Using Two-Stage Chemical Amplification To Determine the Density of Defects in Self-Assembled Monolayers of Alkanethiolates on Gold

This paper describes a technique to measure the surface density of the defects in self-assembled monolayers (SAMs) of hexadecanethiolates on gold that lead to etching of the gold when the system is etched with aqueous ferricyanide solution. This technique uses two steps of amplification through chemical reaction to convert pinhole defects in SAMs into easily imaged, micron-scale pits in an underlying Si support. With this technique, it is possible to evaluate the density of defects in SAMs and in the structures prepared by using SAMs as resists under conditions that may be encountered in lithographic processing. At present, the lowest density of pits that we have measured for SAMs of hexadecanethiolates on 50 nm thick gold is ∼5 pits/mm2. This density is an upper limit for the density of defects that expose bare gold to the etching solution or that comprise regions of SAM sufficiently thin that the SAM is not able to block access of the etchant to the gold.