Mechanism of highly selective SiO2 etching over Si3N4 using a cyclic process with BCl3 and fluorocarbon gas chemistries

A cyclic process using BCl3 and fluorocarbon gas chemistries was investigated for patterning a fine structure with a space width of 20nm level. The relationships between etching parameters and cross-sectional pattern profiles were also analyzed to control the pattern profiles. This process alternately performs two steps: a deposition step using BCl3/N-2/Ar plasma and an etching step using BCl3/CF4/Ar plasma with applying a wafer bias. The mechanism of the cyclic process was investigated by analyzing the surface chemistry at each step. Optical emission was also measured to clarify the reaction between F radicals and BClx radicals. The authors found the BClx layer formed on Si3N4 at the deposition step protected Si3N4 from etching by the reaction of BClx with CFx and F radicals at the etching step. Highly selective etching achieved a fine pattern structure by using BCl3/CF4/Ar plasma at the etching step. F radicals generated by the BCl3/CF4/Ar plasma had been scavenged by BClx radicals forming BFx to inhibit the Si3N4 etching effectively. In addition, B and BClx components adsorbed on the Si3N4 surfaces during the etching step protected Si3N4 from etching by reaction with F and CFx radicals. The adsorbed B and BClx desorbed from the surfaces by forming BFx, BClxF, and CClx, which remained on the Si3N4 surface. In contrast, the BClx layer became thinner on SiO2 than that on Si3N4 to promote ion-assisted etching of SiO2. This is because the BClx component has high reactivity with SiO2, and the remained CClx component was also consumed by the etching reaction with SiO2. The authors also found that ion flux at the etching step should be controlled to etch without shoulder loss, and ion energy at the etching step should be controlled to etch without footing shape at the bottoms of the pattern.