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Nanostructures with controllable geometries are increasingly needed for mechanosensor applications due to their superior capabilities in measuring minute mechanical forces at small scales. Previous nanofabrication technologies are deficient in creating such structures due to high cost, complicated process and poor geometric control. In this paper, we introduce an innovative use of fluorine enhanced oxidation to realize a segment by segment sidewall control strategy for creating silicon nanostructures. This method is based on fluorine reactive ion etching, where the rich amount of fluorine species accumulated on the surface are utilized to enhance surface oxidation during an additional oxygen exposure. The experimental results showed good repeatability. The characteristic feature size of the resulting nanostructures is about 200 nm. The minimum feature size goes down to 50 nm. This method requires minimal use of equipment, and demonstrates good control of sidewall profiles. It thus has technical and economic significance in development of functional nanomechanosensors, which are potent for quantitative mechanical sensing and actuating at small scales.