Title of Invention

RF-PROPERTIES-OPTIMIZED COMPOSITIONS OF (RE) BA2CU3O7-D THIN FILM SUPERCONDUCTORS

Abstract The films of this invention are high temperature superconducting (HTS) thin films specifically optimized for microwave and RF applications. In particular, this invention focuses on compositions with a significant deviation from the 1:2:3 stoichiometry in order to create the films optimized for microwave/RF applications. The RF/microwave HTS applications require the HTS thin films to have superior microwave properties, specifically low surface resistance, Rs, and highly linear surface reactance, Xs, i.e. high JIMD. As such, the invention is characterized in terms of its physical composition, surface morphology, superconducting properties, and performance characteristics of microwave circuits made from these films.
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SPECIFICATION
RF-Properties-Optimized Compositions of (RE)Ba2Cu307.8 thin film
Superconductors
Cross-Reference to Related Applications
[0001] This application claims priority pursuant to 35 U.S.C. § 119(e) to U.S.
Provisional Application Number 60/639,043, filed December 23,2004.
Field of the Invention
[0002] This invention relates to thin films of high temperature superconducting
compositions optimized for RF applications and a method for manufacturing them, more
specifically rare earth compositions of (RE)Ba2Cu3O7-8 deviating significantly from the
1:2:3 stoichiometry.
Background of the Invention
[0003] Rare earth oxide superconductors and their ability to superconduct at
significantly higher temperatures than previously recorded was first reported by J.G.
Bednorz and R.A. Muller in 1986 in regard to mixtures of lanthanum, barium, copper and
oxygen in an article entitled "Possible High Tc Superconductivity in the Ba-La-Cu-O
system." (64 Z. Phys. B. - Condensed Matter, ppl89 -193 (1986)). Bednorz and Muller
described Ba-La-Cu-O compositions that offered a substantial increase in the critical
temperature at which the material becomes superconducting over what had been
previously known for other classes of materials. Here, the composition was La5.
xBaxCu5O5(3-y) where x = 0.75-1, y >0, and the abrupt change in resistivity occurred in the
30 Kelvin range.
[0004] This contribution led to intensive investigation in order to develop materials
having even higher transition temperatures, preferably above 77 Kelvin as this enabled the
use of liquid nitrogen to cool the superconducting equipment. In 1987, C.W. Chu and co-
workers at the University of Houston found that the onset Tc of the La-Ba-Cu-0
compound could by increased to over 50 K by the application of pressure. (Phys. Rev.
Lett. 58.405 (1987); Science 235, 567 (1987)).
[0005] Chu and coworkers at Houston and at the University of Alabama subsequently
discovered a mixed-phase Y-Ba-Cu-O system onset having Tc values near 90 K and a
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zero-resistance state at ~70 K. This compound had the nominal composition
Y1.2Ba0.8CuO4-5 (Phys. Rev. Lett. 58,908 (1987). Chu and coworkers as well as scientists
at AT&T and IBM later showed this compound to consist of two phases of nominal
composition Y2BaCuO5 (the "green" phase) and YBa2Cu3O6+x (the "black" phase). The
latter phase was determined to be the superconducting phase, whereas the former was
semiconducting (Cava et al., Phys. Rev. Lett. 58,1676 (1987); Hazen et al., Phys. Rev. B
35,7238 (1987); Grant et al., Phys. Rev. Lett. 35,7242 (1987).
[0006] Superconductivity near 90 K was also reported in a mixed-phase Lu-Ba-Cu-O
compound by Moodenbaugh and coworkers (Phys. Rev. Lett. 58, 1885 (1987). Chu et al.
also identified superconductivity above 90 K for compounds of the formula ABa2Cu3O6+x,
where A = Y, La, Nd, Sm, Eu, Gd, Ho, Er, or Lu (Phys. Rev. Lett. 58,1891 (1987).
[0007] The data from these differing Rare Earth (RE)BCO (RE = rare earth, B = Ba, C
= Cu) compounds demonstrated that for this class of compounds, the superconductivity is
associated with the CuO2-Ba-CuO2-Ba-CuO2 plane assembly which can be disrupted by
the A cations only along the c-axis.
[0008] Following this discovery, research was focused on the YBCO class of
compounds with high temperature superconducting (HTS) properties. B. Satiogg first
discovered and isolated the single crystallographic phase responsible for the
superconducting properties of the YBCO compound. (B.Batlogg, U.S Patent No
6,636,603). In isolating this single perovskite phase of a composition, Batlogg admonished
that the composition was essential to isolation of the phase and that it must be within 10%
of the M2MCU3O7-8 composition where M is a divalent cation preferably barium and M' is
a trivalent cation preferably yttrium.
[0009] Other studies have investigated both the effects of substitution of various rare
earth elements for yttrium and of varying the 1:2:3 ratio of Y:Ba:Cu on the
superconducting properties of HTS compositions. Multiple studies have shown the ability
to partially or completely substitute rare earth elements except Pr, Ce and Tb and maintain
a Tc of approximately 90 K for the resulting (RE)BCO composition. (S. Jin, Physica C
173, pp 75-79 (1991)). Additionally, further studies show that the c-axis coherence length
and the Tc value increase with increasing ionic radius of the rare earth element substituted
for yttrium (G.V.M. Williams, Physica C 258, pp41-46 (1996)).
[0010] Building on these discoveries, P. Chaudhari and his co-workers at IBM developed
a method for making thin films of high temperature superconducting oxides with a
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nominal composition of (RE)(AE)2Cu3O9-y where RE is a rare earth element, AE is an
alkaline earth element and y is sufficient to satisfy valence demands. (Chaudhari, U.S.
Patent No. 5,863,869 (1999)). The rare earth elements used included Y, Sc and La, and
AE could also be substituted for by Ba, Ca or Sr. Copper was the preferred transition
metal for the oxide due to its high superconducting onset temperature and the smooth,
uniform properties of the copper oxide films. Using this growth process, Chaudhari was
able to obtain YBCO films with superconducting onset temperatures of about 97 Kelvin
that exhibited superconducting behavior from 50 Kelvin to in excess of 77 Kelvin. These
films were within 15% of the targeted (RE)(AE)2Cu3O9-y composition, and Chaudhari
noted that the exact composition was not necessary in order to observe high temperature
superconductivity.
[0011] However, in another study of (RE)BCO cation exchange in thin films, J
MacManus-Driscoll et al. noted that Tc decreased dramatically for off-composition films
with substitutions of rare earth (RE) elements on the Ba site such as RE(Ba2-xREx)Cu3Oy
where RE = Er or Dy and x > 0.1 (14% deviation) and where RE - Ho and x > 0 (any
deviation). (J.L MacManus-Driscoll, Physica C 232, pp 288-308 (1994). J. MacManus-
Driscoll further reported that the oxygen pressure at which the thin films were grown
seemed to have an effect on the structural disordering of the RE and Ba cations as did the
rare earth ion size. Small rare earth cations substituting for the larger Ba cations would
produce large strains on the lattice and therefore an unstable phase which would not likely
occur.
[0012] Another study of varying the 1:2:3 stoichiometry of YBCO thin films noted that
large excesses of yttrium formed ultra small yttrium precipitates leading to increased
surface resistance (R5) and poor microwave quality but that a slightly enhanced copper and
yttrium content lead to minimum surface resistance (E. Waffenschmidt, J. Appl. Phys. 77
(1) pg 438-440). Furthermore, N.G. Chew et al. analyzed the effect of slight changes in
composition on YBCO thin film structural and electrical properties and discovered that
films grown with a stoichiometry close to 1:2:3 or with excess yttrium are smooth while
films with excess barium exhibited surface roughness and growth of a-axis-oriented
grains. (N. Chew, Appl. Phys. Lett. 57 (19) pp 2016-2018 (1990). These authors further
found that there is a well defined YBCO composition where Tc and Jc are maximized and
the c-axis lattice constant, (007) x-ray peak width, and surface roughness are minimized.
These quantities were optimized for a Ba/Y ratio of 2.22 ± 0.05 (subsequently suggested
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to instead be equal to 2) and a Cu/(Y+Ba+Cu) ratio of 0.5. Slight changes in cation ratios
away from this optimized composition caused significant degradation in the parameters
listed above.
[0013] W. Prusseit et al. have created an iso-structural Dy-BCO thin film material with
improved properties compared to their YBCO films. By substituting dysprosium for
yttrium and growing under identical conditions as YBCO, Prusseit created films that
deviated only slightly from the 1:2:3 stoichiometry. Compared to their YBCO films, these
materials exhibited better chemical stability and enhanced transition temperatures (by 2-3
K), and they also had a 20% reduction in surface resistance (R5) at 77 K: ~250 mW vs.
-300 mW at 10 GHz, measured in a microwave cavity (W. Prusseit, Physica C 392-396, pp
1225-1228 (2003)). Hein (High-Temperature Superconductor Thin Films at Microwave
Frequencies (Springer Tracts in Modem Physics, 155), Berlin, 1999) and others have
measured somewhat lower surface resistance, ~200 mW at 10 GHz and 77 K, in cavity
measurements of YBCO thin films.
[0014] The compositions of these (RE)BCO compounds may be altered substantially
from the nominal 1:2:3 stoichiometry in order to optimize their properties for specific
applications. It is the primary object of this invention to provide high temperature
superconducting thin films that have the lowest possible RF surface resistance (R5,) values
as well as the lowest achievable RF nonlinearities. This often requires fabrication of
(RE)BCO films that deviate significantly from the 1:2:3 composition. It is another object
of this invention to provide a thin film superconductor that is optimized for RF/microwave
applications. It is another object of this invention that the film has a low surface resistance.
It is another object of this invention that the film has a highly linear RF/microwave surface
reactance. It is another object of this invention that the stoichiometry of the film deviates
by at least 10% from the standard 1:2:3 stoichiometry and with full substitution for yttrium
by a rare earth element.
Summary of the Invention
[0015] The films of this invention are high temperature superconducting (HTS) thin
films specifically optimized for microwave and RF applications. The prior art (RE)BCO
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films exhibiting high temperature superconducting properties were nominally of the
composition (RE)xBayCu3O7-8 where RE = a rare earth element, preferably yttrium, x = 1,
y = 2 and 0 ≤ 5 ≤1. This 1:2:3 stoichiometry has since been the focus of much study
including varying the rare earth element, full and partial substitutions for RE, for Ba, and
for Cu, oxygen doping, and deviations from the 1:2:3 stoichiometry.
[0016] The present invention focuses on RE HTS films specifically optimized for
microwave and RF applications. The RF/microwave HTS applications require the HTS
thin films to have superior microwave properties, specifically low surface resistance, Rs,
and highly linear surface reactance, X5,, i.e. high JIMD- As such, the invention is
characterized in terms of its physical composition, surface morphology, superconducting
properties, and performance characteristics of microwave circuits made from these films
[0017] In particular, this invention focuses on compositions having a significant
deviation from the 1:2:3 stoichiometry in order to create the films optimized for
microwave/RF applications. These films have a RE:Ba ratio of less than 1.8, which
deviates more than 10% from the typical ratio of 2, and preferably less than 1.7. The
research has shown that the highest quality factor values, Q, representing the surface
resistance of patterned films, peak at a particular Ba:RE ratio for each RE and that these
ratios deviate significantly from the 1:2:3 stoichiometry.
[0018] Additionally, the performance characteristics of the HTS films naturally affect
their efficacy in RF/microwave HTS applications. Specifically desirable are low surface
resistance, R5, ( reactance, Xs, i.e., high JIMD values (>107 A/cm2, preferably >5 x 107 A/cm2 at 77 K). HTS
thin films with such properties permit the fabrication of extremely selective filters (60-dB
rejection within 0.2% relative frequency, to 100-dB rejection within 0.02% relative
frequency, with extremely low in-band insertion loss ( extremely small size ( levels experienced at the front end of a cellular telephone base station receiver (-50 dBm
to -28 dBm, to as high as -12 dBm to 0 dBm or possibly even higher) without producing
undesirable distortion in the passband, particularly intermodulation distortion, and more
particularly intermodulation distortion products comparable to background noise levels (-
173.8 dBm/Hz). Thus, the films of this invention are also characterized by their optimized
microwave and RF properties. These and other objects, features and advantages will be
apparent from the following more particular description of the preferred embodiments.
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Brief Description of the Drawings
[0019] Fig. 1 shows θ-2θx-ray diffraction scans for several (RE)BCO thin films grown
on MgO substrates.
[0020] Fig. 2 displays the 2θ (005) x-ray peak positions as a function of the Ba/RE
ratio for different compositions of the various (RE)BCO thin films indicated.
[0021] Fig. 3 displays the 2θ (005) x-ray peak intensities as a function of the Ba/RE
ratio for different compositions of the various (RE)BCO thin films indicated.
[0022] Fig. 4 shows representative x-ray diffraction j-scans of the (103) peak for
several (RE)BCO films, including Er-, Ho-, and Dy-BCO (left panels). The right panel
plots display the c-scans for (RE)BCO films taken about the (104) peak.
[0023] Fig. 5 shows a higher sensitivity j-scan of the (103) Bragg angle for a Dy-BCO
- thin film.
[0024] Fig. 6 shows an atomic force microscope (AFM) scan of the surface of a Dy-
BCO thin film that is optimized for RF properties.
[0025] Fig. 7 shows an AFM scan of the surface of a Ho-BCO thin film that is
optimized for RP properties.
[0026] Fig. 8 shows an AFM scan of the surface of a Er-BCO thin film that is
optimized for RF properties.
[0027] Fig. 9 shows an AFM scan of the surface of a Nd-BCO thin film that is
optimized for RF properties.
[0028] Fig. 10 displays the room-temperature (300 K) dc resistivity values as a
function of the Ba/RE ratio for different compositions of the various (RE)BCO thin films
indicated.
[0029] Fig. 11 shows the dc resistivity as a function of temperature, p(T), for a Dy-
BCO film optimized for RF properties. A detail of the superconducting transition is shown
in the inset.
[0030] Fig. 12 shows the p(T) curve for a Ho-BCO film optimized for RF properties. A
detail of the superconducting transition is shown in the inset.
[0031] Fig. 13 shows the p(T) curve for a Er-BCO film optimized for RF properties. A
detail of the superconducting transition is shown in the inset.
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[0032] Fig. 14 shows the p(T) curve for a Nd-BCO film optimized for RF properties. A
detail of the superconducting transition is shown in the inset.
[0033] Fig. 15 displays the zero-resistance Tc values as a function of the Ba/RE ratio
for different compositions of the various (RE)BCO thin films indicated.
[0034] Fig. 16 shows the geometry of the quasi-lumped element resonator design used
to measure the Q values of our (RE)BCO films. This resonator has a center frequency of
about 1.85 GHz for films on MgO substrates. The resonator dimensions are 5.08-mm
square.
[0035] Fig. 17 displays the unloaded quality factor (Qu) as a function of the Ba/RE ratio
for different compositions of the various (RE)BCO thin films indicated. These Qu values
were measured at a temperature of 67 K and input power of-10 dBm for lumped-element
RF resonators having a center frequency of about 1.85 GHz. The dotted line at Ba/RE = 2
represents the on-stoichiometric value of the 1:2:3 compound.
[0036] Fig. 18 displays the Qu values as a function of the RE/Cu ratio for different
compositions of the various (RE)BCO thin films indicated. These Qu values were
measured at a temperature of 67 K and input power of -10 dBm for lumped-element RF
resonators having a center frequency of about 1.85 GKz. The dotted line ai RE/Cu - 1/3
represents the on-stoichiometric value of the 1:2:3 compound.
[0037] Fig. 19 displays the Qu values as a function of the Ba/Cu ratio for different
compositions of the various (RE)BCO thin films indicated. These Qu values were
measured at a temperature of 67 K and input power of -10 dBm for lumped-element RF
resonators having a center frequency of about 1.85 GHz. The dotted line at Ba/Cu = 2/3
represents the on-stoichiometric value of the 1:2:3 compound.
[0038] Fig. 20 shows the layout of the 10-pole B-band cellular filter design used for
our IMD tests. The filter dimensions are 18-mm by 34-mm.
[0039] Fig. 21 shows a block diagram for an intermodulation distortion measurement
of an HTS filter.
[0040] Fig. 22 shows the typical S11 response of a 10-pole B-band cellular RF filter
fabricated from a (RE)BCO thin film. The positions of the input frequencies for three two-
tone intermodulation distortion test measurements are shown.
[0041] Fig. 23 shows the results of intermodulation distortion (IMD) test measurements
made at 79.5 K as a function of the Ba/Dy ratio for several 10-pole B-band filters
patterned from Dy-BCO films. The dotted lines indicate the required specification levels.
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[0042] Fig. 24 shows the results of IMD test measurements made at 79.5 K as a
function of the Ba/Ho ratio for several 10-pole B-band filters patterned from Ho-BCO
films. The dotted lines indicate the required specification levels.
[0043] Fig. 25 shows the results of IMD test measurements made at 79.5 K as a
function of the Ba/Er ratio for several 10-pole B-band filters patterned from Er-BCO
films. The dotted lines indicate the required specification levels.
[0044] Fig. 26 shows the results of IMD test measurements made at 79.5 K for four 10-
pole B-band filters patterned from Nd-BCO films. The dotted lines indicate the required
specification levels.
[0045] Fig. 27 displays the unloaded quality factor (Qu) as a function of the Ba/Dy ratio
for different compositions of the various Dy-BCO thin films indicated. These Qu values
were measured at a temperature of 67 K and input power of -10 dBm for lumped-element
RF resonators having a center frequency of about 1.85 GHz, The solid line at Ba/Dy = 2
represents the on-stoichiometric value of the 1:2:3 compound.
[0046] Fig. 28 displays the unloaded quality factor (Qu) as a function of the Ba/Dy ratio
for different compositions of the various Dy-BCO thin films indicated. These Qu values
were measured at a temperature of 77 K and input power of-10 dBm for lumped-element
RF resonators having a center frequency of about 1.85 GHz. The solid line at Ba/Dy = 2
represents the on-stoichiometric value of the 1:2:3 compound.
[0047] Fig. 29 displays the ratio of high input power (+10 dBm) to low input power (-
10 dBm) Q factors for different compositions of the various Dy-BCO thin films indicated
wherein the Qu values were measured at a temperature of 67 K.
[0048] Fig. 30 displays the ratio of high power to low power Q factors for different
compositions of the various Dy-BCO thin films indicated wherein the Qu values were
measured at a temperature of 77 K.
[0049] Fig. 31 displays the 20(005) x-ray peak intensities as a function of the Ba/Dy
ratio for different compositions of Dy-BCO thin films..
[0050] Fig. 32 displays the room-temperature (300 K) dc resistivity values as a
function of the Ba/Dy ratio for different compositions of Dy-BCO thin films..
[0051] Fig. 33 displays the zero-resistance Tc values as a function of the Ba/Dy ratio
for different compositions of Dy-BCO thin films.
[0052] Table I displays the maximum Qu values at -1.85 GHz obtained for several of
our (RE)BCO thin films measured using a patterned test resonator. The measurements
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were made at 67 K and 77 K for an input power of-10 dBm. This table also shows the R5,
values that we have calculated from these Qu values.
Table I. Unloaded Q values of our highest Q films measured with our standard test
resonator at -10 dBm input power. The R5, values are calculated from these measured Q
values. These calculated Rs values of the patterned structures are less than the actual
measured R. values of the bulk films.

Material T=67K T=77 K
Unloaded Q R5(mW) fo(MHz) Unloaded Q Rs(mW) fo (MHz)
YBCO 83599 4.9 1847.94 50470 8.1 1848.23
Dy-BCO 52200 7.8 1851.13 37000 11.1 1850.29
Ho-BCO 70500 5.8 1850.07 39000 10.5 1849.95
Er-BCO 45300 9.0 1840.60 , 18800 21.8 1838.60
Nd-BCO 80866 5.1 1847.09 59341 6.9 1848.14
Detailed Description of the Invention
[0053] As previously mentioned, this invention relates to high temperature
superconducting (HTS) thin films with compositions that are optimized for RF/microwave
applications and methods for reliably producing such films. As such, the invention is
characterized in terms of its physical composition, surface morphology, superconducting
properties, and performance characteristics of microwave circuits made from these films
(filters, delay lines, couplers, etc.; particularly bandpass and bandreject filters, more
particularly bandpass and bandreject preselector filters for cellular telephone base station
receivers). The distinction between HTS (RE)BCO films of the prior art and the (RE)BCO
films of this invention is found both in the composition that deviates significantly from the
1 -.2:3 stoichiometry and the highly optimized RF properties of the new composition.
Definitions
[0054] For our purposes, a thin film may be defined as a layer (generally, very thin) of
a material that is grown, deposited, or otherwise applied to a suitable supporting substrate.
The thickness of this film may range from about one nm (10-9 m) to several microns (>l0-6
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m) thick, The typical range of thin film thickness for many applications is from 100 nm to
1000 nm.
[0055] High temperature superconductors (HTS) encompass a broad class of ceramic
materials, typically oxides, more typically copper oxides or cuprates, that have a transition
temperature or critical temperature, Tc, below which these materials are superconducting.
Above this critical temperature, they generally behave as metallic, or "normal,"
conducting materials. HTS materials are further generally characterized as having Tc
values above about 30 K. Examples of HTS materials include La2CaCu206,
Bi2Sr2CaCu2O8, YBa2Cu3O7, Tl2Ba2CaCu2O8, HgBa2CaCu2O7, etc. These materials must
have a well-defined crystal structure in order to be superconducting, i.e., they must have a
very specific regular and repeated arrangement of their constituent atoms.
[0056] The rare earth (RE) elements are the 15 lanthanide elements with atomic
numbers 57 through 71 that are in Group IIIA of the Periodic Table: lanthanum, cerium,
praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Yttrium (atomic number
39), a Group IIIA transition metal, although not a lanthanide is generally included with the
REs as it occurs with them in natural minerals and has similar chemical properties.
Commonly included with the REs because of their similar properties are scandium (atomic
number 21), also a Group IIIA transition metal, and thorium (atomic number 90), an
element in the actinide series of the Periodic Table.
Composition
[0057] The most ubiquitous HTS material is YBCO, which consists of an ordered
amount and arrangement of Y, Ba, Cu, and O atoms. The fundamental repeated unit of this
material's specific atomic arrangement is known as the unit cell, consisting nominally of
one Y, two Ba, three Cu, and seven O atoms. The size of this compound's orthorhombic
unit cell is about 3.82 x 3.89 x 11.68 Angstroms in the a-, b-, and c-axis directions,
respectively. The atomic ratios needed to form this compound are described by the
chemical formula YBa2Cu3O7-8, where the oxygen content is variable between 6 and 7
atoms per unit cell, or 0 ≤, 6 ≤ 1. For single-phase materials with this composition and
having high crystalline quality and purity, the revalue is determined largely by the value
of S. YBCO is a superconductor for d preferred in order to provide the highest Tc values.
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[0058] YBCO is the most widely studied HTS material, and much is known about how
to make it in single phase form, i.e., consisting of solely the composition mentioned above
and containing no other phases. However, many other similar compounds can also be
fabricated that may have similar or superior superconducting properties, depending on the
application. These compounds may have Y:Ba:Cu ratios that are different from 1:2:3, and
they may also consist of elements other than Y or Ba. A generalized nomenclature for the
makeup of this compound may thus be written as M'xMyCu3O7-8, where M' may in general
be any essentially trivalent ion or combination of ions, and M may be any essentially
divalent ion or combination of ions. The ratios of M':Cu, M:Cu, and M':M may also vary
substantially from the nominal values of 1:3, 2:3, and 1:2, respectively. While the full
range of parameter space has not been explored, it is reasonable to believe that compounds
with cation ratios deviating from the nominal by as much as 50% may still be
superconductors, e.g. 1:6 However, significantly altering the composition from the 1:2:3 stoichiometry does affect
the specific properties of the composition including critical current density (Jc), normal-
state resistivity (p), critical temperature (Te), and surface resistance (R5).
[0059] In order to provide high Tc values, Ba is generally preferred as the divalent
element, or M in the above formula. Full or partial substitutions of many elements for Ba
tend to decrease Tc or destroy superconductivity altogether. These elements include Sr, La,
Pr and Eu. (Y. Xu, Physica C 341-348, pp 613-4 (2000) and X.S. Wu, Physica C 315, pp
215-222 (1999). Similarly, the Cu atoms may be doped with Co, Zn, Mi, etc., the effect of
most of which is to decrease Tc, though the absolute effect (e.g., charge transfer or
disruption of superconductivity on the Cu-0 planes) depends on whether the Cu(l) or
Cu(2) sites are affected. (Y. Xu, Phys. Rev. B Vol 53, No. 22, pp 15245-15253 (1996).
Some partial substitutions on the Y sites may have a similar effect, such as Ca, Ce, and Pr
(L. Tung, Phys. Rev. B Vol 59, No. 6, pp 4504-4512 (1999) and C.R. Fincher, Phys. Rev.
Lett. 67 (20) pp 2902-2905 (1991)). However, there are many known partial or complete
substitutions for Y that lead to similarly high or greater Te values than YBCO. Many of
these known substitutions come from the rare earth family of elements. In general, rare
earth elements that have a larger ionic radius produce higher Tc values for these (RE)BCO
compounds (G.V.M. Williams, Physica C 258, pp41-46 (1996)).
[0060] While it is key to maintain the defining property of superconductivity across the
range of compositions available for these related compounds, our research has shown that
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the compositions may be altered substantially from the nominal 1:2:3 stoichiometry in
order to tailor their properties for specific applications. For example, compositions near
1:2:3 may be preferred for multilayer or active device applications for which smooth thin
film surfaces are of paramount importance. Conversely, optimization of HTS films for RF
applications requires the production of thin films that strike a balance between having the
lowest possible RF surface resistance (R,) values and the lowest RF nonlinearities that are
achievable. This in turn often requires the fabrication of (RE)BCO films that deviate
significantly from the 1:2:3 composition.
[0061] The HTS thin films of this invention are optimized for RF applications, and as such
they have the lowest possible RF surface resistance (R5) values and the lowest possible RF
nonlinearities. In order to achieve this optimization, the film compositions have the
nominal formula (RE)xBayCu3O7-8 where RE is one of the previously defined rare earth
elements, preferably Dy, and where the ratio yvx is preferably between about 1.5 - 1.8,
more preferably between about 1.55-1.75, and most preferably between about 1.6 - 1.7.
Substrates
[0062] The superconducting properties of IITS materials are extremely sensitive to
their degree of crystalline perfection. This places severe constraints on the choice of a
suitable substrate material on which high-quality HTS films may be grown. Some of these
constraints include crystal structure, compatibility with the growth process, chemical
compatibility, compatibility with the application, as well as other requirements imposed by
nature.
[0063] Perhaps the most important requirement is the crystal structure. The substrate
must have an appropriate lattice match with the HTS film such that epitaxial growth of the
film can occur and a well-oriented film will form. A poor lattice match can lead to
dislocations, defects, and misoriented grains in the film. In general, the substrate should be
available in single-crystal form in order to meet these requirements.
[0064] The substrate must be able to withstand the high processing temperatures during
the growth process that are required for the crystallization of the HTS compound. In
addition, structural integrity and a reasonable thermal expansion match with the HTS film
is required in order to prevent strain and cracking of the film during the cool down cycle
from the growth temperature or from any other subsequent thermal cyclings.
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[0065] The substrate must be chemically compatible with (RE)BCO, non-reactive, and
with minimal diffusion into the film at high temperature.
[00661 The substrate must be available in a size large enough for the intended use of
the HTS thin film. For example, certain passive microwave circuits or high-volume
electronics applications require a large substrate size. A minimum substrate of 2" in
diameter is typical for these applications, though larger sizes are often desirable if
available. The substrate may also be required to have physical properties that are
compatible with experimental measurement techniques or applications. For most
applications the substrates should be stable, mechanically robust insulators. Other
requirements may include transparency in the infrared for optical transmission
measurements, constituent elements or structure that do not interfere with spectroscopic
measurements such as Rutherford backscattering (RBS) or energy-dispersive x-ray
analysis (EDX), and a low dielectric constant and loss tangent for microwave
measurements and applications at the intended temperature of operation.
[0067] A handful of single-crystal substrates meet some or all of these requirements.
Examples include MgO, AI2O3, LaA1O3, NdGaO3, (La0.18Sr0.82)(Al0.59Ta0.41)O3, and
SrTiO3. The last four have an excellent lattice match to (RE)BCO. The high dielectric
constant and loss tangent of SrTiO3 make it useless for microwave applications, however.
LaA1O3 and NdGaO3 are better in this regard, though LaA1O3 suffers from the fact that it
tends to twin, and these twin boundaries can be formed and become mobile at typical
processing temperatures. AI2O3 is a low-loss substrate and is widely available in several
different orientations and sizes. However, it reacts strongly with (RE)BCO at high
temperatures, requiring the use of an appropriate buffer layer. In addition, A12O3 has a
poor thermal expansion match to (RE)BCO, causing a tendency for the films to crack upon
cooldown. MgO has relatively low loss and a good thermal expansion match to (RE)BCO,
making it a good choice for RF applications. However, MgO has a much larger lattice
mismatch than the other examples listed above, so that great care must be taken to insure
that the (RE)BCO films grown on MgO are well oriented. In particular, it is relatively
common for (RE)BCO films grown on MgO to contain in-plane-rotated grains and 45°
grain boundaries. (B.H. Moeckly, Appl. Phys. Lett. 57,1687-89 (1990). The minimization
of the amount of these high-angle grain boundaries is mandatory for good microwave
performance, particularly for high RF linearity. Certain MgO substrate surface treatments
may be instituted to help control the number of high-angle grain boundaries, but greater
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effort is required to further suppress formation of these grain boundaries, particularly for
demanding RF applications. The growth method, growth conditions, and particularly the
composition of the (RE)BCO films must all be chosen and adjusted to minimize the
amount of 45° grains in films grown on MgO.
Film morphology and microstructure
[0068] The anisotropic transport properties of (RE)BCO, its orthorhombic crystal
structure, and its small superconductive coherence length mean that the (RE)BCO films
must have excellent crystalline structure and orientation. This is particularly true in order
to obtain good microwave properties. Hence, the films must be substantially free of
secondary phases, they must possess good epitaxy both in-plane (parallel to the substrate
surface) and out-of-plane (perpendicular to the substrate surface). Typically, the c-axis of
(RE)BCO is aligned perpendicular to the substrate surface. All the grains in the film must
be so aligned, and they must be highly aligned with respect to one another. The degree of
this crystalline order is typically characterized by q-2q x-ray diffraction scans, where the
requirements are the existence of only oaxis-oriented (00l) spectral lines having narrow
peak widths, and also narrow peak widths of the so-called w-scan, or rocking curve scan
about a given Bragg angle. The q-2q measurement can also detect the presence of spectral
lines due to a-axis-oriented grains within the film. These grains may also be detected by a
c-scan about an appropriate Bragg angle.
[0069] For a thin film with good microwave properties, the amount of a-axis grains in
the film is ideally zero, so that the intensity of a-axis x-ray peaks relative to c-axis x-ray
peaks for a c-axis-oriented film is ideally zero, and preferably much less than 1%. In
addition, the c-axis-oriented grains should also be in-plane oriented, meaning that they are
in registry with each other and with the substrate crystal structure. Grains that are rotated
with respect to the overall in-plane lattice structure lead to nonzero-degree angle grain
boundaries. The superconducting transport across such nonzero-angle grain boundaries, in
particular high-angle grain boundaries and 45° grain boundaries, is degraded likely due to
strain, the high oxygen mobility, and small coherence length of (RE)BCO (B. H. Moeckly
et al., Phys. Rev. B 47, 400 (1993). Jc, Rs, and the RF nonlinearities may all be adversely
affected by the presence of these high-angle grain boundaries. The presence of these
rotated grains and grain boundaries may be detected by j-scan x-ray measurements taken
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about an appropriate Bragg angle. Ideally, the amount of nonaligned j-scan peaks should
be zero, and preferably less than 0.1% of the magnitude of the aligned peaks, more
preferably less than 0.05%, and most preferably less than 0.02%.
[0070] Fig, 1 shows the q-2q scans for several 700-nm-thick (RE)BCO films made by
us and optimized for RF applications. In addition to YBCO, these films include RE
substitutions of Er (EBCO), Ho (HBCO), Dy (DBCO), and Nd (NBCO). The x-ray scans
display the presence of only (00l) peaks, indicating that the films are single-phase and
highly c-axis aligned, and that no a-axis-oriented grains exist in the films. Note that the
relative peak intensities of the (RE)BCO films are different from YBCO, indicative of the
effect of the different RE ionic radii. Fig. 2 shows the (005) peak positions vs. the Ba/RE
ratio for several (RE)BCO films with different compositions, indicating slightly different
c-axis lattice parameters for these films. Fig. 3 shows the intensities of the (005) peaks for
these films. Fig. 31 shows the intensities of the (005) peaks for DBCO films of varying
compositions as a function of the Ba/Dy ratio. These DBCO films are inclusive of those
shown in Fig. 3, but are not necessarily optimized for RF properties. Here, it is observed
that high peak intensities, indicative of good crystallinity, are obtained for DBCO film
compositions that deviate significantly from the on-stochiometric ratio indicated by the
solid line on the graph. Fig. 4 shows representative j-scans of the (103) peak for several
(RE)BCO films (left hand panels). Note that any peaks at 45° are absent, indicating an
absence of 45° oriented grains and grain boundaries. Fig. 5 shows a higher sensitivity j
scan for one of our Dy-BCO films. The y-axis is plotted on a log scale, and it can be seen
that only very weak peaks occur at 45° relative to the main peak. This scan indicates the
degree to which this film is free from high-angle grain boundaries. The intensity of the
weak lines at 45° is only about 0.012% of the maximum central peak indicating that almost
none of the grains are misaligned. This is important for optimization of the RF properties
of these films, most notably their RF nonlinearities. The right-hand panel of Fig. 4
displays the c-scans for (RE)BCO films taken about the (104) peak; as indicated in Fig. 1,
these scans also demonstrate the absence of a-axis-oriented grains.
[0071] The surface morphology of (RE)BCO thin films is typically measured by
scanning probe profilometry, atomic force microscopy (AFM), and scanning electron
microscopy (SEM). In general, smooth films are preferred for applications, though some
degree of surface roughness may be tolerated in deference to the optimization of other
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important properties such as Jc and R5. Still, it is desirable to have an RMS surface
roughness as determined by AFM, say, which is less than -10 nm.
[0072] Figs. 6-9 show typical AFM images of Dy-, Ho-, Er-, and Nd-BCO films over a
5 mm x 5 mm area. These films have been optimized in terms of their RF properties. The
RMS surface roughness for these films is a few nm. Fig. 6 shows the surface morphology
for a high- DBCO film with very low RF nonlinearities. The grain size of this film can
be seen to be roughly 2 um in diameter. We also observe some sub-micrometer-sized
particles on the surface of the films, seen as the bright dots in the figure. EDX analysis
indicates a high Cu signal for these particles, implying that they composed of Cu oxide.
The grains for the HBCO film of Fig. 7 have a smaller, squarer appearance, and this film
has a general absence of CuO particulates. The optimized EBCO surface depicted in Fig. 8
has a smaller grain size still, and in this case there also exist CuO particulates. The grains
of the NBCO film of Fig. 9 are also square in appearance and have a size of less than 0.5
mm. The different surface morphologies for these optimized (RE)BCO films are in general
reflections of the different composition and growth conditions needed to achieve the best
RF properties for the different RE substitutions.
Film Characterization Methodology
[0073] The (RE)BCO films are further characterized by measuring their composition
and their electrical properties, including the dc resistivity (p) as a function of temperature
[p(T)], Tc value and transition width, critical current density (Je), and RF surface resistance
(Rs). The films are also subsequently patterned into RF circuits for which we measure the
unloaded quality factor (Q) values, intermodulation distortion (IMD), and nonlinear
critical current density (JIMD).
[0074] The composition of the films of this invention was measured using Rutherford
backscattering spectrometry (RBS) and inductively coupled plasma spectroscopy (ICP).
These techniques are both capable of a high degree of accuracy and precision, though
achieving a measurement accuracy of la or 2c equal to 1% is a difficult task and requires
more care than is the norm for these techniques. In the RBS analysis technique, fast, light
ions (typically He ions or alpha particles) are accelerated toward the sample; some of these
ions are backscattered due to Rutherford (Coulomb) scattering from atomic nuclei within
the sample, and the energy spectrum of those backscattered particles is analyzed. The ion
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energies are typically in the range of several hundred to several thousand keV, and the
energy of a backscattered ion depends on the mass of the target atom with which it has
collided. Thus, the energy spectrum of the backscattered ions allows identification of the
elements comprising the sample and their ratios (stoichiometry). In addition, as the
incident ions traverse the sample, they lose energy due to inelastic scattering with
electrons. This energy loss occurs in a known way and therefore allows determination of
sample composition as a function of depth. However, for thick films, the spectral peaks of
the measured constituent elements can overlap, requiring careful fitting of the spectra to
extract the composition, and this procedure involves uncertainty and can introduce error.
Therefore, in order to obtain the highest accuracy by simply counting the number of
counts under each peak, sufficiently thin films must be used so that the peaks due to RE,
Ba, and Cu can be completely separated. We have grown sufficiently thin (RE)BCO films
for this purpose, and the results of these measurements have shown a compositional
accuracy of 2o ≤ ±1%. "Note that this measurement technique is quantitative and does not
require the use of a comparison standard.
(0075] In the ICP technique, the thin films are digested in an acidic solution which is then
introduced into a high-temperature (up to 10,000 °C) plasma discharge. Tne plasma
ionizes and excites the constituent atoms in the solution, and as these atoms decay to a
lower energy state, they emit light of a characteristic wavelength that can be detected by a
high-resolution spectrometer. This is the so-called ICP-AES (atomic emission or optical
emission spectroscopy) technique. ICP hence permits measurement of multiple elements
simultaneously. ICP-AES has detection limits typically at the mg/L level in aqueous
solutions. This technique can be very accurate and precise; an accuracy of 10 obtainable with careful measurement. The method requires the use of a comparison
standard. It does not have an accuracy limitation as a function of thin film thickness,
however, as does RBS. Hence in testing our compositions, we have used RBS and ICP
together. First, we have made careful RBS measurements on very thin films in order to
determine their composition to a high degree of accuracy. We have then confirmed that the
ICP measurements on these same samples agree with the RBS numbers. This allows us to
have confidence that the ICP-AES measurement of thicker (RE)BCO films shares this
same degree of desired accuracy, i.e., 1 o [0076] The dc resistivity p is measured by a standard four-point-probe technique. The
room-temperature resistivity of high-quality (RE)BCO films is typically between 150 and
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300 mW cm, though this value varies as a function of RE element and of film composition.
Fig. 10 shows the room-temperature (300 K) resistivity values of several (RE)BCO films
as a rimction of composition, specifically the Ba/RE ratio. Fig. 32 shows the room-
temperature resistivity values for several DBCO films as a function of the Ba/Dy ratio.
These DBCO films are inclusive of those shown in Fig. 10, but are not necessarily
optimized for RF properties Here, it is observed that the room temperature resistivity
values indicative of high quality films are obtained for DBCO film compositions that
deviate significantly from the on-stochiometric ratio (indicated by the solid line on the
graph), particularly within Ba/Dy ratios ranging between about 1.5-1.8, with particularly
good resistivity values achieved for a Ba/Dy ratio of 1.76. The temperature dependence of
the resistivity is shown in Figs. 11-14 for several (RE)BCO films. The temperature
dependence of p for good films is typically linear or slightly downwardly bowed
indicative of so-called overdoped behavior, as these plots indicate. The measurement of
p(T) is also used to determine both the width (in temperature) of the transition to the
superconducting state and the zero-resistance Tc value. The detail of the superconducting
transition region of these films is shown in the inset of Figs. 11-14. The Tc values for
(RE)BCO films are typically between 87 and 91 K (Figs. 11-13), though the higher ionic
radius RE substitutions may have Tc values as high as 95 K, as shown for the NBCO film
in Fig. 14. The transition from the normal state to superconducting state typically occurs
within 0.5 K for high quality films, as the figures indicate.
[0077] The Tc values of the (RE)BCO samples prepared by the process of this
invention are 88.5(5), 88.9(5), 89,2(5), 89.6(5), and 94.5(8) K for Er, Y, Ho, Dy, and Nd,
respectively. These values were measured immediately following deposition. Since the
films are oxygen overdoped as judged by the slope of the R-T curves, the measured Tc
values are slightly lower than the highest values known for these compounds. Fig. 15 plots
the Tc values for different compositions of our Ho-, Er-, and Dy-BCO films. Fig. 33 plots
the Tc values for different compositions of additional DBCO films. These DBCO films are
inclusive of those shown in Fig. 15, but are not necessarily optimized for RF properties. It
can be seen that high Tc values are obtained even for compositions deviating substantially
from the on-stoichiometric (1:2:3) value, indicated by the solid vertical line.
[0078] The RF surface resistance of (RE)BCO thin films may be measured in a number of
ways, including cavity or parallel plate resonator techniques using bulk (unpatterned)
films. R5 is typically measured at frequencies between a few hundred MHz and 10s of
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GHz. R5, may also be extracted from the Q measurements of patterned resonators of
various kinds, e.g., microstrip, quasi-lumped element, etc. Extraction of R5, from the
measured Q values of these structures requires careful modeling of the resonator
performance to determine the geometric parameter TQ. The relationship between Rs and Q

depends only on the resonator geometry, and Q is the measured unloaded quality factor of
the resonator. The extracted R5, value of patterned structures is typically higher than the Rs
value obtained by direct measurement of the bulk films in an RF cavity. This may be
caused by patterning the film, which may introduce defects that can add additional
Tesistive RF losses in the Q measurement. It may also arise from uncertainties in rQ or the
non-uniformity of the current density in microstrip resonators which is generally not
present in bulk film measurement systems.
Device Performance Characterization
[0079] For evaluation of the RF properties of our (RE)BCO films and for determining
the utility of these materials for microwave filter applications, we have fabricated
microwave resonators and filters from these films. These passive devices require a ground
plane and hence necessitate depositing double-sided films. Quasi-lumped element
resonators were patterned using standard photolithographic processing and inert ion
etching. The geometry of our test resonator is shown in Fig. 16. The materials are
characterized by measuring the unloaded quality factor, Qu of this standard test resonator
which has a center frequency of about 1.85 GHz at 77 K for (RJE)BCO resonators
patterned on MgO substrates. The Qu was measured for a range of composition and growth
conditions of each (RE)BCO material, and the growth conditions and composition of each
material were optimized to achieve maximum Qu. We have demonstrated Qu values that
are sufficient for cellular microwave applications for Dy-BCO, Er-BCO, Ho-BCO and Nd-
BCO thin films. Indeed, for 700-nm-thick films, we have achieved unloaded Q values over
40,000 at 1.85 GHz, 67 K, and -10 dBm input power for our test resonator structure using
each of these materials. We subsequently extracted the R5, value of the films by modeling
the electromagnetic field distribution of the resonator geometry. Good R, values for
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microwave applications are less than -15 mW at 1.85 GHz and 77 K, and more preferably
less than -10 mW, and most preferably less than about 8 mW.
[0080] Fig. 17 shows the unloaded g of our (RE)BCO lumped-element microwave
resonators vs. the relative Ba/RE ratio. These measurements were made at 67 K and -10
dBm input power. The Qu values of these films are slightly lower than our highest gs
obtained with YBCO films. The dotted line at Ba/RE = 2 represents the on-stoichiometric
value of the 1:2:3 compound. It can be seen that the highest g values are obtained away
from this ratio. OurNd-BCO films display higher Qu values at 67 K, reaching 80,000 (not
shown on this plot), comparable to the highest gs obtained with YBCO films. At 77 K, the
g values of Nd-BCO films can exceed those of YBCO. Although there is scatter in the
data, the trend for all three materials shown in Fig. 17 is similar. There exists for each
(RE)BCO film a value of the Ba:RE ratio for which the Q is maximal, and the g values
drop for ratios away from the maximum in a similar way for each RE element. Fig. 18
shows these Qu values measured as a function of the RE/Cu ratio, and Fig. 19 plots these
data as a function of Ba/Cu. The dotted lines indicate the on-stoichiometric ratios of these
quantities, and it is again observed that high g values are obtained for compositions that
deviate significantly from these nominal ratios. Table I displays the maximum Qu values
obtained for test resonators made from our (RE)BCO films measured at 67 K and 77 K for
an input power of -10 dBm. This table also shows the R5, values that we have calculated
from these Qu values.
[0081] Fig. 27 displays additional data on the unloaded quality factor (Qu) as a function
of the Ba/Dy ratio for different compositions of the various DBCO thin films indicated.
These DBCO films are inclusive of those shown in Fig. 17, but are not necessarily
optimized for RF properties. Hence whereas Fig. 17 represents the best Q values
obtainable at each composition, Fig. 27 shows a range of g values at each composition,
because other properties of the films may not be optimized, e.g., growth temperature, film
thickness, surface morphology, or crystallinity. These Qu values were measured at a
temperature of 67 K and input power of -10 dBm for lumped-element RF resonators
having a center frequency of about 1.85 GHz. The solid line at Ba/Dy = 2 represents the
on-stoichiometric value of the 1:2:3 compound. It is again observed that the highest Q
values are obtained for compositions that deviate significantly from the on-stoichiometric
ratio, particularly for Ba/Dy ratios between about 1.5-1.8, more particularly peaking at
between about the 1.6-1.7 ratio. Fig. 28 displays the unloaded quality factor (Qu) as a
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function of the Ba/Dy ratio for the same DBCO thin films of varying compositions
measured at a temperature of 77 K and input power of -10 dBm. While there is more
scatter in the data at this temperature which is nearer Tc the data still clearly show that the
highest Q values are obtained for compositions that deviate significantly from the on-
stoichiometric ratio, particularly for Ba/Dy ratios between about 1.5-1.8, more particularly
peaking around the 1.6 ratio.
[0082] The Q values of (RE)BCO filters can degrade as a function of increasing input
power. The ability of (RE)BCO filters to maintain high Q values as a function of
increasing input power is an important requirement for high performance filter systems.
Fig. 29 displays the ratio of unloaded Q values at 67 K measured at high (+10 dBm) and
low (-10 dBm) input powers for resonators made from DBCO film of different
composition. High values of Q+10dBm / Q-10dBm indicate better power handling capability
and superior performance. It can be seen that increasingly higher ratios are obtained as the
DBCO composition deviates further from the on-stoichiometric value (Ba/Dy = 2),
indicated by the solid vertical line. Fig. 30 plots the ratio of Q measured at high power to
low power at 77 K for several DBCO films of varying composition. This figure also shows
that excellent power handling is obtained even for compositions that deviate substantially
from the on-stoichiometric value.
[0083] The input power levels to the (RE)BCO filter also affects their performance by
generating different amounts of intermodulation distortion, as described below.
[0084] We further evaluated these materials by growing thin films on 2" MgO
substrates and patterning them into 10-pole filter circuits of a type suitable for commercial
cellular communications applications. Fig. 20 shows a layout of the filter design. The
filters were tuned, and their performance was evaluated in terms of insertion loss, return
loss, and out-of-band rejection. In addition, we used these 10-pole filters to measure the
nonlinear properties of these materials in terms of their third-order intermodulation
distortion (IMD). A block diagram of the test setup is shown in Fig. 21. For these
measurements, tones of equal power at two different closely-spaced frequencies// and/2
were combined and applied to the filter at specific power levels. The location of these
input tones is in-band, far from the band edge, or close to the band edge. The output power
of the third-order mixing product at frequency 2f1 — f2 is then measured in a spectrum
analyzer. The magnitude of the output signal from the filter at these frequencies is an
important measure of the RF nonlinearities of the filter and determines its suitability for
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many microwave applications. The presence of intermodulation distortion reflects the
current density dependence of the surface reactance, X5,, of the superconducting thin film
(T. Dahm & D.J. Scalapino, J. Appl. Phys. 81 (4), pp 2002-2009) (1997). In contrast,
nonlinearity in the surface resistance, R5 of the thin film would be reflected in an increase
in the insertion loss of the filter. This type of nonlinearity is not generally a limiting factor
in the application of superconducting thin films to RF and microwave filters.
We have utilized three IMD tests to assess the applicability of our HTS thin film materials
for applications in RF/microwave filters.
1. In-band Test. Two-tone input signals are applied near the center of the AMPS B
Passband (835 MHz to 849 MHz). The input frequencies are at/; = 841.985
MHz and , f2 = 842.015 MHz at power levels of -20 dBm each. The
intermodulation spurious product is measured at 842.045 MHz. The
intermodulation spurious product power at this frequency measured at the
output of the filter must be 2. Near-Band Test. Equal amplitude input signals are applied at 851 MHz and 853
MHz, and the intermodulation spurious product power level is measured at 849
MHz. The specification is the minimum power level of input tones that produce
intermodulation spurious products in the AMPS B Passband with power levels
of -130 dBm at the output. This input power level must be > -28 dBm.
3. Out-of-Band Test. Equal amplitude input signals are applied at 869.25 MHz and
894 MHz, and the intermodulation product is measured at 844.5 MHz. The
requirement is the minimum power level of input tones to cause
intermodulation products in the AMPS B System Passband to reach -130 dBm
at the output of the filter. This input test signal power levels must be > -12
dBm.
[0085] We fabricated B-band cellular microwave filters from several (RE)BCO thin
films which were grown by in situ reactive coevaporation onto 2" MgO substrates. Each
double-sided wafer yields two filters, each having a size of 18 mm x 34 mm. The
patterned (RE)BCO structures are quasi-elliptic 10-pole filters with 3 pairs of transmission
zeros on either side of the frequency passband. Fig. 22 shows the typical response of such
a filter. The positions of the frequencies for the two-tone IMD tests are shown,
[0086] Fig. 23 shows the IMD values measured at 79.5 K as a function of Ba/Dy ratio
for several 10-pole B-band filters patterned from optimized Dy-BCO films. Note that all
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the filters measured meet the requirements, which are indicated by dotted lines. Figs. 24
and 25 show the 1MD values as a function of the Ba/RE ratio measured at 79.5 K for
several 10-pole B-band filters patterned from optimized Ho-BCO and Er-BCO films. Fig.
26 shows the IMD values measured at 79.5 K for four 10-pole B-band filters patterned
from optimized Nd-BCO films.
Intermodulation distortion in HTS filters arises due to nonlinearity of the microwave
surface reactance, X5, of the thin films. (R. B. Hammond et al, J, Appl. Phys. 84 (10) pp
5662-5667 (1998)). In general, at high microwave current densities in HTS thin films X,
ceases to be constant and independent of current density, and begins to increase with
increasing current density. Commonly there is a maximum current density, JIMD. at which
X5 retains its low current density value, and above which Xs increases. In this paper by
Hammond et al, the relationships between measured parameters and the material
parameter JIMD are described. This relationship can be summarized as

the resonant frequency, these two functions depend on the filter function to be realized,
rIMD is a factor which depends only on the geometry of the resonator, and PIN and POUT
are the input and output powers from an intermodulation measurement.
[0087] The out-of-band IMD test requirement corresponds to a minimum JIMD in the
HTS thin film of 1 x 107 A/cm2. The DBCO films surpass the specification by 14 dB,
which here corresponds to a factor of 5. Thus, the DBCO films have a JIMD of 5 x 107
A/cm2. For filter applications JIMD in HTS thin films must be > 1 x 107 A/cm2, more
preferably > 2 x 107 A/cm2, and most preferably > 3 x 107 A/cm2.
Methods of Manufacture
[0088] We have grown our (RE)BCO thin films using an in situ reactive coevaporation
(RCE) deposition technique which has been successfully used to manufacture large-area
YBCO HTS thin films. This is a fabrication technique that readily lends itself to high
volume film production and manufacturability. The yield of high-performance microwave
filters made from films grown by RCE is typically >90%. A key component of this growth
method is the use of a radiative heater that internally maintains an oxygen partial pressure
that is greater than ~10 mTorr. The heater also incorporates a window that allows
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exposure of the rotating substrates to high vacuum, where evaporation and deposition of
the source materials occurs. Our substrates are typically MgO single crystals up to 2" in
diameter that are rotated continuously between the window and the oxidation pocket at
300 rpm. The chamber ambient pressure away from the pocket is ~10-5 Torr. This
configuration provides sufficient oxygen pressure for stability of the high-Tc phase while
the metallic evaporation sources are simultaneously free from oxidation, and the
evaporated species are free from scattering. The rare earth elements Er, Ho, and Dy are
evaporated from electron beam sources, Nd and Cu are evaporated from either electron
beam sources or resistive sources, and Ba is evaporated from a thermal furnace or a
resistive source. The typical deposition rate is ~2.5 A/sec. The deposition temperature for
the films discussed here is 760 to 790 °C, and the film thickness is about 700 nm. The
films were deposited directly onto MgO substrates, with the exception of Nd-BCO, which
presently requires a thin buffer layer in order to achieve the best results.
[0089] Unlike yttrium, which melts readily, some rare earth elements such as Er, Ho,
and Dy sublime during e-beam evaporation, thereby making compositional control more
challenging. We routinely use quartz crystal monitors (QCM) as our primary rate
controllers. However, the subliming materials arc never molten at our evaporation rates;
rather, the electron beam digs a hole in the metallic source material so that the plume
shape changes significantly during the course of the deposition run. Therefore, the QCMs
are not able to correctly monitor the changing amount of RE vapor flux. To alleviate this
difficulty we have employed hollow-cathode-lamp (HCL) atomic absorption (AA)
evaporation flux sensors to monitor and control these subliming materials. Since the AA
light beam passes through the entire plume of evaporated species, this technique can more
accurately monitor the amount of evaporated flux.
[0090] The oxygen pocket pressure and deposition rate used to achieve optimal
results are similar for the (RE)BCO films that we have studied. We have found that the
best substrate temperatures for Er, Ho, Dy, and Nd are 780, 790, 790, and 780 °C,
respectively. These temperatures are significantly higher than the temperature of 760 °C
we use to achieve optimal RF properties for YBCO. The use of different growth
conditions for the (RE)BCO materials compared to YBCO is mandatory in order to
achieve the very best RF properties. For example, higher growth temperatures for the
(RE)BCO materials as compared to YBCO are generally required in order to insure the
absence of deleterious misaligned grains. The composition must also be optimized for this
NBl:672I22.1

WO 2006/071899 PCT/US2005/047147
25
purpose, as we have discussed. In general, many aspects of film growth affect the defect
structure in (RE)BCO thin films, and thus RF properties, including a) growth temperature,
b) growth rate, c) oxygen pressure, and d) stoichiometry. Specific choices for (a), (b), and
(c) may yield different optimized properties and different optimized compositions.
[0091] Although the foregoing invention has been described in some detail by way of
illustration and example for purposes of clarity and understanding, it may be readily
apparent to those of ordinary skill in the art in light of the teachings of this invention that
certain changes and modifications may be made thereto without departing from the spirit
or scope of the appended claims.
N81:672122.l

WO 2006/071899 PCT/US2005/047147
26
We Claim:
1. A superconducting article comprising:
a substrate, and
a thin film disposed on the substrate having the nominal composition of
RE2BayCu3Ox
wherein RE is a rare earth, and wherein y is less than substantially 2.1 and the
ratio of y/z is 1.65±10%.
2. The article of claim 1 wherein y is less than substantially 2.1 and the ratio
of y/z is l.65±6%.
3. The article of claim 1 wherein y is less than substantially 2.1 and the ratio
of y/z is l.65±3%.
4. The article of claim 1 wherein the JIMD at 850 MHz is > 0.8 x 107 A/cm2 at
77K.
5. The article of claim 1 wherein Rs at 1.85 GHz at 77K is less than 15 micro-
ohms.
6. The article of claim 1 wherein the article includes 45-degree grain
boundaries in a concentration 7. The article of claim 1 wherein the thin film is deposited on the substrate by
reactive coevaporation,
8. The article of claim 1 wherein the thin film has a superconducting
transition temperature >87K.
9. The article of claim I wherein the substrate has a surface area > 3 square
inches.
NB!:67212Z1

WO 2006/071899 PCT/US2005/047147
27
10. The article of claim 1 wherein the thin film is patterned into a radio
frequency circuit with high Q circuit elements.
11. A superconducting article with improved microwave qualities comprising:
a substrate, and
a thin film disposed on the substrate having the nominal composition of
REzBayCu3Ox
wherein RE is a rare earth and not yttrium and wherein JIMD > 0.8 X 107 A/ cm2
at 77K.
12. The composition of claim 11 wherein JIMD > 1.5 x 107 A/cm2 at 77K.
13. The composition of claim 12 wherein JIMD > 3 x 107 A/cm2 at 77K.
14. A superconducting article with improved microwave qualities comprising:
a substrate, and
a thin film disposed on the substrate having the nominal composition of
REzBayCu3Ox
wherein RE is a rare earth and not yttrium and wherein R5 measured at 1.85 GHz and 77K.
15. The composition of claim 14 wherein Rs GHz and 77K.
16. The composition of claim 15 wherein R, GHz and 77K.
17. A superconducting article comprising:
a substrate, and
a thin film disposed on the substrate having the nominal composition of
DyzBayCu3Ox
wherein y is less than substantially 2.1 and the ratio of y/z is 1.65±10%.
NBI:672!22.I

WO 2006/071899 PCT/US2005/047147

28
18. The article of claim 17 wherein y is less than substantially 2.1 and the ratio
of y/z is l.65±6%.
19. The article of claim 18 wherein y is less than substantially 2.1 and the ratio
of y/z is l.65±3%.
20. The article of claim 17 wherein JIMD at 850 MHz is > 0.8 x 107 A/cm2 at
77K.
21. The article of claim 17 wherein Rs at 1.85 GHz at 77K is less than 15
micro-ohms.
22. The article of claim 17 wherein the article includes 45-degree grain
boundaries at a concentration of 23. The article of claim 17 wherein the thin film is deposited on the substrate
by reactive coevaporation.
24. The article of claim 17 wherein the thin film has a superconducting
transition temperature >87K.
25. The article of claim 17 wherein the substrate has a surface area > 3 square
inches.
26. The article of claim 17 wherein the thin film is patterned into a radio
frequency circuit with high Q circuit elements.
27. A superconducting composition comprising:
a substrate, and
a thin film disposed on the substrate having the nominal composition of
NdzBayCu3Ox,
wherein y is less than substantially 2.1 and the ratio of y/z is 1.65±10%.
NB!:672122.1

WO 2006/071899 PCT/US2005/047147
29
28. The article of claim 27 wherein JIMD at 850 MHz is > 0.8 x 107 A/cm2 at
77K.
29. The article of claim 27 wherein Rs at 1.85 GHz at 77K is less than 15
micro-ohms.
30. The article of claim 27 wherein the article includes 45-degree grain
boundaries at a concentration of 31. The article of claim 27 wherein the thin film is deposited on the substrate
by reactive coevaporation.
32. The article of claim 27 wherein the thin film has a superconducting
transition temperature >87K.
33. Tne article of claim 27 wherein the substrate has a surface area > 3 square
inches.
34. The article of claim 27 wherein the thin film is patterned into a radio
frequency circuit with high Q circuit elements.
35. A superconducting article comprising:
a substrate, and
a thin film disposed on the substrate having the nominal composition of RE
zBayCu3Ox
wherein RE is a rare earth and not yttrium, and wherein the ratio of y/z is
1.65±10%,
the article including 45-degree grain boundaries in a concentration of NBI:672122.l

The films of this
invention are high temperature
superconducting (HTS) thin films
specifically optimized for microwave
and RF applications. In particular, this
invention focuses on compositions
with a significant deviation from
the 1:2:3 stoichiometry in order
to create the films optimized for
microwave/RF applications. The
RF/microwave HTS applications
require the HTS thin films to have
superior microwave properties,
specifically low surface resistance,
Rs, and highly linear surface
reactance, Xs, i.e. high JIMD. As
such, the invention is characterized
in terms of its physical composition,
surface morphology, superconducting
properties, and performance
characteristics of microwave circuits
made from these films.

Documents:

02476-kolnp-2007-abstract.pdf

02476-kolnp-2007-assignment.pdf

02476-kolnp-2007-claims.pdf

02476-kolnp-2007-correspondence others-1.1.pdf

02476-kolnp-2007-correspondence others.pdf

02476-kolnp-2007-description complete.pdf

02476-kolnp-2007-drawings.pdf

02476-kolnp-2007-form 1.pdf

02476-kolnp-2007-form 3.pdf

02476-kolnp-2007-form 5.pdf

02476-kolnp-2007-gpa.pdf

02476-kolnp-2007-international publication.pdf

02476-kolnp-2007-international search report.pdf

02476-kolnp-2007-pct request form.pdf

02476-kolnp-2007-priority document.pdf

2476-KOLNP-2007-(05-06-2013)-ABSTRACT.pdf

2476-KOLNP-2007-(05-06-2013)-CLAIMS.pdf

2476-KOLNP-2007-(05-06-2013)-CORRESPONDENCE.pdf

2476-KOLNP-2007-(05-06-2013)-DESCRIPTION (COMPLETE).pdf

2476-KOLNP-2007-(05-06-2013)-DRAWINGS.pdf

2476-KOLNP-2007-(05-06-2013)-FORM-2.pdf

2476-KOLNP-2007-(05-06-2013)-FORM-3.pdf

2476-KOLNP-2007-(05-06-2013)-FORM-5.pdf

2476-KOLNP-2007-(05-06-2013)-OTHERS.pdf

2476-KOLNP-2007-(05-06-2013)-PA.pdf

2476-KOLNP-2007-(05-06-2013)-PETITION UNDER RULE 137-1.pdf

2476-KOLNP-2007-(05-06-2013)-PETITION UNDER RULE 137.pdf

2476-KOLNP-2007-(18-06-2013)-ANNEXURE TO FORM 3.pdf

2476-KOLNP-2007-(18-06-2013)-CORRESPONDENCE.pdf

2476-KOLNP-2007-(22-11-2011)-CORRESPONDENCE.pdf

2476-KOLNP-2007-(30-08-2012)-Examination Report Reply Recieved.pdf

2476-KOLNP-2007-(30-08-2012)-PA-CERTIFIED COPIES.pdf

2476-kolnp-2007-form-18.pdf

abstract-02476-kolnp-2007.jpg


Patent Number 257371
Indian Patent Application Number 2476/KOLNP/2007
PG Journal Number 40/2013
Publication Date 04-Oct-2013
Grant Date 27-Sep-2013
Date of Filing 04-Jul-2007
Name of Patentee SUPERCONDUCTOR TECHNOLOGIES, INC.
Applicant Address 460 WARD DRIVE, SUITE F, SANTA BARBARA, CALIFORNIA
Inventors:
# Inventor's Name Inventor's Address
1 PENG SHING-JEN (LUKE) 149 PACCHETTI WAY, MOUNTAIN VIEW, CALIFORNIA 94040
2 GILANTSEV VIKTOR 4221 NORWALK DRIVE, #BB205, SAN JOSE, CALIFORNIA 95129
3 MOECKLY BRIAN 1225 E. COTA STREET, SANTA BARBARA, CALIFORNIA 93103
4 WILLEMSEN BALAM 4563 CALLE NORTE, VENTURA, CALIFORNIA 91320
PCT International Classification Number C04B 35/45
PCT International Application Number PCT/US2005/047147
PCT International Filing date 2005-12-22
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 60/639043 2004-12-23 U.S.A.