HIGH PURITY COBALT SPUTTER TARGET AND PROCESS OF MANUFACTURING THE SAME

Abstract: A high purity cobalt sputter target is disclosed which contains a face centered cubic (fcc) phase and a hexagonal close packed (hcp) phase, wherein the value of the ratio of X-ray diffraction peak intensity, Ifcc(200)/Ihcp (10, 11), is smaller than the value of the same ratio in a high purity cobalt material obtained by cooling fcc cobalt to room temperature from the high temperature at which it is molten. High purity cobalt is defined as having an oxygen content of not more than 500 ppm, a Ni content not more than 200 ppm, an Fe, Al and Cr contents not more than 50 ppm each, and Na and K less than 0.5 ppm. The disclosed sputter target is manufactured by subjecting the material to cold-working treatments (less than 422 DEG C). Annealing the material, at a temperature in the range of 300-422 DEG C for several hours, between cold-working treatments significantly increases the amount of cold work which could be imparted into the material. The high purity cobalt is deformed in such a way so as to cause the (0002) hcp plane to be tilted between 10-35 DEG from the target normal. The aforementioned phase proportions and crystallographic texture significantly improves the sputtering efficiency and material utilization.

Description: HIGH PURITY COBALT SPUTTER TARGET AND PROCESS OF MANUFACTURING THE SAME FIELD OF INVENTION The invention described herein relates to a method for manufacturing high purity cobalt for use in sputter targets and related microelectronics applications. High purity cobalt is defined as having an oxygen content of not more then 500 ppm, a Ni content not more than 200 ppm, an Fe, Al and Cr content not more than 50 ppm each, and Na and K not more than 0.5 ppm. The cobalt target exhibits a low magnetic permeability in the plane of the target and higher magnetic permeability normal to the target surface, i.e. large surface flux leakage. The invention is a method of fabrication of high purity cobalt with a grain structure which has a strong preferred crystallographic orientation in the hep phase and little or no detectable^cc phase. This strong hep crystallographic texture, (tilted (0002) plane), is critical to the sputtering efficiency and material utilization of the target used in the stated application. BACKGROUND OF THE INVENTION High purity cobalt targets are used in sputtering applications to produce thin films on microelectronic devices such as microprocessors, DRAM's, etc. DC magnetron sputtering efficiency relies heavily on the ability of a magnetic field to trap electrons liberated from the target, and direct them back to the negatively charged target where they are repelled. These electrons spiral through the plasma gas (typically argon) in the sputtering ch.amber towards the target. This spiraling motion increases the frequency of collisions with argon atoms, which results in more argon atoms striking the target and ultimately higher deposition rates. The sputtering rate is increased (to some asymptotic limit) by increasing the strength of the component of the magnetic field parallel to the target surface. This in turn relies heavily on the ability of the magnetic field to penetrate the target material. Cobalt, which is ferromagnetic, is very anisotropic in its magnetic properties, and can often resist field penetration. Consequently, when a magnetic substance such as conventionally processed cobalt is used as a target, the magnetic flux tends to pass through the interior of the target and only a low flux can exit into the plasma discharge space. This problem requires the use of very thin cobalt targets which as a result have relatively short service lives. Furthermore, the local cross-sectional decrease of the target during the sputtering (erosion trench) brings about an increasing magnetic flux directly over the erosion trench. This causes a higher ionization probability of the sputtering gas to occur in the region and a higher sputtering rate to occur locally, with the consequence that the erosion trench becomes very narrow, resulting in poor material utilization. The effect is compounded with the use of thin targets because of the exponential relationship between magnetic field strength and the distance from the magnet. A small amount of erosion on a thin target produces a much greater localized magnetic flux strength increase than would an equivalent amount of erosion on a thicker target, due to the fact that the thicker target surface is further from the magnetron magnet. To combat these problems, a low magnetic permeability value in the plane of the high purity cobalt target is needed so that the magnetic flux leakage normal to the target is increased. This will allow target thickness to be increased, thereby extending target life, and improving sputtering efficiency and performance. Cobalt has two crystalline forms - face centered cubic (fee) and hexagonal close packed (hep). The hep phase is the low temperature crystal structure and exists up to 422 °C; above this temperature it transforms into the fee phase. A conventional high purity cobalt sputtering target hitherto used comprises a mixture of these two phases. This conventional target is manufactured by a process wherein high purity cobalt material is heated until it is melted, and then poured into a mold. At this temperature the material it is in the fee single phase. The cast ingot is then allowed to cool immediately or after it has been subjected to hot-working treatment, so that part of the fee single phase is transformed into a martensitic structure which includes a hep phase. The conventional high purity cobalt thus manufactured has been used for magnetron sputtering. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an illustration of the magnetization directions in hep and fee cobalt; Figure 2 is a graph of a (0002) pole figure showing 0-10° tilting of the <0001> hep prism axis; Figure 3 is a graph of a (0002) pole figure showing 20-40° tilting of the <0001> hep prism axis; Figures 4A and 4B are illustrations of pass through flux (PTF) measurements taken on conventional processed cobalt (Figure 4A) and cobalt processed by invention (Figure 4B), both samples were measured with same magnet strength and configuration. DETAILED DESCRIPTION OF THE INVENTION In the case of high purity cobalt, the ratio of fee phase to hep phase has significant influence on the magnitude of the magnetic permeability. The fee phase is much less anisotropic in magnetic properties than the hep phase. Consequently, a target consisting mostly fee phase has very low PTF, and since there are no strongly preferred crystallographic directions the flux flow is dictated by the target geometry. This geometry tends to restrain the flux within the plane of the target and inhibits flux leakage. If the amount of hep phase in the target is increased and the easy magnetization direction <0001> in the hep crystal is aligned normal to the target surface, the permeability in the plane of the target is decreased, and it is easier to generate the leakage magnetic field through the thickness of the target. If it were possible to decrease the fee phase in the cobalt sputter target manufactured by the conventional process, and to increase its hep phase so that the ratio of the fee phase to the hep phase is decreased, then the permeability in the plane of the target could be decreased, resulting in an increase in the magnetic leakage field on the surface of the target. This means that a high purity cobalt target which is thicker than the conventional cobalt can be used, resulting in longer service life and improved material utilization. It is difficult in practice to obtain the ratio of fee phase to hep phase of cobalt by optical volume measurement. It has become common in the industry to consider a ratio of the intensity of X-ray diffraction peaks which are proportional to the volumetric ratio of the two phases. The peaks which are used in the ratio are the (200) in the fee phase and the (10 1 1) peak in the hep phase. These peaks where chosen because of overlap in the stronger fee (111) and hep (0002) diffraction peaks. The inventors have developed a process for manufacturing high purity cobalt sputter targets which is characterized by a ratio of the X-ray diffraction peak intensity for the fee phase to the X-ray diffraction peak intensity for the hep phase, which is considerably smaller than in the conventionally processed high purity cobalt sputter target. The process of this invention comprises the steps of heating conventional high purity cobalt material having a^c single ph.ase until it is melted, and then pouring it into a mold and allowing it to cool from the high temperature. This step can be carried out by any available melting means. However, it is preferable to use a vacuum casting method, such as a vacuum induction melting furnace, or an e- beam furnace. The resultant cast ingot is then cooled to room temperature. In the alternative, the ingot is subjected to a hot-working treatment. The ingot is then allowed to cool to room temperature, so that part of the fee single phase is transformed into a martensite structure comprising a hep phase. Then, in the second stage of the process of this invention, the resultant cobalt material comprising mostly a fee phase with some hep phase is subjected to a cold- working treatment, which preferably results in a thickness reduction of no less than 5% at a temperature less than the hep transformation temperature (422°C). As a result of the cold working, a compression strain is imparted to the cobalt material and a part of the existing fee phase is transformed into a martensite structure comprising a hep phase. Any of the conventional cold-rolling methods, such as rolling, drawing, swaging, forging or general press working may be used for this invention. It is preferable that the high purity cobalt is cold worked until its reduction in thickness is about 10% or more. To increase the reduction, the inventors found that intermediate anneals in the temperature range 300-422°C for several hours allowed substantially more (40-60%) cold work to be imparted into the cobalt. Without such intermediate anneals, the cold ductility limit was found to be 20-25%. High purity cobalt manufactured in the aforementioned way can produce a significantly smaller X-ray diffraction peak intensity ratio, \f cc (200)/I/, C n (10 11), compared to conventionally processed high purity cobalt. The inventors observed that in high purity cobalt which has received about 10% or greater cold deformation, there was no detectable fee phase present in the material, in which case the intensity ratio, lf cc (200)/I/jςp (10 11), becomes about zero. The two crystal phases of cobalt are anisotropic in magnetic properties and have preferred directions for magnetization (1), Figure 1. When the hep prism axis <0001> is tilted between 0- 45° from the target normal via the deformation mechanisms described above, the result is a target with high PTF and better sputtering performance. Figure 2 shows a graph of a (0002) pole figure with the hep prism axis <0001> tilted between 0-10° and Figure 3 shows a (0002) pole figure with the hep prism axis tilted between 20-40°. Sample 1 was conventionally processed by heating cobalt powder until it was melted. The molten cobalt was poured into a mold and allowed to cool to room temperature. An alternative conventional process would involve hot working the solidified ingot while it is still hot, by pressing or rolling the ingot into slab or sheet before allowing it to cool to room temperature. In this example the ingot was first allowed to cool to room temperature and then hot worked in the temperature range 750-900°C to form the sample. Hot working material above 500°C will produce the same metallurgy. According to this invention, samples 2, 3, 4, 5 and 6 were first conventionally processed, but then had an additional cold working treatment. Samples 2, 3, 4, 5 and 6 were hot-worked at a temperature > 750°C and then cold worked in the temperature range 300-422°C. Each sample had a different amount of cold work, as shown in Table 2. Sample 7 did not receive any hot-working treatment. This material was cooled to room temperature from the molten state and cold worked 49%. In samples 4, 5, 6 and 7 where the percentage cold work exceeded 20%, intermediate anneals in the range 300-422 °C were used to stress relieve and recrystalhze the cobalt and allow additional cold working treatments to proceed without the material exceeding the ductility limit and fracturing. This intermediate annealing step is an integral part of the invention and allows substantial quantities of cold work to be imparted into the metal, and in so doing significantly reduces the amount of fee cobalt in the final material. Table 1 shows the composition of the seven high purity cobalt samples used to illustrate the invention. Table 1 Composition of High Purity Cobalt Samples Composition (ppm) The intensity ratio, lf cc (200)/I/,ςp (10 11), of each sample was measured using CuKα radiation. The results of the testing are shown in Table 2. The data shows that the conventionally processed sample (sample 1) has an intensity ratio, lf cc (200)/I/, C r, (10 Tl), of 0.52 compared to lower values for the samples processed according to this invention (samples 2- 7). In samples 4, 5, 6 and 7 there was no fee phase detected thus producing a ratio equal to about zero. The permeability and coercivity of each sample was derived from standard B-H loop measurements. These measurements were made in two directions, parallel to the sample surface (x-direction), and normal to the sample surface (z-direction). A z/x ratio of these measurements is used to illustrate the strength and direction of the magnetic anisotropy. This value, z/x, is defined as the permeability ratio. Magnetic flux effectively takes the path of least resistance, which in most cases is in the direction of highest permeability. In a target it is important that the direction of highest permeability approaches the target normal direction, resulting in a permeability ratio greater than 1. Values significantly greater than 1, and particularly greater than 10, are desirable because of demagnetization effects arising from the target geometrical aspect ratio. This planar shape plays a major role in limiting the amount of magnetic flux leakage into the plasma discharge space and keeping the magnetic flux within the plane of the target. The strength of the magnetic field leaked into the discharge space can be measured by PTF. Figure 4 shows the PTF measurements associated with a conventional cobalt sputtering target (a) and a cobalt sputtering target according to this invention (b). The results, shown in Table 2, confirmed that the permeability in the plane of the conventional sample is multiples higher than it is in the samples of this invention. The permeability ratio, (z-direction / x-direction) is significantly increased with cold work treatment according to this invention. Table 2 Results of Testing on Various Samples % hot and cold work is calculated from the initial material thickness before the working stage and is not calculated

Claims: We claim: 1. A sputtering target of high purity cobalt comprising not more than about 500 ppm oxygen, a Ni content not more than about 200 ppm, an Fe, Al and Cr content not more than about 50 ppm, each, a Na content not more than about 0.5 ppm, a K content not more than about 0.5 ppm and having a magnetic permeability ratio greater than about 10. 2. A sputtering target of high purity cobalt comprising not more than about 50 ppm oxygen, a Ni content not more than about 200 ppm, an Fe and Al content not more than about 20 ppm each, a Cr content not more than about 1 ppm, a Na content not more than about 0.5 ppm, a K content not more than about 0.05 ppm and having a magnetic permeability ratio greater than about 10. 3. A sputtering target as recited in claim 1 or 2, wherein the X-ray diffraction peak intensity ratio, l fcc (200)/I hcp (10, 11), of said cobalt is less than about 0.5. 4. A sputtering target as recited in claim 1 or 2, wherein the X-ray diffraction peak intensity ratio, l fcc (200)/I hcp (10, 11), of said cobalt is 0. 5. A method of manufacturing a sputtering target of composition recited in claim 1 or claim 2, comprising the steps of: a) preparing a high purity cobalt ingot; b) subjecting said high purity cobalt to hot plastic working; c) subjecting the hot worked cobalt to a cold plastic working with a thickness reduction of no less than about 5% at a temperature lower than the hep transformation temperature. 6. A method of manufacturing a sputtering target as recited in claim 5, wherein said high purity cobalt preparation step (a) is carried out by a vacuum casting method or by e-beam melting. 7. A method of manufacturing a sputtering target according to claim 5, wherein said hot plastic working takes place in the temperature range of about 750-900°C and said cold plastic working takes place in the temperature range of about 300-422°C. 8. A method of manufacturing a sputtering target of composition recited in claim 1 or claim 2, comprising the steps of: a) preparing a high purity cobalt ingot; b) subjecting said cobalt ingot to a cold plastic working with a thickness reduction of no less than about 5% at a temperature lower than the hep transformation temperature. 9. A method of manufacturing a sputtering target according to claim 5 or claim 8, wherein said target is subjected to immediate annealing below the hep transformation temperature between cold working treatments. 10. A sputtering target as recited in claim 1 or claim 2, having an X-ray diffraction peak intensity ratio, I cc (200)// Λ (10, 11), of less than about 0.5 and where most of the hexagonal prism axis <0001> is tilted between abut 0-20° from the target normal. 11. A sputtering target as recited in claim 1 or claim 2, having an X-ray diffraction peak intensity ratio, I /CC (200)/7 A (10, 11), of less than about 0.5 and where most of the hexagonal prism axis <0001> is tilted between abut 20-45° from the target normal. 12. A sputtering target as recited in claim 1 or claim 2, having an X-ray diffraction peak intensity ratio, l fcc (2Q0)II hcp (10, 11), of 0 and where most of the hexagonal prism axis <0001> is tilted between abut 0-20° from the target normal. 13. A sputtering target as recited in claim 1 or claim 2, having an X-ray diffraction peak intensity ratio, l fcc (2Q)Q)II hcp (10, 11), of 0 and where most of the hexagonal prism axis <0001> is tilted between abut 20-45° from the target normal. AMENDED CLAIMS [received by the International Bureau on 13 January 1999 (13.01.99); original claims 1-13 replaced by amended claims 1-13 (2 pages)] 1. A sputtering target of high purity ∞bdt comprising not more than about 500 ppm oxygen, a Ni content not more than about 200 ppm, an Fe, Al .and Cr content not more thrø about 50 ppm, each, a Na content not more than about 0.5 ppm, a . content not more than about 0.5 ppm and having a magnetic permeability ratio greater than about 10. 2. A sputtering target of high purity cobalt comprising not more than about 50 ppm oxygen, a Ni content not more th.an about 200 ppm, an Fe and Al content not more th.an about 20 ppm each, a Cr content not more than about 1 ppm, a Na content not more than about 0.5 ppm, a K. content not more than about 0.05 ppm and having a magnetic permeability ratio greater than about 10. 3. A .sputtering target as recited in ctøim 1 or 2, wherein the X-ray diffraction peak intensity ratio, l f 20Q)/J hc/> (10 ϊl) , of said cobalt is less tlian about 0.5. 4. A sputtering target as recited in claim 1 or 2, wherein the X-ray diffraction pe.ak intensity ratio, (10 ϊl) , of said cobalt is 0. 5. A method of manufacturing a sputtering target of composition recited in claim 1 or claim 2, comprising the steps of: a) preparing a high purity cobalt ingot; b) subjecting said high purity cobalt to hot pl ^c working; c) .subjecting the hot worked cob-alt to a cold plastic working with a thickness reduction of no less than about 5% at a temperature lower than the hep transformation temperature. 6. A method of manufacturing a sputtering target as recited in claim 5, wherein said high purity cob t preparation step (a) is carried out by a vacuum casting method or by e-beam melting. 7. A method of manufacturing a sputtering target according to claim 5, wherein said hot plastic working tøkes place in the temperature range of about 750-900°C and said cold plastic working takes place in the temperature range of about 300-422°C. 8. A method of manufacturing a sputtering taiget of composition recited in claim 1 or claim 2, comprising the steps of: a) preparing a high purity cobalt ingot; b) subjecting said cobalt ingot to a cold plastic working with a thickness reduction of no leas than about 5% at a temperature lower th.an the hep transformation temperature. 9. A method of manufacturing a sputtering target according to claim 5 or claim 8, wherein said target is subjected to immediate annealing below the hep transformation temperature between cold working treatments. 10. A sputtering target as recited in claim 1 or claim 2, having an X-ray diffraction peak intensity ratio, 1^(200)/! hep (10 11) , of less than about 0.5 and where most of the hexagonal prism axis <0001> is tilted between about 0-20° from the target normal. 11. A sputtering target as recited in claim 1 or claim 2, having an X-ray diffraction peak intensity ratio, 1^.(200) /^ (10 ϊl) . of less than about 0.5 and where most of the hexagonal prism axis <0001> is tilted between about 20-45° from the target normal. 12. A sputtering target as recited in claim 1 or claim 2, having an X-ray diffraction peaK intensity ratio, 1^2 0)11,^ (1 11) , of 0 and wheϊe most of the hexagon∑U prism axis <000l> is tilted between about 0-20° from the tørget normal. 13. A sputtering target as recited in claim 1 or claim 2, having an X-ray diffraction peak intensity ratio, 1^200)//^ (10 11) , of 0 and where most of the hexagoral prism axis <0001> is tilted between about 20-45° from the target normal.

Inventors: COLE ROBERT S, COOPER MATHEW S, LIU YINSHI, MCCARTY MICHAEL, SCAGLINE RODNEY L, TURNER STEPHEN P, COLE, ROBERT, S., COOPER, MATHEW, S., LIU, YINSHI, MCCARTY, MICHAEL, SCAGLINE, RODNEY, L., TURNER, STEPHEN, P.
Applicants: ALTA GROUP INC, COLE ROBERT S, COOPER MATHEW S, LIU YINSHI, MCCARTY MICHAEL, SCAGLINE RODNEY L, TURNER STEPHEN P, COLE, ROBERT, S., COOPER, MATHEW, S., LIU, YINSHI, MCCARTY, MICHAEL, SCAGLINE, RODNEY, L., THE ALTA GROUP, INC., TURNER, STEPHEN, P.
Application year: 1998
Main classification: C22C19
IPC codes: C22C19, C22F1, C23C14