Case S10: Coating with photoresist for lift-off
This example shows how the shadow masks can be employed to model the self-shadowing effect in a semiconductor manufacturing process: metallization of wafers by means of the lift-off technique.
To achieve good results through the lift-off process, the photoresist, which covers the areas not to be metallized, is often sculpted to have an overhang in its profile. This profile, or any profile of a photoresist, can be modeled in V-Grade 5S; its self-shadowing can be simulated with the help of the shadow-mask function of the program.
Here, a 4-inch-diameter wafer on a lift-off dome (spherical) is modeled through a 4-inch-diamater fixture that revolves around the axis. We study a metallization strip (wire) that situates near the edge of the wafer and runs perpendicular to the radius. A numerical array is created to represent the 3D surface with an overhang, as shown below.
The shadows created by the photoresist during deposition are modeled via two negative shadow masks that choke the two corners of the overhang. As the masks rotate with the wafer, they cast shadows in the same way as the photoresist. The result, thickness maps below, shows the thickness profiles on the wafer - the overhang clearly prevents the photoresist wall from being coated even though the material vapor deviates from perpendicular incidence.*
