Isotropic wet chemical etching of deep channels with optical surface quality in silicon with HNA based etching solutions

Online trace analysis based on UV/Vis spectroscopy requires long detection paths. Therefore an isotropic wet etch process in silicon is developed to fabricate a 300 µm deep channel with low channel wall roughness for desired light guidance application. Four etchant compositions were compared in terms of etching rate, surface roughness and selectivity in a beaker process. The best fitting mixture was selected. To further increase the surface quality (bubble issue) a spin etcher tool is used for producing the channels. The dependence of homogeneity and defect density on media flux, and rotation velocity was investigated. Results show that high rotation velocity and high media flux lead to great defects in the channel wall. Through rotation of the wafer during etching, the etching rate of silicon rises compared to the beaker process due to the rapid removal of etch products and simultaneous supply of fresh etchant. After 38 min of etching, 300 µm deep semi-circular channels with high optical quality (Rq=10 nm±2 nm) over 3 m were produced.