Question

2. In a photolithography process, after the optical exposure and development, the photoresist profile is shown in the figure below, revealing the presence of the ridges along the resist edges. Explain the reason to form the ridges, and how to solve the problem. a2

Answer 1

Ans: After optical exposure and development of the resist, your image shows the presence of the ridges which generally comes due to the standing wave effect.

Standing wave effect: Monochromatic light, when anticipated onto a wafer, strikes the photoresist surface over the scope of points. When the incident light gets reflected back from the resist due to the reflective surface of the resist. There happens to be interference between incident and reflected light which further creates a standing wave outline of different intensity at the different depths of the resist. This pattern creates ridges on the sidewall surfaces of the resist.

The solution of the ridge formation problem follows:

1. The best method to cure this type of defects is the coating of the substrate with a small absorbing layer also called bottom antireflective coating.
2. The diffusion of the photoactive compound at high temperature (100 degrees to 130 degrees) into the resist reduces standing wave ridges. Therefore prebake and post bake conditions should be optimized carefully.
3. The solution to this problem is the use of a chemically amplified resist. These type of resist after exposure generates a small amount of strong acid which changes the solubility of the resin in the resist during the post bake conditons.