Applications and mechanisms of anisotropic two-step Si₃N₄ etching with hydrogen plasma conditioning

The ability to precisely form Si3N4 spacers is critical to the success of dynamic random-access memory and NAND (NOT AND) flash memory technology development. In this study, we investigated the mechanisms and process windows of an innovative two-step nitride (Si3N4) etch consisting of H-2 plasma processing in an inductively coupled plasma chamber followed by either buffered oxide etch (BOE, a mix of NH4F and HF solution) wet clean or in situ NF3 plasma etch. We obtained layer-by-layer removal with each layer's removal capable of more than 10 nm. We revealed that H can penetrate more than 20 nm deep into the nitride film to transform pristine Si3N4 into SiON after air exposure, which can be subsequently removed by BOE wet clean. The H-2 and BOE steps do not need to run back-to-back; the modified SiON layer is stable enough to sustain elevated temperature processing and can be removed by BOE later down-the-line integration. We also demonstrated that using NF3 plasma can have highly selective etch of nitride over oxide due to the incubation time difference between these two types of films. It takes much longer time to initiate the chemical reaction for oxide compared with nitride. Critically, the role of H-2 is not the key for high selectivity; instead, it provides an etch directionality and shortens the incubation time for both nitride and oxide.