A Josephson device and a process for fabricating the same

Abstract: In a Josephson device which can be employed as a sensor including superconductor for measuring an extremely weak magnetic field, a Josephson junction (4a) consisting of superconducting material is formed, and a covering layer (9) consisting of ordinary conducting metal or semiconductor is formed on the Josephson junction (4a). This enables the Josephson junction (4a) to be isolated from the oxidized atmosphere. Further, the covering layer (9) is not to present any deterioration such as cracks even upon being subjected to a thermal hysteresis from very low temperature to ordinary temperature.

Description: The present invention is concerned with a superconductive Josephson junction device comprising in a first embodiment a substrate having a stepped surface portion, a superconductive layer formed on the substrate including the stepped portion, said superconductive layer having a thin film region which forms a Josephson junction. The invention is also concerned with a superconductive device of the type with a Josephson junction, comprising in a second embodiment a substrate having a recessed channel with sidewalls, a bottom of said channel being in parallel with a surface of said substrate, and a layer formed by superconducting material along a side of at least one of said sidewalls on the bottom of said recessed channel. Superconductive Josephson junction devices of the first and the second embodiment are disclosed in EP-A-O 291 050. In general it can be said that a superconductive quantum interference device including a superconductive ring that couples with one or two Josephson junctions, is used for measuring a magnetic field generated from the heart, eye ball, brain, and like organisms of a human body. Such device is mentioned a superconductive quantum interference device or SQUID. The SQUID serves as a sensor for measuring such a extremely weak magnetic field as generated from organisms. However, production of the device including the Josephson junction is conducted with the use of a fine processing technique, and the performance as a sensor directly depends on an error of the shape of the device after the processing. Further, the superconductive material to be used is a material which will be oxidized over time while being used. When the superconductive material in use is oxidized, it affects the performance of the device as a sensor as much as deformation of the Josephson junction. Therefore it is difficult to maintain the initial performance of the device as it is produced, over the time. Conventionally, a surface portion of the Josephson junction is oxidized in advance to protect the Josephson junction from the atmosphere, whereby forming an oxide film on the surface portion. However, in the case where oxide is formed on the surface portion, oxygen progressively diffuses over time in a thin film layer of superconducting material, for example. Nb. As a result, there exists the problem that the properties of the device change due to the reduced thickness of the Nb thin film. It is already known to cover the surface portion with synthetic resin material. In these prior arts, the thermal expansion coefficient of the synthetic resin material differs greatly from that of the superconducting material. Accordingly, upon being subjected to a thermal hysteresis from liquid helium temperature of 4.2° K to ordinary temperature, minute cracks are caused in the synthetic resin material, whereby the SQUID is deteriorated. In order to solve the foregoing problem and isolate the SQUID from the oxidized atmosphere, it can be considered to retain the SQUID in the atmosphere of inert gas, such as N2. However, this is not practical because it is difficult to prevent the leak. Further, in existing SQUIDs, it is difficult to form a uniform and exceedingly thin film of about 100 to 200 Å in consideration of the current fine processing technique. Moreover, in the existing SQUIDs, it is extremely difficult to fabricate with minute length and width accurately and with satisfactory reproducibility in a processing which determines the size and shape of the junction. Therefore, the defect rate in production of the SQUID is extremely deteriorating. It is an object of the present invention to provide a high sensitivity Josephson device that can be readily produced with high accuracy. It is another object of the present invention to provide a Josephson device that can improve property of a Josephson junction by reliably preventing oxidization and aged deterioration of superconducting layers. The object of the invention is reached in that in the above mentioned first embodiment said superconductive Josephson junction device is characterized by superconductive layer including a first superconducting thin film layer formed on said substrate, a second superconducting thin film layer, formed on said first superconducting thin film layer , by using an oblique deposition technique such that said second superconducting thin film layer having an overhanging film edge does not cover said first superconducting thin film layer in a region beside said stepped portion, and a third superconducting thin film layer formed on said first superconducting thin film layer by using an oblique deposition technique from an opposite side, such that said third superconducting thin film layer does not cover said first superconducting thin film layer in said region beside said stepped portion, said third superconducting thin film layer being further formed on the second superconducting thin film layer and the sidewall of the stepped portion, said third superconducting thin film layer being thinner than said second superconducting thin film layer and thicker than said first superconducting thin film layer, the sidewall of the third superconducting film layer being closer to the stepped portion where the sidewall of the third superconducting thin film layer is closer to the substrate, and wherein said substrate has a covering layer consisting of ordinary conducting metal or semiconductor material on said thin film region and a neighboring region thereof. In a further embodiment a layer consisting of ordinary conducting metal is formed between said substrate consisting of semiconducting material and said first superconducting thin film layer. The superconducting material is with preference niobium. The object of the invention is further reached in that in the above mentioned second embodiment said superconductive device of the type with a Josephson junction is characterized by at least one of said sidewalls being perpendicular to a surface of said substrate and by said superconductive layer including a first superconducting thin film layer formed on the bottom surface of said recessed channel, a second superconducting thin film layer, formed on said first superconducting thin film layer, by using an oblique deposition technique such that said second superconducting thin film layer having an overhanging film edge does not cover said first superconducting thin film layer in a region beside said sidewalls, and a third superconducting thin film layer formed on said first superconducting thin film layer, by using an oblique deposition technique from an opposite side, such that said third superconducting thin film layer does not cover said first superconducting thin film layer in said region beside said sidewalls, said third superconducting thin film layer being further formed on the second superconducting thin film layer and a sidewall of the recessed channel, said third superconducting thin film layer being thinner than said second superconducting thin film layer and thicker than said first superconducting thin film layer, the sidewalls of the third superconducting thin film layer at the side of the sidewall of the recessed channel being closer to the opposite sidewall of the recessed channel where the sidewall of the third superconducting thin film layer is closer to the bottom of the recessed channel. Also with this embodiment the superconducting material is with preference niobium. It has to be noted that an oblique deposition technique in the fabrication of a superconductive Josephson junction device is known per se from US-A-4 454 522. According to the invention, the thin film region consisting of superconducting material formed on the substrate is, for example, a part of the Josephson junction, and the covering layer consisting of ordinary conducting metal or semiconductor is formed on the thin film region and the neighboring region thereof. This enables the thin film region to be isolated from the oxidized atmosphere, and thereby preventing the oxidization of the thin film region. In addition, the covering layer is not to present deterioration such as cracks even upon being subjected to a thermal hysteresis from very low temperature of liquid helium to ordinary temperature. Accordingly, the thin film region can be protected over a long period. The ordinary conducting metal constituting the covering layer is, for example, Cr or Ta. The semiconductor constituting the covering layer is, for example, Si. Further, according to the invention, the layer consisting of ordinary conducting material is formed on the substrate, and the thin film region consisting of superconducting material is formed on the ordinary conducting material layer, whereby obtaining a Josephson junction. Accordingly, the thin film region is formed on the clean layer, thereby reliably attaining the characteristics of Josephson effect. Further, since the actual thickness of the thin film region can be made smaller due to the proximity effect, it is made possible to form a layer consisting of superconducting material relatively thick. Therefore, such a thin film consisting of superconducting material can be easily produced. Furthermore, according to the invention, the layer consisting of ordinary conducting material is further formed on the Josephson junction, whereby enabling the Josephson junction to be protected from the external atmosphere. In addition, the actual thickness of the Josephson junction can be controlled to be smaller due to the proximity effect. The ordinary conducting material used for the layer formed on the substrate and the superconducting material used for the layer formed on the Josephson junction may be, for example, Cr or Ta, or may be superconducting material having a lower critical temperature than the superconducting material constituting the Josephson junction. The superconducting material having a lower critical temperature than the superconducting material for the Josephson junction is also included in a concept of the ordinary conducting material. Moreover, according to the invention, the recessed channel is formed in the substrate, for example, by the use of dry etching technique, a bottom of the recessed channel is clean, and a thin film region consisting of superconducting material is formed on the bottom, whereby obtaining the Josephson junction. Further, according to the invention, the thin film region realizing the Josephson junction is formed by depositing the superconducting material from oblique directions using the opposite side walls of the recessed channel, and thereby partially exposing the thin film already formed entirely over the bottom of the recessed channel to obtain the thin film region. In this way, the thin film region realizing the Josephson junction can be formed on the clean bottom of the recessed channel. Also, according to the invention, a covering layer consisting of ordinary conducing metal or semiconductor is formed on the thin film region consisting of superconducting material formed on the substrate and a neighboring region thereof. This enables the thin film region to be prevented from oxidizing. In addition, the covering layer is not to deteriorate over time even upon being repeatedly subjected to a thermal hysteresis from very low temperature to ordinary temperature. In this way, the Josephson device can be realized whose characteristics are not to change over a long period. Furthermore, according to the invention, due to the proximity effect of the layer consisting of ordinary conducting material formed on the substrate and the layer consisting of ordinary conducting material formed on the Josephson junction, the actual thickness of the Josephson junction can be made thinner. This makes the characteristics of the Josephson effect satisfactory, thereby enabling the Josephson junction to have higher sensitivity. Further, since the thin film region forming the Josephson junction may be relatively thick, the production can be carried out easily. Moreover, according to the invention, the recessed channel is formed in the substrate, and the thin film region consisting of superconducting material is formed on the bottom of the recessed channel, whereby obtaining the Josephson junction. Accordingly, the thin film region is formed on the clean bottom of the recessed channel. Therefore, there can be produced a Josephson device having excellent characteristics of the Josephson effect. Since the recessed channel is formed in the substrate and the thin film is partially exposed to form the thin film region forming the Josephson junction by conducting the oblique deposition using the opposite side walls of the recessed channel, the production process of the invention can be simplified and the entire process can be carried out in the same vacuum. This causes the production of the Josephson device to be carried out further more easily, and contamination of the surface of the substrate including the bottom of the recessed channel can be prevented. BRIEF DESCRIPTION OF THE DRAWINGS Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein: Fig. 1 is a sectional view of a first embodiment in accordance with the invention; Figs. 2(1) to 2(6) are sectional views showing a production process of the first embodiment shown in Fig. 1; Fig. 3 is a graph showing temperature dependency of resistance of Si constituting a covering layer 9; Fig. 4 is a sectional view of a modification of the first embodiment in accordance with the invention; Figs. 5(1) to 5(8) are sectional views showing a production process of the embodiment shown in Fig. 4; Fig. 6 is a graph showing I-V characteristics of a superconductive quantum interference device; Fig. 7 is a graph showing relationship between a thickness dll of a Josephson region 14a and a current I1, which is an experiment result obtained by the inventor of this application; Fig. 8 is a graph showing I-V characteristics when the thickness of a first, second, and fifth layers 11, 14, 18 are set 200 Å respectively to d10, d11, d15, which is an experiment result obtained by the inventor of the present application; Fig. 9 is a sectional view of a superconductive quantum interference device of a second embodiment in accordance with the invention; Figs. 10(1) to 10(5) are sectional views showing a production process of the second embodiment shown in Fig. 9; and Fig. 11 is a sectional view showing a superconductive quantum interference device of a prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now referring to the drawings, preferred embodiments of the invention are described below. [Embodiment 1] Fig. 1 is a sectional view partly showing a superconductive quantum interference device including a Josephson junction of a first embodiment of the invention, and Figs. 2(1) to 2(6) are sectional views showing a production process of the SQUID. With reference to these drawings, a substrate 1 consists of quartz, sapphire, Si which is a semiconductor, and like material. A first layer 2 of thin film is formed over an entire upper surface of the substrate 1 by deposition or sputter technique as shown in Fig. 2(1). Material for the first thin film layer 2 may be, for example, Nb which provides superconducting material, or Cr or Ta which provides ordinary conducting material, or may be nonmetallic material. Thickness dll of the first thin film layer 2 is, for example, 100 to 200 Å (1Å = 0.1 nm). The first thin film layer 2 have a portion thereof selectively removed therefrom by the use of etching or the like, so that a long and narrow base 2a in the form of a step can be formed as shown in Fig. 2(2). Fig. 2(3) is a plan view showing the substrate 1 on which the base 2a is formed. The length L1 of the base 2a is, for example, 1 to 5 µm. Next, as shown in Fig. 2(4), superconducting material such as Nb is deposited from a direction 3 perpendicular to the surface of the substrate 1, whereby forming a second layer 4 over the upper surfaces of the substrate 1 and the base 2a. Thickness d2 of the second layer 4 is, for example, 80 to 200 Å, preferably 80 to 150 Å, and more preferably 100 Å. Thereafter, as shown in Fig. 2(5), superconducting material Nb is deposited from an arrow direction 6, whereby forming a third layer 5. Thickness d3 of the third layer 5 is, for example, 1000 Å. The depositing direction 6 makes an angle 1 with respect to the surface of the substrate 1. The angle 1 is, for example, between 25 and 70 degrees, and preferably between 25 and 50 degrees, and particles of superconducting material is supplied from the base 2a. Subsequently, as shown in Fig. 2(6), superconducting material Nb is deposited from an arrow direction 7 on the third layer 5, whereby forming a fourth layer 8. Thickness d4 of the fourth layer 8 is, for example, 500 Å, and a sum of the thickness d3 and d4 is, for example, 200 to 2000 Å. In this way, a thin film region 4a is formed which is not covered by either the third layer 5 or the fourth layer 8. The Josephson junction is formed on the thin film region 4a. At the last stage, a covering layer 9 consisting of conducting or semiconducting material is formed on the fourth layer 8 and the thin film region 4a by the use of sputter technique as shown in Fig. 1. The conducting material constituting the covering layer 9 is, for example, metallic material such as Cr and Ta. On the other hand, the semiconducting material constituting the covering layer 9 is, for example, Si. Particularly, it is preferable that the covering layer 9 consists of Si since temperature dependency of resistance of Si has such characteristics that electric resistance becomes substantially infinite at very low temperatures as shown in Fig. 3. As shown in Figs. 2(5), 2(6), the third and fourth layers 5, 8 are formed by the use of an oblique deposition technique, so to speak, and the thin film region 4a on which the Josephson junction is formed. Thereafter, the covering layer 9 is formed, whereby the thin film region 4a and the third and fourth 5, 8 are isolated by the covering layer 9 from the external atmosphere. The conducting or semiconducting material constituting covering layer 9 has a thermal expansion coefficient similar to that of the second, third, and fourth layers 4, 5, 8. Accordingly, even in the case where the SQUID is repeatedly subjected to a thermal hysteresis from the very low temperature of, such as, liquid helium to the ordinary temperature, there is no such likelihood that minute cracks are caused in the covering layer 9. In the foregoing embodiment, the thin film region 4a is formed using the oblique deposition technique. According to an experiment conducted by an inventor of this application, it is confirmed that the property of the Josephson effect can be maintained over a long period when the thickness d2 of the second layer 4 is set to 80 A, the sum of the thickness d3 and d4 of the third and fourth layers 5, 8 in the neighborhood of the thin film region 4a having the Josephson junction formed thereon is set to 200 to 2000 Å, and the thickness d5 of the covering layer 9 consisting of Cr which provides ordinary conducting material is set equal to 100 Å. On the contrary, an example of comparison wherein the covering layer 9 consists of niobium oxide and the thickness thereof is set to 100 Å exhibits a maintaining period of the characteristics of the Josephson effect shorter than that of the invention. Fig. 4 is a partial sectional view of a superconductive quantum interference device of a modification of the first embodiment in accordance with the invention, and Figs. 5(1) to 5(8) are sectional views showing a production process of the SQUID. A substrate 1 shown in Fig. 5(1) consists of quartz, sapphire, semiconductor such as Si, similarly to the substrate 1 shown in the first embodiment. A first layer 11 consisting of ordinary conducting material is formed on the substrate 1. The ordinary conducting material constituting the first layer 11 may be Cr or Ta. Alternatively, the first layer 11 may consist of superconducting material, such as Pb, having a low critical temperature lower than the critical temperature of a second layer 14 which forms a Josephson junction 14a; a thin film region to be described hereinafter. Thickness d10 of the first layer 11 is, for example, 100 to 200 Å. Subsequently, a thin layer 12 is formed entirely over the first layer 11 as shown in Fig. 5(2). The thin layer 12 may consist of ordinary conducting material, for example, metallic material such as Cr and Ta. Alternatively, the thin layer 12 may consist of nonmetallic material, or super-conducting material such as Nb. Thickness d11 of the thin layer 12 is, for example, 100 to 200 Å. After formation of the thin layer 12, the first layer 12 is process into the form of a long and narrow step by the use of etching or the like, whereby forming a base 12a as shown in Fig. 5(3). Fig. 5(4) is a plan view showing the substrate 11 on which the base 12a is formed. The length L1 of the base 2a is, for example, 1 to 5 µm. Thereafter, as shown in Fig. 5(5), a second layer 14 formed by deposition in an arrow direction 13. The arrow direction 13 is perpendicular to the surface of the substrate 1, i.e., the first layer 11. The second layer 14 consists of superconducting material, for example, Nb. Thickness d12 of the second layer 14 is, for example, 300 to 500 Å, and may be relatively large. Subsequently, as shown in Fig. 5(6), a third layer 15 is formed by deposition from an arrow direction 16. The third layer 15 consists of superconducting material Nb. An angle 11 made by the arrow direction 16 with respect to the surface of the second layer 14 is, for example, between 25 and 70 degrees, and preferably between 25 and 50 degrees. Thickness d13 of the third layer 15 is, for example, 1000 A. As shown in Fig. 5(7), a fourth layer 18 is formed by deposition from an arrow direction 17. The fourth layer 18 consists of superconducting material Nb, and the thickness d14 thereof is, for example, 500 Å. A sum of the thickness d13 and d14 is selected to be 200 to 2000 Å. An angle 2 made by the arrow direction 17 with respect to the surface of the second layer 14 is between 25 to 70 degrees, and preferably between 25 and 50 degrees. In this way, as shown in Figs. 5(6) and 5(7), the third and fourth layers 15, 18 are formed by the oblique deposition technique, so to speak, from the directions 16, 17 from opposing sides, whereby the second layer 14 is partially exposed, forming a thin film region 14a. As a consequence, the Josephson junction can be obtained. At the last stage, as shown in Fig. 5(8), a fifth layer 19 is formed entirely over the surfaces of the Josephson junction 14a, the third and fourth layers 15, 18 by the use of spatter technique. Thickness d15 of the fifth layer 19 is, for example, 100 to 200 Å, and consists of material similar to the first layer 11. More specifically, the fifth layer 19 consists of Cr, Ta, or other ordinary conducting material, or superconducting material such as Pd having a low critical temperature lower than the critical temperature of the superconducting material constituting the second layer 14. In this way, there can be obtained a configuration shown in Fig. 5(8), i.e., a configuration shown in Fig. 4. By conducting the oblique deposition from the two directions 16, 17 shown respectively in Figs. 5(6) and 5(7), it is made possible to fabricate the Josephson junction 14a with high accuracy, so that a width W1 thereof (see Fig. 5(7)) can be set to, for example, 1 µm. Further, as shown in Figs. 5(3) and 5(4), the Josephson junction 14a having a length equal to the length L2 of the base 12a can be fabricated with high accuracy. In this way, the Josephson junction 14a can be fabricated by controlling the width W1 and the length L2 thereof with high accuracy in a simple production process. Further, the width W1 of the Josephson junction 14a can be set to a desired value by changing the angles 11, 12 of the arrow directions 16, 17 with respect to the surface of the second layer 14 in the oblique deposition technique. More specifically, in the oblique deposition technique, the angles 11, 12 made by the arrow directions 16, 17 can be changed easily in a desired manner, and therefore adjustment of an optimum shape of the SQUID can be performed easily. In other words, since the oblique deposition technique is excellent in controllability for forming the Josephson junction 14a of the SQUID with accurate length L2 and width W1, thereby improving processability thereof. The Josephson junction 14a is of a sandwich structure having three layers of the first, second, and fifth layers 11, 14, 19. The invention has an excellent advantage that the oblique deposition technique can be applied to the Josephson device having such a structure. Specifically, this Josephson device provides a construction suitable for drawing advantageous features from the oblique deposition technique. Fig. 6 is a graph showing I-V characteristics of the superconductive quantum interference device shown in Figs. 4 and 5. In order to make smaller a maximum current obtainable in a superconductive state, i.e., a critical current I1, it is required to make the Josephson junction 14a thin. However, the SQUID is more subject to bad influence depending on a stained state thereof when the thickness thereof is made thinner. Moreover, it is very difficult to form such a thin second layer 14. In the foregoing embodiment, the second layer 14 is formed relatively thin, and sandwiched between the first and fifth layers 11, 19. Accordingly, the Josephson junction 14a looks substantially thick due to the proximity effect, thereby enabling the critical current I1 to decrease. Fig. 7 is a graph showing an experiment result obtained by the inventor of this application. A line 25 in Fig. 7 represents characteristics of the SQUID shown in Figs. 4 and 5. A line 26 represents characteristics of another embodiment of the invention in which the fifth layer 19 is omitted, and a Josephson junction 14a is formed on a first layer 11. A line 27 represents characteristics of an existing SQUID for comparison, in which a Josephson junction 54 is formed directly on a substrate 1 shown in Fig. 11, and a first and a fifth layers 11, 19 are omitted. It is generally preferable that the critical current I1 ranges from 10 to 120 µA. In order to attain such a relatively small critical current I1, as represented by the lines 25, 26, it will be understood that a rate of change in the critical current I1 is small even in the thickness d12 of the Josephson junction 14a varies greatly in a construction according to invention. Therefore, according to the invention, it is made possible to easily control the critical current I1 of the SQUID so as to make it smaller. Alternatively, the fifth layer 19 may be omitted from the construction. According to another experiment conducted by the inventor of this application, when the first and fifth layers 11, 19 consist of Ta and Cr, the second, third and fourth layers 14, 15, 18 consist of Nb of purity of 99.9 x, the thickness d10, d15 of the first and fifth layers 11, 19 is set to 100 Å, the thickness of the second layer 14 on which the Josephson junction 14a is formed is set to 100 Å, and the angles 11, 12 are selected to be between 25 and 50 degrees, it is confirmed that a defect rate at the time of production of the SQUID which can obtain a preferable critical current I1 of 10 to 100 µA is improved more than that of the prior arts in which the first and fifth layers 11, 19 as described above are not provided. When the critical current I1 is 100 µA, an output voltage thereof is about 10 µV. When the thickness d10, d11, d15 of the first, second, and fifth layers 11, 14, 19 is set to 200 Å with other conditions being equal to the foregoing ones, it is also confirmed that the defect rate is maintained at a preferable level. At this time, the critical current I1 is 120 µA, and the output voltage thereof is 9 µV. [Embodiment 2] Fig. 9 is a sectional view partially showing a superconductive quantum interference device of a second embodiment of the invention, and Figs. 10(1) to 10(5) are sectional views showing a production process of the superconductive quantum interference device. A substrate 1 consists of quartz, sapphire, and semiconducting material such as Si, similarly to the substrate 1 shown in the first embodiment. Such substrate 1 is prepared as shown in Fig. 10(1). Subsequently, a recessed channel 21 is formed in the substrate 1 as shown in Fig. 10(2). The recessed channel 21 has a bottom 22 and opposite side walls 23, 24. The side walls 23, 24 of the recessed channel 21 are positioned perpendicular to the bottom 22. The recessed channel 21 may be, for example, formed by the use of dry etching technique. Depth d21 of the channel 32 is, for example, 4000 to 5000 Å, width W2 thereof is, for example, 1 µm, and length L3 thereof in a direction vertical to the plane of the drawing in Fig. 10(2) may be, for example, 1 to 5 µm similarly to the lengths L1, L2 of the first and second embodiments. The dry etching technique may be, for example, the spatter technique by which a resist layer having a hole corresponding to the channel 21 is formed on the surface of the substrate 1, and argon ions Ar+ are rendered to crash in a plasma atmosphere. Thereafter, as shown in Fig. 10(3), superconducting material such as Nb is deposited on the recessed channel 21 from above as indicated by an arrow 33, i.e., a direction perpendicular to the bottom 22 of the channel 21, whereby forming a first layer 34. The first layer 34 is also formed on the surface of the substrate 1 besides the surface of the recessed channel 21. Thickness d22 of the first layer 34 is 80 to 200 Å, preferably 80 to 150 Å, and more preferably 100 Å. The superconducting material Nb is deposited on the first layer 34 from an arrow direction 36, whereby forming a second layer 35 as shown in Fig. 10(4). The deposition of the superconducting material Nb from the arrow direction 36 is interrupted by one side wall 23, and therefore the first layer 34 is formed on a part of the first layer 34 on the bottom 22 of the recessed channel 21. An angle 21 made by the arrow direction 36 with respect to the surface of the bottom 22 is, for example, between 25 and 70 degrees, and preferably between 25 and 50 degrees. Thickness d23 of the second layer 35 is, for example, 1000 Å. Thereafter, as shown in Fig. 10(5), superconducting material Nb is deposited on the surface of the first layer 34 from an arrow direction 37 so as to form a third layer 38. In this case, the deposition is interrupted by the other side wall 24, whereby the third layer 38 is formed on another part of the first layer 34 on the bottom 22 where the second layer 35 is not formed. Thickness d24 of the third layer 38 is, for example, 500 Å, and a sum of the thickness d23 and d24 is selected to be, for example, 200 to 2000 Å. An angle 22 made by the arrow direction 37 with respect to the surface of the bottom 22 is between 25 and 70 degrees, and preferably between 25 and 50 degrees. In this way, on the first layer 34 is formed a thin film region 34a; a partially exposed region formed by the second and third layers 35, 38, and thereby a Josephson junction can be obtained. In this manner, an oblique deposition is conducted from opposite sides of the recessed channel 21 as indicated by the arrow directions 36, 37 by using the opposite side walls 23, 24 of the recessed channel 21, whereby the thin film region 34a serving as a Josephson junction is formed. By changing the deposition angles 21, 22, the depth d21 of the recessed channel 21, and the thickness d23, d24 of the second and third layers 35, 38, the shape and characteristics of the Josephson junction can be freely controlled. This production process can be carried out entirely in the same vacuum, which in turn prevents any surface of the substrate 1 including the recessed channel 21 from being stained. Therefore, the production is easier and more convenient. According to further another experiment conducted by the inventor of this application, when a recessed portion 21 having the depth d21 of 4000 Å was formed on a Si substrate 1, Nb was used as superconducting material, and a superconductive quantum interference device was produced with setting the thickness d22, d23, d24 of a first, second, and third layers 34, 35, 38 to 100 Å, 1000 Å, 500 Å respectively, and deposition angles 21, 22 between 30 and 70 degrees, a critical current was 10 µA and an output voltage thereof was 10 µV, whereupon an appropriate characteristics could be obtained. Further, when the thickness d22 of the first layer 34 was set to 200 Å with other conditions being equal to the foregoing ones, the critical current was 18 µA and the output voltage thereof was 7 µV, whereupon an appropriate characteristics could be obtained again. It will be understood that the invention can be embodied not only in a superconductive quantum interference device, but also in other Josephson devices. The present embodiments are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.

Claims: Superconductive Josephson junction device, comprising: a substrate (1), having a stepped surface portion (2a,12a), a superconductive layer (4,5,8;14,15,18) of a superconductive material formed on the substrate including the stepped portion, said superconductive layer having a thin film region (4a,14a) which forms a Josephson junction, characterized by said superconductive layer (4,5,8;14,15,18) including a first superconducting thin film layer (4,14) formed on said substrate (1), a second superconducting thin film layer (5,15), formed on said first superconducting thin film layer (4,14), by using an oblique deposition technique such that said second superconducting thin film layer having an overhanging film edge does not cover said first superconducting thin film layer (4,14) in a region (4a,14a) beside said stepped portion (2a,12a), and a third superconducting thin film layer (8,18) formed on said first superconducting thin film layer (4,14) by using an oblique deposition technique from an opposite side, such that said third superconducting thin film layer (8,18) does not cover said first superconducting thin film layer (4,14) in said region (4a,14a) beside said stepped portion (2a,12a), said third superconducting thin film layer (8,18) being further formed on the second superconducting thin film layer (5,15) and the sidewall of the stepped portion (2a,12a), said third superconducting thin film layer (8,18) being thinner than said second superconducting thin film layer (5,15) and thicker than said first superconducting thin film layer (4,14), the sidewall of the third superconducting film layer (8,18) being closer to the stepped portion (2a,12a) where the sidewall of the third superconducting thin film layer (8,18) is closer to the substrate (1), and wherein said substrate (1) has a covering layer (9,19) consisting of ordinary conducting metal or semiconductor material on said thin film region (4a,14a) and a neighboring region thereof. Superconductive device according to claim 1, characterized in that a layer (11) consisting of ordinary conducting metal is formed between said substrate (1) consisting of semiconducting material and said first superconducting thin film layer (4,14). Superconductive device according to claim 1 or 2, characterized in that said superconducting material is niobium (Nb). Superconductive device of the type with a Josephson junction, comprising a substrate (1) having a recessed channel (21) with sidewalls (23,24), a bottom (22) of said channel (21) being in parallel with a surface of said substrate (1), and a layer (34,35,38) formed by superconducting material along a side of at least one of said sidewalls (23,24) on the bottom (22) of said recessed channel (21), characterized by at least one of said sidewalls (23,24) being perpendicular to a surface of said substrate (1) and by said superconductive layer (34,35,38) including a first superconducting thin film layer (34) formed on the bottom surface (22) of said recessed channel (21), a second superconducting thin film layer (35), formed on said first superconducting thin film layer (34), by using an oblique deposition technique such that said second superconducting thin film layer (35) having an overhanging film edge does not cover said first superconducting thin film layer (34) in a region (34a) beside said sidewalls (23,24), and a third superconducting thin film layer (38) formed on said first superconducting thin film layer (34), by using an oblique deposition technique from an opposite side, such that said third superconducting thin film layer (38) does not cover said first superconducting thin film layer (34) in said region (34a) beside said sidewalls (23,24), said third superconducting thin film layer (38) being further formed on the second superconducting thin film layer (35) and a sidewall (23) of the recessed channel (21), said third superconducting thin film layer (38) being thinner than said second superconducting thin film layer (35) and thicker than said first superconducting thin film layer (34), the sidewalls of the third superconducting thin film layer (38) at the side of the sidewall (24) of the recessed channel (21) being closer to the opposite sidewall (23) of the recessed channel (21) where the sidewall of the third superconducting thin film layer (38) is closer to the bottom (22) of the recessed channel (21). Superconductive device according to claim 4, characterized in that said superconducting material is niobium (Nb).

Inventors: FUJITA SATOSHI, TAMURA ITSURO, WADA MASAO, FUJITA, SATOSHI, TAMURA, ITSURO, WADA, MASAO
Applicants: OSAKA GAS CO LTD, OSAKA GAS CO., LTD.
Application year: 1991
Main classification: H01L39
IPC codes: H01L39