Ph.D Thesis

Ph.D StudentPikhay Evgeny
SubjectSolid State Direct Radiation Sensor Based on Floating Gate
DepartmentDepartment of Electrical and Computer Engineering
Supervisors PROFESSOR EMERITUS Yael Nemirovsky
PROF. Yakov Roizin
Full Thesis textFull thesis text - English Version


Ionizing radiation measurements are required in numerous applications related to  safety, medicine, defense, industry and research. Various types of dosimetry systems, operating on different physical principles are known.   Most of them suffer of such drawbacks as high cost, bulky dimensions, complex and time consuming read-out, etc. Meantime, the emerging approaches, like IoT, require compact, cheap, ultra-low power consuming sensors, that  could be used in precise SoC dosimeters and allow fast and easy read-out. In addition, various Gamma and X-Ray imaging applications require direct sensors of ionizing radiation allowing to overcome the spatial resolution limit, set by scintillator thickness of the competing  indirect radiation imagers.

In this work, we developed, fabricated and investigated a family of novel direct sensors of ionizing radiation, based on Floating Gate (FG) principle and manufactured in standard CMOS technology. The FG is pre-charged (programmed) before the exposure. Irradiation of charged device results in the removal of trapped charge from the FG. Thus, the total dose (TID) can be calculated, based on the threshold voltage shift. The sensor can be re-programmed for multiple sessions of irradiations, while  high sensitivity can be sustained for a wide range of absorbed doses.

With the aim to get insight into the physics  of the proposed sensors,  we designed several types of special test structures and developed  novel techniques for their characterization.  The devices were exposed  to radiation from different sources and sensor response and degradation mechanisms were studied.  This allowed also to propose an  optimal sensor design and outline the roadmap for further device improvement.

As a part of the work, 2D arrays consisting of ionizing radiation sensors were suggested and built.  This  allowed to perform statistical study of sensors and demonstrate direct imaging in Gamma rays. In addition, detection of high-energy ions was performed using the developed 2D arrays.

The scientific importance of the research is the in-depth understanding of the physical mechanisms of the ionizing radiation interaction with Si-SiO2 interfaces and nanometric-scale CMOS devices.  The progress in understanding is to a great extent connected with the   developed in the presented work novel techniques of CMOS device characterization and degradation analyses. 

The technological breakthrough of the research is the provided main building block:  C-Sensor - a compact, ultra-low power, precise on-chip device for monitoring of absorbed radiation dose. The developed sensors are promising in different applications, in particular, radiation safety and security devices and in-vivo dosimeters for precise dose delivery control in radiation therapy.