Paper PS-TuM6
Experimental Evaluation of Ta Film Etching Characteristics by CO⁺ Ion Irradiation

Tuesday, November 11, 2014, 9:40 am, Room 308

Magnetic Random Access Memory (MRAM) is considered as a promising candidate for the next generation memory because of its non-volatility, fast reading or writing speed, and high write-cycle endurance. MRAM consists of magnetic tunnel junctions (MTJ) and CMOS devices. One of the issues concerning MRAM fabrication is the development of a nano-scale anisotropic etching technology for MTJ because an MTJ is used as the memory component of an MRAM device and high integration of MTJs is required for commercially competitive MRAM chips. One approach to MTJ etching is to use CO/NH₃ plasmas with Ta hard masks. With this process, vertical etching profiles with a taper angle of 80° have been achieved with high etching selectivity of magnetic materials over Ta [1].

In previous research [2], etching yields of Ta and Ni films were evaluated with a mass-selected ion beam system with incident ions of Ar⁺, N⁺, O⁺, and CO⁺. It has been found from X-ray photoelectron spectroscopy (XPS), for example, that the Ta film surface was oxidized while the Ni film surface was not oxidized by the same CO⁺ ion irradiation. Therefore, it was concluded that the oxidized layer of Ta, which was formed by CO⁺ irradiation, suppressed the Ta etching yield.

In this study, Ta etching yields by CO⁺ ions were compared with corresponding theoretical values of physical sputtering of Ta to understand why the Ta etching yield by CO⁺ ion irradiation was typically very low. The experimentally obtained sputtering yield is typically about 1 % of the theoretical yield of physical sputtering by non-reactive incident species of the same mass. Our XPS analysis of Ta after CO⁺ ion irradiation indicates that the Ta film is highly oxidized (i.e., to the equivalent degree of Ta₂O₅) by CO⁺ ion irradiation while Ar⁺ ion irradiation of a Ta oxide film reduces its oxidation degree. Therefore the low Ta etching yield by CO⁺ ion irradiation is caused by a larger amount of oxygen supply by incident CO⁺ ions with less oxygen sputtering. The dependence of the Ta sputtering yield by CO⁺ ions on the ion incident angle will be also discussed in terms of the angle dependence of the surface oxidation level.

[1] N. Matsui, et al., Vacuum 66, 479-485 (2002).

[2] H. Li, et al., AVS 60^th Int. Symp (2013).