Abstract
Reference oscillators are used in a wide range of
electronic devices for timing and for providing the
frequency reference signals for wireless communications.
Typically, an oscillator has to be based on a mechanical
resonator, and for many decades, quartz crystals have
served for this purpose. With the progress of
microelectromechanical system (MEMS) technologies,
silicon resonators have been developed for providing
similar functionality as quartz. A silicon MEMS resonator
can offer several advantages over quartz, such as smaller
device size, decreased costs, and integration with other
electronics. This work focuses on two challenges in
silicon resonators: First, electromechanical transduction
of silicon resonators has typically been achieved with
electrostatic coupling, which is inherently quite weak
and requires DC biasing of the devices and tends to
complicate fabrication. Transduction based on a
piezoelectric thin film on top of the resonator has been
investigated as an alternative. Second, the resonance
frequency of a silicon resonator is orders of magnitude
more sensitive to temperature variations than that of a
quartz crystal. Degenerate doping of silicon can be used
to drastically reduce this effect. The first part of the
work concentrates on the design, fabrication and
characterization of piezoelectrically transduced silicon
resonators. An oscillator based on a width extensional
resonator operating at a frequency 24 MHz is demonstrated
to have a phase noise -128 dBc/Hz at a 1-kHz offset from
the carrier. An experimental test is conducted on
piezoelectrically transduced square extensional mode
resonators, whose dimensions are varied so that the main
resonance mode occurs at a frequency range of f = 13 ...
30 MHz. As a result, an anchor coupling effect is
identified and a subharmonic nonlinear coupling mechanism
is discovered. In the second part of the work, the effect
of degenerate doping on the elastic parameters of silicon
is investigated experimentally, with a focus on
temperature compensation. Resonance modes that can be
temperature compensated using doping are identified, and
design rules for the optimization of the frequency
stability are developed. The elastic parameters of
silicon are determined as a function of temperature and
n-type doping up to a level of n = 7.5x1019cm-3, enabling
modelling of the frequency-vs-temperature characteristics
of an arbitrary resonator design. Extrapolation from the
results yields a prediction of full second order
temperature compensation in optimally designed resonators
for n-type doping level above 1020cm-3. The prediction is
experimentally verified by the demonstration of
piezoelectrically transduced resonators with frequency
stability within +/- 10 ppm on a temperature range of T =
-40 ... +85C, on par with the best quartz crystals.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 29 Jan 2016 |
Publisher | |
Print ISBNs | 978-952-60-6615-8 , 978-951-38-8388-1 |
Electronic ISBNs | 978-952-60-6616-5, 978-951-38-8387-4 |
Publication status | Published - 2016 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- MEMS
- silicon resonators
- temperature compensation
- doping
- AlN