JP4841735B2 - Deposition method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a plasma CVD apparatus capable of forming a microcrystalline silicon film or the like at a high speed, and can form a thin film having high crystallinity at a high speed, and of powder (particles generated by an unintended reaction). The present invention relates to a plasma CVD apparatus and a film forming method that are less likely to occur and that can be easily maintained.
[0002]
[Prior art]
Conventionally, a plasma CVD (Chemical Vapor Deposition) apparatus is often used for forming a silicon thin film or a diamond thin film. There are two types of plasma CVD methods, capacitive coupling type and inductive coupling type. In the capacitive coupling type, a vacuum chamber is usually provided with a pair of parallel plate cathode and anode electrodes.
[0003]
A typical example of a silicon thin film is a solar cell. Thin-film silicon solar cells have been studied as solar cells that can replace bulk crystal silicon solar cells from the viewpoint of manufacturing and material cost reduction. A thin-film silicon solar cell usually has a p-layer / i-layer / n-layer or p-layer / n-layer film configuration, and a plasma CVD method is usually used to form these layers. A reactive gas such as monosilane (SiH ₄ ) or disilane (Si ₂ H ₆ ) alone or a mixed gas of these gases and hydrogen (H ₂ ) is introduced into the vacuum chamber, and RF, VHF, or the like is introduced into the cathode. High frequency power is applied to excite the plasma, the reactive gas is decomposed by this plasma, and a silicon film is deposited on the substrate.
[0004]
The substrate is usually placed on the anode, and the substrate temperature during film formation is controlled by a heater in the anode. At this time, the amorphous film and the microcrystalline film can be separately formed by changing the film forming conditions, that is, the flow rate ratio of monosilane or disilane and hydrogen, the substrate temperature, and the like. When diborane (B ₂ H ₆ ) is added to the source gas, boron is added and the film becomes p-type. When phosphine (PH ₃ ) is added, phosphorus is added and the film becomes n-type. )
Amorphous silicon films have been studied extensively from basic studies such as film formation mechanism and film quality to applications such as solar cells, and some of them are already in the industrialization stage. The problem of light degradation remains as an essential problem. An amorphous structure is metastable in terms of energy and is likely to cause structural changes due to external energy. In the case of amorphous silicon, dangling bonds of silicon atoms in the amorphous silicon, that is, the density of defects increases due to light irradiation, so that the conversion efficiency decreases by about 5 to 15% by light irradiation.
[0005]
On the other hand, microcrystalline silicon solar cells have recently attracted attention as materials that do not cause photodegradation. However, with regard to microcrystalline silicon solar cells, at the stage of their development research, there are many issues to be investigated such as crystallization mechanism, film quality, and cell structure optimization, and active development research is currently underway. ing.
[0006]
Since amorphous silicon is a direct transition semiconductor, the absorption coefficient is relatively large. In order to obtain sufficient sunlight absorption, the i layer is usually about 300 to 500 nm, whereas microcrystalline silicon is crystalline silicon. Similarly, since it is an indirect transition semiconductor, the absorption coefficient is smaller than that of amorphous silicon, and a film thickness of about 2 to 3 μm is required.
[0007]
[Problems to be solved by the invention]
A microcrystalline silicon solar cell requires a film thicker than an amorphous silicon film in order to obtain sufficient light absorption. Conventionally, the deposition rate of a microcrystalline silicon film has been about 0.1 nm / second, and therefore, It took several hours to deposit about ˜3 μm, and the productivity was poor.
[0008]
Therefore, at present, increasing the deposition rate of the microcrystalline silicon film in the plasma CVD method is one of the important issues. As a method for increasing the deposition rate, the deposition pressure is increased (several hundred Pa (several Torr)), ion damage is reduced, high RF power or high VHF power is applied, and the source gas is depleted. High pressure depletion method (e.g. Japanese Journal of Applied Physics 37 (1998) Ll 1 1 6 pages, Journal of Non-Crystalline Solids, 266-269 (2000), 84 pages) is proposed. Has been.
[0009]
The above-described method does not require a large change in the apparatus configuration, and a highly crystalline microcrystalline film can be obtained at a film forming rate of about 3 nm / second only by changing the film forming conditions. However, since this method forms a film in a source gas under a high pressure, it is easy to generate powder, and it is necessary to frequently clean the inside of the cathode and the chamber in order to maintain the film formation speed and the reproducibility of the film quality. The maintenance of the equipment took time and productivity was poor. In addition, when the film formation rate was 4 nm / second or more, the crystallinity of the film was remarkably lowered, so that a film with good crystallinity could not be formed.
[0010]
The present invention has been made to solve the above-mentioned problems, and is excellent in productivity, and a microcrystalline film having high crystallinity can be obtained, and generation of powder is small, and plasma CVD with less labor for chamber cleaning is achieved. An object is to provide an apparatus and a film forming method.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 includes a parallel plate type cathode and an anode, and is formed of a conductive mesh between the cathode as the first electrode and the anode as the second electrode. A method of forming a film on a substrate using a plasma CVD apparatus equipped with a third electrode, wherein the substrate is placed in close contact with the anode side, and the distance between the third electrode and the substrate is 10 mm. Hereinafter , film formation was performed in a source gas depletion region of a mixed gas of silane and hydrogen using the High Pressure Depletion method under the condition that the input power was 1.1 W / cm ² or more, and Raman crystallinity (I ( There is provided a film formation method characterized by forming a microcrystalline silicon film having a ratio of 520 cm ⁻¹ ) / I (480 cm ⁻¹ )) larger than 0.5.
[0012]
In the capacitively coupled plasma CVD apparatus suitable for the implementation of the High Pressure Depletion method, the present invention further includes a third electrode made of a conductive mesh such as a metal mesh, and the distance between the third electrode and the substrate is 10 mm or less. Is set. The conductive mesh placed close to the substrate (less than 10 mm) serves to reduce ion damage to the film growth surface. Further, when the input power is increased, the light emission of the plasma becomes stronger near the mesh, and a special plasma state is obtained. Under this plasma state, generation of powder is suppressed, and a microcrystalline film with good crystallinity can be formed with high productivity.
[0014]
Furthermore, as a pre-Symbol cathode, embodiments employing cathode of showerhead that holes for gas injection are formed many preferred. The shower head type cathode can introduce the source gas efficiently, and can increase the deposition rate.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the plasma CVD apparatus and film forming method of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic diagram of an electrode configuration in a plasma CVD apparatus according to an embodiment of the present invention. In the figure, reference numeral 1 denotes a cathode, 2 denotes plasma, 3 denotes a metal mesh, and 4 denotes an anode having a substrate heater installed therein. Reference numeral 11 denotes a glass substrate on which microcrystalline silicon is formed. In this example, the anode 4 is at ground potential, and the glass substrate 11 is placed on the anode 4 in contact with the anode 4.
[0017]
The metal mesh 3 is electrically grounded to be equipotential with the anode 4, and the distance between the metal mesh 3 and the glass substrate 11 is set such that no discharge occurs between them. Hereinafter, the metal mesh 3 is expressed as “mesh electrode” or simply “mesh” as necessary.
[0018]
FIG. 2 schematically shows a plasma potential when a discharge is generated with the electrode configuration of FIG. No discharge occurs between the mesh electrode 3 and the glass substrate 11, and most of the electrons incident from the plasma to the glass substrate 11 are captured by the mesh electrode 3, so that the glass substrate 11 is not charged up. Therefore, the mesh electrode 3, the anode 4 and the glass substrate 11 are electrically equipotential (ground potential), and no potential difference is generated between them.
[0019]
On the other hand, at a film forming pressure of several hundred Pa (several Torr), the mean free path of particles is 0.1 mm or less, and the mesh electrode 3 and the glass substrate 11 have passed through the mesh electrode 3 even if they are separated by several mm. Ions collide several tens of times before reaching the glass substrate 11. Therefore, if the mesh electrode 3 and the glass substrate 11 are equipotential, the ions after collision enter the film surface without being accelerated by the electric field. That is, ion damage to the film surface is suppressed.
[0020]
The distance between the mesh electrode 3 and the glass substrate 11 needs to be 10 mm or less, more preferably 1 to 10 mm, particularly 1 to 5 mm. If the distance between the mesh electrode 3 and the glass substrate 11 is shorter than 1 mm, the mesh pattern may be transferred to the film of the glass substrate 11. If the distance is longer than 10 mm, the mesh electrode 3 and the glass substrate 11 may not be transferred. There is a risk of electrical discharge between them. When discharge occurs, the mesh electrode 3 and the glass substrate 11 do not become equipotential, and ion damage to the film growth surface occurs.
[0021]
The opening ratio of the metal mesh 3 is appropriately in the range of 5 to 95%, and more preferably in the range of 30 to 90%. When the aperture ratio is less than 5%, the film forming speed is remarkably reduced. On the other hand, if the aperture ratio is greater than 95%, electrons entering the glass substrate 11 from the plasma are not captured by the mesh 3 and reach the glass substrate 11, so that the glass substrate 11 may be charged up. Although a metal mesh is used in this example, the material of the mesh is not particularly limited as long as it has conductivity, and not only stainless steel, copper and other metals but also a conductive material such as carbon is used. Can do.
[0022]
Next, the operation of the plasma CVD apparatus configured as described above will be described. In the electrode arrangement shown in FIG. 1, if the input power to the cathode 1 is low, the emission intensity of plasma is strong near the cathode between the cathode and the mesh. However, when the input power is increased, the light is emitted more strongly near the mesh electrode 3 than the cathode 1 above a certain power. Although the cause of this has not been investigated, it is thought to be due to the emission of electrons from the fine line portion constituting the mesh.
[0023]
At this time, when the emission intensity of the plasma was analyzed, it was found that the emission intensity mainly in the excited state of Si (wavelength 288 nm) was stronger near the mesh electrode 3. Further, it was demonstrated that in such a plasma state, the generation of powder in the plasma is suppressed, and the amount of powder adhering to the chamber wall or the like is significantly reduced. This is considered to be because higher order silanes (powder causative substances) are decomposed by discharge occurring near the mesh electrode 3 and the raw materials are used efficiently.
[0024]
So far, both the mesh electrode 3 and the anode 4 have been described as ground potentials. However, the mesh electrode 3 and the anode 4 are not necessarily set to the ground potential, and may be a positive potential or a negative potential. In particular, when the potential is negative, the difference between the plasma potential and the mesh potential becomes large, so that electrons are easily emitted from the mesh electrode 3, and a plasma state in which light emission near the mesh electrode 3 is likely to occur. In addition, the mesh 3 and the anode 4 do not necessarily have the same potential, and the mesh 3 preferably has a negative potential with respect to the anode 4. At this time, ions (positive ions) that have passed through the mesh 3 are not accelerated toward the anode 4.
[0025]
In such an electrode arrangement, if a shower head cathode (a cathode in which a number of gas injection holes are formed at the cathode tip) is further used as the cathode 1, the concentration of the source gas between the cathode and the glass substrate increases. The film formation rate can be significantly increased at the same input power.
[0026]
In the plasma CVD apparatus to which the present invention is applied, although the microcrystalline silicon film is formed at a speed of 5 nm / second or more, the defect density in the film can be suppressed to a low value and the crystallinity of the film can be increased. it can. The cause of this has not been fully elucidated yet, but it is thought to be due to the effect of reducing ion damage by the third electrode or the effect of suppressing the generation of higher order silane by the plasma with strong emission intensity near the third electrode.
[0027]
Up to now, the case of a glass substrate has been described as the film formation object, but the substrate is not limited to glass, and a resin substrate or a metal substrate can also be used. In the former case, the substrate temperature must be low enough that the resin substrate does not deform.
[0028]
Further, the high speed deposition of the crystalline thin film of the present invention have been described silicon thin film so far, not limited to this, for example, a diamond thin film or, such as silicon nitride (Si ₃ N _4), it is formed by a plasma CVD The present invention is also effective for forming other thin films, and a film having good crystallinity can be deposited at a high speed.
[0029]
According to the embodiment of the present invention, the metal mesh 3 (third electrode) arranged close to the substrate functions to reduce ion damage to the film growth surface. As the input power is increased, the emission of plasma becomes stronger even near the mesh, and a special plasma state is obtained. At this time, the generation of powder in the plasma is suppressed. Furthermore, the present invention can significantly reduce the defect density of the deposited microcrystalline silicon film.
[0030]
【Example】
<Example 1>
A cleaned glass substrate having a thickness of 0.7 mm is set in a capacitively coupled plasma CVD apparatus having a shower head cathode and an anode having a diameter of 126 mm as shown in FIG. 1, and is placed on a 1.33 × 10 ⁻⁶ Pa level (1 × 10 ⁻⁸ Torr level). A metal mesh 3 having an opening ratio of 64% as a ground potential was placed at a distance of 2 mm directly above the glass substrate 11. FIG. 3 shows a conceptual diagram of the implementation status. The implementation conditions were a cathode 1 size of 125 cm ² , an anode size of 128 cm ² , a cathode-substrate distance of 17 mm, an excitation frequency of 60 MHz, a deposition pressure of 266 Pa (2 Torr), and a substrate temperature of 250 ° C. FIG. 5 shows the evaluation results of the film when the silicon single layer film is formed under the implementation conditions shown in Table 1 of FIG.
[0031]
A silicon single layer film was formed under the conditions described in the column “Example 1” shown in FIG. The evaluation method of the film is as follows. The same evaluation method was used in Examples 1 to 6 and Comparative Examples 1 to 5.
[0032]
The crystallinity of the film was measured using a Renishaw Ramascope type Raman microscope using He-Ne laser light (wavelength 632.8 nm), and the crystalline silicon peak I _c (520 cm ⁻¹ ) and the amorphous silicon peak in the measurement profile. Evaluation was made using a height ratio I _c / I _a of I _a (480 cm ⁻¹ ). Usually, when I _c / I _a is 0.5 or less, the film is amorphous, and when it is larger than 0.5, the film is microcrystalline, and the larger the value, the better the crystallinity.
[0033]
The film thickness d is measured in the wavelength range from 250 nm to 2500 nm, and the wavelength λmax at the maximum value (peak) of the transmittance on the longest wavelength side and the wavelength λmin at the minimum value (valley) on the longest wavelength side From the refractive index n of the film, the following equation (1)
[0034]
[Expression 1]
d = (λmax × λmin) / (λmax−λmin) / 4n (1)
Determined by Here, the refractive index n is expressed by the following equation from the maximum value Tmax of the transmittance on the longest wavelength side and the minimum value Tmin on the longest wavelength side, the refractive index n _{2 of the} substrate, and the refractive index n ₀ (= 1) of air. (2)
[0035]
[Expression 2]
n = (1/2) × [(n ₀ + n ₂ ) × {(Tmax / Tmin) ^1/2 } + {((Tmax / Tmin) × (n ₀ + n ₂ ) ² −4n ₀ × n ₂ ) ^1/2 }] ... (2)
Determined by
[0036]
The transmittance was measured with a Hitachi U-4000 type spectral transmittance measuring device. The defect density of the film was determined by Electron Spin Resonance (ESR) measurement (Erux ESR measurement device manufactured by Bruker). The amount of the powder was visually observed after adhering to the chamber wall after the film thickness of 1 μm was formed.
[0037]
The film quality of the film formed under the conditions of Example 1 is shown in Table 2 of FIG. The film formation speed of the film according to Example 1 was 5.0 nm / second, and the film was a microcrystalline silicon film. The film had a Raman crystallinity of 3.0. The defect density was 5.5 × 10 ¹⁶ / cm ³ . In practice, it is considered to be preferably 1 × 10 ¹⁷ / cm ³ or less. After film formation, the amount of powder adhering to the chamber wall was very small.
<Examples 2 to 6 and Comparative Examples 1 to 5>
Films were formed in the same manner as in Example 1 except that the conditions shown in the columns of “Example 2” to “Comparative Example 5” in FIG. The film quality of each obtained silicon film is shown in FIG.
[0038]
As the results of Examples 1 to 6 and Comparative Examples 1 to 5 show, a good microcrystalline silicon film can be obtained in a region below the boundary between the mesh-substrate distance of about 10 mm and above 11 mm. It turned out to be amorphous silicon. Although not shown in FIGS. 4 and 5, a microcrystalline silicon film can be obtained under the condition that the distance between the mesh and the substrate is 10 mm.
[0039]
Next, the relationship between the deposition rate and the input power for the Raman crystallinity will be described. 6 shows the film formation rate and Raman crystallinity (I _c / I _a = I (520 cm ⁻¹ ) when film formation is performed using the capacitively coupled plasma CVD apparatus equipped with the parallel plate electrodes shown in FIG. ) / I (480 cm ⁻¹ )) shows the input power dependency. As in the case of Example 1, the operating conditions were a mixed gas of SiH ₄ and H ₂ (flow rate ratio SiH ₄ / H ₂ = 30/270) as a source gas, a shower head cathode size of 125 cm ² , an anode electrode The size was 128 cm ² , the cathode-substrate distance was 17 mm, the excitation frequency was 60 MHz, the deposition pressure was 266 Pa (2 Torr), and the substrate temperature was 250 ° C.
[0040]
In FIG. 6, the solid line shows the film quality of the film formed by the plasma CVD apparatus according to the present embodiment having the mesh electrode, and the dotted line shows the film quality of the film formed by the plasma CVD apparatus not equipped with the mesh electrode for comparison. . Black circle marks (●) and white circle marks (◯) indicate the relationship between the film formation rate and input power, and black triangle marks (▲) and white triangle marks (Δ) indicate the relationship between Raman crystallinity and input power. As shown in the figure, in the plasma CVD apparatus according to the present embodiment, when an input power of 180 W or more is applied, a microcrystalline silicon film having _a Raman crystallinity I _c / I _a of about 3 is 5 to 6 nm / sec. Obtained at the film formation rate.
[0041]
FIG. 7 is a graph showing the results of plasma emission analysis. According to the figure, at a high power of 135 W or more, the intensity of Si (wavelength 288 nm) is remarkably increased as compared with other lines (Si H, Hα, Hβ). That is, it is shown that the emission intensity mainly in the excited state of Si (wavelength 288 nm) is strong near the metal mesh 3 at high power. In addition, as described above, in such a plasma state, the generation of powder in the plasma is suppressed, and the amount of powder attached to the chamber wall or the like is remarkably reduced.
[0042]
【The invention's effect】
As described above, according to the engagement Ru film forming method in the present invention, so that the distance between the substrate and the third electrode becomes 10mm or less, disposed conductive mesh (third electrode) is brought closer to the substrate With this configuration, a special plasma state close to the mesh can be obtained in the high power region, and a microcrystalline silicon film having a low defect density can be deposited at a high speed of 5 nm / second or more. In addition, since the amount of powder generated can be reduced at this time, maintenance for cleaning the apparatus can be reduced, leading to cost reduction.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an electrode configuration of a plasma CVD apparatus according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a plasma potential in the electrode configuration of FIG. 4 is a chart showing the implementation conditions of Examples 1 to 6 and Comparative Examples 1 to 5. FIG. 5 shows the evaluation results of the films when a silicon single layer film is formed under each implementation condition shown in FIG. Chart [Fig. 6] Graph showing deposition power dependence of deposition rate and Raman crystallinity [Fig. 7] Graph showing results of plasma emission analysis [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Cathode, 2 ... Plasma, 3 ... Metal mesh, 4 ... Anode, 11 ... Glass substrate

Claims (1)

A film is formed on a substrate using a plasma CVD apparatus including a parallel plate type cathode and an anode, and a third electrode made of a conductive mesh between the cathode as the first electrode and the anode as the second electrode. A method of forming,
The high pressure depletion method is performed under the condition that the substrate is placed in close contact with the anode side, the distance between the third electrode and the substrate is 10 mm or less , and the input power is 1.1 W / cm ² or more. Is used to form a microcrystalline silicon film having a Raman crystallinity (I (520 cm ⁻¹ ) / I (480 cm ⁻¹ )) greater than 0.5. A film forming method characterized by forming a film.

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