Index of /intel-research/silicon IITC 2002 Xu ppt
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
Intel Corporation
R
1
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.
2002 IITC
• New porous and low k ILDs are very susceptible to cracking
• Tensile residual stress compare to compressive stress of
conventional SiO 2, 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.
• Need quick turn measurements for process improvement and
monitor.
R
2
Introduction
• There will always be a trade-off between mechanical
properties and dielectric constant.
Silicon oxide based dielectric materials
20
2002 IITC
80
15
60
50
10
40
30
5
20
10
0
2
2.5
3
3.5
Dielectric constant, k
R
3
4
0
Hardness (GPa)
Modulus (GPa)
70
Introduction--Challenge
Intrinsic Film Stresses (MPa)
2002 IITC
• 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!
100
0
-100
-200
-300
1
10
100
Modulus (GPa)
R
4
1000
Thin Film Characterization Techniques
• 4 Point Bending
P/2
P/2
2002 IITC
2h
a1
a2
b
G=
–
–
–
–
21( 1 −ν )P l
2
2 2
l
2 3
16Eb h
Basic technique used at Intel for interfacial adhesion energy
o
Mix mode Ψ~45
Only get cohesion data when film is weaker than interfaces.
Need surface analysis to determine failure mode.
R
5
Thin Film Characterization Techniques
• Dual Cantilever Beam(DCB)
P
2h
2002 IITC
a
b
P
G =
12( 1 −ν )P a
2
2 2
Eb2 h 3
– 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.
R
6
Thin Film Characterization Techniques
• Channel Cracking
P/2
P/2
2002 IITC
pa tion
o
r
k p direc
c
a
Cr tion
ga
hf
hs
b
L
G = g el
σ 2f h f ( 1 −ν 2f )
Ef
– Pure M ode I
– Guarantee cohesive failure mode
R
7
Channel Cracking
3 PL 1 − ν s2 E f
Total film stress σ f = σ 0 + 2
h s b 1 − ν 2f E s
2002 IITC
Residual
Stress
Applied stress
through bending
G: Fracture Energy
gel: depends on film-substrate elastic mismatch. Calculated by FEM
σf: Total film stress, including residual stress and applied stress
Ef, Es: Elastic moduli of film and substrate
νf, ν s: Poisson ratio of film and substrate
hf, hs: thickness of film and substrate
P: applied load
L: distance between inner pin and outer pin
b: sample width
R
8
Channel Cracking
• Channel cracking measures cohesive fracture energy
for ILD thin film.
– Fracture energy is measured for a range of crack propagation
velocities.
Crack Velocity, v (m/s)
2002 IITC
1.E-04
ILD1
1.E-05
ILD2
1.E-06
1.E-07
1.E-08
1
1.5
2
Cohesive Fracture Energy, G (J/m2)
R
9
2.5
Concern: Residual Stress
• Residual stress is different! Contribution to G is
different
– ILD#1, σ0=60MPa; ILD#2, σ0=70MPa
2002 IITC
• External (applied) stress is a better metric of reliability
performance.
300
σ applied = σ f − σ 0
( 1 − ν 2f ) g el
1
−σ0
hf
Applied Stress(MPa)
=
GE f
ILD1
Constants (Film
properties)
ILD2
200
Critical Thickness, hc
100
0
0.0
0.5
1.0
1.5
2.0
2.5
Film Thickness, hf(µ
µm)
R
10
3.0
3.5
4.0
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
σ 1.5 µm = σ f
hf
1.5 µm
−σ0
Crack Velocity, v (m/s)
2002 IITC
terms of the maximum external stress the film can
sustain before cracking.1.E-04
ILD1
1.E-05
ILD2
1.E-06
1.E-07
1.E-08
20
25
30
35
40
Cohesive Strength @1.5µm(MPa)
R
11
45
Application of Channel Cracking
• Critical thickness hc can be predicted
– the cracking threshold thickness in absence of external stress
– maximum film thickness that can be deposited before cracking.
σ0
hc = h f 2
σf
2002 IITC
2
• Environmental effect on film property, eg. humidity
– Silica based low-k material is subject to stress corrosion
Crack Velocity(m/s)
1.E-05
30% RH
1.E-06
0% RH
1.E-07
1.E-08
30
40
50
Cohesive Strength @1.5um(MPa)
R
12
60
Cohesive Strength & E, k
2002 IITC
• 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.
• Need to find the sweet spot to meet performance
requirements as well as reliability requirements.
Cohesive Strength(MPa)
80
60
40
20
0
0
5
10
E (GPa)
R
13
15
20
Summary
2002 IITC
• 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 hc—the maximum
deposited thickness before the films cracks from its
own intrinsic stress.
• Humidity effects on cracking threshold may be
studied with channel cracking.
R
14
Cohesive Strength Characterization of
Brittle Low-K Films
G. X u, J. He, E. A ndideh, J. Bielefeld, T. Scherban
Logic Technology Development
Intel Corporation
R
1
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.
2002 IITC
• New porous and low k ILDs are very susceptible to cracking
• Tensile residual stress compare to compressive stress of
conventional SiO 2, 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.
• Need quick turn measurements for process improvement and
monitor.
R
2
Introduction
• There will always be a trade-off between mechanical
properties and dielectric constant.
Silicon oxide based dielectric materials
20
2002 IITC
80
15
60
50
10
40
30
5
20
10
0
2
2.5
3
3.5
Dielectric constant, k
R
3
4
0
Hardness (GPa)
Modulus (GPa)
70
Introduction--Challenge
Intrinsic Film Stresses (MPa)
2002 IITC
• 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!
100
0
-100
-200
-300
1
10
100
Modulus (GPa)
R
4
1000
Thin Film Characterization Techniques
• 4 Point Bending
P/2
P/2
2002 IITC
2h
a1
a2
b
G=
–
–
–
–
21( 1 −ν )P l
2
2 2
l
2 3
16Eb h
Basic technique used at Intel for interfacial adhesion energy
o
Mix mode Ψ~45
Only get cohesion data when film is weaker than interfaces.
Need surface analysis to determine failure mode.
R
5
Thin Film Characterization Techniques
• Dual Cantilever Beam(DCB)
P
2h
2002 IITC
a
b
P
G =
12( 1 −ν )P a
2
2 2
Eb2 h 3
– 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.
R
6
Thin Film Characterization Techniques
• Channel Cracking
P/2
P/2
2002 IITC
pa tion
o
r
k p direc
c
a
Cr tion
ga
hf
hs
b
L
G = g el
σ 2f h f ( 1 −ν 2f )
Ef
– Pure M ode I
– Guarantee cohesive failure mode
R
7
Channel Cracking
3 PL 1 − ν s2 E f
Total film stress σ f = σ 0 + 2
h s b 1 − ν 2f E s
2002 IITC
Residual
Stress
Applied stress
through bending
G: Fracture Energy
gel: depends on film-substrate elastic mismatch. Calculated by FEM
σf: Total film stress, including residual stress and applied stress
Ef, Es: Elastic moduli of film and substrate
νf, ν s: Poisson ratio of film and substrate
hf, hs: thickness of film and substrate
P: applied load
L: distance between inner pin and outer pin
b: sample width
R
8
Channel Cracking
• Channel cracking measures cohesive fracture energy
for ILD thin film.
– Fracture energy is measured for a range of crack propagation
velocities.
Crack Velocity, v (m/s)
2002 IITC
1.E-04
ILD1
1.E-05
ILD2
1.E-06
1.E-07
1.E-08
1
1.5
2
Cohesive Fracture Energy, G (J/m2)
R
9
2.5
Concern: Residual Stress
• Residual stress is different! Contribution to G is
different
– ILD#1, σ0=60MPa; ILD#2, σ0=70MPa
2002 IITC
• External (applied) stress is a better metric of reliability
performance.
300
σ applied = σ f − σ 0
( 1 − ν 2f ) g el
1
−σ0
hf
Applied Stress(MPa)
=
GE f
ILD1
Constants (Film
properties)
ILD2
200
Critical Thickness, hc
100
0
0.0
0.5
1.0
1.5
2.0
2.5
Film Thickness, hf(µ
µm)
R
10
3.0
3.5
4.0
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
σ 1.5 µm = σ f
hf
1.5 µm
−σ0
Crack Velocity, v (m/s)
2002 IITC
terms of the maximum external stress the film can
sustain before cracking.1.E-04
ILD1
1.E-05
ILD2
1.E-06
1.E-07
1.E-08
20
25
30
35
40
Cohesive Strength @1.5µm(MPa)
R
11
45
Application of Channel Cracking
• Critical thickness hc can be predicted
– the cracking threshold thickness in absence of external stress
– maximum film thickness that can be deposited before cracking.
σ0
hc = h f 2
σf
2002 IITC
2
• Environmental effect on film property, eg. humidity
– Silica based low-k material is subject to stress corrosion
Crack Velocity(m/s)
1.E-05
30% RH
1.E-06
0% RH
1.E-07
1.E-08
30
40
50
Cohesive Strength @1.5um(MPa)
R
12
60
Cohesive Strength & E, k
2002 IITC
• 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.
• Need to find the sweet spot to meet performance
requirements as well as reliability requirements.
Cohesive Strength(MPa)
80
60
40
20
0
0
5
10
E (GPa)
R
13
15
20
Summary
2002 IITC
• 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 hc—the maximum
deposited thickness before the films cracks from its
own intrinsic stress.
• Humidity effects on cracking threshold may be
studied with channel cracking.
R
14