Effect of Back-Etching on the residual stress in PZT(30/70) film
Tomoya Ohno, Takeshi Matsuda(Kitami Institute of Technology, Japan)
Babara Malič, Marija Kosec(Jožef Stefan Institute, Slovenia)
Naoki Wakiya, Hisao Suzuki(Shizuoka University, Japan)
Outline
Backgroundabout stress engineeringour concepts
Experimental ProcedureBack-etching processAlkoxide derived PZT
Results & DiscussionsEffect of the back-etching depth on the residual stress and electrical propertiesEffect of the Si thickness on the residual stress and electrical properties
Conclusions
Introduction(1)
20 40 600
2000
4000
6000
8000
10000
PZT(0
02)&
(200)
PZT(0
01)&
(100)
PZT(1
11)
Inte
nsi
ty
2θ
Microstructure Crystal Orientation
-400 -200 0 200 400-60-40-20
0204060
Electric Field (kV/cm)
Pola
rizat
ion
(μC
/cm
2 )103 104 1050
200400600800
10001200
ε"
ε'
diel
ectri
c co
nsta
nt (-
)
Frequency (Hz)-200 -100 0 100 200
-400
-200
0
200
400
Electric Field (kV/cm)
d33
(pm
/V)
Dielectric Property Ferroelectric Property Piezoelectric Property
Residual Stress (Lattice Strain)
Introduction(2)
0 100 200 300 400 500 6000
100
200
300
400
500
600
CaBi4Ti4O15KNbO3
(K,Na)NbO3
(Bi,Na,K)TiO3BaTiO3
conventional lead free materials
d 33 c
onst
ant (
pC/N
)
Curie Temperature (C)
KNN-LiTaO3
KNN-LiTaO3-LiSbO3
PZT familyOur Approach
- Ferroelectric Materials -
Fujito et al. Jpn. J. Appl. Phys., 44 (2005) 6900
Residual stress
Negative Factor for electrical properties
How to release this negative factor ?
But…..
Back Ground-Stress Engineering –
K. J. Choi et al. Science 306 (2004) 1005
P-E hysteresis loops of BTO thin film and single crystal
In the case of compressive stress…..
Positive factor for electrical properties
How to control (induce) the stress to film
Stress Engineering (Strain Engineering)
Our concepts for stress engineering
Single crystal Commercial Si substrate(cost effective process)
Relaxation of the huge tensile stress from Si substrate
Back-etching processUsing thin Si substrateBuffer layer structure ?
1.
2.
Our Previous Results (PTO thin film)
0 100 200 300 400 500
0.8
1.0
1.2
1.4
1.6
1.8
Res
idua
l Stre
ss (G
Pa)
The residual Thickness of Si substrate (μm)
The residual stress in PT thin film as a function of the residual thickness of Si substrate
The residual thickness of Si = 20μm
T.Ohno et al. Jpn . J. Appl. Phys., 43, 6549 (2004) The residual thickness of Si = 160μm
500μm Si
Our Previous Results ((100) oriented PZT(30/70) thin film)
Change in the residual stress as a function of the TSi
0 50 100 150 200 2500.2
0.4
0.6
0.8
1.0
1.2
Res
idua
l Stre
ss (G
Pa)
Residual Si Thickness (μm)
Back-etched part Un-etched part
0 100 200 300 400 500
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Res
idua
l Stre
ss (G
Pa)
Residual Si Thickness (μm)
PTO (100) - PZT(30/70)
In this study.....
PTO film
(100) oriented PZT30 film
Previous Study
(111) oriented PZT30 film
Effect of the crystal orientation degree
on the residual stress
Back-etched substrate
Back-etched substrate Back-etched substrate
(100) oriented PZT30 film
Si substrate with different thick
This study
No stress distribution cases
Case 1
Case 2
Si Wafer
Surface OxidationN2 : 1.0(l/min)
Wet O2 : 1.0(l/min)Dry O2 : 1.0(l/min)
at 1100℃ for 5hr
Patterning
Remove the oxidized layer by BHF
Etching the Si substrate by TMAH (22wt%)
Remove the resist layer
BHF solution
46wt% HF 40wt% NH4F
HF:NH4F=1:6
Tetramethylammonium Hydroxide (TMAH)
Si (100)Oxidation layer
Resist
(840nm/min at 95 C)
Acetone (super sonic wave)
(1000nm/min at R.T.)
Back Etching process (wet chemical etching)
Experimental Procedure
Pb(OCOCH3)2 Absolute Ethanol
Lead Precursor Solution
Lead Zirconate Titanate Precursor Solution
Reflux (78℃ NH3 3hr)
Acetyl acetoneReflux (78℃ 1hr)
Reflux (78℃ 3hr)
Ti(iso-OC3H7)4
Absolute Ethanol
Zr(n-propoxide)
Zr-Ti Precursor SolutionReflux (78℃ 4hr)
Spin coating on Pt/Si subDrying (115℃/10min) n- cycles
Pre-annealing(350℃/10min)Rapid Thermal Annealing (600℃/5min)
Measurements
Si Pt (200nm)
AuPZT(120/30/70)
Back-etching part
Unetched part
Dielectric property
Ferroelectric property
XRD analysis
3mm*3mm
500μm
Case 1 back-etched substrate
20 30 40 50 60
<001
>&<1
00>
<110
>
Pt<1
11>
<111
>
<002
>&<2
00>
<112
>&<2
11>
Inte
nsity
(a.u
.)
2θ (deg.)
XRD Analysis
20 30 40 50 60
<001
>&<1
00>
<110
>
Pt<1
11>
<111
>
<002
>&<2
00>
<112
>&<2
11>
Inte
nsity
(a.u
.)
2θ (deg.)
0 50 100 150 2000
5
10
15
70
80
90
100
(111) (110) (001)&(100)
TSi (μm)
%
TSi=180μm
Change in the crystal orientation as a function of the etching depth
TSi=25μm
Case 1 back-etched substrate
0 50 100 150 2000
200
400
600
800
1000
1kHz
backetching part un-etched part
diel
ectri
c co
nsta
nt (-
)
TSi (μm)
102 103 104 1050
200
400
600
800
1000
Dielectric Property
1kHz backetching part un-etched part
ε"
ε'
diel
ectri
c co
nsta
nt (-
)
Frequency (Hz)
102 103 104 1050
200
400
600
800
1000
1kHz
backetching part un-etched part
ε"
ε'
diel
ectri
c co
nsta
nt (-
)
Frequency (Hz)
TSi=25μm
TSi=180μm
Change in the dielectric constant as a function of the etching depth (1kHz)
Case 1 back-etched substrate
Ferroelectric Property
-200 -100 0 100 200
-40
-20
0
20
40 back-etching part un-etched part
Electric Field (kV/cm)
Pola
rizat
ion
(μC
/cm
2 )
-200 -100 0 100 200
-40
-20
0
20
40 back-etching part un-etched part
Electric Field (kV/cm)
Pola
rizat
ion
(μC
/cm
2 )
TSi=180μm
0 50 100 150 200
40
60
80
100
back-etching part un-etched part
TSi (μm)
Ec (k
V/c
m)
TSi=25μm
0 50 100 150 20015
20
25
30
35
40
45
Ps
Pr
back-etching part un-etched part
back-etching part un-etched part
TSi (μm)
Pr &
Ps (μC
/cm
2 )
Change in the ferroelectric properties as a function of the etching depth
Case 1 back-etched substrate
Effect of the crystal orientation degree on the electrical properties in PZT film on back-etched substrate
0 50 100 150 200
600
800
1000
12001kHz
back-etching part Unetched part
diel
ectri
c co
nsta
nt (-
)
TSi (μm)
0 50 100 150 200 25020
25
30
35
40 Back-etched part Un-etched part
2Pr (μC
/cm
2 )
TSi (μm)
0 50 100 150 2000
200
400
600
800
10001kHz
backetching part un-etched part
diel
ectri
c co
nsta
nt (-
)
TSi (μm)
0 50 100 150 20030
35
40
45
50
55
60
back-etching part un-etched part
TSi (μm)
2Pr (μC
/cm
2 )
(100)&(001) oriented PZT film (111) oriented PZT film
Dielectric Property
Ferroelectric Property
Discussion
0 50 100 150 2000
5
10
15
70
80
90
100
(111) (110) (001)&(100)
TSi (μm)
%
Change in the crystal orientation as a function of the etching depth
0 50 100 150 20015
20
25
30
35
40
45
Ps
Pr
back-etching part unetched part
back-etching part unetched part
TSi (μm)
Pr &
Ps (μC
/cm
2 )
Change in the ferroelectric properties as a function of the etching depth
(111) Orientatied grains
Increase
(110) Orientated grains
Decrease
Improvement of the Ferroelectricity
Why did the remanent polarization increase with decreasing TSi ?
Increasing of the Polarization component
(001)>(111)>(110)
(001)
(111)
The polarization of the (111) orientation is about 57 % for the (001) orientation
(110)
Polarization(Tetragonal)
0 50 100 150 200 2500.2
0.4
0.6
0.8
1.0
1.2
Res
idua
l Stre
ss (G
Pa)
Residual Si Thickness (μm)
Back-etched part Un-etched part
Why the electrical properties decrease?
Existence of the stress distribution between the un-etched part and the back-etched part ?
Damage to the microstructure?
Decrease the electrical properties, even in larger compressive stress which leads to the higher electrical properties
- Our assumption -
How to solve this problem?
Compressive stress
Stress distribution in a filmBut….
Improvement of the electrical properties ?
Decrease the electrical properties
If we use the thin Si substrate
Large compressive residual stress
No stress distribution
We can expect the improvement of the electrical properties by stress engineering
XRD analysis
94 96 98 100 102 104
0
25
50
75
100
125
150
175
200
PZT(
30/7
0) (0
04)
PZT(
30/7
0) (4
00)
Inte
nsi
ty
2θ
20 40 60
Case 2 different substrate thick
50 μm100 μm200 μm300 μm400 μm500 μm
LNO(200)
PZT(001)&(100)
PZT(101)&(110)LNO(100)
PZT(002)&(200)
2θ(deg.)
(004) : 10 %, (400) : 90 %
XRD profile of PZT30 thin film on Si with 50 μm thick
Change in the XRD profiles of PZT30 film with different Si thick
Change in the residual stress with residual Si thickness
0 50 100 150 200 2500.2
0.4
0.6
0.8
1.0
1.2
Res
idua
l com
pres
sive
stre
ss (G
Pa)
Residual Si Thickness (μm)
Back-etched part Un-etched part
0 100 200 300 400 5000.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Res
idua
l com
pres
sive
stre
ss (G
Pa)
Residual Si Thickness (μm)
from XRD from Raman
Back-etched Si sunstratesSi substrates with different thick
When the substrate thick decreased, then the compressive residual stress increase, because the constarin force from the substrate reduce.
Dielectric Properties
102 103 1040
200400600800
100012001400160018002000
▽PZT/LNO/Si(50μm)□PZT/LNO/Si(100μm)◇PZT/LNO/Si(200μm)×PZT/LNO/Si(300μm)△PZT/LNO/Si(400μm)○PZT/LNO/Si(500μm)
ε"
ε'
diel
ectri
c co
nsta
nt (ε
)
Frequency (Hz)
0 100 200 300 400 500
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0 100 200 300 400 5001000
1100
1200
1300
1400
1500
tan δ
(-)
ε at 1
kHz
(-)
Thickness of Si wafer (μm)
Case 2 different substrate thick
Change in the dielectric constant as a function of the thickness of Si substrate
Ferroelectric Properties
-600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600
-80
-60
-40
-20
0
20
40
60
80
○PZT/LNO/Si(500μm)
○PZT/LNO/Si(400μm)○PZT/LNO/Si(300μm)○PZT/LNO/Si(200μm)○PZT/LNO/Si(100μm)○PZT/LNO/Si(50μm)
Electric Field (kV/cm)
Pola
rizat
ion
(μC
/cm
2 )
0 100 200 300 400 500
20
30
40
50
60
60
80
100
120
140
160
Thickness of Si wafer (μm)
2*Pr
(μC
/cm
2 )
2*Ps
(μC
/cm
2 )
Case 2 different substrate thick
-600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600
-80
-60
-40
-20
0
20
40
60
80
0.32 GPa (compressive)
0.96 GPa (compressive)PZT30/LNO/Si (50μm)
PZT30/LNO/Si (500μm)
Electric Field (kV/cm)
Pola
rizat
ion
(μC
/cm
2 )
Change in the remanent polarization as a function of the thickness of Si substrate
Conclusions
Residual Stress in thin film was successfully controlled by the substrate thick
Residual compressive stress leads to increase the electrical properties
Existence of the stress distribution in a film leads to decrease the electrical properties, even if film has large compressive stress.