Piezoelectric Nano Resonators
High-Q Mechanical Flux Gate for CSAC
Gianluca Piazza
Advisor: Prof. A.P. Pisano
Graduate Student Researcher
Berkeley Sensor and Actuator Center (BSAC)
University of California, Berkeley
Berkeley, CA 94720-1740
TEL: (510) 643-8713
FAX: (510) 643-8426
piazza@eecs.berkeley.edu
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
The Chip-Scale Atomic Clock (CSAC)
DATUM
RubiSource 2000
Frequency Electronics
FE-5680A
UC Berkeley
Integrated Nano
Mechanically-Regulated
Atomic Clock
Power: 30 mW
Size: 1 cm3
Accuracy: 1x10-11
Power: ~ 80 W
Power: 11 W
Size: 9050 cm3
Size: 275 cm3
Accuracy: 5x 10-11
Accuracy:1x10-11
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
The Integrated Nano Mechanically-Regulated
Atomic Clock
Frequency
Electronics
Cornell
University
Photodiode array
Electronics
Rb87 cell
Excited state
Thin film magnet
Ni63 film
Rb85 cell
Lenslet array
Ground state
Feedback loop
UC Berkeley
Permanent magnet
Nanomechanical piston
resonator array
Depopulation via
oscillating magnetic
filed (ν = 6.835 GHz)
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Piezoelectric Resonators for CSAC
• Mechanical regulation of magnetic field at 3.4175 GHz
Nanomechanical piston
resonator array
Permanent magnet
• Replace VCXO and VCO in a standard laser-interrogated atomic
clock
Laser
VCO
Laser
Rb Cell
Rb Cell
VCXO
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Aluminum Nitride for the Nano
Mechanically-Regulated Atomic Clock
Optimal Properties of AlN:
• Very high sound
velocity
• Very low permittivity
• High Resistivity
• Good dielectric strength
Property
AlN
ZnO
PZT
Sound Velocity ([km/s])
11.4
5.35
4.5
Piezocoefficient (d33)
[pC/N])
~3.9
~11
~200
Permittivity (ε3)
9
10
~1000
Resistivity ([Ωcm])
1013
107
109
Dielectric Strength
([kV/mm])
20
10
100
Mumetal® Plate
Aluminum or Platinum Electrode
Aluminum Nitride (AlN) Piezoelectric
Silicon Substrate
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
AlN Film Deposition and Characterization
Material Requirement : Low stress C-axis oriented AlN films (500-1000 nm)
AlN(002)
1.2 µm
AlN films deposited on both Si and Pt substrates by a DC magnetron sputtering
• Films are continuous with columnar structure and close pack grains (SEM)
• Orientation: C-axis <002> (X-ray diffraction)
• Residual stress: ~ 20 MPa (Tencor Flex)
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Resonators for CSAC
• Two possible designs are being evaluated for CSAC: pistonmotion and shear-motion nano-mechanical resonators to be
operated at 3.4175 GHz
Mumetal
AlN
Silicon
Mumetal
AlN
Silicon
• The reference frequency of 6.835 GHz is achieved by
mechanically doubling the magnetic flux reversal
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Piston-Motion Resonators
• Suspended membranes envision the proposed piston-motion
resonators that will be used for the modulation of the magnetic field
in the atomic clock
• Membrane suspensions are under study and two possible solutions
are considered
f res =
1
2T
E
ρ
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Shear-Mode Resonators
• Corner-suspended membranes envision shear-mode resonators
• AlN needs to be deposited directly on Si
• Easier to downscale than piston-motion resonator: motional
resistance is primarily set by top electrode spacing
f res =
1
2T
G
ρ
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Bulk Longitudinal Resonators (BLR)
• Resonance frequency of BLRs is set by the device length and
limited by lithographic steps
• This device can cover frequencies up to ~2 GHz and can be
used as building block in RF filters for GSM
f res =
1
2L
E
ρ
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Resonant Disks
• Disk resonators are stiff and potentially high-Q
• The radius is the critical dimension and it is set by lithography
• Can be used for front-end RF filters up to 2 GHz
Input Signal
R
Output Signal
f res ∝
α
Eeq
2R
ρ
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Double-Ended Tuning Forks (DETF)
• DETF cover frequency range from 500 kHz up to 10 MHz
• Potential applications as strain gauges in the automotive
industry and for oscillators at intermediate frequencies (IF)
Input Signal
W
L
f res ∝
W
E
L2
ρ
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.
Output Signal
Future Directions
•
•
•
•
•
Optimize AlN deposition on Si and Pt substrates
Analyze residual stress in AlN films
Complete fabrication process
Characterize AlN piezocoefficients using AFM
Test VHF and UHF Resonators
Acknowledgements
• Prof. Pisano and CSAC group (APP 58 & APP 61)
• Dr. Wijesundara, Mr. Wasilik and Roger Su for help in
AlN deposition
BSAC ©2003 Fall IAB. Confidential Information. Not to be made public without permission from UC Regents.