Reliable superconducting link for superconducting thin film components and a method for fabricating the contacts

Abstract: A reliable superconducting contact for superconducting thin film components, based on soft superconducting materials, and a method for making the contacts. The joint, according to this invention, consists of a thin bonding slab having its end longitudinally cut so that double or multiple parallel joints can be made. In the method, a wire blank (1) is squeezed to a thin plate and then this plate (2) is cut to slabs (3) used in the bonding. The ribbon-like character and the thinness of the bonding slab permit a precise control of the bonding temperature and reduce the strain and bending forces caused by thermal contraction, preventing the contact from peeling off while cooling the superconducting component to the operational temperature.

Description: A reliable superconducting link, resistent to thermal cycling, is necessary in the construction of multicomponent superconducting devices, such as multichannel magnetometers, where simultaneous faultless operation of several superconducting devices is required. Such devices are used, for example, in the detection of feeble biomagnetic signals from the human brain. Superconducting contacts between electronic components fabricated by thin film technology on substrates, such as silicon wafers, quartz, sapphire or glass, is technically problematic for the following reasons. 1) The electrical contact must be superconducting which requires a clean metal-to-metal contact between the bonding wire and the thin film. 2) The differences in thermal contraction of the various construction materials (silicon, lead, tin, niobium, printed circuit board made of fiber glass) lead to considerable thermal stresses at the joints because, in order to achieve superconductivity, the devices must be cooled far below their fabrication temperature. 3) Fixing the broken contacts at the operation temperature is impossible. Therefore, especially in devices comprising a large number of superconducting components, the joints must be extremely reliable and resistent to thermal cycling so that when warming up the device to fix a link one does not break any of the other parallel joints. The prior art of superconducting bonding technology utilizes lead, tin-lead or niobium wires because these materials are sufficiently soft for mechanical bonding and their superconducting transition temperatures are well above the liquid helium temperature (e.g. S. Kiryu et al . in Advances in Biomagnetism, eds. S. J. Williamson, M. Hoke, G. Stroink, M. Kotani, Plenum Press, New York 1990). Lead, tin and their alloys melt at low temperatures (200-400 °C) so that the softening of these materials slightly below the melting point can be utilized in the bonding. Ultrasound rather than heating is used when bonding with niobium wires because the melting point of Nb is very high (2470 °C). Compared to lead, niobium wire is also stiff so that to avoid too high stresses at the joints either long or very thin wires (<10 µm) must be used. Thin wires are difficult to handle and long wires come off easily during further handling of the bonded device. Long wires also require extra space and, especially in the magnetometer applications, form parasitic superconducting pickup loops. Tin and lead are too soft to be bonded with ultrasound. Wires made of these materials are usually pressed against the bonding pad of the thin film device by aid of a small, hot soldering tip. However, because typical substrate materials are good conductors of heat these wires melt easily and get stuck on the soldering tip before the intended proper softening at the wire-thin film interface. The resulting joint may be mechanically weak, come off in cooling or it is not superconducting throughout. Even if the bonding is successful the stress at the joint, mediated by the relatively thick wire (40-100 µm), may break the bond during cooling. GB-A-1322799 describes a method to bond two superconductive materials together. The document deals especially with hard superconductive wires. The joining is performed by welding, followed subsequently by thermal-mechanical treatment. The document does not solve the problem of making a reliable superconducting link to planar thin-film superconductor since it deals with bonding two wires together; and the techniques cannot be applied to a thin-film device. Kokai-no 1-89174 describes a method joining two superconducting ceramic conductors by pasting the conductor ends with superconductive paste and baking the whole at 700 - 1100° C. The document does not solve the problem of making a reliable superconducting link to planar thin-film superconductor since it deals with bonding two high-T c wires together; and the techniques cannot be applied to a thin-film device as such. The cited application does not solve the reliability problem related with peeling-off as characteristic of round conductors bonded on flat substrates. This invention describes a new type of superconducting lead or tin-lead link by which the problems associated with the prior art techniques described above can be avoided. The features characteristic to this superconducting joint are described in claims 1 and 2. The characteristics of the fabrication method are described in claims 4 and 5. When doing thermal bonding with a squeezed, thin strip-like bonding wire characteristic to this invention the temperature of the relatively large planar interface between the flat wire and the bonding pad is essentially equal to the temperature of the bonding tip. Thus, the bonding temperature is easily controlled and can be maintained high enough, to ensure a good metal-to-metal contact at the joint, without melting the wire from the tip side. Splitting of the bonding wire from one or both ends is easy because the wire is flat. Splitting allows making parallel, independent multiple contacts with one wire. This adds to the reliability of multijoint devices essentially: Let us assume that the device consists of N individual elements, and the functioning of each element depends on two joints in series (current in and out). If the failure probability of one contact in cooling is p , and each of the 2N contacts comprises a single bond, the probability of a successful cooling of the entire N -component device is P ₁ = (1- p ) 2N . If double contacts are used instead, the probability of success becomes P ₂ = (1- p ²) 2N . To avoid excessive thermal cycling of the entire device one should succeed in every other cooling, say. That requires a success probability P > 0.5. Thus, by assuming that, with the bonding technique used, one out of hundred contacts fail ( p = 0.01) one gets an upper bound for the feasible number of singly bonded elements N < 34, whereas for double-bonded elements N < 3470. By using a proper pressing force and temperature of the bonding tip one is able to shape the bonding slab described in this invention into a U-beam shape around the joint. This makes the slab stiff at the joint and prevents it from coming off by peeling. When the different parts of the device contract in cooling, the length of the bonding slab and the distance between the joints at the opposite ends of it change causing strain and stress at the joints. These changes in dimensions are compensated by the bending of the thin ribbon-like bonding slab far from the joint region. The force due to bending of a beam is proportional to the width and to the third power of the thickness so that a 10 µm × 100 µm slab mediates a force which is roughly by factor 100/50 × (10/50)³ = 0.02 smaller than that mediated by a typical 50 µm round wire bent in the same way. Therefore, the durability of a link made of a thin slab is considerably increased over a round-wire link, especially when the joints are repeatedly warmed up and cooled down during the maintenance of the cryogenic device. The invention is described in detail below, with reference to Figures 1 to 3, where in Figure 1 the shaping of a contact wire blank into a thin, sliced contact slab is described. Figure 2 shows the mounting of the slab, and Figure 3 illustrates the final double joint. As shown in figure 1, the wire blank 1, of about 100 µm diameter, is pressed or rolled into a plate 2, which is about 10 µm thick and 3-4 mm wide. This plate is sliced with the desired number of partial cuts 4, and then detached from the blank with a long cut 5 into a final bonding slab 3. The slab is then placed to connect the devices located on two different substrates 6 and 7 so that its ends, as shown in figure 2a, rest on the contact pads 8 and 9. The slab is fixed from one of its ends by pressing with the hot soldering tip 10 against the bonding pad 8. After that the slab is bent as shown in figure 2b and bonded from the other end, too. Figure 3 is a cut view of the ready-made double joint showing the U-beam like deformation of the joint regime. The slab adapts to the relative motion of the bonding pads, caused by the differences in the thermal contraction, by bowing at the intentional bend 11 far from the U-beam like bonding regions.

Claims: Superconducting link between a printed circuit board and a superconducting thin film component on a different substrate (6, 7) or between two superconducting thin film components on different substrates (6, 7), characterized in that the link is formed by a thin plate (3) made of soft elastic superconducting material which comprises a flat and a bent part (11) releasing the cooling stress on the bonding joints caused by the differences in the thermal contraction of the parts joined together, and stiff joints deformed into U-beam shape in the thermal bonding process where it is connected to the or each thin film component. Superconducting joint according to claim 1, characterized in that the thin plate is a bonding slab (3) having been longitudinally cut (4) from the end connected with the thin film component or from both ends, into two or more branches and each branch thus formed is separately bonded on bonding pads (8, 9). Superconducting link according to claim 1 or 2, characterized in that the thickness of the thin plate (3) is 5 - 200 times smaller than its width. Method for fabricating a superconducting link according to any of claims 1 to 3, characterized in that the thin plate (3) is pressed against the bonding pad (8, 9) by aid of a soldering tip (10) having a flat small-area end, and at a temperature below the melting point of the bonding slab (3) so that the slab (3) is deformed into a U-beam shape around the joint. Method according to claim 4, characterized in that the bonding slab (3) is made by flattening out a round wire (1).

Inventors: AHONEN ANTTI ILMARI, KAJOLA MATTI JAAKO, SIMOLA JUHA TAPANI ANTERO, AHONEN, ANTTI ILMARI, KAJOLA, MATTI JAAKO, SIMOLA, JUHA TAPANI ANTERO
Applicants: NEUROMAG OY
Application year: 1991
Main classification: H01R4
IPC codes: H01R4, H01R43, H01R12