TUNABLE DIELECTRIC FLIP CHIP VARACTORS

Abstract: The present invention is directed to a method for forming electrically tunable varactors independently of the circuit in which the varactors are to be incorporated. In the method, a tunable dielectric material is deposited on a substrate, subjected to thermal treatment to reduce crystalline defects, metallized, and the layered structure subdivided into a number of varactors. The varactors are bonded using flip chip techniques to a circuit. The present invention further provides varactors having differing configurations. One varactor configuration includes stacked conductor pairs separated by a nontunable insulating material having a low electric permittivity.

Description: TUNABLE DIELECTRIC FLIP CHIP VARACTORS FIELD OF THE INVENTION The present invention relates generally to electrically tunable varactors and specifically to varactors incorporating electrically tunable dielectric materials. BACKGROUND OF THE INVENTION In wireless communications, electrically tunable varactors are employed to alter the characteristics (e.g., phase and wavelength) of electromagnetic energy passing through the varactor. Electrically tunable varactors can be implemented in a wide variety of microwave components, including delay lines, phase shifters, bandpass and bandreject filters, steerable-beam antennas, and voltage- controlled oscillators. Electrically tunable varactors can include semiconducting or tunable dielectric materials. In the latter case, the tunable dielectric material can be a ferroelectric or paraelectric material, which has a dielectric permittivity that is a variable function of a voltage applied to the material, and a pair of spaced apart conductors located on top of the material. To alter the dielectric permittivity, a variable dc voltage is applied to the material via the spaced apart conductors to vary the capacitance across the gap between the conductors. Electromagnetic energy is simultaneously propagated through the gap via the conductors. The portion of the electromagnetic energy propagating through the material in the gap is thereby tuned. In this manner, the circuit containing the varactor can be electronically controlled. In designing a versatile varactor for microwave applications, there are a number of design considerations. First, the varactor should provide a relatively low capacitance between the spaced apart conductors to achieve capacitive impedance values which are compatible with the microwave circuit. Second, the varactor should require a relatively low dc voltage to tune the ferroelectric or paraelectric material. Third, the varactor should have relatively high RF power transmission capability for use in the transmission of microwave and millimeter wave signals. Finally, the electromagnetic losses in the ferroelectric or paraelectric material at frequencies above 800 MHz should be relatively low. SUMMARY OF THE INVENTION These and other design objectives are addressed by the electrically tunable dielectric varactors of the present invention. The varactors are formed separately from the electromagnetic circuit and thereafter bonded to the circuit using flip chip techniques. For electrically tunable thin film and thick film dielectric materials this method is preferably characterized by the following steps: (a) forming, on an upper surface of a substrate, a layer of an electrically tunable dielectric material. (b) thereafter forming a pair of spaced apart conductive layers above the layer of dielectric material to define a layered structure; and (c) cutting the layered structure to form a number of varactors. As noted, "electrically tunable dielectric materials" refers to dielectric materials having a dielectric permittivity that is a function of an electric field applied across the dielectric material. Each of the varactors includes a portion of the substrate, a portion of the layer of dielectric material, and spaced apart portions of conductive layers. In each varactor, the electrically tunable dielectric material is located in or below the gap between the spaced apart portions of the conductive layers. The individual varactors can thereafter be bonded to an electromagnetic circuit at desired locations to provide the desired degree of tuning of electromagnetic energy passing through the circuit. The second and third objectives noted above are realized by designing the varactors so that the magnitude of the dc bias voltage required to vary the capacitance is substantially higher than the voltage level of the electromagnetic energy being passed through the varactor. The electrically tunable dielectric material can be a variety of materials. Preferably, the electrically tunable dielectric material is a ferroelectric or paraelectric thin film or bulk material that is selected from the group of (Ba.Sr^ iOa, (Pbj_ x La x ) (Ti y Zr ! _ y )0 3 , and K(Nb x ,Ta α . x )0 3 , where O≤ X≤ 1 and O≤ y≤ 1. Although the varactors of the present invention are discussed generally with reference to tunable thin film and bulk materials, the application of flip chip techniques to thin film dielectric materials is particularly useful because several desirable varactor properties can be achieved simultaneously. These properties include: capacitance values which typically range from about 0.01 to about 50 picofarads and more typically from about 0.01 to about 10 picofarads, electrode/film/substrate configurations which allow the varactor capacitance to be tuned by greater than about 40 percent via application of a dc voltage of about 300 volts or less, minimal or no hysteresis in capacitance as the tuning voltage is cycled and the capability to decrease the power levels of intermodulation distortion signals generated by passing RF energy through the varactor by increasing the gap distance between electrodes. Intermodulation distortion is highly detrimental to circuit performance and decreases the signal-to-noise ratio of wireless communications systems. Because the varactor is formed independently of the electromagnetic circuit and thereafter attached to the circuit, the dielectric material in the varactor can be formed under substantially optimal conditions for the growth of high quality films of the dielectric material. Optimization of thin film varactor performance requires epitaxial growth of the thin film on the substrate, which in turn necessitates that substrates which have similar lattice parameters and/or crystal structures as the thin film, and similar thermal expansion coefficients be chosen. Furthermore, the substrate surface must be essentially pristine and free of defects prior to film growth so as to avoid the propagation of defects into the film. Film growth conditions such as substrate temperature, gas pressure during growth, and the energy of the atomic and molecular species as they are deposited on the substrate are important to crystal growth in the tunable dielectric material. Further improvements to varactor performance are achieved by post-deposition processing, and these steps generally include a high temperature anneal which removes microstructural defects and increases the grain size of the films from nominally about 70 nanometers to more than about 200 nanometers. The post-anneal is generally performed at about 1000°C or higher. These steps enable the fabrication of varactors with optimal tuning-to-loss ratios. Since the dielectric losses in varactors which are manufactured by this method are essentially unchanged at frequencies ranging from 0.8 to about 20 GHz, this process is an enabling technology for low loss varactors for use in microwave and millimeter wave electronics. By contrast, the dielectric losses of tunable dielectric varactors which have high defect concentrations, and conventional semiconductor varactors , increase more rapidly with increasing frequencies. At frequencies from 20-100 GHz, dielectric losses in varactors fabricated via the disclosed method increase more rapidly because of intrinsic losses in the materials, but the losses remain lower than those of highly-defected tunable dielectric varactors and semiconductor varactors. The optimal conditions for crystal growth include the use of a properly matched substrate. The substrate upon which the dielectric material is deposited is preferably selected based upon the following criteria: (i) the lattice constant of the substrate is matched to the lattice constant of the dielectric material; (ii) the crystallographic structure of the substrate is the same as that of the dielectric material (e.g., the substrate and dielectric material both have the perovskite crystal structure) ; (iii) the coefficient of thermal expansion of the substrate is close to that of the dielectric material; and (iv) the substrate is a dielectric or electrically insulating material having a dielectric permittivity that is less than the dielectric permittivity of the tunable dielectric material. Preferably, the lattice mismatch between the substrate and the dielectric material is no more than about 10%, and the difference between the coefficient of thermal expansion of the dielectric material and substrate is no more than about 30% and more preferably no more than about 2% of the thermal expansion coefficient of the substrate. Examples of tunable dielectric materials which have the perovskite structure are (Ba x Sr 1 _ x )Ti0 3 , (Pb. x La x )Ti0 3 , and K(Nb x ,Ta : _ x )0 3 ) O , Examples of substrate materials having relatively low dielectric permittivities but which have the perovskite structure are LaA10 3 and NdGaθ 3 . Non-tunable substrates which do not have the perovskite crystal structure, but have lattice parameters similar to those of the tunable dielectric films, may also be used as substrates for tunable dielectric varactors. The microwave performance of tunable dielectric varactors fabricated on substrates such as MgO, Y-Zr0 2 , and A1 2 0 3 is generally not as good as the performance of varactors fabricated on perovskite substrates, but may be acceptable for many applications. For ferroelectric and paraelectric materials, the preferred substrate materials include sapphire, magnesium oxide, lanthanum aluminate, neodymium gallate, and yttrium-stabilized zirconia. The use of flip chip techniques to attach a varactor to an electromagnetic circuit further permits the realization of relatively low capacitances for bulk tunable dielectric materials. The bulk materials may be either monocrystalline or polycrystalline. A bulk dielectric material, either in the form of a thick film or self- supporting material, can be reduced in thickness. Use of thinned single crystals offers the microstructural advantages (i.e., the absence of grain boundaries and a low point defect concentration) and consequent low microwave loss of bulk materials with the further advantage that the capacitance of the varactor can be reduced to levels which are attractive for microwave and millimeter wave applications (i.e., preferably no more than about 50 pf, more preferably no more than about 10 pf, and most preferably no more than about 5 pf) . The flip chip varactors of the present invention can be fabricated in a number of different configurations. In one configuration, the varactor includes: (a) an electrically insulating substrate having a substrate dielectric permittivity; (b) a layer of a tunable dielectric material supported by the electrically insulating substrate; (c) a pair of spaced apart conductors located above the layer of tunable dielectric material such that a portion of the layer of tunable dielectric material is located in and/or below a gap between the pair of spaced apart conductors; and (d) a layer of a non-tunable dielectric material. The non-tunable dielectric material has a relatively low dielectric permittivity that is less than the dielectric permittivity of the tunable dielectric material. The non- tunable dielectric material is located above the gap above the layer of the tunable dielectric material to provide a relatively high electric breakdown strength between the spaced apart conductors. In this manner, the varactor can have relatively high power handling capabilities and can significantly increase relative to a varactor having an air-filled gap the maximum dc voltage that can be applied to the tunable dielectric material prior to voltage breakdown across the gap between the conductors. This is especially significant because, in the region over which the tuning range is extended, the dielectric loss is lowered and the power-handling capability improves (i.e., the intensity of the RF signals generated from third-order intermodulation distortion products are lowered) . To ensure that the non-tunable dielectric material completely fills the gap between the spaced apart conductors without also covering the conductors and thereby preventing electrical contact with the electromagnetic circuit when the varactor is bonded to the circuit, the layer of the non-tunable dielectric material is formed not only in the gap but also over the top of the spaced apart conductors. Rather than pattern the non-tunable dielectric material to expose the conductors, a pair of spaced apart secondary conductors is thereafter formed above the layer of the non-tunable dielectric material. Each of the secondary conductors is located above and is capacitively coupled to one of the conductors such that each of the secondary conductors is separated from an adjacent underlying conductor by the non-tunable dielectric material. To provide an electrical short circuit between each of the conductors and the corresponding secondary conductor, the capacitances between each of the secondary conductors and the corresponding underlying conductor is preferably less than about 20% of the capacitance between the conductors themselves. The distance between the secondary conductors is preferably sufficient to inhibit voltage breakdown in the gap between the secondary conductors. An additional advantage of this configuration is that the low dielectric permittivity film is electrically connected in parallel with the high permittivity tunable dielectric layer, thus as the dielectric permittivity of the tunable phase is reduced by application of a dc voltage, a larger fraction of the microwave current will propagate in the lower-loss non- tunable layer, thus lowering the dielectric loss of the varactor. Tuning and loss is improved by designing dielectric film configurations wherein the fraction of RF current, or equivalently, the fraction of electric flux lines in the tunable and non-tunable components of the varactor is controlled by a dc bias voltage. The best embodiment of this concept shows a two-phase film, where the first phase is comprised of a high permittivity phase, patterned into regions with both narrow and broad widths. A second, low permittivity, low loss phase is deposited into the spaces left by the prior patterning step. By constricting the electric flux to flow through narrow structures and increasing the physical path length required for the electric flux to propagate in the higher permittivity phase relative to the low permittivity phase, the probability that a larger fraction of the current will flow in the low permittivity phase increases. This probability is further enhanced via application of a dc bias voltage, which causes the permittivity of the tunable phase to decrease, and thus reducing the differential between the permittivities of the tunable and non-tunable phases. The net result is a simultaneous increase in tuning and decrease in loss relative to values which are achieved using single-phase, homogeneous tunable dielectric film varactors. To reduce the dc voltage required to realize a given degree of tuning of the dielectric material and therefore of the electromagnetic energy propagating through the device, the dc voltage can be applied to opposing surfaces of the dielectric material. In this manner, the distance between electrodes is reduced, thus the voltage required to tune the capacitance is also reduced. In another varactor configuration, first portions of each of the spaced apart conductors, which are adjacent to one another, define a first capacitance and second portions of each of the spaced apart conductors, which are separated from one another by the first portions, define a second capacitance. The first and second capacitances are electrically connected in parallel. To minimize the capacitive contribution of the second, non-tunable portion relative to the overall capacitance of the varactor, the first capacitance exceeds the second capacitance. In yet another varactor configuration, a thin film dielectric material is deposited on a tunable bulk dielectric material, and the pair of spaced apart conductors and the bulk dielectric material are located on opposing sides of the thin film dielectric material. In tuning bulk dielectric materials, electric charge can build up, causing a space charge effect. The space charge effect can cause nonrepeatability and hysteresis of tuning results for a given applied bias voltage over time. Because thin film dielectric materials commonly have significantly more crystalline defects and grain boundaries than bulk dielectric materials and because such defects and grain boundaries can "trap" or immobilize electric charge carriers, the thin film dielectric material can reduce the space charge effect by inhibiting the migration of electric charge carriers from the thin film dielectric material into the bulk dielectric material. The technique of reducing hysteresis and non-repeatability by intentionally introducing defects in the tunable thin film dielectric material contrasts with the prior art, which states hysteresis is reduced by lowering the defect concentration at the surface. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 depicts the attachment of a varactor to a circuit after independent formation of the varactor and the circuit; Fig. 2 is a sectional view of a varactor bonded to a circuit along line 2-2 of Fig. 1; Fig. 3 depicts various embodiments of methods for forming a tunable circuit according to the present invention; Fig. 4 is a plan view of a layered structure; Fig. 5 is a side view of a varactor according to an embodiment of the present invention; Fig. 6A is a plan view of a varactor according to another embodiment of the present invention; Fig. 6B is a side view of the varactor of Fig. 6A; Fig. 7A is a plan view of a varactor according to yet another embodiment of the present invention; Fig. 7B is a side view of the varactor of Fig. 7A; Fig. 8A is a plan view of a varactor according to a further embodiment of the present invention; Fig. 8B is a side view of the varactor of Fig. 8A; Fig. 9A is a plan view of a varactor according to another embodiment of the present invention; Fig. 9B is a side view of the varactor of Fig. 9A; Fig. 10A is a plan view of a varactor according to yet another embodiment of the present invention; Fig. 10B is a side view of the varactor of Fig. 10B; Fig. 11A is a side view of a varactor according to a further embodiment of the present invention; Fig. 11B is a plan view of the varactor of Fig. 11A; Fig. 11C is a perspective view of the varactor of Fig. 11A; Fig. 12 is a side view of a varactor of another embodiment of the present invention; Fig. 13A is a plan view of a varactor of another embodiment of the present invention; Fig. 13B is an exploded view of the interlaced conductors of the varactor of Fig. 13A; Fig. 14 is a plan view of a varactor of another embodiment of the present invention; Fig. 15 is a side view of a varactor of another embodiment of the present invention; Fig. 16 is a side view of a varactor of another embodiment of the present invention; Fig. 17 is a side view of a varactor of another embodiment of the present invention; Fig. 18A is a plan view of a varactor of another embodiment of the present invention operating in a first mode; and Fig. 18B is a plan view of the same varactor operating in a second mode; Fig. 19 is a plot of capacitance (left hand vertical axis) , bias voltage (horizontal axis) , and tanδ (right hand vertical axis) for the varactor of Figs. 7A and 7B; Figs. 20A and 2OB are plots of resonant frequency (left hand vertical axis) , bias voltage (horizontal axis) , and quality factor (right hand vertical axis) of the varactor of Figs. 5A and 5B using a thinned thick film dielectric material; Fig. 21 depicts a two-port resonant structure; Figs. 22A and 22B are plots of resonant frequency (left hand vertical axis) , bias voltage (horizontal axis) , and quality factor (right hand vertical axis) for the varactor of Figs. 5A and 5B using a thin film dielectric material ; Fig. 23 is a plot of capacitance (vertical axis) versus bias voltage (horizontal axis) showing the capability of applying high electric fields to tune the varactor shown in Figs. 11A-C; Fig. 24A is a plot of capacitance (vertical axis) versus temperature (horizontal axis) for a given bias voltage using the varactor of Figs. 11A-C; Fig. 24B is a plot of capacitance (vertical axis) versus bias voltage (horizontal axis) for the varactor of Figs. 11A-C; Fig. 24C is a plot of loss tangent (vertical axis) versus temperature (horizontal axis) for a given bias voltage using the varactor of Figs. 11A-C; Fig. 24D is a plot of loss tangent (vertical axis) versus bias voltage (horizontal axis) for the varactor of Figs. 11A-C; and Fig. 25 is a plot of resonant frequency (left hand vertical axis) , bias voltage (horizontal axis) , and quality factor (right hand vertical axis) of the varactor of Figs. 5A and 5B using a thinned thick film dielectric material. DETAILED DESCRIPTION Fig. 1 depicts a varactor 30 according to the present invention prior to being bonded to a microstrip resonator 34. As shown in Fig. 2, the spaced apart conductors 38 and 42 on the varactor 30 are bonded via a conductive material 54 to spaced apart conductive layers 46 and 50 on the resonator 34 where the gap 58 between the conductive layers 38 and 42 is located. The varactor 30 includes a dielectric or electrically insulating substrate 62 supporting a tunable dielectric material 66 and the pair of spaced apart conductors 38 and 42 separated by the gap 58 containing at least a portion of the tunable dielectric material 66. The resonator 34 includes a ground plane 70, a supporting dielectric or electrically insulating substrate 74 and conductors 46, 50, 78 and 82. This varactor configuration is further described in U.S. Patent 5,472,935, which is incorporated herein by this reference. Although Fig. 1 depicts the application of flip chip techniques to incorporate a varactor into a microstrip resonator, the techniques are equally applicable to numerous other devices. By way of example, the techniques can be employed to incorporate varactors in devices such as delay lines, phase shifters, oscillators, filters, electrically-small antennas, half-loop antennas, directional couplers, patch antennas, and various radiative gratings. Referring to Fig. 3, various methods for forming varactor-containing devices according to the present invention are illustrated. In a first embodiment, an electrically tunable thin or thick film dielectric material is deposited 86 onto a selected substrate 62 to form a tunable dielectric substrate 90. The substrate 62 is selected 94 based on several criteria noted above, namely the degree of lattice matching between the dielectric material and the substrate, the crystallographic structures of the dielectric material and the substrate, the coefficients of thermal expansion of the dielectric material and the substrate, and the dielectric permittivity of the substrate. Preferably, the lattice mismatch between the substrate and dielectric material is no more than about 10% and more preferably no more than about 6%; the substrate and dielectric material have the same crystallographic structures (preferably perovskite) , the difference between the thermal expansion coefficient (TEC) of the dielectric material and the substrate, where the difference, which is defined as TEC difference ( % ) = ( (TEC d ie l ectric m ateria l — TEC 3ubstrate ) /TEC 3ubstrate ) XlOO , is no more than about 30% more preferably no more than about 25% and most preferably no more than about 2%; and the substrate has a lower dielectric permittivity than the dielectric material and more preferably the dielectric permittivity of the substrate is no more than about 10% of the dielectric permittivity of the dielectric material. Unlike the dielectric permittivity of the dielectric material, the dielectric permittivity of the substrate is preferably nontunable (i.e., the dielectric permittivity is not a function of the voltage applied to the substrate) . The electrical impedance of the substrate at microwave frequencies is preferably more than the electrical impedance of the dielectric material, thus causing the majority of the RF current to propagate in the tunable dielectric film. For (Ba x Sr a . x ) Ti0 3 , the preferred substrate materials are LaA10 3 , NdGa0 3 , A10 3 , Y-Zr0 2 and MgO. For (Pb ! _ x La x ) (Ti y Zrι_ y ) 0 3 , the preferred substrate materials are LaA10 3 , NdGa0 3 , MgO, A1 2 0 3 , and Y-Zr0 2 . For K(Nb x ,Ta 1 . x )0 3 , the preferred substrate materials are LaA10 3 , NdGa0 3 , MgO, A1 2 0 3 , and Y-Zr0 2 . To provide support for the dielectric material during later processing steps (e.g., thickness reduction), the substrate 62 should have a sufficient thickness "T s " (See Fig. 2) to provide a relatively high mechanical strength. Preferably, the substrate has a thickness T s preferably ranging from about 125 to about 1,000 microns and more preferably from about 250 to about 500 microns. The dielectric material 66 can be deposited 86 on the substrate 62 by any suitable deposition technique. Such techniques include sputtering, laser deposition, and sol- gel for thin film dielectric materials and sintering, tape casting or doctor-blading for thick film dielectric materials, and Czochralski and hydrothermal for bulk dielectric materials. The thickness "T D " (see Fig. 2) of the layer of dielectric material 66 depends upon the material type. For thin film dielectric materials, the thickness T D preferably is no more than about 5 microns, more preferably ranges from about 0.01 to about 2 microns, and most preferably ranges from about 0.05 to about 1 micron. For thick film dielectric materials, the thickness T D preferably ranges from about 2 to about 100 microns, more preferably from about 5 to about 100 microns, and most preferably from about 7 to about 25 microns. For bulk dielectric materials, the thickness T D preferably is at least about 5 microns, more preferably ranges from about 10 to about 100 microns, and most preferably ranges from about 20 to about 50 microns. The thickness of the dielectric material, particularly the thickness of bulk dielectric materials used in microwave and millimeter wave devices, can be reduced 98 to provide a capacitance of no more than about 50 pf, more preferably no more than about 10 pf, and most preferably ranging from about 0.1 to about 2.0 pf. At such low capacitances, the self resonant frequency of such varactors using thinned dielectric materials is typically no less than about 5 x 10 10 Hz and more typically is no less than about 3 x 10 10 Hz. Although any technique can be used for reducing the thickness of the dielectric material, the preferred method of thickness reduction is mechanically by grinding or polishing the free face of the dielectric material. Preferably, the dielectric material is initially thinned to a thickness of approximately 100 microns using Sic or an alternative abrasive paper. Additional grinding and polishing is preferably performed using a series of progressively finer diamond powders until the a thickness ranging from about 50 to about 75 microns is realized. Final thinning is preferably accomplished using chemical mechanical thinning or a similar technique to thin the dielectric material to the final thickness. Wet chemical etching techniques alone can create pitting and nonuniform etch ratio across the free face of the dielectric material. For bulk dielectric materials, the thickness is commonly reduced by an amount ranging from about 80 to about 99.75%, more commonly from about 90 to about 99.5%, and most commonly from about 95 to about 99%. The initial thickness of the dielectric material commonly ranges from about 125 to about 1,000 microns, more commonly from about 300 to about 800 microns, and most commonly from about 250 to about 500 microns. The final thickness of the bulk dielectric material preferably ranges from about 10 to about 100 microns and most preferably from about 20 to about 50 microns. Whether or not the thickness of the dielectric material is reduced 98, the spaced apart conductors 38 and 42 (See Fig. 2) are next formed 102 on the upper surface of the dielectric material (i.e., the conductors 38 and 42 and substrate 62 are on opposing sides of the dielectric material) to yield a layered structure 106. The conductors can be any conductive or superconductive material, such as a normal metal or YBCO. Referring to Figs. 3 and 4 the layered structure 106 is subdivided 110 into a plurality of varactors. The subdivision is preferably performed by techniques, such as mechanical dicing or laser cutting along dividing lines 114. Each varactor 30 a-n has a portion of the spaced apart conductors 38 and 42, a portion of the dielectric material 66, and a portion of the substrate 62. Each of the varactors 30 a-n is bonded 118 to a circuit 122 to form a tunable circuit 126. The circuit 122, as shown in Fig. 3, is formed independently of the varactor 30 by forming 130 metallization on a substrate 134 for the circuit. The bonding to the circuit 122 can be done by suitable bonding techniques, including solder reflow and gold bump bonding. In these techniques, a sphere of solder is placed either on the conductors of the varactor or circuit at the point where the varactor is to be bonded. The varactor and circuit are mechanically clamped together and the temperature of the solder is increased to a temperature exceeding the melting temperature of the solder, thus causing the solder to flow and form an electrically conductive and mechanically rigid contact between the varactor conductors and the circuit. The conductors can be deposited by any suitable method, including thin film deposition, electroplating, screen printing, and ball bonding. Melting of the metallization is accomplished using elevated temperatures or thermocompression bonding techniques. In another embodiment of the present invention shown in Figs. 3 and 5, a conductor 137 is formed 138 on a self- supporting bulk dielectric material 139 to form a layered bulk dielectric 142. The conductor 137 is thereafter attached 144 by suitable techniques, such as by an epoxy, to a mechanically robust, electrically insulating, low dielectric permittivity substrate 140, such as MgO, A1 2 0 3 , and LaA10 3 to provide a laminated structure 146. The thickness "T B of the bulk dielectric material 139 is reduced 150 as noted above and spaced apart conductors are formed 154 on the free surface of the bulk dielectric material to form the layered structure 106. As will be appreciated, a bulk dielectric material can be subjected to thickness reduction without being attached to a supporting substrate, depending upon the strength of the bulk dielectric material. The flip chip varactors can be in a variety of configurations. The simplest configurations are depicted in Figs. 6A-B and 7A-B. Referring to Figs. 6A and B, the varactor 180 uses a tunable dielectric material 184, preferably a thin or thick film dielectric material, deposited on a substrate 188 and including a pair of spaced apart conductors 192 and 196. Referring to Figs. 7A-B, the varactor 200 uses a self-supporting bulk dielectric material 204 supporting a pair of spaced apart conductors 208 and 212. Another embodiment of a varactor is depicted in Figs. 8A-B. The varactor 220 includes top and bottom conductors 224 and 228 positioned on opposing sides of, and separated by, a dielectric material 232, all of which is supported by the substrate 236. The conductors 224 and 228 are substantially orthogonal to one another to concentrate the tuning of the dielectric material at the area 240 of overlap between the conductors 224 and 228. At this location, the greatest amount of electromagnetic energy will pass through the dielectric material 232. Figs. 7A-B; 8A-B; and 9A-C depict varactors 250, and 300, and 350 using an non-tunable dielectric material 254 to prevent voltage breakdown across the gaps 258, 304, and 354 between the spaced apart conductors 262 and 266 (Fig. 9B) , 308 and 312 (Fig. 10B) , and 354 and 358 (Fig. 11B) . The dielectric permittivity of the non-tunable dielectric material 254 is preferably not a function of applied voltage and preferably is at least about 1200%, more preferably at least about 600%, and most preferably at least about 200% of the dielectric permittivity of air. The dielectric permittivity of the non-tunable dielectric material 254 is preferably no more than about 10% and more preferably no more than about 6% of the dielectric permittivity of the tunable dielectric material 270 (Fig. 9B) , 316 (Fig. 10B) , and 362 (Fig. 11A) . The electric breakdown strength of the non-tunable dielectric material 254 is preferably at least about 1 x 10 7 , more preferably at least about 3 x 10 7 , and most preferably at least about 1 x 10 8 volts/meter. These varactors are capable of handling a broad range of dc voltages before voltage breakdown of the non-tunable dielectric material. Referring to Figs. 9A-C, the varactor 350, unlike the varactors 250 and 300 of Figs. 9A-B and 10A-B, includes a pair of spaced apart secondary conductors 366 and 370 located above the non-tunable dielectric material 254 and the pair of spaced apart conductors 354 and 358. The secondary conductors 366 and 370 enable the non-tunable dielectric material 254 to fill completely the gap 374 between the conductors 354 and 358 while ensuring that the conductors 354 and 358 are in electrical contact (via the secondary conductors 366 and 370) with the metallization of the circuit to which the varactor 350 is attached. The thickness n T τ n of the non-tunable dielectric material is sufficient that the capacitances between the conductors 366 and 370 on the one hand and the conductors 354 and 358, respectively, on the other are preferably less than about 20%, more preferably less than about 10%, and most preferably less than about 5% of the capacitance between the conductors 354 and 358. In other words, the thickness T τ of the non-tunable dielectric material 254 is less than and more preferably less than about 10% of and most preferably less than about 5% of the width "W G " of the gap 374. At such capacitances between the upper and lower conductors, the capacitances between the upper and lower conductors serves as an electrical short circuit at RF and higher frequencies. To prevent voltage breakdown between the upper conductors 366 and 370, the gap 378 between the upper conductors 366 and 370 has a width "W s " that is more than the width "W G " of the gap 374. More preferably, W s is at least about 200% and most preferably at least about 500% of Figs. 12, 13A-B and 14 depict varactors having a small area contact to concentrate the electromagnetic energy to a portion of the dielectric material that is being capacitively tuned. This configuration is particularly useful for varactors utilizing bulk dielectric materials. Referring to Fig. 12, the varactor 400 includes an non- tunable dielectric material 404, a tunable dielectric material 408, and opposing spaced apart conductors 412 and 416. To concentrate the electromagnetic energy in the tuning area 420 between the conductors, the varactor 400 is configured such that C : and C 3 are less than about 80%, more preferably no more than about 20% of, and most preferably no more than about 10% of C 2 . In this manner, Cj and C 3 act as an electrical open circuit to C 2 . This enables the RF energy to be concentrated in the region 420, which has a capacitance C 4 . To accomplish this result, the thicknesses of the conductors "d^" and " d 3 " are each approximately equal to and more preferably within about 70% of the gap width " d 2 ", and the distances ά l r d 2 , and d 3 each are preferably no more than about 10 microns and more preferably ranging from about 2 to about 8 microns. Referring to Figs. 13A-B, the varactor 450 includes a dielectric material 454, contact pads 458 and 462, and interlaced, parallel finger conductors 466 a-d. As shown in Fig. 13B, the thicknesses of the conductors 466 a-d (i.e., tj, t 2 , t 3 , and t 4 ) are approximately the same and are approximately equal to the spacings between adjacent conductors (i.e., s^ s 2 , and s 3 ) . To act as an electrical short circuit to the capacitance between the contact pads 458 and 462, the capacitances between interlaced portion 470 of adjacent conductors 466 a-d are approximately equal and exceed, preferably by at least about 500% and more preferably by at least about 1000%, the capacitances between adjacent conductors 466 a-d in the portions of the conductors that are at a distance from the interlaced portion 470 and the capacitance between the contact pads 458 and 462. Accordingly, the capacitance between the interlaced portions of the conductors dominates the capacitances between the contact pads and portions of the conductors that are at a distance from the interlaced portions. Fig. 14 depicts a varactor 500 that employs parallel conductors 504 and 508 rather than interlaced conductors 466 a-d to concentrate the electromagnetic energy at a desired location, namely in the gap 512 between the parallel segments of the conductors 516 and 520. Referring to Figs. 15 and 16, varactors 550 and 600 are depicted which bias opposing surfaces of the dielectric material 554 and 604. In this manner, the bias voltage, X, is applied substantially throughout the portion of the dielectric material between the opposing conductors 558 and 562 (Fig. 15) and 608 and 612 (Fig. 16) . The varactor 600 of Fig. 16 further includes a substrate 616 located below the dielectric material 600. A via hole 620 containing conductive material 624 passes through the substrate 616 to facilitate biasing of the dielectric material 600. Finally, Fig. 17 depicts a varactor 650 includes spaced apart conductors 654 and 658, tunable thin film dielectric material 662, and tunable bulk dielectric material 666. The numerous grain boundaries and defects in the polycrystalline thin film dielectric material 662 capture charge carriers that would otherwise migrate into the (monocrystalline and substantially defect free) bulk dielectric material 666 and create a space charge effect. It is preferred that the bulk dielectric material 666 and thin film dielectric material 662 have substantially the same chemical composition to optimize lattice matching between the materials and therefore crystal growth in the thin film dielectric material. Yet another embodiment of a varactor is depicted in Figs. 18A and 18B. The varactor 700 is formed on a dielectric substrate (not shown) and includes spaced-apart electrodes 704 and 708, opposing non-tunable, low-loss dielectric materials 712 and 716 and a tunable dielectric material 720 positioned therebetween. Preferably, the tunable and non-tunable dielectric materials are thin film materials. As can be seen from Figs. 18A and 18B, the tunable dielectric material 720 has an hourglass configuration; that is, the material 720 has a width "W m " at its midpoint that is less than the width "W u " and "W," at either end of the dielectric material. As a result, the RF current (and electric flux lines) will be concentrated at the midpoint of the tunable dielectric material. When a dc voltage is applied, the electric flux lines spread latitudinally outward from the midpoint of the tunable material, thereby resulting in a portion of the electric flux lines passing through the non-tunable material. By varying the applied dc bias voltage, the respective amount of the RF current (and electric flux lines) passing through the non-tunable versus the tunable materials can be altered. By way of example, in a first mode shown in Fig. 18A in which no dc bias is applied, the RF current (and electric flux) is concentrated in the tunable material 720. By contrast, in a second mode shown in Fig. 18B in which a dc bias is applied across the electrodes 704 and 708, a larger fraction of the RF current (and electric flux) passes through the low loss, non-tunable materials 712 and 716 and a lesser fraction through the tunable material 720 than in the first mode. The relative amounts of the RF current (and electric flux) passing through the non-tunable versus tunable materials depends on the magnitude of the dc voltage. For higher dc voltages, more of the RF current (and electric flux) is concentrated in the non-tunable material than at lower dc voltages, and at higher dc voltages, a lesser amount of the RF current (and electric flux) is concentrated in the tunable material than at lesser voltages. EXPERIMENTAL Example 1 RF energy having a frequency of approximately 1 MHz was passed through a varactor having the varactor configuration of Figs. 7A-B. The varactor included a tunable bulk strontium titanate material. Fig. 19 shows the capacitance, tuning, and loss of the varactor. The capacitance of the varactor was unacceptably high for microwave applications. Example 2 In another experiment, varactors having the configuration shown in Fig. 14 were fabricated by depositing conductive electrodes directly onto bulk, unthinned strontium substrates which were 0.5 millimeters thick. Fig. 20A shows data obtained by inserting varactors into the microwave circuit of figure 20. Application of a dc bias voltage altered the resonant frequency, but extreme hysteresis and non-reproducibility of the varactor capacitance were observed when the dc tuning voltage was cycled over a range of values. This is a major drawback which renders these varactors useless for practical applications. Hysteresis in varactor capacitance and circuit resonant frequency was substantially lowered by depositing a thin (0.3 micron) strontium titanate film on the bulk strontium substrate prior to electrode deposition. Fig. 20B shows less hysteresis in resonant frequency and varactor capacitance. The changes in resonant frequency tuning were the result of changing the varactor capacitance over a range 9.5 - 17.7 picofarads with a dc bias voltage. This embodiment combines the defect concentration and thus low microwave loss of single-crystal strontium titanate materials with the low hysteresis properties of thin film strontium titanate varactors. Example 3 Varactors having the varactor configuration of Figs. 7A-B were fabricated using 0.3 μm thin film dielectric material with a composition of either SrTi0 3 (STO) or Ba 0 . 4 Sr o.6 Ti0 3 (BSTO) . The STO film was deposited on a pristine NdGa0 3 (NGO) substrate, and the BSTO film was deposited on a pristine LAO substrate. At a temperature of approximately 77°K, electromagnetic energy was passed through a two-port resonant structure (shown in Fig. 21) . Application of dc voltages from 0-50 volts tuned the resonant frequency of the circuit over a range of frequencies ranging from 0.94 - 1.73 GHz. As will be appreciated, tuning at microwave and millimeter-wave frequencies up to 60 GHz could be accomplished by changing the dimensions of the resonant circuit and the range of capacitance values over which the tunable dielectric varactors are tuned. The capacitance and quality factors of the STO varactors at 0.94 - 1.73 GHz were extracted from measurements of resonant frequency and Q values of the varactor-loaded resonators, and are shown in Fig. 22A. It's noteworthy that little or no hysteresis in capacitance as a function of dc voltage is observed. This is significant for tuning devices to the same frequency using a predetermined voltage, when the dc voltage is cycled among different values. Similar experiments were performed at 300 K, using BSTO varactors bonded into the tunable resonant structure near the RF voltage maximums. Figure 22B shows capacitance and quality factor data of the BSTO varactors, and the data was extracted from measured resonant frequency and Q value for the varactor-loaded resonator, where application of 0-50 dc volts across the varactors tuned the resonant frequency of the circuit from 1.27 - 2.02 GHz. As was the case for the STO-tuned circuit, tuning at microwave and millimeter-wave frequencies up to 60 GHz could be accomplished by changing the dimensions of the resonant circuit and the range of capacitance values over which the tunable BSTO dielectric varactors are tuned. The performance, in terms of tuning and microwave Q value, of varactors fabricated on pristine substrates exceeds that of varactors fabricated on substrates where an etching step has taken place. This is attributed to dual contamination and pitting caused by the etching step, which causes reduced microstructural quality in the tunable dielectric films which are subsequently deposited on these substrates. Hence the performance of tunable circuits using flip chip varactors generally exceeds that of monolithic circuits where the tunable dielectric film is deposited directly on a previously etched substrate. Example 4 A varactor having the configuration shown in Figs. 11A-C was fabricated using a strontium titanate thin film material. The varactor could maintain a dc electric field intensity in the strontium titanate film up to 1 x 10 6 V/m. At electric fields of up to about 8 x 10 7 V/m, the varactor did not experience thermal and/or avalanche breakdown of the strontium titanate material. The varactor had a gap width of approximately 2 to 3 microns, a length of approximately 1 millimeter and a thickness of the strontium titanate film of approximately 0.5 microns. With the varactor design, higher breakdown voltages can be obtained by increasing the gas width. Fig. 23 illustrates the dependance of capacitance on applied dc voltage (U) normalized to the width of the gap(s) between conductors measured at the frequency of approximately 1 MHz, where C(O) and C(U) are the measured capacitances at zero and non-zero dc voltage respectively. Curves 1 and 3 illustrate the change of capacitance at temperatures of approximately 300°K and 78°K, and curves 2 and 4 characterize the relative variation of dielectric permittivity under the dc electric field. Figs. 24A-D illustrate the behavior of the varactor when electromagnetic energy having a frequency of about 3 GHz is propagated through the varactor. The dielectric hysteresis of C(U) is relatively weak. Figs. 24C and D demonstrate the total dielectric loss of the varactor in the temperature range of approximately 78.3°K (Fig. 24C) and 78°K (Fig. 24D) . No changes of dielectric properties were observed under the dc electric field having an intensity of approximately 1 x 10 5 V/m. The varactors were able to operate with significantly higher dc voltages than conventional tunable varactors. This ability makes it possible to increase the controllability of the tuning to more than 1.5 to 2 times that of conventional varactors without thermal and/or avalanche breakdown. Example 5 In yet another experiment, a varactor was fabricated having the configuration in Figs. 5A-B using a mechanically thinned strontium titanate material attached to a lanthanum aluminate substrate, using epoxy as an adhesive. The mechanically thinned, bulk strontium titanate material had a thickness of about 50 microns. Varactors fabricated in this manner were inserted into the microwave circuit of Fig. 21, and the resonant frequency and quality factor of the circuit were measured as a function of dc bias. Fig. 25 shows that altering the dc bias from 0 to about + or - 100 volts caused the resonant frequency of the circuit to change from 0.6 to 0.95 GHz. These changes in resonant frequency were due to changes in the capacitance of the varactor from 17.7 to 9.5 picofarads. While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims.

Claims: What is claimed is: 1. A method for forming an electrically tunable circuit, comprising: (a) forming a layer of a dielectric material on a dielectric substrate, the dielectric material having a dielectric permittivity that is a function of a voltage applied to the dielectric material, on a substrate; (b) forming a pair of spaced apart conductive layers above the layer of dielectric material, the substrate being below the layer of dielectric material, to define a layered structure ; and (c) cutting the layered structure to form a plurality of varactors, each varactor having a portion of the substrate, a portion of the layer of dielectric material, and a portion of each of the spaced apart conductive layers such that the portion of the layer of dielectric material is located in a gap between the spaced apart conductive layer portions. 2. The method of Claim 1, wherein the substrate has a substrate lattice constant and the layer of the dielectric material has a dielectric lattice constant and the lattice mismatch between the substrate lattice constant and the dielectric lattice constant is no more than about 10%. 3. The method of Claim 1, wherein the substrate is selected from the group consisting of sapphire, magnesium oxide, lanthanum aluminate, neodymium gallate, and yttrium- stabilized zirconia. 4. The method of Claim 1, further comprising: bonding the spaced apart conductive layer portions of a varactor to a second conductive layer supported by a second substrate. 5. The method of Claim 1, wherein the forming step (a) comprises reducing the thickness of the layer of dielectric material. 6. The method of Claim 5, wherein the reduction is performed by one of mechanical techniques, chemical techniques, or a combination thereof. 7. The method of Claim 1, wherein the substrate is a dielectric material. 8. The method of Claim 7, wherein the substrate has a substrate dielectric permittivity and the layer of dielectric material has a tunable dielectric permittivity and the substrate dielectric permittivity is less than the tunable dielectric permittivity. 9. The method of Claim 1 , wherein the substrate has a substrate crystallographic structure and the layer of dielectric material has a dielectric crystallographic structure and the crystallographic structure and the dielectric crystallographic structure are the same. 10. The method of Claim 1, wherein the substrate has a substrate coefficient of thermal expansion and the layer of dielectric material has a dielectric coefficient of thermal expansion and the difference between the dielectric coefficient of thermal expansion of the dielectric material and substrate ranges from about 2% to about 30% of the substrate coefficient of thermal expansion. 11. A method for forming a tunable electromagnetic device, comprising: mechanically reducing the thickness of a dielectric material. 12. The method of Claim 11, wherein the thickness of the dielectric material is at least about 5 microns. 13. The method of Claim 11, further comprising, after the mechanically reducing step, bonding the dielectric material with a conductor on a substrate. 14. The method of Claim 11, further comprising, before the mechanically reducing step, forming a conductor on the dielectric material. 15. The method of Claim 14, further comprising, before the mechanically reducing step, attaching the conductor to a substrate. 16. The method of Claim 11, wherein the dielectric material has a dielectric permittivity that is a function of a voltage applied to the dielectric material and further comprising forming a pair of spaced apart conductors on the dielectric material to define a capacitance therebetween. 17. A device for electrically tuning electromagnetic energy, comprising: an electrically insulating substrate having a substrate dielectric permittivity; a layer of a dielectric material supported by the electrically insulating substrate, the dielectric material having a tunable dielectric permittivity that is a function of a voltage applied to the dielectric material, the tunable dielectric permittivity being greater than the substrate dielectric permittivity; at least a pair of spaced apart conductors located above the layer of dielectric material, at least a portion of the layer of dielectric material being located in a gap between the at least a pair of spaced apart conductors; and a layer of a second dielectric material, at least a portion of which is located in the gap above the layer of the dielectric material, the second dielectric material having a second dielectric permittivity that is less than the tunable dielectric permittivity. 18. The device of Claim 17, wherein the second dielectric permittivity is at least about 200% of the dielectric permittivity of air. 19. The device of Claim 17 , further comprising a second pair of spaced apart conductors separated by a second gap, each of the conductors in the pair of spaced apart conductors being located above a corresponding conductor in the at least a pair of spaced apart conductors and being separated from the corresponding conductor by the second dielectric material. 20. The device of Claim 17, wherein the gap has a width and the second gap has a second width and the width is less than the second width. 21. The device of Claim 20, wherein the width is no more than about 50% of the second width. 22. The device of Claim 20, wherein the second dielectric permittivity is independent of a voltage applied to the second dielectric material. 23. The device of Claim 20, wherein the second dielectric material has an electric breakdown strength and the electric breakdown strength is at least about 1 x 10 7 volts/meter. 24. The device of claim 22, wherein the at least a pair of spaced apart conductors define a first capacitance across the gap and a conductor in the second pair of spaced apart conductors and the corresponding conductor in the at least a pair of spaced apart conductors define a second capacitance across the second dielectric material located therebetween and the second capacitance is less than about 20% of the first capacitance. 25. The device of Claim 24, wherein the other conductor in the second pair of spaced apart conductors and the corresponding conductor in the at least a pair of spaced apart conductors define a third capacitance across the second dielectric material located therebetween and the third capacitance is at least about 500% of the first capacitance. 26. The device of Claim 17, wherein the tunable dielectric permittivity is varied by applying a bias to opposing surfaces of the layer of dielectric material. 27. A device for electrically tuning electromagnetic energy, comprising: a layer of a dielectric material, the dielectric material having a tunable dielectric permittivity that is a function of a voltage applied to the dielectric material; a layer of a second dielectric material located above the layer of dielectric material, the second dielectric material having a second dielectric permittivity that is independent of a voltage applied to the second dielectric material; and at least a pair of spaced apart conductors located above the layer of second dielectric material, at least a portion of the layer of dielectric material being located in a gap between the at least a pair of spaced apart conductors, wherein the second dielectric permittivity is less than the tunable dielectric permittivity. 28. The device of Claim 27, wherein the second dielectric permittivity is no more than about 10% of the tunable dielectric permittivity. 29. The device of Claim 27, wherein the spaced apart conductors are separated by a gap having a gap width and wherein the gap width is at least about 70% but no more than about 10,000% of the thickness of each conductor in the at least a pair of spaced apart conductors. 30. The device of Claim 29, wherein gap width ranges from about 1 to about 100 microns. 31. The device of Claim 27, wherein the dielectric material is a self-supporting bulk dielectric material. 32. An electrically tunable varactor, comprising: a layer of a dielectric material, the dielectric material having a tunable dielectric permittivity that is a function of a voltage applied to the dielectric material; and a pair of spaced apart conductors located above the layer of dielectric material, at least a portion of the layer of dielectric material being located in a gap between the at least a pair of spaced apart conductors, wherein (a) first portions of each of the spaced apart conductors are adjacent to one another, the first portions being separated by the gap, which has a gap width, and defining a first capacitance, and (b) second portions of each of the spaced apart conductors are separated from one another by the first portions, the second portions being separated by a gap having a second gap width, which is greater than the gap width, and defining a second capacitance, wherein the first capacitance is more than the second capacitance. 33. The electrically tunable varactor of Claim 32, wherein the dielectric material is a self-supporting bulk dielectric material. 34. The electrically tunable varactor of Claim 32, wherein the first capacitance is at least about 500% of the second capacitance. 35. The electrically tunable varactor of Claim 32, wherein the first capacitance is electrically connected in parallel with the second capacitance. 36. The electrically tunable varactor of Claim 32, wherein each of the first portions have a first width and the first width of each of the first portions is at least about 10% and no more than about 200% of the gap width. 37. An electrically tunable varactor, comprising: a bulk dielectric material having a bulk dielectric permittivity that is a function of a voltage applied to the bulk dielectric material; a thin film dielectric material deposited on the bulk dielectric material; and a pair of spaced apart conductors, wherein the bulk dielectric material and pair of spaced apart conductors are located on opposing sides of the thin film dielectric material. 38. The electrically tunable varactor of Claim 37, wherein the dielectric permittivity of the thin film dielectric material is at least about 1,000. 39. The electrically tunable varactor of Claim 37, wherein the dielectric permittivity of the thin film dielectric material is at least about 75% of the dielectric permittivity of the bulk dielectric material. 40. The electrically tunable varactor of Claim 37, wherein the thickness of the thin film dielectric material is no more than about one micron. 41. The electrically tunable varactor of Claim 37, wherein the thin film dielectric material and the bulk dielectric material have substantially the same chemical composition. 1/27 FIG. 1 2/27 FIG. 2

Inventors: GALT DAVID, KOEPF GERHARD A, KOZYREV ANDREY B, MUELLER CARL H, PRICE JOHN C, PRUDAN ALEXANDER, RIVKINA TITIANA, ROGERS CHARLES A, GALT, DAVID, KOEPF, GERHARD, A., KOZYREV, ANDREY, B., MUELLER, CARL, H., PRICE, JOHN, C., PRUDAN, ALEXANDER, RIVKINA, TITIANA, ROGERS, CHARLES, A.
Applicants: GALT DAVID, KOEPF GERHARD A, KOZYREV ANDREY B, MUELLER CARL H, PRICE JOHN C, PRUDAN ALEXANDER, RIVKINA TITIANA, ROGERS CHARLES A, SUPERCONDUCTING CORE TECHNOLOG, GALT, DAVID, KOEPF, GERHARD, A., KOZYREV, ANDREY, B., MUELLER, CARL, H., PRICE, JOHN, C., PRUDAN, ALEXANDER, RIVKINA, TITIANA, ROGERS, CHARLES, A., SUPERCONDUCTING CORE TECHNOLOGIES, INC.
Application year: 1997
Main classification: H03B
IPC codes: H01L27