Goran S. Ristić

Analysis of mechanisms which lead to electrical breakdown in argon using the time delay method

The mechanisms leading to initiation of electrical breakdown in geometrically identical argon-filled tubes at different pressures were investigated by the memory curves. It was shown that the positive ions and metastable states, remaining from the previous discharge, have the dominant role in the initiation of breakdown in early and late afterglow periods, respectively. Contributions to the breakdown of gamma photons from radioactive sources (60Co) and cosmic rays were also analyzed.

Creation and passivation of interface traps in irradiated MOS transistors during annealing at different temperatures

Abstract The temperature dependence of the creation and the passivation of interface traps in irradiated n-channel MOSFETs during annealing has been investigated. The obtained results show that the creation process is directly related to the neutralization of the positive oxide trapped charge, and that hydrogen atoms (ions) and water molecules are directly responsible for the creation process and for the passivation process, respectively. A combined hydrogen-water (HW) model which explains the experimental results is proposed.

MODELLING OF KINETICS OF CREATION AND PASSIVATION OF INTERFACE TRAPS IN METAL-OXIDE-SEMICONDUCTOR TRANSISTORS DURING POSTIRRADIATION ANNEALING

The dependencies of creation–passivation processes of interface traps in irradiated n-channel metal-oxide-semiconductor transistors on the temperature and gate bias during annealing have been investigated. The experimental results, which are explained by the hydrogen–water (H–W) model, show the influence of both the annealing temperature and gate bias on these processes. The modelling of creation–passivation kinetics of interface traps, based on bimolecular theory and numerical analysis, is also performed. Numerical modelling shows that the H–W model can include the temperature and gate bias dependencies of creation of interface traps, latent interface trap buildup and the decrease of interface trap density.

pMOS dosimetric transistors with two-layer gate oxide

Abstract A study of sensitivity and fading of pMOS dosimeters, with a two-layer (thermal+CVD) gate oxide with various thicknesses, is presented. Analysed dosimetric parameters appear not to exhibit any systematic dependence on dose rate. Oxide-trap charge density is not always found to increase with the gate oxide thickness, which is explained in terms of a complex influence of the oxide electric field in the zero-bias regime on the initial e − –h + recombination process and on the spatial distribution of trapped holes. After negligible formation of interface traps during irradiation, interface traps build up significantly in all samples during annealing, and this build-up is found to be in close correlation with oxide-trap charge neutralization. The possibility for increasing the sensitivity and extending the measurable dose range by applying a positive gate bias is also explored. In general, the results support the use of the thick CVD layer for improvement of sensitivity and long-term stability of pMOS dosimetric transistors.

Analysis of postirradiation annealing of n-channel power vertical double-diffused metal–oxide–semiconductor transistors

The behavior of densities of the oxide trapped charge and the interface traps in gamma-ray irradiated n-channel power vertical double-diffused metal–oxide–semiconductor transistors during annealing at different temperatures and gate biases has been investigated. The experimental results have revealed the existence of a latent interface trap buildup (LITB) process. By use of numerical modeling, based on the hydrogen–water (H–W) model, the LITB process has been successfully simulated. The interface trap densities have been determined by both the midgap and the charge pumping methods, and the results have shown good qualitative agreement between these two methods. Isochronal annealing and switching bias experiments have also been performed. The experimental results are consistent with the H–W model.

P-CHANNEL METAL-OXIDE-SEMICONDUCTOR DOSIMETER FADING DEPENDENCIES ON GATE BIAS AND OXIDE THICKNESS

The threshold voltage recovery (‘‘fading’’) of P‐channel metal–oxide–semiconductor dosimeters irradiated to 10 Gy(Si), after 3500 h room‐temperature annealing, has been investigated. The obtained results have shown that fading decreases with the increase of the absolute value of gate voltage as well as with the increase of gate oxide thickness.

Analysis of mechanisms which lead to electrical breakdown in a krypton-filled tube using the time delay method

Contributions of krypton ions and krypton metastables, as well as contributions of gamma-radiation and cosmic rays to the electrical breakdown in a krypton-filled tube have been analysed by use of the time delay method. The experimental results have shown that each of these contributions strictly depends on the afterglow period. The time delay method has enabled the determination of both the recombination time of positive ions and the lifetime of metastables, created in previous discharge. The obtained value of metastable lifetime is in agreement with its theoretical value. The dependence of the electrical breakdown on exposed dose rate of gamma-rays from the radiation source has also been investigated.

Formation and passivation of interface traps in irradiated n-channel power VDMOSFETs during thermal annealing

Abstract The annealing of γ-ray irradiated commerical n-channel power VDMOSFETs at elevated temperature is investigated. Rebound effect, observed in the samples annealed with positive gate bias, is primarily the consequence of a very long-term postirradiation interface-trap buildup. At later annealing times, a significant decrease in the number of interface traps is observed. In order to explain obtained results, we propose a modification to the hydrogen ion transport model for the formation of interface traps. According to our model, H+ ions created during irradiation are responsible only for the initial fraction of the total interface-trap buildup. The other fraction is attributed to H+ ions produced when molecular hydrogen, released at the SiSiO2 interface in interface-trap creation reaction, diffuses back into the bulk of the oxide and is cracked at positive charge centres. Simultaneously with the generation of interface traps, their passivation also takes place. Passivation, probably caused by hydrogen atoms and water molecules, becomes predominant at later annealing times, leading to the decrease in interface-trap density.

Latent interface-trap generation in commercial power VDMOSFETs

Latent interface-trap generation is one of the most controversial post-irradiation effects in MOSFETs, which can have a significant impact on device performance and reliability in radiation environments. In this paper, we present new experimental evidence of latent interface-trap buildup in commercial power VDMOSFETs: its dependencies on dose, temperature and gate bias applied during irradiation and annealing. We discuss several models for latent interface-trap buildup and show that the most consistent is one which involves the diffusion of molecular hydrogen from structures adjacent to the gate oxide (CVD oxide, poly-Si gate), and its cracking on positive charge centers in the oxide. The cracking reaction liberates hydrogen ions, which drift to the Si/SiO/sub 2/ interface to form interface traps. Some hypothesis from the recently proposed H-W model for post-irradiation behavior of interface traps may help resolve the question of the source of hydrogen sufficient to cause up to 800% increase in interface-trap density, experimentally observed. The implications of latent interface-trap generation for hardness assurance test methods are also discussed.

Thermal and UV annealing of irradiated pMOS dosimetric transistors

p-channel metal-oxide-semiconductor (pMOS) dosimetric transistors are unique radiation dosimeters that have an extremely small size (the dimensions of the sensor element are less than 1 mm × 1 mm) and allow the measurement of dose in vivo in real time, which are especially important characteristics for radiotherapy. The isothermal, isochronal and UV annealing of pMOS dosimetric transistors have been investigated. The obtained results have shown that the high temperature annealing for thin and UV annealing for thick oxide, respectively, are very useful tools for radiation defect annealing. The spontaneous annealing is high, as a probable consequence of gate material (Al gate). The investigation has also shown that not only the threshold-voltage behaviour but also the behaviour of radiation defects in the oxide and at the oxide interface has to be known. The modelling of isothermal annealing at different temperatures has been performed.