Characterization and modeling of the thin gate oxide quality and its lifetime

The charge trapping and breakdown characteristics of thin gate oxides (3.8 nm--12 nm) grown on bulk and SIMOX SOI (separation-by-implantation-of-oxygen silicon-on-insulator) substrates were investigated under high current injection conditions. The influence of the starting material and process conditions on gate oxide quality was mainly characterized for gate oxides grown on SIMOX substrates.;The energy of electrons, which depends on barrier height in addition to electric field and temperature, determines the trapping and de-trapping rate of electrons in the bulk oxide and from the pre-existing neutral traps, respectively. De-trapped electrons from pre-existing neutral and interface traps cause the initial gate voltage shift. Although pre-existing neutral traps can explain the sensitivity of initial gate voltage shift to the process and stress conditions, the number of pre-existing traps has almost no correlation with the destructive breakdown time of the gate oxide. This is not only because the number of de-trapped electrons is pre-determined by the process and stress conditions but also because electron trapping is the dominant factor in determining the lifetime and destructive breakdown of gate oxide. It is believed that gate oxide breakdown occurs when damage due to the trapped electrons propagates throughout the gate oxide resulting in a resistive path formation. It is shown that the gate oxide breaks down near the n-LDD region of n-MOSFETs where the injected current is confined under positive gate bias. An empirical model employing the electron transit time can explain the oxide lifetime dependence on barrier height, gate oxide thickness, and temperature.;Although the intrinsic characteristics of gate oxide grown on SIMOX SOI substrate were compatible to those of bulk counterparts, gate oxide grown on SIMOX SOI substrate showed a higher early-failure-rate (EFR), which is sensitive to both starting material and fabrication conditions. The EFR of gate oxides grown on SIMOX SOI can be as low as that of their bulk counterparts by reducing the stress and damage of SOI films with optimized process conditions. Proposed process conditions are low oxygen dose &parl0;4x1017/cm2&parr0; , a 1°C/min. ramp rate for post-implant annealing, shallow trench isolation, and a phosphorous implanted source/drain. Ion contamination (up to 2x1010/cm2 ), surface roughness (up to 1 nm with 20 x 20 mum2 scanned area), and SOI film thickness (150 nm--230 nm) showed almost no impact on the EFR of gate oxides grown on SIMOX SOI.;The empirical oxide breakdown model can be used to predict the lifetime of thin gate oxides as well as to determine the maximum operating voltage for a given gate oxide thickness. Optimized process conditions for SIMOX SOI can push SOI technology into the mainstream for high-speed and low-power applications.