How to spin coat a uniform film on square substrates
How to increase the uniformity of films when spin coating rectangular or square substrates
When spin coating rectangular or square substrates, achieving a uniform film can be tricky. The three main challenges are:
- Edge beads (thicker buildup at the edges)
- Geometrical effects (uneven coating due to substrate shape)
- Bernoulli effects (airflow-induced pressure differences)
Spin speed plays a major role in uniformity. Lower speeds reduce the vacuum effect caused by Bernoulli forces, leading to more even coatings. However, there’s a trade-off: slower speeds usually result in wider edge beads and thicker films, since film thickness is tied to spin speed.
What research says
A study by Gregory Luurtsema at the University of California, Berkeley, found that the most effective way to improve coating uniformity on square substrates is to use:
- A recessed or carrier chuck
- An air barrier plate
- A low spin speed
The results were impressive:
- Using both a recessed chuck and a barrier plate reduced corner buildup on leading edges by 125%
- Most of the improvement (90%) came from the recessed chuck
- The barrier plate, placed 3 – 12 mm above the substrate, provided an additional 35% reduction
