A marked improvement in the reliability of the measurement of trace moisture in gases

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Courtesy of Moisture Control & Measurement Ltd

Introduction
Water is an important substance without which mankind cannot survive. However, it also causes problems such as residual impurity in scientific experiments and in manufacturing processes where a high-vacuum condition or a high-purity gas is required. This is because water is present in the atmosphere in large quantities and therefore can intrude readily everywhere, and can adsorb quickly onto the surfaces of many materials. Because of these characteristics, water is considered to be an extremely troublesome impurity that is difficult to remove. Therefore, in any scientific experiment or manufacturing process, if a high-vacuum condition or a high-purity gas is required, it is necessary to address the effect of residual moisture at some point. In fact, in the field of semiconductor manufacturing, the high purity of the gas used in manufacturing is essential with the rapid progress in the integration and refinement of devices, and the control of trace-level residual moisture in the gas has become a major issue. For example, in the case of the high-purity nitrogen gas used to prevent contamination of the wafer surface, the control of residual moisture to several nmol/mol (ppb) or less in amount-of-substance fraction (mole fraction) is considered necessary' , corresponding to a frost point eno of -100 °C or less at atmospheric pressure. As another example, in the case of a gallium nitride (GaN) light-emitting diode, a significant decrease in light-emitting efficiency was observed in the presence of trace residual moisture of 1 nmol/mol (ppm) or less in an NH3 gas used in the film growth by metal-organic vapor-phase epitaxy (MOVPE). In any case, a highly reliable moisture measurement is essential to control the trace level of moisture and to correctly assess the effect of residual moisture.

Figure 1 shows the humidity range for which the national standard has been established in Japan. Several types of measuring instruments'] for measuring the moisture content in gas (humidity) at various ranges are available commercially and are used in many production sites and scientific experiments. However, in the range of trace moisture of 1 umol/mol (ppm) or less, the measured value of humidity may vary with the instrument used even if the measurement is performed under the same conditions. When such issues arise, they are usually resolved by calibrating the measuring instruments on the basis of the national standard, but there has been no national standard in the trace moisture region until recently, and the calibration of instruments is not straightforward. Hence, when the calibration is essential, the users or measuring instrument companies have had to attempt the calibration by some method that they devised on their own. However, the mismatch of values indicated by the instrument occurred frequently even when the calibration was performed in this way, and the measurements of trace moisture have not been particularly reliable.

To settle this issue, the National Institute of Advanced Industrial Science and Technology (AIST) established the national standard for trace moisture (trace moisture standard); the standard with the highest metrological quality called the primary measurement standard, erm which is directly traceable to the International System of Units (SI), was developed. Furthermore, a system for providing the trace moisture standard (calibration service) was organized, as illustrated in Fig. 2. In the process of developing the trace moisture standard, it was found that a new trace moisture analyzer based on cavity ring-down spectroscopy (CRDS) had excellent capability for measuring trace moisture. Because of these three reasons, namely, the national standard for trace moisture was established, the calibration service system traceable to the SI was organized, and a high-performance measuring instrument became available commercially, the reliability of trace moisture measurement has very recently increased markedly. Simultaneously, the problems concerning the methods conventionally used for measuring trace moisture have been revealed.

This paper describes the scenario adopted by AIST for increasing the reliability of trace moisture measurement, research activities toward achieving this objective, and the recent changes in the reliability of trace moisture measurement brought about by the results of this research.

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