pb-free interconnect metallurgy and...

IBM T. J. Watson Research Center

Stress Workshop 2010 April 14, 2010 © 2003 IBM Corporation

Pb-free Interconnect Metallurgy and Electromigration

Minhua LuIBM T. J. Watson Research Center

Stress Workshop 2010 April 14, 2010 2

Acknowledgement

YKT– Da-Yuan Shih– Sung Kang– Paul Lauro– Robert Polastre

EFK– Charles Goldsmith– Thomas Wassick– Hai Longworth


Outline1.Introduction

– Sn crystal structure and anisotropic properties

1.EM Failure Mechanisms and Grain Orientation– Classification of EM failure modes and associations with Tin grain

orientation and diffusion anisotropy – Solder type and failure mode correlation

1.Kinetic Study and Blech Effect– Activation energy and power exponent for SnAg and SnCu solders

1.Alloy Effect and UBM2.Summary


a=5.83Å

c= 3.18Å

Body Centered Tetragonal <001> view

<010> view

Trade off between mechanical and EM reliability!

CPI C to chip planeEM C in chip planeBieler, ECTC 2006

1. β-Sn Properties – Highly Anisotropic

Yeh and Huntington, PRL 53, pp 1469-1472, (1984)


Failure Mode depends Sn Grain OrientationMode-II: Fast diffusion, IMC movement and barrier layer consumption, c-axis of Sn grain along current direction, common in SnCu

Mode-I: Sn self diffusion, voids between IMC and solder, c-axis of Sn grain not align to current, common in SnAg

Lu et al, APL, 92, 2008, p. 211909

Lu et al. Proc. 58th Electronic Components and Technology Conf., 2008, pp. 360-365.


Bi-Crystal Example in C4 with SnCu solder

Mode-II Mode-I

Diffusion elementDiffusivity (cm2/sec)150°C 25°C

Aga-axis 5.60 x 10-11 5.75 x 10-15

c-axis 3.13 x 10-9 6.76 x 10-12

Cua-axis 1.99 x 10-7 3.85 x 10-9

c-axis 8.57x 10-6 1.16(~2) x 10-6

Nia-axis 3.85 x 10-9 6.04 x 10-12

c-axis 1.17 x 10-4 1.35 x 10-5

Sna-axis 1.24 x 10-12 4.85 x 10-18

c-axis 4.92 x 10-13 1.50 x 10-18

Direct prove of Tin diffusion anisotropy!


Solder and Failure Mode – SnCu is more likely to have mode-II type early fail

Fast diffusion driven mode-II is early failure mode.

SnCu is more likely to have Mode-II than SnAg.

C: Sn0.7Cu mix mode

B: Sn1.8Ag mode-I failure

A: Sn0.7Cu mode-II failure

D: Sn1.8Ag mix mode

555hr

1342 hr

1342 hr

1342 hr

0 200 400 600 800 1000 1200 14000

5

10

15

20

25

30

D

CB

A

Res

ista

nce

Cha

nge

(%)

EM Stress Time ( hours)

A B C D


Multigrain and Twin in SnCu

Cyclic Twinning In SnAg

Solder and Failure Mode – Multigrain and Cyclic Twins mitigate Mode-II Failure

C: Sn0.7Cu mix mode D: Sn1.8Ag mix mode


Time of Operation ( Hours)

Pro

ba

bil

ity o

f Fa

ilu

re (

%)

100 10000001000 10000 1000001.E-1

5.E-11

510

50

99

1.E-1

Activation Energy and Current Exponent

SnCu

SnAg

4, 000

95, 000

T50 at 90°C 200mA (hr)

1.2

2

n

Fast diffusion and grain boundary diffusion, Sn orientation dependent

Sn self diffusion, Blech length effect observed.

Dominate Failure Mechanism

0.54SnCu

0.95SnAg

Ea (eV)Solder

4, 000

95, 000

T50 at 90°C 200mA (hr)

1.2

2

n

Fast diffusion and grain boundary diffusion, Sn orientation dependent

Sn self diffusion, Blech length effect observed.

Dominate Failure Mechanism

0.54SnCu

0.95SnAg

Ea (eV)Solder

20% Failure Criteria

Use level: 90°C 200mA

= −

kTEAjt an exp

Equation sBlack'

50

Lu et al. Proc International Syposium of Reliability Physics, 2009


Blech Effect

Mass Transportation in EM– First, atoms start to move from

cathode towards anode driven by EM force. Resistance will increase due to the formation of voids.

– As atoms accumulate at anode, back stress gradient starts to build up. The diffusion backflow cause by back stress gradient will suppress further EM-induced damage – resistance saturation.

– When electromigration damage process stops.

0* =

Ω−=

dxdjeZ

kTDCJ σρ

Ω−=−=

dxdjeZ

kTDCJJJ EM

σρσ *

Mass transportation

0 200 400 600 800 10000.000

0.005

0.010

0.015

0.020

200mA300mA

500mA

700mA

Res

ista

nce

Cha

nge

(Ω)

Stress Time (Hours)

700mA 500mA 300mA 200mA

Lu et al, Appl. Phys. Lett., 94, 2009, p. 011912


Evidences of Blech Length in Sn solderC4 – Sn1.8Ag

φ=90µm, L=60µm Wire Sample – Sn1.8Ag φ=287µm, L=50-150µm

(JL)c ~ 30A/cm (JL)c ~ 26A/cm

Different test structures gave similar estimation on Blech Length!

100 10000.0001

0.001

0.010

∆Rsa

t (Ω)

Current (mA)20 40 60 80 100 120

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

L/sq

rt(T

TF) (

cm/h

r1/2 )

j*L (A/cm)


Short length effect is observed in SnAg, not in SnCu

Solder modification is key to eliminate mode-II fails, so that short length effect can utilized to extend EM life!

Interstitial diffusion dominated Mode-II fails leads to early interface failure, Blech effect not observed.

In Sn self-diffusion dominated Mode-I fail, back stress gradient can be high enough to slow down or stop further EM damage.

0 200 400 600 800 10000.0000.0020.0040.0060.0080.0100.0120.0140.016 (a)SnAg

500mA150oC

Res

ista

nce

Shift

(Ω)

Time (Hours)0 50 100 150 200 250

0.0000.0050.0100.0150.0200.0250.0300.0350.0400.0450.050 (b) SnCu

500mA150oC

Res

ista

nce

shift

(Ω)

Time (Hours)


Effect Ag and Cu in Sn Crystal Structure

Multigrain and Laminar twins in SnCu and low Ag solder.

Single grain or cyclic twins in high Ag solder


Solder Composition, Texture and Stability

T0 grain size Sn (SnCu) < Sn0.5Ag < Sn1.0Ag < Sn1.8Ag

Grain growth SnCu > Sn > Sn0.5Ag > Sn1.0Ag > Sn1.8Ag

EM Lifetime SnCu < Sn < Sn0.5Ag < Sn1.0Ag < Sn1.8Ag

T0

SnCu Sn Sn0.5Ag Sn1.0Ag Sn1.8Ag

T1525hr 150C 5200A/cm2 e-

Ag CuIMC type Ag3Sn Cu6Sn5IMC partipitate particulate plateIMC melting Temp C 476 415Diffusivity in Sn a-axis 5.60E-11 1.99E-07Diffusivity in Sn c-axis 3.13E-09 8.57E-06


Cu Effect – EM Results

Cu on UBM is better for both SnAg and SnCu solders.

For SnAg solder, about 1% Cu is needed.

EM is insensitive to Cu content in SnCu solder, when there is Cu on UBM.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

200

300

400

500

600

700

800

900

1000Cu Effect on SnCu and SnAg solders

Aver

age

TTF

(Hou

r)

Cu Content (wt.%)

Cu in SnCu solder w. Cu on UBM Cu in SnCu solder wo. Cu on UBM Cu in Sn1.0Ag solder

Lu et al. Proc. 59th Electronic Components and Technology Conf., 2009, pp. 922-929.


Cu Effect – UBM Interfacial Reaction

20 µm

6.50 um

TiW/Ni/Cu(1um) (Sn1.0Ag)TiW/Ni(6um) (Sn1.0Ag)

20 µma b

Cu on Ni UBM can slow down Ni consumption.

5.93µ m4.80µ m

a b

TiW/Ni(6µ m) Sn0.7Cu TiW/Ni(6µ m) Sn2.0Cu

Ni3Sn4

When Cu is absent on UBM, Cu from high Cu solder can reduce IMC spalling.


Cu Effect – Cu as UBM

Cu is not a good barrier layer compare to Ni.

However, unlimited Cu from anode improves the EM lifetime of Ni UBM.

Ni UBM on both sides, limited Cu case, is not as good as Ni UBM as cathode and Cu as anode.

e-

Cu

Ni/Cu

Ni/Cu both sides

Cu anode

Ni/Cu anode


Zn Doping – Effectively reduces early fails

Time, (t)

Un

reli

ab

ilit

y,

F(t)

100 1000010001

510

50

99

Sn1.8Ag

Sn1.0AgSn1.0Ag0.6Zn

Lu, et al. J.A.P. 106 053509 (2009)


IV. Zn doping – Stabilize IMC Network, especially Cu6Sn5

Before

After 200C 8h

Thermal aging at 200°C

Sn1.0Ag0.6Zn Sn1.0Ag


Zn doping – Stabilize IMC Network, especially Cu6Sn5

Before

After EM 969 hr

EM at 5200A/cm2 150°CSn1.0Ag0.6Zn Sn1.0Ag


Zn doping – Zn associates with Cu and Ag

After 969 hours EM test


Zn Doping – EBSD

Zn doped solder appear to be single grain with poly-crystalline structure at UBM interface.

The grains with bad orientation lead to earlier failure.

It seems Zn doping can not eliminate the bad grain orientation, but can slow down or reduce the damage.

T0 394 hr 1525 hr


SummaryEM Failure Mechanisms and Grain Orientation

– Due to the highly anisotropic nature of the Sn crystal, electromigration failure is closely depends on the Sn grain orientation.

– Mode-I is Sn self diffusion dominated that occurs when c-axis is not aligned the current direction. Mode-II is driven by interstitial diffusion that occurs when c-axis is aligned to the current direction, and results early fails.

– The fast diffusion axis, c-axis, should be avoid to be parallel to the current direction. Unfortunately, this is exactly the opposite to the requirement for stress mitigation (CPI). Multi –grain and cyclic twinning is a way to compromise both requirements.


Summary Kinetic Study

– High Ag SnAg alloys showed mode-I dominated fails, with activation energy 0.95eV for lattice diffusion and nucleation dominated n=2.

– More mode –II type failures observed in SnCu solders. The interstitial diffusion lead to activation energy of 0.54eV and growth dominated n =1.2.

Blech Effect and Blech Length– Blech effect is observed in SnAg solders where failure mode is driven by Sn

self diffusion. The Blech length is measured to be about 30A/cm.– Type-1 EM failure mode and reduce current density can prolong EM life.

Alloy Effect– Ag: High Ag SnAg alloys has mode-I dominated fails, better EM

performance than SnCu solders.– Cu: Although Cu is not a good barrier layer, Cu on Ni and Cu as anode

improves EM performance of SnAg solder.– Zn: Minor Zn doping greatly stabilize solder microstructure and slow down

Cu migration, but the wetting is a issue for implementation.

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