Dag Thorstein Wang

Capacitive differential pressure sensor for harsh environments

Abstract A capacitive differential pressure sensor for the pressure range of 0–1 bar has been developed. The primary field of application is hydrodynamic flow measurements in hot petroleum wells. In these harsh environments the sensor has to survive high common mode pressure in the range of 1000 bar and temperatures up to 180°C. The pressure element is formed in a triple-stack of fusion-bonded silicon wafers. A bossed diaphragm etched in the upper wafer bends due to the differential pressure across it. The capacitance to the middle wafer is measured. A reference capacitor insensitive to the differential pressure enables compensation for capacitance shifts caused by ambient pressure and temperature changes. The lower silicon wafer is included to minimise the diaphragm stress from the package. An ASIC, certified for 230°C, which is developed at SINTEF, is used for signal read-out. The sensor is measured to have a low zero pressure signal drift, smaller than 2.5% of full-scale output, when the temperature is varied in the range 0–200°C and the ambient pressure in the range 0–1000 bar. The sensitivity of the sensor is sufficient for the application.

A novel ultra-planar, long-stroke and low-voltage piezoelectric micromirror

A novel piston-type micromirror with a stroke of up to 20 µm at 20 V formed out of a silicon-on-insulator wafer with integrated piezoelectric actuators was designed, fabricated and characterized. The peak-to-valley planarity of a 2 mm diameter mirror was better than 15 nm, and tip-to-tip tilt upon actuation less than 30 nm. A resonance frequency of 9.8 kHz was measured. Analytical and finite element models were developed and compared to measurements. The design is based on a silicon-on-insulator wafer where the circular mirror is formed out of the handle silicon, thus forming a thick, highly rigid and ultra-planar mirror surface. The mirror plate is connected to a supporting frame through a membrane formed out of the device silicon layer. A piezoelectric actuator made of lead–zirconate–titanate (PZT) thin film is structured on top of the membrane, providing mirror deflection by deformation of the membrane. Two actuator designs were tested: one with a single ring and the other with a double ring providing bidirectional movement of the mirror. The fabricated mirrors were characterized by white light interferometry to determine the static and temporal response as well as mirror planarity.

Fabrication and Characterization of CMUTs realized by Wafer Bonding

This paper presents the fabrication process and electrical characterization of Capacitive Micromachined Ultrasonic Transducers (CMUTs) realized by fusion bonding. The transducer array with electrical connections was realized by four photolithography steps. The electrical characterization of two different geometries is presented. The smallest structure have 120 nm deep circular cavities with radius of 5.7 mum separated by a pitch of 12.5 mum and are vacuum-sealed by a 100 nm silicon nitride membrane. The dimensions in the second geometry are twice as large except of the cavity gap which is the same. The electrical measurements of the input impedance were carried out by a vector network analyzer on one row of 52 elements, each with four CMUTs. In non-collapsed mode in air the small CMUTs have a resonance frequency of 30 MHz with a Q value of 70 when DC biased by 30 V. When immersed in oil, the resonance frequency was reduced to 8 MHz and the peak was broadened as expected. The large CMUT structures show somewhat distorted resonances at about 13 MHz in air, with Q-values up to 50 to 60 for the better devices. Simulations indicate resonance frequencies that are about 20% lower. Hence some process is taking place in these devices that are not included in the theory. No hysteresis was observed in any of the measurements

A miniaturized pressure sensor with inherent biofouling protection designed for in vivo applications

The design, fabrication, and measurement results for a diaphragm-based single crystal silicon sensor element of size 820 μm × 820 μm × 500 μm are presented. The sensor element is designed for in vivo applications with respect to size and measurement range. Moreover, it is optimized for longtime operation in the human body through a built-in protection preventing biofouling on the piezoresistors. The sensitivity is about 20 mV/V for a change from 500 to 1500 mbar absolute pressure. This result is comparable to conventional sized micromachined pressure sensors. The output signal is not found to be influenced by exposure to 60 °C for three hours, a normal temperature load for a typical sterilization process for medical devices (Ethylene Oxide Sterilization). The hysteresis is low; < 0.25% of full scale output signal. The sensor element withstands an overload pressure of 3000 mbar absolute pressure. Observed decrease in the output signal with temperatures and observed nonlinearity can easily be handled by traditional electronic compensation techniques.

Chemical analysis of bonded and debonded silicon-glass interfaces

We report on the nature of the interface of anodically bonded glass wafers (Pyrex 7740, Hoya SD-2) and silicon wafers, after having reversed the voltage across the wafer couples. The reaction products and defects discovered at the interface of the wafers are studied with scanning electron microscopy, electron dispersive x-ray spectroscopy and with an electron microprobe. An accumulation, especially of sodium, is found to be closely related to the presence of defects appearing as brown/yellow-brown spots and cracks. Gradients in potassium and zinc concentrations are also observed in regions containing defects. Some of the processes that are believed to take place in the interface region are discussed. ane{abstract

Sensitive and Selective Photo Acoustic Gas Sensor Suitable for High Volume Manufacturing

Sensitive and selective gas measurements are crucial for a large variety of applications. This paper describes the manufacturing and characterisation of a photo acoustic gas sensor system. The system is based on a pressure sensor element with a sensitivity of 10 muV/V/Pa. 12 prototypes for measuring CO2 have been characterised. Detection limits ranging from 92 ppm to below 6 ppm CO2 were obtained, depending on the measurement time and photo acoustic cell design. No cross-sensitivity towards CO, CH4, or humidity could be observed in any of the sensors. The temperature drift of the uncompensated raw signal of two sensor designs was below 117 ppm CO2 in the range from 25degC to 50degC.

Design of micromachined resonators for fish identification

The ID tag presented here was designed to give a tag of small size that could be produced at a low prize, and that could be read remotely in live fish, even in seawater. The last condition precludes use of electromagnetic waves for interrogation of the tags, and acoustic interrogation is then a clear alternative. The solution presented is a passive tag with a set of acoustic resonances that may be detected acoustically. The tag operates in the 200 to 400 kHz range. The identity of the tag is given by a unique combination of resonances in this frequency range. For the tags presented here there are five resonances per tag. If five or more resonances are chosen from a predetermined set of say 17 resonance frequencies, a total number of at least 3000 to 4000 different tags are available. This is adequate for classification of fish at the batch level in fish farms, or of local wild fish tribes. The resonators on a tag consists of a thin, nominally 500 nm thick silicon nitride membrane suspended over separate evacuated cavities, made by bulk silicon micromachining. The resonators were designed to have Q-factors in the range 27 to 35 with viscous losses in the water neglected. The resonators have been measured in water and in dead or live anesthetized fish from distances up to 30 cm. Sharp resonances in fair accordance with the tag design were achieved. Some alterations of the tag response with change of the angular orientation of the tag relative to the ultrasound beam are seen. This is also theoretically expected.

Sensitive and Selective Photoacoustic Gas Sensor Suitable for High-Volume Manufacturing

Sensitive and selective gas measurements are crucial for a large variety of applications. This paper describes the manufacturing and characterization of a photoacoustic gas sensor system. The system is based on a pressure sensor element with a sensitivity of 10 muV/V/Pa. To demonstrate and evaluate the concept, 12 prototypes for measuring CO2 have been manufactured and characterized. Detection limits ranging from 92 ppm to below 6 ppm CO2 were obtained with a path length of 10 cm, depending on the measurement time and photoacoustic cell design. Measurements showed no cross-sensitivity towards CO, CH4, or humidity in any of the sensors. The temperature drift of the uncompensated raw signal of two sensor designs was below 117 ppm CO2 in the range from 25degC to 50degC.

Biofouling on protective coatings for implantable MEMS

Protective coatings can replace traditional packaging methods, which are often voluminous and may spoil the otherwise excellent opportunity for miniaturized implantable medical MEMS. The bio-growth on a selection of biocompatible protective coatings (TiO2, DLC and Parylene) was investigated. The model system for evaluation was a diaphragm based acoustic resonator primary designed for fish identification. By detecting the shift in resonance frequency, we wanted to highlight the following; i) does the amount of biological growth vary for the different coatings? ii) if biofouling occurs, is the growth devastating for the device characteristics? We found that the resonance frequency did not change significantly. From this we conclude that the stiffness, represented by the spring constant for the resonating structure, was not affected. This result is of major importance also for other diaphragm based in vivo devices to be, e.g. pressure sensors, ultrasonic imaging devices, and dosage pumps.

Miniature, low-cost, 200 mW, infrared thermal emitter sealed by wafer-level bonding

Infrared (IR) thermal emitters are widely used in monitoring applications. For autonomous systems, miniaturized devices with low power consumption are needed. We have designed, fabricated and tested a novel device design, packaged on the wafer level by Al-Al thermo-compression bonding. 80 μm wide Aluminium frames on device and cap wafers were bonded in vacuum at 550°C, applying a force of 25 kN for 1 hour. The bond force translated to a bond pressure of 39 MPa. Subsequent device operation showed that the seals were hermetic, and that the emitters were encapsulated in an inert atmosphere. The emitters were optimized for radiation at λ=3.5 μm. Emission spectra by Fourier Transform Infrared Spectroscopy showed high emissivity in the wavelength range 3 – 10 μm at 35 mA driving current and 5.7 V bias, i.e. 200 mW power consumption. The emitter temperature was around 700 °C. The rise and fall times of the emitters were below 8 and 3 ms, respectively. The low thermal mass indicates that pulsed operation at frequencies around 100 Hz could be realized with about 90 % modulation depth. The measured characteristics were in good agreement with COMSOL simulations. Thus, the presented devices have lower power consumption, an order of magnitude higher modulation frequency, and a production cost reduced by 40 – 60%1-4 compared to available, individually packaged devices. The patented device sealing provides through-silicon conductors and enables direct surface mounting of the components.