Some advances have recently been made on several fronts in a continuing effort to develop of means of fabricating electronic and magnetic devices having dimensions of the order of tens to hundreds of nanometers. This effort is a collaboration of members, from three universities, whose interests, expertise, and facilities span synthesis of materials, nanoscale characterization, nanoscale lithography, and non-lithographic processing of nanostructures.

The most substantial body of work addressed the use of a novel inorganic resist material, hydrogen silsesquioxane (HSQ). This material was employed because of its potential utility as both a high-resolution resist and an etch mask. Of particular interest in this work was the use of HSQ in patterning giant-magnetoresistance (GMR) materials and their constituents (e.g., Ni, Fe, Co, and Cu). It is particularly difficult to pattern these materials at high resolution by means of dry etching because of a lack of volatile etch products. It was found that by use of either of two versions of a process involving the use of HSQ, the difficulty can be overcome sufficiently to enable the fabrication of structures in the size range of interest.

The process is summarized as follows:

  1. The GMR material or other material of the device that one seeks to fabricate is coated with HSQ.
  2. In one version of the process, the HSQ is patterned by electron-beam lithography, then etching in an argon plasma. In the other version of the process, the HSQ is patterned by use of a focused Ga+-ion beam, then etching in an argon plasma.
  3. The GMR or other device material exposed by the opening(s) in the patterned HSQ is removed by direct argon-ion-beam sputtering, thereby transferring the desired pattern into the device material.

Rings of a GMR Material having inner diameter of 80 nm and outer diameter of 240 nm were fabricated in the nanolithographic process described in the text.
The figure shows an example of rings of a GMR material fabricated in this process. The best resolution attainable when electron- beam- lithography was used for patterning was found to characterized by a line width of about 30 nm. It was found that patterning by use of the focused Ga+ beam could be accomplished about 10 to 100 times as fast as by electron-beam lithography, but with somewhat poorer resolution characterized by a line width between 30 and 40 nm and rougher feature edges. [It should be noted that a GMR material could also be patterned directly (that is, without a resist mask) to desired shape and size by focused-ion-beam sputtering, but the GMR properties would likely be degraded.]

Another topic addressed in this work was increasing throughput in lithographic processing. One promising approach is ultra-rapid patterning of poly(methyl methacrylate) resist by sputtering induced by focused 30-keV beams of Ga+ ions.

Yet another topic addressed in this work was fabrication of semiconductor nanowires and associated structures. Advances in this area are important for the development of quantum cellular automata and other devices based on quantum dots.

This work was done by R. Hull, L. Harriott, P. Parrish; C. Chen, S. Liu, J. Graham, L. He, M. Smith of the University of Virginia; G. Snider and A. Prager of Notre Dame University; and L. Samuelson, J. Johanson, S. Nilison, and L. Montelius of Lund University for the Naval Research Laboratory.

NRL-0020


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Advances in Fabrication of Nanoscale Devices

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This article first appeared in the December, 2007 issue of Defense Tech Briefs Magazine.

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