The surface resistance of niobium nitride (NBN) thin films was measured at 7.78 and 10.14 GHz in the temperature range of 1.5 to 4.2 K. The films were reactively sputtered on sapphire substrates to a thickness of approximately 1 micron. The surface resistance was determined by measuring the quality factor (Q) of the TE sub 011 mode of a lead-plated copper cavity where the NbN served as one end-cap of the cavity

Bautista, J. J.

Strayer, D. M.

English

NASA Technical Reports Server

TDA Progress Report 42-88
N87-17947
October-December 1986
Microwave Surface Resistance of Reactively Sputtered
NbN Thin Films
J. J. Bautista
Radio Frequency and MicrowaveSubsystemsSection
D. M. Strayer
Microgravity and Flight Experiments ResearchSection
The surface resistance of niobium nitride {NbN) thin films has been measured at 7. 78
and 10.14 GHz in the temperature range of 1.5 to 4.2 K. The films were reactively sput-
tered on sapphire substrates to a thickness of approximately 1 micron. The surface
resistance was determined by measuring the quality factor (Q) of the TEo] 1 mode of a
lead-plated copper cavity where the NbN served as one end-cap of the cavity.
I. Introduction
Although the B1 and AI5 superconductors are of primary
interest for the development of active devices (Josephson junc-
tion and SIS devices), homogenous, high critical temperature
(To) superconductors are also of interest for passive micro-
wave device applications. Dissipative losses and interfering
signals can significantly increase the noise temperature of
cryogenic ultra-low noise microwave amplifiers such as masers
(Ref. 1) and High Electron Mobility Transistor (HEMT) ampli-
fiers (Ref. 2). Microstrip and stripline superconducting filters
and transmission lines can provide protection without sig-
nificantly increasing the amplifier noise temperature (Ref. 3).
Furthermore, the widespread availability of relatively inexpen-
sive commercial, close-cycle refrigerators (10- to 15-K operat-
ing raage) will readily allow the integration of high Tc super-
conducting devices with HEMTs.
For temperatures below 4.2 K, high Tc resonant structures
can significantly improve the performance of maser amplifiers
and cavity-stabilized oscillators. In a traveling wave maser
(TWM) amplifier, dissipative losses in the slow-wave structure
limit the amplifier noise temperature and gain. A supercon-
ducting slow-wave structure having a high second critical
magnetic field would allow the construction of a TWM ampli-
fier with near theoretical response, lowering the noise temper-
ature by as much as 50% and increasing the gain bandwidth
product by as much as 100%. The long-term frequency stabil-
ity of a superconducting cavity-stabilized oscillator (SCSO)
relies on the high Q and good mechanical stability of the
superconducting cavity. A high T c superconductor deposited
on a sapphire substrate to form a superconducting cavity
would allow operation of the oscillator at a higher tempera-
ture. For example, for a cavity having a Q of 109, 10 -17
stability can be obtained using a lead film only by cooling to
24
https://ntrs.nasa.gov/search.jsp?R=19870008514 2020-03-20T12:39:59+00:00Z
0.9 K, while an NbN cavity would allow operation at 2 K
(Ref. 4). Both these systems place requirements on the surface
resistance of the superconducting film at microwave
frequencies.
Surface resistance measurements can also serve as a deposi-
tion process monitor and as a non-destructive means of eval-
uating the homogeneity and quality of the film (Refs. 5 and
6). This work was initiated to determine the feasibility of the
B1 compound NbN for passive microwave applications, since
it can be fabricated with relative ease and, in addition, good
quality thin films have been demonstrated using various
deposition methods (Refs. 7, 8, and 9). This article presents
the preliminary results of measurements of the surface resis-
tance of NbN at X-band frequencies.
II. Sample Preparation
The NbN thin films were dc-magnetron sputtered at an
approximate rate of 1700 A/rain onto flat sapphire substrates
to a thickness of about 1 micron. Throughout the deposition,
the nitrogen partial pressure was maintained at 0.57 Pa and the
argon at 1.56 Pa. The deposition was initiated with the sub-
strates at room temperature and no attempt was made to con-
trol the substrate temperature during the deposition.
The epi-finished sapphire substrates were purchased from
Insaco, Inc. (Quakertown, Penn.). Before the deposition, the
substrates were cleaned with soapy de-ionized water and then
rinsed in de-ionized water. The substrates were then degreased
by rinsing them in acetone (5 min), methanol (5 min), and
freon (5 rain).
A variety of sample sizes were deposited for three different
experiments. Three 5-cm diameter wafers (two 0.025- and one
0.300-cm thick) were made for the Q-cavity measurements
presented in this report. A second set of four rectangular sam-
ples (7.00 by 2.03 by 0.064 cm) were made for calorimetric
surface resistance measurements (Ref. 10). The third set of
rectangular samples (four 9.40 by 0.56 by 0.076 cm and four
12.7 by 2.13 by 0.152 cm) were saved for slow-wave structure
patterning and subsequent insertion loss measurements.
III. Microwave Measurements
The two cylindrical X-band cavities used for the Q measure-
ments were made of OFHC copper electroplated with lead to a
thickness of approximately 3 microns (Ref. 11). The cavities
have diameter to length ratios of 1.0 and 1.75, and were
operated in the TErn1 mode at 7.78 and 10.14 GHz, respec-
tively. The cavity Q's were all determined by pulsing the
incident microwave energy and measuring the decay time of
the stored cavity energy (Ref. 12). For these measurements,
the homogeneous Pb(Cu) cavities were first characterized as a
function of temperature and the surface resistance (Rs) was
determined using the following expression:
G
Rs - Q,, (1)
where G is a geometrical factor (780 and 619 ohms, respec-
tively, for the present geometries) and Qo is the baseline qual-
ity factor for the Pb(Cu) surface only.
The surface resistance at 7.78 GHz of samples 1 and 2
(NbN on 0.025-cm-thick substrates) was determined by plac-
ing them on top of the bottom end-cap of the cavity and
repeating the decay time measurements. At 10.14 GHz, sample
3 (NbN on 0.300-cm thick substrate) replaced the removable
Pb(Cu) end-cap. For this cavity configuration, the relation-
ship between the surface resistance of the thin-film end-cap
(Rx) and the measured quality factor, Qx, is given by the
following expression:
R x : Qo
where f (0.072 and 0.206, respectively) is the fraction of the
total cavity loss associated with the thin-film end-cap.
IV. Experimental Results and Discussion
The results at 7.78 GHz for samples 1 and 2 are plotted in
Fig. 1. Included for comparison are the Stanford calorimetric
data (Ref. 10) for sample 4 at 8.6 GHz. Although the Q-cavity
and calorimetric cavity data appear to be in agreement at
temperatures near 4 K, the measured results for samples 1 and
2 show lots of scatter and appear to be temperature indepen-
dent. Since samples 1 and 2 rest on top of the cavity end-cap,
it is strongly suspected that inadequate heat contact with the
helium bath resulted in the observed thermal heating effects.
To obviate this latter difficulty, the IO.14-GHz cavity was
constructed so that it would accept sample 3 as the end-cap.
This configuration results in a sample that is in direct contact
with the helium bath. Figure 2 contains a plot of the results
for NbN sample 3, along with the data for the Pb(Cu) at
10.14 GHz. Both sets of data clearly exhibit a strong tempera-
ture dependence; however, the expected (BCS) precipitous
drop in surface resistance is absent.
25
The data for sample 3 was fit by linear regression to a
linearized form of the BCS surface resistance for T <0.ST c
given by:
Rs(T)=Aexpl-eg(_)l+Ro (3)
where R o is a residual surface resistance, eg is the normalized
zero temperature energy gap parameter, and T c is the critical
transition temperature. Figure 3 contains a plot of the best fit
to the data of sample 3, assuming Tc = 15.2 K and R o =
10 micro-ohms. The resulting 0.52 value of eg is a factor of
one half smaller than the smallest values determined from
tunneling measurements (Ref. 9).
Although the eg value for NbN sample 3 is anomalously
small, it is in agreement with the results obtained by Isagawa
at 6.5 GHz on samples of NbN reactively sputtered on Nb
(Ref. 13). But in contrast to the NbN results, the linear regres-
sion analysis for the 7.78- and 10.14-GHz Pb(Cu) cavity mea-
surements yielded eg values of 1.98 and 2.00, respectively,
which are approximately 10% less than the tunneling measure-
ment values (Ref. 14).
The large difference between the eg values determined by
RF measurements and those determined by dc tunneling mea-
surements is believed to be associated with the columnar
structure of sputtered NbN and the compounds that form
between the NbN and the 10 A Nb20 s outermost layer. The
smaller eg value obtained from RF measurements is the result
of the formation of a metallic NbN suboxide with a T c of
approximately 7 K adjacent to the serrated NbN (Ref. 15).
The dc tunneling currents flow directly across the serrated
peaks of NbN, while RF currents are induced in both the
NbN and the lossier NbN suboxide, resulting in an averaged
lower eg.
V. Conclusions
In Fig. 1, for comparison to a normal metal, the surface
resistance is plotted for OFHC Cu, calculated from the inser-
tion loss measurements as a function of temperature for an
interdigital filter. Comparison of this Cu data with the calori-
metric data shows that below 12 K, the NbN surface resistance
is at least 1/10th the value of Cu. Future planned insertion
loss measurements of NbN slow-wave structures at 2.3 and
8.4 GHz subjected to large dc magnetic fields will determine
the applicability of NbN to TWMs. Although the question
regarding the applicability of NbN for SCSOs remains unre-
solved, it is evident that this material is clearly applicable for
microstrip and stripline devices operating in the 10- to 15-K
temperature range.
References
1. Bautista, J. J., and Petty, S. M., "Cryogenic Filters for RFI Protection," TDA Progress
Report 42-65, p. 94, July-August, 1981.
2. Pospieszalski, M. W., Weinreb, S., Chao, P. C., Mishra, U. K., Palmeter, S. C.
Smith, P. M., and Hwang, J. C. M., "Noise Parameters and Light Sensitivity of
Low-Noise HEMTs at 300 and 12.5 K," 1EEE Trans. Electron Devices, ED-33, pp.
218-223, February 1986.
3. Bautista, J. J., and Petty, S. M., "Superconducting NbTi and Pb(Cu) Bandpass
Filters," 1EEE Trans. Magn. MAG-21, No. 2, pp. 640-643, 1985.
4. Strayer, D. M., Dick, G. J., and Mercereau, J. E., "Performance of a Superconducting
Cavity Stabilized Ruby Maser Oscillator," Applied Superconductivity Conference
Paper No. MH-4, 1986 Applied Superconductivity Conference, September 29-
October 3, 1986.
5. Allen, L. H., Beasley, M. R., Hammond, R. H., and Turneaure, J. P., "RF Surface
Resistance of High T¢ Superconducting A15 Thin Films," 1EEE Trans. Magn.,
MAG-21, No. 3, p. 1003, 1983.
26
6. Allen, L. H., Anklam, W. J., Beasley, M. R., Hammond, R. H., and Turneaure, J. P.,
"RF Surface Resistance in NbaSn Thin Films," IEEE Trans. Magn., MAG-21, No. 2,
pp. 525-527, 1985.
7. Villegier, J. C., Vieux-Rochaz, L., Goniche, M., Renard, P., and Vabre, M., "NbN
Tunnel Junctions," IEEE Trans. Magn., MAG-21, No. 2, pp. 498-504, 1985.
8. Gurvitch, G., Remekia, J. P., Rowell, J. M., Geerk, J., and Lowe, W. P., "Tunneling,
Resistive and Structural Study of NbN and Other Superconducting Nitrides," IEEE
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9. Bacon, D. D., English, A. T., Nakahara, S., Peters, F. G., Schreiber, H., Sinchair, W. R.,
and van Dover, R. B., "Properties of NbN Thin Films Deposited on Ambient Tem-
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I0. Allen, L. H., Beasley, M. R., and Turneaure, J. P., "RF Surface Resistance of NbaSn ,
NbZr, and NbN Thin Films," ASC Paper No. MG-20, 1986 Applied Superconduc-
tivity Conference, September 29-October 3, 1986.
11. Dick, G. J., and Delayen, J. R., "A New Chemical Polishing Procedure for Lead-
Plated Copper Superconducting Accelerating Resonators," IEEE Trans. Magn.,
MAG-19, No. 3, 1983.
12. Yogi, T., Radio Frequency Studies of Surface Resistance and Critical Magnetic
Field of Type I and Type II Superconductors, Ph.D. Thesis, California Institute of
Technology, 1977.
13. Isagawa, S., "RF Superconducting Properties of Reactively Sputtered NbN," J.
AppliedPhys., 52, pp. 921-927, 1981.
14. Kittel, C., Introduction to Solid State Physics, 5th Ed., John Wiley and Sons, NY,
NY, pp. 412-413, 1976.
15. Halbritter, J., "Superconductivity of Oxidized NbN Compound Surfaces," ASC
Paper No. MC-11, 1986 Applied Superconductivity Conference, September 29-
October 3, 1986.
27
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28


Microwave surface resistance of reactively sputtered NbN thin films

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