Method of making an improved superconducting quantum interference device

An improved superconducting quantum interference device is made by sputtering a thin film of an alloy of three parts niobium to one part tin in a pattern comprising a closed loop with a narrow region, depositing a thin film of a radiation shield such as copper over the niobium-tin, scribing a narrow line in the copper over the narrow region, exposing the structure at the scribed line to radiation and removing the deposited copper.

BACKGROUND OF THE INVENTION 
This invention relates to superconducting quantum interference devices 
(SQUIDs). 
The SQUID is a well-known device that is useful for measuring extremely 
low-level changes in magnetic fields and in measuring magnetic fields of 
extremely low magnitudes. The SQUID is a closed loop of superconducting 
material in which a portion of the loop, referred to as the weak link, is 
caused to have weakened superconductivity or to be a non-superconducting 
substance. This is desirable because a loop that is closed and is strongly 
superconducting is required by the principles of quantum mechanics to 
enclose magnetic fields in units of discrete quanta. One quantum of 
magnetic flux is of the order of 2 .times. 10 exp (-15) webers and it is 
impossible to measure changes of magnetic flux in increments smaller than 
this using a superconducting loop. The introduction of a weak link into 
the superconducting loop destroys the quantization and makes it possible 
to measure magnetic flux in quantities smaller than one quantum of flux. 
The foregoing facts are well known and have been described in numerous 
journal articles and patents. SQUIDs are available commercially for 
measurements of extremely small magnetic fields and for measurements of 
extremely small changes in magnetic fields that are not themselves 
necessarily that small. However, only one SQUID is known to have been made 
to date using superconducting materials that operate at temperatures above 
10 K. This is niobium nitride in single crystal form that is grown 
epitaxially on a substrate of magnesium oxide. Such devices are limited to 
planar geometry and have not appeared to be feasible for commercial 
production because the weak link in niobium nitride can be made only by 
mechanical scribing. This leads to properties that are not readily 
reproduced. All other such devices have the disadvantage that liquid 
helium is required to cool them to operating temperatures. Liquid helium 
is quite difficult to use outside a laboratory, as for example in 
applications such as aerial prospecting, where an extremely sensitive 
magnetometer is of much use. The application of SQUIDs could be extended 
if they could be used at temperatures achievable with closed-cycle 
refrigeration systems instead of liquid helium. 
It is an object of the present invention to provide a method of making an 
improved SQUID. 
It is a further object of the present invention to provide a method of 
making a SQUID that is operable at temperatures achievable by closed-cycle 
refrigeration systems. 
It is a further object of the present invention to provide a method of 
making a SQUID using a thin film of an alloy of 3 parts niobium to one 
part tin. 
Other objects will become apparent in the course of a detailed description 
of the invention. 
SUMMARY OF THE INVENTION 
A thin-film SQUID operable at temperatures above 10 K is produced by 
sputtering a thin film of an alloy of 3 parts niobium to one part tin on a 
substrate in a shape that comprises a closed loop with a narrow portion. 
The sputtered material is then covered with an evaporated coating of a 
radiation shield such as copper which is scratched through at the narrow 
portion in a direction that is transverse to the loop. The area of 
superconductor exposed or nearly exposed by scratching the shield is 
subjected to particle radiation in controlled amounts to make a weak link, 
after which the remaining radiation shield is removed. The resulting 
device comprises a thin-film SQUID.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a view of a thin-film SQUID. In FIG. 1 a thin film 10 of an alloy 
comprising three parts niobium to one part tin has been formed on a 
substrate 12 of a material such as sapphire selected to support the thin 
film and withstand the processes of sputtering and etching and to be 
usable at cryogenic temperatures of the order of 10 K. Thin film 10 is 
placed to a thickness in the range of 20 to 500 nanometers on substrate 12 
in a pattern that forms a loop about opening 14. One portion of the loop 
of thin film 10 is weak-link region 16 which is shown in expanded view in 
FIG. 2. Weak-link region 16 includes a strip 18 of niobium-tin alloy that 
is of the order of 5 to 30 micrometers in width and has a length of the 
order of 250 micrometers. Weak link 20 is a very short portion of material 
that is weakly superconducting. The weak link has a thickness of the order 
of 0.5 to 5 micrometers. This is a distance that is sufficiently small to 
permit tunneling of paired electrons to comprise the conduction of current 
in a closed circuit around the superconducting link. While the 
requirements on the weak link are only that it be extremely short, small 
in cross-section, and weakly superconducting, the desirability of making a 
thin-film SQUID leads to a method of making the weak link that involves 
weakening or destroying the superconductivity of a small region of the 
niobium-tin alloy that is used as the superconductor in the remainder of 
the device. 
FIG. 3 is a representation of an alternate embodiment of an apparatus 
produced by the method of the present invention. This is an apparatus in 
which two loops are in parallel with the same weak link. In FIG. 3 thin 
film 24 of an alloy of 3 parts niobium to one part tin is deposited by 
sputtering on substrate 26 in a pattern that leaves two openings 28 and 
30. Weak-link region 32 is identical to the weak-link region 16 of FIGS. 1 
and 2, and the method of making the structure of FIG. 3 is the same as the 
one to be described below for the SQUID of FIG. 1. The device of FIG. 3 is 
useful for making differential measurements of magnetic fields comparing 
the fields enclosed in openings 28 and 30, whereas the SQUID of FIG. 1 is 
useful for making absolute measurements of the magnetic field through 
opening 14. Structures such as those of FIGS. 1, 2 and 3 are known in the 
art as described above. The present invention comprises a new method of 
making such structures that have the advantage of being operable at 
temperatures achievable by closed-cycle refrigeration systems. This avoids 
the use of liquid helium for achieving the necessary cryogenic 
temperatures and thus extends the use of SQUIDs to places where it is 
impossible or very difficult to use liquid helium. The method of 
manufacture comprises the following steps. A thin film of an alloy of 
three parts niobium to one part tin is sputtered onto a prepared substrate 
such as substrates 12 and 26. A pattern with an open loop, such as that of 
FIG. 1, or two loops, such as that of FIG. 3, or any appropriate such 
pattern is achieved in the thin film either by masking during sputtering 
or by conventional techniques of photoetching after the thin film has been 
sputtered. A thin film of copper is then sputtered over the entire film of 
niobium-tin. A line of width in the range 0.5 to 5 micrometers is 
scratched as with a razor blade at the desired location of the weak link, 
leaving a thin groove in the copper. The surface is exposed to radiation 
of alpha particles in an accelerator. The copper plating should be thick 
enough so that the available energy range of the alpha particles ensures 
that all of the structure is protected against radiation damage except the 
region of the scratch. Radiation will then reach the niobium-tin alloy 
only at the scratch to damage its superconducting properties there. The 
copper is then removed from the surface leaving a completed thin-film 
SQUID. 
SQUIDs have been made according to the method of the present invention at 
the Argonne National Laboratory. An alloy of three parts niobium to one 
part tin was sputtered on a substrate of sapphire to thicknesses of 50 to 
100 nanometers. This deposit was photoetched and the excess alloy was 
removed to leave patterns of niobium-tin similar to those of FIGS. 1 and 
3. Copper was then deposited to cover the niobium-tin to thicknesses of 
the order of 400 nanometers. This material and thickness were chosen 
because of the properties of the available accelerator. The structures 
were then irradiated with accelerated alpha particles at an energy of 50 
KeV to a dosage of the order of 10 exp (14) ions per square centimeter. 
After removal from the accelerator, the structure was treated with nitric 
acid to remove the copper, leaving a SQUID. 
Results of measurements on one such SQUID are shown in FIG. 4, which is a 
plot of observed voltage as a function of temperature. The voltage is 
obtained by exposing the SQUID to a flux change of approximately one 
quantum (2 .times. 10 exp (-15) webers) peak to peak at a frequency of 19 
MHz. The curve of FIG. 4 is seen to achieve a maximum voltage at a 
temperature of approximately 14.36 K. This can be achieved by closed-cycle 
refrigeration systems. As the temperature increases above this point, the 
superconductivity begins to decrease in the main loop of the SQUID, 
reducing the output. Below the peak, the weak link becomes more nearly 
superconducting, thus reducing the output for a flux change of one 
quantum. The signal level could clearly be extended beyond the points 
shown, however, by routine measures obvious to one skilled in the art. 
These measures, characteristic of the measurement of small voltages, 
include especial attention to grounding and to electrostatic shielding to 
reduce the interference of electrical noise. Even without such measures, 
the curve of FIG. 4 shows a SQUID operable over a useful range of 
temperatures that is high enough to be achievable by commercially 
available closed-cycle refrigeration equipment. 
While copper was used as an alpha shield to produce the SQUID of FIG. 4, it 
should be understood that other types of shielding could equally as well 
be used, as could other types of radiation. Bombardment by electrons, 
neutrons, or heavy ions would work as well, requiring only the selection 
of an appropriate material such as aluminum or lead in a thickness 
appropriate to stop bombarding particles everywhere except at the line 
marked to make the weak link. A razor blade was used to mark the line, but 
this was only one choice. A scriber or a ruling machine would function as 
well to remove enough of the protective material to allow the 
superconducting material to be damaged enough by bombardment to generate a 
weak link.