The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Microelectromechanical Resonators for Frequency Reference and Frequency Conversion Applications

Mika Koskenvuori

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Electronics, Communications and Automation for public examination and debate in Large Seminar Hall of Micronova at Helsinki University of Technology (Espoo, Finland) on the 4th of April, 2008, at 12 noon.

Overview in PDF format (ISBN 978-951-22-9279-0)   [2841 KB]
Dissertation is also available in print (ISBN 978-951-22-9278-3)

Abstract

Microelectromechanical systems (MEMS) have been used in sensor applications for more than three decades. With improvements in design and microfabrication, MEMS based solutions are also entering the field of RF electronics.

In this thesis, the main emphasis is laid on two different capacitively coupled resonant micromechanical systems, which are the frequency reference and frequency down-converter. It is shown that a high precision frequency reference with the stability comparable to a macroscopic quartz oscillator can be realised using a micromechanical silicon resonator. An oscillator with a phase-noise of ℒf = −138 dBc/Hz at ∆f = 1 kHz offset from the fc = 13 MHz carrier and with a noise-floor of ℒ = −150 dBc/Hz is realised with a bulk acoustic vibrational mode. It is also shown that the long-term stability of the encapsulated resonators is sufficient for high precision frequency references: the measured frequency stability is ∆f/f < ±1 ppm for a period of t = 1000 h with a drift of |∆f/∆t| < (1.15 - 5.25)×10−4 Hz/h. These results suggest that the demands on the centre frequency stability, which is usually specified for a time scale of one year for various frequency references in wireless communication, can be fulfilled. For a more detailed study of the instability mechanisms of capacitive MEMS devices a fast method to measure the built-in potentials is presented. Two approaches to convert the high frequency input signal down to the mechanical resonance frequency of the MEMS-devices using micromechanical mixers are demonstrated. In the first method a local oscillator signal is used to perform the conversion to the carrier signal at fc = 390 MHz. The second method leads to an intrinsic conversion of an AM-modulated signal without any local oscillator. This conversion is demonstrated up to fc = 1.5 GHz. The second method is used to perform the demodulation of a signal carrying a few bits of information in a micromechanical radio.

Parametric amplification is suggested as a low-noise method to improve the conversion performance of micromechanical mixers. An improvement of the conversion performance by more than 30 dB is demonstrated. Novel approaches to improve the conversion are suggested through mathematical simulations.

A fabrication process combining atomic layer deposition, electron beam lithography and cryogenic etching is demonstrated. The process can be used for fast prototyping of MEMS-devices at least for research purposes. As a result a few micromechanical resonators can be fabricated in a few hours time from the original design without the need of using a metallic photomask but applying a direct electron beam writing in lithography.

This thesis consists of an overview and of the following 7 publications:

  1. T. Mattila, V. Kaajakari, J. Kiihamäki, A. Oja, H. Kattelus, H. Seppä, M. Koskenvuori, P. Rantakari, and I. Tittonen, Silicon micromechanical resonators for RF-applications, Physica Scripta, T114, pp. 181-183 (2004). © 2004 Institute of Physics Publishing. By permission.
  2. M. Koskenvuori, T. Mattila, A. Häärä, J. Kiihamäki, I. Tittonen, A. Oja, and H. Seppä, Long-term stability of single-crystal silicon microresonators, Sensors and Actuators A: Physical, 115, pp. 23-27 (2004). © 2004 Elsevier Science. By permission.
  3. M. Koskenvuori and I. Tittonen, GHz-range FSK-reception with microelectromechanical resonators, Sensors and Actuators A: Physical, 142, pp. 346-351 (2008). © 2008 Elsevier Science. By permission.
  4. N. Chekurov, M. Koskenvuori, V.-M. Airaksinen, and I. Tittonen, Atomic layer deposition enhanced rapid dry fabrication of micromechanical devices with cryogenic deep reactive ion etching, Journal of Micromechanics and Microengineering, 17, pp. 1731-1736 (2007). © 2007 Institute of Physics Publishing. By permission.
  5. M. Koskenvuori, V.-P. Rytkönen, P. Rantakari, and I. Tittonen, Method for the fast measurement of built-in voltage inside closed cavity MEMS-devices, Journal of Physics D: Applied Physics, 40, pp. 5558-5563 (2007). © 2007 Institute of Physics Publishing. By permission.
  6. M. Koskenvuori and I. Tittonen, Improvement of the conversion performance of a resonating multimode microelectromechanical mixer-filter through parametric amplification, IEEE Electron Device Letters, 28, pp. 970-972 (2007). © 2007 IEEE. By permission.
  7. M. Koskenvuori and I. Tittonen, Towards micromechanical radio: overtone excitations of a microresonator through the nonlinearities of the second and third order, Journal of Microelectromechanical Systems, 17, pp. 363-369 (2008). © 2008 IEEE. By permission.

Keywords: micromechanics, resonator, stability, reference oscillator, frequency conversion, fabrication

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© 2008 Helsinki University of Technology

Last update 2011-05-26