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)
Abstract:
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
formation.
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
calculations.
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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