Scanning probe anodization patterning of Si substrates covered with a self-assembled monolayer dependent on surface hydrophilicity

Abstract

Contact-mode atomic force microscopy (AFM)-based anodization patterning was performed on silicon (Si) substrates covered with a self-assembled monolayer (SAM) in order to investigate effects of relative humidity (RH), surface wetability of the SAM, and probe-tip material on widths and heights of drawn lines. Three types of SAMs, that is, methyl-terminated, ester-terminated, and carboxyl-terminated SAMs, were prepared on hydrogen-terminated Si substrates by a thermal activation method. These SAMs were covalently fixed on the Si substrates through Si–C bonds without an interfacial oxide layer between the SAM and Si. Rh-coated and boron-doped conductive diamond-coated AFM probe tips were used for patterning with a positive sample bias of 10 V. Consequently, the region scanned with the AFM probe became protruded due to degradation of the SAM and anodization of Si. When the Rh-coated Si tip was used, the width of the protruded line increased with increasing RH on each SAM sample. The linewidth on the most hydrophobic methyl-terminated SAM was narrowest, while that on the most hydrophilic carboxylic SAM was widest. However there was no distinct difference in the pattern width at the patterning under low RH of 10%. In the case of patterning with the boron-doped conductive diamond-coated tip on the ester SAM, the width of the patterned line showed no clear increase with increasing RH. These pattern width changes were discussed in terms of the size of adsorbed water meniscus at the AFM-tip/sample junction as confirmed by force curve measurements; in the case of the boron-doped conductive diamond-coated tip which is rather more hydrophobic than the Rh-coated Si tip, the size of water meniscus hardly affected RH.