Chi S l Mi f b i t d At iChip-Scale, Microfabricated Atomic ClocksC oc s

Elizabeth Donleyyedonley@nist.gov

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