Maximizing laser solar cell processing precision and efficiency

Aachen, Germany--The annual output of solar cells fabricated with silicon ribbons in Germany amounts to approximately 50 million wafers, and this figure is expected to rise to 500 million wafers by 2012. This trend will require manufacturers to expand their production capacity while maintaining superlative levels of quality and economic efficiency. Companies from the solar cell technology business and laser specialists involved in fine cutting have joined forces to tackle this challenge in the cooperation project KOMET. One of the aims of the project is to use a newly developed laser concept to increase the throughput of the silicon cells by up to 50% while significantly improving the quality of the products.

This collaborative project started in February, funded by the German federal ministry of economics and technology. The name gives a clue as to what the project aims to develop: a "Compact solid-state laser for efficient material ablation with radially polarized light." Participants in the project include Laser-Laboratorium Goettingen (LLG), the Fraunhofer Institute for Laser Technology ILT, and the Chair of Informatics at the University of Erlangen-Nürnberg, as well as seven industry partners. Together, they plan to develop a modular solid-state laser for precision cutting and drilling by 2012 that features significantly improved beam quality and an increase in cutting efficiency of up to 50%.

A key role is played by the polarization state of the radiation beam when it comes to the quality and efficiency of laser materials processing. This dictates various factors including its focusability. Until now, fine cutting of brittle-hard materials such as silicon has made use of a laser with a circularly polarized beam. In contrast to a linearly polarized beam, the quality of the cut is not dependent on the cutting direction: a laser beam with circular polarization can achieve results in industrial applications that represent the state of the art.

In order to further enhance the coupling efficiency and focusability of the laser beam, independent of cutting direction, the partners in the project are now planning to employ radially polarized light which demonstrates up to 30% better absorption than a circularly polarized beam, thereby reducing coupling losses. Radially symmetric polarization leads to significant improvements in cutting quality.

The example of solar technology illustrates some of the concrete benefits that can be obtained using this innovative concept. 200 micrometer-thin silicon cells (silicon ribbon) are currently manufactured with a kerf width of around 10 micrometers. By using a laser with a radially polarized beam, it is possible to significantly optimize this cutting process in terms of both its efficiency and quality: the cutting process can be accelerated by up to 50%, thereby achieving a corresponding boost in production capacity. Moreover, the cutting precision obtained is substantially higher. Under optimum conditions, the focusing point of the radially polarized beam is up to 60% smaller than that of conventional lasers. This allows the usable surface area of the material being processed to be maximized. The new system also holds great interest for laser dicing of silicon wafers.

The first step is being taken by the overall coordinator of the project, LLG, who will develop an external polarizer to generate radially polarized light. A series of preliminary tests are then set to be carried out by the researchers from Göttingen in collaboration with the University of Erlangen-Nürnberg to examine and optimize the polarizer's functionality, subsequent to which the polarizer will be made available to the Fraunhofer ILT for experimental trials.

In Aachen, the intention is then to test the prototype under conditions similar to those of normal production using the equipment available on site. "In collaboration with our project partners from industry, we will be using the radially polarized laser to carry out experimental cutting of workpieces. Thanks to our expertise and equipment in the field of measuring technology we can then certify the components, thereby laying the bridge between research and the end user," explains Dr. Jens Schüttler, the KOMET project leader at the Fraunhofer ILT. In a further step, the consortium is planning to make a powerful solid-state laser available for industrial use, which will not require any external devices to produce radial polarization. At a wavelength of 1064 nm, the laser will be designed with an output power of a few 100 mW (master oscillator) or of up to 30 W (power amplifier), respectively. Medical engineering is a further field of application for this innovative laser concept, in particular the precise machining of stents.

The following industry partners are involved in the KOMET project: InnoLas GmbH, WACKER SCHOTT Solar GmbH, ADMEDES Schuessler GmbH, Advanced Laser Separation International N.V., LAS-CAD GmbH, FEE GmbH and Schumacher Elektromechanik GmbH.

The contact at the Fraunhofer ILT is Dr. Jens Schüttler,
jens.schuettler@ilt.fraunhofer.de.

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