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Register a new userHigh Tc Josephson nanoJunctions made by ion irradiation : characteristics and reproducibility
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Reproducible High Tc Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge 1 to 5 micrometers wide is firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7 film through a gold mask such as the unprotected part becomes insulating. A lower Tc part is then defined within the bridge by irradiating with a much lower dose through a 20 nm wide narrow slit opened in a standard electronic photoresist. These planar junctions, whose settings can be finely tuned, exhibit reproducible and nearly ideal Josephson characteristics. Non hysteretic Resistively Shunted Junction (RSJ) like behavior is observed, together with sinc Fraunhofer patterns for rectangular junctions. The IcRn product varies with temperature ; it can reach a few mV. The typical resistance ranges from 0.1 to a few ohms, and the critical current density can be as high as 30 kA/cm2. The dispersion in characteristics is very low, in the 5% to 10% range. Such nanojunctions have been used to make microSQUIDs (Superconducting Quantum Interference Device) operating at Liquid Nitrogen (LN2) temperature. They exhibit a very small asymmetry, a good sensitivity and a rather low noise. The process is easily scalable to make rather complex Josephson circuits.
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Superconducting Quantum Interference Filters (SQIFs) are arrays of superconducting loops of different sizes including Josephson Junctions (JJ). For a random distribution of sizes, they present a non-periodic response to an applied magnetic field, with an extended linear regime and a sizable field sensitivity. Such properties make SQIFs interesting devices to detect the magnetic component of electromagnetic waves at microwave frequencies. We have used the highly scalable technique of ion irradiation to make High Tc SQUIDs and SQIFs based on commercial YBa2Cu3O7 films, and studied their properties. Both display optimum performances as a function of temperature and bias current, that can be understood in the frame of numerical simulations that we developed. The role of asymmetries and spread in JJ characteristics (routinely found in HTSc technologies) is described : ion irradiation based devices appear robust against them. We finally present results on SQIF made with 2000 SQUID in series, showing a transfer function dV/dB ~ 1000V/T .
High Tc Josephson nanoJunctions (HTc JnJ) made by ion irradiation have remarkable properties for technological applications. However, the spread in their electrical characteristics increases with the ion dose. We present a simple model to explain the JnJ inhomogeneities, which accounts quantitatively for experimental data. The spread in the slits width of the irradiation mask is the limiting factor.Monte Carlo simulations have been performed using different irradiation conditions to study their influence on the spread of the JnJ charcateristics. A universal behavior has been evidenced, which allows to propose new strategies to optimize JnJ reproducibility.
We have investigated the electrodynamic properties of High-Tc strip-lines made by ion irradiation, in order to evaluate the potentialities of such a technology for RSFQ superconductor digital electronic. SQUID loops of different length and width have been fabricated by ion bombardment of 70 nm thick films through e-beam lithographied shadow masks, and measured at different temperatures. The voltage modulations have been recorded by direct injection of a control current in the SQUIDs arms. The corresponding line inductances have been measured and compared with 3D simulations. A quantitative agreement has been obtained leading to typical values of 0.4 pH/microns without ground plane.
Superconductor-Ferromagnet-Superconductor (S-F-S) Josephson junctions were fabricated by making a narrow cut through a S-F double layer using direct writing by Focused Ion Beam (FIB). Due to a high resolution (spot size smaller than 10 nm) of FIB, junctions with a small separation between superconducting electrodes ($leq$ 30 nm) can be made. Such a short distance is sufficient for achieving a considerable proximity coupling through a diluted CuNi ferromagnet. We have successfully fabricated and studied S-F-S (Nb-CuNi-Nb) and S-S-S (Nb-Nb/CuNi-Nb) junctions. Junctions exhibit clear Fraunhofer modulation of the critical current as a function of magnetic field, indicating good uniformity of the cut. By changing the depth of the cut, junctions with the $I_c R_n$ product ranging from 0.5 mV to $sim 1mu $V were fabricated.
Superconducting Quantum Interference Filters (SQIF) are promising devices for Radio- Frequency (RF) detection combining low noise, high sensitivity, large dynamic range and wide-band capabilities. Impressive progress have been made recently in the field, with SQIF based antennas and amplifiers showing interesting properties in the GHz range using the well-established Nb/AlOx technology. The possibility to extend these results to High Temperature Superconductors (HTS) is still open, and different techniques to fabricate HTS SQIFs are competing to make RF devices. We report on the DC and RF response of a High Temperature SQIF fabricated by the ion irradiation technique. It is made of 1000 Superconducting QUantum Interference Devices (SQUIDs) in series, with loop areas randomly distributed between 6 micron2 and 60 micron2. The DC transfer factor is around 450 V/T at optimal bias and temperature, and the maximum voltage swing around 2:5 mV . We show that such a SQIF detects RF signals up to 150 MHz. It presents linear characteristics for RF power spanning more than five decades, and non-linearities develop beyond PRF = -35 dBm in our set-up configuration. Second-harmonic generation has been shown to be minimum at the functioning point in the whole range of frequencies. A model has been developed which captures the essential features of the SQIF RF response.
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