GoKawiilImec, ASML, and TSMC have integrated both n-type and p-type transistors with atomically thin 2D channels on a single 300mm wafer at a 50nm contacted poly pitch, the tightest pitch demonstrated to date for complementary 2D devices and one that lands within range of leading-edge silicon.
The trio presented the work this week at the IEEE/JSAP Symposium on VLSI Technology and Circuits, using a single EUV exposure to print channel lengths as short as 28nm. Imec reported that 94% of the integrated transistors switched correctly, with an on/off current ratio above 100,000. The n-channel devices use molybdenum disulfide (MoS 2 ), while the p-channel devices use tungsten diselenide (WSe 2 ) or tungsten disulfide (WS 2 ).
2D transition metal dichalcogenides have been studied for more than a decade — imec has been fabricating MoS 2 test transistors since the late 2010s — so while it’s not a new material breakthrough, the result is a solid milestone in terms of integration and scaling. What’s changed with this work is that both transistor polarities were built together on a standard 300mm process flow, rather than as isolated single devices patterned with coarser lithography.
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The demonstrated transistors reached active widths down to 75nm and an equivalent oxide thickness near 2nm. Both polarities turned fully off at zero gate voltage, and imec said the WSe 2 p-channel devices performed close to the best lab-scale results reported so far, narrowing the gap on the historically weaker p-type side of 2D CMOS. For perspective on the pitch, 50nm is tighter than the 54nm contacted gate pitch of Intel's 10nm-class node.
Building the transistor upside down
Contact resistance has been the dominant obstacle to scaling 2D transistors because an atomically thin channel carries comparatively little current, and the junction between the metal contact and the 2D film tends to throttle whatever the channel can deliver, partly because the metal pins the semiconductor's Fermi level and raises the Schottky barrier that carriers must cross. Lab devices have compensated by keeping large contact areas, which in turn blocks the pitch scaling that makes the transistors worth pursuing in the first place.
To break that trade-off, the consortium inverted the usual build order: rather than depositing metal onto the fragile film after the channel is in place, the team patterned tungsten-filled contact trenches first and transferred the 2D channel on top, with the gate deposited over it. Imec calls this a “reverse” thin-film-transistor flow, and credits the resulting bottom-contact geometry for the clean off-state behavior, in which both polarities stop conducting at zero gate voltage.
"For the first time, we achieved 50nm CPP — a metric determined by both the gate length and source/drain contact length — without affecting the performance of the 2D n and pFETs," said Gouri Sankar Kar, vice president of R&D for compute and memory device technologies at imec. The single-patterning EUV step, he added, was developed in close collaboration with ASML.
EUV resolution, not High-NA
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