Many applications involve an etching step to transfer the structures produced by means of lithography onto a substrate. Frequently used for this purpose are plasma etching procedures. For certain processes however it is advantageous to etch wet-chemically. These often highly aggressive procedures require a very good resist adhesion and excellent resist stability, especially if hydrofluoric acid solutions are used. Diluted HF solutions with up to about 6 % HF content are well tolerated by almost all resists for a certain amount of time. The stability of the resist is increased even further if buffered HF solutions (BOE process, mixture of HF and NH4F) are used. Even the highly sensitive e-beam resist CSAR 62 can be used as mask for an etching process with HF (BOE, 10:1). Mr. Y. Nori (Lancaster University, Department of Physics, UK) successfully transferred perforation structures with a diameter of 50 nm from the CSAR 62 mask into a SiO2 layer (25 nm SiO2 on Si) with this etch solution. The etching time was 1 minute, the resist film was stable and showed no signs of removal.
Fig.: Perforation structures written in CSAR 62; structures transferred via BOE etching into SiO2