Molybdenum-based target and process for producing a target by thermal spraying

Abstract

A target of a nominal thickness includes molybdenum. The target has a lamellar microstructure and an oxygen content of less than 1000 ppm, preferably less than 600 ppm, and even more preferably less than 450 ppm. An electrical resistivity of the target is less than five times, preferably three times and more preferably twice the theoretical electrical resistivity of the compound.

Claims

1. A target, consisting of molybdenum and inevitable impurities, wherein the target is obtained by a process comprising thermal spraying, and wherein the target has: a lamellar microstructure; an oxygen content of less than 600 ppm; an electrical resistivity less than five times the theoretical electrical resistivity of molybdenum; and a nominal thickness of between 1 and 25 mm.

2. The target of claim 1, having a planar geometry.

3. The target of claim 1, having a tubular geometry.

4. A target comprising the target of claim 3 and additional thicknesses of material at each of its ends.

5. A target comprising the target of claim 2 and a part on which the molybdenum is deposited, wherein the part is a planar support configured to be fitted onto a sputtering machine and optionally an intermediate part that is bonded onto the support.

6. The target of claim 4, wherein the additional thicknesses are 25 to 50% of a nominal thickness of a compound layer.

7. The target of claim 1, having a density of greater than 85%.

8. The target of claim 1, having a nominal thickness between 6 and 14 mm.

9. The target of claim 1, having an iron content of less than 50 ppm.

10. The target of claim 1, having a purity of at least 99.95%.

11. The target of claim 1, wherein the target is obtained by a process comprising thermal spraying and directing cryogenic cooling jets distributed around a torch onto the target during spraying.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) As nonlimiting examples, the invention may be illustrated by the following figures:

(2) FIGS. 1a, 1b and 1c are views showing the microstructure in cross section of an Mo target obtained by the production process according to the invention;

(3) FIGS. 1a and 1b show a very dense structure, the interparticle connections being difficult to distinguish because of the absence of oxide lamellae;

(4) FIG. 1c at high magnification makes it possible to distinguish the typical lamellar structure of thermal spraying processes;

(5) FIGS. 2a and 2b are views showing the microstructure in cross section of an Mo target obtained by conventional production processes, namely by extrusion and sintering respectively, followed by hot forming;

(6) FIG. 2a relates to a tubular target, its hot forming (extrusion) with unidirectional grain texturing along the extrusion direction being clearly revealed; and

(7) FIG. 2b relates to a planar target, its microstructure being conventional for sintering microstructures.

(8) Other features and advantages of the invention will become apparent over the course of the following description.

DETAILED DESCRIPTION OF THE INVENTION

(9) The support on which the target will be constructed may be made of copper, a copper alloy, stainless steel or any other alloy suitably compatible with the production of magnetron targets. In the present invention, no particular requirement associated with the process described in the invention is required that relates to the support such that it only has to meet the usual requirements relating to magnetron targets, in terms of geometry, mechanical strength and chemical inertness with respect to the cooling water.

(10) Surface Preparation of the Support

(11) After having been degreased, the surface of the support is prepared by blasting it with a jet of abrasive grains. These grains may be of various kinds: corundum (fused white alumina) grains, brown corundum grains, alumina-zirconia abrasive grains, abrasive grains produced from fuse-cast slag particles (of the Vasilgrit type), almandine garnet grains or else angular steel or cast iron shot (this list not being exhaustive).

(12) Preferably, the following abrasives are used: corundum (fused white alumina), and alumina-zirconia (for example AZ 24 from Saint-Gobain Coating Solutions) (this material is preferred for its high toughness that limits fracturing of the grains and consequently the inclusion of grain fractions in the surfacesuch inclusions are deleterious to adhesion of the coating). The average diameter of the abrasive grains is preferably between 180 and 800 m, depending on the type of abrasive. The purpose of this operation is to give a surface roughness capable of ensuring correct adhesion of the tie sublayer or of the molybdenum-based compound.

(13) An alternative method consists in machining striations that will also allow good adhesion of the sublayer or the molybdenum compound.

(14) Production of a Tie Sublayer by Thermal Spraying

(15) To optimize the mechanical adhesion of the functional layer of the target, a tie sublayer may be produced by thermal spraying. This operation may employ conventional thermal spraying processes taken from the following: plasma (powder) spraying, electric-arc (wire) spraying, oxy-gas flame spraying (wire or powder depending on the equipment), spraying using the HVOF (high-velocity oxy-fuel) process, the detonation gun spraying process and the cold spray process using an optionally preheated gas into which powder is injected. This operation may be carried out in the ambient air without this impairing the invention.

(16) The tie sublayer material may be chosen from the conventional materials used commonly as sublayers: nickel or nickel-based alloys: NiAl, NiCr or NiCrAl; iron or ferrous alloys: FeCrAl, FeCrC or FeMnC steels, X2CrNi18-9 or X2CrNiMo17-12-2 austenitic stainless steels, etc.; copper or copper alloys, such as CuAl, CuAlFe, CuZn, etc.; molybdenum or molybdenum alloys: MoCu, etc.

(17) The above list is not exhaustive, the choice of sublayer material possibly depending on the material of the support tube and on the spraying equipment (and on the availability of filler material in suitable form).

(18) Formation of the Functional Film of the Target According to the Invention, Preferably by Plasma Spraying

(19) The functional film of the target is formed by thermal spraying, preferably by plasma spraying, under the following particular conditions: plasma spraying carried out in a chamber having an inert atmosphere, that is to say one in which the oxygen and nitrogen content is low, the atmosphere consisting predominantly of inert gas (for example argon), and the pressure in the chamber being between 50 mbar and 1100 mbar; plasma spraying using a reducing plasma gas mixture, making it possible to lower the amount of oxygen initially present on the surface of the powder particles upon melting them and during their flight onto the substrate; use, in the immediate vicinity of the plasma spray torch, of nozzles for blowing powerful liquid or gaseous cryogenic jets of an inert fluid, the jets being distributed around the torch; relative movements between torch and target, allowing possible variation of the thicknesses formed on the target and especially at the ends of the target by forming additional thicknesses commonly referred to as a dog-bone target; use of one or more powder injectors, allowing better distribution of the powder within the plasma jet; and it being possible for the plasma torch to be: either a commercially available DC blown-arc plasma torch; or an inductively coupled RF plasma torch.

(20) The powder used to produce the target has the following typical characteristics: defined particle size distribution such that: D.sub.10% (diameter such that 10% of the particles are smaller in size than this diameter): between 5 and 50 m; D.sub.50% (median diameter): between 25 and 100 m; and D.sub.90% (diameter such that 90% of the particles are smaller in size than this diameter): between 40 and 200 m; purity according to the purity objectives for the target, preferably greater than 99.95%; and oxygen content: <1500 ppm, preferably <1000 ppm or even <500 ppm.

(21) The process according to the invention makes it possible to obtain a target quality superior to that conventionally obtained by spraying and having a lamellar structure (cf. FIGS. 1a, 1b and 1c), especially in the case of pure molybdenum targets, and to obtain a target having an oxygen content of less than 500 ppm directly, without a subsequent step such as a high-temperature heat treatment in a reducing atmosphere.

(22) The fact of not using a subsequent heat treatment step has the advantage of employing any type of material for the support (tube for a tubular target or flat support for planar targets), including supports having an expansion coefficient markedly different from that of molybdenum, such as austenitic stainless steels, which would be proscribed in the case of a subsequent heat treatment for reducing the oxygen content.

(23) Of course, a heat treatment may also be carried out, as an option, so as to further reduce the oxygen content in the target thus produced.

(24) Planar Target Case:

(25) The present invention makes it possible to produce planar targets according to the following procedure: planar target support, suitable for being fitted into the magnetron for use; if the target support has a complex shape and has to be recycled after the target has been used, the target material will not be formed directly on the target support but on one or more intermediate plates (called tiles) which will be bonded onto the support; the target material (molybdenum) will be formed on the support or on the tile(s) following the same procedure as above; and the bonding of the tile(s) may be carried out before formation of the target material (if the support has a high mechanical strength) or after formation of the target material on the tiles in the case in which the support is not strong enough. In the latter case, the dimensions of the tiles will be determined so as to minimize the risk of them being distorted during the operation of forming the target material by plasma spraying.

IMPLEMENTATION EXAMPLE

(26) The implementation example relates to a tubular target intended to be used in magnetron sputtering with a rotating cathode. The following process was carried out: support tube made of austenitic stainless steel such as, for example, X2CrNi18-9 or X2CrNiMo17-12-2; surface preparation of the support tube by AZ grit 24 alumina-zirconia abrasive blasting; production of the keying sublayer by twin-arc wire spraying, carried out in air, the keying sublayer having an NiAl (95% nickel/5% aluminum) composition. In the example described, the thickness of the keying sublayer was a nominal 200 m; formation of the molybdenum active film on the target by plasma spraying under the following conditions: plasma torch imparting particular plasma jet velocity characteristics and consequently sprayed particle characteristics, target placed in a chamber, use of cryogenic cooling jets directed onto the target, these being distributed around the torch, the powder used for producing the target was a molybdenum powder having the following characteristics: agglomerated-sintered molybdenum powder particle size d.sub.50=80 m 99.95% purity, with in particular 20 ppm of Fe and 600 ppm of oxygen and plasma spraying carried out with the following parameters: a plasma torch with the following parameters was used to produce the target of the example:

(27) TABLE-US-00001 Powder Ar flow H.sub.2 flow Arc Spraying flow rate rate current distance rate Parameter (slpm) (slpm) (A) (mm) (g/min) Value 50 14 600 160 160 surface finishing by polishing or machining so as to obtain a roughness such that R.sub.max<15 m.

(28) As indicated above, thanks to the specific process according to the present invention, the oxygen content in the target obtained was 330 ppm, less than the 600 ppm content initially present in the powder. The essential characteristics of the target obtained are given in the following table (Target Example 4).

(29) Additional results according to this protocol with different powder compositions, in comparison with a result without a cryogenic jet according to the invention, are given in the table below:

(30) TABLE-US-00002 Oxygen Nitrogen Oxygen Nitrogen content content content content Trial in the in the in the in the reference Process powder powder target target A According 657 18 340 20 to the invention B According 657 18 240 20 to the invention C According 922 26 340 23 to the invention D According 526 29 360 18 to the invention E According 526 29 360 19 to the invention F According 706 31 580 30 to the invention G No 560 29 960 83 cooling jets

(31) As the above results show, the plasma spraying process with cryogenic cooling jets distributed around the plasma torch makes it possible to reduce the oxygen content in the target compared with the oxygen content in the starting powder. It is thus unnecessary to choose a very pure starting powder, especially since it is not possible in practice to avoid the powder containing a certain amount of oxygen. The process according to the invention is thus particularly advantageous.

(32) Properties and Advantages of the Invention

(33) The targets according to the present invention have the following properties and advantages: better utilization factor of the material used in tubular targets obtained by plasma spraying compared with those obtained by the sintering followed by hot-forming processes because the process according to the present invention offers the possibility of depositing additional thickness at the ends of the targets so as to compensate for the extensive localized erosion in the zones corresponding to the bending, with a small radius of curvature, of the magnetic field created by the cathodes and their magnets. This makes it possible to achieve target material yields greater than 75%, or even 80%, whereas the yields remain below 75% in flat-profile targets. As a corollary to using this type of target, films, especially molybdenum-based films, are obtained whose R.sub.uniformity profile, along a characteristic dimension of the substrate at the surface of which the film was deposited, deviates by no more than 2% (for example on a substrate of 3.20 m width). This measurement is carried out using an apparatus of the Nagy type by contactless measurement; wide material thickness range on the target between 1 and 25 mm: the thickness of the target may be chosen according to the desired lifetime thereof (this thickness being in fact determined by the expected duration of production without stopping the line); in the case of tubular targets, it is possible to bias the target in AC mode or DC mode with power levels in excess of 30 kW/m (increase in deposition rate), without the risk of cracking (due to the thermal gradient between the support tube and the target) or the risk of braze melting; and because the molybdenum thickness is reduced to the amount strictly necessary for the user, it is possible to limit the voltage needed to sustain the high-power discharge and thus make this target compatible with current magnetron power supplies.

(34) In the case of monolithic tubular or planar targets produced using the present invention, and in contrast with targets comprising assembled segments, the following risks are considerably reduced: risk of the appearance of arcing, which generates parasitic particles, and the risk of fragments of the target material being separated from its support, which is known to be a source of contamination of the molybdenum films; risk of sputtering braze material or target support material via the gaps between segments; and risk of thermal or mechanical failure of the bonding (braze or conductive cement) to the support.

(35) The targets according to the invention are particularly intended to be used in a vacuum film deposition installation (magnetron sputtering in an inert or reactive atmosphere, especially by magnetron cathode sputtering, by corona discharge or by ion beam sputtering), for the purpose of obtaining a film based on the material forming said target, this film being molybdenum-based.

(36) This molybdenum-based film may be deposited directly on a substrate or indirectly on another film which is itself in contact with a substrate, it being possible for the substrate to be of organic nature (PMMA or PC) or of inorganic nature (silica-based glass, metal, etc.).

(37) This thin film may form an electrode for a photovoltaic cell or panel, or else it may form part of the constitution (interconnects, etc.) of display screens using TFT, LCD, OLED, ILED or FED technologies, or any other assembly requiring a thin molybdenum film of good quality.

(38) The films forming the subject matter of the following examples were obtained by magnetron sputtering of various targets obtained according to the prior art (Examples 1 and 3) and according to the invention (Examples 4 and 5):

(39) TABLE-US-00003 Deposition Magnetron target process Mo thin film Thickness O Fe Resistivity Power Pressure Thickness Resistivity Example Process (mm) (ppm) (ppm) (ohms .Math. cm) (kW/m) (bar) (nm) (ohms .Math. cm) 1 Sintering 9 <50 60 5.6 30 (AC) 4 88 19.6 2 Sintering 12.5 <50 50 6 10 (DC) 4 180 18.8 3 Plasma 2.2 >700 ? 20 (DC) 2 172 24.7 spray (prior art) 4 Plasma 9 330 9 8.4 30 (AC) 4 88 19.0 spray 5 Plasma 4 300 15 8.5 20 (DC) 2 120 14.0 spray

(40) The thin molybdenum-based films were deposited on extra-clear glass 3 mm in thickness, of the SGG-Diamant extra-clear glass type. These films were deposited in a horizontal magnetron deposition machine provided with a molybdenum target according to the invention, this target being supplied either in AC mode by a Httinger BIG150 power supply or in DC mode by a Pinnacle AE power supply, with an argon plasma of 450 sccm argon in the case of Examples 1 and 4 and 600 sccm argon for Examples 2, 3 and 5.

(41) Comments:

(42) Example 4 versus Example 1 and Example 5 versus Example 2: identical or better performance for the target of the invention compared with a high-purity target of the prior art. For an oxygen content <450 ppm in the target, the oxygen content (and therefore the resistivity) in the film is governed by the limiting vacuum in the deposition chamber (amount of oxygen available under the residual pressure); Example 5 versus Example 3: better performance in the target according to the invention compared with the target according to the prior art. When the oxygen content in the target exceeds 500 ppm, the oxygen content in the film is governed by the purity of the target.

(43) The targets described in Examples 4 and 5 generate a perfectly stable plasma under DC or AC bias without significant arcing throughout the lifetime of the target.

(44) As a variant, if a target possibly obtained by the process according to the invention is sputtered, this target possibly containing at least one metal cation belonging to the (Fe, Ni, Cr, W, etc.) family, a film also having a certain content of these elements is obtained.

(45) The content of cationic impurities in a thin film produced from a rotary target stems practically only from the target. This is because the rotary technology eliminates all components for fastening the target (i.e. clamps) and therefore eliminates any possibility of parasitic sputtering above the glass.

(46) In most applications, the resistivity of the thin Mo film is especially governed by the oxygen content in the film. It is particularly important to minimize this content so as to maintain a minimum level of oxidation of the film and therefore to obtain a resistivity close to that of pure metallic molybdenum.

(47) The oxygen content of the film has two origins: (i) oxygen originating from the residual atmosphere (basic vacuum) before introduction of the sputtering gas and (ii) oxygen originating from the target.

(48) Thus, it is possible to calculate the amount of oxygen theoretically included in the molybdenum film, coming from the residual oxygen partial pressure in the sputter coater, using the following: J.sub.O2 (the oxygen flux reaching the glass during deposition)=3.5110.sup.22 (M.sub.O2T).sup.1/2P, where M.sub.O2 is the molecular weight of the oxygen gas, T is the temperature in kelvin and P is the pressure in torr and J.sub.MO (the amount of MO on the glass that can react with O.sub.2)=V.sub.MoN.sub.Mo, where V.sub.Mo is the Mo deposition rate (in cm/s) and N.sub.Mo is the amount of Mo atoms per cm.sup.3 in a magnetron metal film (in atoms/cm.sup.3).

(49) Assuming that all the oxygen coming into contact with the molybdenum on the substrate reacts, it is possible to calculate the maximum expected oxygen content in the Mo film; for a given deposition rate on sputter coaters of 810.sup.7 cm/s, the residual oxygen contents in the Mo layer as a function of the residual oxygen partial pressure are obtained as given in the following table:

(50) TABLE-US-00004 Calculated oxygen content pO.sub.2 (mbar) in the coming from the vacuum in sputtering atmosphere the Mo film (ppm) 10.sup.7 1000 5 10.sup.8 540 2 10.sup.8 250 1 10.sup.8 110 5 10.sup.9 54

(51) The minimum residual partial pressure measured in the sputter coater is conventionally 510.sup.8 mbar, i.e. about 540 ppm theoretical oxygen. It is therefore unnecessary to use high-purity targets with an oxygen content well below 540 ppm since the influence of the target on the purity of the final film is masked by the oxygen coming from the atmosphere in the sputter coater. The invention consists in choosing a less expensive magnetron technology for producing Mo targets, the oxygen content of which is less than 1000 ppm, preferably less than 600 ppm and even more preferably less than 450 ppm.

(52) The residual content of metal cations (Fe, Ni, Cr, W, etc.) of the thin Mo film obtained within the context of the invention is less than that of the films obtained by conventional targets, for two reasons: the film of the invention is obtained by sputtering a monolithic target (one single segment): no risk of sputtering the backing tube (made of titanium or stainless steel) or the material used for bonding the Mo to the backing tube (for example indium); and the film of the invention is obtained by sputtering a target of high purity in terms of metal cations, this being dependent on the choice of technology for producing the target and on its implementation: choice of a raw material powder of high purity and forming of the target by plasma spraying, i.e. without direct contact between the sprayed molybdenum and metal parts, as in extrusion or hot-rolling techniques, or contact with metal parts based on steel, stainless steel, tungsten, etc. are possible.

(53) The molybdenum film according to the invention typically has: an iron content of less than 50 ppm, preferably less than 40 ppm; and/or a nickel content of less than 10 ppm; and/or a chromium content of less than 20 ppm; and/or a tungsten content of less than 150 ppm.