STM Lithography in an Organic Self-Assembled Monolayer

We describe the suitability of ultra-high vacuum scanning tunneling microscopy (UHV-STM) based nanolithography by using highly ordered monomolecular organic films, called self-assembled monolayers (SAMs), as ultrathin resists. Organothiol-type SAMs such as hexadecanethiol (SH–(CH2)15–CH3) and N-biphenylthiol (SH–(C6H6)2–NO2) monolayers have been prepared by immersion on gold films and Au(111) single crystals. Organosilane-type SAMs such as octadecyltrichlorosilane (SiCl3–(CH2)17–CH3) monolayers have been prepared on hydroxylated Si(100) surfaces as well as hydroxylated chromium film surfaces. Dense line patterns have been written by UHV-STM in constant current mode for various tunneling parameters (gap voltage, tunneling current, scan speed, and orientation) and transferred into the underlying substrate by wet etch techniques. The etched structures have been analyzed by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Best resolution has been achieved without etch transfer for a 20 nm × 20 nm square written in hexadecanethiol/Au(111) with an edge definition of about 5 nm. Etch transfer of the STM nanopatterns in Au films resulted in 55 nm dense line patterns (15 nm deep) mainly broadened by the isotropic etch characteristic, while 35 nm wide and 30 nm deep dense line patterns written in octadecyltrichlorosilane/Si(100) and anisotropically etched into Si(100) could be achieved.