sensors, photosensitive drums, light emitting elements, thin film transistors, etc. and has excellent properties.

The present invention will be explained in more detail below.

The method for forming a multilayer thin film of the present invention uses plasma-excited chemical reactions and photo-excited chemical reactions to alternately and sequentially form thin films with different compositions or band gaps in the same reaction chamber. Either a chemical reaction or a photoexcited chemical reaction is possible or occurs preferentially (a plasma-excited chemical reaction occurs preferentially over a photoexcited chemical reaction, or a photoexcited chemical reaction occurs over a plasma-excited chemical reaction). a first raw material gas (which occurs preferentially),
For these chemical reactions, a mixed gas with a second raw material gas whose reactivity is significantly different from that of the raw material is introduced.

Here, specific examples of this mixed gas include the following.

(1) A raw material gas in which only a plasma-excited chemical reaction (hereinafter referred to as P reaction) is possible or preferentially occurs, and a raw material gas in which only a light-excited chemical reaction (hereinafter referred to as L reaction) is possible or preferentially occurs. mixed gas.

(2) A mixed gas of a raw material gas in which only the P reaction is possible or occurs preferentially, and a raw material gas in which both the P reaction and the L reaction are possible.

(3) A mixed gas of a raw material gas in which only the L reaction is possible or occurs preferentially, and a raw material gas in which both the P reaction and the L reaction are possible.

Here, the layer obtained from the first source gas is different from the layer obtained from the second source gas.

In this case, in the present invention, a discharge plasma capable of excitation with high energy of 10 eV or more, particularly 10 to 20 eV by fast electrons can be suitably used for the P reaction, and an inductively coupled or capacitively coupled plasma using direct current or radio frequency can be suitably used. This can be carried out using a CVD device, a microwave CVD device, or the like. In addition, the L reaction has excellent monochromaticity of excitation energy,
In addition, light sources with a narrow distribution can be used effectively, such as ultraviolet light sources such as low-pressure mercury lamps, excimer lasers, deuterium discharge tubes, rare gas discharge tubes, etc. In particular, photochemical reactions induced by light in the wavelength range of 100 to 300 This can be done by using . Therefore, raw material gases that can or preferentially cause P and L reactions include these reactions,
React depending on the method.

Any material that can form a thin film can be used.

Specifically, considering plasma CVD as a P reaction and mercury-sensitized CVD as an L reaction, raw material gases that are capable of only a P reaction or that have a significant priority over an L reaction include CF, CH1, SiF, .

B F,,NF3. N, etc., and one or more of these may be selected and used depending on the type of thin film to be formed. In addition, as a raw material gas in which only the L reaction is possible or preferentially occurs, GeF4. , GeH4, etc., and one or more of these may be selectively used. Furthermore, as a raw material gas capable of both P reaction and L reaction, S
iH,,Si2H6,SL,Hs*S IHn R(4
-nl l S IHn F (4-n) + A Q
R3t G'e R3tZ n R2y S n R4
(However, n = 1.2 or 3, R represents an alkyl group or an aryl group, and R has 1 to 7 carbon atoms, especially 1 to 4 when R is an alkyl group, and when R is an aryl group. is 6-7
), and one or more of these may be used. In addition, in the present invention, the L reaction includes a dark reaction in which the reaction is excited by light irradiation and continues even if the light irradiation is stopped, and therefore raw material gases capable of dark reaction may also be used. Can be done. Examples of source gases capable of dark reaction include SiH, 10□, and A[(CH,).

etc.

In the present invention, after introducing any of the mixed gases (1) to (3) as described above into a reaction chamber at a pressure of, for example, 0.01 to 100 Torr, a plasma-excited chemical reaction and a photo-excited chemical reaction occur in the reaction chamber. At least one of the chemical reactions is caused to occur intermittently, and the other chemical reaction is caused to occur intermittently or continuously in a phase different from that of the one chemical reaction.

That is, examples of the reaction mode include the following.

(a) As shown in FIG. 1, P reactions and L reactions are performed alternately. In this case, the P reaction and the L reaction may be switched instantaneously to perform the reaction (substantially no non-reactive state), or there may be a non-reactive state between the P reaction and the L reaction. You can also do this. Further, the P reaction and the L reaction may partially overlap.

(b) As shown in FIG. 2, the P reaction is carried out intermittently, during which the reaction is carried out continuously.

(c) As shown in Figure 3, the L reaction is Il)? The P reaction is carried out continuously during this period.

In this case, the reaction time (or non-reaction time) interval is usually 0.
．． The interval is 1 second to 20 minutes, preferably 1 second to 10 minutes, but this interval is not particularly limited and varies depending on the multilayer thin film.

In the reaction modes (b) and (c), the continuous reaction may be carried out continuously during the formation of the multi-layer thin film, or an appropriate rest period may be provided between the formation of the multi-N thin film (i.e. , itself may be performed intermittently). Furthermore, various modifications of the above (a) to (c) are also possible, such as allowing the P reaction or the L reaction to occur during this rest period.

One reactor is thus constructed in such a way that either of these reaction modes takes place within the reaction chamber.

To explain in more detail, the apparatus for forming a multilayer thin film can be constructed of the following. For example, a reaction chamber, a device for holding a substrate in the reaction chamber, a first raw material gas that allows or preferentially performs either a plasma-enhanced chemical reaction or a photo-excited chemical reaction, and On the other hand, a device for introducing into the reaction chamber a mixed gas with a second raw material gas having a different reactivity from the above raw material, a plasma generation device, a light generation device, and one of the plasma generation device and the light generation device. It can be configured with a reaction control device that operates intermittently and controls the other to operate intermittently or continuously in a phase different from that of the other. Specifically, a device such as shown in FIG. 4, which will be described later, can be used, but is of course not limited to this.

In this case, such reaction control can be performed by a computer or a known electric circuit, and for example, the reaction mode (b) can be carried out by using continuous irradiation light and pulsed plasma as an excitation source. can. Here, the order of pulses can be constant or changed during the reaction.

Note that the introduction of the mixed gas serving as the raw material can be carried out continuously without interruption, and thus a multilayer thin film can be formed continuously, but if necessary, it can be carried out intermittently. Often, the mixed gas composition can be changed during the reaction.

Further, the conditions for the P reaction and the L reaction can be respectively known conditions, and the P reaction and the L reaction can be carried out by conventional methods. For example, although not particularly limited, the temperature of the substrate can be from room temperature to 500°C, the reaction gas can be diluted with hydrogen gas or a rare gas, and the total pressure of the gas is o, 01~100 To
It can be rr. In addition, a pulsed high-frequency oscillator (
RF) (13,56MHz) power density is 0.01 to 1
Conditions such as 00 W/rd and mercury vapor pressure of O to 0.2 Torr can be adopted.

In the present invention, the type of substrate is not particularly limited and can be selected from various types, but in the case of obtaining a superlattice film, flexible substrates such as glass, ceramic, metal, polymer films, etc. can be used. can.

Therefore, the present invention allows one of the chemical reactions to be carried out on the substrate by combining the mixed gases (1) to (3) as raw materials and the reaction modes (a) to (C) using the method described above. This method involves sequentially and alternately laminating thin films when the chemical reaction is stopped, and thin films when one side of the chemical reaction is stopped, thereby making it possible to obtain various desired multilayer thin films in which thin films with different compositions or band gaps are stacked. Can be done.

For example, if (2) is used as the mixed gas and the reaction mode (b) is adopted as the reaction mode, when the plasma discharge is stopped, the raw material gas capable of both the P reaction and the L reaction is exposed to light. When the plasma discharge is turned on, the P reaction and the P reaction occur with the source gas, in which only the P reaction is possible or preferentially occurs. Both of the raw material gases capable of both L reactions are excited, and a thin film is formed using the same raw material gases having a different composition or band gap from the above-mentioned thin film.

The number of laminated layers varies depending on the type of multilayer thin film, but is usually 2 to 400 layers, preferably 2 to 200 layers. For example, a multilayer thin film structure can be obtained in which 50 layers obtained from one reaction and 50 layers obtained from the other reaction are alternately laminated to have 100 layers each having a thickness of about 50 layers. In this case, the thickness of each layer is not particularly limited, but
It is preferable that the number of people is 10 to 200 people, particularly 10 to 200 people.

〔Effect of the invention〕

As explained above, according to the present invention, multilayer thin films such as amorphous semiconductor superlattice thin films can be formed continuously, efficiently, simply and reliably in one reaction chamber without repeating the introduction and exhaust of raw material gases. can do. Also,
According to the present invention, a superlattice film having excellent semiconductor properties can be easily formed.

EXAMPLES Hereinafter, the present invention will be specifically explained by showing examples of the present invention, but the present invention is not limited to the following examples.

[Example 1] A multilayer thin film was formed using the reaction apparatus shown in FIG.

Here, in FIG. 4, 1 is a cylindrical reactor, 2 is a reaction chamber formed inside the reactor, 3 is a mixed gas introduction path, 4 is an exhaust port, and a rotating body 5 is inside the reaction chamber 2. is arranged rotatably, and a substrate 7 is placed on a disk 6 of this rotating body 5. Further, on the side of the plasma formation space (upper part of the reaction chamber 2) 2a above the substrate 7, there is a pulsed high frequency oscillator (13,56MHz) 8 which is located outside the reaction apparatus 1 and is controlled on and off by a computer. When the oscillator 8 is turned on, plasma is formed in the plasma formation space 2a, and a thin film formation period is provided above the plasma formation space 2a through a synthetic quartz window 9. Inside is a low-pressure mercury lamp 10 that is always on and emits light. Note that the arrow in the figure indicates the flow direction of the mixed gas. In addition, 11 in the figure is an inert gas inlet.

12 is the flow path, and by introducing this inert gas,
This prevents the quartz window 9 from becoming foggy.

A multilayer thin film was then formed using the above apparatus under the following conditions.

Driving mixture gas 5i2Hr: 3. Osecm
CF4: 60. Osecm Pressure 700mTorr Substrate temperature 300℃ Plasma power 30W Plasma On state: 33 seconds 10 sets repeat Off state: 58 seconds Ultraviolet wavelength 185 nm, 254 nm substrate
Corning 7059 Glass Silicon Wafer (
(single crystal) In the above method, when the plasma is off, 5i2
Only H, was decomposed by light to form a thin film consisting of Si and H, and when the plasma was on, both 5i2HG and CF4 were excited, forming a thin film consisting of SL, C, H, and F.

The physical properties of the obtained membrane were as follows.

Film thickness (1 hour reaction) Composition of 3100 people (
Amorphous silicon) Si: 96%H: 4% Optical bandgap 1.80eV dark conductivity
6.04 O4
Amorphous 5iC) si: 57%C:11
% H: 28% F: 4% Optical bandgap 2.35eV dark conductivity
1.12X10-"5-an-' photoconductivity
2,40X10-7S-an-' film thickness
1000 people optical bandcap 2,05eV dark conductivity 1.69X
10-'5-an-1 Photoconductivity 6.18X
10zuS-■-1 Figure 1 shows the X-ray diffraction results of the obtained superlattice film (X-ray diffraction spectrum of a-Si (50 people) 41 layers/a-8iC (50 people) 40 layers) . The results shown in FIG. 5 clearly show that this superlattice film is stacked in a regulated manner with a set film thickness.

From the above results, we found that by combining a gas that is excited by plasma and a gas that does not decompose by light and a gas that decomposes by light, constantly irradiating light and generating plasma intermittently, it is possible to continuously generate gas without switching gases. It is recognized that a superlattice film can be produced by this process. Further, the superlattice film produced by the above method has semiconductor properties that are superior to a single layer film having the same composition on average.

That is, compared to conventional bulk a-8iC having a similar optical band gap, the photoconductivity is one order of magnitude higher. Moreover, each a-Si.

The photoconductivity is better than that of the a-5iC4 layer, and therefore the present invention can reliably obtain a superlattice film with excellent semiconductor properties based on quantum effects unique to superlattices. is found.

[Example 2] A multilayer thin film was formed using the above apparatus under the following conditions.

Made by JUL Tsui Mixed Gas 5i2H: 3. OsecmC
M4: 60. Osecm Pressure 700 mTorr Substrate temperature
300℃ Plasma power 30W Plasma On state = 94 seconds Off state: 151 seconds Ultraviolet wavelength 185 nm, 254 nm substrate
Corning 7059 glass silicon wafer (single crystal) In the above method, when the plasma is off, Si,
Only H, decomposes by light and only L reaction proceeds), Si
When the plasma is on, both 5i2HG and CH are excited and both L and P reactions proceed), and a thin film consisting of Si, C, and H (amorphous 5iC). was formed.

The physical properties of the obtained membrane were as follows.

Film thickness (90 minutes reaction) 2900 people Optical band cap 1, 78 eV dark conductivity
8.67X10-11S-an-' photoconductivity
2.50X10-'S-am-" film thickness (
120 minutes reaction) 2400 people Optical band cap 2.13eV dark conductivity 3.07
X10-”S-CM-1 Photoconductivity 1.2
2X10-' S-am-1 film thickness
1000 people optical band cap 1.9
0eV dark current rate 7.77X10-'S-c
m-” Photoconductivity 3.25X10-5S-
cm-'' [Example 3] Using the above-mentioned apparatus, a multilayer thin film was formed under the following conditions.

Fraud - Mixed gas 5i2F, : 78secH2
: 7.35secm GeH4: 0.15secm Ar: 15sec Pressure 1.0mTorr Substrate temperature 300℃ Plasma power 40W Plasma On state = 67 seconds 10 sets Repeated off state: 455 seconds Ultraviolet wavelength 185 films m, 254 films m Substrate
Corning 7o59 glass silicon wafer (single crystal) In the above method, when the plasma is off, SiF
4 is not excited by light and forms only a thin film (amorphous Ge) consisting of Ge and H. When the plasma is on, SiF4 is excited and Si, Ge.

A thin film (amorphous 5iGe) consisting of H and F was formed.

The physical properties of the obtained membrane were as follows.

L when plasma is off A fan-a
r-Sore film thickness (120 minute reaction) 1500 people Optical band cap 0.82 eV dark conductivity
2.33 x 10-'S-■-1 photoconductivity
2.35X10-'S-■-1 film thickness (60 minutes reaction) 7200 people Optical band cap
1.566V dark conductivity 1.27×1O-
118-all-1 light guide M rate 3.60xlO-' 5
-an-1 film thickness 1000
Ergonomic band cap 1・25eV dark conductivity
1.56X10-'S-S-Kuniko 1 Photoconductivity 2.76X10-7S-aa-'Example 2
From the results of 3 and 3, it is possible to combine a gas that is excited by plasma and a gas that does not decompose with light and a gas that decomposes with light, and by continuously irradiating light and generating plasma intermittently, it is possible to generate a gas without switching the gas. It is recognized that superlattice films can be fabricated in a continuous process.

Further, the superlattice film produced by the above method has semiconductor properties that are superior to a single layer film having the same composition on average.

[Brief explanation of the drawing]

1 to 3 are diagrams explaining an example of the reaction mode of the present invention, FIG. 4 is a schematic sectional view showing an example of the reaction device used for the cyst of the present invention, and FIG. 5 is a diagram illustrating an example of the reaction mode of the present invention. This is an X-ray diffraction spectrum of an example of a multilayer thin film. DESCRIPTION OF SYMBOLS 1... Reactor, 2... Reaction chamber, 2a... Plasma formation space, 3... Mixed gas introduction path, 4... Exhaust port, 7... Substrate, 8... Pulse high frequency Oscillator, 10...low pressure mercury lamp. Figure 1 9 Biji Q A;, U-p anti-min' ON□ Figure 4

Claims (1)

[Claims] 1. A first raw material gas that is capable of or preferentially undergoes either a plasma-induced chemical reaction or a photo-excited chemical reaction, and a first source gas that has a different reactivity from the above-mentioned raw materials for these chemical reactions. A mixed gas with the second raw material gas is introduced into the reaction chamber, and one of the plasma-excited chemical reaction and the photo-excited chemical reaction is carried out singly or preferentially and intermittently in the reaction chamber, and the other chemical reaction is carried out intermittently. The chemical reactions are carried out intermittently or continuously in different phases, and thin films when one chemical reaction is carried out and thin films when one chemical reaction is stopped are sequentially laminated on the substrate. 1. A method for forming a multilayer thin film, the method comprising: forming a multilayer thin film;