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Numerous studies have been done in studying the etching rates of the isotropic HF/HNO3/CH3COOH (or HNA) etchant for silicon in terms of the compositions of the solution. It is well known that the HNA etchant is an autocatalytic solution. Controlling its reaction rate is more complicated than just fixing the ratios of the reagent components - HF, HNO3, H3COOH. Sometimes, researchers may experience a runaway reaction, surprisingly even with the same composition used before. Researchers should also look into some chemical engineering aspects - such as etch-area or etchant-volume that can affect the dissipation of the heat generated and the self generated catalyst - HNO2. All these are very important in terms of the chemical safety inside a MEMS research laboratory. In this study, HNA solutions with similar compositions were used to demonstrate how the runaway reaction could start as a function of etch area, etchant volume, or etch-area:etchant-volume ratio. A larger area would make the reaction go faster, and might increase the chance of running into an uncontrollable manner easily. Usually, using less amount of HNA solution might be a good practice for waste minimization. However, by over doing it, the heat and the HNO2 generated from the reaction could be accumulated too fast and they might be able to make the reaction out of control. HNA is an autocatalytic etchant and the HNO2 generated is usually required to keep the reaction going. Thus it is desirable to keep the mass transfer low at the beginning of the etching, in order to keep the HNO2. Small amount of heat can also help to accelerate the reaction. However, the mass transfer and the heat transfer need to be controlled properly as reaction proceeds. In the case without mechanical setup for controlling the mass transfer and the heat transfer, the volume of the solution will become the major to carry out these tasks through natural convection and diffusion. In our study, we have demonstrated in simple ways on how to control and safe guard the etching reaction from runaway.
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