Paper PS-TuM2
TaN Etch Mechanisms in BCl₃-based Plasmas

Tuesday, October 21, 2008, 8:20 am, Room 304

TaN is a potential candidate for metal gates. BCl₃ plasma is used to pattern metal gates as it has high selectivity over Si substrate and etches metal oxides (native oxides on metal gates and high-k dielectrics). During metal gate etch in inductively coupled plasma reactor, we found that TaN gate profile depends on the composition of BCl₃-based plasma. Pure BCl₃ results in an undercut of TaN. The undercut can be avoided by addition of 5% O₂, further increase of O₂ concentration (till 10%) does not change the TaN profile. When N₂ is added to BCl₃ plasma, first the undercut disappears (at about 6% of N₂) and then a slope appears as N₂ concentration increases further (toward 10%). To clarify the etch mechanisms, we studied etching of blanket TaN wafers (30 nm film deposited by PVD). To avoid ion bombardment and simulate conditions on the sidewalls of a gate, the substrate bias was set to zero. Etch rate of TaN was measured by spectroscopic ellipsometry; surface composition was examined by XPS. In the case of pure BCl₃ plasma a thick film (deposition rate of 20 nm/min) is formed. The film consists of B (50%), Cl (30%) and O (20%). The oxygen probably comes from the oxidation on air between the etching and the XPS measurements. When 5% of O₂ is added, no film is observed, the surface composition is close to as-deposited TaN (with some B added). We observed strong peaks in emission spectra of BCl₃/O₂ plasma, attributed to BO_x. When 5% N₂ is added to the BCl₃ plasma, a film of the same thickness as for pure BCl₃ is observed, but it contains less Cl (15%). We propose the following etch mechanism. In pure BCl₃ plasma a Cl-containing film is deposited on the sidewalls of the gate. Cl from the film reacts with TaN producing an undercut. When O₂ is added, no film is formed and the TaN profile is straight as B apparently reacts with O in the gas phase, forming volatile BO_x radicals. Further increase of O₂ content does not change the profile as no film is formed (until O₂ concentration reaches 50% when B₂O₃ film is deposited). When N₂ is added to BCl₃, a film is formed but in this case N₂ replaces Cl and the film becomes passivating leading to a straight TaN profile. As more N₂ added, the film passivates TaN more efficiently leading to a sloped profile.