Sensors & transducers Tightening the loop


In high value product vacuum process manufacturing, accuracy is of great importance but so too are repeatability and reproducibility. Jonathan Mitchell, market development manager at HORIBA UK, discusses how best-in-class instrumentation is essential for precision closed-loop control


Figure 1 – Above, how an RGA works.


W


hen fluids play a crucial role - as an ingredient, catalyst or for cleaning purposes - in a vacuum process


manufacturing step, their accurate control, monitoring and analysis are essential. The instrumentation employed tends to be integral to a closed-loop control system which must be precise and highly responsive, certainly if consistent high quality is to be realised. Moreover, when manufacturing high value


products (HVPs) – in the semiconductor and pharmaceutical sectors, for example – there is an increased dependency on not only the accuracy and reliability of all process steps but also their repeatability and reproducibility. In this respect, repeatability relates to variations when measurements are made under identical conditions and reproducibility relates to variations when the measurement conditions are different. Here, think of striving for two production lines with consistent and identical performance. The same product could be made on either line. Understandably, HVP manufacturing


involves increasingly complex process steps, so let us consider some of the


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instrumentation technologies employed. For the provision of a vaporised liquid into a vacuum chamber you would typically have, in order: a mass flow controller (MFC) supplying a liquid precursor to a vaporiser; the vaporiser; a vapour/gas concentration monitor and then another MFC for controlling gas/vapour flow. The gas/vapour would then be fed into the vacuum chamber, which would be fitted with a residual gas analyser (RGA – see figure 1) and, in its exhaust line, a gas concentration monitor. When integrated into a closed-loop control system the above can provide testimony to the fact that the materials are being delivered to the vacuum chamber in correct ratios/concentrations and at the correct flow rates plus, during processing, the intended reactions are taking place; be they deposition, etching or chamber cleaning.


GReAt expectAtIOns


Much is demanded of the instrumentation used in the precision closed-loop control of HVP manufacturing processes. For instance, most MFCs are calibrated to control the flow of a specific liquid over a set range. However, digital


MFCs that are multi-fluid and multi-range are proving increasingly popular. Benefits include easier inventory control plus the MFCs can be easily configured in-line via PROFIBUS, for example. Also, the price point for such devices is falling, and the cost difference makes them a practical alternative to their single-fluid, single-range counterparts. Fortunately, this greater versatility has not come at the expense of accuracy, repeatability or reproducibility. Indeed, users are increasingly demanding that MFCs are calibrated and certified under ISO/IEC 17025. In the same way the ISO/IEC 9000 series is a quality management system, ISO/IEC 17025 specifies the requirements for the technological management necessary for calibration; including traceability (see figure 2) and the technical skills of calibration personnel. One of the standard’s objectives is to provide assurance that recorded test results are within specific levels of uncertainty, i.e. margins of error between what is measured and actual. For example, HORIBA-STEC’s production


unit MFCs are calibrated in-line against master units (or working standards) which


February 2019 Instrumentation Monthly


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