A memory capacitor with a multistacked tunnel layer of Al2O3/HfO2/SiO2 (AHO) has been fabricated together with HfO2 charge trapping layer and Al2O3 control oxide layer. The resulting capacitor exhibits a memory window as large as 7.6 V for +/- 12 V sweep voltage range, a significant flatband voltage shift of 2.1 V after 10 V/100 mu s programing as well as improved charge retention compared with a single SiO2 tunnel layer. The different memory effects in various sweep voltage ranges and enhanced retention characteristics have been explained based on the variable electrical thickness of the AHO stack under different electric fields.