Improvement of gate oxide breakdown through STI structure Modification in DRAM

Abstract

This paper focuses on investigating the origin of the vulnerability and proposing improvement strategies for gate oxide (G_ox) breakdown in the core area of dynamic random access memory (DRAM) products. The shallow trench isolation (STI) area in 15-nm DRAM intricately comprises a triple-layer structure: sidewall oxide, nitride liner, and trench oxide. The existing structure had a high protrusion of the nitride liner, disrupting the gas flow during G_ox deposition and resulting in a relatively thin thickness at the active corners. Additionally, when the gate voltage is applied, the angular shape of the active Si area led to a concentration of the electric field in the corner area. These two structural characteristics were recognized as the causes that render the active corner area vulnerable to G_ox breakdown failure. By developing new wet-etching processes for the active and STI structures, we can significantly improve G_ox breakdown. We applied a phosphoric acid process to improve the high protrusion structure of the nitride liner and used a new solution process to make the active corner more rounded. We validated the enhancement through the application to actual products and verified it by electrical results. Ultimately, this approach serves as a crucial clue for the continued scaling of DRAM core transistors.