Net Negative Charge in low-temperature SiO₂ gate dielectric layers

SiO2 gate dielectric layers (4–60 nm) were grown (0.6 nm/min) by plasma-enhanced chemical vapor deposition (PECVD) in strongly diluted silane plasmas at low substrate temperatures. In contrast to the well-accepted positive charge for thermally grown silicon dioxide, the net oxide charge was negative and a function of layer thickness. Our experiments suggested that the negative charge was created due to unavoidable oxidation of the silicon surface by plasma species, and the CVD component added a positive space charge to the deposited oxide. The net charge was negative under process conditions where plasma oxidation played a major role. Such conditions include low deposition rates and the growth of relatively thin layers.