Walter M. Shedd

The Impact of Total Ionizing Dose on Unhardened SRAM Cell Margins

Static random access memory (SRAM) cells have diminishing read and write margins due to fabrication variations. These variations are a direct result of the small device sizes necessary to maintain scaling commensurate with Moores law. Total ionizing dose (TID) primarily affects NMOS device characteristics, which are the most important to maintaining SRAM cell read stability. A test structure allowing direct measurement of SRAM cell electrical characteristics in an SRAM memory bank is presented. Experimentally measured results from this structure, fabricated on a 90 nm process, show impact of Co-60 irradiation on SRAM cell margins. This test structure is fabricated on the same die as a 1.2 Mbit SRAM, allowing comparison of the individual cell characteristics with the overall leakage impact on the large SRAM. The results indicate that by simply providing sufficient margins during SRAM cell design, functionality at high TID can be achieved without the use of radiation hardening by design (RHBD) techniques. The implications for future unhardened SRAMs operating in TID environments and on design to maintain margins are discussed.

The effect of near-interface network strain on proton trapping in SiO/sub 2/

The buildup of positive charge during annealing in forming gas at 600/spl deg/C was compared for various types of Si/SiO/sub 2/ interfaces. Our data suggest a correlation between the presence of stressed bonds in the SiO/sub 2/ network near the Si/SiO/sub 2/ interface, and the ratio of fixed vs. mobile positive charge (protons) detected near the interface after performing a forming-gas annealing. We further propose that the presence of these stressed bonds near the interface is correlated with the oxygen deficiency at the interface and with the confinement of the oxide due to the presence of a Si cover layer. A model based on first-principles quantum mechanical calculations shows a significant decrease in the overall proton binding energy with increasing network strain near the interface. These calculations support our model of mobile proton generation at Si/SiO/sub 2/ interfaces with large densities of stressed bonds.

Interaction of H+/H0 with O atoms in thin SiO2 films: a first-principles quantum mechanical study

Abstract Ab initio Hartree–Fock calculations have been performed on model SiO 2 +H + H 0 clusters to study the interaction between a bridging O atom and hydrogenic species in SiO 2 films. The binding energy of H + at a bridging O site is calculated to be greater than 8 eV, much larger than previously calculated energy. A neutral H atom is not found to form a stable bond with a bridging O atom. Bonding of a proton at the bridging O atom induces structural changes (increase in the Si–O–Si bond angle and O–Si bond lengths) at the bridging O atom. Subsequent to the addition of an electron, the stable protonated species releases a neutral H atom and relaxes back to the regular oxide structure.

New fundamental defects in a-SiO/sub 2/

The atomic structure and spin properties of two previously undescribed amorphous silicon dioxide fundamental point defects have been characterized for the first time by ab initio quantum mechanical calculations. Both defects are electrically neutral trivalent silicon centers in the oxide. One of the defects, the X-center, is determined to have an O/sub 2/Si/spl equiv/Si/sup /spl dagger// atomic structure. The other defect, called the Y-center, is found to have an OSi/sub 2//spl equiv/Si/sup /spl dagger// structure. Calculated electronic and electrical properties of the new defect centers are consistent with the published characteristics of the oxide switching trap or border trap precursors.

Dose Rate Upset Investigations on the Xilinx Virtex IV Field Programmable Gate Arrays

The following paper describes the results of ionizing dose rate investigations into upset, supply photocurrent, latch-up, and burnout susceptibility of the Xilinx Virtex IV XC4VFX12. All investigations were performed on a commercial version of the device. The maximum no-upset dose rate was 2.8times108 rad(Si)/s. Photocurrent amplitudes as a function of dose rate were recorded.

Theory of H- in SiO2

Abstract We present an ab initio molecular orbital study of H in SiO 2 with special emphasis on H − . We have calculated equilibrium geometries, vibrational spectra, binding energies and electrical levels. For the electrical levels, we have included long-range polarization effects in three approximations. We compare our results to those of Yokozawa and Miyamoto and others. We find that H 0 is unstable in SiO 2 . However, we find that, rather than disproportionate, it prefers to dimerize. We also predict that H 0 is a deep electron trap. Finally, we find that long-range polarization effects are crucially important for obtaining even qualitatively correct values for electrical levels.

The effect of network topology on proton trapping in amorphous SiO/sub 2/

We report the results of first-principles quantum chemical calculations of the interactions of H/H/sup +/ with oxide ring structures of varying sizes. The calculations suggest that the binding and stability of protons in amorphous SiO/sub 2/ depend upon the topology of the interacting network. A neutral hydrogen atom, H/sup 0/, does not bind to bridging O atoms in the rings, but may occupy voids within larger rings.

Heavy Ion Testing of Commercial GaN Transistors in the Radio Frequency Spectrum

Commercial gallium nitride (GaN) high-electron mobility transistors (HEMTs) are tested in the radio frequency (RF) spectrum at heavy ion facilities to explore space environment stresses on these emerging technologies. Findings indicate that gate leakage degradation is a key parameter to consider when selecting devices. Variations in the manufacturing process may drive product selection for use in harsh environments.

Xilinx Virtex V Field Programmable Gate Array Dose Rate Upset Investigations

The results of ionizing dose rate experiments on XC5VLX50T FPGAs demonstrate the most susceptible upset mechanism of commercial devices and provide insight into the effectiveness of dose rate hardening of nano-scale technology by using epi substrates.