CA2085289A1 - Method for patterning a layer on oxide superconductor thin film and superconducting device manufactured thereby - Google Patents

Abstract

Abstract of the Disclosure:
A method for patterning an oxide superconductor thin film, comprising a step of forming a SiO2 layer on the oxide superconductor thin film, patterning the SiO2 layer so as to form the same pattern as that of the oxide superconductor thin film which will be patterned, etching the oxide superconductor thin film by using the patterned SiO2 layer as a mask, and removing the SiO2 layer by using a weak HF solution, a buffer solution including HP or a mixture including HF

Description

SPECIFICATION

Title of the ~vention METHOD ~OR PAll~RNING A LAY~R O~ OXIDE
SUPERCO~DU(: TOR THIN ~LM AND
SIJPERCONDUClING DEVICE MANUFACrURED
- THEREBY

Background of the ~vention 1 0 Field o~ ~e invention 'rhe present invention re~ates to a me~hod ~o~ patteming a layer on a~ oxide supercondurtor thin ~ilm and a superconducting device manu~actured by ~he method, and mo~e specifically to a method for patterning a layer on an oxide superconductor ~in ~i]m wi~out degrading ~he o~ide superconductor thin film, and a superconducting device manufactured by ~he method.

Description o~ related art Devices which u~ e super~onducting phenomena operate rapidly 2 0 with low power consumption so that they have higher perfo~nance t~an conventional semiconductor devices. Particularly, by usin~ an oxide superconductitlg mate~al which has ~en ~ecently ~dvanced in s~udy, it is possible to produce a superconducting device which ope~tes at relatively high temperature.
Joseph~on deviçe is one of well hlown superco~duc~ing devices.
Howe~er, since Josephson device is a two-te~ninal device, a logie ga~e l- ~51~9 2~

which utilize~ Josephson devices becomes complicated, Ther~îore, ~ee-te~ninal super~onducting devices are more practical.
Typical three~terrninal superconduct~ng devices ~nclude two types super-FET (field effec~ transistor). The first ~ype of the super-~l~T
S ~cludes a semiconductor channel, and a superconduc~r source electrode and a superconductor drai~ elec~rode which are formed closely to each other on both side of the semi~onductor chalmel. A po~ion of the semiconductor layer between the superconductor sour~e electrode and ~e superconductor d~ain electrode has a greatly ~eces~çd or undereut rear 10 sur~ace so as to hav~ a reduced thickness. In addition, a ga~e electrode is fo~ned ~rough a gate insulating layer on the por~ion of ~e recessed or undercut rear surface of the semiconductor layer between ~he supercondalctor sollrce ~lect~ode and the superconductor drain electrode.
A superconducting current f1QWS through the sem;conductor layer 15 (channel) between the superconductor source eIectrode and the superconductor d~ain elec~rode due ~o a supe~conducting proximity effect, and is eontrolled by ~n applied ga~e voltage. This type of ~e super-FET
operates at a higher speed with ~ low power consumption.
l~e second type of the super-FET includes ~ chanDel of a 20 supe~onductor folmed between a sol~rce electrode and a drain electrode, so ~hat a cur~ent flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channe].
Both of the super-l~ETs men~ioned a~ove are voi~age contsolled 2 5 devices which are eapable of isolating output signal ~rom anput ol~e and of having a well de~med gain.

~5~g 2~J~3~r~

However, since ~he first type of the super FET utilizes the superconducting proximity ef~ect, the superconductor source electrode and the supercotlductor drain electrode have to be positioned within a distance of a ~ew times the coherencc leng~h of the superconductor materials ~f ~e superconductor source electrode and the superconduc~or drain electr~de. Tn par~icular, since an oxide superconductor has a short coherence length, a distance be~ween the supercvnductor source electrode and ~e superconductor drain electr~de has ~o be made ]ess than about a ~ew ten nanometers, i~ the superconductor source electrode and tl~e 10 superconductor drain electrode a~e formed of the oxide superconduc~or material. However, it is very di~lc~lt to conduct a fine processing such as a fine patte~n etc~ing, so as to satisfy the very short separa~ion distance mentioned above.
On the other hal~d, the super-FET having the superconducting 15 channel has a large cur~ent capability, and the fine processill~ which is required to produc~ the ~irst type of the super-FEl' is not neec1ed to prod~ct t~is type of super-FET.
In order ~ obtaîn a complete ON/OFP operation, both of the superconducting channel and the gate insulating layer should have an 20 extremely thin ~icknes~. Por example, ~he supercorldl~cting charmel formed of an oxide superconduc~r material should have a t~ickness of less than five nanome~ers and ~he gate insulating layer should have a thickness more than ten nanometers which is suf~icient ~o pr~vent a tunnel currellt.
2 5 In ~e super-FET, since the extremely thin superconducting channgl is ~onnested to ~he relatively thick supe~orlduc~ing sour~e regi~n and the super~onducting dMin region at ~eir lower pOl'tiORS, the superconduct~ng 3 1515~

, . .

2~ g current flows substantially horizontally through the superconducting channel and substantially vertically in the supercorlducting sollrce region and ~e ~uperconductillg drain regioll. Since dle oxide superconduc~or h~
dle ~argest critical current density Jc in the direction perpendicular to 5 c-axes of its crystal lattices, the superconducting chamlel is pre~erably formed of a c-axis oriented oxide superconductor thin film and the superconducting source region and ~he superconducting drain region are preferably formed of a a~is oliented oxide supereonduc~or ~in films.
In a prior art, in order to manufacture the super-FET which has ~e 1 o superconducti~g channel of c-axis oriented oxide superconductor thin ~
and ~e superconducting source region and the superconducting drain region of a-ax;s oriented oxide superconductor thin films, a c-axis oriented oxide superconductor thin film is ~o~ned at first and the c-axis oriented oxide superconductor thin film i3 etched and removed excluding 15 a portion which will be the superconducting channel. Then, an a-axis oriented oxide superconductor thin film is deposited so as to form the supe~condue~ing source region and the superconducting drain region.
In another prior art, at first an a-axis oriented oxide superconductor thin ~ilm is deposi~ed and etched so as to ~orm ~he 2 0 superconducting source region and the superconducting drai~ region, and ~ell a c-axis oriented oxide superconductor thin ~ilm is deposited so as to fonn the superconducting ehalmel.
In t~e above rnethods, the oxide superconductor th;n ~llrn is mostly processed by pho~oli~ography. Namely, the oxide supercollductor ~hin 2 5 ~llm is masked by a photoresist and etched by a we~ etchillg proeess using a weak H3PO4 sollltion, or a dry e~ching process sueh as a reactive iOIl etching or an ion-milling using Ar ions. ~ ord~r ~o process the oxide ~ 4 - 161~

, 2~$~cÇ~9 superconductor thm film without degradation, the oxide superconduc~or thin film should be prevented ~rom contacting with water. Since the oxide superconductor has high reac~ivity so as to react with water and i~
degraded. Ther~fore~ ~ese etchi~g process use little water.
S However, an oxide superconductor also reacts wi~h photoresist remover so that a ~urface of the oxide superconductor thin ~IIm on which a photoresis~ is fionned and removed is roughened. It is ve~y di~ficult to deposit ano~er thin film or layer on the rough su~ace of the oxide superconductor thin ~ilm so as to manu~acture a supercondueting device or a superconducting circuit of a multi-layer structure. In addition, if ano~er oxide superconductor thin ~ilm is formed so as to contact the rough sur~ace, an undesirable Josephson junc~ion or a resistance is ~enerated al the inte2face. Purthermore, superconducting characteristics of the reac~ed o~ide superconductor thin film is af~ected, so that she superconducting device does not have an enough performal~ce.

Summary of ~e ~vention Accordingly, it is an object of the present invention to provide a method for patterning a layer on an ox;de superconductor thin film, 20 which have overcome the above mentioned de~ects of the collventiondl ones.
Ano~er object of the present invention is ~o provide a me~hod ~or processing an oxide superconductor thin film, which have overcome the above mentioned defects of ~he conventional ones.
2 5 Still another object o~ dle present inv~ntion is ~o p~wide a med~od ~or mamlfacturing an FET type superconduc~ing device which have overcome ~e above mentioned de~cts of the comrentiQnal ones.

' - 5 - 151S~

5~JI9 Ano~er o~.ject o~ the preseIlt invention is to prov;de a~ PET type superconductillg device having a superconduc~ g region ~o~stitu~ed of an extremely thin oxide superconductor ~lm, which have overcome t~e above men~ioned de~ects of the conventional olles.
The above and other objects of ~e present invention are achieved ~n accordance wi~ the presen~ invention by a method ~or patteming a layer whicl) is folmed on an oxids superconductor thin film characteri~ed in that a weak H~ solution, a buffer solution including HF or a mixture including HF is used for etching the layer.
Preferably, the HF concentration of the weak HF solution, the buffer solution including H~ or the mixture including HP is S to 15 wt%.
An oxide superconductors is nst a~fected by this weak ~F solu~ion so that the exposed portion of the oxide supercondllctor thin film is not roughened.
Ac~ording to another aspect of the present invention, there is provided a method for pat~erning an oxide superconductor thin film, comprising a step OI forming a SiO2 layer on the oxide superconductor thin film, patterning the SiO2 layer so as to fo~n the same pattern ~s tha~
of the oxide superconductor thin film whicb will be pat~e~ed, etching ~e oxide superconductor thin film by using the pa~terned sio2 1ayer as a mas~, and removing the sio2 layer by using A weak HF solu~on, a bl2ffer solution including HF or a mixture including HP.
In ~is mcdlod, ~e HF concentration of the weak HF solution, the bufIer solution including HF or the mixture including HF is preferably S
2 5 ~o 15 wt%. This weak HP solu~ion sel~ctively etches SiO29 ~herefore, ~e ~xide superconductor thin ~llm is not a~fected.

- . - . -In one preferred embodin~ent, the SiO2 layer is also patte~ed by using a weak HP solution, a bu~er solution including HP or ~ mixture including HP.
According to stilI another aspect of th~ present invention, there is s proYided a method of manufacturing a superconducting device, comprising the steps of forming on a principal surface of a sub~trate a non-superconducting o~ide layer ha~ing a similar c~stal structure to that of a c axis oriented oxide superconductor thin film, folming a c-axis oriented ox~de superconductor thin film haYing an extremely thin I O ~hickness on the non-superconducting oxide layer, ~o~ning an insulatillg layer on the c-axis oriented oxide superconductor thin ~llm, ~olming a gate electrode of polycrystaIline silicon on a cen~er portion of the insulating layer, etching th~ insulatihg layer by using the gate electrode so as to folm a ga~e insulati~g layer under the gate electrode and ~ITning an 15 a-axis oriented o~ide superconductor ~hin ~lm so as to embed the gate elec~r~de and ~o îo~n an insulating region by dif~used silicon from the gate electrode, and et~hing back the ~axis oriented oxide superconductor thin film so that all upper surface of the a~axis oriented oxide superconductor ~in ~ilm is planarized and the gate electrode is exposed at 2 0 the planarized upper su~ace of the a-axis oriented oxide superconductor thin film and a superconducting source region and a superconducting drain region are fonned at ~e both sides of the gate eIectrode.
It is pr~ferable th~t the insulating layer is ~tched by using a weak HP solution, a bu~fer solution including HP or a mixture including HP.
25 this case, the superconducting channel of the extremely thin c-axis oriellted oxide superconductor f;lm is not a~fected by tlhe ~ching process.
The~fore, the superconducting device h~s ~ high per~o~ance.

- 7 - ~5 , : .

,9 According to further another aspect of the present invention, there is provided a superconducting device compris~ng a substrate having a principal surface, a non-super~onducting oxide l~yer h~ving ~ similar crystal structure to that of the o~Eide superconductor, an extremely thin S superconducting channel formed of a c~axis oriented oxide xuperconductor thin film on the non-superconducting oxide layer, a superconducting source region and a superconducting drain region fonned of an a-axis oriented oxide superconductor thin ~ilm at the both sides of the superconduc~ing channe} separated from each o~er, which are electrically comlected each other by the superconducting channel, so that superconducting current can flow through the superconducting cham~el between ~e superconducting source region and ~e superconducting drain region, and a gate electrode of a material which includes silicon ~rough a ~ate insulator on the superconducting channel for controlling ~he 1~ superconducting current flowing throllgh the superconducting cbannel, in which the gate elec~rode is embedded between the superconducting source region and the superconducting drain region and is isolated from ~he superconducting ~ource region and ~e superconducting drain region by an insulating ~ion fo~ed ~ di~used silicon from the gate electrode.
In the superconducting device in aceordance with ~e present invention, superconducting current ~ows alozlg the insula~ing reg;on which is formed by diffilsed silicon and has a smoo~ pr~file next ~o the superconducting source region and the superconductirlg drain r~gion, ~he superconducting current ef~ic}ently ~lows into and flows from the superconducting channel. ~her~re, superconduc~;ng curlrent flow into or ~rom ~he supercon~ucting channel efficiently so that the current capabili~y of the super-FET ean be impr~ved.

- 8 ~

, .'9 The gate electrode is preferably ~ormed OI polycrystalline ~ilicon, single crystalline silicon or silicide of a metal.
In the superconducting device in accordance with the present invention, ~e non-superconducting oxide layer prefer~bly has a similar crystal structure to ~at of a c-axis oriented oxide ~upsrcollductsr thin is case, the superconducting channel of a c-axis oriented oxide superconductor thin film can be easily fo~med on the non-superconducting oxide layer~
Preferably, the above non-superconducting oxide layers is ~ormed 1 û of a PrlBa2Cu307 p oxide. A c-axis oriented PrlBa2Cu307 E thin film has almost the same crystal lattice structure as ~at of a c-axis oriented oxide superconductor thin film. I~ compens~tes an oxide superconductor thin lm ~r its cryst:alline incompleteness at the bottom surface. T31erefore, a c-axis oriented oxide superconductor thin f~lm of high c~ystallinity can 1 5 easily formed on the c-axis oriented PrlBa2(:u307 " ~hin ~llm. ~ addition,~e effect of diffusion of ~e constituent elements of PrlBa2Cu307.~ into ~e oxide superconductor thin film i~ negligible and it ~lso pr~vents the diffusion from substrate. Thus, the oxide supercondu~tor thin film deposited on ~e PrlBa2Cu3O7.~ thin ~ilm has a high quality.
In a preferred embodiment, the oxide superconductor is ~rmed of high-TC (high c~i~ical temperature) oxide superconductor, particularly, ~ormed of a high-TC copper-oxide type compound oxide supercondllc~or for example a Y-Ba-Cu-V compound oxide supercor~duc~or material, ~
Bi-Sr-Ca-Cu-O compourld oxide superconductor material, and a 2 ~ Tl-Ba-Ca~ u O compoulld oxide sup rconductor material.
In addition, the substrate can be fonned of an insulating subs~rate, preferably an o~ide single crystalline substrate such as MgO, SrTiO3, 9 ~57~

- , , : , CdNdA104, etc. These substrate materials are very effective in Ionning or growing a crystalline ~ilm having a high degree of cry~tal1ine orientation. However, the supercondllcting device can be ~rmed on a semiconduc~or substrate if an appropriate buf~er layer is deposite~
S ~ereon. For example, ~e buffer layer on the semiconductor substrate can be forrned of a double-layer coating ~o~ned of a Mg~lC)4 layer and a BaTiO3 layer if silicon is used as a subs~rate.
Preferably, the superconduc~ing channel is formed of a c-axis orient~d oxide superconductor thin film and the superconduc~ing source 10 electrQde and the superconducting drairl electrode ar~ ~rmed of a-axis oriented oxide superconductor thin films.
The above and o~er objects~ features and adYantages of the preseI~t invention will be apparent from ~e following descr1ption of preferred embodiments of the invention with reference to the ~ceompanying 1 ~ drawings.

Brief Descrip~doIl of ~e Drawings Figures lA to lF are diagrammatie see~iona~ views for illustrating an embodiment of the method in accordance with the present inven~ion 2 0 for patterning an oxide superconductor thin film; and Figures 2A to 2J are diagramrnat}c section~l views for illustrating an embodiment of t~e method in aceordance with the preserlt inventio for manufacturing ~he super-~ET.

- 10 ~6~6 .

2~$-.

Description of ~e Preferred embodiments Embodiment 1 Referring to Figures lA to lF, the method in accordance with the presen~ invention for patterning an oxide superconductor thin film wilI be described.
As shown in Pigure lA, a YIBa2Cu307 s oxide ~uperconductor thin film 1 is deposited on a MgO ~t00) single crystalline substrate 5 having a subs~antially planar principal surface.
As shown ~n Figure lB, a SiO2 layer 32 having a thiclcness oP 200 10nanometers is fo~ned on ~e YIBa2Cu307 8 oxide superconductor thin film 1 by a CVD. The SiO2 layer 32 is formed under a condition in which the substrate temperature is lower ~an 350 C.
Then, as shown in Figure lC, a photoresist layer 34 having an opening 36 is fo~ned on ~e SiO2 layer 32 and a por~ion of ~e SiQ2 layer 1532 is e~posed at the opening 36. The portion of ~e SiO2 layer 32 exposed at ~e opening 36 is etched by a wet etching using a 10 % ~ solution or a dry etching process such as a reactivP ion etchin~, an ion-milling using Ar ions.
lhe portion of sio2 layer 32 is completely remo~ed so ~at an 20opening 37 is formed and a portion of the Y~Ba2Cu3O7 ~ oxide superconductor thin film 1 is exposed. The~ he photoresist 34 is }emoved, as shown in Figure lD. The portion of the YlBa2Cu307 ~ oxide superconductor ~in fiim I is a~ected by ~e photoresis~ remover at ~his time.
25Thereafter, the por~on ~f the YIBa2Cu3O7.~ oxide supercotlductor ~in film 1 is etched by a wet etching using a ~.1 % H3PO4 solution or a dry e~hing process ~uch as a reactive ion etching, an ion-milling using Ar 151~

ions so ~at the purtion of the YIBa2Cu307~g oxide superconductor thin film 1 i~ completely rernoved and a portion 38 of the su~strate 5 is exposed, as shown in Figure lE. The portion of the Y1~a2Cu307.~ oxide superconductor thin film 1 which is degraded by ~e photoresist remover S is r~moved simultaneously.
Finally, as shown in Figure lF, the remaining sio2 layer 32 is removed by using a 10 % HP solution. This weak HF solution does not afiect the YIBa2Cu307.~ oxide supercondllctor thin ~ilrn 1. lherefore, ~e surface of the YlBa2~u307.~ oxide superconductor thin film is not 10 roughened and is as smooth as that OI an as-grown YlBa2Cu3Q7 ~ oxide superconductor tl~ lm. Also, the superconducting ch~racteristics of ~e YIBa2Cu307 ~ oxide superconductor thin film is not a~fected.
As explained above, if an oxide superconduetor thin ~ilm is patterned in accordance widl ~he embodiment of the me~od of the pre~ellt 15 invention, the surfaee of the oxide superconductor thin ~ilm is not roughened and ~he superconducting characteristics is not affected.
T31erefore, another ~in ~llm or layer can be easily formed on ~he oxide superconductor ~in film so that a supercondllcting device or a circuit of a multi-layer structlire is easily manufactured.
2~
Embodiment 2 Referr~g to Figures 2A to 2J, the process in accordance with the present invention for manufacturing ~e super-FET will be described.
~ s shown in Figure 2A, a MgO (100~ single crystalline subs~rate 5 2 5 having a substantially planar principal sur~ace (~100) sur~ace) is prepa~d.
As shown in Pigure 2B, an oxide layer 20 having a thickness of 50 nanometers composed of a PnBa2Cu3O7.e ~in ~llm is deposite~ on the - ~2 ~ ~5~59 ' .~ , 2~

principal surface of ~ ~ubstrate S, by an MBE. While the PrlBa2cu3o7~e ~in film is growing, the surface mo~phology of the PrlB~2Cu3O7.~ th~n lm is monitored by RHEED. A condition of ~o~nulg ~1~ P~lB~2(~u3o7~e oxide thin film by MBE is as ~ollows:
Molecul~r beam source Pr: 1225~C
Ra: 600C
Cll: 1040C
Pressure 1 x 10 5 Torr TemperatNre of ~e substrate 750C
Then, ~he Pr molecular beam source is exchanged to a Y molecular beam source an~ ~e tempera~ur~ of the substrate is lowered to ~00 ~(: so that a c-axis oriented YIBa2Cu3O7 ~ oxide superGonductor thin film 1 having a thickness of about 5 nanometer is continuously fo~ned on the oxide layer 20 o~ PrlBa2Cu3O7.~ thin film9 as shown in ~igur~ ?C. A
condition of forming the c-axis oriented YlBa2Cu3O7.~ oxide super~onductor thin fi~m 1 by MBE is as follows:
Moleeularbeam sou~e Y: 1250C
Ba: 60ûC
Cu: 1040C
Pressure 1 x 10-5 Torr Temperature of ~e substrate 700C
Then, as shown in ~igure 2D, an insulating layer 17 of SrTiO3 having a tllich~ess of 10 to- 20 nanometers is ~ormed on the c-axis oriented YIBa2Cu3O7 ~ oxide superconduetor thin film 1 by a sputtering A
2 5 polycrystalline silicon layer 14 having a thickness of 200 nanometers is ~med on ~e insulating layer 17 by (:YD, as shown in Figure 2E.

- 13- ~15 .,9 Thereafter, the polycrystalline silicon layer l4 i8 etched by ~
reactive ion etching so as to fo~n a gate elec~rode 4, as shown in Figure 2F. Then, the surfaces of the gate electrode 4 i~ oxidized so ax to form a SiO2 layer havhlg a thickness of 50 to lOO nanometers, as showr 5 in Figure 2~}.
Therea~ter, as shown in Figure 2H, the hlsulating layer 17 of SrTiO3 is etched so as to fo~m a gate insulating layer 7 by us~ng a mixture of HF and NH40H. The mixture of HF ~nd NH40H selectively etched the insulating layer 17 of Srl'iO3 and does not affeet the YlBa2Cu307 ~ ~xide lO superconductor thin film l. The~efore, the characte~istics of the YIBa2Cu307.~-oxide superconductor thin film 1 ;s maintained. A portion of ~e YlBa2Cu307.g oxide superconductor thin film 1 under the gate insulating layer 7 becomes a superconducting channel.
Thereafter, ~e substrate S is hea~ed ~o a temperature of 350 to 400 15 ~C under a pressure lower than 1 x 10-9 Torr so flS to clean ~e exposed surface of ~e Yl}3a2Cu3C)7 ~ oxide supercollductor ~in film 1. 'rhis heat ~reatment is not necessary, if the exposed surface of ~e ~ axis oriented Y~Ba2Cu307 ~ oxide superconductor thin film 1 is clean enough. Then, a a-axis oriented Y1Ba~Cu307 ~ oxide superconductor thin ~llrn 11 having a 20 thic~ess of 500 nanometers is deposi~ed on t~ YlBa2Cu3Q7 ~ oxide s~erconducto~ dlin film 1 by an off-axis sputtering so as to encapsu1a~e ~e gate electrode 4, as shown in Figure 2I. A condition of forming the YlBa2Cu307 ~ sxide superconductor thin filrn 11 by an off-axis sputte is as follows:
2 ~ Tempera~ure of ~e substrate 640 C
Sputten~g ~3a~ Ar: 90%
2: 10%

- 14 - 1~15g 2g'~ 9 Pressure 10 Pa While the YlBa2Cu307.~ oxide superconductor thin film 11 is deposited, silicon diffuses from the gate elec~rode 4 so as to fo~m a insulating region 50 around the gate electrode 4. The insulating region 50 S is folmed of a YIBa2Cu307.~ oxide superconductor which does not show superconductivity by ~e diffused silicon.
Finally, m o~de~ to planarize an upper surface of ~e YIBa2Cu307.~
oxide superconductor thin film 11, a photoresist layer (not shown) is coated on ~e YIBa2Cu307 ~ oxide superconductor thin film 11 in such a 10 manner ~at the deposited photoresist layer has a flat upper swrface~ and ihen, the coated photoresist layer and the Yl B a2Cu 30 7~ oxide superconductor thin film 11 are etched back, until d~e upper surface of the YlBa2Cu307~ oxide superconduc~or ~in film 11 is planar~zed and the gate electrode 4 is exposed at the planarized upper surface of ~he 1 5 YlBa2Cu30~ ~ oxide supercondu~tor thin film 1 llas shown in Figure 2J.
Portions of ~e YIBa2Cu30? ~ oxide superconductor ghin ~llm 11 a~ the bo~ sides of ~e gate ~lectrode 4 become a superconducting source region 2 and a superconducting drain region 3.
Metal electrodes may be fo~ned on the superconducting source 2 0 region 2 and the superconducting drain region 3, if necessary. With this, ~he super-FET in acco~dance wi~ the present inventioll is completed. .
The superconducting channel of ~e above mentioned super-FET
manufactured in accordance with the embodiment of the method of the presel~t invention is formed on an oxide layer which has similar 2 ~ crystalline structure to that ~ the oxide superconductor. There~ore~ the bot~om portion of the superconducting charmel is not degraded so that dle - 15- ~5t59 ~2~ g substantial cross-sectional area of the superconducting charmel ~f the super-FET is l~rger ~an that of a conventional ~uper-F~T.
Additionally, since supercondu~ting current flows ~long the insulating region wbiGh is ~ormed by dif~used silicon next to the S superconducting souree region and the superconducting drain region, the superconducting current efficiently flows into and flows ~rom ~he superconducting channel. By all of ~ese, the current capability of ~he super-FET can be improved.
Furthe~nore, according to the present invention, the oxide layer, l O the superconducting channe~, the gate insulating layer and the gate electrode are self-aligned. The insulating region 50 which isolates the gate electrode from the superconduc~ing source region and the superconducting drain region is also automatically positioned. Therefo~, ~e limitation ~n the fine processing technique required for manufacturing 15 ~e supe~-~T is rela~ed.
Additionally, according to the present invention, ~he gate insulating layer is ~ormed by an etching process using a mixtalr~ of HP and NH40H.
I'he mixture of HP and NH40H selec~ively etched ~e insulating layer of SrTiO3 on the oxide superconductor thin film which will constitutes the 2 0 superconducting channel and does not affect the oxide superconductor thin fi~m. ThereIore, the superconducting charac~eristics of the oxide superc~nductor ~in fi}m is main~ined.
In ~e above mentioned embodiment, ~he oxide supercondlletor ~in film can be formed of not only the Y-Ba-Cu-O compound oxide 2s supe~conductor material, ~ut also a high-TC (high critical temperalture3 o~ide superconductor material~ particularly a h;gh-TC copper-oxide ~ype compound oxide superconductor material, for example a Bi-Sr-Ca~Cu-O

- 16 - t315~

:

2r'~ 9 compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O
compound ~xide superconductor materi~l.
The invention has lhus been shown and described wi~ re~rence ~o the specific embodiments. However, it should be noted ~hat ~e present S invention is in no way limited to the details of the illustrated strucgures bu~ converts aIId modi~lcations may be made wi~hin the scope of the appended claims.

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Claims (14)

1. A method for patterning a layer which is formed on an oxide superconductor thin film characterized in that a weak HF solution, a buffer solution including HF or a mixture including HF is used for etching the layer.

2. A method claimed in Claim 1 wherein the HF concentration of the weak HF solution, the buffer solution including HF or the mixture including HF is 5 to 15 wt%.

3. A method for patterning an oxide superconductor thin film, comprising a step of forming a SiO2 layer on the oxide superconductor thin film, patterning me SiO2 layer so as to form the same pattern as that of the oxide superconductor thin film which will be patterned, etching the oxide superconductor thin film by using the patterned SiO2 layer as a mask, and removing the SiO2 layer by using a weak HF solution, a buffer solution including HP or a mixture including HF.

4. A method claimed in Claim 3 wherein the SiO2 layer is patterned by using a weak HF solution, a buffer solution including HF or a mixture including HF.

5. A 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, forming a c-axis oriented oxide superconductor thin film having an extremely thin thickness on the non-superconducting oxide layer, forming an insulating layer on the c-axis oriented oxide superconductor thin film, forming a gate electrode of polycrystalline silicon on a center portion of the insulating layer, etching the insulating layer by using the gate electrode so as to form a gate insulating layer under the gate electrode and forming an .alpha.-axis oriented oxide superconductor thin film so as to embed the gate electrode and to form an insulating region by diffused silicon from the gate electrode, and etching back the .alpha.-axis oriented oxide superconductor thin film so that an upper surface of the .alpha.-axis oriented oxide superconductor thin film is planarized and the gate electrode is exposed at the planarized upper surface of the .alpha.-axis oriented oxide superconductor thin film and a superconducting source region and a superconducting drain region are formed at the both sides of the gate electrode.

6. A method claimed in Claim 5 wherein the insulating layer is etched by using a weak HF solution, a buffer solution including HF or a mixture including HF.

7. 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, an extremely thin superconducting channel formed of a c-axis oriented oxide superconductor thin film on the non-superconducting oxide layer, 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 superconducting channel separated from each other, which are electrically connected each other by the superconducting channel, so that superconducting current can flow through the superconducting channel between the superconducting source region and the superconducting drain region, and a gate electrode of a material which includes silicon through a gate insulator on the superconducting channel for controlling the superconducting current flowing through the superconducting channel, in which the gate electrode is embedded between the superconducting source region and the superconducting drain region and is isolated from the superconducting source region and the superconducting drain region by an insulating region formed by diffused silicon from the gate electrode.

8. A superconducting device claimed in Claim 7 wherein the non-superconducting oxide layer has a similar crystal structure to that of a c-axis oriented oxide superconductor thin film.

9. A superconducting device claimed in Claim 8 wherein the non-superconducting oxide layer is formed of a Pr1Ba2Cu3O7-.epsilon. thin film.

10. A superconducting device claimed in Claim 7 wherein the gate electrode is preferably formed of polycrystalline silicon, single crystalline silicon or silicide of a metal.

11. A superconducting device claimed in Claim 7 wherein the oxide superconductor is formed of high-Tc (high critical temperature) oxide superconductor, particularly, formed of a high-Tc copper-oxide type compound oxide superconductor.

12. A superconducting device claimed in Claim 11 wherein the oxide superconductor is formed of oxide superconductor material selected from the group consisting of 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.

13. A superconducting device claimed in Claim 7 wherein the substrate is formed of a material selected from the group consisting of a MgO
(100) substrate, a SrTiO3 (100) substrate and a CdNdAlO4 (001) substrate, and a semiconductor substrate.

14. A superconducting device claimed in Claim 13 wherein the substrate is formed of a silicon substrate and a principal surface of the silicon substrate is coated with an insulating material layer which is formed of a MgAlO4 layer and a BaTiO3 layer.