innovating test for better faster introduction to on-wafer

Innovating TestTechnologies

for bettermeasurementsfaster Introduction to

On-Wafer Characterizationat Microwave Frequencies

Chinh DoanGraduate Student

University of California, Berkeley


for bettermeasurementsfaster Introduction to

On-Wafer Characterizationat Microwave Frequencies

Dr. Tariq AlamSenior Applications Engineer

Cascade Microtech, Inc.

E-Mail: [email protected]

presented by: Chinh Doan,University of California, Berkeley


for bettermeasurementsfaster

Presentation Outline

• Microwave Probing Technology– Air CoplanarTM Probes

• On-Wafer Calibration Methods– SOLT, TRL/LRM, SOLR, LRRM

• On-Wafer Verification Methods

• Layout Rules

• Calibration and Measurement Software– WinCalTM



What Are My Measurement Objectives ?

• Now

– Determine S-parameters of on-waferactive devices between 500 MHz to 5 or50 GHz or beyond

• Future?

– ↑ Wafer size (6 to 8 to 12 inch)

– ↑ Frequency range of interest

– ↑ Need for thermal measurements

– ↑ Test automation for throughput


for bettermeasurementsfaster • Vector Network Analyzer

• Cables

• Probes

• Probe positioners

• Probe station

• Controller

• Contact Substrate

• Impedance Standard Substrate (ISS)

What Equipment Do I Need ?

Only Cascade Microtech provides the Total Measurement Solution from the Test Ports of the VNA down to the wafer level



Microwave Probing

• On-wafer fixturing needs:• Electrically

– wide BW transmissionlines

– low contact R• Mechanically

– consistent probe shape

– placement

– durability

Optimize for loss, impedance match, power and current handlingcapability, contact force, tip visibility...



Microwave Probe Transmission Line Contacts

Poor

Better

BestPrecise line impedance rightto the ground-signal-groundcontacts

Long path to single groundcontact limits bandwidth

Variable loop inductanceprevents calibration(Requires repeatable transition)



ACP Probe Technology

Low-loss, low density teflon dielectric coaxMicrowave absorber

- consistent attenuation- termination of coaxial shield energy- provides rigidity

15 W CW at 10 GHz5 A DC current

Absorber

Block

K-Connector

HardAbsorber

Low-LossCable

Air CoplanarWaveguide Tip

SoftAbsorber



Low-loss ACP

• Low-loss andstandard ACPProbes

• Application– Noise

measurements

– Load Pull



Air-Coplanar Tips

• Precision tip fabrication

for tight impedance control

Preferred 23°°contact angle

Clear view ofcontact point

Wide contactarea

l High tip visibility for

consistent placement

on pads

BeCu tips for Au or Cu padsTungsten tips for Al pads



Installing and Using ACP Probes

*definition: planarization -the ability to insure allcontacts are at the sameheight

φ Use Cable strain relief onpositioners

Use Contact Substrate toplanarize probes



Using ISS Alignment Marks

Full skate and overtravelInitial contact

• Used to set ‘skate’ and probe separation

Internal Apex



Maintaining ACP Probes

• Keep tips clean of dirt and debris

• Inspect and clean connectors

• Electrical verification -- Probe Test



Calibration



How Do I Calibrate My VNA ?

•°

•° vv v

° °

° °Forward

Reverse

Switch PerfectReflectometer

Port 1

ErrorAdapter

Port 2

DUT[S]

ao bo

a3 b3

Microwave Errors (Forward) Calibration Standards

Directivity Port-2 Match Open ThruPort-1 Match Transmission Tracking Short LineReflection Tracking Crosstalk Load Etc.



One-port Network Analyzer Model

bO

ED ES

ER

aO

1

ΓΓDUT

Calibrate with three known reflection coefficients

l Short - open - load (SOL)

l Short - offset short1 - offset short2

ED = Directivity of coupler

ER = frequency response of measurement channel

ES = Port match



Two-port Network Analyzer Model(Forward Model)

Calibrate with short-open-load on each portplus a Thru (SOLT) (uses 10 knowns)

bO

ED ES

ER

aO

1 S12

ELS22S11

S21 ET

EX

b3DUT

EL = Models signal reflected back into DUT from P2

ET = Models imperfections in transmission response



VNA CalibrationbO Ideal

NetworkAnalyzeraO

S12 S22S11

S21

a3

DUT

b3

e10e00e11e01

e22e33

e32

e23

N1

N2

N3

One port ortwo port

calibration standards

• Switch modelled and measured separately

– signals simultaneously measured

– two two-port error boxes



GROUND

GROUND

SIGNAL

GROUND

SIGNAL

GROUNDThru

GROUND

GROUND

SIGNAL SIGNAL

ShortGROUND

GROUNDGROUND

GROUND

GROUND

SIGNAL SIGNAL

Loads

GROUND

Calibrating the Probe Tips with CoplanarWaveguide Impedance Standards

ISS

• Electrical behavior of standards

– standard dimensions

– probe pitch

– probe placement (use alignmentmarks)

– cal coefficients supplied with probe



SOLT Calibration

• All standards must beperfectly known– available on virtually every

vector network analyzer (CalKitrequired)

– open has capacitance (oftennegative)

– short and load haveinductance

– sensitive to probe placement

– mathematicallyoverdetermined

– unpredictable behavior

DelayThru

Thru

Short Open(probes in air)

Load

Lterm

Lshort Copen



TRL/LRM Calibration• Thru-Reflect-Line

– requires least info about standards

– S-parameters referenced to line Zo

– reference plane at centre of Thru

– requires multiple probe spacings

– Design rules

– Zo is inherently complex at lowfrequencies

– not suitable for fixed spacing probes(e.g., probe card)

• Line-Reflect-Match– referenced to Zmatch

Thru

Line(s)

Reflect

Match

OR



Multi-line TRL Benchmark CalMethod

• Modified TRL algorithm developed bythe U.S. National Institute ofStandards and Technology

• Benchmark on-wafer calibrationmethod

• Takes an optimal weighted averageof all the line measurements

• On-wafer standards (with DUT)preferred

• Renormalizes the S-parameterimpedance to 50 Ohms

Thru

Line(s)

Reflect



SOLR Calibration

• Short-Open-Load-Reciprocal Thru– reciprocal Thru requires only S12 =

S21

– tolerant to lossy or highly reactiveinsertion standard

– convenient for use with fixed probespacing in probe cards

– Does not require a customThru

– convenient for use when DUTterminals are orientated at 90°

– available in WinCal

Short

Open(probes in air)

Load

Reciprocal

OR



SOLR for Right Angle Measurements

• Carefully constructed rightangle ‘Thru’ standard

• Thru is non-ideal, large dipat 20 GHz

• Errors in standard cal’s

• SOLR immune to Thruerrors

-1.0

-0.5

0.0

0.5

1.0

0 5 10 15 20 25 30 35 40 45 50

|S21|[dB]

[GHz]

Orthogonal SOLT

Orthogonal SOLR

Straight LRRM

Orthogonal LRRM



LRRM

• Cascade Microtech Calibration Research

– Line-Reflect-Reflect-Match Calibration– like TRL, only Match acts as infinitely high loss line

– one transmission line standard only allows fixed probespacing calibration

– Thru (line) delay, Match resistance must be known

– measurements referenced to laser trimmed resistor

– required measurement of only one load standard

– load inductance compensation

– uses off-wafer standards (ISS)

– same standards as SOLT only - no need for cal kit

– available in WinCal

Line

Reflect

Reflect(probes in air)

Match



LRRM Calibration

l System drift baselinel LRRM compares with

system drift limit– best fixed probe

position calibration

l SOLT /LRM– growing error w/freq– possible CalKit error– possible ref plane

error



Popular Calibration Methodsfor Wafer Probing

Z0 Inherently Probe Card Absolute Reference Consistent Support Accuracy

Trimmed Resistor No Fair Fair

Transmission Best (if Lines Yes Poor Corrected)

Trimmed Resistor Yes Fair Good

Trimmed Resistor Yes Best Good

SOLT

TRL

LRM/LRRM

SOLR



Calibration Verification



How Do I Know If My Calibration is Successful ?

GROUND

GROUND

GROUND

SIGNAL SIGNAL

PadOpen stub

GROUND

GROUND

GROUND

SIGNAL

GROUND

SIGNAL

GROUND

GROUND

GROUND

SIGNAL

Line (delay)



Calibration Verification Standards

Thru0.5 1.0

0.1 GHz

40.0 GHz

-0.10

-0.05

0.00

0.05

0.10

0 5 10 15 20 25 30 35 40

[dB

]

[GHz]

•Unity Gain

•1 pS Line

•Phase Lag




•Unity Gain

•Negative Capacitance

Reflect(probes in air)

10 2050

10

2050

-10

-20-50

0.1 GHz

40.0 GHz

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20 25 30 35 40

[dB

]

[GHz]




• Measured S21 when probes placed on 50ohm loads

Match

-80

-70

-60

-50

-40

-30

-20

-10

0

0 5 10 15 20 25 30 35 40

S21 of Standard Loads

[dB]

[GHz]

HPC40 onHPC ISS



Independent Calibration VerificationStandards

OpenstubG

G

S

0.2 0.4 0.6 0.8 1 1.5 2 3 4 5 10 2050

0.2

0.4

0.6

0.81

1.5

2

3

4

5

10

20

50

-0.2

-0.4

-0.6

-0.8-1

-1.5

-2

-3

-4

-5

-10

-20

-50

0.1 GHz

40.0 GHz

-1.5

-1.0

-0.5

0.0

0 5 10 15 20 25 30 35 40

[dB]

[GHz]

• Linear Phase Lag



Common Calibration Errors

• Poor CalibrationVerification/Repeatability

• Calibration Drift

• Narrow Band ‘Suck out’

• Inaccurate Probe Placement

• Inaccurate Probe Definitions

• Poor Probe Contact

• LRM Sensitivity to Differencesin Load Standards

• Incorrect Models for Lines

• Poor Phase Stability of Cable

• Dirty Connectors



Beware of an Unconnected Substrate

GS pads fringe to the ground plane or chuck

GSG pads shield like CPW

• Parasitic couplings to the conductors near DUT

• GSG shields magnetic and electric fields better than GS

Fields terminate onbackside of wafer

on one side



Layout Guidelines



Device Characterization with CoplanarWaveguide Microwave Probes

GROUND SOURCE

SOURCE

DRAINGATE

GROUND

GROUND

GROUND

SIGNAL SIGNAL



Pad Size (Passivation Window)

• Recommended minimum pad size is 80um x 80um forACP Probes when performing automatedmeasurements

• Smaller pad dimensions can be used for manualprobing

• HPC Probe allows 40um x 70um manual probing

• Passivation height must be considered

• Pad height variation must not exceed 25um



Process Control Monitor DeviceLayout

G

S

G

G

S

G

GSG Test Device Open Pads & Metal Shorted Metal(Remove Ypad) (Remove Zmetal)



PPR Corrected H21 Measurement0.25 µm CMOS Transistor

.1 1 10 100FREQUENCY (GHz)

H21

-20

0

20

40

60

As measured FT = 25 GHz

Corrected for pad parasitics FT = 33 GHz



WinCal

• VNA Calibration and Measurement Tool



Calibration Features

• Calibration

– SOL, SOLT, SOLR,LRM, LRRM, TRL

– Stability tests



Setting Up a CalKit

CalKit can be easily defined and downloaded to VNA– Removes one of the most common sources of error



Measurement Features

• Tools

– Read/Save S-Parameters

– Pad ParasiticRemoval

– Probe Test



Monitoring the Calibration

• Stability check checkssystem drift



Pad Parasitic Removal

• Measures intrinsic devices



Calibration Repeatability

LRRM automatic calibration is

very repeatable

Operator dependent probeplacement errors– manual cal’s are not

as repeatable



Summary

• Microwave measurements require carefulcalibration, verifications and attention to detail

• Many new applications to accommodatethe varied needs of growing wireless andhigh-speed digital needs

• Let Cascade Microtech help you keep up withfuture innovations



E-Mail: [email protected]

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