Microfabrication technologies for
highly-laminated thick metallic cores
and 3-D integrated windings
Florian Herrault
Georgia Institute of Technology
Atlanta, GA
florian@gatech.edu
http://mems.gatech.edu/msma
PowerSoc 2012 Workshop
Center for MEMS and Microsystems Technologies
Acknowledgments




Pr. Mark G. Allen (MSMA group - Georgia Tech)
• Dr. Preston Galle
• Dr. Jungkwun Kim
• Minsoo Kim
• Jooncheol Kim
• Richard Shafer
David Anderson (Texas Instruments)
• Jizheng Qiu
Pr. David Perreault (MIT)
• David Otten
Sponsors: Arpa-E, Texas Instruments
2
Center for MEMS and Microsystems Technologies
Outline





Metallic cores for high-frequency magnetics
Highly-laminated magnetic metal cores
•
Concept
•
Fabrication and material selection
•
Core material characterization
•
Core loss measurements at High Fluxes and High Frequency
Integrated Toroidal Inductors
•
Co-packaging of windings and cores
•
Microfabricated winding technology
•
Impedance measurements
High-voltage Power Converter Experiments
Conclusions
3
Center for MEMS and Microsystems Technologies
Metal Core Inductors

Advantages of electroplated metallic alloys






High saturation (High operating flux density  inductor
miniaturization)
Low coercivity (Low loss)
Ability to electroplate in magnetic field to define easy/hard axis
CMOS compatible
Ability to electroplate thick cores (for large power handling)
BUT

Operation in the 1-20 MHz region requires a few m thick film
lamination for conventional ferromagnetic metals (permalloy,
CoNiFe, CoFe,…)
4
Center for MEMS and Microsystems Technologies
Our Approach and Goals – Magnetic Cores

Technology-driven development of thick highly-laminated metallic
alloys

Demonstrate low eddy current losses in metallic alloys at MHz
frequencies

Operate these cores at very high fluxes and high frequencies

Demonstrate high power handling

Integrate cores and microfabricated windings
5
Center for MEMS and Microsystems Technologies
Previously-Reported Microfabricated Laminations
Electrodeposition of
magnetic material



Multiple deposition
steps of seed layer
Multiple
photolithography for
175molds and
plating
m
insulators
Overall core fabrication
time is proportional to
the number of layers
Electrodeposition of
magnetic material


Sputter deposition of
Magnetic material
One deposition of seed
layer
High-aspect-ratio
plating mold is required
for large cross-sectional
core
Magnetic
material
6




Alternate sputtering of
magnetic materials and
insulation layers
Single vacuum step
Process time
Some patterning
complexity
Insulation
material
Center for MEMS and Microsystems Technologies
Micron-Scale Laminations via
Robotically-Assisted Multilayer Plating
Robotic arm
Wafer
Rinse #1
Permalloy
Conventional
copper
Rinse #2
Bright copper
Filtration system
Multilayer plating
through mold
Selective etching
“Electroplating-based approaches for volumetric nanomanufacturing,”
Tech. Dig. Technologies for Future Micro-Nano Manufacturing, Aug. 2011.
7
Center for MEMS and Microsystems Technologies
Core Fabrication Technology
a
b
Permalloymaterial
Magnetic
Copper
PR
SU-8
Ti
Si
Bd
Ac
Sequential electrdeposited
Permalloy/Cu layers
A.
B.
C.
D.
E.
Ce
Selective Cu etch after
removing PR
Df
Applying SU-8 supports
Full Cu etch
Through-mold sequential electroplating of magnetic/sacrificial layers
Mold removal, and partial etching of sacrificial material
Formation of polymer supports
Complete etching of sacrificial material
Polymer infiltration (not shown)
"Nanolaminated Permalloy Core for High-flux, High-frequency
Ultracompact Power Conversion,” TPES, in press.
8
Center for MEMS and Microsystems Technologies
Magnetic Material Selection

Baseline material (Ni80Fe20)
»
»
Bs ~ 0.8 T
Hc ~ 0.7 Oe

High saturation mat.(Fe10Co90)
»
»
Bs ~ 1.9 T
Hc ~ 1 Oe

Low coercivity mat. (NiFeMo)
»
»
Bs ~ 1 T
Hc ~ 0.3 Oe

Low coercivity and high saturation material (CoNiFe)
»
»
Bs ~1.8-2 T
Hc ~ 0.2-1 Oe
9
Center for MEMS and Microsystems Technologies
Highly-laminated Microfabricated Cores
5 μm / 18 layers ~
280 nm thick layers
10 mm
40-layer permalloy cores
with 1-μm-thick laminations
40-layer CoNiFe core
Cross-sectional view: Thick (2 μm) Cross-sectional view: 300-nm-thick
permalloy laminations with thin permalloy laminations with 300-nm(300nm) copper interlayers
tall interlayer gap (300 layers)
40-layer CoNiFe laminations with
lamination thickness ~ 1 μm
10
300-layer CoNiFe laminations with
lamination thickness < 0.3 μm
Center for MEMS and Microsystems Technologies
Wound Test Inductors



Test Core Geometry
» OD:10mm, ID:6~8mm
Packaged with test bobbins
» Characterization with high-power core loss measurement setup
CoNiFe (1.8 T, < 1 Oe) vs. NiFe (0.8 T, 2 Oe)
Wound inductors with inductances
> 1 μH using 7-strand litz wire
11
Center for MEMS and Microsystems Technologies
Manifestation of Eddy Currents in Inductors
Suppressed eddy currents in metal cores manifested
by constant inductance as a function of frequency
No eddy
current
Inductance
Depends on skin depth &
lamination thickness
Eddy currents are
dominant
Frequency
12
Center for MEMS and Microsystems Technologies
In-Situ Measurements of Sacrificial Metal Etching


Inductors packaged and wound before copper core etch
Constant-voltage measurements performed in DI water
Inductor under
test
DI Rinse
Sacrificial
layer etching
time (%)
Z analyzer
connector
In-situ core loss suppression experiment
Inductor inductance as a function of frequency
parameterized by sacrificial layer etching time
13
"Nanolaminated Permalloy Core for High-flux, High-frequency
Ultracompact Power Conversion,” TPES, in press.
Center for MEMS and Microsystems Technologies
High-Frequency Inductance Measurements
Inductors with permalloy cores
Inductance
(uH)
( H) ( H)
inductance
inductance
36-turn inductors
Blue: 0 bias current
Red: 0.5 A bias curent
Green: 0.95 A bias current
1
1
0.5
10
10
frequency
(MHz)
Frequency
(MHz)
10
0.5
2
100
100
100
frequency (MHz)
1000
22
1000
100
1
100
10
1
10
0.5
1
12
10
10
10frequency (MHz)
10
frequency
(MHz)
frequency (MHz)
Frequency
(MHz)
2
2
resistance ()
300 core layers
0.3-0.5 nm thick layers
221
2
Inductance
() ()
resistance
resistance
(uH)
( H)
inductance
Inductors with CoNiFe cores
18-turn inductors
Solid lines: Inductor #1
Dotted lines: Inductor #2
2
100
100
100
100
80
1000
factorfactor
quality
quality
Blue: 0 bias current
80
60
100
Red: 0.5 A bias
60 curent
40
10 bias current
Green: 0.95 A
40
20
10
20
22
0
2
14
10
100
10
100
Center
for MEMS and Microsystems Technologies
frequency (MHz)
frequency
(MHz)
10
100
CoNiFe Cores – Bias Current Measurements
18 turn CoNiFe inductor

300 core layers
0.3-0.5 µm thick layers
10% L
drop
20% L
drop
30% L
drop
#1
0.35
0.55
0.75
#2
0.35
0.5
0.65
At 2 MHz
1
0.5
resistance ()
Resistance
Isat (A)
Batch 3, #1
Batch 3, #2
2
1.5
0
0
0.1
0.2
0.3
0.5
biascurrent
current (A) (A)
DC DC
bias
0.4
0.5
0.6
0.7
0.8
0.9
1
1
1.5
Batch 3, #1
Batch 3, #2
1
0.5
0
factor
Quality
quality factor

2.5
inductance ( H)
Inductance

0
0.1
0.2
0.3
0.5
bias current (A)
DC DC
bias
current (A)
0.4
0.5
0.6
0.7
0.8
0.9
1
1
20
Batch 3, #1
Batch 3, #2
15
Q >20 at 1A bias current
10
5
0
0
0.1
0.2
0.5
DC bias
current (A)(A)
DC bias
current
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1
Impedance measurements
under
bias at Technologies
2 MHz
Center for MEMS
and DC
Microsystems
15
HFHF Characterization Setup
RF Amplifier
input
Capacitor
bank
Scope
connectors
(Vin)
Inductor
under
Test
Scope
connectors
(Vcap)
Inductor Core Loss Test Board with 35nF
capacitor boards
Capacitor boards for frequency-dependent measurements
16
Center for MEMS and Microsystems Technologies
HFHF Core Loss Measurements
Dissipated power in the inductor as a function of
frequency and parameterized by AC peak flux density
"Nanolaminated Permalloy Core for High-flux, High-frequency
Ultracompact Power Conversion,” TPES, in press.
17
Center for MEMS and Microsystems Technologies
Analytical Separation of Eddy Current Losses


Eddy current losses exhibit f^2 dependency
Ptot  Peddy  Physt
Hysteresis losses exhibit f dependency
Ptot /f
Ptot  keddy * f ^2  khyst * f
Ptot / f  keddy * f  khyst
Ptot /f (or Pv_tot/f)
Large eddy
current losses
slope = keddy
Small eddy current losses
Intercept = khyst
No eddy currents
(keddy = 0)
Frequency
Interpretation
Analytical Extraction of Core Losses
18
Frequency
Center for MEMS and Microsystems Technologies
Post-Processed HFHF Core Loss Data
"Nanolaminated Permalloy Core for High-flux, High-frequency
Ultracompact Power Conversion,” TPES, in press.
19
Center for MEMS and Microsystems Technologies
High Flux NiFe Core Loss Distribution


Eddy and hysteresis losses extracted at 1 MHz as a function of flux
At high fluxes, eddy losses have been suppressed and are negligible compared to
hysteresis losses at 1 MHz
"Nanolaminated Permalloy Core for High-flux, High-frequency
Ultracompact Power Conversion,” TPES, in press.
20
Center for MEMS and Microsystems Technologies
Core Lamination Performance
Summary
@ 1 MHz and 0.5 T
Comparison of core loss at 1 MHz and high operating AC peak flux density
21
Center for MEMS and Microsystems Technologies
Microfabricated Inductors
with highly-laminated metallic cores
Hybrid integration process
Independently-fabricated magnetic
cores are integrated halfway through
the winding fabrication process
2 mm
Monolithic process
Co-fabrication of the windings and the cores
through sequential micro-fabrication
steps of electroplating and polymer
insulation
2 mm
“Integrated Toroidal Inductors with Nanolaminated Metallic
Magnetic Cores,” Tech. Dig. PowerMEMS 2012 workshop.
“Monolithically-fabricated laminated inductors with
electrodeposited silver windings,” Tech. Dig. MEMS 2013.
22
Center for MEMS and Microsystems Technologies
Hybrid Integration Concept Overview
Copper
coating
SU-8
core
Key winding
fabrication concept
“Microfabrication of air-core toroidal inductor with
very high aspect ratio metal-encapsulated polymer
vias,” Tech. Dig. PowerMEMS 2012 workshop.
Independent fabrication
of cores and windings
Drop-in pre-insulated
core
• High-aspect-ratio copper-coated polymer vertical vias
• Insulated cores dropped into the winding frame
• Top winding fabricated core integration
23
Microfabrication of
top copper windings
Center for MEMS and Microsystems Technologies
Microfabricated Inductors



50-turn microfabricated inductors (non-optimized geometry)
New generation of integrated cores with CoNiFe layers
Microfabricated conductor heights ~ 0.5 mm
2mm
(a)
Partially-fabricated windings
on a glass substrate
2mm
2 mm
(b)
Batch of dropped-in
cores
Fully-fabricated inductor
“Integrated Toroidal Inductors with Nanolaminated Metallic
Magnetic Cores,” Tech. Dig. PowerMEMS 2012 workshop.
24
Center for MEMS and Microsystems Technologies
Impedance Measurements of
Microfabricated Inductors


50-turn microfabricated inductors with CoNiFe cores
100 layers – 300 nm thick layers
“Integrated Toroidal Inductors with Nanolaminated Metallic
Magnetic Cores,” Tech. Dig. PowerMEMS 2012 workshop.
25
Center for MEMS and Microsystems Technologies
Power Converter Measurements
Power converter circuit board and integrated inductor
Testing board with wirebonded inductor
ZVS buck converter
“A technology overview of the PowerChip development program,” TPES, in press.
26
Center for MEMS and Microsystems Technologies
100 V Power Converter Measurements
- 1.5 µH inductor with CoNiFe cores
- P_out ~ 25-35 W
- V_out = 35 V
Power converter efficiency as a function
of input voltage
Power converter switching as a function
of input voltage
27
Center for MEMS and Microsystems Technologies
Summary

Highly Laminated Metallic Cores: Technology-driven approach





Microfabricated Inductors



Cores and windings are co-packaged
Demonstrated for large inductance inductors and small multi-phase
topologies
Demonstration in 100 V power converter


Negligible eddy current losses
High Saturation flux densities
Low hysteresis losses
Electroplating-based technology compatible with thick magnetic core
fabrication and CMOS manufacturing
Operation at 2-6 MHz and 35W output power
Ongoing work on material reliability (corrosion, stress, packaging)
and in-field material electroplating
28
Center for MEMS and Microsystems Technologies