Microelectromechanical Resonators for Frequency Reference and Frequency Conversion Applications

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Mika Petteri Koskenvuori Name of the dissertation Microelectromechanical resonators for frequency reference and frequency conversion applications Manuscript submitted 07.12.2007 Manuscript revised 03.03.2008 Date of the defence 04.04.2008 Monograph Article dissertation (summary + original articles) Faculty Faculty of Electronics, Communications and Automation Department Department of Micro and Nanosciences Field of research Microsystems Opponent(s) prof. Gabriel Abadal Supervisor prof. Ilkka Tittonen 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 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.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 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.