Xiao Jie Chen

Modeling ionizing radiation effects in solid state materials and CMOS devices

A comprehensive model is presented which enables the effects of ionizing radiation on bulk CMOS devices and parasitic structures to be simulated with closed form functions. The model adapts general equations for defect formation in uniform SiO2 films to facilitate analytical calculations of trapped charge and interface trap buildup in radiation sensitive shallow trench isolation (STI) oxides. An approach whereby defect distributions along the bottom and sidewall of the STI are calculated, incorporated into implicit surface potential equations, and ultimately used to model radiation-induced leakage currents in MOSFET structures and integrated circuits is described. The results of the modeling approach are compared to experimental data obtained on 130 and 90 nm devices and circuits. The features having the greatest impact on the increased radiation tolerance of advanced deep-submicron bulk CMOS technologies are also discussed. These features include increased doping levels along the STI sidewall.

Mechanisms of Enhanced Radiation-Induced Degradation Due to Excess Molecular Hydrogen in Bipolar Oxides

Bipolar junction test structures packaged in hermetically sealed packages with excess molecular hydrogen (H2) showed enhanced degradation after radiation exposure. Using chemical kinetics, we propose a model that quantitatively establishes the relationship between excess H2 and radiation-induced interface trap formation. Using environments with different molecular hydrogen concentrations, radiation experiments were performed and the experimental data showed excellent agreement with the proposed model. The results, both experimentally and theoretically, showed increased radiation induced degradation with H2 concentration, and device degradation saturate at both high and low ends of H2 concentrations.

The Effects of Hydrogen on the Enhanced Low Dose Rate Sensitivity (ELDRS) of Bipolar Linear Circuits

It is experimentally demonstrated with test transistors and circuits that hydrogen is correlated with enhanced low dose rate sensitivity (ELDRS) in bipolar linear circuits. These experiments show that the amount of hydrogen determines the total dose response versus dose rate, both the saturation at low dose rate and the transition dose rate between the high and low dose rate responses. The experimental results are supported with modeling calculations using REOS (radiation effects in oxides and semiconductors).

Estimation and verification of radiation induced N/sub ot/ and N/sub it/ energy distribution using combined bipolar and MOS characterization methods in gated bipolar devices

Complementary bipolar and MOS characterization techniques, specifically the gate sweep (GS) and sub-threshold sweep (SS), are used to estimate the radiation induced oxide charge (N/sub ot/) and interface trap (N/sub it/) buildup in gated bipolar test devices. The gate sweep and sub-threshold sweep data from recent TID testing of gated lateral PNP devices suggests an asymmetric energy distribution of interface traps after ionizing radiation exposure. Charge pumping (CP) experiments were done on the test devices to estimate the energy distribution of interface traps induced by radiation. The CP results are used in this paper to confirm the analytical findings from the GS and SS techniques and solidify the use of the complementary method as a simple way of determining radiation induced interface trap distribution in gated bipolar devices.

Modeling the Dose Rate Response and the Effects of Hydrogen in Bipolar Technologies

A physical model describing the dose rate response and the effect of hydrogen in bipolar technologies is presented. The model uses electron-hole pair recombination and competing hydrogen reactions to explain the behaviors of bipolar devices and circuits at different dose rates. Dose-rate-dependent computer simulations based on the model were performed, and the results provide excellent qualitative agreement with the dose rate data taken on both gated lateral pnp bipolar test transistors and LM193 bipolar dual-voltage comparators. The model presented in this paper can be used to explain a variety of factors that can influence device dose rate response in bipolar technologies.

Nature of Interface Defect Buildup in Gated Bipolar Devices Under Low Dose Rate Irradiation

The buildup of radiation-induced switching states in ELDRS-sensitive bipolar base oxides is measured with dc current-voltage and charge pumping techniques. These states include both faster interface traps (Pb) centers) and slower border traps. After irradiation, border traps and interface traps mostly decrease with annealing time and temperature in devices irradiated at 0 V. However, for devices irradiated at -50 V, there is a decrease in border trap density but an increase in interface trap density. These differences in interface-trap buildup and annealing are attributed to the dependence of defect passivation and depassivation on the concentrations of hydrogen and dangling Si bond defects near the Si/SiO2 interface

Total dose radiation response of CMOS compatible SOI MESFETs

Metal semiconductor field effect transistors (MESFETs) have been fabricated using a silicon-on-insulator (SOI) CMOS process. The MESFETs make use of a TiSi/sub 2/ Schottky gate and display good depletion mode characteristics with a threshold voltage of -0.5 V. The drain current can also be controlled by a voltage applied to the substrate, which then behaves as a MOS back gate. The transistors have been irradiated with 50 keV X-rays to a total ionizing dose in excess of 1 Mrad(Si). After irradiation the threshold voltage of both the top Schottky gate and the back MOS gate shift to more negative values. The shift in threshold is attributed to radiation induced fixed oxide charge at the interface between the SOI channel and the buried oxide.

Post-Irradiation Annealing Mechanisms of Defects Generated in Hydrogenated Bipolar Oxides

Bipolar test structures were irradiated and annealed with various combinations of molecular hydrogen gas ambients, bias, and thermal conditions. The results show that the buildup and annealing behavior of defects in bipolar base oxides depend strongly on hydrogen concentration. Differences observed in trapped oxide charge annealing rates suggest that the charged defects created in hydrogen-rich environments may be attributed to different types of positive charge in addition to trapped holes.

Impact of hydrogen contamination on the total dose response of linear bipolar microcircuits

Several linear bipolar microcircuits commonly used in space have been selected to investigate whether hydrogen contamination has an impact on their total dose response. Results obtained from irradiations performed on the HSYE117 linear voltage regulator from Intersil and the AD590 temperature transducer from Analog Devices show a causal relationship between in-package hydrogen content and total dose response. Silicon nitride passivation has been found to play a key role in this study. A recommended approach to hardening bipolar linear circuits with poor total dose response is to characterize them as processed through metallization.

Radiation Effects on a Potential Scintillation-Based Solid-State Spectrometer Prototype for Compact Monitoring of Space Radiation/Weather Satellite Conditions

New and emerging scintillators, such as DPA (Diphenylanthracene) and CLYC (Cs2LiYCl6:Ce), coupled with Solid-State Photomultipliers (SSPM) provide a lightweight, low voltage, potentially radiation hard spectrometer for real-time monitoring of space weather conditions. This paper presents the results from proton radiation-hardness tests conducted for candidate scintillators and for individual cells of 1 cm2 SSPMs. Decreases from original light outputs were observed for samples of DPA and CLYC by 4% and 28% respectively with a dose of 1 kGy . Following 1 kGy exposure to the SSPMs, the dark current increased by a factor of ~ 32 for a reverse bias of 30 V. The effects of room temperature annealing on the dark current were also monitored. Decreases in quantum efficiency (22% for 420 nm and 7% for 535 nm) and light response (33% for 420 nm and 26% for 535 nm) were observed as a possible result of the formation of surface recombination-generation centers. The energy resolution of a LYSO scintillator sample coupled with a SSPM and measured for a 22Na gamma source (E = 511 kev) decreased from 15% pre-irradiation to 34% post 1 kGy irradiation.