Epitaxially grown polycrystalline silicon (epi-poly) has shown great promise as a MEMS material, offering isotropic material properties with minimal residual stress and stress gradients. While epi-poly has been used previously for MEMS inertial systems or sensors, its use in high precision resonator applications has been restricted to an encapsulation layer for resonators fabricated in single crystal silicon. In this work, encapsulated resonators fabricated with epi-poly as the functional layer were tested alongside single crystal silicon devices with regard to resonant frequency, quality factor, temperature dependence, and stability. Test results indicate that epi-poly resonators have very similar mechanical properties and stability to single crystal silicon resonators when operated in a clean package.
MEMS sensors gained over the last two decades an impressive width of applications:(a) ESP: A car is skidding and stabilizes itself without driver intervention (b) Free-fall detection: A laptop falls to the floor and protects the hard drive by parking the read/write drive head automatically before impact. (c) Airbag: An airbag fires before the driver/occupant involved in an impending automotive crash impacts the steering wheel, thereby significantly reducing physical injury risk.MEMS sensors are sensing the environmental conditions and are giving input to electronic control systems. These crucial MEMS sensors are making system reactions to human needs more intelligent, precise, and at much faster reaction rates than humanly possible.Important prerequisites for the success of sensors are their size, functionality, power consumption, and costs. This technical progress in sensor development is realized by micro-machining. The development of these processes was the breakthrough to industrial mass-production for micro-electro-mechanical systems (MEMS). Besides leading-edge micromechanical processes, innovative and robust ASIC designs, thorough simulations of the electrical and mechanical behaviour, a deep understanding of the interactions (mainly over temperature and lifetime) of the package and the mechanical structures are needed. This was achieved over the last 20 years by intense and successful development activities combined with the experience of volume production of billions of sensors.This chapter gives an overview of current MEMS technology, its applications and the market share. The MEMS processes are described, and the challenges of MEMS, compared to standard IC fabrication, are discussed. The evolution of
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