JP3857024B2 - Method for forming microcrystalline silicon thin film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a microcrystalline silicon thin film on a substrate surface using a chemical reaction by plasma.
[0002]
[Prior art]
In recent years, amorphous silicon has attracted attention as a thin film solar cell material. However, the amorphous silicon has a problem of deterioration due to light irradiation, and has not yet been solved. Therefore, microcrystalline silicon has attracted attention as a material that can replace this amorphous silicon. This microcrystalline silicon has high photoconductivity and no photodegradation due to light irradiation, which is a problem with amorphous silicon, and is expected as a next-generation high-efficiency thin-film solar cell material that surpasses the amorphous silicon ( Applied Physics Vol.68, No.10, p.1133, 1999).
[0003]
Conventionally, as a means for forming such a microcrystalline silicon film, a method using a plasma CVD apparatus in which parallel plate electrodes are installed in a reaction vessel is known. In this method, high frequency power or direct current power is applied to one electrode of the apparatus to generate plasma with the other electrode grounded, and a reactive gas is supplied into the generated plasma. A desired thin film is obtained on the substrate surface by decomposing the reaction gas. Actually, there is currently a method of diverting a large amount of silane gas diluted with hydrogen into the reaction vessel as a reaction gas using a plasma CVD device designed for amorphous silicon film formation. is there.
[0004]
[Problems to be solved by the invention]
In order to put the microcrystalline silicon into practical use as a semiconductor thin film material, the reduction of its manufacturing cost is the most important issue. For example, when microcrystalline silicon is applied to a solar cell module, the microcrystalline silicon has a light absorption coefficient smaller than that of amorphous silicon. Therefore, in order to make the microcrystalline silicon function as a solar cell, it is about 10 times that of amorphous silicon. Therefore, it is essential to increase the formation speed. Furthermore, in order to use as a thin film of a solar cell, it is necessary to form a uniform thin film in a large area, and for this purpose, it is important to supply the reaction gas uniformly and efficiently to the plasma space.
[0005]
However, when a general plasma CVD apparatus is used, the formation rate of microcrystalline silicon is about 1 s / s, which is insufficient industrially. Currently, at the laboratory level using small substrates, VHF with a short wavelength is used for plasma generation (Technical digest of the international PVSEC-11, p.945), and the reaction gas pressure is used to improve the deposition rate. Technology (Technical digest of the international PVSEC-11, p.939) has also been reported, but these technologies are also uniform in a large area due to the influence of a large amount of hydrogen in the atmosphere. It is difficult to generate plasma. Furthermore, particles are generated as the pressure is increased, and gas supply to the plasma space becomes non-uniform, so that uniform film formation on the substrate becomes more difficult.
[0006]
The problem to be solved by the present invention is to form a large-area and uniform thin film of microcrystalline silicon, which has been impossible in the past, at high speed.
[0007]
[Means for Solving the Problems]
As a result of detailed examinations and precise experiments, the present inventors have found that plasma CVD using a rotating electrode can realize a large area and uniform high-speed formation of microcrystalline silicon, which was impossible in the past. did.
[0008]
The plasma CVD method for forming a film using a rotating electrode is described in Japanese Patent Application Laid-Open No. 9-104958. However, the method disclosed in this publication is a method for forming an amorphous silicon thin film. However, there is no disclosure of a method for forming a microcrystalline silicon thin film, particularly a detailed disclosure of film forming conditions.
[0009]
In the CVD method using the rotating electrode, since the atmospheric pressure is high, sufficient crystallization energy cannot be given during film formation only by supplying hydrogen and silane mixed gas. Therefore, in order to form a good microcrystalline silicon thin film by plasma CVD using the rotating electrode, crystallization is performed with the aid of high energy neutral radicals formed by decomposition of inert gas such as helium. Need to be done. On the other hand, when an inert gas having a large mass is excessively mixed in the atmospheric gas, it causes ion bombardment, and damages the surface of the base material to inhibit crystallization.
[0010]
The present invention has been made paying attention to this point, and is a method for forming a microcrystalline silicon thin film on the surface of a substrate, in a reaction vessel in which a rotating electrode having a cylindrical outer peripheral surface is arranged. Introducing a reactive gas containing silane and hydrogen and an inert gas into the substrate, causing the rotating electrode and the substrate to face each other while rotating the rotating electrode, generating plasma in the gap, and A step of causing a chemical reaction to the reaction gas to form a thin film on the substrate and moving the base material relative to the rotating electrode in a direction substantially perpendicular to a rotation center axis thereof, The ratio of the partial pressure of the hydrogen gas to the partial pressure of the silane gas of the reaction gas introduced into the gas is 12 to 100, and the ratio of the partial pressure of the reactive gas to the partial pressure of the inert gas is 0.05 to 10 Than it is.
[0011]
Here, in the narrow sense, “microcrystalline silicon thin film” refers to a silicon film made of crystals of about several tens of nanometers having a peak due to TO phonons in the vicinity of 520 cm −1 by Raman scattering spectroscopy. The term “microcrystalline silicon thin film” refers to a broad concept including, but not limited to, a film made of polycrystalline silicon having a particle size of about 1 nm to 1000 nm, and a film containing nanocrystalline components and amorphous components simultaneously. is there.
[0012]
According to the above-described method, not only can the reactive gas be efficiently introduced into the plasma space along the rotation of the electrode, but also the gas can be drawn into the substrate surface as a laminar flow. Therefore, for example, by using an electrode having a sufficiently wide width as compared with the base material, the reaction gas can be flowed uniformly on the base material. In addition, in this method, plasma can be generated with a gap of several millimeters or narrower. For example, by using a resonator to boost high frequency power and applying it between the electrodes, the electric field strength between the electrodes can be increased. And plasma can be generated even in a high hydrogen concentration state. Furthermore, by using the rotating electrode, it is possible to prevent the temperature of the electrode from rising due to the concentration of plasma.
[0013]
Moreover, in the present invention, it is possible to form a good microcrystalline silicon thin film by introducing a silane gas appropriately diluted with hydrogen into the plasma at an appropriate concentration. Specifically, since the ratio of the partial pressure of hydrogen gas to the partial pressure of silane gas in the reaction gas introduced into the reaction vessel is set to 12 or more, silicon is compared with the case where the partial pressure ratio is less than 12. In addition, the partial pressure ratio is set to 100 or less to suppress excessive supply of hydrogen gas, so that the extraction of hydrogen necessary for crystal growth is improved and excess hydrogen is removed. It is possible to prevent the gas from inhibiting the adsorption of silane radicals on the substrate surface. Furthermore, if the partial pressure ratio is 50 or less, it becomes possible to suppress the etching action by hydrogen and form a better thin film.
[0014]
On the other hand, in the present invention, since the ratio of the partial pressure of the reactive gas to the partial pressure of the inert gas is 10 or less, sufficient crystallization is achieved with the aid of the energetically high neutral radical formed in the decomposition of the inert gas. Can be made. Moreover, since the partial pressure ratio is 0.05 or more, ion bombardment caused by excessive mixing of an inert gas having a large mass into the atmospheric gas is suppressed, and this ion bombardment causes crystallization on the substrate surface. Inhibiting can be prevented.
[0015]
That is, according to the present invention, it is possible to form a good microcrystalline silicon thin film by plasma CVD using a rotating electrode by setting the respective partial pressure ratios.
[0016]
In the present invention, the use of the rotating electrode provides the advantage that the electron temperature can be kept sufficiently low. However, it is known that an inert gas such as helium increases the electron temperature. The introduction of a large amount will halve the above-mentioned advantage. Therefore, the ratio of the partial pressure of the reactive gas to the partial pressure of the inert gas is more preferably 0.2 or more.
[0017]
Here, the inert gas contained in the reaction vessel other than the reaction gas is selected from the group consisting of helium, neon, argon, krypton, and xenon.
[0018]
In the present invention, it is desirable that the total pressure in the reaction vessel is 80 Torr or more and 500 Torr or less (that is, 1.07 × 10 ⁴ Pa or more and 6.67 × 10 ⁴ Pa or less) because the rotating electrode is used. When the total pressure is less than 80 Torr, it becomes difficult to suck the gas into the minute gap formed between the rotating electrode and the substrate surface even if the rotating electrode is rotated. Conversely, the total pressure exceeds 500 Torr. Then, there is a high risk that the radical species generated will be discharged from the reaction vessel before reaching the substrate surface, and the probability that silicon atoms will collide with each other in the gas phase increases and particles are likely to be generated. Because.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a preferred embodiment of the present invention will be described with reference to FIGS.
[0020]
In FIG. 1, a base material carrier 12 is installed in a sealed reaction vessel 10, and a base material 14 is placed thereon, and the base material carrier 12 and the base material 14 are connected to ground. Has been. A rotating electrode 18 is disposed so as to face the substrate 14 with a slight gap. The rotating electrode 18 has a rotating shaft 16 extending through the center of a substantially cylindrical electrode body extending in the depth direction in the figure, and both ends of the rotating shaft 16 are rotatably supported by columns not shown. The end of the rotating shaft 16 is connected to a high-frequency power source 20 (which may be a DC power source) shown in FIG. 2 via a conductive wire 17 and a resonator (impedance matching device) 19 provided on the outer surface of the reaction vessel. .
[0021]
In this apparatus, the inside of the reaction vessel 10 is evacuated, and a high-frequency power (or DC power) may be applied to the rotating electrode 18 while rotating the rotating electrode 18 to generate plasma 22 between the rotating electrode 18 and the substrate 14. At the same time, when a reaction gas (mixed gas of SiH ₄ and H ₂ ) and a dilution gas (for example, an inert gas such as He) are introduced into the reaction vessel 10 from a reaction gas supply source (not shown), these gases are rotated electrodes. By the rotation of 18, it is caught between the rotating electrode 18 and the substrate 14 and guided to the plasma 22, where the reaction gas causes a chemical reaction. As a result of scanning the base material 14 in a predetermined direction (direction orthogonal to the rotational axis direction of the rotary electrode 18) together with the base material transport table 12 while causing such a chemical reaction, a thin film is formed on a large area on the base material 14. 23 can be formed at high speed.
[0022]
In this method, the ratio of the partial pressure of hydrogen gas to the partial pressure of silane gas in the reaction gas is set to 12 or more and 100 or less (more preferably 50 or less), and the partial pressure of the reaction gas with respect to the partial pressure of the inert gas. If the ratio is 0.05 or more (more preferably 0.2 or more) and 10 or less, a good microcrystalline silicon thin film in which the thin film 23 formed on the base material 14 is sufficiently crystallized for the reasons described above. It can be. Further, as described above, it is more preferable that the total pressure in the reaction vessel 10 is 80 Torr or more and 500 Torr or less.
[0023]
In this embodiment, the method of forming the intrinsic semiconductor thin film by introducing only the reaction gas and the inert gas into the reaction vessel 10 is shown, but the present invention does not exclude the addition of other gases. . For example, it is free to add a doping gas typified by diborane or phosphine to form a p-type or n-type semiconductor layer, or to add a gas serving as a carbon source for band gap control.
[0024]
Although the formation temperature of microcrystalline silicon is desirably 300 ° C. or lower, particularly 220 ° C. or lower, the present invention does not exclude film formation at higher temperatures. In general, a glass substrate is often used as the base material, but the material of the base material, particularly the surface material, is not limited to the glass, and for example, it is superimposed on an amorphous silicon or microcrystalline silicon thin film formed on the glass. A microcrystalline silicon thin film may be formed. Further, when single crystal silicon or the like is used as a base material, it is possible to form single crystal or polycrystalline silicon having a large particle size by the method of the present invention.
[0025]
【Example】
Next, specific examples of the present invention will be described.
[0026]
Example 1
As the plasma CVD apparatus, the rotating electrode 18 shown in FIG. 1 is a drum electrode made of aluminum alloy having a width of 30 cm, and a rotating drive means having a high-speed rotating motor connected to the rotating shaft 16 is used. The rotating electrode is connected to the high-speed rotating motor through a vibration-proof magnet coupling and a magnetic fluid seal. A high frequency power source of 150 MHz is used as a power source for generating plasma, and high frequency power is applied to the electrode portion via the impedance matching unit to generate plasma in the film forming gap. A typical value for the deposition gap is 500 microns. A duct for removing generated particles is provided at the rear of the rotating electrode, and the particles are removed by a filter.
[0027]
In this apparatus, a Pyrex glass substrate that has been subjected to a cleaning process and a drying process is set on a sample stage, a film formation gap is adjusted, a vacuum evacuation is performed, and then helium is mixed with a reaction gas composed of hydrogen and silane. Gas was introduced into the chamber through the mass flow controller, and the atmospheric pressure was 400 Torr. Next, plasma was generated while rotating the electrode, and a microcrystalline silicon film was formed on the surface of the substrate. The film formation conditions at this time are: silane partial pressure 1.5 Torr, hydrogen partial pressure 28.5 Torr, substrate temperature 200 ° C., film formation time 60 seconds, total input power about 1000 W (input power per 1 cm of electrode width is about 33W).
[0028]
As a result of Raman spectroscopic analysis of the thin film obtained under these conditions, a Raman shift as shown in FIG. 3A was obtained, and the formed thin film was silicon in which microcrystal and amorphous were mixed, that is, microcrystal as referred to in the present invention. It was confirmed that the thin film was made of silicon.
[0029]
On the other hand, as a comparative example, a film formation experiment was performed with a hydrogen partial pressure of 8.5 Torr and other film formation conditions set to be exactly the same as described above. FIG. 3B shows the result of Raman analysis of the silicon thin film obtained by this experiment. The Raman shift shown in the figure is clearly that of amorphous silicon, and almost no crystallization is observed.
[0030]
Example 2
Next, in order to see the effect of hydrogen dilution, a film formation experiment was performed with the silane partial pressure fixed at 10 Torr and only the hydrogen dilution ratio changed. The total pressure was basically 200 Torr, but when the partial pressure of the reaction gas was high, the pressure was adjusted so that the ratio between the partial pressure and the partial pressure of the inert gas was obtained. Other conditions are the same as in the first embodiment. The results obtained in this experiment are shown in FIG. From this figure, it can be understood that the crystallization rate rapidly increases when the concentration of hydrogen relative to silane is 12 or more.
[0031]
【The invention's effect】
As described above, the present invention forms a microcrystalline silicon thin film on the surface of a substrate by performing plasma CVD while rotating an electrode provided in the reaction vessel, and a reaction introduced into the reaction vessel. The ratio of the partial pressure of hydrogen gas to the partial pressure of silane gas in the gas is 12 or more and 100 or less (more preferably 50 or less), and the ratio of the partial pressure of the reaction gas to the partial pressure of the inert gas is 0. .2 or more (more preferably 0.4 or more) and 10 or less, there is an effect that a microcrystalline silicon thin film can be formed at high speed in a large area.
[Brief description of the drawings]
FIG. 1 is a cross-sectional front view showing an example of a microcrystalline silicon thin film forming apparatus for carrying out the method of the present invention.
FIG. 2 is a schematic perspective view showing a main part of the apparatus.
FIG. 3A is a diagram showing the results of Raman scattering spectroscopy of a silicon thin film obtained in an example according to the present invention, and FIG. 3B is a diagram showing the results of Raman scattering spectroscopy of a silicon thin film obtained in a comparative example. FIG.
FIG. 4 is a diagram showing a relationship between a dilution ratio of hydrogen in a reaction gas and a silicon crystallization rate.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Reaction container 12 Base material conveyance base 14 Base material 18 Rotating electrode

Claims (5)

A method for forming a microcrystalline silicon thin film on a substrate surface, wherein a reaction gas containing silane and hydrogen and an inert gas are introduced into a reaction vessel in which a rotating electrode having a cylindrical outer peripheral surface is arranged. Forming a thin film on the substrate by causing a chemical reaction to the reactive gas with the plasma and generating a plasma in the gap between the rotating electrode and the substrate facing each other while rotating the rotating electrode And the relative movement of the base material in the direction substantially perpendicular to the rotation center axis with respect to the rotating electrode, and the fraction of hydrogen gas relative to the partial pressure of the silane gas of the reaction gas introduced into the reaction vessel A method for forming a microcrystalline silicon thin film, characterized in that a pressure ratio is 12 or more and 100 or less, and a ratio of a partial pressure of the reaction gas to a partial pressure of the inert gas is 0.05 or more and 10 or less. 2. The method for forming a microcrystalline silicon thin film according to claim 1, wherein a ratio of a partial pressure of the reaction gas to a partial pressure of the inert gas introduced into the reaction vessel is 0.2 or more and 10 or less. A method for forming a microcrystalline silicon thin film. 3. The method for forming a microcrystalline silicon thin film according to claim 1, wherein a ratio of a partial pressure of hydrogen gas to a partial pressure of silane gas of the reaction gas introduced into the reaction vessel is 12 or more and 50 or less. A method for forming a microcrystalline silicon thin film. 4. The method for forming a microcrystalline silicon thin film according to claim 1, wherein the total pressure in the reaction vessel is 80 Torr or more and 500 Torr or less. 5. The method for forming a microcrystalline silicon thin film according to claim 1, wherein the inert gas contained in the reaction vessel is at least one selected from the group consisting of helium, neon, argon, krypton, and xenon. There is provided a method for forming a microcrystalline silicon thin film.

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