Development of ball surface acoustic wave trace moisture analyzer using burst waveform undersampling circuit

Abstract

The measurement and control of trace moisture, where the water concentration is lower than 1 ppmv [-76.2 °C for the frost point (°CFP)], are essential for improving the yield rate of semiconductor devices and for ensuring their reliability. A ball surface acoustic wave (SAW) sensor with a sol-gel silica coating exhibited useful characteristics for a trace moisture analyzer (TMA) when the temperature drift of the delay time output was precisely compensated using two-frequency measurement (TFM), where the temperature-compensated relative delay time change (RDTC) was obtained by subtracting the RDTC at the fundamental frequency from that at the third harmonic frequency on an identical propagation path. However, the cost of the measurement circuit was a problem. In this study, a burst waveform undersampling (BUS) circuit based on the theory of undersampling measurement was developed as a practical means. The BUS circuit was useful for precise temperature compensation of the RDTC, and the ball SAW TMA was prototyped by calibrating the RDTC using a TMA based on cavity ring-down spectroscopy (CRDS), which is the most reliable method for trace moisture measurement. The ball SAW TMA outputted a similar concentration to that obtained by the CRDS TMA, and its response time at a set concentration in N with a flow rate of 1 l/min was about half that of the CRDS TMA, suggesting that moisture of -80 °CFP was measured within only 1 min. The detection limit at a signal-to-noise ratio of 3 was estimated to be 0.05 ppbv, comparable with that of the CRDS TMA. From these results, it was demonstrated that a practical ball SAW TMA can be realized using the developed BUS circuit.

Moisture adsorption desorption characteristics of stainless steel tubing measured by ball surface acoustic wave trace moisture analyzer

“…41) Four waveforms of 16 bit with a length of 300 points were simultaneously recorded at a sampling rate of 100 MHz using BUS circuit, representing the signals of 80 and 240 MHz at the third turn and the seventh turn. 45) They were recorded every 3 s at a minimum interval under the current BUS protocol, and then the maximum averaging factor was 1000.…”

“…As R y decreased, t s decreased, where t s was 60, 40, and 15 s for the BA, EP, and ECB tubes, respectively. We succeeded in measuring the transient behavior of the ECB tube for the first time by reducing the measurement interval from 14.2 45,47) to 3 s. There was a significant difference in t s between EP and ECB treatments despite the length being only 100 mm. This result implies that the selection of the inner surface treatment is important for constructing highly pure gas distribution systems.…”

“…42) Here, the twofrequency measurement (TFM) was employed, which can precisely compensate for the temperature drift of the delay time output by subtracting relative delay time changes at two frequencies. 43,44) Moreover, the burst waveform undersampling (BUS) circuit was proposed 24) and implemented 45) for the practical use of TFM and employed in a prototype of a TMA. 46,47) This TMA was designed for the concentration monitoring of process and bulk gases at flow rates higher than 1.0 L=min and a time constant of 10 min, where the measurement interval of 14.2 s for averaging 10000 times was sufficiently short.…”

“…However, high-time-resolution measurement is essential for the application to transient analysis such as the measurement of AD characteristics. Although the interval was reduced by decreasing the averaging factor of the BUS circuit, 45) it was not clear whether the temperature-compensated delay time output was obtained with a significant signal-to-noise ratio or not. Moreover, it was reported that the difference in the inner surface treatment of stainless steel tubes could be evaluated by APIMS, for which the samples were 4 m in length with a 1=4 in.…”

Background gas analysis with leaky attenuation in a trace moisture analyzer using a ball surface acoustic wave sensor

“…We have developed a ball SAW TMA with the twofrequency measurement (TFM) of delay time to compensate for the variation in temperature 14,15) as well as to measure the temperature. 16) In the FP range of −60 to −20 °C, the attenuation of a SAW rather than its velocity is used to measure the moisture content.…”