New pneumatic wafer gripping technology that uses a vacuum level of only 3-5 percent (30-50 millibars) compared to 80 percent for suction cup systems provides fast, reliable handling of photovoltaic wafers and cells. Using a large contact area instead of higher pressure levels to grab wafers, the system also offers extremely fast handling capabilities for one-wafer-per-second applications.
"The new SWG wafer gripper applies completely new gripping technology by combining a very low vacuum level with a large effective vacuum area, and full contact gripping of the wafer surface," says Dr. Mathias Kunz, business manager, Solar/Electronics for Schmalz Inc., a leading supplier of vacuum and gripping technology.
The key to the technology is the very large effective vacuum area, and several hundred vacuum openings in the bottom of the gripper which are all exposed to the low vacuum. Since force is pressure differential multiplied by the total area, the design achieves even higher holding forces than other technologies which operate at much higher vacuum levels.
The low vacuum is achieved by using a series of tubes in the bottom of the gripper where, once around the circumference, the circular opening uses a high-velocity, pressured air stream to suck the air from the vacuum chamber. Typically 0.6-1.0 bars of pressurized air (8-15 psi) are sufficient to power the ejector and generate the vacuum needed to safely grab the wafer.
Kunz says next-generation cell manufacturing lines are specified to produce more than 3,600 wafers or cells per hour, translating into one wafer per second. He says a Bernoulli gripper cannot achieve this level of performance, and two or four Bernoulli grippers are often used in parallel to handle the amount of oncoming wafers in the production line.
The SWG wafer gripper typically requires only 20 msec to safely grab the wafer. The large-area coverage provides an ability to specify a very wide range of acceleration or velocity in any direction. Kunz says they tested wafer slippage by running repetitively into an e-stop using parallel kinematics and a camera to record position measurements. After more than 400 attempts to shake the wafer from the grippers, they stopped the experiment because there was no measurable, compounded slippage detected by the camera.
Along with holding force, another concern with pneumatic grippers is the ability to reliably accelerate and move items in any XY direction without slippage. With a large contact surface that is basically the wafer size minus 5 mm of margin, the SWG gripper's area is an advantage because it is approximately 25 percent openings and 75 percent gripping area. The contact area is made of a material called PEEK (polyetheretherketone) which Kunz says is well-known and established in the micro and semiconductor industry as a method for markless handling of silicon products in high-acidic or high-temperature environments.
Kunz says Bernoulli grippers are marketed as contactless grippers, but moving the wafers requires touching them. Grabbing the edges is a challenge because the edges are the most critical point for any wafer, and transmitting forces using rubber or foam pads on the surface can create impurities and stress in the Z direction. Bernoulli grippers also blow high-pressured air onto the wafer surface and into the working area of the handling unit, compared to the SWG approach that sucks a very high-volume stream (150 to 200l/min) out of the working area.
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