Progress on thin wafer handling
SUNNYVALE, CALIF. - There has been much progress in the past year on wafer thinning, with portable electronics providing the driving force for thin die. A necessary part of this progress has been advances in wafer handling. Several companies presented their work at a symposium titled "The Year of the Ultra-Thin Wafer" sponsored by MEPTEC.
Extensive simulations of 300-mm wafer cassettes resulted in improved designs at ChipPAC. Flynn Carson, engineering director at ChipPAC, presented modeling results for different configurations of cassettes and showed a design that supports very thin 300-mm wafers better than standard cassettes. Similar analysis evaluated design variables in 300-mm wafer handling arms and identified a design that decreased the wafer droop by about 30 percent. Carson stated that these modifications are needed for 300-mm wafers thinned to 150 microns or less. Experiments also showed that polishing is required on the backside of these thinned wafers to remove micro-cracks that weaken the wafers to unacceptable levels.
Another approach to thin wafer handling is to delay the thinning process beyond the dicing process to eliminate the need for handling full wafers that have been thinned. Devin Martin of Disco Hi-Tec presented a "dice before grind" approach in which the wafer is partially diced, laminated with a frontside tape and then singulated by backside grinding. Along with preventing the handling of thin wafers, this approach increases the die strength by preventing backside cracks that can occur when the wafer is diced only by sawing. Phil Marcoux discussed Tru-Si's analogous approach, in which the final thinning is accomplished by etching instead of grinding.
Yet another thinning technology is the spin-processing technology from SEZ. Scott Drews of SEZ discussed this process and had customer data showing diminished levels of wafer breakage due to the smoother backside that results from the process. SEZ's equipment includes a patented Bernoulli chuck that holds the wafer on a cushion of nitrogen during the etching process. It also has a Bernoulli handling system that includes an end effector with N2 tubing inside the robot arm.