There is known a technology of constituting a thin film transistor by using a silicon film formed on a glass substrate. The thin film transistor formed on a glass substrate is utilized in an active matrix type liquid crystal display device. A thin film transistor which has been reduced into practice currently uses an amorphous silicon film formed on a glass substrate by a plasma CVD (chemical vapor deposition) process.
However, the property of a thin film transistor obtained by an amorphous silicon film is poor and more improved property is currently required. Because of the poor property a thin film transistor of P-channel type cannot be manufactured using an amorphous silicon film (the property is considerably low compared with that of N-channel type and is out of practical use) and accordingly, it is not actually possible to constitute a CMOS circuit. Therefore, it is not possible to realize a structure using a CMOS circuit by a thin film transistor when an amorphous silicon film is used which considerably restricts its application.
A thermal annealing at 1000.degree. C. or more can be performed by using a single crystal wafer or a quartz substrate and therefore, it is possible to provide a crystalline silicon film having necessary properties by crystallizing the amorphous silicon film by a thermal annealing process. However, it is not possible to utilize a single crystal wafer wherein a visible light beam cannot be transmitted as a substrate constituting a liquid crystal device. Further, the quartz substrate is expensive which is inconvenient in view of production cost in expanding the area of the liquid crystal display device.
Under such a situation a technology in which an amorphous silicon film is formed on a glass substrate by a plasma CVD process or a low pressure CVD process and a crystalline silicon film is provided by some treatment, is required. As such a treatment a process by heating, a process by laser beam irradiation or the like has been known.
As a process by heating a method of crystallizing an amorphous silicon film by performing a heat treatment at a temperature of 600.degree. C. or more for several tens hours or more has been known. However, the strain point of a Corning 7059 glass substrate which is generally used frequently as a substrate for a liquid crystal display device is 593.degree. C. Deformation or wrinkle of the glass substrate becomes significant by exposing it under a temperature of 600.degree. C. or more for several tens hours which considerably influences on the manufacturing of devices. For example, the mask alignment becomes difficult by the deformation of the glass substrate. Especially, the problem becomes serious in expanding area. The problem is also recognized in case where other glass substrates are utilized.
Meanwhile, there is a superiority in adopting the process by laser beam irradiation in which the amorphous silicon film can be crystallized without causing thermal damage on a glass substrate. However, the utilization of a high power laser beam over a large area is difficult in view of the technology and production cost which lowers its practical use.
There is a technology described in Japanese Unexamined Patent Publication No. 232059/1994 as a technical means solving such a problem. The publication discloses a technology in which a metal element, (for example, nickel) promoting crystallization of silicon is held in contact with the surface of an amorphous silicon film formed on a glass substrate by a plasma processing and a crystalline silicon film is provided by performing a heating treatment at approximately 550.degree. C. for several hours.
With the heating treatment at 550.degree. C. for several hours the deformation or wrinkle of the substrate is not so serious even if Corning 7059 glass substrate is used. Therefore, it is an extremely useful method as a technology for providing a thin film transistor by using a crystalline silicon film. This process is very useful even if a glass substrate other than Corning 7059 glass substrate is used. However, conditions of the plasma processing are delicate in the technology disclosed in Japanese Unexamined Patent Publication No. 232059/1994 and in the actual reduction to practice a metal element having an amount more than necessary is introduced in the silicon film.
For example, when the concentration of nickel element finally remaining in the silicon film is 1.times.10.sup.19 atoms cm.sup.-3 or more, the influence of a nickel silicide component in the silicon film becomes significant and the property as a semiconductor is deteriorated. A means of strictly controlling an amount of doping a metal element is necessary to solve the problem. However, as mentioned above it is difficult to control the concentration of a metal element doped in the silicon film by the technology disclosed in Japanese Unexamined Patent Publication No. 232059/1994.