Airborne molecular contamination: Formation, impact, measurement and removal of nitrous acid (HNO2)

Airborne molecular contamination (AMC) is a significant contributor to the loss of yield in semiconductor processes [1–4]. Impact of weak acids has only been considered for process technologies of 22 nm and below [5–8]. One such weak acid is nitrous acid (HNO2 or HONO), which has no demonstrated direct impact on processes or equipment, but has nevertheless been a target for removal by AMC filtration. HNO2 is commonly formed on all surfaces in all environments from NO2 gas, one of the main oxides of nitrogen formed from combustion processes and ambient air photochemistry. This study investigated the behavior of the NOX/HNOx system around typical AMC filter adsorbents. We find that NO gas passes through AMC filters unchanged, whereas NO2 is converted mostly to NO, but also to HNO2 at the low ppb level, increasing AMC load downstream of filters. Various adsorbents can capture HNO2, but filter lifetimes are short due to the release of the volatile compound over time. The recommendation is to critically evaluate the impact of HNO2 on processes and equipment and adjust AMC filtration needs accordingly.