EP0811257A1

EP0811257A1 - Method and apparatus for increasing power handling capabilities of high temperature superconducting devices

Info

Publication number: EP0811257A1
Application number: EP96911193A
Authority: EP
Inventors: George L. Matthaei; Douglas J. Scalapino; Gregory L. Hey-Shipton
Original assignee: Superconductor Technologies Inc
Current assignee: Superconductor Technologies Inc
Priority date: 1995-02-23
Filing date: 1996-02-07
Publication date: 1997-12-10
Also published as: EP0811257A4; WO1996026555A1; CA2211406A1; JPH11500879A

Abstract

In a stripline transmission system (fig. 1), a center conductor (12) having edges (14) is disposed between generally planar, substantially parallel ground planes (16, 18). A first dielectric (22) supports the center conductor (12) which is disposed between the first and second ground planes (16, 18). Gap portions (20) are formed adjacent the center conductor edges (14), the gap (20) containing a dielectric having a dielectric constant lower than that of the solid dielectric (22). The dielectric in the gap (20) is preferably air or vacuum. In one embodiment (fig. 2), the gap portion (42) extends in the region laterally exterior to the center conductor edges (32) and between the ground planes (34, 36). In a microstrip embodiment (50 in fig. 3), a substrate (52) has substantially parallel first and second faces (54, 56), the first face (54) bearing a center conductor (58) having edges (64) with adjacent troughs (62) formed into the first face (54), and a ground plane (60) on the second face (56). Reduced losses and improved linearity results, thereby providing applications for components such as filters, receivers and transmitters.

Description

DESCRIPTION

Method And Apparatus For Increasing Power Handling Capabilities of High Temperature Superconducting Devices

Field of the Invention

This invention is directed to devices formed from high temperature superconductors. More particularly, it is directed to electronic devices formed from such super- conductors into devices having substantial power handling capability.

Background of the Invention

The discovery of "high-temperature superconductivity" (HTS) in 1987 opened up exciting possibilities for future technology, most of which are yet to be realized. Prior to that time superconductivity could be utilized only at near-absolute-zero temperatures that were achieved by the use of expensive and hard-to-handle liquid helium. Now, with the discovery of HTS, superconductivity can at present be obtained at temperatures of up to 130 K. These temperatures are easily provided by the use of liquid nitrogen which boils at 77 K, and is much cheaper and easier to handle than is liquid helium. In addition, the needed temperatures can also be provided for many applica- tions by electrical cryogenic coolers of relatively small size. Some of these are presently available and, no doubt, more powerful and less expensive models will be available in the future. These developments provide microwave engineers with means for achieving microwave circuits with small size yet with extremely low loss.

Microwave systems have been greatly advanced by the application of planar circuit technology which makes possible the photoetching of complex, compact circuits with many components on a single substrate. These are often referred to as "hybrid microwave integrated cir¬ cuits" if the solid-state devices are produced in the circuit processing. These techniques have made feasible many sophisticated systems that would not have been practical if it had been necessary to use older non-planar circuitry such as circuits using waveguide and coaxial lines which are relatively large and heavy. Examples of non-superconducting, lumped element planar arrangements are shown, for example, in Swanson U.S. Patent No. 4,881,050, issued November 14, 1989. Further, work has been done towards the goal of reducing propagation losses in normal metal coplanar waveguides. For example, in F. Williams, et al, "Reduction of Propagation Losses in Coplanar Waveguide", 1984 IEEE MTT-S Digest, pp 453-4, coplanar waveguide propagation losses are reduced by forming an air-gap between a center conductor and coplanar ground plane portions. Two embodiments are proposed, one in which the coplanar waveguide is formed on a thin dielectric layer in turn formed on a thicker, higher dielectric constant layer, and a second in which trenches are formed into the dielectric substrate which supports the conductive portions of the coplanar waveguide. A local minima in propagation loss is achieved in both embodiments where the thickness h of the first dielectric or trench depth is between approximately 10 to 30% of the spacing between the center conductor and the ground plane.

However, conventional planar microwave circuitry has a serious limitation in that it has high losses compared to, say, waveguide and most coaxial circuitry. This limits the use of conventional microwave integrated circuits to applications where considerable loss in the circuitry can be tolerated. For example, it is not feasible to realize narrow-band microwave filters in conventional integrated circuit form because high-Q (i.e., very low loss with Q's in excess of 400) resonators are required. Also, in many conventional microwave circuits the circuit components are made larger than they would otherwise need to be, in order to reduce the losses. If loss were not a consideration many microwave integrated circuits could be made to be even smaller and lighter.

With the advent of HTS microwave planar circuits it has become feasible to fabricate microwave integrated circuits with very low losses equivalent to and sometimes even lower than are realized by large, heavy waveguide circuits. Very compact "semi-lumped" or "lumped" elements can be used and still obtain phenomenal resonator Q's of 20,000 or more in many situations. This technology opens the way to high performance, compact microwave integrated circuits of types that simply were not possible before.

However, superconductive circuits are not without their problems. For example, use of superconductors as transmitters or receivers requiring any significant power handling capability are problematic. While an ideal superconductor has a perfect crystalline structure which results in total linearity (up to the critical current density J_c) as a function of power, real superconducting crystals have slight irregularities which lead to non- linearity of surface resistance as a function of current, even for currents below the critical current density J_c. Intermodulation effects can arise, resulting in sum and difference frequencies. Thus, for a receiver or transmit¬ ter, interference can result. While this problem may be somewhat handled through the use of filters, the optimal solution in such applications is true linearity and the avoidance of additional components such as filters. Even with the use of filters, the problem is not eliminated. For example, under current technology, if very narrowband filters are used, input power levels on the order of a milliwatt may lead to an increase in transmission loss as well as intermodulation effects.

Despite the high desirability of manufacturing sub¬ stantially linear devices from high temperature super- conductors, problems remain which preclude truly linear devices. Summary of the Invention

This invention relates to methods and apparatus for increasing the power handling capabilities of high temper¬ ature superconductor devices. In a stripline structure, a long, thin center conductor is surrounded by a dielec¬ tric except at the center conductor's thin edges. The thin edges are preferably exposed to a material having a lower dielectric constant than a surrounding dielectric, most preferably air or vacuum. In one embodiment of the stripline structure, the removed dielectric portions provide tunnels or gaps of air adjacent the center conduc¬ tor edges. Yet another stripline embodiment has air gaps extending substantially completely between the ground planes in regions laterally external to the center conduc- tor edges. Optionally, the dielectric between the center conductor and one of the ground planes may be removed. The ground planes may be optionally formed on yet other support substrates.

In an alternative embodiment, a microstrip arrangement consists of a center conductor having a generally long thin shape disposed on a dielectric substrate. Trenches are formed adjacent and laterally exterior to the exterior edges of the center conductor. A ground plane, optionally superconducting, is formed on the side of the dielectric opposite to that of the center conductor.

The trenches or gaps may be formed preferably by milling, such as ion, mechanical or laser milling, or may be etched via isotropic or anisotropic etches. Optional¬ ly, the trench or gap may be undercut beneath the super- conductor, such as through the use of an etch.

Accordingly, it is a principal object of this inven¬ tion to form structures including superconductors which have improved power handling capabilities.

It is yet a further object of this invention to provide superconductive devices having improved linearity of characteristics, especially surface resistance, as a function of current. It is a further object of this invention to minimize the maximum current density being carried through a superconductive device for a given power level.

Brief Description of the Drawings Fig. 1 shows a perspective view of a stripline config¬ uration.

Fig. 2 shows a perspective view of a modified stripline configuration.

Fig. 3 shows a perspective view of a microstrip configuration.

Fig. 4 shows in perspective a detail of dielectric undercut from a center conductor.

Fig. 5 is a cross-section of a stripline transmission structure without use of the instant invention. Fig. 6 shows the current distribution on a center conductor of Fig. 5 as a function of lateral displacement from the center of the conductor.

Fig. 7 shows a cross-section of electric field lines in a stripline configuration. Fig. 8a shows current distribution as a function of lateral position from the center of conductors in the structures of Fig. 2 and Fig. 5.

Fig. 8b shows the current distribution in the vicinity of the right edge of the center conductor in the struc- tures of Figs. 2 and 5.

Detailed Description of the Invention

Fig. 1 shows a perspective view of a stripline config¬ uration of this invention. A center conductor 12 is disposed substantially equidistant from a first ground plane 16 and a second ground plane 18. The center conduc¬ tor 12 generally is wider than it is thick and extends into the plane of the drawing as shown in cross-section. The center conductor 12 terminates in center conductor edges 14. Preferably, the center conductor 12 is formed from high temperature superconductor materials. While any of the superconductive materials may be utilized, the YBCO and thallium superconductors are preferred for their relatively high critical temperature T_c and power handling capabilities. The center conductor 12 is supported by dielectric 22. The dielectric 22 may be of any material compatible with the center conductor 12, such as lanthanum aluminate, sapphire, and magnesium oxide. In this embodiment, the dielectric 22 is disposed between the center conductor 12 and the first ground plane 16, as well as between the center conductor 12 and the second ground plane 18. Gaps 20 are formed in the dielectric 22 adjacent the center conductor edges 14. Preferably, the gap 20 is formed of a material having a dielectric constant which is lower than the dielectric constant of dielectric 22. Most preferably, the dielectric comprising the gap 20 is air or vacuum. The gap 20 runs parallel to the center conductor 12 adjacent the center conductor edges 14. The gap 20 is formed sufficiently large as to improve the power handling capability of the stripline structure 10 but not made so large as to imperil the structural integrity of the overall device. Generally, the larger the gap 20 is relative to the overall stripline structure 10, the better the power handling capabilities. Preferably, the width and depth of the gap should be of the order of size of the width of the center conductor and the substrate thickness, respectively. In one embodiment, the gap depth and width is greater than 20 microns. Ideally, the gap depth would extend to the ground planes as in Fig. 2. The structure of Fig. 1 is preferably constructed by hybridizing two units together. A first unit comprising the upper portion of dielectric 22 has two substantially parallel faces, the first face bearing ground plane 16 and the second face bearing all or part of the gap 20. The lower module consists of substantially planar, parallel faces whereon the first face bears the center conductor 12 and all or part of the gap 20 and the second ground plane on the opposite side of the dielectric. The two units are then hybridized forming an interface 24 of the two mod¬ ules. The modules preferably are held together via pressure clamps. The gap 20 may be formed through any material removal process compatible with the other materials in the stripline structure 10. For example, the gap 20 may be milled, such as by mechanical milling for relatively large structures (e.g., radio frequency devices) or by ion milling or laser milling for smaller structures (e.g., microwave and millimeter wave devices) . Alternatively, the gap 20 may be etched into the dielectric 22. Etching provides the opportunity to undercut the dielectric 22 from the center conductor 12. As shown in Fig. 4, the gap 20 (shown in partial) has an undercut 26. The undercut 26 extends a distance u from the vertical line extending downward from the center conductor edge 14.

Fig. 2 shows a modified stripline structure. A center conductor 30 is generally planar, having a thickness which is much less than its width and length. The center conductor 30 has center conductor edges 32 at the lateral edges of the center conductor 30. First ground plane 34 and second ground plane 36 are disposed generally parallel to the center conductor 30 and arranged with the center conductor 30 being parallel to and equidistant from each of the ground planes 34 and 36. A first dielectric 38 is disposed between the center conductor 30 and the first ground plane 34. Preferably, a second dielectric, opti¬ mally having the same dielectric constant is disposed between the center conductor 30 and the second ground plane 36. In this embodiment, dielectric, other than air, is removed from the regions laterally exterior to the center conductor edges 32, as indicated by arrows 42. Optionally, the first and second ground planes 34 and 36 may be supported by the substrates disposed on the side away from the center conductor 30. Fig. 3 shows a perspective view of a microstrip configuration. The microstrip structure 50 includes a substrate 52 having generally planar, parallel disposed faces, a first face 54 and a second face 56. A center conductor 58 is formed on the first face 54 of substrate 52. Generally the center conductor 58 has a thickness t which is substantially less than its width s. Trenches 62 are formed laterally adjacent to the center conductor edges 64. The trench 62 has a depth h and a width w. Ground plane 60 is disposed on the second face 56 of the substrate 52.

Fig. 4 shows a gap 20 having an undercut portion 26 under the center conductor 1 . The undercut amount of distance u may be as desired, consistent with maintenance of structural integrity.

In the preferred mode of practicing the inventions of this patent, it is preferable that the conductive materi¬ als, namely the center conductor and the ground planes, be formed from superconducting materials. However, replace- ment of one or more of these structures with normal metals, such as gold, high purity copper, or other materi¬ als compatible with transmission of high frequency elec¬ tromagnetic radiation is consistent with this invention. HTS microwave circuits do have limitations in the amount of power they can carry. If the power level in a circuit gets too high the current density in some regions of the circuit will exceed a "critical" level J_c, which depends on the temperature, frequency and microstructure, and the HTS in those regions will no longer operate as a superconductor. This results in markedly increased circuit loss and in nonlinear effects such an intermodulation between signals at different frequencies. While a function of many variables such as temperature, frequency and local structure, this critical current density J_c is typically of the order of 3 x 10⁶ amps/cm² in high quality thin films at 77K. In some planar microwave circuit applications where the transmission lines are well matched (i.e., they do not have reflections at their ends) the HTS transmission lines may be able to carry as much as hundreds of watts (or possibly more depending on the line cross-sectional dimensions) without appreciable effects due to the critical current density being exceeded. However, in cases such as narrow-band filters, the cur¬ rents within the resonators of the filter may be as much as 100 times or more as large as the currents in the lines which connect to the ports of the filter. This is a result of the resonance conditions that exist in the resonators plus the loose couplings between the resona¬ tors. Since power varies as the square of the current, this means that the power rating of the transmission lines used in the resonators may need to be 10,000 times or more greater than is required for the transmission lines leading into or out of the filter. Other parts of an integrated circuit which may have relatively high currents due to high standing wave ratios may also need to have high power ratings. For these reasons apparatus and techniques for increasing the power handling ability of HTS transmission lines is quite important for some micro¬ wave integrated circuit applications which must handle relatively high power.

By way of example, Fig. 5 shows an HTS strip transmis- sion line which consists of a center conductor 60 sur¬ rounded by dielectric 62 with a relative dielectric constant e_r with ground planes 64 at the top and bottom. The center conductor extends into and out of the paper. The current density on the center conductor is distributed as shown in Fig. 6, where there are high peaks of current density at the outer edges of the center conductor. High peaks of current density exist near the center-conductor edges, and it is in these edge regions where current saturation will first occur as the power level is being raised on a transmission line.

Some idea of why the high current density occurs at the strip edges in Fig. 5 can be seen from Fig. 7 which shows the same structure as in Fig. 5 but with electric flux lines D sketched in. The flux lines shown begin on positive surface charge on the center conductor 60 and end on negative surface charge on the ground planes, and the surface charge density at any point on the surface is equal to the electric flux density D associated with that point. As these flux lines and surface-charge distribu¬ tions propagate into (or out of) the paper, the propagat¬ ing charge distributions comprise surface currents on the conductors flowing into or out of the paper. Flux lines from a sharp edge with positive charge will emanate out radially, as shown in Fig. 7, and this requires a high concentration of charge and current along the sharp edges in order to provide the needed electric flux D. Consis- tent with this invention, in order to reduce the high current density along the edges of the center conductor this invention reduces the fringing flux along the edges of the strip.

It is possible to obtain a more complex estimate of the benefits of the structure in Fig. 2 with regard to reducing the current peaks at the edges of the center conductor. If the center conductor in Fig. 5 is infinite¬ ly thin, and if the ground plane spacing h approaches infinity, the current distribution on the center conductor will be exactly of the form

Ax J₁ {x) = (1)

(l-(2x/w)²)⁰"⁵ where A, is a constant which depends on the applied voltage and the structure of the line, x is the distance from the center of the conductor.

It is found that this current-density distribution func¬ tion is still a useful approximation in many cases even when the ground-plane spacing h is finite as in Figs. 5 and 7. (See, e.g., G.L. Matthaei, et. al.. , "A Simplified Means for Computation of Interconnect Distribution Capaci¬ tances and Inductances", IEEE Trans, or Computer-Aided Design, Vol. 11, pp 513-524, April, 1992.) It is easily shown that the function J_x (x) goes to infinity as

(w/2-x)^"°-^s as x approaches w/2. R. Mittra et. al. in

"Analytical Techniques in the Theory of Guided Waves, " at pp 10-11, 1971, analyzes the singularities at the edges of thin conductors at dielectric interfaces and indicates that at the edges of the center conductor in Fig. 2 the current distribution should go to infinity as (w/2-x) ^{~0 128} as x approaches w/2 if e=24 as for LaAl0₃. If we modify (1) to exhibit this behavior at the edges of the strip we get

A₂ J₂(x)= (2)

(1- (2x/w²)-⁰-¹²⁸ which is a good approximation for the current distribution on the center conductor for the case in Fig. 2 where A₂ is a constant which depends on applied voltage and the structure of the line. The dashed line in Figs. 8(a) and 8(b) show a plot of Eq. (1) while the solid lines show plots of Eq. (2) , where the coefficients A-_! and A₂ have been adjusted so that the area under both curves is the same to equalize total current in the two transmission lines. Thus the dashed lines would apply to cases like that in Fig. 5 (the prior art) , while the solid lines apply to the case in Fig. 2 with e=24 (the invention) . As can be seen, particularly in Fig. 8B, the presence of the dielectric discontinuity greatly reduces the intensity of the current singularity at the edges of the center conductor. Thus the structure in Fig. 2 carries considerably more power than that in Fig. 5 without having part of the current density exceed the critical value J_c and cause intermodulation and excess loss. Of course, due to the finite thickness of the HTS and the limited current penetration depth associated with the HTS, the peaks of the current distribution would be rounded off some even before current saturation is included. In practice, the divergence towards extremely high values in the current density is somewhat reduced or cut-off by an effective penetration depth which depends on the wavelength and film thickness t.

The apparatus and methods of this invention are useful in connection with electronic circuits carrying power through waveguide type structures having relatively thin, wide conductors in which fringing affects would otherwise occur. Of particular utility is the application of these techniques to superconductive circuits, especially high temperature superconducting circuits. Because these techniques serve to improve the linearity of device operation, particularly the linearity of the surface resistance as a function of current, the structures are particularly useful in connection with any signal process- ing electronics, such as receivers, transmitters and filters. By reducing the non-linearities, interference is reduced substantially.

Although the foregoing invention has been described in some detail by way of illustration and example for purpos- es of clarity and understanding, it will 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.

Claims

1. A stripline transmission line comprising: a center conductor having a center conductor edge, first and second ground planes arranged substantially parallel to each other and to the center conductor, the center conductor being disposed between the first and second ground planes, a solid dielectric disposed between the first and second ground planes and contacting the center conductor and at least one of the first or second ground planes, the solid dielectric having gap portions comprising a dielec¬ tric having a dielectric constant lower than that of the solid dielectric, the gap contacting the center conductor edge.

2. The stripline transmission system of Claim 1 wherein the center conductor is a superconductor.

3. The stripline transmission system of Claim 2 wherein the center conductor is a high temperature super¬ conductor.

4. The stripline transmission system of Claim 3 wherein the high temperature superconductor is a thallium based superconductor.

5. The stripline transmission system of Claim 3 wherein the high temperature superconductor is a YBCO superconductor.

6. The stripline transmission system of Claim 1 wherein the center conductor is substantially planar.

7. The stripline transmission system of Claim 1 wherein at least one ground plane is formed from superconductive material.

8. The stripline transmission system of Claim 1 wherein the dielectric within the gap is chosen from the group comprising air and vacuum.

9. The stripline transmission system of Claim 1 wherein the gap has a depth greater than 20 microns.

10. The stripline transmission system of Claim 1 in which the gap has a width in excess of 20 microns.

11. The stripline transmission system of Claim 1 wherein the depth of the gap is equal to or greater than 0.5mm.

12. The stripline transmission system of Claim 1 wherein at least one gap undercuts the center conductor.

13. A microstrip transmission system comprising: a substrate having a first face and a second face having substantially planar, parallel surfaces, a center conductor having two edges disposed on a portion of the first face, a ground plane disposed on at least a portion of the second face, trenches formed in the substrate from the first face located laterally adjacent to the edge of the center conductor.

14. The microstrip transmission system of Claim 13 wherein the center conductor is a superconductor.

15. The microstrip transmission system of Claim 14 wherein the center conductor is a high temperature super¬ conductor.

16. The microstrip transmission system of Claim 15 wherein the high temperature superconductor is a thallium based superconductor.

17. The microstrip transmission system of Claim 15 wherein the high temperature superconductor is a YBCO superconductor.

18. The microstrip transmission system of Claim 13 wherein the center conductor is substantially planar.

19. The microstrip transmission system of Claim 13 wherein the ground plane is formed from superconductive material .

20. The microstrip transmission system of Claim 13 wherein the trench has a depth to width ratio greater than 30%.

21. The microstrip transmission system of Claim 13 in which the trench has a depth to width ratio in excess of 50%.

22. The microstrip transmission system of Claim 13 wherein the depth of the trench is equal to or greater than 0.5mm.

23. The microstrip transmission system of Claim 13 wherein the width of the trench is equal to or greater than 0.5mm.

24. The microstrip transmission of Claim 13 wherein at least one trench undercuts the center conductor edge.