Voltage Tunable Radio Frequency Microelectromechanical Resonators And Filters
Advanced radio technology demands for low power consumption and compact architecture that operate at the global range of frequency standards. Using discrete components based on traditional surface acoustic wave (SAW) or thin film bulk acoustic resonator (TFBAR) technology to manufacture quad-band and 7-band radios presently requires an enormous number of power-hungry and large-size filters fabricated on different substrates. On the other hand, employment of Micro Electromechanical Systems (MEMS) technology to create radio front-ends facilitates reductions in size, weight, cost, and power in radio communications. Several research groups from academia and industry have recently demonstrated electrostatically and ferroelectrically transduced microelectromechanical resonators and filters that operate at high frequency and exhibit low insertion loss as potential substitutes for conventional quartz and ceramic devices. However, the modern radio architecture today also requires the ability to discern bandwidths between 0.1 MHz and 5 MHz. This requires frequency-agile intermediate-frequency filters, generating a strong demand for narrow-bandwidth channel-select filters with very high quality factors (Q), which can be formed using high-Q MEMS resonators. This dissertation presents the state of the art transduction mechanisms of MEMS resonators and filters with key innovations in DC voltage tuning schemes, packaging technology and system integration. Electrically coupled, high-Q, tunable channel-select ladder filters comprised of dielectrically transduced thickness shear mode resonators are presented using integrated circuit compatible bulk micromachining technology. The ladder filter consists of shunt and series resonators operating in the half-wave thickness shear vibration mode. Each constituent resonator of the filter can be excited at above 810 MHz resonant frequency with Q of 7, 800 in air and a motional impedance (RX ) of 59 Omega. The ladder filter demonstrates a center frequency tuning range of 8 MHz at 817 MHz and an adjustable bandwidth from 600 kHz to 2.8 MHz, while maintaining an insertion loss less than 4 dB, stop-band rejection greater than 30 dB and pass-band ripple less than 2 dB. By having a tunability feature, radio frequency (RF) MEMS filters can accommodate various signal waveforms with bandwidth range of 0.1 MHz and 5 MHz. In addition, errors due to fabrication can be compensated and capacitive loading in receiver architecture can be minimized. Multi-frequency and multi-band filters on-chip has motivated the design and fabrication of a center frequency and bandwidth tunable RF MEMS filter using a series-coupled array of dielectrically-transduced square-extensional contour mode resonators. The proposed digital tuning scheme provides channel-agility and bandwidth granularity for analog spectral processors and RF spectrum analyzers. A 512 MHz overtone square-extensional mode resonator is demonstrated with a Q of 1, 800 in air and RX of 3.1 k-Omega. An array of four such resonators is coupled mechanically to form a channel-select filter with 1.4 MHz bandwidth at 509 MHz center frequency. By switching the DC-biasing scheme, the filter is split into narrower high and low sub-bands, each 700 kHz wide. Despite of the advances in device performance, packaging technology for MEMS resonators and filters remains a critical challenge. Because of the extreme sensitivity to environment, MEMS resonators and filters need a good vacuum encapsulation technology. The promising on-chip applications also require a CMOS compatible packaging process. The first successful combination of a dielectricallytransduced 200 MHz width-extensional contour mode resonator with the epi-silicon encapsulation process is demonstrated. The fabricated encapsulated-resonator exhibits a resonant frequency of 207 MHz and a Q of 6, 400. The high f x Q(1.2x10 exp(12) Hz) makes this encapsulated resonator an excellent candidate for applications in local oscillators for RF front-ends and frequency references. Dielectric transduction is not only limited to solid dielectric transduction. Lateral contour-mode resonators in which the transduction gaps are filled with a liquid dielectric having much higher permittivity than air are presented. Aqueous transduction is more efficient than air-gap transduction and has a higher frequency tuning range compared to solid dielectric transduction. A 42 MHz poly-SiGe disk resonator with de-ionized (DI) water confined to the electrode gaps is demonstrated. The resonator has a measured Q of 3, 800, RX of 3.9 k-Omega and 3% series frequency tuning range. Ferroelectric materials like lead zirconate titanate (PZT) is of interest because it offers a large electromechanical coupling coefficient (k sub(t) exp(2)) and electric field dependent permittivity and modulus of elasticity. In this effort, PZT transduced resonators with the same lateral dimensions are designed and fabricated with and without a silicon device layer to explore the insertion loss, linearity, tunability and Q trade-offs between the two types of resonators. A novel air-bridge fabrication technology is developed to minimize the pad capacitances and enables the frequency excitation of PZT transduced resonators above 1.8 GHz. PZT transduced fully-differential filters are designed by mechanically coupling two high-overtone width-extensional mode resonators. The fully-differential filter configuration cancels the feed-through capacitance and improves the stop-band floor of the filter. The demonstrated electric field tuning provides channel-agility and bandwidth adjustability for incorporation of analog spectral processors in state of the art radio receiver architectures. The filter demonstrates a center frequency tuning range of 7 MHz at 260 MHz and an adjustable bandwidth from 3 MHz to 6.3 MHz, while maintaining a maximum frequency shift due to hysteresis effects below 0.14% and a stop-band rejection floor of -60 dB. In order to improve immunity to hysteresis and realize the narrow bandwidth channel-select filter, the PZT transduced fully-differential filters are integrated with silicon device layer. A 206.3 MHz high-overtone width-extensional filter is demonstrated with 653 kHz bandwidth, -25 dB insertion loss and -62 dB stop-band rejection in air. Uncompensated temperature coefficient of frequency (TCF) of -16 ppm/degrees C and third-order input intercept point (IIP3 ) of +29 dBm are demonstrated by the filter. The monolithic integration of RF components has long been a goal of researchers and will enable not only more compact and lower cost systems but previously unachievable signal processing functions. This dissertation provides the first experimental demonstration of monolithically integrated piezoelectric RF MEMS switches with contour mode filters. PZT thin films are utilized to enable both low-voltage switch operation and filter tunability. This research leverages previous work using PZT actuators for low-voltage, wide-band switches and PZT transduced silicon resonators. The two device technologies are combined using a hybrid fabrication process that combines the key components of each device fabrication into a single unified process using silicon-on-insulator (SOI) substrates. The voltage tunable and switchable filter array provides a drop-in solution for frequency-agile channel selectivity required in tranceiver front-ends.
Resonators And Filters
Dissertation or Thesis