BACKGROUND OF THE INVENTION

1 . Field of the Invention

Ferroelectric films such as SrTi0 ₃ , Pb(Sr _r Ti)0 ₃ , Sr-Ba-.-TiOg, etc. provide a means for producing tuneable capacitors since the

dielectric constant of the ferroelectric films is tuneable by variation of voltage applied to the ferroelectric films. Because of their intrinsically low losses at high frequencies, these materials can be implemented in a variety of microwave components, including delay lines and phase shifters.

One of the principle applications of delay lines and phase shifters is for phased array antennas. The voltage-controlled ferroelectric film modulates the dielectric constant and hence the time delay of either microstrip or coplanar delay lines. Such phase shifters or tuneable delay lines can be used to phase delay microwave and millimeter wave signals either transmitted or received from the

individual radiative elements of the antenna array.

Such tuneable ferroelectric films can be used in a large family of tuneable microwave components and devices such as phase shifters, matching networks, oscillators, filters, resonators, loop antennas, superconducting film elements, etc.

The invention in general relates to devices which are tuned utilizing the fact that the dielectric constant of a material varies with voltage or some other parameter, and more particularly to such devices that utilize barium strontium titanate as the tuneable dielectric constant material.

2. Description of the Prior Art

Ferroelectric phase shifters are disclosed in "Ceramic Phase Shifters for Electronically Steerable Antenna Systems", Varadan, V.K., et al.. Microwave Journal, January 1992, pp. 116-126.

The use of superconductors for microwave frequency transmission is disclosed in "High-Temperature Superconductivity for EW" Ryan, P.A., Journal of Electronic Defense, August 1992, pp. 48-54.

The dielectric properties of thin films of SrTi0 ₃ used as dielectric layers in superconducting field effect (FET) like devices is disclosed in "Dielectric properties of SrTi0 ₃ thin films used in high T _e superconducting field-effect devices", Walkenhourst, A., et al. Appl. Phys. Lett. 60(14), 6 April 1992, pp. 1744-1746.

A superconductor phase shifter using SQUIDS is disclosed in "Monolithic HTS Microwave Phase Shifter and Other Devices", Jackson, CM., et al. Journal of Superconductivity, Vol. 5 No. 4, 1992, pp. 419-424.

The use of superconductive feed networks and antenna elements of antenna arrays is disclosed in "A Survey of Possible Passive Antenna Applications of High-Temperature Superconductors", Dinger, R.J., et al. IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 9, September, 1991 .

A thin superconducting film antenna is disclosed in "Radiation Efficiency Measurements of a Thin-Film Y-Ba-Cu-0 Superconducting Half-Loop ^

Antenna at 500 MHZ", Dinger, R.J., et al. Paper Presented at MTT-S, Boston,

June, 1991 , pp. 1-4.

The dielectric properties of BST material thin films are disclosed in "Microstructure-lnduced Schottky Barrier Effects", in Barium Strontium Titanate (BST) Thin Films for 16 and 64 MBIT Dram Cells, Scott, J.F., et al., circa 1992.

It is known that dielectric films in which the dielectric constant of the film is tuneable by varying the voltage applied to the film provide a means for producing tuneable capacitors which can be used in a variety of microwave components, including delay lines, phase shifters, and phased array antennas. In delay lines a voltage applied to the dielectric material modulates the dielectric constant and hence the time of transit, or delay, of an electrical signal passing through the material. Delay lines are commonly fabricated as microstrip or coplanar devices. If the time delay is used to shift the phase of a signal, then the device is called a phase shifter. Such phase shifters or tuneable delay lines can be used to phase delay microwave and millimeter wave signals either transmitted or received from individual radiative elements of an antenna array to create a phased array antenna. Because of their intrinsically low losses at high frequencies, ferroelectric materials, which generally are also materials in which the dielectric constant is tuneable, are ^' particularly useful in such devices. Such tuneable ferroelectric films can be used in a large family of tuneable microwave components and devices including the delay lines, phase shifters, and phased antenna arrays mentioned above as well as matching networks, oscillators, filters, resonators, loop antennas, and many other devices.

Ferroelectric phase shifters, and in particular barium strontium titanate (BST) phase shifters which utilize the change in dielectric constant with a biasing field to generate the phase shift, are disclosed in "Ceramic Phase Shifters for Electronically Steerable Antenna Systems", Varadan, V.K., et al., Microwave Journal, January 1992, pp. Hβ-126. However, the dielectric constant changes and phase shifts are

so small that only bulk devices are feasible.

A high temperature superconductor/ferroelectric phase shifter utilizing the change of the dielectric constant with electric field to vary the phase shift is disclosed in "A High Temperature Superconductor Phase Shifter", Jackson, C ., et al. Microwave Journal, Vol. 5 No. 4, December 1992, pp. 72-78. In particular is mentioned as a responsive material for such phase shifters. Such use of SrBaTiθ ₃ in a voltage controlled dielectric constant tuneable phase shift device is also mentioned in "Novel Monolithic Phase Shifter Combining Ferroelectrics and High Temperature Superconductors", Jackson, Charles M., et al., Microwave and Optical Technology Letters. However in both articles, only modest phase shifts of about 10% are disclosed for bulk devices. If these devices were scaled down to microwave integrated circuit (MIC) size, which devices must be made with thin films, the surface effects of the thin films would predominate and the tuneability would become too small to be practically useful. Since MIC devices over many advantages including reliability and low cost, it would be highly desirable to have a dielectric material that provides useful tuneability in such devices.

The dielectric properties of barium strontium titanate (BST) material thin films are disclosed in "Microstructure-lnduced Schottky Barrier Effects", in Barium Strontium Titanate (BST) Thin Rims for 16 and 64 MBit Dram Cells, Scott, J.F., et al., circa 1992.

The prior art tuneable microwave and millimeter wave devices have all involved compromises between the degree of change of the dielectric constant, ■ lossiness, and other parameters. The dielectric material should have a high degree of change in the dielectric constant to permit maximum tuneability of the device. The lossiness should be low since otherwise the power use of the devices is excessive. Thus it would be a significant advance to have tuneable microwave and millimeter wave devices that have both a large degree of tuneability and low lossiness.

SUMMARY OF THE INVENTION The present invention relates to the use of ferroelectric insulating thin films as dielectric layers in superconducting thin film structures for fabrication of microwave and millimeter wave devices which are frequency-tuneable. More particularly, metal oxide ferroelectric and superconducting thin films are used in

combination to produce an entire new class of microwave and millimeter wave devices which can be frequency-tuned by the application of voltage bias signals across various portions of the ferroelectric thin films. In such structures, because

of the low loss performance of superconductors at high frequency, the microwave

and millimeter wave performance of the devices are limited mostly by the loss tangent of the ferroelectric dielectric layers. The fundamental materials compatability of a whole class of metal oxide superconductors with a whole class of metal oxide ferroelectric materials provides a thin film material system with unique flexibility for designing and fabricating a multiplicity of multilayer geometries, including ferroelectric on superconductor structures and superconductor on ferroelectric structures.

The invention is especially useful for producing microwave and millimeter wave devices which have the extreme low loss performance of other superconducting devices, but also have the necessary frequency tuneability required for most practical applications. By this invention, it is possible to realize numerous microwave and millimeter wave devices, including, but not limited to, delay lines, phase shifters, resonators, oscillators, filters, electrically-small antennas, half-loop antennas, directional couplers, patch antennas, and various radiative gratings, which are frequency-tuneable by utilizing voltage-tuneable capacitor structures fabricated from voltage-biased ferroelectric thin films in combination with superconducting thin films.

The invention relates to the utilization of both thin films of high temperature superconductor (HTSC) material and thin films of ferroelectric

materials for achieving tuneability in a variety of microwave structures.

Ferroelectric films such a SrTiO ₃ , Pb(Sr,Ti)O ₃ , Sr _x Ba ₁ . _x TiO ₃ , etc. provide a means for producing tuneable capacitor structures at microwave frequencies since the dielectric constant of the ferroelectric films is tuneable by variation of voltage applied to the ferroelectric films. In accordance

with the invention, a thin film of ferroelectric material is provided with a thin film (or films) of a high temperature superconductor (HTSC) such as Y-Ba-Cu-O (YBCO) or Tl-Ba-Ca-Cu-O (TBCCO) in order to produce low loss microwave and millimeter wave devices which are frequency tuneable by application of voltage to the thin

films of ferroelectric material. The thin film structures of HTSC material and ferroelectric material which are deposited on a substrate result in reduced losses, limited only by the loss tangent of the ferroelectric material and the impedance matching structures. The combination of HTSC thin films and ferroelectric thin

films can be used in a large family of tuneable microwave components and devices such as phase shifter devices which offer a method for obtaining true-time delay, superconducting phase shifters, tuneable delay lines, resonators, filters, matching networks, oscillators and tuneable antennas.

The combination of a HTSC thin film and ferroelectric thin films offers a novel class of microwave devices with unique properties including a substantial frequency tunability, simple construction, the potential for optimization of temperature operation ranges, the ability to use variably thin dielectrics, low cost, high level of integration with other components such as antennas and the increased power handling of the circuit. The extent of the tunability of the structures depends upon the ferroelectric material, its form and geometry, (e.g., film thickness), the temperature, and the applied fields or voltage.

Compatibility of YBCO, TBCCO and other superconductors and a broad class of ferroelectric materials enables the optimization of ferroelectric materials and superconducting materials and the growth of alternating layers of each by a variety of deposition methods including evaporation, sol gel, sputtering physical vapor deposition, chemical vapor deposition, laser deposition and others. The combination of HTSC thin films and ferroelectric films provides a novel class of tuneable microwave and millimeter wave devices.

One of the principle applications of the invention comprises phase

shifters and tuneable delay lines for phased array antennas. The voltage-controlled ferroelectric thin film modulates the dielectric constant and hence the time delay, of either microstrip line or coplanar delay lines. Such phase shifters or tuneable delay lines can be used to phase delay microwave and millimeter wave signals

either transmitted or received from the individual radiative elements of the antenna array. Other applications of the class of tuneable microwave and millimeter wave devices include resonators, filters, matching networks, oscillators, directional couplers and tuneable antennas.

The invention also includes the utilization of a dielectric grating for an individual radiative element in a phased antenna array. Such a grating (with fixed delay between each adjacent element) radiates (or receives) energy at a fixed angle. The angle can be adjusted by biasing the dielectric grating with a single bias voltage. Such a voltage-controlled grating enables steering (in one direction) with the application of a single DC-bias line.

The invention of the dielectric grating can be extended to two- dimensional arrays, with steering in both the x- and y- directions. A simple two- dimensional array of such gratings with a fixed spacing between each grating

"patch" antenna provides a similar fixed radiation angle in the second direction. A second, single voltage bias line modulates the dielectric constant and hence the time delay between each grating structure in the array.

The invention also includes several capacitively coupled devices, including a tuneable superconducting resonator with substantial tunability, a

superconducting fringe-effect capacitor with substantial tunability, and application of the tuneable devices to a broad range of resonant devices, including tuneable loop antennas.

The present invention solves the above problem by providing tuneable microwave and millimeter wave devices that utilize barium strontium titanate (BST)

• as the tuneable dielectric constant material. Preferably the (BST) has the formula

Ba _x Sr ₁ . _x TiO ₃ . Preferably, 0.01 < x < 0.2. As is known in the art, the BST formula can also be written Sr _y Ba^TiO _g .

It has been discovered that BST, especially BST in which the amount of barium is small as compared to the amount of strontium, has extremely high tuneability in microwave and millimeter wave devices. It has been found that the dielectric constant of BST with small amounts of barium as compared to strontium varies typically about 300% and, in some cases, as much as 600% with changes in voltage from zero volts to 25 volts (25 kiiovolts/cm), which voltage range is well within the operating voltage range of typical tuneable microwave and millimeter wave devices. Such high variability of the dielectric constant permits tuneable devices to be fabricated in which the dielectric portions of the circuits are extremely thin, thereby minimizing loss. Since loss in the dielectric portion is minimized, the conductor portion may utilize conventional conductors and still have loss factors that are of the order of the loss factors of devices fabricated with superconductors and conventional dielectric materials.

Preferably the BST microwave and millimeter wave devices are designed to operate at temperatures of between 50 K and 110 K. The BST has been found to have maximum tuneability in this temperature range.

It further has been found that by varying the relative amounts of barium and strontium within the range 0.01 < x < 0.2, the tuneability of the devices can be varied over a wide range. Generally, as the amount of strontium is increased, the peak tuneability shifts to higher temperatures. Thus by varying the relative amounts of barium and strontium in the dielectric portion of the devices, devices with different responses over the same voltage range may be constructed.

In another aspect, the invention provides a tuneable electrical device comprising: a dielectric material; means for maintaining the dielectric material at a temperature between 67 K and 110 K; signal input means for inputting an electrical signal to the dielectric material; and electrical bias means for applying an electrical field to the dielectric material to determine the dielectric constant of the material and tune the electrical signal.

In a further aspect, the invention provides a tuneable electrical device comprising: a thin film of a dielectric material having a dielectric constant that varies with the electric field applied to the material; signal input means for inputting an electrical signal to the dielectric material; and electrical bias means for applying an electrical field to the dielectric material to determine the dielectric constant of the material and tune the electrical signal.

The invention thus, for the first time, provides a tuneable electrical device in which thin film devices have sufficient tuneability for practical applications. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a specific embodiment of the invention, a tuneable delay line, showing a microstrip line of HTSC material deposited on a thin film (substrate) of ferroelectric material, ail deposited on a crystalline substrate;

FIG. 2 is a schematic representation of the tuneable superconductor/ferroelectric thin film device of the invention;

FIG. 3 is a plot of the Magnetic Flux Screening - Temperature characteristic for a multilayer superconductor/ferroelectric thin film structure of the invention after deposition as measured by a mutual inductance technique;

FIG. 4 is a plot of Insertion Loss vs. Frequency at 77 K for the tuneable resonator of the invention;

FIG. 5 is a fragmentary vertical section view of a thin film of HTSC material contiguous with the opposite surface of a film of ferroelectric material;

FIG. 6 is a schematic representation of the phase shifter of the invention in a phased array antenna system;

FIG. 7 is a fragmentary vertical section view of a dielectric grating in accordance with the invention for an individual radiative element in an antenna array;

FIG. 8 is a schematic representation of a two-dimensional array of dielectric gratings in accordance with the invention for an antenna array;

FIG. 9 is a perspective view of a coplanar structure of the invention formed on a LaAI0 ₃ substrate;

FIG. 10 is a perspective view of a resonant cavity formed by coplanar cavities in accordance with the invention;

FIG. 11 is a schematic representation of a capacitively coupled microstrip line resonator with a tuneable ferroelectric capacitor and a capacitively

coupled microstrip line resonator without tunability;

FIG. 12 is a plan view of a superconducting fringe effect capacitor in accordance with the invention; and

FIG. 13 is an elevation view of the capacitor of FIG. 12.

FIG. 14 is a cross sectional view of a specific multilayer structure of the invention, with a HTSC layer on top of a ferroelectric layer.

FIG. 15 is a cross sectional view of a more general multilayer structure

of the invention, with a HTSC layer on top of a ferroelectric layer on top of a substrate.

FIG. 16 is a cross sectional view of a specific structure of the invention, in which a HTSC layer is deposited directly on a ferroelectric substrate.

FIG. 17 is a cross sectional view of a specific multilayer structure of the invention, with a ferroelectric layer deposited on top of a patterned HTSC layer (deposited on a substrate) with the ferroelectric layer also filling in the etched regions of the HTSC layer.

FIG. 18 is a cross section view of a specific structure of the invention in which a ferroelectric layer and HTSC layer lie in the same plane, with the HTSC- ferroelectric interface of the invention being at the edges of the films.

FIG. 19 is a cross sectional view of a general multilayer structure of the invention with various combinations of HTSC and ferroelectric layers on a bulk crystalline dielectric, insulating, or ferroelectric substrate.

FIG. 20 is a fragmentary vertical section view of coplanar thin films of HTSC material contiguous with one surface of a thin film of ferroelectric material and a thin film of HTSC material contiguous with the other, surface of the thin film

of ferroelectric material.

FIG. 21 is a plan view of an interdigitated capacitor of the invention;

FIG. 22 is a fragmentary vertical section view of the interdigitated capacitor of FIG. 21 ;

FIG. 23 is a plan view of a half-loop antenna of the invention;

FIG. 24 is a schematic representation of the resonant circuit of the antenna of FIG. 21 .

FIG. 25 is a cross sectional view of a BST delay/shift element according to the invention showing the application of a bias filed to the element;

FIG. 26 is a block diagram of a microwave/millimeter wave delay/shift device utilitzing the delay/shift element of FIG 25;

FIG. 27 is a cross-sectional illustration of a microwave/millimeter wave delay/shift device utilizing a delay/shift element according to the invention;

FIG. 28 is a perspective view of a portion of a microstrip transmission line embodiment of a delay/shift element according to the invention;

FIG. 29 is a schematic diagram of an artificial trasmission line utilizing a lumped capacitance delay/shift element according to the invention;

FIG. 30 is an illustration of a capacitor device used in obtaining bulk capacitance change data for BST samples;

FIG. 31 is an illustration of a capacitor device used in obtaining thin film capacitance change data for BST samples;

FIG. 32 is an illustration of a coplanar waveguide phase shifter element according to the invention;

FIG. 33 is a cross-sectional view of the phase shifter of FIG. 32 taken through the line 9-9 in FIG. 8;

FIG. 34 shows capacitance versus temperature curves for five electric field strenghts for a BaO.09 Sr0.91 Ti03 capacitor as shown in FIG. 30;.

FIG. 35 shows capacitance versus temperature curves for five electric field strengths for a Ba0.09 Sr0.91 Ti03 capacitor as shown in Fig. 30;

FIG. 36 shows capacitance versus temperature curves for six electric filed strengths for a Ba0.08 Sr0.92 Ti03 thin film capacitor as shown in FIG. 31 ;

FIG.37 shows capacitance versus temperature curves for four electric field strengths for a Ba0.08 Sr0.92 Ti03 thin capacitor as shown in FIG. 31 , which

capacitor is smaller than the capacitor used in obtaining the curves of FIG. 36, and also shows a single point on the capacitance/temperature curve for three other electric field strengths;

FIG. 38 shows loss tangent versus temperature curves for the same capacitor and electric field strengths used in obtaining the curves of FIG. 36;and

FIG. 39 shows the measured change of phase angle and effective dielectric constant versus electric field strength for the phase shifter of FIGS. 32 and 33. DESCRIPTION OF PREFERRED EMBODIMENTS

Discoveries of high temperature superconducting (HTSC) materials such as the 30-K La-Ba-Cu-O superconductor and the 90-K Y-Ba-Cu-O superconductor have stimulated a worldwide race for higher temperature superconductors. Breakthroughs have been made by the discoveries of the 90-K TI-Ba-Cu-0 system, the 1 10-K Bi-Ca-Sr-Cu-0 system, and the 120-K TI-Ba-Ca-Cu-0 (TBCCO) system. The Y-Ba-Cu-0 system is the first rare earth system which reaches zero resistance above the liquid nitrogen boiling point while the Tl-Ba-Ca-

Cu-0 system reaches zero resistance above 100K and has the highest zero- resistance temperature (127K).

A number of superconducting phases, including TI ₂ Ba ₂ Ca ₂ Cu ₃ O _{10+ x} (2223) and TI ₂ Ba ₂ Ca ₁ Cu ₂ 0 _8+x (2212) have been isolated from the TI-Ba-Ca-Cu-0 system. The 2223 superconductor has a 3.85 x 3.85 x 36.25 A tetragonal unit cell. The 2122 superconductor has a 3.85 x 3.85 x 29.55 A tetragonal unit cell. The 2223 phase is related to 2122 by addition of extra calcium and copper layers. In addition, the superconducting phase in the Ca-free TI-Ba-Cu-0 system is TI ₂ Ba ₂ CuO _β+x (2201 ).

The new class of tuneable microwave devices of the invention combines the low loss of a high temperature superconductive (HTSC) material (such as TI-Ba-Ca-Cu-0 and Y-Ba-Cu-O) in a thin film with the variable dielectric properties of a thin film of ferroelectric material.

Ferroelectric thin films such a SrTi0 ₃ , Pb(Sr,Ti)0 ₃ , Sr.Ba^- Ti0 ₃ , etc. provide a means for producing low-loss tuneable capacitor structures at microwave and millimeter wave frequencies since the dielectric constant of the ferroelectric thin films is tuneable by variation of voltage applied to the films. A thin film of ferroelectric material is provided with a thin film of a high temperature superconductor (HTSC) material such as Y-Ba-Cu-O or Tl-Ba-Ca- Cu-O. The HTSC thin film results in a multilayer structure with reduced losses, limited only by the loss tangent of the ferroelectric material and the impedance matching structures. The combination of HTSC thin films and ferroelectric films can be used in a large family of tuneable microwave components and devices such as phase shifter devices which offer a method for obtaining true-time delays in

tuneable delay lines, tuneable filters, matching networks, oscillators, loop antennas, directional couplers, resonators, etc.

The thin film of ferroelectric material, e.g., SrTi0 ₃ and various barium doped strontium titanate materials provide an excellent substrate for the deposition of an HTSC material such as YBa ₂ Cu ₃ 0 ₇ . ₈ (YBCO) or TI-Ba-Ca-Cu-0 (TBCCO). SrTi0 ₃ , and its relatives, have a perovskite crystal structure with an excellent lattice match and thermal expansion coefficient close to that of YBCO, Tl-Ba-Ca- Cu-O, and other thallium copper oxide superconductors. In general other

substrates can also be used, such as lanthanum aiuminate, etc.

The combination of HTSC thin films and ferroelectric thin films offers a novel class of tuneable microwave and millimeter wave devices with unique properties including a substantial frequency tuneability, simple construction, the potential for optimization of temperature operation ranges, the ability to use variably thin dielectrics, low cost, high level of integration with other components such as antennas, and the power handling of the circuit. The extent of the tuneability of the structures depends on the materials, their forms and geometries (e.g. film thickness), the temperature, and the applied voltage.

High quality SrTi0 ₃ , BaSrTi0 ₃ , LaA10 ₃ ' and other dielectric thin films can be deposited on HTSC thin films (e.g., YBCO or Tl-Ba-Ca-Cu-O) films by sol- gel, plasma-spray , sputtering, physical vapor deposition, chemical vapor deposition , laser deposition, and other techniques. Superconducting properties of the HTSC layer depend on the compatability with the ferroelectric thin film and vice-versa, as well as on processing conditions. In practical circuits, a fixed temperature of operation is required to provide good impedance matching. Broadband, compact,

low loss, thin-film, superconducting matching circuits are available. A thin film multilayer implementation of the class of tuneable superconducting/ferroelectric structures of the invention has the added benefit that monolithic devices and integrated circuits can be designed.

As shown in FIG. 1 , a tuneable delay line 20 is formed by a patterned microstrip line 21 of a thin film of HTSC material, for example YBCO material, deposited on a thin film 22 of ferroelectric material which is deposited on ground plane 23. In turn ground plane 23 is deposited on a crystalline substrate such as SrTi0 ₃ . The ferroelectric thin film 22 can be, for example, from 100 A to 10000 A thickness. By applying a voltage between the superconductor or microstrip line 21 and a ground plane 23, a DC bias is provided to the ferroelectric layer, thereby changing the dielectric constant and the wave propagation velocity within the layer. Note that both the microstrip line and the ground plane can be made from HTSC material.

The microstrip line or pattern 21 of a YBCO thin film is deposited on the SrTi0 ₃ thin film substrate 23 by using sol-gel, chemical vapor deposition, physical vapor deposition, sputtering, laser deposition, or other techniques. Sol-gel deposition and chemical vapor deposition of superconducting thin films and ferroelectric thin films is disclosed in U.S. Patent Nos. 5,1 19,760 and 5,138,520, assigned to Symetrix Corporation of Colorado Springs, Colorado.

FIG. 2 shows a schematic representation of the tuneable delay line 20 including the microstrip line or thin film of the (HTSC) YBCO material 21. A normal metal (e.g. silver) or HTSC material ground plane 23 is deposited on the rear surface 22a of substrate or film 22. Variable voltage source 25 enables a variable

voltage to be applied as an electric field to the ferroelectric layer 22 by means of microstrip line 21 and ground plane 23.

SrTiO ₃ belongs to the BaTi0 ₃ group in the classification of ferroelectric materials. BaTi0 ₃ is a well known ferroelectric material and its phase transition from cubic to tetragonal, where a small displacement of Ti in the tetragonal phase under the external electric field, is responsible for ferroelectricity. SrTi0 ₃ has three crystalline phase transitions at lower temperatures (see Table 1 ).

Table 1 LOW TEMPERATURE PHASE TRANSITION OF SrTiO.

TEMPERATURE(K) STABLE PHASE PHYSICAL CONSTANT

> 1 10 cubic a = 3.902A

65-1 10 tetragonal c/a = 1.00056

35-65 orthorhombic a:b:c =

0.9998 : 1 : 1.0002

< 10 Possibly rhombohedral

Other ferroelectric materials which can be used for thin film 22 include BaTi0 ₃ , LiNb0 ₃ , Pb(Sr,Ti)0 ₃ , Sr-Ba^TiO.-.. etc. Other materials from the lll-V, and ll-VI groups could be used with other buffer layers to solve any lattice matching problems which arise should epitaxial deposition be required.

The extent of tuneability of the tuneable delay line 20 depends upon the ferroelectric material, the temperature, and the applied field or voltage.

An advantage of the superconductor/ferroelectric tuneable delay line 20 of the invention is its power handling capability. Distortion can arise in a non-

linear transmission line when the RF voltages become comparable to the DC control voltages. The power handling of a microstrip line can be estimated from the minimum bias voltage to be used. Power is given by P = V ² /Z. Accordingly, for a 5 volt bias and a 1 ohm impedance, 25 watts (i.e. >40 dBm) could be handled before the non-linearities become apparent. The level of high power will set the microstrip line width requirement to insure that the critical current carrying capacity of the line is not exceeded.

FIG. 3 shows changes in resistance with changes of temperature of the multilayer superconductor/ferroelectric thin film structure of the invention after deposition.

FIG. 4 shows the change in insertion loss plotted against frequency at 77K for the tuneable resonator of the invention.

FIG. 5 shows a microstrip line 31 of HTSC material in section. As shown in FIG. 5, the thin film of the HTSC material 32 of microstrip line 31 is deposited upon thin film 34 of the ferroelectric material. Thin film 34 of ferroelectric material is contiguous with thin film 33 of HTSC material. Crystalline substrate 35 supports thin films 33, 34 and 32.

Where size, weight, and drive mechanisms are limited, the traditional moving reflector antenna is being replaced in some applications by phased array antennas. Generally planar in shape, conventional phased array antennas are formed by a substantial number of closely spaced, individual radiators, whose composite beam can be shaped and spacially directed in microseconds, thereby enabling the antenna to track a multitude of targets at one time. This is accomplished electronically by RF phase shifters associated with each individual

radiating element. No moving parts are required.

State of the art passive phased array antennas are limited in their application by cost, more than any other factor. The required phase shifters are not cheap and with a typical array requiring thousands of individual antenna elements, each with its own phase shifter, the price of the total system becomes prohibitive.

The principal difficulty of such phased arrays is that for large arrays, i.e. antenna arrays with a large number of radiative elements, the need to control

each individual time delay to each radiative element requires that each individual phase shifter be independently voltage-biased. Arrays as large as 100x100 radiative elements necessarily require 10,000 independently-controlled bias voltage lines, which can be prohibitive in complexity and cost.

The ferroelectric phase shifter design is based upon a material whose dielectric permittivity can be made to vary by application of a DC electric field, parallel to the polarization of the RF energy, and normal to its direction of propagation. Variations in permittivity alter the RF propagation velocity, and if connected to a waveguide structure, will change the cutoff wavelength and dispersion of the waveguide itself. These two effects translate into propagation phase variation. A short waveguide section containing a phase shifter of ferroelectric material and HTSC material constitutes the key element for accomplishing electronic scanning in a phased array antenna configuration.

A typical application of the thin film HTSC material and ferroelectric thin film technology of the invention is for phase shifters for phased array antennas. The voltage-controlled ferroelectric thin film can be used to modulate

the dielectric constant, and hence the time delay, of either microstrip line or coplanar delay lines. Such phase shifters or tuneable delay lines can be used to phase delay microwave and millimeter wavelength signals either transmitted or received from the individual radiative elements of the antenna array.

As shown in FIG. 6, the phased array antenna 40 of the invention includes a plurality of radiators 41 . The antenna input 42 from a source of a electromagnetic radiation such as that at a microwave frequency is provided with a power distribution network 43. The distribution network connects the antenna input to each of a plurality of phase shifters 44 in accordance with the invention. The thin superconducting film of each of phase shifters 44 is connected by leads 45 to control circuit 46. The control circuit is driven by programmer 47 to control the voltage applied by the thin superconductor film to the ferroelectric film of each of the phase shifters 44. Thus, the dielectric permittivity of the ferroelectric thin film of each of phase shifters 44 can be varied by the application thereto of a variable DC electric field. The DC electric field is applied normal to the direction of propagation of the RF energy and parallel to the polarization of the RF energy being transmitted through the phase shifter to the radiators 41 of phased array antenna 40. Variations in permittivity of the ferroelectric thin film material alter the RF propagation velocity and can change the cutoff wavelength and dispersion of the RF energy being delivered to the radiators 41 of the antenna. As a result there is a variation in the propagation phase of one radiator with respect to another. Thus programmer 47 by means of control circuit 46 can effect electronic scanning of the phase shifters 44 connected to the radiators 41 of antenna 40 and thereby enable the beam produced by antenna 40 to be shaped into a predetermined form

and to be directed in microseconds. In this way the RF energy radiated by the antenna can track a multitude of targets at one time.

Another embodiment of the invention as shown in FIG. 7 comprises a dielectric grating 50 for an individual radiative element in an antenna array. The grating comprises a base portion 50a and a plurality of elements 50b projecting from base portion 50a and spaced apart from one another. Grating 50 which is formed, for example, of a thin film of ferroelectric material with a fixed time delay determined by the spacing between each adjacent element 50b intrinsically radiates

(or receives) RF energy at a fixed angle θ. The angle θ is adjusted by biasing the dielectric grating 50 with a bias voltage from variable bias voltage source 51 . Such a voltage-controlled grating enables changing or steering (in one plane) of the radiation angle θ with the application of a variable bias voltage from the source 51 to the grating 50 by lead 52.

In FIG. 8 there is shown a two-dimensional array antenna with steering of the radiation angle θ in both the x and y planes. A two-dimensional array 53 of gratings 54 is shown in FIG. 8. A fixed spacing between each grating "patch" antenna element 55 formed by a grating 50 as shown in FIG. 7 provides a similar radiation angle in the second plane. Voltage bias lines 56 and 57 connected to arrays 58 and 59, respectively, modulate the dielectric constant of the gratings and thereby the time delay between each antenna element 55 in the arrays 50 and 59 which make up antenna 53.

The thin film superconductor/ferroelectric technology of the invention can also be used with:

(1 ) Tuneable cavities and resonators;

(2) Tuneable frequency and wavelength filters;

(3) Ferrite thin film isolators;

(4) Directional coupler switches;

(5) Phase shift feeder networks f or patch antennas;

(6) Coplanar lines with variable impedance using voltage- tuned meander lines as the center conductor of the coplanar line;

(7) tuneable resonant antennas;

(8) tuneable electrically small antennas; and

(9) Tuneable, one-dimensional confocal resonators which can provide higher Q.

A coplanar structure 60 in accordance with the invention is shown in FIG. 9. Structure 60 includes ferroelectric film 61 overlying LaAI0 ₃ substrate 62. Thin films 63 of HTSC (e.g., YBCO or TBCCO) material are deposited on film 61 of ferroelectric dielectric material such as barium strontum titanate (BST) material, the lower surface of which is contiguous with the upper surface of the substrate of a LaAI0 ₃ (LAO) material.

FIG. 10 shows a structure 15 having a center section 66 of YBCO material forming a half wave-length resonator with coplanar lines 66 and 67 which are disposed between coplanar lines 68 and 69. The lines 66-69 are deposited on a dielectric film 70 of BST material. Changes in the thickness of the substrate 71 which, for example, can be LaAI0 ₃ material, modify the effective dielectric constant of film 70 of BST material and the percentage change of this dielectric constant

with the variable voltage applied to dielectric film 71 by voltage source 72.

FIG. 1 1 shows an capacitively coupled microstrip resonator 80. The dimensions shown on FIG. 1 1 are by way of example. The resonator includes microstrips 81 and 82 of superconductor (HTSC) material. Strip 83 of thin film ferroelectric material extends over gap 84 between strips 81 and 82. Strips 85 and 86 of HTSC material are connected by leads 87 and 88, respectively, to strips 81 and 82, respectively. If the resonator 80 of FIG. 1 1 , by way of example, is deposited on 17 mils of LaAI0 ₃ (LAO) material with e _r = 24.5, then the effective

dielectric constant, e _βff is e, _H ~- 20. inductively coupled resonator 90 in FIG. 1 1 having no tunability includes strip 91 of HTSC material connected by leads 92 and 93 to strips 94 and 95 of HTSC material, respectively, if the untuneable resonator 90 is approximated as a section of transmission line comprising two parallel spaced conductors with open ends, then its resonant frequencies is:

f _n = c n where, for example, = 9mm, n = 1 ,2,3 ... e _β „ 2

This gives f - 3.7 GHz, 7.4 GHz, 11.2 GHz, etc.

The ferroelectric series capacitor 84 in the tuneable resonator 80 will affect those resonant modes with large currents in the region of the ferroelectric capacitor. These are the antisymmetric resonant modes, i.e., modes with odd "n" in the above equation. Transmission line analysis yields a transcendental equation for the increased resonant frequencies of these modes:

- 1 = 1

tan K _ 2/τZoC

2 2 where

C = capacitance

Z ₀ = characteristic impendance — 7

= effective length of the resonator

(open end to open end length) k = 2/τfc for TEM mode,

Ve.„ where

c is the speed of light and f is the frequency of the resonant mode.

It is estimated that the capacitance is C ~ 1 pF. This gives a resonant frequency spectrum of:

6.7 GHz, 7.4 GHz, 13.43 GHz, . . . Under a DC bias, the 6.7 Ghz, 13.43 Ghz, ... modes will increase in frequency due to the voltage tuneable dielectric constant change.

In FIGS. 12 and 13 there is shown a superconducting fringe effect capacitor 100 comprising a thin film of ferroelectric material 101 disposed on a substrate 101a. Thin films 102 and 103 of HTSC material are deposited on ferroelectric thin film 101. Pads 104 and 105 are connected by leads 106 and 107, respectively, to thin films 102 and 103, respectively. Variable voltage source 106 is connected by lead 107 and 108 to pads 104 and 105, respectively, and thereby across thin films 102 and 103. The thin film of ferroelectric material 101

has a fringe effect capacitance region 109 disposed between thin films 102 and 103 of HTSC material. By applying a variable voltage across the thin films 102 and 103, the dielectric constant of the ferroelectric material 101 can be tuned.

As shown in FIG. 14, a HTSC layer A1 is deposited on a ferroelectric layer A2, and the HTSC layer can be subsequently patterned to produce a variety of structures A3. In this geometry, the HTSC/ferroelectric interface A4 is perpendicular to the C-axis of the layers, and lies in the plane of the layers.

As shown in FIG. 15, a HTSC layer A1 is deposited on a ferroelectric

layer A2 which is deposited on a substrate A5. Again, the HTSC/ferroelectric

interface A4, and the ferroelectric/substrate interface A6 lie in the plane of the layers.

As shown in FIG. 16, a HTSC layer A1 is deposited directly on a ferroelectric substrate A7, with the HTSC/ferroelectric interface A4 being coincident with the HTSC/substrate interface, again lying in the plane of the layers.

As shown in FIG. 17, a ferroelectric layer A2 is deposited on top of a patterned HTSC layer A8 (with example etched feature A9 shown), which is deposited on a substrate A7. The ferroelectric layer A9 fills the etched regions A9 and forms HTSC/ferroelectric interfaces A4 between the overlying ferroelectric layer A2 and HTSC layer A8 and interfaces A10 between the ferroelectric layer A2 and the edges of the patterned HTSC film A8. The interface all between the ferroelectric film A2 and the crystalline (or film) substrate A7 (made of dielectric, insulator, ferroelectric, or other material) is also shown.

As shown in FIG. 18, a ferroelectric layer A12 and HTSC layer are deposited in the same plane, with the HTSC/ferroelectric interface A10 lying at the

edges of the layers. Such layers can be deposited on a variety of substrates A7. As shown in FIG. 19, a variety of structures A1 - A9 are shown in combination, representing one example of the more general multi-layer structure of the invention, with various patterned and unpatterned HTSC and ferroelectric layers lying on top of each other, in various orders, and with the entire structure lying on a substrate. In the general structure, the substrate could be made of a bulk crystalline sample or film of any number of materials, including dielectrics, insulators, ferroelectrics, etc.

All the layers described can be deposited by a variety of techniques, including physical vapor deposition, chemical vapor deposition, laser deposition, sputtering, sol gel, etc.

In FIG. 20 there is shown a multilayer component 1 10 of the invention comprising thin films 1 11 and 1 12 of HTSC material contiguous with thin film 1 13 of ferroelectric material. The lower surface of ferroelectric thin film is contiguous with the upper surface of thin film 1 14 of HTSC material. The lower surface of thin film 1 14 is contiguous with the upper surface of substrate 1 15.

FIG. 21 is a plan view of an interdigitated capacitor structure 120 in accordance with the invention. Thin films 121 and 122 of HTSC material have a plurality of projections 121a and 122a, respectively, which nest with respect to one another, thereby forming a meandering slot 123 in structure 120. As shown in FIGS. 21 and 22, a thin film 124 of ferroelectric material overlies the portions of thin films 121 and 122 adjacent meandering slot 123 and extends into the slot itself. The structure 120 enables the overall value of the capacitance of structure

120 to be increased while maintaining tuneability of the capacitance by the application of predetermined voltages to thin film 124.

FIG. 23 shows a tuneable antenna 130 in accordance with the invention. The tuneable antenna comprises half-loop 131 of a thin film of HTSC material. Layers of insulating material 132 and 133 support the half-loop antenna 131. Layers 132 and 133 are separated by thin film 134 of HTSC material. Thin film 135 of HTSC material is connected to half-loop 131. Thin film 136 of ferroelectric material is disposed between thin film 135 and thin film 137 of HTSC

material which is disposed on substrate 138. Thin films 135 and 140 with film 133 therebetween form tuneable capacitor 141.

In FIG. 24 there is shown the equivalent circuit of the half-loop antenna 130 of FIG. 21. Driving signal source 142 is connected to the thin film 140 and thereby to the capacitor 141. As shown in FIG. 24, inductance 143 and resistance 144 are the lumped values of the resistance R and the inductance L of the half-loop antenna. The application of a DC voltage across capacitor 141 tunes the capacitance thereof and hence the frequency of the resonance of the circuit comprising the half-loop antenna 131 and the capacitor 141 .

Directing attention to FIG-2-5 ^' a delay/shift element 216 according to the invention is shown. Delay/shift element 16 comprises a layer 38 of a dielectric material, such as barium strontium titanate (BST), sandwiched between a first coπductor2l9 and a second coπducto^l, which conductors are commonly referred to as electrodes. A DC or low frequency electric field is placed across the BST layer z18 by a variable bias voltage source223. As can best be seen in FIG. ^ the dielectric constant, and thus the capacitance, of the BST layer 18 changes approximately 500% as the electric field strength goes from 0 kV/cm (kilpyolts per centimeter) to 25 kV/cm. This large change in dielectric constant can be used to tune electronic parameters which depend on the dielectric constant. For example, the speed of an electromagnetic wave in a waveguide depends on the dielectric constant of the material that fills the waveguide. Thus tuneable delay lines, phase shifters and other electronic devices that depend on the velocity of an electromagnetic wave in a dielectric medium can be constructed utilizing element

Z.16. In this disclosure such electronic devices shall be referred to as delay/shift devices for simplicity. Also, the terms "waveguide", "transmission line", and "electrical circuit" are conventionally used interchangeably in the field of microwaves and millimeter waves and shall be so used herein. In general the term "transmission line" shall be used to mean either a waveguide, as in the case where the wave length of

• the signal being processed is of the same order or smaller than the size of the system components, or an electrical circuit, as in the case where the wave length of the signal being processed is much larger than the size of the circuit components.

The generalized structure of a microwave/millimeter wave delay/shift device

Z2 utilizing the delay/shift elements is shown in RG.2έ3t comprises a signal input means226, the delay/shift elements 6 which generally forms part of a transmission Iine225 shown by dotted lines, a signal output meaπs 27, and a delay/shift controller. Trie signal input means226 typically comprises a terminal for connecting to an Blecfcica. Sfgπaf, or, in terms of waves, a coupling to a wave source, a

transformer, and a polarizer which introduce a wave228 into transmission iine2l1. It may also include the signal or wave source, though this is not shown in the drawings. The phrase "signal input means" should be interpreted broadly as anything that inputs an electrical signal to the delay shift element 16, and in particular the dielectric material2J8 in the delay/shift element. By "inputting a signal to the dielectric material" is meant that the signal is input so it passes through or otherwise interacts with the dielectric material so that the change in dielectric constant of the material can tune the signal. The delay/shift controlled preferably is a variable voltage source223 as shown in RG.2S"the wave228 is delayed or phase shifted by delay/shift element 16 and then is output from the delay/shift device224 via the signal output means227, which preferably comprises a polarizer, transformer, and a coupling to further elements of a microwave or millimeter wave system. Again, it may also simple comprise a terminal. The delay/shift devicez24 also preferably includes a temperature controller229. Temperature controller229 is preferably a cooling device, such as a liquid nitrogen cryogenic device which maintains the temperature of the delay shift element 2.16 at a predetermined temperature between 50 K and 110 K.

It should be understood that the figures, such as RG.2ξ depicting the delay/shift element 216 and FIGS. ^' 2T7 and % depicting transmission lines or waveguides, are not meant to be actual cross-sectional or other views of any particular portion of an actual electronic device, but are merely idealized representations which are employed to more dearly and fully depict the structure ■ and process of the invention than would otherwise be possible. Likewise it should be understood that the signal input meaπs226, the signal output meaπsZ27, and the delay /shift element*l6 may have many other structures than those discussed above and below. For example, in the case of micro circuits which are relatively small compared to the wavelength of the signal being processed, the input means_?6 and output meaπs227 may each simply consist of an electrical terminal.

The delay/shift element 6 may be used with a wide variety of electronic devices including tuneable delay lines, tuneable filters, matching networks, oscillators, loop antennas, directional couplers, tuneable resonators including one- dimensional confocal resonators, tuneable cavities, tuneable frequency and

wavelength filters, ferrite thin film isolators, directional coupler switches, phase shift feeder networks for patch antennas, coplanar lines with variable impedance using voltage-tuned meander lines as the center conductor of the coplanar line, tuneable antennas such as tuneable resonant antennas and tuneable electrically small antennas, phase shifters for phased array antennas, dielectric gratings for an individual radiative elements in an antenna array, fringe effect capacitors and many other structures, all of which may be fabricated in conventional and hybrid circuit form as well as in the form of monolithic devices and integrated circuits.

In this disclosure "thin film" means a film of a thickness of 50 microns and less and preferably 10 microns and less. In this disclosure the term "electrical" is intended to be a broad term that includes the usual meaning of the term electrical as well as everything included under the terms "electromagnetic" and "electronic".

Turning now to a more detailed description of the invention, the delay/shift element 216 (FIG. 1) preferably comprises dielectric layer 218 and electrical bias means 217 for applying an electrical field to the dielectric materials 8. Electrical bias means 2.17 preferably comprises first conductor7J9, second conductor221, and electrical connecting means 27A for connecting the conductors to variable voltage meansZ23, which typically is an adjustable DC and/or low frequency voltage source.

However, the electrical bias means'37A may include only one electrode2t9 and one connecting wire217A; that is, the delay shift elemeπt216 can be made in the form of single electrode219 and the dielectric Iayer2l8, which eiemeπt2J6, in a later stage of manufacturing the final device to be tuned, is pressed onto a ground plane which acts as the second electrode<21. In the preferred embodiment, the dielectric layer 18 is a ferroelectric material, such as barium strontium titanate (BST), barium titanate (BaTiO^, strontium titanate (SrTiO _g ), lithium niobate (LiNbO _g ), lead strontium titanate (Pb[Sr,Ti]0 ₃₎ lead titanate (PbTiO) ₃ and others. In the embodiment shown in the figures, the dielectric layer is BST, and is in the form of a thin film218. Conductors 19 and 21 are preferably formed as thin film layers in contact with the opposing surfaces 220 aπd222 of BST layer 218 to form a sandwich structure. However, the conductors may be separated from the BST layer 218 by some

distance, providing they are arranged so that they can apply an electric field to the BST layer 18. In the preferred embodiment the conductors^ aπd221 are made of a superconductor such as Y-Ba-Cu-O (YBCO) or TI-Ba-Ca-Cu-0 (TBCCO), though they also may be conventional conductors, i.e. conductors that are not a superconductor, such as platinum, gold, silver, and tungsten. Generally the conductive iayers2J9 aπd221 are made of several thin layers of different conductors, as for example, an adhesion layer of titanium or chromium immediately adjacent the surfaces2ϊ20 and∑22 and a layer of platinum or gold in contact with the adhesion layer. Such multilayered structures for electrodes and other conductive elements are well-known in the art and thus will not be discussed in detail herein. Means27A for connecting conductors^ 9 andZ_1 to a voltage source, typically comprises a wiring layer in an integrated circuit, traces in a hybrid circuit, terminals on the elements 6 which permit a voltage source to be connected to it, or simply solder points.

In the preferred embodiment, BST layer 218 is barium strontium titanate having the formula Ba _x Sr ₁ . _x Ti0 ₃ where, preferably, 0.01 < x < 0.2. A value of x approximately equal to 0.1 has been found to give the greatest change in dielectric constant. A variable bias voltage source 23 connected across coπductors219 and

Z.21 provides a variable electric field, preferably in the range of zero to 25 kV/cm, across BST Iayer218 to vary the dielectric constant. As shown in FIG-2-aWhen the delay/shift element a 6 is placed in a transmission line ^" Z-5, connected between a signal input meaπs226 and a signal output meaπs227, the variable bias voltage sourceZ23 becomes a delay /shift controllerZ23 which controls the delay, shift, or

. other parameter that varies with the dielectric constant of the BST layer.

2.-7

FIG., illustrates a microwave/millimeter wave delay/shift device230 utilizing a delay/shift element ~\ 6A according to the invention in which the wavelength of the signal being processed is of the same order of size or smaller than the electronic components232,735A,236A-233, etc. The device30 includes a housinc£31, a signal input 32 which couples a signal to the device 230, a transmission line.33, which would generally be called a waveguide in this device, a signal outputZ34 which couples the signal output from device230 to an external waveguide (not shown), transformer ^" ≥35A which transforms the input signal to one that will pass easily through the weveguide230, transformer235B which transforms the signal exiting from

waveguide^ to a forr ^s uitable to be used by the external wave ^! de (not shown), and polarizers236A and236B which polarize the signal as it enters and depolarizes it as it exits the waveguide233. In this embodiment the waveguida£_33 comprises a delay /shift eiement2t6A which includes a first conductor219A, a second conductor

^* 221A, and BST layer 2t8A. Wires238A ancTC38B provide a means for connecting conductorsQ-9A aπd221A respectfully to delay shift controller^ (shown in FIG. 2). Varying the voltage across conductors 2J9A and221A by means of coπtroller223 changes the dielectric constant of BST layer 2|8A and thus the delay and/or phase of the signal traveling in waveguide^, thereby tuning the signal. In the delay/shift device ^0 the barium strontium titanate H 8A comprises a dielectric within a distributed capacitance in the transmission Wnej33.

FIG2F? shows a microwave/millimeter wave delay/shift device SO utilizing a delay/shift elements 216B through216E according to the invention in which the wavelength of the signal being processed is of much larger than the electronic components. The device250 comprises a signal inputS2 which couples a signal to be processed to the device 50, a transmission line S3, which would generally be called a circuit£3 in this embodiment, and a signal output 54 which couples the signal output by device250 to and external circuit (not shown). Transmission Iiπe253 comprises a lumped circuit of variable capacitors 216B through216E and inductors

U56; such a circuit is sometimes referred to as an "artificial" transmission line. Each of the variable capacitors2l6B through?!6E is a delay/shift element such asτl6 in RG.ZS ^" having a variable voltage source, such as 23 in RG.2j Tuning the variable

.capacitors2l6B through?! 6E changes the dielectric constant of the capacitors and thus tunes the transmission linθZ-53 to delay and/or phase shift the signal passing along it, thereby tuning the signal.

If either of devices!30 or-SO is implemented as a microwave integrated circuit (MIC) or other micro device, the transmission Iiπes233 and/or 253 may be implemented as a microstrip device2_t0 as shown in FIG-25 Microstrip device240 comprises a ground plane2J9B which is the first conductor in device^iO, microstrip

221 B which acts as the second conductor, and BST layer 218B separating the conductors. In this case the delay shift controller^ (RG26) applies a variable voltage across ground piaπe21SB and microstrip-2.1 B to vary the dielectric constant

of BST layers 8B.

Turning now to FIGS3ό 3?. ^test devices 60. 70 and 80 for testing the BST material according to the invention are shown. RG.3øshows a capacitor devicei2β0 for measuring the bulk capacitance of the BST material according to the invention. It comprises a first conductive eiectrodeZ64 in the form of a semi-circular thin plate and a second conductive electrode266 in the form of a semi-circular thin plate separated by a thin strip262 of BST. A variable bias voltage source2δ8 applies a voltage to electrodes264 andZ66 to create a variable electric field across BST strip 62.

FIG.3 f shows an interdigitated thin film capacitor device'Z70 used for measuring the thin film properties of the BST material according to the invention. It comprises a first conductive electrode274 and a second conductive electrode 76 which have fingers, such as273 and277 respectively, which iπtermesh and are separated by a thin strip_72 of BST. The electrode^74 and 6 and the BST strip272 are thin films formed on a substrates. A variable bias voltage source 78 applies a vottage to electrodesZ74 andZT6 to create a variable electric field across BST strip Z72.

FIGS. ^" 3 3πd 3 ^" 3show a coplanar waveguide phase shifter element 280 according to the invention which was used to test the usefulness of the BST material of the invention in a phase shifter. Phase shifter element2B0 comprises a first thin film coπductor284 and a second thin film coπductor2B6 formed on a substrate289. Rrst thin film conductor234 is formed in two parts ^" B4A and284B. The •conductors*^ aπd236 are formed by depositing a film on substrate,B9 and etching it to form a gap^BJ in the form of a funneQ37A and237B at each end connected by a thin linear sectior^7C. Gap23Zis formed in two partsGB7D and237E separated by conductor2§6. A filrτ _2 of BST is then deposited in the gap portioπs2-37D aπd _< &7E. The BST film overlies the conductors^ and^6 on either side, as can best be seen in FIG.^although this overlap is merely a fabrication artifact to ensure that the gap portions287D and2$7E are filled. In this embodiment the signal input means^δ is a coaxial cable having an inner conductor^ and an outer conductor^. The inner conductor^ is connected to the second coπductorϊδ, while the outer conductor is connected, to the portionsjB4A andS4B of first conductor^. Likewise the signal

output means 27 is a coaxial cable235 having an inner conducto£96 connected to the second coπductor336 and an outer coπductorSS connected to first conductor portions_84A ancβB4B. In the tests described below, a delay/shift controller223 (RG. / comprising a variable bias voltage source was used to apply a DC voltage across conductors^ and2B6 via coaxial cablς O to create an electric field across BST film

282, which DC voltage and field was varied to change the dielectric constant of the fiim--82 and thereby shift the phase of a signal passing through the coplanar waveguide phase shifter elemen^O, thereby tuning the signal.

Turning now to capacitance versus temperature curves for five electric field strengths for a BST capacitor as shown in RG^όare shown. The sample capacitogδO had a thickness, t, of 0.71 mm, a diameter, D, of 0.8 cm, and a BST strip_62 width, d, of 0.08 mm. The electrodes264 an ^δ were made of YBCO superconductor and the BST was Ba _g ^Sr^TiO _j . Electric fields of 0 kV/cm, 3.125 kV/cm, 6.25 kV/cm, 12.5 kV/cm, and 25 kV/cm provided by voltage source 68 were placed across the capacitor2fi0 and the capacitance in picofarads was measured at a frequency of 100 kHz at the points shown as the temperature ranged from approximately 40 K to 150 K. Each of the curves peaked at between 86 K and 88 K, with the peak moving to a higher temperature as the field increased. The peak capacitance decreased from about 2690 pf at 0 kV/cm to about 450 pf at 25 kV/cm. This represents a change of about 500%. This change is enormous compared to the 16% to 50% changes reported in the Varadan article cited above and the changes of 10% to 20% reported to be feasible in the Jackson paper entitled "Novel Monolithic Phase Shifter Combining Ferroelectrics and High Temperature Superconductors" cited above.

RG-jf^shows capacitance versus temperature curves for five electric field strengths for a Ba^Sr,, ₉₁ Ti0 ₃ capacitor as shown in RG^C he sample capacitor

260 had a thickness, t, of 1.05 mm, a diameter, D, of 2.12 cm, and a BST strif£62 width, d, of 0.08 mm. The electrode^β4 aπcQδβ were made of YBCO and the BST was Bao- _jg Sr _jjg .TiO-,. Electric fields of 0 kV/cm, 3.125 kV/cm, 6.25 kV/cm, 12.5 kV/cm, and 25 kV/cm were placed across the capacitoι260 and the capacitance in picofarads 'was measured at a frequency of 100 kHz at the points shown as the temperature ranged from approximately 46 K to 120 K. Each of the curves peaked

the bulk devices in the prior art. Thus, for the first time, the tuneability of the material is large enough for a practical MIC delay line or phase shifter.

FIG.3"7shows capacitance versus temperature curves for four voltages for ^{a Ba} αo8 ^Sr o. ₉₂ ^Ti0 ₃ ^tniπ film capacitor as shown in FIG.^ which capacitor was the same as that described above except the width of the fingers273,77 and the BST strip272 was about 4500 A. Thus this capacitor was about 60% of the size of the capacitor of RG. 36 ^* The figure also shows a single point on the capacitance/temperature curve for three other voltages. Voltages of 0 V, 5 V, 10 V, and 20 V were placed across the capacitor270 and the capacitance in picofarads was measured at a frequency of 100 kHz at the points shown as the temperature ranged from approximately 40 K to 300 K. Each of the curves peaked at aboutS2 K, with little evident shift of the peak as the field increased. Points were also obtained for the capacitance at 82 K for voltages of 30 V, 40 V, and 50 V. The peak capacitance decreased from about 6.67 pf at 0 kV/cm to about 5.96 pf at 66 kV/cm. The percent change of the capacitance for each of the peak capacitance values is shown for each curve and the individual test points for the 30V - 50 volt measurements. The maximum change is about 10.7%.

FIG.3^ shows loss tangent versus temperature curves for the same capacitor, electric field strengths, and frequency used in obtaining the curves of FIG. ? hθ curves show that the loss tangent has a local minimum at about the same temperature as the peak capacitance. Overall the loss tangent is in the range of about 2 x 10 ^"2 and 4 x 10 ^"2 which is about the range reported for prior art GaAs tuneable microwave devices which generally are utilized with normal conductors rather than superconductors. Thus the BST delay/shift elemeπts223 can be used with or without the superconducting electrodes. Since the largest change in the dielectric constant is in the temperature range of the high temperature superconductors, the use of superconductors as the conductors Δ 9, 21 etc. is preferable, though this is not necessary.

FIG. 9 shows the measured change of phase angle and effective dielectric constant versus electric field strength for the phase shifter of RGS.^aπd"^ The conductor 284A, ^" 334B, aπdJBδ were made of YBCO, the BST was Ba _∞ Sr _og2 Tϊθ ₃ , and the substrata789 was made of lanthanum aluminate. The length of the phase

at between 78 K and ol K, with the peak moving to a higher temperature as the field increased. The peak capacitance decreased from about 4150 pf at 0 kV/cm to about 1600 pf at 25 kV/cm. This represents a change of about 150% which is significantly smaller than the change for the Ba _j , ^Sr^TiO _g sample discussed above, but still enormous compared to the prior art. The primary cause for the difference is believed to be due to the 1% change in the BST composition. This indicates that the delay/shift element2l6 can not only be tuned by the applied voltage, but the composition can also be "tuned" to provide the desired delay or phase shift. This is very important in the microwave area where matching parameters is critical. Comparing is evident that as one reduces the barium content and increases the strontium content the peak goes to smaller temperatures. Thus the delay /shift element 216 can also be "tuned" to a desired phase or delay by varying the temperature.

FIG. ^2, ^' shows capacitance versus temperature curves for six electric field strengths for a thin film capacitor as shown in RG3j. ^' The width of the fingers273,277, i.e. the dimension in the vertical direction in RG.Js?., ^' was 7600 A, which was also the thickness of the BST strip272. The thin film was about 600θλ thick, i.e. in the direction out of the paper in RG.JT the electrodes^ and_£6 were made of YBCO and the substrate279 was lanthanum aluminate. Electric fields of 0 kV/cm, 13 kV/cm, 26 kV/cm, 39 kV/cm, 53 kV/cm, and 66 kV/cm were placed across the capacitor270 and the capacitance in picofarads was measured at a frequency of 100 kHz at the points shown as the temperature ranged from approximately 20 K to 300 K. Each of the curves peaked at between 70 K and 90 K, with the peak moving to a higher temperature as the field increased. The peak capacitance decreased from about 6.5 pf at 0 kV/cm to about 5.9 pf at 66 kV/cm. This represents a change of about 10% which is significantly smaller than the change for the bulk samples discussed above, but still is about the same size as the change for the bulk samples described in the prior art. The primary cause for the difference is believed to be due to the thin film. That is, in the thin material surface effects, such as surface charges, predominate. Since it is expected that the bulk results published in the prior art would decrease similarly, the result of RG3 € is a true breakthrough, since the thin film results are nearly equal to the results for

shifter waveguide, i.t le distance across the conductorsZL and 286 in the horizontal direction in RGj was about 13 mm, the thickness of the BST282 in each of gap portioπs237D and2B7E in the horizontal direction in G.^^ as about 5 microns and the thickness of the conductors films34A,2B4B, anc£86, respectively in the vertical direction in G/j was about 6000 A. The phase shift was measured in units of it at a frequency of 10 GHz (gigahertz) using a Hewlett Packard model HP8510 Network Analyzer. The results show a change of about ir or 126 degrees over a change of field from 0 kV/cm to 6 kV/cm. Typically, in phase shifters one seeks to have changes of about 90 degrees or more, so the phase shift is well within the range of a practical device. Over the same range of field strengths, the dielectric constant changes from about 14.5 to about 25 or about 30%.

The BST and other thin films of the invention can be made by methods including physical vapor deposition, chemical vapor deposition, laser deposition, sputtering, sol gel processing, and many other processes and can be patterned by many known MIC patterning processes, such as ion milling, chemical etching, etc. In particular the YBCO of the devices of RG53o,-3|aπd^ vas deposited by laser ablation, while the BST was deposited by a sol-gel process. The devices were patterned using a chemical wet etch. These processes are all well-known in the art and thus will not be discussed in detail herein.

There has been described a novel tuneable dielectric constant electronic device utilizing a thin film BST material having a dielectric constant that is far more tuneable than prior art BST devices. It should be understood that the particular ^• embodiments shown in the drawings and described within this specification are for purposes of example and should not be construed to limit the invention which will be described in the claims below. Further, it is evident that those skilled in the art may now make numerous uses and modifications of the specific embodiments described, without departing from the inventive concepts. For example, now that a BST material having such large tuneability is disclosed, many other applications of the material can be made. Equivalent structures and process may be substituted for the various structures and processes described. Additional features and thin film layers may be added. Or a variety of different dimensions and conductor materials may be used. Many other variations are possible.

The advantages of the present invention, as well as certain changes and modifications of the disclosed embodiments thereof, will be readily apparent to those skilled in the art. It is the applicants' intention to cover in their claims all those changes and modifications which could be made to the embodiments of the invention herein chosen for the purpose of the disclosure without departing from the spirit and scope of the invention.