Passive Wireless Radiation Dosimeters for Radiation Therapy Using Fully-Depleted Silicon-on-Insulator Devices

Abstract

Radiation cancer therapy is one of the most important methods of cancer treatment. The goal of radiation therapy is to apply maximum amount of dose to the tumor region while applying minimum amount of dose to the normal tissues. Being able to measure radiation dose accurately is critical for radiation therapy because it will make sure the radiation is properly delivered to the tumor region as planned. In this dissertation, the development of a passive wireless radiation dosimeter for radiation cancer therapy based on fully-depleted silicon-on-insulator (FDSOI) technology was presented through the following steps. In Chapter 1, the radiation cancer therapy and existing dosimetry technology was introduced. Because of the high demand of modern radiation therapy and the deficiency of existing dosimetry. An ultra-small, passive radiation dosimeter will greatly help the dose verification in today’s radiation therapy. In Chapter 2, the building block of proposed dosimeter technology, FDSOI technology, was introduced and cutting edge development of FDSOI dosimeter was summarized. In Chapter 3, the radiation response of FDSOI metal–oxide–semiconductor field-effect transistors (MOSFETs) over a wide range of therapeutic X-ray beam energies, angles, dose ranges and applied substrate voltages were characterized. The charge collection efficiency was calculated from both technology computer aided design (TCAD) simulation and an analytical model to evaluate the radiation effect quantitatively. Based upon the experimental results, the design of passive wireless sensors using FDSOI varactors was proposed. In Chapter 4, the fabrication process of FDSOI varactors was developed at University of Minnesota. Capacitance vs. voltage (CV), current vs. voltage (IV), and TCAD modeling were used to fully understand the device characterization. The capacitance based sensing of Cobalt-60 (Co-60) gamma radiation was demonstrated using FDSOI varactors. An analytical model was developed to further understand the device response under irradiation. Based on the analytical model, the optimal working condition at different frequencies was discussed and a guide line of improving the device sensitivity was provided. TCAD simulation was also conducted to optimize the design space of FDSOI varactors for wireless sensing. In Chapter 5. 120-finger varactors with 0 V threshold voltage were fabricated using electron beam lithography with small device variability. An impedance measurement setup was built in clinical environment. Completely passive wireless radiation sensing was demonstrated using FDSOI varactor and the device showed good linearity of resonant frequency change under radiation. The possibilities to improve the performance of device were discussed. Chapter 6 summarized the progress of FDSOI dosimeter. The future development towards more reliable dosimeter with smaller sized was proposed. A novel device structure with two-dimensional material that could expend the application of solid-state radiation dosimeter was also proposed.