US20050155677A1

US20050155677A1 - Tantalum and other metals with (110) orientation

Info

Publication number: US20050155677A1
Application number: US11/030,260
Authority: US
Inventors: Charles Wickersham
Original assignee: Cabot Corp
Current assignee: Cabot Corp
Priority date: 2004-01-08
Filing date: 2005-01-06
Publication date: 2005-07-21
Also published as: WO2005071135A3; WO2005071135A2

Abstract

Tantalum metal, niobium metal, alloys thereof and other bcc metals and alloys thereof having a texture of primary or mixed (110) on the surface and/or throughout the thickness of the metal is described. Also described are the processes for making the tantalum metal and other bcc metal with a texture of primary or mixed (110) and the process of making a sputtering target from the tantalum metal or other bcc metal with a texture of primary or mixed (110).

Description

This application claims the benefit under 35 U.S.C. §119(e) of prior U.S. Provisional Patent Application No. 60/535,164 filed Jan. 8, 2004, which is incorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to metals, in particular tantalum, and products made from tantalum as well as methods of making and processing the tantalum.
In the industry, there has always been a desire to form metals having a uniform texture. With respect to tantalum, uniformed textured metals are especially desirable due to tantalum's use as a sputtering target and its use in electrical components such as capacitors. Tantalum sputtering targets are gaining increasing commercial importance as the source for the barrier metallization layer in copper dual-damascene integrated circuit metallization.
In sputtering, the crystallographic orientation, also known as texture, of the material being sputtered has a significant effect on the process parameters and the deposited film characteristics. According to Wehner, Phys. Rev. 102, 690 (1956), incorporated in its entirety herein by reference, in sputtering, the crystallographic orientation (also known as the texture) of the material being sputtered has a significant effect on the process parameters and the deposited film characteristics.
More recently, tantalum has become an increasingly important material as a barrier layer in integrated circuit fabrication. Consequently, there have been a number of publications and patents pertaining to tantalum sputtering targets with (100) and (111) crystallographic orientations. For example, Michaluk, (U.S. Pat. No. 6,348,113 B1), incorporated in its entirety herein by reference, describes, in part, tantalum-sputtering targets which can have a (111) crystallographic texture or mixed textures.
Similarly, there has always been a desire to form higher purity metals for a variety of reasons. With respect to tantalum, higher purity metals are especially desirable due to tantalum's use as a sputtering target and its use in electrical components such as capacitors. Thus, impurities in the metal can have an undesirable effect on the properties of the articles formed from the tantalum.
When tantalum is processed, the tantalum is obtained from ore. The ore is subsequently crushed and the tantalum is separated from the crushed ore through the use of an acid solution. The acid solution containing the tantalum is then separated from the acid solution containing niobium and other impurities by density separation. The acid solution containing the tantalum is then crystallized into a salt and this tantalum containing salt is then reacted with pure sodium in a vessel having an agitator typically constructed of nickel alloy material, wherein tungsten or molybdenum is part of the nickel alloy. The vessel will typically be a double walled vessel with pure nickel in the interior surface. The salt is then dissolved in water to obtain tantalum powder. However, during such processing, the tantalum powder is contaminated by contacting various surfaces such as the tungsten and/or molybdenum containing surfaces. Many contaminants can be volatized during subsequent melting, except highly soluble refractory metals (e.g., Nb, Mo, and W). These impurities can be quite difficult or impossible to remove, thus preventing a very high purity tantalum product.
Accordingly, there is a desire to obtain a uniformed textured tantalum. Also, there is a desire to have a tantalum product having higher purity, and/or a fine grain size. Qualities such as fine grain size can be an important property for sputtering targets made from tantalum since fine grain size can lead to improved uniformity of thickness of the sputtered deposited film. Further, other products containing the tantalum having fine grain size can lead to improved homogeneity of deformation and enhancement of deep drawability and stretchability which are beneficial in making capacitors, capacitor cans, laboratory crucibles, and increasing the lethality of explosively formed penetrators (EFP's). Uniform texture in tantalum containing products can increase sputtering efficiency (e.g., greater sputter rate) and can decrease normal anisotropy (e.g., increased deep drawability), in preform products.

SUMMARY OF THE PRESENT INVENTION

A feature of the present invention is to provide a tantalum metal or other bcc metal having a texture of primary (110) on the surface, or throughout the thickness, or a combination thereof. The tantalum metal or other bcc metals are preferably ingot-derived metals.
Another feature of the present invention is to provide a tantalum metal or other bcc metal exhibiting a fine grain structure with the above texture, which is preferably uniform.
A further feature of the present invention is to provide an increased sputtering yield of a metal target.
An additional feature of the present invention is to provide a tantalum metal having a close-packed crystallographic plane.
Another feature of the present invention is to provide articles, products, and/or components containing the tantalum or other bcc metal having a texture of primary (110).
An additional feature of the present invention is to provide processes to make the tantalum product as well as the articles, products, and/or components containing the tantalum or other bcc metal having a texture of primary (110).
A further feature of the present invention is to provide a tantalum metal, or other bcc metals having a mixed texture of (110) on the surface, and/or throughout the thickness of the metal, wherein the metal is preferably void of textural bands, such as (100) or (111) textural bands.
Another feature of the present invention is to provide a tantalum metal having a mixed texture of (110), on the surface, and/or throughout the thickness of the metal, wherein the mixed texture is uniformly distributed throughout the metal.
Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The objectives and other advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the description and appended claims.
To achieve these and other advantages, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention relates to tantalum metal or other bcc metals having a texture of primary (110). The metal preferably has a fine grain structure and/or uniform texture.
The present invention further relates to an alloy or mixture comprising tantalum, wherein the tantalum present in the alloy or mixture includes a uniform texture. The alloy or mixture (e.g., at least the tantalum present in the alloy or mixture) also preferably has a fine grain structure and/or a purity of at least 99.0% and more preferably at least 99.99%.
The present invention also relates to tantalum metal, e.g., suitable for use as a sputtering target, having a fully recrystallized grain size with an average grain size of about 150 μm or less and/or having a primary (10)-type texture substantially throughout the thickness of the tantalum and preferably throughout the entire thickness of the tantalum metal and/or having an absence of strong (100) and/or (111) textural bands throughout the thickness of the tantalum.
The present invention further relates to forming various components from the above-mentioned tantalum or other bcc metals by casting an ingot with a sufficient diameter so as to avoid any heavy working of the metal so as to maintain the (110) texture in the ingot. The ingot or subsequent shape can be annealed any number of times. Final products such as sputtering targets can be then machined from the unannealed or annealed metal, such as in the shape of a plate or sheet.
The present invention also relates to a sputtering target comprising the above-described tantalum or other bcc metals and/or alloy. The sputtering target can be formed a variety of ways.
The present invention further relates to resistive films and capacitors comprising the above-described tantalum and/or alloys thereof.
The present invention also relates to articles, components, or products which comprise at least in part the above-described tantalum and/or alloys thereof.
Also, the present invention relates to a process of making the above-described tantalum which involves heating the tantalum feedstock to a temperature above its melting point and solidifying the molten drops of the tantalum in a crucible.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates inverse pole figures for a tantalum metal having a texture of primary (110).
FIG. 2 illustrates (111), (110), and (100) pole figures for tantalum sputtering target surface.
FIG. 3 illustrates a grain map of tantalum plate predominantly having (110) crystallographic orientation in the sputtering plane.
FIG. 4 illustrates grain size distribution for a tantalum metal having a texture of primary (110) sputtering target.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to a metal having a (110) texture. The metal, such as tantalum metal, is preferably ingot-derived metal as opposed to powder metallurgy product. This texture is preferably a primary (110), and more preferably this texture is uniform on the surface and/or throughout the thickness of the metal. More specifically, the present invention relates to bcc metals, like tantalum metal. In the present invention, tantalum, which has a body-centered-cubic (bcc) atomic structure, preferably has a texture of primary (110). Given that the close-packed plane in tantalum is the (110) crystallographic orientation, a sputtering target fabricated with a predominately (110) crystallographic orientation parallel to the sputtering surface has an improved sputtering performance over the more typical (100) and (111) orientations. The advantages of using a tantalum metal having a texture of primary (110) include, but are not limited to, higher sputtering rate/yield than other principal orientations, such as the (100) and (111) orientations, and an improved sputter ejection pattern for improved film uniformity. For purposes of discussion only, the preferred metal, tantalum and alloys thereof, shall be discussed below. However, the same discussion including the various parameters apply equally to other bcc metals, including niobium, and alloys thereof.
Preferably, the texture of primary (110) is on the surface and/or throughout the thickness of the metal. Preferably, the tantalum metal of the present invention includes an absence of textural bands. In another embodiment, the present invention relates to a tantalum metal which includes mixed (110) texture throughout its thickness, and preferably is substantially void of (100) and/or

- (111) textural bands.

The tantalum metal or other bcc metal, can have any purity such as 95% or greater. Preferably, the purity of the metal is 99% or greater, 99.95% or greater, 99.99% or greater, and 99.995% or greater. This purity can exclude gases. Preferably, the tantalum metal has a purity of at least 99.999% and can range in purity from about 99.995% to about 99.999% or more. Other ranges include about 99.998% to about 99.999% and from about 99.999% to about 99.9992% and from about 99.999% to about 99.9995%. The present invention further relates to a metal alloy which comprises the tantalum metal, such as a tantalum based alloy or other alloy which contains the tantalum as one of the components of the alloy.
The impurities (e.g., metallic impurities) that may be present in the tantalum metal can be less than or equal to 0.005% and typically comprise other bcc refractory metals of infinite solubility in tantalum, such as niobium, molybdenum, and tungsten. For instance, metallic impurities, like Mo, W, and Nb (in the case of Ta) can be below (individually or combined) 100 ppm, below 50 ppm, below 20 ppm, below 10 ppm, or even below 5 ppm total. The oxygen content can be below 100 ppm, below 50 ppm, below 20 ppm, or below 10 ppm. All other elemental impurities, (including radioactive elements) whether metal or non-metal can be below a combined amount of 200 ppm, below 50 ppm, below 25 ppm, or below 10 ppm or even lower and optionally having 50 ppm or less 02, 25 ppm or less N₂, or 25 ppm or less carbon, or combinations thereof.
The tantalum metal and alloys thereof containing the tantalum metal preferably have a texture which is advantageous for particular end uses, such as sputtering. In other words, when the tantalum metal or alloy thereof is formed into a sputtering target having a surface and then sputtered, the texture of the tantalum metal in the present invention leads to a sputtering target which is easily sputtered and, very few if any areas in the sputtering target resist sputtering. Further, with the texture of the tantalum metal of the present invention, the sputtering of the sputtering target leads to a very uniform sputtering erosion thus leading to a sputtered film which is therefore uniform as well. A texture capable of resulting in a sputtering target which is easily sputtered can be a mixture of textures that are uniformly distributed in the tantalum metal (e.g., (100), (111)), as long as a (110) texture is present in the mixed texture.
The grain size of the tantalum metal can also affect the uniformity of the sputtering erosion and the ease of sputtering. The tantalum metal of the present invention can have any grain size. Preferably, the tantalum metal of the present invention includes an average grain size of about 1,000 microns or less, 750 microns or less, 500 microns or less, 250 microns or less, 150 microns or less, 100 microns or less, 75 microns or less, 50 microns or less, 35 microns or less, 25 microns or less, 20 microns or less, 15 microns or less, or 10 microns or less. Other grain sizes that are suitable in the tantalum metal of the present invention are grain sizes having an average grain size of from about 5 to about 125 microns. Preferably, the tantalum metal of the present invention includes an average grain size of from about 10 to about 100 microns. The tantalum metal of the present invention can include an average grain size of from about 5 to about 75 microns or from 25 to 75 microns, or from about 25 to about 50 microns. Also, in one embodiment, 95% of the grain sizes are 100 microns or less. This can be determined by measuring 500 grain sizes on a sample. Preferably, 95% of the grain sizes are 75 microns or less. Also, 95% of the grains can be less than 3 times the average grain size.
Preferably, the tantalum metal is at least partially recrystallized, and more preferably at least about 80% of the tantalum metal is recrystallized and even more preferably at least about 98% of the tantalum metal is recrystallized. Most preferably, the tantalum metal is fully recrystallized.
Also, it is preferred that the tantalum metal have a fine texture. More preferably the texture is such that the (100) and/or (111) peak intensity within any 5% incremental thickness of the tantalum is less than about 15 random, and/or has a natural log (Ln) ratio of (110):(100) and/or (110):(111) center peak intensities within the same increment greater than about −4.0 (i.e., meaning, −4.0, −3.0, −2.0, −1.5, −1.0 and so on) or has both the (100) centroid intensity and the ratio above or has both the (111) centroid intensity and the ratio above. The center peak intensity is preferably from about 0 random to about 10 random, and more preferably is from about 0 random to about 5 random. Other (100) centroid intensity ranges and/or other (111) centroid intensity ranges include, but are not limited to, from about 1 random to about 10 random and from about 1 random to about 5 random. Further, the log ratio of (110):(100) center peak intensities and/or the log ratio of (110):(111) center peak intensities is from about −4.0 to about 15 and more preferably from about −1.5 to about 7.0. Other suitable ranges of log ratios, include, but are not limited to, about −4.0 to about 10, and from about −3.0 to about 5.0. Most preferably, the tantalum metal of the present invention includes a grain size and preferred texture with regard to the (100) and/or the (111) incremental intensity and the (110):(100) and/or (110):(111) ratio of incremental centroid intensities. The method and equipment that can be used to characterize the texture are described in Adams et al., Materials Science Forum, Vol. 157-162 (1994), pp. 31-42; Adams et al., Metallurgical Transactions A, Vol 24A, April 1993-No. 4, pp. 819-831; Wright et al., International Academic Publishers, 137 Chaonei Dajie, Beijing, 1996 (“Textures of Material: Proceedings of the Eleventh International Conference on Textures of Materials); Wright, Journal of Computer-Assisted Microscopy, Vol. 5, No. 3 (1993), all incorporated in their entirety by reference herein. In one embodiment, the tantalum metal has a) an average grain size of about 50 microns or less, b) a texture in which a (100) pole figure, a (111) pole figure, or a combination thereof has a center peak intensity less than about 15 random or c) a log ratio of (110):(100), (110):(111), or (110):(100):(111) center peak intensities of greater than about −4.0, or a combination thereof.
The tantalum metal of the present invention can be used in a number of areas. For instance, the tantalum metal can be made into a sputtering target or into chemical energy (CE) munition warhead liner which comprises the high purity metal. The metal can also be used and formed into a capacitor anode or into a resistive film layer. The tantalum metal of the present invention can be used in any article or component which conventional tantalum is used and the methods and means of making the various articles or components containing the conventional tantalum can be used equally here in incorporating the tantalum metal into the various articles or components. For instance, the various components and processing used in making sputtering targets, such as the backing plate, one or more interlayers between the target and backing plate, and other components and design options, described in U.S. Pat. Nos. 5,753,090, 5,687,600, 5,522,535, 6,348,113 B1; 6,619,537; 6,605,199; 6,579,431; 6,451,135; 6,444,104; 6,444,100; 6,283,357; 6,183,686; and 6,183,613, can be used here and these patents are incorporated in their entirety by reference herein.
The tantalum of the present invention can be made a number of ways. The tantalum, such as sputtering targets, having a (110) crystallographic orientation can be manufactured by cutting the ingot, such as in the shape of sputtering targets, from ingots such as formed by electron beam melting. In the process of manufacturing tantalum with the (110) crystallographic orientation, tantalum feedstock can be heated to a temperature above its melting point, preferably to a temperature of about 3000° C. by bombardment with energetic electrons in an electron beam device. Molten drops of the tantalum can then be cooled, for example, by contacting the molten drops with a water-cooled crucible to solidify the tantalum into an ingot. The crucible is preferably designed so that as tantalum accumulates in the crucible, the distance between the top surface of the ingot and the tantalum material source preferably remains constant. In one example, to maintain a constant distance between the top surface of the ingot and the tantalum material source, the crucible can be lowered as the tantalum accumulates in the crucible. Other methods known to one skilled in the art can also be used to maintain a constant distance between the top surface of the ingot and a tantalum material source. Maintaining a constant distance between the top surface of the ingot and the tantalum material source allows the ingot length to increase. The produced ingot typically includes a (110) crystallographic orientation in the planes perpendicular to the ingot axis.
In one embodiment, an ingot can be cast by any melting technique such as electron beam melting to form an ingot having the desired diameter of a sputter target such as 12±2 to 13±2 inches. Furthermore, the casting of this ingot can have any thickness (such as the thickness of several sputter targets). For instance, the thickness of the ingot can be the thickness of one target or can be any thickness above the thickness of one target such as several inches or many inches. By this technique, if an ingot is cast having essentially the diameter of a finished target, then the ingot can be sliced or cut to form multiple targets having the desired diameter and thickness of a finished target. Furthermore, by doing so, the (110) texture is maintained. In another embodiment, the ingot can be cast using more conventional EB ingot diameters. For instance, an ingot can be cast having an 11 inch diameter with a thickness, for instance, of 0.563 inch. Then, this cast ingot can be lightly rolled to form a 13.75 inch diameter disc with a 0.36 inch thickness. Essentially, this would be the dimensions of the finished target. This light rolling to form the finished target would, for instance, use a true strain of about 0.447 which is well below the normal true strain applied to metals which are on the order of 2.0 to 3.0 true strain. Thus in one embodiment, the true strain applied in rolling an ingot is preferably less than 1.0 true strain and more preferably less than 0.5 true strain in order to maintain a strong (110) texture. Thus, in these embodiments, the casting of the ingot using the desired mold provides an ingot which can easily be formed into a target with minimal working of the metal in order to preserve the (110) texture. Preferably, when the ingot is formed, the ingot is subjected to very rapid cooling which aids in a more preferred finer grain size such as on the order of below 500 microns as described above with respect to the preferred grain sizes.
In another embodiment, a rolled plate, for instance, having a (111) texture as, for instance, obtained in Michaluk et al. (U.S. Pat. No. 6,348,113 B1) can be cut into slices in order to expose the side edges of the slices. Then, the slices can be rotated 90 degrees in order to expose the cut edges, which provides a (110) texture. Then, these slices or strips can be connected together by any manner to form a mosaic sputtering target. For instance, the cut strips can be ½ inch thick. The connecting together of the various slices can be done by any technique such as mechanical, welding, adhesives, combinations thereof, and the like. For instance, the slices can be cut to have a tongue and groove design or other similar mechanical connecting design to connect each of the slices together to form a mosaic target.
The ingot can be subjected to working (e.g., rolling, forging, and the like), being careful to maintain the preferred texture. The targets from the ingot can be machined-sputtered so that the surface of the sputtering target is parallel to the (110) crystallographic plane. FIG. 1 provides inverse pole figures viewed from the surface of a sputtering target with a texture of primary (110) parallel to the sputtering surface.
FIG. 2 provides (111), (110), and (100) pole figures for the sputtering target surface. According to FIG. 2, the (110) pole figure illustrates a strong central peak characteristic of a predominately (110) texture.
FIG. 3 provides an image of a circular plate showing grains in the metal plate. Although the grain size, as illustrated in FIG. 3 is large, reduction of the average grain size can be accomplished by adjusting the cooling rate during casting and by metal working the plate with or without annealing (one or more times) to retain the (110) crystallographic texture.
FIG. 4 illustrates the grain size distribution for the tantalum plate with a predominately (110) crystallographic orientation. According to FIG. 4, 95% of the grains are less than 3 times the average grain size.

The purity of the sputtering target can be another important attribute. Given that most impurities in tantalum are vaporized at the melting point of tantalum, the electron beam melting of tantalum can purify the tantalum. In one example, a Glow Discharge Mass Spectroscopic (GDMS) analysis was preformed on the sputtering target of the present invention to measure the chemical purity of the sputtering target. The results of the analysis are provided in Table 1. According to Table 1, the overall purity of the material is 99.995%. According to Table 1, the major impurities in the material after the electron beam melting were Fe, Cu, Nb, Mo, and W.

TABLE 1


GDMS analysis of (110) oriented Tantalum sputtering target.

	Concen-
	tration		Concentration		Concentration
Element	(ppmw)	Element	(ppmw)	Element	(ppmw)

Na	0.048	Ca	0.025	Zr	0.003
Mg	0.004	Ti	0.002	Nb	23
Al	0.031	Cr	0.015	Mo	0.35
Si	0.065	Fe	0.69	W	0.16
P	0.007	Co	0.002	Th	0.0003
S	0.007	Ni	0.038	U	0.0006
Cl	0.004	Cu	0.17
K	0.034	As	0.04

Table 1 indicates that the concentration of all other metallic impurities were below the GDMS detection limits, meaning below 10 ppm or even below 5 ppm or below 1 ppm or 0.1 ppm or less, or 0.01 ppm or less.
To measure non-metallic impurities, such as oxygen, nitrogen, hydrogen and carbon, metal fusion and subsequent thermal conductive or infrared measurement of the quantity of gas evolved was used. The results of the determined non-metallic impurities are provided in Table 2. According to Table 2, the oxygen, carbon, hydrogen and nitrogen levels in the tantalum sputtering target material of the present invention are also low.

TABLE 2

Gaseous element analysis of (110) oriented sputtering target materials. All

values are in ppmw.

Carbon Oxygen Hydrogen Nitrogen Sulfur

Avg 37 49 5 24 10

Max 59 60 5 44 10

Min 13 26 5 13 10

StDev 23 20 0 17 0
A refining process, a vacuum melting process, and a thermal mechanical process can also be used to make the tantalum metal of the present invention. In this process or operation, the refining process involves the steps of extracting tantalum metal preferably in the form a powder from ore containing tantalum and preferably the ore-containing tantalum selected has low amounts of impurities, especially, low amounts of niobium, molybdenum, and tungsten. More preferably, the amount of niobium, molybdenum, and tungsten is below about 10 ppm, and most preferably is below about 8 ppm. Such a selection leads to a purer tantalum metal. After the refining process, the vacuum melting process is used to purge low melting point impurities, such as alkyde and transition metals from the tantalum while consolidating the tantalum material into a fully dense, malleable ingot. Then, after this process, a thermal mechanical process can be used which can involve a combination of cold working and annealing of a tantalum.
The tantalum metal preferably may be made by reacting a salt-containing tantalum with at least one agent (e.g., compound or element) capable of reducing this salt to the tantalum metal and further results in the formation of a second salt in a reaction container. The reaction container can be any container typically used for the reaction of metals and should withstand high temperatures on the order of about 800° C. to about 1,200° C. For purposes of the present invention, the reaction container or the liner in the reaction container, which comes in contact with the salt-containing tantalum and the agent capable of reducing the salt to tantalum, is made from a material having the same or higher vapor pressure as tantalum at the melting point of the tantalum. The agitator in the reaction container can be made of the same material or can be lined as well. The liner can exist only in the portions of the reaction container and agitator that come in contact with the salt and tantalum. Examples of such metal materials which can form the liner or reaction container include, but are not limited to, metal-based materials made from nickel, chromium, iron, manganese, titanium, zirconium, hafnium, vanadium, ruthenium, cobalt, rhodium, palladium, platinum, or any combination thereof or alloy thereof as long as the alloy material has the same or higher vapor pressure as the melting point of tantalum metal. Preferably, the metal is a nickel or a nickel-based alloy, a chromium or a chromium-based alloy, or an iron or an iron-based alloy. The liner, on the reaction container and/or agitator, if present, typically has a thickness of from about 0.5 cm to about 3 cm. Other thicknesses can be used. It is within the bounds of the present invention to have multiple layers of liners made of the same or different metal materials described above.
The salt-containing tantalum can be any salt capable of having tantalum contained therein such as a potassium-fluoride tantalum. With respect to the agent capable of reducing the salt to tantalum and a second salt in the reaction container, the agent which is capable of doing this reduction is any agent which has the ability to result in reducing the salt-containing tantalum to just tantalum metal and other ingredients (e.g. salt(s)) which can be separated from the tantalum metal, for example, by dissolving the salts with water or other aqueous sources. Preferably, this agent is sodium. Other examples include, but are not limited to, lithium, magnesium, calcium, potassium, carbon, carbon monoxide, ionic hydrogen, and the like. Typically, the second salt which also is formed during the reduction of the salt-containing tantalum is sodium fluoride. Details of the reduction process which can be applied to the present invention in view of the present application are set forth in Kirk-Othmer, Encyclopedia of Chemical Technology, 3^rdEdition, Vol. 22, pp. 541-564, U.S. Pat. Nos. 2,950,185; 3,829,310; 4,149,876; and 3,767,456. Further details of the processing of tantalum can be found in U.S. Pat. Nos. 5,234,491; 5,242,481; and 4,684,399. All of these patents and publications are incorporated in their entirety by reference herein.
The above-described process can be included in a multi-step process which can begin with low purity tantalum, such as ore-containing tantalum. One of the impurities that can be substantially present with the tantalum is niobium. Other impurities at this stage are tungsten, silicon, calcium, iron, manganese, etc. In more detail, low purity tantalum can be purified by mixing the low purity tantalum which has tantalum and impurities with an acid solution. The low purity tantalum, if present as an ore, should first be crushed before being combined with an acid solution. The acid solution should be capable of dissolving substantially all of the tantalum and impurities, especially when the mixing is occurring at high temperatures.
Once the acid solution has had sufficient time to dissolve substantially, if not all, of the solids containing the tantalum and impurities, a liquid solid separation can occur which will generally remove any of the undissolved impurities. The solution is further purified by liquid-liquid extraction. Methyl isobutyl ketone (MIBK) can be used to contact the tantalum rich solution, and deionized water can be added to create a tantalum fraction. At this point, the amount of niobium present in the liquid containing tantalum is generally below about 25 ppm, and more preferably below 10 ppm or even below 5 ppm.
Then, with the liquid containing at least tantalum, the liquid is permitted to crystallize into a salt with the use of vats. Typically, this salt will be a potassium tantalum fluoride salt. More preferably, this salt is K₂TaF₇. This salt is then reacted with an agent capable of reducing the salt into 1) tantalum and 2) a second salt as described above. This compound will typically be pure sodium and the reaction will occur in a reaction container described above. As stated above, the second salt byproducts can be separated from the tantalum by dissolving the salt in an aqueous source and washing away the dissolved salt. At this stage, the purity of the tantalum can be typically 99.50 to 99.99% Ta.
Once the tantalum powder is extracted from this reaction, any impurities remaining, including any contamination from the reaction container, can be removed through melting of the tantalum powder.
The tantalum powder can be melted a number of ways such as a vacuum arc remelt or an electron beam melting. Generally, the vacuum during the melt will be sufficient to remove substantially any existing impurities from the recovered tantalum so as to obtain an acceptably pure tantalum. Preferably, the melting occurs in a high vacuum such as 10⁻⁴Torr or more. Preferably, the pressure above the melted tantalum is lower than the vapor pressures of the metal impurities in order for these impurities, such as nickel and iron to be vaporized. The diameter of the cast ingot should be as large as possible, preferably greater than 9½ inches. Once the mass of melted tantalum consolidates, the ingot formed can have a purity of 99.995% or higher and preferably 99.999% or higher. The electron beam processing preferably occurs at a melt rate of from about 300 to about 800 lbs. per hour using 20,000 to 28,000 volts and 15 to 40 amps, and under a vacuum of from about 1×10⁻³to about 1×10⁻⁶Torr. More preferably, the melt rate is from about 400 to about 600 lbs. per hour using from 24,000 to 26,000 volts and 17 to 36 amps, and under a vacuum of from about 1×10⁻⁴to 1×10⁻⁵Torr. With respect to the VAR processing, the melt rate is preferably of 500 to 2,000 lbs. per hour using 25-45 volts and 12,000 to 22,000 amps under a vacuum of 2×10⁻²to 1×10⁻⁴Torr, and more preferably 800 to 1200 lbs. per hour at from 30 to 60 volts and 16,000 to 18,000 amps, and under a vacuum of from 2×10⁻²to 1×10⁻⁴Torr.
The tantalum product preferably exhibits a uniform texture of mixed or primary (110) on the surface, throughout its thickness, or a combination thereof as measured by electron backscatter diffraction (EBSD), such as TSL's Orientation Imaging Microscopy (OIM) or other acceptable means. The resulting tantalum can include an excellent fine grain size and/or a uniform distribution. The tantalum preferably has an average recrystallized grain size of about 150 microns or less, more preferably about 100 microns or less, and even more preferably about 50 microns or less. Ranges of suitable average grain sizes include from about 5 to about 150 microns; from about 30 to about 125 microns, and from about 30 to about 100 microns.
The resulting tantalum metal of the present invention, preferably has 10 ppm or less metallic impurities and preferably 50 ppm or less O₂, 25 ppm or less N₂, and 25 ppm or less carbon. If a purity level of about 99.995 is desired, than the resulting high purity metal preferably has metallic impurities of about 50 ppm or less, and preferably 50 ppm or less O₂, 25 ppm or less N₂, and 25 ppm or less carbon.
With respect to taking this ingot and forming a sputtering target, the following process can be used. In one embodiment, the sputtering target made from the tantalum metal can be made by mechanically or chemically cleaning the surfaces of the tantalum metal, wherein the tantalum metal has a sufficient starting cross-sectional area to permit the subsequent processing steps described below. Preferably, the tantalum metal has a cross-sectional area of at least 9½ inches or more. The plate can then be mechanically or chemically cleaned and formed into the sputtering target having any desired dimension. Also, the tantalum can be annealed at a temperature (e.g., from about 950° C. to about 1500° C.) and for a time (e.g., from about ½ hour to about 8 hours) to achieve at least partial recrystallization of the tantalum metal.
With respect to annealing of the tantalum, preferably this annealing is in a vacuum annealing at a temperature and for a time sufficient to achieve complete recrystallization of the tantalum metal. As indicated above, the tantalum ingot and any form of the ingot formed afterwards can be annealed one or more times before and/or after any step mentioned herein. The annealing can be at any conventional annealing temperature, such as a temperature that causes at least partial recrystallization and/or alteration in grain size.
Another way to process the tantalum metal into sputtering targets involves mechanically or chemically cleaning one or more surfaces of the tantalum metal (e.g., the tantalum ingot), wherein the tantalum metal has a sufficient starting cross-sectional area to permit any subsequent processing.
Preferably, the sputtering targets made from the tantalum of the present invention have the following dimensions: a thickness of from about 0.080 to about 1.50”, and a surface area from about 7.0 to about 1225 square inches. Other dimensions can be made.
The tantalum preferably has a primary or mixed (110) texture, and a minimum (100) and/or (111) texture throughout the thickness of the sputtering target, and is preferably sufficiently void of (100) and/or (111) textural bands.
The present invention will be further clarified by the following examples, which are intended to be purely exemplary of the present invention.
The Figures represent analysis done on tantalum metal that was formed as an ingot by EB melting. The ingot was casted essentially in the shape of a sputter target.
Metallurgical Analysis
Grain size and texture were measured along the longitudinal or radial directions of samples taken from the cut ingot or casted ingot. Grain size was measured using ASTM procedure E-112.
Texture Measurement Technique: A limited number of samples (chosen based on metallurgical results) were used for texture analysis. Mounted and polished samples, previously prepared for metallurgical analysis, were employed as texture samples after being given a heavy acid etch prior to texture measurement. EBSD such as Orientation Imaging Microscopy (OIM) was chosen as the method of texture analysis because of its unique ability to determine the orientation of individual grains within a polycrystalline sample. Established techniques such as X-ray or neutron diffraction would have been unable to resolve any localized texture variations within the thickness of the tantalum materials.
For the analysis, each texture sample was incrementally scanned by an electron beam (within an SEM) across its entire thickness; the backscatter Kikuchi patters generated for each measurement point was then indexed using a computer to determine the crystal orientation. From each sample, a raw-data file containing the orientations for each data point within the measurement grid array was created. These files served as the input data for subsequently producing grain orientation maps and calculating pole figures and orientation distribution functions (ODFs).
By convention, texture orientations are described in reference to the sample-normal coordinate system. That is, pole figures are “standardized” such that the origin is normal to the plate surface, and the reference direction is the rolling (or radial) direction; likewise, ODFs are always defined with respect to the sample-normal coordinate system. Terminology such as “a (110) texture” means that the (110) atomic planes are preferentially oriented to be parallel (and the (110) pole oriented to be normal) with the surface of the plate. In the analyses, the crystal orientations were measured with respect to the sample longitudinal direction. Therefore, it was necessary to transpose the orientation data from the longitudinal to sample-normal coordinate system as part of the subsequent texture analysis. These tasks were conducted through use of computer algorithms.
Grain Orientation Maps: Derived from principles of presenting texture information in the form of inverse pole figures, orientation maps are images of the microstructure within the sample where each individual grain is “color-coded” based on its crystallographic orientation relative to the normal direction of the plate of disc from which it was taken. To produce these images, the crystal axes for each grain (determined along the longitudinal direction of the texture sample by EBSD such as OIM) were tilted 90° about the transverse direction so to align the crystal axes to the normal direction of the sample. Orientation maps serve to reveal the presence of texture bands or gradients through the thickness on the product; in tantalum, orientation maps have shown that large, elongated grains identified by optical microscopy can be composed of several small grains with low-angle grain boundaries.
Analysis of the Texture Results: EBSD scans (e.g., OIM scans) were taken along the thickness of each sample provided. The orientation maps were visually examined to qualitatively characterize the texture uniformity through the sample thickness. To attain a quantifiable description of the texture gradients and texture bands in the example materials, the measured EBSD data was partitioned into 20 subsets, with each representing a 5% increment of depth through the thickness of the sample. For each incremental data set, a pole figure was first calculated, then (100), (111), and (110) centroid intensities determined numerically using techniques reported elsewhere. The equipment and procedures described in S. Matthies et al., Materials Science Forum, Vol. 157-162 (1994), pp. 1647-1652 and S. Matthies et al., Materials Science Forum, Vol. 157-162 (1994), pp. 1641-1646 were applied, and these publications are incorporated in their entirety herein by reference. The texture gradients can then be described graphically by plotting the (100), (111), and (110) intensities, as well as the log ratio of the (100):(110), (100):(111), and (100):(111);(110) as a function depth of the sample.
Applicants specifically incorporate the entire contents of all cited references in this disclosure. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.
Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the present specification and practice of the present invention disclosed herein. It is intended that the present specification and examples be considered as exemplary only with a true scope and spirit of the invention being indicated by the following claims and equivalents thereof.

Claims

1. A tantalum metal having a texture of primary (110) on the surface, throughout the thickness, or a combination thereof.

2. The tantalum metal of claim 1, wherein said metal has a) a texture in which a (100) pole figure, a (111) pole figure, or a combination thereof has a center peak intensity less than about 15 random or b) a log ratio of (110):(100), (110):(111), or (110):(100):(111) center peak intensities of greater than about −4.0, or c) both.

3. (canceled)

4. The tantalum metal of claim 2, wherein said center peak intensity is from about 0 random to about 10 random.

5. (canceled)

6. The tantalum metal of claim 2, wherein said log ratio is from about −1.5 to about 7.0.

7. (canceled)

8. The tantalum metal of claim 1, wherein said metal is fully recrystallized.

9-12. (canceled)

13. The tantalum metal of claim 1, having a purity of from 99.99% to about 99.999%.

14. A metal alloy comprising the tantalum metal of claim 1.

15. (canceled)

16. A sputtering target comprising the tantalum metal of claim 1.

17. (canceled)

18. A capacitor comprising the tantalum metal of claim 1.

19. (canceled)

20. A resistive film layer comprising the tantalum metal of claim 1.

21. (canceled)

22. An article comprising at least as a component the tantalum metal of claim 1.

23. (canceled)

24. The tantalum metal of claim 1, wherein the tantalum metal has a substantially fine and uniform microstructure.

25. (canceled)

26. The tantalum metal of claim 1 comprising an average grain size of from about 5 to about 125 microns.

27-28. (canceled)

29. The tantalum metal of claim 28, wherein said tantalum metal includes an average grain size of from about 25 to about 50 microns.

30. The tantalum metal of claim 1, wherein said tantalum metal comprises grains with an average grain size, wherein 95% of said grains are less than three times said average grain size.

31-32. (canceled)

33. A process of making a sputtering target from a tantalum metal of claim 1, comprising casting an ingot having a diameter the same or greater than the diameter of a finished sputter target and having a thickness the same as or greater than a finished target, wherein said casted ingot has a (110) texture.

34-37. (canceled)

38. A process of making a sputtering target from tantalum metal of claim 1, comprising forming a casted ingot having a diameter smaller than the diameter of a finished target and then rolling said casted ingot to form a casted ingot having the diameter of the finished target, wherein the true strain applied in rolling the cast ingot is less than 1.0 true strain.

39. (canceled)

40. The process of claim 38, wherein the amount of true strain is 0.5 or less.

41. The process of claim 38, wherein the true strain is from about 0.4 to about 0.5.

42. A method of making the sputter target of claim 16 having a (110) texture, comprising cutting a plate having a (111) primary texture wherein the plate is cut into multiple strips; rotating the cut strips 90 degrees and then joining together the cut strips to form a mosaic target.

43-48. (canceled)

49. A sputter target comprising a uniform texture of primary or mixed (110) texture on the surface or throughout, wherein said tantalum metal is substantially void of (100) and/or (111) textural bands.

50. The sputter target of claim 49, further comprising a backing plate.

51. The sputter target of claim 50, wherein an interlayer is present between said backing plate and sputter target.

52. A process for making the tantalum metal of claim 1, comprising heating a tantalum feedstock to a temperature above its melting point to create molten drops; and solidifying said molten drops of the tantalum in a crucible.

53-57. (canceled)

58. A bcc metal or alloy thereof having a texture of primary (110) on the surface, throughout the thickness, or a combination thereof.

59. The bcc metal of claim 58, wherein said metal has a) a texture in which a (100) pole figure, a (111) pole figure, or a combination thereof has a center peak intensity less than about 15 random or b) a log ratio of (110):(100), (110):(111), or (110):(100):(111) center peak intensities of greater than about −4.0, or c) both.

60-61. (canceled)

62. The bcc metal or alloy thereof of claim 58, wherein said bcc metal is niobium.

63. The bcc metal or alloy thereof of claim 59, wherein said bcc metal is niobium.