Modeling passives at 60GHz and beyond

Dr. Sharad Kapur

Integrand Software, Inc.

DAC Workshop on CMOS Design at 60 GHz and Beyond: Capabilities and Challenges

Outline

Applications at 60GHz and above

Modeling of passives at high frequencies

3D EM simulation of IC structures

• Methods for accurate and efficient solution

Design issues at high frequencies

Millimeter wave passives

• Couplers, dividers, quadrature-hybrids

Novel IC examples from industry and academia

• Validation and comparison to measurements

• 60GHz to 200GHz

Challenges ahead

Conclusion

Applications at 60GHz and above

Wireless links (60G)

• Chip to Chip

• Base station to Base station

• Wireless HDTV

FMCW imaging (60G)

Collision avoidance radar (77G)

D-band (100G to 170G)

E-band, W-band: (70G-100G)

Doppler Radar (atmospheric monitoring) 180G

Medical applications: 200G+

• Wireless detection of vital signs

Challenges at high frequencies

Modeling of active devices

• Depends a lot on the type of technology, SiGe, BiCMOS, CMOS, GaAs

Modeling of passive components

• Largely dependent on the metallization and substrate

• Does not matter if it is CMOS, SiGe or GaAs

This talk will be about modeling of passive structures and components at very high frequencies

The physics

Behavior of passives described “fully” by Maxwell’s equations

Differential Form

Integral Form

Differential vs Integral

Differential formulations

Finite-element, FDTD

Flexible

Imposes no constraint on shape of metals, dielectric regions

Need to enforce Maxwell’s equations everywhere surrounding the object

Leads to large sparse matrix solve

Integral formulations

MoM, BEM, Integral formulations

Planar dielectrics, conductors

Need to enforce Maxwell’s equations only on conductors (Green’s theorem)

Leads to smaller dense matrix to solve

Many techniques developed recently

For IC passives this approach is the best

6

Tech trends

Thick metals

• 0.1um to 8um copper

High-resistivity substrates

• 10 -cm to 1000-cm

Fine feature sizes

• 0.05m width at 28nm

Many metal layers

• High density capacitors

Easier for MOM/Integral solutions that do not need to discretize dielectric interfaces and only conductors

7

IC processes offer tight tolerances, low variability

When a device can be built with reasonable quality compared to an off-chip or an LTCC structure it will be integrated

It is difficult to build off chip passives at very high frequencies (easier to build on chip).

• Measurement equipment is now using integrated passives

IC processes

8

3D planar EM simulation

EMX is a 3D EM simulator

3D volume integral formulation (time-harmonic)

Unknowns are charges and currents

• Surface charges and volume currents

Includes retardation (time of flight)

Also skin effect is critical at higher frequencies so you need a volume current and meshing of the metals

Matrix formulation

Suppose that N elements in the mesh

Conventional approach O(N3) time and O(N2) memory

• Cost is prohibitive

• Double the size of the problem 8X time

10

(continuous form)

BAx (A is a dense matrix)

Iterative methods (GMRES, Yale, 1986)

Matrix vector products instead of matrix inversion

This reduced the time to O(N2)

The Fast Multipole Method was developed in 1987

Applied to capacitance/EM solution

• FastCap, Fast Henry, (1990s). (MIT/White)

• IES3, Bell Labs, Kapur and Long, (1990s)

Used in EMX O(N)

These sorts of problems can be solved in linear time

Innovations in numerics (GMRES+FMM)

11

bAx

},...,,,{ 2 bAbAAbbK n

n

Time and Memory scaling

Single frequency simulation (including iterative solve)

Compare speed and memory for 1, 2, 4, 8, …, 64 inductors

Very important that it is a linear time solution

1 inductor 64 inductors

12

Issues at high frequencies

At high frequencies the skin effect is a significant issue

Meshes need to be finer to incorporate these effects

This leads to larger problem size (smaller structures?)

1GHz 60GHz

Issues at high frequencies

At high frequencies the skin effect is a significant issue

Meshes need to be finer to incorporate these effects

This leads to larger problem size (smaller structures?)

1um mesh 0.1um mesh

Issues at high frequencies

Coupling at higher frequencies

1GHz 60GHz

Full-wave vs Quasi-static

Full-Wave vs quasi-static

Time of flight becomes important once frequency is “large” compared to the speed of light

• Electrical length of structure is important

• Specially important for electrically long structures

Cannot compromise on using a quasi-static solution

Laplace regime…not yet Helmoltz.

Design issues

Inductors become tiny and getting high Q is easy

Capacitors become the bottle neck at higher frequencies since capacitor Q goes down because of skin effect

Courtesy: TSMC. 65nm RFCMOS, 9LM thick metal technology. Published at RFIC 2009 “Including Pattern-Dependent Effects in Electromagnetic Simulations of On-Chip Passive Components”, Integrand and TSMC

Guard rings

Use of guard rings or seal rings in layout

The ring is a loop of metal

May be a metal line close to the inductor (supply line or bias line)

Simulations

Using a guard ring shows odd behavior at higher frequencies

The ring is a metal line of about 600um long.

At 50Ghz the wavelength is 3000um.

Order of a quarter wavelength line. Couples to the inductor as an LC resonator

Removing the ring or cutting the ring fixes the issue.

Dummy fill simulation

EMX models circulating currents in dummy fill. Eddy current loss increases at higher frequencies

Passive couplers

Couple defined amount of power in transmission line to a port enabling the signal to be used in another circuit

Couple power flowing in one direction. Power entering the output is not coupled to the input port

Directional couplers constructed from two coupled transmission lines set close enough together such that energy passing through one is coupled to the other

3dB couplers, 6dB couplers, etc.

Not “lumped”. Usually ¼ wavelength

6dB coupler

Passive dividers

Wilkinson divider

Achieve isolation between the output ports while maintaining a matched condition on all ports

User also be used as a power combiner because it is made up of passive components and hence reciprocal

Wilkinson Divider

Wilkinson Dividers can be used as power splitters and power combiners

In silicon it is not ideal and all real effects need to be included (ground plane, resistors, etc.)

EM simulation needed

Alcatel Lucent

Circuit Examples at high frequencies

Experimental Validation

Measurement and Circuits for 60GHz-200GHz

1. High-resolution 60-GHz DCO with Reconfigurable Distributed Metal Capacitors in Passive Resonators

2. Amplifiers for D-Band applications

3. W-band data links, phased array receivers and transmitters

All simulations of passives done with EMX

High resolution DCO at 60GHz

60-GHz FMCW imaging (all digital PLL)

Range:

• 3-5m, resolution: 1-5cm

• Through cardboard boxes

60-GHz DCO (L-DCO, T-DCO) • 6-GHz linear freq. tuning range for

modulation

• ~1MHz fine tuning steps

• Moderate power, small area

• Interfacing with divider, PA in TX, mixer in RX

• Mm-wave frequency divider chain

W. Wu, J.R. Long, R.B.Staszewski, J.J. Pekarik (Delft University) "High-resolution 60-GHz DCOs with Reconfigurable Distributed Metal Capacitors in Passive Resonators," IEEE Radio Frequency Integrated Circuits Symposium, June 2012.

Test chip micrographs

26

L-DCODiv-64

60 GHz output

Div-64 output

Digital

control

400 um

36

0 u

m

T-DCODiv-64

Digital

control

60 GHz output

Div-64 output

400 um

38

0 u

m

• Inductor and Transformer-based fine tuning DCOs and divider-by-64 chain

Complete L-DCO tank EM simulation

Including:

• Coarse, mid-coarse tuning bank

• Fine tuning bank

• Interconnection to divider and buffer

• Mimcap for AC coupling

• Ground ring

100+ port EMX simulation

TU-Delft Confidential 27

T-line and inductor tuning

Passive structures with tuned by capacitive loading

• Transmission line with fine and coarse cap banks

• Inductor with shield strips

28

Coarse

tuning bank:

M7

Transmission line

on M8Cin

Mid-coarse

tuning bank:

M6

D0

Lin

CPFine tuning

bank: M6

01

15

Thermometer c

ode index

60-GHz DCO measurements

29

Measured vs. simulated coarse tuning curves of DCOs

-2 0 2 4 6 8 10 12 14 16 18 20

56

57

58

59

60

61

62

63

64

DC

O o

utp

ut fr

eq

ue

ncy (

GH

z)

L-DCO measured T-DCO measured L-DCO simulated T-DCO simulated

Coarse-tune thermometer code index

-2 0 2 4 6 8 10 12 14 16 18 20

-94

-93

-92

-91

L-DCO meas.

T-DCO meas.

L-DCO sim.

T-DCO sim.

DC

O P

N a

t 1

MH

z o

ffse

t (d

Bc/H

z)

Coarse tune thermometer code index

-90.5

-91.5

-92.5

-93.5

L-DCO

T-DCO

Phase noise agreement

180GHz Amplifier

D-band amplifier

6dB coupler

180GHz H2O attenuation window

Atmosphere monitoring

University of Toronto IMS 2012 (private communication) Ionnis Sarkas, Sorin Voinigescu

6dB coupler and amplifier

S-parameter measurements of 6dB coupler up to 180GHz

Silicon based radar imaging

Silicon-based radar and imaging sensors operating above 120 GHz

D-band sensor imaging

Used EMX to model supply ground distribution network up to 200GHz

The on-chip portion of the network consists

of a 200-um long and 100-um wide periodic array of mesh cells of interleaved supply and ground planes

IEEE MIKON 2012 May 21-23,

Warsaw, Poland.

University of Toronto

Silicon based radar imaging

Silicon-based radar and imaging sensors operating above 120 GHz

E-Band and W-Band Data links

Receiver and transmitter arrays designed for steerable-beam and highly spectral efficient data links in both E- and W-band.

4-channel receiver and transmitter array with integrated direct-conversion mixers.

The arrays span newly released commercial bands at 71-76, 81-86 92-95 GHz

Low-noise figure and good output power.

RFIC 2012

RFIn1

RFIn2

RFIn3

RFIn4

LNA

LNA

LNA

LNA

I/Q Down-

converter

IFIOut

IFQOut

LOIn

SPI Control

Ac

tiv

e &

Pa

ss

ive

Po

we

r C

om

bin

er

Phase

Shifter (1)

Phase

Shifter (2)

Phase

Shifter (3)

Phase

Shifter (4)

W-band chip

All passive traces modeled with EMX

Wilkinson Divider

Transformer Hybrid architecture

MM wave passive circuits. Not easy to construct a discrete component equivalent

Need full S-parameter EM simulation

All simulations in the 70GHz to 200GHz range

Measured results

-2.5

0

2.5

5

7.5

10

-15

-10

-5

0

5

10

75 80 85 90 95 100 105 110

Me

asure

d M

agnitu

de

Error (d

B)Me

asu

red

Ph

ase

Err

or

(De

gre

es)

Frequency (GHz)

Single Slice Phase/Mag Accuracy

0

5

10

15

20

25

30

35

40

70 75 80 85 90 95 100 105 110

Me

asu

red

Re

ceiv

er

Gai

n (

dB

)

Input RF Frequency (GHz)

Receiver Gain - Element 1 (H3)Receiver Gain - Element 2 (H3)Receiver Gain - Element 3 (H3)Receiver Gain - Element 4 (H3)Receiver Gain - Element 1 (H2)Receiver Gain - Element 2 (H2)Receiver Gain - Element 3 (H2)Receiver Gain - Element 4 (H2)

5

6

7

8

9

10

11

12

13

14

15

70 75 80 85 90 95 100 105 110

Me

asu

red

Re

ceiv

er

NF

(dB

)

Input RF Frequency (GHz)

Receiver NF - Element 1 (H3)Receiver NF - Element 2 (H3)Receiver NF - Element 3 (H3)Receiver NF - Element 4 (H3)Receiver NF - Element 1 (H2)Receiver NF - Element 2 (H2)Receiver NF - Element 3 (H2)Receiver NF - Element 4 (H2)

Conclusions

Maxwell’s equations used to model passive structures at high frequencies

Need fast 3D Full-wave EM solver to capture relevant effects

Linear time solver like EMX

Design issues

• Coupling to stray lines

• Capacitor Q degradation

• Retardation needs to be modeling

Circuits can and are being built for very high frequency applications

Challenges ahead…

Constant improvement of speed and memory use

Cannot do full-chip level 3D EM extraction

Better characterization of interconnect and substrate

Packaging considerations

• Simple RLC models for packages won’t suffice (seeing this for SERDES applications 30-40GHz).

Measurement difficulties at very high frequencies

Improvements in measurement equipment for D-band and W-band (higher frequencies)

Acknowledgements

TU Delft

• Providing DCO example

University of Toronto

• 6dB coupler example

• Imaging example

Alcatel Lucent

• W-band data link Rx/Tx example

Helic and TSMC for hosting workshop.

Extra Slides

Foundation of EMX

Uses the hierarchical Fast Multipole Method for rapid solution

• Nearby regions are handled directly

• Far regions are approximated to fixed precision

Solution time is O(N) in time and memory, i.e. double the problem size, double the time

• as opposed to N2 or N3

Can solve very large problems with minimal computer resources

41

Fabricated width is different from drawn width according to rules provided by foundry by a “bias” amount

Shaded regions represent original drawn geometry

Lines represents modified “grown” geometry based on local width and spacing

Modifying layout

42

CMOS VCO

43

3D mesh (inductor+ capacitor bank)

Courtesy: Wipro, TSMC90nm, 1P5M