Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – High frequency waveguides – resonators – electrical networks,...
Reexamination Certificate
1997-12-11
2002-10-08
Lee, Benny T. (Department: 2817)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
High frequency waveguides, resonators, electrical networks,...
C333S219000, C333S202000, C333S09900R
Reexamination Certificate
active
06463308
ABSTRACT:
BACKGROUND
The present invention relates to microwave devices and components comprising dielectric substrates and conductors in the form of superconducting films. The tunability of such devices is obtained through varying the dielectric constant of the dielectric material. Examples of devices are for example tunable resonators, tunable filters, tunable cavities etc. Microwave devices or components are important for example within microwave communication, radar systems and cellular communication systems. Of course there are also a number of other fields of application.
The use of microwave devices is known in the art. In “High Temperature Superconducting microwave circuits” by Z-Y Shen, Artech House 1994, dielectric resonators are discussed which are based on TE
011
delta modes. A dielectric resonator is clamped between thin High Temperature Superconducting films (HTS) which are deposited on separate substrates and thus not directly on the dielectric. These resonators fulfill the requirements as to cellular communication losses and power handlings at about 1-2 GHz. It is however inconvenient that the dimensions of the HTS films and the dielectric substrates at these frequencies (e.g. 1-2 GHz) are large and moreover the devices are expensive to fabricate. Furthermore they can only be mechanically tuned which in turn makes the devices (e.g. filters) bulky and introduce complex problems in connection with vibrations or microphonics. WO 94/13028 shows integrated devices of ferroelectric and HTS films. Thin epitaxial ferroelectric films are used. Such films have a comparatively small dielectric constant and the tuning range is also limited and the microwave losses are high. Furthermore there is a highly non-linear current density in thin HTS film coplanar waveguides and microstrips. This results from the high current density at the edges of the strips, D. M. Sheen et al, IEEE Trans. on Appl. Superc. 1991, Vol. 1, No. 2, pp. 108-115. The applicability of these integrated HTS/ferroelectric thin film devices is therefore limited and they are not suitable as for example low-loss narrow-band tunable filters.
Generally tunable filters are important components within microwave communication and radar systems as discussed above. Filters for cellular communication systems for example, which may operate at about 1-2 GHz occupy a considerable part of the volume of the base stations, and often they even constitute the largest part of a base station. The filters are furthermore responsible for a high power consumption and considerable losses in a base station. Therefore tunable low loss filters having high power handling capabilities are highly desirable. They are also very attractive for future broad band cellular systems. Today mechanically tuned filters are used. They have dielectrically loaded volume resonators having dielectric constants of about 30-40. Even if these devices could be improved if materials were found having still higher dielectric constants and lower losses, they would still be too large, too slow and involve losses that are too high. For future high speed cellular communication systems they would still leave a lot to be desired.
In U.S. Pat. No. 5,179,074 waveguide cavities wherein either part of or all of the cavity is made of superconducting material are shown. Volume cavities with dielectric resonators have high Q-values (quality factor) and they also have high power handling capabilities. They are widely used in for example base stations of mobile communications systems. The cavities as disclosed in the above mentioned US patent have been reduced in size and moreover the losses have been reduced. However, they are mechanically tuned and the size and the losses are still too high. WO 94/13028 also shows a number of tunable microwave devices incorporating high temperature superconducting films. However, also in this case thin ferroelectric films are used as already discussed above, and the size is not as small as needed and the losses are too high. Furthermore, the tuning range is limited.
“1 GHz tunable resonator on bulk single crystal SrTiO plated with YBaCuO films.” by O. G. Vendik et al, Electronics Letters, Vol. 31, No. 8, April 1995 shows a tunable resonator on bulk single crystal SrTiO3 plated with YBCO films. This device however suffers from the drawbacks of not being usable above T
c
(the critical temperature for superconductivity). This means for example that no signals could pass if the temperature would be above T
c
which may have serious consequences in some cases. These devices cannot be used unless in a superconducting state.
Furthermore the superconducting films are very sensitive and since they are in no way protected this could have serious consequences as well. In general, in the technical field, only dielectrics e.g. photoresist have been used to protect superconducting films.
SUMMARY OF THE INVENTION
Thus tunable microwave devices are needed which can be kept small, operate at high speed and which do not involve high losses. Devices are also needed which can be tuned over a wide range and which do not require mechanical tuning. Devices are needed which have a high dielectric constant particularly at cryogenic temperatures and particularly devices are needed which fulfil the abovementioned needs in the frequency band of 1-2 GHz, but of course also in other frequency bands. Still further devices are needed which can operate in superconducting as well as in non-superconducting states. Devices are also needed wherein the superconducting films are less exposed. Particularly devices are needed which can be electrically tuned and reduced in size at a high level of microwave power.
Therefore a device is provided which comprises a substrate of a dielectric material with a variable dielectric constant. At least one superconducting film is arranged on parts of the dielectric substrate which comprises a non-linear dielectric bulk material. The substrate comprises a single crystal bulk material and the superconducting film or films comprise high temperature superconducting films. A normal conducting layer is arranged on one or both sides of the superconducting film(s) which is/are opposite to the dielectric substrate. The tuning is provided through producing a change in the dielectric constant of the dielectric material and this may particularly be carried out via external means and particularly the electrical dependence of the dielectric constant used for example for voltage control or also the temperature dependence of the dielectric constant can be used for controlling purposes. Particularly, an external DC bias voltage can be applied to the superconducting film. Alternatively a current can be fed to the films but it is also possible to use a heating arrangement connected to the superconducting film or films and in this way change the electric constant of the dielectric material. Bulk single crystal dielectrics particularly bulk ferroelectric crystals, have a high dielectric constant which can be above for example 2000 at temperatures below 100° K, in the case of high temperature superconducting films below T
c
, which is the transition temperature below which the material is superconducting. Krupka et al in IEEE MTT, 1994, Vol. 42, No. 10, p. 1886 states that bulk single crystal ferroelectrics such as SrTiO3 have small dielectric losses such as 2.6×10−4 at 77° K and 2 GHz and very high dielectric constants at cryogenic temperatures.
However, according to WO 94/13028 and “A High Temperature Superconducting Phase Shifter” by C. M. Jacobson et. al in Microwave Journal Vol. 5, No. 4, December 1992 pp 72-78 states that the electrical variation to change the dielectric constant of bulk material is small and thus far from satisfactory. Moreover, microwave integrated circuit devices are exclusively made by thin film dielectrics which according to the known documents is necessary.
The dimensions of the devices according to the invention can be very small, such as for example smaller than one centimeter at frequencies of about 1-2 GHz and still the total
Gevorgian Spartak
Kollberg Erik
Vendik Orest
Wikborg Erland
Burns Doane Swecker & Mathis L.L.P.
Lee Benny T.
Telefonaktiebolaget LM Ericsson (publ)
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