MBE Grown AlN Films on SiC for Piezoelectric MEMS Sensors
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MBE Grown AlN Films on SiC for Piezoelectric MEMS Sensors Dharanipal Doppalapudi1, Richard Mlcak1, Jeffrey Chan1, Harry Tuller1, Anirban Bhattacharya2, and Theodore Moustakas2 1 Boston MicroSystems Inc., Woburn, MA 01801; 2Boston University, Boston MA 02215 Abstract Miniaturized piezoelectric sensors based on Microelectromechanical Systems (MEMS) offer the advantages of reduced size, reduced power consumption, increased sensitivity coupled with the ability to form compact multi-sensor arrays. Fabrication of such sensors from single crystal materials further insure more highly reproducible and stable devices with improved performance. In this paper, we describe the integration of MBE grown AlN films onto photoelectrochemically machined SiC microcantilevers and membranes. AlN exhibits excellent piezoelectric properties, including an electromechanical coupling coefficient of 0.088 and a high in-plane acoustic velocity (~5700m/sec) as well as excellent thermal-mechanical compatibility with SiC. The fabrication of AlN-SiC-based microresonators and flexural plate wave devices, and their application to chemical, biological and fluid sensing, are reported. Introduction III-V Nitrides (AlN in particular) have excellent piezoelectric properties and their potential use in SAW devices operating over the gigahertz frequency has long been appreciated, due to their high in-plane acoustic velocity in polycrystalline [1,2] and single crystal (epitaxial layer) forms [3,4,5]. Integration of III- nitride materials with microelectromechanical systems (MEMS) creates exciting and new opportunities in the miniaturization of various devices including arrays of optical devices, sensors (chemical, biological, gas and fluid), actuators and RF MEMS due to reduced size, reduced power consumption, coupled with the ability to form multi-element arrays. In addition, MEMS platforms have demonstrated remarkable sensitivity to various stimuli [6], often orders of magnitude greater than those achieved with competing technologies. While silicon based MEMS are readily available, silicon is not an ideal substrate for epitaxial growth of single crystal III- nitrides due to the high thermal expansion and lattice mismatches between these materials. The common use of polycrystalline films in surface micromachining results in less reproducible device characteristics due to difficulties in replicating, from run-torun, the polycrystalline morphology, composition, uniformity, and internal stress of the films. In contrast, epitaxial films on single crystal micromechanical elements can be expected to provide more highly reproducible and stable devices. SiC is currently the most suitable substrate (in lattice and thermal match) for epitaxial growth of III- nitrides. Furthermore, SiC has excellent mechanical strength (Young’s Modulus of 448 GPa is 2.5 times that of Si), chemical and thermal stability and an exceptionally high thermal conductivity (5 W/cm-°C), making it an ideal material for harsh environment applications. AlN and SiC also have excellent aco
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