JP3007579B2 - Manufacturing method of silicon thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a silicon thin film, and more particularly to a method for manufacturing a silicon thin film having characteristics as a semiconductor using a solid Si raw material.

[0002]

2. Description of the Related Art Silicon thin films are used in various applications such as solar cells. Conventionally, silicon thin films used in solar cells have been produced on glass substrates by chemical vapor deposition (CVD). This silicon thin film was amorphous silicon containing hydrogen (a-Si: H).

However, an a-Si: H film solar cell has a problem that photodeterioration occurs and the power generation cost increases with use. It is known that crystallization of a silicon thin film by heat treatment is effective in preventing light deterioration. Since CVD uses a dangerous gas such as silane as a silicon raw material, equipment for safety measures is required.

As a simpler method for producing a silicon thin film, an evaporation method using an electron beam is known. By heating and evaporating metallic silicon with an electron gun, a silicon thin film is deposited on the substrate. However, the silicon thin film manufactured in this manner contains a large amount of dangling bonds, and significantly degrades semiconductor properties such as variability in conductivity, variability in conductivity type, and light response capability.

It is known that terminating with hydrogen or the like is effective in reducing dangling bonds. For example, if all dangling bonds are terminated with hydrogen atoms, the inconvenience due to the presence of dangling bonds can be greatly reduced.

FIG. 4 is a schematic sectional view showing a method of manufacturing a silicon thin film according to the prior art. An electron beam (EB) evaporation source 53 is arranged in a film forming chamber 51 which can be evacuated, and a substrate 61 which can be heated by a heater 60 is arranged above the evaporation source 53. Beside the EB evaporation source 53,
An ion gun 55 is provided to supply hydrogen ions.

The inside of the film forming chamber 51 is evacuated to a predetermined degree of vacuum through an exhaust pipe 57, an electron beam is generated from an EB evaporation source 53, and is irradiated on silicon to evaporate silicon.
Hydrogen ions obtained by ionizing hydrogen from the ion gun 55 are directed to the substrate 61. On the substrate 61, the EB evaporation source 5
The Si from No. 3 and the hydrogen from the ion gun 55 arrive at the same time, forming a thin film of a-Si: H.

When hydrogen ions are not assisted, dangling bonds of about 10 ²⁰ cm ⁻³ are present in the Si thin film, but the dangling bonds are reduced to 3 × 10 ¹⁷ cm ⁻³ by the assist of hydrogen ions by an ion gun. It has been reported that it can be reduced to a degree.

[0009]

In the above-mentioned technology, hydrogen ions are generated by an ion gun.
For this reason, the hydrogen ion current density is small, and the film formation rate is limited by the hydrogen ion current.

Although the film quality is greatly improved as compared with the case without hydrogen ion assist, further improvement of the film quality is desired.

[0011]

According to one aspect of the present invention, a step of arranging a substrate in a film forming chamber, introducing at least one kind of inert gas into the film forming chamber,
Generating a primary DC arc discharge plasma, generating hydrogen gas after generating the primary DC arc discharge plasma, and forming a secondary DC arc discharge plasma; Heating and evaporating the metal silicon in the second direct current arc discharge plasma to reach the substrate, the step of forming a silicon thin film on the substrate, the step of forming the silicon thin film,
A method of manufacturing a silicon thin film is provided, wherein the pressure in the film forming chamber is set to 10 to 500 mTorr to grow a polycrystalline silicon film.

In a conventional ion gun, hydrogen is ionized by discharge generated inside the gun, and ions are electrically extracted through a grid. The ion current that can be extracted is limited by the space charge, and a large current cannot be extracted.

On the other hand, since plasma is not limited by space charge, a large current can be taken out. Further, a large amount of activated hydrogen such as ions and radicals is present in the plasma. This phenomenon is the same for glow discharge and arc discharge, but as shown in FIG. 1B, arc discharge plasma AG has a lower voltage and a larger current than glow discharge plasma GD. That is, there are few high-energy particles that damage the film quality, and a large number of particles having appropriate energy exist.

Therefore, the film quality of the silicon thin film can be improved by the high-density active hydrogen, and the film forming speed can be improved.

When the evaporated silicon reaches the substrate through the plasma of the DC arc discharge, the silicon is also activated and the film quality can be improved.

[0016]

In the step of forming a silicon thin film, a polycrystalline silicon thin film can be obtained by setting the pressure in the film forming chamber to 10 to 500 mTorr. At this time, it is preferable that the DC arc discharge plasma is generated between two electrodes of an anode and a cathode.

As described above, according to the present invention, by increasing the pressure in the film forming chamber, the amount of active hydrogen can be increased and a polycrystalline silicon thin film can be obtained.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a graph illustrating a film forming apparatus for producing a silicon thin film and a DC arc discharge.

In FIG. 1A, a cathode 2 and an anode 3 are arranged in a film forming chamber 1 of the film forming apparatus.
Also serves as a crucible. In the crucible of the anode 3, metal Si
8 are accommodated. A susceptor 9 for holding a substrate is disposed above the anode 3. The susceptor 9 can be heated to a desired temperature by the heater 10. A substrate 11 such as a glass substrate is held by the susceptor 9 and is arranged to face the crucible of the anode 3.

The electrode plate 12 of the cathode 2 is made of, for example, Ta-La
B ₆ is formed of a composite material. A pipe 5 for introducing hydrogen gas is provided through the electrode plate 12 of the cathode. The electrode plates 12 of the anode 3 and the cathode 2 are connected to the variable DC voltage source 4 while being electrically separated from the film forming chamber 1. The film forming chamber 1 is grounded.

An exhaust pipe 6 is connected via a valve V to a side facing the pipe 5 for introducing hydrogen gas. By introducing hydrogen gas from the pipe 5 and exhausting the gas from the exhaust pipe 6, the inside of the film forming chamber 1 can be maintained at a predetermined hydrogen gas pressure. A voltage of, for example, several hundred volts is applied between the anode 3 and the cathode 2 from the variable DC power supply 4 to start discharge in a hydrogen gas atmosphere. Charged particles (hydrogen ions, electrons) generated by the discharge are accelerated toward the cathode 2 or the anode 3. The cathode 2 is heated by the colliding positive ions, and eventually generates thermoelectrons.

When thermionic electrons are generated, the density of the plasma further increases, and the plasma changes from a glow discharge to an arc discharge. When an arc discharge state occurs, the voltage between the anode 3 and the cathode 2 decreases. Such a variable voltage source 4 can be formed, for example, by a constant current source capable of controlling a current value.

FIG. 1B is a graph showing the relationship between the applied current and the current flowing between the anode and the cathode. The region GD where the current is low indicates a region where glow discharge occurs. In the glow discharge region, the current value _IG is set to the maximum value, and when the current amount further increases, the glow discharge region transitions to the arc discharge region AG via the transition region. Current value of the arc discharge region to the minimum value of I _A. Here, the current value I _A, higher than 2 orders of magnitude than the normal current value I _G.

That is, the DC arc discharge is a discharge state in which the current density is higher than the glow discharge by about two digits or more. Therefore, when hydrogen plasma is generated using DC arc discharge, the amount of active hydrogen in this plasma can be estimated to be about two orders of magnitude or more higher than hydrogen plasma generated by glow discharge.

In the arc discharge, the voltage between the anode and the cathode decreases. For example, it is about 100V.
In such a state, the electron temperature in the plasma P is also about 100 eV.

In the glow discharge, the applied voltage is high,
The electron temperature in the plasma also increases. In order to activate the Si atoms evaporated from the Si source 8, it is preferable to collide electrons with about the ionization energy of Si.
In this regard, the electrons in the arc discharge are suitable for activating Si atoms. When glow discharge is used, the energy of electrons is too high, and it becomes impossible to activate Si efficiently. In order to form a high-quality Si film, electrons having an electron temperature of about 0.1 to 10 eV must have a density of about 5 × 10 ¹¹ to 1
It is preferably present at 0 ¹⁴ / cc.

When a direct current arc discharge occurs, a large area in the film forming chamber 1 emits light, and particularly, an area P indicated by a broken line emits light brightly. High-density plasma exists in this region P.

It is considered that the following various reactions proceed in the plasma. _{^{H 2 → 2H + + 2e -}} H 2 → H 2 + + e - H 2 → H * + H * H 2 → H * + H + + e - where H ^* represents a radical of H.

A large amount of electrons in the plasma P collide with the metal Si8, thereby heating and evaporating the Si8. Evaporated Si atoms (or clusters) form plasma P of the DC arc discharge.
It passes through the inside and collides with electrons and active hydrogen (ions, radicals), and reaches the surface of the substrate 11. The substrate 1
Active hydrogen also collides with one surface.

As described above, a high-quality Si film is formed by generating high-density active hydrogen by DC arc discharge and activating the evaporated Si to reach the substrate.

FIG. 1C shows a configuration of a thermoelectron generating type cathode which can be used in place of the cold cathode 2 shown in FIG. 1A. The cathode includes a hot filament 14 and a power supply 15 (for example, a DC power supply) for supplying a current to the hot filament. When using a thermoelectron source, the DC power supply 4 can generate a DC arc discharge by applying a predetermined low voltage from the beginning.

Although the case where DC arc discharge is generated by introducing only hydrogen gas has been described, hydrogen gas and an inert gas (for example, He, Ar, Kr, Xe, etc.) may be simultaneously introduced.

FIG. 7 schematically shows a configuration example of a film forming apparatus for introducing a hydrogen gas and an argon gas as an inert gas.

FIG. 7A is a cross-sectional view of a film forming apparatus for forming a silicon thin film. Cathode 2 of deposition chamber 1
In the vicinity of the electrode plate 12, a pipe 16 is provided. The argon gas is introduced from one end of the pipe 16, and the hydrogen gas is introduced from the pipe 5 connected in the middle of the pipe 16. First, a primary DC arc discharge plasma is generated in an atmosphere of an argon gas introduced from a pipe 16. Thereafter, hydrogen gas is introduced from the pipe 5 to form a secondary arc discharge plasma.

As shown in FIG. 7 (B), the hydrogen is introduced at the wall of the film forming chamber 1 as well as at the plasma P as at the pipe 17 or at the Si 8 as at the pipe 18. It may be sprayed on the substrate 11 like the pipe 19. The Si thin film formed in these cases has less oxidation.

Furthermore, in order to generate stable and high-density plasma, a pinch effect by a magnetic field can be used.

FIG. 2 schematically shows a configuration example when a magnetic field is used. A permanent magnet 21 is embedded under the crucible of the anode 3, and a plurality of sets of electromagnets 22 a, 22 b, 23 a, and 23 b are arranged around the cathode 2 of the film forming chamber 1. These electromagnets 22 and 23 can adjust the shape and intensity of the generated magnetic field by adjusting the current. Note that a magnet having another configuration may be used.

Although the case where Si is heated by electrons in the plasma has been described, heating of Si can be performed by other means.

FIG. 3 shows a case in which Si is evaporated using the EB evaporation source 25 to separately generate arc discharge plasma.

In FIG. 3A, an EB evaporation source 25 is disposed below the film forming chamber 1 and a substrate 11 is disposed above the EB evaporation source 25. A cathode 12 and an anode 13 are arranged below the substrate 11 so as to generate an arc discharge plasma AP in a direction substantially parallel to the surface of the substrate 11. The Si atoms jumping out of the evaporation source 25 are activated in the plasma,
Immediately reaches the surface of the substrate 11.

In FIG. 3B, the anode 13 is
One susceptor is also used, and is disposed above the film forming chamber 1. The cathode 12 is arranged below the film forming chamber 1 together with the EB evaporation source 25. According to this configuration, E
Arc discharge plasma is generated between the cathode 12 and the anode 13 such that the Si beam generated from the B evaporation source 25 overlaps a path that flies toward the substrate 11 for a long time.

According to these configurations, the plasma density and S
i and the amount of evaporation can be controlled independently. Note that a Si thin film was formed using the apparatus of FIG. 1A under the following conditions.

Cathode current: 100 A Cathode voltage: 100 V Hydrogen flow rate: 120 sccm Argon flow rate: 10 sccm Pressure: 7 × 10 ^-4 Torr Substrate temperature: 400 ° C. Under the conditions of not less than 3 × 10 ¹⁶ at a film formation rate of 10 nm / s. c
An amorphous silicon film having a dangling bond density of m ⁻³ was obtained.

For comparison, a film was formed also by the ion gun method shown in FIG. In this case, the film formation rate is 3 nm / s
And the dangling bond density was 3 × 10 ¹⁷ cm ⁻³ .

In the above example, the dangling bond density is reduced by about one digit, even though the film forming speed is about three times as fast. This indicates that a Si thin film having good film quality can be obtained at a high film forming rate. It is considered that the reason why the film quality is improved in this way is that the plasma density is at least partially high and the electron temperature in the plasma is almost equal to the ionization energy of Si.

An example of polycrystalline Si film formation using the apparatus shown in FIG. Cathode current: 200 A Cathode voltage: 100 V Hydrogen flow rate: 120 sccm Argon flow rate: 30 sccm Pressure: 7.5 × 10 ⁻² Torr Substrate temperature: 300 to 500 ° C.

FIG. 5 shows a Raman spectrum of the Si film formed under the above conditions. FIG. 6 shows an X-ray diffraction spectrum of the same Si film. It can be seen from both spectra that polycrystalline Si has been formed.

In the apparatus shown in FIG. 7A, evaporation of metal Si and generation of active hydrogen are performed simultaneously by one plasma.
The ratio between the amount of evaporation of i and the amount of active hydrogen is important. FIG. 8 is a graph obtained by changing only the pressure among the above conditions.

FIG. 8A shows the emission of 657 nm by active hydrogen H and the formation of 63
6 is a graph showing a relationship between an intensity ratio with 4.7 nm light emission and a pressure change in a film forming chamber. In the graph, the horizontal axis indicates pressure (unit is Torr), and the vertical axis indicates the emission intensity ratio of hydrogen (hydrogen / Si) to Si. As shown in the graph, the emission intensity ratio is 0.008 to 0.02T.
It increases in the range of orr, and is kept substantially constant in the range beyond that. That is, 0.008 to 0.02T
It can be considered that in the range of orr, the ratio of the active hydrogen to the evaporated Si increases, and at a pressure higher than that, the ratio between the two is kept constant.

FIG. 8B is a graph showing an X-ray diffraction spectrum of the formed Si film. The pressure in the film forming chamber is set to 0.006, 0.02, 0.05, 0.07 To.
The X-ray diffraction spectrum of the Si film obtained under the four conditions of rr is shown. From the spectrum, under the condition of 0.006 Torr, almost no crystalline peak can be confirmed. On the other hand, at a pressure of 0.02 Torr or more, a clearer peak of crystallinity can be confirmed as the pressure increases. That is, it can be determined that polycrystalline Si is formed at a pressure higher than 0.006 Torr.

FIG. 8C is a graph showing the relationship between the pressure in the chamber and the deposition rate. The growth rate decreases as the pressure in the chamber increases, and hardly grows at a pressure of 500 mTorr or more. This is because, as the pressure increases, the number of molecules in the gas (hydrogen gas, inert gas) increases, and energy is consumed to activate the molecules.
This is considered to be because it becomes difficult to evaporate.

As described above, the film forming chamber is set to 0.01 Torr.
When the pressure is equal to or higher than r, polycrystalline Si is obtained because the amount of active hydrogen increases. Also, it has been found that the upper limit pressure is preferably about 500 mTorr because the growth rate becomes slower as the pressure increases. That is, the pressure in the film forming chamber is preferably 10 to 500 mTorr, and is approximately 20 to 500 mTorr.
mTorr has been found to be more preferred. On the other hand, 0.01
At a pressure equal to or lower than Torr, the evaporation amount of Si is relatively increased as compared with the amount of active hydrogen, and an amorphous silicon film is obtained.

The present invention has been described in connection with the preferred embodiments.
The present invention is not limited to these. For example,
It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

[0056]

As described above, according to the present invention,
A high-quality Si polycrystalline thin film can be manufactured.

A method for manufacturing a silicon thin film that is easy to take environmental measures is provided.

[Brief description of the drawings]

FIG. 1 is a sectional view, a diagram, and a graph for explaining an example of the present invention.

FIG. 2 is a schematic sectional view for explaining another embodiment of the present invention.

FIG. 3 is a schematic sectional view for explaining another embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view for explaining a conventional technique.

FIG. 5 shows a Raman spectrum of the formed Si film.

FIG. 6 shows an X-ray diffraction spectrum of the formed Si film.

FIG. 7 is a cross-sectional view for explaining another embodiment of the present invention.

FIG. 8 is a graph for explaining another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Film-forming chamber 2 Cathode 3 Anode 4 DC power supply 5 Inlet pipe 6 Exhaust pipe 8 Si9 susceptor 10 Heater 11 Substrate 12 Cathode plate 14 Hot filament 15 DC power supply 16-19 Piping 21 Permanent magnet 22, 23 Electromagnet 25 EB evaporation source

Continuation of the front page (72) Inventor Kenichi Kondo 2-33-1 Chitosedai, Setagaya-ku, Tokyo (56) Reference JP-A-60-223113 (JP, A) JP-A-4-252020 (JP, A) JP-A-4-252018 (JP, A) JP-A-59-96718 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21/203

Claims (2)

(57) [Claims] A step of disposing a substrate in a film forming chamber; a step of introducing at least one kind of inert gas into the film forming chamber to generate a first DC arc discharge plasma; Generating a primary DC arc discharge plasma, introducing hydrogen gas to form a secondary DC arc discharge plasma; heating and evaporating metallic silicon in a film forming chamber; Forming a silicon thin film on the substrate by reaching the substrate through the arc discharge plasma. In the step of forming the silicon thin film, the pressure in the film forming chamber is set to 10 to 500 mTorr, and the polycrystalline silicon film is formed. A method for producing a silicon thin film, comprising: growing silicon. 2. The step of generating a DC arc discharge plasma is a step of generating plasma between two electrodes of an anode and a cathode, wherein the introduction of the hydrogen gas and the inert gas is performed from near the cathode by piping. The method for producing a silicon thin film according to claim 1, wherein: