Chi S l Mi f b i t d At iChip-Scale, Microfabricated Atomic ClocksC oc s
Elizabeth [email protected]
Time and Frequency Division, NIST, Boulder, COime and equency ivision, N S , oulde , CO
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I. Introduction to Atomic Clocks• Applications• Basic description
II. Chip-Scale Atomic Clocks (CSACs)• Basic science • Examples of components (not an exhaustive review)• Examples of components (not an exhaustive review)
III. State of the Art from Industry**• Status of performanceStatus of performance • Ongoing research and development
IV. Technology Spinoffs from Chip-Scale Atomic ClocksV. ec o ogy Sp o s o C p Sc e o c C oc s
** Products or companies named here neither constitute nor imply
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endorsement by NIST or by the US government.
Applications for higher-performance portable clocks
• Navigation.• Security/encryption.• RF/microwave broadcasts.• Telecommunications network
synchronization.y• Sensors (magnetic fields, gravity, etc.)• Unpredicted applications• Unpredicted applications.
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One Specific Application:p pp
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Chip-scale atomic clocks can now exceed this performance specification.
What is an atomic clock?
Atomic resonance is intrinsically more stable than quartz local oscillator.
“N l” i i f i h i d f RF LO“Natural” atomic microwave resonance frequency is synthesized from RF LO.Control Loop continuously steers LO frequency to atomic resonance.RF output (10 MHz) embodies stability of atomic resonance.
ITSF 2008Courtesy of R. Lutwak 2008 FCS Tutorial
Since 1967, the SI second has been defined by the frequency of a microwave transition in Cesium atoms.
Resonance frequencies of atomic transitions – Can be measured with high precision and accuracy
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g p y– Are relatively stable over time
Cesium Microwave Frequency Standardsq y
• 0.30 x 10-15 in house frequency uncertainty.• 0.6 x 10-15 uncertainty reported to BIPM.• Best in the world in house and reported
• Lower performance• MUCH lower power• MUCH smaller size
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• Best in the world, in house and reported uncertainties.
MUCH smaller size
Electronmagnetic
Nuclearmagnetic
To realize one SI second, count 9 192 631 770 l f th di ti
N NSS
magneticmoment
magneticmoment
6S
F=4 9,192,631,770 cycles of the radiation absorbed by this transition.
SS
6S1/2 9.192631770 GHz
NNS
S
F=3
Transition is between hyperfine ground state energy levels
NS
States correspond to different relative orientations of the valence electron and nuclear spins
Magnetic-field dependent substructure of energy levels has to be accounted for
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Magnetic field dependent substructure of energy levels has to be accounted for
Clock Stability:Set by the properties of the atomic resonance
5.674
ννΔ
= 0Q5.666
5.670
[V] S
QNS≈τσ
)/(1)(
νΔ
5.658
5.662
Sign
al Δν = 7.1 kHz
Nν QNS ⋅τ)/(
40 50 60 70 80 905.654
f 9 192 631 770 H [kH ]
ν0
• (S/N)τ is the signal-to-noise ratio at an averaging time of τ.
f - 9,192,631,770 Hz [kHz]
• ν0 is the center frequency of the atomic resonance• Δν is the width of the atomic resonance• A clock’s precision improves with lower values of stability
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• A clock s precision improves with lower values of stability
Miniaturized Atomic Frequency Referencesq y• Small size, low power dissipation BUT retain long-term
frequency stability typical of atomic standardsfrequency stability typical of atomic standards– Portability, battery operation
• State-of-the-art (as of very recently):– V ~ 100 cm3 DATUM/Symmetricom
– P ~ 5 W– Stability ~ 10-11 @ 1 sec. → 1 day
Frequency ElectronicsKernco
Stanford Research SystemsNorthrop-GrummanA bT
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Stanford Research SystemsNorthrop Grumman Physics Package
AccubeatTemex
Standard Portable Clock Technology Size/Power Limitations :
• Size– Microwave cavity (~ 1 cm3)– Control electronics (~ 10’s cm3)– Local oscillator/synthesizer
• Power– Lamp (watts)– Cell heating (100’s mW)– Local oscillator/synthesizer
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Chip-Scale Atomic Clocks (CSACs)Chip-Scale Atomic Clocks (CSACs)
• DARPA-funded effort to produce accurate timing sources f t bl i t tfor portable instruments– Began in 2001. Initially funded:
– NIST/U of ColoradoNIST/U. of Colorado– Symmetricom/Draper/Sandia– Teledyne Scientific/Rockwell Collins/Agilent
ll– Honeywell– Sarnoff/Princeton/Frequency Electronics
– Still ongoing with industry participants (Symmetricom & Teledyne)g g y p p ( y y )
• Key Specifications– Device Volume: < 1cm3
– Total Power Consumption: < 30 mW– Stability: 11101 hr) 1( -
y ×<=τσ
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• 100× smaller and lower power than current atomic clock technology.
(A Very Rough) Oscillator Comparison( y g ) p10-6 105
Day TCXO
MCXO ay [μ
s]
10-8 103
at O
neMCXO
OCXO One
Da
10-10 101
CompactRubidiumab
ility
a OCXO
Mi f b i t d Ove
r O
10-12 10-1
cy In
st
RubidiumMicrofabricatedRubidium (CSAC)
Better
tabi
lity
10-14 10-3
requ
en Cesium
H-maser "Battery ng In
st
10-3 10-2 10-1 100 101 102 10310-16 10-5Fr
H-maserOperated"
Tim
i
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Electrical Power Dissipated [W]Adapted from figure by M. Garvey, Symmetricom
“Classical” Compact Frequency Standardsp q y
E. Jechart, Efratomearly 1970searly 1970s
S ll l l t th i l il bl• Smaller, lower power, lower cost than previously available clocks
ITSF 2008Figure courtesy of R. Lutwak
Rubidium Miniaturization: Philosophyp y
Magnetic shield
1. Lamp → laser2. All-optical excitation g
“C-Field” Absorptioncell
3. Cell miniaturization4. Compact integration
Ph t Detector
87Rblamp
85Rb+ buffer
gasRb-87
+ buffergas
LightDiodelaser
CavityPhoto
cellDetectoroutput
F
FilterCell
Power suppliesfor lamp, filter
C-fieldpower
Frequencyinput
6.834,685 GHzrf
lamp for lamp, filterand absorptioncell thermostats
powersupply
lampexciter
ITSF 2008Adapted from figure by John Vig, Tom Parker
Laser TechnologyReplaces Lamp for small size low powerReplaces Lamp for small size, low power
• Requirements:Single freq enc operation– Single-frequency operation
– Wavelengths: Rb – D2: 780 nm, D1: 795 nmCs - D2: 850 nm, D1: 894 nm
– Low-power operation – High modulation efficiency at GHz frequencies 50 μmHigh modulation efficiency at GHz frequencies
• Vertical-cavity, surface-emitting lasers (VCSELs)– Typical ith < 1 mA; Pop ~ 4 mW– Modulation bandwidth > 5 GHz– Highly reliableHighly reliable
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All-Optical (CPT) ExcitationCyr Tetu and Breton IEEE Trans Instrum Meas 42 640 (1993)Cyr, Tetu, and Breton, IEEE Trans. Instrum. Meas. 42, 640 (1993)
ModulateModulate
smis
sion
DiodeLaser
Opticalsbsb
Tran
s
Modulation Frequency
E1
E2fields
carrier
• Non-linear process in atoms drives hyperfine oscillation at difference frequency between optical fields
• Absorption of light drops when modulation frequency equals half of the hyperfine splitting
• Advantage: no microwave cavity required
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• Advantage: no microwave cavity required
Buffer Gas CellsR.H. Dicke, Physical Review 89, 472 (1953)
Buffer Gas (N2 Ne Ar )Buffer Gas (N2, Ne, Ar, …)
Alkali Atom (Cs, Rb, …)
• Buffer gas• Buffer gas – confines Cs atoms to small volumes away from the cell walls and each
other– Eliminates first order Doppler Shift
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Scaling of Clock Stability with Sizeg y
• Allan deviation: τσ)/(
1)(NSQy ⋅
≈
Kitching, Knappe, & Hollberg, Appl. Phys. Lett. 81, 553 (2002)
• Both Q and S/N are functions of cell size
τ)/( NSQ
– Q-factor: collisions of atoms with walls of cell– S/N: smaller size ⇒ smaller optical power ⇒ shot noise
1E-7
1E-6
1E-5
Wall coated cell Buffer gas cell (100 kPa) Buffer gas cell (10 kPa)e
Seco
nd
Smaller size ⇒worse performance
1E-10
1E-9
1E-8
Northrop-FTS/Datum NIST 1
tion
at O
n pBUT
σ (1 sec) ~ 3×10-11
1E 13
1E-12
1E-11
1E 10 pGrumman
NIST 2Agilent
Alla
n D
evia σy (1 sec) 3×10
for 1 mm cell
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1E-6 1E-5 1E-4 1E-3 0.01 0.11E-13A
Characteristic Cell Size (m)
Cell Fabrication: Basic Structure
Excite/detect atoms
GlassBond
Silicon ~ 1 mm
GlBond
Buffer gas
Glass
Hole
Alkali atoms
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Cell Fabrication: Micromachining Process
<100> Si wafer DSP
LPCVD nitride dep
PR spin on
PR exposure
CF4 plasma etch
PR strip
Anisotropic KOH etch
Nitride stripReservoirs etched in Si
Glass bonding
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Cell Fabrication: Anodic Bondingg
• Preform created by KOH etching or DRIE of Si 1.0 DRIE of Si
• Pyrex bonded on one side with anodic + Cesium + buffer gas
(a) (b)
0.6
0.8
nsm
ittan
ce
bonding• Cell preform filled with Cs
BaN + CsCl → BaCl + Cs + 3N
V
300 oC
-
(c) (d)
0 2
0.4
mal
ized
Tra
n
– BaN6 + CsCl → BaCl + Cs + 3N2
@ 150 ºC
• Diced cells made at NIST using the V
+
-15 -12 -9 -6 -3 0 3 6 9 12 150.0
0.2
Nor
m
F (GH )• Diced cells made at NIST using the anodic bonding technique– Interior: 1 mm x ∅ 0.9 mm
300 oC
-
(e)
(f) Frequency (GHz)
– Exterior: 1.33 mm x (1.45 mm)2 Pyrex 7740 (125 µm)Silicon (375 µm)Pyrex 7740 (200 µm)
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Integration: Heater Fabricationg
H t f b i t d b d iti thiGold busbars
(2 mm)Heaters fabricated by depositing a thin
layer of Indium-Tin-Oxide onto glass
ITO layer(300 Å)
( )
glassGlass substrate
(125 mm)
– ITO: transparent, conductive material– Uniform heating of windowsg– Reduces solid alkali buildup– Gold contacts deposited via e-beam
ti bl i b devaporation enable wire-bonds
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Integration: Optics Assembly• Micro-refractive lens
– Inkjet deposition of optical epoxy
g p y
– Inkjet deposition of optical epoxy– Commercially available in arrays
• ND filters (opaque glass): – Total thickness = 1 mm– Total OD = 1.66
S250 μm
11 μW• Spacers– For light collimation,
thermal isolation QuartzND
11 μW
– Glass or SU-8• Waveplate
Q t 70 thi k GlassNDSiQuartz
Lens
VCSEL– Quartz 70 μm thick ⇒ λ/4 @ 850 nm Alumina
1.5 mm
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NIST Chip-Scale Atomic Clock
Volume: 9.5 mm3
Cell volume:0.81 mm3
Power:75 mW(Physics Package)Package)
Stability:σy(1 sec.) =σy(1 sec.) 3×10-10
ITSF 2008S. Knappe et al., App. Phys. Lett. 85, 1460 (2004)
CPT Resonance and Frequency Instabilityq y y
• Short term instability:Sh t i l AM i
5.6685.6705.672
gnal
[V]
– Shot noise, laser AM noise, FM-AM conversion
• Long-term instability: 5 6585.6605.6625.6645.666
otod
iode
Sig
7.1 kHz
Long term instability:– Drift, temperature fluctuations
of cell 40 50 60 70 80 905.6545.6565.658
Ph
Frequency Detuning from 9 192 631 770 H [kH ]
10-9 With linear drift present With linear drift removed
σy60540
60560
ncy
[Hz]
H
z Drift: -1.6x10-8/day
9,192,631,770 Hz [kHz]
10-10
Dev
iatio
n,
60460
60480
60500
60520
put F
requ
en92
,631
,770
1 10 100 1000 1000010-11A
llan
Integration Time τ [s]0 10000 20000 30000 40000 50000
60420
60440
60 60
Clo
ck O
utp
- 9,1
9
Time (s)
Counter gate time = 1 s
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Integration Time, τ [s]Time (s)
Improvements on Initial Performancep• Change from D2 to D1 laser excitation
– Switch to Rb atoms because of laser wavelength availability– >3× improvement in line contrast – 5× improvement in clock stability– S. Knappe, et al., Opt. Exp. 13, 1249, (2005).
I ll filli t h i t d d ift f t i ti• Improve cell filling techniques to reduce drift from contamination– Alkali beam filling: S. Knappe et al., Opt. Lett. 30, 2351 (2005)– Cesiated cell walls: F. Gong et al., Rev. Sci. Instrum. 77, 076101 (2006)g , , ( )
Cs D2Cs D2 Rb D1
Rb D1Rb D1Cleaner Cells
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Commercial Chip-Scale Atomic Clocks
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Symmetricom Physics PackageMescher, Lutwak, & Vargese., IEEE Solid-State Sensors and Actuators (2005)
Pdiss < 10 mWTcell ~ 80 °C, Tamb ~ 25 °CR di i d i dRadiation dominated
Excellent thermal isolation• Tensioned polyimide suspension
– Excellent thermal & mechanical properties
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• Vacuum-packaged to eliminate convection/conduction
Symmetricom Synthesizer• Modulation via digital control of PLL• Tuning via digital control of PLL • Output is 10 0 MHz
4596 MHzto PhysicsAttenuatorMicrowave
Power Adjust
• Output is 10.0 MHz• Power adjust 0 to -10 dBm• Good phase noise (-75 dBc @ 1 kHz)
4596 MHzVCO
• High Power (≈ 50 mW)Tuning
LoopFilter
E i16-bit DAC
Err in
MicroControllerTuning
&Modulation
Fractional-NPLL
10 MHzClock Output
18-bit DAC
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10.0 MHzTCXO
TCXO TuningSlide courtesy of R. Lutwak (Symmetricom)
Electronics Block DiagramR Lutwak et al., Proc. 2007 Joint EFTF Frequency Control Symposium (Symmetricom & Draper Labs)R Lutwak et al., Proc. 2007 Joint EFTF Frequency Control Symposium (Symmetricom & Draper Labs)
SignalConditioning
ADC122
4596 MHz
4596 MHzVCO
2 8 V Bi
C-FieldBias
Attenuator
DAC16
32110
Tuning
+2.8 V Bias
Wave Pow
LoopFilter
Err in
DAC16
DAC16
Temp Coarse
Temp Fine
TemperatureController
DAC16
DAC12
VCSEL Coarse
VCSEL FineDAC16
1
1 1
5
16
Fractional-N
Wave Pow
Vreg 3.0 VAnalog 20
1 1 1
2PLL
MicroController
ACMOSBUFFER
3.3 VSupply
Vreg 2.8 VuWaves
202011
10.0 MHzClock Output
10,.0 MHzTCXO
DAC16
DAC12
TCXO Coarse
TCXOFine 7Microwave System: 60 mW
11
Control System: 40 mW 1Physics: 10 mW
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TCXO
Total: 125 mW
Regulators & Passives: 15 mWPhysics: 10 mW
Slide courtesy of R. Lutwak (Symmetricom)
Symmetricom Clock Stability1E-9
y y
σ y(τ)
, σy(τ
)
1E-10
Dev
iatio
n,
1E-10
Dev
iatio
n
1E-11ping
Alla
n D
SN309 SN310 SN312SN313pi
ng A
llan
Ove
rlapp
SN313 SN314 SN317 SN318 SN321SN322
1E-11
Ove
rlapp
SN084SN084 Drift Removed
100 101 102 103 104
1E-12
SN322 SN323
100 101 102 103 104 105 106 107
A i Ti
Specification
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10 10 10 10 10Averaging Time, τ
R. Lutwak et al., Proceedings of the 2007 PTTI Conference
Averaging Time, τ
Symmetricom Retracey25
15
20
trace
s RMS = 2.3x10-11
5
10Ret
-6 -4 -2 0 2 4 60
Offset [x10-11]
ITSF 2008Courtesy of R. Lutwak 2008 FCS Tutorial
CSAC Activities at Teledyne Scientific
U d Ph III f CSAC
yJ.F. DeNatale et al., PLANS 2008, IEEE/ION (2008)
Under Phase III of CSAC program, achieved DARPA’s program goals
Compact Physics
DARPA s program goals
– Size reduced to 1cc (0.7cc physics package and 0 3 cc Compact Physics
Package on Test Boardphysics package and 0.3 cc electronics
– Power of physics package d b l 30measured below 30 mW
– Stability of Phase-III physics package measured < 1x10-11 at
Compact Control ElectronicsPhoto of Fully Integrated
p g1 hour
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Photo of Fully Integrated 1cc CSAC with compact
control electronics
Teledyne Physics Package FeaturesJ.F. DeNatale et al., PLANS 2008, IEEE/ION (2008)
DetectorCell / Heaters
QWPVCSEL
LensND / Mirror
DetectorCell / Heaters
QWPVCSEL
LensND / Mirror
DetectorCell / Heaters
QWPVCSEL
LensND / Mirror
Dual-pass optical configuration
Compact Compact Vapor Cells
Microfabricated Optics
reduce size, power and improve robustness / improve robustness /
assembly
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Phase-III Physics Package
Honeywell CSAC Physics Packagey y gD.W. Youngner, et. al., Transducers & Eurosensors, 2007.
top capld fl ttitani m getter
optics
top cap wafervacuum
gold reflectortitanium getter
soldersolder
cavity wafer
rubidium +AR/N2
bench wafer
optical path
Photo-detector
vcsel
Trans-Impedance Amp
3 wafer stack
“Honeywell has attempted to differentiate itself … by making clocks that are easy to assemble and package.”
– more integration and critical alignment at the wafer level and less at the
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more integration and critical alignment at the wafer level, and less at the package level.
Current Status of DARPA-Funded CSAC Research
• Currently in Phase IV of DARPA CSAC program.– Funds Symmetricom and Teledyne Scientific
• The main Phase-IV objective: “Demonstrate compliant functionality in a• The main Phase-IV objective: Demonstrate compliant functionality in a relevant military environment.”
– Deliver a quantity of units (currently 35) to the U.S. Army – Power will be roughly 100 mW– Units will be tested over the full military range of temperature, shock,
vibration, humidity, EM susceptibility, etc.
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Evolution of CSAC TechnologyKitching et al Proc 2007 IEEE Intl Freq Cont SympKitching et al., Proc. 2007 IEEE Intl. Freq. Cont. Symp.
One of the 10 emerging technologiesOne of the 10 emerging technologies most likely to change the way we live.
- MIT Technology Review (2008)
SSpin-O
ff
NMR Gyroscopes• Add spin-polarized noble-gas nuclei to the cell
IntegratedMicro Biomedical ApplicationsMicroPrimaryAtomicClock MEMSMTO
DARPADARPApp
• Magnetocardiography• Magnetoencephalography• Low-field MRI
Other Sensors• Magnetic Anomaly• Geophysical
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ClockTechnology
• etc…• Geophysical• etc…
AcknowledgementsgNIST and CUVladislav Gerginov Leo Hollberg
SymmetricomRobert Lutwak and Mike Garvey Vladislav Gerginov, Leo Hollberg,
John Kitching, Svenja Knappey
HoneywellLisa Lust and Dan YoungnerLisa Lust and Dan Youngner
Teledyne
DARPA
Jeff DeNatale
DARPAClark Nguyen and Amit Lal
DARPADARPA
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MEMSMTO
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