Microwave phase shifter with planar capacitors using strontium titanate films
A. Kozyrev, A. Ivanov, O. Soldatenkov, E. Gol’man, A. Prudan, and V. Loginov
St. Petersburg State Electrotechnical University
Submitted May 18, 1999
Pis’ma Zh. Tekh. Fiz. 25, 78–83 October 26, 1999
A design of a microwave phase shifter based on a loaded microstrip line using planar SrTiO
3
capacitors as nonlinear elements is described. The microwave phase shifter at T =300 K
demonstrated a continuous phase shift between 0 and 55 deg in the frequency range 8.6–9.0 GHz
with a phase error not exceeding 5 deg. The quality parameter of the phase shifter in the
operating frequency range was 110 deg/dB. © 1999 American Institute of Physics.
S1063-78509902910-9
The nonlinear properties of ferroelectric materials such
as strontium titanate SrTiO
3
and barium strontium titanate
Ba,SrTiO
3
can be used to design electrically controllable
microwave devices tunable resonators and filters, phase
shifters, and so on.
1,2
Previous studies of ferroelectrics have
demonstrated their high speed
3
and their capacity to operate
at high microwave powers without any substantial degrada-
tion of the dielectric properties.
4
Bearing in mind the rela-
tively low cost of ferroelectric nonlinear elements compared
with similar semiconducting and ferrite components, we note
that these may be potentially useful for the microwave range.
Ferroelectric nonlinear elements for microwave devices op-
erating at room temperature are usually based on
Ba,SrTiO
3
films. However, the fairly strong temperature
dependence of the permittivity and the relatively high level
of microwave dielectric losses in these materials
5
limit their
application in the microwave range at T =300 K. Strontium
titanate films are used at cryogenic temperatures ( T 77 K
Ref. 2. Nevertheless, the nonlinearity of the permittivity of
SrTiO
3
in high static electric fields was recently demon-
strated by the authors of Refs. 6–8 at T =300 K and low
frequencies. The present study demonstrates for the first time
the use of SrTiO
3
films in electrically controlled microwave
devices at T =300 K. Here we report results of an investiga-
tion of the characteristics of a microwave phase shifter using
planar SrTiO
3
capacitors.
MICROWAVE PROPERTIES OF SrTiO
3
CAPACITORS
Strontium titanate films were prepared by rf magnetron
sputtering of a stoichiometric target in an 0.7 O
2
– 0.3 Ar at-
mosphere at pressure P =4 Pa. The substrates for the planar
capacitors were prepared using single-crystal sapphire
( r -cut. The substrate temperature ( T =700°C was kept
constant during the film deposition process. After the re-
quired thickness of SrTiO
3
film had been reached ( h
=0.8 m, deposition ceased and the samples were cooled to
room temperature at a rate of 10 deg/min. Before deposit-
ing the SrTiO
3
film, platinum Pt electrodes were formed on
the substrate surface by photolithography. The gap between
the electrodes was g =4 m. After the film had been depos-
ited, the multilayer SrTiO
3
/Ptelectrodes of the planar
capacitor/Al
2
O
3
system was annealed in air for 2 h at
T =1050°C. The technology used to fabricate the multilayer
structures containing a thin SrTiO
3
film was described in
greater detail in Ref. 8. The nonlinearity and dielectric losses
of the SrTiO
3
capacitors were estimated by a resonance
technique
5
at frequency f 10 GHz and T =300 K. It was
established that as the dc bias U
b
applied to the capacitors
increased to U
b
=400 V, their capacitance decreased by a
factor of 1.8. The Q factor of the capacitors Q 70 ( Q
=1/tan , where tan are the dielectric losses increased
slightly, by no more than 10%, with increasing bias.
TOPOLOGY OF MICROWAVE PHASE SHIFTER
The topology of the microwave phase shifter is shown in
Fig. 1. Two parallel LC circuits containing planar ferroelec-
tric capacitors are incorporated in a microstrip line ( Z
0
=50 ). The LC circuits are connected by a section of mi-
crostrip line having a lower wave impedance ( Z
01
=40 )
which is required to improve the matching of the phase
shifter with the external microwave circuits. The length of
the microstrip line connecting the LC circuits is approxi-
mately 3/4 near the central frequency f 8.8 GHz in the
operating range of the phase shifter. The planar SrTiO
3
ca-
pacitors C were placed in the gaps between the microstrip
line and radial quarter-wave loops which ensure that the
short-circuiting condition is satisfied for the microwave sig-
nal in this design of phase shifter. The inductances L were
sections of microstrip line having a wave impedance
Z =50 and lengths less than /4 and were shorted with
respect to the microwave signal using the radial quarter-wave
loops. A dc bias U
b
is supplied to the planar capacitors via
thin 10 m silver Ag wires soldered to the radial loops
and the microstrip line of the phase shifter.
MICROWAVE CHARACTERISTICS OF PHASE SHIFTER
The frequency dependences of the transmission coeffi-
cient S
21
and the phase shift of the microwave phase
shifter are plotted in Figs. 2a and 2b. The measurements
were made at T =300 K. A dc bias U
b
= 0–400 V was ap-
plied to the planar SrTiO
3
capacitors. At the central fre-
quency of the operating range, the insertion losses of the
TECHNICAL PHYSICS LETTERS VOLUME 25, NUMBER 10 OCTOBER 1999
836 1063-7850/99/25(10)/2/$15.00 © 1999 American Institute of Physics