Higher capacitance density is achieved by increasing a surface area of a capacitor. A larger surface area may be obtained by forming isotropic ball shapes (a concave surface) in the trenches on the semiconductor die. The concave surfaces are fabricated by depositing bilayers of amorphous-silicon and silicon oxide. Openings are patterned in the silicon oxide hard mask for trenches. The openings are transferred to the amorphous-silicon layers through isotropic etching to form concave surfaces. Conducting, insulating, and conducting layers are deposited on the concave surfaces of the trenches by atomic layer deposition.