Method for forming recessed oxide isolation containing deep and shallow trenches

Recessed isolation oxide is deposited in shallow trenches simultaneoulsy with oxide deposition in deep isolation trenches. A single planarization of both trench fillings provides efficient recessed isolation oxide without bird's beak or bird 's head problems of LOCOS isolation oxide. Self-aligned trench filling by successive conformal depositions of oxide and polysilicon followed by planarization to remove polysilicon away from the trenches. The the remaining polysilicon may be used as an oxide etch mask to remove all of the oxide except in the trenches.