Sub 10-nm Nanopantography and Nanopattern Transfer Using Highly Selective Plasma Etching

Date

2015-05

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Abstract

Nanopantography is a new patterning method for massively parallel writing of nanofeatures over large areas. Billions of electrostatic lenses are first fabricated on top of a Si wafer using conventional semiconductor manufacturing processes. A broad area, collimated, monoenergetic ion beam is then directed towards the wafer surface. By applying an appropriate DC voltage to the lens array with respect to the wafer, the ion beamlet entering each lens converges to a fine spot focused on the wafer surface that can be 100 times smaller than the diameter of the lens. By controlling the tilt of the wafer with respect to the ion beam, the focused ion beamlets can “write” a desired pattern in a massively parallel fashion, in selected areas of the substrate. A new high density inductively coupled plasma source was designed and built to extract a higher current of a nearly mono-energetic Ar+ ion beam. The extracted ion beam was space-charge neutralized using a set of 8 yttria coated-iridium electron emitting filaments. It was found that beam neutralization enhanced the ion beam current density by several hundred fold in the cases of continuous wave plasma with continuous DC bias on a boundary electrode, and pulsed plasma with continuous DC bias on a boundary electrode. Such enhancement wasn’t observed for synchronous DC bias applied during the afterglow of a pulsed plasma, however. To improve throughput and resolution of nanopantography, a highly selective plasma etching pattern amplification method was developed. In this method, nanopantography was used to remove the native oxide on Si and then using the patterned native oxide as a hard mask, patterns were etched deep into Si (aspect ratio > 6) using a chlorine plasma, with sub-threshold ion energy. Such latent pattern amplification improved the resolution and the throughput of nanopantography by a factor of 30. To explain VUV photo-assisted etching of subwavelength features, EM wave propagation in such features was simulated. It was found that VUV light can propagate into subwavelength features in Si, aided by collective oscillations of free electrons interacting with the EM waves (plasmons). Timed etching of 23±7 nm holes in p-type Si using chlorine plasma, under photo-assisted etching conditions, showed that the etching rate was highly dependent on feature aspect ratio. The profile of light wave energy as a function of depth in the feature was similar to the etching rate vs. depth profile supporting the hypothesis that a photo-assisted mechanism dominated etching. Millions of interlocking “UH” logo features (225nm×250nm, area 0.056 µm2) with minimum linewidth ~13 nm were simultaneously written in silicon as a demonstration that nanopantography can write any desired (but non-reentrant) pattern. Finally, this work has demonstrated that nanopantography can produce ~3nm holes and ~7nm trenches in Si, in a massively parallel fashion, by simultaneous exposure to a monoenergetic Ar+ ion beam and Cl2 gas. The resolution of this top down patterning method is beyond the current state of the art.

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Keywords

Nanopantography, Plasma Etching

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