Preparation of wafer-scale highly conformalamorphous hafnium dioxide thin films by atomic layer deposition using a thermally stable boratabenzene ligand-containing hafnium precursor

Abstract

In the present study, HfO2 thin films were fabricated via atomic layer deposition (ALD) using a novel heteroleptic metal organic precursor [tris (dimethylamino) dimethylaminoboratabenzene hafnium] [η6:η1-(C5H5BNMe2)Hf(NMe2)3; (BBHf)] along with O2 as the oxygen source at a range of growth temperatures (i.e., 150–350 °C). This novel precursor is a heteroleptic complex synthesized by the introduction of a boratabenzene ligand (BB) into the parent Hf metal sphere to achieve an enhanced thermal stability. In this system, O2 is used as a mild oxygen-containing reactant to replace the typically employed ozone (O3). Distinctive self-limiting deposition was established with a comparatively high growth per cycle value of 0.068 nm, and linear growth was observed as a function of the ALD cycle number. Thermal decomposition was not detected at or below 350 °C, thereby indicating the improved thermal stability compared to when frequently used Cp (cyclopentadienyl)-amide precursors are employed. Under the ALD deposition conditions employed herein (275 °C), a complete step coverage was achieved with a good conformality on high aspect ratio dual trenches [top and bottom widths = 40 and 15 nm, respectively, aspect ratio (AR) ≈ 6.3], and uniformity was obtained on the large planar substrate (15 cm diameter). Upon annealing at 700 °C, the as-grown film formed an amorphous structure with a slightly enhanced crystallinity, while annealing at 850 °C led to the generation of nanocrystalline HfO2 films with amorphous structures, as indicated by X-ray diffraction measurements. The as-grown films were determined to be slightly rich in oxygen compared to the stoichiometry of HfO2, although they also contained significant amounts of residual impurities, such as H, B, and C (∼6, 6, and 7 at.%, respectively), as confirmed by Rutherford backscattering spectrometry and elastic recoil detection analyses. The impurity levels were further reduced by increasing the growth temperature and by subsequent post-annealing, as evidenced by X-ray photoelectron spectroscopy and secondary-ion mass spectrometry analyses. Finally, ellipsometry analysis was performed to measure the optical properties of the prepared ALD-HfO2 thin films. It is expected that the described process may be of significance in the preparation of high-k films wherein thermally stable amorphous films with extremely conformal and uniform coatings are required to fabricate next-generation electronic devices.