MICRO: Critical Materials

Andrew O. Wright and Clayton D. Wood, and Kimberly J. Reynolds and Mark L. Malczewski, n increasingly important chemical used in the manufacture of electronic devices such as high-intensity light-emitting diodes (LEDs) is ultrahigh-purity (UHP) anhydrous ammonia. However, the presence of residual trace moisture in UHP ammonia is a matter of concern. Especially in blue and white LEDs, there is a strong correlation between device performance and moisture content in the process ammonia used during manufacturing. In addition, increasing chemical consumption is forcing many facilities to use bulk storage tanks as a single supply source instead of cylinders as supply sources for individual tools. In light of these trends, it is imperative to have a robust, continuous process instrument with a single-digit parts-per-billion detection limit that can measure moisture in ammonia. The challenge posed by moisture in ammonia has driven the development of continuous-detection instrumentation. Because ammonia and water are chemically similar, spectroscopic methods have proven to be the most promising option for detecting trace moisture in ammonia down to parts-per-billion levels. Since the mid-1990s, Fourier-transform infrared spectroscopy (FTIR) has been the method of choice for detecting these low levels of moisture. The performance of FTIR-based moisture measurement in ammonia has been enhanced by improved methods for handling ammonia samples and optimized spectral collection and data processing techniques. Dual-cell near-infrared absorption spectroscopy using a tunable diode laser over a narrow spectral bandwidth is another method for detecting trace spectroscopic levels of moisture in ammonia. This article describes the design and operation of a novel IR spectroscopic instrument, the DF-740 moisture analyzer, from Delta F (Woburn, MA). It also discusses the design of a calibration system for the generation of low levels of moisture in ammonia standards. Tunable Diode Laser Absorption Spectroscopy For use in process applications in which maintenance and user