Optical lithography (photolithography) is a key process in microfabrication for transferring a geometrical pattern to a thin film substrate. The optical lithography mainly includes: wafer cleaning, photoresist spin-coating, soft backing, mask alignment and exposure, photo resist developing, hard backing, etching and finally resist removal. The first step of photolithography process is chemically cleaning (mainly by acetone and isopropanol) of wafer (silicon or glass substrate) in order to remove any contaminants from the surface of it. In the next step, a photoresist (i.e. a material sensitive to light) spin-coated to the surface by using a rotational speed of 1000-8000 rpm which is followed by heating the substrate for some minutes to dry the photoresist (soft backing). Once the resist is spin-coated to the surface, by using a lithography mask which has the geometrical features on it, the sample is partially exposed to the UV or Extreme-UV light source (this is simply done by locating the mask between light source and the wafer). In this step based on the mask design and polarity, some parts of the resist exposed to the light while the other one does not receive any light energy. Once the exposure is done then the resist is developed by a developer liquid which will remove either the exposed areas or non-exposed section (depends on resist type). In order to make the resist harder normally it is backed for some few minutes and then the wafer is etched by chemical or physical etching processes like ion-beam etching. In this case the areas without any resist will be etched while the areas protected by resist will remain. Finally the residual resit on the wafer will be removed. Following figure shows different steps of photo-lithography process. Electron beam lithography are explained in another post.