Sub 5 nm thick doped metal oxide high k films for future generations of VLSI devices

Dr. Yue Kuo


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 5 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 5 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 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.