High-quality Junctions Research Articles

Fabrication of submicron Nb/AlO/sub x//Nb Josephson junctions using ECR plasma etching technique

It is important to develop a high-yielding and reproducible fabrication process of submicron Nb/AlO/sub x//Nb Josephson junctions to improve the integration level and the operating speed of Josephson LSI circuits. For this purpose, we have developed a junction fabrication process by introducing an electron cyclotron resonance (ECR) plasma etching technique with CF/sub 4/ gas. In the ECR plasma etching technique, highly anisotropic etching of Nb was achieved. Over-etching was reduced by 86%. We have successfully fabricated Nb/AlO/sub x//Nb junctions with critical current density of 10/sup 4/ A/cm/sup 2/ using the cross-line patterning (CLIP) method and the electron beam (EB) lithography technique, where the size of the junctions was varied from 2 /spl mu/m to 0.5 /spl mu/m at 0.1 /spl mu/m intervals. High-quality submicron junctions for integrated circuits with small spread of critical current Ic was obtained. High uniformity of Ic was achieved. The characteristics of the fabricated junctions are discussed and compared with the junctions fabricated by RIE technique.

Influence of Depth Position of End-of-Range Defects on Current-Voltage and Noise Characteristics of Shallow (p+/n) Junctions

The goal of this work is to study the relationship that exists between a population of end of range (EOR) defects, measured by TEM, and electrical properties of the Ge preamorphized ( p+/n) junctions. In this paper, we report current-voltage (I-V) and low frequency (L-F) noise measurements, performed on crystalline and Ge preamorphized diodes, as a function of temperature. The I-V measurements reveal the conduction process which is dominant in the shallow ( p+/n) junctions, while L-F noise characteristics confirm the presence of generation-recombination centers in direct relation to the presence of EOR defects in the n region. Moreover, temperature-L-F noise measurements have shown a drawback of this shallow ( p+/n) junction formation procedure, independent of the preamorphization step. Nevertheless, high-quality ( p+/n) junctions can be obtained by low-energy boron implantation into germanium preamorphized Si substrates using rapid thermal annealing for dopant activation through the judicious choice of Ge implantation energy.

Deep “cold” junctions by porous silicon impregnation

A new doping process for silicon power device technology, based on the electrochemical impregnation of low porosity silicon, is presented. N-type Si substrates were electrochemically etched in order to obtain pores ∼100 μm deep with controlled density and morphology. Electrochemical impregnation by AlCl 3, and thermal diffusion were then employed to dope the porous layer. Spreading resistance and scanning electron microscopy characterisations revealed deep high-quality junctions with sharp doping profiles. Deep p-wells are known to improve the breakdown voltage characteristics of Si power devices. The porous silicon technology represents an alternative way to the damaging and time-consuming procedures of deep diffusion currently used for producing Si power devices. © 1997 Elsevier Science S.A.

Intrinsic Josephson junctions in high temperature superconductors

The high temperature superconductors form natural superconducting multilayers where adjacent superconducting layers are weakly coupled by the Josephson effect. As a consequence, single crystals act intrinsically as vertical stacks of Josephson junctions. At present, intrinsic Josephson junctions are fabricated and investigated by several groups. There has been considerable progress both in production of high quality junctions and in theoretical understanding of the intrinsic Josephson effect. In this paper the basic physics of intrinsic Josephson junctions is outlined. Some examples of the present status of research are given.

NbN/AlN/NbN tunnel junctions with high current density up to 54 kA/cm2

We report on progress in the development of high current density NbN/AlN/NbN tunnel junctions for applications as submillimeter wave superconductor-insulator-superconductor mixers. A very high current density up to 54 kA/cm2, roughly an order of magnitude larger than any reported results for all-NbN tunnel junctions, was achieved in the junctions with about 1 nm thick AlN barriers. The magnetic field and temperature dependence of critical supercurrents were measured to investigate the Josephson tunneling behavior of critical supercurrents in the high-Jc junctions. The junctions showed high-quality junction characteristics with a large gap voltage of 5 mV and sharp quasiparticle current rise (ΔVg=0.1 mV). The Rsg/RN ratio was about 5 with a Vm value of 14 mV measured at 4.2 K.

Electrical Defects of Shallow (P+/N) Junctions Formed by Boron Implantation into Ge-Preamorphized Si-Substrates

ABSTRACTIn this paper, we report Current-Voltage (I-V), Capacitance-Voltage (C-V) and Deep Level Transient Spectroscopy (DLTS) measurements on ultra-shallow (p+/n) junctions obtained by boron implantation into crystalline and Ge preamorphized Si-substrates. Germanium implantations were carried out at the energies of 30, 60 and 150 keV at a dose of 1015 cm−2. Boron was then implanted at an energy of 3 keV at a dose of 1015 cm−2. Dopant activation was obtained by Rapid Thermal Annealing (RTA) performed at 950°C for 15s in a nitrogen ambient. The aim of this work is to study the relation that exists between a population of End-Of-Range (EOR) defects, measured by TEM, and electrical properties of the Ge preamorphized diodes. The electrical measurements allow us to give the conduction mechanism which dominates in the diodes. Moreover, DLTS measurements showed the presence of two majority-carrier traps in direct relation with the EOR defects, measured at Ec-0,22eV and Ec-0,47eV. The presence of energy-distributed G-R centers independent upon the preamorphization stage is also discussed. Nevertheless, high quality (p+/n) junctions can be obtained by this technology with a judicious choice of Ge implantation energy.

C-axis tunneling in multilayers NdBa2Cu3O7−δ junctions with PrBa2Cu3O7−δ barriers

This letter reports c-axis tunneling spectroscopy investigations of high quality junctions fabricated from c-axis oriented NdBa2Cu3O7−δ/PrBa2Cu3O7−δ/NdBa2Cu3O7−δ multilayers deposited by pulsed laser deposition. The conductance spectra as a function of temperature show a smooth crossover from tunneling to metallic junction behavior. A temperature dependent BCS-like gap giving 2Δ/kBTc=6 was observed. The current–voltage characteristics exhibit supercurrent and quasiparticle tunneling that is commonly observed for superconductor–insulator–superconductor junctions.

Electric field controllable Josephson junctions of high quality in high-T c superconductors

Three-terminal Josephson junctions, the third terminal of which can be used to control the electronic properties of the device, are the key to a number of exciting device applications of high-Tc superconductors. To this end a novel device structure has been developed, which consists of an inverted metal-insulator-superconductor field-effect transistor into which a bicrystal grain boundary junction is embedded. These devices are excellent Josephson junctions as exemplified by their current–voltage characteristics and the magnetic field dependencies of the critical currents. The critical current of the junctions can be changed by 8% per volt applied to the gate electrode, the gate currents being negligible. Because they are sensitive to electric as well as magnetic fields, the junctions can be controlled by two independent inputs.

Basic Aspects of High-Tc Grain Boundary Devices

Grain boundaries are extensively used as high-quality Josephson junctions in high-T c superconductors. Their superconducting characteristics can generally be well described by conventional models of strongly coupled Josephson junctions. Here, we report on highly anomalous critical current vs. magnetic field dependencies of grain boundaries in YBa 2 Cu 3 O 7-x . Direct imaging with scanning SQUID microscopy provides evidence of magnetic flux generated by single grain boundaries. Conventional Josephson junction models cannot explain these effects if a superconducting order parameter with a pure s-wave symmetry is assumed. The results have significant implications for our understanding of the properties of grain boundaries in high-T c superconductors and for their applications.

C-axis tunneling in YBa 2Cu 3O 7−δ trilayer junctions with PrBa 2Cu 3O 7−δ barrier

This paper reports c-axis tunneling spectroscopy investigations of high-quality junctions fabricated from YBa 2Cu 3O 7−δ/PrBa 2Cu 3O 7−δ/YBa 2Cu 3O 7−δ (YBCO/PBCO/YBCO) trilayers deposited by off-axis laser ablation. We observe a temperature-dependent BCS-like giving 2 Δ/ k B T C = 1.3 and a finite subgap structure. The conductance spectra display parabolic behaviour at higher bias. These results indicate that even by using a 60 n, thick PBCO barrier it is possible to obtain SIS-like junctions in c-axis oriented YBCO. We obtain high I c R N values and an almost exponential dependence of junction resistance with temperature indicating that we are not observing transport through the PBCO via shorts. We show that quasiparticulate transport in these junctions can be described by the Glazman-Matveev theory for hopping transport via localised states.

Superconducting niobium and niobium nitride processes for medium-scale integration applications

A 10 layer circuit process is described that is based on relatively reproducible, high quality, all-niobium (Nb) and niobium nitride (NbN) Josephson junctions. The junctions are fabricated from in-situ trilayer films of S/M-MOx/S using a selective niobium etching process (SNEP), where S is superconducting Nb or NbN and M is aluminium (Al) to form the barrier for Nb junctions, or magnesium (Mg) to form the barrier for NbN junctions. These junctions have been incorporated into a 10 level process that has been used to fabricate numerous analogue and digital circuits. This paper describes this superconducting process and its application to circuit fabrication for demonstrating superconducting circuits of medium-scale complexity.

Ta-based tunnel junctions for energy resolving X-ray detection

We report the results of a programme to develop an energy-resolving superconducting tunnel junction based on a Ta X-ray absorber. The design is based on a vertical heterostructure consisting of a Nb underlayer to act as a quasiparticle reflector, a thick Ta absorber layer and an Al trap layer. The upper surface of this trap layer is oxidised to form the tunnel barrier. The counterelectrode consists of an Al and thin Nb layer with thicknesses controlled to give approximately equal energy gaps in both electrodes and minimal absorption in the Nb counterelectrode. The effect of electro-magnetic barrier resonances on detection performance has been evaluated. Very high quality junctions have been fabricated, tested and evaluated for X-ray energy resolution.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fabrication of high quality, deep-submicron Nb/AlO/sub x//Nb Josephson junctions using chemical mechanical polishing

A reliable process based on Chemical Mechanical Polishing (CMP) has been developed for the fabrication of high quality, deep-submicron Nb/AlO/sub x//Nb Josephson junctions on 2 inch wafers. The Nb counter electrode is defined using low pressure SF/sub 6/ reactive ion etching (RIE) with a mask of SiO, which is thermally evaporated through a bilayer resist stencil patterned by electron beam lithography. After RIE, the entire wafer is coated with SiO, which is then planarized using CMP (which also removes the etch mask) to expose the counter electrode. This technique has produced high quality (V/sub m//spl sime/60 mV for J/sub c/ of 2 kA/cm/sup 2/) junctions with areas as small as 0.003 /spl mu/m/sup 2/ demonstrating that the process does not degrade the junction quality. Junctions with critical currents of 22 /spl mu/A and areas of 0.006 /spl mu/m/sup 2/ have been fabricated from trilayers with J/sub c/>300 kA/cm/sup 2/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An improved etching process used for the fabrication of submicron Nb/AlO/sub x//Nb Josephson junctions

It is important to develop a reliable and reproducible fabrication process of submicron Nb/AlO/sub x//Nb Josephson junctions to improve the integration level and the operating speed of the Josephson LSI circuit. For this purpose, we have developed an improved etching process by introducing a dummy etching process and using a scanning electron microscope (SEM) as an etching end-point detector. The dummy etching process improves the anisotropy of a reactive ion etching (RIE) process using CF/sub 4/. The etching residue around a junction is detected easily and correctly by the SEM observation. We have successfully fabricated Nb/AlO/sub x//Nb Josephson junctions with critical current density of 10/sup 4/ A/cm/sup 2/ using the cross-line patterning (CLIP) method and electron beam lithography, where the junction size was varied from 2 /spl mu/m to 0.5 /spl mu/m at 0.1 /spl mu/m intervals. High-quality submicron junctions for integrated circuits with suitable critical current variations were obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurements of the dc Josephson current in proximity systems.

Junctions modeled as S-N/I/S and S-N/I/N-S proximity systems (where S, I, and N indicate a superconductor, an insulator, and a normal metal, respectively) are widely discussed in literature from both the experimental and theoretical point of view. In the present paper experimental aspects concerning Nb/Nb junctions including a proximity bilayer are considered. Nb-M/I/Nb and M-Nb/I/Nb structures (where M is a normal metal, a semimetal, or a superconductor) are investigated by using a semimetal (bismuth) and a superconductor (aluminum) as the M layer. In particular, how the deposition of a back layer influences the behavior of Nb/I/Nb high-quality junctions is discussed, focusing the interest on measurements of the temperature dependence of the maximum dc Josephson current. Experimental data are discussed in the framework of Kresin's theoretical calculations based on the thermodynamic Green's function method.

Formation of Raised Source/Drain Junctions by Rapid Thermal Chemical Vapor Deposition

AbstractIn this paper, we present alternative uses of rapid thermal chemical vapor deposition (RTCVD) in forming junctions for the raised source/drain MOSFET. The results will include applications of epitaxial silicon, SixGe1−x and TiSi2 all selectively deposited in dedicated coldwalled, lamp heated high or ultra high vacuum RTCVD reactors. Two general approaches will be considered : 1) ultra shallow junction formation in silicon followed by a selective deposition process to form a raised contact, 2) selective deposition to obtain a layer that can be used as a solid diffusion source and as a sacrificial layer for self-aligned silicide formation. In the first approach, junctions are formed typically by low energy ion-implantation. In this paper, we present rapid thermal vapor phase doping (RTVPD) as an alternative to ion-implantation to form defect free ultra-shallow junctions in Si. The method involves exposing a silicon wafer to a dopant gas (such as B2H6) at a moderate temperature (∼600°C) for a short time and subsequent annealing for drive-in. This is followed by either selective epitaxy and conventional self-aligned TiSi2 formation or selective deposition of a low-resistivity C54 TiSi2 from TiCl4 and SiH4. In the second approach, first, a semiconductor (Si, polysilicon or SixGe1−x) is deposited selectively. If the material is undoped, doping can be achieved by ion-implantation. In-situ doping is also possible as will be shown with p- and n-type SixGe1−x at temperatures as low as 625°C using B2H6 or PH3. The doped layer is then used as a solid diffusion source to form the junctions by out-diffusion. Using these different approaches, we present examples of high quality junctions in Si as shallow as a few hundred angstroms. The techniques are compared based upon their robustness, complexity, equipment and thermal budget requirements.

Thermal stability of highly Sb-doped molecular beam epitaxy silicon grown at low temperatures: Structural and electrical characterization

The structural and electrical properties of highly Sb-doped molecular beam epitaxy grown silicon have been investigated as function of rapid thermal annealing (RTA) temperature. Doping levels of 3×1020 cm−3 were obtained using low temperature epitaxy (LTE) performed at a growth temperature of 300 °C. Ion channeling and transmission electron microscopy (TEM) measurements showed that the as-grown samples were of very high quality. The combination of Hall-effect profiling and Rutherford backscattering spectroscopy revealed an electrically active Sb fraction of 0.8. Short time RTA processing improved the electron mobility and the activation: RTA at 600 °C for 10 s yielded unity activation and RTA at 800 °C gave mobilities matching phosphorus doped bulk values, thus significantly exceeding previously reported values for highly doped LTE material. A degradation of the crystalline quality was observed for higher RTA temperatures: RTA at 1000 °C for 10 s reduced both the Sb-substitutional fraction and electrical activation to 0.6 due to precipitation of Sb, and lead to the formation of a high density of dislocation loops as observed by TEM. A large fraction of the precipitates decorated these dislocation loops. Mesa isolated diodes were fabricated to evaluate the use of LTE material for device production. Current–voltage measurements on these diodes revealed high quality junctions with low reverse currents and near-ideal forward characteristic.

Fabrication and characterization of all-refractory NbCN/Al/AlO/sub x//Al/Nb junctions

High-quality AlO/sub x/ tunnel barriers have been fabricated on epitaxial niobium carbonitride (NbCN) base layers by the deposition of an Al layer followed by thermal oxidation. By careful control of its uniformity, the thickness of the Al layer has been reduced to less than 3 nm, which results in an average gap voltage, V/sub g/(NbCN), of up to 2.65 mV. Using a self-aligned whole-wafer processing route, high-quality NbCN/Al/AlO/sub x//Al/Nb junctions as small as 0.6 mu m/sup 2/ have been made. These junctions offer considerable advantages over directly deposited barriers in terms of minimal subgap leakage, good control of the barrier conductance, and simple processing procedures. It was shown that submicron junctions can be fabricated with no gap smearing or reduction in quality. Using only Nb counterelectrodes total gap voltages up to 4.0 mV, with widths of 0.6 mV, have been demonstrated in high-quality junctions. >

Investigation of etching techniques for superconductive Nb/Al-Al/sub 2/O/sub 3//Nb fabrication processes

Wet etching, CF/sub 4/ and SF/sub 6/ reactive ion etching (RIE), RIE/wet hybrid etching, Cl-based RIE, ion milling and liftoff techniques have been investigated for use in superconductive Nb/Al-Al/sub 2/O/sub 3//Nb fabrication processes. High-quality superconductor-insulator junctions have been fabricated using a variety of these etching methods; however, each technique offers distinct tradeoffs for a given process an wafer design. In particular, it was shown that SF/sub 6/ provides an excellent RIE chemistry for low-voltage anisotropic etching of Nb with high selectivity to Al. The SF/sub 6/ tool has greatly improved the trilevel resist junction insulation process. Excellent repeatability, selectivity with respect to quartz, and submicron resolution make Cl/sub 2/+BCl/sub 3/+CHCl/sub 3/ RIE a very attractive process for trilayer patterning. >

Performance of DC SQUIDs fabricated on 4-inch silicon wafer

A Nb/AlO/sub x//Nb Josephson process has been developed to fabricate DC superconducting quantum interference devices (SQUIDs) for biomagnetic measurements on 4-in silicon wafers. The purpose of the process is to fabricate high-quality junctions and to keep the quality uniform over a silicon wafer. All the films were deposited using magnetron sputtering. The components of the SQUID, except the junctions, were formed by taper etching to improve the step coverage. The uniformity of the SQUID characteristics (critical current, modulation depth, and mutual inductance between SQUID and modulation coil) was measured, and an excellent value of less than +or-5% was obtained. Three different configurations of SQUIDs, single washer (SW), double washer series (DWS), double washer parallel (DWP) have been fabricated with the same characteristics. The SW (single washer) had a white noise level of 6.3*10/sup -6/ Phi /sub 0// square root Hz, lower than those of DWS and DWP. 1/f crossover frequencies were below 1 Hz. >