Case 16: Terraced Wafers in Lift-off Coater
An alternative to the use of correction masks in lift-off coaters is wafer terracing, if some thickness nonuniformity can be tolerated. In this example we examine what result we may expect from adopting this technique.
The need of maintaining small incident angles of the vapor means that the wafers can only be repositioned in such way that they remain facing the source. In other words, they lack the freedom to pivot. Here we reposition the 1st and the 2nd wafer so that they are off the 100-cm-radius sphere while maintaining their orientations.
The terracing equalizes the average coating thickness among the three wafers. There remains, however, considerable intra-substrate thickness nonuniformity: the overall nonuniformity for the three wafers are 1.1%, 2.2% and 3.5%, respectively. Their thickness distributions are shown in the contour plots below.
