Embedded Integrated With A Single Layer : A Realistic Option

- Bridging the gap between discrete inductors and planar spiral inductors -

Dok Won Lee, LiangLiang Li, and Shan X. Wang Department of Materials Science and Engineering Department of Electrical Engineering

1 Outline

I. Introduction

II. Analytical Models and Design

III. Fabrication of Integrated Inductors

IV. Measurement of Fabricated Inductors

V. Analysis of Magnetic Inductors on Si

VI. Conclusion

2 Use of Inductors in Our Daily Lives

• Traffic light • Metal detector • RFID tag •Red-light camera

Texas Instruments Tag-itTM

• Voltage regulator module • Cell phone

3 Why Integrated Inductors?

Fully integrated electronics

R.K. Ulrich and L.W. Schaper, “Integrated Passive Component Technology”, 2003 4 Example: Power Management

DC-DC converter

Passive components are discrete and occupy large areas

IC containing Inductor gate driver and power MOSFETs Capacitors

Enpirion EN5330 PSoC (Power-System-on-a-Chip)*

70% of pc-board area saved

* R. Allen, Electronic Design, May 2004 5 Requirement for Power Management

Discrete inductors 10 uH ☺ Large inductance Large volume Poor AC performance 1 uH

100 nH

Planar spiral inductors 10 nH ☺ Small area consumption Inductance Small inductance Limited performance in 1 nH sub-GHz applications

100 pH

100 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz

From A. Ghahary, Power Electronics Technology, Aug. 2004 6 Schematics of Magnetic Inductor Designs

Transmission line Spiral inductor Solenoid inductor

☺ Small resistance ☺ Close to the planar spiral ☺ Magnetically efficient Small inductance Limited inductance gain Relatively complex Usually two magnetic One or two magnetic structure layers needed layers ☺ One magnetic layer

7 Brief Rev of Integrated Magnetic Inductors

Intel, Tyndall ≥

Tohoku

CEA-LETI

Taken from Lee et al, Embedded Inductors with Magnetic Cores, Book Chapter in press (Springer) 8 Magnetic Inductors from Stanford & Cowork

P. K. Amiri, et al. Intermag 08, CV 01

Planar transmission line A.M. Crawford, et al. inductor with CoTaZr core, IEEE Trans. Magn. Q = 6 @ 700 MHz 2002, p.3168-70 Planar spiral inductor with CoTaZr core, CMOS compatibility, Q ~ 2.7 @ 1 GHz

L. Li, D. W. Lee, et al. D. W. Lee, et al. IEEE Trans. Advanced Intermag 08, AG 01 (invited) Packaging (accepted 2008) Planar solenoid inductor with On-package solenoid CoTaZr core, inductor with CoFeHfO core, Inductance enhancement over Q = 22 @ 200~300 MHz, air core = 34x R ~ 10 mΩ dc Q>6 @ 26 MHz 9 II. Analytical Models and Inductor Design

I. Introduction

II. Analytical Models and Inductor Design • Analytical models for key device properties • Material selection • Optimization of design parameters • Inductor design concepts III. Fabrication of Integrated Inductors

IV. Measurement of Fabricated Inductors

V. Analysis of Magnetic Inductors on Si

VI. Conclusion

10 Schematics of Integrated Solenoid Inductor

• Solenoid inductor design was mainly considered in this work.

Top view Cross-section view

wM

lC t tC wV w tM A C sV

g lA, lM

gV

sV wA

11 Key Device Properties

• Inductance L

• Resistance R

Energy stored ωL • Quality factor Q Q = 2π = Power dissipation ⋅T R

• Device area

• Useful bandwidth

12 Inductance of Magnetic Inductor LMC

Inductance of magnetic inductor LMC: = + ∆ 5x10-8 LMC LAC µ L µ Slope = 1.01 ± 0.01 2 N 2w t 4x10-8 ∆ = 0 r N wM tM = 0 eff M M where L µ µ µ l [1+ N ( −1)] l -8 M d r M (H) 3x10 N = Demagnetizing factor of rectangular prism d -8 µ simulated 2x10 L µ ≡ r ∆ HFSS simulations eff + µ − -8 1 Nd ( r 1) 1x10 Linear fit

0 • For a finite-sized magnetic core, there is a -8 -8 -8 -8 -8 0 1x10 2x10 3x10 4x10 5x10 demagnetizing field inside the magnetic core, ∆L (H) µ calculated which effectively reduces r. • Demagnetizing field is not uniform inside the magnetic core, µ and the numerical solutions should be used for r > 1.*

Inductance enhancement:

AMC AAC,eff ∆L L − L A µ = MC AC ≈ µ MC Much less than r eff but still significant LAC LAC AAC,eff

* D.-X. Chen et al., IEEE Trans. Magn., 41, 2077 (2005) 13 Resistance of Magnetic Inductor RMC

Resistance of magnetic inductor RMC:

• From the classical electromagnetism:* = 1 2 µ Magnetic contribution to the energy stored EMagnetic ' H dV ω ⎛ " ⎞ 2 ∫∫∫ P ≈ 2 ⎜ ⎟E Magnetic ⎜ µ' ⎟ Magnetic Magnetic power loss P = ωµ" H 2 dV ⎝ ⎠ Magnetic ∫∫∫ µ • Representing in terms of the device properties: P = (R − R )I 2 = ∆RI 2 where ∆R ≡ R − R Magnetic MC AC MC AC ω µ ∆ = ⎛ " ⎞∆ 1 1 1 R ⎜ ⎟ L E = E − E = L I 2 − L I 2 = ∆LI 2 ⎝ µ' ⎠ Magnetic Magnetic inductor Air core inductor 2 MC 2 AC 2 ω ⎛µ " ⎞ ∴R = R + ∆R = R + ⎜ ⎟∆L MC AC AC ⎜ µ' ⎟ ⎝ ⎠ Both ω and (µ”/µ’) increase with frequency. Hence ∆R becomes ∆R for large ∆L significant as the frequency increases. R The more inductance enhancement ∆ ∆ R for small L we obtain by using a magnetic core, R the more resistive losses we introduce AC at high .

Freq. * R.F. Harrington, “Time-Harmonic Electromagnetic Fields”, 1961 14 Quality factor Q

L Quality factor of air core inductor Q : Q = ω AC AC AC ω RAC ω L L + ∆L Q = MC = AC Quality factor of magnetic inductor QMC: MC ω µ RMC + ⎛ " ⎞∆ RAC ⎜ ⎟ L ⎝ µ' ⎠

10 ∆ L/LAC=1 ∆ L/LAC=10 ∆ 8 L/LAC=30

QAC µ'/µ" ∆L << L Q ~ Q at low frequencies 6 AC MC AC µ µ ∆L >> LAC QMC ~ ’/ ” at high frequencies 4

fMC can be considered as the useful bandwidth Quality factor Quality of the magnetic inductor. 2

0 106 107 108 109 fMC Frequency (Hz)

15 Material Selection

Conductor: due to its low electrical resistivity

Magnetic core: 0.2 μmCoTaZrmagnetic film • Desirable properties: - High permeability 2500 - Soft magnetic material (low ) µ' 2000 µ - High resistivity " - High ferromagnetic resonance (FMR) 1500 frequency

1000 • Amorphous Co90Ta5Zr5 (at. %) alloy: - µ’ ~ 600 500 Relative permeability - Hc < 1 Oe - ρ ~ 108 µΩ-cm 0 106 107 108 109 - fFMR ~ 1.5 GHz Frequency (Hz)

16 Inductor Designs

Planar spiral inductor “Spiral” with or without magnetic plane “Scale-down” “Standard”

N = 4.5, 8.5, 17.5 N = 4.5 Solenoid inductor with lateral parameters scaled down by a factor of 2 while maintaining vertical parameters unchanged

N = 4.5, 8.5, 17.5

“Series” “Closed core” Solenoid inductor with different magnetic core arrangement or shape

N = 4.5, 8.5, 17.5 17 III. Fabrication of Integrated Inductors

I. Introduction

II. Analytical Models and Inductor Design

III. Fabrication of Integrated Inductors • Fabrication steps • Images of fabricated inductor devices • Magnetic properties of processed magnetic core

IV. Measurement of Fabricated Inductors

V. Analysis of Magnetic Inductors on Si

VI. Conclusion

18 Image of Fabricated Wafer

Wafer (4”-dia.) map Die map “Air core inductors”

“De-embedding “Magnetic inductors” structures” 19 SEM Images of Fabricated Inductor Devices

“Spiral” “Standard” “Scale-down”

400 µm 400 µm

400 µm

“Series” “Closed core”

400 µm 400 µm 20 FIB Cross-section Images of Fabricated Inductors

Top Cu layer 6.6 um

Polyimide 0.5 um Magnetic core 2.2 um Polyimide 2.0 um

Bottom Cu layer 4.4 um

Thermal oxide 50 µm Si substrate 2 µm

• FIB images confirm the successful fabrication of multi-layered inductor devices.

• The successful polyimide planarization is also confirmed, resulting in the continuous magnetic core layer.

21 Magnetic core shape affects permeability!

B-H loops Permeability spectra Kerr microscope image

µ', blanket Bottom conductor µ", blanket 1.0 600 Easy µ', processed Magnetic core Hard µ", processed 0.5 400 0.0

-0.5 200 Normalized B Normalized

-1.0 permeability Relative 0 -150 -100 -50 0 50 100 150 107 108 109 Easy

Field (H) Frequency (Hz) axis 100 µm

• Magnetic test structures identical to the actual magnetic cores were included in the wafer layout and processed in parallel with the inductor fabrication.

• Magnetic measurements confirm that the magnetic core in the fabricated inductor maintains the desired soft magnetic properties.

• The permeability spectra of blanket film and processed magnetic core structures are not identical to each other. 22 On-package Inductors on 8-inch Substrate

Easy axis of CoFeHfO

12 13

21 22 23 24

31 32 33 34

42 43

Notch 23 Surface roughness affects permeability!

Ra=128.6 nm

Permeability spectra of patterned CoFeHfO Surface roughness of bars on material dielectric material

The rough surface of dielectric material degrades the magnetic properties of CoFeHfO deposited on it (even before patterning).

24 IV. Measurement of Fabricated Inductors

I. Introduction

II. Analytical Models and Inductor Design

III. Fabrication of Integrated Inductors

IV. Measurement of Fabricated Inductors • Measurement method • Circuit model of integrated inductor • Measurement results of “Standard” inductors V. Permeability of CoTaZr Magnetic Cores

VI. Conclusion

25 Device Properties of “Standard” Inductors - L

Air core inductors Magnetic inductors

-7 -7 1x10 1x10 N = 4.5 N = 4.5 N = 8.5 N = 8.5 -8 -8 N = 17.5 8x10 N = 17.5 8x10

6x10-8 6x10-8

4x10-8 4x10-8 Inductance (H) Inductance (H) 2x10-8 2x10-8

0 0 107 108 109 106 107 108 109 Frequency (Hz) Frequency (Hz)

• With the use of magnetic core, inductance is 70.2 nH for N = 17.5, and the inductance enhancement is as high as 34×.

• The device area for N = 17.5 is 0.88 mm2, corresponding to an inductance density of 80 nH/mm2. 26 Device Properties of “Standard” Inductors - R

Air core inductors Magnetic inductors

10 10 N = 4.5 N = 4.5 8 N = 8.5 8 N = 8.5 N = 17.5 N = 17.5 ) ) Ω Ω 6 6

4 4 Resistance ( Resistance Resistance ( Resistance 2 2

0 0 6 7 8 9 106 107 108 109 10 10 10 10 Frequency (Hz) Frequency (Hz)

• Resistance at low frequencies is less than 1 Ω.

• Resistance of magnetic inductors increases greatly at high frequencies due to the magnetic power losses.

27 Device Properties of “Standard” Inductors - Q

Air core inductors Magnetic inductors

10 10 N = 4.5 N = 4.5 8 N = 8.5 8 N = 8.5 N = 17.5 N = 17.5

6 6

4 4 Quality factorQuality factorQuality 2 2

0 0 106 107 108 109 106 107 108 109 Frequency (Hz) Frequency (Hz)

• Quality factor of magnetic inductor is above 6 at 20 MHz for N = 17.5, and the enhancement over air core is well above 10×. However, it starts to decrease as the frequency increases due to the magnetic power losses.

28 Five-Turn Magnetic Inductor on Package

29 V. Analysis of Measurement Results

I. Introduction

II. Analytical Models and Inductor Design

III. Fabrication of Integrated Inductors

IV. Measurement of Fabricated Inductors

V. Analysis of Magnetic Inductors on Si • Comparison with analytical models • Effect of magnetic core shape • Effect of scaling down

VI. Conclusion

30 Comparison with Analytical Models (I)

Inductance (@ 10 MHz) Coil resistance

1x10-7 1.0 Air core, measurement -8 Air core, measurement 1x10 Air core, simulation -8 Magnetic, measurement Air core, calculation 8x10 ) 0.8 -9 Ω Calculation 8x10 Magnetic, measurement Magnetic, simulation 6x10-8

L 0.6 -9 6x10 Magnetic, calculation MC (H) 4x10-8 (H) AC 34× L 0.4 4x10-9 2x10-8 2x10-9 0.2

0 Resistance ( @ 1MHz 0 0.0 0 5 10 15 20 25 0 5 10 15 20 25 Number of turns Number of turns

• The good agreements confirm that the analytic models can accurately describe the of air core and magnetic inductors and their coil resistances.

• It indicates that the demagnetization effect plays a major role in determining the effective permeability of the magnetic inductors.

• The calculated inductance enhancement is about 30× for N = 17.5, which is very close to the observed enhancement of 34×. 31 Comparison with Analytical Models (II)

Trade-off between ∆L and ∆R “Standard” with N = 17.5

12 10 10 R measured 20 MHz R calculated 10 40 MHz ⎛ µ" ⎞ 8 Q measured 8

60 MHz ∆ = ω ∆ ) R ⎜ ⎟ L Q calculated Quality factor

⎜ ⎟ Ω 8 ⎝ µ' ⎠ Measurement 6 6 ) Calculation Ω 6

R ( 4 4 ∆ 4 Resistance ( 2 2 2

0 0 0 0.0 5.0x10-8 1.0x10-7 1.5x10-7 106 107 108 109 ∆L (H) Frequency (Hz)

• Permeability spectra of the processed magnetic core are used for the calculations of resistance and quality factor of the magnetic inductor.

• The excellent agreements between the calculation and measurement results directly confirm the validity of the proposed analytical models.

32 Effect of Magnetic Core Shape (I)

Inductance (@ 10 MHz) “Closed 2.0x10-7 core” Standard, measurement Standard, simulation Series, measurement 1.5x10-7 Series, simulation Closed core, measurement “Series” Closed core, simulation 1.0x10-7

-8 Inductance (H) 5.0x10

“Standard”

0.0 0 5 10 15 20 Number of turns • For a given number of turns, the inductance of the “series” inductor is nearly doubled from those of the “standard” inductor, indicating that the “series” inductor can be viewed as two “standard” inductors connected in series. 33 Effect of Magnetic Core Shape (II)

Inductance (@ 10 MHz) “Closed core” “Two bars” 2.0x10-7 Closed core, measurement Closed core, simulation 1.5x10-7 Two bars, simulation

1.0x10-7

-8 Inductance (H) 5.0x10

0.0 0 5 10 15 20 µ’= 600 Number of turns

• Simulation results indicate that the effective shape of the closed magnetic core should be viewed as two parallel magnetic bars closed by two “bad” soft .

• Hence, the closed magnetic core is not effective in improving the magnetic closure significantly, and it can be explained by the tensor nature of permeability of the magnetic core.

34 Effect of Scaling Down

Inductance Resistance

6x10-8 10 N = 4.5 N = 4.5 N = 8.5 5x10-8 N = 8.5 N = 17.5 8 N = 17.5 -8 4x10 ) Ω 6 3x10-8 4 2x10-8 Inductance (H) Resistance ( 2 1x10-8

0 0 106 107 108 109 106 107 108 109 Frequency (Hz) Frequency (Hz)

• Inductance is 48.4 nH at 10 MHz for N = 17.5, and the device area is reduced by a factor of four to 0.22 mm2, resulting in the inductance density to 219 nH/mm2.

• The coil resistance is not affected by the scale-down and is measured to be 0.57 Ω for N = 17.5 at 1 MHz.

35 Bridging the Gap

10 uH Discrete inductors 1 uH

100 nH Integrated magnetic inductors 10 nH Inductance

1 nH Planar spiral inductors 100 pH

100 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz Frequency

36 Summary

• High-performance integrated magnetic inductors were successfully designed and fabricated: For the coil resistance less than 1 Ω and the device area below 1 mm2, the inductance as high as 70.4 nH was obtained on Si, corresponding to the inductance enhancement of 34× over the air core equivalent, and the inductance density reached 219nH/mm2. For DC resistance ~ 10 mΩ and device area of ~14 mm2: Q ~ 25 at 200 MHz for magnetic inductor on package.

• An analytical model can accurately describe the actual device properties:

The fundamental trade-offs (∆L vs ∆R) of the integrated magnetic inductors are well understood. The inductor device properties can be further optimized (by materials or design) for a given application or frequency range.

37