Embedded Integrated Inductors With A Single Layer Magnetic Core: A Realistic Option

Embedded Integrated Inductors With A Single Layer Magnetic Core: 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 Inductor Design III. Fabrication of Integrated Inductors IV. Measurement of Fabricated Inductors V. Analysis of Magnetic Inductors on Si VI. Conclusion

Use of Inductors in Our Daily Lives Traffic light Red-light camera Metal detector RFID tag Texas Instruments Tag-it TM Voltage regulator module Cell phone 3

Why Integrated Inductors? Fully integrated electronics R.K. Ulrich and L.W. Schaper, Integrated Passive Component Technology, 003 4

Example: Power Management DC-DC converter Passive components are discrete and occupy large areas IC containing gate driver and power MOSFETs Inductor Capacitors Enpirion EN5330 PSoC (Power-System-on-a-Chip)* 70% of pc-board area saved * R. Allen, Electronic Design, May 004 5

Inductance Requirement for Power Management 10 uh 1 uh Discrete inductors Large inductance Large volume Poor AC performance Inductance 100 nh 10 nh 1 nh Planar spiral inductors Small area consumption Small inductance Limited performance in sub-ghz applications 100 ph 100 khz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz Frequency From A. Ghahary, Power Electronics Technology, Aug. 004 6

Schematics of Magnetic Inductor Designs Transmission line Spiral inductor Solenoid inductor Small resistance Small inductance Usually two magnetic layers needed Close to the planar spiral Limited inductance gain One or two magnetic layers Magnetically efficient Relatively complex structure 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 A.M. Crawford, et al. IEEE Trans. Magn. 00, p.3168-70 Planar transmission line inductor with CoTaZr core, Q = 6 @ 700 MHz Planar spiral inductor with CoTaZr core, CMOS compatibility, Q ~.7 @ 1 GHz L. Li, D. W. Lee, et al. IEEE Trans. Advanced Packaging (accepted 008) On-package solenoid inductor with CoFeHfO core, Q = @ 00~300 MHz, R dc ~ 10 mω D. W. Lee, et al. Intermag 08, AG 01 (invited) Planar solenoid inductor with CoTaZr core, Inductance enhancement over air core = 34x Q>6 @ 6 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 w M l C w V w C s V t M t A t C g l A, l M g V s V w A 11

Key Device Properties Inductance L Resistance R Quality factor Q Energy stored Q = π = Power dissipation T ωl R Device area Useful bandwidth 1

Inductance of Magnetic Inductor L MC Inductance of magnetic inductor L MC : L MC = L AC N µ d eff + L where L = l M µ 0µ rn [1 + N = Demagnetizing factor of rectangular prism µ r 1+ N ( µ 1) d wmt µ 0µ M eff N = ( µ 1)] l d r r M w M t M L simulated (H) 5x10-8 4x10-8 3x10-8 x10-8 1x10-8 Slope = 1.01 ± 0.01 HFSS simulations Linear fit For a finite-sized magnetic core, there is a demagnetizing field inside the magnetic core, which effectively reduces µ r. 0 0 1x10-8 x10-8 3x10-8 4x10-8 5x10-8 L calculated (H) Demagnetizing field is not uniform inside the magnetic core, and the numerical solutions should be used for µ r > 1. * Inductance enhancement: L L AC = L MC L L AC AC µ eff A A MC AC, eff Much less than µ r but still significant A MC A AC,eff * D.-X. Chen et al., IEEE Trans. Magn., 41, 077 (005) 13

Resistance of Magnetic Inductor R MC Resistance of magnetic inductor R MC : From the classical electromagnetism: * Magnetic contribution to the energy stored Magnetic power loss P Magnetic = ωµ " H E dv Magnetic = 1 µ ' H dv P µ " ω µ ' Magnetic E Magnetic Representing in terms of the device properties: R P E Magnetic R Magnetic MC = ( R = E = R AC MC R AC ) I Magnetic inductor + R = R = RI E AC where Air core inductor µ " + ω L µ ' = 1 R R L MC R for large L I MC R for small L R AC 1 R L AC AC I = 1 LI µ " R = ω L µ ' Both ω and (µ /µ ) increase with frequency. Hence R becomes significant as the frequency increases. The more inductance enhancement we obtain by using a magnetic core, the more resistive losses we introduce at high frequencies. Freq. * R.F. Harrington, Time-Harmonic Electromagnetic Fields, 1961 14

Quality factor Q Quality factor of air core inductor Q AC : Q AC L = ω R AC AC Quality factor of magnetic inductor Q MC : Quality factor 10 8 6 4 L/L AC =1 L/L AC =10 L/L AC =30 Q AC µ'/µ" L << L AC L >> L AC Q MC L = ω R MC MC = ω R AC LAC + L µ " + ω L µ ' Q MC ~ Q AC at low frequencies Q MC ~ µ /µ at high frequencies f MC can be considered as the useful bandwidth of the magnetic inductor. 0 10 6 10 7 10 8 10 f 9 MC Frequency (Hz) 15

Material Selection Conductor: Copper due to its low electrical resistivity Magnetic core: Desirable properties: - High permeability - Soft magnetic material (low coercivity) - High resistivity - High ferromagnetic resonance (FMR) frequency Amorphous Co 90 Ta 5 Zr 5 (at. %) alloy: - µ ~ 600 - H c < 1 Oe - ρ ~ 108 µω-cm - f FMR ~ 1.5 GHz Relative permeability 0. μm CoTaZrmagnetic film 500 000 1500 1000 500 µ' µ" 0 10 6 10 7 10 8 10 9 Frequency (Hz) 16

Inductor Designs Spiral Planar spiral inductor with or without magnetic plane Standard Scale-down N = 4.5 N = 4.5, 8.5, 17.5 Solenoid inductor with lateral parameters scaled down by a factor of while maintaining vertical parameters unchanged N = 4.5, 8.5, 17.5 Solenoid inductor with different magnetic core arrangement or shape Series Closed core 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 structures Magnetic inductors 19

SEM Images of Fabricated Inductor Devices Standard Spiral Scale-down 400 µm 400 µm 400 µm Series Closed core 400 µm 400 µm 0

FIB Cross-section Images of Fabricated Inductors Top Cu layer 6.6 um Polyimide 0.5 um Magnetic core. um Polyimide.0 um Bottom Cu layer 4.4 um Thermal oxide 50 µm Si substrate 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. 1

Magnetic core shape affects permeability! B-H loops Permeability spectra Kerr microscope image Normalized B 1.0 Easy Hard 0.5 0.0-0.5-1.0-150 -100-50 0 50 100 150 Field (H) Relative permeability 600 400 00 0 µ', blanket µ", blanket µ', processed µ", processed Easy Frequency (Hz) axis 100 µm 10 7 10 8 10 9 Bottom conductor Magnetic core 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.

On-package Inductors on 8-inch Substrate Easy axis of CoFeHfO 1 13 1 3 4 31 3 33 34 4 43 Notch 3

Surface roughness affects permeability! R a =18.6 nm Permeability spectra of patterned CoFeHfO bars on dielectric material Surface roughness of dielectric material The rough surface of dielectric material degrades the magnetic properties of CoFeHfO deposited on it (even before patterning). 4

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 5

Device Properties of Standard Inductors - L Air core inductors Magnetic inductors 1x10-7 N = 4.5 N = 8.5 8x10-8 N = 17.5 1x10-7 N = 4.5 N = 8.5 8x10-8 N = 17.5 Inductance (H) 6x10-8 4x10-8 x10-8 Inductance (H) 6x10-8 4x10-8 x10-8 0 10 7 10 8 10 9 Frequency (Hz) 0 10 6 10 7 10 8 10 9 Frequency (Hz) With the use of magnetic core, inductance is 70. nh for N = 17.5, and the inductance enhancement is as high as 34. The device area for N = 17.5 is 0.88 mm, corresponding to an inductance density of 80 nh/mm. 6

Device Properties of Standard Inductors - R Air core inductors Magnetic inductors 10 10 8 N = 4.5 N = 8.5 N = 17.5 8 N = 4.5 N = 8.5 N = 17.5 Resistance (Ω) 6 4 Resistance (Ω) 6 4 0 10 6 10 7 10 8 10 9 Frequency (Hz) 0 10 6 10 7 10 8 10 9 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. 7

Device Properties of Standard Inductors - Q Air core inductors Magnetic inductors 10 10 8 N = 4.5 N = 8.5 N = 17.5 8 N = 4.5 N = 8.5 N = 17.5 Quality factor 6 4 Quality factor 6 4 0 10 6 10 7 10 8 10 9 0 10 6 10 7 10 8 10 9 Frequency (Hz) Frequency (Hz) Quality factor of magnetic inductor is above 6 at 0 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. 8

Five-Turn Magnetic Inductor on Package 9

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 L AC (H) 1x10-8 8x10-9 6x10-9 4x10-9 x10-9 Air core, measurement Air core, simulation Air core, calculation Magnetic, measurement Magnetic, simulation Magnetic, calculation 34 1x10-7 8x10-8 6x10-8 4x10-8 x10-8 0 L MC (H) Resistance @ 1MHz (Ω) 1.0 0.8 0.6 0.4 0. Air core, measurement Magnetic, measurement Calculation 0 0 5 10 15 0 5 0.0 0 5 10 15 0 5 Number of turns Number of turns The good agreements confirm that the analytic models can accurately describe the inductances 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 R (Ω) 1 10 8 6 4 0 MHz 40 MHz 60 MHz Measurement Calculation µ " R = ω L µ ' Resistance (Ω) 10 8 6 4 R measured R calculated Q measured Q calculated 10 8 6 4 Quality factor 0 0.0 5.0x10-8 1.0x10-7 1.5x10-7 L (H) 0 0 10 6 10 7 10 8 10 9 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. 3

Effect of Magnetic Core Shape (I) Closed core Series Inductance (H).0x10-7 Standard, measurement Standard, simulation Series, measurement 1.5x10-7 Series, simulation Closed core, measurement Closed core, simulation 1.0x10-7 5.0x10-8 Inductance (@ 10 MHz) Standard 0.0 0 5 10 15 0 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) Closed core Two bars Inductance (@ 10 MHz).0x10-7 Closed core, measurement Closed core, simulation 1.5x10-7 Two bars, simulation Inductance (H) 1.0x10-7 5.0x10-8 µ = 600 0.0 0 5 10 15 0 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 magnets. Hence, the closed magnetic core is not effective in improving the magnetic flux closure significantly, and it can be explained by the tensor nature of permeability of the magnetic core. 34

Effect of Scaling Down Inductance (H) 6x10-8 N = 4.5 5x10-8 N = 8.5 N = 17.5 4x10-8 3x10-8 x10-8 1x10-8 Inductance Resistance (Ω) 10 8 6 4 Resistance N = 4.5 N = 8.5 N = 17.5 0 10 6 10 7 10 8 10 9 Frequency (Hz) 0 10 6 10 7 10 8 10 9 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. mm, resulting in the inductance density to 19 nh/mm. 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 Inductance 100 nh 10 nh Integrated magnetic inductors 1 nh 100 ph 100 khz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz Frequency Planar spiral inductors 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 mm, 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 19nH/mm. For DC resistance ~ 10 mω and device area of ~14 mm : Q ~ 5 at 00 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