Research On The Microstructure Evolution Of High Purity Tantalum Plate During Cold Rolling And Subsequent Annealing Process

As a sputtering target, high purity tantalum(Ta) has been widely used in integrated circuit, and its sputtering performance is strongly affected by grain size and crystal orientation. In order to improve the sputtering performance of sputtering target and ensure the quality of the as-deposited film, in this paper, the microstructure of clock-rolled and annealed Ta plate was characterized by EBSD, XRD and DSC. The main results and conclusions can be drawn:① The grain size and crystal orientation distribution of the original Ta plate was extremely inhomogeneous. The grain size in the surface layer was larger than that in the center layer, and the grains were dominated by {100} orientation near the surface compared to the primary {111} orientation near the center layer. The grains of clock-rolled Ta plate were broken down more seriously with the increasing deformation. Compared with other position, the grain refinement in the surface layer was more serious for a strong shear force; Typical θ-fiber and γ-fiber textures were found during clock rolling and they varied periodically with rolling passes.② Clock-rolled Ta plate with 87% thickness reduction produced a through-thickness texture gradient. Strong θ-fiber and weak γ-fiber textures were found in the surface layer. The θ-fiber had a slight change from the surface to the center layer, and yet the γ-fiber increased monotonously with the position nearing the center. In the center layer, the intensity difference of the two orientations was minimum. Triple focused ion beam could make a much better sample surface for EBSD analysis and reveal the substructure in clock-rolled Ta availably. The density of low angle boundaries(LABs) and related average misorientation in {111} grains were higher, and the subdivision of {111} grains was more serious than {100} grains.③ The {111} grains formed during the low or moderate annealing were larger than {100} grains, and the grain size changed slightly even for a long holding time. However, the grain coarsening behaviour of {111} and {100} grains could occur in a short time after annealing at a high temperature. The through-thickness texture gradient of clock-rolled Ta plate could not be eliminated effectively by 1050 oC annealing, while 30 min annealing at 1200 oC showed a relatively random texture distribution and homogeneous grain size distribution.④ After 87% deformation, the stored energy of unidirectional-rolled Ta plate was larger than clock-rolled Ta. The stored energy released by recovery and recrystallization was 1.429 J/g and 0.517 J/g in unidirectional-rolled Ta,while it was 1.325 J/g and 0.452 J/g in clock rolling. Overly high heating rate was detrimental for the recovery process, which would promote the recrystallization and finally lower the crystallization temperature. The stored energy of {111} grains and {100} grains was 2.28 J/mol, 5.38 J/mol and 2.85 J/mol, 5.43 J/mol for unidirectional rolling and clock rolling, respectively. The difference of stored energy between {111} and {100} grains was reduced evidently in clock rolling.