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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
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 2
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
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 3
Lecture Outline
• Reading: Senturia Chpts. 10, 6• Lecture Topics:
Project DescriptionEnergy Conserving Transducers
Charge ControlVoltage ControlLinearizing Capacitive Actuators
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 4
Project Description
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 5
Go Through the Project Handout
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 6
Micro-Scale Power Generation
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 7
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
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 8
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.
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 9
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]
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 10
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
H2 generatedwhen H2Oreaches
LiAlH4 powder
Pressure risesMembrane deflectsValve closes
Pressure risesMembrane deflectsValve closes
H2O generatedH2O generated
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 11
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!!!
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 12
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]
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 13
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
H2 generatedwhen H2Oreaches
LiAlH4 powder
H2 generatedwhen H2Oreaches
LiAlH4 powder
Pressure risesMembrane deflectsValve closes
Pressure risesMembrane deflectsValve closes
H2O generatedH2O generated
H2O
H2O
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 14
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!
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 15
Radioisotope Power Sources?
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 16
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!
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 17
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
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 18
• 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
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 19
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
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 20
Go Through the Project Handout
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 21
Energy Conserving Transducers
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 22
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
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 23
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
EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 24
Charge-Control Case
• Having found stored energy, we can now find the force acting on the plates and the voltage across them:
+ -V
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EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 25
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)