Plasma-enhanced atomic layer deposition and etching of high-k gadolinium oxide

Atomic layer deposition (ALD) of high-quality gadolinium oxide thin films is achieved using Gd(iPrCp)3 and O2 plasma. Gd2O3 growth is observed from 150 to 350 °C, though the optical properties of the film improve at higher temperature. True layer-by-layer ALD growth of Gd2O3 occurred in a relatively narrow window of temperature and precursor dose. A saturated growth rate of 1.4 Å/cycle was observed at 250 °C. As the temperature increases, high-quality films are deposited, but the growth mechanism appears to become CVD-like, indicating the onset of precursor decomposition. At 250 °C, the refractive index of the film is stable at ∼1.80 regardless of other deposition conditions, and the measured dispersion characteristics are comparable to those of bulk Gd2O3. XPS data show that the O/Gd ratio is oxygen deficient at 1.3, and that it is also very hygroscopic. The plasma etching rate of the ALD Gd2O3 film in a high-density helicon reactor is very low. Little difference is observed in etching rate between Cl2 and pure Ar plasmas, suggesting that physical sputtering dominates the etching. A threshold bias power exists below which etching does not occur; thus it may be possible to etch a metal gate material and stop easily on the Gd2O3 gate dielectric. The Gd2O3 film has a dielectric constant of about 16, exhibits low C–V hysteresis, and allows a 50 × reduction in gate leakage compared to SiO2. However, the plasma enhanced atomic layer deposition (PE-ALD) process causes formation of an ∼1.8 nm SiO2 interfacial layer, and generates a fixed charge of −1.21 × 1012 cm−2, both of which may limit use of PE-ALD Gd2O3 as a gate dielectric.