The control of water vapor (moisture) contamination in process gases is critical to the successful manufacture of semiconductor devices. As specified moisture levels have become more stringent, there is a growing demand for more sensitive analytical methods. Instrumental methods currently being used or in development for measuring trace moisture at ppbv levels include Fourier transform infrared spectroscopy, tunable diode laser absorption spectroscopy, cavity ringdown spectroscopy, intracavity laser spectroscopy, electron impact ionization mass spectrometry, and atmospheric pressure ionization mass spectrometry. In addition, sensor-based technologies such as oscillating quartz crystal microbalances, and chilled mirror-, capacitor-, and electrolytic-based hygrometers operate in this regime. These approaches are presented and reviewed. As the success of each trace moisture method is dependent on the degree to which the different process gases interfere with the measurement process, important process gas applications of the techniques are highlighted. Advantages and disadvantages as well as future developments and trends are also presented.

Techniques for the measurement of trace moisture in high-purity electronic specialty gases

A significant improvement to the classical least squares (CLS) multivariate analysis method has been developed. The new method, called prediction-augmented classical least squares (PACLS), removes the restriction for CLS that all interfering spectral species must be known and their concentrations included during the calibration. The authors demonstrate that PACLS can correct inadequate CLS models if spectral components left out of the calibration can be identified and if their spectral shapes can be derived and added during a PACLS prediction step. The new PACLS method is demonstrated for a system of dilute aqueous solutions containing urea, creatinine, and NaCl analytes with and without temperature variations. The authors demonstrate that if CLS calibrations are performed using only a single analyte's concentration, then there is little, if any, prediction ability. However, if pure-component spectra of analytes left out of the calibration are independently obtained and added during PACLS prediction, then the CLS prediction ability is corrected and predictions become comparable to that of a CLS calibration that contains all analyte concentrations. It is also demonstrated that constant-temperature CLS models can be used to predict variable-temperature data by employing the PACLS method augmented by the spectral shape of a temperature change of the watermore » solvent. In this case, PACLS can also be used to predict sample temperature with a standard error of prediction of 0.07 C even though the calibration data did not contain temperature variations. The PACLS method is also shown to be capable of modeling system drift to maintain a calibration in the presence of spectrometer drift.« less

/pdf/new-prediction-augmented-classical-least-squares-pacls-5y3uif0mrz.pdf

New prediction-augmented classical least squares (PACLS) methods: Application to unmodeled interferents

Humidity is a physical quantity that evaluates the concentration of water vapor in the air. The detection of humidity has received extensive attention in various fields such as industry, agriculture, meteorology, and national defense. Among the many reported humidity detection methods, humidity sensors stand out due to their advantages of portability, easy fabrication, low cost, high sensitivity, and fast response. It is worth noting that the humidity detection limit is becoming more and more stringent in some special industries, and the dynamic range of humidity measurement can be extended from low relative humidity to ppm or ppb or even ppt level. However, the development of high-performance low humidity sensors still faces great challenges due to the very small amount of water molecules that the sensor can adsorb and the limitation of some harsh environments. So far, the literature on low humidity sensors is scarce, and there is no review on its research progress. In this case, a comprehensive and in-depth understanding of the research progress of low humidity sensors is of great significance, and it may provide new insights to promote the development of low humidity sensors. Therefore, this paper reviewed the research progress of low humidity sensors for nearly three decades. Firstly, the necessity of low humidity detection, trace moisture generators and various low humidity measurement methods were introduced. Then, the research status of different types of low humidity sensors was highlighted, including capacitance, resistance, QCM, MCL, SAW, optical fiber, fluorescence and other types. 

An overview: Sensors for low humidity detection

Water vapor is measured by use of a near-infrared diode laser and wavelength-modulation absorption spectroscopy. Humidity levels as low as 5 nmol/mol [1 nmol/mol = 1 ppb (1 ppb equals 1 part in 109)] of water vapor in air are measured with a sensitivity of better than 0.2 nmol/mol (3σ). The sensitivity, linearity, and stability of the technique are determined in experiments conducted at the National Institute of Standards and Technology, Gaithersburg, Maryland, by use of the low frost-point humidity generator over the range from 5 nmol/mol to 2.5 µmol/mol of water vapor in air. The pressure-broadening coefficients for water broadened by helium [0.0199(6) cm-1 atm-1 HWHM] and by hydrogen chloride [0.268(6) cm-1 atm-1 HWHM] are reported for the water line at 1392.5 nm.

/pdf/wavelength-modulation-laser-hygrometer-for-ultrasensitive-394lg7tj6a.pdf

Wavelength-modulation laser hygrometer for ultrasensitive detection of water vapor in semiconductor gases

A system and method for detection of impurities in gases comprising a Fourier transform infrared spectrometer and computation devices for system control, spectral analysis, and chemometrics.

Specialty gas analysis system

A commercial Fourier transform infrared spectrometer (modified to improve the purge) and a long-path gas cell were used to demonstrate a detection limit of about 10 ppb for water in N 2 , HCl - , and HBr. This technology is being developed as an in-line monitor to extend the life of gas delivery systems and improve wafer yields in the semiconductor industry. The reported detection limit includes an improvement of about a factor of three achieved by applying quantitative multivariate calibration to the problem. Methods are discussed to compensate for background moisture in the beam path. Also, discussed are the choice of operating parameters to optimize the instrumental performance. Surprisingly, resolving the narrow water absorption bands (FWHM 0.20 cm -1 ) is not necessary to achieve optimal sensitivity. In fact, optimal sensitivity is achieved at 2 to 4 cm -1 resolution, allowing the use of an inexpensive interferometer in the final instrument. A calibration over the range of 40 to 2000 ppb of water in N 2 has been performed and found to be in good agreement with published spectral data. Finally, an improved instrument design is discussed which is projected to have a lower detection limit and a shorter collection time.

Trace Water Vapor Detection in Nitrogen and Corrosive Gases by FTIR Spectroscopy

To extend the life of gas delivery systems and improve wafer yields, there is a need for an in-line monitor of H{sub 2}O contamination. Goal of this project is to develop such an instrument, based on infrared spectroscopy, that has a detection limit of 30 ppB or better and costs $$50K or less. This year`s work considered the application of Fourier transform infrared (FTIR) spectroscopy to H{sub 2}O detection in N{sub 2} and HCl. Using a modified commercial FTIR spectrometer and a long-path gas cell, a detection limit of about 10 ppB was demonstrated for H{sub 2}O in N{sub 2} and HCl. This includes about a factor of three improvement achieved by applying quantitative multivariate calibration methods to the problem. Absolute calibration of the instrument was established from absorptivities of prominent H{sub 2}O bands between 3600 and 3910 cm{sup {minus}1}. Methods are described to minimize background moisture in the beam path. Spectral region, detector type, resolution, cell type, and path length were optimized. Resolving the narrow H{sub 2}O bands (FWHM {approx} 0.20 cm{sup {minus}1}) is not necessary to achieve optimal sensitivity. In fact, optimal sensitivity is achieved at 2 to 4 cm{sup {minus}1} resolution, allowing the use of an inexpensive interferometer. A much smaller, second generation instrument is described that will have a conservatively estimated detection limit of 1 ppB. Since the present laboratory instrument can be duplicated in its essential parts for about $$90K, it is realistic to project a cost of $50K for the new instrument. An accessory for existing FTIR spectrometers was designed that may be marketed for as little as $10K.

/pdf/trace-water-vapor-determination-in-corrosive-gases-by-4stsocm4rc.pdf

Trace water vapor determination in corrosive gases by infrared spectroscopy

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Trace Water Vapor Detection in Nitrogen and Corrosive Gases by FTIR Spectroscopy

Trace water vapor determination in corrosive gases by infrared spectroscopy