Ball Surface Acoustic Wave Sensor Research Articles

Dynamic calibration method for a trace moisture analyzer based on the quick response of a ball surface acoustic wave sensor

The measurement of trace moisture is important in industry to maintain the quality and yield of products, such as semiconductors, lithium-ion batteries, and organic electroluminescence devices. In the field, on-site calibration of a trace moisture analyzer is required to maintain its reliability. Given these circumstances, we previously developed a ball surface acoustic wave (SAW) sensor using SAW on spherical piezoelectric elements and reported its quick response as its advantage over other trace moisture analyzers. However, the ultimately short response time of less than 1 s has not been verified, and it has not yet been calibrated on-site. In this study, we developed a system for the injection of saturated water vapor to evaluate the quick response of a trace moisture analyzer, and we verified the quick response time of as short as 0.64 s. It is the shortest response time reported so far. In addition, to realize its on-site calibration, we developed a dynamic calibration method, which takes only 10 min, by the injection of saturated water vapor, in contrast to the presently available static calibration method that takes 10 h. Since this system consists of simple components, it can be downsized. Moreover, because it uses a small amount of saturated water vapor as the calibration gas, it is easy to prepare calibration gases in the field and may be applied to on-site calibration.

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

A method is described for background gas (BG) analysis in surface acoustic wave (SAW) gas sensors using two frequencies. The gas parameter G is defined as the square root of molecular weight times the ratio of specific heat at constant pressure to specific heat at constant volume, which is a representative property of BGs. In an experiment using varying BGs, the measured G values agreed with the true values. The concentrations of mixed BGs were successfully measured. This method will be useful in a trace moisture analyzer in natural gases and in the optimization of the purge process of glove boxes.

Simultaneous measurement of gas concentration and temperature by the ball surface acoustic wave sensor

We have developed a ball surface acoustic wave (SAW) trace moisture sensor with an amorphous silica sensitive film and realized wide-range measurement from 0.017 ppmv [a frost point (FP) of −99 °C] to 6.0 × 103 ppmv (0 °C FP). However, since the sensitivity of the sensor depends on the temperature, measurement results are disturbed when the temperature largely changes. To overcome this problem, we developed a method to simultaneously measure temperature and gas concentration using a ball SAW sensor. Temperature and concentration is derived by solving equations for the delay time change at two frequencies. When the temperature had a large jump, the delay time change was significantly disturbed, but the water concentration was almost correctly measured, by compensating the sensitivity change using measured temperature. The temperature measured by a ball SAW sensor will also be used to control the ball temperature. This method will make a ball SAW sensor reliable in environments of varying temperatures.

Deep Sub-micro $$\hbox {mol}{\cdot }\hbox {mol}^{-1}$$ mol · mol - 1 Water-Vapor Measurement by Dual-Ball SAW Sensors for Temperature Compensation

A collimated surface acoustic wave (SAW) circles around the equator of a sphere hundreds of times. Because of the long distance travel of the collimated SAW, a small change in the SAW propagation caused by the environment of the sphere can be accumulated as a measurable range in amplitude and/or in delay time. So, a spherical SAW device enables highly sensitive water-vapor measurements. In this paper, deep sub \(\upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}\) water-vapor detection by 1 mm diameter quartz crystal ball SAW sensors is described. To measure such a low water-vapor concentration in real time, it is necessary to compensate the temperature dependence of the ball SAW sensor, which is about 20 \(\hbox {ppm}{\cdot }^{\circ }\hbox {C}^{-1}\) in delay time change. A dual-frequency burst analog detector was developed for the temperature compensation in real time. By using a harmonic SAW sensor, which was excited by 80 MHz and 240 MHz at the same time, it was confirmed that the delay time drift for a temperature range of \(21.0\, ^{\circ }\hbox {C} \pm 1.0\,^ {\circ }\hbox {C}\) became less than 0.05 ppm in delay time change. By using dual-ball SAW sensors (which included a 150 MHz sensor with a water-vapor sensitive layer and a 240 MHz sensor as a reference), water-vapor concentrations from 0.1 \(\upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}\) to \(5\; \upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}\) were successfully measured. It appears that the delay time change is proportional to the square root of the water-vapor concentration. The detection limit determined by the electrical noise of the system was estimated at \(0.01\; \upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}\).

Temperature compensation of ball surface acoustic wave sensor by two-frequency measurement using undersampling

To realize a practical two-frequency measurement (TFM) system for precise temperature compensation in a ball surface acoustic wave (SAW) sensor, the application of undersampling (US) was investigated. The subtraction coefficient for the temperature compensation in US was theoretically explained. The principle of the TFM system using US was simulated by the decimation of the oversampling (OS) waveform after applying a narrow band-pass filter, and the delay time was measured using a wavelet transform. In the application of the method to trace moisture measurement, the delay time response due to US matched that due to OS with a correlation coefficient higher than 0.9999. Although rms noise was increased by US, the response to the concentration change of 4–17 nmol/mol was measured with a signal-to-noise ratio higher than 20. From these results, it was shown that the precise output of the ball SAW sensor could be obtained even when using US, which was equivalent to that using OS.

Highly sensitive trace moisture ball surface acoustic wave sensor using SiOx film

Regarding the gases for the manufacturing process of semiconductor devices, fast and sensitive sensors are needed to control trace moisture to less than 1 µmol/mol. A ball surface acoustic wave (SAW) sensor is promising technology for this purpose. In this study, we verify the response of a sol–gel SiOx film-coated ball SAW sensor to extremely low-concentration moisture using a cavity ring down spectroscopy (CRDS), the most sensitive commercially available moisture sensor, as a reference sensor. We clearly measured 17 nmol/mol moisture by developing a sensor cell using metal O-rings reducing the leakage of moisture. The response time from 390 to 790 nmol/mol was one tenth of that for the CRDS, and the detection limit was estimated to be as low as 0.2 nmol/mol.

Detection of Trace Water Vapor Using SiO<i><sub>x</sub></i>-Coated Ball SAW Sensor

In manufacturing process of semiconductor, blue light emitting diode (LED), and lithium ion battery, detection and monitoring of trace water vapor (<1μmol/mol) are significantly important techniques. We have been researching about detection of trace water vapor with high speed and sensitivity by using ball surface acoustic wave (SAW) sensor. In this paper, we tried to detect trace water vapor by coating silicon oxide (SiOx) on ball SAW sensor using sol­gel method. It was confirmed that SiOx-coated ball SAW sensor could detect water vapor in a wide range from 195 to 0°C in frost point (0.035­6.0 © 103 μmol/mol in concentration) and its response time to frost point change from 170 to 160°C was one half of that of a chilled-mirror hygrometer. [doi:10.2320/matertrans.I-M2014807]

Highly Sensitive Ball Surface Acoustic Wave Hydrogen Sensor with Porous Pd-Alloy Film

In order to commonly use explosive H2 gas as an energy source, fast, sensitive, and low-power consumption sensor is required. We developed a ball surface acoustic wave (SAW) sensor with porous Pd­Pt alloy film to realize such a sensor. The sensor with 20% Pt alloying film was useful for the suppression of a phase transition of Pd and the detection of ppm order concentration at 35°C. The amplitude response was proportional to the square root of the concentration, which was demonstrated for the first time in the field of SAW sensor, resulting in the detection limit of 3.7ppm at signal to noise ratio of 3. The response time decreased to 1/5, compared with those of pioneering H2 sensors working at room temperature. From these results, it was shown that the fastest and most sensitive hydrogen sensor working at room temperature could be realized using the ball SAW sensor with porous Pd alloy film. [doi:10.2320/matertrans.I-M2014816]

Continuous Measurement of Multiple Gases Using Ball Surface Acoustic Wave Gas Chromatograph

Although portable gas chromatographs (GCs) have been developed for the monitoring of volatile organic compounds (VOCs) in working environments, they still need high power consumption for the heating column. Thus, we previously developed a portable surface acoustic wave (SAW) GC equipped with a ball SAW sensor and a micro-electromechanical-system column (ball SAW GC) and proved the usefulness of the forward flush (FF) method for realizing the fast analysis of multiple gases without a heater. However, its ability to measure ten kinds of VOCs at ppm order and automatic continuous measurement were not demonstrated. In this study, a ball SAW GC employing the FF method and equipped with a gas sampler for continuous injection was developed. Then, the performance of monitoring multiple gases in working environments was verified by measuring ten kinds of VOCs with maximum acceptable concentrations. Moreover, real-time monitoring of seven kinds of VOCs with a linear change in the response value to concentration changes was demonstrated.

Development of High Precision Metal Micro-Electro-Mechanical-Systems Column for Portable Surface Acoustic Wave Gas Chromatograph

In the field of environmental measurement and security, a portable gas chromatograph (GC) is required for the on-site analysis of multiple hazardous gases. Although the gas separation column has been downsized using micro-electro-mechanical-systems (MEMS) technology, an MEMS column made of silicon and glass still does not have sufficient robustness and a sufficiently low fabrication cost for a portable GC. In this study, we fabricated a robust and inexpensive high-precision metal MEMS column by combining diffusion-bonded etched stainless-steel plates with alignment evaluation using acoustic microscopy. The separation performance was evaluated using a desktop GC with a flame ionization detector and we achieved the high separation performance comparable to the best silicon MEMS column fabricated using a dynamic coating method. As an application, we fabricated a palm-size surface acoustic wave (SAW) GC combining this column with a ball SAW sensor and succeeded in separating and detecting a mixture of volatile organic compounds.

Development of Multiple-Gas Analysis Method Using the Ball Surface Acoustic Wave Sensor

In the fields of energy exploitation, and for the safety and security of society, portable multiple-gas sensors are essential. Although the gas chromatograph (GC) is applicable to multiple-gas analysis, it is not portable. As a result, we have proposed the ball surface acoustic wave (SAW) GC using a ball SAW sensor, which realizes high sensitivity by the ultramultiple roundtrip propagation of a SAW and a separation column fabricated by micro-electro-mechanical-system technology. However, it takes too long for a portable GC such as the ball SAW GC to analyze mixed gases of high and low molecular weights at the same time. In this study, we developed the forward-flush (FF) method, involving two (or more) separation columns with a ball SAW sensor inserted after each column, to solve the above problem. Using the ball SAW GC with the FF method, we analyzed mixed gases of natural gas components and volatile organic compounds.

Fabrication of Thin Sensitive Film of Ball Surface Acoustic Wave Sensor by Off-Axis Spin-Coating Method

In environmental assessment and security, it is necessary to detect hazardous gases at low concentrations. The ball surface acoustic wave (SAW) sensor can have significantly improved sensitivity when the ultramultiple roundtrips (e.g., >100 turns) of the SAW is achieved. However, it is hindered by the thick and coarse organic sensitive film fabricated by the conventional spin-coating method. We propose an “off-axis spin coating method” in order to fabricate thin and highly uniform organic sensitive film with high reproducibility. Langasite ball SAW sensors were coated by polyisoprene, polyisobutylene, and polydimethylsiloxane sensitive films and were applied in detecting normal saturated hydrocarbons (pentane, hexane, heptane, octane, and nonane), a nerve gas simulant (dimethyl methylphosphonate: DMMP), and sick-house gases (benzene, toluene, and xylene). With the proposed method, we succeeded in realizing multiple roundtrips and in detecting DMMP at a sub-ppm concentration without preliminary condensation of the sample.

Development of Packed Column for Surface Acoustic Wave Gas Chromatograph Using Anodically Bonded Silicon–Glass Structure with a Compression Jacket

Gas or liquid chromatographs (GC/LCs) are frequently used for multiple-compound sensing, but they are too large and heavy to be portable. To provide a handy GC/LC, many groups have proposed micro GC/LCs in which large columns are replaced by small micro-electro-mechanical-system (MEMS) columns. However, because of the high packing pressure, a packed MEMS column using an anodically bonded structure has not been realized so far. In this study, we realized such a MEMS column for the first time by introducing a compression jacket to reduce the tensile stress at the silicon–glass boundary. Consequently we succeeded in the separation and detection of lower hydrocarbons using a ball surface acoustic wave sensor.

Principle and application of ball SAW devices

AbstractA thin beam of waves usually diverges due to diffraction, which is a limitation of any device using such waves. However, a surface acoustic wave (SAW) on a sphere with an appropriate aperture does not diverge but is collimated, realizing ultra‐multiple roundtrips along an equator of the sphere. This effect is caused by the balance between diffraction and focusing on a spherical surface, and it enables realization of high‐performance ball SAW sensors. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(1): 41–51, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20817

Principle and application of ball surface acoustic wave (SAW) sensor

Detection of hydrogen gas is a crucial task for establishing safety and reliability of fuel cells, a key technology for the environment and our society. However, hydrogen is difficult to detect and various hydrogen sensors have many drawbacks. Here we report a novel hydrogen gas sensor, the ball surface acoustic wave (SAW) sensor, using Pd or PdNi sensitive film. The ball SAW sensor is based on a novel phenomenon, diffraction-free propagation of collimated beam along an equator of sphere. The resultant ultra-multiple roundtrips of SAW makes it possible to achieve highest sensitivity among SAW sensors. Moreover, it enables to use a very thin sensitive film, and consequently the shortest response time (2 s) was realized. In terms of the sensing range, it has the widest range of 10 ppm to 100% among any hydrogen sensors including FET or resistivity sensors. The response time was less than 1 s for 3.0% hydrogen concentration in nitrogen gas, evaluated by using a newly developed digital quadrature detector.

Development of Microseparation Column for Ball Surface Acoustic Wave Gas Chromatograph

For the safety and security of a society, ubiquitous multiple-gas sensors are required. The gas chromatograph (GC) based on the separation of gases according to the elution time is frequently used for multiple-gas sensing, but it is not portable. In this situation, we have developed a ball surface acoustic wave (SAW) sensor, where SAW on a sphere with a specific width is naturally collimated and makes multiple roundtrips without diffusing by diffraction. In this study, in order to develop a portable GC, we developed a gas separation column using micro-electro-mechanical-system (MEMS) technology and evaluated it, combining it with a ball SAW sensor.

Observation of Surface Acoustic Wave Propagation on a Quartz Ball with Concave Proximate Electrodes

We evaluated the characteristics of roundtrip propagation on a ball surface acoustic wave (SAW) device by controlling the approach of electrodes to the quartz ball. In order to investigate the relationship between the interdigital transducer (IDT) position and the exponential attenuation in the roundtrips of SAW, we brought a concave glass plate with an IDT towards the equatorial belt (Z-axis cylinder) of the quartz ball, and transmitted and received a SAW. In this study, the precision of the apparatus was improved, and as a result, it was found that roundtrip of SAW attenuates exponentially when the IDT is at a certain position. The specific positions were not always at the Z-axis cylinder but distributed around it with the meander belt with a period of 120°. By fabricating an IDT at such positions, it is expected that the performance and reproducibility of ball SAW sensors will be enhanced.

Analysis of Ball Surface Acoustic Wave Sensor Response to Wide Variety of Gases Using Gas Chromatography

It is necessary to detect a wide variety of dangerous gases for environment assessment and security. Gas chromatography using a gas separation column has been used for this purpose; however, a compact and high-performance system has not yet been established. Thus, we propose a new approach to measure gases using a ball surface acoustic wave (SAW) sensor, after separating them using gas separation columns. In this work, amplitude responses were observed for mixed alcohol gases. With water–ethanol mixed gas, both delay time and amplitude responses were observed, and different response and recovery times were found for these responses. Also, an amplitude response reflecting the leaky loss of SAW was observed and an approximate equation of leaky loss was derived to quantitatively evaluate the roles of leaky loss and other parameters observed in the ball SAW sensor. These results suggest that the proposed approach is promising for developing compact and high-performance multigas sensors.

Acoustic dispersion in a ball-shaped surface acoustic wave device

Theoretical model for ball surface acoustic wave (SAW) sensors has been developed. Wave front distortion encountered in planar SAW sensors caused by diffraction or reflection can be neglected because of the diffraction-free propagation of a naturally collimated beam. Wave form at each turn is expressed by an integral of terms defined by the array factor and the elementary charge density of electrodes as well as the frequency-dependent velocity and attenuation caused by the sensitive film. Irrespective of the significant simplicity of the theory, excellent agreement has been observed between the simulated and experimental wave forms of a double-electrode ball SAW sensor.

Principle and Application of Ball SAW Devices

A thin beam of wave usually diverges due to diffraction, which is a limitation of any device using such waves. However, a surface acoustic wave (SAW) on a sphere with an appropriate aperture does not diverge but is collimated, realizing ultramultiple roundtrips along an equator of the sphere. This effect is caused by the balance between diffraction and focusing on a spherical surface, and it enables realization of high-performance ball SAW sensors.