The process to fabricate a needle-shaped silicon crystal (silicon needle) has been developed. These silicon needles are fabricated by highly selective anisotropic dry etching, where the etching mask is oxygen precipitation in the silicon substrate. In order to apply these silicon needles to field emitters, the shape of the silicon needle should be controlled. This is because a high aspect ratio can lead to electric field enhancement around tips and a flared base of the silicon needle can lead to mechanical and thermal stability. In this article we studied how to control the shape of the silicon needles. We found that the silicon needles' with a high aspect ratio could be obtained by either lowering the deposition rate of sidewall passivation film or increasing etching depth, unlike conventional silicon cones fabricated by isotropic dry etching. Furthermore, a high aspect ratio could induce a flared base of the silicon needle, since incident ions were reduced in the vicinity of the base due to a shadowing effect by the silicon needle with a high aspect ratio.