A motion marvel

The fabrication of solar-cell wafers involves coating them using a special plasma deposition process. The wafers are held in graphite 'boats' while undergoing the required thermal process. Extremely high precision is required in loading and unloading these boats. This is a demanding task that can only be mastered using specialised system engineering, high-precision cleanroom robots and special gripper technology. Jonas & Redmann Automationstechnik, of Berlin, is a system house that specialises in automation products for solar cell manufacturing and has developed a system that can fully unload and reload the solar-cell wafers in a vertical boat in 7.5 minutes. In the system a handling device takes wafers from an externally supplied magazine and provides them to a robot for handling. A high-precision Stäubli six-axis long-arm robot (model RX 90 L) takes the wafers from a supply cassette and places them precisely into a graphite boat, in which they will pass through the actual coating operation. Unloading works in the reverse order. "The key feature of this application is the absolute security of the loading and unloading processes," says Lutz Redmann, general manager of the Berlin system house. "Until recently, automatic boat loading and unloading was not considered to be practically feasible. Now we have progressed so far that compared with other systems in this field we do not have any remaining technical problems." And technical progress continues. "As recently as around a year ago, a single boat had a 100 wafer capacity. Now we have reached 144 wafers. This increase was made possible by a tighter arrangement of the plates and narrower spacing in the boat, which naturally places high demands on the precision of the handling."

Optimised process The robot works with a repeatability of 25µm. Over the entire operating envelope of the robot, the required positioning accuracy is 0.1mm. Thanks to the robot's excellent repeatability and the possibility of compensating for the positioning error of the six-axis robot by programming, this requirement can be easily met. The process is as follows: an input buffer is located at the entry port of the system. A handling system upstream of the system places a carrier holding 100 solar-cell wafers on a conveyer belt at the entry port, which transports it to the carrier lift. This is accompanied by non-contact identification using a transponder reader. The solar-cell wafers are located 4.7 mm apart in wafer separator battens. A handling system picks them up one at a time and places them in a supply cassette. The positioning of the wafers is verified by optical sensors. On completion of this step, the robot takes the group of wafers from the supply cassette. For this purpose, it is fitted with a special 12-way gripper system, which enables it to hold 12 wafers using individually controllable groups of suction grippers. The robot gripper is made from a special aluminum alloy originally used in the aircraft construction industry that combines high strength with small dimensions. The wafers are slid against precisely located reference edges in two planes and held in place by vacuum. Precise handling makes it possible to use very small mushroom-head retainer posts in the graphite boat. This allows shadowing of the solar cells in the region of the support points to be almost completely avoided. Before the robot places the solar-cell wafers into the boat, it checks the actual position of the boat using optical sensors. The boat is made up of a stack of 2.5mm thick plates. Each of the plates has fixtures on the right and left to hold the wafers. Extremely high precision is required to place the wafers into the boat. The highly fragile discs are positioned above the fixture positions, rotated by a few degrees, lowered between the plates of the boat and slid in opposite directions until they contact the retainer posts. The striking features of this highly demanding handling task are its precision and the astonishingly gentle and highly accurate motions of the Stäubli robot. The structural features of the 12-way gripper, which is also an in-house product of the Berlin system builder, also come into play in performing this delicate task. The gripper is fitted with a mechanism that detects a possible collision while it is entering into the boat. This is necessary because the boats holding the wafers also receive a coating during the plasma deposition process, which can produce stresses that may distort the boats. Any warpage of the boat could cause a collision inside the very narrow gap that the gripper must enter and major damage would be unavoidable. "After all, a boat like that costs around 15,000 Euros," says Redmann. To rule out the possibility of damage, a displacement mechanism is continually monitored by a sensor along the direction of entry. If there is a risk of hard contact between the gripper and the boat, robot motion is stopped. Once a boat has been fully loaded, it is sent off to the coating system. Following the coating operation, the wafers are removed from the boat by reversing the sequence of motions used to place them in the boat. The robot transfers the unloaded wafers to a removal cassette, in which they are again oriented. A walking-beam transfer device removes the wafers from the cassette and conveys them to the subsequent stations. The wafers can be transported in carriers or blisters or on a conveyor belt. The inlet and outlet ports of the system can be matched to any desired material transport concept. It is also possible to integrate the system into a higher-level batch tracking system. It can also be flexibly configured for several different wafer formats. Formats smaller or larger than the specified sizes are also possible. It is also conceivable to adapt the system to other types of carriers. It is fundamentally possible to build test systems into the system. For example, image processing systems could be used with the wafers to detect things such as colour faults, edge chipping, mechanical damage and scratches. Electrical test stations can also be integrated if necessary, for example in order to make film resistance measurements.

The decisive factors The technology of the system is not dependent on a particular robot manufacturer. "We are not tied to Stäubli," says Redmann, "but we decided on this robot for various reasons. The decisive factors were its compact construction, its standard suitability for use in a cleanroom environment, the high precision of the arm and the multitude of programming options. It's true that these devices are in the upper price class, but they make up only a fraction of the total system cost." The system needs a certain amount of time to take in a loaded boat, handle it inside the system and prepare an empty boat for loading. This means that the time required for preparation must be subtracted when calculating the possible number of processes per hour. The remaining time is used for loading and unloading, which presently takes around 7.5 minutes for a complete boat. In total, approximately 1000 wafers per hour pass through the system. In this regard, the availability of the systems is well above 90%. The Berlin specialists allocate about five months for planning and implementing such a system, since a large amount of work is required to adapt the system to a specific situation. Installation and commissioning at the customer's site can be completed in around three to four days. A few days are also needed to programme the robot. This is normally followed by a one-week trial operation period before the system is ready for provisional customer acceptance. It goes without saying that downtime must be avoided as much as possible with such high-tech systems. Lutz Redmann assures us that measures have been taken to avoid expensive production outages. "In the worst case, we can be on site within 24 hours. However, this is only necessary in extremely rare cases, since we have set up an online help service and most problems can be solved immediately in this manner."