Reminder: University Ph.D. Oral Examination - Dok Won Lee

Dok Won Lee dwlee at
Thu Jul 24 08:43:36 PDT 2008

"Integrated Inductor with Magnetic Core: A Realistic Option"

Dok Won Lee
Department of Materials Science and Engineering
Advisor: Prof. Shan X. Wang

Tomorrow (Friday, July 25th, 2008)
9:00 AM (Refreshments served at 8:45 AM)
CIS-X Auditorium (Rm. 101)


Nowadays cell phones and laptop computers are playing important roles  
in our everyday lives, and the demand for more portable electronic  
devices continues to increase rapidly. This is currently driving the  
integration or embedding of passive components, which would replace  
off-chip discrete modular assemblies. However, poor properties of  
integrated inductors have been a critical factor limiting the overall  
performance of radio-frequency (RF) circuits and hence the realization  
of system-on-a-chip (SoC) or system-in-package (SiP) circuits for  
portable electronics.

Use of magnetic core with high permeability in the integrated inductor  
was proposed decades ago to significantly increase the inductance by  
the relative permeability of the magnetic material used. However, the  
inductance enhancements reported so far have been limited and not well  
explained. In addition, the use of magnetic core comes with the cost  
of introducing magnetic power losses. This needs to be well understood  
in order to make the magnetic inductor practical and useful.

In this talk, I present the high performance integrated inductors  
using a solenoid design with a magnetic layer. A set of analytical  
models was developed to describe the device properties of integrated  
solenoid inductors. Using the models, design parameters were optimized  
to achieve a high inductance while maintaining the lateral device area  
< 1 mm^2 and the coil resistance < 1 ohm. The integrated inductors  
were fabricated on Si wafers using copper as the conductor layer and  
CoTaZr alloy as the magnetic core layer. A polyimide planarization  
process was developed as the preceding step for the magnetic core  

The inductance of the fabricated inductor was as high as 70.2 nH  
measured at 10 MHz with DC resistance of 0.67 ohm and the device area  
of 0.88 mm^2. This is an enhancement by a factor of 34 from the air  
core inductor with the identical geometry, and the resulting  
inductance density was 80 nH/mm^2. By shrinking the lateral dimensions  
while maintaining the vertical dimensions unchanged, the inductance  
density further increased to 219 nH/mm^2 without affecting the coil  
resistance significantly. The measured device properties and the  
calculations using the analytical models show good agreements. The  
resistance of the magnetic inductor increased significantly with the  
frequency due to the introduction of magnetic power losses at high  
frequencies, and the frequency-dependent resistance and quality factor  
of the magnetic inductor were also in excellent agreement with the  

The device properties of the integrated magnetic inductors are well  
understood with the analytical models developed and can be further  
optimized for applications and frequency ranges of interest. The  
integrated magnetic inductors can now be reliably designed and  
fabricated for various applications, enabling the realization of the  
RF integrated electronics.

Dok Won Lee, Ph.D candidate
Materials Science and Engineering, Stanford University
McCullough BLDG Rm.208, 476 Lomita Mall, Stanford, CA 94305-4045
Phone:650-723-4015 | Fax:650-736-1984

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