Transcript High Q Inductor for RFIC

『Microwave Device』 Term Project
High Q, 3D Inductors for RF-IC
Bioelectonic Systems Lab.
Choongjae Lee (2001-21538)
[email protected]
Introduction
 RF-IC requires high performance passive components
as an integrated form to reduce the total system size
and assembly cost.
 High performance RF inductors are the key components
for implementing critical building blocks such as
low noise RF VCOs, low loss impedance matching
networks, passive filters, and inductive loads for power
amplifiers.
 High Q inductors are critical for lowering power
dissipation, and improving the performance of personal
RF communication devices such as cellular phones.
Inductor Q Factor
 Definition of Q Factor
f
: Operating freq.
 Rm(f) : Real Parts of the inductor impedance
 Xm(f) : Imaginary parts of the inductor Impedance
 Degradation of Q
 Losses from coil Resistance - Frequency dependent
(∵ skin depth effects)
 thick metallization
 Self resonance of the coil
 parameter control
(ex. turn number, turn to turn spacing, metal width)
 Losses from eddy currents circulation
below the spiral in the silicon substrate
(proximity effects)
Techniques for High Q
< Ohmic loss reducing >
 Thick metallization, multilayer metallization
< Eddy current reducing >
 Patterned ground shield below the inductor
 Thick or low dielectric layer to separate the spiral
from the substrate
 Using high resistivity (>4kΩ) silicon or ceramic, glass
as the substrate
 Etching away the underlying substrate
 Fabrication a suspended inductor
 3D on chip inductor
minimize the device capacitive coupling to the substrate
and eddy current loss
Ex1) Suspended Spiral Inductor-[1]
 Using MEMS technology.
 The inductor is sustained with the T-shaped pillars.
Continued
 Separation between the substrate and the inductor
 Great improvements in Q-Factor
 Qmax is 37 for the L of 4.2nH
with a suspended height of 60㎛
Idea) 3D Solenoid Design
 In comparison to planar spiral inductors, 3D solenoid
inductors have less substrate parasitic capacitance since
only partial parts of the coil, the bottom conductors, are
facing or touching the substrate
 3D solenoid inductors have significantly less eddy current
induced substrate loss than planar spiral inductors since
the core center is in the direction parallel to the substrate
 Core-loss can be minimized using low loss-tangent core
such as alumina.
 For post IC integration approach processing temperature
must not exceed approximately 450℃
Ex2) 3D on Chip Inductor–[2]
 3D
 minimize the device capacitive
coupling to the substrate
and eddy current loss
 minimize coil area
 Thick copper
 reduce the series resistance
 Core = alumina
(negligible loss tangent
at high freq.)
Continued
1.
2.
3.
4.
5.
6.
7.
Wafer passivation with 10㎛ Low
Temp. Oxide
Cu(3000Å)/Ti(500Å) Sputtering
8㎛ thick electroplated resist and
patterning
To prevent oxidation, Cu is
passivated with Au/Ni
The PR and Cu/Ti seed layer
removing
Core forming from alumina sheet
Side and top cu patterning with
direct write laser lithography
tool
Continued
 Standard Si sub
(10Ωcm)
 Amenable to
monolithic integration
in a standard IC
process due to
its low thermal budget
 Q is 30
for the L of 4.8nH
at 1GHz
Ex3) 3D solenoid inductor–[3]
 Utilizing deformation of a sacrificial thick polymer and
conformal photoresist electrodeposition techniques.
Continued
(a)
(b)
(c)
(d)
(e)
(f)
Substrate Insulation,
sputtering of electroplating
base (2000ÅCu/1000ÅTi)
15㎛ thick SU-8 resist coating
(polymeric mold for bottom
conductor electrodeposition),
Metal electroplating
Deposition a polymeric mesa,
SJR5740
120℃ Hard cure for 6hours to
deform bell-shaped profile,
sputtering of electroplating
base, PEPR2400 photoresist is
applied using
electrodeposition
Ni-Cu electroplating through
the PEPR2400 mold to form
top conductor
Sacrificial layers and
electroplating base removing
Ex4) Surface Micromachined
Solenoid Inductor–[4]
 Using an ordinary IC process having several metal layers
 20 turn, all-copper solenoid inductor
Continued
 On-Si with a 15㎛ thick
insulating layer
 Q = 16.7 at 2.4GHz
with L of 2.67nH
 On-glass
 Q = 25.1 at 8.4GHz
with L of 2.3nH
 The inferior Q factor
performance of the on-Si
inductor originates from the
relatively large increase in
the parasitic capacitance to
the substrate.
Idea) Toroidal Geometry
 Higher Q compared to planar coils and lower
interference with surrounding circuits
 Most of the electromagnetic field is concentrated
inside the torus.
 Magnetic flux away from ground planes
and semiconducting substrates
 Little eddy current is induced.
 Toroidal geometry have optimal electromagnetic
characteristics
Toroidal Inductor [5]
 p=100㎛
 t=8.3㎛
 w=70㎛
 r=440㎛
 a=170㎛
 hs=500㎛
 hox=0.5㎛
 N=15
Continued
 Post-processing technology for monolithically ICs
 Fabrication without requiring changes to the silicon process
 Surface micromachining technology
 Confining the magnetic fields
 Optimize the tradeoff between flux linkage and
turn-to-turn parasitic capacitance
 For the same inductance, toroidal structures consume
significantly less area
 A concentrated magnetic field along the core results in
less noise coupling and electromagnetic interference with
the neighboring components
Continued
Si sub 20Ωcm, 500㎛ thickness
(a) Input output lead lines
patterning
(b) Silver – seed layer for
electroplating
(c) Thick PR deposition
(d) anchoring pints patterning
(e) gold gold/palladium layer
deposition
(f) suspended metal bridge
patterning
(g) PR removing and metal
thickening by electroplating
copper and gold
Continued
 Micromachined implementation of the toroidal inductor
 Low resistivity
silicon wafer
 Q Factor: 22
(At 1.5GHz)
 L=2.45nH
 self–resonant freq.
> 10GHz
 11 Turns
Toroidal Inductor [6]
Dout=Outer diameter of the coil
δskin=Skin depth
μ0=Vacuum permeability, μr=relative permeability,
N=Number of turns, rcoil=radius of the coil
rtorus=radius of the torus cross section
α(f) = current crowding effect coefficient
ρ = resistivity, Scoil = seperation between coils
∴
Continued
Copper
Gold
Freq.=0.9GHz
Dout=1mm,
Scoil=15㎛
Metal Thickness=8㎛
L=5nH
Freq.=1GHz
2rtorus/Dout=0.2
Gold Thickness=8㎛
L=5nH
Continued
 Using polymer replication processes
 accuracy and small features available
+ production economy
Continued
 6.0nH Inductance, Q Factor 50 (freq.: 3GHz)
This implementation is not intended
for direct integration with RF ICs
Metal Thickness = 4㎛, 15 turns
Summary
 RFIC requires high performance on chip inductor for small size,






low cost, high performance.
For high Q inductor, we must reduce ohmic loss and induced
current.
To reduce the induced current, suspended spiral inductor, 3D
solenoid inductor and 3D toroidal inductor were suggested.
With the increase of the suspended heights, the maximum Q
factor increases gradually.
3D inductors have significantly less eddy current induced
substrate loss since the core center is in the direction parallel
to the substrate.
3D inductors have less substrate parasitic capacitance since
only partial parts of the coil are facing or touching the
substrate.
In toroidal inductor design, most of the electromagnetic field
is concentrated inside the torus. Thus higher Q and lower
interference with surrounding circuits can be achieved.
References
[1] Xi-ning Wang, “Fabrication and Performance of a novel
suspended RF spiral inductor” IEEE Tran. On Electron
Devices, Vol. 51, No. 5, May. 2004.
[2] D. J. Young, “Monolithic high-performance threedimensional coil inductors for wireless communication
applications” Int. EDM 97, 1997, pp 67-70
[3] N.Chomnawang, “On-chip 3D air core micro-inductor for
high-frequency applications using deformation of
sacrificial polymer” Proc. SPIE, Vol. 4334, pp. 54-62, 2001
[4] J.B.Yoon, “Surface micromachined solenoid on-Si and
on-glass inductors for RF applications” IEEE Electron
Device Lett., Vol. 20, pp 487-489, Sep. 1999.
[5] Wai Y. Liu, “Toroidal inductors for radio-frequency
Integrated Circuits” IEEE Tran. On Microwave Theory and
Techniques, Vol. 52, No2, Feb. 2004.
[6] Vladimir Ermolov, “Microreplicated RF toroidal inductor”
IEEE Tran. On Microwave Theory and Techniques, Vol. 52, No.
1, Jan. 2004.