Sigurd T. Moe

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.

Development of cost-effective high-density through-wafer interconnects for 3D microsystems

High-density through-wafer interconnects are of great interest for fabricating real 3D microsystems. A complete solution for realizing through-wafer interconnects is presented. The proposed solution is believed to be cost effective and easy to integrate in a device process flow. A deep reactive ion etch process was developed to etch 20 x 20 μm 2 via holes through 300 μm thick silicon wafers. Thermal oxide is used to insulate the vias from the bulk silicon and heavily doped polysilicon is used as the conductor. Aluminum metallization is provided on both sides of the wafer. The electrical resistance of a single through-wafer via is close to 30 Ω.

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.

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.

Two-state Optical Filter Based on Micromechanical Diffractive Elements

We have designed a robust two-state filter for infrared gas measurement, where the filter transmittance alternates between a single bandpass function, and a double-band offset reference. The device consists of fixed and movable diffractive sub-elements, micromachined in the device layer of a bonded silicon on insulator (BSOI) wafer. Switching between the two states of the filter is obtained by actuation of the movable sub- elements between idle and pull-in positions, which affects the interference of reflected light The characteristics of the filter are defined by a diffractive microrelief pattern etched on top of the sub-elements and by the position of the movable sub-elements at pull-in, the latter mechanically defined by the buried oxide layer. Thus, no accurate electrical control is needed to operate the filter. The first test components operate at 2 mum wavelength using a displacement of 500 nm and an actuation voltage of 5 V. No sticking or change in filter characteristics have been observed after repeated pull-in operations. The simplicity of fabrication and operation is likely to make the two-state filter an attractive component for sensors such as non-dispersive infrared gas detector.

Design and processing of a cost-effective piezoresistive MEMS cantilever sensor for medical and biomedical use

In this special section article, cost-effective methods for fabrication of a piezoresistive cantilever sensor for industrial use are focused on. The intended use of the presented cantilever is a medical application. A closer description of the cantilever design is given. The low-cost processing sequence is presented and each processing step is explained in detail. The processing sequence is also compared to other low-cost fabrication techniques. Results from the electrical probing and mechanical strength test are given. The results demonstrate that the chosen low-cost processing route results in high yield and a mechanical robust device.

Diffusion at anodically bonded interfaces

The diffusion of gas molecules into cavities closed by anodic bonding is quantified by the annealing of specially designed test structures. Annealing is performed at temperatures in the range 150-430 °C for several days. An increased concentration of molecules within the closed cavities after heat treatments is verified both electrically and optically. The diffusion of gas into the cavities is found to be substantial at temperatures above 300 °C. A diffusion parameter, the diffusion coefficient times the height of the bonded interface, is found from curve fitting of experimental data with an analytic expression for diffusion.

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.

The effect of true human synovial fluid on the functionality of an in vivo pressure sensor element

This paper presents a study on the feasibility of packaging a sensor element by a thin biocompatible coating. The goal of the work was twofold; Firstly to investigate the possible impact of the coating on sensor element performance; Secondly to examine the sensor element functionality after soaking into true human synovial fluid for more than 30 days. Sensor elements with two different structures of TiO2, the amorphous and the anatase, were examined and compared to uncoated elements. The device under test was a piezoresistive pressure sensor element designed for in vivo applications. Pressure characteristics were measured before and after Atomic Layer Deposition of the TiO2 coatings. Sensor signals were examined and visual inspection of the sensor element surfaces were done after more than 30 days soaking in true human synovial fluid. Throughout the soaking period the shift in output signal was higher and varied more for uncoated elements than for coated ones. Our results indicate that a 20 nm thick TiO2 coating can provide good protection towards the harsh synovial fluid.

Al-Al wafer-level thermocompression bonding applied for MEMS

Wafer-level thermocompression bonding (TCB) using aluminum (Al) is presented as a hermetic sealing method for MEMS. The process is a CMOS compatible alternative to TCB using metals like gold (Au) and copper (Cu), which are problematic with respect to cross contamination in labs. Au and Cu are commonly used for TCB and the oxidation of these metals is limited (Au) or easily controlled (Cu). However, despite Al oxidation, our experimental results and theoretical considerations show that TCB using Al is feasible even at temperatures down to 300–350 °C using a commercial bonder without in-situ surface treatment capability.