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The most common and available method used to fabricate needles from single-crystal silicon wafers is anisotropic wet etching. The dependence of the shape and sizes of the needles on the etching solution concentration when using a mask of one type (round, square, or some other shape) has not been fully studied. There are barely any data on the shape, size, and aspect ratio of the obtained needles for high concentrations of KOH aqueous solution (more than 50%). In this work, the dependence of the shape, sizes, and aspect ratio of pyramidal needles fabricated by anisotropic etching of silicon in an aqueous solution of KOH on the solution concentration in the range of 30–75% has been studied. The Miller indices (
hkl)
for the planes forming the needle faces are calculated. It is shown that the use of a KOH aqueous solution with a high concentration (70 or 75%) makes it possible to obtain needles in the form of octagonal pyramids, retaining their shape over the entire height. Defects of microblade type are practically absent on the tips of these needles, whereas at lower KOH concentrations (30 and 50%) “microblades” are observed on almost all needles. It is found that the needles formed during silicon etching in a 75% KOH aqueous solution have the largest height (12–14 µm) and the maximum aspect ratio, equal to 1.72 µm; the side faces of the needles are formed by eight planes from the {411} and {141} families.