L. Sambuco Salomone

Experimental evidence and modeling of two types of electron traps in Al2O3 for nonvolatile memory applications

Al2O3-based dielectrics are currently considered as promising materials to use in nonvolatile memories. The electron trap density in this material is much higher than in conventional SiO2, being their characteristics critical for the application. Conventional capacitance-voltage (C-V) techniques were used to study the main effects of the electron traps on the electrical characteristics of MOS capacitors with atomic layer deposited Al2O3 as insulating layer. More detailed information about the trapping kinetics was obtained through the study of the constant capacitance voltage transient. Two different types of traps were found. One is responsible for the instabilities observed in C-V measurements, the other has characteristic trapping times three orders longer. A physical model is presented to explain the observed trapping kinetics exhibiting good agreement between experiments and simulations. The energy levels of the studied traps were determined at 2.2 and 2.6 eV below the Al2O3 conduction band, with den...

Field Oxide n-channel MOS Dosimeters Fabricated in CMOS Processes

This paper presents a new technique to build MOS dosimeters using unmodified standard CMOS processes. The devices are n-channel MOS transistors built with the regular Field Oxide as a thick radiation-sensitive gate. The devices were fabricated in two different commercial 0.6 μm CMOS processes, gate oxide thicknesses of ~600 nm and ~400 nm. Responsivities up to 4.4 mV/rad with positive bias, and 1.7 mV/rad with zero gate bias were obtained in the thicker oxides. The effect of charge trapped in the oxide and interface states on the shift in the threshold voltage are analyzed.

Switched Bias Differential MOSFET Dosimeter

This paper presents a differential MOSFET sensor reading technique based on the bias controlled cycled measurement. The circuit was implemented, and tested with gamma radiation from a 60-cobalt source. Temperature rejection performance was assessed during the exposure in real-time measurements. The results show that in comparison with a single MOSFET dosimeter the thermal drift is 20 times smaller and the radiation sensitivity is approximately 10% higher. The switched biasing allows to extend the measurement range beyond MOSFETs threshold voltage saturation.

Long Term Effects of Charge Redistribution in Cycled Bias Operating MOS Dosimeter

Techniques based on bias switching during the irradiation allow to extend the measurement range of MOS dosimeters. The response of the REM RFT300 RADFET dosimeter during bias cycled measurements shows a slow shift of the quasi-steady state threshold voltage value during radiation-induced charge neutralization. This phenomenon was previously explained as due to the presence of border traps. In this work, a recently developed numerical model which included the main physical processes leading to hole trapping and neutralization in MOS oxides was used to reproduce this experiment. The application of the model shows that the slow shift of the quasi-steady state threshold voltage during neutralization stages is a consequence of the spatial redistribution of trapped charge within the oxide. The effect this phenomenon has on MOS dosimetry is analyzed.

Numerical Modeling of MOS Dosimeters Under Switched Bias Irradiations

Techniques based on bias switching during the irradiation allow to extend the measurement range of MOS dosimeters. To well predict the response of these sensors under operation conditions it is mandatory to understand the physical phenomena involved. A physics-based numerical model is presented here to reproduce the response of MOS dosimeters under switched bias irradiations. Reported non-monotonic responses after a bias switch were previously explained by the presence of two types of hole traps with different characteristic times. The model presented in this work shows that this behavior can be explained by the spatial charge redistribution using a single trap. The electric field dependences of hole capture and neutralization rates are analyzed and compared with previous experimental results and models in the literature. The spatial distribution of hole traps within the oxide is also analyzed.

Radiation and bias switch-induced charge dynamics in Al2O3-based metal-oxide-semiconductor structures

Charge trapping dynamics induced by exposition to γ-ray (60Co) radiation and bias switching in MOS capacitors with atomic layer deposited Al2O3 as insulating layer was studied. Electrical characterization prior to irradiation showed voltage instabilities due to electron tunneling between the substrate and preexisting defects inside the dielectric layer. Real-time capacitance-voltage (C-V) measurements during irradiation showed two distinct regimes: For short times, the response is strongly bias dependent and linear with log(t), consistent with electron trapping/detrapping; for long times, the voltage shift is dominated by the radiation-induced hole capture being always negative and linear with dose. A simple model that takes into account these two phenomena can successfully reproduce the observed results.

Electron trapping in amorphous Al2O3

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 (VC) 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 VC-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−3 eV−1 to 9.6 × 1018 cm−3 eV−1, with a uniform spatial distribution within the first 2 nm from the semiconductor interface for the hysteresis traps.

Simplified model for radiation effects in MOS devices

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.

Quantized bands model for the determination of the dielectric constant of high-κ layers

A quantized bands model capable to generate capacitance-voltage (C-V) curves of MOS capacitors was implemented and numerical details are discussed. This model is applied to the extraction of the dielectric constant of Al2O3 layers with known physical thicknesses by fitting experimental results. A comparison with a continuum band model is presented.

Application of a CMOS differential and amplified dosimeter with Field Oxide n-channel MOSFETs to Diagnosis X-Ray beams

We present in this paper a new circuit embedded sensor for ionizing radiation based on MOSFET dosimeters. The circuit presents differential sensing and inherent amplification, making it much more sensitive than standard MOS dosimeters. An n-channel Field Oxide transistor pair is used as the sensitive module of the circuit. The circuit was tested against γ-radiation and showed a non-linear responsivity and a maximum 58× amplification, corresponding to a sensitivity of 25.6 V/Gy. Then the circuit was tested against X-Ray beams and the response was corrected using the previous characterization, showing to be linear with the beam charge and dependent with the energy of the beam. These results indicate that the proposed circuit is suitable for X-Ray dosimetry.