Well known are direct writing techniques and graytone lithography. In direct writing, a laser or an e-beam shapes the surface of the microlens in photoresist. In the case of graytone lithography, the photoresist layer is exposed by UV-light passing through a mask that has different graytone densities over the surface. Another promising technique for the fabrication of refractive microlenses is the so-called melting resist or reflow method. Photoresist cylinders are formed by photolithography and subsequent melting.
Surface tension forms plano-convex microlenses.
Because of the physical properties of the fabrication step, the possible geometry of the microlenses is limited. In addition, a minimum photoresist volume is required to enable the formation of a spherical surface. Dry etching plays an important role in the fabrication of micro-optical elements. The transfer of photoresist structures into rigid materials, e.g. fused silica, shows several advantages, such as the robustness of the elements and the higher transmission at UV wavelengths. In addition, the dry etching transfer step allows the modification and control of the dimensions and shapes of refractive microlenses.