The method of planarizing polysilicon-filled trenches involves first filling the trenches (12) with an undoped polysilicon (14) until the upper surface (18) is substantially planar. The polycrystalline silicon (14) is then heavily doped by means of diffusion of a dopant from the upper surface. The time and temperature of the diffusion are carefully controlled providing for the dopant to penetrate the polysilicon to a depth (16), level with the tops of the trenches. A selective etchant is then utilized which removes the heavily doped polysilicon (16) and leaves the undoped polysilicon (17) untouched in the trenches (12).