Reliability of MOSFET gate insulator layers at high temperatures.

Abstract

This dissertation describes the high temperature reliability performance of reoxidized nitrided oxides serving as MOSFET gate insulator layers and the applications of these layers to high-field devices. The lightly-nitrided oxides with an optimum reoxidation process were proven to be the most stable insulator layers under high temperature operation. Bias-temperature (B-T) stress on metal gate MIS capacitors demonstrated that the reoxidized nitrided oxides give lower charge trapping and less interface state generation than the oxide and the nitride oxides. Among the reoxidized nitrided oxides, the film nitrided at 700$\sp\circ$C for 1 hour and then reoxidized at 1000$\sp\circ$C for 2-3 hours offers the most stable characteristics. This superior performance was related to the reduced nitrogen concentration and the absence of a new oxide layer at the silicon interface. Poly gate capacitors give different B-T results than the metal gate capacitors. This is attributed to the modification of layer composition or structures caused by the post-insulator device fabrication steps. MOSFET structures with injection rails were used to study the B-T degradation mechanisms. The carriers passing through the silicon/insulator interface proved not able to generate insulator defect centers; this indicates that the gate electric field and temperature are more important in causing B-T degradation than the passing carriers. High-field electron injection stress tests showed that the lightly-nitrided reoxidized nitrided oxides are more suitable for high-field applications since they offer less charge trapping and moderate interface state generation. Appropriate annealing treatment was found to restore device characteristics to their original values. The inversion layer mobilities were also studied. Very little mobility degradation occurs to the most stable insulator layers, the lightly-nitrided oxides with an optimum reoxidation process. Overall, a conclusion was made that pre-existing bulk defects cause B-T charge and interface state buildup. The films with low bulk defect densities present stable high-temperature characteristics.