A new MOS radiation-induced charge - Negative fixed interface charge

The irradiation behavior (up to about 1.3 Mrad SiO2) of MOS capacitors (50 nm thick dry grown SiO2, Al, or poly-Si gate) with or without postoxidation anneal (POA) in Ar at 1000 C has been studied by conventional capacitance-voltage (CV) analysis and thermally stimulated current techniques. The most important finding is that all of the samples processed for radiation hardness, i.e., without POA, exhibited a charge compensation effect. The radiation-induced positive oxide charge is partially compensated by a negative fixed interface charge which lies either outside the energy range accessible by CV and similar techniques or extends spatially away from the interface into the oxide without acquiring the characteristics of a 'bulk' oxide charge. The existence of this fixed charge may jeopardize long-term device reliability as an interface charge is more prone to change than an oxide charge.