Etchingless Microfabrication of a Thick Metal Oxide Film on a Flexible Polymer Substrate

Abstract

Well-defined microstructures of metal oxide, i.e., tin oxide (SnOx), were formed on a flexible polymer substrate from an aqueous solution of tin chloride without the use of any chemical or physical etching. First, through the chemical vapor deposition of tetraethoxysilane and subsequent photooxidation using 172 nm vacuum-ultraviolet (vacuum-UV) light, an extremely thin SiO2 layer about 1 nm thick, which we call an “oxide nanoskin” (ONS), was formed on a polyimide (PI) substrate. Next, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trimethoxysilane (FAS) molecules were chemisorbed onto this ONS-covered PI (ONS/PI) surface from the vapor phase. As a control, a hydrophilized PI substrate was also treated under the same conditions. Both substrates were then photolithographically micropatterned using the same vacuum-UV light. Finally, the fabricated micropatterns composed of hydrophobic FAS- and hydrophilic SiOH-covered regions served respectively as chemically inert and active sites to provide a template for the area-selective deposition of SnOx film. Although the SnOx film was initially deposited on the entire substrate, the physisorbed film on the FAS-covered regions was easily eliminated from the surface by sonication in absolute toluene, while that on the SiOH-covered regions remained tightly adhered to the substrate surface. As confirmed by atomic force microscopy, square-shaped SnOx micropatterns composed of 25 × 25 μm2 features were formed on the micropatterned FAS/ONS/PI substrates. However, on the substrate without the ONS layer, pattern resolution became significantly worse and elimination of the physisorbed SnOx on the FAS-covered regions was incomplete. The ONS interlayer was effective both in forming a highly ordered FAS monolayer on the PI substrate and in obtaining excellent adhesion of the SnOx film to the PI substrate. Furthermore, our process enabled us to control the thickness of the SnOx microstructures within the range of a few hundreds nanometers to over 1 μm without distortion of their fine patterns. The SnOx microstructures we obtained on the PI substrates were free of cracks or peeling even after thermal treatment and showed excellent mechanical properties.