Abstract:
A process for the manufacture of sputtering target comprises the steps of i) providing a substrate; ii) plasma melting of a material selected to form the sputtering target, yielding droplets of molten material; and iii) deposition of the droplets onto the substrate, yielding a sputtering target comprised of the coated layer of the material on the substrate. In some application, it might be preferable that the substrate be a temporary substrate and iv) to join the coated temporary target via its coated layer to a permanent target backing material; and v) to remove the temporary substrate, yielding a sputtering target comprised of the coated layer of the material on the permanent target backing material. The plasma deposition step is carried out at atmospheric pressure or under soft vacuum conditions using, for example, d.c. plasma spraying, d.c. transferred arc deposition or induction plasma spraying. The process is simple and does not require subsequent operation on the resulting target.

Description:
FIELD OF THE INVENTION 
     The present invention relates to sputtering targets. More specifically, the present invention is concerned with a process for the manufacture of sputtering targets and with an apparatus therefore. 
     BACKGROUND OF THE INVENTION 
     Sputtering targets are essential components in the thin film coating industry. They are used as a source of high purity materials, which are released from the target surface as a result of its bombardment with energetic projectile particles such as an ion beam. The particles that are released, in the form of vapours, are subsequently directed towards the surface of a substrate where they are deposited in the form of a thin film of controlled thickness and purity. The most striking characteristic of the sputtering process is its universality. Virtually any material can be a coating candidate since it is passed into the vapour phase by a physical momentum-exchange rather than a chemical or thermal process. 
     Thin film deposition using sputtering techniques is an essential step in a wide range of applications such as aluminium alloy and refractory metal microcircuit metallization layers, microcircuit insulation layer, transparent conduction electrodes, amorphous optical film for integrated optics devices, piezo-electric transducers, photoconductors and luminescent film for displays, optically addressed memory devices, amorphous bubble memory devices, thin film resistors and capacitors, video discs, solid electrolytes, thin film laser and microcircuit photolithographic mask blanks. 
     Target materials vary depending on the application. They can be formed of pure metals such as aluminium, copper, iron, silver, chromium, silicon, tantalum, gold, platinum, rhenium; alloys and compounds, such as cadmium sulfate, gallium arsenate, gallium phosphate; a wide range of ceramics such as silica, alumina, silicon carbide; polymers such as PTFE (Teflon™); or even a mosaic of different materials. The performance of the target is strongly dependant on the purity of the target material, its apparent density and microstructure. 
     Sputtering targets have traditionally been manufactured through the use of different powder metallurgical techniques for the formation of the target plate made of high purity materials, which are subsequently mounted on the target backing material for proper heat management under its final operating conditions. The technique is relatively tedious and requires a number of steps for the powder preparation and densification, followed by powder compaction at room temperature and subsequent sintering to the required high-density level. In certain cases, room temperature compaction is not sufficient to achieve the required density of the final product. In such cases, it is necessary to resort to the considerably more complex and expensive Hot Iso-static Pressing (HIP) sintering techniques. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is therefore to provide improved process and apparatus for the manufacture of sputtering targets. 
     SUMMARY OF THE INVENTION 
     The present invention deals with a novel technique that can be used for the production of sputtering targets in a series of simple steps using plasma spraying/deposition technology. 
     According to a first aspect of the present invention, there is provided a process for the manufacturing of sputtering targets through the plasma deposition of the target material directly on the target support, or on a temporary substrate, from which the deposit is latter transferred to the final baking of the target material. 
     More specifically, in accordance with a first aspect of the present invention, there is provided a process for the manufacture of a sputtering target comprising: 
     i) providing a substrate having a coating-receiving surface; 
     ii) plasma melting in-flight of a target material selected to form the sputtering target in powder form, yielding droplets of molten target material; and 
     iii) deposition of the droplets onto said coating-receiving surface of the substrate, yielding a sputtering target comprised of a coated layer of the target material on the coating-receiving surface of the substrate. 
     The deposited target material can be, for example, a metal, or an intermetalic alloy, a ceramic, a mosaic of different metals and/or mixture of metals and ceramics, or a polymer. 
     The plasma deposition step is carried out at atmospheric pressure or under soft vacuum conditions using, for example, d.c. plasma spraying, d.c. transferred arc deposition or induction plasma spraying. 
     More specifically, in accordance with a second aspect of the present invention, there is provided an apparatus for the manufacture of a sputtering target comprising: 
     a plasma torch for melting of a material selected to form the sputtering target, yielding droplets of molten material and for deposition of these droplets onto a coating-receiving surface of a substrate, yielding a sputtering target comprised of a coated layer of the material on the coating-receiving surface of said substrate. 
     Other objects, advantages and features of the present invention will become more apparent upon reading the following non restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the appended drawings: 
         FIG. 1  is flowchart of a process for the manufacture of sputtering targets according to an illustrative embodiment of the present invention; 
         FIG. 2  is a schematic side elevation view illustrating the various steps from  FIG. 1 ; 
         FIG. 3  is a schematic top plan view illustrating the various steps from  FIG. 1 ; 
         FIG. 4  is a schematic top plan view of a temporary substrate according to a second illustrative embodiment of the present invention; 
         FIG. 5  is a schematic sectional view of a d.c. plasma apparatus according to a first illustrative embodiment of a plasma apparatus allowing to perform the plasma deposition step from the process of  FIG. 1 ; 
         FIG. 6  is a schematic sectional view of a d.c. transferred arc according to a second illustrative embodiment of a plasma apparatus allowing to perform the plasma deposition step from the process of  FIG. 1 ; 
         FIG. 7  is a schematic sectional view of a r.f. induction plasma apparatus according to a third embodiment of a plasma apparatus allowing to perform the plasma deposition step from the process of  FIG. 1 ; 
         FIG. 8  is a schematic sectional view of a supersonic r.f. induction according to a fourth embodiment of a plasma apparatus allowing to perform the plasma deposition step from the process of  FIG. 1 ; 
         FIG. 9  is a graph of radial profiles of silicon target material deposited on a graphite temporary substrate for two experimental runs (no.  1  and no.  2 ); 
         FIGS. 10   a  and  10   b  are electron micrographs of the deposited material for experimental runs no.  1  and no.  2 ; and 
         FIG. 11  is a graph of apparent density profile for the silicon deposit following experimental run no.  1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A process  100  for the manufacture of sputtering targets according to an illustrative embodiment of a first aspect the present invention will now be described with reference to  FIGS. 1-3 . 
     As illustrated in  FIG. 1 , the process  100  includes the following steps: 
       102 —providing a temporary substrate  200  having a coating-receiving surface  204 ; 
       104 —plasma melting of the material selected to form the sputtering target, yielding droplets of molten material; 
       106 —deposition of the droplets onto the coating-receiving surface  204  of the temporary substrate  200 , yielding a coated layer  206  of the material on the coating-receiving surface  204  of the temporary target  200 ; 
       108 —joining the coated temporary target via its coated layer  206  to a permanent target backing material  208 ; and 
       110 —removing the temporary substrate  200 , yielding a sputtering target  210  comprising the coated layer of the material  206  on the permanent target backing material  208 . 
     Each of these steps will now be described in more detail with reference to  FIGS. 2 and 3 . 
     In step  102 , a temporary substrate  200  having a coating-receiving surface  204  is provided (see first step on  FIGS. 2 and 3 ). Even though the temporary substrate  200  is illustrated in  FIGS. 2 and 3  as being a flat disk having two opposite sides, one being a coating-receiving side  204 , the temporary substrate can have other shape providing a coating-receiving surface. 
     Proper selection of the temporary substrate material and control of its temperature during the deposition step  106  allows minimizing internal stresses and cracks in the target material  206  to be deposited (steps  104 - 106 ). Examples of substrate materials that can be used include: refectory metals such as molybdenum or tungsten; ceramics such as silica, alumina, zirconia, silicon nitride, silicon carbide or boron nitride; or graphite or carbon-based composite materials. 
     The temporary substrate material is selected for resistance to the high temperature involved in the deposition process and so as to have a coefficient of thermal expansion similar to the one of the target material. Whenever the coefficients of thermal expansion of the target material  206  and that of the temporary substrate  200  are too different, it has been found that providing the substrate materials with markings allows to minimize the development of internal stresses into the deposit. Such a temporary substrate  200 ′ including spiral markings  202 ′ is illustrated in  FIG. 4  of the appended drawings. The markings can have many forms, such as spiral or radial cuts or sections giving the substrate  200 ′ a sufficient level of flexibility to avoid the development of internal stresses in the deposited coating  206 . 
     In certain application, the temporary substrate  100  is pre-coated by a thin diffusion barrier, or non-sticking, control layer of an inert material in order to facilitate the sub-sequential removal of the target material  206  from the temporary substrate  200  and/or avoid the contamination of the deposited target material  206  due to diffusion between the temporary substrate  200  into the deposited target material  206 . The use of standard sputtering techniques or Chemical Vapor Deposition (CVD) techniques for the deposition of such an adhesion/diffusion-control layer (not shown) could be effectively used for such a purpose. The proper control of the surface finish of the substrate  200  could also allow for the easy removal of the target material  206  from the substrate  200  without sacrificing the substrate  200  which could be reused in these cases for further target material deposition. 
     The process  100  then includes the plasma melting of the material selected to form the sputtering target, which yields droplets of molten material (step  104 ) and then the deposition of those droplets onto the coating-receiving surface  204  of the temporary substrate  200  (step  106 ). The deposit  206  is built up to thickens of a coated layer of the material  206  require by the sputtering target application, as illustrated in the second step illustrated in both  FIGS. 2 and 3 . 
     Once the coating-formation step  106  has been completed, the resulting target plate  206  with its temporary support  200  is joint to a permanent target backing material  208  (step  108 ) via its coated layer  206 , as illustrated in the third step of  FIGS. 2 and 3 , using either soldering, epoxy or other appropriate bonding means or techniques. 
     A heat treatment of the target material deposit  206  can be performed before the soldering step  108  onto the permanent target material support  208  in order to relief the deposited target material  206  from internal stresses. Alternatively, the target material deposit  206  can be provided with stress relieving markings similarly to those described hereinabove with reference to the temporary substrate  200 . 
     The temporary substrate  200  is then removed for example by etching, by machining it off (step  110 ) or using another material removing method, yielding a sputtering target  210 . 
     The surface of the target  210  can then be appropriately finished through grinding and polishing, for example. 
     In some applications, the plasma deposition of the material  206  can be executed directly onto the permanent target backing material  208 . Of course, in those cases, steps  108 - 110  of method  100  are not performed. An application of the process  100  can in this particular case be the rebuilding of used and depleted targets. 
     In cases where the deposit steps  104 - 106  are performed directly on the target support  208 , thermal protection of the target support  208  is provided during these steps. For example, target support  208  can be made of molybdenum or of an appropriate refractory material that would allow for the deposition of the target material  206  on the hot substrate  208 . 
     Steps  104 - 106  are achieved using a plasma spraying/deposition apparatus. 
     A first illustrative example of such a plasma apparatus, in the form of a direct current (d.c.) plasma jet  300 , is shown in  FIG. 5 . 
     Using the plasma jet  300 , steps  106 - 108  from the method  100  are performed by injecting the target material in powder form into a d.c. plasma jet under atmospheric or vacuum deposition conditions. The individual powder particles are molten, in-flight, and are accelerated towards the substrate  200  on which they impact forming a flat splat of the target material  206 . The coating  206  is built up through the successive deposition of those splats until the required deposit thickness is reached. The deposit quality, in terms of purity and apparent density, depends on the purity of the feed material and the deposition conditions such as, the plasma torch-to-substrate distance Z S , the substrate temperature, plasma gas compositions and plasma power. Typical operating conditions for such an apparatus  300  operating under vacuum deposition conditions are: 
     plasma gas flow rate=40 to 50 slpm; 
     plasma gas composition=Ar/H 2  with 10 to 20% vol. H 2 ; 
     plasma power=30-40 kW; 
     deposition chamber pressure=50 to 100 Torr; 
     plasma substrate deposition distance=10 to 20 cm; 
     powder feed rate=10 to 20 g/min; and 
     powder mean particle diameter=30 to 50 micrometers. 
       FIG. 6  illustrates a second illustrative example of a plasma apparatus  310  in the form of a d.c. plasma transfer arc. Since, the d.c. plasma arc apparatus  310  is very similar to the d.c. plasma jet apparatus  300  from  FIG. 5 , only the differences between the two apparatuses will be described herein in more detail. 
     With the d.c. transferred arc apparatus  310 , the transfer of the plasma arc is achieved between the plasma torch cathode and the substrate  200 , which act in this case as the anode. This technique is more prone to target material contamination by the substrate material  200 , though higher target material densities are generally achieved using this technique. 
       FIG. 7  illustrates a radio frequency (r.f.) induction plasma spraying apparatus  320  that can be used to perform steps  106 - 108  from the process  100 . With this particular apparatus, the plasma jet is generated through the inductive electromagnetic coupling of the energy into the plasma gas, which guarantees a considerably higher level of purity of the plasma environment compared to d.c. plasma technology. The target material  206  in powder form is melted through, in-flight, heating by the plasma gas and is accelerated towards the substrate  200  on which the deposit  206  is built up through the successive formation of individual particle splats. 
     According to a more specific embodiment of vacuum induction plasma spraying, the induction plasma torch is mounted on the top of an appropriate vacuum deposition chamber with the plasma jet directed vertically downwards. The substrate  200  on which the target material  206  is to be deposited is placed inside the vacuum deposition chamber, mounted on a translating and rotating mechanism (not shown), which allows the substrate  200  to be maintained in a near right angle orientation with respect to the direction of the plasma jet. According to this specific embodiment of the vacuum induction plasma spraying apparatus, the motion of the substrate  200  is set in such a way as to maintain a fixed deposition distance between the exit nozzle of the plasma torch and the substrate surface. While the size and shape of the individual splats formed depend on the particle diameter, its temperature and velocity prior to impacting on the substrate, the deposition thickness depends on the relative speed between the substrate and the plasma torch. The microstructure of the formed deposit depends, in turn, on the particle parameters prior to their impact on the substrate and the substrate conditions. It has been found that the substrate temperature, its linear velocity and angle of impact of the particles on the substrate influence the properties of the deposits  206  obtained. 
     Typical operating conditions of an inductively coupled r.f plasma deposition step using a PL-50 Tekna induction plasma torch are as follows: 
     plasma gas flow rates; 
     Sheath gas=90 slpm (Ar)+10 slpm (H 2 ); 
     Central gas=30 slpm (Ar); 
     Powder gas=9 slpm (He); 
     plasma plate power=80 kW; 
     chamber pressure=100 Torr; 
     powder feed rate=30-40 g/min; 
     powder material=silicon; 
     powder particle size distribution=40-90 micrometers; 
     plasma torch-to-substrate distance=20 cm; 
     substrate rotation=20 rpm; and 
     total deposition time=30 min. 
       FIG. 8  illustrates a radio frequency (r.f.) induction plasma spraying apparatus  330  similar to the apparatus  320  in  FIG. 7  with the addition of a Laval contraction/expansion nozzle, which allows for a significantly increase to the velocity of the particles prior to their impact on the substrate  200  and gives rise to the formation of a deposit  206  with a significantly finer grain structure compared to that obtained using subsonic plasma deposition conditions (see  FIG. 7 ). Nanostructured coating can also be generated using the plasma apparatus  330  through the use of nanosized feed powder. 
     Experiments have been conducted to demonstrate the possibility of using the process and apparatus from the present invention for the manufacture of silicon sputtering targets, through the vacuum induction plasma deposition of silicon powder on a graphite substrate. These experiments showed that target densities in excess of 99% of the theoretical density of elemental silicon could be obtained with a relatively uniform deposit thickness profiles, and a fine grain microstructures as can be seen on  FIGS. 9-11 . 
       FIG. 9  shows the radial profile of the silicon target material deposited on a 20 cm graphite substrate for two different experimentation runs. 
       FIGS. 10   a  and  10   b  are electron micrographs of the deposited material resulting from respectively run no.  1  and run no.  2 . 
     Finally,  FIG. 11  illustrates the apparent density profile for the silicon deposit for the run no.  1 . It is to be noted that the theoretical density of silicon is 2.33 g/cm 3 . 
     Although the present invention has been described hereinabove by way of preferred embodiments thereof, it can be modified without departing from the spirit and nature of the subject invention, as defined in the appended claims.