ABSTRACT
Polymer-based micro/nanoelectromechanical systems open a new area of research interest for their possible use in a variety of domains such as physical sensing, chemical sensing, bio-sensing, and inertial sensing applications. Polymer micro-electromechanical system (MEMS) devices have potential to find their place in mainstream sensing area because of their low processing cost, versatility to adapt to different sensing applications such as from thin films to high-aspect-ratio structures, low stiffness, and biocompatibility. In a broader view, micro-electromechanical systems also encompass micro-actuators. 2.1 Different Transduction MechanismsIn this section, a few fundamental concepts and methods about transduction are reviewed. Most widely used transduction
mechanisms are piezoresistive, piezoelectric, capacitive, thermal, and tunneling resonant frequency-based techniques. 2.1.1 Piezoresistive Transduction
Piezoresistive effect is a widely used transduction mechanism for sensors. This is the effect of change in resistance in an electrical resistor upon application of an external strain. Such an effect is seen in various materials from metals to semiconductors and composites. The change in resistance can happen mainly in two ways: (1) geometrical change due to strain, which is particularly dominant in the case of metal strain gauges and (2) change in the resistivity of the material due to internal atomic position and their motion because of strain.1,2 These days, piezoresistive effect is used in the MEMS field for a wide variety of sensing applications, including accelerometers, pressure sensors, cantilever-based sensors, tactile sensors, gyros, flow sensors, and many other chemical and biological sensors. 2.1.1.1 Metal strain gaugeThe most well-known piezoresistor for sensing purposes is a metal strain gauge; metal strain gauges were used even before semiconducting materials such as silicon came into picture. The origin of piezoresistivity in metals lies in the geometry changes and in a change of the internal atomic positions upon application of stress. Due to stress, the energy band of metals undergoes a slight distortion resulting in a change in conduction, leading to the piezoresistive effect. In MEMS fabrication processes, metal thinfilm resistors are generally deposited using sputter or evaporation. Elemental metal thin films used as strain gauges in MEMS usually have their gauge factors ranging from 0.8 to 3. Strain gauges made of thin-film metals do not compare favorably with semiconductor strain gauges in terms of their gauge factor values. However, the main reason for use of metal strain gauges instead of semiconductor materials is that this would eliminate the need for doping. Metals can also generally sustain a much greater elongation before fracture. As such metal resistors can be integrated with polymer materials for polymer MEMS device applications.3,4To meet the requirements of aeronautics and aerospace
applications, where the stress gradients encountered are high, thin-film strain gauges (TFSGs) using palladium-13% rhodium as piezoresistive materials have been reported.5 Gold piezoresistors have also been used for fabrication of an SU-8 three axis sensor for measuring tactile sensitivity and force exerted during locomotion of small biological organisms.6 Doped silicon piezoresistors are preferred over metal strain gauges for sensor applications because of the low gauge factor for the metals. However, metal thinfilm piezoresistors are desirable in devices fabricated using polymer MEMS technology, since low processing temperatures are involved. 2.1.1.2 Doped silicon piezoresistorThe piezoresistivity in silicon arises in the same way as in metal strain gauges, though the dominant mechanism is often different. When a stress is applied to a semiconductor, the band energies change by a small amount and unlike in metals, this small shift in energy band causes a significant change in conductivity in semiconductors. This gives a large piezoresistive gauge factor. Relative orientation of crystallographic direction of silicon affects the piezoresistive coefficients. Considering a rectangular coordinate system having arbitrary orientation with respect to the crystallographic axes of a homogeneous semiconductor, the electric field components Ei and current density components ii are related by a symmetric resistivity matrix Eq. 2.1. 1 6 56 2 45 4 3=
E i
E i
E i
r r r r r r r r r (2.1) There are six independent stress components in 3D-space, three normal stresses (sxx, syy, szz) and three shear stresses (txy, tyz, tzx).
