Liquid film thickness measurement underneath a gas slug with miniaturized sensor matrix in a microchannel

Measurement of liquid film thickness is essential for understanding the dynamics of two-phase flow in microchannels. In this work, a miniaturized sensor matrix with impedance measurement and MEMS technology to measure the thin liquid film underneath a bubble in the air–water flow in a horizontal microchannel has been developed. This miniaturized sensor matrix consists of 5 × 5 sensors where each sensor is comprised of a transmitter and a receiver electrode concentrically. The dimension and performance of the sensor electrodes were optimized with simulation results. The maximum diameter of the sensor ring is 310 µm, allowing a measurable range of liquid film thickness up to 83 µm. These sensors were distributed on the surface of a wafer with photolithography technology, covering a total length of 8 mm and a width of 2 mm. A spatial resolution of 0.5 × 2.0 mm² and a temporal resolution of 5 kHz were achieved for this sensor matrix with a measurement accuracy of 0.5 µm. A series of microchannels with different heights were used in the calibration in order to achieve the signal-to-thickness characteristics of each sensor. This delicate sensor matrix can provide detailed information on the variation of film thickness underneath gas–water slug directly, accurately and dynamically.     

Fabrication of micro sensors on a flexible substrate

Flexible micro temperature and humidity sensors on parylene thin films were designed and fabricated using a micro-electro-mechanical-systems (MEMS) process. Based on the principles of the thermistor and the ability of a polymer to absorb moisture, the sensing device comprised gold wire and polyimide film. The flexible micro sensors were patterned between two pieces of parylene thin film that had been etched using O₂ plasma to open the contact pads. The sacrificial Cr spacer layer was removed from the Cr etchant to release the flexible temperature and humidity sensors from the silicon substrate. Au was used to form the sensing electrode of the sensors while Ti formed the adhesion layer between the parylene and Au. The thickness of the device was 7 ± 1 μm, so the sensors attached easily to highly curved surfaces. The sensitivities of the temperature and humidity sensor were 4.81 × 10⁻³ °C⁻¹ and 0.03 pF/%RH, respectively. This work demonstrates the feasibility and compatibility of thin film sensor applications based on flexible parylene. The sensor can be applied to fuel cells or components that must be compressed.     

Assessing the effect of hydrocarbon oil type and thickness on a remote sensing signal: A sensitivity study based on the optical properties of two different oil types and the HYMAP and Quickbird sensors

We measured the light absorption properties of two naturally occurring Australian hydrocarbon oils, a Gippsland light crude oil and a North West Shelf light condensate. Using the results from these measurements in conjunction with estimated sensor environmental noise thresholds, the theoretical minimum limit of detectability of each oil type (as a function of oil thickness) was calculated for both the hyperspectral HYMAP and multispectral Quickbird sensors. The Gippsland crude oil is discernable at layer thickness of 20 µm or more in the Quickbird green channel. The HYMAP sensor was found to be theoretically capable of detecting a layer of Gippsland crude oil with a thickness of 10 µm in approximately six sensor channels. By contrast, the North West Shelf light condensate was not able to be detected by either sensor for any thickness up to 200 µm. Optical remote sensing is therefore not applicable for detecting diagnostic absorption features associated with this light condensate oil type, which is typical of the chemistry of many hydrocarbon oils found in the Australian Northwest Shelf area and condensates world wide. We conclude that oil type is critical to the applicability of optical remote sensing for natural oil slick detection and identification. We recommend that a sensor- and oil-specific sensitivity study should be conducted prior to applying optical remote sensors for oil exploration.The oil optical properties were obtained using two different laboratory methods, a reflectance-based approach and transmittance-based approach. The reflectance-based approach was relatively complex to implement, but was chosen in order to replicate as closely as possible real world remote sensing measurement conditions of an oil film on water. The transmittance-based approach, based upon standard laboratory spectrophotometric measurements was found to generate results in good agreement with the reflectance-based approach. Therefore, for future oil- and sensor-specific sensitivity studies, we recommend the relatively accessible transmittance-based approach, which is detailed in this paper.     

Remote flood monitoring system based on plastic optical fibres and wireless motes

This paper summarises an effort in the development of a remote flood monitoring system based on plastic optical fibre (POF) sensors and a wireless mote network. The wireless mote, comprising of a network of MICA2DOT™ units, was used as a platform to monitor and record the signal from the POF sensors and transmit this information to a base station wirelessly. A prototype of the integrated wireless POF sensor unit has been constructed, rendering it possible to deploy the autonomous unit remotely at multiple monitoring points as required. A flood monitoring simulation was carried out in a 24 m × 10 m × 0.9 m wave basin where four of these wireless optical fibre mote sensors were used to detect the rising water level in the basin. The novelty of the work lies in the successful integration of the wireless platform to a POF-based liquid level sensor and the subsequent demonstration of the prototype of the system for the purposes of flood monitoring applications.The sensing principle of the POF sensor developed here is well-known and is based on the loss of total internal reflection of the optical signal as the sensor probe comes in contact with the liquid. Compared to optical fibre-based sensors reported previously in the literature, the probe profile used in this study differs in terms of its simplicity in design, while exhibiting an excellent signal intensity loss ratio without the need for additional attachments to the probe such as optical prisms. The tests carried out showed that the POF sensor is capable of detecting a variety of fluids. Exhibiting good signal stability, the sensor also detects the liquid level reliably when the liquid rises or falls to the predetermined level. The responsiveness of the optical fibre sensor was evaluated by simulating different rates at which the liquid rises by immersing the sensor tip into the liquid and vice-versa at various speeds ranging from 1 mm/min to 500 mm/min.     

Thermal conductivity measurement of submicrometer-scale silicon dioxide films by an extended micro-Raman method

The micro-Raman method is a noncontact and nondestructive method for thin film thermal conductivity measurements. To apply the micro-Raman method, however, the thickness of the film must be at least tens of micrometers. An analytical heat transfer model is presented in this work to extend the micro-Raman measurement method to measure the thermal conductivity of thin films with submicrometer- or nanometer-scale thickness. The model describes the heat transfer process in the thin film and substrate considering the effects of thin film thickness, interface thermal resistance, thermal conductivity of the thin film and substrate. From this heat transfer model, an analytical expression for the thermal conductivity of the thin film is derived. Experiments were successfully performed to measure the thermal conductivity of 200, 300 and 500 nm thickness silicon dioxide films using the extended micro-Raman measurement method, with results confirming the accuracy and validity of the extended model.     

Design and fabrication of micro-nano fusion gas sensor based on two-beam micro-hotplatform

Micro-nano fusion gas sensors integrating two-beam micro-hotplatform with nanostructured porous film were fabricated in this study. The micro-hotplatform (MHP) was manufactured using standard micro-electro-mechanical systems technology in wafer runs. Based on a colloidal crystal template method, highly ordered porous tin dioxide films were in situ grown on the MHP. The as-fabricated sensors achieved the highest response at 250 °C with power consumption only 24 mW. Due to the low thermal capacity and ordered porous thin films of the sensor, the response time was about 2 s. The sensors are sensitive to ethanol in a large range from 0.1 to 250 ppm. The developed sensor here with high performance is an excellent candidate which can be incorporated into portable devices for alcohol detection such as breath analyzers.

     

A T-junction device allowing for two simultaneous orthogonal views: application to bubble formation and break-up

A novel design for the classical microfluidic device known as T-junction is proposed with the purpose of obtaining a simultaneous measurement of the in-plane velocity components in two orthogonal planes. A crucial feature of the proposed configuration is that all three velocity components are available along the intersection of the two planes. A dedicated optical set-up is developed to convey the two simultaneous views from the orthogonal planes into the sensor of a single camera, where a compound image is formed showing on either half one of the two views. A commercial micro-particle image velocimetry system is used to measure the velocity in the two planes. Feeding the T-junction with a liquid continuous phase and a dispersed gas phase, the velocity is measured by phase averaging along the bubble formation and break-up process showing the potentialities of the new design. The accuracy analysis shows that the error is dominated by a systematic component due to the thickness of the measurement slice. The error can be reduced by applying confocal microscopy to the present system with no further modifications so as to reduce the thickness of the measurement slab thereby reducing the error. Moreover, by sweeping the planes across the region of interest, a full three-dimensional reconstruction of the velocity field can be readily obtained. Finally, the simultaneous views offer the possibility to extract the principal curvatures of the bubble meniscus thereby providing access to the Laplace pressure.     

Sensor properties and surface characterization of the metal-deposited SPR optical fiber sensors with Au,Ag, Cu,and Al

Metal-deposited optical fiber sensors with Cu and Al with a film thickness of 45 nm based on surface plasmon resonance (SPR) were fabricated for the first time. The response curves and the properties of these sensors were investigated with a comparison of those of the sensors with Au and Ag. The reflection properties of thin films of Au, Ag, Cu, and Al due to the SPR phenomenon were also measured and considered. The metal-deposited SPR optical fiber sensors with Au, Ag, and Cu have high sensitivities and good responses. Though the sensor with Al shows a lower sensitivity, it has a wider response range in the refractivity. The response curve of the sensor with Au calculated from SPR theoretical equations agreed well with that obtained by the experiment. However, the response curves of the sensors with Ag, Cu, and Al have the effects of the surface oxide layers. The surface characterization of these metal films by X-ray photoelectron spectroscopy (XPS) showed the presence of oxide layers on the films of Ag, Cu, and Al. A very thin (about 0.3 nm) oxide layer is present on Ag, while thick (about 2 nm) oxide layers are present on Cu and Al.     

Silicon-based miniature sensor for electrical tomography

The paper describes the fabrication of a novel miniature sensor for electrical tomography. The sensor comprises a number of copper electrodes that are fabricated around a small hole that is etched through a silicon wafer. Copper electrodes are electroplated to fill channels that are formed in thick photo-resist on top of the silicon wafer. Electrodes with a thickness of 60 μm, surrounding a hole of diameter 300 μm, have been realised. Initial measurements have been made using a commercial LCR meter applied to an eight-electrode sensor and images of a 80 μm diameter wire have been obtained. Future work will consider the integration of measurement circuitry alongside the electrodes in order to reduce parasitic capacitances.     

Surface-micromachined capacitive differential pressure sensor withlithographically defined silicon diaphragm

A capacitive surface-micromachined sensor suitable for the measurement of liquid and gas pressures was fabricated. The structure consists of a polysilicon stationary electrode suspended 0.7 μm above a 20-μm-thick lightly doped silicon diaphragm formed by a patterned etch stop. The a priori patterning of the buried etch stop yields diaphragm widths independent of wafer thickness variations with excellent alignment. The design described here has a pressure range of 100 PSI, a nominal capacitance of 3.5 pF with a full scale span of 0.8 pF, and a temperature coefficient of 100 ppm°C^-1. Each device, including a matched reference capacitor, occupies 2.9 mm² , yielding approximately 2000 devices per 100-mm wafer     

Fabrication of thermal microbridge actuators and characterization of their electrical and mechanical responses

This study presents thermal silicon microbridge actuators which have been made by a novel fabrication process utilizing dry processes for all critical steps. The fabrication process results in microbridges which are fully oxide covered, with excellent surface quality and dimensional control. The microbridges are made in the device layer of a silicon-on-insulator (SOI) wafer which ensures uniform doping profile and accurate thickness control. The electrical and mechanical responses of the bridges were measured upon rapid heating up to near the melting point of silicon. Up to 12 μm mechanical deflection due to thermal expansion was detected by white light interferometry (WLI) which allowed accurate measurement. Mechanical deflection has previously not been measured for silicon microlamps. Thermal conduction in the air gap between the actuator and the neighbouring solid silicon parts was analysed and shown to be more important than convection or radiation, even at very high operation temperatures.     

Wafer-level reliability characterization for wafer-level-packaged microbolometer with ultrasmall array size

For the development of a small and low-cost microbolometer, wafer-level reliability characterization techniques for vacuum-level packaged wafers are introduced. Amorphous-silicon-based microbolometer-type vacuum sensors fabricated on an 8-inch wafer are bonded with a cap wafer by using an Au–Sn eutectic solder. Membrane deflection and integrated vacuum sensor techniques are independently used to characterize the hermeticity at the wafer level. For a packaged wafer with a membrane thickness below 100 μm, it is possible to determine the hermeticity via a screening test performed using an optical detector. An integrated vacuum sensor having the same structure as a bolometer pixel shows a vacuum level below 100 mTorr. All steps from the packaging process to the fine hermeticity test are implemented at the wafer level to verify that high-volume and low-cost production of the microbolometer is possible.     

Passive single chip wireless microwave pressure sensor

A novel micromachined passive wireless pressure sensor is presented. The device consists of a tuned circuit operating at 10 GHz fabricated on to a SiO₂ membrane, supported on a silicon wafer. A pressure difference across the membrane causes it to deflect so that an antenna circuit detunes. The circuit is remotely interrogated to read off the sensor data wirelessly. The chip area is 5 mm × 4 mm and the membrane area is 2 mm² with a thickness of 4 μm. Two on-chip passive resonant circuits were investigated: a meandered dipole and a zigzag antenna. Both have a physical length of 4.25 mm. The sensors show a shift in their resonant frequency in response to changing pressure of 10.28–10.27 GHz for the meandered dipole, and 9.61–9.58 GHz for the zigzag antenna. The sensitivities of the meandered dipole and zigzag sensors are 12.5 kHz/mbar and 16 kHz/mbar respectively.     

Enhancing the sensitivity of ionic liquid sensors for methane detection with polyaniline template

Flammable gas sensors are essential in occupational health and safety to prevent fire or explosion in gas facilities and underground mining. Our early study demonstrated that ionic liquid (IL)/quartz crystal microbalance (QCM) gas sensors and sensor arrays were excellent for the detection of various organic vapors at both room temperature and elevated temperatures. In this paper, we developed a general method that significantly enhanced the sensitivity of the IL/QCM sensors for flammable gases detection by immobilizing IL on a conductive polymer polyaniline (PAn) template. Studies were performed to optimize the PAn oxidation states, thickness, and IL concentrations. Results showed that the sensitivity increased with increasing the PAn film thickness and the amount of IL immobilized within the PAn film. The sensitivity depended also on the oxidation state and doping state of PAn. With doped and partially oxidized PAn (emeraldine salt) the IL/QCM sensor showed the best performance. The current detection limit for methane was as low as about 115 ppm at room temperature. The sensitivity also depended on the structure of the IL used. Among the four ILs tested, two of them showed excellent sensitivities after being immobilized in the PAn film.     

Bulk property of 1/f noise for piezoresistive Ni/Cr thin films in pressure sensors on flexible substrate

In the current paper, we report the 1/f noise measurement of nichrome Ni/Cr (80/20 %) thin films for two types of pressure sensors: relative pressure sensors and absolute pressure sensors. The normalized Hooge coefficient for nichrome thin film was found to be 1.89 × 10^?10 for the relative pressure sensors and 4.64 × 10^?11 for the absolute pressure sensors. We demonstrated that the normalized Hooge coefficient multiplied by the volume of the thin film become constant regardless of the sensor types and discuss the complexities arise for the miniaturization of MEMS sensors due to the bulk noise properties of piezoresistive thin films.     

In-situ optical monitoring of single-crystal silicon membrane etching

We present a simple yet efficient technique to monitor membrane thickness during etching of silicon in anisotropic etching bath. This technique uses a mechanical holder to protect the front side of the wafer and the measurement of light absorption to obtain remotely the thickness of a reference zone in the etched wafer. The original feature in our set-up is that we measure the absorption in two different bands of wavelength, one where the silicon is strongly absorbing and the other where it is not, improving the robustness of the measurement. Actually, this principle allows for effectively compensating the fluctuation in the optical path, and after calibration provides real-time information on the membrane thickness, which proves to be particularly useful for fabricating membranes below 40 \(\upmu\)m.     

Fabrication and characterization of MEMS-based flow sensors based on hot films

The purpose of this paper is to propose two types of airflow velocity measurement modules, double-chip and single-chip, of MEMS-based flow sensors that consisting of heating resistors and sensing resistors on alumina substrates. In this study, MEMS techniques are used to deposit platinum films on the substrate to form resistors which are to regard as heaters and sensing elements. As air flows through the heater and the sensor, the temperature of the sensing resistor on the hot film decreases and the changes of the local temperature determine the airflow rate. The experimental results show the resistance linearly varies as airflow velocity changes from 5 to 28 ms⁻¹. Finally the experimental data indicate that sensing performance of the single-chip type is better than that of the double-chip type with its higher sensitivity (0.7479 Ω/ms⁻¹) due to the more rapid heat conduction from the heating resistor to the sensing resistor.     

Simulation and fabrication of piezoresistive membrane type MEMS strain sensors

Two piezoresistive (n-polysilicon) strain sensors on a thin Si₃N₄/SiO₂ membrane with improved sensitivity were successfully fabricated by using MEMS technology. The primary difference between the two designs was the number of strips of the polysilicon patterns. For each design, a doped n-polysilicon sensing element was patterned over a thin 3 μm Si₃N₄/SiO₂ membrane. A 1000×1000 μm² window in the silicon wafer was etched to free the thin membrane from the silicon wafer. The intent of this design was to fabricate a flexible MEMS strain sensor similar in function to a commercial metal foil strain gage. A finite element model of this geometry indicates that strains in the membrane will be higher than strains in the surrounding silicon. The values of nominal resistance of the single strip sensor and the multi-strip sensor were 4.6 and 8.6 kΩ, respectively. To evaluate thermal stability and sensing characteristics, the temperature coefficient of resistance [TCR=(ΔR/R₀)/ΔT] and the gage factor [GF=(ΔR/R₀)/] for each design were evaluated. The sensors were heated on a hot plate to measure the TCR. The sensors were embedded in a vinyl ester epoxy plate to determine the sensor sensitivity. The TCR was 7.5×10⁻⁴ and 9.5×10⁻⁴/°C for the single strip and the multi-strip pattern sensors. The gage factor was as high as 15 (bending) and 13 (tension) for the single strip sensor, and 4 (bending) and 21 (tension) for the multi-strip sensor. The sensitivity of these MEMS sensors is much higher than the sensitivity of commercial metal foil strain gages and strain gage alloys.     

The optimisation of a paired emitter-detector diode optical pH sensing device

With recent improvements in wireless sensor network hardware there has been a concurrent push to develop sensors that are suitable in terms of price and performance. In this paper a low-cost gas sensor is detailed, and significant improvements in sensor characteristics have been achieved compared to previously published results. A chemical sensor is presented based on the use of low-cost LEDs as both the light source and photodetector, coupled with a sensor slide coated with a pH sensitive colorimetric dye to create a simple gas sensor. Similar setups have been successfully used to detect both acetic acid and ammonia. The goal of this work was to optimise the system performance by integration of the sensing technique into a purposely deigned flowcell platform that holds the colorimetric slide and optical detector in position. The reproducibility of the sensor has been improved through this arrangement and careful control of deposited film thickness. The enhanced reproducibility between sensors opens the potential of calibration-free measurement, in that calibration of one sensor can be used to model the characteristics of all sensors in a particular batch.     

Low temperature wafer bonding for thin silicon film transfer

In this work, we investigate the low temperature (<200 °C) wafer bonding using wet chemical surface activation and we demonstrate high bonding strength sufficient to achieve the transfer of a thin silicon film of thickness less than 400 nm on top of another silicon wafer using spin-on-glass (SOG) film as an intermediate layer. The process developed is the first critical step that can enable three-dimensional (3D) integration and wafer level packaging of MEMS with electronic circuits.