Q Enhancement in Spiral Inductors
 Why Inductors need?
 Inductors Used in RFIC, MMIC
 Spiral Inductor Modeling
 Degradation of Q
 Q Enhancement Techniques
 Conclusion
Microwave Devices Term Project
Unha Kim (2004-21475) edmaun1@snu.ac.kr
RF and Millimeter-wave Integrated Systems Lab.
U. Kim
1
RF and Millimeter-wave Integrated Systems Lab.
Why Inductors Need?
 Impedance matching
 DC biasing (RF choke)
 Phase shifting
 Filtering
 LC tank
Typical Design Example
 A single-chip GPS Receiver
 CMOS technology
 Freq = 1.57542GHz
 Used more than 10 inductors
 About 25% of chip area
U. Kim
2
RF and Millimeter-wave Integrated Systems Lab.
Inductors Used in RFIC, MMIC
Inductors
 Ribbon Inductor
 Loop Inductor
 Meandered Inductor
 Spiral Inductor
 Bondwire Inductor
 Active Inductor
Considerations
 Inductance
 Quality Factor
 Self Resonant Frequency
U. Kim
3
RF and Millimeter-wave Integrated Systems Lab.
Some Types of Inductors
Ribbon inductor
 Less than 1nH
 High Z0 needed
 Relatively ‘pure’ inductance (low parastics)
 Often used in distributed amplifiers
Loop inductor
 Used extensively in the early days of MMICs
 Inefficient use of chip area
 Recently, it is used very little
Meandered track inductor
 Can get more than 1nH
 Lmeandered < Lstraight track with same length
U. Kim
4
RF and Millimeter-wave Integrated Systems Lab.
Some Types of Inductors
Bondwire inductor
 Diameter = 1mil (0.001 inches)
 More surface area per length than spirals
 Less resistive loss, Higher Q
 L = 1nH / mm
Active inductor
 Higher noise
 Power consumption
 Limited linearity - Distortion
 L = C / (gm1gm2)
Discrete inductor
 L = 2 ~ 100nH with 2 ~10% tolerance
 Q = 50 to 200 (1 to 2GHz)
 SRF = 4 to 10GHz
U. Kim
5
RF and Millimeter-wave Integrated Systems Lab.
Spiral Inductor
 The most frequently used
 High inductance per unit area
 Square, octagon, circular type
 Qcircular > Qoctagon > Qsquare
 Air bridge crossover or dielectric spaced underpass
 Din : Inner dimension
 Dout : Outer dimension
 S : Spacing
 W : Width
 t : Thickness
 n : Number of turns
U. Kim
6
RF and Millimeter-wave Integrated Systems Lab.
Spiral Inductor Modeling
Ls : Mutual Couplings
C.Patrick Yue, “Physical Modeling of Spiral
Inductors on Silicon”
Rs : DC & AC resistance (skin effect)
Cs : Series Capacitance
Cox : Oxide Capacitance
Csi : Si Substrate Capacitance
Rsi : Si Substrate Ohmic Loss
U. Kim
7
RF and Millimeter-wave Integrated Systems Lab.
Degradation of Q
Problems
 Limitation on the number of turns
 Occupies large area
 Series (DC + AC) resistance
 Substrate loss
Some Proposed solutions
 Patterned ground shield
 Differentially driven inductor
 Copper metallization
 Three dimensional inductor
U. Kim
8
RF and Millimeter-wave Integrated Systems Lab.
Dominant Effects on Spiral Inductor
SRF
Rs, Cs effect
dominant
Csi , Rsi effect
dominant
 Low frequency : series resistance effect
 High frequency : substrate loss effect
 Conductive Si substrate have a defect!
 How can we reduce the substrate loss?
U. Kim
9
RF and Millimeter-wave Integrated Systems Lab.
Other Dimensional Effects on Spiral Inductor
1
2
3
1. Size dependency
larger size, larger substrate loss
2. Oxide thickness dependency
thicker oxide, lower substrate loss
3. Metal thickness dependency
thicker metal, lower Rs
Or, using Cu instead of Al, lower Rs
U. Kim
10
RF and Millimeter-wave Integrated Systems Lab.
Solid Ground Shield
 Severe substrate loss at high freq.
 Si substrate is vulnerable
Usually ρ < 20Ω·cm
 GaAs substrate is less vulnerable
Semi-Insulating Substrate
SGS
 To reduce substrate loss
 Conductive ground shield
between oxide and substrate
 Metal or polysilicon deposition
 Eddy current ☞ L↓ Q↓
 Capacitance increases ☞ SRF↓
U. Kim
11
RF and Millimeter-wave Integrated Systems Lab.
Eddy Current
 Eddy current occurs when a conductor is subjected to time-varying-magnetic
field and is governed by Faraday’s law.
 Eddy currents produce their own magnetic fields to oppose the original field
 Eddy currents reduce the net current flow in the conductor
 Increase the ac resistance
U. Kim
12
RF and Millimeter-wave Integrated Systems Lab.
Patterned Ground Shield
PGS
 Orthogonal to spiral (block eddy current)
Avoid attenuation of the magnetic field
C.Patrick Yue, “On-Chip Spiral Inductors
with Patterned Ground Shields for Si-Based
RFICs”
 Isolates between inductor and ground
termination of the electric field
 Aluminum metal or polysilicon
(better)
 Capacitance increases ☞ SRF↓
U. Kim
13
RF and Millimeter-wave Integrated Systems Lab.
Patterned Ground Shield (cont’)
 Q factor up to 33%
 SRF decrease
SRF
U. Kim
14
RF and Millimeter-wave Integrated Systems Lab.
Patterned Ground Shield (cont’)
2
1
1. Parallel LC resonator at 2GHz
There are many advantages in designing oscillator.
2. Reduce the substrate coupling b/w two adjacent inductors by 25dB
Using PGS has both advantages and disadvantages.
U. Kim
15
RF and Millimeter-wave Integrated Systems Lab.
Differentially Driven Inductors
 Differential circuits have robustness and superior noise rejection properties
 Can get greater Q without altering the fabrication process
 Differential signal path requires extra chip area compared to a single-ended
 Symmetrical inductor has better performance than asymmetric inductor.
 Adjacent conducting strips : voltage (anti-phase), current (same direction)
Reinforces the magnetic field by the parallel groups of conductors
Increases the overall inductance per unit area
High V difference
Same I direction
Low V difference
Same I direction
asymmetric
U. Kim
symmetric
16
RF and Millimeter-wave Integrated Systems Lab.
Differentially Driven Inductors (cont’)
Lseries.
Spiral inductor modeling
Lsub.s
Lseries.d
s
Single-ended
Lsub.d
 Lseries.d and Lseries.s are similar
low-freq. dominant factor
 Lsub.d is less than L sub.s up to 2 times
high-freq. dominant factor
 Low freq. performance is similar
 (c) is superior at high freq.
Differential excitations
Mina Danesh, “Differentially Driven
Symmetric Microstrip Inductors”
U. Kim
17
RF and Millimeter-wave Integrated Systems Lab.
Differentially Driven Inductors (cont’)
common
node
 Less affected by substrate parastics
 50% grater Q factor than single-ended
 Broader range of operating frequencies
U. Kim
18
RF and Millimeter-wave Integrated Systems Lab.
Circular Shaped Inductors
1GHz
S. Chaki, “Experimental Study on Spiral
Inductors”
 Rcircular and Roctaogonal is smaller by 10% than Rsquare
 Decreasing conductor spacing is better than increasing conductor width
CIW ↑
U. Kim
Cox, C si ↑
19
RF and Millimeter-wave Integrated Systems Lab.
Reducing Line Resistance
 AC resistance ☞ W, t ＞ 2δs
 DC resistance
 The four best conducting metal resistivities are
Silver
: 1.62 μΩ·cm
Copper
: 1.72 μΩ·cm
Gold
: 2.44 μΩ·cm
Aluminum : 2.62 μΩ·cm
 If we use Cu instead of Al, Rs would be reduced significantly
 Some paper proposed that ( 3um-thick Al ) = ( 1um-thick Cu )
 But thicker metal, larger CIW
 Damascene Cu metallization
 Cu metallization is not mature in RFIC & MMIC
U. Kim
20
RF and Millimeter-wave Integrated Systems Lab.
Cu Damascene Interconnects
(a) Etch trenches and via holes
(d) CMP Cu and Ta, CVD nitride
Example : Cu metallization in VLSI technology
 Better conductor than aluminum
 Higher speed and less power consumption
(b) Ta barrier layer and Cu seed layer
 Higher electomigration resistance
 Diffusing freely in silicon and silicon dioxide,
causing heavy metal contamination, need
diffusion barrier layer
 Hard to dry etch, no simple gaseous
chemical compounds
(c) Electrochemical plating Cu
U. Kim
21
RF and Millimeter-wave Integrated Systems Lab.
High Q Inductor in Single Damascene
 Al sheet resistance : 20~100
 Q factors up to 24 at 2.8nH
by using think metal layer
 Non-effective unless the substrate
losses are lowered sufficiently
Snezana Jenei, “High Q Inductor Add-on
Module in Thick Cu/SiLKTM single damascnene”
U. Kim
22
RF and Millimeter-wave Integrated Systems Lab.
Conclusion
 Inductors are needed in RFICs & MMICs
 High Q inductors are required for high performance
 Spiral inductors are mostly used
 The Q of spiral inductor is very low
 Substrate loss and series resistance are major effects on Q
 Some Q-enhancement techniques are suggested
 PGS + Cu-metal + Octagonal shaped inductor is best performance
 It will be trade-off relation between high-Q process and cost
U. Kim
23
RF and Millimeter-wave Integrated Systems Lab.
References
 C. Patrick Yue, “Physical Modeling of Spiral Inductors on Silicon”
 Mina Danesh, “Differentially Driven Symmetric Microstrip Inductors”
 C. Patrick Yue, “On-Chip Spiral Inductors for Silicon-Based RFICs”
 Snezana Jenei, “High Q Inductor Add-on Module in Thick Cu/SiLK single damascene”
 Daniel C. Edelstein, “Spiral and Solenoidal Inductor Structures on Silicon Using
Cu-Damascene Interconnects”
 Joachim N. Burghartz, “On the Design of RF Spiral Inductors on Silicon”
 S. Chaki, “Experimental Study on Spiral Inductors”
 C. Patrick Yue, “On-Chip Spiral Inductors with Patterned Ground Shields for Si-Based RFICs”
 John Rogers, “Radio Frequency Integrated Circuit Degisn”, Artech House
 Thomas H. Lee, “The Design of CMOS RFICs”, Cambridge Univ. press
 I. D. Robertson, “MMIC Design”, IEE press
U. Kim
24
RF and Millimeter-wave Integrated Systems Lab.