Superconducting field-effect transitor having an extremly thin superconducting channel formed of oxide superconductor material

Abstract: A method for patterning an oxide superconductor thin film, comprising a step of forming a SiO₂ layer on the oxide superconductor thin film, patterning the SiO₂ layer so as to form the same pattern as that of the oxide superconductor thin film which will be patterned, etching the oxide superconductor thin film by using the patterned SiO₂ layer as a mask, and removing the SiO₂ layer by using a weak HF solution, a buffer solution including HF or a mixture including HF

Description: Background of the Invention Field of the invention The present invention relates to a superconducting field-effect transistor wherein source and drain regions and an extremly thin channel are formed of an oxide superconductor material. Description of related art Devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconducting material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature. Josephson device is one of well-known superconducting devices. However, since Josephson device is a two-terminal device, a logic gate which utilizes Josephson devices becomes complicated. Therefore, three-terminal superconducting devices are more practical. Typical three-terminal superconducting devices include two types of super-FET (field effect transistor). The first type of the super-FET includes a semiconductor channel, and a superconductor source electrode and a superconductor drain electrode which are formed closely to each other on both side of the semiconductor channel. A portion of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode has a greatly recessed or undercut rear surface so as to have a reduced thickness. In addition, a gate electrode is formed through a gate insulating layer on the portion of the recessed or undercut rear surface of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode. A superconducting current flows through the semiconductor layer (channel) between the superconductor source electrode and the superconductor drain electrode due to a superconducting proximity effect, and is controlled by an applied gate voltage. This type of the super-FET operates at a higher speed with a low power consumption. The second type of the super-FET includes a channel of a superconductor formed between a source electrode and a drain electrode, so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channel. Both of the super-FETs mentioned above are voltage controlled devices which are capable of isolating output signal from input one and of having a well defined gain. However, since the first type of the super-FET utilizes the superconducting proximity effect, the superconductor source electrode and the superconductor drain electrode have to be positioned within a distance of a few times the coherence length of the superconductor materials of the superconductor source electrode and the superconductor drain electrode. In particular, since an oxide superconductor has a short coherence length, a distance between the superconductor source electrode and the superconductor drain electrode has to be made less than about a few ten nanometers, if the superconductor source electrode and the superconductor drain electrode are formed of the oxide superconductor material. However, it is very difficult to conduct a fine processing such as a fine pattern etching, so as to satisfy the very short separation distance mentioned above. On the other hand, the super-FET having the superconducting channel has a large current capability, and the fine processing which is required to product the first type of the super-FET is not needed to product this type of super-FET. In order to obtain a complete ON/OFF operation, both of the superconducting channel and the gate insulating layer should have an extremely thin thickness. For example, the superconducting channel formed of anoxide superconductor material should have a thickness of less than five nanometers and the gate insulating layer should have a thickness more than ten nanometers which is sufficient to prevent a tunnel current. In the super-FET, since the extremely thin superconducting channel is connected to the relatively thick superconducting source region and the superconducting drain region at their lower portions, the superconducting current flows substantially horizontally through the superconducting channel and substantially vertically in the superconducting source region and the superconducting drain region. Since the oxide superconductor has the largest critical current density J c in the direction perpendicular to c -axes of its crystal lattices, the superconducting channel is preferably formed of a c -axis oriented oxide superconductor thin film and the superconducting source region and the superconducting drain region are preferably formed of a -axis oriented oxide superconductor thin films. European Patent Application N° A 1 -0 545 801 describing the state of the art in accordance with Article 54(3) EPC discloses a superconducting device comprising a substrate, a non superconducting oxide layer of PrBa 2 Cu 3 O 7-x , a superconducting channel and a gate electrode of a material which includes silicon, in which the gate electrode is embedded between superconducting source and drain regions and is isolated therefrom by an insulating region formed by diffused silicon from the gate electrode. In a prior art, in order to manufacture the super-FET which has the superconducting channel of c -axis oriented oxide superconductor thin film and the superconducting source region and the superconducting drain region of a -axis oriented oxide superconductor thin films, a c -axis oriented oxide superconductor thin film is formed at first and the c -axis oriented oxide superconductor thin film is etched and removed excluding a portion which will be the superconducting channel. Then, an a -axis oriented oxide superconductor thin film is deposited so as to form the superconducting source region and the superconducting drain region. In another prior art, at first an a -axis oriented oxide superconductor thin film is deposited and etched so as to form the superconducting source region and the superconducting drain region, and then a c -axis oriented oxide superconductor thin film is deposited so as to form the superconducting channel. In the above methods, the oxide superconductor thin film is mostly processed by photolithography. Namely, the oxide superconductor thin film is masked by a photoresist and etched by a wet etching process using a weak H 3 PO 4 solution, or a dry etching process such as a reactive ion etching or an ion-milling using Ar ions. In order to process the oxide superconductor thin film without degradation, the oxide superconductor thin film should be prevented from contacting with water. Since the oxide superconductor has high reactivity so as to react with water and is degraded. Therefore, these etching process use little water. However, an oxide superconductor also reacts with photoresist remover so that a surface of the oxide superconductor thin film on which a photoresist is formed and removed is roughened. It is very difficult to deposit another thin film or layer on the rough surface of the oxide superconductor thin film so as to manufacture a superconducting device or a superconducting circuit of a multi-layer structure. In addition, if another oxide superconductor thin film is formed so as to contact the rough surface, an undesirable Josephson junction or a resistance is generated at the interface. Furthermore, superconducting characteristics of the reacted oxide superconductor thin film is affected, so that the superconducting device does not have an enough performance. Summary of the Invention Accordingly, it is an object of the present invention to provide a FET type superconducting device having a superconducting region constituted of an extremely thin oxide superconductor film, which have overcome the above mentioned defects of the conventional ones. According to the present invention, there is provided a superconducting device comprising a substrate (5) having a principal surface; a non-superconducting oxide layer (20) having a crystal structure similar to that of an oxide superconductor on the principal surface of the substrate (5); a superconducting channel (10) formed of a c -axis oriented oxide superconductor thin film (1) on the non-superconducting oxide layer (20); a superconducting source region (2) and a superconducting drain region (3) far thicker than the superconducting channel (10), formed of the same oxide superconductor as the one of the superconducting channel (10) but a -axis oriented, at both ends of the superconducting channel (10) separated from each other but electrically connected through the superconducting channel, so that superconducting current can flow through the superconducting channel (10) between the superconducting source region (2) and the superconducting drain region (3); and a gate electrode (4) of a material including silicon above a gate insulating layer (7) on the superconducting channel (10) for controlling the superconducting current flowing through the superconducting channel, which gate electrode is embedded between the superconducting source region (2) and the superconducting drain region (3) and isolated from the superconducting source region (2) and the superconducting drain region (3) by an insulating region (50) formed by diffused silicon from the gate electrode (4) and a SiO 2 layer (40) located between the gate electrode (4) and the superconducting source region (2) and between the gate electrode (4) and the superconducting drain region (3). In the superconducting device in accordance with the present invention, superconducting current flows along the insulating region which is formed by diffused silicon and has a smooth profile next to the superconducting source region and the superconducting drain region, the superconducting current efficiently flows into and flows from the superconducting channel. Therefore, superconducting current flow into or from the superconducting channel efficiently so that the current capability of the super-FET can be improved. The gate electrode is preferably formed of polycrystalline silicon, single crystalline silicon or silicide of a metal. In the superconducting device in accordance with the present invention, the non-superconducting oxide layer preferably has a similar crystal structure to that of a c -axis oriented oxide superconductor thin film. In this case, the superconducting channel of a c -axis oriented oxide superconductor thin film can be easily formed on the non-superconducting oxide layer. Preferably, the above non-superconducting oxide layers is formed of a Pr 1 Ba 2 Cu 3 O 7-ε oxide. A c -axis oriented Pr 1 Ba 2 Cu 3 O 7-ε thin film has almost the same crystal lattice structure as that of a c -axis oriented oxide superconductor thin film. It compensates an oxide superconductor thin film for its crystalline incompleteness at the bottom surface. Therefore, a c -axis oriented oxide superconductor thin film of high crystallinity can be easily formed on the c -axis oriented Pr 1 Ba 2 Cu 3 O 7-ε thin film. In addition, the effect of diffusion of the constituent elements of Pr 1 Ba 2 Cu 3 O 7-ε into the oxide superconductor thin film is negligible and it also prevents the diffusion from substrate. Thus, the oxide superconductor thin film deposited on the Pr 1 Ba 2 Cu 3 O 7-ε thin film has a high quality. In a preferred embodiment, the oxide superconductor is formed of high-T c (high critical temperature) oxide superconductor, particularly, formed of a high-T c copper-oxide type compound oxide superconductor for example a Y-Ba-Cu-O compound oxide superconductor material, a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide superconductor material. In addition, the substrate can be formed of,an insulating substrate, preferably an oxide single crystalline substrate such as MgO, SrTiO 3 , CdNdAlO 4 , etc. These substrate materials are very effective in forming or growing a crystalline film having a high degree of crystalline orientation. However, the superconducting device can be formed on a semiconductor substrate if an appropriate buffer layer is deposited thereon. For example, the buffer layer on the semiconductor substrate can be formed of a double-layer coating formed of a MgAlO 4 layer and a BaTiO 3 layer if silicon is used as a substrate. Preferably, the superconducting channel is formed of a c -axis oriented oxide superconductor thin film and the superconducting source electrode and the superconducting drain electrode are formed of a -axis oriented oxide superconductor thin films. The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. Brief Description of the Drawings Figures 1A to 1J are diagrammatic sectional views for illustrating an embodiment of the method in accordance with the present invention for manufacturing the super-FET. Description of a preferred embodiment. Referring to Figures 1A to 1J, the process in accordance with the present invention for manufacturing the super-FET will be described. As shown in Figure 1A, a MgO (100) single crystalline substrate 5 having a substantially planar principal surface ((100) surface) is prepared. As shown in Figure 1B, an oxide layer 20 having a thickness of 50 nanometers composed of a Pr 1 Ba 2 Cu 3 O 7-ε thin film is deposited on the principal surface of the substrate 5, by an MBE. While the Pr 1 Ba 2 Cu 3 O 7-ε thin film is growing, the surface morphology of the Pr 1 Ba 2 Cu 3 O 7-ε thin film is monitored by RHEED. A condition of forming the Pr 1 Ba 2 Cu 3 O 7-ε oxide thin film by MBE is as follows: Molecular beam source Pr: 1225°C Ba: 600°C Cu: 1040°C Pressure 1.333 x 10 -3 Pa (1 × 10 -5 Torr) Temperature of the substrate 750°C Then, the Pr molecular beam source is exchanged to a Y molecular beam source and the temperature of the substrate is lowered to 700 °C so that a c -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 having a thickness of about 5 nanometer is continuously formed on the oxide layer 20 of Pr 1 Ba 2 Cu 3 O 7-ε thin film, as shown in Figure 1C. A condition of forming the c -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 by MBE is as follows: Molecular beam source Y: 1250°C Ba: 600°C Cu: 1040°C Pressure 1.333 × 10 -3 Pa (1 × 10 -5 Torr) Temperature of the substrate 700°C Then, as shown in Figure 1D, an insulating layer 17 of SrTiO 3 having a thickness of 10 to 20 nanometers is formed on the c -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 by a sputtering. A polycrystalline silicon layer 14 having a thickness of 200 nanometers is formed on the insulating layer 17 by CVD, as shown in Figure 1E. Thereafter, the polycrystalline silicon layer 14 is etched by a reactive ion etching so as to form a gate electrode 4, as shown in Figure 1F. Then, the surfaces of the gate electrode 4 is oxidized so as to form a SiO 2 layer having a thickness of 50 to 100 nanometers, as shown in Figure 1G. Thereafter, as shown in Figure 1H, the insulating layer 17 of SrTiO 3 is etched so as to form a gate insulating layer 7 by using a mixture of HF and NH 4 OH. The mixture of HF and NH 4 OH selectively etched the insulating layer 17 of SrTiO 3 and does not affect the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1. Therefore, the characteristics of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 is maintained. A portion of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 under the gate insulating layer 7 becomes a superconducting channel. Thereafter, the substrate 5 is heated to a temperature of 350 to 400 °C under a pressure lower than 1.333 × 10 -7 Pa (1 × 10 -9 Torr) so as to clean the exposed surface of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1. This heat treatment is not necessary, if the exposed surface of the c-axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 is clean enough. Then, a a -axis oriented Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 having a thickness of 500 nanometers is deposited on the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 1 by an off-axis sputtering so as to encapsulate the gate electrode 4, as shown in Figure 1I. A condition of forming the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 by an off-axis sputtering is as follows: While the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 is deposited, silicon diffuses from the gate electrode 4 so as to form a insulating region 50 around the gate electrode 4. The insulating region 50 is formed of a Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor which does not show superconductivity by the diffused silicon. Finally, in order to planarize an upper surface of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11, a photoresist layer (not shown) is coated on the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 in such a manner that the deposited photoresist layer has a flat upper surface, and then, the coated photoresist layer and the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 are etched back, until the upper surface of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 is planarized and the gate electrode 4 is exposed at the planarized upper surface of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11,as shown in Figure 1J. Portions of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 11 at the both sides of the gate electrode 4 become a superconducting source region 2 and a superconducting drain region 3. Metal electrodes may be formed on the superconducting source region 2 and the superconducting drain region 3, if necessary. With this, the super-FET in accordance with the present invention is completed. The superconducting channel of the above mentioned super-FET manufactured in accordance with the embodiment of the method of the present invention is formed on an oxide layer which has similar crystalline structure to that of the oxide superconductor. Therefore, the bottom portion of the superconducting channel is not degraded so that the substantial cross-sectional area of the superconducting channel of the super-FET is larger than that of a conventional super-FET. Additionally, since superconducting current flows along the insulating region which is formed by diffused silicon next to the superconducting source region and the superconducting drain region, the superconducting current efficiently flows into and flows from the superconducting channel. By all of these, the current capability of the super-FET can be improved. Furthermore, according to the present invention, the oxide layer, the superconducting channel, the gate insulating layer and the gate electrode are self-aligned. The insulating region 50 which isolates the gate electrode from the superconducting source region and the superconducting drain region is also automatically positioned. Therefore, the limitation in the fine processing technique required for manufacturing the super-FET is relaxed. Additionally, according to the present invention, the gate insulating layer is formed by an etching process using a mixture of HF and NH 4 OH. The mixture of HF and NH 4 OH selectively etched the insulating layer of SrTiO 3 on the oxide superconductor thin film which will constitutes the superconducting channel and does not affect the oxide superconductor thin film. Therefore, the superconducting characteristics of the oxide superconductor thin film is maintained. In the above mentioned embodiment, the oxide superconductor thin film can be formed of not only the Y-Ba-Ca-O compound oxide superconductor material, but also a high-T c (high critical temperature) oxide superconductor material, particularly a high-T c copper-oxide type compound oxide superconductor material, for example a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide superconductor material. The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but converts and modifications may be made within the scope of the appended claims.

Claims: A superconducting device comprising a substrate (5) having a principal surface; a non-superconducting oxide layer (20) having a crystal structure similar to that of an oxide superconductor on the principal surface of the substrate (5); a superconducting channel (10) formed of a c -axis oriented oxide superconductor thin film (1) on the non-superconducting oxide layer (20); a superconducting source region (2) and a superconducting drain region (3) far thicker than the superconducting channel (10), formed of the same oxide superconductor as the one of the superconducting channel (10) but a -axis oriented, at both ends of the superconducting channel (10) separated from each other but electrically connected through the superconducting channel, so that superconducting current can flow through the superconducting channel (10) between the superconducting source region (2) and the superconducting drain region (3); and a gate electrode (4) of a material including silicon above a gate insulating layer (7) on the superconducting channel (10) for controlling the superconducting current flowing through the superconducting channel, which gate electrode is embedded between the superconducting source region (2) and the superconducting drain region (3) and isolated from the superconducting source region (2) and the superconducting drain region (3) by an insulating region (50) formed by diffused silicon from the gate electrode (4) and a SiO 2 layer (40) located between the gate electrode (4) and the superconducting source region (2) and between the gate electrode (4) and the superconducting drain region (3). A superconducting device as claimed in Claim 1, characterized in that the non-superconducting oxide layer (20) has a similar crystal structure to that of a c -axis oriented oxide superconductor thin film. A superconducting device claimed in Claim 1 or 2, characterized in that the non-superconducting oxide layer (20) is formed of a Pr 1 Ba 2 Cu 3 O 7-ε thin film. A superconducting device claimed in anyone of Claims 1 to 3, characterized in that the gate electrode (4) is formed of polycrystalline silicon, single crystalline silicon or a silicide of a metal. A superconducting device claimed in anyone of Claims 1 to 4, characterized in that the oxide superconductor is formed of high-'Γ c (high critical temperature) oxide superconductor. A superconducting device claimed in Claim 5, characterized in that the oxide superconductor is formed of a high-T c copper-oxide type compound oxide superconductor. A superconducting device claimed in Claim 6, characterized in that the oxide superconductor is formed of oxide superconductor material selected from the group consisting of a Y-Ba-Cu-O compound oxide superconductor material, a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide superconductor material. A superconducting device claimed in anyone of Claims 1 to 7, characterized in that the substrate (5) is formed of a material selected from the group consisting of a MgO (100) substrate, a SrTiO 3 (100) substrate and a CdNdAlO 4 (001) substrate, and a semiconductor substrate. A superconducting device claimed in Claim 8, characterized in that the substrate (5) is formed of a silicon substrate and a principal surface of the silicon substrate is coated with an insulating material layer which is formed of a MgAl 2 O 4 layer and a BaTiO 3 layer.

Inventors: IIYAMA MICHITOMO, INADA HIROSHI, TANAKA SO, IIYAMA, MICHITOMO, INADA, HIROSHI, TANAKA, SO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: H01L39
IPC codes: H01L39