A novel approach to modulating the inductance of a superconducting microstrip is described. This approach could be the basis for numerous practical applications, such as phase-shifters and high-frequency tuning elements. The physical mechanisms involved are quasiparticle injection, gap suppression, and penetration-depth modulation. The authors have investigated the modulation of the penetration depth of niobium and niobium nitride films by excess quasiparticle injection. To this effect, all niobium and all-niobium-nitride SQUID (superconducting quantum interference device) circuits were designed and fabricated. These circuits allow quasiparticle injection into the inductive element of the SQUID. This injection is achieved by optical irradiation through an opening in a Nb reflective layer which partially masks the rest of the circuit or electronic current injection through a tunnel junction overlaid on the microstrip inductance. Penetration-depth modulation is achieved with both methods. The magnitude of the effect varies from 10% to over 200% change in inductance.>
A novel approach to modulating the inductance of a superconducting microstrip is described. This approach could be the basis for numerous practical applications, such as phase-shifters and high-frequency tuning elements. The physical mechanisms involved are quasiparticle injection, gap suppression, and penetration-depth modulation. The authors have investigated the modulation of the penetration depth of niobium and niobium nitride films by excess quasiparticle injection. To this effect, all niobium and all-niobium-nitride SQUID (superconducting quantum interference device) circuits were designed and fabricated. These circuits allow quasiparticle injection into the inductive element of the SQUID. This injection is achieved by optical irradiation through an opening in a Nb reflective layer which partially masks the rest of the circuit or electronic current injection through a tunnel junction overlaid on the microstrip inductance. Penetration-depth modulation is achieved with both methods. The magnitude of the effect varies from 10% to over 200% change in inductance.>
Turnable superconducting millimeter-wave phase shifters are discussed. The tuning mechanism is based on inductance modulation of NbN microstrips by excess quasiparticle injection. When measured by a DC method, using the interference patterns and resonances of specially designed DC SQUIDs, the inductance of NbN microstrips can be varied by up to 300%. Based on these results, superconducting microstrip interferometers to operate as low-power 100 GHz phase shifters were designed and fabricated. The 100 GHz signal is coupled into and out of the interferometers by finline antennas. Amplitude modulation of the output is used as the criterion for phase shift in one of the interferometer branches. Thin superconducting films (thinner than the penetration depth) and dielectric layers are needed to achieve tunability of the inductance. The conflicting requirements of tunability and efficient signal coupling are quantified in the interferometer testing, and the range of parameter optimization is bracketed. The elements of this required optimization are presented.
In recent years, rapid single flux quantum (RSFQ) circuits, which are promising for the development of high-speed digital electronics, have attracted widespread interest. Niobium nitride (NbN) films have also attracted significant attention for high-frequency applications in superconducting electronics because of their superior material parameters, including their high critical temperature Tc and large gap voltage Vg. To enable the development of all-NbN SFQ circuits, we have developed a fabrication process based on NbN junctions and inductors made of NbN strip lines. It is important to evaluate the inductance because it can affect the performance of an RSFQ circuit and is essential for the RSFQ design. We measured the inductance properties of both epitaxial and polycrystalline NbN strip lines using a superconducting quantum interference device modulation technique at 4.2 K and obtained the width dependence on the inductance of the NbN films. The penetration depth was then calculated from these measurements. The NbN inductance values were calculated numerically with respect to the penetration depth and were then compared with the measured values from the experiments. It was demonstrated that the differences between these values were within 5%. The results reported here will be useful for the design and fabrication of all-NbN RSFQ circuits.
Abstract In this paper, we report on a systematic study of the inductance and magnetic field penetration depth (λ) of polycrystalline NbN superconducting thin films. By employing a four-metal-layer fabrication process specifically designed for all-NbN single-flux-quantum circuits, we constructed a superconducting quantum interference device loop composed of two parallel NbN SNS junctions, NbN microstrips, and NbN ground planes for precise inductance measurement. At 4.2 K, as the linewidth increases from 1 μm to 30 μm, the inductance per unit length (L u) of NbN microstrips significantly decreases, for example, the L u of 250 nm thick NbN microstrips drops from 0.907 pH/μm to 0.047 pH/μm. Compared to Nb, the L u of polycrystalline NbN microstrips is approximately two to three times that of Nb, offering an advantage for manufacturing smaller superconducting inductors. Furthermore, we conducted simulation analysis using InductEx software to extract the λ of NbN films of varying thicknesses. The results indicate that as the film thickness increased from 45 nm to 600 nm, λ initially decreased sharply and then stabilized, with values ranging from 430 nm to 323 nm. Notably, once the film thickness exceeded 200 nm, λ remained essentially constant, even at a temperature of 10 K, where it showed good stability, albeit with a slight increase (about 50 nm) compared to 4.2 K. This dependence of λ on thickness is reasonably explained by considering the effects of NbN film thickness on the superconducting critical temperature and residual resistivity. These research findings not only deepen our understanding of the characteristics of superconducting films but also lay a solid foundation for the future design and manufacture of more compact superconducting circuits at the higher temperature of 10 K.
Abstract Superconducting quantum interference devices (SQUIDs) with 3D nano-bridge junctions can be miniaturized into nano-SQUIDs that are able to sense a few spins in a large magnetic field. Among all device parameters, the inductance is key to the performance of SQUIDs with 3D nano-bridge junctions. Here, we measured the critical-current magnetic flux modulation curves of 12 devices with three design types using a current strip-line directly coupled to the SQUID loop. A best flux modulation depth of 71% was achieved for our 3D Nb SQUID. From the modulation curves, we extracted the inductance values of the current stripe-line in each design and compared them with the corresponding simulation results of InductEX. In this way, London penetration depths of 110 and 420 nm were determined for our Nb (niobium) and NbN (niobium nitride) films, respectively. Furthermore, we showed that inductances of 11 and 119 pH for Nb and NbN 3D nano-bridge junctions, respectively, dominated the total inductance of our SQUID loops which are 23 pH for Nb and 255 pH for NbN. A screening parameter being equal to one suggests optimal critical currents of 89.6 and 8.1 μ A for Nb and NbN SQUIDs, respectively. Additionally, intrinsic flux noise of 110 ± 40 n Φ 0 /(Hz) 1/2 is calculated for the Nb SQUIDs with 3D nano-bridge junctions by Langevin simulation.
At frequencies above the gap frequency of niobium (710 GHz), the performance of Nb/A10x/Nb SIS mixers degrades and the losses in the niobium films of the superconductive Nb/SiO/Nb microstrip circuits used to provide a good RF impedance match severely limit the performance. Therefore, we are investigating the properties of NbN mixers, and in particular of NbN/SiO/NbN microstrip circuits integrated with submicron IlbN/Mg0/NbN tunnel junctions. The very large penetration depth of NbN (3000-4000 A) makes the circuit design difficult and also puts critical constraints on alignment during fabrication. We present here the design of several rf tuning circuits for 630 GHz including novel symmetric designs which ease the alignment tolerances. In addition, a new approach for determining experimentally the penetration depth in the NbN films is presented.
Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current-voltage characteristics in the absence of any shunting resistance.