research activities in 2007 - CSEM

In order to optimize the performances of the resonator and
guarantee long-term frequency stability, one needs to perform
hermetic packaging under reduced pressure. The rear side of
the resonators is closed hermetically by low temperature
fusion bonding (with Si wafer) or by anodic bonding (with
Pyrex). Both methods require an extremely smooth surface of
the wafer.
The resonator chips are then vacuum encapsulated using
silicon caps (ultimately working ICs) and AuSn (80% wt Au)
soldering technology. Metallic alloy materials provide both
low-permeability sealing characteristics as well as electrical
conduction for the resonator driving interconnects. The AuSn
electroplating process was carried out at the Fraunhofer
Institute for Reliability and Microintegration (IZM FhG).
Vacuum sealing of the resonator chips is done through a twostep
process:
• Tacking of the sealing cap on the resonator chip at a
temperature below the AuSn melting point using flip-chip
• Reflow under vacuum in a dedicated oven
The tacking methodology proved to be successful. Taguchi
runs using thermo-compression parameters (temperature,
force, dwelling time) as variable experimental factors were
carried out.
Reflow process development, where resulting vacuum level is
monitored through Q factor measurements, is on-going.
A differential oscillator structure has been chosen to minimize
the circuit power dissipation despite the large shunt
capacitance of the resonator (~10 pF). A programmable
fractional divider is used to generate a thermally compensated
32768 Hz clock from the 960 kHz oscillator signal that drifts by
-28 ppm/°C. The output of a high resolution temperature
sensor integrated on the same die is used by a sequencer to
implement an open-loop compensation algorithm that requires
initial calibration of the resonator absolute frequency and
thermal drift. The state machine has been implemented on an
external FPGA to yield greater flexibility. Communication with
the IC to read the thermal sensor indication and update the
fractional divider ratio is ensured via a serial bus interface.
Figure 4 shows a photograph of a miniature packaged
resonator that has been glued above the IC mounted over a
printed circuit board. The close vicinity of the resonator and
the thermal sensor located within the IC minimize any thermal
gradient that would affect the compensation accuracy.
Extensive testing of the IC with the miniature packaged
resonators will be initiated once satisfactory vacuum levels are
reached within the micro-cavity to assess the performance of
the thermo-compensated time-base.
Figure 4: Photograph showing a chip on board assembly of a
miniature time-base
[1] J. Baborowski, et al., “Piezoelectrically Activated Silicon
Resonators”, IEEE Frequency Control Symposium, 1210-1213
(June 2007)
[2] B. Kim, et al., “Si-SiO2 Composite MEMS Resonators in CMOS
Compatible Wafer-Scale Thin-Film Encapsulation”, IEEE
Frequency Control Symposium, 1214-1219 (June 2007)
[3] J.M. Mayor, et al., “Micro-Vibration Analysis Setup for MEMS
and MOEMS Characterization”, in this report, page 72
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