Patent Application
20080146449
The present invention relates to an electrical device and a method of manufacturing the same; particularly but not exclusively the invention relates to a superconductor device or a magnetic circuit device and methods of making the same.
Superconductivity is commonly known as the complete loss of electrical resistance of a material at a well defined temperature. The transition temperature below which a material begins to demonstrate superconductivity is commonly known as the superconducting critical temperature Tc and is usually of the order of a few degrees Kelvin.
An example of a device relying on superconductivity is a Superconducting Quantum Interference Device (SQUID). A SQUID is generally seen as a magnetic flux to voltage transducer characterized by its function transfer dV/dφ (V is the voltage across the SQUID and φ is the magnetic flux through the loop). A SQUID can be used as a sensor of magnetic flux, current, voltage or energy, in a broad range of applications including susceptometry, voltmetry, non-destructive evaluation, nuclear magnetic resonance, geophysics and bio magnetism. Currently, SQUIDs made of superconducting metals or alloys are the most widely developed superconducting devices.
Nb/Al2O3/Nb trilayer junction technology is currently used for most applications. Such SQUIDs have achieved impressive sensitivity (a few fT/Hz−1/2). However, the very low transition temperature Tc of superconducting metals and alloys make them inappropriate for many applications.
The discovery of superconductivity in metal oxides, such as Lanthanum-based oxides, by J. G Bednorz and K. A Mueller in 1986 resulted in a major improvement in the superconducting transition temperature. It was followed by the discovery of a superconducting compound (YBa2Cu3O6+x), where 0≦x≦1 which demonstrates superconductivity above 77 K, the boiling temperature of liquid nitrogen. Since the critical or transition temperatures Tc of these new compounds are much greater than the Tc of superconducting metals and alloys, they are generally referred as High Tc superconductors (HTSc) and belong to a family referred to as “oxide superconductors”. A majority of them are copper oxides, their main characteristics being the presence of CuO2 layers which provide most of their electronic properties.
This major improvement in the transition temperature Tc of superconductors resulted in further development of superconductor applications operating at temperatures that could be obtained easily by means of a cryo-cooler or liquid nitrogen. In particular, there has been intensive effort to make SQUIDs operable at such temperatures.
A Josephson Junction is a weak connection between two superconductors. Josephson Junctions can be used to make a range of devices. Single Josephson Junctions can be used as photon detectors; arrays of Josephson Junctions in series can be used to build voltage standards; complex arrangements of Josephson Junctions can provide logical devices known as Rapid Single Flux Quantum (RSFQ) devices, comparable to semiconductor arrays of transistors, with four orders of magnitude less power consumption and a hundred times more rapid. A DC SQUID consists of two Josephson junctions connected in parallel on a superconducting loop.
Given the short characteristic length scale of a few nanometers in HTSc materials, making Josephson junctions for superconductor devices based on these materials on a scale comparable thereto can be rather challenging.
Efforts have been invested in the development of Josephson junctions with artificial barriers. Most high Tc SQUIDs are made with bicrystal grain boundary junctions which are fabricated by epitaxial growth of a high Tc thin film on a bicrystal substrate with a given misorientation angle. Although these junctions have yielded good performance, reproducibility from junction to junction is poor, due to difficulties in controlling grain boundary characteristics. The variability in the bicrystal substrates also increases the spread of junctions' parameters from chip to chip. In addition, the long-term stability of these devices is not guaranteed, due to oxygen diffusion along the grain boundary. Moreover, they are serious design constraints since the junctions have to be aligned along the grain boundary. It is therefore difficult to make arrays or more complex structures including a great number of SQUIDs. The cost of the bicrystal substrates is an obstacle for mass production of HTSc SQUIDs.
U.S. Pat. No. 5,026,682, incorporated herein by reference, describes a method of making a SQUID using high Tc superconductors. A superconducting loop having superconducting weak links is formed to comprise the SQUID device. The superconducting weak links are formed of the same superconductive material as the loop but have a narrower current path. This is a major issue: the width of the narrow region has to be of the order of the coherence length, i.e. 1 to 2 nm for HTSC. These weak links are difficult to form on complex material and thus unstable. A major drawback of the SQUIDs described in this document is that the devices have low sensitivity and do not demonstrate controllable and reproducible properties.
A first aspect of the invention provides a method of making a superconductor device, the method comprising forming, in a vacuum, a layer of superconductive material; forming, in situ, a mask over part of the layer of superconductive material; irradiating the layer of superconductive material through the mask with ions such that a first portion having superconductive properties and a second portion having non superconductive properties are formed in the layer of superconductive material, the mask overlying the first portion.
A second aspect of the invention provides a method of making a superconductor device having at least one Josephson Junction, comprising the steps of forming a layer of superconductive material, forming a first mask over part of the layer of superconductive material, irradiating the layer of superconductive material through the first mask with first ions such that a first portion having superconductive properties and a second portion having electrical insulating properties are formed in the layer of superconductive material, the first mask overlying the first portion, forming a second mask over a part of the first portion of the superconductive layer, defining a slit in the second layer of masking material, and irradiating the layer of superconductive material through the second mask with second ions to disorder atoms in a portion of the layer of superconductive material underlying the slit such that the critical superconducting temperature of the part of the first portion of layer of superconductive material exposed through the slit is lowered relative to the critical superconducting temperature of a part of the first portion of the superconductive layer protected by the second mask.
A third aspect of the invention provides a superconductor device comprising a layer of superconductive material having at least one first region formed therein exhibiting superconductive properties and at least one second region formed therein exhibiting non superconductive properties or electrical insulating properties relative to the first region, at least one connector for passing a superconducting electrical current through the at least one first region, and at least one junction formed within the at least one first region, the junction having a lowered transition temperature relative to the transition temperature of the first region.
A fourth aspect of the invention provides a superconducting quantum interference device (SQUID) comprising a layer of superconductive material having at first region therein forming a loop exhibiting superconductive properties and a second region surrounding the loop exhibiting electrical insulating properties relative to the first region; at least one connector for passing a superconducting electrical current through the first region; and at least one Josephson junction formed within the loop, the junction Josephson having a lowered critical superconducting temperature relative to the critical superconducting temperature of the first region.
A fifth aspect of the invention provides a method of making a magnetic circuit device, the method comprising: forming a layer of manganite material; forming a mask over part of the layer of manganite material; and irradiating the layer of manganite material through the mask with ions such that a portion of the layer of manganite material not underlying the mask has its conductive properties altered by the ions.
A sixth aspect of the invention provides a magnetic circuit device comprising: a layer of manganite material having at least one first region formed therein exhibiting electrical conductive properties and at least one second region formed therein exhibiting electrical insulating properties relative to the first region; at least one connector for passing an electrical current through the at least one first region; and at least one junction formed within the at least one first region, the junction having a higher resitivity relative to the resistivity of the first region.
Embodiments of the invention will be described, by way of example only, with reference to the following drawings in which:
A method of making a SQUID according to a first embodiment will now be described with reference to
In the same vacuum chamber, without breaking the vacuum, i.e. in situ, a gold layer 13 having a thickness in a range for from 100 nm to 500 nm, for example approximately 250 nm is deposited on the YBCO film 12 such that it covers the top surface of the YBCO film 12. The presence of the gold layer 13 protects the SQUID during the process of manufacture. Moreover, applying the gold layer in situ ensures a good electrical contact between the gold layer 13 and the YBCO film 12 resulting in both reproducible characteristics and low contact resistances to the resulting superconductor device through its contact pads, leading to a low noise device.
A layer of polymethylmethacrylate (PMMA) resist 14 is then deposited on the layer of gold 13. The thickness of the photo resist layer may be a range of from 500 nm to 1000 nm, for example, 800 nm. Electronic lithography is then used to pattern the desired SQUID geometry. The SQUID geometry of the present embodiment corresponds to the geometry illustrated in
The YBCO film 12 is then irradiated with high energy ions through the gold mask 20. 100 keV oxygen ions at a high fluence F of approximately 5×1015 at/cm2 may be used. The gold mask 20 prevents the implantation of the ions in regions of the YBCO film 12 corresponding to the SQUID geometry i.e. in regions of the YBCO film 12 underlying the gold mask 20. The atomic disorder induced by ion irradiation in the regions of the YBCO film 12 which are unprotected by the gold mask 20 lowers the transition temperature of the superconductive material 12 driving the oxide superconductors in the exposed regions towards a non superconducting and to an electrical insulating state. Although in this embodiment oxygen ions at an energy of 100 keV are used, in alternative embodiments different type of ions may be used. For example, in some embodiments, He, Ne, Cu, Ar, Xe or Kr ions may be used. The energy of irradiation may be adjusted to the nature of ions used in order to give the desired amount of defects in the unprotected region of the YBCO film 12. For example, ion energies ranging from 10 keV to 1 MeV may be used. The thickness of the gold layer 13 can be adjusted to be greater than the maximum penetration depth of ions with a given energy. After irradiation, the gold mask 20 is removed by a suitable technique such as chemical wet etching or by ion beam etching through a suitable resist mask, leaving the contact pads. Since no superconducting material is removed during the process, oxygen diffusion out of the resulting superconducting device is prevented thereby ensuring long term stability and cycling.
Since the resulting structure is a planar structure, no superconducting matter is removed during the process thereby preventing oxygen diffusion out of the resulting SQUID ensuring its long-term stability and cycling.
A layer of PMMA photo resist 15 is deposited on the device 30 and two slits 38A and 38B, each approximately 20 nm wide, are defined in the photoresist 15 by electronic lithography across opposing arms 36A and 36B. The structure is than irradiated with 100 keV oxygen atoms with a typical fluence F of a few 1013 at/cm2 e.g. 6. 1013 at/cm2. Ion mass and energy, and photo resist thickness can be chosen such that the ions can be stopped by the photoresist layer thereby protecting the superconducting layer below.
The atomic disorder induced by ion irradiation drives superconductors oxide in the region 40 under the slit towards a non superconducting state thereby lowering the local superconducting transition temperature Tc in the region to a temperature Tc, and increasing the resistivity of the region 40 in a controllable and reproducible manner.
In this way, a superconducting-normal-superconducting junction 40 at temperatures between Tc′ and Tc is formed in the regions under the slits 38A. In this range of temperature, a clear Josephson coupling occurs at a temperature Tj.
In
For the manufacture of effective SQUIDs, it is necessary to make pairs of junction with identical characteristics. Using the method described above, the variation of characteristics from junction to junction on the same chip as well as variations of junctions from chip to chip can be small, for example less than 5%. Another property of the Josephson junctions manufactured by this method, compared to grain boundary junctions, is the ability to position the junction on the thin film without any geometrical constraints, allowing the fabrication of a high density of devices on a single substrate.
Regarding this aspect, it is worth mentioning that the methods described here allow highly reproducible Josephson Junctions to be made. Thus very complex circuits, as for example needed for RSFQ logic devices, can be made based on junctions having the very similar characteristics. This is a key point for the development of this promising technology, which has not yet emerged with HTSC, due to the spread in the junctions' characteristics (critical current, critical current density, normal state resistivity, Josephson coupling energy).
The junctions made in this way can carry high current densities (greater than 50 KA/cm2) giving high IcRn products (in the mV range), as required for RSFQ applications In absence of truly metallurgic interfaces in this type of junction, fluctuations of the critical current appear to be reduced which can enable SQUIDs with low noise (<10−10 V/Hz at 1 kHz) to be manufactured.
By choosing the irradiation characteristics (ion, energy, dose), the geometry of the SQUID and the geometry of the slits, the operating temperature, the critical current and the normal resistance of a SQUID manufactured according to this method can be finely tuned, in order to match the requirement of specific applications. In addition, the process can be highly scalable, without adding specific constraints for the manufacture of arrays and complex structures including numerous SQUIDs or other superconductor devices. Moreover, flux transformers and different controlled lines can be made using the first step of irradiation presented in the invention.
In an alternative embodiments a number of different layers of gold may be applied. An embodiment using a so called “lift-off technique” is illustrated in
A layer of polymethylmethacrylate (PMMA) resist 14 is then deposited on the remaining portion of layer of gold 43 and exposed regions of the gold layer 41 as illustrated in
It will be appreciated that the electronic lithography steps of the so-called “lift-off technique” can be also made by mean of optical lithography (using UV deep UV, double exposure technique, phase-shift mask technique or X-rays).
An example of such a technique is described in document “High Tc superconducting quantum interference devices made by ion irradiation”—APL 89, 112515 (2006), which is incorporated herein by reference. The application of such a technique for the manufacture of Josephson junctions is described in the document “High quality planar high-Tc Josephson junctions”—APL 87, 102502 (2005), which is also incorporated herein by reference.
Although YBCO film was used as superconducting material in the embodiment described above, it will be appreciated that the above-described methods can be applied to a SQUID made of any oxide superconductor film material and not only to SQUIDs formed of a yttrium based compounds. This includes SQUIDs formed of other copper oxide type compound oxide superconductor thin film, including the so called Bismuth type compound oxide superconductor and thallium type compound oxide superconductor. Moreover, the method is not restricted to the use of oxide superconductors, other suitable superconductive materials may be used.
In addition, although in these embodiments the substrate used was a single crystal, SrTiO3 substrate, it will be appreciated that any insulating substrate which is suitable for growing c-axis oriented oxide superconductors may be used. Other examples of substrates include perovskites such as LaAlO3, MgO, CeO2, NdGaO3, sapphire, Y-stabilized Zirconia etc or thin layers (ranging from 10 to 100 nm) of these materials deposited on top of single crystals of the others, for example CeO2/MgO, or even SrTiO3/CeO2/MgO etc. . . .
It will also be appreciated that instead of using a superconductor film on a substrate the superconductor material may be bulk material.
It will be appreciated that different geometries can be used to define SQUIDS and superconducting other devices. Some example of SQUID geometries made according to this method are a SQUID having a superconducting loop of approximately 1000 μm2 with a 5 μm arm width corresponding to an inductance of L1=32 pH and a SQUID having a superconducting loop of approximately 36 μm2 with a 2 μm arm width corresponding to an inductance of L2=17 pH.
While in the embodiments described above PMMA photoresist is used to define the geometry of the device, it will be appreciated that any suitable masking material for defining a pattern may be used. Other suitable photoresists, for example, include AZ type, Shippley Type, and trilayers AZ/Ge/PMMA materials.
It will also be appreciated that in alternative embodiments of the invention the layer of gold may be replaced by other suitable materials exhibiting suitable properties of electrical conductivity and/or masking, for example, silver or copper. The thickness of the layer may be varied accordingly.
The above-described methods employing high Tc superconductivity can be used to manufacture a wide range of novel electronic devices having advantageous and unique features. The lossless conductivity can be employed to make interconnections and passive devices such as high Q value filters, transition edge photon or current detectors.
In these cases, a technology suitable for enabling thin films of High Temperature Superconductors (HTSc) to be easily patterned is of great interest. Standard lithography suffers from lack of reproducibility and long term stability, when it comes to small dimensions typically in the range of microns. The above-described method can also employ the quantum nature of superconductivity to make active devices based on the control of the quantum phase of electrons through Josephson Junctions (JJ), and on the quantization of the magnetic flux in a superconductor (Φ0=h/2e).
Although methods of making an electrical device was described above with reference to the manufacture of a SQUID, it will be understood that the methods may be applied to the manufacture of various superconductor or electronic devices with or without Josephson Junctions. Such superconductor devices may include interconnecting circuits, High Q value filters, transition edge photon or current detectors, voltage standards and RSFQ devices, magnetometers and voltmeters. These devices will be operated at temperatures below the Tc of the chosen superconductor. The operating temperature (or temperature range) itself, can be finely tuned by choosing the ion irradiation parameters: this is specific to this method of making superconductive electronic devices.
It will also be appreciated that the method of making a superconductor device and the method of making a Josephson junction can be applied independently. The method may be used to make a superconductor device not having a Josephson junction, and a Jospephson junction may be formed in a layer of superconductive material formed by another technique.
Furthermore, steps of the method can be applied to the manufacture of magnetic circuits. In as further embodiment of the method, a manganite film, for example LaxSr1−xMnO3 or LaxCa1−xMnO3, (with 0≦x≦1) is formed on a single crystal substrate such as SrTiO3. A gold mask is used, as previously described to design the desired circuit geometry and the structure is irradiated with ions which may be oxygen ions having an energy of 100 keV and a fluence of 5×1015 at/cm2. The ions causes a degree of disorder in the manganite film not protected by the gold mask and thus exposed to the ion beam thereby altering the properties of the manganite material in these regions rendering it insulating so that current can be concentrated in the areas of manganite material protected by the gold mask.
Such circuits may find applications in fields such as spintronics. Spintronics is the manipulation of information from electron spins as opposed to their charges.
In some examples of magnetic circuits manufactured according to an embodiment, a tunnel junction or equivalent, for example, a magnetic tunnel junction may be formed in the circuit. Such a tunnel junction may be manufactured in the a similar way to the manufacture of a Josephson junction as described above by irradiating the manganite film with ions though a photoresist mask (e.g. PMMA) having a slot of approximately 20 nm, using an ion fluence in a range of approximately 1013 or 1014 at/cm2.
It will be appreciated that the methods described here to make superconductive and/or magnetic electrical devices are compatible with the current industrial technological processes used in the semiconductor electronic industry (lithography, patterning, etching, layer deposition, ion-irradiation . . . )
Further modifications lying within the spirit and scope of the present invention will be apparent to a skilled person in the art.
