Superconducting Technology Assessment - nitrd

MRAM Future Projections
MRAM is a very attractive RAM option with a potentially high payoff. Compared to SFQ RAM, MRAM is
non-volatile, non-destructive readout, and higher density, albeit slower. Compared to hybrid JJ-CMOS, monolithic
RSFQ-MRAM does not require signal amplification to convert mV-level SFQ input signals to CMOS operating voltage,
which should improve relative access and cycle times. If 50psec SMT switching is achieved, memory cycle times
should be between 100 and 200ps. A 200ps cycle time MRAM—where RSFQ-based floating point units and vector
registers are communicating with multiple 64-bit wide memory chips—translates to a local memory bandwidth of
~40GBytes/s. Thus, cryogenic superconducting-MRAM could well meet the minimum local memory bandwidth
requirement of ~2 Bytes/FLOP for a 200-GFLOP processor.
High-density, stand-alone superconducting-MRAM main memory—even with 500 ps to 1 ns cycle times—could be
a big win since this would allow the entire computer to be contained within a very small volume inside the cryostat,
reducing the communication latency to a bare minimum in comparison with external main memory. Such an
arrangement would expose the entire MRAM memory bandwidth to the RSFQ logic, providing RSFQ processor
arrays access to both extremely fast local as well as global main memory. Superconductive-MRAM mass memory
should also dissipate less power than RT CMOS, since all high-speed communication would be confined to the cryostat.
RSFQ MRAM density should be able to track that of commercial MRAM with some time lag. Monolithic 4 Mb
RSFQ-MRAM chips operating at 20 GHz would provide the low-latency RAM needed for 100 GHz RSFQ processors.
It will, however, require investment beyond 2010 to bring this high-payoff component to completion.
Understanding and optimizing the cryogenic performance of MRAM technology is important. Fortunately, device
physics works in our favor.
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■ MRAM has a higher MR at low temperatures due to reduced thermal depolarization
of the tunneling electrons. A 50% increase in MR from room temperature to 4 K is typical.
Higher MR combined with the lower noise inherent at cryogenic operation provides faster
read than at room temperature and lower read/write currents.
■ Since thermally induced magnetic fluctuations will also decrease at low temperatures,
the volume of magnetization to be switched can be reduced, with a concomitant reduction
in switching current and power dissipation.
■ Resistive metallization, such as Cu used in commercial MRAM chips, can be replaced with
loss-less superconducting wiring. This will dramatically reduce drive voltages and total
power dissipation, since ohmic losses are at least an order-of-magnitude higher than the
dynamic power required to switch the bits. Calculations indicate that the field energy per pulse
approximates 60 fJ per mm of conductor, or 240 fJ to write a 64-bit word. Without ohmic
losses, the energy required to write a FS-MRAM at 20 GHz is simply 240 fJ times 20 GHz,
or 5 mW at room temperature. Reading would require approximately one-third the power
or ~1.7 mW. For SMT with 50 ps switching, power per bit reduces to ~10 fJ. If switching currents
can be further reduced below 1 mA, as expected at low temperatures, the dynamic power
dissipation in the memory would go down even further. A more accurate assessment of total
power dissipated in the memory depends on memory arrangement and parasitic loss mechanisms.