In-line measurement of “trench structures” caused by the texturization of mc-silicon solar cells

Acidic texturing of multicrystalline silicon (mc-Si) wafers often leads to rough surfaces with strong etch attacks (trench structures), especially at sites with crystal defects. The appearance of trench structures on the wafer surface has been cited in earlier publications and has been often recognised as being harmful to solar cell performance [Mathijssen et al., 2009]. In this work, an in-line measurement method for these structures is presented, using a line camera system with diffuse illumination. The presented method can be used for the in-line quality control of the acidic texturization. The number of trench structures is quantified in the images via a newly developed algorithm by an adaptive threshold method. With the help of AFM images the measurement method could be regarded in detail. It has been shown that the true area fraction of trench structures only lies between 0.3% and 1.8% instead of 2–12%, which is estimated from measurements. As expected, the number of trench structures strongly depends on the texture strength and the number of crystal defects in as-cut material. Therefore, if the texturization method remains constant, it is possible to easily measure material quality by measuring the fraction of trench structures and the presented measurement method could be used as cheap alternative to photoluminescence measurements. The reliability of the algorithm is demonstrated by the correlation between material quality, texture strength and the resulting number of trench structures.