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Title:
METHOD FOR DEPOSITING A FILM OF SUPERCONDUCTING MATERIAL
Document Type and Number:
WIPO Patent Application WO/2004/077581
Kind Code:
A1
Abstract:
Method for depositing a film of a superconducting material, preferably of MgB2, comprising at least an elemental precursor volatile at the deposition conditions, by an as-grown method starting from elemental precursors.

Inventors:
GRASSANO GIUSEPPE (IT)
BOFFA VINCENZO (IT)
CELENTANO GIUSEPPE (IT)
MANCINI ANTONELLA (IT)
Application Number:
PCT/EP2003/002051
Publication Date:
September 10, 2004
Filing Date:
February 28, 2003
Export Citation:
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Assignee:
PIRELLI & C SPA (IT)
GRASSANO GIUSEPPE (IT)
BOFFA VINCENZO (IT)
CELENTANO GIUSEPPE (IT)
MANCINI ANTONELLA (IT)
International Classes:
C23C14/06; C23C14/24; H01L39/24; (IPC1-7): H01L39/24; C23C14/24
Foreign References:
US20030130130A12003-07-10
Other References:
SHIMAKAGE H ET AL: "Preparation of as-grown MgB2 thin films by co-evaporation method at low substrate temperature", 2002 APPLIED SUPERCONDUCTIVITY CONFERENCE, HOUSTON, TX, USA, 4 - 9 AUGUST 2002, vol. 13, no. 2, IEEE Transactions on Applied Superconductivity, June 2003, pages 3309 - 3312, XP002260961, ISSN: 1051-8223
UEDA K ET AL: "Growth of superconducting MgB2 thin films", ARXIV.ORG E-PRINT ARCHIVE, 18 March 2002 (2002-03-18), XP002260962, Retrieved from the Internet [retrieved on 20031111]
SAITO A ET AL: "As-grown MgB2 thin films deposited on Al2O3 substrates with different crystal planes", SUPERCONDUCTOR SCIENCE AND TECHNOLOGY, vol. 15, no. 9, September 2002 (2002-09-01), pages 1325 - 1329, XP002260963, ISSN: 0953-2048
Attorney, Agent or Firm:
Bottero, Carlo (Viale Sarca 222, Milano, IT)
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Claims:
CLAIMS
1. Method for depositing a film of a superconducting material, comprising the steps of a) positioning a first source of flux of at least an elemental precursor volatile at the deposition conditions and a second source of flux of at least an elemental precursor nonvolatile at the deposition conditions so as to minimize interference volume of fluxes before the growth substrate; b) simultaneously generating the flux of said at least a volatile elemental precursor and of said at least a nonvolatile elemental precursor.
2. Method according to claim 1 wherein the at least a volatile elemental precursor is selected from magnesium, calcium, lithium, sodium, potas sium, strontium, barium.
3. Method according to claim 1 wherein the at least a nonvolatile elemen tal precursor is selected from boron, carbon, silicon, germanium.
4. Method according to claim 1 for depositing a film superconducting of magnesium diboride.
5. Method according to claim 1 wherein the flux of said at least a volatile elemental precursor is thermally generated.
6. Method according to claim 1 wherein said first source is placed at a dis tance from the growth substrate lower than 4 cm.
7. Method according to claim 6 wherein said first source is placed at a dis tance from the growth substrate lower than 2 cm.
8. Method according to claim 4 wherein the growth substrate is heated at a temperature ranging between 250°C and 700°C.
9. Method according to claim 8 wherein the growth substrate is heated at a temperature ranging between 400°C and 500°C.
Description:
METHOD FOR DEPOSITING A FILM OF SUPERCONDUCTING MATERIAL The present invention relates to a method for depositing a film of su- perconducting material comprising at least a volatile elemental precursor, for example, of magnesium diboride (MgB2).

Nagamatsu J. et al., 2001, Nature, 410,63, report the superconductiv- ity of MgB2. This compound has the highest critical temperature (Tc = 39K) known for'non-oxide compounds. As critical temperature is intended the tem- perature under which the compound shows a null resistivity.

As reported by X X Xi et al., Supercond. Sci. Technol., 15 (2002) 451- 457, Josephson junctions of MgB2 could have the performance of conven- tional superconductor junctions, such as Nb and NbN, but operating at a much higher temperature.

The deposition of MgB2 films raises various technical problems due to the high volatility, at the deposition conditions, of magnesium (Mg), melting at about 650°C (atmospheric pressure), versus boron (B), melting at about 2200°C (atmospheric pressure).

X X Xi et al., supra, teaches that MgB2 is thermodynamically stable only under fairly high to very high Mg partial pressures at the temperature range appropriate for yielding superconducting properties. This requirement favours deposition techniques that can maintain a large Mg flux over those where a large Mg flux is impractical.

Three types of deposition processes have been mainly used. The first type employs ex situ annealing in Mg vapour of a precursor film deposited at room or low temperature (typically lower than about 300°C). The precursor can be boron or amorphous non-superconducting MgB2 or a mixture of MgB2 and magnesium. The precursor film is sealed under argon in a not reactive tube, out of the deposition system, with Mg pellets and heated to about 900°C. The resultant films exhibit bulk-like Tc of about 39K and extremely high critical current density (about 107 A/cM2 at low temperature). However, the high-temperature ex situ annealing is unlikely to be compatible with multi- layer device fabrications, such as Josephson junctions.

The second type is an in situ process. Thin films or multilayers of Mg+B or Mg+MgB2 are deposited at low temperature, then annealed in the same deposition chamber at about 600°C. The annealing procedure can take place in Ar atmosphere or in vacuum or in presence of Mg vapour variously supplied. This process is potentially more compatible with junction fabrica- tions, however the superconducting properties of such films are poorer than in the ex situ annealed films.

The third type is the so-called"as grown"deposition technique. In this technique, the elements are supplied from separated sources on the surface of the growth substrate, where they react and form the superconducting phase, if the thermodynamic conditions are appropriate.

Among the"as grown"deposition techniques, X. H. Zeng et al., In situ epitaxial MgB2 thin films for superconducting electronics, cond-mat/0203563 (provided by http ://xxx. lanl. gov/archive/cond ;-snat) relates to a HPCVD (High Pressure Chemical Vapor Deposition) technique of MgB2 films. This proce- dure employs pressure of 100-700 torrs (about 0.1-0. 9 bar), at temperature/of 730-760°C for Mg heating, and a mixture of diborane (B2H6) in hydrogen (H2) as boron precursor gas.

Diborane is extremely hazardous to health, very flammable and capa- ble of explosive decomposition in the presence of moisture in the air. Be- cause of these harmful features, particular and expensive precautions are needed for its use in industrial applications.

Kenji Ueda, Michio Naito, Growth of Superconducting MgB2Thin Films, cond-mat/0203181 (provided by http://xxx. lanl. gov/archive/cond-mat) report an overview of the results of the so-called"as grown"synthesis. This method is said to produce films that have lower To than the bulk value, but this ap- proach makes multilayer deposition feasible. Table III of this paper summa- rizes the film preparation recipes and physical properties of films grown by"as grown"synthesis. The deposition methods employed are molecular beam epi- taxy (MBE), carrousel-type sputtering and pulsed laser deposition (PLD).

MBE is an expensive deposition technique working in High Vacuum (HV) condition and Ultra High Vacuum (UHV) condition and providing very small material flux. In the case of codeposition, as reported by Kenji Ueda su- pra, this entails operating at low temperature (of about 300°C) to avoid signifi- cant Mg re-evaporation and the growth rate of the film is low. The sources of elemental precursors are provided at the same distance.

Carrousel-type sputtering deposition technique does not imply co- deposition of the elemental precursors of the film. It provides very weak flux, typically in the range of some nanometers per second as maximum. Films produced by this method have poor superconducting properties (Tc of about 28 K), and the same applies for those obtained by PLD"as grown"method (Tconset of about 25K and Tco of about 22.5K). In this latter case, a unique source is used, formed by magnesium and boron powder mixed together.

Two critical temperatures may be reported for a superconducting mate- rial. Tco is the temperature at which the resistance is zero and Tconset is the temperature at which the resistance significantly differs from the resistance in normal state. In a good quality film, these temperatures can differ by few tenths of degree, while they can differ by many degrees, decreasing the qual- ity of the sample.

Kenji Ueda et al., supra, underlines that important in terms of obtaining as-grown superconducting MgB2 films is the growth temperature that must be kept lower than about 300-350°C to avoid significant Mg loss. None of the "as-grown"films reported at the date of the article (march 2002) are single- crystalline or epitaxial. The reason is apparent in view of what taught by X X Xi et al., supra, saying that there has been no observation of MgB2 epitaxy for a deposition temperature lower than 400°C.

As from the pressure-temperature phase diagram for MgB2 shown by X X Xi et al., supra, temperature increasing has to be accompanied by an in- creasing of pressure, but deposition techniques used for the"as grown"syn- thesis showed to be unable to provide sufficient pressure as they generate

poor fluxes of material. In film deposition technique pressure and flux are linked by the following equation wherein F is the flux, P is the pressure, m is the mass of the material, k is the Boltzman's constant, and T is the temperature.

Applicant perceived that the deposition of a film of superconducting material, for example magnesium diboride, comprising at least an elemental precursor volatile at the deposition conditions, could be accomplished by an as-grown method starting from elemental precursors rather than molecular precursors such as diborane.

Such a goal is attained by providing the elemental precursors of the superconducting material in distinct fluxes emanating from sources. positioned so as to minimize the interference volume of said fluxes and to maximize the deposition pressure of the elemental precursor volatile at the deposition con- ditions.

The present invention relates to a method for depositing a film of a su- perconducting material, comprising the steps of a) positioning a first source of flux of at least an elemental precursor volatile at the deposition conditions and a second source of flux of at least an elemental precursor non-volatile at the deposition conditions so as to mini- mize interference volume of fluxes of said elemental precursors before the growth substrate; b) simultaneously generating the flux of said at least a volatile elemen- tal precursor and of said at least a non-volatile elemental precursor.

In the present invention, as"source"it is intended a device to produce a flux of atoms of elemental precursor, which defines an area from which the flux freely emanates.

In the present invention, as"deposition conditions"it is intended the conditions of pressure and temperature on the surface of the growth sub- strate.

In the present invention, as"interference volume"it is intended the amount of space where the fluxes of the elemental precursors mingle.

In the present invention, as"distance"it is intended the shortest seg- ment joining any point of said substrate surface to any point of said second source.

Volatile elemental precursors according to the invention can be mag- nesium, calcium, lithium, sodium, potassium, strontium, barium, preferably magnesium. Non-volatile elemental precursors can be boron, carbon, silicon, and germanium, preferably boron.

Preferably the flux of said at least a volatile elemental precursor is ther- mally generated.

In particular the method of the invention is directed to the deposition of film of superconducting magnesium diboride.

Advantageously said first source is positioned at a distance from the growth substrate lower than about 4 cm, more preferably lower than about 2 cm.

The deposition method of the invention can be carried out in a sealed environment under vacuum or in an inert atmosphere containing a buffer gas, such as argon. Said buffer gas can be at a pressure lower than 10-3 torr.

The growth substrate can comprise sapphire (Al203), MgO and/or SiC.

The method of the invention allows providing flux of the at least a vola- tile elemental precursor corresponding to pressure comprised between about 10-4 torr and about 1 torr (1. 3-10-4-1. 3 mbar) on the growth substrate. This pressure range makes possible to operate at higher growing temperatures (i. e. the temperature on the growth substrate) than those of other as-grown methods known in the art and discussed above.

In the case of the deposition of MgB2 films, the growth substrate can be heated at a temperature ranging between about 250°C and about 700°C,

preferably ranging between about 400°C and about 500°C. Such temperature ranges (growth temperature, i. e. temperature on the surface of the growth substrate) are in accordance with the temperature-pressure phase diagram set forth by X X Xi et al. supra, for example.

The invention will be better illustrated by means of the following exam- ple and figures, wherein - Figure 1 illustrates a schematic set-up for the deposition method of the invention; - Figure 2 illustrates results of a test carried out on a film obtained ac- cording to the invention.

An example of set-up useful for the deposition method of the invention is schematically depicted in Figure 1. Said set-up (1) comprises a heater (2), a growth substrate (3), a first source (4) and a second source (5).

The fluxes originated by sources (4) rld (5) are schematically depicted as (B) and (A), respectively.

The flux of the volatile elemental precursor can be created, by a thor- mal evaporator, for example, by evaporation from the surface of pellet of said volatile elemental precursor. In a particular set-up, said elemental precursor may originate from a ring-shaped thermal source yielding a flux towards the centre of the ring where the growth substrate is provided.

Generally, the evaporation takes place perpendicularly to the surface, originating, at the beginning, a flux with a high density of atoms. This flux ex- pands because of internal scattering of the atoms, according to the law 1/r2 wherein r is the distance from the evaporation source, thus reducing its den- sity during the expansion with the same 1/r2 law.

One or more screens can be placed around the first source for hinder- ing the volatile elemental precursor from escaping the substrate-centred di- rection, in particular towards the incoming flux of the non-volatile elemental precursor.

Both these features of the first source (position and configuration) can help in reducing the region of the intense scattering of the non-volatile ele-

mental precursor flux impinging on the substrate to a small region, localised on the substrate.

Example 1 Deposition of MgB2 superconducting film A sapphire substrate was horizontally glued on a heater by silver paint.

The deposition chamber was at about of 10 6 mbar. An electron gun for boron deposition was located in front of the substrate surface at a distance of about 15cm.

The evaporation rate of boron was measured and checked by a thick- ness monitor located inside the deposition chamber. Such rate was kept in the range of some 10-1 nanometer/second.

A standard thermal evaporator comprising an alumina basket contain- ing granules of magnesium and enveloped by a tungsten heater, was placed at a distance of about 2 cm from substrate, inclined in such a way to avoid the shadowing of the incoming boron flux on the growth substrate.

Magnesium evaporation rate was modulated by the feeding current of the tungsten heater.

Magnesium flux was calibrated as follows. On a test substrate, kept at room temperature, a thick magnesium film was deposited at a constant feed- ing rate. During the deposition, a thickness monitor, placed inside the deposi- tion system in such a way to intercept a part of the Mg flux, allows a real time measurement of the rate of growth. After deposition, the thickness of the film was measured in order to obtain the real thickness of the film. This procedure was repeated using different rates of growth, in order to obtain the required reproducibility and control of the Mg flux.

After calibration, different deposition procedures were carried out.

Table 1 sets forth experiments carried out at different deposition tem- perature and magnesium flux.

Table 1 Deposition Magnesium Test temperature flux MgB2 film critical temperature (C°) (nm/s) (K) Mg only 400 130 No film is deposited (Mg evaporates) 1) Mg+B 400 130 Tco=36 TConset = 38 2) Mg+B 450 130 Non superconducting film of boron-rich phases only (insufficient Mg flux) 3) Mg+B 450 250 Tco=34 TConset = 37

Co-deposition test effected at 400°C with a magnesium flux of 130 nm/s, but positioning the evaporation source at higher distance provided films with insulating properties, enriched of boron'.

Figure 2 sets forth resistance vs. temperature of the test film 1). The ; ;<BR> graphic inset illustrates a particular of the superconducting transition of the same product. The resistance of the sample decreases by reducing tempera- ture thus exhibiting metallic behaviour. The sample has a superconducting TConsetOf about 38K, evidencing a value close to that of bulk samples.