The electrical instabilities during ultra-fast S-N switching in high-Tc thin film microstrip

PUYSlgA Physica C 235-240 (1994)3377-3378 North-Holland (~ T h e e l e c t r i c a l i n s t a b i l i t i e s d u r i n g u l t r a - f a s t S - N s w i t c h i n g in h i g h - T , t h i n f i l m m i c r o s t r i p S.BaleviOus ~, F.Anisimovas ~, V.Bal~it~nas ~, R.Butkute ~, B.Vengalis ~'and A.S.Flodstr0m ~ ~'Semiconductor Physics Institute, A.Gogtauto 11, 2600 Vilnius, Lithuania ~Materials PhysiCs, Royal Institute of Technology, S-10044 Stockholm, Sweden Voltage(V) - Current(I) characteristics and time response measurements were carried out on laser deposited thin film Y-Ba-Cu-O microstrips, using 3-80 ns duration pulses. Two types of electricalinstabilities appearing at currents significantlyhigher then critical (I~) were investigated. It was shown that first of them, which takes place during several tens of nanoseconds, is the result of heating induced by flux flow and is fully reversible process. The second- manifesting at more higher currents causes during subnanosecond time the upset of the microstrip. The behaviour of the instabilitiesis explained in terms of a bundle-like motion of magnetic vortices. 1.INTRODUCTION The investigationof S-N switching in highT~ superconducting microstrips showed, that in a high overcurrent (I> >Ic) short pulse regime, in addition to an ultra-fast (10 -118) superconducting to normal states transition, several new phenomenon takes place [1,2]. They are: the electrical instability, which manifests as relatively slow (10-~-10Ss) increase in resistance during switching pulse and fast (10-1°s) spruce-like damaging of microstrip material I1]. Both these processes are very important, if the superconducting microstrip is to be used as a protector against high-power short rise time fault current pulse [3]. The first of them determines the current clamping characteristic of the device, the second - the highest possible operating power. In this paper we present a summary of detailed studies of the electrical instabilities and propose an explanation for these phenomenon. 2. E X P E R I M E N T A L The samples for investigationwere in the form of microstriplines95 or 195 I~m wide, 3 mm long and {).054).3 ~m thick. The ends of the microstrips were connected to Ag film sections 2 I~m thick and 2 mm wide, prepared by DC magnetron sputtering. MgO(100) plates served as substrates. The YBazCu30 7 films themselves were fabricated by laser deposition. During the deposition the substrate was held at 800°C in an oxygen atmosphere at 0.2 Torr. Later the O, pressure was increased up to 760 Torr and the film was slowly cooled to room temperature. The prepared microstrips had a Tc=75-78K, AT~=5K, a total resistance at T<T~ of less then 0,5-0.7 ohm and critical current density of j~=104-10 s A/cm-" at 70K. 3. R E S U L T S Typical V-I dependence for the microstrip, measured using 81) ns duration pulses are presented in Fig.1. As can be seen, the V-1 curves obtained at different instants of time are identical only up to the current (Id), at which point the time-dependent increase in sample resistance (instability)begins. The V-I curve in the latter case consists of several lines with different slopes. For I > Ic~straight lines are typical only if VI dependence is measured immediatelyat the end of pulse rise time (I).4 ns). V-I curves representing longer times are more curved and does not exhibit straight line shape. However current-induced resistance of the microstrip was always less than the smallest resistance in the normal state at T > T c (dotted line in Fig.l). The measurements also showed that the critical power P=IclXV, which is needed for the appearance of the instability, decreases nonlinearly as the strip temperature 0921-4534/94/$07.00© 1994 - ElsevierScienceB.V. All fights reserved. SS£'I 0921-4534(94)02254-2 3378 S. Bele~i~ius e/a/./Physica C 235 240 (19941 3377 337~ 200 4/, 3 // o2 // // /// / I 0.2 0.4 0.6 0.8 (Ampers) Figure 1. Voltage-current dependence at different instants of time 1 - 0.5ns; 2 - 10ns; 3 - 30ns; 4 80ns. Microstrip wide - 1951~m, thickness - 0,21~m. approaches T c. It was obtained that the instability (i.e. time dependent resistance increase) appears without any delay, immediately following the switching pulse rise time. This process is most noticeable at the beginning (5-20 ns) of the pulse and becomes more rapid if current increase. For longer times the changes in resistance are less pronounced. We used double pulse method, described in 14] and obtained that the instabilityis always accompanied by an increase in microstrip temperature, but the increase is not large and T~ is never reached. It enable, considering the unilorm power dissipation [4], to suppose that instabilityis not the result of normal "hot" region expansion along the microstrip [21. Most probably it is due to flux flow induced temperature increase in the paths of movingvortices during several tens of nanoseconds. This can cause the penetration of additional moving vortices from the edges of the microstrip and consequently the time-dependent increase of resistance. The further increase in temperature is slopped by heat flow to the substrate of the sample as a result changes of resistance during time disappeared. It is evident that such type of instability cannot induce irreversible upset of superconducting film. In order to investigate this process, we studied SME pictures of damaged regions. It was obtained that damaging evolution can be divided to three stages. At the first stage - two narrow channels start to move from the opposite edges of the microstrip against each other. After joining they form normal Figure 2. SME picture (x4000) of damaged region. First stage of instabilily. region ("basic" channel) crossing the sample (Fig.2). The moving is accompanied by melting of thin film material and is slightly oriented to current flow direction. At the second stage the spruce-like structures grow from one side of the basic channel along the microstrip. Simultaneously the wide of the basic channel expanded. The final stage appears then in result of heating the liquid material is splashed out of the basic channel and microstrip is irreversibly damaged. All this process lasted no more thai several hundreds of picoseconds. Such high speed of the process and the behaviour of instabilityaI first stage shows thai il is also related with rapid magnetic flux flow. REFERENCES 1. S.Balevieius el al., Mat. Res. Soc. Syrup. Proc., 275 (1992) 589. 2. A.B.Kozirev, T.B.Samoilovaand S.Y.Shaferova, Supercon.: physics, chemisl~, technique, 6(4) (1993) 823. 3. R.C.Callarotiand P.E.Schmidt, Thin Solid Film, 90 (1982) 379. 4. S.Balevieius et al., IEEE transactions on Magnetics, 29, No. 6 (1993) 3589.