Plasma-Assisted ALD for the Conformal Deposition of SiO₂: Process, Material and Electronic Properties

Plasma-assisted atomic layer deposition (ALD) was used to deposit SiO2 films in the temperature range of Tdep = 50–400°C on Si(100). H2Si[N(C2H5)2]2 and an O2 plasma were used as Si precursor and oxidant, respectively. The ALD growth process and material properties were characterized in detail. Ultrashort precursor doses (~50 ms) were found to be sufficient to reach self-limiting ALD growth with a growth-per-cycle of ~1.2 Å (Tdep = ~200°C) leading to SiO2 films with O/Si ratio of ~2.1. Moreover, the plasma ALD process led to a high conformality (95–100%) for trenches with aspect ratios of ~30. In addition, the electronic (interface) properties of ultrathin ALD SiO2 films and ALD SiO2/Al2O3 stacks were studied by capacitance-voltage and photoconductance decay measurements. The interface quality associated with SiO2 was improved significantly by using an ultrathin ALD Al2O3 capping layer and annealing. The interface defect densities decreased from ~1×1012 eV-1 cm-2 (at mid gap) for single layer SiO2 to <1011 eV-1 cm-2 for the stacks. Correspondingly, ultralow surface recombination velocities <3 cm/s were obtained for n-type Si. The density and polarity of the fixed charges associated with the stacks were found to be critically dependent on the SiO2 thickness (1–30 nm).