U. Kim RF and Millimeter-wave Integrated Systems Lab. 1 Q Enhancement in Spiral Inductors q Why Inductors need? q Inductors Used in RFIC, MMIC q Spiral

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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) [email protected]

RF and Millimeter-wave Integrated Systems Lab.


Why Inductors Need?

Typical Design Example

A single-chip GPS Receiver

CMOS technology

Freq = 1.57542GHz

Used more than 10 inductors

About 25% of chip area

Impedance matching

DC biasing (RF choke)

Phase shifting

Filtering

LC tank


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


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


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)

d

Discrete inductor

L = 2 ~ 100nH with 2 ~10% tolerance

Q = 50 to 200 (1 to 2GHz)

SRF = 4 to 10GHz


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


Spiral Inductor Modeling

Ls : Mutual Couplings

Rs : DC & AC resistance (skin effect)

Cs : Series Capacitance

Cox : Oxide Capacitance

Csi : Si Substrate Capacitance

Rsi : Si Substrate Ohmic Loss

C.Patrick Yue, “Physical Modeling of Spiral Inductors on Silicon”


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


Dominant Effects on Spiral Inductor

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?

SRF


Other Dimensional Effects on Spiral Inductor

1 2

3 1. Size dependencylarger size, larger substrate loss

2. Oxide thickness dependencythicker oxide, lower substrate loss

3. Metal thickness dependencythicker metal, lower Rs

Or, using Cu instead of Al, lower Rs


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↓


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


Patterned Ground Shield

PGS

Orthogonal to spiral (block eddy current) Avoid attenuation of the magnetic field

Isolates between inductor and ground termination of the electric field

Aluminum metal or polysilicon (better)

Capacitance increases SRF↓

C.Patrick Yue, “On-Chip Spiral Inductors with Patterned Ground Shields for Si-Based RFICs”


Patterned Ground Shield (cont’)

Q factor up to 33%

SRF decrease

SRF


Patterned Ground Shield (cont’)

1. Parallel LC resonator at 2GHzThere 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.

1

2


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

asymmetricsymmetric

High V difference

Same I directionLow V difference

Same I direction


Differentially Driven Inductors (cont’)

Mina Danesh, “Differentially Driven Symmetric Microstrip Inductors”

Lsub.sSpiral inductor

modelingSingle-ended

Lsub.d

Differential excitations

Lseries.d

Lseries.s

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.


Differentially Driven Inductors (cont’)

Less affected by substrate parastics

50% grater Q factor than single-ended

Broader range of operating frequencies

common node


Circular Shaped Inductors

1GHz

Rcircular and Roctaogonal is smaller by 10% than Rsquare

Decreasing conductor spacing is better than increasing conductor width

CIW ↑ Cox, C si ↑

S. Chaki, “Experimental Study on Spiral Inductors”


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


Cu Damascene Interconnects

(a) Etch trenches and via holes

(b) Ta barrier layer and Cu seed layer

(c) Electrochemical plating Cu

(d) CMP Cu and Ta, CVD nitride

Example : Cu metallization in VLSI technology

Better conductor than aluminum

Higher speed and less power consumption

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


High Q Inductor in Single Damascene

Snezana Jenei, “High Q Inductor Add-on Module in Thick Cu/SiLKTM single damascnene”

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


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


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

Documents

U. Kim RF and Millimeter-wave Integrated Systems Lab. 1 Q Enhancement in Spiral Inductors q Why Inductors need? q Inductors Used in RFIC, MMIC q Spiral