| 22 February 2011
A 50 kilometre ride from the Dresden Airport brings you the photovoltaic valley of Saxony. German photovoltaic giant SolarWorld has set up a series of production facilities around the city of Freiberg, covering the whole production chain from wafer to cell to module production, and even recycling.
Also some fifty kilometres away from an airport – this time the one of Cologne/Bonn – is Hennecke Systems a supplier of machine vision quality inspection machines for the PV industry. Hennecke, which today is part of the Swiss-based Meyer Burger AG, started in 1985 and has grown with the industry from 10 employees to a current staff of 100 people.
Both companies must have used the flight connections out of the two airports regularly for now more than a decade. “Our first contact into the solar business was as early as 1998. A year later, we delivered the first measurement machine to Bayer Solar GmbH in Freiberg, which later became part of SolarWorld”, says Jürgen Großer, CEO of Hennecke.
Jürgen Großer, CEO of Hennecke“SolarWorld clearly has a leading position in quality and therefore constantly pushes the boundaries of what is technically feasible. We have a lasting relationship to SolarWorld”. A statement that is being confirmed by Dr. Gunter Erfurt, responsible for technology transfer within the SolarWorld group: “Hennecke is part of the success of SolarWorld. There is no other machinery supplier that has such a longer history with us. In this partnership we realized several applications that didn’t exist before and now are standard in the industry, the micro crack detection being the first one to name.” According to Dr. Erfurt, the already existing prime quality level at SolarWorld was advanced once again by the common developments.
Typically, Hennecke machines are at the end of the first production line in the manufacture of wafers, or at the very beginning of the production line of solar cells to perform optical inspection of the wafers. The first system delivered by Hennecke in 1999 measured the parameter thickness, saw marks, and edge defects of the wafers. In a next step, geometry measurement was added to the machine vision system. A milestone was set in 2007 with the common development of one of the first measurement devices to detect micro cracks in the wafers. Detection of relevant micro cracks is a key issue, because the cell production line cannot be stopped immediately in the event of a wafer break. Collateral damage such as the breakage of other wafers is likely.
Vision system
Detection of relevant micro cracks is a key issue, because the cell production line cannot be stopped immediately.
The vision system to detect micro cracks consists of a high resolution infrared CCD-camera, an LED lighting system that is able to penetrate the wafer, and software which can separate the structures of micro cracks from the poly-crystalline structures of the wafer. The software is also able to detect SiC inclusions and separate them from micro cracks. Pixel resolution for the micro crack detection is 50mu/pixel. Because micro cracks never run at right angle in the wafer but take a sharp angle, the correct lighting can magnify and thus detect even cracks with an actual amplitude of 3-4mu in the shadowing.
Microcrack detection in solar photovoltaic wafers
Image courtesy of Hennecke Metrology Systems
For Jürgen Großer, challenges in the development were the speed (production can go as fast as one wafer per second) and finding the right LED lighting. But an even greater challenge was programming the algorithms to create software that is able to separate a micro crack from the poly-crystalline wafer structure.
About 0.5-1.5% in the total wafer production has micro cracks with a dimension that can lead to problems in the production line and cause quality issues. Thus, the first prototype of the micro crack detection system delivered by Hennecke to SolarWorld had configurable software. It enabled the wafer producer after an extensive in-house research process to adjust the parameters according to their specific needs, and by this find the optimal balance of best possible quality and production yield.
Dr. Gunter Erfurt - SolarWorld AGA critical task, according to Großer, because with the increasing sensitivity of the system more and more A-quality wafers are being sorted out. “If the sensitivity gets to high, the error rate rises drastically.”
Therefore it is up to the wafer and cell manufacturers to define the parameters. But because there is as yet no defined standard, a certain conflict of interests regarding the sensivity of these systems remains between wafer and cell manufacturer. SolarWorld is in the position to balance these interests internally and always sees the overall context: As a manufacturer of the total production chain it is important, according to Dr. Erfurt, that the data generated by quality inspection systems is being constantly analysed. “By using statistic process control we can get clear information about micro cracks and SiC inclusions and have precise feedback for production optimisation.”
Stain detection in solar wafers
Another example that reflects this overall thinking is the cooperation between Hennecke and SolarWorld on the issue of stain detection. If the wafers are too dirty, the chemical bath in the cell production wears out too soon. To avoid these costs, another inspection step has been developed to detect stain. Besides the additional software development, a specific lighting solution was developed that lowers the contrast of the poly-crystalline structure and highlights stain. According to Großer, quality improvement in wafer inspection continues, as wafers are becoming thinner and thinner.
Stain detection in solar photovoltaic wafers
Image courtesy of Hennecke Metrology Systems
A specific lighting solution was developed that lowers the contrast of the poly-crystalline structure and highlights stain.
The close cooperation between SolarWorld and Hennecke is seen in their accomplishments: The pv company has set quality standards in the industry and introduced applications to detect almost non-detectable characteristics. And the supplier of high-end metrology with machine vision now sells globally. With an estimated worldwide market share of 70% in wafer inspection, Hennecke now wants to conquer new markets and has an eye on the inspection of solar cells. Großer: “We clearly want to get a good piece of the cake here, too”.Written by Andreas Breyer, Contributing Editor--Germany
For additional information on machine vision in solar wafer inspection, see the related Solar Novus Today exclusive feature article "Solar Cell Inspection: Taking a Closer Look".
