• 한국초전도학회:학술대회논문집
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• 한국초전도학회 2001년도 High Temperature Superconductivity Vol.XI
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• pp.35-35
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• 2001

• Progress in Superconductivity
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• 제3권1호
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• pp.49-55
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• 2001

Josephson junction arrays were fabricated by DC magnetron sputtering, self-aligning and reactive ion etching technique. The Al native oxide, formed by thermal oxidation, was used as the tunneling barrier of Nb/$Al-A1_2$$O_3$Nb trilayer. The arrays have 2,000 Josephson junctions with the area of $14\mu\textrm{m}$ $\times$ $46\mu\textrm{m}$. The gap voltages were in the range of 2.5 ~2.6 mV and the spread of critical current was $\pm$11~14%. When operated at 70~94 ㎓, the arrays generated zero-crossing steps up to 2.1~2.4 V. To improve transmission of microwave power and prevent diffusion of oxygen into Nb ground-plane while depositing $SiO_2$dielectric, we applied a plasma nitridation process to the Nb ground-plane. The microwave power was well propagated in Josephson junction arrays with nitridation. The difference in microwave transmission 7an be interpreted by the surface impedance change depending on nitridation.

• Progress in Superconductivity
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• 제3권2호
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• pp.172-177
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• 2002

Superconductive digital to analog converters (DAC) based on Josephson effect produce the voltage steps with high precision and good stability Therefore, they can be applied to obtain a very accurate ac voltage standard. In this paper, we made a simulation study of Rapid Single Flux Quantum (RSFQ) DAC. RSFQ DAC was composed of Non-destructive Head Out (NDRO) cells, T flip-flops, D flip-flops, Splitters, and Confluence Buffers. Confluence Buffer was used in resetting the DACs. We also obtained operating margins of the important circuit parameters in simulations.

• 한국초전도ㆍ저온공학회논문지
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• 제22권4호
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• pp.62-66
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• 2020

We fabricated superconducting quantum interference devices (SQUIDs) based on Nb Josephson junctions, and characterized the key parameters of the SQUIDs. The SQUIDs are double relaxation oscillation SQUIDs (DROSs) having larger flux-to-voltage transfer coefficient than the standard DC-SQUIDs. SQUID sensors were fabricated by using Nb junction technology consisted of a DC magnetron sputtering and a conventional photolithography process. In multichannel SQUID systems for whole-head magnetoencephalography measurement with a helmet-type SQUID array, we need about 336 SQUID sensors for each system. In this paper, we fabricated a few hundred SQUID sensors, measured the critical current, flux modulation voltage and decided if each tested SQUID can be used for the multichannel systems. As the criterion for the acceptance of the sensors, we chose the critical current and amplitude of the modulation voltage to be 8 ㎂ and 80 ㎶, respectively. The average critical current of the SQUIDs was 10.58 ㎂. The typical flux noise of the SQUIDs with input coil shorted was 2 μΦ0/√Hz at white region.

• Progress in Superconductivity
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• 제2권2호
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• pp.71-75
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• 2001

The effect of annealing step-edges of LaAlO$_3$ and MgO single crystal substrates on YBa$_2$Cu$_3$O$_{7}$ junction has been studied. The step-edge was fabricated by argon ion milling and was annealed at 105$0^{\circ}C$ in 1 attn oxygen pressure. We compared AFM image near step-edge of the substrates between before and after annealing process. And YBa$_2$Cu$_3$O$_{7}$ thin film was deposited on the step-edge by a standard pulsed laser deposition. The step-edge junctions were characterized by current-voltage curves at 77 K. The annealing of step-edges of MgO substrate improved the current-voltage characteristic of Josephson junction: double steps in the current-voltage characteristic disappeared. However the annealing for LaAlO$_3$ did not improve the junction property.rty.

• Progress in Superconductivity
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• 제2권2호
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• pp.81-85
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• 2001

Uniformity of critical currents of YBa$_2$Cu$_3$O$_{7}$ step-edge Josephson junctions on SrTiO$_3$(100) substrates have been studied at various step-line angles. 15 identical junctions were made in series on each substrate that has a long straight step-edge line. Step-line angles studied were 0$^{\circ}$, 15$^{\circ}$, 30$^{\circ}$, and 45$^{\circ}$with respect to the crystal major axes of the substrate. Scattering of junction critical currents among the junctions on the same substrate increased with the step-line angle. Current-voltage curves showed standard resistively-shunted-junction (RSJ) characteristics in most of the 0$^{\circ}$junctions. However, the number of junctions showing RSJ behavior decreased with increasing step-line angle. Variations of detailed microstructure of the step-edge among junctions, which are coupled with the d-wave symmetry of YBa$_2$Cu$_3$O$_{7}$, are believed to be the main cause for the nonuniformity in the critical current.ent.

• Progress in Superconductivity
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• 제4권1호
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• pp.1-9
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• 2002

Transferring single flux quanta across a Josephson junction at an exactly determined rate has made highly precise voltage measurements possible. Making use of self-shunted Nb-based SINIS junctions, programmable fast-switching DC voltage standards with output voltages of up to 10 V were produced. This development is now extended from fundamental DC measurements to the precise determination of AC voltages with arbitrary waveforms. Integrated RSFQ circuits will help to replace expensive semiconductor devices for frequency control and signal coding. Easy-to-handle AC and inexpensive quantum voltmeters of fundamental accuracy would be of interest to industry. In analogy to the development in the flux regime, metallic nanocircuits comprising small-area tunnel junctions and providing the coherent transport of single electrons might play an important role in quantum current metrology. By precise counting of single charges these circuits allow prototypes of quantum standards for electric current and capacitance to be realised. Replacing single electron devices by single Cooper pair circuits, the charge transfer rates and thus the quantum currents could be significantly increased. Recently, the principles of the gate-controlled transfer of individual Cooper pairs in superconducting A1 devices in different electromagnetic environments were demonstrated. The characteristics of these quantum coherent circuits can be improved by replacing the small aluminum tunnel Junctions by niobium junctions. Due to the higher value of the superconducting energy gap ($\Delta_{Nb}$ ＝ $7\Delta_{Al}$), the characteristic energy and the frequency scales for Nb devices are substantially extended as compared to A1 devices. Although the fabrication of small Nb junctions presents a real challenge, the Nb-based metrological devices will be faster and more accurate in operation. Moreover, the Nb-based Cooper pair electrometer could be coupled to an Nb single Cooper pair qubit which can be beneficial for both, the stability of the qubit and its readout with a large signal-to-noise ratio..

• Progress in Superconductivity
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• 제11권1호
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• pp.36-41
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• 2009

Superconducting quantum phenomena are getting attention from the field of metrology area. Following its first successful application of Josephson effect to voltage standard, piconewton force standard was suggested as a candidate for the next application of superconducting quantum effects in metrology. It is predicted that a micron-sized superconducting Nb ring in a strong magnetic field gradient generates a quantized force of the order of sub-piconewtons. In this work, we studied the design and fabrication of Nb superconducting quantum interference device (SQUID) on an ultra-thin silicon cantilever. The Nb SQUID and electrodes were structured on a silicon-on-insulator (SOI) wafer by dc magnetron sputtering and lift-off lithography. Using the resulting SOI wafer, we fabricated V-shaped and parallel-beam cantilevers, each with a $30-{\mu}m$-wide paddle; the length, width, and thickness of each cantilever arm were typically $440{\mu}m,\;4.5{\mu}m$, and $0.34{\mu}m$, respectively. However, the cantilevers underwent bending, a technical difficulty commonly encountered during the fabrication of electrical circuits on ultra-soft mechanical substrates. In order to circumvent this difficulty, we controlled the Ar pressure during Nb sputtering to minimize the intrinsic stress in the Nb film and studied the effect of residual stress on the resultant device.

• 한국자기학회지
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• 제2권3호
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• pp.277-281
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• 1992

입력코일과 변조코일이 박막 형태로 집적화된 planar DC SQUID를 제작하고 그 특성을 조사하였다. SQUID loop은 균일한 외부자장의 영향을 받지 않도록 두개의 ring이 '8'자형으로 직렬로 연결된 구조를 하고 있으며, 조셉슨 접합으로는 Pd shunt 저항을 가지는 Nb/Al-oxide/Nb 턴넬 접합을 이용하였다. 소자는 포토리소그라피 기술과 RF 마그네트론 스퍼터링 및 양극산화 방법으로 제작하였으며, 제작된 SQUID는 4.2 K에서 기본적인 특성을 측정한 결과 standard readout 시스템에 적합한 매끄러운 특성을 보였으며 전압 잡음은 측정할 수 없을 정도로 작아 white noise는 최대 $10^{-4}\;{\phi}_o/\;\sqrt{H_z}$ 이하로 나타났다. 현재 matching 코일을 사용하여 잡음 특성을 측정중에 있으며 앞으로 SQUID parameter를 최적화할 경우 매우 고감도의 자기센서로 활용될 수 있을것으로 기대된다.

• Progress in Superconductivity
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• 제10권2호
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• pp.74-78
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• 2009

Inter-grain nanobridges of the $MgB_2$ superconductor have been fabricated by focused-ion-beam(FIB) and their electrical transport properties were studied. The $MgB_2$ film was prepatterned into microbridges by a standard argon ion milling technique and then FIB-patterned into 100 nm$\times$100 nm bridges. Current-voltage characteristics showed a strong flux-flow type behavior at all temperatures with a trait of Josephson coupling near $T_c$. At low temperatures, the curves showed a two-step resistance-doubled transition with occasional hysteresis. The resistance-doubling transition is believed to be due to a two-channel flux-flow effect. The temperature-dependent critical current data showed $I_c(T){\propto}(1-T/T_c)^2$ near $T_c$, same as a normal barrier junction, and $I_c(T){\propto}(1-T/T_c)^{1.2}$ at low temperatures, similar to that of a film.