Fabrication of hollow silicon microneedles using grayscale lithography and deep reactive ion etching

Microneedles are a promising technology for drug delivery. They are minimally invasive, painless, and easy to use for various applications. In this paper, a process for fabricating hollow microneedles on a silicon substrate using maskless direct writing grayscale lithography and deep reactive ion etching (DRIE) is investigated. During the process, two approaches are employed to smoothen the needle tip and reduce the roughness of the needle bevel. First, a grayscale lithography process is used to create a 3D photoresist structure with discrete height steps. This structure is then transferred into silicon using DRIE. An SF6 isotropic etching process is utilized to create a smooth surface on the upper portion of microneedles by smoothing out the steplike structures. Alternatively, after grayscale lithography, resist thermal reflow is used to smooth out the resist structures. The resist patterns are then transferred into silicon microneedles using the DRIE technique. The morphology of the microneedles can be adjusted by varying the etching selectivity of silicon over the resist. Hollow microneedles with a height of 360 μm are produced, which offers numerous practical applications.