YIELDANALYSIS
Statistically improved
There are many jokes about statistics and how such information can be manipulated to meet any ends but analysing manufacturing statistics effectively is no laughing matter. An enormous amount of information is collected by industrious metrology tools during the manufacturing process leaving operators with a mountain of information that is relatively useless unless appropriate analysis is applied. Eddie Renouf of Thermco Systems UK, a division of Tetreon Technologies, discusses the use of statistical analysis to improve PV device yields.
emiconductor device manufacturers whether they produce simple discrete transistors or advanced microprocessors with millions of transistors and highly complex multi metal layer designs, will use every tool available to help fine tune device processes, improve yield and prevent scrap events. This process is driven by the increasing WIP value of a wafer as it progresses through the various productions steps. Reducing process problems has as a result always been the number one priority for a device yield engineer. A failed process step can mean a scrapped batch of wafers with each wafer having borne the cost of production up to the point of the scrap event. As a consequence, there are huge savings to be made by reducing these catastrophic events. Whilst the number of steps and complexity of a Photovoltaic cell are much lower than for example a microprocessor, the batch size and
S throughput are much greater.
As a result the potential loss resulting from product which is out of specification or equipment downtime is significant.
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It is therefore unsurprising that PV cell manufacturers are now showing increasing interest in tools that track process reliability and predict potential problems. Statistical Process Control (SPC) is a statistics based methodology, measuring process performance within pre-defined limits, traditionally known as the upper control limit (UCL) and lower control limit (LCL). The Motorola ‘Six Sigma’ (6Û) program is a well known example of standard setting employing SPC methodology. A six sigma process is defined as one in which
www.solar-pv-management.com Issue VII 2011
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