Protection for Satellite Electronics
Ruth DeJule, Associate Editor -- Semiconductor International, 10/1/1998
F ailures such as the recent nationwide satellite/pager breakdown in the United States are rare. However, when they do occur the results can be inconvenient and at times devastating. Traditional reliability tests suitable for consumer and automotive applications have not been transferable to satellite electronics that must endure long-term radiation exposure. For the past seven years, a team at Sandia National Laboratories (Albuquerque, N.M.), a Department of Energy (DOE) national security research lab headed by physicist Dr. Dan Fleetwood and colleague Dr. Peter Winokur, and a consortium headed by the Defense Special Weapons Agency (DSWA) have been developing tests to determine if bipolar electronics commonly used for analog signal processing in satellites can survive long-term radiation exposure in space.
Bipolar electronics found in many satellites are manufactured commercially without special consideration for possible application in space. Until 1991, it generally was thought that bipolar electronics could withstand radiation exposure on the basis of tests using high doses of radiation delivered for short periods of time at room temperature, Fleetwood said. The devices functioned well under those circumstances. In fact, many parts easily survived testing to levels of radiation well past the levels expected to be received in space. It was thought that longer exposure times in space actually would improve their response due to the annealing of radiation-induced defects, Fleetwood said.
However, confidence in bipolar electronics' reaction to long-term radiation began to erode following a discovery that bipolar electronics failed prematurely at considerably lower radiation levels than laboratory testing would suggest. The problem was that the active "base" region of the bipolar transistor typically is covered by a thick layer of insulation that is adequate for performance in applications where there is no radiation exposure or where radiation exposure occurs so rapidly that the amount of damage to the device is limited by radiation-induced space-charge screening within the insulator. Ironically, at the lower dose rates encountered in space, this protective charging effect does not occur, thus leaving the active base of the transistor open to damage.
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| Fig. 1. The illustration of charge transport in the oxide region indicates that at high rates, positive charge is neutralized and prevented from reaching the base. |
The challenge was to devise tests that qualify a process lot's long-term performance in just a few weeks of testing. The Sandia/DSWA consortium developed a procedure that tests three groups of devices, each irradiated under different conditions. In the first group, bipolar electronics are irradiated at high dose rates at room temperature, taking only a few hours. The response is compared to a second group that receives the same dose of radiation delivered at a much lower dose rate. A third group receives high dose rate irradiation at higher temperatures, ~100°C. The purpose of the lower-rate and higher-temperature irradiations, groups two and three respectively, are to approximate more closely the charge-transport phenomena found in space. If these devices perform significantly worse than those in the high rate group, the devices are replaced or a subsequent series of very-low-dose rate tests may be required.
This procedure has been adopted as the official test standard for
satellite electronics parts by the American Society for Testing of
Materials (ASTM). For some space systems, this has meant new designs to
avoid potential failures. What this means to the consumer is more
reliable satellite communications systems. 
