Paper TF-TuA1
Infrared Characterization of Atomic Layer Deposition and Post Annealing of Lanthanum Oxide Films

Tuesday, October 16, 2007, 1:40 pm, Room 613/614

La₂O₃ is a potential material for high-k gate dielectrics in microelectronics due to its relatively large permittivity, thermal stability in contact with Si, suitable band gap and conduction band offset. Uniform and conformal thin layers of metal oxides can be realized by a self-limiting atomic layer deposition (ALD) process. Surface nucleation and overall interface formation mechanisms are unfortunately not well understood since most of film characterization has been performed with ex-situ measurements. We report on in-situ Fourier transmission infrared (FTIR) studies of ALD growth and post-deposition annealing of La-oxide thin films on Si substrates using a metal-amidinate precursor¹ and D₂O. Film growth rates were determined from ex-situ RBS measurements. Atomically flat hydrogen-terminated Si(111) surfaces were used as substrates for La-oxide film growth. IR spectra were obtained at every half-cycle of the initial stages and post-deposition annealing was studied up to 800°C by performing IR measurements after 20 cycles of ALD process at 100°C increments. Between 200 and 300°C substrate temperatures, the first La-precursor exposure consumes approximately 30% of surface hydrogen, as measured by the intensity loss of the monohydride Si-H stretching mode (2083 cm^-1). A D₂O exposure is used each time to complete the cycle. More than 90 % hydrogen is consumed after the third metal-precursor dosing (2.5 cycle) at 300°C deposition temperature. La₂O₃ grown at 200°C shows higher concentration of carbonate species (modes in the 1390 ~ 1470 cm^-1 region) incorporated into the film compared to 300°C grown films, probably due to incomplete elimination of the metal precursor by D₂O. The intensity of these modes decreases with increasing annealing temperature and almost completely disappears at 400 ~ 500°C annealing. Carbonate groups in the films grown at 300°C are lower, but profound ligand decomposition is observed around 2000 cm^-1. Si-O-Si modes (900 ~ 1200 cm^-1) are observed to increase from the very initial ALD cycle, consistent with interfacial SiO_x formation at 300°C. Films grown at 200°C show much slower interface oxide formation, but even a mild (400°C) post-deposition anneal triggers substantial interfacial SiO_x formation.

¹ B. S. Lim et al., Appl. Phys. Lett. 84, 3957 (2004).