A method includes forming a first photoresist layer on a front side of a device substrate and having first trenches spaced apart from each other. A first implantation process is performed using the first photoresist layer as a mask to form first isolation regions in the device substrate. A second photoresist layer is formed on the front side and has second trenches. A second implantation process is performed using the second photoresist layer as a mask to form second isolation regions in the device substrate and crossing over the first isolation regions. A third photoresist layer is formed on the front side and has third trenches spaced apart from each other. A third implantation process is performed using the third photoresist layer as a mask to form third isolation regions in the device substrate and crossing over the first isolation regions but spaced apart from the second isolation regions.