Radio frequency (RF) surface resistance measurement experiments on high T(sub c) thin films were performed. The method uses a microstrip resonator comprising a top gold conductor strip, an alumina dielectric layer, and a separate superconductivity ground plane. The surface resistance of the superconducting ground plane can be determined, with reference to a gold calibration standard, from the measured quality factor of the half-wave resonator. Initial results near 7 GHz over the temperature range from 25 to 300 K are presented for YBa2Cu3O(7-x) and Bi2Sr2CaCu2O(8-y) thin film samples deposited by an electron beam flash evaporation process. The RF surface resistance at 25 K for both materials in these samples was found to be near 25 milliohms

Davis, Matthew

Lichtenberg, Christopher L.

Wolfe, J. C.

Wosik, Jaroslaw

English

NASA Technical Reports Server

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c; 40025 
NASA Technical Memorandum 102 159 
J . 
A 
Microwave Microstrip Resonator 
Measurements of Y Ba2 Cu3 0 7 - x  and 
Bi2 Sr2 Cal  Cu2 08-y Thin Films 
Christopher L. Lichtenberg, Jaroslaw Wosik, Matthew Davis, and 
J.C. Wolfe 
November 1989 
NASA 
https://ntrs.nasa.gov/search.jsp?R=19900004388 2020-03-19T23:49:44+00:00Z
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NASA Technical Memorandum 102 159 
Microwave Microstrip Resonator 
Measurements of Y 1 Ba2 CuQ 0 7 - x  and 
Biz Sr2 Cal Cu2 08-y Thin Films 
Christopher L. Lichtenberg 
Lyndon B. Johnson Space Center 
Houston, Texas 
Jaroslaw Wosik, Matthew Davis, and J.C. Wolfe 
University of Houston 
Department of Electrical Engineering and 
Texas Center for Superconductivity 
Houston, Texas 
ABSTRACT 
Radio frequency (RF) surface resistance measurement experiments on high- 
T, thin films were performed between July and November 1988. The 
method uses a microstrip resonator comprising a top gold conductor strip, 
an alumina dielectric layer, and a separate superconducting ground plane. 
The surface resistance of the superconducting ground plane can be deter- 
mined, with reference to  a gold calibration standard, from the measured 
quality factor of the half-wave resonator. Initial results near 7 GHz over the 
temperature range from 25 to 300 K are presented for Y 1 Ea2 Cu3 0 7 - x  and 
Biz Sr2 Cal Cu2 08-y thin film samples deposited by an electron beam flash 
evaporation process. The RF surface resistance at 25 K for both materials in 
these samples was found to  be near 25 milliohms. 
High-T, superconducting thin films have potential applications in analog signal processing circuits 
with bandwidths of tens of terahertz.1 Characterizing the microwave surface resistance of these thin 
films is  vital to  assessing and predicting their performance in microwave devices. Several methods 
have now been employed to  measure microwave surface resistance. Low-T, material cavities have 
produced some of the most sensitive and widest dynamic range measurements of ceramic, thin film, 
and single crystal high-T, samples.2 Another powerful method uses electro-optical techniques to  
determine attenuation and distortion of sub-picosecond pulses on coplanar transmission lines.' 
Loaded-waveguide methods have also been used with some success.3 The use of a superconducting 
stripline (tri-plate) resonator has been successfully used in the microwave band.4 A low impedance 
microstrip resonator, excited by an impedance mismatch method has also been used.5 Finally, a disk 
resonator structure has been used for measuring thin films and bulk material.6 
In this work, a simple open-ended, gap-coupled microstrip resonator method7.8 adapted for 
determining the RF surface resistance of a material is  discussed. The method does not require dc 
electrical contact or patterning and is  insensitive to the film supporting substrate material. This 
technique applies a frequently used microwave circuit element, i.e., a half-wave resonator.7 
Preliminary measurements of the RF surface resistance of Y1 BazCu3O;r-, (YBCO) and BizSrzCal CuzO8-,, 
(BSCCO) thin films are presented. 
The cryogenic and microwave test equipment was specially assembled for these experiments. A 
commercially available two-stage, closed-cycle helium cryopump was modified for cooling the test 
resonator package. Vacuum system flanges and electrical feedthroughs were added onto the basic 
cryopump vacuum shroud. This allowed us to connect heater wires and stainless steel microwave 
coaxial cables to the test fixture. Base temperatures of approximately 16 K were possible. The 
micro\ivave response (magnitude of S21) was measured using an HP8510A automatic vector network 
analyzer and the S21 data were used to  calculate the quality factor of each resonance. 
Figure 1 illustrates the microstrip resonator test configuration. During measurements, high-T, film is 
used as a ground plane. A reference or known material, such as gold, is used for the top strip of the 
resonator. A known material ground plane is used in place of the high-T,fiIm during calibration 
1 
measurements. Excitation of the resonator arises by capacitive gap coupling from open-ended 
coaxial connectors in proximity to the resonator ends. The weakly coupled condition (insertion loss 
greater than 30 decibels) is achieved by empirically adjusting the spacing between the center conduc- 
tor end and the resonator end. Radiation loss from the resonating microstrip is controlled by choos- 
ing an appropriately small resonator geometry.9 Dielectric loss effects are avoided through the use of 
low microwave loss dielectrics such as alumina or sapphire. 
The high-T, films reported here were deposited by a novel electron beam flash evaporation tech- 
nique which reliably produces YBCO and BSCCO films whose dc zero resistance temperatures were 
typically 75 and 80 K, respectively.10 This process is unique in that the starting material is high purity 
superconducting powder. The films were deposited on heated, 0.5 by 0.5 by 0.02 inch MgO single 
crystal (100) substrates. The films were approximately 0.5 micrometer thick. The YBCO and BSCCO 
films required a post-deposition annealing at 875" and 860" C, respectively, followed by a slow, 6-hour 
cooling to room temperature in flowing oxygen. No special treatment of the film surface was per- 
formed to reduce RF losses. The BSCCO films were highly oriented with the c-axis perpendicular to  
plane of the substrate. The YBCO films were not preferentially oriented. 
The measurement technique involves first measuring a reference gold metal resonator, then replac- 
ing the gold reference ground plane with the high-T, thin film to be tested. Using standard analytical 
relations for a microstrip transmission line model, an RF surface resistance can be assigned to  the 
high-T, thin film ~ample.7.8~11 The total Q of the resonator can be expressed by the equation 
where Qc, Qd, Qr, and Qe are the quality factors corresponding to  the conductor, the dielectric, radia- 
tion, and external losses, respectively. Other terms may be added to  this equation. However, in this 
work additional terms were not considered because of their negligible influence on the measured Q. 
In transmission line theory, the total conductor loss is a function of the surface resistances of the top 
strip line and ground plane. By measuring Q for a resonator in which both conductors are gold, one 
can solve for the surface resistance of gold under the assumption that the top gold strip and the 
ground plane have equal surface resistance. Replacing the gold ground plane by the high-T,ground 
plane to be tested, then performing again the Q versus temperature measurements, one can then 
calculate the microwave surface resistance of the high-T, ground plane. 
In i t s  present form this technique can be used for measuring thin films whose resistance is  of the same 
order or higher than the reference metal. Raw microwave data are available from approximately 
5 to 26.5 GHz; however, only the data from approximately 5 to 7 GHz are presently used due to 
cabling and connector interfering reflections which become worse at higher frequencies. Methods 
are available to  improve the test fixture and to  extend the useful measurement frequency range. 
Figure 2 demonstrates typical frequency response as viewed on the network analyzer around the 
fundamental microstrip resonance for a BSCCO thin film ground plane. One can notice an increas- 
ingly sharper peak with lowering temperature which indicates lessening surface resistance. 
2 
Figure 3 shows the dependence of Q and the calculated microwave resistance on temperature for a 
high-T,ground plane. For gold the Q value increases smoothly to approximately twice the room 
temperature value at 25 K. However, the Q value of the high-T,films increases typically by orders of 
magnitude in the same range of temperature and shows the characteristic rapid increase below T,. A 
dramatic drop in RF resistance below the critical temperature is similar to the dc resistance, but unlike 
the dc resistance the RF surface resistance in these samples is not negligible below the critical temper- 
ature. Surface resistance values around 25 milliohms were encountered in our samples a t  25 K. The 
YBCO and BSCCO samples here showed, by coincidence, similar RF surface resistance values. 
A microstrip resonator method useful for determining the r f  surface resistance of high-T, samples was 
described. Some initial results were presented. We believe this test method is attractive because it is 
simple to  apply and should be especially useful for quickly characterizing high-T, samples for their RF 
losses. Improvements to our method might include using a very low-loss superconductive top strip 
and reducing the microstrip dielectric height further. These improvements should increase the sen- 
sitivity of measurement. 
REFERENCES 
- 1. D. R. Dykaar, R. Sobolewski, J. M. Chwalek, 1. F. Whitaker, T. Y. Hsiang, G. A. Mourou, D. K. 
Lathrop, 5. E. Russek, and R. A. Buhrman, Appl. Phys. Lett. 52, 1444 (1988). 
2. D. L. Rubin, K. Green, 1. Gruschus, J. Kirchgessner, D. Moffat, H. Padamsee, 1. Sears, Q. 5. Shu, L. F. 
Schneemeyer, and J. U. Waszczak, Laboratory of Nuclear Studies, Cornell University, 
Report # CLNS 88/840, May 1 1 , 1988. 
3. J. Tateno and N. Masaki, Jap. J. Appl. Phys. 26, L1654(1987). 
4. E. R. McAvoy, G. R. Wagner, J. D. Adam, 1. Talvacchio, and M. Driscoll, IEEE Trans. Magn. 
MAG-25, (1989) (preprint). 
5. 1.5. Martens and J. B. Beyer, Microwaves and RF 7, 150 (1988). 
6. A. Fathy, D. Kalokitis, and E. Belohoubek, Microwave Journal 31,75 (1988). 
7. R. K. Hoffmann, lntegrierte Mikrowellen Schaltungen, Springer-Verlag, Berlin, (1983). 
8. J. D. Woermbke, Microwaves 21,89 (1982). 
9. R.-bavidheiser, Procs. 34th Ann. Freq. Control Symposium, 252 (1980). 
10. M. Davis, J. Wosik, J. C. Wolfe, and C. L. Lichtenberg, Procs. of the 35th Nat'l Symposium of the 
AVS, (1 988). 
11. C. L. Lichtenberg, Universityof Houston, M. 5. Thesis, May 1989. 
3 
=Top Gold Stripe 
I ]Alumina 
High-Tc Film 
MgO Substrate 
Figure 1 .- Microstrip resonator configuration. 
4 
-33 
-53 
-73 
Figure 2.- Typical frequency response around the fundamental microstrip resonance for a BSCCO 
thin film ground plane sample. 
5 
i 225 
50 - 
25 - 
200 i 
1 175 
K 150 1 
0 
751 
4 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 QUALITY FACTOR 
0 MICROWAVE RESISTANCE 
0 
0 0 
B 
7 
6 
5 
4 
3 
2 
' 1  
- 0  
TEMPERATURE [K] 
Figure 3.- Quality factor Q and RF surface resistance versus temperature at 7 G H t  for a BSCCO thin 
film determined by the microstrip resonator technique. 
6 
1. Report No. 2 Government Accession No 
NASA TM 102 1 59 
4. Title and Subtitle 
Microwave Microstrip Resonator Measurements of 
Y1 Ba2 Cu3 07-x and Biz Sr2 Ca1 Cu2 Thin Films 
7. Author(s) 
Christopher L. Lichtenberg, Jaroslaw Wosi k, Matthew Davis, 
and J.C. Wolfe 
3. Recipient's Catalog No 
5 Report Date 
November 1989 
6. Performing Organization Code 
8. Performing Organization Report No. 
5-60 1 
9. Performing Organization Name and Address 
Lyndon B. Johnson Space Center 
Houston, Texas 77058 
12. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 
10. Work Unit No. 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
Washington, DC 20546 
17. Key Words (Suggested by Author(s1) 
Thin films Microwave measurements 
Resonators Cryogenic measurements 
Microwave resonance 
Microstrip devices 
High tem peratu re superconductors 
14: Spahsoring Agency Code 
18. Distribution Statement 
Unclassified - Unlimited 
Subject Category: 33 
I 
15. Supplementary Notes 
Christopher L. Lichtenberg - Lyndon B. Johnson Space Center 
Jaroslaw Wosik, Matthew Davis, and J.C. Wolfe - University of Houston, Department of Electrical Engineering and 
Texas Center for Superconductivity, Houston, Texas 
9. Security Classification (of this report) 20 Security Classification (of this page) 
Unclassified Unclassified 
16. Abstract 
Radio frequency (RF) surface resistance measurement experiments on high-Tc thin films were performed between July 
and November 1988. The method uses a microstrip'resonator comprising a top gold conductor strip, an alumina 
dielectric layer, and a separate superconductivity ground plane. The surface resistance of the Superconducting 
ground plane can be determined, with reference to a gold calibration standard, from the measured quality factor of 
the half-wave resonator. Initial results near 7 GHz over the temperature range from 25 to 300 K are presented 
forY1 Ba2 Cu3 07-x'and Biz Sr2 Cal Cu2 thin film samples deposited by an electron beam flash evaporation 
process. The RF surface resistance at 25 K for both materials in these samples was found to  be near 25 milliohms. 
21. No. of pages 22 Price 
9 
. .
. -  
JSC Form 1424 (Rev Aug 89) (Ethernet Jan 88) NASA-JSL 


Microwave microstrip resonator measurements of Y1Ba2Cu3O(7-x) and Bi2Sr2Ca1Cu2O(8-y) thin films

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