We showed that STM selectively exposes SAMs and defined conditions necessary for pattern formation in gold at the 20 nm level in a practical manner. Three conditions describe our system: (a) An imaging mode used for alignment of new patterns with features already present on the surface; this condition does not expose the SAM even after multiple images. (b) A minimum linewidth mode that writes continuous lines with high reliability and (c) an area write mode used to fill in large patches without leaving unexposed parts.
Figure 1: SEM images of patterns produced by STM lithography and subsequent cyanide etching of SAMs. (a) Set of individual lines with a width of 20 nm and a separation of 400 nm. The linewidth in this case was limited by the thickness and crystallinity of the gold and by the resolution of the SEM. (b) Complex pattern demonstrate the are write mode.
We developed a positive lithographic system based on e-beam activation of CH3-CH2NHCOCH2-SH self-assembled on gold. This monolayer on its own does not provide an efficient barrier towards etchants for its gold substrate. UV [2] and e-beam exposure under appropriate conditions disrupt locally the film so that in these exposed regions the gold substrate becomes accessible for adsorption of hexadecanethiol. The latter molecules form monolayers in the processed regions that are effective stops of a cyanide etchant. After exposure of the SAM to the e-beam, its replacement by hexadecanethiol, only those regions of gold initially exposed remain protected in an etch (Figure 2). We demonstrated the effects of several parameters relevant to this lithographic scheme such as the energy of the e-beam, the vacuum of the SEM chamber, the hyderophobicity of the surface, and the doses applied to write patterns. Interestingly, the best results were obtained using STM lithography in air because here the proximity of the electron source with the substrate and the low energy of the electrons minimized the polymerization of contaminants ont the surface as was usually the case during SEM experiments. With this and similar other studies we gain insight into the structure and barrier properties of nanometer thin resists which is an important prerequisite for their technological application.
Figure 2: Optical micrographs of 5 square micron patterns of 15 nm thick gold on silicon. The monolayer of these residual gold regions were exposed to electrons from an STM tip with increasing energy (from left to right). Increasing exposure of electrons allowed progressively more headecanethiol to be added to the effected regions thus causing their protection from the etch.
[1] Kumar, A., Biebuyck, H.A. and Whitesides, G.M. J. Am. Chem. Soc. 114 (1992) 9188-9189.
[2] Tam-Chang, S.W., Biebuyck, H.A., Whitesides, G.M., Jeon, N. and Nuzzo, R.G. Langmuir 11 (1995) 4371-4382.