Process for making masks with structures in the submicron range

Abstract

In the course of the process for making masks with structures in the submicrometer range, initially structures of photoresist or polymer material with horizontal and substantially vertical sidewalls are produced on a silicon substrate covered with an oxide layer. This is followed by a layer of silicon nitride which is deposited by LPCVD. The resultant structure is planarized with a photoresist which is etched back until the start of the vertical edges of the sidewall coating formed by the nitride layer is bared on the photoresist structures. In a photolithographic step, a trimming mask is produced on the surface of the nitride layer and the planarizing resist. The bared regions of the nitride layer are then removed by isotropic etching. The dimensions A-B of the openings defined after removal of the nitride layer from the vertical surfaces of the photoresist structures are transferred to the oxide layer by anisotropic etching. Concurrently with these structures of minimum line width, registration marks are generated which allow the adjustment necessary for a further photolithographic step to be carried out with maximum accuracy. After removal of the trimming mask, the planarizing resist, the photoresist structures and the remainder of the nitride layer, structures with coarser line widths are defined in a further photolithographic step, which are also transferred to the oxide layer. Using the oxide layer as a mask, trenches of the desired depth are produced in the silicon substrate by anisotropic etching. The mask is thinned by anisotropic etching, and the oxide layers are removed from the front and the back side by wet etching.