Tantalum
exists in nature exclusively as an oxide, Ta
2O
5. Although tantalum can be extracted as a byproduct from the smelting of cassiterite, or from columbite, the primary source is commercially mined tantalum ores, the principle mineral of which is tantalite, (Fe,Mn)Ta
2O
6.1,2 The quality of ore varies among regions in both Ta
2O
5 concentration and levels of specific impurities, both of which can influence the ore benefication costs. Mines in western Australia are the largest producers of high-grade tantalite, and such concentrates typically are provided directly to the primary tantalum metal manufacturers for processing. Low-grade concentrates and tin slags must be upgraded to remove much of the bulk minerals in order to elevate the Ta
2O
5 content.
Known tantalum reserves, estimated to exceed 100 M pounds, are sufficient to satisfy worldwide demand for tantalum for at least the next several decades. The tantalum ore resources are distributed widely about the global regions (
Fig. 1). The largest single producer of tantalum ore is the Sons of Gwalia, whose two mines in western Australia account for more than 40% of the reserves. In addition to known reserves in operating or developing mines, there are considerable additional potential resources elsewhere, primarily in western Australia, the Rift Valley region of Africa, Greenland, Canada, South America and Asia.
Fine chemical compounds, capacitor-grade powder and mill products are the primary market segments for tantalum; the relative size of each segment is illustrated in Figure 2. Combined, these segments consumed nearly 4.2 M pounds of Ta2O5 in 1999, and their demand is expected to grow at a moderate rate (nominally 5% annual average growth rate) over the next five years.1,3 Ore is not the only source of tantalum: the turning of existing inventories through recycling is commonplace in the tantalum industry. Today, nearly 25% of the total tantalum production is recycled. The percentage of tantalum reclaimed is expected to increase significantly with growth of the tantalum sputtering target market. Should the market for high-quality tantalum sputtering targets outpace the rate of increase in tantalite production, then it might become necessary for fabricators to return spent targets to the tantalum manufacturers in order to be assured of future availability.
Tantalum manufacturing
1. Worldwide distribution of confirmed tantalum ore reserves.
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2. High-performance tantalum products, including sputtering targets, represent less than 10% of the tantalum mill product and wire business.
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Like other refractory metals, the extraction and refinement processes for tantalum conclude with a chemical precipitation reaction that yields the consolidated metal in the form of a powder. The reduction parameters are controlled to produce high-surface-area, capacitor-grade powders, or high-tap-density metallurgical-grade powders. The latter powders can be processed into powder metallurgy wire and strip, or blended with capacitor and recycled scrap as feed for vacuum melting. Cabot Corp.'s Performance Materials Division (Boyertown, Pa.), H.C. Starck (Gosslar, Germany) and Ningxia Non-Ferrous Metals Smeltery (Shizuishan City, Ningxia, China) are the primary tantalum manufacturers that refine ore into metal. These organizations manufacture tantalum powders for use as capacitor anodes, or for subsequent fabrication into powder metallurgy (P/M) wire and strip or ingot-derived mill products. Cabot and Starck are the prominent suppliers of tantalum powder and mill products; aside from extensive manufacturing capabilities, both of these primary manufacturers have long-term contracts with the tantalite mines that serve to secure the availability of raw material and help stabilize the price of tantalum.
3 Tantalum differs from the more common refractory metals such as tungsten and molybdenum in that it can be readily vacuum-cast and fabricated into a variety of flat and round forms. The manufacture of tantalum mill products also provides the industry with an economical means of consuming its scrap. The primary tantalum manufacturers employ tantalum recycle sources to supplement their melt stock, whereas the smaller secondary metal manufacturers must rely solely on purchasing Ta scrap from the open market to remelt and process into mill forms (
Fig. 3). Clearly, the cost, availability and integrity of tantalum produced by secondary manufacturers are highly sensitive to fluctuations in the spot market price and impurity content of scrap available for remelting.
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3. Tantalum supply chain showing primary (black) and secondary (red) raw material flow.
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The largest applications for ingot-derived tantalum products are as additives for superalloys, and as corrosion-resistant linings and conduits for the chemical processing industry; for these product lines, functionality is independent of material attributes such as purity or microstructure. Hence, for the majority of the tantalum mill product market, material requirements are not stringent and can be achieved easily by most manufacturers' offerings. Yet for demanding applications such as ballistics, deep-drawing, and sputter deposition, product performance is directly dependent on the quality and consistency of the tantalum, which is recognized to vary significantly between the different manufacturers' material. Fortunately, the manufacturing capabilities and much of the knowledge base needed to produce high-reliability tantalum already have been developed.4 Although the demand for high-performance tantalum currently accounts for less than 10% of the total mill product market, its share is expected to grow as consumption of tantalum sputtering targets escalates.
Melting and purification
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4. Microstructure of improperly processed Ta sputtering target showing polygomized bands and non-uniform grain structure. Metallurgical inhomogeneities have been reported to hinder the performance of tantalum sputtering targets.
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From a feedstock composed of low-purity tantalum powder and/or scrap, high-purity tantalum ingots are cast using electron beam (EB) and/or vacuum arc remelting (VAR) techniques. Because tantalum exhibits a great affinity for oxygen, nitrogen and carbon, the metal usually is worked at ambient temperature, and any subsequent thermal treatments must be conducted in a vacuum or inert gas atmosphere. Typically, tantalum ingots are melted several times to purge most of the impurities from the metal. EB melting is more efficient than VAR for purifying tantalum; the greater vacuum level and melt temperature achieved during EB melting better allow for the volatilization of interstitial gases (O, N, C and H) and most metallic impurities. However, due to the relatively high vapor pressures of the refractory metal elements, niobium, molybdenum and tungsten are not volatized during vacuum melting operation but remain as a soluble impurity in tantalum. The refractory metal contaminants can be minimized through specialized front-end chemical extraction processes. Cabot employs its 1200 KW EB furnaces to triple-melt high-purity tantalum powder feedstock to produce 300 mm-diameter ingots of 99.999% tantalum weighing nearly 2000 kg.
5 VAR is a lower-cost alternative to EB melting and often is used for casting small tantalum ingots. The finer as-cast grain size intrinsic to VAR tantalum ingots helps reduce the critical amount of cold work necessary to achieve a recrystallize grain structure in the finished annealed product. In addition, VAR enables small amounts of transition metal and rare earth elements to be retained in the form of second-phase dispersions, which then act as potent grain refiners in tantalum.
4 Microstructural uniformity
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5. Microstructure of Ta target fabricated by thermomechanical processing of 300 mm ingot revealing a uniform and equiaxed grain structure. Metallurgical homogeneity is desirable for consistent performance through the life of the tantalum sputtering target.
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Tantalum is unusual among metals in that it can be cold rolled to a true strain (epsilon) of -2.5 without intermediate annealing. However, imparting an excessive amount of cold work alone will not assure that the finished recrystallized tantalum blank will contain a fine, homogeneous grain structure as deemed desirable for sputtering targets. In polycrystalline tantalum, the extent to which individual grains convert strain energy into stored cold work depends on the orientation of the crystallites: (111) orientations effectively generate new dislocation line length and will recrystallize readily, whereas (001) crystals do not sufficiently store cold work and will only polygomize after exposure to a high annealing temperature.
6 Relics of as-cast grains are evident in the microstructure of a tantalum target presented in
Figure 4; any misoriented, as-cast ingot grains now appear as isolated bands of polygomized or unrecrystallized regions within the wrought product. Microstructural inhomogeneities such as bimodal grain size distribution, large and elongated grains, and bands of unrecrystallized or recovered material are common in high-purity tantalum products rolled or upset-forged directly from small ingots. For sputtering targets and other high-technology applications, specialized equipment and processes must be used to achieve the chemical and metallurgical integrity critical to the product's performance.
Figure 5 reveals the uniform microstructure of a tantalum target that was manufactured using a thermomechanical process that included controlled deformation schedules amid multiple annealing operations. Starting with large-diameter tantalum ingots and employing intermediate anneal operations are key to controlling the microstructural uniformity in the finished tantalum target. The target material shown in
Figure 5 was processed from a 300 mm-diameter, triple-EB-melted ingot of 99.99% pure tantalum; this starting ingot size allows for a sufficient amount of cold work to be imparted to the material prior to each thermal treatment.
Texture control and sputtering performance
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6. Inverse pole figure map of improperly processed Ta target revealing [red] (001) and [blue] (111) texture bands. (011) Orientations are represented as green.
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In general, the bulk recrystallization texture of wrought tantalum will lie on or near the (111)-(001) symmetry line of the standard orientation triangle.
7 The proportion of (111) to (001)-type texture components is governed primarily by deformation history and is less sensitive to melting process or final annealing temperature.
4,8,9,10 Several researchers have reported that annealed tantalum plate contains a texture gradient that varies from (001) at the surface to a primary (111)-type along the mid-plane of the plate.
11,12,13 Furthermore, most recent studies of commercial Ta plate reveal that atop the texture gradient lie several narrow bands of sharp, localized texture.
14,15 While the origin of the texture gradients in tantalum remains the subject of debate, it is hypothesized that textural bands, like microstructural discontinuities, are the remnants of as-cast ingot grains.
15 Much of the research on microstructural and textural banding in tantalum has focused on its impact on plastic deformation;
10,14,16,17,18,19 only of late have metallurgical inhomogeneities been shown to influence the performance of tantalum sputtering targets. Indeed, it has been demonstrated that low-quality tantalum targets containing large (001) orientation clusters are highly resistant to sputter erosion.
15 Both sputtering theory and experimental observations confirm that the sputter yield of metals varies with planar orientation.
20,21 For body-centered cubic (BCC) metals such as tantalum, the erosion rate of (111) incident planes is expected to be far greater than that for (001) crystals.
22 Figure 6 and
Figure 7 present the Inverse Pole Figure (IPF) maps for the two tantalum targets whose microstructures are shown in
Figure 4 and
Figure 5, respectively. Comparing the optical photomicrographs to the IPF maps of each sample indicates that microstructural and textural homogeneity of tantalum sputtering targets are indeed correlated. Regions comprised of a large recovered grain structure, as evident in
Figure 4, correspond to primary (001) texture bands in
Figure 6. These observations support the claims of others regarding an interrelationship between grain structure and texture in tantalum.
6,10,15 | 
7. Inverse pole figure map of Ta target fabricated by thermomechanical processing of 300 mm ingot revealing an absence of localized texture bands. (001), (111) and (011) orientation represented as red, blue and green, respectively.
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Evidence of textural banding is readily observed in the sputter-eroded surface of tantalum targets that exhibit a non-uniform grain structure. Large areas of sputter-resistant (001) tantalum will appear as highly lustrous bands or surface regions, while the portions of the surface sputtered from fine grains of mixed or primary (111) orientations contain a rough, matte finish. The intertwining of lustrous and matte surface finishes often shown in lesser-quality tantalum targets has been defined as "marbleizing;" and the tantalum thin films deposited from said targets have been reported to exhibit excessive thickness variability. Hence, attaining textural homogeneity in tantalum through use of proper thermomechanical processing is critical for assuring the performance of tantalum sputtering targets.
Summary
The level of quality typical of commercial-grade tantalum, while sufficient for the vast majority of mill product applications, fails to fulfill the requirements of high-performance products such as sputtering targets. The purity and metallurgical attributes necessary to assure the utmost reliability of tantalum sputtering targets can be attained through proper thermomechanical processing of large-diameter ingots of high-purity tantalum. Superior sputtering performance is realized from tantalum targets that possess microstructural and textural homogeneity, and are void of sharp bands of local (001) texture. Film deposited from said tantalum targets exhibits exceptional thickness uniformity as desired for semiconductor wafer manufacturing. •
Richard O. Burt is the director of Cabot Mineral Development, a position he’s held since 1997. After graduation from the Royal School of Mines, London, in 1963, he spent time in Uganda and in Cornwall (U.K.) prior to joining Tantalum Mining Corp. of Canada (Tanco) in 1977. He was appointed general manager of Tanco in 1983 and retained the position following Cabot’s purchased of Tanco in 1993. He has specialized in the gravity concentration of minerals, having written the authoritative text on the subject. Awarded the Selwyn Blaylock Medal for his contributions to Canadian Mining in 1998, he is currently Overseas Member of Council of the Institution of Mining & Metallurgy in the U.K.David P. Lewis is senior marketing analyst for Cabot Corp. A 1976 graduate of Penn State University with a B.S. in chemical engineering, he worked nearly 20 years for Dupont, initially as an engineer, then advancing into business and market research. He joined Cabot as a market analyst in 1994.
e-mail: david_lewis@cabot-corp.com
Christopher A. Michaluk is technical marketing manager of mill products for the Performance Materials Division of Cabot Corp. He spent eight years working for Picatinny Arsenal as an armaments materials researcher, primarily supporting the U.S. Army’s tantalum warhead development efforts, prior to joining Cabot in 1990. He was project leader for tantalum metallurgical products at Cabot Performance Materials before accepting his current position in 1996. He is an author of 17 technical publications on the subject of tantalum processing and characterization. He holds B.S. and M.S. degrees in materials engineering from Drexel University.
REFERENCES- J. Linden, T.I.C Bulletin, No.100, Tantalum-Niobium International Study Center, Brussels, Dec. 1999, pp.2.
- E.E. Foord, MAC Short Course Handbook, Mieralogical Association of Canada, Toronto, 8, 1982, pp. 187.
- L.D. Cunningham, Mineral Commodity Summaries, U.S. Geological Survey, February 2000.
- C.A. Michaluk, Tantalum, E. Chen et.al. (eds.), TMS, Warrendale, Pa., 1996, pp. 205.
- Tantalum on Target, Cabot Corporation, 2000.
- R.A. Vandermeer and W.B. Snyder, Met. Trans. A, 10A, August 1979, pp. 1031.
- C. Feng, T. Chatterjee, and L. Ting, High Strain Rate Behavior of Refractory Metals and Alloys, R. Asfahani, E. Chen, and A. Crowson (eds.), TMS, Warrendale, Pa.., 1992, pp. 45.
- J.B. Clark, R.K. Garrett, Jr., T.L. Jungling, R.A. Vandermeer and C.L. Vold, Met. Trans. A, 22A, 1991, pp. 2039.
- J.B. Clark, R.K. Garrett, Jr., T.L. Jungling and R.I. Asfahani, Met. Trans. A, 22A, 1991, pp. 2959.
- J.B. Clark, R.K. Garrett, Jr., T.L. Jungling and R.I. Asfahani, Met. Trans. A, 23A, August 1992, pp. 2183.
- S.I. Wright, Computer-Assisted Microscopy, 5 (3), 1993, pp. 207.
- H. Klein, C. Heubeck and H.J. Bunge, Mat. Sci. Forum, 157-162, H.J. Bunge (ed.), 1994, pp. 1423.
- C.A. Michaluk, C. Heubeck and H. Klein, Tantalum, E. Chen et.al. (eds.), TMS, Warrendale, Pa.., 1996, pp. 123.
- S.I. Wright, A.J. Beaudoin and G.T. Gray, Mat. Sci. Forum, 157-162, H.J. Bunge (ed.), 1994, pp. 1695.
- C.A. Michaluk, D.B. Smathers and D.P. Field, Proc. 12th Int. Conf. Texture of Materials, J.A. Szpunar (ed.), National Research Council of Canada, 1999, pp. 1357.
- S.I. Wright, G.T. Gray and A.D. Rollett, Metall. Mat. Trans, A, 25A, 1994, pp. 1025.
- A.J. Schwartz, J.S. Stölken, W.E. King, G.H. Cambell, D.H. Lassila, S. Sun and B.L. Adams, J. Eng. Mat. and Tech., 121, April 1999, pp 001.
- J.F. Bingert, P.B. Desch, S.R. Bingert, P.J. Maudlin and C.N. Tome, Tungsten Refractory Metals and Alloys 4, A. Bose and R.J. Dowding (eds.), Metal Powder Industries Federation, Princeton, 1998, pp.169.
- R.J. DeAngelis and J.W. House, Proc. 12th Int. Conf. Texture of Materials, J.A. Szpunar (ed.), National Research Council of Canada, 1999, pp. 659.
- G.D. Magnuson and C.E. Carlston, J. Appl. Phys., 34 (11), 1963, pp. 3267.
- D. Onderdelinden, Can. J. Phys., 46, 1968, pp. 739.
- H.E. Roosendaal, Sputtering by Particle Bombardment I: Physical Sputtering of Single-element Solids, Behrisch (ed.), Springer-Verlag, Berlin, 1981, pp. 219-256.
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