ELSEVIER Microelectronic Engineering 46 (1999) 149-152 Nonorganic evaporation mask for superconducting nanodevices T.Hoss, C.Strunk and C.Schhnenberger Institut fiir Physik, Universit£t Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland MICI~ELECTgONIC We describe a novel technique to produce submicron thin film structures of high melting superconducting materials (Nb). The method is based on a nonorganic evaporation mask (Si3N4) to avoide any outgassing of the mask material during the metal deposition which would deteriorate the superconducting properties of the Nb. The mask has a large offset from the substrate so that clean interfaces of different materials (e.g. normal metal/ superconductor (NS)) can be achieved by angle evaporation in one single process step. By this means we have prepared narrow Nb wires with high transition temperature and NS structures with high quality interfaces. Recently, several groups [1] have encountered the problem that submicron niobium structures patterned with ordinary PMMA resist had a sig- nificantly suppressed transition temperature Tc compared to the Tc of a coevaporated niobium film. The most important reason is the out- gassing of the PMMA during the evaporation of the high melting Nb due to radiation heat- ing from the evaporation source. Most experi- ments which involve superconductors have there- fore been performed with low melting materials (e.g. A1). However niobium based junctions are very interesting because of the larger supercon- ducting gap (A) which allows to study a wider energy range of temperature and magnetic field dependent effects. Also for future technical ap- plications, e.g. bolometers [2], Nb would be advantageous. An established method to fabri- cate junctions with well defined interfaces is the use of angle evaporation through a suspended mask [3]. By varying the angle between the sub- strate and the evaporation source one can fabri- cate very clean interfaces with small overlap or wires of very narrow linewidth. The commonly used method for shadow evaporation is the lift off technique. Two different photoresists with different sensitivity provide high resolution pat- tern with a suitable undercut profile. The double layer resist of polymethylmethacrylate (PMMA) as mask and a co-polymer (PMMA-MA) as sup- port has shown to be successful for low melting materials [4]. For high melting materials more complicated techniques were developed like a two layer metal mask which was differentially etched [5] or a four layer resist system [1]. The first is not applicable for angle evaporation of submicron structures because of the rough edges of the mask. The latter still had the problem of deterioration of the superconducting properties. The Tc was suppressed up to 1/5 of its bulk value. Our in- vestigation showns that not only the background pressure during evaporation but in particular the outgassing of the organic resist mask is the main source of impurities in the deposited material. This is most important for small devices where the whole structure is in the vicinity of the mask material. We have developed a nonorganic evaporation mask to avoid any outgassing of the stencil during evaporation. Figure 1 shows a chart of the pro- cess. We start with a 800nm SiO2 and a 200nm Si3N4 layer on a Si wafer. Both materials were de- posited by Low Pressure Chemical Vapor Depo- sition (LPCVD). On the Si3N4 a 600nm PMMA layer is spun. The PMMA top layer is structured by conventional e-beam lithography (Jeol JSM- IC848). After development in 1:3 MibK:IPA the PMMA is used as an etchmask for the Si3N4. The pattern is transferred to the Si3N4 by a CHF3 anisotropic Reactive Ion Etching (RIE). The etch ratio of Si3N4 to PMMA is 1/3 at 0.03 mbar and 50W. At this step we have to accept a little widening of the structure because the width of the opening in the Si3N4 will correspond more to the bottom width of the PMMA undercut profile, because the overhanging PMMA is etched more 0167-9317/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0167-9317(99)00047-7