Mechanical MEMS

2.4.1 Mechanical sensors

MEMS mechanical sensors are very popular because of their easy integration procedure. The sensing mechanism utilises the following methods and principles,

• Cantilever beam sensor

• Capacitive sensing

• Accelerometers

• Microphones

• Gyroscopes

• Piezoelectricity

2.4.2 Accelerometer, Cantilever and Capacitive Measurement

Cantilever sensors can be used for the detection of physical, chemical and biological analytes with relatively good sensitivities and selectivity. The vast application areas include: acoustic applications, vibration monitoring, detection of viscosity and density, infrared and UV radiation, magnetic and electric fields, detection of chemical vapours including medical and biological agents, contaminants in water, explosive vapours such as RDX, PETN and TNT, nuclear radiation and the detection of DNA, antibodies and pathogens.

Fig. 2.2. Cantilever beam for the measurement of static and dynamic acceleration

Fig. 2.2 shows a typical cantilever sensor that can measure acceleration. The body of the sensor (proof mass) could be about 0.5-0.7 micrograms. The proof mass moves in the X- and Y-axes. Polysilicon springs suspend the MEMS structure above the substrate facilitating the proof mass to move freely. Acceleration causes deflection of the proof mass from its centre position. There could up to 32 sets of radial fingers around the four sides of the square proof mass. The fingers (middle one), shown in the figure, are positioned between two plates that are fixed to the substrate. Each finger and pair of fixed plates constitutes a differential capacitor, and the deflection of the proof mass is determined by measuring the differential capacitance. This sensing method has the ability of sensing both dynamic acceleration such as shock or vibration, as well as static acceleration such as inclination or gravity.

Many accelerometers employ piezoelectric sensing techniques. Thin film piezoelectric materials such as lead zirconate titanate (PZT) are promising materials for MEMS applications due to their high piezoelectric properties (Davis 2004: at http://www.nnf.cornell.edu/1999REU/ra/Meteer.pdf). Piezoelectric polymers are now being used for sensor applications. Piezoelectric polymeric sensors offer the advantage of strains without fatigue, low acoustic impedance and operational flexibility. The PZT converts mechanical disturbances to electrical signals. The starting material for the front-side process (FSP) is a silicon wafer that has silicon dioxide, lower metal electrode (Ti/Pt), and deposited PZT films. Industrial applications of MEMS accelerometers include airbag release mechanisms, machinery failure diagnostics, and navigational systems.

Fig. 2.2. Cantilever beam for the measurement of static and dynamic acceleration

2.4.3 Microphone

Acoustic MEMS are air-coupled, and can offer a wide range of applications such as detection, analysis and recognition of sound signals. The basic component is the microphone, called micro-microphone or simply MEMS microphone. A simple definition of a microphone is that it is an electromechanical acoustic transducer that transforms acoustical energy into electrical energy. It is an ultrasonic microsensor, which takes advantage of miniaturisation and also consumes low power. When multiple microphones are arranged in an array, the device is referred to as smart system as it can offer more reliable and intelligent operations. The basic challenge that is encountered in designing the MEMS microphone is the formulation, design, and implementation of signal processing circuitry that can adapt, control and utilise the signal in noisy environments. MEMS microphones are also very suitable for outdoor acoustic surveillance on robotic vehicles, wind noise flow turbulence sensing, platform vibration sensing, and so on.

Fig. 2.3. Cross-sectional diagram of piezoelectric microphone; plan view (Arnold 2001)

The ultrasonic sensor is based on the mechanical vibration of micro membrane or diaphragm realised in the silicon platform. The diaphragm is a thin, circular membrane held in tension and clamped at the edge. Many designs use a flat freeplate that is held in proximity to the back plate by electrostatic attraction. The free-plate makes up a variable capacitor with the back plate. Its value changes during the vibration caused by the sound signal. The deformation or deviation of the membrane (free-plate) from the normal values depends on the amplitude of the incident pressure. Fig. 2.3 shows the schematic as well as a micromachined SEM (Scanning Electron Microscope) picture of a MEMS microphone. The microphone can have a very low stray capacitance, is self-biasing, mass producible, arrayable, integrable with on-chip electronics, structurally simple and extremely stable over time in an ordinary environment. The typical dynamic range is from 70 to 120 dB SPL and the sensitivity can be in the order of 0.2 mV/Pa over the frequency range 100-10 kHz.

2.4.4 Gyroscope

MEMS gyroscopes (Fig. 2.4) are typically designed to measure an angular rate of rotation. A measurement of the angle is useful in many applications. A very common application is the measurement of the orientation or tilt of a vehicle running in a curved path. The MEMS gyroscope design introduces sophisticated and advanced control techniques that can lead to measure absolute angles. Some design is based on the principle of measuring the angle of free vibration of a suspended mass with respect to the casing of the gyroscope (Rajamani, 2000). The gyroscope can accurately measure both the angle and angular rate for low bandwidth applications. The measurement of orientation, for instance, is very useful in the computer-controlled steering of vehicles as well as for differential braking systems for skid control in automobiles. A typical gyroscope consists of a single mass, oscillating longitudinally with rotation induced lateral deflections being sensed capacitively. The iMEMS ADXRS gyroscope from Analog Devices, Inc., integrates both an angular rate sensor and signal processing electronics onto a single piece of silicon. Mounted inside a 7x7x3 mm BGA package, the gyro consumes 5 mA at 5 V and delivers stable output in the presence of mechanical noise up to 2000 g over a reasonably wide frequency range. A full mechanical and electronic self-test feature operates while the sensor is active. Mechanical sensors inherit many drawbacks as follows (www.sensorland.com),

• The overall silicon area is generally larger

• Multi-chip modules require additional integration steps

• Existence of larger signals from the sensor output

• Stray capacitance of the interconnections

• Comparatively larger volume with respect to packages

2.4.5 Mechanical Actuators

One of the fundamental components in MEMS technology is the actuator. A mechanical actuator is a device that usually converts electrical signals into mechanical motion. For instance, if a voltage is applied to a quartz crystal, it will change its size in a very precise and predictable way. Cantilever structures are used for actuation purposes. Cantilevers bend when pressure is applied to them and oscillate in a way similar to a spring when properly put into place. The important point that is considered in electrostatically actuated cantilever based MEMS devices is their stability.

Fig. 2.4. (a) Conceptual schematic of a torsional micromachined gyroscope with non-resonant drive (Source: Acar 2004); (b) Schematic diagram of a vibratory gyroscope (Source: Piyabongkarn 2005)

2.5 Thermal MEMS

Microsystems whose functionalities rely on heat transfer are called thermal MEMS. Thermal MEMS can be sensors and actuators. When a physical signal to be measured generates a temperature profile, the principle is called thermal sensing. The physical signals could be heat radiation, non-radiant heat flux or a reaction heat. Gas pressures, masses, volume fluxes, and fluidic thermal conductivities are measured using the principle of thermal sensing. As always, sensing or actuation is a process of energy conversion, called transduction. The transduction effect is realised in three ways: the thermoelectric effect (Seebeck effect), the thermoresistive effect (bolometer effect) and the pyroelectric effect.

In 1821, Thomas Johann Seebeck discovered that a compass needle was deflected when it was placed in the vicinity of a closed loop formed from two dissimilar conductors and the junctions maintained different temperatures. This is as good as saying that a voltage (and hence the current) is developed in a loop containing two dissimilar metals, provided the two junctions are maintained at different temperatures. The magnitude of the deflection is proportional to the temperature difference and depends on the material, and does not depend on the temperature distribution along the conductors. The effect is called the thermoelectric effect, and is the basis of the thermocouple, a real temperature measuring device. The opposite of the Seebeck effect, in which current flow causes a temperature difference between the junctions of different metals, is the Peltier effect. The reversed Seebeck effect is thus the Peltier effect. A thermopile is a serially interconnected array of thermocouples. Thermopiles are used for achieving better sensitivity.

Pyroelectricity is the migration of positive and negative charges to opposite ends of a crystal's polar axis as a result of a change in temperature. The cause is referred to as electric polarisation. The polarisation phenomenon within the material further states that below a temperature, known as the Curie point, crystalline materials or ferroelectric materials exhibit a large spontaneous electrical polarisation in response to a temperature change. Materials, which possess this property, are called pyroelectric materials. The change in polarisation is observed as an electrical voltage signal and appears if electrodes are placed on opposite faces of a thin slice of such material. The design can be thought of as a typical form of a capacitor. Cooling or heating of a homogeneous conductor resulting from the flow of an electrical current in the presence of a temperature gradient is known as the Thomson effect. It is hence defined as the rate of heat generated or absorbed in a single current carrying conductor, which is subjected to a temperature gradient. Arne Olander observed that some alloys such as NiTi, CuZnAl, CuAlNi, etc. change their solid-state phase. The property is reflected as pseudo-elasticity and the shape memory effect. After alloying and basic processing, the alloy can be formed into a desired shape, a coil for example, and then set to that shape by a heat treatment. When the shape is cooled, it may be bent, stretched or deformed and then with subsequent re-heating (which should be the heat setting temperature) the deformation can be recovered. Some of the main advantages of shape memory alloys (SMA) are that they are biocompatible with good mechanical properties such as strong and corrosion resistant and can be applied to diverse actuator applications.

2.5.1 Thermometry

Rigorous thermal analyses and experiments that assist in designing MEMS structures are in progress. Some effort has been made on liquid-crystal thermometry of micromachined silicon arrays for DNA replication. This work is being carried out at Perkin-Elmer Applied Biosystems. Replication is achieved through what is known as the polymerase chain reaction (PCR) process. PCR requires accurate cycling of the liquid sample temperature with an operational range between 55 and 95oC. PCR that makes use of micromachined structures is shown in the Fig. 2.5(a). The structure assures uniformity as far as temperature and cycle timing is concerned. It also utilises less reagent and sample volumes. Thermal design requires measurement of the temperature distribution in the reacting liquid. The measurement is possible by encapsulating the liquid crystals suspended in the liquid. This in turn led to the measurement of the temperature uniformity and the time constant for about 20 vessels in a micromachined silicon array. Two separate sets of crystals are used to image temperature variations near the two processing temperature thresholds with a resolution of 0.1oC. While the thermometry technique described above is useful for characterising microfabricated PCR systems, it can also support thermal designs of a broad variety of MEMS fluidic devices.

Silicon vessel arrays for DNA PCR

Fig. 2.5. (a) Liquid-crystal thermometry based micromachined vessel array for thermal processing of DNA using polymerase chain reaction (PCR) (Source: Woudenberg and Albin, Perkin Elmer Applied Biosystems)

Silicon vessel arrays for DNA PCR

Fig. 2.5. (a) Liquid-crystal thermometry based micromachined vessel array for thermal processing of DNA using polymerase chain reaction (PCR) (Source: Woudenberg and Albin, Perkin Elmer Applied Biosystems)

Thermally activated systems are mostly employed in the bio-analytical microsystems or lab-on-a-chip (loac) devices. Several key issues in existing and emerging bio-analytical microsystems are directly related to thermal phenomena. Research and development on loac technology is essentially directed toward miniaturisation and the integration of chemical and biochemical analysis tools for manipulation, handling, processing and analysis of samples in a single integrated chip. In order to develop a highly sensitive and responsive thermal device, the thermal mass of the transducer element is kept as low as possible. This is only achieved by using thin film structures made by micromachining techniques.

2.5.2 Data Storage Applications

There are many other applications for thermal designs. One of the more important examples is the design of silicon cantilevers for high-density thermo-mechanical data storage applications. The design method can use the principle of the atomic force microscope (www.stanford.edu/group/microheat/seven.html). MEMS based scanning probe data storage devices are emerging as potential ultra-high-density, low-access-time, and low-power alternatives to conventional data storage devices. The implementation of a probe based storage system uses thermo-mechanical means, as described below, to store and retrieve information in thin films. The design and characterisation of a servomechanism to achieve precise positioning in a probe based storage device is extremely critical. The device includes a thermal position sensor that provides position information to the servo controller. The research work by Kenny and Chui at Stanford University in collaboration with IBM is based on the design of a microcantilever tip that exerts a constant force on a polycarbonate sample and induces localised softening and deformation during the heating phase. This is caused by a bias current along the cantilever. The resulting serrations serve as data bits, which are read by the use of another separate cantilever integrated with a piezoresistive displacement sensor (Fig. 2.5(b)) followed by measuring circuitry. The cooling time constant of the heated cantilever tip governs the rate at which it can achieve sub micrometer writing. One of the challenges in building such devices is the accuracy and the latency required in the navigation of the probe.

Fig. 2.5. (b) AFM based single-crystal silicon cantilevers tip for data storage (Source: JMEMS Vol.7, pp69 (1998)).

2.5.3 Microhotplate Gas Sensors

Data storage cantilever

Fig. 2.5. (b) AFM based single-crystal silicon cantilevers tip for data storage (Source: JMEMS Vol.7, pp69 (1998)).

Semiconducting SnO2 films when deposited on microhotplate platforms can detect gas species such as hydrogen and methanol (Semancik, 2001; Panchapakesan,

2001). Microhotplates are thermally isolated micromachined platforms with integrated temperature sensing and actuation for closed-loop thermal control. Typical dimensions of a thermal platform have lateral dimensions of less than 100 micrometer, and are suspended over a bulk-etched cavity for thermal isolation. The physical architecture of each microhotplate consists of a polysilicon heater, a thermoresistive film for temperature measurement, and a semiconducting SnO2 film, which exhibits a change in conductance with the adsorption of chemical species. With aluminum as the temperature sensing film, the microhotplate can operate up to 500oC. Thermal response time for a 100-micron wide microhotplate has been measured at around 0.6 ms, with a thermal efficiency of 8 C/mW (Semancik 2001; DeVoe 2003).

2.5.4 Thermoactuators

Thermal actuation is another scenario with respect to its counterparts including electrostatic and piezoelectric types. Thermal actuation is based on electrothermal energy density transformation which is given by, E=V2/pL2, where V is the applied voltage, p is resistivity and L is the effective length of the actuator element. Microactuators based on electrothermal principles can offer significant energy density compared to electrostatic microactuators. It is worth mentioning that for some configurations, electrothermal actuation provides 1 to 2 orders of magnitude higher energy density than piezoelectric actuation, and 4 orders of magnitude higher energy density than electrostatic transduction (DeVoe 2003). Several thermal microactuator geometries have been investigated, including U-beam and V-beam geometries, respectively (Schreiber 2001; Maloney 2000). Piezoelectric microactuators usually provide a fraction of the energy density of electrothermal elements.

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