J.W. Tao

Extraction of material parameters in NbN multilayer technology for RSFQ circuits

Abstract NbN/MgO/NbN Josephson tunnel junctions and multilayers have been achieved on 3 inch. Si and SOI substrates in order to develop high-speed RSFQ logic gates with terahertz cut-off frequency junctions and high operating temperature (10 K). Small area junctions (∼1 μm 2 ) with 20 kA/cm 2 J C and low sub-gap leakage current ( V m >15 mV) at 4.2 K (and J C ∼10 kA/cm 2 at 12 K) are reproducibly obtained from multilayers post-annealed at 250°C. Deposition of a thin MgO buffer layer underneath the NbN base electrode is critical to insure lower NbN surface resistance values ( R s =7 μΩ at 10 GHz and 5 K) and lower London penetration depth λ 0 (∼270 nm). NbN penetration depth and intrinsic capacitance of NbN/MgO/NbN junctions have been investigated by measuring resonant Fiske steps in the I – V curves of junctions coupled to an integrated resonant stub. The extension of our present NbN technology for fabricating highly integrated planarized RSFQ circuits, based on self-shunted 0.5 μm 2 area, 20 kA/cm 2 junctions is now made possible on large area substrates (up to 8 inch).

On-chip high-frequency diagnostic of RSFQ logic cells

Hilbert Transform spectroscopy is described as a method to analyze the spectrum of pulse trains generated by RSFQ circuits. Simulations are carried out using parameters appropriate for NbN Josephson junctions for both generation and detection of SFQ pulse trains. It is shown that the pulse shape along with the pulse train repetition rate can be extracted through the use of a Josephson junction, used as a spectrometer, and located on-chip with the RSFQ circuit to test.

A simple and systematic method for the design of tapered nonlinear transmission lines

In this paper, we present an original method, simple to implement, rapid and systematic for the conception of Nonlinear Transmission Lines (NLTL) for shock wave generation. This method is based on SPICE simulations and allows the synthesis of hybrid or monolithic NLTLs.

Multimodal variational analysis of quasiplanar waveguide discontinuities

Multimodal variational analysis will be presented for step discontinuity of some commonly used quasiplanar structures. Appreciable reduction in the computation time and computer memory space can be obtained by handling differently the modes which can be accessible from a certain distance to the discontinuity plane and those localized. An analysis program developed in FORTRAN on a PC computer will be applied to microstrip, finline and coplanar discontinuities, and compared to previously published results and experiments.

Some rules for the choice of the C(V) characteristic for the design of frequency triplers with symmetrical varactors

In this paper we consider multipliers designed with varactors that have a symmetric C(V) capacitance-voltage characteristic, i.e. triplers, quintuplers, ... We show that for a tripler the optimal C(V) characteristic is not the most abrupt one, as stated in much works, but rather a cosine-like one. Our work is validated with the design of a frequency tripler based on the use of HBVs non-linear transmission lines. We obtained a significant improvement for the maximum conversion efficiency when a cosine C(V) is used instead of an abrupt one, for a 15 HBVs NLTL frequency tripler.

FDTD and SPICE Simulations for Lossy and Dispersive Nonlinear Transmission Lines Used for Pulse Compression : a Comparison

For the first time, we demonstrate the simulation of lossy and dispersive non linear transmission lines (NLTLs) used for pulse compression, by two different time-domain approaches: SPICE and a full wave 3D FDTD. Results show a good agreement between the two approaches. Output pulse risetime is very affected by DC and skin-effect losses.

Comparison of Fully Distributed and Periodically Loaded Nonlinear Transmission Lines

Two different approaches to realizing nonlinear transmission lines (NLTLs) are compared in detail: those using fully-distributed and those employing periodically-loaded nonlinearities. Hetrostructure-barrier varactor measured characteristics are used as the nonlinear elements in the pulse-compression and harmonic-generation (20-60GHz tripler) simulations. We point out that the choice of simulation step size is critical in the case of fully-distributed NLTLs. It must be chosen so that no numerical Bragg cutoff frequency appears. For the frequency tripler, simulations show that with periodically loaded NLTLs, 21% efficiency at 210mW output power and 30% bandwidth can be obtained, whereas only 4.8% efficiency is possible using fully-distributed NLTLs. For pulse compression, the results are comparable but in the fully-distributed case the NLTL cannot be matched and significant reflections occur at the NLTL far end.

Fullwave frequency dependent inductance estimation for RSFQ applications in NbN multilayer technology

A frequency dependent inductance estimation has been carried out for Rapid-Single-Flux Quantum (RSFQ) applications in NbN multilayer technology up to frequencies above the NbN gap frequency. For this purpose an extension of the fullwave modified transverse resonance method (MTRM) has been used, in which each superconducting layer is characterized by its complex conductivity according to the BCS derived model of Mattis and Bardeen. Our estimation shows significant influence of the gap frequency on the NbN multilayer structure inductance at high frequencies.

Fullwave description of propagation and losses in quasiplanar transmission lines by quasi-analytical solution

The authors present a full-wave description of propagation and losses for some quasi-planar transmission lines using a quasi-analytical solution. The latter is derived from a modified transverse resonance method (MTRM), in which analytical preprocessing has been introduced. The quasi-static contribution is obtained by an entirely analytical solution, making the resultant system of linear equations very efficient. The resistive boundary conditions and the complex substrate permittivity are taken into account in an intrinsic manner, leading to an accurate determination of dielectric and conductor losses in lossy transmission lines. Theoretical and experimental results are presented for a lossless coplanar waveguide and a lossy microstrip line, respectively.<<ETX>>

Accurate and fast characterization of a class of quasiplanar structures

Propagation and losses for some quasiplanar transmission structures will be characterized by using an accurate and fast quasi-analytical solution. This latter is derived from a recently proposed modified transverse resonance method (MTRM), in which an analytical preprocessing has been introduced. The quasistatic contribution is obtained by an entirely analytical solution, so the resultant system of linear equations is very efficient. Furthermore, the resistive boundary conditions as well as the complex substrate permittivity are taken into account in an intrinsic manner, leading to an accurate determination of dielectric and conductor losses in lossy transmission lines.