Highly selective SiO2 etching over Si3N4 using a cyclic process with BCl3 and fluorocarbon gas chemistries

Abstract

A cyclic process for highly selective SiO2 etching with atomic-scale precision over Si3N4 was developed by using BCl3 and fluorocarbon gas chemistries. This process consists of two alternately performed steps: a deposition step using BCl3 mixed-gas plasma and an etching step using CF4/Ar mixed-gas plasma. The mechanism of the cyclic process was investigated by analyzing the surface chemistry at each step. BClx layers formed on both SiO2 and Si3N4 surfaces in the deposition step. Early in the etching step, the deposited BClx layers reacted with CFx radicals by forming CClx and BFx. Then, fluorocarbon films were deposited on both surfaces in the etching step. We found that the BClx layers formed in the deposition step enhanced the formation of the fluorocarbon films in the CF4 plasma etching step. In addition, because F radicals that radiated from the CF4 plasma reacted with B atoms while passing through the BClx layers, the BClx layers protected the Si3N4 surface from F-radical etching. The deposited layers, which contained the BClx, CClx, and CFx components, became thinner on SiO2 than on Si3N4, which promoted the ion-assisted etching of SiO2. This is because the BClx component had a high reactivity with SiO2, and the CFx component was consumed by the etching reaction with SiO2.