Radiation Effects on Commercial 4-Kilobit NMOS Memories

The 4K NMOS dynamic RAM is one of the least radiation tolerant of modern semiconductor devices. Samples from seven manufacturers exhibit a uniformly high susceptibility to total dose with the failure threshold starting at 1700 rads (Si) and the parts completely dead by 3500 rads (Si). The entire circuit is dynamic including the peripheral circuitry with the sense amps detecting 200 millivolt signals. In the tested population, most failed for VGT shifts. The design will tolerate <0.2 volt change. Test transistors on the die were measured confirming this conclusion. Circuit analysis and electrical autopsy indicate the X and Y decoders are stuck in "1" state causing failure. Most 4K NMOS devices are processed for optimization of electrical performance, commercial reliability and yield. These processes utilize chlorine oxides for stability, ¿ 1000 Å thick gate oxides for static discharge reliability, and silicon gate techniques for threshold and speed requirements. Changes in any of these processes affect at least one of the "optimized" performance criteria adversely so that the devices no longer meet commercial price/performance requirements. Increased complexity NMOS devices, such as 16K RAM's and microprocessors, utilize extensions of the existing NMOS technology and will also be very susceptible to ionizing radiation.