Date on Master's Thesis/Doctoral Dissertation

12-2018

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Electrical and Computer Engineering

Degree Program

Electrical Engineering, PhD

Committee Chair

McNamara, Shamus

Committee Co-Chair (if applicable)

Alphenaar, Bruce

Committee Member

Alphenaar, Bruce

Committee Member

Walsh, Kevin

Committee Member

Sumanasekera, Gamini

Author's Keywords

MEMS; radiation effect; UV; x-ray; proton; native oxide charging

Abstract

We explored UV, X-ray and proton radiation damage mechanisms in MEMS resonators. T-shaped MEMS resonators of different dimensions were used to investigate the effect of radiation. Radiation damage is observed in the form of resistance and resonance frequency shift of the device. The resistance change indicates a change in free carrier concentration and mobility, while the resonance frequency change indicates a change in mass and/or elastic constant. For 255nm UV radiation, we observed a persistent photoconductivity that lasts for about 60 hours after radiation is turned off. The resonance frequency also decreases 40-90 ppm during irradiation and slowly recovers at about the same time scale as the resistance during annealing. For X-ray radiation, the resonance frequency decreases with radiation, but the resistance increases. To investigate X-ray dose-rate dependence, we irradiated the resonators at three different dose rates of X-ray: 5.4, 10.9 and 30.3 krad(SiO2)/min. The change in resonance frequency and resistance both showed a dose rate dependence where a lower dose-rate X-ray caused a larger shift in resonance frequency than the higher dose-rate. We attributed the observed shift in resonance frequency to the change in carrier concentration—using Keyes’ theory of electronic contribution to elastic constant—for both X-ray and UV radiation. The resistance change is explained by the net effect of the carrier concentration and mobility change. We proposed that the carrier concentration changes through two differing mechanisms for X-ray and UV radiation. For X-ray, dopant depassivation is primarily responsible for the carrier concentration change since an X-ray is known to dissociate the hydrogen-boron complex and it penetrates through the 15μm thick Si resonator affecting the whole bulk of Si. On the contrary, the 255nm UV gets absorbed near the surface (within 10nm) and charges the native oxide. The mirror charge on adjacent silicon is responsible for the carrier concentration change. The mirror charges drive the silicon surface to accumulation, depletion or strong inversion depending on the type and amount of charge trapped in the oxide. Since the carrier concentration only changes near the surface, it was predicted that higher surface-to-volume ratio devices will show a greater shift in resonance frequency. This was proven by radiating three devices with differing widths (1, 2 and 8μm), and therefore differing surface-to-volume ratios. This experiment verified that the UV light effect is surface dominated. The dimensional dependence is also observed for X-ray radiation damage. We found that a reduction in the surface-to-volume ratio enhances the X-ray radiation damage and we proposed a hydrogen diffusion-based model that fits the observed dimensional dependence of X-ray radiation damage. For proton radiation, the direction of resonance frequency change depended on the energy of radiated proton. Two proton energies were tested: 0.8MeV and 2MeV. The proton with 0.8MeV energy stops inside the resonator, causing greater displacement damage than the proton with 2MeV energy, which readily passes through the resonator. The 2MeV proton causes more ionization damage than the 0.8MeV protons. So, the observed energy dependence of resonance frequency shift comes from the competing effects of displacement damage and ionization damage since resonance frequency decreases due to ionization damage but increases due to displacement damage. The result agrees with our theory since the 0.8MeV proton radiation showed net resonance frequency increase during radiation and more permanent damage after annealing compared to the 2MeV proton radiation.

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