Striation defects are a thickness pattern of coating material on the wafer that has an apparent form similar to a sinusoid. The ridges and valleys are lined up parallel to the major fluid flow direction of coating solution during the spin-coating process. As part of our effort to understand and prevent striation defects, we have studied how the striation spacing depends on position and spin-speed on the wafer.

We used reflectance interferometric profile mapping (Veeco Inc.) to capture the following images. Samples were made using a commercial Spin-on-glass and deposited onto 4" silicon wafers at three different RPM rates. These pictures show the striation patterns that formed. At the center location there is no significant fluid flow outward in any direction while all other pictures are oriented approximately so that fluid is flowing from left to right.
 

RPM	Center	R=1 cm	R=2 cm	R = 3 cm
2000
3000
4000

The most interesting feature of these patterns is that the striation spacing is fundamentally self-similar at all radii on a specific wafer. That means that the striations do NOT form simply as "cells" or dimples at the center that then spread out and elongate as spinning progresses. Instead they must also be splitting and adapting to the local conditions. For example at R=3 there is 3 times as much circumference, meaning that since the striation spacing is about the same, there will be approximately 3 times as many striations around the wafer at that radius than would be found at R=1.

This work has been submitted for publication and is currently under review.