Strontium titanate (STO) is a promising material that offers the possibility of achieving large dielectric permittivity (k) constants for gate dielectric and other applications. Thin (~28 nm) STO films were deposited by remote plasma-assisted atomic layer deposition on the native oxide of n-type Si substrates in an Oxford Instruments FlexAL reactor at ~250°C using Bis(Tris-IsopropylCyclopentadienyl) Strontium, Tetrakis(dimethylamido)titanium as metal precursors, and a remote oxygen plasma as oxidizer. The general approach to deposit the ternary perovskite SrTiO3 is by alternating the ALD of the constituents TiO2 and SrO in a specific ratio to control such properties as the stoichiometric composition and dielectric constant [1,2]. The deposition temperature was chosen to give optimum growth uniformity for both SrO and TiO2 and to avoid decomposition of the precursors. The growth rate of the STO films was ~ 0.14 nm/cycle, which is slightly higher than the combined growth rate of the individual components. Also, the STO growth rate and stoichiometry are highly dependent on the temperature of the strontium precursor.

 Initial capacitance-voltage (C-V) and current-voltage (I-V) results on 50-250 µm diameter circular capacitors patterned on ALD SrO:TiO2 (1:1) films were obtained. These dielectrics exhibited a moderate (~ 20) dielectric constant with reduced reverse-biased leakage current of ~1.6x10-5 A/cm2but larger forward bias leakage current ~ 1.2x10-3 A/cm2at 1V. Hysteresis in forward and reverse C-V sweeps was not observed, suggesting that these films are high-quality with limited or no slow time-constant charge trapping. After annealing the STO films at 550ºC in N2 for 5 min, a reduction in the oxide thickness by ~ 9% was measured, as well as a small increase in the dielectric constant to ~ 28 as a result of crystallization [3].

 During this study, the overall thickness of the dielectric was held relatively constant, while the relative ratio of SrO:TiO2 was varied to tune the stoichiometry of the films and to monitor changes in the dielectric constant, optical band gap, Eg, and the electrical performance of the resulting oxides. The thickness, g rowth rates, and ALD mode behavior of STO, SrO, and TiO2 oxides were evaluated by spectroscopic ellipsometry measurements. We will p resent electrical measurements of STO oxides of various compositions with optimized deposition conditions for the chosen precursors, as well as stoichiometry assessments by x-ray photoelectron spectroscopy.

1. Vehkamäki,et.al. Chem. Vap. Deposition, 7, 75 (2001)

2. Popovici,et.al. J. Electrochem. Soc., 157, G1  (2009)

3. Langereis,et.al. J. Electrochem. Soc., 158, G34-G38  (2011)