TOP > Report & Column > The Forefront of Space Science > 2007 > Quality Assessment of Semiconductor Substrates for Solar Cells Using Photoluminescence - Identify Microscopic Distribution of Defects in Less Than One Second -
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Achieving ultra high-speed, high-accuracy evaluation by the HF-solution immersion method Although the PL imaging method is a powerful one, a little more thought is necessary to monitor entire manufacturing processes. In the ingot or block state at the early-manufacturing stage of cells or in the substrate cut from them, the surface is raw with no special treatment. The surface condition of these samples is poor and photoexcited electrons and holes recombine without emitting light. This makes the PL intensity weak and high-speed, high-accuracy evaluation difficult. As a countermeasure, we noted that hydrofluoric acid (HF) solution, generally used in device manufacturing processes, delivered a good surface condition. We developed the “HF immersion PL imaging method,” where test pieces were immersed in dilute HF-solution as shown in Fig. 2 and quality was assessed by maintaining good surface condition. This method eliminates troubles related to vibration and/or leaks of liquids/gases because the light is illuminated and the PL image is photographed through a transparent plastic container. It also enables short-time measurement with dilute solution, freeing us from safety concerns. Thus, we are able to assess various states of cells including ingot, block, and substrate with high-speed and high-spatial resolution. 
Figure 3 shows a comparison of evaluation results of multicrystalline Si substrate at the same spatial resolution by two methods - traditional method (left) and HF immersion PL imaging method (right). The darker areas indicate poor quality. Dark lines are due to defects called “dislocation clusters,” which particularly degrade the substrate quality. Although it boasted the world’s highest performance, our conventional PL mapping equipment took 40 minutes to examine one test sample. The HF immersion PL imaging method shortens the measuring time drastically to 0.1 second. Furthermore, since the new method is hardly affected by surface condition, it enables much more accurate quality evaluation. 
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