Shielding for space microelectronics needs to provide an acceptable dose rate with minimum shield mass. The analysis presented here shows that the best approach is, in general, to use a graded-Z shield, with a high-Z layer sandwiched between two low-Z materials. A graded-Z shield is shown to reduce the electron dose rate by more than sixty percent over a single-material shield of the same areal density. For protons, the optimal shield would consist of a single, low-Z material owever, it is shown that a graded-Z shield is nearly as effective as a single-material shield, as long as a low-Z layer is located adjacent to the microelectronics. A specific shield design depends upon the details of the radiation environment, system model, design margins/levels, compatibility of shield materials, etc. Therefore, we present here general principles for designing effective shields and describe how the computer codes are used for this application.
budgets, there has been a drastic decrease in the dollar value of the radiation-hardened integrated circuit The radiation hardness of commercial Floating (IC) market, and a concomitant reduction in the Gate 256K E2PROMs from a single diffusion lot number of viable suppliers of radiation-hardened was observed to vary between 5 to 25 krad(Si) when ICs. This trend is expected to continue for the foreirradiated at a low dose rate of 64 mrad(Si)/s. Addi-seeable future. In addition, the capability of radiational variations in E'PROM hardness were found tion-hardened technologies has traditionally lagged to depend on bias condition and failure mode (Le., one to two generations behind commercial technolinability to read or write the memory), as well as the foundry at which the part was manufactured. This variability is related to system requirements, and it is shown that hardness level and variability affect the allowable mode of operation for E2PROMs in space applications. The radiation hardness of commercial 1-Mbit CMOS SRAMs from Micron, Hitachi, and Sony irradiated at 147 rad(Si)/s was approximately 12, 13, and 19 krad(Si), respectively. These failure levels appear to be related to increases in leakage current during irradiation. Hardness of SKAMs from each manufacturer varied by less than 20%, but differences between manufacturers are ogies, leading to reduced system performance. This has prompted a renewed interest in the use of commercial technology -with its enhanced performance and yield, and reduced cost -in military and space systems. But unlike the vendors of hardened technologies, the commercial manufacturer has no interest in identifying and controlling the technology parameters that affect radiation hardness. As such, we expect lower hardness levels for commercial technologies and, perhaps even more importantly, greater variability of commercial technologies in a radiation environment.significant. The Qualified Manufacturer's List approach to radiation hardness assurance is suggested as a way to reduce variability and to improve the hardness level of commercial technologies.In Figure 1 we show the total-dose requirements for pieceparts used on several space-based vehicles, and payloads located on space-based vehicles, as a function of orbit altitude. The vehicle and payload
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