A silicon nitride film is used as an insulating film, a protection film or the like in various semiconductor devices. The silicon nitride film may be formed by a plasma CVD method using a silicon-containing gas such as silane (SiH4) and a nitrogen-containing gas such as nitrogen or ammonia as source gases (see, e.g., Japanese Laid-open Publication No. 2000-260767).
In the silicon nitride film formed by a conventional plasma CVD method, it is important to suppress stresses, i.e., tensile and compressive stresses, of the film, which have a bad influence on device characteristics. For instance, if a compressive stress of the silicon nitride film is large, stress migration may occur to cause disconnection of metal lines right under the film. Accordingly, the compressive stress needs to be reduced to prevent such disconnection. In the plasma CVD method, the direction of stress (tensile stress or comprehensive stress) of the silicon nitride film or the magnitude of stress depends on process conditions including pressure, temperature, gas species, etc. Thus, conventionally, process conditions are set such that a strong stress is not applied to the silicon nitride film, and a silicon nitride film having no stress is formed by a plasma CVD method (e.g., Maeta Gazuo ┌VLSI and CVD┘ Tenshoten, published on Jul. 31, 1997).
Recently, attempts have been made to take advantage of the stress of silicon nitride film to improve the device characteristics. However, for example, a parallel plate type or inductively coupled plasma CVD apparatus utilizes plasma having a relatively high electron temperature. Accordingly, when film formation conditions such as high-frequency output, pressure and temperature are changed to introduce a high stress, plasma damage may occur in the silicon nitride film. Thus, it is difficult to obtain a high-quality silicon nitride film and it is also difficult to obtain a silicon nitride film having a high stress. Further, since there is a limitation in the plasma process conditions, it is also difficult to precisely control a stress.