**4. Graytone lithography**

Graytone lithography [9,10] is another inexpensive 3D micro-fabrication technology. This technology has been evolved to one step mask-less fabrication using SU-89. For the applica‐ tions not requiring or not suited with no access to using an expensive micro-stereolithography machine, gray tone lithography is an interesting alternative to micro-stereolithography. Graytone lithography is a modification of conventional 2D optical lithography. It works by exposing a positive photoresist to a UV light through a grayscale mask which defines the patterns of 3D micro-structures to be formed. The UV light through the grayscale mask produces local intensity modulation. Following a UV exposure, the 3D profile on the surface of the positive photoresist can be formed on the substrate by stripping off the UV-exposed photoresist. While this technology is well known for its potential for mass production, the technology itself is not without limitation. One such limitation is that grayscale lithography based on a positive photoresist does not support fabrication of hollow or suspended microstructures without change in the technology.
