Isotropic etching of silicon in fluoride containing solutions as a tool for micromachining

μTAS is hot in micromechanics today. All μTAS devices contain channels to connect the different components together. Channels can also be used as chromatography columns. Isotropic wet chemical etching of silicon can be a suitable process to construct (hemi)circular channels with very smooth surfaces. Wet etching of silicon can be done chemically, using aqueous solutions of HF and HNO , or electrochemically, using aqueous HF solutions. Both processes suffer from the loading-effect: due to extra diffusion of fluoride from above masked areas, the edges of structures etch faster than the middle. This problem was solved by the placement of extra structures which ensured a uniform consumption of HF over the whole wafer. Two channels with a length of 10 m and diameters of 93 and 35 μm, to be used as gas chromatography columns, were constructed using chemical etching: one by etching two mirror images of one half of the channel (hemicircular cross section), and bonding them to each other, and the other with the Buried Structure Technology (BST), respectively. The BST process developed in this project makes the etching of cavities beneath the surface possible, thus eliminating the need for bonding, as well as bonding-related problems. The surface roughness of the constructed channels was �?�5 nm. Electrochemical etching of silicon in aqueous HF solutions offers more possibilities. Hemicircular channels were etched electrochemically in silicon, using high current densities (> 45 mA/cm2 when 5% HF is used). At lower current densities porous silicon forms. This can be etched free by increasing the current density. In this way porous silicon sieves with a pore size of �?�7 nm can be made. Another possibility of electrochemical etching of silicon is maskless pattern etching. In n-type silicon, electrochemical etching must be induced by the excitation of an electron into the conduction band. This means that a structure can in principle be etched by projecting a mask pattern on an n-type silicon substrate, and etching electrochemically. Structures were succesfully etched this way. The resolution is limited by the diffusion length of the holes (around 11 μm) and the resolution of the optical equipment. In our case the theoretical resolution was 27 μm. In practice the resolution was lower due to focussing difficulties.