A method of minimizing a seam effect of a deposited TEOS oxide film during a trench filling process performed on a semiconductor substrate in a semiconductor substrate plasma processing apparatus comprises supporting a semiconductor substrate on a pedestal in a vacuum chamber thereof. Process gas including TEOS, an oxidant, and argon is flowed through a face plate of a showerhead assembly into a processing region of the vacuum chamber. RF energy energizes the process gas into a plasma wherein TEOS oxide film is deposited on the semiconductor substrate so as to fill at least one trench thereof. The argon is supplied in an amount sufficient to increase the electron density of the plasma such that the deposition rate of the TEOS oxide film towards the center of the semiconductor substrate is increased and the seam effect of the deposited TEOS oxide film in the at least one trench is reduced.