Inline Metrology and Quality Assurance

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In light of the meteoric growth in production capacity and ever more efficient cell concepts and processes, quality assurance is becoming increasingly significant in solar cell manufacture and, internationally, is becoming a determining competitive factor. Because of the high level of automation and rising throughput of manufacturing lines, efficient quality control requires rapid inline metrology and powerful systems for data acquisition and statistical data analysis.

At Fraunhofer ISE, in our Photovoltaic Technology Evaluation Centre (PV-TEC), we test many different inline measurement methods with regard to their suitability for use in quality and process control under industrial conditions. Using available methods, we are in a position to measure all significant quality parameters on starting wafers, solar cell precursors and finished solar cells, and assign them at wafer level to a wafer tracking system. On the one hand, this enables efficient control of the individual processes and, on the other hand, the provision of comprehensive QC data records on the experiments performed. A direct link between the characterization laboratory and the manufacturing line additionally permits the measurement of random samples for detailed analyses in parallel to the process using many different offline measurement methods.
In addition to quality assurance for research operations in PV-TEC, our research can be divided into three fields:

Metrology evaluation

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PV-TEC provides excellent conditions for qualifying new measuring instruments from manufacturers for use in the photovoltaics industry. Test samples of any form can be produced, such that the full range of samples which occur in practice can be included in measurement suitability testing. Numerous reference methods are available for determining measurement accuracy. Reliability and reproducibility may be determined by temporarily incorporating inline measuring systems into automatic measuring stations and so testing them under industrial conditions. This allows problems to be efficiently identified and eliminated jointly with the manufacturer.

Metrology development

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At Fraunhofer ISE we are developing metrology and methods in various areas, both independently and with partners. Examples include:

  • Photo- and electroluminescence imaging
    Electro- and photoluminescence images are of very great interest for the purposes of 100% inspection during production, owing to the short measurement time and the possibility to determine different cell parameters in a spatially resolved manner. A fundamental problem for industrial use, however, is the ambiguity of the image information. In addition to metrology, Fraunhofer ISE is developing methods for the industrial characterization of wafers and solar cells. The focus here is on the development (i) of inline-capable image recording methods for contrast differentiation and quantitative determination of physical parameters and (ii) of evaluation algorithms for the automated identification of process and material defects.
  • Wafer identification methods
    There is an urgent need for single-wafer tracking on research lines due to the great diversity of products, but it is also becoming increasingly significant in mass production owing to the greater insight it is expected to provide into process interrelationships. Since purely logistical wafer tracking has limited robustness, at Fraunhofer ISE we are developing marking methods and investigating recognition systems for their reliability depending on the code structure used.
  • Measurement blocks for back contact solar cells
    One essential structural element of high efficiency solar cell concepts are the current conveyance contact structures located on the back surface. Electrical measurement of such solar cells requires single-side contacting measurement blocks. Suitable measurement block concepts are being developed at Fraunhofer ISE and produced according to customer requirements.

Production control concepts

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From a production engineering standpoint, research lines such as the PV-TEC line, with its great product diversity and comparatively small batch sizes, place particular demands on a central management system. For example, it must be possible to define complex experimental designs with great depth of variation (job planning) and to implement them in an automated manufacturing process (job performance). Furthermore, the large number of product variants must also be taken into account in statistical data analysis and in process and system monitoring. Because they are tailored for mass production, conventional products do not satisfactorily reflect these requirements. At Fraunhofer ISE, we are therefore developing innovative concepts for efficient operation of research lines, right up to independent software modules, and are trialing these in conjunction with a commercial process and production control system.