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Tunable high-quality-factor interdigitated (Ba, Sr)TiO
Tunable high-quality-factor interdigitated (Ba, Sr)TiO
3
capacitors fabricated
on low-cost substrates with copper metallization
Dipankar Ghosh
a,
*, B. Laughlin
a
, J. Nath
b
, A.I. Kingon
a
, M.B. Steer
b
, J.-P. Maria
a
a
The Electroceramic Thin Film Group, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA
b
Electrical and Computer Engineering Department, North Carolina State University, Raleigh, NC 27695, USA
Received 17 December 2004; received in revised form 2 September 2005; accepted 6 September 2005
Available online 4 October 2005
Abstract
Interdigitated capacitors containing the field-tunable ferroelectric Ba
0.75
Sr
0.25
TiO
3
, polycrystalline alumina substrates, and copper metallization
have been fabricated. Dielectric layers were prepared by magnetron sputtering, while the Cu metallization was evaporated. The dielectric
tunability of the Ba
0.75
Sr
0.25
TiO
3
was 40% at an applied electric field of 12 V/
Am. This corresponds to a 3-Am electrode gap width and a 35 V dc
bias. Low-frequency (1 MHz) loss tangent measurements indicate a dielectric Q (quality factor) of
¨100 while microwave measurements reveal a
zero bias device Q of
¨30 at 26 GHz. These values are comparable or superior to numerous reports of barium strontium titanate interdigitated
capacitors prepared using single crystalline substrates and noble metallization. As such, this technology is significantly less expensive and more
amenable to large-volume manufacturing.
D 2005 Elsevier B.V. All rights reserved.
PACS: 77; 85; 89
Keywords: Thin films; Copper electrodes; Dielectric properties; Interdigitated capacitors
1. Introduction
Developing ferroelectric solid-state tunable microwave
devices like phase shifters, filters, and antennae necessitates
the fabrication of ferroelectric thin film elements offering high
tunability, low dielectric loss, and large quality factor
[1 5]
.
Barium strontium titanate, Ba
1
Àx
Sr
x
TiO
3
, 0
x 1, (BST), a
solid solution perovskite, is the most interesting current
candidate for RF (radio frequency, 1 Hz 100 MHz) and MW
(microwave, 300 MHz 300 GHz) tunable devices due to its
attractive figure of merit and thermal stability when compared
to many other ferroelectric perovskites which contain volatile
constituents
[2,6,7]
.
BST thin films have been deposited by various techniques
such as metal organic chemical vapor deposition
[2,6,8]
, pulsed
laser deposition
[9 11]
, radio frequency sputtering
[1,4,5]
and
sol gel processing
[12,13]
. To date, though, most BST thin film
microwave devices have been fabricated on single crystal
substrates such as MgO
[14,15]
, LaAlO
3
[14,15]
, and Al
2
O
3
(sapphire).
[1,16]
This approach has been adopted because
these substrates offer very low loss tangent values and
conventional processing teaches that epitaxial BST thin films
offer the optimal ferroelectric properties. These substrates,
however, are expensive and many are available only in small
dimensions. If the technology of tunable ferroelectric micro-
wave devices is to be realized, materials solutions allowing the
optimal properties in economical or manufacturable embodi-
ments must be developed. In this investigation we have chosen
polycrystalline alumina (Al
2
O
3
) substrates. These are compar-
atively low in cost and available in dimensions suitable for
large-area film deposition; for instance 152.4 mm polished
alumina wafers which are compatible with tooling for 150 mm
Si wafers. Alumina is also attractive for its excellent
microwave properties; when prepared with high purity,
polycrystalline alumina exhibits a loss tangent of 10
À 4
in
microwave frequencies. Furthermore, the thermal expansion
coefficient of alumina (
¨9 ppm @ RT) is similar to BST, thus
annealing to temperatures above 700
-C is possible without
film cracking. This is typically not the case for Si or SiO
2
. The
authors note that BST films have been prepared on ceramic
0040-6090/$ - see front matter
D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2005.09.025
* Corresponding author.
E-mail address: dghosh2@unity.ncsu.edu (D. Ghosh).
Thin Solid Films 496 (2006) 669 673
www.elsevier.com/locate/tsf alumina in the report of Delprat et al.
[17]
; however, in that
work, Q
dielctric
values were < 10 and dielectric tunability was
strongly dispersive, thus comparison to other data and overall
property evaluation is not straightforward.
BST-based and other thin film perovskite devices usually
incorporate noble metallization like Pt, Au, or Ir
[2,7,18]
because they are in most instances non-reactive in contact with
oxides and their large work functions provide blocking, or
schottky contacts. Though these properties are attractive, the
expense associated with these choices and the difficulty in
patterning provide limitations, especially for volume applica-
tion. Furthermore, the high resistance values of Pt and Au
necessitate multiple micron layer thicknesses for suitably low
sheet resistances: this only exacerbates the patterning issues. To
overcome these difficulties, we have investigated copper
metallization. Though Cu has recently been introduced in the
semiconductor integrated chip (IC) industry for interconnect
lines, limited work has been reported using it as an electrode
material for thin film oxide-based devices. This is due to its
inherently poor adhesion and its tendency to oxidize
[19]
.
Some recent reports, however, have shown that, provided the
proper synthesis conditions, copper can be used as a reliable
electrode with BST. Cu was chosen as the top electrode metal
in the current study since it provides the highest conductivity of
any base metal, it is inexpensive, and it can be etched with
accuracy and reproducibility.
For these reasons, we have pursued the deposition of the
BST thin films by radio frequency magnetron sputtering using
only inexpensive substrate and metallization materials. In
addition, we demonstrate a single etch-step fabrication of
prototype tunable microwave devices. Again we note that
conventional technology suggests that this complement of
materials may not provide suitable electrical properties
primarily due to the fine-grained and randomly oriented
microstructure. We will however demonstrate that a tuning
response comparable to or better than many literature examples
can be achieved.
2. Experimental procedure
In this work we have used radio frequency magnetron
sputtering to deposit 600-nm-thick (Ba
0.75
Sr
0.25
)TiO
3
thin films
on 625-Am-thick polished alumina substrate (Intertec South-
west Inc., Tucson, AZ) using a 10.16-cm stoichiometric
ceramic BST (Ba
0.75
Sr
0.25
)TiO
3
target (Super Conductor
Materials Inc., NY). A substrate surface roughness of 50 nm
(root mean square) was measured by atomic force microscopy.
Sputter deposition was performed at two different substrate
temperatures, 130 and 300
-C, for 60 min using a 30- off-axis
geometry in an argon/oxygen mixture (Ar : O
2
= 5 : 1) to obtain
uniform film thickness and optimal stoichiometry. Note that in
this sputter arrangement, despite the 30
- incident angle, the gun
normal pointed at the center of the substrate. Sputtering
pressure was varied between 0.666 and 1.6667 Pa in incre-
ments of 0.333 Pa. After deposition the sample was annealed in
air at 650, 750 and 900
-C, respectively, to crystallize and
densify the BST films. Annealing time was varied between 1
and 20 h. Film crystallinity was characterized by X-ray
diffraction (XRD) using a 4-circle Bruker AXS D-5000
diffractometer with a CuK
a
radiation source.
An array of IDCs (interdigitated capacitors) was fabricated
on the BST film surfaces by photolithography and a metal lift-
off process. A bilayer technique, using positive imaging
photoresist Shipley 1813 and Microchem LOR 5A, was used
to develop the interdigitated patterns. Once suitable patterns
were prepared, a two-layer metallization stack was deposited.
Initially, a thin layer of Cr (30 nm) was dc magnetron sputter-
deposited. Subsequently, 500 nm of Cu was deposited by
thermal evaporation. To complete the capacitor fabrication, lift-
off was performed by immersion into room temperature
Microchem PG remover. The IDCs had 6 fingers, 3 Am wide,
3 Am apart, and 50 Am long.
The voltage-dependent IDC capacitance and loss tangent
were measured at a frequency of 1 MHz using a Hewlett
Packard 4192 A impedance analyzer. The initial bias was set at
À 35 V and swept to +35 V in 2 V increments. The ac
oscillation level