The design of a lift for fragile 300-mm semiconductor wafers gets stiffness just right to synthesize smooth motion.
Robert Repas Associate Editor
When the semiconductor industry moved to 300-mm wafers a few years ago, it changed the whole manner of working within chip fabs. In earlier times, it was not uncommon to see cleanroom workers manually carrying racks of wafers from one processing station to another. Semiconductor makers basically used batch-fabrication methods. There was little automation when it came to moving inprocess wafers from one fabrication step to the next.
The 300-mm wafer made this mode of work impractical on several levels. The wafers themselves were physically more fragile and, of course, larger than those of previous chip generations. And line widths shrank as well. The net result was that wafers became too valuable to be handled manually by even the most careful cleanroom technicians.
In response, semiconductor makers automated their fabs. Robotics now handles the chore of moving wafer racks from one processing station to another. Wafer carriers are sealed as they move between stations and sealed to the processing chamber during use to keep out contaminants.
The move to robotic wafer handling changed the way semiconductor-equipment builders approached material handling in fabs. Time was when equipment builders would make all the hardware in the fab, including that used for moving wafers between processing stations. It soon became clear that robotic wafer handling was a field by itself. Semiconductor-equipment makers eventually concluded that their best course of action was to focus on core processes that differentiated their equipment from that of competitors. They now frequently farm out robotic handling work to specialists.
The design of an automated waferlift subassembly is an example of this trend. A wafer lift measures roughly 10 x 12 x 24 in., weighs about 20 lb, and consists of components that include a linear actuator, wafer platform, motor, support and mounting hardware. Lift stations transfer wafers between robots and through the load lock of stations that serve as transitions between atmospheric and vacuum areas. They are also used internally by robots that move wafers between chambers in wafer-processing equipment.
In this case, the wafer lift was to replace an older design built inhouse by a semiconductor OEM manufacturer. Most OEMs want to be sure the savings are significant before they change a design to cut costs. Typically, they’re looking for a target savings of 20 to 30%.
The key parameters for the new lift station were smooth motion and reliability over a lifetime that exceeds 7 million cycles while still hitting the savings mark.
The wafer lift uses a custom electromechanical assembly that employs a Size 15 ball rail driven by a precision 12-mm-diameter ball screw in the T5 accuracy class. A servodrive mounted remotely on the tool controls a 100-W servomotor that powers the lift. The drive uses a field bus interface for intratool communications. The platform mount travels over a 2.25-in. stroke at an average speed of 2.25 ips, so the entire lift takes only 1 sec. The duty cycle is relatively low at one cycle every 2 min.
The lift handles a mass load of 12 lb and a vacuum load of 108 lb. While the lift is in atmosphere, it is linked to the hoop holding the wafer inside a vacuum chamber. The vacuum load comes from the diameter of the lift coupling-shaft that penetrates the vacuum orifice. The vacuum “pulls” constantly against the lift. A bellows isolates the atmosphere from the vacuum environment. As it compresses it presents an additional 20.9-lb/in. load.
It’s critical that the materialhandling equipment provide a smooth, vibration-free movement to protect the unfinished wafer from damage. The stiffness of the linear actuator is the key to that smooth motion. An actuator with too much slop vibrates excessively. On the other hand, too much stiffness lets any small vibrations propagate and resonate through the entire assembly. Either case degrades smooth motion. The engineers optimized smoothness of motion in the lift station by calculating the proper balance of preload on the ball screw for the best level of stiffness.