A capacitor structure includes a semiconductor substrate, a first vertical diffusion plate disposed in the semiconductor substrate, a first shallow trench isolation (STI) structure disposed in the semiconductor substrate and surrounding the first vertical diffusion plate, and a second vertical diffusion plate disposed in the semiconductor substrate and surrounding the first STI structure. The first vertical diffusion plate further includes a first lower portion that is part of the semiconductor substrate. The first lower portion is surrounded and electrically isolated by a first wafer-backside trench isolation structure. The first wafer-backside trench isolation structure is in direct contact with a bottom of the first STI structure.