research activities in 2007 - CSEM

PackTime – Zero-Level Packaging of Silicon Time-base
D. Ruffieux, J. Baborowski, M. Fretz, S. Grossmann, C. Henzelin, I. Kjelberg, T.C. Le, J.-M. Mayor, A. Pezous, A.-C. Pliska,
A. Schifferle, G. Spinola Durante, Y. Welte
The development of a thermally compensated silicon time-base and the associated packaging to yield a miniature vacuum-sealed cavity around the
resonator requires multidisciplinary competences in the fields of IC, MEMS design/fabrication, packaging, finite element modeling and metrology.
The low power frequency and timing market relies almost
exclusively on quartz resonators to derive precise and low
aging references thanks to the availability of crystal cuts with
null first order temperature coefficient on frequency (TCF).
Silicon on the other hand appears quite attractive from a
miniaturization perspective but suffers from a severe
drawback with a TCF close to -30 ppm/°C whatever the
crystal orientation. Consequently, electronic compensation
that can possibly be combined with structural
compensation [1, 2] appears as one of the most promising
workarounds to reach performances similar or better than that
of AT-cut quartz crystal achieving null first and second TCF.
The development of such a high performance, miniature and
low power real time silicon clock (RTC) together with the
associated packaging technology entails multidisciplinary
competences in IC and MEMS design/fabrication, packaging,
FEM and metrology. The activities ongoing in each of these
fields are described in the following sections after an overview
of the system is presented. Figure 1 shows a cross-section of
the envisioned miniature whole silicon time-base that consists
of a rear side packaged silicon resonator integrated on a SOI
substrate that is assembled and interconnected by a flip-chip
and reflow process to an IC capping die generating the
thermally compensated clock. The reflow process is
performed under vacuum and should ensure hermeticity of the
cavity that is formed around the resonator to take advantage
of the high quality factor of the latter and minimize any aging
of the time-base.
Figure 1: Cross-section of the miniature zero-level packaged silicon
timebase with a vacuum-sealed cavity around the resonator
A FEM CAD model of the complete resonator including its
package has been developed to help determine the sensitive
parameters that affect the resonator frequency during and
following the assembly process and could then be responsible
for excessive aging. Thermo-mechanical simulations are also
very valuable to predict the performance of the compensated
time base that relies on a good matching of the resonator and
sensor temperature and that may be affected during thermal
transients. The lack of availability of precise data for the
stiffness of the materials involved in the fabrication of the
resonators and their temperature dependency has motivated
the development of an optical metrology bank [3] and dedicated
test structures that should help future resonator design and
allow more precise FEM analysis once measurement and
extraction is completed.
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The resonator exploits a high-Q in-plane, longitudinal,
extensional mode and is formed of a T-shaped silicon beam,
typically 1000 x 250 µm 2 , anchored at its base end, with an
inertial mass at each extremity. Figure 2 shows some
extensional resonators after processing. The driving voltage is
applied on the piezoelectric layer only in the central part of the
beam. The resonators are built from a (100) oriented Silicon
on Insulator (SOI) substrate. Structure of the resonator
consists mainly of single crystal silicon that is oxidized on both
sides, and that is topped by AlN and its electrodes.
Polycrystalline piezoelectric (002) AlN films are deposited by
magnetron sputtering on Pt (111) electrode. A metal ring is
patterned around the resonators for subsequent assembly
with the capping wafer.
Figure 2: Microphotograph of fabricated resonators
Si resonators in extensional mode, oriented along with
a thickness of 105 microns, and activated by 2 micrometers of
AlN, exhibit a Q factor under vacuum of 140000 and k 2 eff
around 0.05%. Q factor at atmospheric pressure is up to
20000, and increases linearly when the pressure decreases.
In order to obtain the maximum Q factor the pressure must be
below 0.1 mbar. The measured impedance in air and under
vacuum is plotted in Figure 3. The resonance frequency of
these resonators is close to 960 kHz, the motional resistance
is in the range of 200 Ohm and the linear TCF is -28 ppm/°C.
Figure 3: Impedance plot of high Q resonator in air and vacuum