Capacitive sensors for the detection of mechanical quantities all rely on a displacement measurement. The movement of a suspended electrode with respect to a fixed electrode establishes a changing capacitor value between the electrodes. This effect can be measured and if the mechanical quantity controls the movable electrode, a sensor is realized. Since the value of the capacitor is directly related to its size, and a small capacitor means high noise susceptibility, capacitive sensors should be as large as possible. Capacitive pressure sensors have been developed with success for industrial applications, where large membrane sizes are not a critical issue. However, most centres of expertise in silicon sensors show an interest in exploiting silicon technology to produce capacitive pressure sensors as well. From the above, this miniaturization trend appears to be an unsound idea. On the other hand, the principle of capacitive sensors allows the realization of measuring systems with so far unknown performance. Indeed, the capacitive sensor reveals distinct advantages when compared to its piezoresistive counterpart: high sensitivity, low power consumption, better temperature performance, less sensitive to drift, etc. Nevertheless, only a minor fraction of the market for pressure sensors is taken up by capacitive-type sensors. When observing the characteristics of capacitive sensors, it may seem surprising to encounter so few devices in real-world applications. The reasons for the lack in breakthrough can be found in the design complexity and the requirements for a matched sensing circuit. This paper will extensively discuss the justification of the choice for this research effort, and will elaborate on the techniques to fabricate the devices based on electronic manufacturing procedures. Basically, silicon capacitive sensors differ from piezosensors in that they measure the displacement of the membrane, and not its stress! This has important implications on the final assembled device: less package-induced problems can thus be expected. However, a far more important issue is their extremely high sensitivity, together with a low power consumption. These issues make them especially attractive in biomedical implant devices, or in other telemetry applications, where power is not randomly available. So far, this is the only field of success for these sensors. However, due to the interesting detection principle, new fields of application emerge, offering unique and superior performance when compared to available sensors. Uniaxial accelerometers are a good example, where extremely high cross-sensitivity reduction can be obtained.

/pdf/capacitive-sensors-when-and-how-to-use-them-49vq9dr8ba.pdf

Capacitive sensors: When and how to use them☆

Micromechanical smart sensor and actuator systems of high complexity become commercially viable when realized as a multi-wafer device in which the mechanical functions are distributed over different wafers and one of the wafers is dedicated to contain the readout circuits The individually-processed wafers can be assembled using wafer-to-wafer bonding and can be combined to one single functional electro-mechanical unit using through-wafer interconnect, provided that the processes involved comply with the constraints imposed by the proper operation of the active electrical and micromechanical subsystems This implies low-temperature wafer-to-wafer bonding and through-wafer interconnect Au/Si eutectic bonding has been investigated as it can conveniently be combined with bulk-micromachined through-wafer interconnect The temperature control in eutectic bonding has been shown to be critical

Low-temperature silicon wafer-to-wafer bonding using gold at eutectic temperature

A single-chip gas detection system fabricated in industrial CMOS technology combined with post-CMOS micro-machining is presented. The sensors rely on a chemo-sensitive polymer layer, which absorbs predominantly volatile organic compounds (VOCs). A mass-sensitive resonant-beam oscillator, a capacitive sensor incorporated into a second-order /spl Sigma//spl Delta/-modulator, a calorimetric sensor with low-noise signal conditioning circuitry and a temperature sensor are monolithically integrated on a single chip along with all necessary driving and signal conditioning circuitry. The preprocessed sensor signals are converted to the digital domain on chip. An additional integrated controller sets the sensor parameters and transmits the sensor values to an off-chip data recording unit via a standard serial interface. A 6-chip-array has been flip-chip packaged on a ceramic substrate, which forms part of a handheld VOC gas detection unit. Limits of detection (LOD) of 1-5 ppm n-octane, toluene or propan-1-ol have been achieved.

CMOS single-chip gas detection system comprising capacitive, calorimetric and mass-sensitive microsensors

A new approach to vacuum packaging of micro-machined resonant, tunneling, and display devices is covered in this paper. A multi-layer, thin-film getter, called a NanoGetter, which is particle free and does not increase the chip size of the microsystem has been developed and integrated into conventional wafer-to-wafer bonding processes. Hermetic electrical feedthroughs are also provided as part of this total-solution technology. Experimental data taken with silicon resonators is presented in which Q values in excess of 21,000 have been obtained. Applications for this technology include gyroscopes, accelerometers, displays, flow sensors, density meters, infrared (IR) sensors, microvacuum tubes, radio frequency microelectromechanical systems (RF-MEMS) and pressure sensors.

Chip-level vacuum packaging of micromachines using NanoGetters

Integrated circuit technology and its later development, microsystem technology, make good use of wafer bonding. An increased interest in bond adhesion quantification can be anticipated when wafer bonding is optimized for complex microelectro-mechanical systems where the bonding process must take every other component into consideration when it comes to cost, temperature budget and process compatibility. Adhesion quantification methods for evaluation of bonded brittle material, especially direct wafer bonded, are reviewed in this paper. The most commonly utilized methods, namely the double cantilever beam, tensile, chevron and blister test methods, are thoroughly covered and miscellaneous techniques are mentioned. The physics background and modeling presented by different authors are examined. Based on models and experiences made in neighboring research fields improvements are suggested. Practical capabilities and limitations and origin and mitigation of measurement errors are addressed. Experimental foundation on fundamental knowledge in solid mechanics and the statistical nature of brittle fracture behavior is emphasized. The methods’ adequacy are compared and ranked for three types of uses: general understanding, bonding process optimization and quality control. It is shown that the quality of all commonly used methods for adhesion quantification of wafer bonding can be dramatically improved by small means.

Adhesion quantification methods for wafer bonding

A process for silicon-to-thin film anodic bonding with polysilicon, silicon oxide, silicon nitride or aluminium as the thin film materials has been developed. Silicon wafers covered with these thin films have been sealed together by anodic bonding using thin sputter-deposited glass layers as sealing material. The bond strengths of the samples have been tested by pull tests. Some samples were exposed to water for 300 h to test the media compatibility. IR microscopy has been shown to be a good method to uncover bonding voids. Bond strength tests of three-inch silicon-to-silicon anodic bonded wafers are shown to be in excellent agreement with the bonding yield expected from IR-microscope inspection.

https://iopscience.iop.org/article/10.1088/0960-1317/2/3/002/pdf

Silicon-to-thin film anodic bonding

This paper describes a novel high-precision, low-noise, high-stability, calibrated and compensated digital oscillatory gyroscope with SPI interface, housed in custom-made ceramic packages The device is factory-calibrated and compensated for temperature effects to provide high-accuracy digital output over a broad temperature range Optimized tuning of the excitation and detection frequencies, as well as optimized mechanical and electrical balancing result in low sensitivity to shock and vibrations By utilizing a unique sealed cavity technology, the vibrating elements of the gyroscope are contained within the low-pressure hermetic environment needed for high Q factors Further on, improved stability of the device is achieved by full design symmetry, high thermal efficiency and choice of crystalline materials in the entire structure

SAR500 - A high-precision high-stability butterfly gyroscope with north seeking capability

In the project IRMA-EU (IRMA: integrated resonant accelerometer microsystem for automotive applications), a project sponsored by the European Commision under ESPRIT, SensoNor and project partners Autoliv and SINTEF are developing a resonant-structure two-chip accelerometer silicon microsystem for automotive applications, the SA30 Crash Sensor for front impacts, with range ± 50g. The project is focusing on the development of key process technologies, product designs and manufacture of functional prototypes. The IRMA project is coordinated with other activities performed by the partners to cover all aspects of research and technology development as well as establishment of high-volume production capabilities needed for successful product innovation of this new generation of crash sensors. The sensing principle is an acceleration-sensitive resonant structure, with an ASIC for resonance control and signal conditioning. Prototypes in silicon of both the sensor chip and the ASIC chip have confirmed the feasibility of the concept. Ramp up to high-volume production will be started as soon as fully functional microsystems are demonstrated.

An integrated resonant accelerometer microsystem for automotive applications

In the project "IRMA-EU" (IRMA: Integrated Resonant Accelerometer Microsystem for Automotive Applications), a project sponsored by the European Commision under ESPRIT, SensoNor and project partners Autoliv and SINTEF are developing a resonant structure two-chip accelerometer silicon microsystem for automotive applications, the SA30: Crash Sensor for front impacts, with range /spl plusmn/50 g. The project is focusing on the development of key process technologies, product designs and manufacture of functional prototypes. The IRMA project is coordinated with other activities performed by the partners to cover all aspects of research and technology development as well establishment of high volume production capabilities needed for successful product innovation of this new generation of crash sensors. The sensing principle is an acceleration sensitive resonant structure, with an ASIC for resonance control and signal conditioning. Prototypes in silicon of both the sensor chip and the ASIC chip have confirmed the feasibility of the concept, and ramp up to high volume production has now started.

Mechanical stress introduced in a silicon diaphragm chip from the support chip or the substrate affects the performance of silicon pressure sensors. The influence of this effect on long-term stability, thermal zero shift, and common mode pressure sensitivity has been evaluated on differential piezoresistive sensors. The following different structures have been tested and compared: (1) unmounted diaphragm chips as reference; (2) silicon support chips sealed to diaphragm chips by silicon-to-silicon anodic bonding; (3) silicon support chips sealed to diaphragm chips with screen printed solder glass; and (4) Pyrex 7740 borosilicate glass substrates sealed to diaphragm chips by anodic bonding. The results from these tests show that all these methods can be used for high-performance piezoresistive pressure sensors. Silicon-to-silicon anodic bonding is shown to be the best method with respect to thermal zero stability and common mode pressure stability. >

Reidar Holm

Papers

Silicon-to-thin film anodic bonding

SAR500 - A high-precision high-stability butterfly gyroscope with north seeking capability

An integrated resonant accelerometer microsystem for automotive applications

An integrated resonant accelerometer microsystem for automotive applications

Stability and common mode sensitivity of piezoresistive silicon pressure sensors made by different mounting methods