Laser processing method of semiconductor device

DETAILED DESCRIPTION However, it is very difficult for only the laser annealing to produce a crystalline silicon film having such a large mobility as is required for a liquid crystal display device.
Therefore, it has conventionally been proposed to form a crystalline silicon film by using both of the thermal annealing and laser annealing.
First a silicon film is crystallized by heating it at about 550.
degree.
C.
for several hours (thermal annealing), and then the crystallinity of a resulting crystallized silicon film is improved by illuminating it with laser light.
Thin-film transistors produced by using a crystalline silicon film obtained by this method have small off-currents of about 10.
sup.
-12 A with a small variation (in the same order), as well as a mobility (about 20 cm.
sup.
2 /Vs) as required for thin-film transistors in the pixel area.
Therefore, the thin-film transistors thus produced are suitable for use in the pixel area.
However, it is difficult for this annealing technique to realize a mobility larger than 100 cm.
sup.
2 /Vs, which is required for thin-film transistors in the peripheral circuits.
The present inventors have already proposed a technique of producing a crystalline silicon film having a high mobility in which an amorphous silicon film is crystallized by using a metal element for accelerating crystallization.
Various metal elements can be used as the metal element for accelerating crystallization because they are merely required to serve as nuclei when amorphous silicon is crystallized.
Our experiments have revealed that the addition of Ni produces the best crystallinity.
One method of adding Ni to an amorphous silicon film is to apply a nickel acetate salt solution to its surface.
After the introduction of Ni into the amorphous silicon film, it is crystallized by holding it in an atmosphere of 550.
degree.
C.
for about 4 hours.
The crystallinity of a crystallized silicon film (crystalline silicon film) is improved by illuminating it with linear laser light.
The laser light illumination is performed such that weaker pulse laser light is preliminarily applied before application of stronger pulse laser light