Semiconductor Manufacturing and Design – Does the transition to 450mm wafers offer the ultimate opportunity to switch to maskless lithography (ML2)? That was the suggestion made by Burn Lin, senior director of micropatterning at TSMC in his keynote for the SPIE Alternative Lithography Conference in San Jose Feb. 14. The 450mm transition would appear to require expensive development of a variety of patterning tools and resists if a conventional mix-and-match strategy were employed. Since multi-electron beam lithography can be used to write any layer, one 450mm e-beam direct-write tool could pattern them all, and for 30% lower cost at any production volume. Only one machine would have to be engineered and only a few resists formulated.
The architecture of Lin’s tool would be similar to a multi-column version of KLA-Tencor’s ReBL system, with several wafers on a rotating stage and up to 36 columns, each supporting one million independent beams. Direct write with continuous stage motion on all levels would allow devices to break out of the standard 26 x 33mm field, with consequent efficiency improvements. The issue, as always in electron beam lithography, would be throughput. Lin claimed that could be addressed by scaling the number of columns and stages. Mass production would reduce tool cost to $500,000 per wafer per hour. Lin claimed that by the time 450mm came along, the CD would be 10nm or below, making his suggested ML2 architecture the least challenging and most economical of the available options.
Other advocates of ML2 and alternative lithographies seemed less visionary. The KLA-Tencor presentations on ReBL emphasized the steps needed to re-target the DARPA-funded 45nm program towards commercial viability at 16nm or so in 2015. Regina Freed described how returning to linear stage motion reduced risk and complexity with the lower stage velocity needed for 1nm CDU and overlay at 16nm. There still could be 36 columns and the throughput target would be 100 wph.
Later in the conference, Kenji Abe described more esthetic 22nm and 11nm line-and-cut results achieved on a multi-column cell projection prototype by a team from eBI member Advantest and TEL. In the first stage, the line grating was written by TEL’s self aligned double or quadruple patterning process. Then one beam of the Advantest MCC-POC EdBW tool cut line ends with 5nm overlay accuracy using either a positive or negative tone process. Development etching and subsequent processing resulted in controllable line end spacings down to 13nm. Abe pointed out that such a tool could cut the gate and M1 patterns anticipated for the 11nm node. Other papers from Advantest recommended design and resist innovations that would facilitate EbDW with character projection. The goal is 100 WPH at the 14nm node in a 150 beam, 10 cluster system with 100B shots/wafer, and 75mC/cm2 resist. The drastic shot reduction required may first prove feasible in complementary lithography.