Prompt and Total Dose Response of Hard 4k and 16k CMOS Static Random Access Memories (SRAMs)

The evolution of hardened CMOS memories has recently reached a milestone where the achieved bit density (16K bits/chip) is sufficiently large to warrant their application as an integral part of hardened systems, often replacing older memory technologies (e.g., magnetic core, plated wire, etc.) and achieving substantial performance (weight, power, volume) advantages. This paper discusses total dose and prompt dose rate effects in three types of SRAMs: Harris 6504RH (CMOS/ bulk) and, RCA 11121 (CMOS/SOS), which are commercially available 4K memories, and developmental samples of a more recent hardened 16K CMOS/SOS SRAM (RCA TA 12702). Major process improvements have resulted in enhancing the radiation hardness of CMOS/bulk and CMOS/ SOS integrated circuits. The total dose hardness had been increased to more than 100 Krads (Si) and the latchup sensitivity of CMOS/bulk has been eliminated for dose rates of 10/sup 12/ rads (Si)/s. The use of sapphire substrates in CMOS/SOS RAMs has resulted in increasing the upset threshold beyond 10/sup 11/ rads (Si)/s (45 ns pulse width.) In addition, the incorporation of cross coupled resistors in the inverters of CMOS/bulk memory cells has reduced the sensitivity to single event upset (SEU).