Proximal probe study of self-assembled monolayer resist materials

Abstract

A scanning tunneling microscope (STM) has been used to investigate organosilane self-assembled monolayer films (SAMs) as imaging layers for low voltage e-beam lithography. We have studied three different SAMs [(aminoethylaminomethyl)phenethyltrimethoxysilane (PEDA), 4-chloromethylphenyltrichlorosilane (CMPTS), and n-octadecyltrichlorosilane (OTS)] deposited on the native oxide of Si. We have found OTS to act as a positive resist in wet etch processing upon exposure to 50 keV electrons, in agreement with Lercel [J. Vac. Sci. Technol. B 11, 2823 (1993)]. Identically processed samples patterned with low voltage (−10 to −25 V tip–sample bias) electrons in the STM exhibit a negative tone. STM biases below −10 V were insufficient to expose the film. An improved etch for STM-generated patterns, leading to smoother surfaces, was developed. STM exposure of PEDA and CMPTS SAMs leads to a destruction of ligand functionality. An exposure threshold of −4 V bias for CMPTS and −8 V for PEDA has been established. As an example of post-exposure processing, these latent images were metallized with an aqueous Pd(II) catalyst solution followed by an electroless Ni plating bath. The patterned, thin (25 nm) Ni layers grown in this way are shown to be excellent masks for reactive ion etching (RIE) with SF6, exhibiting at least a 1:200 etch selectivity on Si. Linewidths after metallization of 20 nm and etched (CBrF3/O2 RIE) trench widths of 25 nm are shown.