Josephson device and fabrication process thereof

Details Josephson device and fabrication process thereof Josephson device and fabrication process thereof

2003-03-18

2004-09-14

Niebling, John F. (Department: 2812)

Semiconductor device manufacturing: process

Having superconductive component

C257S031000, C257S032000, C257S033000, C257S036000, C029S599000

Reexamination Certificate

active

06790675

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is based on Japanese priority application No.2002-086896 filed on Mar. 26, 2002, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention relates to fabrication method of Josephson devices that use a superconductor, and more particularly to the method of fabricating a Josephson device that uses an oxide superconductor while minimizing the variation of operational characteristics of the Josephson junction formed therein.
A superconductor has a unique property characterized by: 1) zero electric resistance; 2) complete diamagnetism; 3) Josephson effect, and application thereof is expected in various fields including electric power transfer, electric power generation, confinement of nuclear fusion plasma, magnetically levitated trains, magnetic shield, high-speed computers, and the like.
In 1986, Bednorz and Mueller discovered a copper oxide superconductor (La
1-x
Ba
x
)
2
CuO
4
that has a very high superconducting transition temperature Tc of about 30K. Thereafter, numerous reports followed, reporting observation of superconducting transition occurring at high temperatures particularly in the systems of YBa
2
Cu
3
O
7-y
(Tc=90K), Bi
2
Sr
2
Ca
2
Cu
3
O
y
(Tc=110K) Tl
2
Ba
2
Ca
2
Cu
3
Oy (Tc=125K), HgBa
2
Ca
2
Cu
3
Oy (Tc=135K), and the like. Currently, investigations are being made on the method of producing these materials as well as the properties and applications of these materials.
Particularly, the superconductor of YBa
2
Cu
3
Oy is thought as the most promising material in relation to the application to electron devices or conductor wires, in view of the preferable nature of the material such as absence of toxic elements such as Tl or Hg and small anisotropy of superconductivity.
In order to use Josephson effect in electron devices, on the other hand, it is necessary to establish the process of forming a Josephson junction by using a thin-film technology.
Generally, a Josephson junction constituting the essential part of a Josephson device is formed by a physical vapor deposition process in which a source material is dispersed into a gaseous phase by exciting the source material in a vacuum vessel, such as sputtering process; laser ablation process; vacuum evaporation deposition process; molecular beam epitaxy, and the like. Further, various structures are proposed for the Josephson devices that use a copper oxide superconductor, such as bicrystal structure, biepitaxial structure, step-edge structure, ramp-edge structure, laminated structure, and the like (S. Takada, Oyo-Buturi, 62, pp.443, 1993). Particularly, the Josephson device of the ramp-edge structure is thought most promising in view of the fact that a large driving power is achieved at the time of switching and that the critical current can be changed by controlling the thickness of the tunneling barrier layer (M. Hidaka, et al., Oyo-Buturi, 67, pp.1167, 1998).
An Ic.Rn product is an index representing the performance of a Josephson junction. Larger the Ic.Rn product, the better the operational speed of the Josephson junction. An Ic.Rn product is defined as a product of a critical current Ic and a resistivity Rn, wherein the critical current Ic is the maximum current possible in the superconducting state of a superconductor at a certain temperature, while the resistivity Rn is the resistivity for the case the superconducting state is lost and the superconductor has returned to a normal conducting state.
It should be noted that the Ic.Rn product is normalized by the size of the Josephson junction. Qualitatively, the Ic.Rn product represents the magnitude of the signal achieved at the time of the switching of the Josephson junction. By using the Josephson device of the ramp-edge structure, an Ic.Rn product larger than other device structures is obtained. Typically, YBa
2
Cu
2
Oy is used for the upper and lower superconducting electrodes. On the other hand, investigations are made also on the Josephson devices of the laminated structure, which is thought advantageous for formation of future large-scale integrated circuits. In both cases, the Josephson junction uses any of a PrBa
2
Cu
3
Oy layer, an Nb doped SrTiO
3
layer or a damaged layer formed at the time of processing, for the junction barrier.
Recently, active investigation is being made on the certain type of Josephson devices that use a damaged layer formed at the time of processing, for the formation of the Josephson junction. This type of Josephson junction is called IEJ (Interface-Engineered, Junction). Reference should be made to B. H. Moeckly, et al., Appl. Phys. Lett.71, pp.2526, 1997). In the Josephson device having the ramp-edge structure, formation of a very thin layer having a thickness of 1-2 nm is confirmed by transmission electron microscopy. It is believed that this very thin layer functions as the Josephson junction, while detailed mechanism thereof is not yet understood (J. G. Wen et al. “Advances in Superconductivity XII”-Proc. ISS '99, 10/17-19, pp. 984, 1999 in Morioka, Y. Soutome, et al., “Advances in Superconductivity XII”-Proc. ISS '99, 10/17-19, pp.990, 1999, in Morioka).
In the foregoing IEJ Josephson devices, there can occur short-circuit in the Josephson junction at various locations in the case the process parameter at the time of formation of the Josephson junction is not appropriate. For example, the thickness of the damaged layer is so thin that control of thickness of the damaged layer is difficult.
It should be noted that the I-V characteristics of a Josephson device changes depending on the thickness of the junction layer therein. When the thickness of the Josephson junction is too large, a superconducting current cannot flow through the junction. When the thickness is appropriate, the superconducting current can flow through the junction by way of tunneling, without causing voltage difference across the junction, provided that the superconducting current is within the predetermined critical current Ic. When the magnitude of the current has exceeded this critical current Ic, there suddenly appears a voltage across the junction. In the state there is caused such a voltage, the I-V characteristic of the Josephson device approaches a straight line that crosses the origin. The I-V characteristics pertinent to such a Josephson device are called RSJ (Resistivity Shunted Junction) characteristics.
In the case there is a short-circuit in the Josephson junction due to the excessively small thickness of the barrier layer, on the other hand, there gradually appears a voltage across the junction when the current has exceeded the critical current Ic. In this case, the voltage is induced as a result of movement of the magnetic flux, and thus, the foregoing characteristics are called FF (Flux Flow) type I-V characteristics.
In order to realize a superconducting electron device that uses a Josephson device, it is necessary to produce a large number of Josephson junctions and devices having the foregoing RSJ characteristics and having a suitable critical current Ic. Further, the Josephson device is required to have a suitable Ic.Rn product. Particularly, the quantity Ic is sensitive to the junction structure or fabrication process, and thus, it is very important to establish the technology of suppressing the variation of the critical current Ic.
In order to achieve operation of a Josephson integrated circuit including therein 100 or more Josephson junctions, there is an estimate that the variance 1&sgr; of the junction characteristics has to be suppressed to 10% or less (J. Talvacchio, et al., IEEE Trans. Appl. Supercond.7, pp.2051, 1997) for the Josephson junctions included in the circuit.
Recently, there has been a report announcing the success of achieving the 1&sgr; value satisfying the foregoing requirement for an IEJ ramp-edge Josephson device.
More specifically, Satoh et al. achieved the variance 1&sgr; of 8% (1&sgr;=8%) over 100 Josephson junctions by using YBa
2
Cu
3

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