JP4289017B2 - Method for producing crystalline silicon thin film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a crystalline silicon thin film applied to a semiconductor element substrate, a semiconductor element and the like.
[0002]
[Prior art]
In recent years, active research has been conducted on forming a crystalline silicon thin film on a non-conductive dissimilar substrate such as a glass substrate. The crystalline silicon thin film formed on the glass substrate is widely used for TFTs for liquid crystal devices, thin film photoelectric conversion elements, and the like.
[0003]
In a thin-film photovoltaic power generation element, a crystalline silicon thin film having good crystallinity is formed on an inexpensive substrate by a low temperature process, and this is used as a photoelectric conversion device, thereby reducing cost and improving performance. By using a crystalline silicon thin film as a photoelectric conversion element, it is possible to prevent light degradation which is a problem in an amorphous silicon photoelectric conversion element, and long wavelength light in which the amorphous silicon photoelectric conversion element has no sensitivity. Can also be converted into electrical energy. Such a technique is expected to be applied to photoelectric conversion devices other than photoelectric conversion elements, such as optical sensors.
A crystalline silicon photoelectric conversion element is generally manufactured by a method of depositing a crystalline silicon thin film directly on a substrate by plasma CVD. This method is advantageous for cost reduction because a crystalline silicon film can be formed on a substrate at a low temperature. However, with this method, it is difficult to form polycrystalline silicon having a large crystal grain size. In addition, the quality of the i layer, which is the basis of the power generation function, is drastically lowered when doping is performed to optimize the device structure. For these reasons, with the above method, it is difficult to achieve an efficiency exceeding 10% in a single cell advantageous for cost reduction.
[0004]
On the other hand, various methods for crystallization by scanning with a solid-state laser have been studied. For example, Patent Document 1 discloses a method using a continuous wave.
[0005]
[Patent Document 1]
In this method, amorphous silicon is formed on a heterogeneous substrate, and the amorphous silicon is melted by scanning a band-like continuous wave light source to recrystallize polycrystalline silicon. Yes, it is possible to grow crystal grains that are long in the scanning direction.
In the method of crystallization using a continuous wave, it has been attempted to use Nd: YAG, Nd: YVO ₄ or the like as a solid-state laser. By using these solid-state lasers, the running cost of the manufacturing apparatus is greatly reduced, and at the same time, high-quality polycrystalline silicon can be formed.
[0006]
However, in the above method, when the shape of the laser beam is processed in order to adapt it to the crystallization of the silicon film, the laser beam after processing is only about several hundred μm in the long direction, so it is irradiated once. The laser light irradiation area that can be produced is limited, and it takes a lot of time to crystallize the entire surface of a large substrate.
Moreover, since solid state lasers such as Nd: YAG have a relatively large apparatus size, they cannot be irradiated side by side in parallel. Therefore, it is necessary to repeat scanning using one laser beam, and it is difficult to reduce the cost.
Furthermore, in order to obtain sufficient characteristics as a photoelectric conversion element, the film thickness of the crystalline silicon needs to be 1 μm or more, preferably 2 μm or more. However, in the case of a solid-state laser, a second harmonic having a center wavelength of 532 nm is required. Since it is used, laser light penetrates only to a depth of about 0.1 μm from the surface of the amorphous silicon layer and to a depth of about 1 μm from the surface of the crystalline silicon layer. Therefore, when 532 nm laser light is irradiated to amorphous silicon with a thickness of several μm, the laser light is absorbed rapidly in the vicinity of the surface, resulting in a temperature exceeding the melting point of silicon. Because the laser beam does not reach inside, the temperature does not reach a sufficient temperature for melting. If the surface is further heated to melt the entire silicon film, ablation may occur.
[0007]
On the other hand, a method for melting an amorphous or crystalline silicon film using a semiconductor laser instead of a solid-state laser has been proposed. Since semiconductor lasers are small in size, the entire surface of a large-area substrate can be crystallized in a lump by irradiating laser beams in parallel. When silicon is crystallized, the second harmonic is normally used in a solid-state laser, whereas the fundamental wave can be used in a semiconductor laser. For example, a semiconductor laser having an AlGaAs active layer generates a fundamental wave having a central wavelength of about 800 nm. In this wavelength region, the penetration length into the amorphous silicon layer is around 1 μm, which is an optimum wavelength for crystallizing an amorphous silicon layer of about several μm. Thus, since the penetration length is long in the case of a semiconductor laser, energy can be given to the silicon layer relatively uniformly. It is also possible to positively optimize the film quality of the precursor film by changing the absorption coefficient of the precursor film that is the basis of the crystalline silicon layer. For these reasons, in the case of a semiconductor laser, stable crystallization can be performed without causing ablation even in a thick film.
[0008]
Furthermore, the semiconductor laser can oscillate rectangular laser light and is suitable for scanning the laser light in a straight line, which is advantageous for crystallization of amorphous silicon. And there is no need to shape the laser beam as in the case of a solid laser.
In addition, in the case of a semiconductor laser, in addition to the AlGaAs system, there are an InP / InGaAsP system, an InGaAlP system, an InGaAsP system, a GaN system, and the like, and various oscillation wavelengths can be selected. As a result, the degree of freedom in the absorption coefficient and thickness of the precursor film that can be crystallized increases, and the degree of freedom in the manufacturing process increases.
[0009]
[Problems to be solved by the invention]
However, in the manufacturing method of the crystalline silicon thin film using the conventional semiconductor laser, all the semiconductor lasers that can obtain high output at present have a long wavelength and the penetration length of the laser light into the silicon film is relatively long. When crystallizing a relatively thin silicon film, there is a problem that the energy of the laser beam to be irradiated is not effectively absorbed by the silicon film and the irradiation efficiency is lowered. For example, in the above-described AlGaAs semiconductor laser that generates a fundamental wave with a center wavelength of about 800 nm, when a crystalline silicon film with a thickness of 1 μm is irradiated with laser light (when the reflected light from the back surface of the film is ignored), The energy absorbed by the film is only about 10% of the energy of the entire irradiation light. Thus, when a relatively thin silicon film is crystallized using a long wavelength semiconductor laser, the irradiation efficiency may be lower than when a short wavelength solid state laser is used.
[0010]
[Means for Solving the Problems]
The present invention has been made in view of the above problems, and in order to improve the irradiation efficiency of the semiconductor laser light when a relatively thin silicon film is crystallized, a crystalline silicon thin film is formed on a different substrate different from the crystalline silicon. In the method for producing a crystalline silicon thin film to be formed , a heterogeneous substrate having periodically formed V-grooves parallel to each other on a part or the entire surface is prepared, and a diffusion prevention layer is formed on the heterogeneous substrate. An amorphous silicon layer, a polycrystalline silicon layer, or a layer containing crystalline silicon is formed on the diffusion prevention layer, and the surface of the heterogeneous substrate is irradiated with laser light using a semiconductor laser as a light source to be parallel to the V groove. performs scanning in the direction, the amorphous silicon layer, comprising the steps of melting and recrystallizing a layer including a polycrystalline silicon layer or a crystalline silicon, the diffusion barrier layer, the heterogeneous substrate The amorphous silicon layer material is melted by laser irradiation, there is provided a method for producing a crystalline silicon thin film to prevent the diffusion in the layer containing polycrystalline silicon layer or a crystalline silicon.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The method for producing a crystalline silicon thin film according to the present invention includes a method for melting and recrystallizing an amorphous silicon film or a crystalline silicon film formed on a different substrate using a semiconductor laser, on a substrate having a groove structure on the surface. An amorphous silicon film or a crystalline silicon film is formed on the substrate, and laser light is irradiated and scanned in a direction parallel to the groove structure.
[0012]
A preferred embodiment of the present invention will be described below. First, a heterogeneous substrate such as a glass substrate having a fine groove structure, particularly a V-groove formed uniformly on the surface is prepared. In the case of a glass substrate, the groove structure can be formed by embossing, mechanical polishing, or the like. The V-groove has a lateral period smaller than the size of the laser beam in the longitudinal direction.
[0013]
Next, a precursor film as a base of the crystalline silicon thin film is formed on the substrate. As the precursor film, amorphous silicon, polycrystalline silicon, microcrystalline silicon, or amorphous silicon containing a crystal component can be used. In addition, carbon, germanium, or the like can be mixed to control the band gap of the crystalline silicon thin film. The thickness of the precursor film may be several tens nm to several μm.
[0014]
In the case where impurities diffused from the substrate into the crystalline silicon thin film after crystallization become a problem, a diffusion preventing layer can be formed between the substrate and the precursor film. The diffusion prevention layer may be made of any material as long as it can prevent the diffusion of impurities. For example, it is made of silicon oxide, silicon nitride, silicon oxynitride, zinc oxide, tin oxide, ITO (Indium Tin Oxide), titanium oxide or a compound thereof. The diffusion prevention layer may have a laminated structure composed of two or more layers, each layer made of a different material. For example, a silicon nitride or silicon oxynitride layer having a dense film structure can be formed on the substrate side, and a silicon oxide layer can be formed thereover. According to this structure, since silicon oxide matches well with crystalline silicon, the surface recombination rate on the backside of the crystalline silicon can be reduced while preventing diffusion of impurities from the substrate by the silicon nitride or silicon oxynitride layer.
[0015]
Further, when forming a crystalline silicon thin film for a photoelectric conversion element, a layer having high reflectance and conductivity is formed on the substrate side, and a layer having transparency and conductivity (in contact with the crystalline silicon thin film) is formed thereon. Can be formed. One example is a structure in which an Ag layer is formed on the substrate side and a tin oxide layer is formed thereon. According to this layer configuration, the short-circuit current density of the photoelectric conversion element can be improved.
Further, a high melting point material layer can be formed on the substrate, and a precursor film can be formed thereon. Examples of the high melting point material include tungsten and molybdenum.
On the other hand, in order to prevent ablation upon laser light irradiation, silicon oxide may be formed on the precursor film.
[0016]
The silicon film on the substrate thus formed is irradiated with semiconductor laser light. When the inclined surface of the V-groove makes a predetermined angle or more with the back surface of the substrate, the amount of irradiation light absorbed is significantly increased as compared with the case where laser light is irradiated onto a flat surface (with an inclination angle of zero). This is because the light irradiated on the slope of the V-groove includes a component that is reflected twice on the surface of the silicon film above a predetermined tilt angle. Specifically, when the laser beam is incident perpendicularly to the substrate, if the tilt angle is 30 ° or more, reflection occurs twice on the silicon film surface, and if the tilt angle is 45 ° or more, all incident light is reflected on the silicon film surface. Two reflections occur. Moreover, since the laser beam is incident on the silicon film surface obliquely at a predetermined inclination angle or more, the optical path of the light propagating slightly inside the silicon film as compared with the case where the laser beam is incident perpendicularly on the silicon film surface. The length is increasing.
[0017]
By making the semiconductor laser light to be irradiated p-polarized light (electric field vector is parallel to the incident surface) and optimizing the incident angle, the reflection on the silicon film surface can be made close to zero. In this way, by reducing reflection on the surface of the silicon film, the amount of light entering the silicon film can be increased, and the amount of light absorption in the silicon film can be increased. The optimum value of the incident angle for minimizing the reflectance when p-polarized light is such that the transmitted light and the reflected light form an angle of 90 °. This incident angle is called the Brewster angle. For example, assuming that the laser beam wavelength is 800 nm, the refractive index of air is 1, and the refractive index of silicon film is 3.64, the Brewster angle (optimum incident angle) is 75 °. Further, if the refractive index of the silicon film takes a value of about 3.5 to 6.8 in the wavelength range of the laser beam, the Brewster angle (optimum incident angle) is 74 to 82 °.
[0018]
On the other hand, when silicon oxide is formed on the surface of the silicon film, the laser beam to be irradiated is s-polarized (the electric field vector is perpendicular to the incident surface), and the incident angle and the silicon oxide film thickness are optimized. As in the case of polarized light, reflection on the silicon film surface can be reduced to increase the amount of light entering the silicon film and increase the amount of light absorption in the silicon film. In order to minimize the reflectance when s-polarized light is used, it is necessary to optimize both the silicon oxide film thickness and the laser light incident angle with respect to the incident laser light wavelength. For example, the silicon oxide film thickness is 110 to 116 nm and the incident angle is 72 to 74 ° for the laser beam wavelength of 500 nm, and the silicon oxide film thickness is 175 to 186 nm and the incident angle is 68 to 72 for the laser beam wavelength of 800 nm. For a laser beam wavelength of 1000 nm, the silicon oxide film thickness is 218 to 233 nm and the incident angle is 67 to 71 °.
[0019]
With respect to the scanning of the semiconductor laser light, crystal grains grown in the scanning direction can be obtained by scanning in the direction parallel to the grooves. Therefore, a crystalline silicon thin film composed of large crystal grains can be obtained without being affected by the unevenness of the substrate.
Laser beam scanning is performed by moving the laser beam or the substrate.
In a semiconductor laser, the size of a single laser is small, and a plurality of lasers can be arranged and simultaneously irradiated and scanned, so that the throughput is improved in the production of a crystalline silicon thin film.
In the semiconductor laser device, it is known that the output wavelength slightly changes when the temperature of the active layer changes. In order to cope with this, by providing the semiconductor laser device with a temperature holding function, Can be made more stable.
[0020]
【Example】
Examples according to the invention are described below. In addition, these Examples show an example of this invention and this invention is not limited to these Examples.
Example 1
In this embodiment, a precursor film (amorphous silicon layer) is formed using plasma CVD. 1A to 1C are cross-sectional views perpendicular to the extending direction of the V groove 12 on the substrate 1.
First, as shown in FIG. 1A, a glass substrate having a periodic V-groove structure 12 on the entire surface was prepared as a heterogeneous substrate 1. The period of the V groove 12 was 100 μm, and the inclination angle of the slope of the groove was 45 °. A first diffusion prevention layer 2 made of SiN and having a thickness of 200 nm was formed on the substrate 1, and a second diffusion prevention layer 3 made of SiO was formed thereon (FIG. 1B). Further, as shown in FIG. 1C, an amorphous silicon layer 4 having a thickness of 1 μm was formed on the second diffusion prevention layer 3 by using plasma CVD. The formation conditions of the amorphous silicon layer 4 by plasma CVD are a mixed gas of SiH ₄ : 10 ccm and PH ₄ : 0.01 ccm, a substrate temperature of 500 ° C., and a plasma power source using an RF frequency. Under this condition, amorphous silicon containing an n-type dopant is formed. After the amorphous silicon layer 4 was formed, the amorphous silicon layer was crystallized by irradiating a laser beam of a semiconductor laser perpendicularly to the substrate. A GaAlAs laser was used as the semiconductor laser, and it was scanned linearly in a direction parallel to the V-groove at a speed of 10 cm / sec. The semiconductor laser device was water-cooled so that the temperature during oscillation was constant. All the amorphous silicon having a thickness of 1 μm was melted and crystallized by the irradiation of the laser beam, and crystal grains that grew long in the scanning direction of the semiconductor laser were obtained.
FIG. 2 is a cross-sectional view in a direction parallel to the extending direction of the V-groove 12 on the substrate, and shows the positional relationship between the melted silicon layer 6 and the laser beam 5. As shown in the drawing, the silicon crystallized by melting becomes a polycrystalline silicon layer 4 ′.
Further, when ZnO, SnO ₂ , ITO, TiO ₂ , or SiON was used as the diffusion prevention layer instead of SiO or SiN, the same results as above were obtained.
[0021]
Example 2
In this example, the configuration was the same as that of Example 1 except that the precursor film was formed of microcrystalline silicon. In the same manner as in Example 1, the diffusion preventing layers 2 and 3 were formed on the substrate 1 having the V-groove 12, and then a 2 μm microcrystalline silicon layer 4 was formed by plasma CVD. When this microcrystalline silicon layer 4 was irradiated with laser light under the same conditions as in Example 1 and scanned linearly in the direction parallel to the V-groove, all the microcrystalline silicon having a thickness of 2 μm was melted without causing ablation. The crystal grains were crystallized and grown long in the scanning direction of the semiconductor laser.
[0022]
Example 3
In this embodiment, the precursor film is formed as polycrystalline silicon. A quartz substrate having a V-groove 12 on the surface was used as the substrate 1, and diffusion prevention layers 2 and 3 similar to those in Example 1 were formed thereon, and then a 3 μm polycrystalline silicon layer 4 was formed by thermal CVD. . When this polycrystalline silicon layer 4 was irradiated with laser light under the same conditions as in Example 1 and scanned linearly in the direction parallel to the V-groove, all the polycrystalline silicon having a thickness of 2 μm was melted without causing ablation. The crystal grains were crystallized and grown long in the scanning direction of the semiconductor laser. In this embodiment, ablation may occur depending on the laser intensity, but as a countermeasure, crystallization is stabilized without causing ablation by forming a SiO film on the polycrystalline silicon layer 4. It was confirmed.
[0023]
Example 4
In this example, a laminate having an electrode structure on the back side of crystalline silicon was produced. 3A to 3D are cross-sectional views perpendicular to the extending direction of the V groove 12 on the substrate 7. As shown in FIG. 3A, a glass substrate having a periodic V-groove structure on the entire surface was used as the heterogeneous substrate 7. The period of the V groove 12 was 50 μm, and the inclination angle was 25 °. An Ag electrode was formed as the first electrode layer 8 on the substrate 7, and a ZnO electrode was formed as the second electrode layer 9 thereon (FIG. 3B). At this time, a layer in contact with silicon among the plurality of electrode layers is preferably formed using zinc oxide, tin oxide, titanium oxide, or a compound thereof. Further, an amorphous silicon layer was formed as a precursor layer 10 on the second electrode layer 9 (FIG. 3C). This was irradiated with a laser beam of a semiconductor laser perpendicularly to the substrate under the same conditions as in Example 1 and scanned linearly in a direction parallel to the V-groove 12. Here, the laser light was p-polarized with respect to the inclined surface of the silicon layer. As a result, the entire portion inside the layer was crystallized at the location irradiated with the laser beam. Thereafter, an Al electrode 11 was formed on the crystallized silicon layer 10 ′ (FIG. 3D). Furthermore, a part of the electrode layer on the back surface side of the crystalline silicon layer 10 ′ was exposed by dry etching, and IV measurement was performed. As a result, it was confirmed that the electrode layer on the back side of the crystalline silicon layer 10 ′ and the crystalline silicon layer 10 ′ were in ohmic contact.
In addition, when the back side electrode is made of only tungsten, a crystalline silicon layer was produced in the same manner as the above method. Even when laser irradiation was performed, tungsten did not melt and the electrode structure was maintained and ohmic contact with the crystalline silicon layer was achieved. It was confirmed that
Further, when SnO ₂ , ITO, TiO ₂ was used as the second electrode layer 9 instead of ZnO, the same results were obtained.
[0024]
【The invention's effect】
As described above, according to the method for producing a crystalline silicon thin film of the present invention, in the crystalline silicon thin film producing method for forming a crystalline silicon thin film on a different substrate different from the crystalline silicon, a part of the surface or the entire surface are parallel to each other. A heterogeneous substrate having a groove structure is prepared, an amorphous silicon layer, a polycrystalline silicon layer, or a layer containing crystalline silicon is formed on the surface of the heterogeneous substrate, and a laser beam using a semiconductor laser as a light source on the surface of the heterogeneous substrate Is performed in a direction parallel to the groove structure, and includes the steps of melting and recrystallizing the amorphous silicon layer, the polycrystalline silicon layer, or the layer containing crystalline silicon. In the case of crystallizing a thin silicon film, the irradiation efficiency of the semiconductor laser light can be improved.
[Brief description of the drawings]
FIGS. 1A to 1C are cross-sectional views showing an embodiment of a method for producing a crystalline silicon thin film of the present invention.
FIG. 2 schematically shows a process of melting and recrystallizing a silicon layer by irradiating a semiconductor laser beam in the embodiment of FIG.
FIGS. 3A to 3D are cross-sectional views showing another embodiment of the method for producing a crystalline silicon thin film of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1, 7: Different board | substrate 2: 1st diffusion prevention layer 3: 2nd diffusion prevention layer 4: Precursor film | membrane 4 ': Crystalline silicon layer 5: Laser beam 6: Molten silicon layer 8: 1st electrode layer 9 : Second electrode layer 10: Precursor layer 11: Al electrode

Claims (11)

In the method for producing a crystalline silicon thin film, the crystalline silicon thin film is formed on a different substrate from the crystalline silicon.
Prepare a heterogeneous substrate having periodically formed V-grooves parallel to each other on a part or the entire surface of the surface,
Forming a diffusion preventing layer on the heterogeneous substrate;
Forming an amorphous silicon layer, a polycrystalline silicon layer or a layer containing crystalline silicon on the diffusion preventing layer;
The surface of the heterogeneous substrate is irradiated with a laser beam using a semiconductor laser as a light source and scanned in a direction parallel to the V-groove to melt the amorphous silicon layer, the polycrystalline silicon layer, or the layer containing crystalline silicon. With each step of recrystallization,
The diffusion prevention layer is a method for producing a crystalline silicon thin film, in which a substance in the heterogeneous substrate is prevented from diffusing into the amorphous silicon layer, the polycrystalline silicon layer, or the layer containing crystalline silicon melted by laser irradiation.   2. The method for producing a crystalline silicon thin film according to claim 1, wherein the scanning of the laser beam is performed by moving the laser beam or the dissimilar substrate linearly in a direction parallel to the V-groove.   2. The method for producing a crystalline silicon thin film according to claim 1, wherein the V groove has a lateral period smaller than a size of the laser beam in a longitudinal direction.   The method for producing a crystalline silicon thin film according to claim 1, wherein the V groove has an inclined surface that forms an angle of 30 ° or more with the back surface of the substrate.   The method of manufacturing a crystalline silicon thin film according to claim 1, wherein the scanning with the laser light is performed by irradiating the inclined surface of the V-groove so as to be p-polarized light.   6. The method of manufacturing a crystalline silicon thin film according to claim 5, wherein the scanning of the laser light is performed at an angle between the inclined surface of the V-groove and the laser light so that the reflectance is lower than that in the case of normal incidence. .   2. The method for producing a crystalline silicon thin film according to claim 1, wherein the diffusion prevention layer is made of any one of silicon oxide, silicon nitride, silicon oxynitride, titanium oxide, zinc oxide, tin oxide, ITO, or a compound thereof.   2. The method for producing a crystalline silicon thin film according to claim 1, wherein the diffusion preventing layer has a laminated structure including two or more layers each made of a different material.   9. The crystalline silicon thin film according to claim 8, wherein the different materials are each selected from silicon oxide, silicon nitride, silicon oxynitride, titanium oxide, zinc oxide, tin oxide, ITO, or a compound thereof. Production method. The method for producing a crystalline silicon thin film according to claim 1, further comprising a step of forming a silicon oxide layer on the amorphous silicon layer, the polycrystalline silicon layer, or the layer containing crystalline silicon. The method of manufacturing a crystalline silicon thin film according to claim 10, wherein the scanning of the laser light is performed by irradiating the inclined surface of the V groove so as to be s-polarized light.

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