The effect of annealing temperature on the ferroelectricity of HfAlO_x with Al concentration of 4.5% is physically and electrically investigated by metal–ferroelectric–insulator–semiconductor (MFIS) platform. HfAlO_x with 600 °C annealing is confirmed to possess ferroelectricity by the formation of non-centrosymmetry orthorhombic III phase, clockwise capacitance–voltage hysteresis, significant polarization–electric field hysteresis curve, and in-depth analysis of current component as compared with non-ferroelectric HfO₂. Compared to commonly discussed MFM devices from which thermal annealing of 800 °C is required to induce ferroelectricity, the relatively lower thermal budget to form ferroelectric HfAlO_x in MFIS devices can be attributed to the compressive thermal stress caused by the difference in thermal expansion between the Si substrate and HfAlO_x during thermal annealing, and it is the stress that helps lower the annealing temperature to crystallize HfAlO_x in the orthorhombic phase. In addition, MFIS devices with 600 °C-annealed HfAlO_x hold the prospect of becoming a promising candidate for memory applications by demonstrating a memory window of 0.6 V with 3 V operation voltage which is comparable with or superior to those with conventional ferroelectric films. Furthermore, the process to from ferroelectric HfAlO_x-based devices can be fully integrated into incumbent mass production fabs, empowering next-generation memory technology.