controlling pad damage - cleaning technologies€¦ · basic damage control strategies – probe...

International Test Solutions

Impact and Control of Pad Damage at Wafer Sort

International Test SolutionsJerry Broz, Ph.D.

VP of WW ApplicationsReno, NV USA

Probing Process Workshop

October 13 and 14, 2014Ieper, Belgium

J. BrozOct-2014


Overview• International Test Solutions Profile

• Pad Damage at Wafer Sort– Defining the Problem– Packaging and Assembly Considerations

• Basic Damage Control Strategies– Probe card parameters– Operational parameters

• Summary / Discussion

2J. BrozOct-2014






3J. BrozOct-2014


International Test Solutions Corporate Profile

• Global supplier of high quality yield and utilization improvement products for wafer sort and package test since 1997

– Cost-effective cleaning solutions and industry leading technical services.– Strong IP position for front-end, wafer-sort, and back-end test.

• Managed by highly experienced semiconductor industry personnel with a combined experience of +100 years.

– ITS Branch Offices in Taiwan, Japan, Korea, China, and Singapore.– World-wide sales support network of authorized agents.

• Manufacturing Center for advanced polymer materials research and development– Controlled Compliance Manufacturing methods. – Materials characterization, development, and testing laboratories.

• Award winning Test Analysis Center for electrical test and process characterization– Analytical laboratory focused on probe technology and contactor performance testing.– Cleaning recipe assessment and optimization

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ITS – Increasing Yields and Reducing Costs

Maximized wafer yield– Controlled and stable CRES– Reduced site-to-site failure

Increased throughput– Minimize off-line cleaning– Extend probe card lifetime

Improved tool uptime– Reduce operator intervention– Reduce spare inventories

Increased litho tool output– Clear “hot spots” w/o downtime– Perform regular PM cycles

Higher etch tool output– Clean ESC with closed chamber– Accelerate wet clean recovery

Lower operating costs– Extend time between wet cleans– Lower process kit part usage

Improved first-pass yields– Improved contact– Reduced rescreen

Greater throughput– Minimize off-line cleaning– Maintaining high UPH

Reduced Cost of Test– ACC for low downtime– Tri-temperature capable

Probe Card Clean Test Socket Clean Chuck Cleaning Wafer

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6J. BrozOct-2014


Introduction• Probe card technologies are more advanced; however,

the contact basics of wafer sort really have not changed.

• ALL probe technologies have a contact area substantially harder than the pads, solder balls, or pillars.

• “Contact and slide” CRITICAL for electrical contact, but results in localized plastic deformation.

• Volume of material displaced during the slide is a function of probe mechanics, metallic interactions, frictional effects, and surface properties.

7J. BrozOct-2014


Controlled Contact is One of the TOP GOALS

• To control wafer test is to control the mechanical contact and the electrical contact between the probes and the DUT.

• Process Monitors• Probe Yield• Binout Metric• Contact Resistance• Probe Mark• Re-Probe / Re-Test• I/O Damage

Apply the least mechanical contact that ensures a reliable electrical contact.

• Control Variables• Probe Force• Overtravel• Probe Placement (XYZ)• Current / Duration• Temperature• Cleaning Execution

8J. BrozOct-2014


Copper Bonding Wire Implementation• Cost Benefit?

– Au price at $400: little adoption– Au price at $700: some movement– Au price at $1000: significant motivation

• Advantages– High thermal conductivity– Low electrical resistance– Higher breaking stress– Smaller wire diameters for higher density– Excellent intermetallic formation

• Challenges– Bonding temperature can increase

oxidation– High pad Impact Force – Bonding force and ultrasonic energy

control is critical

9J. BrozOct-2014


Local Plastic Deformation (a.k.a., Probe Marks)

• Stress distribution during the scrubbing has been shown by cause underlying damage and cracking.

• Repeated probing has been shown to displace material and create under pad damage.

Probe Mark Anatomy

Pile-Up Created DuringForward Motion

Pile-Up Created DuringBackward Motion

ForwardMotion

BackwardMotion

Depth CreatedDuring Scrub

10J. BrozOct-2014


“Traditional” Definition of Pad Damage• Large and multiple probe marks affect ball bond adhesion

and cause long term reliability issues.

Damaged Area Due to Probe

• Pad Area = pad-X dimension × pad-Y dimension

Pad Damage (%) = 𝑷𝑷𝑷𝑷𝑷𝑷 𝑨𝑨𝑨𝑨𝑨𝑨𝑷𝑷𝑨𝑨𝑷𝑷

K. Karklin, J. Broz, and B. Crump, SW Test 2008

AaA sTD

nnd ∑

=−=

11

1Where:

Ad - disturbed area

TD - touchdowns

a - scaling coefficient

As - scrub mark 2D size

• Modeling the Disturbed Area due to Probe:

11J. BrozOct-2014


Damage Area Affects Bondability• Probe damage area positively correlated to bondability issues.

– Reduced ball shear strength and wire pull strength– Increased NSOP (no stick on pad) and LBB (lifted ball bond)

Assembly Parameter vs. Probe Mark Area

% L

BB R

ejec

ts%

NSO

P Re

ject

s

% AREA Pad Damage

Ball

Shea

r (gr

ams)

Wire

Pul

l (gr

ams)

Sources …Tran, et al., ECTC -2000Tran, et al., SWTW-2000Langlois, et al, SWTW-2001Hotchkiss and Broz, ECTC-2001Hothckiss and Broz., IRPS-2001Among others …

Critical ValueDamage = 25%Ball Shear

Wire Pull

% LBB% NSOP

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Probe Mark Area Impact on Bond Integrity

0

278

8333

19444

0 5,000 10,000 15,000 20,000 25,000

30%

40%

50%

60%

Lifted Balls

Damage Area vs. Au Wire Bond FailureInter-metallic

CoveragePad

Damage

80-90%

70-80%

70-80%

50-60%

Gillaed, et al., SW Test 2007

Source: TIPI C027 Study

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Damage Area vs. Cu Wire Bond Failure

• Ball shear and pull strength showed no significant differences vs. pad damage.• Ball shear and ball pull strength values were more than twice the required strength. • Non-stick on pad (NSOP) bonding failure was not observed with 15,840 wire bonds at

each level of pad damage.

Beleran, et al., ECTC 2013

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Specification RequirementSpecification Requirement


Damage Depth Affects Bondability!• A probe mark can have a small damage

area, but exceed critical depth.– % Damage < 9 % (which is within limits)– Depth = 10kÅ (which is excessively deep)

• Multiple touchdowns displace the pad material and expose the barrier metal

6.0kÅ aluminum + 5.5kÅ thermal oxide = 11kÅ

Probe Depth = 10kÅ

Miller, et al., SWTW-2007

Exposed Barrier Metal

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No. Touch Downs Impact on Bond Integrity

0

5556

8333

13889

0 5,000 10,000 15,000 20,000 25,000

1

4

8

12

Lifted Balls

Depth Damage vs. Au Bond Failure

Inter-metallic Coverage

No. ofTouchdown

Source: TIPI C027 Study

60-70%

60-70%

65-75%

80-90%

Gillaed, et al., SW Test 2007

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Damage Area vs. Au Wire Intermetallic (%)• Insufficient aluminum-gold intermetallics form at the deepest portion of

the probe mark.• Bonding to pads with > 25% probe damage produces a higher incidence

of lifted balls during production.

3X TDs 6X TDs

Regions of little or no intermetallic formation and voidsmatch the locations of the probe marks

1X TD

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Damage Area vs. Cu Wire Intermetallic (%)

• No significant differences of intermetallic formation vs. pad damage.

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Beleran, et al., ECTC 2013

J. BrozOct-2014

Specification Requirement


OT = 30um

OT = 60um

Probe MarkHeel

Metallization Affects Probe “Pile-up”

• OT = 30um• Length = ~10 to ~14um• Depth = ~3.5 to ~3.8kÅ

• OT = 60um• Length = ~18 to ~20um• Depth = ~5.0 to 5.4kÅ

• OT = 30um• Length = ~12 to ~14um• Depth = ~8.0 to ~8.3kÅ




6kÅ Wafer 15kÅ Wafer 30kÅ Wafer

OT = 30um

OT = 60um

Probe MarkHeel

OT = 30um

OT = 60um

Probe MarkHeel

Bischoff, et al., SWTW-2012

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“Pile-up” Height Effects• Pile-up has been correlated to bondability issues.

– Reduced ball shear strength and wire pull strength– Increased NSOP (no stick on pad) and LBB (lifted ball bond)

Assembly Parameter vs. Aluminum Pile-Up

% L

BB

Rej

ects

% N

SO

P R

ejec

ts

Height of Pile - Up

Bal

l She

ar (g

ram

s)W

ire P

ull (

gram

s)

Ball ShearWire Pull

% LBB% NSOP

Critical Value“unknown”

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Low Sensitivity of Cu Wire to “Pile-up”

• With Cu wire bonding the soft Al pad material readily deforms.

• The pile-up and depth of the probe mark are flattened under high force.

• Under high loads, the Al pad material gets squeezed out from under the ball bond.

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Beleran, et al., ECTC 2013Breach, Gold Bulletin, 2010

J. BrozOct-2014


Depth Effects Create Hidden Damage• Probe induced cracking of underlying structures and the impact on

package long term reliability is an ongoing test industry issue.

• Damage to Low-k and CUP / BOAC during probe and assembly– Low fracture toughness = high probability of cracking– CUP damage = leakage or shorts electrical nodes.

• Cu wire bonding requires higher forces and more ultrasonic energy exacerbating the problems of pad damage and underlying cracks.– A weak pad structure cannot withstand robust copper wire bonding process – In fact, some unprobed pads will crater under high force and energy

requirements of a Cu wire bond.

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Hidden Damage due to Probe• Under-layer micro-scratches and cracking attributed to probe.

Slight Medium Serious

OD = 65μm TD = 6 times

Tip Dia.= 8μmBCF = 4gw/mil

Hwang, et al., SW Test 2006

ForwardScrub Backward

Scrub

• TaN Crack > Underlying Deformation > Pad Void• z-Force is well determined from probe needle properties.• Transverse loading conditions were not characterized.





23J. BrozOct-2014


Assessing Transverse Loading Conditions

Pad TD X-Axis Video

Pad TD Y-Axis Video

y-Force vs. OT

z-Force vs. OT

z-Force and y-Transverse Force vs. OT are clearly differentiated

Khavandi, et al., SW Test 2014

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Damage Control Strategies• Probe card parameters …

– Probe technology– Tip shape / Geometry– Scrub “mechanism” – Low force

• Operational parameters …– Optimized probe to pad interactions– Z-stage motion control– Number of Touchdowns

• Probe pad properties … – Robust BEOL stacks designed to resist cracking– Alternate pad materials, for example, Ni-Pd or Ni-Pd-Au– Thicker pad metal layers to protect the underlying stack

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Pad Damage Depth vs. Technology

27

Cantilever

Vertical

MEMS

Pad Surface

2000

1000

500SCRUB HEIGHT

- 400

SCRUB DEPTH

- 500- 600 - 800- 1200

nm

Horn, SW Test 2008

Metal Thickness

J. BrozOct-2014


Probe Tip Geometry Optimization• Reduce probe tip diameter• Reduce spring force and overdrive

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Benefits:• Smaller probe mark• Minimize probe mark size• Reduce probe mark depth

Concerns:• CRES stability • Probe process control• Probe card maintenance• Reduced card life

J. BrozOct-2014


Probe Tip Texture / Surface Roughness(Example: 15kA Al-thickness)

29

Horizontal Force Horizontal Force

Textured Probe TipSmooth Probe Tip

Vertical ForceVertical Force

Moment

• Textured probe tips “dig into” the aluminum layer; while smooth probe tip “skate” across.• As the textured surface resists the forward scrub of the tip, a moment is generated and the

heel of the probe penetrates the surface of the pad.

Khavandi, et al., SW Test 2014

J. BrozOct-2014


Advanced Scrub Control

• Macroscopically, punch through level was found to be a direct function of tip pressure (FORCE / AREA)

– Tip area– Spring constant– Planarity – Over travel

30

Tip Size

Low High

Standard

Large

KWang, et al., SWTW-2007

J. BrozOct-2014


Probe Count and Total Probe Force• Probe force needs to stay in a manageable region• Probe force per probe has to decrease• Certain probe force is needed for stable CRES

Probe Force [g]

C re

s [O

hm]

Characterization of Different Probe Types

Type 1Type 2Type 3

M. Huebner, SWTW 2009

Unstable CRES Stable CRES

31J. BrozOct-2014


What Operational Steps Can I Take ?

• Can reasonable steps be taken with existing technologies (e.g., an existing probe card and a prober) to reduce pad damage in a cost-effective manner?

• Is it possible to identify an optimized combination of prober operational settings to reduce the overall area and volumetric probe damage?

32J. BrozOct-2014


Factors (Prober Operational Settings)• Number of Touchdowns

Single vs. Double

• Overtravel MagnitudeLow (50um) vs. Middle (63um) vs. High (75um)

• Undertravel MagnitudeLow (0um) vs. Middle (10um) vs. High (20um)

• Pin-Update Execution– Abbreviated pin alignment to compensate for thermal movement – On vs. Off

• Wafer Chuck SpeedLow (6000 um/sec) vs. High (18000 um/sec)

• Chuck Revise Execution– Re-zero of the wafer chuck to compensate for thermal movement– On vs. Off

33J. BrozOct-2014


Significant Factor Estimates13.3180

1.6657

-1.1663

-0.9465

-0.7554

-0.6623

2.0472

-1.9320

1.0323

-0.0079

-12.4100

0 2 4 6 8 10 12 14 16

ut[20]

ut[10]

cr[Off]

speed[Low]*cr[Off]

pu[Off]

speed[Low]

speed[Low]*ot[50]

ot[63]

speed[Low]*ot[63]

ot[50]

td[Double]

Probe Mark Volume

Scaled Estimates

1.6605

2.0336

2.7873

5.0350

5.4209

39.9630

-38.75266

-1.5891

-4.8920

-7.1279

-9.0581

0 5 10 15 20 25 30 35 40 45

ut[10]

ut[20]

cr[Off]

pu[Off]

speed[Low]*cr[Off]

speed[Low]*ot[63]

speed[Low]*ot[50]

ot[63]

speed[Low]

ot[50]

td[Double]

Probe Mark Area

Scaled Estimates

• Single vs. Double Touchdown• Minimum vs. Maximum Overtravel

Primary Responses Secondary Responses• No clear chuck speed dependency for volume was surprising.• Speed was the third largest factor probe mark area response.

34J. BrozOct-2014


1.3116

1.5082

1.8716

1.9195

1.9346

2.5774

20.6415

-4.4643

-11.6636

-1.4899

-0.4748

0 5 10 15 20 25

speed[Low]*ot[50]

speed[Low]*ot[63]

ot[63]

ut[10]

cr[Off]

pu[Off]

speed[Low]

speed[Low]*cr[Off]

ut[20]

ot[50]

td[Double]

Pile Up Area

Scaled Estimates

13.3180

1.6657

-1.1663

-0.9465

-0.7554

-0.6623

2.0472

-1.9320

1.0323

-0.0079

-12.4100

0 2 4 6 8 10 12 14 16

ut[20]

ut[10]

cr[Off]

speed[Low]*cr[Off]

pu[Off]

speed[Low]

speed[Low]*ot[50]

ot[63]

speed[Low]*ot[63]

ot[50]

td[Double]

Pile-up Volume

Scaled Estimates

Significant Factor Estimates

• Single vs. Double Touchdown• Minimum vs. Maximum Overtravel

Primary Responses Secondary Responses• Speed was a low level contributor to pile-up volume.• No clear huck speed dependency was surprising.• Undertravel was a more contributing factor than chuck speed.

35J. BrozOct-2014


50100150200250300

Mea

n(Sc

rub

Area

)64

.055

95±1

9.72

275

Doub

le

Sing

le

Singletd

Low

High

Lowspeed

Off

On

Offcr

50 63 75

50ot

10 20 off

10ut

Off

On

Onpu

0

20

40

60

Mea

n(D

ivot

Vol

ume)

8.73

2232

±8.4

3241

3

Dou

ble

Sing

le

Singletd

Low

Hig

h

Highspeed

Off

On

Oncr

50 63 75

50ot

10 20 off

20ut

Off

On

Onpu

Off

Best Case Combinations• Modeled response data can be used to investigate the effects of changing one

parameter and keeping the other constant.

36

Miller, Broz, Robinson, SWTW-2007

Number of TDs Overtravel

J. BrozOct-2014






37J. BrozOct-2014


Two-Pronged Strategy for Reduced Damage

• Well-controlled probe card metrics can mitigate some of the damage effects.– Tip size / shape– Scrub length– Probe force– Contact material

• Reasonable steps can be taken with “existing” hardware to pad damage in a cost-effectively.– Reduced overtravel– Reduced touchdown counts

38

Recall a GOAL for wafer test …Apply the least contact that ensures reliable electrical connection.

J. BrozOct-2014


Summary / Discussion• I/O pad damage has been aggravated by smaller pads,

sharper needles, and new process node technologies.

• Changes and improvements to probe card specification have been developed to mitigate some of the problems.

• Significant new probe methods, new probe card technologies, and design and layout tricks are now being implemented.

• Reasonable steps can be taken with “existing” hardware to reduce pad damage in a cost-effective manner.

39J. BrozOct-2014


“I'm smart enough to know that I'm dumb as the next guy.”- Richard Feynman

Nobel Winner in Physics, 1965

Questions ???

40J. BrozOct-2014


IEEE SW Test Workshop – www.swtest.orgJune 7 - 10, 2015, in San Diego, CA, USA• Want to Learn More !

– All aspects of wafer level testing– Three Day Technical Program – EXPO that showcases key suppliers

• EXPO does not compete with technical program

• Topics include but are not limited to:

– New probe card and contractor technologies– Challenges of 300-mm wafer probing– Monitor and reduction of chip I/O pad damage– Productivity improvements for production– Probe data collection, analysis, and management– Cleaning and cost of ownership – e-Test, Parametric, and Test Structure Testing

41

25 Years of Probe Technology

J. BrozOct-2014


About the AuthorJerry Broz, Ph.D., has been the Applications Engineering Team Leader and VP of Applications at International Test Solutions since 2003. Dr. Broz is responsible for the ITS branch office teams located in Taiwan, Korea, Japan, China, and Singapore that are focused on optimal on-line cleaning solutions for wafer sort and package test. Previously, Dr. Broz was a Member of Technical Staff with the Worldwide Probe Development Team at Texas Instruments, Inc. He has authored numerous publications and presentations in the areas of wafer level test, package test, and IC packaging. Dr. Broz holds a number of US and International patents as well as several pending patent applications related to wafer sort, package test, and front-end processes. Dr. Broz earned a Ph.D. in Mechanical Engineering from the University of Colorado at Boulder and has over 20 years of experience in various high volume manufacturing and applied research environments.

Dr. Broz is the General Chair for IEEE SW Test Workshop and a Sr. Member of the IEEE as well as an IEEE Golden Core member. The SW Test web site http://www.swtest.org is an on-line repository for many probe technology presentations.

Jerry Broz, Ph.D.VP World Wide ApplicationsInternational Test SolutionsReno, NV 89502

4242J. BrozOct-2014

Impact and Control of Pad �Damage at Wafer SortOverviewOverviewInternational Test Solutions �Corporate ProfileSlide Number 5OverviewIntroductionControlled Contact is One of the TOP GOALSCopper Bonding Wire ImplementationLocal Plastic Deformation �(a.k.a., Probe Marks)“Traditional” Definition of Pad DamageDamage Area Affects BondabilityDamage Area vs. Au Wire Bond FailureDamage Area vs. Cu Wire Bond FailureDamage Depth Affects Bondability!Depth Damage vs. Au Bond FailureDamage Area vs. Au Wire Intermetallic (%)Damage Area vs. Cu Wire Intermetallic (%)Metallization Affects Probe “Pile-up”“Pile-up” Height EffectsLow Sensitivity of Cu Wire to “Pile-up”Depth Effects Create Hidden DamageHidden Damage due to ProbeAssessing Transverse Loading ConditionsOverviewDamage Control StrategiesPad Damage Depth vs. TechnologyProbe Tip Geometry OptimizationProbe Tip Texture / Surface Roughness�(Example: 15kA Al-thickness)Advanced Scrub ControlProbe Count and Total Probe ForceWhat Operational Steps Can I Take ?Factors (Prober Operational Settings)Significant Factor EstimatesSignificant Factor EstimatesBest Case CombinationsOverviewTwo-Pronged Strategy for Reduced DamageSummary / DiscussionSlide Number 40IEEE SW Test Workshop – www.swtest.org�June 7 - 10, 2015, in San Diego, CA, USAAbout the Author

controlling pad damage - cleaning technologies€¦ · basic damage control strategies – probe...

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