JP3692870B2 - 2-SQUID magnetometer and driving method thereof - Google Patents

Description

【０００７】
ここで、Ａ_effは有効面積、Ｓ_Φ ^1/2は回路のノイズを含めた磁束換算雑音、Φ_SQはＳＱＵＩＤに固有の雑音、Ｖ_nは回路から発生する雑音電圧である。
【０００８】
また、ｄＶ／ｄΦは磁束−電圧変換係数で、Ｖ−Φ特性の最大傾きで与えられ、変調電圧振幅ΔＶを用いてπΔＶで近似できる。磁場分解能Ｓ_B ^1/2を下げるためには回路のノイズを含めた磁束換算雑音Ｓ_Φ ^1/2を下げ、有効面積Ａ_effを増加させることが必要となる。また、Ｓ_Φ ^1/2はＳＱＵＩＤに固有の雑音と回路から発生する雑音からなっているが、上記数１からわかるように雑音は二乗和の平方根の形で加算されるため、支配的な雑音を下げることが磁場分解能の向上につながる。最高の感度を得るためには、Φ_SQとＶ_n／（π・ΔＶ）が同程度になるように設計する。
【０００９】
変調電圧振幅ΔＶは、熱雑音を考慮した数値計算により導出された数２から数４で見積もることができる（たとえば、円福ら、ジャパニーズ・ジャーナル・オブ・アプライド・フィジックス、第３２巻（１９９３年）Ｌ１４０７頁（K.Enpuku, Jpn. J. Appl. Phys. 32 (1993) L1407））。
【００１０】
【数２】

【００１１】
ただし
【００１２】
【数３】

【００１３】
【数４】

【００１４】
ここでＬはＳＱＵＩＤリングのインダクタンス、Ｉ_cとＲ_nはそれぞれ弱結合の臨界電流と接合抵抗、Ｔは動作温度、Ｋ_Bはボルツマン定数、δΦ_nは雑音磁束と呼ばれ、熱雑音によりインダクタンスに生じる雑音を示す。つまり、数２の第２項は熱雑音によるΔＶの減衰を示す。δΦｎは温度ＴとインダクタンスＬの関数であるが、７７Ｋではインダクタンスのみで決まり、インダクタンスが小さいほど熱雑音の項は小さくなる。また、βはインダクタンスパラメーターと呼ばれており、通常１〜５程度になるように設計する。数２〜数４より、インダクタンスを小さくすることでΔＶが増加することが判るが、インダクタンスを小さくすると検出コイルとの結合が弱くなり有効面積が減少する。
【００１５】
別のアプローチとして、同じ特性のＳＱＵＩＤを直列に接続することで有効面積が同じまま大きなΔＶを実現することが可能である。理想的な場合、ｎ個のＳＱＵＩＤを直列にした場合の出力は、単体ＳＱＵＩＤのΔＶのｎ倍になる。ただし、個々のＳＱＵＩＤの臨界電流および磁場−電圧特性（図２）の周期と位相が揃っている必要がある。
【００１６】
一方、直列に接続することで個々のＳＱＵＩＤの電圧ノイズも積算されるが、電圧ノイズの位相はランダムであるためＳＱＵＩＤに固有の雑音ΦＳＱは√ｎ倍にしかならない。したがって、直列にすることにより磁束計の磁場分解能を向上させることができる。
【００１７】
Ｓｃｈｕｌｔｚｅらは検出コイル７が直接ＳＱＵＩＤリング４に接続された構造のダイレクトカップル型磁束計において、図１（ｂ）に示すように２つのＳＱＵＩＤ６を直列に接続することでＳＱＵＩＤが一つのもののほぼ２倍の変調電圧振幅が得られることをアイ・イー・イー・イー、トランザクションズ オン アプライド スーパーコンダクティビティ，第９巻、３２７９頁（１９９９年）（V. Schultze et al, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY VOL.9 P.3279 (1999)）で報告している。
【００１８】
ダイレクトカップル型磁束計では３つ以上のＳＱＵＩＤを直列に接続することができないが、単層の高温超電導薄膜で構成できる。彼らの磁束計（２−ＳＱＵＩＤダイレクトカップル型磁束計９と呼ぶことにする）は検出コイル７を備えており磁場分解能も大きいが、ＳＱＵＩＤ６を２つしか直列にしていないため、含まれる２つのＳＱＵＩＤの臨界電流および磁場−電圧特性（図２）の周期と位相がほぼ完全に揃っていないと直列にしたことの効果、即ちΔＶの増加を得ることができない。
【００１９】
磁場−電圧特性（図２）の周期は２つのＳＱＵＩＤのインダクタンスを同じ値にすることで実現でき、これは通常の微細加工技術を用いて比較的容易に実現できる。一方、ＳＱＵＩＤの臨界電流および磁場−電圧特性（図２）の位相を一致させるためには接合特性（接合の臨界電流と接合抵抗）をそろえる必要がある。
【００２０】
しかし、現在、弱結合として最も広く高温超電導ＳＱＵＩＤの作製に用いられている粒界ジョセフソン接合ではよくても２０−３０％程度の接合特性のばらつきがあるため、接合特性が揃った磁束計の歩留まりが低いのが現状である。
【００２１】
【発明が解決しようとする課題】
上記従来技術で述べたように、高温超電導薄膜により作製されたＳＱＵＩＤおよびそれらで構成される磁束計は様々な改良がなされてきた。本発明はその延長線上にあり、ＳＱＵＩＤおよび磁束計の性能を一層向上させることを課題としている。
【００２２】
本発明では大きな変調電圧振幅ΔＶを実現することで電圧−磁束換算係数を増加させ、最終的にＳＱＵＩＤの磁場分解能を向上させることを可能とするものである。特に、ΔＶを約２倍に向上させることを狙いとした単層の高温超電導薄膜を用いて作製可能な２−ＳＱＵＩＤダイレクトカップル型磁束計において、含まれる２つのＳＱＵＩＤの臨界電流および磁場−電圧特性（図２）の周期と位相のばらつきにより変調電圧振幅ΔＶの増加が抑制されるという問題を解決するものである。
【００２３】
【課題を解決するための手段】
本発明では、単層の高温超電導薄膜で形成された２つの弱結合を含む超電導リング４と電流、電圧端子５で構成される２つのＳＱＵＩＤ６を直列に接続するとともに、それぞれのＳＱＵＩＤ６が共通の検出コイル７と直接結合させられた２−ＳＱＵＩＤダイレクトカップル型磁束計９において、あらかじめその検出コイルの異なる位置に複数の電極を設ける。そして、２つのＳＱＵＩＤの臨界電流および磁場−電圧特性の位相がたとえずれていても、その特性に応じて適した電極を使用し複数の経路でバイアス電流を流すことで、変調電圧振幅ΔＶを最大で２つのＳＱＵＩＤのΔＶを足し合わせた値にまで増加できる。
【００２４】
【発明の実施の形態】
以下、実施例をあげて本発明をより具体的に説明するが、以下の開示は本発明の一実施例にすぎず、本発明の技術範囲を何ら限定するものではない。
【００２５】
（実施例１）
以下、本発明の第１の実施例を述べる。図３（ａ）に本実施例の磁束計の上から見た模式図、（ｂ）にその断面模式図、（ｃ）に等価回路を示す。１０は基板であり、たとえば、ＳｒＴｉＯ₃（１００）バイクリスタル基板である。上記基板１０上には、超電導リング４、切り欠き２、２つの弱結合３および電流・電圧端子１４，１５よりなるＳＱＵＩＤ１２，１３が超電導薄膜１により２つ形成され、その２つのＳＱＵＩＤ１２，１３が同じ超電導薄膜１により形成された検出コイル７と直接結合している。
【００２６】
すなわち、検出コイル７の一部である入力コイル１１と超電導リング４とが兼用されている。２つのＳＱＵＩＤ１２，１３から外側に形成された電流・電圧端子１４，１５には外部接続線を接続するための貴金属薄膜よりなる端子１６，１７が形成される。また、検出コイル７に上にも外部接続線を接続するための貴金属薄膜よりなる端子１８〜２３が複数個形成される。ここで、便宜上、図の上側のＳＱＵＩＤ１２をＱ１、下側のＳＱＵＩＤ１３をＱ２と呼び区別することにする。
【００２７】
本発明の第１の実施例では２つのＳＱＵＩＤＱ１，Ｑ２を、検出コイル７を２分割する位置に配置し、２つのＳＱＵＩＤを結ぶ線を中心として左右それぞれに３個、合計６個の電極１８〜２３を検出コイル上に形成した。等価回路には検出コイルのインダクタンス２４よりはるかに小さいＳＱＵＩＤのインダクタンスは省略している。ちなみに、検出コイルのインダクタンスは数ｎＨ、ＳＱＵＩＤのインダクタンスは５０−１００ｐＨ程度である。
【００２８】
図４は、図３に示した実施例の作製プロセスを説明する図である。図３（ａ）に示すＡ−Ａ’の位置で矢印方向に見た作製プロセスの各過程での断面図の概要である。
【００２９】
まず、基板４１として１０mm角のＳｒＴｉＯ₃（１００）バイクリスタル基板を用いた。接合角としては２４゜、３０゜、３６゜および４５゜などが使用できるが、本実施例では３０゜を使用した。上記以外の接合角も作製可能であるが、特注品となるため価格が高い。
【００３０】
基板上に膜厚１５０nmのＹＢａ₂Ｃｕ₃Ｏ_y（以下ＹＢＣＯと略す）超電導膜４２をＫｒＦエキシマレーザを用いたレーザ蒸着法で作製した。ターゲットには直径２０mmの円盤状のＹＢＣＯ焼結体を用いた。基板温度は８００℃、成膜時の雰囲気は３００ｍTorrの純酸素とし、レーザエネルギ７０ｍＪ、レーザエネルギ密度１〜２Ｊ／ｃｍ²、レーザ発信周波数２Ｈｚ、基板とターゲットの間隔を約４ｃｍとして成膜を行った。この際、ターゲットがなるべく均一に削れるように、ターゲットを自転させるとともに、膜厚の分布を抑えるためレーザ光線をターゲット上でラスタースキャンさせ、また基板も回転させた。
【００３１】
成膜後、酸素分圧を５Torrまで増加させ、７６０℃から１０℃／分の速度で２００℃以下まで薄膜試料を冷却し、成膜装置から取り出した。ここで、４３は基板の結晶粒界、４４は超電導膜４２の結晶粒界である（図４（ａ））。
【００３２】
つぎに、抵抗加熱蒸着法で電極パッド用の貴金属電極層４５としてＡｕ薄膜を、メタルマスク４６を使用して必要な部分にのみ蒸着した。その後、電気炉で純酸素フロー中、５００℃の熱処理を行い、ＹＢＣＯ薄膜を十分に酸化するとともにＡｕとＹＢＣＯの界面抵抗を低減した（図４（ｂ））。
【００３３】
つぎに、作製した薄膜に膜厚６００nmの電子線描画用のレジスト（製品名ＺＥＰ５２０、日本ゼオン製）をスピンコートした。１６０℃、１０分のプリベークを行った後、電子線描画法で下部ＹＢＣＯ超電導膜用のパターンを露光し、キシレンで現像することでレジストパターン４７を形成した。このパターンは、図３（ａ）に示す磁束計のパターンである（図４（ｃ））。
【００３４】
つぎに、ＡｒイオンビームエッチングでレジストパターンをＹＢＣＯ超電導薄膜に転写した。Ａｒのガス圧は１．１×１０^-4Torr、加速電圧は５００Ｖとした。また、イオン衝撃による試料温度の上昇を緩和するため、３分間のエッチングと３分間の休憩を交互に繰り返した。この条件でのＹＢＣＯ薄膜とレジストのエッチング速度はそれぞれ１２nm／分および１６nm／分程度であった。下部ＹＢＣＯ薄膜の膜厚１５０nmに対してエッチング深さは２００nmとし、約５０nmのオーバーエッチングを行った。残ったレジストは２−ブタノン中で超音波洗浄することで除去した（図４（ｄ））。
【００３５】
実施例１で作製した磁束計に含まれる２つのＳＱＵＩＤの特性を表１に示す。どちらのＳＱＵＩＤも同じ膜厚、同じ接合幅の粒界ジョセフソン接合からなっているが、接合特性のばらつきのため臨界電流２Ｉ_cが異なっている。この表１で、Ｉ_c、Ｒ_nは接合１つ当たりの臨界電流と接合抵抗を表している。ＳＱＵＩＤは２つの接合からなるので、ＳＱＵＩＤの臨界電流は２Ｉ_cである。
【００３６】
ΔＶ_obsとＩ_b ^optはそれぞれのＳＱＵＩＤの最大の変調電圧振幅とそのときのバイアス電流値（最適バイアス電流値）である。この最適バイアス電流値はほぼ臨界電流値に等しい。また、Ａ_eff、Ｌ_s、Ｌ_pは有効面積、ＳＱＵＩＤのインダクタンス、検出コイルのインダクタンスである。
【００３７】
【表１】

【００３８】
図５（ａ）に従来方法での２−ＳＱＵＩＤ磁束計の測定回路を示す。測定回路には磁場印加用のコイルとその電源は省略した。図５（ｂ）に測定した電圧−磁束特性（Ｖ−Φ特性）を示す。Ｑ１（１２）からでている電極１６に電源５１の正極を可変抵抗５０を介して接続し、Ｑ２（１３）からでている電極１７に電源５１の負極を接続した。これは検出コイルの電極を使用しない従来の方法である。また、Ｑ１からでている電極１６とオシロスコープの正の入力を、Ｑ２からでている電極１７とオシロスコープの負の入力を接続した。
【００３９】
図５（ｂ）はバイアス電流値を変えて測定した電圧−磁束特性を多重露光で一枚の写真に撮影したものであり、バイアス電流が大きいほど電圧が大きくなっている。
【００４０】
バイアス電流が小さい場合（下側）では、臨界電流が小さい方のＳＱＵＩＤの特性、つまりＱ１のＶ−Φ特性が現れる。バイアス電流を増加させると出力電圧が増加するとともに臨界電流が大きいＱ２の特性が現れ始める。しかし、Ｑ２の変調電圧振幅ΔＶが最大になるバイアス電流では、Ｑ１に対してはバイアス電流が過剰となり、そのΔＶは小さくなっている。また、ＳＱＵＩＤの出力電圧ピークの磁場に対する位置（位相）がＱ１とＱ２で異なっている。
【００４１】
図５（ｃ）にΔＶのバイアス電流依存性（実線）を示すが、グラフからわかるようにＱ１、Ｑ２それぞれの最適バイアス電流値でΔＶが２つの極大値を持っており、その値は表１に示したそれぞれのＳＱＵＩＤ単体のΔＶ程度であり、ΔＶは増加していない。
【００４２】
つぎに、検出コイルの電極を使用した本発明の方法で特性を評価した。まず、図５（ａ）と同じ電極配置で、電流を臨界電流が少ないＱ１の最適バイアス電流まで流した。バイアス電流の値は４７μＡであった。図５（ｃ）からわかるように、このときのΔＶは３５μＶである。
【００４３】
つづいて、図６（ａ）に示すように第２のバイアス電源５２として別のバイアス電源を用いてＱ２から直接でている電極１７と検出コイル上の電極のいずれかを接続してＱ２のジョセフソン接合に電流を流した。検出コイル上の電極には正極を接続した。このように電流を流すことで、Ｑ１のジョセフソン接合部分には電流を流すことなく、Ｑ２のジョセフソン接合部分に電流を流すことができ、Ｑ１、Ｑ２の両方に最適なバイアス電流を流すことができる。
【００４４】
検出コイル上の電極を順番に第２のバイアス電源５２の正極に接続して６通りの電流経路で調べた。図６（ａ）に示すように左上の電極２１とＱ２から直接でている電極１７を接続した場合に最も大きなΔＶが得られた。
【００４５】
このときのΔＶと第２のバイアスの関係を図６（ｂ）に示す。第２のバイアス電流が２つのＳＱＵＩＤの臨界電流の差にほぼ等しい２６μＡのときにΔＶが最大値の６５μＶになった。これは図６（ａ）に矢印で示したように第２のバイアス電流の一部が、臨界電流が小さいＱ１のＳＱＵＩＤリングを流れ、その電流が生成する磁束によりＱ１のＶ−Φ特性の位相がＱ２のＶ−Φ特性の位相と一致するように変化したためである。
【００４６】
この場合、検出コイルのインダクタンスがＳＱＵＩＤ自体のインダクタンスよりもはるかに大きなことを考えるとＱ１を経由する経路とＱ１を経由しない経路のインダクタンスがほぼ等しいので、流した第２のバイアス電流の約半分がＱ１を経由して流れていると考えられる。
【００４７】
本発明の磁束計では検出コイルに複数の電極を形成しており、第２のバイアス電源５２の接続位置を変えることで臨界電流が小さい方のＳＱＵＩＤリングに流れる電流の量と方向を変えることができる。
【００４８】
例えば、第２のバイアス電源５２の正極と検出コイルの右中の電極１９を接続した場合には臨界電流が小さいＱ１のＳＱＵＩＤリングを流れる電流は図６（ａ）とは逆向きになる。また、検出コイルの左中の電極２２から直接Ｑ２に流れる経路の方のインダクタンスがＱ１を経由する経路よりインダクタンスが小さいので、Ｑ１に流れる電流の絶対値も図６（ａ）より小さくなっている。
【００４９】
このように、本発明の磁束計では第２のバイアス電源の接続位置を変えＱ１あるいはＱ２に流れる電流の向きと大きさを制御することで、たとえ２つのＳＱＵＩＤの臨界電流とＶ−Φ特性の位相がずれていても、そのずれを補正して大きなΔＶを得ることができる。本実施例では検出コイルに６個の電極を作製したが、より多くの電極を作製することでより細かく２つのＳＱＵＩＤの特性のずれを補正できることは明らかである。
【００５０】
（実施例２）
以下、本発明の第２の実施例を述べる。磁束計としては実施例１と同じものを用いた。実施例１では２つのバイアス電源を用いたが本実施例では図７に示すように１つのバイアス電源５１を用いて、可変抵抗５０で分割することで２つの経路で電流を流した。流した電流値は実施例１と同じになるように可変抵抗５０の値を調整した。その結果、実施例１と同じように６５μＶのΔＶを得ることができた。
【００５１】
（実施例３）
以下、本発明の第３の実施例を述べる。磁束計としては実施例１と同じ構造であるが１５×１５mm²の基板を用いて一回り大きな磁束計を作製した。大きな基板を使用することで検出コイルの面積を大きくでき、より感度が高い磁束計を作製できる。
【００５２】
本実施例は、実施例１とはパターンが異なるが、作製プロセスは同じであるので説明を省略する。
【００５３】
表１に実施例３の磁束計に含まれる２つのＳＱＵＩＤの特性を示す。ＳＱＵＩＤの臨界電流は４０μＡと３８μＡとほぼ一致しており、最適バイアス電流もほぼ一致していた。しかしながら、ＳＱＵＩＤを形成する２つの接合特性が異なっていたため、２つのＳＱＵＩＤのＶ−Φ特性は位相が大きくずれており、図５（ａ）のように電源を接続して３８から４０μＡの電流を流しても最高で３８μＶのΔＶしか得られなかった。これは２つのＳＱＵＩＤのΔＶの合計値６０μＶよりも小さな値である。
【００５４】
つぎに、図６（ａ）のように２つのバイアス電源を用いて測定したが、２つのＳＱＵＩＤの臨界電流がほぼ等しいため第２のバイアス電流が数μＡと小さく、どの電極を用いてもあまり変化が見られなかった。
【００５５】
そこでつぎに第１のバイアス電源５１はＱ１の接合にだけ電流が流れるように、第２のバイアス電源５２はＱ２の接合にだけ電流が流れるように電源を接続した。図８にその一例を示す。この場合、第１のバイアス電源の負極を検出コイル上の６個の電極１８から２３のいずれかと接続し、第１のバイアス電源の正極は可変抵抗５０を経由してＱ１から直接でている電極１６と接続する。
【００５６】
したがって、第１のバイアス電源５１について６通りの接続の仕方がある。また、第２のバイアス電源５２の正極を可変抵抗５０を経由して検出コイル上の６個の電極１８〜２３のいずれかと接続し、第２のバイアス電源の負極はＱ２から直接でている電極１７と接続する。第２のバイアス電源５２についても６通りの接続の仕方がある。したがって、第１のバイアス電源５１はＱ１の接合にだけ電流が流れるように、第２のバイアス電源５２はＱ２の接合にだけ電流が流れるように電源を接続する方法として６通りかける６通りで３６通りの異なる接続が可能である。
【００５７】
異なる接続では電流の分布が異なるため、２つのＳＱＵＩＤのＶ−Φ特性は３６通りの異なる位相差を持つ。その中から最も位相差が少ないもの、すなわち最もΔＶが大きくなる組み合わせを選択した。実施例のＳＱＵＩＤでは図８のように電源を接続することで２つのＳＱＵＩＤのΔＶの合計値６０μＶにほぼ一致する５９μＶのΔＶを得た。
【００５８】
このように、本発明によると２つのＳＱＵＩＤの臨界電流が異なっていても、一致していても大きなΔＶを得ることができる。本実施例では２つのバイアス電源を用いているが、実施例２のように周辺回路を工夫して電流の分布をうまく制御すれば、１つのバイアス電源でも本実施例と同じように電流を流すことができる。
【００５９】
（実施例４）
以下、本発明の第４の実施例を述べる。図９に本実施例の磁束計の模式図を示す。１０は基板であり、実施例１と同様、ＳｒＴｉＯ₃（１００）バイクリスタル基板である。上記基板１０上には、超電導リング４、切り欠き２、弱結合３および電流・電圧端子１４，１５よりなるＳＱＵＩＤ１２，１３が超電導薄膜により形成される。
【００６０】
第１の実施例と同様に、その２つのＳＱＵＩＤ１２，１３が同じ超電導薄膜１により形成された検出コイルと直接結合している。すなわち、検出コイル７の一部である入力コイル１１と超電導リング４とが兼用となっている。２つのＳＱＵＩＤ１２，１３から外側に形成された電流・電圧端子１４，１５には外部接続線を接続するための貴金属薄膜よりなる端子１６，１７が形成される。また、検出コイルの上にも外部接続線を接続するための貴金属薄膜よりなる端子２５〜３３を複数個形成している。
【００６１】
本実施例では２つのＳＱＵＩＤ１２，１３を隣接して配置している。図１（ｂ）に示した従来から知られている磁束計と似ているが、本発明の磁束計では検出コイルに上に多数の電極が形成される。実施例では９個の電極２５〜３３を作製した。本実施例は、実施例１とはパターンが異なるが、作製プロセスは同じであるので説明を省略する。
【００６２】
表１に実施例４の磁束計に含まれる２つのＳＱＵＩＤの特性を示す。
【００６３】
図１０は本実施例の磁束計の等価回路と測定回路である。等価回路には検出コイルのインダクタンスよりはるかに小さいＳＱＵＩＤのインダクタンスは省略している。実施例２と同様に１つのバイアス電源５１でふたつの経路にバイアス電流を流した。
【００６４】
１つの経路ではＱ１側の電極１６からＱ２側の電極１７へと、バイアス電流が２つのＳＱＵＩＤの両方に流れている。この経路で流した電流は臨界電流が少ないＱ１のＳＱＵＩＤの最適バイアス電流である４０μＡとした。
【００６５】
ふたつめの経路では検出コイル上の電極２５〜３３のいずれかから、Ｑ２側の電極１７へと、バイアス電流をＱ２の接合にのみ流した。流すべき電流量はＱ１とＱ２の最適バイアス電流値の差に相当する２５μＡである。
【００６６】
本実施例の構造では、検出コイル上の電極から流した電流はＱ１のＳＱＵＩＤリングを必ず反時計回りに流れる。しかしながら、検出コイル上には９個の電極２５〜３３があるので、Ｑ１のＳＱＵＩＤリングを流れる電流値を９段階で変化させることが可能である。Ｖ−Φ特性は磁束量子Φ₀を周期とした周期関数なので、２つのＶ−Φ特性の位相差を０から２πまで調節できれば必ず２つのＳＱＵＩＤのＶ−Φ特性の位相を合わせることができる。
【００６７】
位相差を０から２πまで調節するのに必要な電流量はＳＱＵＩＤのインダクタンスのうち、実際に位相調整のための電流が流れる部分のインダクタンス、すなわち検出コイルと重なっている部分のインダクタンス、Ｌ_sq.cで決まる。
【００６８】
本実施例ではＬ_sq.cは約５０ｐＨである。この場合、位相差を２π変化させるのに必要な電流はΦ₀／Ｌ_sq.c＝２×１０^-15／５０×１０^-12＝４０μＡである。本実施例の磁束計ではＱ１とＱ２の最適バイアス電流値の差が２５μＡであったので、図１０の測定系では位相差を０から２πまで調節することはできなかったが、図１０の電極配置でＱ１とＱ２のΔＶの和に相当する約６５μＶのΔＶを得た。図１０の電極配置で位相を０からΦ₀まで調節するためには、２つのＳＱＵＩＤの臨界電流の差がΦ₀／Ｌ_sq.cよりも大きい必要がある。また、Ｌ_sq.cを大きくすることで少ない電流で大きく位相を変化できることは明らかである。
【００６９】
（実施例５）
以下、本発明の第５の実施例を述べる。作製した磁束計は図９に示した実施例４と基本的に同じ構造である。しかし、ＳＱＵＩＤインダクタンスを１００ｐＨと大きくし、またＱ１の接合幅を１．５μm、Ｑ２の接合幅を３μmとして、２つのＳＱＵＩＤの臨界電流を意図的に変えた。その結果、表１に示すようにＱ１、Ｑ２の臨界電流はそれぞれ３２μＡ、６０μＡとなり、その差は２８μＡであった。
【００７０】
また、Ｌ_sq.cは約９０ｐＨ、位相を２π変化させるのに必要な電流は２２μＡとなり、１つのバイアス電源を用いた図１０の測定系でも、位相差を０から２πまで調節することが可能となった。実際に最適な電極配置を調べることで、２つのＳＱＵＩＤのΔＶの和に相当する４７μＶの変調電圧振幅を得た。
【００７１】
以上の５つの実施例からわかるように、本発明によれば、いずれの実施例でも２つのＳＱＵＩＤの直列接続が効果的に作用していることが分かる。また、ＳＱＵＩＤの重要な構成要素である弱結合は、実施例では、バイクリスタル基板を用いた粒界接合としたが、段差基板でも、ランプエッジ型や積層型のＳＮＳ接合でも、またこれら以外の構造の弱結合でもよいのは言うまでもない。
【００７２】
【発明の効果】
本発明によれば、接合特性にばらつきがある２−ＳＱＵＩＤ磁束計でも、高い磁場分解能の磁束計を提供できる。
【図面の簡単な説明】
【図１】従来例のＳＱＵＩＤと検出コイルを含む磁束計の構成を示す平面図。
【図２】磁束計の典型的な電圧−磁場特性を示す波形図。
【図３】本発明の第１の実施例の磁束計の平面図（ａ）、断面模式図（ｂ）および等価回路図（ｃ）。
【図４】本発明の第１の実施例の磁束計の作製プロセスを示す断面図。
【図５】図３の磁束計と従来例の測定回路の回路図（ａ）、種々のバイアス電流値で測定した上記磁束計の電圧−磁束特性図（ｂ）および変調電圧振幅ΔＶのバイアス電流依存性の測定図（ｃ）。
【図６】本発明の第１の実施例の磁束計にと本発明の実施例における測定回路の回路図（ａ）および変調電圧振幅ΔＶの第２のバイアス電流依存性の測定図（ｂ）。
【図７】本発明の第２の実施例の測定回路図。
【図８】本発明の第３の実施例の測定回路図。
【図９】本発明の第４の実施例の磁束計の構造を示す平面図。
【図１０】本発明の第４の実施例の測定回路図。
【符号の説明】
１…超電導薄膜、２…切り欠き、３…弱結合、４…超電導リング、５…電流・電圧端子、６…ＳＱＵＩＤ、７…検出コイル、８…磁束計、９…２−ＳＱＵＩＤダイレクトカップル磁束計、１０…基板、１１…入力コイル、１２…Ｑ１、１３…Ｑ２、１４…Ｑ１のＳＱＵＩＤから外側に形成された電流・電圧端子、１５…Ｑ２のＳＱＵＩＤから外側に形成された電流・電圧端子、１６…Ｑ１のＳＱＵＩＤから外側に形成された電流・電圧端子の電極、１７…Ｑ２のＳＱＵＩＤから外側に形成された電流・電圧端子の電極、１８−２３…検出コイル上に形成した電極、２４…検出コイルのインダクタンス、２５−３３…検出コイル上に形成した電極、４１…バイクリスタル基板、４２…超電導薄膜、４３…基板の結晶粒界、４４…超電導薄膜の結晶粒界、４５…貴金属薄膜、４６…貴金属薄膜用のメタルマスク、４７…レジスト、５０…可変抵抗、５１…第１のバイアス電源、５２…第２のバイアス電源。[0001]
BACKGROUND OF THE INVENTION
The present invention is a high-sensitivity magnetic field measurement system using a supersensitive magnetometer using a SQUID (Superconducting Quantum Interference Device) among application fields of high-temperature superconducting materials, such as a magnetocardiogram and a magnetoencephalogram. It is related with the element structure of SQUID and a magnetometer which are useful when applied to a biomagnetic measurement system and a nondestructive inspection.
[0002]
[Prior art]
Since the discovery of high-temperature superconducting materials having a critical temperature higher than the boiling point of liquid nitrogen, the application of superconducting elements using liquid nitrogen or refrigerators for cooling has been studied. SQUID is an ultra-sensitive magnetic sensor that cannot be obtained by other elements such as semiconductors. It can perform non-invasive measurement of biomagnetism generated from the brain, heart, etc., and non-destructive inspection of aircraft aircraft defects. it can.
[0003]
Currently, high-temperature superconducting SQUIDs can be obtained from YBa because high-quality thin film samples can be obtained.₂Cu_ThreeO_ySo-called 123 series superconductors are mainly used. As for the structure of the magnetometer, an integrated SQUID magnetometer manufactured by using a multilayer process is the mainstream in a magnetometer of a low-temperature superconductor such as Nb, but a high-temperature superconductor can be formed of a single-layer superconducting thin film. Direct couple type magnetometers are the mainstream. This is because a sufficiently reliable multilayer structure forming process has not yet been established for high-temperature superconductors.
[0004]
As shown in FIG. 1A, the SQUID includes a superconducting ring 4 including two notches 2 and two weak couplings 3 of the superconducting thin film 1 and a current / voltage terminal 5. The element area of the SQUID 6 is about several tens μm × several tens μm to several hundreds μm × several hundreds μm. Since the element area is small, the magnetic flux capture area of the SQUID unit 6 is small. Usually, the detection coil 7 made of a superconducting thin film with a size of several tens of mm × several tens of mm is coupled to the SQUID 6 to increase the magnetic flux trapping area and improve sensitivity. Although the SQUID including the single SQUID and the detection coil may be referred to as SQUID, in the present invention, the SQUID single 6 is simply referred to as SQUID, and when the detection coil is included, it is referred to as the magnetometer 8.
[0005]
Magnetic field resolution S of the magnetometer_B ^1/2Is given by equation (1).
[0006]
[Expression 1]

[0007]
Where A_effIs the effective area, S_Φ ^1/2Is magnetic flux equivalent noise including circuit noise, Φ_SQIs the noise inherent to SQUID, V_nIs a noise voltage generated from the circuit.
[0008]
DV / dΦ is a magnetic flux-voltage conversion coefficient, which is given by the maximum slope of the V-Φ characteristic, and can be approximated by πΔV using the modulation voltage amplitude ΔV. Magnetic field resolution S_B ^1/2To reduce the magnetic flux conversion noise S including the circuit noise_Φ ^1/2Effective area A_effNeed to be increased. S_Φ ^1/2Is composed of noise inherent to the SQUID and noise generated from the circuit. As can be seen from Equation 1, the noise is added in the form of the square root of the sum of squares. It leads to improvement. For best sensitivity,_SQAnd V_nIt is designed so that / (π · ΔV) is approximately the same.
[0009]
The modulation voltage amplitude ΔV can be estimated by Equations 2 to 4 derived by numerical calculation considering thermal noise (for example, Enfuku et al., Japanese Journal of Applied Physics, Vol. 32 (1993)). ) L1407 (K. Enpuku, Jpn. J. Appl. Phys. 32 (1993) L1407)).
[0010]
[Expression 2]

[0011]
However,
[0012]
[Equation 3]

[0013]
[Expression 4]

[0014]
Where L is the inductance of the SQUID ring, I_cAnd R_nAre the critical current and junction resistance of the weak coupling, T is the operating temperature, K_BIs the Boltzmann constant, δΦ_nIs called noise magnetic flux and indicates noise generated in the inductance due to thermal noise. In other words, the second term of Equation 2 indicates the attenuation of ΔV due to thermal noise. δΦn is a function of the temperature T and the inductance L, but at 77K, it is determined only by the inductance. The smaller the inductance, the smaller the thermal noise term. Β is called an inductance parameter, and is usually designed to be about 1 to 5. From Equations 2 to 4, it can be seen that ΔV increases when the inductance is reduced. However, when the inductance is reduced, the coupling with the detection coil is weakened and the effective area is reduced.
[0015]
As another approach, it is possible to realize a large ΔV while maintaining the same effective area by connecting SQUIDs having the same characteristics in series. In an ideal case, the output when n SQUIDs are connected in series is n times the ΔV of a single SQUID. However, the period and phase of the critical current and magnetic field-voltage characteristics (FIG. 2) of each SQUID must be aligned.
[0016]
On the other hand, voltage noises of individual SQUIDs are integrated by connecting them in series. However, since the phase of the voltage noise is random, the noise ΦSQ inherent to the SQUID is only √n times. Therefore, the magnetic field resolution of the magnetometer can be improved by making it in series.
[0017]
Schultze et al. In a direct-coupled magnetometer having a structure in which the detection coil 7 is directly connected to the SQUID ring 4, two SQUIDs 6 are connected in series as shown in FIG. Double modulation voltage amplitude can be obtained. IEE, Transactions on Applied Superconductivity, Vol. 9, p. 3279 (1999) (V. Schultze et al, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY VOL. 9 P.3279 (1999)).
[0018]
A direct-coupled magnetometer cannot connect three or more SQUIDs in series, but can be composed of a single-layer high-temperature superconducting thin film. Their magnetometer (referred to as 2-SQUID direct-coupled magnetometer 9) has a detection coil 7 and a large magnetic field resolution, but only two SQUIDs 6 are connected in series, so the two SQUIDs included. If the period and phase of the critical current and magnetic field-voltage characteristics (FIG. 2) are not almost perfectly aligned, the effect of serial connection, that is, an increase in ΔV cannot be obtained.
[0019]
The period of the magnetic field-voltage characteristic (FIG. 2) can be realized by setting the inductances of the two SQUIDs to the same value, which can be realized relatively easily by using a normal microfabrication technique. On the other hand, in order to match the phases of the SQUID critical current and the magnetic field-voltage characteristics (FIG. 2), it is necessary to align the junction characteristics (the junction critical current and the junction resistance).
[0020]
However, at present, the grain boundary Josephson junction, which is the most widely used for the production of high-temperature superconducting SQUID as a weak coupling, has a variation in junction characteristics of about 20-30%. Currently, the yield is low.
[0021]
[Problems to be solved by the invention]
As described in the above prior art, various improvements have been made to SQUIDs made of high-temperature superconducting thin films and magnetometers composed of them. This invention exists on the extension line, and makes it the subject to improve the performance of SQUID and a magnetometer further.
[0022]
In the present invention, by realizing a large modulation voltage amplitude ΔV, it is possible to increase the voltage-magnetic flux conversion coefficient and finally improve the magnetic field resolution of the SQUID. In particular, the critical current and magnetic field-voltage characteristics of two included SQUIDs in a 2-SQUID direct-coupled magnetometer that can be fabricated using a single-layer high-temperature superconducting thin film aimed at improving ΔV by about twice. This solves the problem that the increase in the modulation voltage amplitude ΔV is suppressed by the variation in the period and phase in FIG.
[0023]
[Means for Solving the Problems]
In the present invention, a superconducting ring 4 including two weak couplings formed of a single-layer high-temperature superconducting thin film and two SQUIDs 6 composed of current and voltage terminals 5 are connected in series, and each SQUID 6 is commonly detected. In the 2-SQUID direct couple type magnetometer 9 directly coupled to the coil 7, a plurality of electrodes are provided in advance at different positions of the detection coil. Even if the critical current and magnetic field-voltage characteristics of the two SQUIDs are out of phase, the modulation voltage amplitude ΔV can be maximized by flowing a bias current through a plurality of paths using electrodes suitable for the characteristics. Thus, ΔV of two SQUIDs can be increased to a value obtained by adding them.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described more specifically with reference to examples. However, the following disclosure is only an example of the present invention and does not limit the technical scope of the present invention.
[0025]
Example 1
The first embodiment of the present invention will be described below. FIG. 3A is a schematic diagram viewed from above the magnetometer of the present embodiment, FIG. 3B is a schematic sectional view thereof, and FIG. 3C is an equivalent circuit. 10 is a substrate, for example, SrTiO_ThreeA (100) bicrystal substrate. Two SQUIDs 12 and 13 comprising a superconducting ring 4, a notch 2, two weak couplings 3, and current / voltage terminals 14 and 15 are formed on the substrate 10 by the superconducting thin film 1, and the two SQUIDs 12 and 13 are formed. It is directly coupled to a detection coil 7 formed by the same superconducting thin film 1.
[0026]
That is, the input coil 11 that is a part of the detection coil 7 and the superconducting ring 4 are also used. Current / voltage terminals 14 and 15 formed outside the two SQUIDs 12 and 13 are formed with terminals 16 and 17 made of a noble metal thin film for connecting an external connection line. A plurality of terminals 18 to 23 made of a noble metal thin film for connecting an external connection line are also formed on the detection coil 7. Here, for the sake of convenience, the upper SQUID 12 in the figure is called Q1, and the lower SQUID 13 is called Q2.
[0027]
In the first embodiment of the present invention, two SQUIDs Q1 and Q2 are arranged at a position where the detection coil 7 is divided into two, and three on each of the left and right around the line connecting the two SQUIDs, a total of six electrodes 18 to 23 was formed on the detection coil. In the equivalent circuit, the inductance of the SQUID that is much smaller than the inductance 24 of the detection coil is omitted. Incidentally, the inductance of the detection coil is several nH, and the inductance of the SQUID is about 50-100 pH.
[0028]
FIG. 4 is a diagram for explaining a manufacturing process of the embodiment shown in FIG. It is the outline | summary of sectional drawing in each process of the preparation process seen in the arrow direction in the position of A-A 'shown to Fig.3 (a).
[0029]
First, 10 mm square SrTiO as the substrate 41_ThreeA (100) bicrystal substrate was used. As the joining angle, 24 °, 30 °, 36 °, 45 °, and the like can be used, but in this embodiment, 30 ° was used. Join angles other than the above can be produced, but the price is high because it is a custom-made product.
[0030]
YBa with a film thickness of 150 nm on the substrate₂Cu_ThreeO_yA superconducting film 42 (hereinafter abbreviated as YBCO) was produced by a laser vapor deposition method using a KrF excimer laser. A disk-shaped YBCO sintered body having a diameter of 20 mm was used as the target. The substrate temperature is 800 ° C., the atmosphere during film formation is 300 mTorr pure oxygen, the laser energy is 70 mJ, and the laser energy density is 1 to 2 J / cm.²The film was formed with a laser transmission frequency of 2 Hz and a distance between the substrate and the target of about 4 cm. At this time, the target was rotated so that the target was shaved as uniformly as possible, and the laser beam was raster scanned on the target to suppress the film thickness distribution, and the substrate was also rotated.
[0031]
After film formation, the oxygen partial pressure was increased to 5 Torr, the thin film sample was cooled from 760 ° C. to 200 ° C. or less at a rate of 10 ° C./min, and taken out from the film forming apparatus. Here, 43 is a crystal grain boundary of the substrate, and 44 is a crystal grain boundary of the superconducting film 42 (FIG. 4A).
[0032]
Next, an Au thin film was deposited as a noble metal electrode layer 45 for an electrode pad by a resistance heating vapor deposition method only on a necessary portion using a metal mask 46. Thereafter, heat treatment was performed at 500 ° C. in a pure oxygen flow in an electric furnace to sufficiently oxidize the YBCO thin film and reduce the interface resistance between Au and YBCO (FIG. 4B).
[0033]
Next, an electron beam drawing resist (product name: ZEP520, manufactured by Nippon Zeon Co., Ltd.) having a film thickness of 600 nm was spin-coated on the prepared thin film. After prebaking at 160 ° C. for 10 minutes, a pattern for the lower YBCO superconducting film was exposed by an electron beam drawing method, and developed with xylene to form a resist pattern 47. This pattern is the pattern of the magnetometer shown in FIG. 3A (FIG. 4C).
[0034]
Next, the resist pattern was transferred to the YBCO superconducting thin film by Ar ion beam etching. The gas pressure of Ar is 1.1 × 10^-FourTorr and acceleration voltage were 500V. Further, in order to mitigate the rise in the sample temperature due to ion bombardment, etching for 3 minutes and break for 3 minutes were repeated alternately. The etching rates of the YBCO thin film and the resist under these conditions were about 12 nm / min and 16 nm / min, respectively. The etching depth was 200 nm with respect to the film thickness of 150 nm of the lower YBCO thin film, and over-etching of about 50 nm was performed. The remaining resist was removed by ultrasonic cleaning in 2-butanone (FIG. 4D).
[0035]
Table 1 shows the characteristics of the two SQUIDs included in the magnetometer manufactured in Example 1. Both SQUIDs are composed of grain boundary Josephson junctions with the same film thickness and the same junction width._cIs different. In this Table 1, I_c, R_nRepresents the critical current and junction resistance per junction. Since the SQUID consists of two junctions, the critical current of the SQUID is 2I_cIt is.
[0036]
ΔV_obsAnd I_b ^optIs the maximum modulation voltage amplitude of each SQUID and the bias current value (optimum bias current value) at that time. This optimum bias current value is approximately equal to the critical current value. A_eff, L_s, L_pIs the effective area, the inductance of the SQUID, and the inductance of the detection coil.
[0037]
[Table 1]

[0038]
FIG. 5A shows a measurement circuit of a 2-SQUID magnetometer in the conventional method. In the measurement circuit, the coil for applying the magnetic field and its power source are omitted. FIG. 5B shows the measured voltage-magnetic flux characteristics (V-Φ characteristics). The positive electrode of the power source 51 was connected to the electrode 16 extending from Q1 (12) via the variable resistor 50, and the negative electrode of the power source 51 was connected to the electrode 17 protruding from Q2 (13). This is a conventional method that does not use the electrodes of the detection coil. Further, the electrode 16 coming from Q1 and the positive input of the oscilloscope were connected, and the electrode 17 coming from Q2 and the negative input of the oscilloscope were connected.
[0039]
FIG. 5B shows the voltage-magnetic flux characteristics measured by changing the bias current value in a single photograph by multiple exposure. The voltage increases as the bias current increases.
[0040]
When the bias current is small (lower side), the SQUID characteristic with the smaller critical current, that is, the V-Φ characteristic of Q1 appears. When the bias current is increased, the output voltage increases and the characteristic of Q2 having a large critical current starts to appear. However, in the bias current at which the modulation voltage amplitude ΔV of Q2 is maximized, the bias current is excessive with respect to Q1, and the ΔV is small. Further, the position (phase) of the output voltage peak of the SQUID with respect to the magnetic field is different between Q1 and Q2.
[0041]
FIG. 5 (c) shows the bias current dependency (solid line) of ΔV. As can be seen from the graph, ΔV has two maximum values at the optimum bias current values of Q1 and Q2, and the values are shown in Table 1. It is about (DELTA) V of each SQUID shown in (5), and (DELTA) V has not increased.
[0042]
Next, the characteristics were evaluated by the method of the present invention using the electrodes of the detection coil. First, with the same electrode arrangement as in FIG. 5A, the current was passed to the optimum bias current of Q1 with a small critical current. The value of the bias current was 47 μA. As can be seen from FIG. 5C, ΔV at this time is 35 μV.
[0043]
Subsequently, as shown in FIG. 6A, another bias power source is used as the second bias power source 52 to connect either the electrode 17 directly coming from Q2 and the electrode on the detection coil to connect the Josephson of Q2. A current was passed through the Son junction. A positive electrode was connected to the electrode on the detection coil. By flowing current in this way, current can be passed through the Josephson junction portion of Q2 without passing current through the Josephson junction portion of Q1, and an optimum bias current is passed through both Q1 and Q2. Can do.
[0044]
The electrodes on the detection coil were sequentially connected to the positive electrode of the second bias power source 52, and the six current paths were examined. As shown in FIG. 6A, the largest ΔV was obtained when the upper left electrode 21 and the electrode 17 directly from Q2 were connected.
[0045]
FIG. 6B shows the relationship between ΔV and the second bias at this time. ΔV reached a maximum value of 65 μV when the second bias current was 26 μA, which was almost equal to the difference between the critical currents of the two SQUIDs. As shown by the arrows in FIG. 6A, a part of the second bias current flows through the SQUID ring of Q1 having a small critical current, and the phase of the V-Φ characteristic of Q1 is generated by the magnetic flux generated by the current. Is changed so as to coincide with the phase of the V-Φ characteristic of Q2.
[0046]
In this case, considering that the inductance of the detection coil is much larger than the inductance of the SQUID itself, the inductance of the path that passes through Q1 and the path that does not pass through Q1 are almost equal. It is thought that it flows via Q1.
[0047]
In the magnetometer of the present invention, a plurality of electrodes are formed in the detection coil, and the amount and direction of the current flowing through the SQUID ring having the smaller critical current can be changed by changing the connection position of the second bias power source 52. it can.
[0048]
For example, when the positive electrode of the second bias power supply 52 and the right middle electrode 19 of the detection coil are connected, the current flowing through the SQUID ring of Q1 having a small critical current is opposite to that shown in FIG. Further, since the inductance of the path flowing directly from the left middle electrode 22 of the detection coil to Q2 is smaller than that of the path passing through Q1, the absolute value of the current flowing through Q1 is also smaller than that in FIG. .
[0049]
Thus, in the magnetometer of the present invention, by changing the connection position of the second bias power supply and controlling the direction and magnitude of the current flowing through Q1 or Q2, the critical current and the V-Φ characteristics of the two SQUIDs are controlled. Even if the phase is shifted, a large ΔV can be obtained by correcting the shift. In this embodiment, six electrodes are produced in the detection coil, but it is clear that the deviation in the characteristics of the two SQUIDs can be corrected more finely by producing more electrodes.
[0050]
(Example 2)
The second embodiment of the present invention will be described below. The same magnetometer as in Example 1 was used. In the first embodiment, two bias power sources are used, but in this embodiment, as shown in FIG. 7, a single bias power source 51 is used, and current is passed through two paths by dividing the variable resistor 50. The value of the variable resistor 50 was adjusted so that the value of the current passed was the same as in Example 1. As a result, a ΔV of 65 μV could be obtained in the same manner as in Example 1.
[0051]
(Example 3)
The third embodiment of the present invention will be described below. The magnetometer has the same structure as that of Example 1, but 15 × 15 mm.²A larger magnetometer was made using the substrate. By using a large substrate, the area of the detection coil can be increased, and a magnetometer with higher sensitivity can be manufactured.
[0052]
Although the pattern of this example is different from that of Example 1, the manufacturing process is the same, and the description thereof is omitted.
[0053]
Table 1 shows the characteristics of two SQUIDs included in the magnetometer of the third embodiment. The critical current of SQUID was almost the same as 40 μA and 38 μA, and the optimum bias current was also almost the same. However, since the two junction characteristics forming the SQUID are different, the V-Φ characteristics of the two SQUIDs are greatly out of phase, and a current of 38 to 40 μA can be obtained by connecting a power source as shown in FIG. Even when flowing, only a maximum ΔV of 38 μV was obtained. This is a value smaller than the total value 60 μV of ΔV of two SQUIDs.
[0054]
Next, measurement was performed using two bias power sources as shown in FIG. 6A. However, since the critical currents of the two SQUIDs are almost equal, the second bias current is as small as several μA, and no matter which electrode is used. There was no change.
[0055]
Then, the first bias power source 51 is connected to a power source so that current flows only through the junction of Q1, and the second bias power source 52 is connected so that current flows only through the junction of Q2. An example is shown in FIG. In this case, the negative electrode of the first bias power source is connected to one of the six electrodes 18 to 23 on the detection coil, and the positive electrode of the first bias power source is an electrode directly from Q1 via the variable resistor 50. 16 is connected.
[0056]
Therefore, there are six ways of connection for the first bias power supply 51. In addition, the positive electrode of the second bias power source 52 is connected to one of the six electrodes 18 to 23 on the detection coil via the variable resistor 50, and the negative electrode of the second bias power source is an electrode directly from Q2. 17 is connected. The second bias power supply 52 can be connected in six ways. Accordingly, the first bias power source 51 has six ways of connecting the power source so that the current flows only to the junction of Q1 and the second bias power source 52 so that the current flows only to the junction of Q2. Different street connections are possible.
[0057]
Since the current distribution is different in different connections, the V-Φ characteristics of the two SQUIDs have 36 different phase differences. Among them, the combination having the smallest phase difference, that is, the combination having the largest ΔV was selected. In the SQUID of the example, by connecting a power source as shown in FIG. 8, a ΔV of 59 μV that substantially matches the total value 60 μV of the ΔV of two SQUIDs was obtained.
[0058]
Thus, according to the present invention, a large ΔV can be obtained regardless of whether the critical currents of the two SQUIDs are different or coincident. In the present embodiment, two bias power supplies are used. However, if the current distribution is controlled well by devising the peripheral circuit as in the second embodiment, a single bias power supply allows a current to flow as in the present embodiment. be able to.
[0059]
Example 4
The fourth embodiment of the present invention will be described below. FIG. 9 shows a schematic diagram of the magnetometer of the present embodiment. Reference numeral 10 denotes a substrate, which is SrTiO as in Example 1._ThreeA (100) bicrystal substrate. On the substrate 10, SQUIDs 12 and 13 including a superconducting ring 4, a notch 2, a weak coupling 3, and current / voltage terminals 14 and 15 are formed by a superconducting thin film.
[0060]
Similar to the first embodiment, the two SQUIDs 12 and 13 are directly coupled to a detection coil formed by the same superconducting thin film 1. That is, the input coil 11 which is a part of the detection coil 7 and the superconducting ring 4 are combined. Current / voltage terminals 14 and 15 formed outside the two SQUIDs 12 and 13 are formed with terminals 16 and 17 made of a noble metal thin film for connecting an external connection line. A plurality of terminals 25 to 33 made of a noble metal thin film for connecting an external connection line are also formed on the detection coil.
[0061]
In this embodiment, two SQUIDs 12 and 13 are arranged adjacent to each other. Although it is similar to the conventionally known magnetometer shown in FIG. 1B, in the magnetometer of the present invention, a number of electrodes are formed on the detection coil. In the example, nine electrodes 25 to 33 were produced. Although the pattern of this example is different from that of Example 1, the manufacturing process is the same, and the description thereof is omitted.
[0062]
Table 1 shows characteristics of two SQUIDs included in the magnetometer of the fourth embodiment.
[0063]
FIG. 10 shows an equivalent circuit and a measurement circuit of the magnetometer of the present embodiment. In the equivalent circuit, the inductance of the SQUID that is much smaller than the inductance of the detection coil is omitted. In the same manner as in the second embodiment, a bias current was passed through two paths with one bias power source 51.
[0064]
In one path, a bias current flows from both the Q1 side electrode 16 to the Q2 side electrode 17 in both of the two SQUIDs. The current passed through this path was set to 40 μA, which is the optimum bias current of the SQUID of Q1 having a small critical current.
[0065]
In the second path, a bias current was supplied from only one of the electrodes 25 to 33 on the detection coil to the electrode 17 on the Q2 side only at the junction of Q2. The amount of current to flow is 25 μA corresponding to the difference between the optimum bias current values of Q1 and Q2.
[0066]
In the structure of this embodiment, the current flowing from the electrode on the detection coil always flows counterclockwise through the SQUID ring of Q1. However, since there are nine electrodes 25 to 33 on the detection coil, the value of the current flowing through the SQUID ring of Q1 can be changed in nine steps. V-Φ characteristic is flux quantum Φ₀Therefore, if the phase difference between the two V-Φ characteristics can be adjusted from 0 to 2π, the phases of the V-Φ characteristics of the two SQUIDs can always be matched.
[0067]
The amount of current necessary to adjust the phase difference from 0 to 2π is the inductance of the SQUID inductance where the current for phase adjustment actually flows, that is, the inductance of the portion overlapping the detection coil, L_sq.cDetermined by.
[0068]
In this embodiment, L_sq.cIs about 50 pH. In this case, the current required to change the phase difference by 2π is Φ₀/ L_sq.c= 2 × 10^-15/ 50 × 10^-12= 40 μA. In the magnetometer of this example, the difference between the optimum bias current values of Q1 and Q2 was 25 μA, so the phase difference could not be adjusted from 0 to 2π in the measurement system of FIG. A ΔV of about 65 μV corresponding to the sum of ΔV of Q1 and Q2 was obtained in the arrangement. Phase is changed from 0 to Φ in the electrode arrangement of FIG.₀The critical current difference between the two SQUIDs is Φ₀/ L_sq.cNeed to be bigger than. L_sq.cIt is clear that the phase can be changed greatly with a small current by increasing.
[0069]
(Example 5)
The fifth embodiment of the present invention will be described below. The produced magnetometer has basically the same structure as that of Example 4 shown in FIG. However, the critical current of the two SQUIDs was intentionally changed by increasing the SQUID inductance to 100 pH, the junction width of Q1 to 1.5 μm, and the junction width of Q2 to 3 μm. As a result, as shown in Table 1, the critical currents of Q1 and Q2 were 32 μA and 60 μA, respectively, and the difference was 28 μA.
[0070]
L_sq.cIs about 90 pH and the current required to change the phase by 2π is 22 μA, and the phase difference can be adjusted from 0 to 2π even in the measurement system of FIG. 10 using one bias power source. By actually examining the optimum electrode arrangement, a modulation voltage amplitude of 47 μV corresponding to the sum of ΔV of two SQUIDs was obtained.
[0071]
As can be seen from the above five embodiments, according to the present invention, it can be seen that the serial connection of two SQUIDs works effectively in any of the embodiments. In the embodiment, the weak coupling, which is an important component of the SQUID, is a grain boundary junction using a bicrystal substrate, but it may be a stepped substrate, a lamp edge type or a stacked type SNS junction, or other than these. Needless to say, the structure may be weakly coupled.
[0072]
【The invention's effect】
According to the present invention, even a 2-SQUID magnetometer with variations in bonding characteristics can provide a magnetometer with high magnetic field resolution.
[Brief description of the drawings]
FIG. 1 is a plan view showing a configuration of a conventional magnetometer including a SQUID and a detection coil.
FIG. 2 is a waveform diagram showing typical voltage-magnetic field characteristics of a magnetometer.
FIG. 3 is a plan view (a), a schematic cross-sectional view (b), and an equivalent circuit diagram (c) of the magnetometer of the first embodiment of the present invention.
FIG. 4 is a cross-sectional view showing a manufacturing process of the magnetometer according to the first embodiment of the present invention.
5A is a circuit diagram of the magnetometer of FIG. 3 and a conventional measurement circuit, FIG. 5B is a voltage-magnetic flux characteristic diagram of the magnetometer measured at various bias current values, and a bias current having a modulation voltage amplitude ΔV. Dependency measurement diagram (c).
FIG. 6 is a circuit diagram (a) of the measurement circuit in the first embodiment of the present invention and the measurement circuit in the embodiment of the present invention, and a measurement diagram (b) of the second bias current dependency of the modulation voltage amplitude ΔV. .
FIG. 7 is a measurement circuit diagram of a second embodiment of the present invention.
FIG. 8 is a measurement circuit diagram of a third embodiment of the present invention.
FIG. 9 is a plan view showing the structure of a magnetometer according to a fourth embodiment of the present invention.
FIG. 10 is a measurement circuit diagram of a fourth embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Superconducting thin film, 2 ... Notch, 3 ... Weak coupling, 4 ... Superconducting ring, 5 ... Current / voltage terminal, 6 ... SQUID, 7 ... Detection coil, 8 ... Magnetometer, 9 ... 2-SQUID direct couple magnetometer DESCRIPTION OF SYMBOLS 10 ... Board | substrate, 11 ... Input coil, 12 ... Q1, 13 ... Q2, 14 ... Current / voltage terminal formed outside from SQUID of Q1, 15 ... Current / voltage terminal formed outside from SQUID of Q2 16... Current / voltage terminal electrodes formed outward from the SQUID of Q1, 17... Current / voltage terminal electrodes formed outward from the SQUID of Q2, 18-23... Electrodes formed on the detection coil, 24. Inductance of detection coil, 25-33 ... Electrode formed on detection coil, 41 ... Bicrystal substrate, 42 ... Superconducting thin film, 43 ... Grain boundary of substrate, 44 ... Connection of superconducting thin film Grain boundary, 45 ... noble metal thin film, 46 ... metal mask for noble metal thin film, 47 ... resist, 50 ... variable resistor, 51 ... first bias power supply, 52 ... second bias power.

Claims (4)

A thin film made of a high-temperature superconducting material formed on a substrate is patterned with two SQUIDs composed of two superconducting rings including two weak couplings and current / voltage terminals, and a common detection coil is connected to the two SQUIDs. The 2-SQUID magnetometer is provided with a plurality of current / voltage terminals for adjusting the phase difference between the V-Φ characteristics of the two SQUIDs in the detection coil. The 2-SQUID magnetometer according to claim 1, wherein the optimum bias current values of the two SQUIDs are different. By supplying bias currents to multiple current paths and controlling the distribution of the bias currents, the optimum bias current value between the two SQUIDs and the phase shift of the V-Φ characteristics are simultaneously adjusted, and generated at both ends of the two SQUIDs. The magnetometer driving method according to claim 1, wherein a sum of voltages to be output is used as an output. 4. The method of driving a magnetometer according to claim 3, wherein a bias current is caused to flow through a plurality of current paths by dividing the current using a single bias power source.

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