We fabricated Al/Al2O3/SiO2/Si and Al/HfO2/Si structures to optimize the passivation layer of a backside-illuminated (BSI) complementary metal oxide semiconductor (CMOS) image sensor (CIS), with the key properties of the newly developed high- $k$ passivation layer analyzed via border traps, interface traps, and fixed charges. In the first experiment using Al2O3/SiO2 bilayer-based structures, different thicknesses of SiO2 were applied from 0 to 15 nm. The improvement in their properties was confirmed by applying forming gas annealing (FGA), a type of post-treatment, to all experimental systems. The first experiment results indicated that both the SiO2 layer and FGA were effective for chemical passivation. However, a tradeoff occurred in the degree of improvement of the interface trap density ( $\text{D}_{\mathrm {it}}$ ) and fixed-charge density ( $\text{Q}_{\mathrm {f}}$ ) according to the SiO2 layer thickness. Subsequently, in the second experiment using HfO2 single-layer-based structures, FGA improved the border trap to a relatively poor extent compared to the first experiment. Nevertheless, FGA improved the electrical characteristics of the HfO2 films without any side effects and results in optimal $\text{D}_{\mathrm {it}}$ and $\vert \text{Q}_{\mathrm {f}}/\text{q}\vert $ values of $2.59 \times 10^{11}$ eV $^{-1}$ cm $^{-2}$ and $1.00 \times 10^{12}$ cm $^{-2}$ , respectively, demonstrating its potential for the passivation layer in BSI CIS applications.
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