imaps presentation

© 2009 IBM Corporation

Assessment of XRF Technique as a Method to Measure Percent Ag in SnAg Solders for

Flip Chip ApplicationsJennifer D Schulera Chia-Hsin Shihb, Charles L Arvina, KyungMoon Kimc, Eric Perfectoa

a: Semiconductor Research and Development Center, IBM, Hopewell Junction, New York 12533

b: STATS ChipPAC Taiwan Ltd, Hsin-Chu Hsien Taiwan, R.O.C 307

c: STATS ChipPAC Korea Ltd, Kyoungki-do, 467-814 South Korea

© 2009 IBM Corporation2

Introduction

X-Ray Fluorescence (XRF) is a non-invasive method of measuring Ag% composition in Pb-free solders.

1. Sample preparation - Experimental bumping variables

2. Tool configuration - XRF configuration, calibration, optimized measuring methodology and the importance of having known standards with the same dimensions of the bumps being measured

3. XRF recipe setting - Measuring accuracy and correlation with ICP and DSC

4. Data interpretation - Ag distribution study in the die and wafer level


Why XRF?

One area of interest for Pb-free solder manufacturing is the ability to control and measure the %Ag composition and its variation from wafer to wafer, chip to chip, and C4 to C4.

Invasive Non-Invasive

Atomic Absorption (AA) X-Ray Fluorescence (XRF)

Differential Scanning Calorimetry (DSC)

Inductively Coupled Plasma (ICP)

Electron Probe Micro-Analyzer (EPMA)

Methods to Measure Solder CompositionWafer to Wafer Chip to Chip C4 to C4

Ag% Composition ControlPb-free SnAg solder has become the industry standard for fabricating flip chip interconnects utilizing C4 (controlled collapse chip connection) technology.


Bum

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bles

Wa

fer

Va

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les

Underlying Metallurgy

Bottom Layer Metallurgy (BLM) Size

UBM Stack

C4 Height

Ag% Target

Mechanically Good (MG)

Electrically Good (EG)

90 um

110 um

Thick Cu

No Cu

70 um

90 um

0.6%

1.7%

Sample Preparation – Test Vehicle ListV

ari

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les

Imp

act

ing

XR

F M

easu

rem

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t


Tool Configuration

# BLM Size

C4 Height

Type Known Bump Composition

UBM Stack

SnAg Solder Process

A 90 70 MG 0.5%Ag Ni/SnAg C4NP

B 90 70 MG 1.8%Ag Ni/SnAg C4NP

C 110 90 MG 0.5%Ag Ni/SnAg C4NP

D 110 90 MG 1.8%Ag Ni/SnAg C4NP

C4NP is a solder transfer process

which controls the Ag% composition to

± 0.1%Ag.

XRF Calibration Standards

Tool Description

XRFAnode material is made of tungsten or

molybdenum

ICP-OESInvasive method to get C4 composition

on global zone

DSCInvasive method to get C4 composition

on global zone

Verification Methods

Through data comparison with ICP and DSC, a non-invasive XRF

method was created and calibrated.


Mass Transport Effect The need for the correct geometry comes from the learning that the %Ag is mass transport

controlled. The concentration of Ag in a larger test structure on the edge of a wafer may have a different and most likely higher concentration of Ag than the Ag in an actual C4.

C

Site A has a largest height controlled by diffusion plating compared to B or C

Site A has the lowest Ag%

Diffusion Controlled Plating Convection Controlled Plating

10um

Site C has the smallest height controlled by diffusion plating. Compared to A or B

Site C has the highest Ag%


Optimized collection time creates a stronger XRF feedback generating more stable readings.

From C. Shih

XRF Performance by Collection Time


Process time 180sec on C4NP Standards

20 XRF Readings on Randomly C4s

# BLM Size

C4 Height

Ag% Target

Mean Std Dev Range

A 90 70 0.5% 0.66% 0.44% 1.84%

B 90 70 1.8% 1.77% 0.16% 0.53%

C 110 90 0.5% 0.48% 0.06% 0.19%

D 110 90 1.8% 1.79% 0.05% 0.21%

Remeasure on flattened C4NP Standards

Process time 180sec on flattened C4NP standards

20 XRF Readings on Randomly C4s

# BLM Size

C4 Height

Ag% Target

Mean Std Dev Range

A 90 70 0.5% 0.50% 0.05% 0.20%

B 90 70 1.8% 1.89% 0.06% 0.27%

XRF inspection on C4NP Standards

Tool issues such as stage movement accuracy and laser alignment were investigated and then eliminated as possible causes.

The X-ray spot size is 40µm -> a C4 geometry issue was hypothesized.

Reliability dramatically improved after the flattening, proving that C4 geometry impacts the intensity of the signal. Flattening is recommended if the bump height is less than 70um

What caused the instability in A and B?

Bump Geometry


FlattenedUn-Flattened

SEM photos comparing flattened and un-flattened C4’s

The spot size of the X-ray opening is 40 µm.

The detector is collecting photoelectrons from any region that is excited by the X-rays. Flattening reduces the chance of exciting larger regions which would cause artificially higher readings and thus a larger standard deviation.

From S. McLaughlin


The spectrum data shows a stronger feedback of X-ray fluorescence radiation on larger C4s.

Ag%

Un-reflowed Bump

From C. Shih

90um, 1.8% 70um, 1.8%


Wafer 1

-thick Cu

Wafer 2

-thick Cu

Comparison Post Flattening

Wafer 3

-thin Cu


XRF readings on MG wafers are comparable with ICP data (within 0.2%Ag), but the XRF readings are not reliable on EG wafers due to high background noise

To alleviate this, the XRF collection time was shortened to mitigate primary X-ray penetration.

Collection Time on EG Wafers

From C. Shih

EG/MG Spectrum Comparison


EG spectrum pre-flattening EG spectrum post-flattening


Conclusion

XRF is a popular, non-invasive inspection method for bump composition.

Several challenges were encountered when establishing this technique.

Balancing X-ray power voltage and current settings to obtain suitable X-ray penetrating ability.

Mitigating background noise, especially from EG chips

Finding an appropriate collection time

Characterizing a tool ability limitation for very low Ag% detection (less than 0.2% Ag).

Through XRF, ICP, DSC comparison, XRF recipes can be developed as an excellent monitor for non-invasive bump composition evaluations. As in all methods, XRF has a unique set of challenges and limitations which can be overcome through proper calibration and verification.

As the packaging trend moves toward finer pitched products, studies of small diameter C4s will become pervasive. If XRF is preferred to measure the bump composition of smaller C4s, geometry will become a key issue. C4 flattening helps mitigate geometry issues on smaller C4s, but may be insufficient for micro-bump features. Future work will involve how to define C4 geometry and the possible creation of composition test sites which will need to have extensive correlation established.


References

Acknowledgements

J. Sylvestre, et al.,”The Impact of Process Parameters on the Fracture of Brittle Structures During Chip Joining on Organic Laminates,” 2008 ECTC.

E. Perfecto, et. al, “C4NP Technology: Present and Future,” 2008 IMAPS Device Packaging Conference.

B.. Beckhoff B. Kanngießer N. Langhoff R.Wedell H.Wolff (Eds.) in Handbook of Practical X-Ray Fluorescence Analysis 2006.

Special thanks go to Chia-Hsin Shih and KyungMoon Kim from STATSChipPAC as well as Charles Arvin and Eric Perfecto from IBM for their guidance and support. Thanks also go to John Pennacchia, Stephen McLaughlin and the entire C4 team.

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