University takes cue from Aladdin for 'magical' wafer levitation
By MARK A. DeSORBO
MUNICH—When it came to finding a new way to handle meticulously patterned wafers without ever touching the somewhat sterile substrates, the Technical University of Munich found inspiration in Aladdin's magic carpet and developed a method of levitating them on a posh pillow of ultrasonic vibration.
"The technique allows complete non-contact handling when used for mere transportation of the wafer," says Michael Schilp, a member of the university's assembly technology team at the Institute for Machine Tools and Industrial Management (IWB). "No particles are produced by the handling process."
Moreover, Schlip told CleanRooms via e-mail, the process does not disrupt the cleanroom environment, and process gases can be used. Wafers also can be transferred in and out of cassettes and buffers, and moved between tools while levitated on their 1-mm to 2-mm cushions.
"As the ultrasonic cushion does not need additional air, the airflow is not disturbed by air blown into the cleanroom, like with Bernoulli grippers or air tracks," Schilp says.
According to the team of researchers, "a near-field or squeeze-field" levitation is applied to lift a wafer by direct radiation of its underside within the near field of a high intensity ultrasonic transducer. The wafer runs along a track, levitated by a 24 kHz beam vibrator on each side, and kept on track by side barriers. Speed and direction are controlled by using smaller, diagonally positioned ultrasound generators, or by gravity so as to tilt the track slightly in the desired direction.
Calculations are simplified by using a vibrating foil on top of the solid unit, instead of vibrating the entire base unit. Wafers can remain floating in buffer storage in cassettes of stacked flexible vibrators, or loaded and unloaded from standard cassettes with a non-contact gripper. The gripper picks them up without touching their surface by a counterbalancing combination of vacuum and ultrasonic forces.
But the gripper does touch the edge of the wafer, as do the side barriers, which are the simplest means to keep the wafer centered on the track. Researchers point out that the contacts serve only to center and guide the floating wafer in the desired direction, so they only touch the outer edge very lightly, leaving very few particles.
"If you need higher precision, wafers have to be aligned by touching the edges very slightly, only to move the wafer on the frictionless surface," Schilp says. "Compared to edge-gripping technologies, the whole wafer surface is supported. Also, in most applications, particle generation is reduced."
Schilp says the system has the advantage of being able to apply even support under an entire ultra-thin 300-mm wafer so it does not sag. Because they are so thin, 300-mm wafers can actually wrinkle up, and may have to be flattened back out by another layer of ultrasonic vibration pushing down from the top. The system also cannot be operated at the natural frequency of any of the materials on the wafer.
"It is possible to transport wafers over long distances and store them without any [human] contact," Schilp says. "Wafer cassettes, and the bound capital inside them, can be avoided. Also, people carrying these cassettes are not necessary anymore. Process stations can be connected in a manner that the wafer is not leaving the local environment at all."
At press time, the Technical University was seeking partners to test and assist in the further development of the technology. "As we are not originally from the semiconductor industry...we do not know all the requirements of the different processes," says Schilp. "The technology is at a state now where we need to find applications and develop specific prototypes for special processes."