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.