A two-step electrochemical etch-stop to produce freestanding bulk-micromachined structures

The paper introduces a processing scheme to produce freestanding micromechanical beams by bulk micromachining silicon substrates in aqueous KOH. The release of the structures is done by wet-chemical etching exclusively. Standard MOS process steps are used to generate two adjacent etch-stop regions of different depths. During the anisotropic etching of the substrate in a protective chuck, membranes of two different thicknesses are formed by the electrochemical etch-stop mechanism. A short time-controlled etch of these regions in KOH releases the final beam by removing the thinner membrane areas around it. A layer of thermal oxide with low stress supported by a thin film of copolymer will keep the etchant away from the frontside of the wafer. It can be removed easily by BHF subsequent to micromachining. Resonance measurements with a laser vibrometer were used to determine the mechanical behaviour of the created structures under varying gas pressure.