Post-Fab Melt Perfects Microchip Details Nanofabrication

Improved fabrication techniques have made it possible to control the size and shape of nanostructures with greater precision. However as the required feature size shrinks, defects caused by physical limits in the nanofabrication process will begin to dominate the structures produced, compromising performance. It may also become impossible to reduce the size of features any further. Tackling this issue requires a paradigm shift. Instead of struggling to improve existing nanofabrication methods, imperfections to desired components should be dealt with as a second step.

Chou's proposed method, developed with graduate student Qiangfei Xia, involves selectively melting flawed nanostructures for a short period of time (hundreds of nanoseconds) while guiding the molten material into the required shape prior to re-solidification.

The pair tested this method, termed self-perfection by liquefaction (SPEL), using a 20-ns excimer (λ = 308 nm) laser pulse to perform the melt. With open SPEL (no guide) they reduced line-edge roughness on 70 nm-wide chromium grating lines from 8.4 nm to less than 1.5 nm. Placing a quartz plate in contact with the top surface (capped SPEL) kept the sidewalls and top surface flat during the melt. On adding spacers between the surface and the plate (guided SPEL), they reduced the width of a silicon line from 285 nm to 175 nm, while increasing its height from 50 nm to 90 nm.

Scanning electron micrographs of nanostructures before (left) and after (right) treatment with a single excimer laser pulse.

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Donald Tennant, director of operations at the NanoScale Science at Cornell University, notes that the techniques: “may be a way forward when nanofabricators bump up against the limits of lithography and pattern transfer.” The SPEL cannot be applied when the dimensions of the defect are comparable with the nanostructure's dimensions, and it cannot fix defects without sufficient total material. Applicability to complex structures has yet to be studied. A series of investigations using large (20 cm) wafers are planned.