Minimizing feature width in atom-optically fabricated Cr nanostructures

One of the central questions about laser-focused atomic deposition is how small the focused features can be made in practice. While theoretical calculations predict that ideal atom beams (no divergence and no velocity spread) can be focused into flux distributions with widths of order a few nanometers, practical limitations that occur in any real situation will in general result in larger features. We have undertaken a detailed study [EPG Pub# 691] of these practical issues and found that a significant contributor to feature size is the growth characteristics of the Cr on the surface.

Our approach has been to conduct very carefully controlled experiments in which the atom optical parameters of the deposition are well characterized. Knowing the temperature of the atom beam source, the degree of collimation produced by the transverse laser cooling, and the power and spatial distribution of the standing wave, it is possible to calculate with a fair amount of confidence the flux distribution of atoms striking the surface. To investigate the relation between this flux and the resulting deposited feature shape, we have compared these calculations with atomic force microscope images of deposited features. Interestingly, good qualitative agreement is seen for the variation of the feature width as a function of the standing wave intensity and location relative to the substrate. However, the experimentally measured widths are significantly larger that the calculated flux widths. (See figure 1)

Such a variation is indicative of the influence of surface growth phenomena, such as diffusion or grain formation. To investigate this further we have studied linewidth as a function of substrate temperature, which was found to have no influence on the linewidth, and as a function of total amount of material deposited. This latter study showed an interesting variation, in which the linewidth decreased at low coverages, then increased steadily as more material was added. (See figure 2)

From these studies we have learned that (1) the narrowest features attainable for Cr deposition without extraordinary surface preparation is 29 nm, and (2) the feature shape of laser-focused atomic deposition nanostructures is significantly influenced by surface growth phenomena. This work calls attention to some aspects of laser-focused atomic deposition that up to now have not been considered in much detail. It is clear, though, that further research is called for in an environment where surface contamination can be controlled (i.e., in UHV conditions), where the true influence of surface growth phenomena can be more fully investigated.

Minimizing Feature Width in Atom Optically Fabricated Chromium Nanostructures

Jabez J. McClelland - NIST
Robert J. Celotta - NIST

William Anderson - Lighthouse, Inc.
Curtis Bradley - Petroleum Institute, Dubai

Online: July 1999
Last update: February 2008

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