Design and fabrication considerations in developing high-Q MEMS capacitors and inductors

Design and fabrication considerations in developing high-Q MEMS capacitors and inductors,10.1109/SIRF.2011.5719335,Mina Rais-Zadeh

Design and fabrication considerations in developing high-Q MEMS capacitors and inductors  
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Despite great advancement in micromachining techniques, design and fabrication of high-Q tunable capacitors and inductors remain to be a challenging task. This paper discusses the design and fabrication considerations in developing high-Q fixed and tunable micro- electromechanical capacitors and inductors. Electrostatic actuation mechanism is employed for tuning the value of passives. The measurement results of several high-Q capacitors and inductors fabricated on silicon substrate are presented, and a number of research directions to improve the performance of tunable passives are proposed. Continuously tunable micro-electromechanical systems (MEMS) and micro-machined passives (i.e., inductors and capacitors) are critical elements for miniaturized low- power cognitive radios. Based on their actuation type, tunable passives can be classified as: electrostatic, thermal, piezoelectric, or magnetic. Among them, electrostatic actuation can offer the highest actuation speed and the lowest DC power consumption. In addition, the biasing network of electrostatic actuators is relatively straightforward. As such, there has been extensive research on developing electrostatically tunable passives and particularly capacitors and several devices with wide tuning range have been reported in the literature (1) (3). However, for successful insertion of these devices into RF systems, several other performance aspects of the device are equally important and need to be addressed; some or none of which is considered for most reported tunable capacitors. These include quality factor (Q), self- resonance frequency (SRF), temperature stability, travel range, power handling, tuning speed, and tuning linearity. Development of a continuously tunable inductor that satisfies all of the above mentioned criteria has been even more challenging. Therefore, most reports are focused on discrete tuning mechanisms for altering the inductance value (4). This paper presents the design and fabrication considerations in developing high-performance continuously tunable capacitors as well as tunable inductors with emphasis on the electrostatic actuation scheme. On the other hand, the size and the low-Q of fixed inductors have been the major barriers in increasing the performance and reducing the size of RF integrated circuits (RF ICs) such as filters, voltage controlled oscillators (VCOs), and power amplifiers (PAs). Several researchers proposed inductor-less approaches to reduce the size of these modules at the cost of increased power consumption and reduced dynamic range and linearity. This paper presents a detailed analysis of the loss mechanisms that limit the quality factor of lumped passives on silicon. The measurement results of a number of high-performance fixed inductors are also presented in this paper.
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