Microstrip line resonator composed of oxide superconductor material

Abstract: A microwave resonator includes a ground conductor (2a,b) formed on an under surface of a dielectric layer (3a) and a signal conductor (1a-c) formed on an upper surface of the dielectric layer separately so that the signal and ground conductors cooperate to form a microstrip line. The signal conductor has a launching pad portion (1b,c) for receiving a signal, and a resonating conductor portion (1a) forming an inductor. The resonating conductor portion is formed separated from the launching pad portion so that a gap (4a) between the launching pad portion and the resonating conductor portion forms a capacitor. Thus, the inductor formed by the resonating conductor portion of the signal conductor and the capacitor formed by the gap between the launching pad portion and the resonating conductor portion form a resonator circuit. The resonating conductor portion (1a) of the signal conductor and a portion of the ground conductor (2a) positionally corresponding to the resonating conductor portion of the signal conductor are formed of a compound oxide superconductor material, and the launching pad portion (1b,c) of the signal conductor and the remaining portion (2b) of the ground conductor are formed of a metal which is of a normal conductor.

Description: The present invention relates to microwave resonators, and particularly to microwave resonators which are passive devices for handling electromagnetic waves having a very short wavelength such as microwaves and millimetric waves, and which have conductor layers, a portion of which is formed of an oxide superconductor material. Description of related art Electromagnetic waves called "microwaves" or "millimetric waves" having a wavelength in a range of a few tens centimeters to a few millimeters can be said from a viewpoint of a physics to be merely a part of an electromagnetic wave spectrum, but have been considered from a viewpoint of an electric engineering to be a special independent field of the electromagnetic wave, since special and unique methods and devices have been developed for handling these electromagnetic waves. Microwaves and millimetric waves are characterized by a straight-going property of radio waves, reflection by a conduction plate, diffraction due to obstacles, interference between radio waves, optical behavior when passing through a boundary between different mediums, and others. In addition, some physical phenomena which were too small in effect in a low frequency electromagnetic wave and in light and therefore could not be utilized in practice, will remarkably appear in the microwaves and millimetric waves. For example, there are now actually used an isolator and a circulator utilizing a gyro magnetic effect of a ferrite, and medical instruments such as plasma diagnosis instrument utilizing interference between a gas plasma and a microwave. Furthermore, since the frequency of the microwaves and millimetric waves is extremely high, the microwaves and millimetric waves have been used as a signal transmission medium of a high speed and a high density. In the case of propagating an electromagnetic wave in frequency bands which are called the microwave and the millimetric wave, a twinlead type finder used in a relative low frequency band has an extremely large transmission loss. In addition, if an inter-conductor distance approaches a wavelength, a slight bend of the transmission line and a slight mismatch in connection portion will cause reflection and radiation, and is easily influenced from adjacent objects. Thus, a tubular waveguide having a sectional size comparable to the wavelength has been actually used. The waveguide and a circuit constituted of the waveguide constitute a three-dimensional circuit, which is larger than components used in ordinary electric and electronic circuits. Therefore, application of the microwave circuit has been limited to special fields. However, miniaturized devices composed of semiconductor have been developed as an active element operating in a microwave band. In addition, with advancement of integrated circuit technology, a so-called microstrip line having an extremely small inter-conductor distance has become used. In 1986, Bednorz and Müller discovered (La, Ba)₂CuO₄ showing a superconduction state at a temperature of 30 K. In 1987, Chu discovered YBa₂Cu₃O y having a superconduction critical temperature on the order of 90 K, and in 1988, Maeda discovered a so-call bismuth (Bi) type compound oxide superconductor material having a superconduction critical temperature exceeding 100 K. These compound oxide superconductor materials can obtain a superconduction condition with cooling using an inexpensive liquid nitrogen. As a result, possibility of actual application of the superconduction technology has become discussed and studied. Phenomenon inherent to the superconduction can be advantageously utilized in various applications, and the microwave components are no exceptions. In general, the microstrip line has an attenuation coefficient that is attributable to a resistance component of the conductor. This attenuation coefficient attributable to the resistance component increases in proportion to a root of a frequency. On the other hand, the dielectric loss increases in proportion to increase of the frequency. However, the loss of a recent microstrip line particularly in the range of microwaves and millimetric waves is almost attributable to the resistance of the conductor, since the dielectric materials have been improved. Therefore, if the resistance of the conductor in the strip line can be reduced, it is possible to greatly elevate the performance of the microstrip line. As well known, the microstrip line can be used as a simple signal transmission line. However, if a suitable patterning is applied, the microstrip line can be used as an inductor, a filter, a resonator, a directional coupler, and other passive microwave circuit elements that can be used in a hybrid circuit. Applied Physics Letter, vol. 55, no 10 (1989), page 1029-1031 describes a coplanar waveguide transmission line resonator patterned from a perovskite high Tc superconducting film. Proceedings of the 42nd Annual Frequency Control Symposium 1-3 June, 1988 (IEEE Catalogue No. 88CH2588-2), pages 556-558 discloses a superconducting resonator comprising a dielectric substrate having on one surface a one-half wavelength resonator and having on an opposite surface a ground plane layer. EP-A2-0 357 507 published on March 7, 1990 discloses microwave waveguides using an oxide superconductor material. However, a practical microwave resonator utilizing an excellent property of the oxide superconductor material has not yet been proposed. Summary of the Invention Accordingly, it is an object of the present invention to provide a high performance microwave resonator utilizing an oxide superconductor material of a good superconduction characteristics. The above and other objects of the present invention are achieved in accordance with the present invention by a microwave resonator including a dielectric layer, a first conductor formed on the dielectric layer and functioning as a ground conductor, a second conductor formed on the dielectric layer separately from the first conductor so that the first and second conductors cooperate to form a microwave line. The second conductor has at least a launching pad portion for receiving a signal, and a resonating conductor portion forming an inductor. The resonating conductor portion is formed separated from the launching pad portion so that a gap between the launching pad portion and the resonating conductor portion forms a capacitor, and the inductor formed by the resonating conductor portion of the second conductor and the capacitor formed by the gap between the launching pad portion and the resonating conductor portion forms a resonator circuit. The resonating conductor portion of the second conductor and a portion of the first conductor positionally corresponding to the resonating conductor portion of the second conductor are formed of a compound oxide superconductor material, and the launching pad portion of the second conductor and the remaining portion of the first conductor are formed of a metal which is of a normal conductor. Preferably, the conductors in the microwave resonator in accordance with the present invention are formed in the form of a thin film deposited under a condition in which a substrate temperature does not exceed 800°C throughout a whole process from a beginning until a termination. As seen from the above, the microwave resonator in accordance with the present invention is characterized in that only the portions of the first and second conductors constituting a resonating circuit are formed of oxide superconductor material, and the other portions of the first and second conductors are formed of a normal conduction metal. Since the portions of the first and second conductors constituting a resonating circuit are formed of oxide superconductor material, propagation loss in a microwave line constituting the microwave resonator is remarkably reduced, and a usable frequency band is expanded toward a high frequency side. In addition, since the conductor is formed of the oxide superconductor material, the superconduction condition can be realized by use of inexpensive liquid nitrogen, and therefore, the microwave resonator of a high performance can be used in increased fields of application. On the other hand, since the conductors excluding the resonating circuit, for example, the launching pad portion for guiding a signal to the resonator from an external circuit and a conductor for supplying a signal from the resonator to an external circuit, are formed of a normal conductor metal, the existing materials and methods can be used for connecting the resonator in accordance with the present invention to another circuit or a package. In addition, since the resonating conductor portion and the launching pad portion of the second conductor are separated from each other, the resonating conductor portion and the launching pad portion of the second conductor can be easily formed of different materials, respectively. The conductors of the microwave resonator in accordance with the present invention can be formed of either a thin film or a thick film. However, in the case of the superconductor forming the conductor portion of the resonating circuit, the thin film is more excellent in quality than the thick film. The oxide superconductor thin films constituting the conductor layers can be deposited by any one of various known deposition methods. However, in the case of forming the oxide superconductor thin films used as the conductor layers of the microwave resonator, it is necessary to pay attention so as to ensure that a boundary between the dielectric layer and the oxide superconductor thin films is maintained in a good condition. Namely, in the microwave components, an electric current flows at a surface of the conductor layer, and therefore, if the surface of the conductor layer is disturbed in a physical shape and in an electromagnetic characteristics, a merit obtained by using the oxide superconductor material for the conductor layer would be lost. In addition, if the dielectric layer is formed of Al₂O₃ or SiO₂, it is in some case that Al₂O₃ or SiO₂ reacts with the compound oxide superconductor material by a necessary heat applied in the course of the oxide superconductor film depositing process, with the result that the superconduction characteristics of a signal conductor is deteriorated or lost. The matters to which attention should be paid at the time of depositing the oxide superconductor material are: (1) The material of the oxide superconductor material and the material of the dielectric layer or substrate have a less reactivity to each other, and (2) a treatment which causes the materials of the oxide superconductor layer and the dielectric layer to diffuse to each other, for example, a heating of the substrate to a high temperature in the course of deposition and after the deposition, should be avoided to the utmost. Specifically, it is necessary to pay attention so as to ensure that the temperature of the substrate in no way exceeds 800°C in the process of the oxide superconductor material deposition. From the viewpoint as mentioned above, a vacuum evaporation or a laser evaporation are convenient, since there is less restriction to the substrate temperature in the course of the deposition and therefore it is possible to easily and freely control the substrate temperature. In addition, a so-called post-annealing performed after deposition is not convenient not only in the above deposition processes but also in other deposition processes. Therefore, it is important to select a deposition process ensuring that an as-deposited oxide superconductor material layer has already assumed a superconduction properly without treatment after deposition. The dielectric layer can be formed of any one of various known dielectric materials. For example, SrTiO₃ and YSZ are greatly advantageous from only a viewpoint of depositing the superconductor thin film. However, a very large dielectric loss of these material would cancel a benefit of a decreased conductor loss obtained by using the superconductor. Therefore, in order to improve the characteristics of the microwave line, it is advantageous to use a material having a small dielectric dissipation factor "tan δ", for example, Al₂O₃, LaAlO₃, NdGaO₃, MgO and SiO₂. Particularly, LaAlO₃ is very convenient, since it is stable until reaching a considerably high temperature and is very low in reactivity to the compound oxide superconductor material, and since it has a small dielectric loss that is one-tenth or less of that of SrTiO₃ and YSZ. In addition, as the substrate which has a small dielectric loss and on which the oxide superconductor material can be deposited in a good condition, it is possible to use a substrate obtained by forming, on opposite surfaces of a dielectric plate such as a sapphire and SiO₂ having a extremely small dielectric loss, a buffer layer which makes it possible to deposit the oxide superconductor material in a good condition. For forming the conductor portions of the resonating circuit, a yttrium (Y) system compound oxide superconductor material and a compound oxide superconductor material including thallium (Tl) or bismuth (Bi) can be exemplified as the oxide superconductor material which has a high superconduction critical temperature and which becomes a superconduction condition with a liquid nitrogen cooling. However, the oxide superconductor material is not limited to these materials. The compound oxide superconductor material can be formed in any pattern by a lift-off process in which a resist pattern is previously formed on a substrate and then a thin film of oxide superconductor material is deposited on the resist pattern. Alternatively, the compound oxide superconductor material layer deposited on a whole surface of the substrate can be patterned by a wet etching using a hydrochloric acid or other etching agents. The microwave resonator in accordance with the present invention can be in the form of a linear resonator which is formed of rectangular conductor layers having a predetermined width and a predetermined length, or in the form of a circular disc resonator or a ring resonator which is constituted of a circular conductor having a predetermined diameter. The above and other objects, features and advantages of the present invention wilt be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. However, the examples explained hereinafter are only for illustration of the present invention, and therefore, it should be understood that the present invention is in no way limited to the following examples. Brief Description of the Drawings Figures 1A, 1B and 1C are diagrammatic sectional views of various microwave transmission lines which can form the superconduction microwave resonator in accordance with the present invention; Figure 2 is a diagrammatic plan view illustrating a patterned signal conductor of a superconduction microwave resonator in accordance with the present invention; and Figures 3A to 3D are diagrammatic sectional views illustrating various steps of a process for fabricating the microwave resonator in accordance with the present invention. Description of the Preferred embodiments Referring to Figures 1A to 3C, there are shown sectional structures of microwave transmission lines which can constitute the microwave resonator in accordance with the present invention. A microwave transmission line shown in Figure 1A is a so called microstrip line which includes a dielectric layer 3, a center signal conductor 1 formed in a desired pattern on an upper surface of the dielectric layer 3, and a ground conductor 2 formed to cover a whole of an undersurface of the dielectric layer 3. A microwave transmission line shown in Figure 1B is a so called balanced microstrip line which includes a center signal conductor 1, a dielectric layer 3 embedding the center signal conductor 1 at a center position, and a pair of ground conductors 2m and 2n formed on upper and under surfaces of the dielectric layer 3, respectively. A microwave transmission line shown in Figure 1C is a so called coplanar guide type microwave line which includes a dielectric layer 3, and a center signal conductor 1 and a pair of ground conductor 2m and 2n formed on the same surface of the dielectric layer 3, separately from one another. The various microwave lines as mentioned above can constitute a microwave resonator by appropriately patterning the center conductor 1. In this embodiment, in view of the degree of freedom in the patterning and an excellent characteristics of the microwave line itself, the microwave resonator was fabricated by adopting the structure of the balanced microstrip line shown in Figure 1B. Figure 2 shows a center signal conductor pattern of the microwave resonator fabricated in accordance with a process which will be described hereinafter. Figure 2 also shows a section taken along the line X-X in Figure 1B. As shown in Figure 2, the center signal conductor pattern of the microwave resonator includes a pair of center conductors 1b and 1c aligned to each other but separated from each other, and another center conductor 1a located between the pair of center conductors 1b and 1c and aligned to the pair of center conductors 1b. The center conductor 1a is separated from the pair of center conductors 1b and 1c by gaps 4a and 4b, respectively. With this arrangement, the center conductor 1a forms an inductor, and each of the gaps 4a and 4b forms a coupling capacitor, so that a series-connected LC resonating circuit is formed. Therefore, the center conductor 1a forms a resonating conductor in the microwave resonating circuit, and each of the pair of center conductors 1b and 1c forms a launching pad in the microwave resonating circuit. Specifically, the center conductor 1a has a width of 0.26 mm and each of the gaps 4a and 4b is 0.70 mm. The launching pads 1b and 1c forms a microstrip line having a characteristics impedance of 50 Ω at 10 GHz. On the other hand, the resonating conductor 1c is in a rectangular pattern having a width of 0.26 mm and a length of 8.00 mm. Here, the dielectric layer 3 was formed of LaAlO₃, and the resonating conductor 1a of the resonating circuit is formed of a YBa₂Cu₃O y (6<y≦7) thin film. The launching pads 1b and 1c and the ground conductor (not shown in Figure 2) are formed of an Al (aluminum) thin film. Referring to Figures 3A to 3D, a process of fabricating the embodiment of the microwave resonator in accordance with the present invention is illustrated. Figures 3A to 3D show a section taken along the line Y-Y in Figure 1B and in Figure 2. First, a LaAlO₃ plate 3a having a thickness of 0.5 mm was used as the dielectric substrate. YBa₂Cu₃O y thin films were deposited on an upper surface and an undersurface of the LaAlO₃ dielectric substrate 3a by an electron beam evaporation process. Thereafter, the oxide superconductor thin films were patterned by a wet etching using an etching agent of hydrochloric acid, so that a resonating conductor 1a is formed on the upper surface of the dielectric substrate 3a, and a ground conductor 2a is formed on the undersurface of the dielectric substrate 3a, as shown in Figure 3A. The YBa₂Cu₃O y thin films were of a thickness 6000 Å. The ground conductor 2a has a width which is three times the width of the resonating conductor 1a, and a length which is one and one-fifth of the length of the center conductor 1a. Thereafter, an aluminum thin film of a thickness 6000 Å was formed on the upper surface and the undersurface of the dielectric substrate 3a by a lift-off process, so as to form the launching pads 1b and 1c and a ground conductor 2b, as shown in Figure 3B. The ground conductor 2b was formed to completely cover the whole of the undersurface of the dielectric substrate 3a. Then, as shown in Figure 3C, a mask 5 was deposited on the resonating conductor 1a and the launching pads 1b and 1c, and an LaAlO₃ thin film 3b of a thickness 6000 Å was grown on an uncovered portion of the substrate 3a. On the other hand, an LaAlO₃ plate 3c having a YBa₂Cu₃O y thin film ground layer 2c and an aluminum thin film ground layer 2d formed on an upper surface thereof were prepared with the same process as that shown in Figures 3A and 3B. As shown in Figure 3D, the LaAlO₃ plate 3c was closely stacked on the conductors 1a, 1b, and 1c and the LaAlO₃ thin film 3b of the LaAlO₃ plate 3a after the mask layer 5 was removed. Thus, the microwave resonator having substantially the same basic structure as the sectional structure shown in Figure 1B was completed. The resonating conductor 1a, the ground conductor layers 2a and 2b and the dielectric layer 3b were deposited in the following conditions: When the YBa2Cu3Oy thin films as mentioned above were deposited, an O₃ gas was blow onto a deposition surface by a ring nozzle located in proximity of the deposition surface. The blown O₃ gas was obtained by gasifying a liquefied ozone refrigerated by a liquid nitrogen. Namely, the blown O₃ gas was a pure O₃ gas. This O₃ gas was supplied at a rate of 40 cm²/minute. The microwave resonator fabricated as mentioned above was connected to a network analyzer in order to measure a frequency characteristics of a transmission power in a range of 2 GHz to 20 GHz. To evaluate a frequency selectivity of a microwave resonator, it is an ordinary practice to indicate, as Q factor, a ratio of a resonance frequency "fo" and a band width "B" in which the level of the transmission power does not drop below a level which is lower than a maximum level by 3 dB. (Q = fo / B) In addition, as a comparative example, there was prepared a microwave resonator having the same specification as that of the above mentioned microwave resonator in accordance with the present invention, other than the fact that all of the conductors are formed of aluminum. Q factor of the embodiment of the microwave resonator of the present invention and the comparative example was measured. The result of the measurement is shown in the following TABLE. As seen from the above, the present invention can give the microwave resonator capable of operating at a liquid nitrogen temperature and having a remarkably high Q factor, since the resonator constituting conductor portions of a microstrip line are formed of an oxide superconductor material layer having an excellent superconduction characteristics. In addition, since the conductors other than the resonator constituting portions are formed of a normal conduction metal, the microwave resonator in accordance with the present invention can be connected to the existing package or parts by means of a conventional manner. The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but changes and modifications may be made within the scope of the appended claims.

Claims: A microwave resonator including a dielectric layer (3a), a first conductor (2a,b) formed on said dielectric layer (3a) and functioning as a ground conductor, a second conductor (1a,c) formed on said dielectric layer (3a) separately from said first conductor (2a,b) so that said first and second conductors cooperate to form a microwave line, said second conductor (1a,c) having at least a launching pad portion (1b,c) for receiving a signal, and a resonating conductor portion (1a) forming an inductor, said resonating conductor portion (1a) being formed separated from said launching pad portion (1b,c) so that a gap (4a,b) between said launching pad portion (1b,c) and said resonating conductor portion (1a) forms a capacitor, said inductor formed by said resonating conductor portion (1a) of said second conductor and said capacitor formed by said gap (4a,b) between said launching pad portion (1b,c) and said resonating conductor portion (1a) forming a resonator circuit, characterized in that said resonating conductor portion (1a) of said second conductor and a portion of said first conductor (2a) positionally corresponding to said resonating conductor portion (1a) of said second conductor being formed of a compound oxide superconductor material, and said launching pad portion (1b,c) of said second conductor and said remaining portion of said first conductor being formed of a metal which is of a normal conductor. A microwave resonator claimed in Claim 1 wherein said dielectric layer (3a) is formed of a single dielectric substrate, and wherein said first conductor (2a,b) is formed to cover a whole surface of one of opposite surfaces of said dielectric layer (3a), and said second conductor (1a,c) is formed on the other of said opposite surfaces of said dielectric layer (3a), and shaped in a determined pattern. A microwave resonator claimed in Claim 1 wherein said first conductor (2a,b) is formed to cover a whole surface of one of opposite surfaces of said dielectric layer (3a) and said second conductor layer (1a,c) is embedded within said dielectric layer (3a), and shaped in a determined pattern, and further including a third conductor (2d) formed to cover a whole surface of the other of said opposite surfaces of said dielectric layer (3a) and functioning as a ground conductor. A microwave resonator claimed in Claim 1 wherein both said first (2a,b) and second (1a,c) conductors are formed on one of said opposite surfaces of said dielectric layer (3a), and said first conductor (2a,b) is divided into a pair of half portions in parallel to each other and separated from each other, and said second conductor (1a,c) is located in a space formed between said pair of half portions of said first conductor (2a,b) and separated from each of said pair of half portions of said first conductor (2a,b). A microwave resonator claimed in Claim 1 wherein said second conductor (1a,c) also includes a second launching pad portion (1b,c) formed separated from said resonating conductor portion (1a) so that a gap (4a,b) between said resonating conductor portion (1a) and said second launching pad portion (1b,c) forms a capacitor, and wherein said first second launching pad portion, said resonating conductor portion and said second launching pad portion of said second conductors are located on a straight line. A microwave resonator claimed in Claim 1 wherein said dielectric layer (3a) is formed of a material from a group consisting of Al₂O₃, LaAlO₃, NdGaO₃, MgO and SiO₂. A microwave resonator claimed in Claim 1 wherein said compound oxide superconductor material is YBa₂Cu₃Oy (6<y≦7). A microwave resonator claimed in Claim 1 wherein said first conductor (2a,b) includes an oxide superconductor layer (2a) formed on a surface of said dielectric layer (3a) at a position corresponding to said resonating conductor portion (1a) of said second conductor (1a,c) and having a size sufficiently larger than that of said resonating conductor portion (4a) of said second conductor (1a,c) and a normal conductor metal layer (2b) formed to cover said oxide superconductor layer (2a) and said surface of said dielectric layer (3a) uncovered by said oxide superconductor layer (2a).

Inventors: HIGAKI KENJIRO CABINET BALLOT-, ITOZAKI HIDEO CABINET BALLOT-S, TANAKA SABURO CABINET BALLOT-S, HIGAKI, KENJIRO, CABINET BALLOT-SCHMIT, ITOZAKI, HIDEO, CABINET BALLOT-SCHMIT, TANAKA, SABURO, CABINET BALLOT-SCHMIT
Applicants: JAPAN RES DEV CORP, SUMITOMO ELECTRIC INDUSTRIES, RESEARCH DEVELOPMENT CORPORATION OF JAPAN, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: H01P7
IPC codes: H01L39, H01P7, G01N22