Adrian Faigon

A physics-based numerical model is proposed as a simulation tool to predict the total ionizing dose response of CMOS integrated circuits. The model includes the radiation-induced charge buildup within both the gate oxide of the standard transistors and the bird’s beak parasitic transistor due to LOCOS isolation method. The zero bias radiation response of standard MOSFETs is simulated, showing the difference between n- and p-channel transistors. For high dose levels, the charge buildup in the parasitic transistor leads to an increase of the off-state leakage current for nMOSFET. The radiation response of an inverter is shown as an example of the consequence this increment of the leakage current may cause to the simple CMOS circuit. The model showed to successfully predict the increment in the delay and the failure of the circuit for high absorbed doses.

This paper presents the design, fabrication and characterization of an integrated differential dosimeter based on the mismatch of two identical FOXFETs. This dosimeter was fabricated in a high-voltage 0.35 µm CMOS process, where the FOXFET and the biasing circuit were integrated in the same chip. The FOXFET as a single device and the whole circuit as an integrated differential sensor were characterized regarding its response to both radiation and temperature. The differential sensor showed low temperature sensitivity, 320 times lower than the single FOXFET temperature sensitivity, while it also showed a reduction in radiation sensitivity only in a factor of 1.6. These results drastically improved the temperature error factor, calculated to 23mrad/°C. Moreover, the Bias Controlled Cycled Measurement technique was successfully implemented improving dose range up to 9.4 krad. Finally, the temperature rejection performance was assessed in real-time measurements during exposure to radiation, and sensitivity of the dosimeter showed no change with temperature.

In this paper, a Floating Gate transistor designed and fabricated to be used as a radiation sensor is presented, and the charge injection process through an injection electrode is studied. The device was successfully charged, and the injection process was simulated in a SPICE software, considering that the injection mechanism was Fowler-Nordheim tuneling. The parameters for the Fowler-Nordheim current were found and simulations successfully reproduced the measurements. These simulations will allow to improve the desing and the charge injection setup.

Temperature variations can reduce accuracy in the radiation measurements of MOS dosimeters, so it is necessary to have an accurate temperature characterization of the dosimeter. This paper presents a temperature control system based on Peltier modules that enables automated temperature adjustment of the device under test (DUT). At first, the design and features of the system is presented. In a second part, measurements of some characteristics of the device are shown. Finally, the zero temperature coefficient (ZTC) of a MOS radiation dosimeter is measured to show the controller application.

The electron trapping in MOS capacitors with amorphous Al₂O₃ as an insulating layer was studied through pulsed capacitance-voltage technique. A positive shift of the voltage value corresponding to a constant capacitance (V_C) was observed. The dependences of the voltage instability with the applied bias and the charging time were investigated. Two different contributions could be distinguished: a hysteresis phenomenon observed on each measurement cycle, and a permanent accumulated V_C-shift to which each measurement cycle contributes. A physical model based on tunneling transitions between the substrate and defects within the oxide was implemented. From the fitting procedure within the energy range covered in our measurements (1.7–2.7 eV below the conduction band edge), the trap density was found to decrease exponentially with trap energy depth from 3.0 × 10²⁰ cm⁻³eV⁻¹ to 9.6 × 10¹⁸ cm⁻³eV⁻¹, with a uniform spatial distribution within the first 2 nm from the semiconductor interface for the hysteresis traps.

The offset of a Miller amplifier is mainly due to an imperfect matching between its inputs. When the input transistors are MOSFETs, the exposure to ionizing radiation affect the threshold voltage of both transistors. With proper polarization, the shift in the threshold voltage impoverishes the matching in the input, increasing the offset. Hence, measuring the change in the offset during irradiation, one can estimate the absorbed radiation dose. In this work, a theoretical analysis is presented in order to establish the effect of ionizing radiation on the different blocks of the Miller amplifier. The design of a Miller amplifier and its fabrication on an integrated circuit for testing is described. The design and assembly of a dedicated hardware and software for data acquisition is also described. First results on the fabricated dosimeter in a 0.6μm commercial CMOS technology show a sensitivity of 60 mV/Gy and a resolution of 4 cGy.

A floating gate dosimeter was designed and fabricated in a standard CMOS technology. The design guides and characterization are presented. The characterization included the controlled charging by tunneling of the floating gate, and its discharging under irradiation while measuring the transistor drain current whose change is the measure of the absorbed dose. The resolution of the obtained device is close to 1 cGy satisfying the requirements for most radiation therapies dosimetry. Pending statistical proofs, the dosimeter is a potential candidate for wide in-vivo control of radiotherapy treatments.

The spatial distribution of trapped holes in the gate oxide of MOS exposed to ionizing radiation plays a significant role in the dynamics of the threshold voltage shift with dose, specially when switched bias measurements are considered. In this work, we analyze currently available simplified models stressing out their limitations when dealing with complex measurements. Therefore, we present a simplified model based on the fitting of the evolution with dose of the spatial distribution of trapped holes obtained from a physics-based numerical model. For that, a two-exponential expression was considered, with four fitting parameters. Then, the change with dose of these parameters is considered. The effect the variation of the capture and neutralization rates has on the spatial distribution of trapped holes is analyzed to better understanding radiation effects in MOS devices and also to contribute to the development of simplified numerical models capable of reproducing complex dynamics.