A monolithic thin film DC-SQUID

Abstract

Superconducting quantum interference devices with tunnel junctions were reported by Clarke et al. /l/, Ketchen et al. /2, 3/, and Ehnholm et al. /4/ Using a cylindrical thin film SQUID in which'is placed a wire wound input coil Clarke et al. have reached the highest sensitivity up to now. Disadvantages in further enhancement of the sensitivity of this construction are firstly the cylindrical shape which leads to an undesirable influence by external magnetic perturbation fields and secondly the wire wound input coil. This coil allows only relatively small values for the coupling factor between coil and SQUID, especially in the case of high sensitive devices with low inductances. The thin film SQUID with thin film gradiometer antenna reported in /2, 3/ is not extremely sensitive because of the mismatch between the high gradiometer inductance and the low inductance of the SQUID. We have solved the problem of coupling the input signal into the SQUID with a construction which uses consequently the flat thin film technique. A sufficiently low SQUID inductance and number of windings of the input coils required for matching are realizable by means of the modarn thin film technology only, if a superconducting screening film is used. A schematic of our device is shown in Fig. 1. The SQUID cons i s t s of a first superconducting film which forms the contacts 1 to 6, the input coils represented by the dashed lines in the figure, and the ground electrode of the tunnel junctions 7 in the middle part of the device. A second superconducting film insulated from the f i r s t with the exception of the windows for the tunnel junctions f o r m s a double hole loop which is closed in the middle part by the junctions and the first superconducting film. The tunnel junctions are shunted by two normal conducting thin film stripes 8 connecting the second superconducting film with the lower contact 6 in Fig. 1. The arrangement is completed by a second insulating film 9 and a superconducting screening film 10 which cover the whole SQUID with exception of the contacts.

The critical current of miniature thin film Josephson junctions with high current densities

“…With the film thicknesses d p b = 160 nm and dNb = 95 nm and using (11) we have estimated jiPb(0) = 39 nm and &b(O) = 80 nm. This is in agreement with results obtained previously with our films [17]. The values of d' calculated with these data using (14) are shown in Fig.…”

Critical current versus magnetic flux characteristic of DC tunnel SQUIDS

“…2a shows the dependences of the relative modulation depth AIc/Icm (curve 1) and y-l (curve 2) on the value of LIcm/Go(L = Ll + L,, I,, = Iol + IO2). Assuming symmetrical feeding of the SQUID (Ll = L,) the curves were obtained from (3) by numerical calculation of the maximum critical current of the SQUID at QX = O.6@,.…”

“…Fig. 3 shows calculated Ic(CDz) characteristics with different asymmetries of current feeding into the loop (LIcm = Go) carried out with (3). The curves are shifted against one another.…”

“…The other structure is a SQUID gradiometer with an integrated input coil of 15 turns and superconducting screening to reduce the inductance of the SQUID to the H range described in [3]. Instead of the photoresist insulation between input coil and the SQUID loop a SiO insulation, about 1 pm thick, was used.…”

Development of a high sensitive receiver system for transient electromagnetics

“…In the multiloop magnetometer, as described in detail in [132][133][134][135], the superconducting pickup loop is divided into a number of separate loops connected in parallel to reduce the total SQUID inductance. This allows a lower SQUID inductance while maintaining a large effective area.…”