During semiconductor fabrication, wafers undergo numerous processing steps — coatings, bakings, etches, rinses — and inspections, creating electrical circuits that stretch across their surface. In many cases, wafers must be flipped, manually or mechanically, for further processing on their backside. Mechanical flipping is faster, cleaner, and less prone to error. It's also a shining example of motion engineering.
One of the more clever automated wafer flippers, developed by Owens Design, Fremont, Calif, integrates directly onto existing loading stations. The flipper accepts a wafer from a robot and gently grips it with pads that touch only the wafer's edge. A controller commanding the “wrist” tells the robot to retract, and when all's clear, the flipper rotates the wafer 180°. Upper and lower gripper arms then open to release the wafer, and the robot returns it to the loading station.
The mechanics in the module work like this: Gripper arms mount on independent slides that precisely maintain their distance while rotating with the wafer. When they are horizontal, a cam linkage — driven by a dc gearmotor — engages cam blocks that connect to the arms. Mechanically linked and spring loaded, the arms move together with the wafer captured safely inside.
A dc servomotor rotates the entire assembly, driving it through a low-backlash, 10:1 gearbox and external pulley-belt system. On the output pulley, an adjustable index flag sets and indicates the horizontal home position for start-up. An integral encoder monitors trajectory and ensures accurate positioning.
One challenge in the flipper's design was determining the correct force to adequately hold a wafer without crushing sensitive material. Another was the arms' off-center mass, which produces a force under acceleration that operates against the springs. To solve both challenges, designers added counterweights to each arm, moving its center of gravity to the center of rotation.
Total exchange time for wafer placement, grip, rotation, ungrip, and pickup is less than 5 seconds. The flipping motion itself takes less than 2 seconds, and the wafer can be held in this new position indefinitely until the robot picks it up.
This month's handy information provided by Tony Smith of Owens Design Inc., Fremont, Calif.