Direct and pulsed current annealing of p-MOSFET based dosimeter: the “MOSkin”

Abstract

Contemporary radiation therapy (RT) is complicated and requires sophisticated real-time quality assurance (QA). While 3D real-time dosimetry is most preferable in RT, it is currently not fully realised. A small, easy to use and inexpensive point dosimeter with real-time and in vivo capabilities is an option for routine QA. Such a dosimeter is essential for skin, in vivo or interface dosimetry in phantoms for treatment plan verification. The metal-oxide-semiconductor-field-effect-transistor (MOSFET) detector is one of the best choices for these purposes, however, the MOSFETs sensitivity and its signal stability degrade after essential irradiation which limits its lifespan. The accumulation of positive charge on the gate oxide and the creation of interface traps near the silicon-silicon dioxide layer is the primary physical phenomena responsible for this degradation. The aim of this study is to investigate MOSFET dosimeter recovery using two proposed annealing techniques: direct current (DC) and pulsed current (PC), both based on hot charged carrier injection into the gate oxide of the p-MOSFET dosimeter. The investigated MOSFETs were reused multiple times using an irradiation-annealing cycle. The effect of the current-annealing parameters was investigated for the dosimetric characteristics of the recovered MOSFET dosimeters such as linearity, sensitivity and initial threshold voltage. Both annealing techniques demonstrated excellent results in terms of maintaining a stable response, linearity and sensitivity of the MOSFET dosimeter. However, PC annealing is more preferable than DC annealing as it offers better dose response linearity of the reused MOSFET and has a very short annealing time.

Comparative evaluation of modern dosimetry techniques near low‐ and high‐density heterogeneities

“…It is a real‐time dosimeter and has good characteristics linearity and decent reproducibility 53 , 54 . MOS kin detector has always been used in its linear dose range by using current annealing technique 60 , 61 for recovery of its initial threshold values after about 30 Gy accumulated dose that warrants its linearity. However, care needs to be taken to minimize measurement uncertainties, such as voltage creep‐up effect that could introduce up to 2% error in a typical clinical dose of 2 Gy (62) .…”

Application of pMOS Dosimeters in Radiotherapy

“…The MOSFET needs to be replaced when the upper limit of linearity is achieved. Although, recently, the possibility of MOSFET reuse after recovering for a certain period of time at room or elevated temperature [22] or by current annealing [23] has been studied.…”

P-channel MOSFET as a sensor and dosimeter of ionizing radiation

“…Once the upper limit of linearity is achieved, the RADFETs need to be replaced. However, recent studies have shown that such RADFETs can be recovered for reuse by storing at room or elevated temperature for a sufficient time [22], [23] or by annealing with current [24], [25].…”