EP0485806B1

EP0485806B1 - Superconducting microwave parts

Info

Publication number: EP0485806B1
Application number: EP19910118488
Authority: EP
Inventors: Higaki C/O Itami Works Of Sumitomo Kenjiro; Itozaki C/O Itami Works Of Sumitomo Hideo
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1990-10-29
Filing date: 1991-10-29
Publication date: 1997-03-05
Anticipated expiration: 2011-10-29
Also published as: DE69124922D1; DE69124922T2; EP0485806A3; EP0485806A2

Description

This invention relates to superconducting microwave components. More specifically, this invention relates to high frequency parts for treating electromagnetic waves having short wavelengths, such as microwaves, millimeter waves or others, and especially to new constitutions of microwave components having the conductor layers formed of oxide superconducting materials.
Although the electromagnetic waves having wavelengths from tens centimeters to some millimeters and called microwaves, millimeter waves or others are theoretically only a part of the electromagnetic wave spectrum, these electromagnetic waves are, in many cases, specially studied independently in the engineering field because special means and parts have been developed for treating these electromagnetic waves. The microwave line for guiding the electromagnetic waves in this band comprises a pair of conductor lines arranged through a dielectric and having one of the conductor lines grounded.
On the other hand, in 1986 (La,Ba)₂CuO₄ which exhibits superconductivity at 30 K was discovered by Bedbirz, Mueller, et al. In the next year 1987 YBa₂Cu₃Oy having a critical superconducting temperature in the order of 90 K was discovered by Chu, et al. In 1988 Maeda, et al. discovered the so-called Bi-based composite oxide superconducting material which exhibits a critical superconducting temperature exceeding 100 K. Since these composite oxide superconductors can realize superconductivity by the cooling by inexpensive liquid nitrogen, the possibility of practical applications of the superconducting technique has been suddenly noted.
Unexceptionably the microwave components also enjoy the characteristic phenomena of superconductivity. That is, generally in a strip line the attenuation constant of a conductor due to a resistance is proportional to a square root of a frequency. The dielectric loss also increases with an increase of a frequency. The dielectric loss in the recent strip lines is almost attributed mainly to a resistance of a conductor layer especially in the band equal to or lower than 10 GHz owing to the improvement of dielectric materials. Accordingly it much improves the efficiency of the strip line to decrease the resistance of a conductor layer of the strip line. That is, by making a conductor line superconducting, the propagation loss is much reduced while the applicable frequency band is expanded toward the higher frequency side.
Microwave strip lines can be not only used as mere transmission lines, but also can be patterned suitably to be microwave components, such as inductors, filters, resonators, delay lines, directional couplers, etc. Accordingly the improvement of strip lines leads to the improvement of the characteristics of such microwave components.
Since the use of oxide superconducting materials as superconducting materials enables superconductivity to be realized by use of inexpensive liquid nitrogen, it is possible that microwave components of higher performance will prevail in more various fields.
But it is impossible to obtain microwave components which sufficiently take advantage of the characteristics of superconductors, by simply replacing the metal conductors of microwave components with oxide superconductors.
One reason for this is that further decrease of the dielectric loss is necessary. That is, in the conventional microwave lines using metal conductors the dielectric loss in comparison with the conductor loss of the metal conductor has been sufficiently decreased. In the case where superconductors are used as the conductor lines, the decrease of the dielectric loss is again brought up as a problem to be solved since the conductor loss can be minimized to the extremity.
On the other hand, it is known that oxide superconductors can have good characteristics when the superconducting films are formed on specific substrates, as of MgO, SrTiO₃, etc. But all the oxides of MgO, SrTiO₃, etc. do not have good characteristics of dielectrics. But when oxide superconducting films are formed on substrates, as of sapphire, SiO₂, etc., having very low dielectric losses, the superconductive characteristics of the superconducting films are deteriorated or lost. Thus it is substantially impossible to form oxide superconducting films which are to be conductor lines, directly on these dielectric substrates of low dielectric losses. In short, it is impossible to fabricate microwave components which exhibit effective characteristics simply by replacing the conductor portions of the conventional microwave components formed of metal conductors with oxide superconductors.
In the article "Electrical behavior of a 31 cm, thinfilm YBaCu = superconducting microstrip" by Hornak et al., published in Journal of Applied Physics, vol. 66, no. 10, pages 5066ff, a superconducting microwave component is disclosed which component has a first and a second layer made of the same material with a sapphire layer between.
However, this microwave component has a superconducting microstrip on the upper layer only whereas the ground plane layer is connected to a simple Cu-fixture only with several disadvantages such as fabricating, mounting, and the electric and mechanic behavior of such a device.
In the article "Evaluating superconducting resonator materials" by McAvoy et al., published on the 42nd Annual Frequency Control Symposium, 1988, Baltimore, Maryland (IEEE Catalog No. 88CH2588-2), page 556ff., a similar stripline design with a film conductor is disclosed. However, according to the disclosure of this reference, the ground contact to the lower substrate in a sandwich design is achieved by contact to bulk ground plane, too.
It is one object of this invention to provide microwave components which can solve the above-described problem, and which have innovational constitutions which can make sufficient use of the characteristics of the oxide superconductors.
It is another object of the present invention to provide a superconducting microwave component comprising a first substrate of a dielectric material with a conductor line of an oxide superconductor formed in a required pattern on the surface, a second substrate of a dielectric with a grounding conductor of an oxide superconductor formed on the surface, and a third substrate of a dielectric which is laid on the first and the second substrates with the third substrate sandwiched between the first and the second substrates.
The above problems are solved by a semiconducting microwave component as defined by the characterizing part of claim 1. The semiconducting devices as set forth in the following dependent claims are developments with further advantages.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only.

Brief Description of the Drawings

FIG. 1 is a sectional view of the structure of a microwave component according to one embodiment of this invention;
FIG. 2 is sectional views of a microwave component according to another embodiment of this invention; and
FIG. 3 is a view showing the configurations of the members of the microwave component of FIG. 2.

FIG. 1 is a sectional view schematically showing the structure of the microwave component according to one embodiment of this invention.
The microwave component of FIG. 1 comprises a first substrate 2 having a conductor line 1 formed of an oxide superconducting film depicting a required pattern, a dielectric strip 4, and a second substrate 6 having a superconducting grounding conductor 5 formed of a superconducting film, which are laid on each other in a package 7, and the package is sealed with covers 8a, 8b. Although not shown, a lead for connecting the superconductor line 1 to the outside of the package 7 is actually provided through the package 7, or through the covers 8a, 8b.
In this microwave component, the first substrate 2 and the second substrate 6 have different sizes. A step 7a is formed on the inside of the package 7 for accommodating the size difference. That is, the second substrate 6 has a larger size than the first substrate 2, and the grounding superconductor 5 on the second substrate 6 is in contact at the boundary portion with the step 7a on the inside of the package 7. A rib 8c is formed on the underside of the cover 8a for pressing down the first substrate 2.
In the microwave component of the above-described structure, the conductor line 1 and the superconducting grounding conductor 5 are formed respectively of Y-based, Bi-based, Tl-based or others-based oxide superconducting films. The substrates 2 and 6 are formed of oxides, such as MgO, SrTiO₃ or others, which permits those oxide films to be well formed. The dielectric strip 4 is formed of a material, e.g., Sapphire, whose dielectric loss is very small.

(Fabrication Example 1)

A microwave resonator, which is one of the microwave components, having the sectional structure of FIG. 1 was fabricated.
As the first substrate 2, a single MgO crystal substrate which is a 18 mm-square having a thickness 0.1 mm. As the second substrate 6, an MgO single crystal substrate which is 20 mm-square having a thickness of 1 mm.
The conductor line 1 and the superconducting grounding conductor 5 formed respectively on the substrate were formed of thin films of Y-Ba-Cu composite oxide. Table 1 shows the film preparation conditions. Table 1

Evaporation source Y,Ba,Cu (metal)

Gas pressure 2 x 10^-4 (Torr)

Substrate temperature 600 (°C)

Film thickness 6000 (Å)
When the oxide films were formed, O₃ gas was blown onto the film forming surfaces of the substrates from a ring nozzle positioned near the film forming surfaces. The blown O₃ gas was vaporized liquid ozone cooled by nitrogen gas and was substantially pure O₃ gas. The feed amount of the O₃ gas was 20 cc/min.
That of the thus-formed oxide superconducting films which was formed on the first substrate 2 was patterned into the conductor line 1. The patterning was performed by wet-etching using hydrochloric acid as the etchant. A straight conductor line having 1.1 mm-width and 8.0 mm-length was formed, and a pair of pads for leading microwaves was formed in the conductor line.
The dielectric strip 4 was prepared by machining Sapphire plate. This dielectric strip 4 had the same size as the first substrate 2 and had 0.9 mm-thickness.
The package 7, and the covers 8a, 8b were made of brass. By making the package 7 and the covers 8a, 8b of a metal, the cooling was facilitated and efficient.
The thus-prepared members were fabricated into a microwave resonator of the structure shown in FIG. 1.
For comparison, a microwave resonator was prepared. That is, a conductor line was formed of the same oxide superconducting film in the same size and the material except that the thickness of the first substrate 2 was 1.0 mm. This conductor line was housed in the same package, But this sample as a control did not include the dielectric strip 4, and the first substrate 2 was laid directly on the superconducting grounding conductor 5.
The thus-fabricated example sample and control sample were measured by a network analyzer with respect to the frequency dependency of the transparent power, and Q-values of the respective samples as resonators. The measured results are shown in Table 2. It is shown that Q-value of the resonance can be made larger by thinning the first substrate 2 and disposing the dielectric strip 4 between the first an the second substrates 2 and 4. Table 2

Frequency (GHz) 6.9 13.7

Q-value Example 1610 1270

Control 1390 1012
FIGs. 2A and 2B are sectional views schematically showing the structure of a microwave component according to another embodiment of this invention. FIG. 3 is a view showing the members of the microwave component of FIG. 2A. The members of the second embodiment which are common with the first embodiment have same reference numerals.
As shown, the microwave component according to the second embodiment comprises a first substrate 2 having a conductor line 1 formed on the underside, a dielectric strip 4 having a pair of waveguides 3a, 3b, and a second substrate 6 having a superconducting grounding conductor 5 formed the surface which are laid on each other and housed in a package 7. The package 7 is sealed with covers 8a, 8b. Although not shown, a lead is actually provided through the package 7, or through the covers 8a, 8b for connecting the conductor line 1 to the outside of the package 7.
In this microwave component, the first substrate 2, the dielectric strip 4, and the second substrate 6 have different sizes from one another. To accommodate a size difference there is formed a step 7a on the inside of the package 7. That is, the size of the second substrate 6 is larger than that of the first substrate 1 and that of the dielectric strip 4. The superconducting grounding conductor 5 is in contact at the boundary portion with the step 7a on the inside of the package 7. On the underside of the cover 8a there is provided a rib 8c for pressing down the first substrate 2. Since the first substrate 2 and the dielectric strip 4 have different sizes, in the laid state the first substrate 2 is superposed on a part of the dielectric strip 4 with parts of the surface of the dielectric strip 4 exposed. In these exposed parts a pair of waveguides 3a, 3b are formed. These waveguides 3a, 3b of a metal film are coupled with the conductor line 1 of an oxide superconductor by statically electric coupling.
In this microwave component, the conductor line 1, and the grounding conductor 5 are formed of Y-based, Bi-based, Tl-based or others-based oxide superconducting films. The substrates 2 and 6 are provided by insulating substrates of MgO, SrTiO₃ or others on which the above-mentioned oxide superconducting films can be well formed. The dielectric strip 4 is formed of a material, such as Sapphire or others, having small dielectric loss. For the metal film of the waveguides 3a, 3b a stable material, such as Au or others, is used.

(Fabrication Example 2)

Microwave resonators of FIGs. 2A, 2B and 3 were fabricated.
As the first substrate 2, MgO single crystal substrate having 0.2 mm-thickness, 18 mm-width and 10 mm-length was used. As the second substrate 6, MgO single crystal substrate 1 mm-thickness, 20 mm-width and 20 mm-length was used. As the dielectric strip 4 is 0.5 mm thickness, 18 mm-width and 18 mm-length Sapphire strip was used.
The conductor line 1 and the grounding conductor 5 were formed of Y-Ba-Cu composite oxide film on the respective substrates. The film preparatin conditions are shown in Table 3. Table 3

Evaporation source Y,Ba,Cu (metal)

Gas pressure 2 x 10^-4 (Torr)

Substrate temperature 600 (°C)

Film thickness 6000 (Å)
When the oxide films were formed, O₃ gas was blown onto the film forming surfaces of the substrates from a ring nozzle positioned near the film forming surfaces The blown O₃ gas was vaporized liquid ozone cooled by nitrogen gas and was substantially pure O₃ gas. The feed amount of the O₃ gas was 20 cc/min. That of the thus-formed oxide superconducting films which was formed on the first substrate 2 was patterned into the conductor line 1. The patterning was performed by wet-etching using hydrochloric acid as the etchant. A straight conductor line having 0.56 mm-width and 8 mm-length was formed.
The waveguides 3a,3b were formed of Au by evaporation. The patterning was conducted by lift-off.
The package 7, and the covers 8a, 8b were made of brass.
The thus-prepared members were fabricated into the microwave resonator of FIG. 2.
As a control, a microwave resonator was prepared. That is, a conductor layer and a waveguide were formed of Au on one dielectric substrate, and a grounding conductor layer of Au was formed on the entire underside of the substrate. The thus-prepared microwave resonator was housed in a package of substantially the same package structure.
The thus-prepared example sample and control sample were measured by a network analyzer with respect to the frequency dependency of the transparent power, and Q-values of the respective samples as resonators. The measured results are shown in Table 4. The measuring temperature was 77 K. Table 4

Frequency (GHz) 7.1 13.9

Q-value Embodiment 1780 1420

Control 550 720
The microwave components according to this invention is characterized mainly in that a conductor line and a grounding conductor both of oxide superconducting films are formed on respective optimal substrates, and then the substrates are laid together with the dielectric strip or are set through a vacuum layer or an air layer, whereby a microwave line is formed.
As described above, an oxide superconducting film cannot be formed directly on a dielectric strip having a dielectric loss corresponding to a lower conductor loss of a superconductor. Then in the microwave components according to this invention, an oxide superconducting film, which is the conductor layer, is formed on a specific substrate which can provide good superconducting properties, and this conductor layer is superposed on the dielectric strip formed cf a material having a small dielectric loss or the substrate is opposed to the dielectric strip with a certain gas therebetween, whereby a microwave line having good characteristics is realized. It is not essential that the waveguide for guiding a microwave from the outside is superconducting. The waveguide may be formed of a metal film formed on the dielectric strip.
When the thickness of the substrates for the oxide superconducting films is increased because an oxide substrate material of the substrates, such as YSZ, SrTiO₃, MgO, LaAlO₃, NdGaO₃, Y₂O₃ or others, do not have especially superior properties, the influence of the substrates as dielectrics becomes unnegligible. Accordingly it is preferable that these substrates are thin as much as possible when the faces of the substrates opposite to the faces with the conductor line and the grounding conductor respectively formed on are positioned on the side of the dielectric strip or a gap as a substitute for the dielectric strip. By this arrangement it is not necessary to make the substrates especially thin.
In order to reduce as much as possible the influence of material of the substrates for the oxide superconducting films, preferably a couple of the substrates with the conductor layer and the grounding conductor layer respectively formed on are so arranged that the oxide superconducting films are opposed to each other so as to hinder the direct contact of the respective oxide superconducting films with the dielectric strip.
As oxide superconducting materials of the conductor layer and the grounding conductor layer, oxide superconducting materials which have especially high superconducting critical temperatures and become superconductive by cooling with liquid nitrogen are exemplified by Y-based composite oxides, and composite oxides containing Tl and/or Bi. But in this invention the materials of the conductor layer and the grounding conductor layer are not limited to them. For example, Ln-Ba-Cu-O (Ln: Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu)-based, Bi-Sr-Ca-Cu-O-based, Bi-Pb-Sr-Ca-Cu-O-based, Tl-Ba-Ca-Cu-O-based, or Tl-Bi-Ca-Sr-Cu-O-based etc. are usable.
The dielectric material which is preferably used in the microwave components according to this invention are exemplified by Sapphire LaAlO₃, NdGaO₃, beryllia and borosilicate glass, etc. having a small dielectric tangent tanδ. Sapphire is especially preferable because its dielectric loss is lower by more than one place compared with LaAlO₃ and YSZ. In the microwave components according to this invention, the third substrate (the dielectric strip) is preferably formed of the above-mentioned dielectric materials, but may be formed of any dielectric material because no oxide superconducting film is formed thereon.
The conductor line formed on the first substrate, the grounding conductor on the second substrate, and the dielectric strip (the third substrate), which are formed respectively of the above-mentioned materials, are laid on each other and housed in a suitable package, and a microwave line can be readily fabricated.
The conductor line can be formed in an optional pattern by lift-off in which a resist mask is prepared on the substrate before the formation of the superconducting film. The patterning of the conductor line can be performed also by wet-etching the conductor layer formed on the entire surface of the substrate with an etchant, such as hydrochloric acid or others. A suitable patterning is formed by these methods, and various microwave components can be fabricated as described above.
The microwave components according to this invention have very low transmission loss and has a wide usable frequency band. Furthermore, the microwave components exhibit good properties by cooling with liquid nitrogen.
From the invention thus described, it will be obvious that the invention may be varied in many ways.

Claims

A superconducting microwave component comprising:
a first substrate (2) composed of a first dielectric material and having a first surface and a conductor line (1) comprised of an oxide superconductor disposed on the first surface thereof in a required pattern;

a second substrate (6) comprised of the first dielectric material and having a grounding conductor (5) disposed on a surface thereof; and

a third substrate (4) comprised of a second dielectric material different from the first dielectric material, the third substrate (4) being sandwiched between the first and second substrates (2, 6);
characterized in that
the grounding conductor (5) is comprised of an oxide superconductor; and that

the second substrate (6) is so arranged that the grounding conductor (5) formed thereon faces the third substrate (4) .
A superconducting microwave component according to claim 1,
wherein the conductor layer pattern (1) is formed of a metal.
A superconducting microwave component according to any one of the preceding claims,
wherein the oxide superconductor is Ln element-Ba-Cu-O based, Bi-Sr-Ca-Cu-O based, Bi-Pb-Sr-Ca-Cu-O based, Tl-Ba-Ca-Cu-O based or Tl-Bi-Ca-Sr-Cu-O based.
A superconducting microwave component according to any one of the preceding claims,
wherein the first and the second substrates (2, 6) are formed of MgO, SrTiO₃, LaAlO₃, NdGaO₃, or Y₂O₃.
A superconducting microwave component according to any one of the preceding claims,
wherein the third substrate (4) is formed of MgO, SrTiO₃, or Y₂O₃.
A superconducting microwave component according to one of claims 1 to 4,
wherein the third substrate (4) is formed of Al₂O₃, SiO₂, beryllia, or borosilicate glass.
A superconducting microwave component according to one of the preceding claims,
characterized by a holding member for holding the first (2), the second (6), and the third (4) substrates in a laminar structure.
A superconducting microwave component according to any one of the preceding claims,
wherein the first and the second substrates (2, 6) are so arranged that the respective surfaces opposite to the surfaces thereof on which the conductor line (1) and the grounding conductor (5) are respectively formed face to the third substrate (4); and the first and the second substrate (2, 6) are sufficiently thinner compared with the third substrate (4).