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Improved interconnect properties for nano-twinned copper: Microstructure and thermal stability
Department of Materials Science and Engineering,University of California at Los Angeles
K. N. Tu, Di Xu, and Hsin-Ping Chen
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Acknowledgement
Nanotwinned Cu work is supported by NSF/NIRT-0506841: "Nanostructured Materials for Interconnect and Packaging Technology”
Our collaborators:
UCLA: Prof. Vidvuds OzolinsAMD Dresden: Ehrenfried Zschech, Inka Zienert, Holm Geisler, Petra HofmannNIST: Dr. Gery R. StaffordIBM: Dr. C. K. HuNational Tsing Hua University : Prof. L.J. Chen, Prof. Chien-Neng LiaoNational Chiao Tung University: Prof. Wen-Wei Wu
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Outline
•Background and motivation• Processing and Characterization• Nanotwin formation mechanism• Effect of nanotwins on physical properties of Cu
Hardness and microstructure Grain growth behavior of nanotwinned Cu Thermal stability of (111) and (112) twins Electromigration behavior
• Summary
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Background on Nano-twinned copper
Lu L. et al, Science 304 (2004), 422-426
Nanotwinned Cu shows veryhigh mechanical strength andgood electrical conductivity
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In-situ TEM observation of atom diffusion at twin boundaries in Cu
Twin boundaries can effectively block the EM induced atomic migration.
K.C Chen, W.W.Wu, C.N. Liao, L.J.Chen and K.N.Tu, Science 321, (2008).
Promising applications in Cu interconnects and other fields
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Processing: Pulse electroplating
• On TimeHigh current density high volume of nuclei
• Off-time Recrystallization Grain Growth Nano twins may form
Cu Ni Ti Si Si
Cu column
Cu thin film (1~5 μm thick) Cu TSV (10~50 μm diameter) Cu lines (100nm~3 μm wide)
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Nanotwin Characterization
Advantage Disadvantage
X-ray diffraction
crystallinity, preferred orientationNon-destructive, large penetration depth
No microstructure imaging, cannot show twin boundaries
SEM Morphology information, easy access
small penetration depth, low contrast for twinned grains
TEM Accurate identification of twin, high resolution
Time consuming sample preparation process
EBSD Simple sample preparation,Grain and boundary info over large surface area
Only surface information (~50 nm depth)
FIB Cross section microstructure,TEMsample preparation
Destructive, Gacontamination
100nm
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Stoney’s equation:
In Situ Stress MeasurementNational Institute of Standards and Technology
)/,/(6
22
mJmNfRtEF ss
w
avg Fw
t f
f
winc dt
Fd
Fw (t )dt0
t f
(film growth)
Stress relaxation occurs at pulse off time during pulse electrodeposition
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Nanotwin formation mechanism• Recrystallization occurs during pulse off time• Effect of biaxial stress on nanotwin formation by First principles
calculations
When the strain is high enough (either compressive or tensile) in Cu, it’s easy to form twinning structure during stress
relaxation
Di Xu et al., Appl. Phys. Lett., 91, 254105 (2007)
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In-situ stress measurements
The energy (per atom) of a system with m layers separating twin boundaries can be given by :
%)2.0()( twinFCC E
mAEmE
Twin spacing about 28 nm
400 MPa stress
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Comparison with twin in Al [strain in (111) plane]
-0.020 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020
-3.696
-3.694
-3.692
-3.690
-3.688
-3.686
-3.684
-3.682
-3.680
-3.678
Tota
l ene
rgy
(eV/
atom
)strain in (111) plane
Al with twinAl FCC
It’s much easier to form twinning structure in Cu than in Al
-0.020 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020
-3.766
-3.764
-3.762
-3.760
-3.758
-3.756
-3.754
-3.752
-3.750
-3.748
Tota
l ene
rgy
(eV/
atom
)
strain in (111) plane
Cu with twinCu FCC
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Microstructure investigation by FIB
Di Xu et al., J. Appl. Phys. 105, 023521 (2009)
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Microstructure studies on nano-twinned Cu
• Nano-indentation study
• Nanotwin effect on Grain growth of Cu
• Thermal stability of (211) incoherent twin
• Electromigration study
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Nanotwin effect on hardness
bottom middle topL.Xu, K.N. Tu et al., Applied Physics Letters, 90, 033111 2007
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Nanotwin effect on hardness
Combination of EBSD & nanoindentation:To study effect of twin boundaries on hardness of Cu at different locations
Twin effects on mechanical strength:• The density of nanotwins•The twin spacing•Twin intersection with grain boundaries
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Current studies on nano-twinned Cu
• Nano-indentation study
• Nanotwin effect on grain growth of Cu– Grain growth at 200C for 1 hr
– Self annealing at RT for 1 year
• Thermal stability of (211) incoherent twin
• Electromigration study
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Grain growth and abnormal grain growth
Abnormal grain growth
1 year at RT
Grain growth at 200 C
2 um
DC electroplated samples with few twins after deposition
Xu D., Sriram V., Tu K.N. et al Microelectronics Engineering. 85(2008), 2155
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Grain growth and abnormal grain growthPulse electroplated samples with many twins after deposition
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Thermal stability of (111) and (112) twin boundary
Time=0, 10.5 nm Time=15 minutes, 9.31 nm, 1.19 nm moved
Time=45 minutes, 7.4 nm, 3.1 nm moved
Temperature=473K
The distance of the observed two 111 twin boundary is
2.72 nm, or about 13{111} spacing
Move of 112 twin boundary
111
111
111
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(112) Incoherent twin boundaries
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Possible (111) twin nucleation mechanism
Twin grows
Twin disappear
Stress at grain boundaries
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Current studies on nano-twinned Cu
• Nano-indentation study
• Nanotwin effect on grain growth of Cu
• Thermal stability of (211) incoherent twin
• Electromigration study– Electromigration test on IBM samples
– In situ TEM observation of 112 boundary motion under EM
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*j xZ e
23
e-
Electron wind force direction
Back stress direction
Al strip10 m
Critical product:
jeZkTDC
dxd
kTDCJ *
Higher yield stress could give longer critical length.
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Nanotwin effect on Electromigration (sample from IBM, Dr. C. K. Hu)
Power+ ‐
V
e‐Cu line
Segment 1
Segment 2
Segment 3
100 um
e‐
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Possible effect of twins on electromigration
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Summary
• Nanotwinned Cu thin films and interconnects have been produ-ced by pulse electroplating and illustrated by various characterization techniques
• Effect of stress on nanotwin boundary formation has been inve-stigated by first principles calculations, in situ stress measurements and microstructure characterization
• Effect of twin boundaries on hardness, grain growth and electromigration behavior have been studied by various measurement methods combined with microstructure characterization techniques
• Cu with high density of nanotwins can improve the mechanical properties and electromigration reliability of metal interconnects
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Thank you for your attention!
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