|
Monolithic Integration of High-Temperature
Superconductor Quanxi Jia Superconductivity Technology Center
Since the discovery of high-temperature superconducting (HTS) materials, electrically tunable and active microwave device applications of these materials have been explored both experimentally and theoretically. Passive microwave elements incorporating HTS films have started to find application in high quality filters, resonators, delay lines, etc., operating at or around the liquid nitrogen temperature. Another emerging area where the HTS materials play a big role is in tunable microwave electronics. It is desirable for many microwave applications to tune the device characteristics. The fundamental aspect of a tunable microwave device is based on the change of the microwave propagation factor by physical, electrical, or magnetic means. For example, electric tuning employs the nonlinearity of ferroelectric materials under DC bias, whereas magnetic tuning utilizes the nonlinear magnetization of the ferrite under a magnetic field. When an electromagnetic wave travels in a medium of dielectric constant e and magnetic susceptibility m, its phase speed is vp = (em)-1/2 and its intrinsic impedance is Z = (m/e)1/2. When the media include a nonlinear dielectric, such as SrTiO3 (STO), applying an electric field will change its e. Similarly, when a ferrimagnetic material, such as ferrite, Y3Fe5O12 (YIG), is incorporated in the media, applying a magnetic field will change its m. The frequency and phase characteristics of the microwave device can thus be tuned. The drawback of these devices operated at room temperature, however, is their higher signal losses from the patterned metal circuits (typically Cu). Microwave propagation can be dramatically improved if superconductors can be used to replace the normal conductors. From a materials point of view, the successful demonstration of high performance active microwave devices relies on the full integration of high quality tunable media with the superconductors. In other words, the ultimate goal of monolithic integration of superconductors with tunable media is to combine dissimilar materials with complementary functionalities on a single platform and to have optimum performance from each layer. YBCO/SrTiO3 on a single crystal substrate for electrically tunable devices Ideal performance of functional materials is best achieved by minimizing crystallographic imperfections such as grain boundaries, and chemical imperfections such as variations in the stoichiometry. To this end, single crystal substrates are often used as the starting templates. For example, excellent structural and chemical compatibilities make it possible to grow high quality STO on LaAlO3 or MgO substrates. YBCO can be epitaxially grown on STO thereafter. Figure 1 shows the microwave reflection S11 and transmission S21 vs frequency at 4 K under varying bias voltages (average values applied at each pole) for a 3-pole half-wave bandpass coplanar waveguide (CPW) filter incorporating a 1.2 mm-thick STO layer and a 0.4 mm-thick YBCO electrode layer on a LaAlO3 substrate, where the gap between the centerline and the ground-plane is 30 mm [1]. If the broadband tuning of the filter is not required, one would be better off using much thinner STO films, still allowing fine tuning of the filter profile. The use of thinner STO films as well as large dc biases should reduce dielectric losses significantly. Also, the required bias voltages can be reduced by designing circuits with smaller centerline-to-ground plane gaps.
YBCO on polycrystalline YIG for magnetically tunable devices Ferrite components have traditionally played important roles in microwave systems. Their inherent capability to meet high power-handling requirements with high efficiency has made them the preferred technology for numerous radar applications. It has been demonstrated that the optimum coupling of rf signal in the superconductor to the ferrite occurs where a monolithic structure is used in which the superconductor is deposited directly on the ferrite substrate [2]. This configuration also simplifies device design and packaging. The deposition of high quality YBCO on polycrystalline YIG represents an advance of great technical significance. Ceramic substrates are not only readily available in sizes needed for devices, but they are dramatically less expensive than single-crystal substrates that for many chemical compositions may not even be attainable by known processing methods. However, the deposition of device quality YBCO on a polycrystalline YIG substrate can be a formidable challenge because of the inherent crystallographic incompatibility of the two materials and the lack of an epitaxial template for the growth of well-oriented YBCO. Recently, it has been demonstrated that highly oriented YBCO films can be grown on a biaxially textured YSZ layer that is deposited by an ion-beam-assisted-deposition (IBAD) process [3]. Figure 2 schematically shows the filter fabricated from YBCO on polycrystalline ferrite YIG with an IBAD-YSZ template as described in [3]. The tunability is what has been expected from the YIG, and the excess loss is believed to be from the ferrite [4].
YBCO/SrTiO3 on polycrystalline YIG for dual-tuning devices Ferrite and ferroelectric materials individually provide the magnetic and the electrical tunability for adaptive microwave devices, respectively. The adaptability will increase significantly if a microwave device can be simultaneously tuned by both magnetic and electrical techniques. In the tunable filter case, for example, the dual-tuning allows not only for fine tuning of the filter profile to achieve symmetric and optimum filter characteristics electrically but also for broadband magnetic tuning of the filter passband to demonstrate adaptive filter response over a wide frequency range. Figure 3 shows a cross-sectional view of the architecture used for integration among polycrystalline YIG, STO, and YBCO. Using a template of biaxially oriented MgO, deposited by an IBAD technique, device quality STO and YBCO films on polycrystalline YIG substrates have demonstrated [5]. The effort to fabricate high performance devices based on this monolithic integrated system is underway.
|
