Negative Charge in Plasma Oxidized SiO₂ Layers

SiO2 gate dielectric layers were deposited on n-type Si by inductively-coupled plasma-enhanced chemical vapor deposition (ICPECVD) at 150°C. In contrast to the well-accepted positive charge for thermally grown SiO2, the net oxide charge was negative and a function of the layer thickness. Our experiments suggested that the negative charge was created due to unavoidable oxidation of the silicon surface by plasma species. The CVD component added a positive space charge to the oxide. Additional measurements showed that the negative charge in SiO2 also persisted on p-type substrates. We suggest that plasma oxidation of the silicon surface results in SiO2 with a surplus of oxygen, which is able to accumulate a negative charge. This assumption is addressed by a series of experiments wherein oxygen is implanted into thermal SiO2. It is shown that the implantation can result in a negative charge to the bulk oxide layer.