Microwave Power Handling Research Articles

Engineering nonlinear response of superconducting niobium microstrip resonators via aluminum cladding

In this work, we find that Al cladding on Nb microstrip resonators is an efficient way to suppress nonlinear responses induced by local Joule heating, resulting in improved microwave power handling capability. This improvement is likely due to the proximity effect between the Al and the Nb layers. The proximity effect is found to be controllable by tuning the thickness of the Al layer. We show that improving the film quality is also helpful as it enhances the microwave critical current density, but it cannot eliminate the local heating.

A comparative study of microwave power handling capabilities of Tl-1212 and Hg-1212superconducting microstrip lines

The microwave power handing capabilityPc of the microstrip lines of the isomorphic ‘1212’ pair is studied for the superconductorsTlBa2CaCu2O7 andHgBa2CaCu2O6+δ at≥77 K.The Pc for the pair follows the same temperature dependence at a reduced temperature scale(t = T/Tc). A comparison of the dc critical current densityJc and theradio-frequency one Jrf derived from the Pc of the ‘1212’ pair reveals a similar reduced temperature dependence, suggesting that the depinningof the magnetic vortices, which is dictated by the ‘1212’ structure, pays a similar role in limitingJc andJrf in theproximity of Tc.

Simultaneous electro‐thermal experimental analysis of RF‐MEMS switches for high microwave power handling

AbstractAn experimental setup for the characterization of electromagnetically induced heat on MEMS devices operating in high RF power regimes (> 5 W) is here proposed. The technique is based on infrared (IR) imaging of on‐probe DUT, in working conditions. The present work focuses on switches. Undoubtedly the switch represents the most emblematic and strategic component across the RF‐MEMS device portfolio. The capacitive ones can withstand more RF power with respect to DC contact (or ohmic) counterparts. Here, both serial and shunt configurations will be considered in the two working states (ON‐OFF). The approach presented here is based on a non‐intrusive real‐time IR imaging method capable to provide high resolution thermal maps and thermal transients of DUTs undertaking up to 6.3 W of CW incident power at 10 GHz. This technique represents a very powerful approach toward design optimization on the one hand and fast testing and product quality assessment against RF induced failures in production line for integrated RF MEMS and MMIC front‐end. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The mechanism of nonlinear microwave behaviour inhigh-Tc superconducting devices

First, this paper gives an overview of possible origins and mechanisms that might beresponsible for experimentally observed nonlinear effects. Focusing on well-known physicalloss mechanisms with nonlinear characteristics, their impact on microwave properties andtheir relevance in microwave devices are discussed. Secondly, an experimental investigationof the power handling capability of coplanar resonators in additionally applied dc magneticfields in zero field cooled, field cooled, and field sweep experiments is presented. Theseexperiments demonstrate that the microwave power handling capability of optimizedhigh-Tc thin film devices is limited by a maximum current density, andsuggest that the limitation is caused by the dc critical current densityJc of the superconducting material.

Microwave properties of MgB/sub 2/ thin films

The microwave surface impedance Z/sub s/=R/sub s/+j/spl omega//spl mu//sub 0//spl lambda/ of MgB/sub 2/ thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth /spl lambda/ measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to T/sub c/ by the standard BCS integral expression assuming the reduced energy gap /spl Delta/(0)/kT/sub c/ to be as low as 1.0-1.1 assuming /spl lambda/(0)=100--110 nm. These results clearly indicate the s-wave nature of the order parameter. Similar good fits were achieved by an anisotropic one gap and an isotropic two-gap model. Second, the temperature dependence of surface resistance R/sub s/, as measured with a rutile puck, indicates an exponential behavior below about T/sub c//2 with a reduced energy gap being consistent with the one determined from the /spl lambda/ data. The R/sub s/ value at 4.2 K was found to be as low as 19 /spl mu//spl Omega/ at 7.2 GHz, which is comparable with that of a high-quality high temperature thin films of YBa/sub 2/Cu/sub 3/O/sub 7/. A higher-order mode at 17.9 GHz was employed to investigate the frequency f dependence of R/sub s//spl alpha/f/sup n(T)/. Our results revealed an decrease of n with increasing temperature ranging from n=2 below 8 K to n=1 close to T/sub c/. Finally, the microwave power handling of MgB/sub 2/ films was deduced and compared with values for YBa/sub 2/Cu/sub 3/O/sub 7/ films. We found that the power handling of MgB/sub 2/ is comparable or even better than that of YBa/sub 2/Cu/sub 3/O/sub 7/ films for temperature below 30 K.

Nonlinear microwave properties of HTS thin film coplanar devices

It is demonstrated that the combination of vortex matter and rf measurements yields new insight into the microwave properties of superconducting thin film devices, both in small magnetic fields and zero field. The comparison of field-cooled and different types of field sweep experiments on coplanar high-T/sub c/ thin film resonators shows that the microwave properties strongly depend on magnetization and vortex distribution in the superconducting film as well as on the way, the magnetic field is approached. Thus, using vortices as a kind of local probe for the microwave properties leads to a consistent explanation of the microwave power handling in nonzero and zero magnetic field. In a model that is derived from the experiments, the nonlinear microwave behavior is explained by the limitation of the total current density in the device considering contributions of the rf field and the screening of the magnetic field and vortices to the current. The limiting current value seems to be related to the dc critical current of the superconductor.

Probing microwave properties of high-Tc films via small dc magnetic fields

It is demonstrated that the combination of vortex matter and rf measurements yields new insight into the microwave properties of superconducting thin-film devices, both in small magnetic fields and zero field. The comparison of field-cooled and different types of field-sweep experiments on coplanar high-Tc thin-film resonators shows that the microwave properties strongly depend on magnetization and vortex distribution in the superconducting film as well. Thus, using vortices as a kind of local probe for the microwave properties leads to a consistent explanation of the microwave power handling in nonzero and zero magnetic fields. In a model that is derived from the experiments, the nonlinear microwave behavior is explained by the limitation of the total current density in the device considering contributions of the rf field and the screening of the magnetic field and vortices to the current. The limiting current value seems to be related to the dc critical current of the superconductor.

Microwave characterization of HgBa 2CaCu 2O 6+δ thin films

We assessed the potential of Hg-based high-T c superconducting thin films for microwave applications by measuring its microwave surface resistance R s and microwave power handling capability. HgBa 2CaCu 2O 6+δ (Hg-1212) epitaxial thin films were fabricated using cation-exchange. They are c-axis oriented with thickness of about 0.3 μm. Their T c s are typically ∼ 120 K. On un-patterned films, we obtained an R s of 0.3 mΩ at 10 GHz and 100 K using Nb cavity perturbation technique. For power handling measurements, we fabricated Hg-1212 microstrip structure and determined the maximum power it could handle at 1 GHz. At 100 K, output power was very stable up to 19 dBm (critical operating power level) and above that level, non-linearity between input and output power was observed. When temperature was increased to 112 K, critical power level dropped to 16 dBm. These microwave properties suggest that Hg-1212 films are very promising for microwave device applications above 100 K.

Microwave power handling capability of HTS superconducting thin films: weak links and thermal effects induced limitation

Microwave power-handling capabilities of YBCO thin superconducting films, up to 150 W of the input power, have been investigated using a 14 GHz shielded dielectric cavity. The YBCO films were deposited by the thermal reactive co-evaporation and the dc sputtering method on LaAlO/sub 3/, NdGaO/sub 3/, and Al/sub 2/O/sub 3/ substrates. A simple nonlinear R(P)-L(P)-equivalent circuit model was used to simulate a nonlinear cavity response. The correlation of nonlinearity with global and local heating is discussed. The analysis of experimental data confirms, that for all films, the heating of weak links switched to normal state is one of the rf power handling capability major limitations.

Microstructure and microwave surface resistance oftypical YBaCuO thin films on sapphire andLaAlO3

Epitaxial c-oriented YBCO films laser deposited onto 3 in diameter CeO2-buffered sapphire wafers and LaAlO3 cylinders as well as sputter deposited onto 2 in diameter LaAlO3 wafers were characterized by integral and spatially resolved measurements of the critical current density jc and the microwave surface resistance Rs, by microstructure investigations using optical and electron microscopy and by x-ray diffraction. Epitaxial misorientations of in-plane-rotated as well as of a-axis-oriented grains were found in amounts up to 10%. The in-plane rotation seriously degraded Rs while the a orientation mainly lowered jc. Moreover, a degradation of Rs and of the microwave power handling could be clearly correlated with the density of microcracks occasionally found in YBCO films on sapphire. Inhomogeneities like a-axis-oriented grains were observed to `disperse' microcracks, probably in favour of the electrical properties. The impact of further microstructure imperfections on Rs, in particular of the typical twin lamellae and their domains, is discussed in view of findings from transmission electron microscopy.

The microwave power handling of an FIB-generated weak link in a YBCO film

We have measured the power-dependent microwave properties of a weak link in a YBa2Cu3O7- thin film formed by writing a line of damage using a focused ion beam. The measurement was made using a parallel plate resonator at 5.5 GHz with the weak link written across the width of one of the plates. The ion-induced damage was characterized using a TRIM computer simulation and the dc properties of similar weak links were measured. Using a 200 eV Si ion dose of 2 × 1013 cm-2, Tc of the damaged region was reduced by 5.5 K and the normal resistivity was doubled. Surprisingly, the microwave measurements did not show any Josephson junction characteristics. Rather, the ion-damaged region exhibited a greatly increased microwave resistivity that was constant as a function of microwave power up to rf fields of 20 mT at 21 K.

Microwave power handling in engineered YBa2Cu3O7−δ grain boundaries

Microwave-frequency (rf) power-dependence measurements performed on thin-film YBa2Cu3O7−δ grain boundaries engineered on sapphire bicrystal substrates with misorientation angles of θ=2°, 5°, 10°, and 24° are presented. The data are compared to measurements on films grown on single-crystal substrates. A stripline-resonator measurement technique is employed. The rf results are compared to dc measurements performed on a four-point test structure on the same substrate as the resonator. The measurements demonstrate that low-angle grain boundaries (θ⩽10°) have little effect on the rf power handling, while the high-angle grain boundaries (θ=24°) cause large nonlinear losses due to Josephson vortices created by rf currents.

Microwave applications of high-temperature superconductors

The major advance in the area of high temperature superconducting microwave devices has been in the incorporation of high performance devices (based on planar transmission line structures or shielded dielectric resonators) into microwave receiver systems with performances equivalent or better than those of conventional systems. These systems include reliable closed-cycle coolers that can allow access to the low temperatures required for efficient operation (<80K). Nonlinear effects at high powers have previously presented a drawback for these novel devices, but superconducting thin film disc resonators, operating in modes with radial current flow (in addition to compact, shielded dielectric resonators), do not suffer from the very high microwave edge current densities present in planar structures, and have demonstrated very encouraging microwave power handling capabilities.

Tunnel diode with asymmetric spacer layers for use as microwave detector

A novel single-barrier, intraband tunnel diodes with asymmetric spacer layers has been designed and fabricated. The structural asymmetry leads to asymmetric current-voltage characteristics and makes the diodes suitable for use as zero-bias microwave detectors. The voltage sensitivity as 9.375 GHz shows a weak temperature dependence (typically less than 1 dB variation over − 40° –+ 80°C, compared with 3 dB for a zero-bias Schottky diode). The maximum microwave power handling is also superior to a germanium back diode (1 dB rolloff typically at + 10 dBm compared with –10 dBm for the Ge back diode) and similar to a zero-bias Schottky diode.