MEMS Technology - Enabling Design Flexibility for Fine ...

MEMS Technology - Enabling Design Flexibility for Fine Pitch Probing

Bahadir Tunaboylu, PhD& Gerry Back

SV Probe, Inc.2120 W Guadalupe Rd Ste 112

Gilbert, AZ 85233

June 7-10, 2009San Diego, CA

June 7 to 10, 2009June 7 to 10, 2009 IEEE SW Test WorkshopIEEE SW Test Workshop 22

Outline Introduction

Emerging fine pitch peripheral & array test requirements at 60µm pitch Design perspective & probing multiple DUTs by cantilever vs. vertical probe Contact model for vertical probe contacts to control bond pad damage

Method & Systems for Characterization Hertzian contact mechanics & Holm electrical contact model Instrumentation & software

Test Results & Analysis MEMS-based vertical technology contact performance for various contact

metallurgies on wafers Contact on aluminum, copper & lead-tin Contact resistance as a function of contact force & current

Summary & Follow-on Work


Fine Pitch Probing Cantilever probing approaches, both traditional

& MEMS-cantilever, have limitations for multidut probing at 60µm-pitch : Number of rows of bond pads are limited, dependent

heavily on pad density Corner keep-out in device layouts Requires skip-DUT configurations, compromising test

stepping efficiency Vertical probing technology approaches allow

more rows of peripheral pads & array patterns


CantileverCantilever

VerticalVertical

Probing Technology & Design CapabilityMEMSMEMS--VerticalVertical

1, 2

Row

Per

iphe

ral L

ayou

ts

Arr

ay L

ayou

ts

1-row Peripheral Multi-dut 3

Row

Per

iphe

ral l

ayou

ts


Cantilever TechnologiesCantilever Technologies Vertical- Buckling BeamVertical- Buckling Beam MEMS Fine Pitch VerticalMEMS Fine Pitch Vertical

Probing Technology & Scrub on Pads

MEMS Cantilever


Fine Pitch Probing

Fine pitch probing requires precise control of alignment at pad sizes of 45µmx45µm Contact model for vertical probe contacts

is different than cantilever style beams Scrub marks generated by cantilever beams

by design is typically longer than marks by vertical probes

Accurate guiding of probes permits finer controls & precise scrub marks for Vertical. The tolerances on guiding holes as well as probes are critical for positions

Probe action, scrub mark size & depth must be precisely controlled to prevent damage to bond pads & low-k dielectrics Study scrub behavior, determine scrub

length, width, depth & also the debris pile created

Vertical- Buckling BeamVertical- Buckling Beam

MEMS Fine Pitch VerticalMEMS Fine Pitch Vertical


Methodology for Analysis Contact Model

Hertzian Contact Mechanics Software model is developed for predictive scrub behavior on various

wafer pad metallurgies, based on VB code Simplified Holm electrical contact model

Test systems for scrub mark & contact resistance characterization Instrumentation

Probe: TEL P12 XLn Keithley Tester & Source Meter Nikon Optical Inspection System Veeco Profilometer Test Wafers: Al, Cu, PbSn Probing Technology: MEMS-Fine Pitch Vertical Technology

(LogicTouch™)


MEMS-Fine Pitch Vertical Probing Technology for Contact Study

1

3 24

1. Probes2. Space Transformer (MLC)3. Interposer4. PCB

ProbesProbes

60/30µm layout is shown Technology scalable to 50µm & 40µm pitch Supports much higher speeds & bandwidth compared to cantilever technologies


Probe Contact Contact Model

Hertzian Contact Mechanics: Hertz’s classical solution provides the foundation in contact mechanics of solid pairs (of two surfaces). The size & depth of an indentation of a probe into a flat surface can be estimated by Hertz contact stresses. GW model based on Hertz theory is assumed where the probe tip of radius rindents a flat plane to depth d, creates a contact area of radius a = √rd . The force equation

F = 4/3 Er1/2 (zs – d)3/2

Where zs is the normalized summit height & elastic modulus E of the equivalent surface is given as

1/E = (1-ν12)/E1 + (1- ν2

2)/E2Where v is the Poisson’s ratio & two bodies of 1 & 2

Surfaces are rough & the apparent contact area between a probe tip & the pad is not the actual load bearing area due to asperities. The real area of contact is found as, Ar/Aa = 1 - 3%

Metallic surfaces also have insulating films. Real intimate contact & load bearing area is actually much smaller & the electrical conduction is achieved through these a-spots, conducting contact areas. Holm defined the electrical contact model using this constriction resistance, Rf = ρ/Ac , between contacting members by extension of Ohm’s law.

Predict scrub mark by known properties of probe materials, pad materials & geometry

Radius a of Conducting Spot

Probe Force

Spherical Tip of radius r

Wafer Pad

Model of Surface Roughness


Contact Model Results for Aluminum

Scrub depth as a function of probe force. Assumes a hemispherical probe tip.

Input ParametersFPV Probe tip: 4um radius

Overdrive: 63.5um

Force: 7 g

Pad: Al

Results


Contact Model Results on Copper

Input ParametersProbe tip: 4um radius

Overdrive: 63.5um

Force: 7 g

Pad: Cu

Results



Contact Model Results on PbSn

Input ParametersProbe tip: 4um radius

Overdrive: 63.5um

Force: 7 g

Pad (Bump): PbSn

Results



Experimental Scrub Characterization Scrub marks by standard cantilever & vertical

technologiesFPV Scrub Characterization

Comparative study of multiple TDs on Al & Cu padsScrub dimensions were measuredTwo different tip diameters were studied

Contact resistance behavior was also investigatedContact resistance (Cres) was measured per TD & as a function

of overdrive to determine the onset of frittingCres was measured during lifecycle experiments monitoring

stability for Al, Cu as well as PbSn


Conditions:

50µm O.D

Force: 2.3 g @ 50µm

3-milØ WRe

10µm Tip Diameter

Aluminum Wafer

Cantilever Technology Scrub Marks

Veeco ProfilometerVeeco Profilometer

Scrub: 25umx10µm


Vertical Technology Scrub Marks

125 µm O.D

3-milØ Pointed Probe

13 µm Tip

Aluminum Wafer

Conditions:


Test Results for FPV: Resistance Comparison for Different Pad Materials

0.5

0.75

1

1.25

1.5

Aver

age

Ohm

s

Aluminum Copper SolderMaterial

AluminumCopperSolder

Level444

Number1.081460.868410.96307

Mean0.066560.066560.06656

Std Error0.930900.717850.81250

Lower 95%1.23201.01901.1136

Upper 95%

Means for Oneway Anova

Oneway Anova

Oneway Analysis of Average By Material

Baseline ResistanceBaseline Resistance

Contact resistance values are path resistance measurements & not normalized


Cres Behavior on Al Resistance vs Overdrive on Aluminum

0

2

4

6

8

10

12

14

16

18

20

0.5 1 1.5 2 2.5 3 2.5 2 1.5 1 0.5

Overdrive (mils)

Ohm

s

1 mA 50 mA 100 mA 200 mA

Contact resistance as a function of overdrive for current values of 1, 50, 100 & 200 mA. It appears that the fritting takes place below 1 mil OD, the fritting ratio drops as the OD increases.


Cres Testing on Al

Contact resistance results up to 1M TDs. Resistance is the path resistance including the Cres.

LIFE TEST RESISTANCE ON ALUMINUM WAFER 2.5 MIL OD

0.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.5

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

PROBE NUMBER

RES

ISTA

NC

E (O

hm)

0 (Ohm)50K (Ohm)150K (Ohm)288K (Ohm)313K (Ohm)338K (Ohm)363K (Ohm)388K (Ohm)413K (Ohm)438K (Ohm)463K (Ohm)488K (Ohm)513K (Ohm)563K (Ohm)628K (Ohm)673K (Ohm)773K (Ohm)873K (Ohm)1000K (Ohm)


Cres Behavior on Cu

Contact resistance as a function of overdrive for current values of 1, 50, 100 & 200 mA. Resistance is the path resistance including the Cres.Cres unstable below 1 mil OD & stabilizes at higher OD.

Resistance vs. Overdrive on Copper

0

1

2

3

4

5

6

7

8

9

10

0.5 1 1.5 2 2.5 3 2.5 2 1.5 1 0.5

Overdrive (mils)

Ohm

s

1 mA 50 mA 100 mA 200 mA


Cres Testing on Cu

Contact resistance results up to 100K TDs. Resistance is the path resistance including the Cres.

Path Resistance on Copper Wafer

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

50

5000

1000

0

1500

0

2000

0

2500

0

3000

0

3500

0

4000

0

4500

0

5000

0

5500

0

6000

0

6500

0

7000

0

7500

0

8000

0

8500

0

9000

0

9500

0

1000

00

Touchdowns

Ohm

s

After Clean Before Clean


Cres Testing on PbSnPath Resistance on Solder Lead-Tin

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0

5000

1000

0

1500

0

2020

0

2540

0

3000

0

3500

0

4000

0

4500

0

5000

0

5500

0

6000

0

6500

0

7000

0

7500

0

8020

0

8500

0

9000

0

9500

0

1000

00

Touchdowns

Ohm

s

After Clean Before Clean

Contact resistance results up to 100K TDs. Resistance is the path resistance including the Cres.


Comparing Means of Scrub Depth for Al & Cu

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Scru

b D

epth

Al

Mic

rons

1 4 8 12Touchdow ns

14812

Level10101010

Number0.5110000.6232000.7706000.884800

Mean0.049340.049340.049340.04934

Std Error0.410930.523130.670530.78473

Lower 95%0.611070.723270.870670.98487

Upper 95%


Oneway Anova

Scrub Depth versus Touchdowns on Aluminum

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

Scru

b D

epth

Cu

Mic

rons

1 4 8 12Touchdow ns

14812

Level10101010

Number0.5421000.5687000.6144000.677000

Mean0.023890.023890.023890.02389

Std Error0.493650.520250.565950.62855

Lower 95%0.590550.617150.662850.72545

Upper 95%


Oneway Anova

Scrub Depth versus Touchdowns on Copper

Fit Y by X Group

Scrub depth on Al & Cu for 1, 4, 8 & 12 TDs on the same spot. Probe tip diameter is 8 µm.


Comparing Means of Scrub Depth

Scrub depth on Al & Cu for 1, 4, 8 & 12 TDs on the same spot. Probe tip diameter is 10 µm.

0.3

0.4

0.5

0.6

0.7

0.8

Scru

b D

epth

Al

mic

ron

1 4 8 12Touchdow ns

Missing Row s 2

14812

Level109

109

Number0.4598000.5062220.5401000.531111

Mean0.031470.033170.031470.03317

Std Error0.395840.438810.476140.46370

Lower 95%0.523760.573640.604060.59853

Upper 95%


Oneway Anova

Scrub Depth versus Touchdowns on Al (10 um Tips)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Scru

b D

epth

Cu

Mic

ron

1 4 8 12Touchdow ns

Missing Row s 1

14812

Level10109

10

Number0.4292000.5754000.5458890.564800

Mean0.027140.027140.028610.02714

Std Error0.374110.520310.487820.50971

Lower 95%0.484290.630490.603960.61989

Upper 95%


Oneway Anova

Scrub Depth versus Touchdowns on Cu (10 um Tips)

Fit Y by X Group


Comparing Means of Debris Pile Height

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Deb

risi H

eigh

t Al

Mic

rons

1 4 8 12Touchdow ns

Missing Row s 2

14812

Level8

101010

Number0.3310000.4678000.5527000.781600

Mean0.057110.051080.051080.05108

Std Error0.214940.363990.448890.67779

Lower 95%0.447060.571610.656510.88541

Upper 95%

Std Error uses a pooled estimate of error variance


Oneway Anova

Scrub Debris Pile Height on Aluminum

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Deb

ris H

eigh

t Cu

Mic

rons

1 4 8 12Touchdow ns

14812

Level10101010

Number0.3367000.3914000.4498000.355800

Mean0.040230.040230.040230.04023

Std Error0.255110.309810.368210.27421

Lower 95%0.418290.472990.531390.43739

Upper 95%



Oneway Anova

Scrub Debris Pile Height on Copper

Fit Y by X Group

Scrub pile height on Al & Cu for 1, 4, 8 & 12 TDs on the same spot. Probe tip diameter is 8 µm.


Comparing Means of Scrub Diameter

9

9.5

10

10.5

11

11.5

12

Scr

ub D

iam

eter

Al

mic

rons

1 4 8 12Touchdowns

14812

Level10101010

Number9.79009.67009.8200

10.1800

Mean0.129550.129550.129550.12955

Std Error9.52739.40739.55739.9173

Lower 95%10.0539.933

10.08310.443

Upper 95%



Oneway Anova

Scrub Diameter versus Touchdowns on Aluminum

9

9.5

10

10.5

11

11.5

12

Scr

ub D

iam

eter

Cu

mic

rons

1 4 8 12Touchdowns

14812

Level10101010

Number10.020010.580010.980011.0400

Mean0.124860.124860.124860.12486

Std Error9.767

10.32710.72710.787

Lower 95%10.27310.83311.23311.293

Upper 95%



Oneway Anova

Scrub Diameter versus Touchdowns on Copper

Fit Y by X Group

Scrub diameter on Al & Cu for 1, 4, 8 & 12 TDs on the same spot. Probe tip diameter is 8 µm.


Scrub Optical Images on Al at 1 vs 4 TDs

Scrub marks on Al imaged optically


3D Scan for Multiple Touchdowns on Al

1 TD1 TD 4 TD4 TD

12TD12TD8 TD8 TD


DepthDepth

DiameterDiameter

Debris HeightDebris Height

2D Scan 1TD Case for Al


Scrub Optical Images on Cu at 1 vs 4 TDs

Scrub marks on Cu imaged optically.


3D Scan for Multiple Touchdowns on Cu

1 TD1 TD 4 TD4 TD

12TD12TD8 TD8 TD


Summary For fine pitch multidut requirements, vertical probe technologies

provide advantages over cantilever approaches with design flexibilities MEMS-based vertical technology has an edge over buckling beam technologies

for design flexibility for highly parallel peripheral devices as well as accuracy of scrub signatures required for smaller pad sizes

Contact mechanics for MEMS-based fine pitch vertical technology is studied on various contact metallurgies. Calculations for scrub depth correlate well for aluminum and copper pad contacts

in experimental results. It appears that the modeling can also predict contact resistance for these pad metallurgies. This allows predictive performance of contact pin & pad materials of choice.

Contact resistance is studied as a function of test parameters. Stable contact resistance is achieved for three types of pad/bump metallurgies.

Initial results were presented for solder bump probing. More scrub analysis and characterization on different solder metallurgies on

copper pillars are needed.


1. Handbook of Micro/Nano Tribology, 2nd ed., edited by B. Bhushan,19992. Electric Contacts, R. Holm, 4th ed. 3. Electrical contacts-2001, Proceedings of the 47th IEEE Holm

Conference on Electrical Contacts4. SWTW 2006, C. Degen et al, “Parametric Study of Contact Fritting for

Improved CRes Stability”.5. SWTW 2006, J. Martens, on “Fritting”

Acknowledgments Authors gratefully acknowledge the analytical support from Jeff

Hicklin & Habib Kilicaslan

References

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