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Japanese Journal of Applied Physics

Dry Etching of Nb and Fabrication of Nb Variable-Thickness-Bridges

Takeshi Harada, Kenji Gamo, Susumu Namba

Published Jan 1, 1981• 27 citations
Publisher

Abstract

Several reactive dry etching methods; ion etching, sputter etching and plasma etching, were studied in order to fabricate a niobium variable-thickness type thin film bridge (VTB). Niobium was etched by CF4 reactive sputter etching with submicron accuracy and no significant influence on the superconducting transition temperature except very near the surface (<200 Å). The niobium VTB behaved as an S-S'-S bridge, and a reduced self-heating effect was realized by a variable-thickness geometry. The DC Josephson effect was observed in the temperature range without thermal hysteresis. The quality factor of the I c R n product obtained was at least 0.42 mV.

DOI
10.1143/jjap.20.259

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Several reactive dry etching methods; ion etching, sputter etching and plasma etching, were studied in order to fabricate a niobium variable-thickness type thin film bridge (VTB). Niobium was etched by CF4 reactive sputter etching with submicron accuracy and no significant influence on the superconducting transition temperature except very near the surface (<200 Å). The niobium VTB behaved as an S-S'-S bridge, and a reduced self-heating effect was realized by a variable-thickness geometry. The DC Josephson effect was observed in the temperature range without thermal hysteresis. The quality factor of the I c R n product obtained was at least 0.42 mV.
A novel method of fabrication, as described in the following, was developed that ensures a good reproducibility of the junction. Namely, poly-Si was deposited over a Si substrate whose surface had been previously thermally oxidized. The poly-Si overlaid surface was then thermally oxidized. The bridge configuration of the junction was microprocessed by means of electron beam lithography. The fabricated configuration served as a mask in depositing the Nb layer on the substrate by means of rf sputtering to conclude the fabrication of a microbridge Josephson junction. The final process of the formation of the variable thickness microbridge configuration was readily carried out by placing another properly shaped mask over the masking bridge or by forming an SiO2 layer over the poly-Si layer. The SiO2 layer was processed by selective plasma etching so that it would act as a mask over the bridge portion. The stabilization of the characteristics of the junction has been achieved due to the passivation which takes place during the heat treatment in atmosphere by forced oxidation of the Nb film surface and to the annealing of the film itself.
To realize antenna-coupled Josephson detectors for microwave and millimeter-wave radiation, planar-type Nb nanobridge Josephson junctions were fabricated. Nb thin films whose thickness, the root mean square roughness and the critical temperature were 20.0 nm, 0.109 nm and 8.4 K, respectively were deposited using a DC magnetron sputtering at a substrate temperature of 700°C. Nanobridges were obtained from the film using 80-kV electron beam lithography and reactive ion-beam etching in CF 4 (90%) + O2 (10%) gases. The minimum bridge area was 65 nm wide and 60 nm long. For the nanobridge whose width and length were less than 110 nm, an I-V characteristic showed resistively-shunted-junction behaviour near the critical temperature. Moreover, Shapiro steps were observed in the nanobridge with microwave irradiation at a frequency of 6 – 30 GHz. The Nb nanobridges can be used as detectors in the antenna-coupled devices.
Amplification, down-conversion and detection of the microwaves in thin-film Josephson variable thickness bridges of submicron dimensions have been investigated experimentally. The results for indium and tin bridges are discussed taking into account the influence of the nonequilibrium electrons in a weak link region. The same experiments were carried out with SNS variable thickness Nb-Al-Nb bridges. They showed that the properties of SNS bridges agreed better with the theoretical predictions of the resistively shunted junction model in a wide temperature range. Moreover the parameters of the Nb-Al-Nb bridges have possessed better stability and reliability.
Superconducting Hot-electron Bolometric (HE.B) mixers employing thin films ofNbN have become the devices of choice for heterodyne detection at tera,hertz frequen- cies. HEB mixers are inherently insensitive to the bandgap frequency of the superconducting material used in the device, in contrast to SIS mixers,. We have developed a fabrication pro- cess for thin NbN films at the National Institute of Standards and Technology in Boulder, CO. A two-fold approach can be taken. The first is to maximize the critical temperature of the superconducting device by growing thicker films at the expense of IF bandwidth, whereas the second approach focuses on maximizing the IF bandwidth at the expense of the critical tem- perature. So far, we have developed a film-deposition process utilizing our DC reactive mag- netron sputtering chamber. By current-biasing the RF plasma in a mixture of Ar and N2, while using a Nb target, we can control the film stoichiometry and produce films with thicknesses of 5 nm. The films are deposited on MgO substrates that are heated to about 800 °C during depo- sition. A typical critical temperature (TO is about 10 K and the transition width is very small (0.5 K). The films are evaluated by measuring their superconducting characteristics as well as their thickness and surface roughness by means of AFM analysis. Phonon-cooled HEB devices are fabricated from the films to study their performance as HEB mixers. The device fabrication process at UMass/Amherst involves lift-off lithography of the antenna (gold), and Reactive ion etching (R1E) or wet etching of the NbN. The I-V char- acteristics and noise temperatures of the devices were measured in order to determine their quality for PHEB applications.
Nano-meter(nm)-bridges with high-Tcmaterials hold great technological interest because of their smaller capacitance and expected higher IoRn-product. They are promising especially for applications such as high speed logic and high frequency radiation detectors. Also they can operate over a wide temperature range and are stable against thermal cycles as are incorporating the refractory high-Tcsuperconductors. One of the essential ingredient for high quality bridges is to have NbN films. Ultra-thin NbN films have been prepared by rf reactive-sputtering. NbN films deposited epitaxially on rf sputtered MgO films have high superconducting transition temperature Tc. Tcvalue of the film with thickness of about 5nm was about 14K, and is much higher than those deposited on Al2O3films and Si substrates. Two types of nm-bridges were reproducibly fabricated with NbN films deposited epitaxially on MgO films. The obtained IoRn-products were in a range of 0.5 to 3.6mV. The microwave-induced voltage steps were observed up to the voltage comparable to IoRn- product. The dc and ac quantum effects of the dc SQUIDs were observed in quite wide range of temperature, 4.2 to 11.4K.
For high-quality superconducting tunnel junctions (STJs), it is necessary to reduce leakage current as much as possible. We describe the fabrication of niobium STJs using the selective niobium (Nb) etching process and various ways to minimize the leakage current. The experiment shows that the leakage current mainly comes from shorts in the tunnel barrier layer rather than those around the junction edges. Through systematic analysis of the thin film stress, surface morphology and modified junction structures, we fabricate high-quality Nb STJs with a gap voltage of 2.8 mV and a leakage current at 1 mV as low as 8.1% and 0.023% at 4.2 K and 0.3 K, respectively.