A method of fabricating a semiconductor structure including the following steps is provided. A mask layer is formed on a semiconductor substrate. The semiconductor substrate revealed by the mask layer is anisotropically etched until a cavity is formed in the semiconductor substrate, wherein anisotropically etching the semiconductor substrate revealed by the mask layer comprises performing a plurality of first cycles and performing a plurality of second cycles after performing the first cycles, each cycle among the first and second cycles respectively includes performing a passivating step and performing an etching step after performing the passivating step. During the first cycles, a first duration ratio of the etching step to the passivating step is variable and ramps up step by step. During the second cycles, a second duration ratio of the etching step to the passivating step is constant, and the first duration ratio is less than the second duration ratio.