JP5277848B2 - Method for forming photoexcitable substance - Google Patents

Description

  The present invention relates to a method for forming tin oxide using a vapor phase chemical growth reaction.

  Tin oxide is excellent in transparency, electrical conductivity, and chemical durability, and is used in various applications including transparent conductive films for solar cells. When a tin oxide film is used as a transparent conductive film for solar cells, tin oxide is formed on a transparent substrate such as glass by vapor phase chemical growth reaction (hereinafter referred to as CVD method), sputtering method, spray method, etc. It is common to do. Among these, CVD is capable of film formation at atmospheric pressure, so production costs are low, and various surface shapes can be created by devising manufacturing conditions, making it suitable for film formation over a large area. It is a film forming process.

  In order to increase the conductivity of the tin oxide film, it is effective to increase crystallinity and reduce crystal grain boundaries. In particular, when a film is formed on glass, Patent Document 1 describes that it is effective to increase the crystal orientation of the (200) plane.

  In addition, when a tin oxide film is formed on a glass as a solar cell substrate, it is possible to achieve good battery characteristics by setting the orientation of the (100) plane together with the (200) plane to a crystal strength ratio of 100 to 20 to 120. It is described in Patent Document 2 that it is easy. Also in this case, it is described that higher crystallinity is preferable. Thus, when utilizing a tin oxide film as an electrically conductive film, the higher the crystallinity, the better.

  On the other hand, to produce a tin oxide film, it is also important to improve productivity. One method for improving the productivity of the tin oxide film is to increase the deposition rate. In order to increase the film formation rate by the CVD method, it is effective to increase the concentration of the raw material to be supplied. However, when the raw material concentration is increased, the crystallinity tends to be low, and there is a problem that the improvement of the crystallinity and the improvement of the film forming speed are not compatible. As a method for increasing the film formation rate, it is well known that a method for increasing the film formation temperature is also effective. However, in the case of a low melting point material such as glass, the substrate is thermally deformed when the film forming temperature is raised. There is a problem that easily diffusing components such as sodium in the glass are mixed in the film and the conductivity of the tin oxide is remarkably deteriorated.

In Patent Document 3, there is a description that ultraviolet rays are applied when an oxide film is formed by CVD as a process using vacuum ultraviolet light. In addition, in the examples, there is a description that the film formation rate is increased by irradiating ultraviolet rays (xenon excimer lamp: 12 mW / cm ² ) when forming an organic tin oxide raw material (TEOS). As described in the paragraph (paragraph 0007) that the molecular bond of the material gas is broken by photon energy in the gas phase and adsorbed on the wafer surface to become an oxide (paragraph 0007), it becomes a mechanism for decomposing the gas in the gas phase with ultraviolet rays. Yes. Then, in order to decompose the gas in the gas phase, for example, a xenon excimer lamp is used to generate an expensive ultraviolet ray that emits strong ultraviolet rays having a peak wavelength of less than 200 nm (specifically, peak wavelengths of 126, 135, 146, 165, 172 nm, etc.) The apparatus requires a very strong ultraviolet irradiation intensity of 12 mW / cm ² .

Patent Document 4 states that “a photodecomposition reaction is caused to a source gas in a space to form a thin film of a photodecomposition product of the source gas on the substrate” (Claim 1). In the example, as a condition for ultraviolet rays, ultraviolet light (peak wavelengths 184.9, 253.7 nm) was irradiated to the source gas Sn (CH ₃ ) ₄ with a low-pressure mercury lamp, and the irradiation gas was 470.0 nm over 1 hour. There is a description of forming a SnO ₂ film. There is a description in the specification that the intensity of ultraviolet rays needs to be at least 0.34 W / cm ² .

In non-patent literature, a TiSiOx film is formed by using TTIP + TEOS raw material as a raw material gas with ultraviolet rays (222 nm, 20 mW / cm ² ). Although there is a description that uses ultraviolet rays with strong ultraviolet irradiation intensity, there is only a description that the effect of ultraviolet rays accelerates the decomposition of TTIP, there is no description about raw materials other than Ti, especially tin oxide.

JP-A 61-227945 JP-A-5-67797 JP 2001-274156 A JP-A-62-74081 Thin Solid Films 453. 454 (2004) 167.171

  Provided is a method for forming a tin oxide film by a CVD method which improves crystallinity without changing the film formation rate.

A thin film forming method using a chemical vapor deposition reaction to form a tin oxide film on a substrate, and at the same time heating the substrate during the formation, ultraviolet irradiation intensity 0.03 mW / cm ² or more 1 mW / cm ² Provided is a thin film forming method characterized by irradiating a substrate with the following ultraviolet rays.

  By heating the substrate during film formation and irradiating ultraviolet rays at the same time, the crystallinity can be increased without reducing the film formation rate of the formed film.

  Further, since the crystallinity is increased, the resistance is lowered, and when used as an electrode of a solar cell, the electrical loss of the solar cell can be reduced. Alternatively, when a material having the same resistance value as in the prior art is used, a thin transparent conductive film can be used, so that the cost of the raw material can be reduced, and the high transmittance due to the thin film itself can be expected.

  Further, since the crystallinity is increased, the haze ratio is increased, and it can be expected that the power generation efficiency is increased when used in a solar cell.

The present invention provides a forming method for forming a tin oxide film on a substrate using a CVD method.
Here, the tin oxide film may contain impurities as long as it has a photoactive effect described later. In order to develop conductivity, fluorine or antimony may be included.

Here, a thin film for improving optical characteristics may be provided between the substrate and the tin oxide film. For example, for the purpose of reducing the reflection of light when light enters from the substrate side, it is preferable to dispose a film on the substrate in the order of a high refractive index film, a low refractive index film, and a tin oxide film. The high refractive index film is a transparent material made of a material having a refractive index of 2.0 or more. Among them, titanium oxide (TiO ₂ ) is particularly preferable. The low refractive index film is a material having a refractive index smaller than that of the tin oxide film, and silicon oxide (SiO ₂ ) is particularly preferable.

  In order to improve the power generation efficiency of the solar cell, it is necessary to form irregularities on the surface of the tin oxide. As unevenness, for example, when the height difference is 0.2 to 0.5 μm and the convex pitch is 0.3 to 0.75 μm, the visible light is appropriately scattered to the solar cell installed on the tin oxide and the power generation efficiency Is known to improve.

  In the metal oxide film formation by the CVD method, a method is generally used in which a metal compound as a raw material is vaporized, and some energy is given to the vaporized raw material gas to decompose and oxidize the metal compound on the substrate. As the energy added to the source gas, there are a thermal CVD method for heating the substrate to a high temperature and a plasma CVD method for activating the source gas with plasma. The thermal CVD method is easy to perform uniform film formation with a simple apparatus. There are benefits. However, since the thermal CVD method is a method for uniformly heating the entire substrate, energy cannot be selectively supplied to the crystal grains, and crystallization cannot be promoted efficiently.

  On the other hand, the inventors have intensively studied the film formation of the tin oxide film. As a result, the substrate was heated at the same time as the tin oxide film was formed and simultaneously irradiated with ultraviolet rays to crystallize the tin oxide film. I found a way to promote it efficiently. This is because the tin oxide film has a photoactive effect on ultraviolet rays. When ultraviolet rays are irradiated during film formation, the surface of the tin oxide film is selectively activated by the photoactive effect of the tin oxide film, and the source gas is decomposed on the surface of the tin oxide film, thereby promoting the oxidation reaction. It is thought that. In the present invention, the photoactive effect is a phenomenon including a photocatalytic effect.

In Patent Document 3, in order to decompose a gas in a gas phase, a xenon excimer lamp (peak wavelength is less than 200 nm) and ultraviolet light having a strong photon energy and a very strong ultraviolet irradiation intensity of 12 mW / cm ² are required. Become. Patent Document 3 describes a mechanism for decomposing gas in the gas phase with ultraviolet rays. In the present invention, the reaction is different from the reaction on the surface of the tin oxide film due to the photoactive effect of the tin oxide film.

Patent Document 4 describes that oxygen gas in space is excited as an effect of ultraviolet irradiation, and the necessary ultraviolet irradiation intensity required at this time is a very strong ultraviolet irradiation intensity of 0.34 W / cm ^{2 at} a minimum. It is stated that it is necessary. In Patent Document 4, the effect of ultraviolet rays is to decompose oxygen and increase the deposition rate, but in the present invention, it differs from the reaction on the tin oxide film surface due to the photoactive effect of the tin oxide film.

  The photoactive effect of the tin oxide film is greater in the crystalline portion of the tin oxide film than in the non-crystalline portion of the tin oxide film. For this reason, the decomposition of the source gas and the oxidation reaction are promoted on the surface of the crystallized tin oxide film, and the effect is manifested by irradiation with ultraviolet rays having a low ultraviolet irradiation intensity.

  As a light source for irradiating ultraviolet light having a peak wavelength of 220 to 400 nm, black light (peak wavelength 365 nm), low pressure mercury lamp (peak wavelength 254 nm), high pressure mercury lamp (peak wavelengths 250 to 320 nm, 365 nm), excimer lamp (peak wavelength 172 nm). , 222 nm, 308 nm, etc.), and these may be used alone or in combination. Here, in order to distinguish the xenon excimer lamp (peak wavelength less than 200 nm) described in Patent Document 3 from the excimer lamps having peak wavelengths of 222 nm and 308 nm, the former is a xenon excimer lamp and the latter is simply an excimer lamp. Call. Note that the shorter the wavelength of ultraviolet light, the higher the energy.

  Since the excitation wavelength of tin oxide is 326 nm, the surface can be activated efficiently by applying light of 326 nm or less. For this reason, a lamp that generates light having a wavelength of 326 nm or less is preferable as the light source to be used, and it is preferable to use a low-pressure mercury lamp and a high-pressure mercury lamp in consideration of the lamp life and the cost of the apparatus. The peak wavelength is 220 nm to 400 nm, and more preferably 220 to 365 nm. A light source that emits ultraviolet light having a peak wavelength of less than 200 nm is not preferable because the ultraviolet light emitting device itself is expensive and has an unnecessarily high energy with respect to the excitation wavelength of tin oxide.

  The excitation wavelength of tin oxide is 326 nm as described above. Even when ultraviolet light having a peak wavelength of 326 nm or more is used, tin oxide sufficiently absorbs ultraviolet light in the wavelength band of the short wavelength side from the peak wavelength. Then, the photoexcitation effect appears.

  In the present invention, the substrate is preferably glass in order to heat the substrate. There are no particular restrictions on the type of glass, for example, colorless and transparent soda lime silicate glass, aluminosilicate glass, borate glass, lithium aluminosilicate glass, quartz glass, borosilicate glass, alkali-free glass, and other transparent glasses A glass plate or the like can be used.

  Among these, it is preferable to use soda lime silicate glass from the viewpoint that the price of the present invention is low. In addition, when using soda lime silicate glass, if a thin film is formed by the method of the present invention on the glass ribbon of the manufacturing process by the float method, no new equipment cost is required for heating, and the manufacturing cost is reduced. Can be cheap.

  Further, for example, when using soda lime silicate glass, there is a problem that alkali metal ions such as sodium in the glass diffuse into the tin oxide film. Therefore, it is particularly preferable to install a thin film (hereinafter referred to as a sodium prevention layer) made of a material different from that of the tin oxide film between the glass and the tin oxide film. As the sodium prevention layer, silicon oxide is particularly suitable. However, the composition of the thin film, the total number of thin films, and the thickness of the thin film are appropriately selected as long as the purpose is achieved. Further, the above-described thin film for improving optical characteristics may be used as the sodium prevention layer.

  In the present invention, the substrate temperature during film formation is preferably 450 ° C. or higher. This is because when the substrate temperature is lower than 450 ° C., the film formation rate becomes slow and the productivity deteriorates, and the substrate temperature is more preferably 500 ° C. or more from the viewpoint of the film formation rate. The upper limit of the substrate heating temperature is not particularly limited from the viewpoint of film formation, but in the case of a glass substrate, it is preferably 750 ° C. or lower so as not to cause thermal deformation.

The ultraviolet irradiation intensity is measured using an ultraviolet irradiation intensity measuring device at a film forming position (surface on which tin oxide is formed). In intensity measurements at wavelengths of 220 nm to 400 nm, measurement is usually performed by combining a sensor having sensitivity at 254 nm and a sensor having sensitivity at 365 nm. The ultraviolet irradiation intensity described in the present invention is the total value of both sensors. It is essential that the ultraviolet irradiation intensity is 0.03 mW / cm ² or more. It is because sufficient crystallinity cannot be obtained when the ultraviolet irradiation intensity is lower than 0.03 mW / cm ² . The relationship between the ultraviolet irradiation intensity and the crystallinity is considered to reach almost parallel at 0.03 mW / cm ² or more. For this reason, the upper limit of the ultraviolet irradiation intensity is preferably 1 mW / cm ² or less in consideration of the price and life of the ultraviolet irradiation device. The ultraviolet irradiation intensity is particularly preferably 0.05 mW / cm ^{2 to} 0.3 mW / cm ² .

The installation position of the light source for irradiating ultraviolet rays is not particularly limited as long as the substrate position on which the tin oxide film is formed can be irradiated with an intensity of 0.03 mW / cm ² or more. It is preferable to install a light source lamp directly above or below the substrate position because the apparatus is simple. Irradiation may be performed using an optical fiber capable of transmitting ultraviolet light such as quartz fiber from a light source. When the tin oxide film is formed, the surface of the tin oxide film is activated by irradiating the tin oxide film deposited on the substrate with ultraviolet rays, and the source gas in contact with the surface of the tin oxide film is Decomposition and oxidation are promoted. In addition, irradiation at the substrate position where the tin oxide film is formed means that after a part of a predetermined thickness of the tin oxide film is formed, ultraviolet light is irradiated without forming the film, and then the photoexcitation effect is obtained. The tin oxide film may be stacked again while it continues. You may continue irradiating an ultraviolet-ray when forming into a film continuously. Moreover, if the ultraviolet irradiation intensity | strength can permeate | transmit a glass base | substrate and can satisfy | fill the above-mentioned intensity | strength on the surface which forms a film | membrane, you may irradiate from the back surface of a glass base | substrate.

  Further, the present invention is characterized in that the reaction proceeds on the substrate using the photoexcitation action of the tin oxide film. For this reason, the present invention irradiates ultraviolet rays on the substrate by exciting the source gas in the gas phase as in Patent Documents 3 and 4 and comparing the excited source gas deposited on the substrate. The source gas is excited by ultraviolet rays only on the surface of the substrate, and the film is deposited only on the substrate. On the other hand, when the source gas is excited in the gas phase as in Patent Documents 3 and 4, it is deposited on the surface first touched. The tin oxide film adhering to unnecessary portions needs to be periodically cleaned.

  As a tin raw material that can be used in the present invention, inorganic tin compounds such as tin tetrachloride, and organic tin such as monomethyltin chloride, dimethyltin chloride, tetramethyltin, and monobutyltin chloride can be used. Of these, tin tetrachloride is preferred as a raw material to be used because it is less likely to be polluted with respect to the environment like organotin, is easily crystallized, and has a large effect of promoting crystallization by ultraviolet irradiation.

  Examples of the present invention will be described below.

Example 1
This was performed using a CVD apparatus as shown in FIG. Anhydrous tin tetrachloride was used as a tin raw material. The raw material tin tetrachloride was put into the syringe 1 and introduced into the vaporizer 3 by the syringe pump 2 at a rate of 1.2 cc / hr. The introduced tin tetrachloride was vaporized by a vaporizer heated to 150 ° C., and nitrogen gas 2.5 NL / min was added from the transfer nitrogen line 4 and introduced into the CVD reactor. Oxygen gas 0.3 NL / min was introduced into the CVD reactor from the oxygen line 5, mixed with the previous raw material line, and then heated to 550 ° C. with a heater 3 having a diameter of 3 cm under atmospheric pressure, and 2.5 cm square quartz. It sprayed on the base | substrate 7 for 1 minute. Ultraviolet rays are generated using an ultraviolet irradiation device 8 (MUV-250UL, manufactured by Moritex Corp .: high pressure mercury lamp type: peak wavelength 250 to 320 nm, 365 nm), and passed through quartz fiber 9 to form 0.24 mW / Irradiated with an intensity of cm ² . The intensity of ultraviolet rays was measured on the substrate surface using a measuring instrument (UVR-254 and UVR-365 manufactured by Tokyo Optical Machinery Co., Ltd.) before heating the substrate. The haze value was measured with a haze meter (HZ-2 manufactured by Suga Test Instruments). The resistance value was measured using a four-terminal measuring instrument. The physical property values of the prepared tin oxide film are shown in Tables 1 to 3. Here, NL of NL / min represents a liter unit in a standard state.

(Examples 2 to 4)
A tin oxide film was prepared in the same manner as in Example 1 except that the ultraviolet irradiation intensity was changed as shown in Table 1. The physical property values of the prepared tin oxide film are shown in Tables 1 to 3.

(Comparative Example 1)
A tin oxide film was prepared in the same manner as in Example 1 except that the ultraviolet irradiation intensity was 0.01 mW / cm ² . The physical property values of the prepared tin oxide film are shown in Tables 1 to 3.

(Comparative Example 2)
Except not irradiating with ultraviolet rays, a tin oxide film was prepared in the same manner as in Example 1, and the physical properties thereof are shown in Tables 1 to 3.

  When the film thickness of the formed tin oxide film was measured with a stylus-type film thickness meter DEKTAK manufactured by Beco, the film thickness was about 300 nm and there was no significant change. The crystallinity of the film was measured using a thin film X-ray diffractometer (RU200B manufactured by Rigaku Corporation). As shown in Table 3, since the (200) plane was the main peak in any of the formed tin oxide films, the integrated intensity of the (200) plane was calculated. Here, the integrated intensity of the (200) plane is defined as crystallinity, the ultraviolet irradiation intensity and crystallinity are summarized in Table 1, and the graph is shown in FIG.

In Table 1, it was found that as the ultraviolet irradiation intensity increases, the film formation rate does not substantially change, but the integrated intensity (crystallinity) of the (200) plane increases. From FIG. 2, the integrated intensity of the (200) plane has an almost constant value for the integrated intensity of the (200) plane, particularly at an ultraviolet irradiation intensity of 0.03 mW / cm ² or more. For this reason, in Examples 1 to 4 having an ultraviolet irradiation intensity of 0.03 mW / cm ² or more, it was found that the tin oxide film was sufficiently crystallized and the crystallinity was increased as compared with the comparative example.

  In addition, it has been found that a conventional method that does not irradiate ultraviolet rays can obtain a highly crystalline tin oxide film when the film formation rate is slowed down. In the present invention, it is considered that a tin oxide film having the same degree of crystallinity as that of the conventional method can be formed at a high film formation rate. This results in a reduction in manufacturing cost, and a low-cost tin oxide film can be produced according to the present invention.

Table 2 shows the measurement results of the physical properties of the substrates with tin oxide films of Examples 1 to 4 and Comparative Examples 1 and 2. The values were normalized and displayed based on the haze ratio and sheet resistance of Comparative Example 2 (an example in which no ultraviolet ray was irradiated). Moreover, the result of Table 2 is shown with a graph in FIG. The haze rate increases with an increase in ultraviolet irradiation intensity, and becomes substantially constant when 0.03 mW / cm ² or more. Further, the sheet resistance value decreases as the ultraviolet irradiation intensity increases, and becomes substantially constant when 0.03 mW / cm ² or more.

  As shown in Table 1, when compared at the same film formation rate, a tin oxide film having a higher haze ratio was obtained when the method of the present invention was used, and when this substrate was applied to a solar cell, the power generation efficiency was Can be raised.

  In addition, when compared at the same film formation rate, a tin oxide film having a lower sheet resistance is obtained when the method of the present invention is used compared to the comparative example, and when this substrate is applied to a solar cell, the power generation efficiency is improved. Can do. Further, in order to obtain the same sheet resistance, the film formation time can be shortened, so that the film formation cost of the tin oxide film can be reduced.

  When a tin oxide film is formed on a glass substrate, a film having good crystallinity can be formed at a high film formation speed, and the productivity of the tin oxide film film formation process can be improved.

Schematic diagram of a CVD apparatus for carrying out the present invention Relationship between UV irradiation intensity and (200) integral intensity Relationship between UV irradiation intensity, sheet resistance value and haze ratio of tin oxide film

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Syringe 2 Syringe pump 3 Raw material vaporizer 4 Nitrogen for conveyance 5 Oxygen 6 Substrate heater 7 Base 8 Ultraviolet irradiator 9 Quartz fiber 10 Exhaust line 11 CVD apparatus

Claims (4)

A thin film forming method for forming a tin oxide film on a substrate using vapor phase chemical growth reaction,
Simultaneously heating the substrate during the formation, a thin film forming method characterized in that ultraviolet radiation intensity is irradiated with 0.03 mW / cm ² or more 1 mW / cm ² or less of ultraviolet substrate.   The thin film forming method according to claim 1, wherein the heating temperature of the substrate for heating is 450 ° C. or higher.   The method for forming a thin film according to claim 1 or 2, wherein the ultraviolet rays are ultraviolet rays having a peak wavelength of 220 nm to 400 nm.   The thin film forming method according to claim 1, wherein a tin raw material as a main raw material of the tin oxide film is tin tetrachloride.

Images