Physical and electrical characteristics of physical vapor-deposited tungsten for bit line process

We attempted to explain the phenomenon that the electric resistivity of tungsten film increases as the thickness decreases and that physical vapor-deposited (PVD) tungsten shows a much lower resistivity than chemical vapor-deposited (CVD) tungsten. The crystallinity and electric conductivity of an under-layer does not affect the electric resistivity of tungsten film. The low resistivity of PVD tungsten originates from a large grain size. PVD tungsten with large grains is free from grain boundary scattering, while CVD tungsten with small grains exhibits grain boundary scattering. As film thickness decreases down to the mean free path of tungsten, the surface scattering effect surpasses the grain boundary scattering effect. Consequently, the resistivity of PVD and CVD tungsten becomes equal because surface scattering increases the resistivity of both large- and small-grained films. The same rule is adaptable for the resistance change of a narrow line structure. CVD tungsten shows a high resistance because of grain boundary scattering originating from a small grain size. However, if the line-width is reduced to the mean free path, grain boundary scattering disappears, making surface scattering only the factor that increases electric resistivity. Thus, CVD tungsten shows the same resistance as PVD tungsten in a very narrow line structure.