JP2817613B2 - Method for forming crystalline silicon film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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.

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

A polycrystalline silicon film is mainly formed by a low pressure chemical vapor deposition (LPCVD) method, but a high-temperature process of 600 ° C. or more cannot be used, so that a glass substrate cannot be used. Therefore, a method of irradiating the amorphous silicon film with a laser beam to perform polycrystallization 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.
At a growth temperature of about 0 ° C., the amorphous silicon film 2 is
Deposit 00 nm. After that, the silicon film is irradiated with a short-wavelength laser having a large extinction coefficient, for example, XeCl excimer laser light. In the amorphous silicon film 2, laser light is almost absorbed in a surface region 3 within 10 nm from the film surface, and the temperature in this region rises and melting starts.

[0006] When the laser beam irradiation further continues, FIG.
As shown in FIG. 4, when the melting region 4 is enlarged and the irradiation energy of the laser beam is sufficiently large, the entire film is melted, and FIG.
As shown in the above, the entire film becomes a molten silicon layer.

When the laser beam irradiation is completed, the film is cooled and the molten silicon is crystallized, as shown in FIG.
A polycrystalline silicon film 5 is formed.

The polycrystalline silicon film 5 formed by the above steps has fewer crystal defects than those formed by the ordinary LPCVD method, and by using this as an active layer of a thin film transistor, the transistor characteristics are improved. Can be improved. Almost the same process is used in the case of an image sensor or the like.

[0009]

When the amorphous silicon film is laser-annealed, the molten silicon layer moves when it is crystallized, and as shown in FIG.
A surface roughness or undulation of about 30 nm occurs in the polycrystalline silicon film 5.

Conventionally, as a method for preventing surface roughness, FIG.
(A) As shown in FIG.
A method has been proposed and implemented in which a silicon oxide film 6 which is transparent and easy to remove at a low temperature as described below is deposited, capped, and then laser-annealed to prevent molten silicon from moving on the surface during crystallization. However, in this method, as shown in FIG. 5B, when the amorphous silicon film is melted, oxygen 9 diffuses from the cap oxide film 6 into the molten silicon layer 4 and the oxygen concentration in the polycrystalline silicon film is reduced. Increases, and the transistor characteristics are degraded.

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. It is necessary to crystallize the entire surface of the wafer. Shots overlap four times at the corners of the irradiation region. However, in the region where the shots overlap, the diffusion of oxygen increases, so that the characteristics of the polycrystalline silicon film are deteriorated, which 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 method of forming a crystalline silicon film, for example, a polycrystalline silicon film according to a reference example of the present invention, an amorphous silicon film formed on a substrate is crystallized by irradiating a laser beam. The method for forming a crystalline silicon film is characterized by using laser light having a wavelength whose extinction coefficient in an amorphous silicon film is larger than that in a crystalline silicon film.
It is a sign.

The method for forming a crystalline silicon film, for example, a polycrystalline silicon film according to the present invention is the same as the method for forming a crystalline silicon film in which an amorphous silicon film formed on a substrate is irradiated with a laser beam to be crystallized. After irradiating the amorphous silicon film with a laser beam having a large extinction coefficient in the amorphous silicon film to crystallize only the surface of the amorphous silicon film, the amorphous silicon film has a small extinction coefficient in the crystalline silicon film. And irradiating a laser beam having a large extinction coefficient to crystallize the entire amorphous silicon film.

[0016]

Next, the present invention will be described with reference to the drawings. FIG.
(A), (b) is sectional drawing of the board | substrate for demonstrating the reference 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 with 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. Since the extinction coefficient of the amorphous silicon film is larger than the extinction coefficient of the crystalline silicon film between the wavelengths of 400 nm and 500 nm, when the polycrystalline silicon film is irradiated with laser light in this wavelength region, it is hardly absorbed. When the light passes through the underlying substrate and irradiates the amorphous silicon film, it is almost absorbed at a depth of about 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 beam 7 was performed so that the irradiation areas overlapped. That is, the irradiation energy is set to 400 mJ / cm ² so that the polycrystalline silicon film in the overlap region of the laser beam 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 the polycrystalline silicon film 5A, and by the second irradiation, the amorphous silicon film 2 in the region 8B becomes the polycrystalline silicon film 5A as shown in FIG. As shown in FIG. 6, a polycrystalline silicon film 5B is obtained.

In this case, since the polycrystalline silicon film in the region where the laser irradiation overlaps does not melt, 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 decrease. In other words, even if the laser irradiation regions are overlapped with each other, 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 manufactured using this polycrystalline silicon film, variations in transistor characteristics can be reduced.

FIGS. 3A to 3C are cross-sectional views of a substrate for explaining an embodiment of the present invention .

First, as shown in FIG. 3A, an amorphous silicon film 2 is formed on a glass substrate 1 by LPCVD.
After depositing to a thickness of 0 nm, KrF with a wavelength of 248 nm
An excimer laser was irradiated with light 7A. In the laser light of this wavelength, as shown in FIG. 2, the extinction coefficients of the amorphous and crystalline silicon films are large, and the entire energy is absorbed within about 10 nm from the surface. Irradiation energy,
By setting it at 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 this polycrystalline silicon film is relatively flat and less rough since the entire film does not melt.

In the second irradiation, a KrF excimer laser beam 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,
Laser light 7 passes through polycrystalline silicon film 5C. However, since the extinction coefficient of the amorphous silicon film is large, 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 temperature of the amorphous portion on the base side can be selectively increased, so that the amorphous portion melts and crystallizes.
As shown in FIG. 3C, a polycrystalline silicon film 5D with a small surface roughness can be formed.

As described above, in this embodiment , the polycrystalline silicon film 5C on the surface formed by the first irradiation is used.
Becomes a cap layer, and surface roughness can be suppressed to 10 nm or less. In addition, 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. Also, in the first irradiation,
Using laser light with a high extinction coefficient in crystalline silicon,
Even if the irradiation is repeated, the crystallized region can be prevented from spreading, so that the variation in crystallinity in the region where the irradiation is large can be reduced.

The method of this embodiment takes advantage of the feature that laser light absorbed only in the amorphous portion of silicon is used, and the film quality of a polycrystalline silicon film having poor crystallinity and an amorphous phase is used. It can be applied to improvement. For example, plasma CV
In the polycrystalline silicon film formed by the method D, there is a region near the undersubstrate having poor crystallinity close to amorphous. By irradiating the polycrystalline silicon film with a KrF excimer laser having a wavelength of 486 nm, it is possible to selectively anneal only a region close to an amorphous phase and improve crystallinity.

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

[0028]

As described above, according to the reference example of the present invention, only the amorphous silicon is used by using laser light having a wavelength larger than the extinction coefficient of the crystalline silicon film in the amorphous silicon film. The amorphous silicon film formed on the substrate is laser annealed at the energy density at which it melts, so that only the amorphous silicon film is crystallized without melting the region of the polycrystalline silicon film where laser irradiation overlaps. Because you can
Since diffusion of oxygen from the cap oxide film into the silicon film can be prevented as in the related art, it is possible to easily form a polycrystalline silicon film having small variations in characteristics. Therefore, variations in transistor characteristics can be reduced.

According to the present invention , in the step of crystallizing the amorphous silicon, the amorphous silicon film is irradiated with laser light having a large extinction coefficient to crystallize the surface of the amorphous silicon film. By irradiating a laser beam 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 and crystallize the entire film, There is an effect that a polycrystalline silicon film with little roughness can be easily formed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a substrate for describing a reference example of the present invention.

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

FIG. 3 is a cross-sectional view of a substrate for explaining an example of the present invention .

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

FIG. 5 is a cross-sectional view of a substrate for describing 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 Melted area 5, 5A-5D polycrystalline silicon film 6 Cap oxide film 7, 7A Laser light 8A, 8B Irradiation area 9 Oxygen

Claims (1)

(57) [Claims] 1. An amorphous silicon film formed on a substrate
Method of forming crystalline silicon film that crystallizes by irradiating laser light
The extinction coefficient of the amorphous silicon film
The amorphous silicon film is irradiated with
After crystallizing only the surface of the silicon film,
In amorphous silicon film with small extinction coefficient
Irradiation of laser light with a large coefficient
Crystallized silicon film characterized by crystallizing the entire film
Forming method.