Index of /intel-research/silicon IITC 2002 Xu ppt

(1)

2002 IITC

Cohesive Strength Characterization of

Brittle Low-K Films

G. X u, J. He, E. A ndideh, J. Bielefeld, T. Scherban

Logic Technology Development

(2)

2002 IITC

Introduction-Motivation

• TM Characterization challenge

– Mature techniques available for hardness, modulus (indentation) and adhesion (4 point bending).

– Thin film cracking is an important reliability problem.

• New porous and low k ILDs are very susceptible to cracking • Tensile residual stress compare to compressive stress of

conventional SiO₂, SiOF.

– Need a technique to quantity cohesive energy and critical thickness for cracking.

• At early development stage thin blanket films are often the only ones available.

• Failure criteria for material screening/ selection.

(3)

2002 IITC

Introduction

• There will always be a trade-off between mechanical properties and dielectric constant.

0 10 20 30 40 50 60 70 80 0 5 10 15 20

2 2.5 3 3.5 4

Silicon oxide based dielectric materials

Modul u s (GPa) Ha rd n ess ( G P a)

Dielectric constant, k 0 10 20 30 40 50 60 70 80 0 5 10 15 20

2 2.5 3 3.5 4

Silicon oxide based dielectric materials

Modul u s (GPa) Ha rd n ess ( G P a)

(4)

2002 IITC

Introduction--Challenge

• Film residual stress becomes tensile for materials with lower modulus (and k) while conventional SiOx and SiOF have compressive residual stress. If not

properly comprehended, film is subjected to cracking!

-300 -200 -100 0 100

1 10 100 1000

Modulus (GPa)

In

tri

n

sic

F

ilm

S

tre

ss

e

s (MP

(5)

2002 IITC

Thin Film Characterization Techniques

• 4 Point Bending

– Basic technique used at Intel for interfacial adhesion energy

– Mix mode Ψ~45o

– Only get cohesion data when film is weaker than interfaces. – Need surface analysis to determine failure mode.

P/2 P/2

l

b

a₁ a₂

2h

3

2

h

Eb

16

l

P

)

1

(

21

(6)

2002 IITC

• Dual Cantilever Beam(DCB)

– Alternative technique for interface adhesion measurement. – Mainly Mode I.

– Only get cohesion data when film is weaker than interfaces. – Need surface analysis to determine failure mode.

Thin Film Characterization Techniques

3

2

h

Eb

a

P

)

1

(

12

G

=

−

ν

a P

P

2h

(7)

2002 IITC

Thin Film Characterization Techniques

• Channel Cracking

– Pure M ode I

– Guarantee cohesive failure mode

f

2 f f

2 f el

E

)

1

(

h

g

G

=

σ

−

ν

h_f

h_s

P/2 P/2

L

b

Crac k pro

pa

-gatio n dire

(8)

2002 IITC

Channel Cracking

G: Fracture Energy

g_el: depends on film-substrate elastic mismatch. Calculated by FEM

σ_f: Total film stress, including residual stress and applied stress

E_f, E_s: Elastic moduli of film and substrate

ν_f, ν _s:Poisson ratio of film and substrate

h_f, h_s: thickness of film and substrate

P: applied load

L: distance between inner pin and outer pin

b: sample width

s f 2

f 2 s 2

s 0

f

E

1

b

h

PL

3

ν

σ

−

+

=

Residual

Stress Applied stress through bending

(9)

2002 IITC

Channel Cracking

• Channel cracking measures cohesive fracture energy for ILD thin film.

– Fracture energy is measured for a range of crack propagation velocities.

1.E-08 1.E-07 1.E-06 1.E-05 1.E-04

1 1.5 2 2.5

Cohesive Fracture Energy, G (J/m2)

C

rack

V

el

o

cit

y

,

v

(m/

s) ILD1

(10)

2002 IITC

0 100 200 300

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Film Thickness, hf(µµµµm)

A ppl ie d S tr e s s (M P a) ILD1 ILD2

Critical Thickness, h_c

• Residual stress is different_! Contribution to G is

different

– ILD#1, σ₀=60MPa; ILD#2, σ₀=70MPa

• External (applied) stress is a better metric of reliability performance.

Concern: Residual Stress

(11)

2002 IITC

1.E-08 1.E-07 1.E-06 1.E-05 1.E-04

20 25 30 35 40 45

Cohesive Strength @1.5µm(MPa)

Cr

ac

k

V

elo

city

,

v

(m

/s

)

ILD2

ILD1

New Metric--Cohesive Strength

• Applied stress has to be normalized to fixed film

thickness(ex. 1.5µm) to compare films of different

thickness.

• Cohesive strength-- the new metric for ILD ranking in

terms of the maximum external stress the film can sustain before cracking.

0 f

f m

5 . 1

m

5

.

1

h

σ

µ

σ

(12)

2002 IITC

1.E-08 1.E-07 1.E-06 1.E-05

30 40 50 60

Cohesive Strength @1.5um(MPa)

C

rack V

el

o

ci

ty(

m

/s)

30% RH

0% RH

Application of Channel Cracking

• Critical thickness h_c can be predicted

– the cracking threshold thickness in absence of external stress – maximum film thickness that can be deposited before cracking.

• Environmental effect on film property, eg. humidity – Silica based low-k material is subject to stress corrosion

2 f 2

f c

0

h

σ

(13)

2002 IITC

0 20 40 60 80

0 5 10 15 20

C

o

h

esi

ve S

tr

e

ng

th

(M

P

a

)

Cohesive Strength & E, k

• Cohesive strength is found to linearly increase with film modulus.

• Cohesive strength will decrease as k decreases — Trade-off between mechanical properties and performance always exists.

(14)

2002 IITC

Summary

• A new thin film characterization metric “ Cohesive Strength” is defined.

• “ Cohesive Strength” is measured by the channel cracking technique and is defined as the external

stress applied through bending at fixed film thickness and fixed crack velocity.

• Can predict critical thickness h_c—the maximum

deposited thickness before the films cracks from its own intrinsic stress.