Optical lithography (also termed photolithography or UV lithography) is the patterning of masks and samples with photoresist prior to other processing steps (e.g. deposition, etching, doping).

For optical lithography a physical pattern is required with clear and dark areas typically on a mask. Most often the mask is a glass plate with a UV opaque material on it such as chromium.

There are a series of steps that are common to all types of optical lithography:

1. Sample preparation –

   Prior to photoresist application and exposure, the sample should be cleaned and free of moisture.

2. Photoresist application –

   Once the sample is prepared, photoresist is applied to the sample. This is most commonly achieved by spinning it on as a liquid and then baking the sample to remove the solvent. The thickness of the layer is determined by the speed at which it is spun. The film thickness is inversely proportional to square root of spin rate.

3. Soft bake –

   After coating the slice is baked to drive out all traces of solvent from photoresist. Temperature is usually $90-100^0C$. About 20-30 minutes are required for this process, if it carried out in a convection oven. During this process, film thickness shrinks to about 85% of its spun on value.

4. Mask Alignment –

   In manual system, a mask consisting of a gelatin photographic emulsion on a glass plate is placed over slice and brought in contact with it and then backed off slightly to produce an air gap. It is then manipulated in its desired position by micrometer adjustment and finally the exposure is made.

5. Exposure –

   After the photoresist is applied and baked, it is exposed to UV light to generate the desired pattern. The UV light causes a chemical reaction in the photoresist. In positive photoresist, the reaction makes the photoresist acidic, so that it will dissolve in an alkaline developer solution. With negative photoresist, the exposed polymer cross-links, making it impervious to the developer, which only removes the areas that are unexposed. The exposure may be directly written on the mask with a laser, or the entire wafer can be exposed through a mask. The latter is a much faster and more cost-effective process when multiple samples are desired. Mask exposure can further be divided into three categories: contact, proximity and projection.

6. Development –

   After exposure, the photoresist is placed in a developer solution which dissolves parts of the photoresist on the wafer. For positive photoresist, the areas that were exposed dissolve, and for negative photoresist, the areas that were un-exposed dissolve. For most standard resists, this is performed by soaking the sample in an alkaline solution, although some use solvent based developers.

7. Hard bake –

   Some photoresists recommend hard-baking the resist after development. This will densify the resist, improve the adhesion to the surface, and make it more resistant to wet chemical etching. It will reduce the undercut of the resist during wet chemical etching.