Characterization of masking materials for deep glass micromachining

In this paper the characterization of different masking materials for the fabrication of flow channels or thin diaphragms in aluminosilicate glass substrates (Coming 1737) is presented. Materials such as photoresist, polysilicon and gold were investigated with concentrated hydrofluoric acid, HF 48% used as an isotropic etchant. The use of single material masks restricts the useable etch depth to less than 250 mum. Surface and material imperfections result in weaknesses in the masking layer and subsequent penetration by the etchant. An etch depth of greater than 300 mum was achieved using a combination of thick SU-8 photoresist and polished polycrystalline silicon as the masking material. The two materials act as double protection to the glass substrate and the etch depth obtained is approximately three to six times larger than those published for standard photoresist or SU-8 etch mask.