Patent Application
20080011392
The present invention relates to a method of making a sputtering target and, in particular, to a method of casting a metallic sputtering target to have an equiaxed, cellular, non-dendritic microstructure.
A current process employed to make metallic sputtering targets comprises crushing a slab of the metallic material, screening and sorting the crushed particles to appropriate particle sizes, hot isostatic pressing (HIP'ing) particles of certain sizes in an evacuated, sealed can to from a target body, and then machining the HIP'ed body to produce the desired target shape.
Another method currently used to make a large molybdenum sputtering target is to cold isostatic press (CIP) Mo powder, sinter the cold pressed body to reduce the oxygen content, and then hot roll the sintered body to a flat plate or disk of desired length/width/thickness. The plate or disk then is machined to final tolerance.
These processes involve numerous processing steps and considerable cost to make the sputtering target.
The present invention provides a method for making a fine grain, cast sputtering target. The present invention provides in an embodiment a method of making a sputtering target by melting a metallic target material, controlling the temperature of the melted target material in a manner that the melted target material has almost no superheat, introducing the melted target material into a mold having interior walls forming a mold cavity in the shape of the desired target, and solidifying the melted target material in the mold by extracting heat therefrom at a rate to solidify it to form a sputtering target having substantially equiaxed, cellular nondendritic microstructure uniformly throughout the target. The mold optionally can be heated to a high enough elevated mold temperature that prevents substantial columnar grain formation directly adjacent interior walls of the mold
The present invention also provides in another embodiment a metallic sputtering target having a substantially equiaxed, cellular nondendritic microstructure uniformly throughout the target. The sputtering target can be used in the as-cast condition without further post-cast treatments other than finish machining or after the as-cast target is hot isostatically pressed to densify the as-cast target.
The invention is advantageous to provide a cast sputtering target without the need for numerous processing steps employed in the art and to provide a sputtering target with beneficial microstructural properties for sputtering.
The invention also provides grain size control of the target, reduces manufacturing lead times from material selection to target manufacture, and increased material selection flexibility such as more alloying options.
Other advantages, features, and embodiments of the present invention will become apparent from the following description.
The present invention provides a method of making a sputtering target comprising a metallic target material. The metallic target material can comprise a metal or an alloy of two or more metals. For purposes of illustration and not limitation, the target material can comprise molybdenum, tungsten, and other metals and high temperature melting alloys such as nickel based, chromium based, cobalt based, iron based, tantalum based, molybdenum based, tungsten based, and other alloys materials. For purposes of illustration and not limitation, a target alloy can comprise a cobalt based alloy including an alloying element selected from the group consisting of boron, chromium, platinum, tantalum, ruthenium, niobium, copper, vanadium, silicon, silver, gold, iron, aluminum, zirconium, and nickel. For example, the target can comprise cobalt based alloys including, but not limited to, a Co—Ta—Zr alloy, Co—Ta—B alloy, Co—Cr—Pt—B alloy, Co—Cr—Pt—B—Cu alloy and others. Such target metals or alloys can be obtained commercially from raw materials suppliers with the appropriate purity for particular sputtering target applications. The target metals or alloys are supplied in the form of briquets, powder, chunks, etc.(shown as INPUT: ALLOY CONTROL in
Referring to
A particular conventional vacuum induction melting furnace used in the Example employs a melting crucible that pours directly into an underlying mold M. However, the invention envisions use of a pouring vessel, such as a pouring crucible, optionally as an intermediate vessel between the melting vessel and the mold to be cast.
Preferably, the melted target material in the melting vessel or in the pouring vessel is held in a substantially quiescent state to allow any low density non-metallic inclusions to float to the surface where they can be disposed of or eliminated from the melt. For example, when vacuum induction melting is used to melt a charge of target material, a susceptor such as graphite can be placed between the induction coil IC and the melting vessel such that the susceptor is heated and in turn heats the charge and such that the melted target material is not stirred. Alternately, very high frequencies or resistance heating may be employed to achieve the same results.
Furthermore, use of a bottom pouring crucible allows melted target material to be introduced into a mold without entraining the floating non-metallic inclusions on the melt surface. Alternately, a teapot crucible can be used to block non-metallic inclusions floating on the melt from entering the mold. Other techniques for minimizing the amount of non-metallic inclusions entering the mold are described in U.S. Pat. No. 4,832,112 which is incorporated in its entirety herein by reference.
The invention further involves controlling the temperature of the melted target material TM in the melting or pouring vessel in a manner that the melted target material has almost no superheat prior to introduction into the mold. The temperature of the melted target material is reduced to remove up to substantially all of the superheat in the melted target material. This reduced temperature should be substantially uniform throughout the melted target material and, for most target materials, is controlled to be within 0 degree to 20 degrees F. above the measured melting point of the particular metal or alloy target material, although the range may be adjusted in dependence on the particular target metal or alloy. The measured melting point can be determined as described in U.S. Pat. No. 4,832,112.
The temperature of the melted target material in the melting vessel can be reduced by gradually reducing the power or energy supplied to the melting furnace in which the melting vessel is located. For example, when the charge of target material is melted by vacuum induction melting as described in the example below, the electrical power supplied to the induction coil IC can be gradually reduced to reduce the temperature of the melted target material so that substantially all of the superheat is removed prior to introduction of the melted target material into the mold. The temperature of the melted material can be measured (shown as TEMPERATURE MEASUREMENT) using the infrared pyrometer shown or other temperature measuring device.
The mold M can include a metal or ceramic mold that includes interior walls defining a mold cavity having the shape of the desired sputtering target. Typical shapes of sputtering targets that can be made include, but are not limited to, plates of rectangular, square or other polygonal shape and circular discs.
Except when making investment cast sputtering targets, the invention envisions optionally generating turbulence in the melted target material after it is introduced into the mold. For most target materials, it is sufficient to pour the melted target material directly into the mold. The turbulence alternately can be imparted to the melted target material in the mold by electromagnetic stirring, mechanical stirring, and comminuting the melt as it is poured in to the mold such as by breaking the melt into multiple streams or droplets as it enters the mold as described in U.S. Pat. No. 4,832,112.
In accordance with the invention, the melted target material is solidified in the mold by extracting heat therefrom at a rate to obtain a substantially equiaxed, cellular, nondendritic grain structure throughout the sputtering target. The as-solidified (as-cast)sputtering target preferably has an equiaxed, cellular ASTM grain size of 3 or less throughout the sputtering target. The rate of heat extraction is controlled to achieve such equiaxed, cellular grain structure. In some instances, the initial temperature gradient between the melted target material and the relatively cold mold is sufficiently high to produce a zone of dendritic columnar grains at the interface. The invention envisions optionally heating the mold to a high enough elevated mold temperature (shown as Controlled Preheat Process and PREHEATED MOLD) that prevents substantial columnar grain formation directly adjacent interior walls of the mold. The solidified target has a net or near net shape of the desired target and requires only minimal machining prior to use as a target.
As the aspect ratio of the mold increases, it is increasingly important to extract heat more rapidly from the solidifying target material to maintain the fine grain size and associated cellular microstructure and to minimize the increasing tendency for porosity and possible segregation. Improved heat extraction can be facilitated by the previously disclosed comminution of the melted target material as it is poured into the mold.
In the event the solidified, as-cast sputtering target has some porosity, this porosity can be removed by various techniques including by hot isostatic pressing (HIP'ing) the as-cast sputtering target using conventional hot isostatic gas pressing processes whose parameters of gas pressure, temperature and time will depend on the particular target metal or alloy employed. Control and removal of as-cast porosity of the sputtering target is described in U.S. Pat. No. 4,832,112.
For purposes of further illustrating the invention and not limiting it in any way, a rectangular sputtering target having dimensions of 27 inches length by 4.25 inches width by 0.2 inches thickness can be cast in a conventional preheated ceramic investment mold, which is positioned in a lower chamber of a conventional vacuum induction furnace. The preheated investment mold will include a mold cavity that closely replicates the desired shape of the sputtering target. The target metal or alloy comprising for example a cobalt based alloy of the type described above can be heated in an upper chamber of the furnace under vacuum conditions below 10 microns to a temperature about 20-50 degrees F. above its melting point to melt it in a zirconia crucible. Power to the induction coil of the furnace can be gradually reduced until the melted target material is within 0 to 20 degrees F. of the melting point. The melted target material then can be poured into the mold which can contain a constriction at the top of the mold that forces rapid local solidification at the center line of the mold cavity. This can prevent the formation of interconnected porosity at the center line and allowed densification of the as-cast sputtering target, when necessary, by HIP'ing the target at 2100 degrees F. at 29 KSI gas pressure for 1 hour. The resultant HIP'ed sputtering target exhibits a fine grain, equiaxed cellular grain structure.
Although certain embodiments of the invention have been described above, those skilled in the art will appreciate that the invention is not limited to these embodiments and that modifications and changes can be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.
This application claims benefits and priority of U.S. provisional application Ser. No. 60/831,521 filed Jul. 17, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60831521 | Jul 2006 | US |
