CA2047139C

CA2047139C - Method for manufacturing superconducting device composed of oxide superconductor material and superconducting device manufactured thereby

Info

Publication number: CA2047139C
Application number: CA002047139A
Authority: CA
Inventors: Mitsuchika Saitoh; Sou Tanaka; Michitomo Iiyama
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1990-07-16
Filing date: 1991-07-16
Publication date: 1997-12-09
Anticipated expiration: 2011-07-16
Also published as: EP0467777B1; US5504058A; DE69125425T2; EP0467777A3; DE69125425D1; EP0467777A2; CA2047139A1; US5215960A

Abstract

In a method of manufacturing a superconducting device which has a first thin film of oxide superconductor material formed on a substrate and a second thin film formed on the first thin film of oxide superconductor material, after the second thin film is deposited on the first thin film of oxide superconductor material, a multi-layer structure formed of the first and second thin films is patterned so that a side surface of the first thin film is exposed. In this condition, the whole of the substrate is heated in an O2 atmosphere or in an O3 atmosphere so that oxygen is entrapped into the first thin film of oxide superconductor material. Thereafter, the patterned multi-layer structure is preferably covered with a protection coating.

Description

Z(~ 3L3 SPE~FICATION

Title of ~e l~vention METHOD FOR MANU~ACTURING SUPERCC~NDUCIIN~
DEVICE COMPOSED O~ OXIDE SUPERCONI)UCTOR
MATERIAL AND SUPERCONDUCTING DEVICE
MANUFACI'UREI) THEREBY

Backgr~und of the Invention Pield of the invention The present inverltion relates to a method for manufacturing a superconducting device and a superconducting device manufactured thereby. More specifically, the present invention relates to a method for "~n~f~cturing a superconducting device by depositing a thin ~llm such as non-superconductor material thin ~ilm or a semiconductor material thin ~ilm on an o~ide su~ercol~ductor material thin film folmed on a substrate, and a novel sLIpclcollducting device rn~nuf~ctllred thereby.

Desc~ n of related art In:the case of using an oxide supercorlductor material in a ~u~a~e6l~lductirlg device, it it nÇcessary to form and stack a thin ~ilm of oxide superconductor material. For example, when a superconductor/non superconduc~or/superconductor junction called a "tunnel type" Josephson junction is formed by using an oxide su~ol~luctor material, it is nece~s~ry to seq~lent~ y sta~k a first thin lm of oxide superconductor material, a thin fllm of non-sllperconductor .

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material, and a second thin film of oxide superconduet~r material in the named order In this tunnel type Josephson ju~ction, a thickness of th~
non-superconductor material ~hin film is ~enerally determined by a coherence length of the superconductor material Since the oxide superconductor materials have a very short coherence length, it i~
~ecess~ry to cause the non-superconductor ma~eria~ thin film to have ~e thirkness o~ the order of a few nanometers.
On the other hand, considering the charac~eristics of the device, each of ~he above mentioned three thin films has to have good cryst~11inity. Namely, all of the three thin fiLrns are preferred to be a single crystal, and if any one of ~he three thin ~llms is polycrystalline or amorpholls, a Josephson device cannot have a stable ~llolJ~ ce The good ~rystalline thin film is required not only in the above mentioned tunnel type Josephson junction device, but also in other devices including a superconducting t~ansistor composed of an oxide sup~o~d~etor material and a semiconductor material in combination However, if the oxide superco~ c~0r material thin film is exposed to air, it will lost ~t~ superconductiYity and cryst~ ty in the extent of about 1 nm depth from its surface In ordinary cases, in order to deposit a second thin film on ~he oxlde supe~onductor material tbin film, a de~Gsilion ~a~dtus different fro~ that used ~or folmation of ~he o~ide superconductor material thin ~ilm is used, and therefore, the oxide sup~rconductor material thin film is inevitably expo~ed to air in the cour~o of feeding from a deposition apparatus to another deposition apparatus. Because of this, i~ has been a conventional practice to heat the oxide s~l elco~lductor material thin film at about 700~C under an ultra Z~ 3~3 high vacuum on the order of I x 10-9 Torr, before the second t~in film is deposited on the oxide sul,clcol-du~tor materi~l thin film.
The oxide superconductor materi~l thin film su~jected ~ the ~bove mentioned heat-trea~nent can l~ave a surface of an improved crystaIlinity, and in addition, it become possible ~o epitaxially grow the second thin film on the oxide superconductor mater~al thin film. Howevcr, the h~ting of the oxide superconduc~or material thin film Imder ~he above mentioned ultra high vacuum will result in loss of oxygen ~rom the oxide supelcol)dwtQr material crystal thin film, which will deteriorate or lose dle sup~i~ol~duction characteristjcs.
On the other hand, if the heat treatment is perforrned in oxygcn a~nospherc, no deterioration will oceur in superconduction charaeteristics of the oxide superconductor mate~ial thin film, but the crystallinity of the thin film surface is not improved. Therefore, it may be considered to perform the heat treatrnent in oxygen or ozone atmosphere after the second film is deposited on the oxide superconductor material thin film.
Ho~ er, even if the oxide superconductor material thin film is treated in this method~ af~er the oxide superconductor material thin ~1lm i9 heated in a later step, oxygen wili in some cases be lost again from the crysta] of thé
oxide S~JpC~ tiUClOl' mater~al thin S-Jmms~y of ~e Invention Accordingiy, it is an object of the present invention to provide a method of manu~acturing a superconducting device, which method has OV~ ~ the above mentioned defect of the conventional one.
Another object of the present invention is to provide a method of manufacturing a superconductin~g devi~e having an excellent .. . .

:-ch~racteristlcs, without losing the superconductivity of an oxide su~ co~ductor rnaterial thin ~ilm.
Still another object of the pre~ent invention is to provide a superconducting device having an excellent characte~stics and includi~g an oxide superconductor material thin film of excellent superconductivity.
The above and other objects of the present inven~ion are achieved in accordance with the present invention by a me~od of manufac~uring a superconducting dcvice which has a firxt thin fi]m of oxide superconductor material forrned on a substra~e and a second thin ~ilm stacked OR the first thin film of oxide superconductor material, the method including the s~eps of depositing the second ~in film on ~e first ~in filrn of oxide superconductor material, proeessing the first thin film so as to cause a side surface of the first thin film to be exposed, and thereafter, hP~ the whole of the substrate in an ~2 atmosphere or in an 03 a~ os~hele.
According to a second aspect of the present invention, after the whole of the substrate is heated in an 02 atmosphere or in an 03 atmosphere so as to cause oxygen to be entrapped into an oxide supercollductor matedal crystal ~orming the first thin film of oxide su~x~ol--luctor maeerial, the whole of the first and second thin films is covered with a protection coating. Thls pro~ection co~in~ is preferably made of a noble metal.
According to a third aspect of the present invention, the pro~ection coating is made of an oxide supl.en.~lu~tQr material layer forrned of an oxfde ~u~fcollductor material crystal different in orielltation from the oxide su~clcollductor material crystal forming the ~irst thin ~llm of oxide p~ O-ductormaterial.

Purthermore, according to the present invention there is ,vrovided a superconducting device comprising a first oxide superconductor material thin film formed on a subs~rate, a non-superc~nductor material ~hin film formed on the first oxide superconductor material thin ~llm, and a second oxide superconductor material thin film formed on the non-supe~conductor material thin ~ilm, side surfaces of the ~Irst oxide superconductor material thin film, the non-superconductor matenal thin fiLm and ~e second oxide superconc~uctor material thin ~llm being coated with a thin film of noble metal, which functions as a resistor connec~ing between the first oxide superconductor material thin film and the second oxide ~ll~feonductor material thin film.
As seen from the aboYe, if a supercon~luctin~ device, which includes an oxide superconductor material thin film ~ormed on ~ substrate and a thin film such as an insulator thin film made of for example MgO
(m~gnesium oxide) or a semiconductor thin film made of for example Si (silicon) forrnçd on the oxide superconductor material thin film, is manufactured in accordance with the present invention, the following is a first important fe~ture: A~ter the stacked deposited thin films are patterned until- a side sur~ace of a lvwermost oxide superconductor material thin film is exposed, t~e heating is perfotmed in an ~2 atmosphere or in an 03 atmo~phere A second important fe~ture of the invention is that: A~ter the sta~ed thin fi~s so patterned ~hat a side su~face of a lowe~nos~ oxide superconduc~or material thin film is exposed is heat-treated in an 02 atmosphere or in an 03 atmosphere so as to cause oxygen to be e~ Al>~d into the oxide superconductor material crystal forming the oxide superconductor material thin film, the whole of the patterned stacked thin 3~

films iS covered wit}l a pro~ection coating which is ~ormed of f~r example a noble metal. A third imp~rtant feature of the invention is ~hat ~he pro~ection coating is constituted of an oxide superconductor material layer formed of an oxide superconductor material crystal different in orientation f~m the oxide supereQnductor material crystal ~orming the first named oxide superconductor material thin film.
The above mentioned method in aceordance with the present invention is effective particularly in restoring an oxide superconductor material thin film in which oxygen in a crystal has been lost and superconductivity ha~ been deteriorated. Specifically, the method in acGordance with the present invention is very eff*ctive in the case of manufacturing a superconducting device, by heating in high vacuum an oxide superconductor material thin film having a surface crystallinity deteriorated for example due to exposure to air, so as to restore the surface crystallinity, and tbereafter by epitaxially growing another thin fiLIn on the ~hin fi~n having the restored sur~ace crystallinity.
In the method of the present invention, since oxygen is elllr~ped or supplied into the inside of the thin film from the side surface exposed as ~he result of th~ patterning, the superconducting device m~nl?factured in accordance wi~h the me~hod of the present invention has to have the pattemed oxide supereonductor matenal ~hin film of a shape and a size which allow a su~ficient amount of oxygen to be supplied to a center portion of the oxide supercollductor rnaterial t}lin film. ~or example, if ~e ~atlell,ed oxide superconductor material thin film is rectangular in a plan view, a width of the patterned oxide superconductor mater~al thin ~llm (distance between opposite exposed side surfaces of the patterned oxide superconductor material thin film) is preferably not greater than 3 mm, more preferably not greater than 1 mm. However, t~is width of the patterned thin film is very general in current electronic devices.
Therefore, superconducting devices which can be manu~actured in accor~ance with the present invention are not substantially subjected to any limitations in size attributable to the present invention On the o~er hand, ~he oxide superconductor mater~al has a general property in which oxygen is easier to move in a direction perpendicular to a "c"-axis of the crystal. Therefore, not only when oxygen is lost from the ery~tal, but also when oxygen is diffused and entrapped into t~e crystal, the oxygen moves mainly in a direction perpendicular to a "c"-a~is. Accordingly, the superconducting device manufactured in ~ccordance with the present invention is l)refel..,d to have a "c"-axis oriented oxide superconductor material thin fillm, which has a "c"-axis perpendicular to a surface (such as a surf~ce of a substrate) on which the ~in film is to be deposited In the method of the present invention, the ~ieating ~n~pe.dture is p~f~l~l)ly not less than 400~C but not greater ~han 500~C. In addition, when the oxide superconductor material thin film is heat-treated in C)2 atrnosphere, a p~rtial pressure of ~2 is preferably not less than 5 Torr, and when the o7~ide supercond~ctor material thin film is heat-treated in O3 a~nosphere, a partial pressu~e of O3 is preferably not less than 0.1 Torr If the he~ting temperatu~ Is less than 400~C, the oxide ~up~l~CQ~ tor material thin film cannot entrap a sufficient amount of oxygen, ~nd if the heatin~ tel~-,ucrdl~re is greater than 500~C, diffusion will in ~ome cases occur between adjacerlt thin films, Furthermore, the partial pressures of ~2 and O3 have a closed relation to the heating temperature. It can be generally said that if the 3~

he~ing temperatu~e is high, the partial pressures of O2 and O3 rnust also be high. E~or example~ if the oxide superco~ductor material thin film is heated at 400~C in the ~2 atmo~phere, the pa~ial pressure of C)2 is prefer~d to be within a range of 5 Tolr to 20 Torr, HoweYer, when the oxide ~uperconductor material thin film is heated at 500~(~ in the O2 ~trno~phere, if the partial pressure of C)2 were less than 10 Torr, it is not possible to cause oxygen to be sumciently entrapped into the oxide superconductor material ~hin film. In addition, when the oxide superconductor material thin film is heated at 500~C in the O3 atmosphere, if the partial pressure of O3 were less than 10 Torr, a su~ficient amount of oxygen cannot be entrapped into the oxide sul)c.coi.d~lctor material th~n film.
As mentioned above, according to the second aspect of the present invention, the whole of the stacked thin films is covered with a protection co~ting after oxygen has been entrapped into the oxide superconductor mater~al thi~ film. The protection coating is formed of for examp]e a noble metal, preferably Au (gold) or Ag (silver) which has a low reactivity to the oxide superconductor material. In addition, the noble met~l coating is pl~felYed to have a ~hickness of not less than 10 nm but not gre~ter than 50 nm.
With the stacked thin films being covered with thc protection coating, even if the oxide superconductor materîal thin film is heated in a later step, oxygen is not lost from the oxide superconductor material crystal. In addition, it is possible to prevent impurity from entering into the oxide superconductor material thin film fron- materials in contact with or surrounding the oxide superconductor material thin film.
Th~f~ole, the superconducting device manufactured in accordance with . ~

~7~L3~3 the second aspect of the present inven~ion can effectively utilize an e~cellent character1stics of the oxide superconductor mater~al thin film without being deter~or~ted, and thefefore, can have a high per~rmance, The noble metal coating can ~e formed by a sputtering, a vacuum evaporation, or any other method. Ln this cormection, attention sbould be given to ensure that, when the protection coating is fonned, the oxide sul~erconductor material thin film is aYoided from passing through or att~ining ~0 a temperature zone in which oxygen is lost from the oxid~
~upe~GIlductor material crystal (for example, about 400~C in the case of Y-Ba-~u-O type oxide su~er~onductor material) In other words, it is necess~ry to deposit the protection coating of for example a noble me~al under a t~n-pc~ature which is below the above mentioned tell~pe~ture zone.
According to the third aspect of the present invention~ after the oxide super~onductor material bas been caused to entrap oxygen, the whole of the stactred thin films is covered with a layer formed of an o~ide s~l~es~Gn~nçtor material crystal dirr~le~lt in orien~ation ~rom the oxide ~u~rc~lductor matsrial crys~al thin film. This layer fo~ned of the oxide ~u~r~ hlct~r m~terial crystal different in orientation from the oxide superconductor material crystal ~hin film behaves as an electrical insulator, ~irrc.e.~t from the oxide superconduetor mater;al crystal thin film, and therefore, filnctions as a protection coating. In the case of manufacturing the ~uper'col-ducting device by using the "c"-axis orien~ed oxide superconductor material crystal thin film as mentioned hereinbefore, ~he protection coating OI ~e oxide superconductor n~ate,;al layer is preferably formed of ~ so called "a" axis oriented oxide superconductor material crys~al thin film, which has an "a"-axis :
:

ZO~L3~

perpendicular to the surface on which the "c"-axis oriented oxide superconductor material crystal thin film is deposited, Theref~re~ ~e "c"-axis of the "a"-axis oriented oxide superconduct~r m~teri~l crystal thin ~llm ;s pe~pendicular to the "c"-~xis of t~e "c"~axis oriented oxide superconductor material crystal thin fiim. Preferably, this oxide su~el~ollductor material protection coating has a thickress in a range of lOnm to SOnm.
As mentioned hereinbefore, oxygen in the oxide superconductor material cry~tal is easier to move in a direction perpendicular to the "c"-axis, but hardly moves in a direction paralle1 to the "c"-axis.
Accordingly, oxygen in the "c"-axis oriented oxide supercondu~tor material crystal (forming the oxide superconduc~or material thin film) covered with the "a"-axis oriented oxide superconductor material crystal proteetion layer is di~ficult to pass through the "a"-axis oriented oxide superconduc~or materia1 crystal protection layes, and therefore, oxygen will not almost lost from the "c"-axis oriented oxide supereonductor material crystal thin f;~m. In addition, impurity is prevented from entel~ng into the o~ide supereondwtor material thin fi1m ~rom materials in colltact wi~ ~he oxide ~u~o~ uctor mat~rial ~in film.
F~l~le~ ore, ~he "a"-axis oriented o~ide superconductor material crystal protection layer has the "c"-axis perpet dic~ r ~o the side surface of ~e "c"-axis oriented oxid~ superconductor material crystal thin film.
Since the oxide sul)ercol~ductor material crystal is apt to easily deform in the "c"-axis direction. distortion caused by a difference in thermal SiQ.1 coefficient between the oxide superconcl~lctor material thin ~
and materials in contact with or ~lrvul~ding the oxide sup~rcoi1ductor material thin fi]m will be a~sorbed by the "a"-axis oriented oxide su~eleo~ductor material crystal protection layer.
Thus, even if the oxide supercondwctor material thin film i~ heated in a later step, the charac~eristics of the oxide ~upercondu~tor material ~hin film is m~int~ined.
As seon from the above, par~icularly, ~e "a"-a~is oriented oxide superconductor ma~erial crystal protection layer functions as (1) a buffer for absorbing a mçch~nical slress, (2) a barrier for prcventing the mutual diffusion between adjacent layers and the loss of oxygen~ and (3) an electrical insnl~tor. Therefore, the degree of freedom of working ar processing in later steps of the manufacturing process can be increased.
~ccordingly, the superconducting device manufactured in accordance wi~b the ~hird aspect of the present invention can ef~ectively utilize an excellent characteristics of the oxlde supereonductor material tllin ~llm without being deteriorated, and the~gfore, can have a high per~ormance.
The"a"-~xis oriented oxide superconductor rnaterial crystal protection layer c~n be forrned by a spultering or other various method~
having a growth isotropy. Por the ~pull~r;l~ haYing a growth isotropy, a formation tempera~urff~tgenerally, t~e substrate temperature) is about 6S0~C: or less in the case of the Y-Ba-Cu-O type oxide superconductor material. Accordingly, it is preferred th~t immediately after the processing for causing oxygen to be elltrapped into ~he oxide superconductor material thin film has been completed, the substrate temperature is elevated and the a"-axis oriented oxide superconductor material crystal protection layer is ~nned The novel ~uperconducting device, which can be formed in acco~dal~ce wi~ ~e method of the present invention, can be realized in , . .
.. .. ~ . .... .

3~3 the form of, for example, a Josephson device or a superconducting transistor, both of which have the first oxide superconductor material thin film, the non-supersonductor ~aterial thin film and the second oxide superconductor material thin ~lm formed or stacked on the substrate in the nam~d order, a side surface of the s~cked layers being covered wi~ a noble metal layer which electrically interconnects between the ~lrst and second oxide superconductor material ~in films and which acts as a resistive element. Since the first and second oxide superconductor material thin fi~rns separated by the non-sul~ercol-dll~tor material thin ~
are elec~rically inierc~ ected by the nob]e metal layer, a hysteresis which has been expressed without exception in conventional superconducting devices of ~bis type will be cancelled. Accordingly, it is possible to easily construct a superconducting q~l~ntllm intelre.ence device (SQUID) when the ~u~trconducting device is inco,polaled in m~ etic circuits.
The me~od of the present invention can be applied to any arbitrary o~ide supercondu&tor material. But, Y1Ba2Cu307 x (OSx<1) type oxide superconductor is ~-bfe-r,d, since a thin film having a high ~uality and a good crys~llinity can be stably obtained. In addition, Bi2Sr2Ca2Cu30y (75y~11) type ~xide supcrconductor is p~f~ d, since it }~a~ a hi~h supG~ol~duction critica~ te~ eldtule Tc.
The above and other o~ject~, features and advantages of the present invention will be a~>~afe.lt ~rom the following description of pr~ferred embodim~nts of the invention with reference to the accompanying drawings.

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7~3 Brief Dcs~ri~lion ~f the Drawings Figures lA to lGG are dia~r~rnA~ic sectional Yiews And plan views for illustrating various steps of the first embodiment of the method in accordance with the present inven~i~n for manu~acturing a Su~l~Q~ Ctin~ devic~;
Figure~ 2A to 2FF are diagr~mrr ~fic sectional ~iews and plan views ~or illlls~r~ting various steps of the second embodirnent of the method in accordance with the present invention for manufacturing a su~c~-~ducting device;
~ igures 3A and 3B are a graph showing the current-voltage char~ctelistics of the superconduc~ing device manufactured in acco~al~ce with ~e second embodiment of the method of ~e present inventio~; and Figures 4A to 4~F are diagr~mm~tic sectio~l views and plan views for illustrating various steps of the third embodimen~ of ~he method in aceordance with the present invention for manufacturing a ~.l~r~o,~d"csin~ device, D~sc~ ion of she ~ ,d e~ n F:.ml~o~lim~nt 1 A ~osephson device was formed in accordance wi~h each of the method of the pl'~30il~ invention and the prior art method, by depositing or st~kine a ~Irst oxide super~o~d~ctor material thin film, a SrTiO3 thin film, a second oxide superconductor material thin film on a MgO
substrate in ~e named order. The process will be described with ~,fc.~ cc to Figures lA to l(:;G.
First, a Yl~Ba2cu3o7~x superconducting thin film (which will become a first oxide superconduçtor ma~erial thin ~llm 1) was deposited :

on a (lQ0) sur~ace of an MgO substra~e 4 by a sputte~ng. The deposi~ion condi~ion is as follows:
substrate temperature 700 ~C
s~ g g~s Ar 90 %
O2 10%
pressure 5 x 10 ~ Torr film thickness 4~0 nm Then, ~he ~fgO substr~te having the deposited first oxide superconductor material thin film 1 was rnoved or placed in a vacuum e~apora~ion appara~us from a sputtering apparatus, and a surface crystallinity restoring treatment (heat-tre~tment) was per~ormed under the following condition:
pressure I x 10-9 Torr subs~ate t~ e~ re 700 ~C
t~ nent time 3 min1ltes After the above tre~tme~t, the substrat~ is allowed to cool until 400~C, and thereafter, an SrTiO3 thin film (which will become a non-superccnductor ma~erial thin film 3) was deposi~ed on the oxide ~u?~rcq.-~luçt~r material thin ~lm 1 by an ion~eam sputtering under the following condition:
subs~ate ~ ye,~dtUle 700 ~C
oxygen pressure 4 x 10-6 Torr lm thic~rnes~ 3 nm ~ I~I.e~ ore, a YIBa2cu3o7~x superconducting thin ~ilm (which will ~come a second oxide supercoilductor material thin ~llm 2) was deposited on ~e S~iO3 thin film 3 by a laser ablatinn method under dle fol1ow~ng condition:

~ 14 .
- -, : .: .
. , , ~- :
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-substrate temperatllre 700 ~C
oxygen pressure l~ Torr ~llm ~ickness 400 nm Thus, a three layer structure as shown in Figure lA was formed on the MgO substrate 4. At this stage, the oxide supereonductor matenal of d~e second oxide superconductor material thin film 2 had a co~ o3ilion of Y~Ba~Cu306 9, but the oxide superconductor material of the first oxide superconduc~or material thin ~11m 1 had a composition of YlBazCu306 4, because of influence of ~e above mentioned heat treatrnent. In addition, the first and second oxide superconductor material thin films 1 and 2 were folmed of a "c"-axis oriented crystal, and the SrTiO3 layer 3 was sl1bst~nti~11y formed of a single crystal.
Thereafter, the three-layer structure as shown in Figure lA ~rmed of ~he first oxide superconduc~or material thin film ll the SrTiO3 thin ~llm 3, and the second oxide super~ot~uctor material thin ~llm 2 stacked in the named order, was patterned by using a photo1ithography and an Ar-ion etching, so that a linear pattern having a wi~th of I ,um is forrned as shown in Figures lB and lBB. Figu~e lBB is a top plan view of the substrate shown in Figure lB.
After the above mentioned p~tterning, the heat treatment was performed in oxygen a~nosphere. The following is ~he heat treatment cotlditiotl:
substrate le~e-a~re 400 ~C
02 partial pressure l0 Torr t~ent time 4hours ~3~ 3 As the result vf this heat treatment, the oxide superconductor material of the first oxide superconduetor ma~erial thin film 1 had become a composition of YIBa2~ll306,9.
After the above mentioned treatment, a SiO~ la~er S having a thickness of 800 nm was deposited to cover the whole of the substrat-e, and a resist layer 6 was deposited to cover the SiO2 l~yer 5, as shown in Figure lC. Then, the deposi~ed resist layer 6 and SiO2 layer 5 were etched back by a reactive ion etching process, until an upper su~ce of the second oxide superconductor material thin ~llm 2 is exposed as shown in Figure lD, Furthe~more, an Au layer 7 was deposited so as to cover the whole of an upper surface of the substrate ~y a v~cuum evaporation, as sbown in Figure 113. As shown in Figures lF and IFF, a patterned linear resist 8 was formed on the deposited Au layer 7 so as to intersect the linear patterned three~layer structure formed of the oxide superconductor material thin filrns 1 and 2 and the SrTiO3 thin film 3, in a plan view shown in ~igure lFF~ The patterned linear resist 8 was 1 ~m in wid~.
An Ar-ion milling was ~e~ ed using the pattemed linear resist 8 as a mask, until the linear patte~ned three-layer struc~ure wh}ch was not cove~d with ~he patterned linear resist ~ becomes about a half in dlickness, namely until the lower or ~irst oxide superconductor material thin fi~ 1 has an exposed upper surface ~1, as shown in Figures lG and lGG~ In addition, a m~tAlli7~tion layer was fo~ned on the exposed upper surface 11 of ~e lower or ~Irst oxide superconductor material thin film 1~
Thus, a Josephson device was completed, in which the l?a~erned Au layer 7 forrns one of a pair of electrodes, and the metallization layer formed on the exposed upper surface 11 of the lower or first oxide ': :

7~ 3 superconductor material thin film I fortns the other of the pair of electrodes.
After the comple~ion of the device, crystallinity cQndition of the first and second oxide swperconductor material thin films :1 and 2 and the SrTiO3 layer 3 was examined. All of ~he three layers in tlle Josephson device manufactured in accord~llce with the present in~/ention h~d good crystallinity. ~n addition, both of the first and secand oxide superconductor material thin ~ilm~ I and 2 were ~nned of a "c"-axis or,ented crystal. The critical temperatures Tc of ~he first and second oxide superconductor material thin films 1 and 2 were 90K and 89K, respeetively.
In ti e Josephson device manufactured in the conventional process which does not inelllde the step of the he~t treatment in the oxygen atmosphere, on the other hand~ the first oxide superconductor ma~erial thin film 1 had a composition of YIBa~Cu3o6.4~ and did not show s~ col-dllctivity at a liquid nitr~gen tcn~e,alulre.

Embo.lil,lcnt 2 l'he same processings as tho~e of the Embodiment 1 were pc.f~ ed until the three-layer stluctur~ etched to haYe a linear pa~ters having the width of 1 ~m as shown in Figures lB and IBB has been heat-treated in the oxygen atmosphere.
After ~e heat treatment, an Au layer 9 is deposited by a sputtering so as to cover the linear pat~erned ~hree-layer structure formed of the ~lrst and second oxide superconductor material thin ~llms 1 and 2 and ~e SrTiO3 layer 3, as shown in Pigure 2A. The deposition condition is as fo~lows:

substrate temperature 340 ~C
s~ eling gas Ar 1~ ~o pressure S x 10-~ Torr ~l~m thirkness 30 nm In this case, the substrate te~npe~ re i5 made as low as possibl¢ in ~e range of not greater than 40~ ~C. The reason for this tbat, at abou~
400~C and and in the pro~imity thereof, oxygen in the YlBa2cu3o6~9 super~onductor crystals, which constitute the first and second oxide superconductor material thin films 1 and 2, respectively~ is easiest to diffuse and lost.
After the linear patterned three-layer structure formed of the first and second oxide superconducto~ material thin ~lms 1 and 2 and tl~e Srl'iO3 layer 3 was covered with the Au layer 9, a SiO2 layer 5 having a thickness of 800 nm was deposited ~o cover the whole of the substrate, and a resist layer 6 was deposited to eover the SiO2 layer 5, as shown in Figuro 2B. Then, the deposited resis~ layer 6 and SiO2 layer 5 were etched ~back by a reactive ion etching process, until an upper sur~are of ~e secnnd oxide supe.co~ ctor material tllin film 2 is exposed as shown ~n Figure 2C.
~ .
Furthe~ore, as shown in ~igure 2D, an Au layer 7 was deposi~ed by a vacuum evaporation so as to c~ver the whole of an upper surfaee of .
~e:~u~s~nat~. As shown in Figures 2E and 2EE, a pat~erned linear resist 8 was formed on the deposited Au l~yer 7 so as to intersect the linear pattemed ~hree-layer structure ~ormed of the oxide superconductor ate.;a1 ~in films 1 and 2 and the in~ul~or layer 3, in a plan view shown in Figwe 2EE. l~ne patterned linear resist 8 was 1 ~lm in wid~h.

- ~ ~
- . - - . . ~ . . , .. . ~.... :. .. . :
:
.
. . : :

. , . . - . .

Z~ 3~
An Ar-ion milling was perforrned using t}le pattemed linear resist 8 as a mask, until the linear patterned three~layer struc~ure wh~ch was not covered with the patterned linear resist 8 becomes about a half in ~ickness, namely until the lower or first oxide superconductor material thin ~llm 1 has an exposed upper surface 11~ as sbown in Figures 2F and 2FI~ In addition, a metallization layer was fotmed on the exposed upper sur~ace 11 of ~e lower or first oxide superconductor material thin film 1 Thus, a Josephson device was completed, in which the patterned Au layer 7 folms one of a pair of electrodes, and the meta]lization layer formed on the exposed upper surface 11 of the lower or first oxide superconductor material thin film 1 forms th~ other of the pair of elec~odçs After the completion of the device, crystallinity condition of the first and seeond oxide superconductor material thin ~lims 1 and 2 and the SrTiO3 layer 3 in the Josephson device m~n~lfachlred in accordance with the p~s~llt invention was ex~mine~. All of the three layers had good crys~allinity, In addition, bo~h of the first and second oxide supercondl~ctor material thin films 1 and 2 were ~ormed of a "c"-axis oiiel~te~ crys~al. The first and second oxide superconductor material thin films 1 and 2 had the critical temperatures Tc of 90K and 89K, ~ccli~ely.
~ the Josephson device manufactured in the conventional proce~s wh~ch does not inelude the step of the heat treatment in the oxyg~n atmosphere1 on the other hand, the first oxide superconduc~or material ~hin film 1 had a composition of YIBa2cu3o6 4, and did not show ~uyer~ ductiYity at a liquid nitrogen te~ ture.

3~

Purthermore, the characteristics of a superconducting device (Josephson device) m~nufactured in accordance with the ~mbodiment 2 was measured, The result is shown in Fi~ures 3A and 3B. In e~ch ~f Figures 3A and 3B, a curved dotted line shows a current-voltage characteristics of a Josephson device in the case that the protection coating 10 is not provided, The curved dotted line 20 shows the current-voltage charact~lis~ics of the Josephson device when dle voltage is increased, and thc curved dotted line 21 shows the current-voltage characteristics of the Josephson device when the voltage is decreased A strai~ht do~ted line shows a current-voltage characteristics of ~e protection coating 10. As seen ~rom comparison between Fig~res 3A and 3B, the protection coating 10 shown in Figure 3B has a resistanee lower than that of the protection coating l 0 shown in F~gure 3A. A solid line a current-voltage characteristics of a Josephson device in ~e case that the protection coating 10 is provided in accordance wi~ the Embodiment 2 of ~he invention.
I~ the case that ~e pro~e~tion coating 10 having ~e current-voltage characteristics as shown by the straight dotted line in Figure 3A is provided, ~e Josephson device manufactured in accordance with the Embo~i..,c1-l 2 has a current-vol~ge characteristics as shown by the solid line, which is determined by a com~ination of the current~voltage characteristics of the Josephson device and the current voltage characteristic~ of the protection coating 10. When the voltage is increased, the current increases in accordance with the current-voltage characteristics curve 2~, and when ~he voltage is decreased, the current decreases in accordance with the current-Yoltage characteristics curve 23~
On ~e o~er hand, when the protectjon coating 10 having a low resist~r~ce as shown by the straight dotted line in Figure 3B is provided, the .

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Josephson device manufactured in accordance with the Embodiment 2 ~a.s a current-voltage characteristics shown in the solid line, in which when the voltage is increased, the current increases in accordance with the current volta~e characteristics curve 24, and when the voltage is decreased, the current decreases in accordance with ~he current-voltage ch~racteristies curve 25.
As seen from the above, the Josephson device manufac~ured in accordance wi~ the Embodiment 2 has a curr~nt-voltage charaeteristics having less hysteresis, and thereforet has high possibility of practical applicxtion. Further resistance adju~tment will compensate the hysterics r~ ~u at all r~ g~ 3 The s~me processings as those of the ~mbodiment 1 were pe~ ...cd until the three-layer structure etched to have a linear pattern haYing the width of l ~lm as shown in Figures lB and lBB has been heat~ ed in ~e ~xygen atmo~phere.
A~ter the heat treatment, an "~"-axis oriented layer 10 of Yll3a2cu3c)7-x is deposited by a sputtering so as to cover ~e linear pattemed three-layer stru~tur~ fo~rned ~ the first and second oxide superconductor material thin films 1 and 2 and the SrTiO3 layer 3, as shown in Figur~ 4A. The deposition condition is as follows:
substrate t~ c~dture 640~C
sputter~ng gas Ar g0 %
~2 10%
pressure 4 x 10-2 Torr film thickness 30 nm ~t~ 3~

In the case of forrning the "a"-axis oriented YIBa2Cu307 x thin film, it is preferred that the substrate temperah~re is not ~re~t~r than about 650 ~C, and the p~essure is in the range of 10 mTorr to 50 mTorr After the linear patterned three-layer structure fonned of the first and second oxide superconductor material thin films 1 and 2 and the SrTiO3 layer 3 was covered with the "a"-axis oriented Y1Ba~Cu3O7.x layer 10, a SiO2 layer S having a thickness of 800 nm was deposited to ~over the whole of the substrate, and a resist layer 6 was deposited to cover the SiO2 layer 5, as sho~vn in Pigure 4B, Thereafter, the same processings as those of the Embodiment 2 were exe~uted as seen from Figures 4D, 4E and 4EE, and a ~osephson device as shown in Fi~,ures 4F and 4PF was completed After the con pletinn of the devi~e, crystal~inity condition of the first and second oxide ~ulJe~ollductor material thin filrns 1 and 2 and the SrTiO3 layer 3 in the Josephson device manufac~red in accordance with the ~ el-t invention was examined. All of the three layers had good crystallinityO In addition, both of the first and second oxide ~uper~ol~ductor material thin films 1 and 2 were formed of a "c"-axis oriented crystal, and the YIBa2Cu3O7 x thin film 10 was ~ormed of ~n "a"-axis oriented crys~al having an "a"-axis pelpendicular to the sur~ace of the substra~e 4 The first and seGond oxide superconductor material thin films I and 2 had the critical temperatures of 90K and 89K, ~sp~cli~ely.
In the Josephson device nl~n~chlred in tlle conventional proccss which does not include the step of the heat treatment in the oxygen atmosphere, on the other hand, the iFirst oxide superconductor material ... - .
. ~ , - ..

. ~- .
.
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thin film 1 had a composition of YIBa2Cu306 4, and did no~ show superconductivity at a liquid nitrogen temperature, As seen from the above, according to the method of the pre~ent invention, it is possible to manufacture a superc~nducting device such as tunnel type Josephson device having a high peRormance, lltilizing the advantages of the oxide superconductor m~terial without being deteriorated. In addition, it is possible to manu~acture a Josephson device having an improved hysteresis. Accordingly, it would be expected to promote application of oxide superconductor materials to electronic devices.
~ n the above mentioned embodiments, the non-superconductor material sandwiched between the first and second oxide superconductor material thin films 1 and 2 was formed of an ins~ tor material such as SrTiO3, and therefore, a so-called SIS type Josephson junction device was realized. However, it should be noted that the non-superconductor material sandwiched between the first and second oxide superconductor material thin films I and 2 can be formed of any other non-su~rcol~ductor material such as a semiconduc~or material and a normal conductor material which haYe a low or poor reactivity ~o the oxide ~-lp~rcol~ductor matenal being used.
The invention has thus been showrl and described with reference to dle specifilG embodimen~s. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but changes and modifications may be rnade within the scope of the appended claims.

~ 23 -

Claims

1. A method of manufacturing a superconducting device which has a first thin film of oxide superconductor material formed on a substrate and a second thin film formed on the first thin film of oxide superconductor material, the method including the steps of depositing the second thin film on the first thin film of oxide superconductor material, processing the first thin film so as to cause a side surface of the first thin film to be exposed, and heating the whole of the substrate in an O2 atmosphere or in an O3 atmosphere so as to cause oxygen to be entrapped into an oxide superconductor material crystal forming the first thin film of oxide superconductor material.

2. A method claimed in Claim 1 wherein after the whole of the substrate is heated in the O2 atmosphere or in the O3 atmosphere, the whole of the first and second thin films is covered with a protection coating.

3. A method claimed in Claim 2 wherein the protection coating is formed of a noble metal.

4. A method claimed in Claim 1 wherein after the whole of the substrate is heated in the O2 atmosphere or in the O3 atmosphere, the whole of the first and second thin films is covered with a layer formed of an oxide superconductor material crystal different in orientation from the oxide superconductor material crystal forming the first thin film of oxide superconductor material.

5. A method claimed in Claim 1 wherein the second thin film is formed of a non-superconductor material, and a third thin film of oxide superconductor material is formed on the second thin film of non-superconductor material, so that a three-layer structure is formed of the first thin film of oxide superconductor material, the second thin film of non-superconductor material and the third thin film of superconductor material, and wherein the three-layer structure is etched to have a given pattern so that side surfaces of the first thin film of oxide superconductor material, the second thin film of non-superconductor material and the third thin film of superconductor material are exposed, and then, the three-layer structure is heat-treated in the O2 atmosphere or in the O3 atmosphere so that a sufficient amount of oxygen is entrapped into the first thin film of oxide superconductor material through the exposed side surface of the first thin film of oxide superconductor material.

6. A method claimed in Claim 5 wherein the given pattern of the three-layer structure has a width not greater than 3 µm.

7. A method claimed in Claim 5 wherein each of the first and third thin films of oxide superconductor material is formed of a "c"-axis oriented superconductor material crystal.

8. A method claimed in Claim 5 wherein the heat-treatment is executed at a heating temperature in a range of not less than 400°C but not greater than 500°C.

9. A method claimed in Claim 5 wherein the heat-treatment is executed in the O2 atmosphere having a partial pressure not less than 5 Torr.

10. A method claimed in Claim 9 wherein the heat-treatment is executed at 400°C in the O2 atmosphere having a partial pressure in a range of not less than 5 Torr but not greater than 20 Torr.

11. A method claimed in Claim 9 wherein the heat-treatment is executed at 500°C in the O2 atmosphere having a partial pressure of not less than 10 Torr.

12. A method claimed in Claim 5 wherein the heat-treatment is executed in the O3 atmosphere having a partial pressure not less than 0.1 Torr.

13. A method claimed in Claim 5 wherein the heat-treatment is executed at 500°C in the O3 atmosphere having a partial pressure of not less than 10 Torr.

14. A method claimed in Claim 5 wherein each of the first and third thin films of oxide superconductor material is formed of a Y-Ba-Cu-O
type oxide superconductor material single crystal.

15. A method claimed in Claim 5 wherein after the three-layer structure etched to have the given pattern is heat-treated in the O2 atmosphere or in the O3 atmosphere so as to cause oxygen to be entrapped into the first thin film of oxide superconductor material, the whole of the three-layer structure is covered with a protection coating.

16. A method claimed in Claim 15 wherein the protection coating is deposited at a temperature lower than a temperature zone in which oxygen is lost from the first thin film of oxide superconductor material.

17. A method claimed in Claim 16 wherein each of the first and third thin films of oxide superconductor material is formed of a Y-Ba-Cu-O
type oxide superconductor material single crystal, and the protection coating is deposited at a temperature not greater than 400°C.

18. A method claimed in Claim 15 wherein the protection coating is formed of a noble metal layer having a thickness of not less than 10 nm but not greater than 50 nm.

19. A method claimed in Claim 18 wherein the protection coating is formed of a Au or Ag layer having a thickness of not less than 10 nm but not greater than 50 nm.

20. A method claimed in Claim 15 wherein the protection coating is formed of an oxide superconductor material crystal layer having an orientation different from that of each of the first and third thin films of oxide superconductor material.

21. A method claimed in Claim 20 wherein each of the first and third thin films of oxide superconductor material is formed of a "c"-axis oriented superconductor material crystal, and the oxide superconductor material crystal layer of the protection coating is formed of a so called ".alpha."-axis oriented superconductor material crystal.