EP1787311A1

EP1787311A1 - Cylindrical target obtained by hot isostatic pressing

Info

Publication number: EP1787311A1
Application number: EP05717113A
Authority: EP
Inventors: Wilmert De Bosscher; Ruben Vermeersch
Original assignee: Bekaert Advanced Coatings NV
Current assignee: Soleras Advanced Coatings BV
Priority date: 2004-07-16
Filing date: 2005-03-21
Publication date: 2007-05-23
Also published as: WO2006008197A1; CN1985345B; CN1985345A; KR101147941B1; KR20070033425A

Abstract

A method for producing a rotatable target and the resulting target thereof is described. The target forming material is poured in the hollow formed between a cylindrical mold wherein an inner tube is coaxially held by a top and a bottom closing body. The assembly is subjected to hot isostatic pressing at elevated temperature and pressure. The method differs from the state of the art in that during the hot isostatic pressing, the inner tube deforms and presses the target forming material against the substantially undeformed outer mold.

Description

CYLINDRICAL TARGET OBTAINED BY HOT ISOSTATIC PRESSING

Field of the invention.

The invention relates to a method to manufacture a rotatable sputtering target by means of outward Hot lsostatic Pressing (HIP).

Background of the invention.

The advantages of rotating cylindrical targets - such as increased material usage and less arcing to name just a few - make them more and more attractive to use with non-conventional materials such as ceramics. The high temperatures needed to melt or vaporise these materials inhibit conventional target manufacturing methods such as casting. At present the two most popular routes to manufacture a rotatable ceramic target directly on a backing tube are:

- Plasma spraying whereby a powder comprising the ceramic of interest is intensely heated by a gas plasma and is sprayed at high speed onto a backing tube in a controlled gas atmosphere. See e.g. US 6,461 ,686 where the target material is TiO_x (x<2). However, this method cannot be applied if the powder is difficult to inject into the nozzle jet if, for example, it is too fine or tends to stick to the feeder.

- Hot lsostatic Pressing method (or HIPping). Here the target forming material is brought into a relatively thin-walled, deformable, cylindrical can having an undeformable core mounted coaxially to said cylindrical can. After evacuation of the intergranular gas followed by gastight sealing, the can is subjected to a high pressure (50 to 200 MPa, usually by means of a fluidum notably Ar) while being maintained at a high temperature (250 to 1500⁹C). After subsequent cooling and pressure normalisation, the can is mechanically removed from the target material. The pressure and the temperature density the powder into a solid, glass-like material with a density very close to the theoretical density.

Rotatable sputtering targets obtained by the HIP method have been described in US 5,354,446 and US 5,435,965. The undeformable core is a solid metal cylinder onto which the target material is fused in the hot isostatic pressing. The solid metal cylinder can be coated with an intermediate layer to alleviate the thermal stresses between the core and the target material during operation of the target/US 5,354,446). Also the solid metal cylinder can be mechanically treated (threaded or sandblasted) in order to enhance the adhesion between the backing tube and the target material (US 5,354,446). Instead of a solid cylinder, the use of a circular tube is suggested in US 5,435,965.

The walls of the outer can should not be too thick, so that the walls do not impede the isostatic pressure action on the powder. The degree of compression of the outer can depends on the dimensions of the can, the final thickness of the target material layer and the degree of compaction of the powder. If the compression of the outer can material exceeds a critical threshold, the outer can will 'buckle'. I.e. the compression of the outer can is not longer uniform and the outer surface wrinkles together. When buckling occurs, the compression of a non-melting target material is not longer uniform leading to an inhomogeneous material. In case the target material does melt during the HIP process, the material will still be uniformly compressed. In both cases the irregular shape of the can makes it difficult to remove the can from the ingot and necessitates the machining of the outer surface (as illustrated in table 2 of US 5,435,965). Such machining is not only an additional process step, but also leads to a considerable loss of expensive material.

Summary of the invention.

The object of the present invention is to eliminate the mentioned drawbacks of the prior art. More specifically it is an object of the invention to eliminate the additional process step of machining the outer surface of the ingot. Additionally the loss of material due to this machining step is also impeded. Other problems of the existing art can also be solved as mentioned in the summary below. A first aspect of the invention relates to a method of manufacturing of a rotatable sputtering target as described in the combination of features of claim 1 and the claims 2 to 7 depending thereof. The method comprises the steps of:

(A) Providing an inner tube. In the end, this inner tube will be the carrier of the target material. The outer surface of said inner tube may or may not be provided with a surface coating or surface treatment (such as threading, brushing or grit blasting) to enhance the adhesion of the target material to the inner tube.

Or said inner tube may be treated with a coating having a coefficient of thermal expansion intermediate between that of the inner tube and the target material to abate thermal residual stresses after cooling. These coatings can be applied by the known techniques such as plasma spraying.

(B) In a second step an outer mold is mounted around said inner tube. This mold has an internal cavity in the shape of a body of revolution having a central axis of revolution. The mold is mounted with the central axis of revolution coaxial to the axis of the inner tube. The hollow, which thus forms between the inner tube and the outer mold, serves to hold the target forming material. The inner side of the mold may or may not be covered with an anti-stick layer such as an AI₂O₃ thermal spray layer, or foil, or have been subjected to a surface treatment.

(C) A bottom annular closing body is provided between the inner tube and the mold. The closing body is sealed to the mold and the inner tube so that the seam holds under the extreme conditions of temperature and pressure in the steps to follow. Sealing can be done by means of welding or brasing. Sealing can also be obtained by mechanical means such as by a thread-locking connection e.g. by threading outside of the ends of the inner tube and the inner mantle -A-

of the annular closing body. The same can be done for the outer mantle of the annular closing body and the inside of the mold ends. Or the inner tube can be threaded into the centre of an end-flange which is subsequently bolted to a rim attached to the mold with an appropriate sealing in between the flange and the rim.

(D) Filling said hollow with a target forming material. Typically the method can be used for any kind of target forming material that can be supplied in a powdery form. Powders can be provided in the hollow between inner tube and mold e.g. by pouring.

Non-exhaustive examples of powders are ceramic powders, more particular oxides, nitrides or carbides of metals such as indium, tin, zinc, gallium, copper, titanium, aluminium to name just a few. Compound mixtures of these ceramic powders are also possible for example zinc oxide (ZnO) maybe mixed with aluminium oxide (AI₂Os). Mixtures of these ceramic powders with pure metal powders are also possible in order to obtain the desired properties of the rotatable target, a notable example being indium sesquioxide (In₂O₃) mixed with tin (Sn) powder. The powders may also be alloyed prior to pouring them into the hollow. One way of alloying - namely mechanical alloying - is e.g. described in EP 0 871 793. The skilled person knows that the powders must be properly densified before proceeding to subsequent steps. Most conveniently this is done by vibration.

(E) Providing a top annular closing body and sealing said top annular closing body to said inner tube and said mold. The sealing is done with any technique such as described in step (C) of the process for sealing the bottom closing body.

(F) Evacuating and sealing said hollow. Evacuation is usually done through an evacuation tube at the top closing body. During evacuation, the densification may proceed. An elevated temperature (>1 OO¹C) during evacuation may help to desorb water and other volatile contaminants out of the powder. The evacuation tube is sealed when a sufficient vacuum has been reached Together the mold, the inner tube and the top and bottom closing bodies form a closed container having a hole in the middle. (G) As next step in the manufacturing method, the container is subjecting to a hot isostatic pressure treatment. Such a treatment is generally performed under pressures between 50 and 200 MPa and at elevated temperatures between 25O⁰C and 1500"C in an inert atmosphere. The target material to be produced dictates the exact processing conditions of temperature, pressure and duration. The treatment cycle can be complex with different levels of temperature and pressure being maintained over different periods in order to obtain optimum target material.

The steps (A) through (G) are known in the art. All parts of the container are made of a suitable metal or alloy selected for its use. Typical materials are: stainless steel, titanium and its alloys, aluminium and its alloys, Hastalloy, lnconel to name just a few. All parts of the container may be made of the same metal or alloy, but this is no necessity for the invention.

The contribution over the state-of-the-art of the inventors is that they have found that when during the hot isostatic pressure treatment the inner tube is deformed and presses the target forming material against the substantially undeformed outer mold, the problems with the known processes are overcome.

In order to guarantee this, the strength requirements for the inner tube and outer mold are to be reversed compared to the existing art. Indeed, in order to allow the inner tube to deform, the pressure must be able to enter the inner tube and to deform it. Although the outer mold is equally well subjected to an identical isostatic pressure, it must withstand the pressure substantially undeformed. With "substantially undeformed" is meant that the mold deforms less than the inner tube: the largest decrease in diameter of the mold as measured along its longitudinal axis, must be smaller than the largest increase in diameter of the inner tube. With "substantially undeformed" the case of a buckled mold is explicitly excluded.

The role of the top and closing body during this deformation is of less significance: whether they are deformed or not in the process does not make a substantial difference for the invention.

The difference in deformability between inner tube and mold can be implemented in a number of ways:

- the most obvious way is to use dimensional features of the inner tube and mold to alter the degree of formability. For example the inner tube is made substantially thinner than the mold. In the HIP process, the inner tube will therefore more easily expand while the target material is pressed against the outer mold. The inner tube can - by way of example - be one half or one third of the material thickness of the mold.

- the inner tube can be made of a different, more ductile material than the mold. 'Ductility' relates to the plastic deformability of the material. In this invention, the ductility at the elevated temperatures of the process is of particular relevance. As a rule of thumb the ductility of a metal or a metal alloy greatly increases above one third of its melting point.

- the inner tube can have regions of lower resistance to deformation while the mold has a substantially uniform strength. This can for example be achieved by machining grooves in the lengthwise direction of the outer or inner surface of the inner tube. The material below the grooves will expand easier than the material on top of the grooves. Machining of the grooves can also be alternated on the inner tube side and on the outer side of the inner tube so that the thickness of the material remains substantially the same thus forming a lengthwise corrugated tube. Under pressure the tube will be partially or totally stretched out in the circumferential direction. Such an approach can result in an improved adhesion of the target material to the tube and to an increased stiffness of the inner tube.

It will be clear to the person skilled in the art that the different ways of implementing the degree of formability can be combined.

As a limiting case the mold can be made so undeformable and strong that it can act as the high pressure container for the hot isostatic pressure step. In this case pressure is applied through the ends of the inner tube. The heating can be applied through the mold or through the pressure fluidum.

After the hot isostatic pressure step, the mold (dependent claim 2) and the bottom and top annular closing body is removed (dependent claim 3). In order to ease this operation a mold composed out of a number of lengthwise split shells e.g. two that are held tightly together by removable bands can be used. The regions where the shells contact one another must also be treated with an anti-adhesive in order to ease the removal of the target after the HIP process. A seal between the shells has to be foreseen in order to prevent fluid ingress during hot isostatic pressing. Such a seal can e.g. be implemented by means of a copper or indium gasket.

As the outer shape of the rotatable sputtering target is substantially determined by the shape of the mold cavity, there is a large degree of freedom in the outer shape of the sputtering target. One can make the cavity in the form of a cylinder that is coaxial to the inner tube (dependent claim 4).

The cavity can also comprise a cylindrical medial part and two cylindrical end portions adjacent to said top and bottom closing body wherein the outer diameter of the medial part is smaller than the diameter of the end portions (dependent claim 5). Such an outer shape of the target helps to increase the target usage as described in US 6,264,803.

As the inner tube may deform to a lesser extent in the vicinity of the top and bottom annular closing bodies, these differently deformed end sections may or may not be machined away (dependent claim 6). In any case - whether the end sections are cut away or not - adaptor pieces will be necessary to make the rotatable target connectable to the drive system of a sputtering machine because the inner tube may not keep its engineering tolerance in the hot isostatic pressing step (dependent claim 7).

According a second aspect of the invention, the rotatable sputter target resulting from the method described distinguishes itself from the prior-art targets with the characterising features as set forth in claim 8 to 16.

By the nature of the process, the inner tube is elongated in the circumferential direction (independent claim 8). Depending on the outer diameter of the inner tube respectively before - called 'd₀ '- and after - called 'di¹- the hot isostatic pressing, the elongation ε will be different:

ε = In(ChAJ₀) The compaction ratio 'c' (i.e. the ratio of the final target material density to the powder material density), depends not only on d₀ and d_λ but also on the outer diameter 'D' of the rotatable target according:

c = (D²-do²)/((D²-d₁ ²)

for the case of a pure cylinder. Preferably the method to produce the target will result in an elongation of at least 2% (dependent claim 9). The method will be even more beneficial when an elongation of 5% is needed to form the material (dependent claim 10).

Another feature by which the inventive rotatable sputtering target distinguishes itself from the state-of-the-art rotatable targets is noticeable when the end portions of the target are still present. The inner diameter of the innertube is than larger than the inner diameter of the end sections (independent claim 11). The elongation to which the inner tube has been subjected is then equal to relative difference between the original inner diameter - as measured at the end sections of the target - and the final inner diameter - as measured in between the end sections of the target. Preferably this difference is larger than 2% (dependent claim 12). Even more preferred is that this difference is larger than 5% (dependent claim 13).

The method is preferentially used on materials that have a high compaction ratio. Preferably the compaction ratio to apply the method is larger than 1.5 (dependent claim 14), most preferably larger than 2 (dependent claim 15). Rotatable sputtering targets for which this method is particularly preferred are made with the following target forming materials:

- Indium Sesqui Oxide alloyed with tin (Indiumtinoxide ITO) (dependent claim 16) - Titanium oxide (TiO_x, x < 2) (dependent claim 17)

- Impurity doped ZnO:AI or ZnO:Ga (dependent claim 18)

The material to be used for the inner tube is by preference titanium or one of its alloys as the properties of this material in many cases matches best with the ceramics that are formed on it (claim 19).

Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawings wherein

FIGURE 1 : describes the prior art mold configuration for forming rotatable targets by means of hot isostatic pressing. FIGURE 2: (a) describes the built-up of a preform according a first preferred embodiment, prior to hot isostatic pressing

(b) describes the shape of the configuration of the first preferred embodiment, after hot isostatic pressing. FIGURE 3: (a) describes the built-up of a preform according a second preferred embodiment, prior to hot isostatic pressing

(b) describes the shape of the configuration of the second preferred embodiment, after hot isostatic pressing. FIGURE 4: (a) describes the built-up of a preform according a third preferred embodiment, prior to hot isostatic pressing

(b) describes the shape of the configuration of the third preferred embodiment, after hot isostatic pressing.

Description of the preferred embodiments of the invention.

FIGURE 1 describes the prior art can 100 that is used to make a target according the hot isostatic pressing method. A thick walled inner tube 102 is used as a carrier tube that is threaded in order to increase the adhesion of the target material to the tube. First, a bottom 108 is welded between the inner tube 102 and the outer tube 104. The powdery target forming material 110 is poured in the hollow between inner tube 102 and outer tube 104. The outer tube is coated with an anti-sticking paper 112. After densifying the powder, a top cover 106 is welded between the inner tube 102 and the outer tube 104. After evacuation of all remaining gas, the can is subjected to the hot isostatic pressing. Thereafter the outer can 104, the bottom and the top cover are mechanically removed.

FIGURE 2a depicts a first preferred embodiment of the invention wherein the inner tube is preferentially deformed while the mold remains substantially undeformed. The HIP can 200 comprises the inner tube 202 and an outer mold 204 where in between a bottom annular closing body 206 is welded between the inner tube 202 and the outer mold 204 with welding seams 212. The can 200 is axial symmetric around the axis 220. The dimensions of this first embodiment are summarised in table 1 (all dimensions in mm):

Table 1

The length of the can is 200 mm. All pieces are made out of titanium. A cylindrical cavity is created in this way: inner diameter 50 mm, outer diameter 60 mm and with a length of 150 mm. The cavity is partly filled (up to a height of 100 mm) with ISOT powder (indium sesqui oxide mechanically alloyed with tin) prepared according EP 0 871 793. The filled can is tapped and vibrated to achieve a tapping density of typically 3.5 g/cm³. After filling a second titanium top annular closing body 210 is positioned on top of the powder. The entire structure is welded together. A degassing tube 214 is provided so that gasses can be evacuated. After filling the can, the powder is degassed at a temperature of at least 400O. Hereafter the degassing tube 214 is closed gastight. The can 200 is now ready for hot isostatic pressing.

During hot isostatic pressing, the pressure is slowly increased to 200 MPa, while at the same time the temperature is increased to 700O. The can is left under hot isostatic pressure for about 4 hours. After this 'dwell time' the can is slowly cooled, while the pressure is decreased.

After hot isostatic pressing, the can has been deformed as depicted in FIGURE 2b. The inner diameter of the inner tube 202' has changed from 45 mm to 48 mm when measured in the medial part of the tube. The outer diameter of the mold 204 has changed from 70 to 68.5 mm. The density of the final ITO target was 7 g/cm³ that is about 98 % of the density of massive ITO (7.14 g/cm³). The compaction ratio is 2. The elongation of the inner tube is 6.5 %.

After the hot isostatic pressing, the outer can is removed and a smooth surface remains. The end sections 218 and 216 that are less deformed than the medial part are cut off from the target. After insertion of a connector piece, the target is ready for mounting in a sputtering apparatus.

As a second preferred embodiment, a larger version of the first embodiment was made. Again a titanium inner tube was used with an initial inner diameter of 135 mm, an outer diameter of 141 mm - hence a wall thickness of 3 mm - with a length of 600 mm. The outer mold had an outer diameter of 165 mm and an inner diameter of 153 mm, the thickness thus being 6 mm. As a bottom closing body a titanium ring with an inner diameter of 141 mm, an outer diameter of 153 mm and a thickness of 100 mm was used. The cavity is filled with ISOT powder up to a height of 400 mm. By vibration, a tapping density of 3.5 g/cm³ can be achieved. After filling a top closing body identical to the bottom closing body is inserted. The entire structure is welded together with 3 degassing tubes in such a way that the gasses can be evacuated through the degassing tubes. Degassing is performed at a temperature of at least 400"C. Hereafter the tubes are vacuum sealed and the can is ready for hot isostatic pressing at a pressure of 200 MPa and at a temperature of 500¹C. In this way the inner tube is deforming, while the outer diameter of the can is limited. The inner diameter of the inner tube deforms from 135 to 139 - i.e. a circumferential elongation of 3% - while the outer diameter of the can has become 163.5. The density of the material has increased to 6.4 g/cm³ yielding a compaction ratio of 1 .8. Very little material is lost while the mold is removed from the final target material.

The influence of the thickness of the outer mold was further investigated in a third preferred embodiment. Here an identical procedure was followed as in the second embodiment with the same materials but with different dimensions of the outer mold. The dimensions before and after the hot isostatic pressing is summarised in table 2:

Table 2

All numbers are in mm. 'ID' and 'OD' stand respectively for 'Inner' and 'Outer' Diameter. Numbers between brackets have been calculated, by taking the thickness of the tube or mold in account. The thicker outer mold (20 mm) results in less deformation. The inner tube has increased 12 mm in diameter i.e. the material has been circumferentially elongated by 9%. No buckling has occurred, and the densification was homogeneous over the length of the tube. The outer surface of the target was smooth after removal of the outer mold and no further machining was necessary. At both ends of the inner tube, the inner diameter of the inner tube was less (ab. 130 mm) than in the middle due to the transition between the less compressed annular closing bodies and the expanding inner tube (as shown in fig. 2b, claim 11 ). In this way the target prepared by this inventive method can be distinguished from other targets made by other methods.

According a fourth preferred embodiment 300 depicted in FIGURE 3 (a) and (b), the bottom and top annular closing bodies 310 have a specific shape that ensures an adequate compression of the powder in the vicinity of the end sections after hot isostatic pressing 310'. In a fifth preferred embodiment 400 as shown in FIGURE 4, the outer mold 404 can be locally weakened 420 by making it thinner, so that the target surface obtains a specific shape after hot isostatic pressing.

Claims

Claim 1

A method to manufacture a rotatable sputtering target comprising the steps of:

- providing an inner tube

- mounting an outer mold around said inner tube, said outer mold forming a cavity, said cavity having the shape of a body of revolution, said cavity sharing the same axis with said inner tube so that a hollow forms between said inner tube and said mold

- providing a bottom annular closing body in said hollow and sealing said bottom closing body to said inner tube and said mold

- filling said hollow with a target forming material

- providing a top annular closing body and sealing said top annular closing body to said inner tube and said mold

- evacuating and sealing said hollow, said inner tube, said mold and said top and bottom closing body thus forming a container having a hole in the middle

- subjecting said container to a hot isostatic pressure treatment characterised in that during the hot isostatic pressure treatment said inner tube deforms and presses said target forming material against said substantially undeformed outer mold.

Claim 2

The method as in claim 1 followed by the step of removing said mold from said container.

Claim 3

The method as in claim 2 followed by the step of removing said top and bottom annular closing body. Claim 4

The method as in any one of claim 1 or 3 wherein said cavity in said mold is a cylinder

Claim 5

The method as in any one of claim 1 or 3, said cavity having a cylindrical medial part and two cylindrical end portions adjacent to said top and bottom closing body wherein said medial part has a first diameter and said end portions have a second diameter, said second diameter being larger than said first diameter.

Claim 6

The method as in any one of claim 1 to 5 followed by the step wherein the end sections of said rotatable sputter target containing those parts of said inner tube that are less deformed than the medial part of said inner tube are cut off.

Claim 7

The method as in any one of claim 1 to 6 followed by the step of assembling one or two adapter pieces to said inner tube, said adapter pieces for connecting said tubular target to a magnetron sputtering apparatus.

Claim 8

A rotatable sputtering target comprising an inner tube and target material, said target material disposed on the outer surface of said inner tube, said inner tube being made of an inner tube material, said inner tube having a symmetry axis characterised in that the inner tube material in a plane perpendicular to said axis has been circumferentially elongated Claim 9

The rotatable sputtering target according Claim 8 wherein the circumferential elongation is at least 2%.

Claim 10

The rotatable sputtering target according Claim 9 wherein the circumferential elongation is at least 5%.

Claim 11

A rotatable sputtering target comprising an inner tube, said inner tube having a first end section and a second end section, said target further comprising target material disposed on the outer surface of said inner tube in between said first and second end section, said inner tube having a first inner diameter at the end sections, said inner tube having a second inner diameter in between said first and second end section, characterised in that said second diameter is larger than said first diameter

Claim 12

The rotatable sputtering target as in claim 11 wherein said second diameter is 2% larger than said first diameter

Claim 13

The rotatable sputtering target as in claim 11 wherein said second diameter is 5% larger than said first diameter

Claim 14

The rotatable sputtering target obtained by the method of any one of claim 1 to 7 wherein said target forming material has been compacted with a factor of at least 1.5 Claim 15

The rotatable sputtering target obtained by the method of any one of claim 1 to 7 wherein said target-forming material has been compacted with a factor of at least 2.0.

Claim 16

The rotatable sputtering target according any one of claims 8 to 15 wherein said target-forming material is indium sesqui oxide alloyed with tin.

Claim 17

The rotatable sputtering target according any one of claims 8 to 15 wherein said target-forming material is titanium oxide.

Claim 18

The rotatable sputtering target according any one of claims 8 to 15 wherein said target-forming material is zinc oxide impurity doped with aluminium or gallium.

Claim 19

The rotatable sputtering target according any one of claims 8 to 18 wherein said inner tube is made of titanium or of a titanium alloy.