JPS62104180A - Josephson element composed by using superhighpurity niobium - Google Patents

Abstract

PURPOSE:To improve the characteristics by constructing a Josephson element using usperhigh-purity niobium as the evaporation or sputtering target, thereby minimizing the trap of the magnetic field, reduction of the transition temperature, and change in the coherence length epsilon. CONSTITUTION:Niobium 2 becoming a grand plane for shielding the element and device from the floating magnetic field is grown into a film on a Si substrate 1. Further, in the same vacuum, SiO2 3 becoming an insulating layer is formed, and then, after making the resist pattern 4 of the lower electrode of a Josephson element, niobium 5 is evaporated, and lift-off is performed, forming the lower electrode. Thereafter, the resist pattern of the upper electrode is formed, the lower electrode surface of the exposed connecting portion if cleaned, thermal oxidation and plasma oxidation are applied to make in insulating barrier 6, and thereafter an upper electrode 7 is formed by evaporation and lift-off. That is, by using superhigh-purity niobium, ununiformity due to the impurities existing in the film can be reduced, enabling the trap center causing the trap of the magnetic field to be reduced.

Description

【発明の詳細な説明】 〔産業上の利用分野〕 本発明は格子欠陥が極めて低減されたニオブ系の超高純
度薄膜を用いたジ１セフソン素子に関スる０ 〔従来の技術〕 特性の良いジヲセフソン素子を作製するためＫは次に挙
げる条件を満足せねばならない。第１に２つの超伝導体
を何らかの方法によってその材料によって決まるコヒー
レント長（ξ０）の数倍の距＃ＩＫ近接させることが不
可欠である。ξ。は材料によってまちまちであるが幾ね
１００　ｎｍ未満でありこの値を達成する為には高度の
微細加工技術が要求され、これまで活発な検討がなされ
ているがまだ決定的な方法を見出すまでには致っていな
い。[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a di-1 Cefson device using a niobium-based ultra-high purity thin film with extremely reduced lattice defects. In order to produce a good Josephson device, K must satisfy the following conditions. First, it is essential to bring the two superconductors close to each other by some method at a distance #IK several times the coherence length (ξ0) determined by their materials. ξ. Although it varies depending on the material, it is often less than 100 nm, and in order to achieve this value, advanced microfabrication technology is required. It hasn't happened yet.

その他にも動作範囲を広くし、操作を容易にするために
高い遷移温度（Ｔｃ）を有することも必要である。さら
に液体ヘリウム温度と室温の温度サイクルに強いことも
求められる。以上の特徴を考慮して鉛合金系及びニオブ
系が選ばれ検討されてきた。Additionally, it is necessary to have a high transition temperature (Tc) in order to widen the operating range and facilitate operation. It is also required to be resistant to temperature cycles between liquid helium temperature and room temperature. In consideration of the above characteristics, lead alloy type and niobium type have been selected and studied.

鉛合金系はξが比較的長い（約１００　ｎｍ）ことから
当初から現在に致るまで最も多く使われた材料で文献数
も多い。しかし、ソフトメタルであるので本質的に温度
サイクルに弱くヒロックが生じやす（安定性に問題があ
ることや模表面の平担性。Since lead alloys have a relatively long ξ (approximately 100 nm), they have been the most commonly used materials from the beginning to the present, and have a large number of publications. However, since it is a soft metal, it is inherently susceptible to temperature cycles and prone to hillocks (problems with stability and flatness of the patterned surface).

膜の不均一性に問題があり、磁束をトラップしやすいな
どの欠点があった。この点を改善する検討もかなり行な
われているが根本的な対策は得られなかった。そこで温
度サイクルに強く、遷移温度も高いニオブ系が再び注目
されてきた。ニオブはゲッタ作用が強（超伝導特性が劣
化しやすいという欠点があるが、今後の主流になると思
われる。There was a problem with the non-uniformity of the film, and there were drawbacks such as the tendency to trap magnetic flux. Considerable efforts have been made to improve this problem, but no fundamental countermeasures have been found. Therefore, niobium-based materials, which are resistant to temperature cycles and have a high transition temperature, are attracting attention again. Although niobium has a strong getter effect (it has the disadvantage that its superconducting properties tend to deteriorate easily), it is expected to become mainstream in the future.

そこで膜質の良好なニオブ又はニオブ系薄膜の作製が非
常に重要になってくる。Therefore, the production of niobium or niobium-based thin films with good film quality becomes very important.

薄膜作製法は種々あるが、広（用いられている方法は蒸
着法とスパッタリング法の２種類である。There are various methods for producing thin films, but two are widely used: vapor deposition and sputtering.

これらの方法を用いたニオブ系薄膜の製膜はいづれも真
空室内の残留ガスとニオブの反応を防ぐために室内の脱
ガスを充分に行なった上で１０−’Ｐａ以下の超高真空
に排気し、さらに膜形成中の蒸着原子又はスパッタ原子
が基板に到達するまでにガス成分と反応しない様高速で
行なっていた。When forming a niobium-based thin film using these methods, the chamber must be thoroughly degassed and evacuated to an ultra-high vacuum of 10-'Pa or less to prevent the reaction between the residual gas in the vacuum chamber and niobium. Furthermore, the film formation process was carried out at high speed so that the evaporation atoms or sputtered atoms during film formation did not react with gas components before reaching the substrate.

しかし、蒸着源及びターケラト用のニオブは純度の高い
ものが存在せず、それらに混入している不純物が膜特性
に影響を及ぼしている。すなわちこれら膜を使って作製
したジョセフソン素子又はそれを含む超伝導回路及び受
動素子で構成される超伝導デバイスの特性評価が好結果
とならない原因として蒸着源及びスパッタ用ターゲット
中に含まれる不純物が挙げられていた。However, there is no highly pure niobium for vapor deposition sources and tarcerates, and impurities mixed in them affect film properties. In other words, the impurities contained in the evaporation source and sputtering target are the reason why the characteristics evaluation of Josephson devices fabricated using these films, or superconducting devices including superconducting circuits and passive devices, do not yield good results. It was mentioned.

〔発明が解決しようとする問題点〕[Problem that the invention seeks to solve]

超伝導薄膜特性に悪影響を及ぼす薄膜中の不純物の混入
を極力避けることにより磁束のトラップ。Trapping of magnetic flux by avoiding as much as possible the inclusion of impurities in the thin film that adversely affect the properties of the superconducting thin film.

遷移温度の低下、コヒーレンス長ξの変化を最小限にと
どめ、ジョセフソン素子の特性を改善することである。The objective is to minimize the reduction in transition temperature and the change in coherence length ξ, thereby improving the characteristics of the Josephson element.

〔問題点を解決するための手段〕[Means for solving problems]

本発明は前記の問題点を解決するためになされたもので
あり、ニオブ系の超伝導薄膜特性を改善するためにガス
成分、アルカリ金属１重金属、半導体の不純物となる元
素が極めて微量か又は検出限界以下である超高純度ニオ
ブを蒸着又はスパッタリングターゲットに用いたジョセ
フソン素子を構成することである。ニオブ化合物として
は例えばＮ１）Ｎがニオブ単体の場合と同様に用いるこ
とができる。The present invention was made in order to solve the above problems, and in order to improve the properties of niobium-based superconducting thin films, gas components, alkali metals, heavy metals, and elements that become impurities in semiconductors are detected in extremely small amounts or detected. The objective is to construct a Josephson device using ultra-high purity niobium, which is below the limit, as a vapor deposition or sputtering target. As the niobium compound, for example, N1) can be used in the same manner as when N is niobium alone.

本発明で用いられる超高純度ニオブは本特許出願人が先
に出願した特許出願＆６０−１１８７７４号に記載した
方法により得られるが、その代表的な分析値は表−１に
示す通りであり、ニオブの純度は９９．９９チ以上であ
る。The ultra-high purity niobium used in the present invention is obtained by the method described in patent application No. &60-118774 previously filed by the applicant of this patent, and its typical analysis values are as shown in Table 1. The purity of niobium is 99.99% or higher.

〈表−１〉 超伝導嗅の膜特性は蒸着、スパッタリングのどちらでも
良好なものが得られるが前者の方がより好ましい。<Table 1> Superconducting membrane properties can be obtained by both vapor deposition and sputtering, but the former is more preferable.

次にこの超高純度ニオブを用いた蒸着法によるジョセフ
ソン素子の作製例を示す（図−１）。Next, we will show an example of fabricating a Josephson device using this ultra-high purity niobium vapor deposition method (Figure 1).

まず浮遊磁場から素子及びデバイスを遮蔽する目的のグ
ランドブレーンとなるニオブ２を８１基板１上にスパッ
タ法圧より２００〜３００　（ｎｍ）製膜する。さらに
同一真空中におい【、絶縁層となるＳｉ０，３を同様に
スパッタ法を用いて２００〜５００　（ｎｍ）製膜する
。次にジョセフソン素子の下部電極のレジストパタン４
を作製した後、蒸着によりニオブ５を１５０〜２００　
（ｎｍ）　］＃模しリフトオフして下部電極を形成する
。その後、上部電極のレジストバタンを形成する。そし
て露出している接合部の下部電極表面をプラズマクリー
ニングし、熱酸化及びプラズマ酸化をほどこし絶縁障壁
６を作製した後、蒸ＬＩＪ７トオフにより上部電極７の
厚さを約４００　ｎｍにする。First, a film of 200 to 300 (nm) of niobium 2, which will serve as a ground brain for shielding elements and devices from stray magnetic fields, is formed on the 81 substrate 1 by sputtering pressure. Furthermore, in the same vacuum, a film of Si0,3, which will become an insulating layer, is formed to a thickness of 200 to 500 (nm) using the same sputtering method. Next, resist pattern 4 of the lower electrode of the Josephson element
After producing niobium 5, 150 to 200
(nm)] #Make lift-off to form a lower electrode. After that, a resist baton for the upper electrode is formed. Then, the surface of the lower electrode at the exposed joint portion is plasma cleaned, thermal oxidation and plasma oxidation are performed to produce an insulating barrier 6, and then the thickness of the upper electrode 7 is reduced to about 400 nm by vapor LIJ7 removal.

〔発明の効果〕〔Effect of the invention〕

本発明により以下の効果が得られる。すなわち超高純１
ｆニオブを用いたことにより、模中に存在する不純物に
よる不均一性を極めて少なくすることができる。これに
より磁束のトラップの原因となるトラップセンタを極め
て少な（することが可能となる。The present invention provides the following effects. In other words, ultra-high purity 1
By using f-niobium, non-uniformity due to impurities present in the mold can be extremely reduced. This makes it possible to extremely reduce the number of trap centers that cause magnetic flux traps.

従来までの超伝導薄膜では、多数個のジョセフソン素子
を作製する場合、最大ジョセフソン素子に対する磁束の
トラップの影響と除去することが難かしかったが本発明
による薄膜を用いることにより、磁界の影響を極めて小
さくできる。また不純物が原因となる薄膜構造上の不完
全性による散乱も極めて低くすることができるために残
留抵抗比（−ρ。Ｖρア、〜、とは３００　Ｋ、ρ掃ま
デバイ温度に比べ充分に低い温度）を小さくすることが
可能となり、その結果より薄い膜で従来と同じＴｃ値を
維持でき、多層の積層構造をとる場合、有利である。こ
れは、熱サイクルに対しても、より強くできるという幅
時的効果も生む。With conventional superconducting thin films, when producing a large number of Josephson elements, it was difficult to eliminate the effect of magnetic flux trapping on the maximum Josephson element, but by using the thin film of the present invention, it was difficult to eliminate the effect of magnetic flux trapping on the maximum Josephson element. The impact can be extremely small. In addition, since scattering due to imperfections in the thin film structure caused by impurities can be extremely low, the residual resistance ratio (-ρ. As a result, the same Tc value as before can be maintained with a thinner film, which is advantageous when a multilayer structure is adopted. This also has the temporal effect of making it more resistant to thermal cycles.

これらの効果はニオブ薄膜で用いるのみならずニオブ化
合物薄膜の場合及びスパッタリングによる薄膜化の場合
でも同様の効果が得られる。These effects can be obtained not only with a niobium thin film, but also with a niobium compound thin film and when the film is thinned by sputtering.

【図面の簡単な説明】[Brief explanation of drawings]

るジョセフソン素子製作例の一例を示すものである０ １・・・基板、２・・・グランドプレーン、３・・・絶
縁層。 ４・・・レジストステンシル、５・・・下部電極。 ６・・・絶縁障壁、７・・・上部電極 特許出願人　　東洋曹達工業株式会社 図−１を工、不兄明超尚縄腿ン用〜また黒眉云艮よ′へ ↓↓　↓　↓↓ □□−７” ４　ｊルｊ　↓↓ □□□−一一フ７− ↓ 一 □□−一二し 図−１0 1...Substrate, 2...Ground plane, 3...Insulating layer. 4... Resist stencil, 5... Lower electrode. 6...Insulating barrier, 7...Top electrode patent applicant Toyo Soda Kogyo Co., Ltd. Figure-1 was constructed, and it was used for a long time. □□−7” 4 jruj ↓↓ □□□−11F7− ↓ 1□□−12Fig.−1

Claims (1)

【特許請求の範囲】[Claims] ガス成分、アルカリ金属、重金属、半導体不純物元素が
極めて微量か又は検出限界以下である超高純度ニオブ単
体又はその化合物を用いてなるジョセフソン素子。A Josephson element using ultra-high purity niobium or a compound thereof, in which gas components, alkali metals, heavy metals, and semiconductor impurity elements are present in extremely small amounts or below detection limits.