Polymer masks for structured surface and plasma etching
Résumé
Silica and silicon structures have been prepared at the sub-micrometer length-scale, using laterally phase-separated thin films of poly(styrene) (PS) and poly(lactic acid) (PLA) homopolymer blends. The selectiveremoval of one polymer and the filling of the released space by silica precursor solution led, after calci-nation, to silica structures on silicon such as arrays of bowl-shape features or pillars, layers with throughor non-through cylindrical holes, which has not been observed for some of them. The control of themorphology of the initial polymer film was a key point to achieve such type of structures. Particularlyrelevant was the use of solvent vapor annealing (vs thermal annealing) of the initial spin-coated films thatfavored and stabilized laterally phase-separated morphologies. Characteristic dimension of the domainswere shown to be coupled with the thickness of the film, thinner films giving smaller domain sizes.Despite a relatively high incompatibility of the two polymers, a macro-phase separation was preventedin all the studied conditions. Sub-micrometric domains were formed, and for the thinner films, nano-metric domains as small as 74 nm in size can be obtained. The silica structures formed by the infiltrationof the polymer templates were used as hard masks for the cryogenic etching of underlying silicon. Newstructured surfaces, arrays of silicon pillars which can be plain or hollow at the upper part or arrays ofcylindrical holes were formed. A selectivity as high as 21 was obtained using this type of mask for 1.5 um deep holes having a typical diameter of 200 nm.