Preparation of microbridges and small area tunnel junctions

2014 An evaporation technique for preparation of thin-film weak links with a width smaller than 1 03BCm and tunnel junctions with an area smaller than 1 03BCm x 20 03BCm is described. Some examples of I-V characteristics and the influence of microwaves on the characteristics are given. REVUE DE PHYSIQUE APPLIQUÉE TOME 9, JANVIER 1974, PAGE Various techniques for preparation of thin-film weak links have been discussed in the literature [1]. Often the thin-film is evaporated onto the substrate utilizing a suitable masking [2], at times including a photo-etching [3] or a razor-blade cutting procedure [4]. In the latter case the dimensions of the bridge region are of the order of 0.5 ym wide by 0.2 gm long, and typical resistances below Tc are reported to be of the order of 0.5 Q. In the present note we report on an evaporation technique for preparation of thin-film weak links. The new feature consists of a specially designed mask the main elements of which are two precision hemispheres or nearly hemi-spheres mounted as discussed below. The width of the thin-film weak links is normally below 1 ym and typical resistances obtained below Tc are 70 Q. It is, further, possible to use the device for making small area tunnel junctions of the crossed film type. Typically areas of the order of 1 gm x 20 gm are obtained. The technique has turned out to be very reliable in the sense that nearly all weak links and tunnel junctions produced in this way do show the usual 1 v characteristics and with a given setting of the mask one obtains very nearly identical weak links and tunnel junctions. Examples of 1V characteristics obtained with and without incoming microwaves will be presented below. In principle the weak links are prepared by evaporating a metal film (marked F) between two hemispheres (marked S) placed on a glass substrate (marked G), as shown in figure 1, where the evaporation is thought to take place from above. For preparation of a tunnel junction the substrate is removed from FIG. 1. The principle for evaporation of a weak link. The hemi-spheres are marked S, the substrate is marked G and the metal film is marked F. the hemi-spheres and a cross-strip is evaporated after and oxidation process. In the following a short description of the mechanical arrangement allowing one to mount the two hemi-spheres on the glass substrate and to separate them less than 1 pm apart is given. The hemi-spheres are made out of two precision spheres (chromium steel) for ball-bearings with diameter 390 ± 0.1 ym (also spheres with a diameter of 0.35 mm have been used). The spheres are ground and polished to hemi-spheres with an equatorial diameter about 0.35 mm and a height about o.15 mm, The last grinding and polishing is performed with diamond powder in order to secure a plane bottom Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:0197400901019100