Challenges in the Processing and Handling of Silicon Wafers
Photovoltaic systems or solar modules, which consist of the interconnection of individual solar cells, are known to play an important role in renewable energies. The potential through the energy output of the sun is enormous and through the further development of solar technology, the efficiency could already be significantly increased. The most important semiconductor material for the manufacture of solar cells is silicon.
Damage often occurs during the production and further processing of silicon wafers. In addition to downtimes and additional cleaning steps as well as readjustments, there are also increased material and process costs. The smoother the handling, the less likely the material is to crack or break.
Furthermore, both sides of the wafer are now frequently coated, as for example in heterojunction technology, in order to generate higher efficiencies. As a result, the handling processes are also becoming more and more demanding to avoid marks, scratches, or particles. Using ZS-Handling's patented ultrasonic bearing, substrates can float evenly on a film of air generated by vibrations and thus be held without contact during handling. Through a combination of negative pressure and ultrasound, attractive and repulsive forces act simultaneously on the workpiece, thus keeping it at a distance even during transport from above.
How Does the Ultrasonic Technology Work?
The ultrasonic movement of the so-called sonotrode creates a supporting gas film (air or process gas) between the sonotrode surface and the substrate. The substrate floats on the resulting gas film at distances of 10 - 150 micron, depending on the application. In this way any mechanical surface contact is avoided.
In combination with negative pressure and the resulting equilibrium of forces (attractive due to the negative pressure and repulsive due to the ultrasound and the weight force), handling or gripping from above without contact is made possible.The physics of the ultrasonic bearing results from the flow dynamics. The gas pressure in the gap between the workpiece and the vibrating surface increases due to the cyclic compression and decompression of the thin gas film. It is therefore necessary to realize a uniform oscillation pattern in order to generate constant floating forces over the entire sonotrode. The vibrations are not transmitted into the substrates and do not lead to any impairment of the substrate material.
With the repulsive forces of the ultrasonic bearing, the substrate can be supported without any friction even at very high speeds. In combination with vacuum, attractive forces can be applied simultaneously, which allows handling from above. In addition, flexible materials can be "smoothed" without contact by this technique, i.e. they can be kept in a uniform, flat position.The principle of operation of ZS-Handling handling systems is similar to that of a conventional air bearing, but no compressed air supply is required. This means, for example, that in a clean room environment the laminar air flow - unlike with Bernoulli grippers - is not disturbed by high flow velocities and no particles can penetrate through external air or via pipes. Also, the costs for the compressed air supply can be saved in production lines.By avoiding surface contact and without dynamic turbulence in the ambient gas, no damage, micro-scratches, micro-cracks, or contamination can damage the substrate. Handling on machined or coated surfaces is possible without contact, allowing more degrees of freedom in process and machine design. During handling, a high level of flatness of the substrate is achieved as well.
The systems can be used in all atmospheric processes and in up to 20% partial vacuum processes. This requires fewer resources, such as energy or compressed air, than a standard air bearing. This has a positive effect on the energy and cost balance for the handling systems of ZS-Handling.
Exemplary Requirements of a Use Case:
In a cleanroom environment of ISO 6, silicon solar wafers are to be separated from a stack without contact and handled from above before they can be inspected and sorted. They are then placed on matrix trays and coated in the subsequent process. The wafers have a size of 156x156 mm and a thickness of 120-180 µm. A cycle time of 1.8s (2,000 wafers per hour) is to be achieved for the gripping process, with a time for picking up and placing of 0.1s each. For transport via linear belts (from above and below), speeds of up to 7,200 wafers per hour are to be achieved. At the same time, the breakage rate should be minimal with the highest possible machine availability.
Check out the slideshow for more in-depth information on the case study.