Microsoft PowerPoint - Electromigration

© D.K. Schroder, Semiconductor Characterization

ElectromigrationElectromigration


Failure TimesFailure TimesConsider n products that fail after successively longer times t1, t2, t3….tnThe mean time to failure, MTTF, is defined as

MTTF is most commonly used to describe or model physical failure times.The median time to failure, t50, is the time when 50% have failed. It is most commonly used in the statistical or mathematical analysis of failure distributions.The mean time between failures, MTBF, is defined as

nttttMTTF n....321 +++

=

nttttttMTBF nn )....()()( 12312 −−+−+−

=


Acceleration FactorsAcceleration FactorsFailure measurements under normal operating conditions take very long timesMany physical/chemical processes are temperature dependent; follow Arrhenius equationAcceleration factor

For EA = 1 eV, T1 = 250°C and T0 = 30°C ⇒ AFT = 107, by increasing the temperature from 30°C to 250°C, the time is reduced by 107. Instead of 10 years, test takes 30 seconds !For voltage acceleration

)/exp()( kTEATt A=

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−===

101

0

1

0 11exp)/exp()/exp(

)()(

TTkE

kTEkTE

TtTtAF A

A

AT

)exp()( VBVt γ−=

( ))(exp)()(

011

0 VVVtVtAFV −== γ


Electromigration (EM)Electromigration (EM)Electromigration is the motion of atoms in response to the electron “wind”, i.e., momentum transfer with the electrons pushing the atomsIt can lead to circuit failure through metal line resistance increase or in the extreme the line opens

J = 23 MA/cm2, T = 160°C

Electron Motion Void

Extrusionor Hillock Metal Line

Image courtesy of T. Alford, Arizona State University


ElectromigrationElectromigrationMotion of atoms dominated by changes in mobility, e.g., at grain boundariesCollision of electrons with atoms ⇒ momentum transfer

Grain BoundaryTriple Point

CurrentFlow

ElectronMotion

Void startsat triplepoint

Grain

Micrographs courtesy of P. Nguyen, Arizona State University

Grain BoundaryGrain

1 µm


Median TimeMedian Time--ToTo--Failure, Failure, tt5050Stress a number of test lines to failure

Produce a resistance increase of x% (e.g., x = 2, 10)Plot time-to-failure vs. cumulative % failure Determine t50 as a function of temperature and current density

A = constant, J = current density, n = 1- 3; EA=activation energy(EA ≈ 0.4-0.6 eV for Al; 0.8-0.9 eV for Al/2% Cu)

nA

JkTEAt )/exp(

50 =

F.M. d’Heurle, “Electromigration and Failure in Electronics: An Introduction,” Proc. IEEE 59, 1409-1418, Oct. 1971.

Black’s Equation


ElectromigrationElectromigration

0 10 20 30 40.01.115

102030507080909599

99.999.99

Failure Time (h)

Cum

ulat

ive

% F

ailu

ret50

10-1

100

101

102

1.4 1.6 1.8 2 2.2 2.4 2.6

t 50

(h)

2.5 µm Line Width

1000/T (K-1)

Al / 2% Cu

Al / 0.5% Cu

Al

10-1

100

101

102

105 106 107

t 50 (

h)

0.7 µm Line Width

Current Density (A/cm2)

Al / 0.5% Cu


Effect of CopperEffect of CopperGrain boundaries/triple points are the weak points in Al metal linesStrengthen them by adding impurities, e.g., CuCu by itself is even better

I. Ames, F. M. d'Heurle, and R. E. Horstmann, IBM J. Res. Dev. 14, 461-463, July 1970.

D. Edelstein et al., IEEE IEDM, 1997, p. 773

0.1 1 10 102 103.01.11510

2030507080909599

99.999.99

Failure Time (h)

Cum

ulat

ive

% F

ailu

re

Al/Cu Cu

0.3 µm wide2.5 MA/cm2

1 10 102 103.01.11510

2030507080909599

99.999.99

Failure Time (h)

Cum

ulat

ive

% F

ailu

re

Al Al - 4% Cu


Test StructuresTest StructuresElectromigration test structures:

1, 2, 7, 8; 10, 3, 6, 14; 9, 10, 15, 16

Van der Pauw test structure: 2, 3, 10, 11Test lines should be at least 800 µm long (R ≈ 20-30 Ω); otherwise develop temperature gradients along the linesSome test structures contain extrusion detectors

I V V IExtrusion (short)detector

1 2 3 4 5 6 7 8

9 10 11 12 13 14 15 16


Effect of Line Length / WidthEffect of Line Length / WidthFor short lines: atoms migrate due to electron push, then the atoms flow back due to stress gradientBelow a critical length (Blechlength) mass transport vanishes

Be careful in choosing correct length for electromigration measurements

For narrow lines: triple points disappear and bamboo structure is formedFailure time increases

C.K. Hu et al., IBM J. Res. Dev. 39, 465-497, July 1995; C.V. Thompson and H. Kahn, J. Electron. Mat. 22, 581, June 1993.

0

10

20

30

40

50

0 100 200 300 400 500 600

∆R

(Ohm

s)

Line length 25 µm

Time (h)

50 µm

100 µm Al/2% Cu/3% SiJ = 1.2x106 A/cm2

1

10

102

103

0 0.5 1 1.5 2

t 50

(h)

Al/2% Cu

Line Width (µm)

T=305CJ=2.75x106 A/cm2

Bamboo

Near Bamboo


Contact (Via) ElectromigrationContact (Via) Electromigration

Contact or viaelectromigration is an important electromigration failure mechanism

Different materials e.g., W plugs/Al linesAl electromigrates and is not re-supplied by W

Metal Depletion

Current Flow

CurrentCrowdingMetal 1

Barrier

Electron Flow

M. Tada et al., IEEE Trans. Electron Dev. 54, 1867, 2007.

Cu

Cu

Ta/TaN

Metal 2


Via EM Test StructureVia EM Test StructureForce current through viaChanging current direction allows upper and lower interfaces to be probedCan combine with poly-Si resistance heater to change temperature

I

I

V

V

Lower LevelMetal

Upper LevelMetal

Via

IElectrons

Tungsten Plug

Stresses Lower Interface

ElectronsFIB Cut

FIB Cut


Contact (Via) ElectromigrationContact (Via) ElectromigrationCurrent continues to flow through the Ti/TiN and Al3Ti layers when there is a void, but the resistance increases

TEM Micrographs Courtesy of T.S. Sriram and E. Piccioli, Compaq Computer Corporation

Ti/TiN ARC

Al3Ti

Al

Liner

W plug

Void Al

GrainBoundary

0.5 µmVoid

Electrons

Electron Flow

Al

AlW Plug

Al Depletion

Al Accumulation

Electrons


Review QuestionsReview QuestionsWhat is electromigration?What is a triple point?What is Black’s equation?Why does the electromigration lifetime depend on line length and width?What is contact electromigration?Why does a small percentage copper in aluminum line strengthen the Al line?Why is Cu more electromigration resistant than Al?

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Microsoft PowerPoint - Electromigration