JPS58108739A - Josephson junction device - Google Patents

Abstract

PURPOSE:To obtain the junction structure having excellent Josephson and superconductive tunnel characteristics for the Josephson junction element in which Nb is used as a lower electrode. CONSTITUTION:A metal film, whereon only an oxide which turns into an insulating layer in an oxidizing process will be formed, is coated on the surface of the Nb film 1 which will be turned to the lower electrode. Ta is used as the metal film having the above characteristics. The thickness of the Ta is to be within the range of 1-20nm. When the thickness is in excess of 3nm, a non-oxidized Ta layer remains between the Ta oxides layer 7 and the Nb layer 1. In the case of a junction having a Ta oxide as a barrier layer, as the low grade oxide which will be formed between a Ta2O5 oxide film and metal Ta layer is thinly formed, there exists almost no superconduction deteriorating layer and a normal conductive layer (within 10Angstrom ). Accordingly, a Josephson junction having little leak current and high quality can be formed.

Description

【発明の詳細な説明】 本発明はジョセフソン接合装置の構造に関し、とくに高
速計算機用スイッチング素子に好適なサンドインチ形ジ
ョセフノン接合素子特性を向上させ、かつ安定化するの
に好適な接合障壁層の構造に関する。DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the structure of a Josephson junction device, and in particular to a junction barrier layer suitable for improving and stabilizing the characteristics of a sandwich-type Josephson non-junction device suitable for switching elements for high-speed computers. Regarding the structure of

従来のＮｂ膜を下部電極とするＮｂ系ジョセフソン接合
の接合部障壁層はＮｂ膜表面の酸化層によって形成され
ていた。しかしながら、このような障壁層を有する接合
および、接合の製造方法には以下のような問題点がおっ
た。The junction barrier layer of a conventional Nb-based Josephson junction using a Nb film as the lower electrode was formed by an oxide layer on the surface of the Nb film. However, the following problems arose in the junction having such a barrier layer and the method for manufacturing the junction.

ｆｌｌＮｂは酸素などと活性であるため、バタンニング
等のプロセス工程において表面汚染層が１００人の深さ
まで形成される。Since flNb is active with oxygen and the like, a surface contamination layer is formed up to a depth of 100 nm during process steps such as battening.

＋２１Ｎｂ膜表面を熱酸化、あるいは酸素雰囲気中の高
周波放電によって酸化させた場合、絶縁層となるべきＮ
ｂ、ＯＬ１酸化物以外に常伝導的性質を示すＮｂＯある
いはＮｂＯ，などが形成される。Ｎ、０゜層の厚みはス
イッチング素子用接合の場合２〜３ｎｍ程度であるが、
第１図に示すごとく同時にＮｂ０層とＮｂＯ２層が３ｎ
ｍ以上の厚さで形成される（ＸＮｂ／Ｎｂ　□ｘｉｄｅ
／ｐｂ−ＡＩｌｏｙ　ＪｏｓｅｐｈｓｏｎＴｕｎｎｅｌ
　　Ｊｕｎｃｔｉｏｎｓ”、　　Ｓ、　　１．　　Ｒａ
１ｄｅｒ　　　ａｎｄ　　Ｒ，。When the +21Nb film surface is oxidized by thermal oxidation or high-frequency discharge in an oxygen atmosphere, the Nb film that should become an insulating layer is
b. In addition to the OL1 oxide, NbO, NbO, etc. exhibiting normal conductivity are formed. The thickness of the N,0° layer is approximately 2 to 3 nm in the case of a junction for a switching element.
As shown in Figure 1, the Nb0 layer and NbO2 layer are 3n at the same time.
Formed with a thickness of m or more (XNb/Nb □xide
/pb-AIloy JosephsonTunnel
Junctions”, S, 1. Ra
1der and R,.

Ｅ、　Ｄｒａｋｅ、　ＩＥＥＥ　Ｔｒａｎｓ、　Ｍａｇ
ｌＭＡＧ　ＩＬ２９９（１９８１））。とくにＮｂ０層
は常伝導のトンネル電１流湯となり、この部分がＮｂ系
接合のトンネル特性を低下させる。E, Drake, IEEE Trans, Mag
lMAG IL299 (1981)). In particular, the Nb0 layer becomes a normal conducting tunnel current, and this portion deteriorates the tunneling characteristics of the Nb-based junction.

ｆ３１Ｎｂ膜はｐｂ合金膜と比較して、障壁層として必
要な酸化膜厚が短時間で容易に形成されるため、酸化膜
厚の正確な制御が逆に困難となる。Compared to the pb alloy film, the f31Nb film can easily form the oxide film thickness required as a barrier layer in a short time, so that accurate control of the oxide film thickness becomes difficult.

本発明の目的は下部電極をＮｂとするジョセフソン接合
素子において、ジョセフソンおよび超電導トンネル特性
が優れ、かつ制御性よくトンネル障壁層を形成できる接
合構造を与えることにある。An object of the present invention is to provide a Josephson junction device having Nb as the lower electrode, which has excellent Josephson and superconducting tunneling properties and which allows formation of a tunnel barrier layer with good controllability.

上記目的を達成するために、下部電極となるＮｂ膜表面
に、酸化プロセスにおいて絶縁層となるべき酸化物のみ
が形成される金属被膜をコーティングする。このような
性質を有する金属膜としてＴａを用いる。Ｔａの膜厚と
して１〜２０ｎｍの範囲とする。Ｔａの厚みを３ｎｍ以
上とした場、合、’ｆａ酸化物層とＮｂ層との間に未酸
化Ｔａ層が残る。Ｔａの臨界温度は４．５にであり、Ｎ
ｂの臨界温度９．３によシ低いが、Ｔａの極薄層が超電
導トンネル特性の劣化をもたらし難いことを以下に示す
。In order to achieve the above object, the surface of the Nb film that will become the lower electrode is coated with a metal film in which only an oxide that will become an insulating layer is formed in an oxidation process. Ta is used as a metal film having such properties. The Ta film thickness is in the range of 1 to 20 nm. When the Ta thickness is 3 nm or more, an unoxidized Ta layer remains between the 'fa oxide layer and the Nb layer. The critical temperature of Ta is 4.5 and N
Although the critical temperature of b is lower than 9.3, it will be shown below that an ultrathin layer of Ta is unlikely to cause deterioration of superconducting tunneling properties.

一般に超電導体に常電導体を接続したサンドインチ構造
の金属においては、超電導体から常電導体への超電導電
子（いわゆるクーパ一対）のしみこみ、および常電導体
から超電導体への常電導電子のしみこみ現象、いわゆる
プロキシミテイ効果及存在する。この結果、超電導体内
部における常電導電子の割合が増加して超電導特性が劣
化するとともに、常電導体における超電導電子の存在に
よって超電導性が生じる。ここで超電導トンネル特性の
優れた接合を得るためには、プロキシミテイ効果によっ
て常電導体の超電導特性を下部電極超電、導体の特性に
近づける必要がある。このための条件は、（１）超電導
体と常電導体間の電子透過係数が大きいこと。（２）常
電導体の膜厚が薄いことである。（１）に関しては結晶
構造の異なる２種類の物質の界面において、電子が散乱
されやすく、透過係数が大きい。ＮｂとＴａの場合、両
方共結晶構造が等しく体心立方晶であり、格子定数は３
桁目まで等しい（０，３３００ｍ　）。したがって清浄
なＮｂ膜表面上にＴａ極薄膜を真空蒸着した場合、＋ｐ
　ａ結晶はＮｂ上にエピタキシアルに成長する。In general, in metals with a sandwich structure in which a normal conductor is connected to a superconductor, superconducting electrons (so-called Cooper pairs) seep into the normal conductor, and normal conductive electrons seep in from the normal conductor to the superconductor. A phenomenon, the so-called proximity effect, exists. As a result, the proportion of normal conducting electrons inside the superconductor increases and the superconducting properties deteriorate, and superconductivity occurs due to the presence of superconducting electrons in the normal conductor. In order to obtain a junction with excellent superconducting tunneling properties, it is necessary to bring the superconducting properties of the normal conductor closer to those of the lower electrode superconductor through the proximity effect. The conditions for this are: (1) the electron transmission coefficient between the superconductor and the normal conductor is large; (2) The film thickness of the normal conductor is thin. Regarding (1), electrons are easily scattered at the interface between two types of substances with different crystal structures, and the transmission coefficient is large. In the case of Nb and Ta, both crystal structures are equally body-centered cubic, and the lattice constant is 3.
Equal to the digits (0,3300m). Therefore, when an extremely thin Ta film is vacuum-deposited on a clean Nb film surface, +p
The a-crystal grows epitaxially on Nb.

この結果ＮｂとＴａ界面における電子の透過係数間 は異なる結晶構造の金−一り面の場合（透過率はほぼ０
．１）より大きくなる。（２）に関してはすべての金属
の組合せに共通であシ、Ｔａの場合、この条件により膜
厚が２０ｎｍ以下に制限される。Ｔａに関しては臨界温
度が４．５にであり、上に述べたプロキシミテイ効果に
よる臨界温度の上昇効果により超電導特性が向上し、実
効的な臨界温度はＮｂ層値に近くなる。As a result, the transmission coefficient of electrons at the Nb and Ta interface is different for the case of a single gold plane with a different crystal structure (the transmittance is almost 0).
．． 1) Become larger. Regarding (2), it is common to all metal combinations, and in the case of Ta, the film thickness is limited to 20 nm or less due to this condition. Regarding Ta, the critical temperature is 4.5, and the superconducting properties are improved due to the effect of increasing the critical temperature due to the proximity effect described above, and the effective critical temperature becomes close to the value of the Nb layer.

本発明に基づいて製造したジョセフソン接合に関する実
施例を以下に示す。Examples of Josephson junctions manufactured according to the present invention are shown below.

まず以下に示す工程によってＮｂ系接合素子の作製を行
なった。First, an Nb-based junction element was manufactured through the steps shown below.

ｆｌｌ　　８ｉウエハｅ１４Ｆ’とＨ２Ｏの混合液によ
って洗浄する。Clean with a mixture of fll 8i wafer e14F' and H2O.

＋２＋ｓｉウエハを真空蒸着装置に装着し、真空装置を
ｔｏ−’ｐａの真空度に排気する。なおこの蒸着装置に
は２個のルツボを備えた電子ビーム銃加熱装置を装備し
、それぞれＮｂおよびＴａの塊を挿入した。The +2+si wafer is mounted on a vacuum evaporation apparatus, and the vacuum apparatus is evacuated to a degree of vacuum of to-'pa. This vapor deposition apparatus was equipped with an electron beam gun heating device equipped with two crucibles, into which Nb and Ta lumps were inserted.

ｆ３１　　Ｓｉウェハを加熱して４００Ｃに保チ、Ｎｂ
を電子ビーム銃によって加熱蒸発させて、Ｓｉウェハ上
に２　ｎ　ｍ　／　ｓの蒸着速度で蒸着する。膜厚は２
００１ｍとする。f31 Heating Si wafer and keeping it at 400C, Nb
is heated and evaporated using an electron beam gun, and is deposited on a Si wafer at a deposition rate of 2 nm/s. Film thickness is 2
001m.

１４）Ｎｂ膜薫蒸着後基板温度を一定に保ち、電子ビー
ム銃のルツボ位置を移動して、Ｔａ膜の蒸着（５） を行なった。膜厚は６ｎｍとした。14) After the Nb film was vapor-deposited, the Ta film was deposited (5) by keeping the substrate temperature constant and moving the crucible position of the electron beam gun. The film thickness was 6 nm.

（５１ＮｂとＴａの積層膜が全面に付着したＳｉウェハ
にレジスト膜をコーティングし、ジョセフソン接合の下
部電極パターンを形成した。(A resist film was coated on a Si wafer on which a laminated film of 51Nb and Ta was adhered to the entire surface, and a lower electrode pattern of a Josephson junction was formed.

（６１ＨＦとＨＮＯ，とＨｌＯの混合液によってＮｂ−
Ｔａ膜のエツチングを行ない、下部電極パターンの形成
を行なった。引続いて接合開口用ＳｉＯ膜のためのレジ
ストパターンニングを行なった。(Nb-
The Ta film was etched to form a lower electrode pattern. Subsequently, resist patterning for the SiO film for the junction opening was performed.

（７）　ＳＩＯ膜の真空蒸着、リフトオフによるＳｉＯ
膜パターン形成を行なう。続いて上部電極用レジストパ
ターンの形成を行なった。(7) Vacuum deposition of SIO film, SiO by lift-off
Perform film pattern formation. Subsequently, a resist pattern for an upper electrode was formed.

＋Ｓ＋Ｓｉウェハを別の真空蒸着装置の高周波電極に装
着し、０．４　Ｐ　ａのＡｒ雰囲気中で高周波放電を行
なう。これによりパターンニング工程における汚染物を
除去した。The +S+Si wafer is attached to a high frequency electrode of another vacuum evaporation device, and high frequency discharge is performed in an Ar atmosphere of 0.4 Pa. This removed contaminants during the patterning process.

（９）０□２％−Ａｒ混合ガス雰囲気中で高周波放電を
行なうことによりＴａ酸化膜層を形成する。(9) A Ta oxide film layer is formed by performing high frequency discharge in a 0□2%-Ar mixed gas atmosphere.

Ｔａｍ化膜の形成条件は以下の通シとした。つまり、酸
素とアルゴンの混合ガス（酸素２　ｖｏ１％）を３ｍ’
ｐｏｒｒの圧力で導入し、高周波電圧２００、（６） ■（ピーク間電圧）の下で１０分間高周波放電を行なっ
た。この条件下では通常表面から２０〜３０人の厚さで
酸化膜が形成される。引続いて７ＸＩＯ−’Ｐａに真空
排気し、Ｐｂ−Ｉｎ合金膜の蒸着を行なう。リフトオフ
を行なうことによシ接合を完成した。この結果、接合構
造は第２図に示すごとく形成された。この接合の電流−
電圧特性を測定したところ、従来のＮｂ酸化膜を障壁層
とする接合よりすぐれた特性が得られた。すなわち。The conditions for forming the Tam film were as follows. In other words, a mixed gas of oxygen and argon (oxygen 2 vol 1%) is
porr pressure, and high-frequency discharge was performed for 10 minutes under a high-frequency voltage of 200, (6) (peak-to-peak voltage). Under these conditions, an oxide film is usually formed to a thickness of 20 to 30 people from the surface. Subsequently, the vacuum was evacuated to 7XIO-'Pa, and a Pb-In alloy film was deposited. The joint was completed by performing lift-off. As a result, a bonded structure as shown in FIG. 2 was formed. Current in this junction −
When voltage characteristics were measured, characteristics superior to those of conventional junctions using Nb oxide films as barrier layers were obtained. Namely.

エネルギーギャップ以下における抵抗と以上における抵
抗の比が、Ｎｂ酸化膜を障壁層とする接合＾ の聯合、約１０であったのに対し、Ｔａ酸化膜を障壁Ｊ
−とする接合では１５以上であった。以上の酸化条件で
作製した接合のジョセフソン臨界電流密度は１０００Ａ
／Ｃ１ｎ２であった。The ratio of the resistance below the energy gap to that above the energy gap was about 10 for a junction with a Nb oxide film as a barrier layer, while it was about 10 for a junction with a Nb oxide film as a barrier layer,
It was 15 or more in the case of -. The Josephson critical current density of the junction fabricated under the above oxidation conditions was 1000 A.
/C1n2.

本発明によれば実施例に述べたごとく以下のような効果
を有する。According to the present invention, as described in the embodiments, the following effects are achieved.

１）Ｔａ酸化物を障壁層とする接合は、’ｒａ、ｏ、酸
化膜と金属Ｔａ層との間に形成される低級酸化物の厚さ
が薄いので対応する超電導劣化層および常電導層がほと
んど存在しない（１０層以内）。したがってリーク電流
の少ない、質の高いジョセフソン接合を形成することが
できる。常電導トンネル抵抗とリーク電流の比は、Ｎｂ
酸化膜をバリアとする接合の場合、通常１：１０であっ
たのに対して、Ｔａｇ化膜をバリアとする接合の場合、
１：１５あるいはそれ以上の値が得られた。1) In a junction using Ta oxide as a barrier layer, since the lower oxide formed between the oxide film and the metal Ta layer is thin, the corresponding superconducting degraded layer and normal conducting layer are Almost non-existent (within 10 layers). Therefore, a high-quality Josephson junction with low leakage current can be formed. The ratio of normal conducting tunnel resistance to leakage current is Nb
In the case of bonding using an oxide film as a barrier, the ratio was usually 1:10, whereas in the case of bonding using a tag film as a barrier,
Values of 1:15 or higher were obtained.

２）Ｔａ酸化物を障壁層とする接合は、Ｎｂ酸化物を障
壁層とする接合と比較して、同一の酸化条件に対してト
ンネル抵抗値が低くなる。したがってＮｂ酸化物障壁層
の接合と比較して、容易に高い臨界電流密度（、：１０
７Ａ／ｍ２　）を実現できるとともに、障壁層形成時に
おけるトンネル抵抗値の制御が容易となる。2) A junction using Ta oxide as a barrier layer has a lower tunnel resistance value under the same oxidation conditions than a junction using Nb oxide as a barrier layer. Therefore, compared to the junction of Nb oxide barrier layer, it is easy to obtain a high critical current density (,:10
7A/m2), and the tunnel resistance value can be easily controlled when forming the barrier layer.

３）ＴａはＮｂと比較して、パターン形成工程を施すこ
とによる表面からの汚染層の侵入深さが浅い。したがっ
て下部電極表面酸化層形成前の、表面スパッタクリーニ
ング深さが、Ｎｂ膜の場合１００人必要なのに対して、
Ｔａ膜の場合は５０Å以下でよい。3) Compared to Nb, Ta has a shallower penetration depth of a contaminant layer from the surface when subjected to a patterning process. Therefore, the surface sputter cleaning depth before forming the lower electrode surface oxide layer is 100 people required for Nb film, whereas
In the case of a Ta film, the thickness may be 50 Å or less.

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

第１図はＮｂ−Ｎｂ酸化物−ｐｂ金合金ｐｂ−Ｉｎ）接
合の断面図である。第２図はＴａ酸化物を障壁層とする
Ｎｂ−障壁層−ｐｂ合金（Ｐｂ−Ｉｎ）接合の断面図で
ある。 １・・・Ｎｂ膜、２・・・ＮｂＯおよびＮｂＯ，層、３
・・・Ｎｂ、Ｏ，層、４・　Ｐｂ−Ｉｎ膜、５−ｓ　ｉ
ウニノー、６・・・’ｐａ層、７・・・Ｔａ、０６層、
８・・・ｓｉｏ膜。 烹　１　図 夕 ！　Ｚ　図FIG. 1 is a cross-sectional view of a Nb-Nb oxide-pb gold alloy pb-In) junction. FIG. 2 is a cross-sectional view of an Nb-barrier layer-pb alloy (Pb-In) junction using Ta oxide as a barrier layer. DESCRIPTION OF SYMBOLS 1...Nb film, 2...NbO and NbO, layer, 3
...Nb, O, layer, 4-Pb-In film, 5-s i
Unino, 6...'pa layer, 7...Ta, 06 layer,
8...Sio membrane.烹 1 Illustration! Z diagram

Claims (1)

【特許請求の範囲】[Claims] １、Ｎｂ膜を下部電極とするジョセフソン接合装置にお
いて、Ｎｂ膜表面を覆った厚さ１〜２０ｎｍのＴａの極
薄膜の全部もしくは表面部を酸化してなる酸化膜を接合
障壁層とすることを特徴とするジョセフソン接合装置。1. In a Josephson junction device using a Nb film as the lower electrode, an oxide film formed by oxidizing the entire or surface portion of an extremely thin Ta film with a thickness of 1 to 20 nm covering the Nb film surface is used as the junction barrier layer. A Josephson joining device featuring: