Dc SQUID element with quasi-planar-type Josephson junction

In a dc SQUID element having two quasi-planar-type Josephson junction portions, as obtained by laminating a plurality of superconducting thin films on a substrate, a SQUID ring and a counter electrode on either of which quasi-planar-type Josephson junction portions are to be formed, are respectively formed at the lowermost layer and the uppermost layer, or at the uppermost layer and the lowermost layer, so that the value of critical current can be adjusted. The arrangement above-mentioned assures good flatness and film quality of a barrier layer interposed between the lower and upper electrodes of the quasi-planar-type Josephson junction portions. In a method of manufacturing the dc SQUID element having the arrangement above-mentioned, a protective film is formed on the thin film of the lowermost layer at the position where Josephson junction portions are to be formed, after which layers except for a thin film of the uppermost layer are formed, and the protective film is then removed immediately before the uppermost-layer thin film is formed, thus preventing damages to the Josephson junction forming area in the course of production of the dc SQUID element.

BACKGROUND OF THE INVENTION 
The present invention relates to a dc SQUID to be used for measurement of a 
micromagnetic field such as instrumentation of biomagnetism, magnetic 
airborne detecting or the like. 
There is known a Josephson junction of the quasi-planar type in which an 
upper electrode is formed on a lower electrode through a barrier layer, 
and the surface of the lower electrode is weak-linked to the surface of 
the upper electrode by a bridge. 
In such a Josephson junction of the quasi-planar type, the weak-link length 
is determined by the thickness of the barrier layer interposed between the 
upper electrode and the lower electrode. Thus, the Josephson junction of 
the quasi-planar type is advantageous in that the weak-link length is 
determined by the adjustment of te film thickness of which control is 
relatively easy, as compared with a Josephson junction of the planar type 
requiring micro-machinings on the planar face. 
FIG. 7 shows an example of the arrangement of a dc SQUID element having a 
conventional Josephson junction of the quasi-planar type with interlaminar 
insulation and barrier layers seen in a perspective manner. FIG. 7 (a) is 
a general plan view, while FIG. 7 (b) is an enlarged view of a portion B 
in FIG. 7 (a). 
In this example, the dc SQUID has an arrangement of total four 
superconducting thin films in lamination in which a modulation coil 1 and 
an input coil 2 are formed at the lowermost layer on a substrate, and an 
input coil leading electrode 21 and a groundplane 3 are formed at the 
layer above the lowermost layer. A SQUID ring 4 is formed at the layer 
above the layer of the electrode 21 and the groundplane 3, and a counter 
electrode 5 is formed at the uppermost layer. 
The SQUID ring 4 and the counter electrode 5 are weak-linked at two 
portions to each other, through a barrier layer (not shown) interposed 
therebetween, by a bridge 6 striding over the counter electrode 5 at both 
lateral edges thereof. Thus, two Josephson junctions 71, 72 are formed. 
The arrangement above-mentioned may be produced, for example, according to 
steps as shown in plan views of FIGS. 8 to 11, in which the interlaminar 
insulation and barrier layers are not shown as seen in a perspective 
manner. 
First, a superconducting thin film of the Nb type or the like is deposited 
on the substrate, which is then patterned to obtain the modulation coil 1 
and the input coil 2. This state is shown in FIG. 8. 
After an interlaminar insulation layer and contact holes are formed, a 
superconducting thin film is deposited from above. This film is patterned 
to form the groundplane 3 and the input coil leading electrode 1. This 
state is shown in FIG. 9. 
An interlaminar insulation layer and contact holes are then formed, and a 
superconducting thin film is deposited from above. This film is patterned 
to form the SQUID ring 4 as shown in FIG. 10. 
A barrier layer is formed on the entire surface of the SQUID ring 4 serving 
as the lower electrode. A superconducting thin film is then deposited on 
this barrier layer. This film is patterned to form the counter electrode 5 
also serving as the upper electrode. This state is shown in FIG. 11. 
Finally, a superconducting thin film is deposited from above and the bridge 
6 having the pattern as shown in FIG. 7 is formed. 
In a tunnel-type Josephson junction element or the like, it is not possible 
to adjust the value of critical current after the element has been formed. 
Accordingly, the order of preparing the respective layers above-mentioned 
does not particularly cause trouble. In a quasi-planar-type Josephson 
junction element, however, the value of critical current can be adjusted 
while monitoring the same by anodic oxidation or the like. To utilize such 
advantage, the bridge 6 is preferably formed at the last step. 
In this point of view, the conventional element arrangement and 
manufacturing method mentioned earlier are reasonable in that the SQUID 
ring 4 and the counter electrode 5 respectively serving as the lower and 
upper electrodes for forming Josephson junction portions, are formed on 
the modulation coil 1 and the input coil 2, and the bridge 6 is formed at 
the uppermost layer. 
When a number of films are laminated in the manner above-mentioned, it is 
inevitable that the surface flatness and quality of each of films of 
layers at upper positions are deteriorated more. 
In the quasi-planar-type Josephson junction element, the film flatness and 
quality of the lower electrode (the SQUID ring 4 in the example 
above-mentioned) exerts a great influence upon the quality of the barrier 
layer which is formed above the lower electrode and which not only assures 
the insulation with respect to the upper electrode (the counter electrode 
5 in the example above-mentioned), but also determines the weak-link 
length. In view of the foregoing, if such advantage of the 
quasi-planar-type Josephson junction element as to adjust the value of 
critical current is sacrificed, it may be considered rather preferable to 
dispose the lower electrode and the upper electrode at layers as lower as 
possible in the element, and in an extreme case, to dispose the Josephson 
junction portions at the lowermost layer.

OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a dc SQUID element 
which simultaneously satisfies two contradictory requirements of a 
quasi-planar-type Josephson junction element, and to provide a method of 
manufacturing such a dc SQUID. 
To achieve the object above-mentioned, the dc SQUID element in accordance 
with the present invention is arranged such that a SQUID ring and a 
counter electrode are respectively formed at the uppermost layer and the 
lowermost layer.on a substrate, or at the lowermost layer and the 
uppermost layer on the substrate. A modulation coil and an input coil are 
formed between these two layers. The top surface of the film deposited at 
the lowermost layer (the SQUID ring or counter electrode) presents an area 
where the modulation coil, the input coil and an interlaminar insulation 
layer are not being formed. At this area, the upper-most-layer film (the 
counter electrode or SQUID ring) is deposited through a barrier layer and 
two quasi-planar-type Josephson junction portions are formed there by a 
bridge. 
In the element having the arrangement above-mentioned of the present 
invention, the barrier layer which determines the weak-link length in the 
quasi-planar-type Josephson junction element, is formed on the thin film 
deposited on the lowermost layer of the substrate. This prevents the 
barrier layer from being deteriorated in film flatness and quality. 
Further, the bridge is formed on the thin film at the uppermost layer, 
thus enabling to adjust the value of critical current by anodic oxidation 
or the like, which is the structural advantage of the quasi-planar-type 
Josephson junction. 
According to the present invention, the method of manufacturing the dc 
SQUID element having the arrangement above-mentioned comprises the steps 
of forming the SQUID ring or counter electrode on the substrate and 
covering, with a protective film, the film forming the SQUID ring or 
counter electrode at predetermined area thereof including and in the 
vicinity of the area where the Josephson junction portions are to be 
formed. Then, the modulation coil and the input coil are formed at a film 
above the lowermost layer, and the protective film is removed immediately 
before a superconducting thin film is deposited at the uppermost layer. 
The thin film at the uppermost layer is then deposited and patterned, and 
the bridge is formed to form two Josephson junction portions. 
According to the manufacturing method above-mentioned, while that 
predetermined area of the thin film of the lowermost layer including a 
portion where the Josephson junction portions are to be formed, is covered 
with the protective film, the superconducting thin film for the modulation 
coil and the like is deposited and patterned, and the films for the 
interlaminar insulation layers and the like are then deposited. This 
prevents damages to that portion of the superconducting thin film where 
the Josephson junction portions are to be formed. 
According to the manufacturing method of the present invention, RIE 
(Reactive Ion Etching) or sputtering may be suitably used for removing the 
modulation coil and the input coil made of a superconducting thin film of, 
for example, Nb formed on the protective film and for removing the 
interlaminar insulation layers of SiO.sub.2 or the like. 
The protective film should be made of a material which can resist such 
treatments and can be readily removed before the thin film at the 
uppermost layer is deposited. The protective film is preferably removed by 
wet-etching. In view of the foregoing, Al or MgO is suitable as the 
material satisfying the conditions above-mentioned. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The following description will discuss a preferred embodiment of the 
present invention with reference to the attached drawings. 
As shown in FIG. 1 (a) to (c), a SQUID ring 4 is formed on the surface of a 
substrate 9, i.e., at the lowermost layer. A modulation coil 1, a 
modulation coil leading electrode 11 and an input coil 2 are formed on the 
SQUID ring 4 through an insulation layer electrode 21 are formed on the 
modulation coil 1 and the electrode 11 through an insulation layer 102. 
A barrier layer 103 is formed directly on the thin film of the lowermost 
layer at one end of the SQUID ring 4, and a counter electrode 5 is formed 
through the barrier layer 103. The surface of the counter electrode 5 and 
the surface of the SQUID ring are weak-linked to each other by a bridge 6 
striding thereover. There are thus formed two quasi-planar-type Josephson 
junction portions 71, 72. 
In the arrangement above-mentioned, attention should be placed on the fact 
that the barrier layer 3 which determines the link length of the Josephson 
junction portions 71, 72, is formed immediately above the SQUID ring 4 
which is the lowermost film directly deposited on the substrate 9. This 
improves not only the flatness of the barrier layer 103 but also the film 
quality thereof. Further, the bridge 6 is formed at the uppermost layer of 
the element as done in the conventional element mentioned earlier. It is 
therefore possible to carry out the adjustment of the value of critical 
current by anodic oxidation or the like which is the advantage of the 
Josephson junction of the quasi-planar type. 
The following description will discuss a suitable method of manufacturing 
the element having the arrangement above-mentioned with reference to plan 
views of FIGS. 2 to 6. For convenience sake, the interlaminar insulation 
layers 101, 102 are not shown in FIGS. 2 to 6. 
First, a superconducting thin film is deposited on the substrate 9 and the 
SQUID ring 4 as shown in FIG. 2 is patterned. As shown in FIG. 3, a 
protective Al film 8 is formed on the SQUID ring 4 at a position thereof 
on which the Josephson junction portions 71, 72 are to be formed. 
Then, the interlaminar insulation layer 101 is deposited on the entire 
surface of the substrate 9 from above and necessary contact holes are 
formed. A superconducting thin film is deposited on the insulation layer 
101 and then patterned to form the modulation coil 1, the modulation coil 
leading electrode 11 and the input coil 2. This state is shown in FIG. 4. 
In the state shown in FIG. 4, the interlaminar insulation layer 102 is 
deposited on the entire surface of the modulation coil 1, the electrode 11 
and the input coil 2 from above, and necessary contact holes are formed. 
Then, those portions of the insulation layers 101, 102 on the protective 
film 8, located on the positions where the Josephson junction portions are 
to be formed, are completely removed by RIE or sputter-etching. In the 
etching, the protective film 8 serves as a stopper. 
Then, the protective film 8 is removed by wetetching and the barrier layer 
103 is formed by oxidation or the like of the surface of that portion of 
the SQUID ring 4 corresponding to the protective film 8. 
A superconducting thin film is deposited on the barrier layer 103 and 
patterned to form the counter electrode 5. This state is shown in FIG. 6. 
In order that the SQUID ring 4 and the counter electrode 5 come in 
superconducting contact with the bridge 6, the entire surface of the 
element is sputter-etched and a superconducting thin film is then 
deposited in order to form the bridge 6. 
A resist is then applied to the element surface. The resist is exposed to 
an electronic beam in a bridge pattern and then developed. With the resist 
film serving as a mask, the element is subjected to sputter-etching or RIE 
to pattern the bridge 6. There is thus obtained the dc SQUID element 
having the arrangement shown in FIG. 1. 
In the manufacturing method above-mentioned, attention should be placed on 
the fact that, while that portion of the SQUID ring 4 (which is located at 
the lowermost layer and serves as the lower electrode) on which the 
Josephson junction portions are to be formed, is protected by the 
protective film 8, the modulation coil 1 and the input coil 2 are 
patterned and the interlaminar insulation layers 101, 102 or the like are 
removed by etching. This prevents damages to that portion of the 
superconducting thin film where the Josephson junction portions are to be 
formed. 
Accordingly, the protective film 8 should be made of a material which 
presents a certain measure of durability with respect to RIE or 
sputter-etching and which can be readily removed by wet-etching before the 
counter electrode 5 is formed. As such a material, there may be suitably 
used MgO, in addition to Al mentioned earlier. 
The description has been made of the embodiment in which the SQUID ring 4 
is used as the lower electrode and the counter electrode 5 is used as the 
upper electrode. However, the present invention may be embodied as a dc 
SQUID element in which the counter electrode 5 is formed as the lower 
electrode at the lowermost layer and the SQUID ring 4 is formed as the 
upper electrode at the uppermost layer. Such a dc SQUID element produces 
the same effects as those produced by the embodiment above-mentioned. This 
may also apply to the steps of the manufacturing method. That is, when the 
alternate arrangement above-mentioned is adopted, the SQUID ring 4 and the 
counter electrode 5 may be mutually replaced with each other in the 
description in connection with the manufacturing method mentioned earlier. 
It is a matter of fact that the layer including the modulation coil 1 and 
the input coil 2 and the layer including the groundplane 3 may be mutually 
replaced with each other in position.