Superconducting device having an extremely thin superconducting channel formed of oxide superconductor material

A superconducting device comprises a substrate having a principal surface and a non-superconducting oxide layer having a similar crystal structure to that of the oxide superconductor, which has a projection at its center portion. A superconducting source region and a superconducting drain region formed of an .alpha.-axis oriented oxide superconductor thin film are positioned at the both sides of the projection of the non-superconducting oxide layer separated from each other and an extremely thin superconducting channel formed of a c-axis oriented oxide superconductor thin film is positioned on the projection of the non-superconducting oxide layer. The superconducting channel electrically connects the superconducting source region to the superconducting drain region, so that superconducting current can flow through the superconducting channel between the superconducting source region and the superconducting drain region. This superconducting device further includes a gate electrode through a gate insulator on the superconducting channel for controlling the superconducting current flowing through the superconducting channel. In the superconducting device the upper surfaces of the superconducting source region and the superconducting drain region have the same level as that of the superconducting channel.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a superconducting device and a method for 
manufacturing the same, and more specifically to a superconducting device 
having an extremely thin superconducting channel formed of oxide 
superconductor material, and a method for manufacturing the same. 
2. Description of Related Art 
Devices which utilize superconducting phenomena operate rapidly with low 
power consumption so that they have higher performance than conventional 
semiconductor devices. Particularly, by using an oxide superconducting 
material which has been recently advanced in study, it is possible to 
produce a superconducting device which operates at relatively high 
temperature. 
Josephson device is one of well-known superconducting devices. However, 
since Josephson device is a two-terminal device, a logic gate which 
utilizes Josephson devices becomes complicated configuration. Therefore, 
three-terminal superconducting devices are more practical. 
Typical three-terminal superconducting devices include two types of 
super-FET (field effect transistor). The first type of the super-FET 
includes a semiconductor channel, and a superconductor source electrode 
and a superconductor drain electrode which are formed closely to each 
other on both side of the semiconductor channel. A portion of the 
semiconductor layer between the superconductor source electrode and the 
superconductor drain electrode has a greatly recessed or undercut rear 
surface so as to have a reduced thickness. In addition, a gate electrode 
is formed through a gate insulating layer on the portion of the recessed 
or undercut rear surface of the semiconductor layer between the 
superconductor source electrode and the superconductor drain electrode. 
A superconducting current flows through the semiconductor layer (channel) 
between the superconductor source electrode and the superconductor drain 
electrode due to the superconducting proximity effect, and is controlled 
by an applied gate voltage. This type of the super-FET operates at a 
higher speed with a low power consumption. 
The second type of the super-FET includes a channel of a superconductor 
formed between a source electrode and a drain electrode, so that a current 
flowing through the superconducting channel is controlled by a voltage 
applied to a gate formed above the superconducting channel. 
Both of the super-FETs mentioned above are voltage controlled devices which 
are capable of isolating output signal from input one and of having a well 
defined gain. 
However, since the first type of the super-FET utilizes the superconducting 
proximity effect, the superconductor source electrode and the 
superconductor drain electrode have to be positioned within a distance of 
a few times the coherence length of the superconductor materials of the 
superconductor source electrode and the superconductor drain electrode. In 
particular, since an oxide superconductor has a short coherence length, a 
distance between the superconductor source electrode and the 
superconductor drain electrode has to be made less than about a few ten 
nanometers, if the superconductor source electrode and the superconductor 
drain electrode are formed of the oxide superconductor material. However, 
it is very difficult to conduct a fine processing such as a fine pattern 
etching, so as to satisfy the very short separation distance mentioned 
above. 
On the other hand, the super-FET having the superconducting channel has a 
large current capability, and the fine processing which is required to 
product the first type of the super-FET is not needed to product this type 
of super-FET. 
In order to obtain a complete ON/OFF operation, both of the superconducting 
channel and the gate insulating layer should have an extremely thin 
thickness. For example, the superconducting channel formed of an oxide 
superconductor material should have a thickness of less than five 
nanometers and the gate insulating layer should have a thickness more than 
ten nanometers which is sufficient to prevent a tunnel current. 
In the super-FET, since the extremely thin superconducting channel is 
connected to the relatively thick superconducting source region and the 
superconducting drain region at their lower portions, the superconducting 
current flows substantially horizontally through the superconducting 
channel and substantially vertically in the superconducting source region 
and the superconducting drain region. Since the oxide superconductor has 
the largest critical current density J.sub.c in the direction 
perpendicular to c-axes of its crystal lattices, the superconducting 
channel is preferably formed of a c-axis oriented oxide superconductor 
thin film and the superconducting source region and the superconducting 
drain region are preferably formed of .alpha.-axis oriented oxide 
superconductor thin films. 
In a prior art, in order to manufacture the super-FET which has the 
superconducting channel of c-axis oriented oxide superconductor thin film 
and the superconducting source region and the superconducting drain region 
of .alpha.-axis oriented oxide superconductor thin films, a c-axis 
oriented oxide superconductor thin film is formed at first and the c-axis 
oriented oxide superconductor thin film is etched and removed excluding a 
portion which will be the superconducting channel. Then, an .alpha.-axis 
oriented oxide superconductor thin film is deposited so as to form the 
superconducting source region and the superconducting drain region. 
In another prior art, at first an .alpha.-axis oriented oxide 
superconductor thin film is deposited and etched so as to form the 
superconducting source region and the superconducting drain region, and 
then a c-axis oriented oxide superconductor thin film is deposited so as 
to form the superconducting channel. 
However, in the prior art, the oxide superconductor thin film is degraded 
during the etching so that the superconducting characteristics is 
affected. In addition, the etched surface of the oxide superconductor thin 
film is roughened, therefore, if another oxide superconductor thin film is 
formed so as to contact the rough surface, an undesirable Josephson 
junction or resistance is generated at the interface. 
By this, the super-FET manufactured by the above conventional process does 
not have an enough performance. 
Summary of the Invention 
Accordingly, it is an object of the present invention to provide an FET 
type superconducting device having a superconducting region constituted of 
an extremely thin oxide superconductor film, which have overcome the above 
mentioned defects of the conventional ones. 
Another object of the present invention is to provide a method for 
manufacturing an FET type superconducting device which have overcome the 
above mentioned defects of the conventional ones. 
The above and other objects of the present invention are achieved in 
accordance with the present invention by a superconducting device 
comprising a substrate having a principal surface, a non-superconducting 
oxide layer having a similar crystal structure to that of the oxide 
superconductor, which has a flat-top projection at its center portion, a 
superconducting source region and a superconducting drain region formed of 
an .alpha.-axis oriented oxide superconductor thin film at the both sides 
of the projection of the non-superconducting oxide layer separated from 
each other, an extremely thin superconducting channel formed of a c-axis 
oriented oxide superconductor thin film on the projection of the 
non-superconducting oxide layer, which electrically connects the 
superconducting source region to the superconducting drain region, so that 
superconducting current can flow through the superconducting channel 
between the superconducting source region and the superconducting drain 
region, and a gate electrode through a gate insulator on the 
superconducting channel for controlling the superconducting current 
flowing through the superconducting channel, in which the upper surfaces 
of the superconducting source region and the superconducting drain region 
are at the same level as that of the superconducting channel. 
In the superconducting device in accordance with the present invention, 
upper surfaces of the superconducting channel, the superconducting source 
region and the superconducting drain region are at the same level. 
Therefore, superconducting current flows into or flows from the 
superconducting channel efficiently so that the current capability of the 
super-FET can be improved. 
In the superconducting device in accordance with the present invention, the 
non-superconducting oxide layer preferably has a similar crystal structure 
to that of a c-axis oriented oxide superconductor thin film. In this case, 
the superconducting channel of a c-axis oriented oxide superconductor thin 
film can be easily formed on the projection. 
Preferably, the above non-superconducting oxide layers is formed of a 
Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide. A c-axis oriented 
Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. thin film has almost the same 
crystal lattice structure as that of a c-axis oriented oxide 
superconductor thin film. It compensates an oxide superconductor thin film 
for its crystalline incompleteness at the bottom surface. Therefore, a 
c-axis oriented oxide superconductor thin film of high crystallinity can 
be easily formed on the c-axis oriented Pr.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-.epsilon. thin film. In addition, the effect of diffusion of the 
constituent elements of Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. into 
the oxide superconductor thin film is negligible and it also prevents the 
diffusion from substrate. Thus, the oxide superconductor thin film 
deposited on the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. thin film 
has a high quality. 
In a preferred embodiment, the oxide superconductor is formed of 
high-T.sub.c (high critical temperature) oxide superconductor, 
particularly, formed of a high-T.sub.c copper-oxide type compound oxide 
superconductor for example a Y-Ba-Cu-O compound oxide superconductor 
material, a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a 
Tl-Ba-Ca-Cu-O compound oxide superconductor material. 
In addition, the substrate can be formed of an insulating substrate, 
preferably an oxide single crystalline substrate such as MgO, SrTiO.sub.3, 
CdNdAlO.sub.4, etc. These substrate materials are very effective in 
forming or growing a crystalline film having a high degree of crystalline 
orientation. However, the superconducting device can be formed on a 
semiconductor substrate if an appropriate buffer layer is deposited 
thereon. For example, the buffer layer on the semiconductor substrate can 
be formed of a double-layer coating formed of a MgAl.sub.2 O.sub.4 layer 
and a BaTiO.sub.3 layer if silicon is used as a substrate. 
Preferably, the superconducting channel is formed of a c-axis oriented 
oxide superconductor thin film and the superconducting source electrode 
and the superconducting drain electrode are formed of .alpha.-axis 
oriented oxide superconductor thin films. 
According to another aspect of the present invention, there is provided 
method of manufacturing a superconducting device, comprising the steps of 
forming on a principal surface of a substrate a non-superconducting oxide 
layer having a similar crystal structure to that of a c-axis oriented 
oxide superconductor thin film and a flat-top projection at its center 
portion, forming a c-axis oriented oxide superconductor thin film having 
an extremely thin thickness on the non-superconducting oxide layer so as 
to form a superconducting channel on the projecting portion of the 
non-superconducting oxide layer, forming an insulating layer on the c-axis 
oriented oxide superconductor thin film so as to form a gate insulating 
layer on the superconducting channel, and forming an .alpha.-axis oriented 
oxide superconductor thin film so as to form a superconducting gate 
electrode on the gate insulating layer and a superconducting source region 
and a superconducting drain region of which upper surfaces have the same 
level as that of the superconducting channel. 
In one preferred embodiment, the flat-top projection of the 
non-superconducting oxide layer is formed by a process which does not 
comprise any etching but lift-off. The process comprises the steps of 
forming on a principal surface of a substrate a lift-off layer excluding a 
portion on which the projection will be placed, which can be removed 
without degrading the principal surface, forming a first 
non-superconducting oxide layer over the exposed portion of the principal 
surface and the lift-off layer, removing the lift-off layer so that a 
portion of the first non-superconducting oxide layer which will be the 
projection remains on the principal surface and the principal surface is 
exposed at the both sides of the remaining portion of the first 
non-superconducting oxide layer, and forming a second non-superconducting 
oxide layer having a thinner thickness than that of the first 
non-superconducting oxide layer over the exposed portion of the principal 
surface and the first non-superconducting oxide layer. 
In this case, the lift-off layer is preferably formed of a CaO layer of 
which surface is covered with a Zr layer. This lift-off layer can be 
removed by utilizing water and following reaction: 
EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2 
In the above process, no reactive agent is used but water. Therefore, if 
the flat-top projection is formed by the above process, the substrate and 
the non-superconducting layer are not degraded. 
The above and other objects, features and advantages of the present 
invention will be apparent from the following description of preferred 
embodiments of the invention with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1A to 1J, the process in accordance with the present 
invention for manufacturing the super-FET will be described. 
As shown in FIG. 1A, a MgO (100) single crystalline substrate 5 having a 
substantially planar principal surface 52 ((100) surface) is prepared. 
As shown in FIG. 1B, a photoresist 24 is patterned on a center portion of 
the principal surface 52. Then, as shown in FIG. 1C, a lift-off layer 26 
of CaO layer covered with Zr layer having a thickness of 500 nanometers is 
deposited over the photoresist 24 and the exposed portion of the principal 
surface 52. This lift-off layer 26 is preferably formed by a sputtering in 
which temperature of the substrate 5 is room temperature. 
Thereafter, the photoresist 24 is removed so as to remove a portion of the 
lift-off layer 26 so that a portion of the principal surface 52 is 
exposed, as shown in FIG. 1D. Then, the substrate 5 is heated to a 
temperature of 350.degree. to 400.degree. C. under a pressure lower than 
1.times.10.sup.-9 Torr so as to clean the exposed portion of the principal 
surface 52. 
Thereafter, as shown in FIG. 1E, a Pr.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-.epsilon. oxide thin film 50 having a thickness on the order of 
about 300 nanometers is deposited on the exposed portion of the principal 
surface 52 and the lift-off layer 26. The Pr.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-.epsilon. oxide thin film 50 is preferably c-axis oriented and 
formed by an MBE (molecular beam epitaxy). A condition of forming the 
Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin film 50 by an MBE 
is as follows: 
______________________________________ 
Molecular beam source 
Pr: 1225.degree. C. 
Ba: 600.degree. C. 
Cu: 1040.degree. C. 
Pressure 1 .times. 10.sup.-5 Torr 
Temperature of the substrate 
700.degree. C. 
______________________________________ 
Then, the lift-off layer 26 is removed so as to remove the portion of the 
Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin film 50 so that 
the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin film 50 is 
remained on the center portion of the principal surface 52 and the 
principal surface 52 is exposed at the both sides of the Pr.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.7-.epsilon. oxide thin film 50, as shown in FIG. 1F. This 
lift-off process utilizes water and a following reaction: 
EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2 
Since the lift-off process does not use an agent of high reactivity but use 
water, the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin film 50 
and the substrate 5 are not degraded. 
Thereafter, the substrate 5 is again heated to a temperature of 350.degree. 
to 400.degree. C. under a pressure lower than 1.times.10.sup.-9 Torr so as 
to clean the exposed portion of the principal surface 52. 
As shown in FIG. 1G, a Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide 
thin film 53 having a thickness of 50 nanometers is again deposited on the 
Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin film 50 and the 
exposed portion of the principal surface 52 by an MBE. A condition of 
forming the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin film 
53 is the same as that of the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. 
oxide thin film 50. 
Thereafter, a c-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. 
oxide superconductor thin film 1 having a thickness of 5 nanometers is 
deposited on the Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin 
films 50 and 53 by an MBE, as shown in FIG. 1H. A condition of forming the 
Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. oxide superconductor thin film 1 
by an MBE is as follows: 
______________________________________ 
Molecular beam source 
Y: 1250.degree. C. 
Ba: 600.degree. C. 
Cu: 1040.degree. C. 
Pressure 1 .times. 10.sup.-5 Torr 
Temperature of the substrate 
700.degree. C. 
______________________________________ 
A portion of the deposited Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. oxide 
superconductor thin film 1 on the Pr.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-.epsilon. oxide thin film 50 becomes a superconducting channel 10. 
In this connection, a portion of the second Pr.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-.epsilon. oxide thin film 53 which is deposited on the Pr.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin film 50 functions as a 
buffer layer on which a Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. oxide 
thin film of high crystallinity grows easily. 
Then, an insulating layer 17 of MgO or SrTiO.sub.3 is deposited on the 
Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. oxide superconductor thin film 
1, as shown in FIG. 1I. The insulating layer 17 is formed so as to have a 
thickness of 10 to 15 nanometers by an MBE. A portion of the insulating 
layer 17 on the superconducting channel 10 becomes a gate insulating layer 
7. 
Finally, as shown in FIG. 1J, an .alpha.-axis oriented Y.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.7-.delta. oxide superconductor thin film is deposited on 
the insulating layer 17 so as to form a superconducting source region 2 
and a superconducting drain region 3 at the both sides of the Pr.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide thin film 50 and a 
superconducting gate electrode 4 on the gate insulating layer 7. The 
.alpha.-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. oxide 
superconductor thin film is formed by an MBE so that the upper surfaces of 
the superconducting source region 2 and the superconducting drain region 3 
have the same level as that of the superconducting channel 10. A condition 
of forming the .alpha.-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-.delta. oxide superconductor thin film 1 by an MBE is as follows: 
______________________________________ 
Molecular beam source 
Y: 1250.degree. C. 
Ba: 600.degree. C. 
Cu: 1040.degree. C. 
Pressure 1 .times. 10.sup.-5 Torr 
Temperature of the substrate 
640.degree. C. 
______________________________________ 
Metal electrodes may be formed on the superconducting source region 2 and 
the superconducting drain region 3, if necessary. With this, the super-FET 
in accordance with the present invention is completed. 
The above mentioned super-FET manufactured in accordance with the 
embodiment of the method of the present invention has no undesirable 
resistance nor undesirable Josephson junction between the superconducting 
channel and the superconducting source region and between the 
superconducting channel and the superconducting drain region. Since the 
upper surfaces of the superconducting source region and the 
superconducting drain region are at the same level as that of the 
superconducting channel, superconducting current efficiently flows into 
and flows from the superconducting channel. In addition, in the above 
mentioned method in accordance with the present invention, the flat-top 
projecting portion of Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.epsilon. oxide 
thin film on the substrate is formed without using etching process. 
Therefore, the portion has a high crystallinity so that the 
superconducting channel formed on the portion has a good superconducting 
characteristics. By this, the current capability of the super-FET can be 
improved. 
In the above mentioned embodiment, the oxide superconductor thin film can 
be formed of not only the Y-Ba-Cu-O compound oxide superconductor 
material, but also a high-T.sub.c (high critical temperature) oxide 
superconductor material, particularly a high-T.sub.c copper-oxide type 
compound oxide superconductor material, for example a Bi-Sr-Ca-Cu-O 
compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide 
superconductor material. 
The invention has thus been shown and described with reference to the 
specific embodiments. However, it should be noted that the present 
invention is in no way limited to the details of the illustrated 
structures but converts and modifications may be made within the scope of 
the appended claims.