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Aspect ratio dependent etching and microloading effects are two mechanisms leading to non-uniformities in the etching of silicon using deep reactive ion etching technology. This paper focuses on the apparent presence of a critical aspect radio when etching three-dimensional MEMS structures. A mask designed to separate the effects of microloading from aspect radio dependent etching was made, and various sizes of features (3 to 1000 /spl mu/m) are etched simultaneously for different etching times (10 to 180 min) using a Bosch etching process. Experimental data exhibiting three distinct regimes for the etch rate, the corresponding critical aspect ratios, and the dependence on microloading effects are presented. Possible mechanisms governing these regimes are also postulated.