The passivation property of $Al_2O_3$ thin film formed using atomic layer deposition (ALD) for the application of crystalline Si solar cells was investigated using microwave photoconductance decay (${mu}$-PCD). After post-annealing at $400^{circ}C$ for 5 min, $Al_2O_3$ thin film exhibited the structural stability having amorphous nature without the interfacial reaction between $Al_2O_3$ and Si. The post-annealing at $400^{circ}C$ for 5 min led to an increase in the relative effective lifetime of $Al_2O_3$ thin film. This could be associated with the field effective passivation combined with surface passivation of textured Si. The capacitance-voltage (C-V) characteristics of the metal-oxide-semiconductor (MOS) with $Al_2O_3$ thin film post-annealed at $400^{circ}C$ for 5 min was carried out to evaluate the negative fixed charge of $Al_2O_3$ thin film. From the relationship between flatband voltage ($V_{FB}$) and equivalent oxide thickness (EOT), which were extracted from C-V characteristics, the negative fixed charge of $Al_2O_3$ thin film was calculated to be $2.5{ imes}10^{12}cm^{-2}$, of which value was applicable to the passivation layer of n-type crystalline Si solar cells.