(a) Field of the Invention
The present invention relates to a method for forming a gate insulating film in a semiconductor device and, more particularly, to the technique for forming a nitrogen-containing gate insulating film on a semiconductor substrate in the semiconductor device.
(b) Description of the Related Art
Along with wide spreading of mobile terminals, it is desired to further reduce the power dissipation in semiconductor devices such as a DRAM (Dynamic Random Access Memory) device. For achieving the lower power dissipation in the semiconductor devices, a technique for introducing nitrogen into the gate insulating film (gate oxide film) of MOSFETs (Metal-Insulator-Semiconductor Field Effect Transistors) is employed to reduce the thickness of the gate insulating film. The nitrogen introduced into the gate insulating film raises the dielectric constant of the gate insulating film, and prevents impurities from penetrating through the gate insulating film, to thereby improve the characteristics of MOSFETs.
Upon introduction of nitrogen into the gate insulating film, it is desired to control diffusion of the introduced nitrogen toward the interface between the gate insulating film and the silicon substrate. This is because the nitrogen accumulated in the vicinity of the interface with the silicon substrate generates an impurity level, to cause degradation in a variety of characteristics of MOSFETs, such as shift or variation in the threshold voltage (Vth) and reduction in the mobility of careers. In order to prevent the accumulation of nitrogen in the vicinity of the interface with the silicon substrate, a two-layer gate insulating film is employed, wherein a silicon oxide (SiO2) layer and an overlying silicon nitride (SiN) layer are formed on the silicon substrate by using a CVD (Chemical Vapor Deposition) technique.
The technique for forming the two-layer gate insulating film including SiO2 and SiN layers is described in Patent Publication JP-2002-203961A, for example.
It may be considered to use an ALD (Atomic Layer Deposition) technique for forming the above silicon nitride layer in the two-layer gate insulating film to obtain an improved reliability thereof. In the ALD technique, the silicon nitride layer may be formed by the steps of depositing a monoatomic silicon layer, and nitriding the deposited monoatomic silicon layer to form a silicon nitride layer having a monoatomic-level thickness, and iterating the process for formation of the monoatomic-level silicon nitride layer to obtain a desired thickness of the silicon nitride film. The ALD technique generally provides a superior film quality to the resultant silicon nitride film.
However, in the ALD technique for forming the silicon nitride layer on the SiO2 layer, the concentration of nitrogen in the silicon nitride layer is considerably reduced compared to the concentration of the stoichiometric composition of SiN layer, whereby a desired concentration of nitrogen is not achieved in the two-layer gate insulating film. It is desired to raise the nitrogen concentration in the silicon nitride layer in the vicinity of the interface between the gate insulating film and the gate electrode, for obtaining MOSFETs having desired characteristics.