JP2730693B2 - Thin film formation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Technical Field The present invention relates to an amorphous silicon (a-Si: amorphous silic) formed by a plasma CVD method under a pressure near the atmospheric pressure.
on) and a method of forming a thin film such as titanium nitride (TiN).

For example, an amorphous silicon a-Si film that usually contains several at% to several tens at% (atomic percent) of H in a film is promising as a material for a low-cost solar cell.
There are also other uses such as image sensors, optical sensors, thin film transistors, and photosensitive materials for copying machines. There is an advantage that it is cheaper than single-crystal Si and a large-area one is easily obtained.

Further, TiN is important as a surface protective film having abrasion resistance and the like.

Such thin film formation methods include thermal CVD and plasma CVD.
The law is known.

Since the thermal CVD method requires heating the substrate, it can be used only for heat-resistant materials.

On the other hand, the plasma CVD method can form a thin film at a lower temperature than the thermal CVD method.

Therefore, a thin film can be formed on a low-cost glass substrate, a polymer film, or the like having poor heat resistance, and is widely used.

In the plasma CVD method, the excitation energy is provided not in the form of heat but in the form of kinetic energy of electrons and ions in the plasma and chemical energy of neutral radicals. For this reason, the temperature of the substrate can be lower than that of the thermal CVD method.

As an example, amorphous silicon a-Si is
Invented a method of forming a thin film by glow discharge, and it was possible to incorporate an appropriate amount of H into the film and to reduce the defect density in the film, thereby producing a device that can withstand device applications such as solar cells and sensors. Is now available.

WESpear, PG Lecomber: Solid Commun., 17, p1993 (197
5) This involves applying an alternating voltage of 100 kHz to 13.56 MHz to a parallel plate type electrode, and applying SiH ₄ / H ₂ , SiH ₄ -S at a low pressure of 0.1 to ₂ Torr.
Glow discharge is caused in a mixed gas such as iF ₄ / H ₂ .

Of course, a dopant may be added. This is the PH
It is performed by mixing gas such as ₃ / H ₂ and B ₂ H ₆ / H ₂ .

(B) Prior art Since the invention of Spear, a-Si manufacturing equipment has been continuously improved, but basically glow discharge was performed at a low pressure.

Glow discharge does not occur unless the pressure is as low as about 0.1 to 10 Torr. If the pressure is higher than this, the discharge shifts to local arc discharge, and film formation on a substrate having poor heat resistance and uniform film formation over a large area cannot be performed. Thus, such a pressure is chosen.

Therefore, the container required an expensive vacuum chamber and a vacuum pump had to be installed.

In particular, in the case of a photoelectric conversion material such as a solar cell using a-Si or the like, or a surface protective film such as TiN, it is strongly demanded from the viewpoint of cost that a large-area thin film can be formed at once.

However, the plasma CVD method maintains a stable plasma by maintaining a glow discharge. Glow discharge is carried out in a vacuum (0.1-1
Only about 0 Torr).

Since a film can be formed only in a vacuum, if a large-area product is to be manufactured, the entire vacuum container must be enlarged.

 A high-power evacuation device is required.

Then, the equipment becomes extremely expensive.

(C) Atmospheric-pressure plasma CVD method A large-area uniform film and uniform processing are absolutely necessary for cost reduction, but if the equipment cost is high, nothing will happen.

However, recently, under atmospheric pressure, plasma CVD
An invention was made that enabled the law.

 JP-A-63-50478 (published in S.63.3.3).

This is to make a thin film of carbon C. For example, CH ₄ , C
The F ₄ as a raw material gas, but to which is added 90% He gas.

Because of the large amount of He gas, glow discharge can be maintained even at atmospheric pressure. Since it is under atmospheric pressure, a vacuum chamber and a vacuum exhaust device are unnecessary. This is a great invention that can significantly reduce the cost of forming a thin film.

An important point of this method is that glow discharge occurs even at atmospheric pressure and stays stable when He gas is used.

The inventor explains why He is as follows.

(A) He is easily excited by discharge.

(B) He has many metastable states and can produce many active particles in an excited state.

(C) He active particles dissociate hydrocarbons and hydrogen halides.

(D) In He, ions are easily diffused. Therefore, the discharge is likely to spread.

Naturally, the mixing ratio of He and CH ₄ becomes extremely important.

According to the description of the specification, at 92: 8, the spread of the glow discharge becomes narrower, and at 90:10, the corona discharge becomes
When it becomes 9.5: 10.5, there is spark discharge.

FIG. 2 shows an apparatus disclosed in Japanese Patent Application Laid-Open No. 63-50478.

A cylinder 12 is suspended from above in a vertically long reaction vessel 11.

Below the cylinder 12 is the electrode 14. RF oscillator 16, cylindrical
An RF voltage is applied to electrode 14 via a metal rod that passes through 12.

A support substrate (conductor) 17, an insulator 18, and a sample substrate 19 are provided below the container. There is also an annular external electrode 20.

A mixed gas of He and CH ₄ (the case of He, CH _4, CH ₄ also)
Is supplied from the gas inlet 21 at the upper end of the cylinder 12. This gas flows down the inside of the cylinder, passes by the side of the electrode 14, hits the sample substrate 19, partially reacts to form a thin film, and the rest is discharged from the side gas outlet 22.

Glow discharge occurs between the electrode 14 and the supporting substrate (sample electrode) 17.

According to this specification, the present invention also states that "the present invention can be similarly applied to the formation of other thin films such as a silicon nitride film, amorphous silicon, and a silicon carbide film."

(D) Problems to be Solved by the Invention According to the invention of Japanese Patent Application Laid-Open No. 63-50478, "at a pressure in the range of about 200 Torr to 2 atmospheres, about 90% or more of rare gas and film component A method of forming a thin film on a substrate by forming a mixed gas with a gas containing a gas into a plasma state by glow discharge, and forming a thin film on a substrate.

(1) The inventor tried to form a C-based thin film on a 10 cm × 10 cm substrate using a mixed gas of CH ₄ and He gas according to this disclosure.

The pressure is atmospheric pressure. However, according to this disclosure, although a glow discharge can be obtained, the discharge is unstable (or non-uniform) depending on conditions. Also, because of the atmospheric pressure, gas replacement in the center of the plasma is not effectively performed, and the raw material gas is decomposed only in the outer peripheral portion of the plasma. Almost no film could be formed at the center of the substrate, and a uniform film could not be formed over a large area.

(2) Further, the present inventor tried to make a-Si based on this disclosure.

In order to produce a-Si, a mixed gas of SiH ₄ gas and He gas was used. The pressure is atmospheric pressure. Since it is sufficient that the He gas is 90%, SiH ₄ : He = 10: 90 (volume ratio).
When this was attempted, an arc discharge occurred and no glow discharge occurred.

When the ratio of SiH ₄ / He was further reduced, a stable glow discharge could be generated between the electrodes.

However, since the SiH ₄ gas is extremely easily decomposed, it does not enter the plasma region, and the SiH ₄ is decomposed at the outer peripheral portion. Dust consisting of fine powder having a particle size of about 0.05 to 0.5 μm was only deposited on the outer peripheral portion of the plasma region.

An a-Si thin film could not be formed on the sample substrate.

That is, from these facts, it can be seen that the invention of Japanese Patent Application Laid-Open No. 63-50478 cannot be used as it is for uniform film formation of a large area even if it can be used for plasma formation at atmospheric pressure.

(E) Objective It is an object of the present invention to provide a method for uniformly forming a thin film of a-Si, TiN, or the like over a large area under the atmospheric pressure using a plasma CVD method.

(F) Method of the Present Invention In order to form a stable glow discharge under atmospheric pressure, a large amount of a raw material gas for film formation is diluted with He. In addition, in order to form a stable glow discharge and form a uniform thin film over a large area, a sample substrate placed directly on one of the opposing surfaces of two electrodes facing each other and an electrode facing the sample substrate are required. And the distance g between them are 10 mm or less and 0.1 mm or more.

Furthermore, a mixed gas consisting of a source gas for film formation and He,
The value Q / S obtained by dividing the total gas flow rate Q by the deposition S in the discharge space is 1 to 1
The gas is supplied to the discharge space on the sample substrate between the opposing electrodes so as to be 0 ² sec ⁻¹ so that the gas in the discharge space is replaced in 10 ^{−2 to 1} sec.

 As described above, the present invention has three features.

(1) Dilute the source gas with He in large quantities.

(2) The distance between the sample substrate and the electrode facing the sample substrate, g is 0.1 mm ≦ g ≦ 10 mm. (3) The gas flow rate Q supplied to the discharge space (volume S) is 1 sec ⁻¹ ≦ Q / S ≦ 10 ² sec ⁻¹ Hereinafter, the method of the present invention will be described with reference to FIG.

FIG. 1 shows an example of a thin film forming apparatus for carrying out the present invention, but the present invention is not limited by FIG.

In the film forming chamber 1, electrodes 2 and 3 facing each other are provided. One is grounded, and this is called a ground electrode 3. The other is referred to as a non-ground electrode 2.

A sample substrate 4 is placed on the electrode 3. Here, the electrode 2 also serves as a gas supply port to the discharge space, and the surface of the electrode 2 facing the sample substrate 4 is a porous plate.

Here, the reason why the electrode 2 is a perforated plate and the gas supply port is to effectively perform gas replacement in the central portion of the plasma and obtain a uniform film with a large area.

Here, the distance g between the sample substrate 4 and the electrode 2 is 10 mm to 0.1 m.
m.

A high frequency power supply 6 is connected to the non-ground electrode 2. This can use, for example, a 13.56 MHz RF oscillator and amplifier.

A mixed gas obtained by diluting a source gas with a large amount of He gas is introduced from a nozzle 5, supplied to a discharge space via an electrode 2, and discharged out of a film forming chamber 1 through a gas outlet 8. Further, the flow rate Q of the mixed gas with respect to the volume S of the discharge space is:
Q / S is set to 1 sec ^-1 to 10 ² sec ^-1 .

(G) Function A mixed gas of a source gas and He is introduced from the nozzle 5, and a high-frequency voltage is applied to the electrode 2. The pressure is at or near atmospheric pressure.

Glow discharge occurs between the electrodes. Since the proportion of He is large, glow discharge occurs even at atmospheric pressure, and is maintained stably.

The mixed gas is excited by glow discharge to become plasma.

The sample substrate 4 is heated in advance by the heater 7. A thin film is formed on the substrate 4.

Here, the reason why the distance g between the sample substrate 4 and the electrode 2 is set to 10 mm or less is to expand the range in which the glow discharge occurs, prevent localization of the discharge, and make the intensity of the discharge uniform. .

Unreacted gas and reaction products are removed from the gas outlet 8 together with He.

Since the gas is supplied to the discharge space via the electrode 2 and the Q / S is 1 to 10 ² / sec, the source gas can reach the center of the electrode. That is, the discharge occurs widely and uniformly, and the thin film is uniformly formed even when the sample substrate is wide.

If the gas flow rate Q is insufficient, the raw material gas is decomposed as soon as it is supplied to the glow discharge region, and the deposition rate on the substrate is reduced. In addition, easily decomposed gases such as SiH ₄ immediately cause a polymerization reaction and become fine dust.
Thus, the gas flow rate Q must be such that it replaces the volume S of the discharge space in at least one second.

Conversely, if the gas flow rate Q is too large, not only is the gas wasted unnecessarily, but also the film forming speed is reduced.

For this reason, Q / S is 1 to 10 ² / sec.

 Next, the ratio between the source gas and He will be described.

Since the source gas is diluted with He, the discharge sustaining voltage is low. If He is 100%, glow discharge can be maintained at atmospheric pressure. Since the mixing amount of the source gas is small, glow discharge can be performed even under atmospheric pressure.

By the action of He, transition to arc discharge can be prevented.

Even at the same pressure, the mean free path of gas molecules is long in He. Therefore, the plasma is easily spread.

If the raw material gas / He ratio δ exceeds a certain value,
Glow discharge cannot be maintained. Transition to arc discharge. According to an experiment conducted by the present inventors, the value of δ that shifts to arc discharge differs depending on the ease with which the source gas is decomposed. In order to suppress the transition to the arc and obtain a stable glow discharge, it is necessary that δ ≦ 10 ⁻¹ , provided that SiH ₄ ,
_{Si 2 H 6, C 2 H} 4, C 2 H 2, GeH 4, N 2 O, it is preferred in the case of easily decomposed gases such as O ₂ is δ ≦ 10 ^-2.

Conversely, if the ratio δ of the raw material gas / He is smaller than 10 ⁻⁴ , the film forming rate decreases, which is not desirable.

Spreads plasma uniformly, prevents localization of discharge, and
In order to make the film thickness distribution uniform, it is better to narrow the gap g between the sample substrate 4 and the ungrounded electrode 2.

The smaller the value of g, the more stable and uniform glow discharge occurs in the electrode surface. The effect is particularly large when the sample substrate 4 is conductive.

 The value of g is desirably 10 mm or less.

However, if they are too close, it is difficult to provide a uniform distance between the electrode 2 and the sample substrate 4. This is because a slight inclination or unevenness becomes a problem.

 Practically, the value of g should be 0.1 mm or more.

Dust due to decomposition of the source gas may adhere to the vicinity of the non-ground electrode 2. Such dust forms the sample substrate 4
Causes pinholes. Device characteristics,
This may cause variations in thin film characteristics.

In order to prevent this, it is preferable to provide a heating means or a cooling means (not shown) on the ungrounded electrode 2. that way,
No decomposition reaction of the raw material gas occurs in the vicinity of the non-grounded electrode, thereby preventing dust from adhering.

The pressure P may be atmospheric pressure P ₀ or the vicinity thereof.

The fact that there is no need to apply a vacuum is the greatest advantage of the present invention.

The pressure P, when slightly higher than the atmospheric pressure P ₀ can prevent contamination of impurity gases into the deposition chamber 1 from the outside.

The frequency of the high frequency power supply may be between 100 kHz and 100 MHz. The optimum values of the frequency and the power can be determined by the film to be formed and the gap between the electrodes.

However, glow discharge becomes unstable below 1 KHz in terms of discharge stability. Therefore, it should not be lower than 1KHz.

The power of the high frequency power supply is 10 ^-2 W / cm ² to 10 ² W / cm ²
And When it exceeds 10 ² W / cm ₂ , the electrode 2 is sputtered by ions. Therefore, impurities are mixed into the thin film.

If the power is lower than 10 ^-2 W / cm ₂ , a substantial deposition rate cannot be obtained.

(H) Example I (Q / S Dependence) A thin film was formed using the apparatus shown in FIG. 1 while changing the gas flow rate. The source gas used was SiH ₄ , CH ₄ , TiCl ₄ + NH ₃ .

 Table 1 shows the film forming conditions.

The value Q / S obtained by dividing the discharge space volume S by the supply gas flow rate Q is 10
^-1, shown in ^{10 0, 10 1, 10 2} , 10 3 sec each deposition rate of the central portion of the thin film at the time of changing ^-1 Table 2.

As can be seen from the table, when Q / S is 10 ^-1 , the supplied source gas is immediately decomposed, and the film formation rate on the substrate is reduced.

In particular, in the case of a-Si, since SiH ₄ is easily decomposed, polymerization occurs in the gas phase, and dust is formed.

In addition, it can be seen that when Q / S is increased to 10 ³ , the film formation rate is reduced.

This result, Q / S is able that is 10 ⁰ ~10 ² sec ^-1 is good.

(G) Example II (Distance between Electrode Substrates and Discharge State) In order to form a uniform film over a large area, a stable and uniform plasma is required. For that purpose, the distance g between the electrode and the substrate is required. is important. Therefore, the discharge state was examined by changing g at 15 mm, 10 mm, and 5 mm under the conditions shown in Table 3. The results are shown in FIG.

Table 3 Deposition conditions Source gas SiH ₄ source gas / He SiH ₄ / He = 10 ^-3 Q / S 10 ¹ sec ^-1 Pressure Atmospheric pressure RF power 100W RF frequency 13.56MHz Electrode area 40mm × 40mm Substrate SUS ( (40mm x 40mm) Quartz glass (40mm x 40mm) When the substrate is SUS, when g = 15mm, no discharge occurs at 100W RF power, and when the RF power is further increased, the arc changed from local discharge to arc. . On the other hand, g ≦
At 10 mm, uniform plasma is obtained as a whole, and it can be seen that localization of discharge can be prevented and plasma can be formed uniformly by reducing g.

When the substrate is quartz glass, plasma discharge is possible even at g = 15 mm. However, as g is smaller, the plasma spreads more easily, and uniform plasma is obtained as a whole.

(III) Example III (Distance g between electrode substrates and film thickness distribution) Next, the film thickness distribution on the substrate was evaluated by changing the electrode substrate distance g.

Q / S is 10 ¹ sec ^-1 and g is 15mm, 10mm, 5mm, 3mm, 1mm
Was changed. Table 4 shows the results of the film thickness distribution in the central portion of 8 cm × 8 cm of each substrate. Other film forming conditions were the same as those in Example I.

In the case of an insulating substrate, it is possible to form a film even when the distance g between the electrode substrates is 15 mm. On the other hand, when g ≦ 10 mm, the variation is small, and it can be said that the value of g is preferably 10 mm or less.

(C) Example IV (discharge state and source gas / He ratio) The state of glow discharge was examined by changing the ratio of the source gas used for forming a thin film to He. Q / S is 1 under the same conditions as in Example I, such as the electrode area and the distance between the electrodes.
0 ¹ / sec ^-1 . The results are shown in Table 4.

In this table, the crosses indicate that the transition to arc discharge occurs. _{SiH 4, Si 2 H 6,} C 2 H 4, C 2 H 2, N 2 O, O 2, GeH 4
If a gas other than the above is used, a stable discharge can be obtained by setting He to 90% or more, but SiH ₄ , Si ₂ H ₆ , C ₂ H ₄ , C
_{When using 2} H ₂ , N ₂ O, O ₂ , and GeH ₄ , these gases He
It is understood that the ratio to is desirably set to 10 ^-2 or less.

Subsequently, under the same conditions as in Example I, SiH ₄ / He = 10 ⁻³ , Q
As a result of producing various thin films containing Si at a gas flow rate shown in Table 5 with / S to 10 ² sec ^-1 , excellent mechanical and electrical properties were obtained.
A Si thin film was obtained.

(I) Comparative Example For comparison, an a-Si, aC, and TiN thin film was formed under the same conditions as in Example I using the apparatus shown in FIG.

The thin film formation was attempted by variously changing the distance g between the sample substrate electrode and the flow rate Q.

In the case of aC and TiN, when g was larger than 10 mm, a film could be formed at the center of the substrate, but plasma did not spread over the entire electrode, and a uniform film could not be formed on a 10 cm × 10 cm substrate.

In order to expand the plasma, it was only necessary to reduce the ratio of the source gas / He, but the film formation rate was reduced, which was not practical.

Conversely, if g is reduced, the plasma can be spread, and it spreads over the entire electrode at 10 mm or less, but at this time, the deposition rate at the center of the plasma decreases, and the center of the 10 cm × 10 cm substrate
The film thickness distribution of 8 cm × 8 cm was ± 30% or more, so that a uniform film could not be formed.

Further, in the case of a-Si, dust is formed on the outer peripheral portion of the plasma under any conditions, and only a film having a very low film forming rate (0.1 ° / sec or less) is formed outside the plasma. Could not be formed at all.

(S) Thin film characteristics The characteristics of the a-Si thin film formed in Example I were examined.

Band cap Eg-1.77 eV Photoelectric conductivity Δσ _ph = 6 × 10 ⁻⁵ S / cm Dark conductivity σ _d = 8 × 10 ⁻⁹ S / cm. The band gap is a value calculated by measuring the transmittance in the visible light region and performing a Tauk plot of the measured value.

The photoelectric conductivity is a value measured using a light source of AM1.5 100 mW / cm ² .

The larger the ratio between Δσ _ph and σ _d, the more promising as a solar cell material.

A photoconductivity Δσ _ph almost the same as that of the a-Si film formed by the conventional low-pressure plasma CVD method is obtained.

(C) Effect According to the present invention, plasma CVD is performed at a pressure near atmospheric pressure.
According to the method, a thin film such as an a-Si or TiN thin film can be formed.

Since it is near the atmospheric pressure, no vacuum chamber or vacuum exhaust device is required.

In the formation of an a-Si film of a solar cell and the coating of TiN that require a large area of film formation, the cost required for equipment can be significantly reduced.

Also, since the pressure is high, compared to low pressure plasma CVD,
The film forming speed can be increased.

The volume S of the discharge space is defined as the area A of the electrode.
And the distance g between the electrode 2 and the sample substrate 4. That is, S = Ag.

[Brief description of the drawings]

FIG. 1 is an example of a schematic sectional view of an apparatus used for the thin film forming method of the present invention. FIG. 2 is a sectional view of a thin film forming apparatus disclosed in Japanese Patent Application Laid-Open No. 63-50478. FIG. 3 is a view showing a difference in discharge due to a difference in distance between a sample substrate and an electrode in the method of the present invention. DESCRIPTION OF SYMBOLS 1 ... Film-forming chamber 2 ... Non-ground electrode 3 ... Ground electrode 4 ... Sample substrate 5 ... Gas inlet 6 ... RF power supply 7 ... Heater 8 ... Gas outlet

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuuji Tomikawa 1-1-1, Koyokita, Itami-shi, Hyogo Prefecture Inside Itami Works, Sumitomo Electric Industries, Ltd. (72) Inventor Norihiko Fujita 1 Kunyokita, Itami-shi, Hyogo No. 1-1, Sumitomo Electric Industries, Ltd. Itami Works (56) References JP-A-2-73978 (JP, A) JP-A-63-50478 (JP, A)

Claims (1)

(57) [Claims] 1. A sample substrate is directly installed on one of opposing surfaces of two electrodes facing each other, and a distance between the sample substrate and the electrode facing the sample substrate is 10 mm or less, 0.1 mm or more, A value Q / S obtained by dividing a gas flow rate Q by a volume S of a discharge space of a mixed gas composed of a film forming gas and He is 1 to 10 ² sec ^−1.
Is supplied to the discharge space on the sample substrate, and under a pressure near the atmospheric pressure, the high-frequency voltage applied to the counter electrode causes
A thin film forming method comprising: causing a glow discharge between a sample substrate and an electrode facing the sample substrate to form a thin film on the sample substrate.