1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device in which semiconductor elements are formed on a substrate. The present invention also relates to a method of manufacturing a display device such as a liquid-crystal panel including switching elements for driving pixels.
2. Description of the Related Art
Recently, thin film transistors (hereinafter referred to as TFTs) using polycrystalline Silicon films (referred to as p-Si films) acting as active layers formed on a transparent insulating substrate have been employed as pixel drive elements in an active matrix LCD (Liquid Crystal Display).
The poly silicon thin film transistor (referred to as a p-Si TFT) advantageously has a larger mobility and higher drive capability, as compared with the a-Si thin film transistor in which an amorphous silicon film is used as an active layer. Hence, the use of p-Si TFTs allows a high-performance LCD to be realized and peripheral drive circuits to be integrated on the same substrate in addition to the pixel area.
In the p-Si TFT, the source region and the drain region are formed in a p-Si film acting as an active layer by implanting ions into both p-Si films and then annealing them for activation.
FIG. 1 is a cross sectional view illustrating a transistor element which is subjected to a conventional activating step after ion implantation to source region and drain regions.
The following steps are performed prior to the step shown in FIG. 1. First, a gate electrode 2 of a high-melting point metal (refractory metal) is formed on an insulating substrate 1. Insulating thin films 3 and 4, and an a-Si film are subsequently formed on the gate electrode 2. The p-Si film 6 is formed by melting and recrystallizing the a-Si film with a laser beam. Next, a SiO.sub.2 film is formed on the entire surface of the p-Si film 6, so that a stopper 7 is formed by using the photolithographic technique and the dry-etching technique. The p-Si film 6 is subjected to an ion doping, with the stopper 7 used as a mask. Thus, the source region 6s and the drain region 6d are formed in the p-Si film 6.
Thereafter, a heating process is performed to activate doped ions, as shown in FIG. 1. Known heating methods include RTA (Rapid Thermal Annealing), furnace annealing, and the like. RTA methods include lamp RTA methods using a lamp and ELA (Excimer Laser Annealing) methods using an excimer laser.
ELA uses a laser beam with a relatively small beam size of 0.5 mm.times.150 mm, thus providing small throughput. The laser beam has short oscillation wavelengths and tends to be easily absorbed by the gate electrode material. Furthermore the laser beam outputted from excimer laser source has a relatively short oscillation pulse width of 10 to 30 ns. Hence, ELM methods cannot sufficiently activate impurities because the film temperature rise time is very short. For sufficient activation, the p-Si film needs to increase its annealing temperature so that ELA tends to be easily affected by the material, size, and pattern density of the gate electrode.
Particularly, with a top-gate structure, the gate electrode may be melted during the activation annealing process or may be ablated. RTA uses a xenon arc lamp which radiates light of relatively broad oscillation wavelengths and a large beam size of 10 mm wide.times.400 mm long. Hence, unlike ELA, a light absorption efficiency according to materials is small difference, and it is not needed to rising the temperature of the p-Si film because an irradiation time is relatively long. This method can provide high throughput and is not affected by the influence of the gate electrode structure.
However, since the activation of a semiconductor layer through RTA requires large irradiation beam size and long irradiation time, the glass substrate on which a semiconductor layer is formed, increases to a very high temperature. Hence, there is a disadvantage in that, since the substrate temperature at a light irradiated portion rises with a larger lamp output, the difference in temperature between the light irradiated portion and non-irradiated portions becomes too large, and the resulting thermal strain often causes a substrate to crack.