U.S. patent application number 14/255129 was filed with the patent office on 2015-02-19 for rf-properties-optimized compositions of (re) ba2cu3o7- thin film superconductors.
The applicant listed for this patent is SUPERCONDUCTOR TECHNOLOGIES, INC.. Invention is credited to VIKTOR GLIANTSEV, BRIAN MOECKLY, SHING-JEN PENG, BALAM WILLEMSEN.
Application Number | 20150051080 14/255129 |
Document ID | / |
Family ID | 36615496 |
Filed Date | 2015-02-19 |
United States Patent
Application |
20150051080 |
Kind Code |
A1 |
MOECKLY; BRIAN ; et
al. |
February 19, 2015 |
RF-PROPERTIES-OPTIMIZED COMPOSITIONS OF (RE) Ba2Cu3O7- 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,
R.sub.s, and highly linear surface reactance, X.sub.s, i.e. high
J.sub.IMD. 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.
Inventors: |
MOECKLY; BRIAN; (Santa
Barbara, CA) ; GLIANTSEV; VIKTOR; (San Jose, CA)
; PENG; SHING-JEN; (Mountain View, CA) ;
WILLEMSEN; BALAM; (Newbury Park, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUPERCONDUCTOR TECHNOLOGIES, INC. |
Santa Barbara |
CA |
US |
|
|
Family ID: |
36615496 |
Appl. No.: |
14/255129 |
Filed: |
April 17, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12974771 |
Dec 21, 2010 |
8716187 |
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14255129 |
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11317889 |
Dec 22, 2005 |
7867950 |
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12974771 |
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60639043 |
Dec 23, 2004 |
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Current U.S.
Class: |
505/237 ; 427/62;
428/141; 428/212; 428/337; 428/697; 505/150; 505/473 |
Current CPC
Class: |
H01L 39/2461 20130101;
Y10T 428/26 20150115; Y10S 505/78 20130101; H01L 39/2438 20130101;
C04B 2235/3215 20130101; H01P 1/20381 20130101; H01L 39/2454
20130101; Y10T 428/24355 20150115; C04B 35/4508 20130101; H01L
39/128 20130101; C04B 2235/96 20130101; Y10S 505/778 20130101; H01L
39/143 20130101; C04B 2235/3224 20130101; H01L 39/2458 20130101;
H01L 39/2432 20130101; H01L 39/126 20130101; H01P 1/20336 20130101;
Y10T 428/24942 20150115; Y10S 505/776 20130101; C04B 2235/85
20130101; Y10S 505/777 20130101; Y10S 505/779 20130101; Y10T
428/266 20150115 |
Class at
Publication: |
505/237 ;
505/150; 505/473; 427/62; 428/697; 428/141; 428/337; 428/212 |
International
Class: |
H01L 39/12 20060101
H01L039/12; H01L 39/24 20060101 H01L039/24 |
Claims
1. A superconducting article comprising: substrate, at least one
buffer layer supported by the substrate, and a thin film disposed
on the substrate having the nominal composition of
RE.sub.zBa.sub.yCu.sub.3O.sub.x wherein RE is a rare earth, wherein
the ratio of y/z is 1.65.+-.10% and x is between 6 and 7 inclusive,
and the article including 45-degree grain boundaries in a
concentration of <1%.
2. The article of claim 1 wherein the thin film is deposited on the
substrate by reactive coevaporation.
3. The article of claim 1 wherein the thin film has a
superconducting transition temperature >87K.
4. The article of claim 1 wherein the substrate is lattice matched
to the thin film.
5. The article of claim 1 having an RMS surface roughness of less
than about 10 nm.
6. The article of claim 1 wherein the topmost buffer layer is
lattice matched to the thin film.
7. The article of claim 6 wherein one of the buffer layers is
MgO.
8. The article of claim 6 wherein one of the buffer layers is
Al.sub.2O.sub.3.
9. The article of claim 1 wherein the substrate is rigid.
10. The article of claim 1 wherein the substrate is a single
crystal.
11. The article of claim 1 wherein the substrate is selected from
the group MgO, Al.sub.2O.sub.3, LaAlO.sub.3, NdGaO.sub.3,
(La.sub.0.18Sr.sub.0.82)(Al.sub.0.59Ta.sub.0.41)O.sub.3, and
SrTiO.sub.3.
12. The article of claim 1 wherein the substrate has a thermal
expansion match to the thin film.
13. The article of claim 1 wherein the substrate has a surface area
>3 square inches.
14. The article of claim 1 containing a-axis-oriented grains in a
concentration of <1% relative to c-axis-oriented grains.
15. A superconducting article comprising: a substrate, and a thin
film disposed on the substrate having the nominal composition of
RE.sub.zBa.sub.yCu.sub.3O.sub.x wherein RE is a rare earth, wherein
the ratio of y/z is 1.65.+-.10% and x is between 6 and 7 inclusive,
and wherein the thin film deposited by reactive coevaporation.
16. A superconducting article comprising: a rigid substrate, and a
thin film disposed on the substrate having the nominal composition
of RE.sub.zBa.sub.yCu.sub.3O.sub.x wherein RE is a rare earth,
wherein the ratio of y/z is 1.65.+-.10% and x is between 6 and 7
inclusive, and the article including 45-degree grain boundaries in
a concentration of <1%.
17. The article of claim 17 wherein the thin film is deposited on
the substrate by reactive coevaporation.
18. The article of claim 17 wherein the thin film has a
superconducting transition temperature >87K.
19. The article of claim 17 wherein the substrate is lattice
matched to the thin film.
20. The article of claim 17 wherein the substrate has a thermal
expansion match to the thin film.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. patent
application Ser. No. 12/974,771, filed Dec. 21, 2010, which is a
Continuation of U.S. patent application Ser. No. 11/317,889, filed
Dec. 22, 2005, now U.S. Pat. No. 7,867,950, which claims priority
to U.S. Provisional Patent Application No. 60/639,043, filed Dec.
23, 2004, the contents of which are all incorporated by reference
herein in their entirety as if fully set forth herein.
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)Ba.sub.2Cu.sub.3O.sub.7-.delta. 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 T.sub.c Superconductivity in
the Ba--La--Cu--O system." (64 Z. Phys. B.--Condensed Matter, pp
189-193 (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 La.sub.5-xBa.sub.xCu.sub.5O.sub.5(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
T.sub.c of the La--Ba--Cu--O 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 T.sub.c values near 90 K and a zero-resistance state
at .about.70 K. This compound had the nominal composition
Y.sub.1-2Ba.sub.0.8CuO.sub.4-.delta.. (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 Y.sub.2BaCuO.sub.5 (the "green" phase) and
YBa.sub.2Cu.sub.3O.sub.6+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
ABa.sub.2Cu.sub.3O.sub.6+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
CuO.sub.2--Ba--CuO.sub.2--Ba--CuO.sub.2 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. Batlogg first discovered and isolated the single
crystallographic phase responsible for the superconducting
properties of the YBCO compound. (B. Batlogg, U.S. Pat. No.
6,635,603). 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
M.sub.2M'Cu.sub.3O.sub.7-.delta. 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 T.sub.c 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 T.sub.c value increase with increasing
ionic radius of the rare earth element substituted for yttrium (G.
V. M. Williams, Physica C 258, pp 41-46 (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 nominal composition of
(RE)(AE).sub.2Cu.sub.3O.sub.9-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. Pat. 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).sub.2Cu.sub.3O.sub.9-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 T.sub.c decreased
dramatically for off-composition films with substitutions of rare
earth (RE) elements on the Ba site such as
RE(Ba.sub.2-xRE.sub.x)Cu.sub.3O.sub.y 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
(R.sub.s) 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 T.sub.c and J.sub.c 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 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 (R.sub.s) at 77 K: .about.250
.mu..OMEGA. vs. .about.300 .mu..OMEGA. 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 Modern Physics, 155),
Berlin, 1999) and others have measured somewhat lower surface
resistance. .about.200 .mu..OMEGA. 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 (R.sub.s) 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 films
exhibiting high temperature superconducting properties were
nominally of the composition
(RE).sub.xBa.sub.yCu.sub.3O.sub.7-.delta. where RE=a rare earth
element, preferably yttrium, x=1, y=2 and
0.ltoreq..delta..ltoreq.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, R.sub.s, and
highly linear surface reactance, X.sub.s, i.e. high J.sub.IMD. 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,
R.sub.s, (<15 micro-ohms at 1.85 GHz and 77 K) and highly linear
surface reactance, X.sub.s, i.e., high J.sub.IMD values
(>10.sup.7 A/cm.sup.2, preferably >5.times.10.sup.7
A/cm.sup.2 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 (<1-dB,
preferably <0.2-dB) in an extremely small size (<10-cm.sup.2
filter chips), which can handle the interference power 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows .theta.-2.theta. x-ray diffraction scans for
several (RE)BCO thin films grown on MgO substrates.
[0020] FIG. 2 displays the 2.theta. (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 2.theta.(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 .phi.-scans of
the (103) peak for several (RE)BCO films, including Er-, Ho-, and
Dy-BCO (left panels). The right panel plots display the .chi.-scans
for (RE)BCO films taken about the (104) peak.
[0023] FIG. 5 shows a higher sensitivity .phi.-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 RF 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 de resistivity as a function of
temperature, .rho.(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 .rho.(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 .rho.(T) curve for a Er-BCO film optimized
for RF properties. A detail of the superconducting transition is
shown in the inset.
[0032] FIG. 14 shows the .rho.(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 T.sub.c 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 (Q.sub.u) as a
function of the Ba/RE ratio for different compositions of the
various (RE)BCO thin films indicated. These Q.sub.u 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 Q.sub.u values as a function of the
RE/Cu ratio for different compositions of the various (RE)BCO thin
films indicated. These Q.sub.u 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 RE/Cu=1/3 represents the on-stoichiometric value of
the 1:2:3 compound.
[0037] FIG. 19 displays the Q.sub.u values as a function of the
Ba/Cu ratio for different compositions of the various (RE)BCO thin
films indicated. These Q.sub.u 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 S.sub.11 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.
[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 (Q.sub.u) as a
function of the Ba/Dy ratio for different compositions of the
various Dy-BCO thin films indicated. These Q.sub.u 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 (Q.sub.u) as a
function of the Ba/Dy ratio for different compositions of the
various Dy-BCO thin films indicated. These Q.sub.u 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 Q.sub.u 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 Q.sub.u values were measured at a temperature
of 77 K.
[0049] FIG. 31 displays the 2.theta.(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) de 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 T.sub.c values as a
function of the Ba/Dy ratio for different compositions of Dy-BCO
thin films.
[0052] Table I displays the maximum Q.sub.u values at .about.1.85
GHz obtained for several of our (RE)BCO thin films measured using a
patterned test resonator. The measurements were made at 67 K and 77
K for an input power of -10 dBm. This table also shows the R.sub.s
values that we have calculated from these Q.sub.u values.
TABLE-US-00001 TABLE 1 Unloaded Q values of our highest-Q films
measured with our standard test resonator at -10 dBm input power.
The R.sub.s values are calculated from these measured Q values.
These calculated R.sub.s values of the patterned structures are
less than the actual measured R.sub.s values of the bulk films. T =
67K T = 77K Material Unloaded Q R.sub.s (.mu..OMEGA.) f.sub.0 (MHz)
Unloaded Q R.sub.s (.mu..OMEGA.) f.sub.0 (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.sup.-9 m) to several
microns (>10.sup.-6 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, T.sub.c, 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 T.sub.c
values above about 30 K. Examples of HTS materials include
La.sub.2CaCu.sub.2O.sub.6, Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8,
YBa.sub.2Cu.sub.3O.sub.7, Tl.sub.2Ba.sub.2CaCu.sub.2O.sub.8,
HgBa.sub.2CaCu.sub.2O.sub.7, 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.times.3.89.times.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 YBa.sub.2Cu.sub.3O.sub.7-.delta., where the oxygen
content is variable between 6 and 7 atoms per unit cell, or
0.ltoreq..delta..ltoreq.1. For single-phase materials with this
composition and having high crystalline quality and purity, the
T.sub.c value is determined largely by the value of .delta.. YBCO
is a superconductor for .delta.<.about.0.6, with values of
.delta. near 0 being generally preferred in order to provide the
highest T.sub.c values.
[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'.sub.xM.sub.yCu.sub.3O.sub.7-.delta., 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<M':Cu<1:2,
1:3<M:Cu<1:1, and 1:4<M':M<3:4. However, significantly
altering the composition from the 1:2:3 stoichiometry does affect
the specific properties of the composition including critical
current density (J.sub.c), normal-state resistivity (.rho.),
critical temperature (T.sub.c), and surface resistance
(R.sub.s).
[0059] In order to provide high T.sub.c 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
T.sub.c 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, Ni, etc., the effect of most
of which is to decrease T.sub.c, though the absolute effect (e.g.,
charge transfer or disruption of superconductivity on the Cu--O
planes) depends on whether the Cu(1) 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
T.sub.c 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 T.sub.c
values for these (RE)BCO compounds (G. V. M. Williams, Physica C
258, pp 41-46 (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 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 (R.sub.s) values and the lowest possible RF
nonlinearities. In order to achieve this optimization, the film
compositions have the nominal formula
(RE).sub.xBa.sub.yCu.sub.3O.sub.7-.delta., where RE is one of the
previously defined rare earth elements, preferably Dy, and where
the ratio y:x 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 HTS 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.
[0065] The substrate must be chemically compatible with (RE)BCO,
non-reactive, and with minimal diffusion into the film at high
temperature.
[0066] 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, Al.sub.2O.sub.3,
LaAlO.sub.3, NdGaO.sub.3,
(La.sub.0.18Sr.sub.0.82)(Al.sub.0.59Ta.sub.0.41)O.sub.3, and
SrTiO.sub.3. The last four have an excellent lattice match to
(RE)BCO. The high dielectric constant and loss tangent of
SrTiO.sub.3 make it useless for microwave applications, however.
LaAlO.sub.3 and NdGaO.sub.3 are better in this regard, though
LaAlO.sub.3 suffers from the fact that it tends to twin, and these
twin boundaries can be formed and become mobile at typical
processing temperatures. Al.sub.2O.sub.3 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, Al.sub.2O.sub.3 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.degree. 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
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.degree. 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
.theta.-2.theta. x-ray diffraction scans, where the requirements
are the existence of only c-axis-oriented (001) spectral lines
having narrow peak widths, and also narrow peak widths of the
so-called .omega.-scan, or rocking curve scan about a given Bragg
angle. The .theta.-2.theta. 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 .chi.-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.degree. 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). J.sub.c, R.sub.s, 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 .phi.-scan x-ray measurements taken
about an appropriate Bragg angle. Ideally, the amount of nonaligned
.phi.-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 .theta.-2.theta. 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 (001) 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-stoichiometric ratio indicated by the solid line on the
graph. FIG. 4 shows representative .phi.-scans of the (103) peak
for several (RE)BCO films (left hand panels). Note that any peaks
at 45.degree. are absent, indicating an absence of 45.degree.
oriented grains and grain boundaries. FIG. 5 shows a higher
sensitivity .phi.-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.degree. 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.degree. 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 .chi.-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 important properties such as J.sub.c and R.sub.s. Still, it
is desirable to have an RMS surface roughness as determined by AFM,
say, which is less than .about.10 nm.
[0072] FIGS. 6-9 show typical AFM images of Dy-, Ho-, Er-, and
Nd-BCO films over a 5 .mu.m.times.5 .mu.m 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-Q DBCO film with very low RF nonlinearities.
The grain size of this film can be seen to be roughly 2 .mu.m 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 .mu.m. 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 (.rho.) as a function of temperature [.rho.(T)],
T.sub.c value and transition width, critical current density
(J.sub.c), and RF surface resistance (R.sub.s). 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 (J.sub.IMD).
[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 1.sigma. or 2.sigma. 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 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
2.sigma..ltoreq..+-.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.degree. C.) plasma discharge. The 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 .mu.g/L level in aqueous solutions. This technique can be very
accurate and precise; an accuracy of 1.sigma.<.+-.1% is
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.sigma.<.+-.1%.
[0076] The dc resistivity .rho. is measured by a standard
four-point-probe technique. The room-temperature resistivity of
high-quality (RE)BCO films is typically between 150 and 300
.mu..OMEGA.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 function 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-stoichiometric 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 .rho. for good films is typically linear or slightly
downwardly bowed indicative of so-called overdoped behavior, as
these plots indicate. The measurement of .rho.(T) is also used to
determine both the width (in temperature) of the transition to the
superconducting state and the zero-resistance T.sub.c value. The
detail of the superconducting transition region of these films is
shown in the inset of FIGS. 11-14. The T.sub.c values for (RE)BCO
films are typically between 87 and 91 K (FIGS. 11-13), though the
higher ionic radius RE substitutions may have T.sub.c 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 T.sub.c 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 T.sub.c values are slightly lower than the highest values
known for these compounds. FIG. 15 plots the T.sub.c values for
different compositions of our Ho-, Er-, and Dy-BCO films. FIG. 33
plots the T.sub.c 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 T.sub.c 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. R.sub.s is
typically measured at frequencies between a few hundred MHz and 10
s of GHz. R.sub.s may also be extracted from the Q measurements of
patterned resonators of various kinds, e.g., microstrip,
quasi-lumped element, etc. Extraction of R.sub.s from the measured
Q values of these structures requires careful modeling of the
resonator performance to determine the geometric parameter
.GAMMA..sub.Q. The relationship between R.sub.s and Q can be
written as
R s = .omega. 0 .GAMMA. Q 1 Q ##EQU00001##
where w0 is the resonant frequency, .GAMMA..sub.Q is a parameter
that depends only on the resonator geometry, and Q is the measured
unloaded quality factor of the resonator. The extracted R.sub.s
value of patterned structures is typically higher than the R.sub.s
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 resistive RF losses in
the Q measurement. It may also arise from uncertainties in
.GAMMA..sub.Q 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, Q.sub.u, of this standard
test resonator which has a center frequency of about 1.85 GHz at 77
K for (RE)BCO resonators patterned on MgO substrates. The Q.sub.u
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 Q.sub.u. We have
demonstrated Q.sub.u 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 R.sub.s value of the films by modeling
the electromagnetic field distribution of the resonator geometry.
Good R.sub.s values for microwave applications are less than
.about.15 .mu..OMEGA. at 1.85 GHz and 77 K, and more preferably
less than .about.10 .mu..OMEGA., and most preferably less than
about 8.mu..OMEGA..
[0080] FIG. 17 shows the unloaded Q 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 Q.sub.u
values of these films are slightly lower than our highest Qs
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 Q values are obtained away from this ratio. Our Nd-BCO
films display higher Q.sub.u values at 67 K, reaching 80,000 (not
shown on this plot), comparable to the highest Qs obtained with
YBCO films. At 77 K, the Q 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 Q values drop for ratios away from the maximum in a similar
way for each RE element. FIG. 18 shows these Q.sub.u 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 Q values are obtained for compositions that
deviate significantly from these nominal ratios. Table I displays
the maximum Q.sub.u 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 R.sub.s values that we have
calculated from these Q.sub.u values.
[0081] FIG. 27 displays additional data on the unloaded quality
factor (Q.sub.u) 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 Q 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 Q.sub.u 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 (Q.sub.u) as a 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 T.sub.c, 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.sub.+10dBm/Q.sub.-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 f.sub.1 and f.sub.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 2f.sub.1-f.sub.2 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
many microwave applications. The presence of intermodulation
distortion reflects the current density dependence of the surface
reactance, X.sub.s, 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, R.sub.s, 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.
[0085] 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 f.sub.1=841.985 MHz and f.sub.2=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 <-105 dBm. [0086] 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. [0087] 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.
[0088] 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.times.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.
[0089] 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 the filters
measured meet the requirements, which are indicated by dotted
lines. FIGS. 24 and 25 show the IMD 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, X.sub.s, 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.sub.s ceases to be constant and independent of
current density, and begins to increase with increasing current
density. Commonly there is a maximum current density, J.sub.IMD, at
which X.sub.s retains its low current density value, and above
which X.sub.s increases. In this paper by Hammond et al, the
relationships between measured parameters and the material
parameter J.sub.IMD are described. This relationship can be
summarized as follows
j IMD = Q L 2 .omega. 0 1 .GAMMA. IMD P IN 3 P OUT ##EQU00002##
here Q.sub.L is the loaded quality factor of the resonator,
.omega..sub.0 is the resonant frequency, these two functions depend
on the filter function to be realized, .GAMMA..sub.IMD is a factor
which depends only on the geometry of the resonator, and P.sub.IN
and P.sub.OUT are the input and output powers from an
intermodulation measurement.
[0090] The out-of-band IMD test requirement corresponds to a
minimum J.sub.IMD in the HTS thin film of 1.times.10.sup.7
A/cm.sup.2. The DBCO films surpass the specification by 14 dB,
which here corresponds to a factor of 5. Thus, the DBCO films have
a J.sub.IMD of 5.times.10.sup.7 A/cm.sup.2. For filter applications
J.sub.IMD in HTS thin films must be >1.times.10.sup.7
A/cm.sup.2, more preferably >2.times.10.sup.7 A/cm.sup.2, and
most preferably >3.times.10.sup.7 A/cm.sup.2.
Methods of Manufacture
[0091] 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 .about.10 mTorr. The heater also
incorporates a window that allows 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 .about.10.sup.-5 Torr.
This configuration provides sufficient oxygen pressure for
stability of the high-T.sub.c 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 .about.2.5 .ANG./sec. The deposition
temperature for the films discussed here is 760 to 790.degree. 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.
[0092] 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 are 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.
[0093] 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.degree. C.,
respectively. These temperatures are significantly higher than the
temperature of 760.degree. 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
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.
[0094] 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.
* * * * *