Tunable Dielectric Properties of BST Thin Films

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Tunable Dielectric Properties of BST Thin
Films for RF/MW Passive Components
Jeffrey Bellotti, E.Koray Akdogan, and A.
Safari Rutgers University Department of Ceramic
and Materials Engineering Electroceramics
Group IFFF 2002 Nara, Japan 14th International
Symposium on Integrated Ferroelectrics
Acknowledgements This project is supported
by the Glenn N. Howatt Foundation. Wontae
Chang, Steve Kirchoefer, Jeffrey Pond -- U.S.
Naval Research Laboratory
Visit us at www.rci.rutgers.edu/ecerg
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Objectives
Fabricate low-loss frequency-agile paraelectric
thin films by Pulsed Laser Deposition (PLD) for
RF/microwave wireless communication devices by
controlling the composition, defect structure,
and metallization.
Processing (PLD)
Thin film structure
Microwave Dielectric Properties
The dielectric properties will be correlated with
the structural quality and the processing
conditions.
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Systems of Interest Design
Thin Film Compositions Ba(1-x)SrxTiO3 (x 0.4)
Substrates (All single crystal) LaAlO3
(Compressive strain) MgO (Tensile
strain) NdGaO3 (Compressive strain)
Pre-Annealed before deposition 1hr.
Vacuum 1hr. Flowing O2
GHz Interdigitated capacitor High Frequency
Measurements
Electrodes Cr/Ag/Au (IDC) Pt (Parallel plate)
Film Thickness 22 - 1150 nm
PLD with KrF Excimer Laser (248 nm) Deposition
Conditions 700-800 C, 100 mTorr, 2 J/cm2, 2
Hz, z 8.25 cm Post-anneal 500 C, 1 Atm O2
MHz Parallel plate capacitor Low Frequency
Measurements
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Interdigitated Capacitor Fabrication
Field-emission SEM
3 x 4 grid Interdigitated electrode
patterned with multilayer lift-off
process developed by the Naval Research
Laboratory. Device dimensions Gap size
6, 8, 10, and 12 ?m
Finger length 40, 60, and 80 ?m BST Film
Thickness 22 nm to 1150 nm Cross-section
FE-SEM shows uniform film coverage down to 22 nm.
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Rutherford Backscatter Spectroscopy
Ideal stoichiometry of Ba0.6Sr0.4TiO3 determined
from simulation within 5
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XRD Omega Scan with Area Detector
Width 40 In Omega
Film FWHM 0.40 /- 0.03
LA (012)
LA (024)
BST (100)
BST (200)
BST 60/40 film deposited on LaAlO3 Epitaxial
growth -- near perfect single crystal.
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Variation of BST Cell Constants with Film
Thickness
abulk 3.965 Å

• BST cell is body-centered
• tetragonal due to misfit strain.
• Most pronounced in thinner films.

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Evolution of Strain with Film Thickness on LaAlO3
Distortion of BST unit cell resulting in large
in-plane, compressive strain (up to -0.25)
which affects the dielectric properties of the
material ?Decreased permittivity and tunability
for BST on LaAlO3
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Variation of BST Cell Constants with Film
Thickness
abulk 3.965 Å

• BST cell shows pseudo-tetragonal
• distortion, where c-axis changes
• from in-plane to normal at 150 nm.

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Evolution of Strain with Film Thickness on MgO
Distortion of BST unit cell resulting in large
in-plane, tensile strain (up to 0.5) which
affects the dielectric properties of the material
?INCREASED permittivity for BST on MgO !!!
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MW Dielectric Measurements on BST 60/40 on LaAlO3
Capacitance and Q-Factor vs. DC Bias 1 to 20 GHz
LaAlO3 substrate
IDC dimensions 80 ?m fingers 6 ?m gap
Q-factor 4 - 20 for all films
Extracted ?r 50 - 1200
For thickest films capacitance decreases with
increasing DC bias Q-factor increases with
increasing DC bias. 825 nm film shows
tunability of 65 with 0 to 40 V DC bias.
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MW Dielectric Measurements on BST 60/40 on LaAlO3
Capacitance and Q-Factor vs. DC Bias 1 to 20 GHz
BST Film thickness 22 nm
LaAlO3 substrate
0 to 40 V DC bias
IDC dimensions 80 ?m fingers 6 ?m gap
With decreasing film thickness the effect of DC
bias becomes small.
22 nm film shows 3 tunability with 40 V bias,
compared to 65 at 1?m
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MW Dielectric Measurements on BST 60/40 on MgO
Capacitance and Q-Factor vs. DC Bias 1 to 20 GHz
BST Film thickness 825 nm
MgO substrate
0 to 40 V DC bias
Capacitance (pF)
Q-Factor
IDC dimensions 80 ?m fingers 6 ?m gap
Q-factor 4 - 20 for all films
Extracted ?r 1600 ? 1000
Frequency (GHz)
Capacitance decreases with increasing DC bias
Q-factor increases with increasing DC
bias. Thickest films have lower dielectric
constant (?r ) than thinner films. 825 nm film
shows tunability of 65 with 0 to 40 V DC bias.
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MW Dielectric Measurements on BST 60/40 on MgO
Capacitance and Q-Factor vs. DC Bias 1 to 20 GHz
BST Film thickness 22 nm
MgO substrate
0 to 40 V DC bias
Capacitance (pF)
Q-Factor
IDC dimensions 80 ?m fingers 6 ?m gap
Frequency (GHz)
For tensile strain in BST films on MgO, high
tunability is maintained for thinnest films, and
an increase in permittivity is observed.
22 nm film shows 30 tunability with 40 V bias,
compared to 65 at 1?m
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MW Dielectric Measurements on BST 60/40
Capacitance vs. DC Bias at10 GHz
BST on (100) LaAlO3
BST on (100) MgO
Capacitance (pF)
Capacitance (pF)
DC Bias (V)
DC Bias (V)
Capacitance vs. DC bias shows tunability
increases with increasing film thickness. Max.
BST 60/40 tunability 65 for thickest films BST
films on LaAlO3 show significantly less
tunability for very thin films compared to BST on
MgO. Due to the tensile strain in the BST films
deposited on MgO substrates, the tunability
remains high at 30 even at 22 nm.
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MW Dielectric Measurements on BST 60/40
In-Plane, Field-Induced Charge vs. E-Field with
Thickness
BST on (100) MgO
BST on (100) LaAlO3
1150 nm
1150 nm
400 nm
400 nm
22 nm
22 nm
Charge calculated from integration of C-V curves
shows typical paraelectric behavior. Charge
decreases with film thickness ? Lower
Polarization? Non-linearity becomes small for
thinner films lt 160 nm ? Size effects??? Polariza
tion yet to be computed. Electromagnetic field
modeling in progress to determine the effective
dielectric length and depth of penetration
of microwave field as a function of film
thickness.
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Thickness In-plane Strain Dependence of
Tunability and ?r
Frequency 10 GHz
BST on (100) LaAlO3
BST on (100) MgO
Tunability ()
Tunability ()
Average In-Plane Strain ()
Average In-Plane Strain ()
Thickness (nm)
Thickness (nm)
LaAlO3 series
MgO series
Tunability is constant with film thickness down
to 150 nm, followed by sharp decrease. Tunability
in-plane strain show complementary behavior
with decreasing film thickness.
Tunability increases with thickness and levels
off at around 800 nm. Tunability in-plane
strain show same behavior with increasing film
thickness.
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Dielectric Constant of BST on LaAlO3 and MgO
Substrates
Frequency 10 GHz
MgO series
Dielectric Constant
LaAlO3 series
Dielectric constant extracted using conformal
mapping methods proposed by S. Gevorgian.
Thickness (nm)
Permittivity of BST on LaAlO3 increases with
thickness as compressive strain decreases, and
levels off around 800 nm. For BST on MgO,
permittivity increases with decreasing film
thickness due to large in-plane tensile strain
causing large ionic displacements in unit cell.
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Conclusions
BST 60/40 grown epitaxially on (100) LaAlO3 and
(100) MgO by PLD from 22 nm to 1150 nm. Epitaxy
confirmed by XRD omega scans. RBS shows
stoichiometry to be ideal (within 5
insturmental resolution for BST on LaAlO3). BST
cubic lattice exhibits pseudo-tetragonal (4/mmm)
distortion in normal direction of film with
decreasing film thickness for compressive strain,
and both in-plane and normal cell elongation for
tensile strain (crossover point of 150
nm). Microwave response of BST films from 1 to
20 GHz shows a weak frequency dependence of
capacitance. Dielectric constant varies with
thickness and strain. Q-factor 4 20 for all
films limited by metallization. Max. tunability
65 for both LaAlO3 and MgO series. Compressive
strain in BST/LaAlO3 films significantly
decreases the tunability and permittivity as
films become thinner. Tensile strain in BST/MgO
films allows high tunability even in very thin
films, and increased permittivity with decreasing
film thickness. Crossover point in unit cell
dimensions matches the point at which tunability
sharply decreases.
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Work in Progress
Kramers-Kronig Transform of ? ? j ? to
extract the loss directly from the real part of
the complex permittivity. Cole-Cole analysis
(? vs. ?) to extract defect information as
a function of frequency, temperature, film
thickness, and state of strain. Electromagnetic
field modeling of microwave field in BST film as
a function of film thickness. Phenomenological
studies of BST films to model thickness-dependent
nonlinear dielectric behavior (high and low
fields).