JPH07235498A - Formation of crystalline silicon film - Google Patents

Abstract

PURPOSE:To prevent the roughness of the surface when crystallizing amorphous silicon by laser annealing to form a polycrystalline silicon film. CONSTITUTION:An amorphous silicon film 2 on a glass substrate 1 is irradiated with KrF excimer laser light 7A, 248nm in wavelength, and its uppermost part approx. 30nm from the surface is crystallized. In the second irradiation, KrF excimer laser with a wavelength of 486nm is used. This wavelength makes the laser light 7 pass through the polycrystalline silicon film 5C and allows almost all of it to be absorbed in the amorphous silicon film 2. That selectively rises the temperature of the amorphous silicon film 2 on the base side, and melts and crystallizes it, and consequently obtains a polycrystalline silicon film 5D having a flat surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a crystalline silicon film, and more particularly to a method for forming a crystalline silicon film using a laser annealing method.

[0002]

2. Description of the Related Art A crystalline silicon film, for example, a polycrystalline silicon film has been widely used in liquid crystal display devices, image sensors, general semiconductor devices and the like.

A liquid crystal display device forms a thin film transistor by using a polycrystalline silicon film formed on a transparent substrate as a drive circuit as an active layer, and in order to obtain a high quality liquid crystal display device, a crystal of the polycrystalline silicon film is used. It is essential to improve the sex. Further, from the viewpoint of manufacturing cost, since it is necessary to use an inexpensive glass substrate having a heat resistant temperature of about 600 ° C., a so-called low temperature process in which the process temperature is 600 ° C. or less has been studied and implemented.

A polycrystalline silicon film is mainly formed by a low pressure chemical vapor deposition (LPCVD) method, but since it is a high temperature process of 600 ° C. or higher, a glass substrate cannot be used. Therefore, a method of irradiating the amorphous silicon film with laser light to polycrystallize it is generally used. This will be described below with reference to FIG.

First, as shown in FIG. 4 (a), a glass substrate 1 having a heat resistant temperature of about 600.degree.
The amorphous silicon film 2 has a thickness of about 1 at a growth temperature of about 0 ° C.
00 nm is deposited. After that, a short wavelength laser having a large extinction coefficient with respect to the silicon film, for example, XeCl excimer laser light is irradiated. In the amorphous silicon film 2, most of the laser light is absorbed in the surface region 3 within 10 nm from the film surface, the temperature of this region rises, and melting starts.

When the laser light irradiation is further continued, FIG.
As shown in FIG. 4, when the melting region 4 expands and the irradiation energy of the laser light is sufficiently large, the entire film melts, and FIG.
, The entire film becomes a molten silicon layer.

When the irradiation of the laser beam is finished, the film is cooled and the molten silicon is crystallized, as shown in FIG.
Polycrystalline silicon film 5 is formed.

The polycrystalline silicon film 5 formed by the above steps has less crystal defects than that formed by a normal LPCVD method, and by using this as an active layer of a thin film transistor, the transistor characteristics are increased. Can be improved. Similar steps are used in the case of an image sensor and the like.

[0009]

When the amorphous silicon film is laser-annealed, the molten silicon layer moves when crystallized. Therefore, as shown in FIG.
Surface roughness or waviness of about 30 nm occurs in the polycrystalline silicon film 5.

As a method for preventing surface roughness, the conventional method shown in FIG.
As shown in (a), 500 ° C. is formed on the amorphous silicon film 2.
The following method has been proposed and implemented in which a transparent and easy-to-remove silicon oxide film 6 is deposited at a low temperature and capped, and then laser annealing is performed to prevent migration of molten silicon on the surface during crystallization. However, in this method, as shown in FIG. 5B, oxygen 9 diffuses from the cap oxide film 6 into the molten silicon layer 4 when the amorphous silicon film is melted, and the oxygen concentration in the polycrystalline silicon film is increased. Is increased and the transistor characteristics are deteriorated.

Further, when the cap oxide film 6 is used, the mobility of the molten silicon is greatly reduced as the number of laser beam irradiations (shots) is increased. Therefore, continuous shots are used to move the irradiation region. It is necessary to crystallize the entire surface of the wafer. Shots overlap four times in the corners of the irradiation area, but oxygen diffusion increases in the area where the shots overlap, which deteriorates the characteristics of the polycrystalline silicon film and causes variations in transistor characteristics.

An object of the present invention is to provide a method for forming a crystalline silicon film for reducing variations in transistor characteristics.

Another object of the present invention is to provide a method for forming a crystalline silicon film without surface roughness.

[0014]

According to a first aspect of the present invention, there is provided a method of forming a crystalline silicon film, for example, a polycrystalline silicon film, in which an amorphous silicon film formed on a substrate is irradiated with laser light to crystallize. The method for forming a silicon film is characterized in that laser light having a wavelength whose extinction coefficient in the amorphous silicon film is larger than that in the crystalline silicon film is used.

The method of forming a crystalline silicon film, for example, a polycrystalline silicon film, of the second invention is a method of forming a crystalline silicon film in which an amorphous silicon film formed on a substrate is irradiated with laser light to be crystallized. After irradiating the amorphous silicon film with laser light having a large extinction coefficient in the amorphous silicon film to crystallize only the surface of the amorphous silicon film, the extinction coefficient in the crystalline silicon film is small and amorphous. It is characterized in that the entire amorphous silicon film is crystallized by irradiating a laser beam having a large extinction coefficient on the silicon film.

[0016]

The present invention will be described below with reference to the drawings. Figure 1
(A), (b) is sectional drawing of the board | substrate for demonstrating the 1st Example of this invention.

First, as shown in FIG. 1A, an amorphous silicon film 2 is formed on a glass substrate 1 by LPCVD.
After depositing to a thickness of 00 nm, a cap oxide film 6 was deposited thereon to a thickness of 100 nm. Next, laser light having a wavelength having a large extinction coefficient in the amorphous silicon film is irradiated.

FIG. 2 shows the extinction coefficient k for the crystalline silicon film and the amorphous silicon film. In the wavelength range of 400 nm to 500 nm, the extinction coefficient of the amorphous silicon film is larger than that of the crystalline silicon film. Therefore, when the polycrystalline silicon film is irradiated with laser light in this wavelength region, it is hardly absorbed. When it is transmitted to the underlying substrate and irradiated to the amorphous silicon film, it is almost absorbed at a depth of several tens nm from the surface.

Therefore, the amorphous silicon film 2 was crystallized using KrF excimer laser light having a wavelength of 486 nm.
The irradiation of the laser light 7 was such that the irradiation areas overlap. That is, the irradiation energy is set to 400 mJ / cm ² so that the polycrystalline silicon film in the overlapping region of the laser light irradiation does not melt but only the amorphous silicon film 2 melts and crystallizes.
Set to about.

By the first irradiation, the amorphous silicon film 2 in the region 8A becomes a polycrystalline silicon film 5A, and by the second irradiation, the amorphous silicon film 2 in the region 8B becomes as shown in FIG. As shown in, the polycrystalline silicon film 5B is formed.

In this case, since the polycrystalline silicon film in the region where the laser irradiation overlaps is not melted, oxygen does not diffuse from the cap oxide film 6 to the polycrystalline silicon films 5A and 5B, and the characteristics of the polycrystalline silicon film. Does not occur. That is, even if the laser irradiation areas are overlapped and irradiated, the characteristic variation of the polycrystalline silicon film does not occur. Therefore, when a transistor of a liquid crystal display device or an image sensor is formed by using this polycrystalline silicon film, it is possible to reduce variations in characteristics of the transistor.

3 (a) to 3 (c) are sectional views of the substrate for explaining the second embodiment of the present invention.

First, as shown in FIG. 3 (a), an amorphous silicon film 2 is formed on a glass substrate 1 by LPCVD to form 10
After depositing to a thickness of 0 nm, KrF with a wavelength of 248 nm
The excimer laser was irradiated with light 7A. As shown in FIG. 2, the laser light of this wavelength has a large extinction coefficient in the amorphous and crystalline silicon films, and all the energy is absorbed within about 10 nm from the surface. Irradiation energy,
By setting about 200 mJ / cm ^2, about 30 nm was crystallized from the surface of the amorphous silicon film 2 to form a polycrystalline silicon film 5C. The surface of the polycrystalline silicon film is relatively flat and is not rough because the entire film is not melted.

Further, in the second irradiation, KrF excimer laser light 7 having a wavelength of 486 nm was used. At this wavelength, the extinction coefficient of the crystalline silicon film is small and almost transparent, so
The laser light 7 passes through the polycrystalline silicon film 5C. However, since the amorphous silicon film has a large extinction coefficient, almost all of the laser light 7 is absorbed in a region within 40 nm from the interface between the amorphous silicon film 2 and the polycrystalline silicon film 5C. In other words, in the second irradiation, the amorphous portion on the base side can be selectively raised in temperature, melted and crystallized,
As shown in FIG. 3C, a polycrystalline silicon film 5D having a small surface roughness can be formed.

As described above, in the second embodiment, the polycrystalline silicon film 5C on the surface formed by the first irradiation serves as a cap layer, and the surface roughness can be suppressed to 10 nm or less. Moreover, there is an advantage that oxygen is not mixed into the silicon film, which is a problem when the conventional cap oxide film is used. Further, when laser light having a high extinction coefficient in crystalline silicon is used for the first irradiation, the crystallized region can be prevented from expanding even if irradiation is repeated, so that the crystallinity in a region where irradiation is large can be prevented. Variation can be reduced.

The method of the second embodiment takes advantage of the fact that the laser beam absorbed only in the amorphous portion of silicon is used, and the polycrystalline silicon film having an amorphous phase with poor crystallinity. It can also be applied to improve the film quality. For example, in a polycrystalline silicon film formed by the plasma CVD method, a region having poor crystallinity close to amorphous exists on the base substrate side.
By irradiating this polycrystalline silicon film with a KrF excimer laser having a wavelength of 486 nm, only the region close to the amorphous phase can be selectively annealed to improve the crystallinity.

In the above embodiment, the case where the polycrystalline silicon film is formed on the glass substrate has been described.
Of course, other insulating substrates such as quartz may be used.

[0028]

As described above, according to the first aspect of the present invention, only amorphous silicon is obtained by using laser light having a wavelength whose extinction coefficient in the amorphous silicon film is larger than that of the crystalline silicon film. The amorphous silicon film formed on the substrate is laser-annealed with an energy density that melts the laser beam to crystallize only the amorphous silicon film without melting the region of the polycrystalline silicon film where laser irradiation overlaps. Because you can
Since oxygen can be prevented from diffusing into the silicon film from the cap oxide film as in the conventional case, a polycrystalline silicon film with less variation in characteristics can be easily formed. Therefore, variations in transistor characteristics can be reduced.

In the second aspect of the present invention, in the step of crystallizing amorphous silicon, the amorphous silicon film is irradiated with laser light having a large extinction coefficient, and the surface is crystallized. After crystallization, irradiate laser light with a small extinction coefficient in the crystalline silicon film and a large extinction coefficient in the amorphous silicon film to selectively anneal the amorphous silicon film to crystallize the entire film. As a result, there is an effect that a polycrystalline silicon film with less surface roughness can be easily formed.

[Brief description of drawings]

FIG. 1 is a sectional view of a substrate for explaining a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a wavelength of light and an extinction coefficient for a crystalline silicon film and an amorphous silicon film.

FIG. 3 is a sectional view of a substrate for explaining a second embodiment of the present invention.

FIG. 4 is a cross-sectional view of a substrate for explaining a conventional method for forming a polycrystalline silicon film.

FIG. 5 is a sectional view of a substrate for explaining another conventional method for forming a polycrystalline silicon film.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Amorphous silicon film 3 Surface area 4 Melting area 5, 5A-5D Polycrystalline silicon film 6 Cap oxide film 7, 7A Laser light 8A, 8B Irradiation area 9 Oxygen

Claims (2)

[Claims] 1. A method of forming a crystalline silicon film, comprising irradiating a laser beam to an amorphous silicon film formed on a substrate to crystallize the amorphous silicon film, wherein the extinction coefficient of the amorphous silicon film is equal to that of the crystalline silicon film. A method for forming a crystalline silicon film, which comprises using a laser beam having a wavelength larger than a coefficient. 2. A method for forming a crystalline silicon film in which an amorphous silicon film formed on a substrate is irradiated with a laser beam to crystallize the amorphous silicon film, the laser beam having a large extinction coefficient in the amorphous silicon film is amorphous. After irradiating the surface of the amorphous silicon film to crystallize only the amorphous silicon film, laser light having a small extinction coefficient in the crystalline silicon film and a large extinction coefficient in the amorphous silicon film is irradiated, A method for forming a crystalline silicon film, characterized by crystallizing the entire crystalline silicon film.