Constructive Nanolithography

The subject of this volume, Scanning Probe Microscopy, by its nature brings us to the molecular or atomic realm, or minimally into nanometer-scale phenomena. This chapter deals with the fabrication of small features on solid surfaces, aided by the scanning probe microscope (SPM) tip. Whereas lithography was developed to create small structures, and is continually undergoing improvements to reduce the size scale and improve fidelity, reproducibility, and speed, most lithographic procedures treat the surface as a formable mold, into which features are etched or grown with spatial resolution aided by the blocking properties of a resist. The procedures described here use the surface as template for subsequently grown structures – the surface is built upon rather than carved into. Further, in contrast to conventional lithographic processes, this can essentially be thought of as resistless lithography, since the organic coating on the silicon substrate is not used to “block” chemical access to the silicon, but rather serves as a template for the growth of surface structures, by chemical specificity of subsequent reactions. In this case, the organic coating consists of a self-assembled monolayer (SAM) film. The wide range of surface chemical modifications that can be controllably performed on such films lends flexibility to this technique that is not available with other lithographic processes.
In order to provide a sound overview of this field, brief discussions of the principle ingredients will be made - silane-based SAMs, and scanned-probe based anodic oxidation at a silicon surface. Subsequently, several examples of the application of this technique will be given, in order to give a feeling for the extent of different possibilities, and to demonstrate the philosophy of the approach. Particular care will be given to the experimental aspects of the work, such as the non-trivial problem of performing surface
analytical chemistry at the nanoscale. The goal of this chapter is thus to present the scientific basis behind a new approach for performing surface chemistry at the nanoscale, rather than to propose a technologically sophisticated lithographic procedure.