Full text of DOI:10.1557/JMR.2002.0411

Improvement in tunability and dielectric loss of
(Ba0.5Sr0.5)TiO3 capacitors using seed layers on
Pt/Ti/SiO2/Si substrates
Young-Ah Jeon, Woong-Chul Shin, Tae-Suck Seo, and Soon-Gil Yoon
Department of Materials Engineering, Chungnam National University, Daeduk Science Town,
305-764, Taejon, Korea
(Received 9 April 2001; accepted 5 August 2002)
The absence of a low dielectric constant layer at the barium strontium titanate
(BST)/Pt interface and a decreased roughness are critical issues in the production of
(Ba0.5Sr0.5)TiO3 thin films with high tunabilities and low losses. An improvement in
dielectric properties was achieved by the insertion of seed layers at the BST/Pt
interface by pulsed laser deposition. The higher tunability can be attributed to (100)
texturing of the BST films, which is independent of grain size and grain morphologies,
thus leading to a variation in seed layer thicknesses. The tunability and dielectric
constant of 1600-Å-thick BST films showed a maximum of 53% and 720, respectively,
at a seed layer thickness of 100 Å. Dielectric loss is dependent on the roughness of
BST films and reached a minimum of 0.8% at a root mean square roughness of 28 Å.
The maximum figures of merit, defined as the ratio of tunability to dielectric loss, of
approximately 58 at 100 kHz and 198 kV/cm were obtained at a seed layer thickness
of 70 Å. The optimized seed layer thickness for BST deposition onto Pt/Ti/SiO2/Si
substrates plays an important role in maintaining the high tunabilities and low loss,
which are suitable for microwave device applications.
I. INTRODUCTION
interfacial layers deposited onto Pt/Ti/SiO2/Si substrates
Barium strontium titanate, Ba1−xSrxTiO3 (BST), is be-
ing investigated and is of considerable interest for use as
a new dielectric material for the generation dynamic
random-access memories (DRAMs)1,2 in the future and
more currently in tunable microwave applications.3,4 The
large electrical field dependent dielectric constant can be
used for tunable microwave devices, such as tunable os-
cillators, phase shifters, and varactors.5,6 In such devices,
it is desirable to have a high dielectric tunability over a
given electric field range and a low dielectric loss. BST
films have been deposited by sputtering,7 chemical vapor
deposition (CVD),8,9 and pulsed laser deposition
(PLD).10,11 PLD provides unique advantages for multicom-
ponent oxide films because it allows easy reproduction of
the stoichiometry of the target in the deposited films.
Various factors, such as the lattice mismatch and dif-
ferences in thermal expansion between the film and the
substrate, as well as the partial pressure of oxygen during
the deposition affect the microwave dielectric properties
of BST films.12 Park et al. systematically controlled the
stress states of BST films by depositing a very thin BST
interlayer between the main layer and a MgO (001) sub-
strate.11 The Ba0.6Sr0.4TiO3 films grown on a Ba0.7Sr0.3TiO3
interlayer exhibited a small tensile stress and the best
dielectric values. The (Ba,Sr)RuO3 (BSR) conductive
by metalorganic chemical vapor deposition offered an
appropriate match in structure with BST and increased
the tenability of (100)-textured BST thin films.13 How-
ever, BST films with BSR interfacial layers showed an
increase of dielectric loss compared with those without
BSR layers, and the origin for that was not identified
in detail.14 The abrupt decrease of electrical properties, in
particular dielectric constant, with thinner film thickness
can be attributed to the formation of an initial low di-
electric constant layer and a small grain size.15 Earlier
studies reported that the overall performance of BST thin
film capacitors can be improved by using a bilayer struc-
ture composed of a thin BST seed layer followed by a
relatively thick BST layer.16,17 In these studies, the effect
of seed layer thickness on tunability of BST films for
microwave tunable device was not reported in detail.
In this study, ultrathin seed layers having the same
composition as the main layer [(Ba0.5Sr0.5)TiO3] were
deposited onto Pt/Ti/SiO2/Si substrates at 300 °C and
annealed at 600 °C for 10 min in O2 to minimize residual
stress and to suppress the formation of a low dielectric
constant layer. Improvements in tunability and dielec-
tric loss were examined through the correlation between
the microstructure and crystallinity of the main BST lay-
ers deposited onto various thicknesses of seed layers.
J. Mater. Res., Vol. 17, No. 11, Nov 2002
© 2002 Materials Research Society
2831
http://journals.cambridge.org
Downloaded: 13 Mar 2015
IP address: 131.204.172.32

Y-A. Jeon et al.: Improvement in tunability and dielectric loss of (Ba0.5Sr0.5)TiO3 capacitors using seed layers on Pt/Ti/SiO2/Si substrates
TABLE I. Conditions used for the deposition of seed and main layer
(Ba0.5Sr0.5)TiO3 thin films by pulsed laser deposition.
II. EXPERIMENTAL
BST films were grown by PLD using a KrF excimer
Deposition parameter
Target
BST main layer
BST seed layer
laser (␭ ס 248 nm) and a (Ba0.5Sr0.5)TiO3 target. The
films were deposited at a laser repetition rate of 4 Hz
with a laser pulse energy density of 1.5 J/cm2 on Pt/Ti/
SiO2/Si substrates. The Pt/Ti/SiO2/Si substrates with a
Ti buffer layer 30 nm thick were used to investigate the
impact of Ti on the tunability and dielectric loss of BST
films. (Ba0.5Sr0.5)TiO3 seed layers, at various thick-
nesses, were deposited onto Pt (100 nm)/Ti (30 nm)/
SiO2/Si substrates at 300 °C and then crystallized at
600 °C for 10 min in an O2 ambient. The main BST films
were deposited onto the seed layers at 650 °C. The total
thickness of BST films, including the seed layer, was
maintained at approximately 160 nm. The conditions em-
ployed for the deposition of seed and main layers are
summarized in Table I.
The crystalline structures of the BST films were ana-
lyzed by x-ray diffraction (XRD) using Cu K␣ radiation
and a Ni filter. The microstructure and roughness of the
main BST films were observed by scanning electron
microscopy (SEM) and atomic force microscopy (AFM),
respectively. The thickness of the ultrathin seed layers
was controlled by reference to the deposition conditions
of the thick BST film as obtained by SEM cross-sectional
images. For electrical measurements, Pt top electrodes
(A ס 2.0 × 10−4 cm2) were deposited through a shadow
mask onto the BST thin films by direct current magne-
tron sputtering. After the deposition of the top electrode,
the samples were annealed in a quartz tube furnace at
600 °C for 20 min in an O2 ambient to improve the top
electrode/BST interface. The dielectric constant and loss
of Pt/BST/Pt capacitors as a function of applied electric
field were measured at 100 kHz using a Hewlett-Packard
4194A impedance analyzer. To maintain the reproduc-
ibility of tunability and dielectric loss of BST films, data
reported for each seed layer thickness represented the
average of the three samples.
(Ba0.5Sr0.5)TiO3
ceramics
650 °C
(Ba0.5Sr0.5)TiO3
ceramics
300 °C
Deposition temperature
Deposition pressure
Energy density
Repetition rate
Substrate
0.3 torr
0.3 torr
1.5 J/cm2
4 Hz
1.5 J/cm2
4 Hz
Pt/Ti/SiO2/Si
…
Pt/Ti/SiO2/Si
at 600 °C, 10 min in O2
Post-annealing
irrespective of BST seed layer thickness and can be char-
acterized by the appearance of (100), (110), (111), and
(211) peaks in the XRD spectra. In previous work, BSR
conductive interfacial layers influenced the growth mode
of BST films and showed a (100)-textured film proper-
ties leading to an enhancement in an in-plane orientation
of the polar axis.13 The inset in Fig. 2 shows the varia-
tion of the full-width half maximum (FWHM) of (110)
peak as a function of the BST seed layer thickness. The
FWHM of BST thin films rapidly decreases with increas-
ing seed layer thickness up to 100 Å and then slowly
increases with increasing thickness above 100 Å thick-
ness. The crystallinity of the BST films increased as
the result of the presence of seed layers and showed the
largest crystallinity in a seed layer thickness of 100 Å.
This result suggests that seed layer thickness plays an
important role in improving the crystallinity of BST
films and a critical thickness exists for crystallinity. The
growth characteristics of BST films deposited onto seed
layers below a 100 Å thickness was mainly influenced by
the Pt bottom electrode and the effect of seed layers
gradually disappeared above 100 Å thickness because the
BST bulk characteristics appeared and increased above a
critical thickness. The ultrathin seed layers suppressed
the formation of low dielectric layers such as an amor-
phous structure at BST/Pt interface. Figure 3 shows SEM
surface and cross-sectional morphologies of BST thin
films deposited onto the bare substrates and seed layers
with a thickness of 100 Å. The grain size and morpholo-
gies of the BST films were not affected by the existence
of seed layers. The variation of grain morphologies was
mainly controlled by the nucleation and growth rate,
which were influenced by the deposition temperature and
system pressure. Figure 4 shows the field-dependent di-
electric properties of BST capacitors as measured at
100 kHz. Hysteresis was not observed when the bias
voltage was increased or decreased within the region of
the applied electric field 198 kV/cm. Furthermore, the
electric field corresponding to maximum capacitance did
not shift for the deposition of films with various seed
layer thicknesses. This indicates that the dielectric films
contain no mobile ions. The maximum capacitance at
zero voltage changed with various thicknesses of seed
III. RESULTS AND DISCUSSION
Figure 1 shows SEM surface and cross-sectional im-
ages of BST seed layers having various thicknesses,
which were annealed at 600 °C for 10 min in an O2 am-
bient. The hillocks (BST with excessive Ti content) on
the BST seed layers increased with a decrease in the
thickness of the seed layers, as shown in Fig. 1. Ti, which
diffused through the Pt, reacted with the thinner seed
layers during the 600 °C annealing step. These hillocks
formed by Ti buffer layers are considered to serve as a
source of nucleation sites during the deposition of vari-
ous materials.18
Figure 2 shows XRD patterns of BST thin films with
various seed layer thicknesses. The BST films depos-
ited at 650 °C exhibited a polycrystalline structure
2832
J. Mater. Res., Vol. 17, No. 11, Nov 2002
http://journals.cambridge.org
Downloaded: 13 Mar 2015
IP address: 131.204.172.32

Y-A. Jeon et al.: Improvement in tunability and dielectric loss of (Ba0.5Sr0.5)TiO3 capacitors using seed layers on Pt/Ti/SiO2/Si substrates
FIG. 1. SEM surface and cross-sectional images of BST seed layers of (a) 100 Å, (b) 200 Å, and (c) 300 Å thickness annealed at 600 °C for 10 min
in an oxygen ambient.
layer compared with that of a no seed layer. The tunabil-
ity and texturing ratio of the BST films as a function of
seed layer thickness are shown in Fig. 5. Tunability is de-
fined as (⑀max − ⑀min)/⑀max at 100 kHz and 198 kV/cm,
where ⑀max and ⑀min are the maximum and minimum
measured permittivity. The texturing ratio is defined as
I(100)/[I(100) + I(110)]. I(hkl) is the integrated intensity of the
(hkl) peaks recorded in ␪–2␪ scans. The tunability and
texturing ratio of BST films monotonically increased
with increasing seed layer thickness up to 100 Å and then
decreased with a further increase in thickness above
100 Å. The tunability of the BST films showed a close
relationship with a texturing ratio of (100). The tunability
also showed a maximum when the texturing ratio was
maximum in the case of a seed layer thickness of 100 Å.
Figure 6 shows the relationship between dielectric
constant and tunability as a function of seed layer thick-
ness. The dielectric constant also showed a similar ten-
dency with respect to the variation in tunability for
different seed layer thicknesses. The tunability and di-
electric constant of 1600-Å-thick BST films showed a
maximum of 53% and 720, respectively, at a seed layer
thickness of 100 Å. These results suggest that the (100)
textured-BST films lead to an enhancement in the in-
plane oriented polar axis. The relationship between the
dissipation factor and root mean square (rms) roughness
as a function of seed layer thickness is shown in Fig. 7.
As shown in Fig. 7, a close relationship is evident be-
tween the dissipation factor and the roughness of BST
films. The dielectric loss of BST films decreased slightly
J. Mater. Res., Vol. 17, No. 11, Nov 2002
2833
http://journals.cambridge.org
Downloaded: 13 Mar 2015
IP address: 131.204.172.32

Y-A. Jeon et al.: Improvement in tunability and dielectric loss of (Ba0.5Sr0.5)TiO3 capacitors using seed layers on Pt/Ti/SiO2/Si substrates
with increasing seed layer thickness up to 70 Å, reach-
ing a maximum of 1.5% at 100 Å. The dielectric loss
and roughness of BST films showed a minimum of
approximately 0.8% and 28 Å, respectively, at a seed
layer thickness of 70 Å. When BST films were deposited
onto Pt/Ti(30 nm)/SiO2/Si substrates at a high tempera-
ture in an oxygen ambient, TiOx was formed on platinum
surface by titanium diffusion through the platinum grain
boundary. The TiOx formed on the platinum at the initial
stage of BST deposition was thought to act as a nucleation
FIG. 2. XRD patterns of BST thin films deposited on (a) no seed layer,
seed layers (b) 70 Å, (c) 100 Å, (d) 200 Å, and (e) 300 Å in thickness
(the inset shows the variation of FWHM as a function of seed layer
thickness).
FIG. 4. Capacitance–voltage characteristics of BST films as a func-
tion of seed layer thickness.
FIG. 3. SEM surface and cross-sectional images of BST films deposited on (a) no seed layer and (b) seed layer 100 Å in thickness.
2834
J. Mater. Res., Vol. 17, No. 11, Nov 2002
http://journals.cambridge.org
Downloaded: 13 Mar 2015
IP address: 131.204.172.32

Y-A. Jeon et al.: Improvement in tunability and dielectric loss of (Ba0.5Sr0.5)TiO3 capacitors using seed layers on Pt/Ti/SiO2/Si substrates
site for BST growth.18 The TiOx in an ultrathin BST seed
layer up to 70 Å promoted the BST nucleation and pro-
duced dense and smooth morphologies in finial BST
structure. However, above 100 Å, influence of TiOx on
BST growth decreased, and the BST films showed an
increase of surface roughness, resulting in an increase of
dielectric loss. Thus an improvement in roughness plays
a critical role in decreasing dielectric loss.
Figure 8 shows the variation of the figure of merit
(K) as a function of seed layer thickness. The figure of
merit is frequently used as design criteria for the micro-
wave tunable devices and is defined as K ס tunability/
tan ␦.19 The tan ␦ value used in the computation of K was
measured at an electric field of 198 kV/cm. The highest
K value was found for BST films with a seed layer thick-
ness of 70 Å, even though tunability at 100 Å was the
highest. Even though BST films deposited onto BSR
interfacial layers showed an increase of tunability,13 an
increase of dielectric loss as compared with films without
BSR layers produced a decrease in K value.14 This dem-
onstrates that the BST films with BST seed layers may
offer superior performance for microwave tunable device
applications.
FIG. 7. Relationship between dissipation factor and rms roughness as
a function of seed layer thickness.
FIG. 5. Tunability and texturing ratio of BST films for various seed
layer thicknesses.
FIG. 6. Relationship between dielectric constant and tunability as a
function of seed layer thickness.
FIG. 8. Figure of merit of BST films deposited on various thicknesses
of seed layers.
J. Mater. Res., Vol. 17, No. 11, Nov 2002
2835
http://journals.cambridge.org
Downloaded: 13 Mar 2015
IP address: 131.204.172.32

Y-A. Jeon et al.: Improvement in tunability and dielectric loss of (Ba0.5Sr0.5)TiO3 capacitors using seed layers on Pt/Ti/SiO2/Si substrates
3. F.A. Anda, F.W. Van Keuls, R.R. Romanofsky, and G. Subramanyam,
IV. CONCLUSIONS
Integr. Ferroelectr. 22, 269 (1998).
(Ba0.5Sr0.5)TiO3 thin films with high tunabilities and
low losses can be grown using pulsed-laser deposition
with various seed layer thicknesses when deposited onto
Pt/Ti/SiO2/Si substrates. The higher tunability can be at-
tributed to the (100) texturing of the BST films irrespec-
tive of grain size and grain morphologies for achieving a
variation in seed layer thicknesses. The tunability and
dielectric constant of 1600-Å-thick BST films showed a
maximum of 53% and 720, respectively at a seed layer
thickness of 100 Å. Dielectric loss is dependent on the
roughness of BST films and shows a minimum of 0.8%
at rms roughness of 28 Å. The maximum figure of merit
of about 58 was obtained at seed layer thickness of 70 Å.
4. W.J. Kim, W. Chang, S.B. Qadri, J.M. Pond, S.W. Kirchoefer,
D.B. Chrisey, and J.S. Horwitz, Appl. Phys. Lett. 76, 1185 (2000).
5. W. Chang, J.S. Horwitz, A.C. Carter, J.M. Pond, S.W. Kirchoefer,
C.M. Gilmore, and D.B. Chrisey, Appl. Phys. Lett. 74, 1033
(1999).
6. S. Zafar, R.E. Jones, P. Chu, B. White, B. Jiang, D. Taylor,
P. Zurcher, and S. Gillepsie, Appl. Phys. Lett. 72, 2820 (1998).
7. S.G. Yoon and A. Safari, Thin Solid Films 254, 211 (1995).
8. H.J. Cho and H.J. Kim, Appl. Phys. Lett. 72, 2820 (1998).
9. C.S. Chern, S. Liang, Z.Q. Shi, S.G. Yoon, A. Safari, P. Lu,
and B.H. Kear, Appl. Phys. Lett. 64, 3181 (1994).
10. S.G. Yoon, J.C. Lee, and A. Safari, J. Appl. Phys. 76, 2999
(1994).
11. B.H. Park, E.J. Peterson, Q.X. Jia, J. Lee, X. Zeng, W. Si, and
X.X. Xi, Appl. Phys. Lett. 78, 533 (2001).
12. S.T. Lee, N. Fujimura, and T. Ito, Jpn. J. Appl. Phys. Part 1 34,
5168 (1995).
13. Y.A. Jeon, E.S. Choi, T.S. Seo, and S.G. Yoon, Appl. Phys. Lett.
ACKNOWLEDGMENT
79, 1012 (2001).
This work was supported by the Korea Science and
Engineering Foundation through the Research Center for
Advanced Magnetic Materials at Chungnam National
University.
14. T.S. Seo, Y.N. Oho, and S.G. Yoon (unpublished).
15. W.J. Lee, S.G. Yoon, and H.G. Kim, J. Appl. Phys. 80, 5891
(1996).
16. Y.C. Choi, J. Lee, and B.S. Lee, Jpn. J. Appl. Phys. Part 1 36,
6824 (1997).
17. L. Kinder, X.F. Zhang, I.L. Grigorov, C. Kwon, Q.X. Jia, L. Luo,
and J. Zhao, J. Vac. Sci. Technol. A 17, 2148 (1999).
18. J.C. Lee and S.G. Yoon, J. Vac. Sci. Technol. B 17, 2182 (1999).
19. A.B. Kozyrev, V.N. Keis, G. Koepf, R. Yandrofski, O.I. Soldatenkov,
K.A. Dudin, and D.P. Dovgan, Microelectron. Eng. 29, 257
(1995).
REFERENCES
1. S.G. Yoon, J.C. Lee, and A. Safari, Integr. Ferroelectr. 7, 329
(1995).
2. A.I. Kingon, S.K. Streiffer, C. Basceri, and S.R. Summerfelt,
Mater. Res. Bull. 21, 18 (1995).
2836
J. Mater. Res., Vol. 17, No. 11, Nov 2002
http://journals.cambridge.org
Downloaded: 13 Mar 2015
IP address: 131.204.172.32