Microstrip ring resonators operating at 35 GHz were fabricated from laser ablated YBCO thin films deposited on lanthanum aluminate substrates. They were measured over a range of temperatures and their performance compared to identical resonators made of evaporated gold. Below 60 Kelvin the superconducting strip performed better than the gold, reaching an unloaded Q approximately 1.5 times that of gold at 25 K. A shift in the resonant frequency follows the form predicted by the London equations. The Phenomenological Loss Equivalence Method is applied to the ring resonator and the theoretically calculated Q values are compared to the experimental results

Bhasin, K. B.

Chorey, C. M.

Heinen, V. O.

Itoh, T.

Kong, K. S.

Lee, H. Y.

Romanofsky, R. R.

Warner, J. D.

English

NASA Technical Reports Server

NASA Technical Mem&andum i025:26
Performance and Modeling of
;rconducting Ring, Supe _sonators ....
............ at Millimeter-Wave-FreqUencies
• : : : : : _ -:7
K.B. Bhasin
National Aeronatuics and Space Administration_i
Lewis Research Center
Cleveland, Ohio
C.M. Chorey
......... Sverdrup Technology, Inc. ::::
NASA Lewis Research Center Group
Cleveland, Ohio
J.D. Warner, R.R. Romanofsky, and V.O. Heinen
National Aeronautics and Space Administration
Lewis Research Center -
Cleveland, Ohio
n
K.S. Kong, H.Y. Lee_ and T. Itoh .....
The University of Texas at Austin
Austin, Texas
Prepared for the
1990 IEEE MTT-S International Microwave Symposium
Dallas, TexaS, May 8-10, 1990
(NASA-TM-I0252(_) PERFORMANCE AND MOOELING NgO-18634
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OF SUPERCONDUCTING RING RESONATORS AT
MILtIMETER-WAVE FREQUENCIES (NASA) 9 p
CSCL 09A Unclas
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_ _ _ _ ____r r........... _
PERFORMANCE AND MODELING OF SUPERCONDUCTING RING RESONATORS AT
MILLIMETER-WAVE FREQUENCIES.
K.B. Bhasin
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
C.M. Chorey
Sverdrup Technology, Inc.
Lewis Research Center Group
Cleveland, Ohio 44135
J.D. Warner, R.R. RomanoFsky, and V.O. Heinen
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
K.S. Kong, H.Y. Lee, and T. Itoh
Department of Electrical and Computer Engineering
The University of Texas at Austin
Austin, TX 78712
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t.C)
f
LIJ
ABSTRACT
Microstrip ring resonators operating at 35 GHz have been
fabricated from laser ablated YBCO thin films deposited on
lanthanum aluminate substrates. They were measured over a range
of temperatures and their performance compared to identical
resonators made of evaporated gold. Below 60 ° Kelvin the
superconducting strip performed better than the gold, reaching an
unloaded 'Q' _1.5 times that of gold at 25 ° K. A shift in the
resonant frequency follows the form predicted by the London
equations. The Phenomenological Loss Equivalence Method is applied
to the ring resonator and the theoretically calcu]ated Q values are
compared to the experimental results.
INTRODUCTION
Recent observations of low surface resistance at microwave and
millimeter wave frequencies in thin superconducting films [i]
suggest their use for low loss/high Q microstrip circuits. Of
interest is the surface resistance exhibited by these films across
a wide frequency range. To date, measurements of surface resistance
in the Ka band and above have been by the cavity technique. This
technique fails to model microstrip losses completely because it
neglects substrate losses and fails to adequately probe the film-
substrate interface. Microstrip resonators patterned from thin
films on microwave substrates allow direct measurement of
microstrip losses. Several groups have made such measurements at
lower microwave frequencies.[2,3,4] In this paper we report on the
direct measurement of losses by Ka band microstrip resonators made
from laser ablated YBCO films on lanthanum aluminate. Also, we
calculate the Q values of the structure using the Phenomenological
Loss Equivalence Method and invoking superposition of the internal
impedances of the strip and ground plane of the microstrip line.
The calculated Q value of the ring resonator with a superconducting
strip and a normal conducting ground plane is compared with the
experimental results.
GROWTHAND PATTERNING
The superconducting films were deposited by laser ablation of a
sintered YBCO pellet onto a heated (700°C) lanthanum aluminate
substrate in a 100 mtorr oxygen atmosphere and then slowly cooled
to room temperature in 1 atmosphere of oxygen.[5] Films with very
smooth surfaces and Tc's of 89.8 have been produced; X-ray analysis
has shown that they are c-axis aligned. The microstrip resonators
were patterned by standard photolithography using negative
photoresist and a 'wet' chemical etchant. This etchant was either
a 3% solution of bromine in ethanol or dilute phosphoric acid in
water. A metal ground plane was deposited by first evaporating 100
of Ti for adhesion followed by 1 micron of gold. In addition to
the resonator, each chip also had a test bar for direct Tc testing
of the patterned film. Identical resonators were fabricated
entirely from gold (both strip and ground plane) using evaporation
and lift-off to define the strip.
The resonators were measured using a Hewlett-Packard 8510 Network
Analyzer, operating in WR-28 waveguide. The microstrip circuit was
mounted in a tapered ridge waveguide to microstrip test fixture
which was mounted at the second stage of a two stage, closed cycle
helium refrigerator. Circuit temperatures reached approximately
20°K and were monitored by a silicon diode sensor mounted in the
test fixture. The entire cold finger and test fixture were
enclosed in a custom designed vacuum can. Microwave coupling to
the test fixture was through 6 inch sections of WR-28 waveguide
made of thin wall stainless steel to minimize heat conduction.
Vacuum was maintained at the waveguide feedthroughs by means of
'0' rings and mica sealing windows.
THEORETICAL CALCULATION OF Q
The theoretical Q values were calculated using the
Phenomenological Loss Equivalence Method (PEM).[6] This method is
applicable to cases where the strip conductor thickness is on the
order of a skin depth (for a normal metal) or a penetration depth
(for a superconductor). The Incremental Inductance Rule, which is
often used to calculate microstrip losses, can only be applied in
the case of shallow field penetration, which is not satisfied in
this study. Also, PEM has the advantage of simple calculation
compared with other numerical techniques such as the Finite Element
Method. The technique proceeds on the basis of separately
calculating the internal impedances of the strip and the ground
plane through use of an equivalent isolated strip, and then adding
these impedances to the external impedance of the microstrip
structure. First, the ground plane is assumed to be a perfect
conductor so that there is no magnetic field penetration into it
as shown in figure i. A geometric factor (GI) for the strip line
is then obtained from the magnetic field penetration into it. This
G1 factor is used to obtain an equivalent strip; from which the
internal impedance of the microstrip line under the assumption of
perfect ground plane can be obtained as
Zi, = G,.Z_,.coth(Z,1._,-A.G1)
ORIGINAL PAGE IS
OF POOR QUALITY
where Z,I , G I and A are the surface impedance, the conductivity of
the material and the cross sectional area of the strip,
respectively. Next we consider the strip as a perfect conductor
as shown in figure i. Then a geometric factor (G2) is obtained for
the field penetration into the ground plane. With the value G2,
we obtain the internal impedance of the ground plane based on the
assumption of a perfect strip,
where Z.2 and _2 are
surface impedance and
conductivity of the
ground, respectively.
The internal
impedance of the
microstrip line is
obtained by adding Z n
and Zi2. We add this
internal impedance to
the external
inductance and
calculate the
propagation constant
of the microstrip
line by using a
transmission line
G 2.z,2.coth(z 2.a 2.A.(32)
Field penemadon
Perfect conductor
Ri I Lil
PerfectConductor
m
Ri2 Li2
t
Nit Lit
Figure I. Field Penetration in the strip and
ground plane; for PEM calculation.
model. It should be emphasized that (I) and (2) are applicable to
any field penetration depth.
The conductor losses of the structure in fig. 2 were calculated
by applying the method explained above. Then, the Q values of each
resonator were calculated by additional consideration of substrate
SUPERCONDUCTING
STRIP LINE --_
\
\\ ,,-- RING = 3k
\ ,,
w= t43 p.m
t I = 0.5l.gn
ts = 254 I.tm
tt = 0.1 bLrn
t2 = 1.0 $,tm
/
GROUND , - II
PLANE --" [:i:!_,_%;_)_;_;_i_ _ _i:ii_: _.
t2 I_,_,_y:_._._._:_,,i - _ tt ( Titanium Layer )
* /
Au
Figure 2. 35 GHz Ring Resonator.
loss; radiation loss
was assumed
negligible. For the
calculation, the
value of 5.8xi0-4 was
used for the loss
tangent. Since the
current is more
concentrated on the
strip, the
implementation of a
superconductor in the
strip has more
influence on the
loss. The extent of
the effect of
implementing a
superconductor in the
microstrip line can
be different for
different geometries.
Log Mug
18 dB/ SII REFLECTED POHER
0 !
G_ K
f
78 K
STRRT: 33 GHz
STOP: 37 GHz
RESULTS AND DISCUSSION Figure 3. Sll of the superconducting
In figure 3 are resonator in its test fixture; at three
shown plots of SII temperatures.
for a superconducting
resonator at several
temperatures. This plot is of the reflected power from the
resonator in the test fixture and is thus a measure of the loaded
'Q'. Two features are apparent; I) the coupling changes with
temperature (in this case, starting at near critical coupling just
below Tc and going to overcoupled at lower temperatures), and 2)
the resonant frequency shifts with temperature. The change in the
resonant frequency vs temperature is plotted in figure 4 along with
the resonant frequencies of a gold resonator. The variation
observed in the gold resonator follows the form expected from
thermal contraction in the substrate. But since accurate data on
lanthanum aluminate is not readily available, ],recise comparisons
are not possible. The variation seen in the superconducting
resonator is a consequence of the dependence of the internal
impedance of the strip on the changing normal/superconducting
electron densities. The internal inductance of a superconducting
strip over a ground plane is given by:[7]
Lin t = #o,k,coth(t/k)
Assuming the Gorter-Casimir temperature dependence of k:
X(T) = Xo
[ i- (T/Tc)'] _
the form of the resonant frequency variation based on the changing
ORIGINAL PAGE IS
OF POOR QUALITY
line inductance matches the experimental observations. However,
attempts at numerical fitting to extract ko, result in k o in excess
of 1 micron, indicating that the film quality may not be at its
highest.
RESONRNT FREQUENCY v= TEHPERRTURE
S5.5
_|l.J
85.1
S4°9
TEMPERRTURE (KEI.VZN)
Fisure 4. Resonant frequencies vs temperature for superconducting
and normal strips.
The best circuit to date has been from a 6500 A film with a post 7
processing Tc of 79°K. The unloaded Q of this circuit is plotted
against temperature in figure 5 along with the unloaded Q of an
identical gold resonator. The Q of the superconducting circuit
rises sharply below Tc, exceeding the Q of the gold circuit at
~60°K and reaching a value of 1.5 times that of the gold resonator
at 25°K. Comparing the experimental results with the calculated
values in the same figure, we see that for the gold resonator, the
PEM calculation matches the experimental fairly well. The measured
superconducting 'Q', however, is much lower than the calculated
values. Several reasons can be given for this. First, the values
for the complex conductivity of the superconductor used in the PEM
calculation were obtained by microwave reflectance/transmission
measurements on separate laser ablated films.[8] It is likely that
the quality of those films was higher than the resonator film, in
part because these films were unpatterned. In addition, substrate
losses in the PEM were calculated on the basis of tan6=5.8×10E-4
but accurate values for lanthanum aluminate are not available so
the actual value may be higher or lower. It seems likely that
improvements in the measured Q are possible with increased film
quality.
ORFGINAL PAGE FS
OF POOR QUALITY
CONCLUSIONS
Ring resonator circuits were fabricated from laser ablated YBCO
superconducting films on lanthanum aluminate to determine
transmission line losses at millimeter wave frequencies. At 25°K
the unloaded Q of the superconducting resonator was 1.5 times the
Q of _denLical resonators made of gold. A shift in the resonant
freque_,,_y _i_IL uempera_ure follows _he form predicted by the London
equanion. Using nhe PEM we calculated the Q values of the ring
resonator with a thin YBCO strip and a gold ground plane. The
theoretical results were compared with experimental results of the
ring resonator of that structure. The calculated results predict
higher values of Q than those actually observed, but improved film
quality should increase measured Q values.
I|m
|Im
?Im
Q
m
|IR
HERSURED Ik MODELED "Q"
-,._ s'r8
I I , |
211 51 75
TEHPERRTURE (KELVTN)
llu
Figure 5. Measured and calculated values of unloaded Q for
superconducting and normal resonators.
REFERENCES
I. N. Klein, G. Muller, H. Piel, B. Roas, L. Schultz, U. Klein and
M. Peiniger, "Millimeter Wave Surface Resistance of Epitaxially
Grown YBCO Thin Films," Appl. Phys. Lett. Vol. 54, pp 757-759.
2. A. A. Valenzuela and P. Russer, "High Q Coplanar Transmission
Line Resonators of YBCO on MgO," Appl. Phys. Lett., Vol. 55, pp.
1029-1031, 1989.
3. B. R. McAroy, G. R. Wagner, J. D. Adam, J. Talvacchio and M.
Driscoll, "Superconducting Stripline Resonator Performance," IEEE
Trans. Magn., Mag. Vol. 25, pp. 1104-1106 (1989).
4. J. H. Takemoto, F. K. Oshita, H. R. Fetterman, P. Kobrin, and
E. Sovoro, "Microstrip Ring Resonator Technique for Measuring
Microwave Attenuation in High-Tc Superconducting Thin Films," IEEE
Trans. Microwave Theory and Tech., Vol. MTT-37, pp. 1650-1652,
1989.
5. J. D. Warner, K. B. Bhasin, N. C. Varaljay, D. Y. Bohman and C.
M. Chorey, "Growth and Patterning of Laser Ablated Superconducting
YBCO Films on LaAIO3 Substrates," NASA TM-I02336.
6. H. Y. Lee, and T. Itoh, "Phenomenological Loss Equivalence
Method for Planar Quasi-TEM Transmission Line with a Thin Normal
Conductor or Superconductor," IEEE Trans. Microwave Theory and
Tech., Vol. MTT-37, no. 12, pp. 1904-1909, 1989.
7. James Swihart, "Field Solution for a Thin-Film Superconducting
Strip Transmission Line," Journal Appl. Phys., Vol 32, no. 3, pp.
461-469, 1961.
8. F. A. Miranda, W. L. Gordon, K. B. Bhasin, V. O. Heinen, and J.
Valco, "Millimeter Wave Transmission Studies of YBCO Thin Films in
the 26.5 to 40 GHz Frequency Range," Proc. Third Annual Conf. on
Superconductivity and Applications, to be published by Plenum Press
1990, and NASA TM-102345.
7
National Aeronauticsand Report Documentation Page .
Space Administration
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TM-102526
4. 5. Report Date
12.
Title and Subtitle
Performance and Modeling of Superconducting Ring Resonators at
Millimeter-Wave Frequencies
7. Author(s)
K.B. Bhasin, C.M. Chorey, J.D. Warner, R.R. Romanofsky, V.O. Heinen,
K.S. Kong, H.Y. Lee, and T. Itoh
Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
6. Performing Organization Code
8. Performing Organization Report No.
E-5334
10. Work Unit No.
307-51-00
11. Contract or Grant No.
13. Type of Reportand PeriodCovered
Technical Memorandum
14. Sponsoring Agency Code
i15. Supplementary Notes
Prepared for the 1990 IEEE MTT-S International Microwave Symposium, Dallas, Texas, May 8-10, 1990.
K.B. Bhasin, J.D. Warner, R.R. Romanofsky, and V.O. Heinen, NASA Lewis Research Center. C.M. Chorey,
Sverdrup Technology, Inc., NASA Lewis Research Center Group, Cleveland, Ohio 44135. K.S. Kong,
H.Y. Lee, and T. Itoh, Department of Electrical and Computer Engineering, The University of Texas at Austin,
Austin, Texas 78712.
16. Abstract
Microstrip ring resonators operating at 35 GHz have been fabricated from laser ablated YBCO thin films
deposited on lanthanum aluminate substrates. They were measured over a range of temperatures and their
performance compared to identical resonators made of evaporated gold. Below 60* Kelvin the superconducting
strip performed better than the gold, reaching an unloaded "Q" - 1.5 times that of gold at 25* K. A shift in the
resonant frequency follows the form predicted by the London equations. The Phenomenological Loss Equivalence
Method is applied to the ring resonator and the theoretically calculated Q values are compared to the
experimental results.
17. Key Words (Suggested by Author(s))
Superconducting thin films
Microwave passive circuits
Laser ablation
18. Distribution Statement
Unclassified - Unlimited
Subject Category 33
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages
Unclassified Unclassified 8
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Performance and modeling of superconducting ring resonators at millimeter-wave frequencies

Performance and modeling of superconducting ring resonators at millimeter-wave frequencies

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