CA2103634A1 - Dc superconducting quantum interference device - Google Patents

Abstract

ABSTRACT

A dc superconduction quantum interference device (dc SQUID) with a ground plane which covers the Josephson device is capable of stable operation. The ground plane which is formed of a superconducting film shields the Josephson device from interlinkage of external magnetic flux, and then stable operation of the dc SQUID can be realized without external magnetic flux trap. The ground plane can be simultaneously formed with the washer cover.

Description

2~.~3~J3'~
DC SUPER O~DUCTI~G QUANTUM I~TERFERE~C~ DEVICE
BACKGROUND OF THE I~VE~TIO~
The present invention relates to a dc superconducting quantum ln~erference device (which, for brevity3 will her~i~after be referred to as a "dc SQUID"), which i3 u~eful as a highly sensltive magnetic sen~or, ~ ammeter, a displacement meter, A high-~requency ~ignal amplifier or the like.
The prior art dc SQUID, (which i8 hereinafter descxlbed in ~ore detailj i8 operationally un~table due to the lack of a grou~d plane, because i~terlinXage ln the external ~agnetic flux ofte~ oceurs, whlch tend~ to invite magnetic flux ~rap.
Because the interli~kage trap in the magnetlc flu~ is generated i~
the ~eighborhood of the Jo~eph~on device~ the operation of the dc SQUID
becomes u~stable.
lS SUMMARY OF THE IFVE~TIO~
It is an ob~ect of the pre3ent invention to pro~ide a dc ~QUID wi~h s~able operation.
In order to achieve the above ~peclfied ob~ect, a supercoQducting layer grou~d plan~ 1~ provlded, in ~uch manner that the ground plane overlay~ the area contalning the Jo~eph~on devlce. For producinæ thl~
layer, the number o~ production step~ does not require to be lncrea~ed because the ground plane is simultaneously formed in ~he same proces~
step during which the ~asher cover of the washer coil 18 processed.
Since the dc SQUID in the afores~id structure iB superconduc~lvely shielded ~y the ground plane, in the neighborhood o~ the JosephRon de~ice, lnterlinkage of external magnetic flux can be avoided) and s~able operatlon can ~hen be realiæed without external ~agnetie flux tr~p.
The invention will now be de~crlbed further by way of example ~nly and with reference to the accompanyiDg drawings.
BRIEF DESCRIP~ L5~ LIeL~YI~
Fig. 1 is a top plan view ~howing a dc SQUID accordlng to a preferred e~bodiment of the present inventioll;
Fig. 2 is a top plan ~iew showing a prior art dc SQUI~;
Fig. 3 is a sectional YieW along the line A-Ai of Fig~ 1, showing th~ fabrication structure of the dc SQUID;
Fig. 4 iQ ~ top plan view of a dc SQUID w~th double ~sher coil~
~onnec~ed in parallel; and : ,:

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

, . .. . .

~ ~ t~

Fig. 5 i5 a top plan view o a dc SQUID ~ith double washer coils con~ected i~ series.
ETAILED DESCRIPTIO~ OF TH~ PREF~RB~D ~MBODIMRNTS
Con~iderin8 firstly Fig. 2, there ~9 ~hown ~ prlor art dc SQ~ID
having a washer coil 4 with a dumping re~l~tor 3, Jo3ephson dev~ces l, ~hu~ting resistors 2, input coll 5~ feedback modul~ltio~ coil 6, And superconductive layer 8 whlch functlon~ as a washer cover layer.
Because there i~ no ~hielding in the neighborhood of the Josephson device, ehe aorementloned operational ins~ability due to esternal ~ag~etic flu~ interlinXage occurs.
Such lnstabill~y gi~e ri~e to flicker of the output 3ignal3 ln the superconduc~ivity ~tate operation of the device because ~agnetic flu~
become~ trapped or s~ifted. Such fluctuation~ become larger in effect, e3pecially due to the existence of trapping in the neighborhood of Jo~ephson device l.
Fig. 1 ~llustrates a structure accordlng to the invention, where~n ground plane 7, which is an electrically insulated superconductive layer, is for~ed to cover broadly enough the field of the Joseph~on devices ln t~e shape of the tOp plan view of Fig. 1. Wlth this layer, the traps of magnetic flux become hard to generate9 and then the behaviour of the device becomes stable. In the production of the ground pl~ne 7, ~he number of proces3 steps doe3 not need to be increa~ed, because the ground plane 7 can be simultaneou~ly formed in the ~me procesq for formln8 washer cover 8, whlch i~ also a superconductive layer.
~eferri~g now to Fi8. 3, which shows a cro~s-nection alon8 line ~-A' of Fi8. 1, the ~anufacturing proce~s will be de~cribed as follows Usine a thin ~ilm photolithography process, ground plane 7 and washer cover 8 are formed as a superconductive fll~ In the fir3t phaqe, and lnsulated by inter-lsyer in~ulating film 11. A~ the ~uperconductive film, usually Nb i~ depo~ited by DC magnetron sputterlng. The aforesaid ilm should be a~
thic~ a~ nece~sary to provide ~uperconduetive shielding. Therefore, about 100 nm or more will be enough in ~he cas~ of ~b. Ground pl~ne 7 can also be electrically grounded to the SQUID.
~ext, after the fesi~eance fll~ fo~ming the 3hunting resistor 2 and dumplng resi~tor 3 of the dc SQUID are de~osited, the resistor films are insulated by means oP an inter-layar insulatlng film, ~nd the re~ist~ce value~ are fixed at the required designed level~. The resl~tor film8 oan , . , . . :
~' ' . . ` , ' ' ' . ,, ' :
.. . . . .
' ' '' :: . ' ': ' ', . . . ' ~. : ' be made of a metal such as Mo, MoN, Pd, Au, Cu~ AQ, Pd, Ti and the like, any of which can be deposited by a sputtering or e~aporation proce~s.
In the present case, AQ is deposi~ed to a depth o lO0 nm by DG
Magnetron sput~erlng and ~he pattern i~ ~ormed to ~he required layout and size by photolithography. For the purpoa~ of AQ etching, both wet ~nd dry etchine methods cnn be used. As ~n e~a~ple of wet etchin8, there may be used a mixture of malnly phosphoric ncid and nitrlc acid. As for a~
example of dry etching, there may be used reac~ive ~o~ etching (~
using a chloride 8as ~uch as CCQ4 or-a mixture of s~loride ga~e~s. In the present ca~e, the AQ i~ etched by a wet etching met:hod. Inter-layer insulating film 11 can be made of SiO2, SiO, Si, MgO or the like. Any of these ~aterial3 can be deposited by sputtering, e~aporation or CVD and 80 on. The deposited film i8 set to be 1.5 to 2 times as thick as the resi~tance film, 80 as to i~sulate the resi~tance film completely.
Here, after the SiO2 ls deposited to 100 to 200 nm by RF magnetron sputteri~g~ the SiO2 i~ etched to form contact hole~ to the resista~ce fllm by the photolithographic step. Both the wet and dry etchi~g methods can be u3ed for the etching of the SiO~. The wet etching i8 esemplified by a method uslng a mixture of hydrofluoric acid. The dry etching is exempllfied by reactive ion etching (U ~) using a mixture of CF4 or CEF3, and oxygen. In thl~ case, the SiO2 is etched by ~IE using a mixture of CHF3 and oxygen.
~ he ~ext ~tep will be to ~orm the Joseph~on device 1 comprising of lower electrode 12, barrier layer 13, and upper electrode 14. ~ach of those ~hould be deposited, and the electrode 14 and the barrier layer 13 etched by photolithography. The Jo9eph90~ device 1 can be e~emplifled not only by the Nb/AQOx/~b structure but also by a variety o~ structures, such as ~b~/MgO/~b~, ~b/Si/~b7 ~b/Nb - oxide/Nb and the like. In the present case, the Nb/~QOx/Nb structure i8 deposited by ~putteri~.
~xamples of the depositlon proce~ are as follows.
An ~r Bas i8 introduced into a reaction chamber, whlch hAs been e~acuated to ~ high ~acuum of 10-5 Pa or less. The ~b fil~ formin~ the lower electrode 12 ls depo~lted under a pres~ure of 0.1 to 4 Pa by DC
magnetron sputterlng. The film i~ deposited to a depth of 100 to 300 nm. Then, the introduction of the argon gas is ~nterrupted, and the reaction chamber is evacuated agaln to a high ~acuum Qf 10-5 Pa or les~.
~P~er thl~, the argon ga~ i8 introduced so that the ~Q i~ deposted to a .. . .
. . , . . :
. , - ~ - :, '. :
. :: , .
'' . ' . . : . ~ ., ., :
:

i 3 ~
depth of 1 to 20 n~ snder a pre~sure of 0.1 to 4 Pa hy ~he DC magnetron sputtering. If the AQ spu~tering pre~sure is lower than that of the ~b9 the film becomes flner and its uniformlty will be increa~ed on the Nb surace.
~o problem arises hereg even ~hough the evacuat~on to a high vacuum i~ not elaborately performed before the ~puttering of the AQ. The reactio~ ~hamber is evacuated to a high vae~um of 10-5 Pa or less, and oxygen ga3 or a mi~ture of oxyg4n and argon is lntroduced to ad~ust the pres3ure to ~he se~ level. Then, the AQ ~urface l~i o~idized to form the barrier layer 13 of AQOxJAQ. The reactlo~ c~a~ber i8 evacuated to a high vacuum of 10-5 Pa or less, and the uppe~ electrode 14 is deposited to a depth of 100 to 300 nm under ~he aforementloned ~b depositin8 condition.
~ ex~, the upper electrode 14 and the barrier layer 13 are etched by the photoll~hographic step to form the Josephson device 1. The etching method used i~ generally the dry etchil~ method using plas~a. The ~b of the upper electrode 14 is sub~ected to reactive ion etching tRIE) ~sing CF4 or a ~ixture of CF4 and oxygen. The A~ of the barrier layer 13 is removed by either the wet etching method using an acid or ~1~ u~ing an A~
ga~. ~ere, the barrier layer 13 need not necessarily be etched.
~ext, the wa~her coil 4 i9 $ormed by patterning the superconductlve film depos~ted as t~e lower electrode 12 of the Joseph~on de~lce 1 by photol~thography. The etching method u ed i~ generally the dry etching method u8ing plasma. The ~b of the lower electrode 12 i~ sub~ected ~o the pla~ma etching or reactlve lon etching (RI~) u8ing a gas ~ixture of CF4 and oxygen. Here, the etchin8 i~ ~o different from that of t~e a~orementloned upper electrode 14 that the i~otropic etching i8 effected by increaslng the amount of oxygen, ~nd the re~lst film at the periphery of t~e pattern i~ etched with the oxygen in~o a tapered ~hape. As an example~ the etching can be perEormed by uqing the plas~a etching apparatus wlth a gss which i~ prepared by adding lOX of oxygen to the CF4, under a pre~sure of 133 Pa and ~ith a power of 50 W.
~ e~t, after the lnter-layer insulating film 11 i8 depo~lted, a contact hole i8 then formed by pho~olithography. A superconductive fil~
1~ then deposlted agaln to forn a feedback modulatlon coil 6 and an input coil 5 by photolithography. The superconductlng film i~ exemplified by ~b, NbN, Pb-In or Pb-In-Au which 1g depo~ited by ~puttering or evaporation.

.' . ' :. ' ,.
' ,,, 1 ;,1 ~ ~ 3 ~
In the present ca~e, the Nb film i~ deposlted to 250 - 600 nm by DC
magne~ron sputtering, Just a3 the Josephson ~nctlon electrode ~8 for~ed. Before this deposltion, the subs~rate should be rever~e ~puttered u~ing Ar gas so as for ~he sub3~rate to be in superconductive contact. ~e~t~ the wa~her coll 4 and counter elec~rode~ and ~ther wiring elementa are formed by photollthography. The etching of ~he above ~s by plasma etching, as in the forming of ~he aforementioned lower electrode 12.
In the ca3e that the de~ectlon coil which detects ~he ternal magnetic field is not for~ed s1multaneously with the process of SQUID
formation, it i~ neceRsary t~a~ the external de~ection coil ~hould be superconductively connected to the SQUID for use in mea~uring. For this purpose, the buffer ~etal 9 may be nece~sary for the superconductive contact on the pad 10. In the case where a Pb-In alloy ~unction wire 13 used for the ~uperconductive con~act on the pad 10 by ~b, Au ~ deposi~ed to 10 - 100 nm for the buffer metal by ~puttering. The patte~ling for the above can be done ea~ily by wet etching uqing acid or the like.
For ~he exfoliation of the re3ist in the photolithographic proce~s, a wet or dry method can be applied. In the wet method, an organlc solvent such as acetone and t~e like, alkaline detachment liquid, concentrated nitric acid, hea~ed concentrated ~ulfuric acid~ etc. i8 used. In the dry method, oxygen plasma or W radiation i~ u~ed.
~xfollatlon can be done by the combination of one or both method~ or one or more of the abo~e ~aterials.
The fabrlcation of the dc SQUID~according to the pre~ent inven~io~
can be do~e as mentioned above. The se~uence of each layer formation can be inte~changeable or co~verted, 30 long as the clrcuitry i8 not changed. Table 1 i9 an example of design parameter~ for the fabrication of a SQUID having the shape and pattern as shown in Fig. 1.

.
. ., .. . . .. . . .

..

.

7J l C~3 ~s ~ 3 ~
TABLE l Washer hole size 50 ~m 0 Washer inductance 79 p~
Input coil inductance 64 n~
5 input coil ~urns 27 Feedback ~odulatlon coil ~urn Josephson ~unction slze 5 ~m~
Josephson ~unction critical current 16 ~A
ShuntlDg re~istor 4 10Damping reslstor 2 Q

So far, the inventio~ applied to a slngle washes coil ha~ been exemplified. Mul~lpl~ was~er coll ~ypes of ~he device ~aving the circultry the same a~ that of t~e first embodiment> but wherein the lS mul~iple washer coils are ~oined, can be alsQ fabricated ~o form ~he SQUIDS shown in Fig~ 4 or Fig. 5. Thus; these figures show seeQnd and third embodiments of the inYention, which examples of double washer coil type de~ices with two washer coils.
In a dc SQUID with a washer coil as shown in Fig. 1, there is still a possibility of the output ~ignals tending to flicker, since lt detects even a uniform ~agnetic field of flux linkage which the washer coil link8 to. Since SQUIDs with two washer coll~ can cancel the u~ifor~ ~agnetic flux of each other, the flicker of their outpu~ signal~ becomes smaller~
and the~ the operation becomes more stable.
Two ~ays of eonnecting two ~asher c0118 are possible, that 1~ in parallel or in ~eries. Fig. 4 ls an example of parallel connection and Fig. 5 is of serie~ connection. Each double washer coll SQUID has two feedback modulation coil~ 6, and such SQUID can be utillzed in msny different way3. For feedback modulation purpose, either one coil or both coils can be used. Two coll~ ~ay be used in such a way that one ~ for feedbacX purpose and another for modulation purpose. Also, two coil~ may be used only ~or modulatlon purpo~e, and in thi~ ca~e, ~he feedback function may be realized through the usage of a detection coil which i~
operatively coupled not only for detection but also for feedback.

-, In the devlce according to ~he pre~ent inventlon as hereinbefore de~crlbed, magnetic flux trap become~ hardly generated, ths fllcker o~
outputs owing to magnetlc flux trap can be reduced, and the~ s~able operation of the SQUID can be realized~ by setting up and overlaying the ground plane on ~he dc S~UID Jo~ephson device.

.

- : . ::

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

Claims (2)

1. A dc superconducting quantum interference device comprising:
a Josephson device coupled to both ends of a washer coil forming a superconducting ring;
a shunting resistor connected in parallel to said Josephson device;
a damping resistor coupled to both ends of said washer coil;
an input coil and a feedback modulation coil, both of which are magnetically coupled to said washer coil;
a ground plane formed of a superconducting film to cover said Josephson device; and a washer cover formed of a superconducting film to cover a slit portion of said washer coil;
wherein said ground plane and said washer cover are formed in the same layer.

2. A dc superconducting quantum interference device according to claim 1, wherein said input coil is formed in a spiral shape upon said washer coil.