Targeted sacrificial layer etching for MEMS release using microfluidic channels

A microfluidics based targeted etchant delivery and masking approach to wet etching has been used to control the etch progression of a MEMS sacrificial layer during the release of silicon nitride (SiNx) microbeams. A reusable 3-input open-channel polydimethylsiloxane (PDMS) microfluidic cassette was used to form a dynamically controllable fluid etch mask to control the location of the etchant during the wet release process. In contrast conventional release techniques which use solid masking and homogeneous etching environments, microfluidic devices can utilise laminar flows to generate heterogeneous etching conditions which can be controlled in real-time by altering the composition and flow rates of the fluids passing through specific inlets. The fluid nature of the heterogeneous flow can be used to target etch specific areas of sacrificial material or conversely, dynamically mask specific areas both above and below suspend structures. As a result of this control, structures with anchor geometries not achievable using conventional release techniques were created. Not only does this method require small volumes of etchant fluid, it is also suitable for use on samples which may be sensitive to the chemical and/or physical rigors of photolithographic patterning, such as porous silicon. Microfluidic based release etching, using dynamically controlled fluid masks, provides a valuable addition to the suite of microchannel based fabrication techniques.