By Pete Singer
Nitrous oxide (N2O) has a variety of uses in the semiconductor manufacturing industry. It is the oxygen source for chemical vapor deposition of silicon oxy-nitride (doped or undoped) or silicon dioxide, where it is used in conjunction with deposition gases such as silane. It’s also used in diffusion (oxidation, nitridation, etc.), rapid thermal processing (RTP) and for chamber seasoning.
Why these uses – and more importantly what happens to the gas afterward — may soon becoming under more scrutiny because it is being included for the first time in the IPPC (Intergovernmental Panel on Climate Change) GHG (Greenhouse Gas) guidelines. The IPCC has refined guidelines released in 2006 and expect to have a new revision in 2019. “Refined guidelines are actually up and coming and the inclusion of nitrous oxide in them is a major revision from the 2006 document,” said Mike Czerniak, Environmental Solutions Business development Manager, Edwards. Czerniak is on the IPPC committee and lead author of the semiconductor section.
Although the semiconductor industry uses a very small amount of N2O compared to other applications (dentistry, whip cream, drag racing, scuba diving), it is a concern because after CO2and CH4, N2O is the 3rd most prevalent man-induced GHG, accounting for 7% of emissions. According to the U.S. Environmental Protection Agency, 5% of U.S. N2O originates from industrial manufacturing, including semiconductor manufacturing.
Czerniak said the semiconductor industry been very proactive about trying to offset and reduce its carbon dioxide footprint. “The aspiration set by the world’s semiconductor council to reduce the carbon footprint of a chip to 30 percent of what it was in 2010, which itself was a massive reduction of what it used to be back in the last millennium,” he said. Unfortunately, although that trend had been going down for the first half of the decade, it started going up again in 2016. “although each individual processing step has a much lower carbon footprint than it used to have, the number of processing steps is much higher than they used to be,” Czerniak explain. “In the 1990s, it might take 300-400 processing steps to make a chip. Nowadays you’re looking at 2,000-4,000 steps.”
There are two ways of abating N20 so that it does not pollute the atmosphere: reduce it or oxidize it. Oxidizing it – which creates NO2and NO (and other oxides know as NOx) — is not the way to go, according to Czerniak. “These oxides have their own problems. NOx is a gas that most countries are trying to reduce emissions of. It’s usually found as a byproduct of fuel combustion, particularly in things like automobiles and it adds to things like acid rain,” he said.
Edwards’ view is that it’s much better to minimize the formation of the NOx in the first place. “The good news is that it is possible inside a combustion abatement system where the gas comes in at the top, we burn a fuel gas and air on a combustor pad and basically the main reactant gas then is water vapor, which we use to remove the fluorine effluent, which is the one we normally try to get rid of from chamber cleans,” Czerniak said.
The tricky part is that information from the tool is required. “We can — when there is nitrous oxide present on a signal from the processing tool — add additional methane fuel into the incoming gas specifically to act as a reducing agent to reduce the nitrous oxide to nitrogen and water vapor,” he explained. “We inject it at just the right flow rate to effectively get rid of the nitrous oxide without forming the undesirable NOx byproducts.”
Figure 1 showshowcareful control of combustion conditions make them reduce rather than oxidizing during the N2O step by the addition of CH4. 30 slm N2O represents two typical process chambers.
“It’s not complicated technology,” Czerniak concluded. “You just have to do it right.”