No abstract

Changes in interface trap density Dit have been determined in MOSFETs as a function of time during hydrogen annealing at 295K. Large increases in Dit are observed during H, annealing in MOSFETs previously stressed by either @CO irradiation or Fowler-Nordheim electron injection. The annealing behavior is very similar for both types of stress, which suggests that the Dit creation mechanism involves similar chemistry for hydrogen reactions. Studies of the time dependence of Dit creation as a function of MOSFET gate length show that the time dependence is limited primarily by lateral diffusion of molecular hydrogen (H,) through the gate oxide. An activation energy of 0.57 eV, which is consistent with H, diffusion, is obtained from the temperature dependence.

Substrate hot-hole injection (SHI) induced hole trapping and interface trap generation have been characterized at 295 and 77 K. At both temperatures, the trapping is independent of the injection conditions in the silicon, and is fairly insensitive to the oxide field. Initial trapping efficiencies are about 2.5 times higher at 77 K. The experiments show that essentially the same traps are being filled at the two temperatures, and that the increase in trapping efficiency can be attributed to a larger effective cross section of the traps at 77 K. Hot-hole induced interface trap generation is observed to be independent of the injection conditions in the silicon, and to decrease with increasing oxide field magnitude. More interface traps are generated at 77 K for the same injected fluence. This is in contrast to the characteristics of irradiation-induced interface trap generation. The presence of holes at the Si-SiO2 interface is the key factor in the direct interface trap generation process acting during hot-hole injection. Following low-temperature SHI, an additional temperature-activated generation mechanism, attributed to the migration of H+, is observed in isochronal anneal experiments. This delayed mechanism is identical to the one that accounts for most of the irradiation-induced interface trap generation.

A very large format neural stimulator device, to be used in future retinal prosthesis experiments, has been designed, fabricated, and tested. The device was designed to be positioned against a human retina for short periods in an operating room environment. Demonstrating a very large format, parallel interface between a 2-D microelectronic stimulator array and neural tissue would be an important step in proving the feasibility of high resolution retinal prosthesis for the blind. The architecture of the test device combines several novel components, including microwire glass, a microelectronic multiplexer, and a microcable connector. The array format is 80 times 40 array pixels with approximately 20 microwire electrodes per pixel. The custom assembly techniques involve indium bump bonding, ribbon bonding, and encapsulation. The design, fabrication, and testing of the device has resolved several important issues regarding the feasibility of high-resolution retinal prosthesis, namely, that the combination of conventional CMOS electronics and microwire glass provides a viable approach for a high resolution retinal prosthesis device. Temperature change from power dissipation within the device and maximum electrical output current levels suggest that the device is acceptable for acute human tests.