Photolithography is a sequence of process steps which allows to replicate or create a pattern on a substrate before performing any additive (lift-off) or substractive (etching) steps. This is achieved by using a photosensitive polymer, the photoresist, that reacts with blue-UV (350nm – 440nm) or DUV (248nm) light to become soluble. The photoresist is generally diluted with a solvent, dispensed on the wafer, and spin-coated to reach its final thickness. Then, it is baked to remove the residual solvent content.
Patterning can be done using different methods: 1) Using a mask that locally blocks the UV light with a thin layer of chromium in order to replicate (mask-aligner) or project (stepper) the mask pattern on the wafer, 2) Using a focussed UV laser spot that writes the pattern directly on the wafer (direct laser writing), 3) Using interferences to create periodic intensity patterns (interference lithography), etc…
Photolithography steps
Here is the basic sequence of steps in a photolithography process. Make sure to follow the links to learn about each step:
- Create your layout/pattern
- Fabricate a mask (mask-aligner or stepper lithography only)
- Select your photoresist
- Use the recommended equipment (to coat, expose and develop the photoresist)
- Find your recipe name (using the nomenclature document)
In general, it is recommended to perform photolithography (and all microfabrication steps) at the wafer-scale. Most of the automatic photolithography equipment (Süss ACS 200, EVG 150, TEL ACT 8, ASML PAS 5500/350C, HIMT VPG 200) are compatible only with 100mm (or 150mm) SEMI wafers.
However, in CMi, several equipment (Sawatec SM-200, MLA-150) can be configured and used to perform photolithography on small rectangle-shape substrates (chips), which are typically diced from 100mm wafers.
The minimum sample size that can be used on the these tools without additional precautions is 5mm * 5mm. Please read the following section if your samples are even smaller for possible work-around options: Working with small samples.
Photolithography definitions
Some additionnal information/definitions about photolithography can be found here: Definitions.