A method of providing isolation between element regions of a semiconductor memory device (200). Isolation trenches (211) are filled using several sequential anisotropic insulating material (216/226/230) HPD-CVD deposition processes, with each deposition process being followed by an isotropic etch back to remove the insulating material (216/226/230) from the isolation trench (211) sidewalls. A nitride liner (225) may be deposited after isolation trench (211) formation. A top portion of the nitride liner (225) may be removed prior to the deposition of the top insulating material (230) layer.