A 4-Mbit Non-Volatile Chalcogenide-Random Access Memory Designed for Space Applications

A 4 Mbit non-volatile chalcogenide-random access memory (C-RAM^TM) has been designed and fabricated in RH 25, a radiation hardened CMOS technology. The top-down design focused on accommodating chalcogenide process variations and satisfying space system specifications. The optimized band-gap circuit supplies reference current and voltage that meet temperature and voltage requirements. The innovative write circuitry supplies appropriate currents (amplitude and shape) to the chalcogenide memory cells to allow them to be programmed either in amorphous state (write "0") or crystalline state (write "1"). The on-chip pulse generator circuit can provide multiple pulse widths for write "0" and write "1". The write circuits have a dedicated power supply, which can be removed to place the part in a read only mode. The read circuitry includes a voltage limiting circuit, an adjustable current reference, an adjustable pre-charge circuit, and a sense amplifier to accurately sense the current difference between cells programmed as "0" or "1". A localized, redundant cell architecture is implemented with shared read/write circuits to improve yield without impacting access times. The redundant cells can be tested prior to laser fusing or used to monitor endurance. Considerations for testability such as direct chalcogenide cell access, margin testing, analog monitors, and endurance acceleration have been implemented. Noise and power reduction techniques have also been used globally.