Self-shadowing of 3D Objects in PVD and its Modeling

When PVD is employed to coat a 3-dimensional object, certain areas of the surface may come into the shadows cast by its own protruding structures. Here, we speak of the self-shadowing. Self-shadowing commonly occurs in coating of photoresist-patterned semiconductor wafers, automobile parts, medical devices etc. To correctly predict the thickness distribution on these surfaces, a realistic model of PVD must take into account the obstruction of vapor due to the self-shadows.

Example 1: Coating of a deep parabolic reflector in a magnetron sputtering chamber

The parabolic reflector to be coated is 10 cm in diameter and 5 cm deep, and is mounted on a planetary-rotation fixture. As it revolves in its orbit, the center of the parabolic reflector spends much of the time shielded from the material vapor by its own wall. The correct thickness distribution, shown below, is a result of modeling the self-shadowing with a rotational and negative shadow mask that is "affixed" to the rim of the bowl. An erroneous prediction by withdrawing the shadow mask is also displayed, on the left side, for contrast.
Example 2: Photoresist-covered wafer

In a common technique of semiconductor manufacturing, i.e. the lift-off process, a layer of photoresist is patterned on a wafer to cover areas not to be metallized. To achieve good results the photoresist 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, we study the deposition of a metal wire that situates near the edge of a 4-inch-diameter wafer and runs perpendicular to its radius. The wafer is mounted tangentially on a spherical dome. 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, exhibited in the false-colored and value-labeled maps below, shows the wire profiles on the wafer - the overhang clearly prevents the photoresist wall from being coated even though the material vapor deviates significantly from perpendicular incidence.