EE C245 – ME C218Introduction to MEMS Design

Fall 2008Fall 2008

Prof Clark T C NguyenProf. Clark T.-C. Nguyen

Dept of Electrical Engineering & Computer SciencesDept. of Electrical Engineering & Computer SciencesUniversity of California at Berkeley

Berkeley, CA 94720y

L t 3 B fit f S li II

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 1

Lecture 3: Benefits of Scaling II

Lecture Outline

• Reading: Senturia, Chapter 1g p• Lecture Topics:

Benefits of MiniaturizationExamplesExamples

GHz micromechanical resonatorsChip-scale atomic clockpMicro gas chromatograph

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 2

NIST F1 Fountain Atomic Clock

Vol: ~3.7 mVol: ~3.7 m33

Power: ~500 WPower: ~500 WAcc: Acc: 11××1010––1515

Stab: 3.3x10Stab: 3.3x10--1515/hr/hr

After 1 sec Error: 10-15 sec

Loses 1 sec every y30 million years!

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 3

Physics Package

1st Chip-Scale Atomic Physics PackageNIST’s Chip-Scale Atomic

Physics Package

1.5 mmPhotodiode

Package

Cell

4.2 mm

Optics

Q tND

1.5 mm Laser 1 mm

GlassND

SiQuartz

Lens

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 4

Total Volume: 9.5 mm3 Stability: 2.4 x 10-10 @ 1sCell Interior Vol: 0.6 mm3 Power Cons: 75 mW

Glass

Alumina

VCSEL

Tiny Physics Package Performance• Experimental Conditions:

Cs D2 ExcitationExternal (large) Magnetic Shielding

Dime

NIST’s Chip-Scale

External (large) Magnetic ShieldingExternal Electronics & LO Cell Temperature: ~80 ºCCell Heater Power: 69 mWC p Sca e

Atomic Physics Package

Cell Heater Power: 69 mWLaser Current/Voltage: 2mA / 2VRF Laser Mod Power: 70μW

Open Loop Resonance: Drift to Be Removed

10-9σ y

Stability Measurement:Drift IssueCs (D2)

5.67

[V]

7.1 kHz

Removed in Phase 3

Sufficient to meet

10-10

Dev

iatio

n,

Q =1.3x106

5.66

PD S

igna

l

Contrast: 0.91% 2.4e-10 Allandeviation @ 1 s

to meet CSAC

program goals

10-11

Alla

n D

Rb (D1) 1 day1 day1 hour1 hour

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 5

40 50 60 70 80 905.65

Frequency Detuning, Δ [kHz] from 9,192,631,770 Hz

100 101 102 103 104 10510-12

Integration Time, τ [s]

1 day1 day1 hour1 hour

Atomic Clock Fundamentals

• Frequency determined by an atomic transition Energy Band Diagram

energy

E it t

ΔE = 1.46 eV

ΔE/hExcite e- to the next orbital

ν = ΔE/h= 352 THz852.11 nm

m = 1 ΔE = 0.000038 eV

ν = ΔE/ħ 133Cs

ν = ΔE/ħ = 9 192 631 770 Hz

m = 0 m = 0

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 6

m = 0f = 4

m = 0f = 3Opposite

e- spins

Miniature Atomic Clock Design

Atoms become transparent to

Carrier(852 nm)

ν = ΔE/ħ

Sidebands

4.6GHz

transparent to light at 852 nm

/= 9 192 631 770 Hz

HyperfineS

9.2GHzλ

Splitting Freq.

ModulatedLaser

PhotoDetector

133Cs vapor at 10–7 torrLaser Cs vapor at 10 torr

Mod fVCXOv

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 7μwave osc

VCXO4.6 GHz Close feedback

loop to lockvo

Chip-Scale Atomic ClockLaser 133Cs vapor at 10–7 torr

GHzGHzResonatorResonator

Mod fVCXOv Photo

Atomic Clock Concept Cs or RbCs or RbVCSELVCSEL

ResonatorResonatorin Vacuumin Vacuumμwave osc

4.6 GHzvo PhotoDetector

p Cs or RbCs or RbGlassGlassDetectorDetectorSubstrateSubstrateMEMS andMEMS and SubstrateSubstrate

Photonic Photonic TechnologiesTechnologies

• Key Challenges:thermal isolation for low power

ll d i f i Q Vol: 1 cmVol: 1 cm33

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 8

cell design for maximum Qlow power μwave oscillator

Vol: 1 cmVol: 1 cm33

Power: 30 mWPower: 30 mWStab: Stab: 11××1010––1111

Chip-ScaleAtomic Clock

Challenge: Miniature Atomic CellLarge Vapor Cell Tiny Vapor Cell

1,000XVolumeVolumeScaling

SurfaceVolume

More wall collisions stability gets worse

tylowest Q

Atomic Resonance

Inte

nsit

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 9

Wall collision dephases atoms lose coherent state

I

Mod f9.2 GHzlower Q

Challenge: Miniature Atomic CellLarge Vapor Cell Tiny Vapor Cell

1,000XVolumeVolumeScaling

Buffer Gas

Soln: Add a buffer gas

Lower the mean free path of the atomic vapor

tyAtomic Resonance Return to

higher QIn

tens

it

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 10

I

Mod f9.2 GHz

Chip-Scale Atomic ClockLaser 133Cs vapor at 10–7 torr

GHzGHzResonatorResonator

Mod fVCXOv Photo

Atomic Clock Concept Cs or RbCs or RbVCSELVCSEL

ResonatorResonatorin Vacuumin Vacuumμwave osc

4.6 GHzvo PhotoDetector

p Cs or RbCs or RbGlassGlassDetectorDetectorSubstrateSubstrateMEMS andMEMS and SubstrateSubstrate

Photonic Photonic TechnologiesTechnologies

• Key Challenges:thermal isolation for low power

ll d i f i Q Vol: 1 cmVol: 1 cm33

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 11

cell design for maximum Qlow power μwave oscillator

Vol: 1 cmVol: 1 cm33

Power: 30 mWPower: 30 mWStab: Stab: 11××1010––1111

Chip-ScaleAtomic Clock

Micro-Scale Oven-Control AdvantagesMacro-Scale Micro-Scale300x300x300 μm3

Atomic Cell @ 80oCHeater

Macro-Oven(containing heater

and T sensor)

LaserInsulationAtomic Cell @ 80oC

Laser

T = P x Rth

Laser25oC

P Rth

T P x Rth

Long, Thin Polysilicon

T Sensor(underneath)

Thermally Isolating Feet

Rth= 38 K/WC = 22 J/K Rth= 83,000 K/W

Cth

R ~ support length

Tethers( )

Cth= 22 J/K thCth= 6.3x10-6 J/K

Cth ~ volume

Rth ~ X-section area

P (@ 80oC) = 2 6 mWP (@ 80oC) = 1 5 W 550x lower power550x lower power

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 12

P (@ 80oC) = 2.6 mW

Warm Up, τ = 0.1 s

P (@ 80oC) = 1.5 W

Warm Up, τ = 16 min.

550x lower power550x lower power

7,300x faster warm up7,300x faster warm up

Physics Package Power Diss. < 10 mW

Heater/Sensor SuspensionC i ll

• Achieved via MEMS-based thermal isolation

SuspensionCesium cell

VCSEL / 20 i LCC

7 mm

Frame Spacer

VCSEL

VCSEL / Photodiode 20 pin LCC

Only ~5 mW heating power VCSEL

Suspension

10

12

g pneeded to

achieve 80oC cell temperatureSymmetricom /

D Ph i

6

8

10

wer

[mW

] MeasuredModel

Draper Physics Package Assembly

0

2

4Pow

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 13

00 20 40 60 80 100 120 140

Temperature [oC]

Thermal Circuit Modeling

Macro-Scale Macro-Oven(containing heater

and T sensor)

Insulation

Laser

Atomic Cell @ 80oC

Laser25oC

Thermally Isolating Feet

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 14

Thermal Circuit Modeling

Insulation

Laser

Atomic Cell @ 80oC

Laser25oC

Thermally Isolating Feet

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 15

Thermal Circuit Modeling

EE C245: Introduction to MEMS Design Lecture 3 C. Nguyen 9/4/08 16