This paper presents the microwave properties of barium strontium titanate (BST) thin films on r-plane sapphire substrates. A series of films with thickness 25-400 nm was prepared by pulsed laser deposition (PLD). Microwave properties of the films, including capacitance tunability and loss tangent, were extracted by patterning interdigitated capacitors (IDCs) on the film surface. The highest tunability of 64% was observed in the 200 nm film. These results demonstrate the possibility of integrating BST into the silicon on sapphire process

Fardin, E

Ghorbani, K

Holland, A

RMIT Research Repository

Barium strontium titanate thin film varactors on r-plane sapphire 
Ernest A. Fardin
1
, Anthony S. Holland
1
, Kamran Ghorbani
1
, William K. Simon
2
, E. Koray Akdogan
2
, and 
Ahmad Safari
2
1
School of Electrical and Computer Engineering, RMIT University, GPO Box 2476V, Melbourne, 3001, 
Australia
2
Department of Ceramic and Materials Engineering, Rutgers, The State University of New Jersey, 
Piscataway, New Jersey, 08854, U.S.A. 
Tel: +61 3 9925 3250, Fax: +61 3 9925 2007, E-mail: efardin@ieee.org 
Abstract  —  This paper presents the microwave 
properties of barium strontium titanate (BST) thin 
films on r-plane sapphire substrates. A series of films 
with thickness 25 – 400 nm was prepared by pulsed 
laser deposition (PLD). Microwave properties of the 
films, including capacitance tunability and loss 
tangent, were extracted by patterning interdigitated 
capacitors (IDCs) on the film surface. The highest 
tunability of 64% was observed in the 200 nm film. 
These results demonstrate the possibility of integrating 
BST into the silicon on sapphire process. 
Index Terms  —  Ferroelectric thin films, silicon on 
sapphire, microwave varactors.
I. INTRODUCTION
Barium strontium titanate (BST) thin films are 
currently being studied for applications such as high 
density dynamic random access memories 
(DRAMS) [1], small footprint DC blocking 
capacitors [2] and tunable microwave circuits, such 
as phase shifters [3] and bandpass filters [4]. An 
important step towards the commercialization of 
BST-based devices is the integration of BST films 
with semiconductors. Despite the large volume of 
research, there has been limited progress towards 
the integration of BST with semiconductor 
technology.  
One of the difficulties associated with BST thin 
film deposition is the high substrate temperature, 
typically 450 – 700°C [5] required to form the 
tunable perovskite phase of the material. The design 
of a low loss bottom electrode on silicon which is 
stable at high deposition temperatures is not 
straightforward. Several layers are generally 
required, for example Pt/Au/Pt/TiO2/SiO2/Si [6]. 
Problems with hillock formation in the Pt layer have 
been reported [7], due to the difference in thermal 
coefficients between the Pt and Si layers. Improved 
bottom electrodes have been achieved using c-plane 
sapphire, [8] however this substrate material is not 
compatible with silicon-based processes.  
Silicon on sapphire (SoS) technology offers a 
potential solution to the problem of BST integration. 
The SoS process uses r-plane sapphire as the 
substrate material [9]. In recent work [10], BST was 
deposited on a SoS wafer, and the underlying Si 
layer was subsequently converted to SiO2 by 
annealing in oxygen for 48 hours at 1000ºC. Since 
no Si remains after annealing, BST-based devices 
and semiconductors cannot be fabricated on the 
same chip using this process.   
In this paper, the microwave properties of BST 
thin films deposited directly on r-plane sapphire are 
studied. Several different film thicknesses, in the 
range of 25 – 400 nm were investigated, so as to 
map out the relationship between the film thickness 
and microwave properties of varactors formed from 
these films. Interdigitated varactors were formed by 
electroplating a seed pattern on the surface of the 
BST film, as described below. 
II. BST THIN FILM DEPOSITION AND VARACTOR
FABRICATION
Pulsed laser deposition (PLD) was used to deposit 
the BST material on 10 x 10 x 0.5 mm sapphire 
substrates (Techno Chemics, Inc.) The PLD system 
employed a KrF excimer laser (Ȝ = 248 nm) with 2 
Hz repetition rate focused to 2 J·cm-2 energy 
density. Prior to deposition, the substrates were 
annealed inside the deposition system at 600°C in 
100 mTorr of flowing oxygen, in order to remove 
any residual surface contamination.  
A series of depositions were performed from a 
stoichiometric Ba0.6Sr0.4TiO3 circular target, for BST 
films thickness of 25, 50, 100, 200 and 400 nm. The 
substrate temperature was maintained at 700ºC for 
all depositions. After deposition, ~1 Atm of oxygen 
was admitted to the deposition chamber, and the 
films were in-situ annealed at 700ºC for 6 hours 
before cooling to room temperature. 
Proceedings of Asia-Pacific Microwave Conference 2006
Copyright 2006 IEICE
Authorized licensed use limited to: RMIT University. Downloaded on June 8, 2009 at 19:54 from IEEE Xplore.  Restrictions apply.
Thick interdigitated capacitor (IDC) structures 
were formed on the surface of the BST film by 
electroplating.  Seed metal layers of Ti, Ni and Au 
were successively deposited by e-beam evaporation 
and patterned by lift-off. Interdigitated Au electrode 
structures were electroplated to a thickness of ~2.5 
μm, after which the seed layer was removed by wet 
etching. The finger length of the varactors ranged 
from 80 – 120 μm and the gap between the fingers 
was 2 – 8 μm, while the finger width was fixed at 5 
μm. Each varactor consisted of two sets of 
interdigitated electrodes (8 fingers each), and was 
designed for probing in the ground-signal-ground 
(GSG) configuration. Open and short circuit 
calibration standards were also patterned next to 
each varactor, as shown in Fig. 1. 
Fig. 1. Array of interdigitated capacitors (IDCs) 
with adjacent open and short circuit calibration 
standards.
III. DEVICE CHARACTERISATION
X-ray diffraction (XRD) measurements showed 
that all of the films were polycrystalline and single 
phase.  A detailed description of the material results 
and their relation to the microwave properties of the 
devices will be reported elsewhere [11]. The method 
and results of the microwave characterization 
procedure are given below. 
A. On-wafer calibration 
Open-circuit and short circuit calibration 
standards, with the same finger / pad dimensions as 
the varactors, were patterned next to each varactor 
device on the BST film. This allowed the extraction 
of the parasitic inductance, capacitance and 
resistance associated with the electrode 
metallization, following the procedure given in [5, 
12]. A schematic of the BST varactor with 
associated parasitic elements is given in Fig. 2.  
Fig. 2. Equivalent circuit of BST varactor, incorporating 
pad parasitics Lp, Cp and Rp, based on [5]. 
One port reflection (S11) measurements of the 
open circuit were used to calculate Cp. Values for Lp
and Rp were then extracted by measuring the short 
circuit standard. The Q-factor of the devices was 
calculated using 
BSTBST CR
Q
Z
1 .          (1) 
A Matlab optimisation routine was used to calculate 
the frequency dependent values of the circuit 
elements in Fig. 2. 
B. Measured Results 
Microwave characterization of the BST varactors 
was performed on an Anritsu 37369A vector 
network analyzer (VNA). The VNA was connected 
to a wafer probe station equipped with 200 μm pitch 
GSG wafer probes. The bias voltage for the 
varactors was supplied via the internal bias tee of 
the VNA, and was therefore limited to ±40 V.  
Fig. 3. Extracted BST varactor capacitance (CBST) with 
and without modeling the pad parasitics Lp, Cp and Rp.
L R
BST
Cp
p p
R
C
BST
1 5 10 15 20
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
Frequency (GHz)
Ca
pa
ci
ta
nc
e 
(p
F)
Lp, Cp, Rp Modelled
Lp, Cp, Rp Not Modelled
Authorized licensed use limited to: RMIT University. Downloaded on June 8, 2009 at 19:54 from IEEE Xplore.  Restrictions apply.
Fig. 4. Measured zero bias performance of BST varactors 
as a function of film thickness at (a) capacitance CBST, and 
(b) Q-factor. 
For each device, three measurements were 
performed (open circuit, short circuit and varactor). 
Typical values for Cp and Lp were 6 fF and 10 pH, 
respectively. The effect of not modeling the pad 
parasitics is shown in Fig. 3. Failing to model the 
parasitics causes a large variation in the extracted 
capacitance CBST in wide band measurements. Since 
the calculations for tunability and dielectric constant 
are generally based on the value of CBST, it is 
important to account for the pad parasitics. 
Zero-bias measurements for a series of varactors 
with different BST film thickness are shown in Fig. 
4. Figure 5 shows measured results for the same 
devices at 40 V bias. The data provided is for 2 μm 
gap devices with the highest capacitance tunability 
for a given film thickness. Note that the capacitance 
tunability is calculated using  
%100
)0(
)40()0(
u

 
VC
VCVC
Tun .       (2) 
Fig. 5. Measured 40 V bias performance of BST varactors 
as a function of film thickness: (a) capacitance CBST, and 
(b) Q-factor. 
Values for the device tunability at each film 
thickness are given in Table 1. Note that the lowest 
tunability 2 μm gap devices on each film had a 
capacitance variation ~5% lower than that of the 
highest tunability devices. Devices with larger gaps 
between the electrodes showed lower tunability, due 
to the lower bias field across the BST film. The Q-
factor is shown only above 2 GHz due to large 
fluctuations in the extracted resistance RBST at lower 
frequencies. 
C. Discussion 
A high tunability of 64% or 2.8:1 was measured 
for the 200 nm film, at a bias field of 200 kV·cm-1.
However, this device had the lowest Q-factor of 
~7.8 at 10 GHz. Devices with lower tunability had a 
higher Q-factor, with the best Q-factor observed in 
the 25 nm film. Interestingly, the 400 nm film 
showed a lower tunability than the 200 nm film. In 
epitaxial BST films, the highest tunability is 
1 5 10 15 20
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Frequency (GHz)
Ca
pa
ci
ta
nc
e 
(pF
)
400 nm
200 nm
100 nm
50 nm
25 nm
2 5 10 15 20
0
10
20
30
40
50
Frequency (GHz)
Q
400 nm
200 nm
100 nm
50 nm
25 nm
1 5 10 15 20
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Frequency (GHz)
Ca
pa
ci
ta
nc
e 
(pF
)
400 nm
200 nm
100 nm
50 nm
25 nm
2 5 10 15 20
0
50
100
150
200
Frequency (GHz)
Q
400 nm
200 nm
100 nm
50 nm
25 nm
(a)
(b)
(a)
(b)
Authorized licensed use limited to: RMIT University. Downloaded on June 8, 2009 at 19:54 from IEEE Xplore.  Restrictions apply.
generally observed in thicker films due to strain 
relaxation [13]. However, it appears that this is not 
the case in polycrystalline BST films on r-plane 
sapphire.  
These results compare favourably with other 
reports of interdigitated BST varactors [14]. 
Importantly, the process temperature does not 
exceed 700°C. Depending on proprietary details of 
the SoS process, this may allow BST to be 
incorporated in the SoS-based circuits.
VI. CONCLUSION
A series of BST films with thickness in the range 
of 25 – 400 nm was deposited by PLD. Thick gold 
IDCs were patterned on the BST films by 
electroplating, and the microwave properties of the 
devices were measured from 1 – 20 GHz. The best 
tunability of 64% was observed in a varactor on the 
200nm film. Since the processing temperatures are 
below 700°C, BST films grown on r-plane sapphire 
are potential candidates for integration with SoS 
technology. 
ACKNOWLEDGEMENT
The authors wish to acknowledge the support of 
the Australian Research Council Linkage 
International Award LX0666659. 
REFERENCES
[1] S. Rios, et al., “Phase transitions in epitaxial 
Ba0.5Sr0.5TiO3 thin films,” J. Appl. Phys., vol. 99, pp. 
024107(1)-024107(8), 2006. 
[2] H. Xu, et al., “Integration of BaxSr1-xTiO3 thin films 
with AsGaN/GaN HEMT Circuits,” Proc. EDMO 
2003, pp. 273-278, November 2003. 
[3] T. O’Sullivan, R.A. York, B. Noran, and P.M. 
Asbeck, “Adaptive duplexer implemented using 
single-path and multipath feedforward techniques 
with BST phase shifters,” IEEE Trans. Microwave 
Theory & Tech., vol. 53, no. 1, pp. 106-114, January 
2005.
[4] J. Nath, et al., “An electronically tunable microstrip 
bandpass filter using thin film barium strontium 
titanate (BST) varactors,” IEEE Trans. Microwave 
Theory & Tech., vol. 53, no. 9, pp. 2707-2712, 
September 2005. 
[5] B. Acikel, Ph.D. thesis, University of California, 
Santa Barbara, 2002.
[6] A. Boroviev, P. Rundqvist, K. Khamachane, and S. 
Gevorgian, “Microwave loss mechanisms in 
Ba0.25Sr0.75TiO3 thin film varactors,” J. Appl. Phys.,
vol. 96, no. 8, pp. 4642-4649, October 2004. 
[7] S.R. Summerfelt, D. Kotecki, A. Kingon, and H.N. 
Al-Shareef, “Pt hillock formation and decay,” Mat.
Res. Soc. Symp. Proc., vol. 361, pp. 257-262, 
December 1994. 
[8] B. Acikel, T.R. Taylor, P.J. Hansen, J.S. Speck, and 
R.A. York, “A new high performance phase shifter 
using BaxSr1-xTiO3 thin films,” IEEE Microwave 
Wireless Compon. Lett., vol. 12, no. 7, pp. 237-239, 
Jul, 2002. 
[9] S. Cristoloveanu, “Silicon films on sapphire,” Rep.
Prog. Phys., vol. 50, no. 3, pp. 327-371, 1987. 
[10] H. Li, J. Finder, Y. Liang, R. Gregory, and W. Qin, 
“Dielectric properties of epitaxial Ba0.5Sr0.5TiO3
films on amorphous SiO2 on sapphire,” Appl. Phys. 
Lett., vol. 87, pp. 072905(1)-072905(3), August 
2005.
[11] E.A. Fardin, A.S. Holland, K. Ghorbani, W.K. 
Simon, E.K. Akdogan, and A. Safari, “Dielectric and 
microwave properties of Ba0.6Sr0.4TiO2 thin films on 
r-plane sapphire”, Appl. Phys. Lett., submitted for 
publication.
[12] J. Park, J. Lu, S. Stemmer, and R.A. York, 
“Microwave dielectric properties of tunable 
capacitors employing bismuth zinc niobate thin 
films,” J. Appl. Phys., vol. 87, pp. 084110(1)-
084110(4), 2005. 
[13] J. Bellotti, E.K. Akdogan, A. Safari, W. Chang, and 
S. Kirchoefer, “Tnable dielectric properties of BST 
thin films for RF/MW passive components,” 
Integrated Ferroelectrics, vol. 49, pp. 113-122, 
2002.
 [14] D. Ghosh, et al., “Tunable high-quality-factor 
interdigitated (Ba,Sr)TiO3 capacitors fabricated on 
low-cost substrates with copper metallization,” Thin
Solid Films, vol. 496, no. 2, pp. 669-673, February 
2006.
TABLE I 
SUMMARY OF ELECTRICAL RESULTS
 Capacitance Q-Factor (1/tanį)
(10 GHz) 
Film Thickness [nm] C(0 V)
[pF]
C(40 V)
[pF]
Tun [%] Q(0 V) Q(40 V)
25 0.19 0.14 24 22.9 180
50 0.45 0.26 43 13.4 78.3
100 0.53 0.24 55 9.90 37.8
200 0.96 0.34 64 7.80 32.2
400 0.99 0.49 50 11.8 26.9
Authorized licensed use limited to: RMIT University. Downloaded on June 8, 2009 at 19:54 from IEEE Xplore.  Restrictions apply.


Barium strontium titanate thin films on r-plane sapphire

Abstract

Similar works

Full text

Available Versions

RMIT Research Repository