Sub 5 nm thick doped metal oxide high k films for future generations of VLSI devices
New gate dielectric materials with k values much higher than that of SiO2 are required for better device performance and reliability of sub 100 nm complementary metal-oxide-semiconductor (CMOS) transistors.19 Metal oxides, such as Ta2O5, HfO2, ZrO2, Y2O3 and Al2O3 are potential candidates for this purpose. However, a metal oxide thin film usually suffers from a high leakage current after being processed through a high temperature. Most metal oxides are thermally unstable and easily form a thin oxide interface layer at the contact area with the silicon wafer at a raised temperature.[i] The high leakage-current of the metal oxide is caused by the formation of the polycrystalline phase in the film. It was proved that the amorphous-to-polycrystalline transition temperature can be increased by the doping method, i.e., adding a third element into the oxide.[ii],[iii] Therefore, the leakage-current of the amorphous film is lower than that of the polycrystalline film.
Recently, our group and other researchers reported that the properly doped TaOx thin film could have a higher apparent k value than that of the undoped TaOx film.49,50,[iv] The doping process probably changed the film structure such as the realignment of the local metal-oxygen bond.[v] Since the formation of a 2-3 nm thick interface layer between the high k film and the silicon wafer is unavoidable, when the high k film is very thin, this interface layer directly affects the apparent k value and the leakage current. We further presented a new method to solve the interface problem, i.e., introducing a 5Å thick tantalum nitride (TaNx) interface layer.[vi] The result shows that the leakage current is reduced and the k value is greatly enhanced, as shown in Fig. 11. Apparently the non-stoichiometry TaNx thin film was converted into a non-conductive TaOxNy film during high temperature O2 annealing. The TaOxNy film has a much higher k value than that of SiO2 and can block electron transport.
Currently, this new type of doped TaOx film with the 5Å TaOxNy interface layer has been successfully integrated into capacitors with a geometry larger than micrometers. In order to investigate the material and electrical properties of this kind of structure, in the particularly interesting sub mm regime e-beam lithography is required.
Figure 11: Influence of 5 Å TaNx interface layer on the dielectric constant k (y-axis) of Hf-doped TaOx films.53
[i] K. J. Hubbard and D. G. Schlom, J. Mater.Res., 11, 2757 (1996).
[ii] R. F Cava, W. F. Peck, Jr., and J. J. Krajewski, Nature, 337, 215 (1995).
[iii] Y. Kuo, J-Y. Tewg and J. P. Donnelly, Procs. Intl. Semiconductor Technol. Conf., ECS, 2001-17, 324 (2001).
[iv] Y. Kuo, J. Lu, P. C. Liu, F. M. Daby, and J-Y Tewg, IEEE 2nd NANO 2002 Symp. Procs., 251 (2002).
[v] G. Lucovsky, J. L. Whitten, and Y. Zhang, Solid-State Electron., 46, 1687 (2002).
[vi] Y. Kuo, J. Lu, and J.-Y. Tewg, submitted to Appl. Phys. Letts., January 2003.