title: Total Ionizing Dose Response of High-k Dielectrics on MOS Devices
Creators: name: Mu, Y
Publisher: name: University of Liverpool Repository
Year: 2017

description: As advanced Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) technology continues to minimize the gate oxide thickness, the exponential increase in gate leakage current poses a major challenge for silicon dioxide (SiO2) based devices. In order to reduce the gate leakage current while maintaining the same gate capacitance, alternative gate insulator materials with higher dielectric constant (high-k) became the preferred replacement of SiO2 gate dielectrics. Germanium (Ge) MOSFETs have been regarded as promising candidates for future high-speed applications because they possess higher carrier mobility when compared to silicon based devices. At present, advanced microelectronics devices and circuits are used in aerospace engineering, nuclear industry, and radiotherapy equipment. These applications are unavoidably exposed to space-like radiation, which has a relative low radiation dose rate at 10-2-10-6 rad(Si)/s. For these reasons, it is necessary to understand the low-dose-rate radiation response of high-k materials based on Si and Ge MOS devices. The radiation response of high-k materials such as radiation-induced oxide and interface trap density have been typically examined by carrying out off-site capacitance-voltage (CV) measurements. However, the conventional and off-site radiation response measurements may underestimate the degradation of MOS devices. In this study, a semi-automated laboratory-scale real-time and on-site radiation response testing system was developed to evaluate the radiationresponse. The system is capable of estimating the radiation response of MOS devices whilst the devices are continuously irradiated by -rays raysrays. Moreover, the complete CV characteristics of MOS capacitors were measured in a relatively short time. The pulse CV measurement reduces the impact of charge trapping behavior on the measurement results, when compared to conventional techniques. The total ionizing dose radiation effect on HfO2 dielectric thin films prepared by atomic layer deposition (ALD) has been investigated by the proposed measurement system. The large bidirectional ΔVFB of the irradiated HfO2 capacitor was mainly attributed to the radiation-induced oxide trapped charges, which were not readily compensated by bias-induced charges produced over the measurement timescales of less than 5 ms. Radiation response of Ge MOS capacitors with HfO2 and HfxZr1-xOy gate dielectrics was also investigated. It was found that radiation-induced interface traps were the dominant factor for Flat-band Voltage shift (ΔVFB) in HfO2 thin films, whereas the radiation response for Zr-containing dielectrics under positive bias was mainly affected by oxide traps. Under positive biased irradiation, the Zr-doped HfxZr1-xOy exhibited smaller ΔVFB than that of HfO2. This is attributed to the de-passivation of Ge-S bonds in capacitors incorporating HfO2 thin films, resulting in the build-up of interface traps. Under negative biased irradiation, ΔVFB was attributed to the combined effect of the net oxide trapped charges and the passivation of Ge dangling bonds at the Ge/high-k interface.