H.L. Hughes

Radiation effects and hardening of MOS technology: devices and circuits

Total ionizing dose radiation effects on the electrical properties of metal-oxide-semiconductor devices and integrated circuits are complex in nature and have changed much during decades of device evolution. These effects are caused by radiation-induced charge buildup in oxide and interfacial regions. This paper presents an overview of these radiation-induced effects, their dependencies, and the many different approaches to their mitigation.

Hole and electron trapping in ion implanted thermal oxides and SIMOX

Optically assisted methods of injecting either electrons or holes into SiO/sub 2/ layers were used to determine the effect of ion implantation on charge trapping in oxides. Dry-grown thermal oxides and the buried oxides of material grown by the SIMOX (separation by implantation of oxygen) process were studied. Al, Si, and P ions were implanted into the oxides at doses of 1/spl times/10/sup 13/ to 1/spl times/10/sup 16/ and the oxides were annealed at 700, 900, or 1050/spl deg/C after implantation. High dose implantations were found to create electron traps having high capture cross sections, the density of which depends on the implant species, suggesting that electron trapping is related to chemical aspects of the implanted ion. This was supported by measurements on an oxide implanted with a large dose of Ar, which showed no increase in electron trapping. It was found that the shift in the flatband voltage resulting from hole trapping could be reduced by high dose implantations, and that this effect is only weakly dependent on implant species. The hole trapping results are explained in terms of the effect of implantation on the oxide structure.

Total dose radiation hard 0.35 /spl mu/m SOI CMOS technology

This paper presents the total dose radiation performance of 0.35 /spl mu/m SOI CMOS devices fabricated in a radiation hard full dose SIMOX technology. The radiation performance is characterized by transistor threshold voltage shifts, transistor array leakage currents, and 256 K SRAM standby currents as a function of total dose up to 10 Mrad(SiO/sub 2/). The worst case threshold voltage shifts of front channels are less than 60 mV for PMOS transistors at 1 Mrad(SiO/sub 2/) and less than 10 mV for NMOS transistors. No significant radiation induced leakage currents are observed in small transistor arrays to 10 Mrad(SiO/sub 2/). Standby currents of 256 K SRAMs are less than the 1.5 mA specification over the total dose range of 1 Mrad(SiO/sub 2/). The results suggest high density SRAMs and ASIC fabricated in this technology will perform well in harsh radiation environment.

Annealing of total dose damage: redistribution of interface state density on [100], [110] and [111] orientation silicon

The annealing of interface states after an X-ray dose of 10 Mrad (SiO/sub 2/) under 1-MV/cm bias is studied on [100], [110], and [111] silicon. During annealing the bias is 1 MV/cm. Annealing times range from under an hour to hundreds of hours, and the temperature ranges from 75 to 175 degrees C. Using charge pumping, the energy distribution of interface states within the bandgap is determined. After annealing, the shape of the interface-state density curve implies that one or more defects other than P/sub b0/ and P/sub b1/ are present. Comparison of the interface-state density curves before and after annealing shows that a redistribution of interface-state density occurs over a large portion of the time-temperature range studied. The density near 0.4 eV above the valence band decreases and the density near 0.7 eV increases although the average density does not significantly change. Based upon the time scale and activation energy of the redistribution, a model is proposed in which the rate-limiting step is water diffusion within the gate oxide to the interface. This model provides a framework for a transformation among interface defects that accounts for the observed redistribution. Further tests for this model are discussed. >

Reduction of charge trappings and electron tunneling in SIMOX by supplemental implantation of oxygen

The reduction of excess-silicon related defects in SIMOX (separation by implantation of oxygen) by the supplemental implantation of oxygen is examined. The supplemental implant is 6% of the oxygen dose used to form the buried oxide, and is followed by a 1000 degrees C anneal, in contrast to the >1300 degrees C anneal used to form the buried oxide layer of SIMOX. The defects examined include shallow electron traps, deep hole traps, and silicon clusters. The radiation-induced shallow electron and deep hole trapping is measured by cryogenic detrapping and isothermal annealing techniques. The low-field (3 to 6 MV/cm) electron tunneling is interpreted as being due to a two-phase mixture of stoichiometric SiO/sub 2/ and Si clusters at a few nm in size. Single and triple SIMOX samples have been examined. All of the defects are reduced by the supplemental oxygen processing. Shallow electron trapping is reduced by an order of magnitude. The low-field electron tunneling due to Si clusters is also significantly reduced. >

H/sup +/ motion in SiO/sub 2/: incompatible results from hydrogen-annealing and radiation models

Two models that incorporate the same mobile H/sup +/ entity are the radiation model by McLean [1980] and the mobile charge model of hydrogen-annealed oxide by Vanheusden et al. [1997]. Mobile charge in hydrogen-annealed silicon-on-insulator (SOI) buried oxides before and after irradiation was studied to investigate discrepancies between the two models. We examined Unibond, low-dose SIMOX, and single- and triple-implant standard-dose SIMOX as well as single- and triple-implant SIMOX with supplemental oxygen implantation. To measure H/sup +/ motion as fast as 0.01 sec we developed a gate pulse method and combined it with standard I-V techniques to measure the full range of H/sup +/ response times. All but single-implant SIMOX exhibit mobile H/sup +/ that can be cycled between the Si/SiO/sub 2/ interfaces without reacting or being trapped. Trapping near the top interface attenuates the cycling of H/sup +/ in single-implant SIMOX. The transit H/sup +/ times were strongly affected by defects in the oxides and varied by an order of magnitude in oxides with the same thickness. The transit time varied linearly with oxide thickness. The effects of irradiation on the mobile H/sup +/ was studied to see if the irradiation would introduce defects that modify the H/sup +/ behavior and that bring the two models into agreement. No convergence was observed, After irradiation, H/sup +/ could be cycled between the Si/SiO/sub 2/ interfaces without reacting and its transit time across the oxide was not altered.

STRUCTURAL INHOMOGENEITY AND SILICON ENRICHMENT OF BURIED SIO2 LAYERS FORMED BY OXYGEN ION IMPLANTATION IN SILICON

The microstructure and electrical properties of buried SiO2 layers produced in silicon by the implantation of oxygen ions are analyzed in terms of implantation parameters and supplemental incorporation of oxygen. The buried oxides show inhomogeneous etching in aqueous HF, revealing the presence of a crystalline oxide phase and Si-enriched regions. Silicon enrichment in SiO2 is found in the form of Si inclusions and oxygen deficient network defects. The former are found to be sensitive to the oxygen implantation profile, and may arise as a result of a blockage of Si outdiffusion by crystalline oxide inclusions. The network defects, in turn, are predominantly generated during high temperature postimplantation annealing, caused possibly by some mechanism of silicon transport from the interfaces into the bulk of oxide. The electron trapping and electrical conduction characteristics of buried oxides are found to correlate with the density and size of the inhomogeneities. By contrast, hole trapping and the gene...

The role of nanoclusters in reducing hole trapping in ion implanted oxides

At the 2000 IEEE Nuclear and Space Radiation Effects Conference, it was shown that the negative shift in flatband voltage that results from hole injection is reduced in oxides that have been implanted with large doses of Al, Si, or P ions. In the present paper, we study the basic mechanism responsible for this reduced shift in the flatband voltage in more detail by comparing electron and hole trapping in Si and Ar implanted oxides. We find that in Si implanted oxides, the reduction in the shift of the flatband voltage is accompanied by the formation of entities in the oxide that have a large electron capture cross section, and that can become positively charged by photoemitting electrons. Photoluminescence studies indicate that these entities are Si nanoclusters. Oxides implanted with large doses of Ar do not form clusters, and these oxides show neither a reduction in the shift of the flatband voltage nor the formation of large capture cross-section electron traps. We show evidence that the nanoclusters reduce the shift of the flatband voltage by trapping protons formed during hole injection.

Total dose hardening of SIMOX buried oxides for fully depleted devices in rad-tolerant applications

A total dose hardening treatment is applied to SIMOX buried oxides. Total ionizing dose radiation testing is performed on fully-depleted transistors Fabricated on both hardened and non-hardened substrates. At 200 krads X-ray dose, the front gate shift is reduced from -0.7 to -0.2 V for FETs built on the hardened wafers.

Reduction of radiation induced back channel threshold voltage shifts in partially depleted SIMOX CMOS devices by using ADVANTOX/sup TM/ substrates

Excessive total dose radiation induced back channel threshold voltage shifts often observed in fully depleted and partially depleted NMOS transistors fabricated in full dose SIMOX wafers can be greatly reduced by use of new low dose ADVANTOX/sup TM/ substrates.