METAL CUTTING - Waterjet-guided laser cuts solar cells

Synova SA (Lausanne, Switzerland) continues to explore new applications for its waterjet-guided laser-cutting process. The Laser Microjet technique, originally developed at the Swiss Polytechnical Federal Institute (Lausanne), has already proven its processing capabilities in otherwise problematic laser-cutting materials, such as shape memory alloys, Inconel, and silicon wafers (see Laser Focus World, Dec. 1998, p. 26). Now it has taken on solar-cell production.

In a recent application study, Synova worked with ITW Institute for Innovative Technologies (Chemnitz, Germany) and Solarwatt GmbH (Dresden, Germany) to study different ways of processing silicon solar cells. Generally, companies cut these cells by scribing, followed by breaking-a procedure restricted to straight-line cutting. Even with limited processing flexibility, end users still require cells in various shapes for assembly into photovoltaic cells.

Alternative manufacturing methods examined by the researchers included waterjet cutting, conventional laser cutting, and waterjet-guided laser cutting. Results varied widely among the different techniques. Waterjet cutting caused damage such as cracks and chips. Conventional laser cutting produced side effects such as slugs. Waterjet-guided laser cutting generated clean and narrow cuts, even across superficial strip conductors. It also allowed cutting in arbitrary contours. The technique had no effect on the electrical functions of the treated cells.

How it works

The Microjet process couples a laser beam into a very small waterjet via a special interface. The waterjet then guides the laser power directly to the workpiece without further focusing. Radiation with a power density up to 50 MW/cm² can be guided. The laser is typically a pulsed (up to 1 kHz) Nd:YAG laser with average power of 300-500 W and a peak power of 10-20 kW.

The waterjet is generated by a hydraulic unit that operates at pressures of 20-500 bars. Diamond nozzles establish a laminar flow in the jet to provide stable light-guiding. The maximum flow rate is 1 l/min.The jet guiding the laser beam can have a diameter as thin as 0.1 mm while traversing a distance of 100 mm or 0.05 mm over 50 mm. Hence, the depth of focus is extremely high. A video camera monitors the performance to control the parameters necessary for flawless beam-guiding. Several processing parameters are under full computer numerical control. A worktable, for example, can position the beam with accuracy of 5 µm, even with a high travel velocity.

According to Synova, typical materials that can benefit from Microjet cutting include stainless steel and aluminum sheets up to 1 mm thick, ceramics to 5 mm thick, polyethylene to 10 mm thick, and fiber composites to 6 mm thick. Cutting speeds tend to exceed or equal those of conventional laser cutting in thin materials-up to 1 mm-because of more-efficient expulsion of the liquified material. Speeds will be somewhat lower for thicker sheets, but the company believes cuts should provide higher edge quality.