Strength and Fracture Standards at Micro and NanoScales Workshop

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Fatigue and Tensile Properties of Thin Films through Electrical Testing

R. R. Keller and N. Barbosa IIINational Institute of Standards and Technology,

Materials Reliability DivisionBoulder, Colorado, U.S.A.

http://www.boulder.nist.gov/div853

• Why an Electrical Test?• Measurement Principle• Microstructure• Fatigue Lifetimes• Strength

Acknowledgements:NIST Office of Microelectronics Programs

NRC Research Associateship ProgramR. Mönig, Forschungszentrum Karlsruhe, Germany

C. A. Volkert, U. Göttingen, GermanyD. T. Read and R. Geiss, NIST

Strength and Fracture Standards at Micro and NanoScales Workshop

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Why Develop an Electrical Test?

• Method for testing “real” structures:

150 nm-wide Cu, Ag(NIST, Sematech)

• Testing such structures is difficult

• Special specimen requirements for more common methods usually don’t match actual conditions.

Airgap microprocessorCourtesy of International Business Machines Corporation. Unauthorized use not permitted. 5/07.

Strength and Fracture Standards at Micro and NanoScales Workshop

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Measurement Principle: Cyclic Joule Heating

• Four-point probe method• Low frequency, high alternating currents• No electromigration• Controlled joule heating• Thermal mismatch ⇒ cyclic strain!

Reference: R. Mönig, R. R. Keller, C. A. Volkert, Rev. Sci. Instrum. 75, 4997 (2004).

t

σ

tj

t

T

100 Hz

200 Hz

200 Hz

(100 to 200) °C

(0 to 100) °C

(100 to 200) MPa

∆εthermal = ∆α ∆T(between metal and substrate)

(5 to 30) MA/cm2

i

i

V

VSi

Metal

Strength and Fracture Standards at Micro and NanoScales Workshop

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What about Non-Conducting Films?

• Use a proximity approach:

Thermal cycling occurs in film that doesn’t carry current.

Not yet demonstrated on non-conductors, though

Strength and Fracture Standards at Micro and NanoScales Workshop

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Temperature from Resistance

0( ) dRR T R TdT

= + ∆

• R vs. T (calibration):

( )0( )R T RT

dRdT

−⇒ ∆ =

⎛ ⎞⎜ ⎟⎝ ⎠

V and i vs. time: T vs. time:

Strength and Fracture Standards at Micro and NanoScales Workshop

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Fatigue Lifetime Curves - Patterned Cu Lines

200

300

400

500

600

700

800

900

1,000

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

AC load plus unload cycles to failure

Tem

pera

ture

rang

e, C

Electroplated, damascene, narrowPVD (e-beam), wide, uncovered

Strength and Fracture Standards at Micro and NanoScales Workshop

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Fatigue Lifetime Curves

Strain Amplitude

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

0 1 2 3 4 5 6 7 8

Log(reversals to failure)

Stra

in a

mpl

itude

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 1 2 3 4 5 6 7 8 9

Log(reversals to failure)

Stre

ss a

mpl

itude

, GP

a

Stress Amplitude

Strength and Fracture Standards at Micro and NanoScales Workshop

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Estimates of Thin Film Ultimate Strength

Ultimate strength:• Use Basquin equation, where

σa = stress amplitudeE = Young’s modulus∆εe = elastic strain rangeσ′f = fatigue strength coefficient

(~ ultimate strength, σUTS)b = fatigue strength exponentNf = number of reversals to

failure

( )' 22

bea f

EN

εσ σ

∆= =

~ σUTS

f

Strength and Fracture Standards at Micro and NanoScales Workshop

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UTS Measurements on Aluminum

0

20

40

60

80

100

120

140

160

180

200

1000 10000 100000 10000002Nf

σ [M

Pa]

σm

σa

Extrapolate to one reversal to estimate strength

σUTS(Al) = 250 ± 40 MPa electrical

σUTS(Al) = 239 ± 4 MPa microtensile

Tests performed on specimens fabricated on same wafer

Approach needs more qualification

N. Barbosa III, R. R. Keller, D. T. Read, R. H. Geiss, and R. P. VinciMet. Mater. Trans. A, 38A, 2160 (2007)

Strength and Fracture Standards at Micro and NanoScales Workshop

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Microstructural Changes

• SEM/EBSD, test for t1, SEM/EBSD, test for t2, …• Al-1Si, t = 0.5 µm, w = 3.3 µm• jrms = 12 MA/cm2 ⇒ ∆T ≈ 200 °C, ∆ε ≈ 0.4 %• ti = 0, 10, 20, 40, 80, 160, 320, 697 s

0 s:

80 s:

320 s:

Strength and Fracture Standards at Micro and NanoScales Workshop

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Commentary: Standardization of Small-Scale Tests

• Concern:– Different methods usually require different processes– For thin films, even slight variations in processing can cause

dramatic changes in microstructure (& properties)

• Suggested Action:– Include microstructural parameters when reporting results

• Concern:– Even with films from same wafer, different specimen needs can

change mechanical constraint• Suggested Action:

– Exercise caution when comparing one type of test to another

Strength and Fracture Standards at Micro and NanoScales Workshop

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Summary

• An electrical method for applying cyclic thermal strain to patterned thin films on substrates has been developed.

• This approach does not require special specimen geometries.– wide range of structure dimensions;– buried structures → properties under constraint;

• Test naturally provides fatigue lifetime data

• Ultimate strength of patterned thin films can be estimated through use of modified Basquin equation.

• Test method requires more demonstration and refinement on non-conductors, unusual film patterns, buried structures

Contact information:bob.keller@nist.gov