Photolithography, also termed optical lithography or UV lithography, is a process used in micro-fabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate. A series of chemical treatments then either engraves the exposure pattern into, or enables deposition of a new material in the desired pattern upon, the material underneath the photo resist. For example, in complex integrated circuits, a modern CMOS wafer will go through the photolithographic cycle up to 50 times.
Photolithography shares some fundamental principles with photography in that the pattern in the etching resist is created by exposing it to light, either directly (without using a mask) or with a projected image using an optical mask. This procedure is comparable to a high precision version of the method used to make printed circuit boards. Subsequent stages in the process have more in common with etching than with lithographic printing. It is used because it can create extremely small patterns (down to a few tens of nanometers in size), it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface cost-effectively. Its main disadvantages are that it requires a flat substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.
Processing Steps in Photo-Lithography
- Dehydration: Removes the water molecules from the substrate surface, makes the surface hydrophobic, which increase the adhesion between substrate and photo-resist
- Priming: In this step, adhesion promotor known as primer is added to the substrate surface
- Application of photo-resist: Photo-resist is applied to substrate surface through either spin coating or spray coating
- Soft Bake / Pre-bake: Excess solvent in photo-resist is driven out of the film by heat
- Pattern Alignment: Pattern of mask is aligned with the pattern on substrate
- Photo-resist Exposure: Photo-resist undergoes cross-linking under the illumination of UV light
- Post-exposure Bake: In this step, the photo-resist is further cross-linked by heating
- Photo-resist Development: Exposed positive resist or unexposed negative resist is dissolved in developer leaving needed part on substrate surface
- Hard Bake: Photo-resist is further baked to remove pin-holes and make it denser to facilitate the later etching step
- Phot-Resist Removal: Photo-resist is removed by chemicals that can either dissolve or oxidize photo-resist