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Wafer placement repeatibility and robot speed improvements for bonded wafer stacks used in 3D integration

Robotic wafer handling of bonded wafer stacks (BWS) brings new challenges in placement repeatability and robot speed. Current standard SEMI M1.15 [1] does not contemplate wafer stacks >775 microns thick. Varying BWS thickness of 800-microns (top wafer thinned to 25-microns bonded to a 775 micron carrier wafer) to 1550 microns thick (two full thickness 775 micron wafers bonded together) creates additional mass and thus additional momentum during wafer movement. Without a corresponding increase in holding force, wafer sliding on the end effector is likely. One solution is to reduce robot velocity and acceleration, but this can lead to tool throughput reductions. In this paper we explore wafer handling issues for BWS and the application of a new end effector technology, called Gravity Edge Hold (GEH). Wafer sliding issues will be described in terms of wafer and BWS momentum, lateral holding force, and robot acceleration. It will further describe the wafer placement repeatability issues encountered and the corresponding decrease in robot speeds implemented to counteract this problem. Laboratory experiments were conducted to compare the performance of traditional end effectors used in current 300mm tools to that of the GEH end effector. An evaluation based on lateral holding force is included.