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 (HNO
₂
or HONO), which has no demonstrated direct impact on processes or equipment, but has nevertheless been a target for removal by AMC filtration. HNO
₂
is commonly formed on all surfaces in all environments from NO
₂
gas, one of the main oxides of nitrogen formed from combustion processes and ambient air photochemistry. This study investigated the behavior of the NO
_X
/HNOx system around typical AMC filter adsorbents. We find that NO gas passes through AMC filters unchanged, whereas NO
₂
is converted mostly to NO, but also to HNO
₂
at the low ppb level, increasing AMC load downstream of filters. Various adsorbents can capture HNO
₂
, but filter lifetimes are short due to the release of the volatile compound over time. The recommendation is to critically evaluate the impact of HNO
₂
on processes and equipment and adjust AMC filtration needs accordingly.