EE C245 – ME C218 Introduction to MEMS Design Fall 2007ee245/fa07/... · •Hand back graded...

1

EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 1

EE C245 – ME C218Introduction to MEMS Design

Fall 2007

Prof. Clark T.-C. Nguyen

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

Berkeley, CA 94720

Lecture 20: Lossless Transducers


Announcements

• Hand back graded midterm today•Midterm Statistics:

• Come to my office if you would like to see the details of your Z-score

62Median

13Standard Deviation

62Average

101Top Score

115Max. Possible Score

2


Lecture Outline

• Reading: Senturia Chpts. 10, 6• Lecture Topics:

Project DescriptionEnergy Conserving Transducers

Charge ControlVoltage ControlLinearizing Capacitive Actuators


Project Description

3


Go Through the Project Handout


Micro-Scale Power Generation

4


Micro-Scale Power Generation

• Goal: generate power at the micron scale with superior energy density compared to batteries

•Motivation: enable standalone micro sensors and micro actuators with wireless communication pursuant to realizing large wireless sensor networks

Sensors

Fuelstorage

Actuators ASIC/CPURF/Optical

Comm

Heat engine/Fuel reformer

Thermal/Exhaustprocessor 1 mm

TE Converter/Fuel cell

• Approach: harness fuels with higher energy density

0 2 4 6 8 10 12 14Energy Density (kW-hr/kg)

PropaneMethaneGasoline

DieselEthanol

MethanolLi Battery


Approach: Fuel Cell• Common elements among fuel cells:

fuel storage/deliveryanode and cathode electrodescatalyst to dissociate fuel (e.g., into H+ and e-) at anode and combine products at cathodeion exchange medium (i.e., electrolyte)

Fuel Storage

Vout

ChemicalEnergy

FuelDelivery

ChemicalEnergy

ElectricalEnergyElectrolyte

PorousAnode

Electrode

PorousCathode

Electrode

Catalyst(e.g., platinum)

Load

+

-H+

e-

H+

e-

CO2O2

H2O

ElectricalEnergy

20–50% eff.

5


Metal-Hydride Micro Fuel Cell

•Objective: provide 0.5mA @ 3.2V output continuous power with 30mA 100ms pulses for comms for 3 month operation

• Specific energy: 0.95 W-hr/g•Water and metal hydride powder fuel kept separate until power is needed

shelf life > 10 years

• For 3 month operation:need 0.9g of LiAlH4 (1.4cc)Need 1.6g of H2O (1.6cc)

Tiny Fuel CellTiny Fuel Cell

RegulatingCheck ValveRegulating

Check ValveLiAlH4 FuelLiAlH4 Fuel

PolymerBlock

PolymerBlock

[Honeywell]


e-

O2

H2

H2O

H2

H2

H2 H+

H+

H+

H+

H+

Vout

+

-

e-

Hydrogen Generator/Regulator• LiAlH4 + H2O reaction rate regulated by a pneumatic valve

a completely mechanical feedback system requiring no electrical power

Water Chamber

LiAlH4 PowderChamber

Valve DiskSeal

Diaphram

H2 here consumed by fuel cellPressure dropsValve opens again

H2 here consumed by fuel cellPressure dropsValve opens again

Waterevaporates

Waterevaporates

H2 generatedwhen H2Oreaches

LiAlH4 powder


LiAlH4 powder

Pressure risesMembrane deflectsValve closes


H2O generatedH2O generated

6


Metal-Hydride MFC Performance

• Right: actual AMPGen hydride fuel cell, including fuel storage

• Performance: (as advertised)steady hydrogen regulatorsteady 3.2V output voltage under a 0.5mA load

-6

-4

-2

0

2

4

6

8

0 1 2 3

Time (days)

Vfc

(Vol

ts),

H2

Pres

sure

(psi

)

-200

-150

-100

-50

0

50

100

150

200

250

300

Air

RH

(%),

Air

T (°

C),

Valv

e Po

sitio

n (u

m)

H2 over-pressure

Output Volts (Load current 70uA)

Air Humidity

Air Temperature

[Honeywell]

Series Connectionof 5 Micro Fuel CellsSeries Connection

of 5 Micro Fuel Cells

0.90 W-hr/ccEnergy Density:0.95 W-hr/gSpecific Energy:

3.19 W-hrsTotal Energy:3.55 ccTotal Volume:3.36 gTotal Mass:

• Compare: CR2430 Li Battery4.6g, 1.3cc, 0.83 W-hrs0.65 W-hr/cc, 0.18 W-hr/g

• Compare: CR2430 Li Battery4.6g, 1.3cc, 0.83 W-hrs0.65 W-hr/cc, 0.18 W-hr/g

5X Better!!!5X Better!!!


Metal-Hydride Micro Fuel Cell

•Objective: provide 0.5mA @ 3.2V output continuous power with 30mA 100ms pulses for comms for 3 month operation

• Specific energy: 0.95 W-hr/g•Water and metal hydride powder fuel kept separate until power is needed

shelf life > 10 years

• For 3 month operation:need 0.9g of LiAlH4 (1.4cc)Need 1.6g of H2O (1.6cc)

Tiny Fuel CellTiny Fuel Cell

RegulatingCheck ValveRegulating

Check ValveLiAlH4 FuelLiAlH4 Fuel

PolymerBlock

PolymerBlock

[Honeywell]

7


e-

O2

H2

H+

H+

H+H2

H2

H2

Vout

+

-

e-

Empty Water Chamber Operation• LiAlH4 + H2O reaction rate regulated by a pneumatic valve

a completely mechanical feedback system requiring no electrical power

Water Removed

LiAlH4 PowderChamber

Valve DiskSeal

Diaphram

get very dry condition on leftwater diffuses towards left

get very dry condition on leftwater diffuses towards left


LiAlH4 powder


LiAlH4 powder



H2O generatedH2O generated

H2O

H2O


Water-Less Operation Improves AMPGen Performance

Extracting water from exit fuel air can still yield >0.2mW

Extracting water from exit fuel air can still yield >0.2mW

Specific energy 2.7xLiAlH4 AMPGen 2.6 W-hr/g

Specific energy 2.7xLiAlH4 AMPGen 2.6 W2.6 W--hr/ghr/g

Volumetric Energy density 2.4xLiAlH4 AMPGen 2.1 W-hr/cc

Volumetric Energy density 2.4xLiAlH4 AMPGen 2.1 W2.1 W--hr/cchr/cc

01234567

CR2430 LiBattery

LiAlH4w/ Water

LiBH4w/ Water

LiAlH4No Water

LiBH4No Water

Power Source Type

Spec

ific

Ener

gy

[W-h

r/g]

Water-less Operation of AMPGen prototype with LiAlH4 fuel

0

50

100

150

200

250

0 2 4 6 8 10 12

Time (days)

Pow

er (u

Wat

ts)

Water-less operation for 12 days, stepping up power level

from 0.05, 0.16, and to 0.21 mW

Water-less operation for 12 days, stepping up power level

from 0.05, 0.16, and to 0.21 mW

5x12x 14x

35xThis is 14x higher than the CR2430

Li Battery!

This is 14x higher than the CR2430

Li Battery!

8


Radioisotope Power Sources?


Why Radioisotope Fuels?

• For micro-scale systems, energy and power density are paramount

0 2 4 6 8 10 12 14

Specific Energy Density [W-hr/g]

Propane

Methane

Gasoline

Diesel

Ethanol

Methanol

Li Battery

Micro-Scale Power Generation (MPG)

~5-10 W-hr/g~5-10 W-hr/g

1 10 100

1,00

0

10,0

00

100,

000

1,00

0,00

0

10,0

00,0

00

100,

000,

000

MethanolDiesel

GasolinePropane

Pu-238 (80% Pu)Pm-147

Tl-204Sr-90U-235

Deuterium

Specific Energy Density [W-hr/g]

Logarithmic ScaleLogarithmic Scale

Radio Isotope Micro-Scale Power Sources (RIMS)

~500-10,000 W-hr/g

~500-10,000 W-hr/g

Radioisotopes can store orders of magnitude more energy in a given volume!

Radioisotopes can store orders of magnitude more energy in a given volume!

9


Radioisotopes: Tiny Suns

The Sun

p

n

i E-Field

Fixed Charge

h+

e-

Fixed Charge

hν

Radioisotope

betaalpha

~1.1eV~1.1eV

~10-250 keV~10-250 keV

e- e- e- e-

h+ h+

Load

e-

h+ h+h+Photon

Half-life up to 500 yrs!

Half-life up to 500 yrs!

Equivalent to a tiny sunEquivalent

to a tiny sun

~5 MeV~5 MeV

Problem: High αenergy can damage the semiconductor

greatly reduces converter lifetime

Problem: High αenergy can damage the semiconductor

greatly reduces converter lifetime


• Problem: past radioisotope power sources had been plagued by numerous deficiencies (e.g., low efficiency, low damage threshold) that limited them to low output power

• Example: betavoltaic (attaining efficiency ~ 1.7%)

Previous Radioisotope Power Sources

p

n

Radioisotope

i E-Field

Poor EfficiencyLow Output PowerPoor Efficiency

Low Output Power

Fixed Charge

Fixed Charge

h+

e-

βEnergy lost to heat

Energy lost to heat

Significant leakage current

Significant leakage current

Increase in recombinationIncrease in

recombinationHigh energy particle can

damage the semiconductorHigh energy particle can

damage the semiconductor

Betas absorbed within radioisotopeBetas absorbed

within radioisotope

Silicon pnjunction

Silicon pnjunction

β

Betas going upward unusedBetas going upward unused Energy lostEnergy lost

10


Benefits of MiniaturizationConventional Betavoltaic Cell:

Result: higher output power using smaller radioisotope volume

Result: higher output power using smaller radioisotope volume

p

n

Radioisotope

i E-Field

Fixed Charge

h+

e-

β Fixed Charge

n

p Radioisotope

i

MEMS-Based Betavoltaic Cell:

Micro-Scale Plate Spacingallows high E-fieldreduces ion-electron recombination

Micro-Scale Plate Spacingallows high E-fieldreduces ion-electron recombination

Alpha Particles Allowed5 MeV particle energygas absorbs energyhigher power density

Alpha Particles Allowed5 MeV particle energygas absorbs energyhigher power density

Tiny Chamber Volumeallows high pressurebetter ionization eff.

Tiny Chamber Volumeallows high pressurebetter ionization eff.

Miniaturize & Miniaturize & Use Higher Use Higher

Energy AlphasEnergy Alphas

Miniaturize Miniaturize & Retain & Retain

Safe BetasSafe Betas

Deep RIE Trencheshigh surface-to-volumehigher power density

Deep RIE Trencheshigh surface-to-volumehigher power density

III-V Semiconductorhigher efficiencybetter damage resilience

III-V Semiconductorhigher efficiencybetter damage resilience

Low Work Function Metal

High Work Function Metal

Charging Gas Cell:

High Press. Gas

Radioisotope

iαe-

+ion

E-Field


Go Through the Project Handout

11


Energy Conserving Transducers


Basic Physics of Electrostatic Actuation

• Goal: Determine gap spacing g as a function of input variables

• First, need to determine the energy of the system

• Two ways to change the energy:Change the charge qChange the separation g

ΔW(q,g) = VΔq + FeΔg

dW = Vdq + Fedg

•Note: We assume that the plates are supported elastically, so they don’t collapse

12


Charge-Control Case

• Here, the stored energy is the work done in increasing the gap after charging capacitor at zero gap

• Find force and voltage:Need stored energyCan find by recognizing that the energy in the final state is just the energy stored in capacitor charged to q


Charge-Control Case

• Having found stored energy, we can now find the force acting on the plates and the voltage across them:

+ -V

13


Voltage-Control Case

• Practical situation: We control VCharge control on the typical sub-pF MEMS actuation capacitor is difficultNeed to find Fe as a partial derivative of the stored energy W = W(V,g) with respect to g with V held constant? But can’t do this with present W(q,g) formulaSolution: Apply Legendre transformation and define the co-energy W′(V,g)

EE C245 – ME C218 Introduction to MEMS Design Fall 2007ee245/fa07/... · •Hand back graded...

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Transcript of EE C245 – ME C218 Introduction to MEMS Design Fall 2007ee245/fa07/... · •Hand back graded...