Nano pillar collapse enables 10nm e-beam lithography patterns

September 2, 2011 — Nanoscale transistor and optical chips as well as biosensors, microfluidics and micromirror chips are limited by photolithography, specifically, light wavelengths. Electron beam lithography (e-beam litho) cannot expose an entire chip at once. Research from MIT’s Research Laboratory of Electronics and Singapore’s Engineering Agency for Science, Technology and Research (A*STAR) demonstrates a new technique that could produce 10nm chip features using plastic pillar deposition and predetermined pillar collapses.

The work evolved from research on preventing nanopillar collapse, which afflicts lithography processes at the 10nm level, according to Karl Berggren, the Emanuel E. Landsman (1958) Associate Professor of Electrical Engineering and Computer Science.

Etching a pillar into the resist requires focusing an e-beam on a single spot, unlike e-beam lithography techniques. Scattering sparse pillars across the chip and allowing them to collapse into more complex patterns could increase e-beam lithography efficiency.

The layer of resist deposited in e-beam lithography is so thin that, after the unexposed resist has been washed away, the fluid that naturally remains behind is enough to submerge the pillars. As the fluid evaporates and the pillars emerge, the surface tension of the fluid remaining between the pillars causes them to collapse.

Berggren and Huigao Duan, a visiting student from Lanzhou University in China, published a paper last year in Nano Letters showing how two pillars will collapse toward each other if they are very close. In a follow-up paper, appearing in the Sept. 5 issue of the nanotech journal Small, Berggren, Duan (now at A*STAR) and Joel Yang (who did his PhD work with Berggren, also joining A*STAR after graduating in 2009) show that by controlling the shape of isolated pillars, they can get them to collapse in whatever direction they choose.

Slightly flattening one side of the pillar will cause it to collapse in the opposite direction. In experiments, the partially flattened pillars collapsed in the intended direction with about 98% reliability, which is a good "starting point" to build toward industrial yields, said Berggren.

If pillars are too close together, they’ll collapse toward each other, no matter their shape. That restricts the range of patterns that the technique can produce on chips with structures packed tightly together.

Joanna Aizenberg, the Amy Smith Berylson Professor of Materials Science at Harvard University, has been using the controlled collapse of structures on the micrometer scale to produce materials with novel optical properties for several years. The sub-100-nanometer scale will create new applications, said Aizenberg.

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