Electrochemical Etch-StopBy Harry Chiu & Zachary FjeldheimWhat is an Etch-Stop?` Etch-stop is defined as a technique that allows termination of the etching process at a controllable depth` Selectivity: S = δ1/ δsEtch-Stop Techniques` Timing Method` Boron Etch-Stop` Electrochemical Etch-StopTiming Method: V-Groove` Method to control depth of etch` At the precise moment the V-groove develops, the membrane has reached the correct thicknessTiming Method` Advantages` Simplest of all etch-stops` Cheap` Disadvantages` Low Accuracy` Not very reliableBoron Etch-Stop` Heavily B doped silicon has a very low etch rate` Membranes can be created by doping the top of the wafer etching from the backBoron Etch-Stop` Advantages` Relatively simple process` High selectivity and reliability` Disadvantages` Microstructures subjected to high tensile stress` Not compatible with CMOS processIntroduction` P - n junction` Electrochemical etch rate modulation` ElectrodlessP - n junction` Reverse biased` Anisotropic etch` High selectivity` Passivation potential` TMAH/KOH` Three electrode vs. four electrode systemElectrochemical etch rate modulation` Open circuit potential` Passivation potential` SiO2 etch stop` Anodic current flow` KOH/TMAHElectrodless` Au/Cr/n-Si/TMAH` Gold/chrome layer` Anodic oxide` TemperatureElectrochemical Etch-Stop` Advantages` Doesn’t require as high doping level as boron` Better thickness control` Less stress on wafer` Batch fabrication *(Electrodless)` Disadvantages` Requires cumbersome fixtures to insulate electrical wiring from etchantTypical Applications` Thin membranes` Pressure Sensors` Actuators` Micro-needles used by neurophysiologist as a minimally invasive tool to monitor neural
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