Anisotropic etch rates of silicon in KOH solutions were studied as a function of an externally applied potential. A combination of three micromachined samples consisting of predry-etched wagon-wheel patterns and masked trench offset patterns was used to measure the etch rates at a large number of crystal orientations simultaneously. The measured data was described in terms of microscopic properties, including step velocities, terrace roughening, and step anisotropy, using the kinematic wave etch model. All parameters show distinct changes due to the applied potential and resulting additional electrochemical reaction path. A decrease in step velocity shows the electrochemical oxidation and subsequent passivation of the Si surface. Trends in terrace roughening show a minimum in roughness and a corresponding change in anisotropic etch-rate ratio at the non-open-circuit potential of −1250 mV vs saturated calomel electrode. The observed decrease in step anisotropy and subsequent step-anisotropy reversal at more positive potentials indicates an anisotropy in not only chemical etching but also electrochemical oxidation of ͑111͒ surface steps.Anisotropic wet chemical etching of silicon in concentrated alkaline solution has proven to be a simple, reliable, and widely used bulk micromachining process for microelectromechanical systems ͑MEMS͒ technology. 1 Using the difference in etch rates of various crystal orientations in a silicon single crystal allows for a relatively simple manufacturing process of complex structures where etchants such as potassium hydroxide ͑KOH͒ and tetramethyl ammonium hydroxide ͑TMAH͒ solutions are most commonly used. The fundamental basis of the etching mechanism, however, still presents many questions. On a molecular/chemical level it seems relatively clear how the etching process can be described in terms of chemicalreaction mechanisms. How these mechanisms translate on a microscopic scale, where properties such as surface roughening and step anisotropy are important, is still unclear. This is of particular interest when including additional electrochemical reaction paths. Previous studies 2-4 have shown that the overall anisotropic etch rate is a function of the electrochemistry. Electrochemical oxidation caused by factors such as chemical oxidizers 5,6 or applied potentials can reduce overall etch rates. Detailed in situ scanning tunnel microscope ͑STM͒ experiments by Allongue et al. show similar results on an atomic scale. 7,2 Other electrochemical studies have also shown that the electrochemical oxidation rate also depends on the crystal orientation of the exposed surface. 8 These results, however, typically deal with stable crystal surfaces such as ͑100͒ and ͑111͒. In this work we have measured the anisotropic etch rate of a large number of orientations as a function of a constant applied potential. This was done by performing etch experiments using micromachined wagon-wheel patterns on two different substrate types and a masked trench offset pattern. This combination resulted in etch-rat...

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