Superconducting Technology Assessment - nitrd

0.08
0.06
0.04
0.02
30 40 50 60 70 80 90
Figure 4. Estimated switching currents for SMT devices at various lithography nodes. The device is designed to maintain a fixed energy barrier,
as shown by the red line (right scale), for sufficient stability against thermally-activated errors at operating temperatures above room temperature.
Resistance matching and compatibility of the MRAM write current requirements to RSFQ electronics are topics of
research for developing embedded MRAM-in-RSFQ circuits. Since high-density MRAM always employs the currentperpendicular-to-plane
(CPP) device geometry, the resistance of a device scales as the inverse of the device area.
Typically the material is characterized by its resistance-area product (RA), so that the device resistance (R) is given
by R=RA/area. Since the resistance of the pass transistor used for MRAM-on-CMOS is in the kΩ range, it is desirable
to have R in the kΩ or tens of kΩ range. This leads to a requirement for RA of several kΩ-µm 2 for the current
generation of MRAM, and scaling lower with subsequent technology generations. Very high-quality MTJ material
can be made for this RA range, enabling progress of MRAM-on-CMOS. At the same time MTJ material for hard
disk drive (HDD) read heads has been developed for a much lower RA, in the < 10-Ω-µm 2 range, to meet the
requirement for a 100-Ω sensor in the head. Products with such heads have recently begun shipping, indicating
some level of maturity in this type of material. While the requirements for the MTJ material used in HDD sensors
are significantly different from MRAM requirements, these developments indicate that MTJ material development
for lower RA is progressing at a rapid pace.
CPP-GMR material is not of practical use for MRAM-on-CMOS because the material is metallic, and therefore has
very low RA < 1 Ω-µm 2 . In addition, typical MR values are ~10%, compared to ~50% for standard MTJ material
at room temperature. However, such material would easily provide a bit resistance on the order of 10Ω at advanced
lithography nodes, providing a natural match for RSFQ circuitry. The lower MR may be acceptable as long as the
bit-to-bit resistance uniformity is superior to that for MTJ bits, and given the low thermal noise available at
cryogenic temperatures. It is not unreasonable to expect that the resistance distributions of GMR bits would be
narrower than that for MTJ bits since the tunneling resistance depends exponentially on the local barrier thickness,
while the resistance of the Cu barrier used in GMR material is a small part of the device resistance. In addition,
defects in the tunnel barrier can cause dramatically lower resistance, while defects in the metal layers of a GMR
device make only minor contributions. Of course, several other criteria must be met before a definitive choice
between the MTJ and GMR approaches can be made. However, from these basic arguments it is apparent that
GMR materials should be considered seriously for MRAM-in-RSFQ circuits.
182
Write I (mA)
0.10
Technology Mode (nm)
120
100
80
60
40
20
0
Energy E 0 /kT