The non-destructive characterization of surface structures on solar wafers is of increasing importance, because it can offer solutions for many problems and concrete applications. From development to production: The demand for high-resolution measurement technology extends along the entire supply chain. As a rule of thumb, the more the structuring progresses in the manufacturing process stepping forward, the more information is needed about details on the surface.
Modern optical surface measuring tools with micro- and nanometer resolution provide contact-free, non-destructive and optionally fully automatic information about the properties of a solar wafer on the front and back as well as on the edge and allow statements about thickness, topography, waviness, film thickness, edge processing and many other parameters. The measurements allow conclusions to be made about surface quality, fragility and efficiency – three parameters that are important not only for wafers that are becoming thinner and thinner, such as thin-film cells with amorphous silicon. This opens up opportunities for wafer producers to save material and increase efficiency, which is an important market advantage for this expensive material. The material savings also pay off during production, as the modules are easier to install due to their lower overall weight and at the same time have less static influence on roof constructions.
Especially in the final inspection of outgoing products from raw wafer manufacturers and the incoming goods inspection of their respective customers, previously agreed specifications must be checked and fulfilled. Wafers that deviate from the specifications, for example due to strong waviness, can lead to problems with structuring and contacting in downstream process steps. For such investigations, the MicroProf® 300 with TTV option is suitable for simultaneous measurement of both sides of the sample. With modular hardware components such as film thickness sensors, the standard system for inspecting thicknesses and surfaces can be adapted to many other complex customer requirements.
Another case in which optical measuring technology is used for production control is the investigation of electrical conductors and contacts (so-called fingers). These are typically silk-screen printed onto the wafers. It is important to have a uniform height and width, since irregularities have an immediate effect on the electrical properties of the cells.
In order to get the last “efficiency percent” out of the conventional cells, the manufacturers are looking for further optimization. Non-destructive surface measurement can help to develop new structures, reduce reflection losses and thus further increase efficiency. In a pyramid shape of the cell surface, for example, the incident light hits the surface several times and thus increases its efficiency.
Thin-film solar cells are an emerging and promising technology alternative to conventional crystalline cells because they require less silicon resources to produce them. Due to the high light absorption of thin film materials and light trap structures as well as back side reflector layers, coatings of a few micrometers of silicon are sufficient for the conversion of sunlight. The task of measuring optically active layers that absorb as much light as possible in a non-destructive way using optical sensors is technically very difficult. Particularly tricky is the optical measurement of anti-reflection coatings, which are supposed to absorb all incoming light. Modular surface measuring tools are the solution. With its multi-sensor technology, FRT offers the possibility of combining point and area sensors with atomic force microscopy, depending on the individual task, to create a measuring tool that suits the application perfectly.
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