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No Interfacial
IMC
Maureen Williams, NISTECTC, Reno, NV
May 29, 2007
1
NIST NIST SnSn Whisker TeamWhisker Team
Metallurgy Division Gaithersburg, Maryland
William J. Boettinger
Kil-Won Moon
Gery Stafford
Maureen Williams
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Motivation for this experimentMotivation for this experimentMeasured Deflection Curves for 3 Alloys (16 µm) 2005 work
-160
-120
-80
-40
0
40
-40
-32
-24
-16
-8
0
8
1000 104 105 106
Can
tilev
er D
efle
ctio
n (µm
) Initial Film Stress (M
Pa)
time (s)
Sn
Sn-Pb
Sn-Cu
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How to separate the IMC effect from the deposit?How to separate the IMC effect from the deposit?
• To eliminate interfacial IMC electrodeposit Sn on a non-reactive substrate.– Sn on Tungsten (W)
Phase diagram constructed by U. Kattner, NIST, 2005
- based on Binary Phase Diagram, Massalski
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GoalGoal
• Electrodeposit Sn, SnCu, and SnPb on W, a substrate that does not form an interfacial intermetallic.
• Determine if IMC is needed for hillock & whisker growth.
• Measure– whisker growth
– Stress as a function of time
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OvOverview of erview of ApproachApproach
• Part One – experiments done in 2006 - Williams M.E., et al., JEM (36) 3, pp.214-219 (2007)
– 1 mm thick pure tungsten (W) substrates– Vapor deposited 0.2 µm (5-9’s) Sn with e-beam– Electroplated 15 µm Sn (made with 18 Mohm cm water)
• Part Two – experiments done in 2007– 150 µm phosphor bronze (Cu) cantilever substrate– Vapor deposited 0.02 µm (4-9’s) W with e-beam on both sides– Vapor deposited 0.2 µm (5-9’s) Sn with e-beam– Electroplated 15 µm Sn (made with 18 Mohm cm water)
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ResultsResults
Surface eruptions form without interfacial intermetallic (IMC) between Sn and substrate.
Both Cu & W substrates:
•Similar compressive stress values for Sn
•Sn deposits have similar hillock formation.•SnCu deposits have similar whisker formation.•SnPb deposits are clean (some plating bumps).
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ResultsResults
Avg. Stress 15 minutes after electroplatingmeasured by cantilever beam deflection (2007)
Pure Sn Sn-3 wt% Cu Sn-2 wt% Pb
W - 28.2 MPa - 57.6 MPa No data
Cu -31.2 MPa No data No data
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Surface Defects on Pure Surface Defects on Pure SnSn Deposits Deposits
W substrate (2006)
10 µm
100 µm
500 µm
141 days oldebeam W on Cu substrate (2007) Cu substrate (2006)
256 days old
500 µm
100 µm
10 µm
26 days old
SEM photos of deposit surface
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Surface Defects on Surface Defects on SnSn-- 3 wt% Cu Deposits3 wt% Cu Deposits
20 µm
100 µm
500 µm
Cu substrate176 days old
ebeam W on Cu substrate (2007)21 days old
500 µm
100 µm
10 µm
W substrate (2006)
10 µm
500 µm
100 µm
133 days old
SEM photos of deposit surface
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Experimental OutlineExperimental Outline
• Sample – preparation
– FIB cross sections of the deposit - substrate interface
• Stress Measurement– Deflection of cantilever as a function of time
• Sn and SnCu on W• Sn on Cu and W
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For Sn electroplating, need to overcome oxide barrier on W substrate surfacePreparation of a Preparation of a NNonon--reactive reactive Cantilever BeamCantilever Beam
Electron beam vapor deposition – Plasma etched the surface of diamond polished Cu cantilever beams in high vacuum (10-8 torr)
– vapor deposited 0.02 um of Tungsten (W) in high vacuum (10-6 torr) to both sides of cantilever beam
– vapor deposited 0.2 um of Tin (Sn) in high vacuum (10-6 torr) to one side of cantilever beam
–Performed ASTM adherence test
Surface of e-beam deposited Sn (0.2 um) on
e-beam deposited W layer (0.02 um)
10 um5 um
Surface of e-beam deposited W layer (0.02 um)
on Cu substrate
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Cantilever BeamsCantilever Beams
• Sample Mounting– Mask is cut and placed on substrate– Sample is attached to cathode
• Plating Conditions **(same as 2005 experiments)– 1 liter of electrolyte (made with 18 Mohm cm water)
• 100 rpm rotating cathode• 5-9s pure Sn anode• current density of 60 mA/cm2
** Boettinger W.J., et al., Acta Materialia 53, 5033 (2005)
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Pure Pure SnSn Deposit / Substrate InterfaceDeposit / Substrate Interface
Top FIB photo courtesy of
A. Deal, Lehigh Univ.
Bottom FIB photo courtesyof
G. Galyon & M. Palmer, IBM
3.0 µmSn / Cu interfaceCu substrate
Sn deposit
Cu6Sn5
Sn / W interface
W substrate
Sn deposit
1.0 µm
Sn on W substrateCu6Sn5
Sn on Cu substrate
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SnSn--3 wt% Cu Deposit / Substrate Interface3 wt% Cu Deposit / Substrate Interface
Top FIB photo courtesy of
A. Deal, Lehigh Univ.
2 µmSn-Cu / W interface
Cu6Sn5
W substrate
Sn-Cu deposit
Bottom FIB photo courtesy of
G. Galyon & M. Palmer, IBM
1.5 µm
Sn-Cu deposit
Cu substrateSn-Cu / Cu interface
Cu6Sn5
SnCu on W substrate
SnCu on Cu substrate
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-70
-60
-50
-40
-30
-20
-10
0
0 5 10 15 20 25
Sn-Cu
Sn
Time (days)
Stress Calculated from Stress Calculated from Cantilever Beam DeflectionCantilever Beam Deflection
Stoney’s single layer equation is valid - No interfacial IMC
Stre
ss (M
Pa)
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Comparison of 16 um thick Comparison of 16 um thick SnSn deposit on deposit on TungstenTungsten (W) and (W) and phosphor bronzephosphor bronze (Cu) substrates(Cu) substrates
100
-80
-60
-40
-20
0
0 5 10 15 20 25 30
Sn on P-Bronze
Sn on W
Time (days)
Bea
m D
efle
ctio
n (u
m)
-
Without IMC Sn deflectionrelaxes to constant value
With IMC reaction Sndeflection moves in
positive direction
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SummarSummaryy• Successfully plated Sn on a non-reactive cantilever beam• Measured residual stress in deposits without interfacial IMC
formation– Sn-Cu deposit shows higher initial stress than pure Sn– Sn has hillocks, and Sn-Cu has filament whiskers– deposits maintain compressive stress initially and after
relaxation• Comparison of Sn deposits on Cu and W cantilever beams
– with IMC, Sn deflection moves in positive direction– without IMC, Sn deflection relaxes to constant compressive
value• Interfacial IMC is not required to initiate whisker growth
