lifecycle analyses of greenhouse gas emissions consider both direct and indirect sources. Indirect CO2 emissions, attributed to electricity and other forms of energy purchased by the fab, are the semiconductor industry’s single largest environmental impact. Of those emissions, a large fraction are attributable to plasma-based etch and deposition steps, the most energy-intensive processes in the fab.
Those same deposition and etch processes are also major contributors to direct, Scope 1, fab emissions. Up until 1999, perfluorocarbons (PFCs) such as CF4 and C2F6 were routinely used in dry etch and post-deposition chamber cleaning steps. With good stability and relatively low toxicity, they gave fab managers a versatile library of potential etch and cleaning chemistries.
Trace amounts, big impact
Unfortunately, PFCs also have substantially greater global warming potential than CO2. As the name suggests, greenhouse gases in the upper atmosphere absorb infrared radiation, trapping it near the Earth’s surface rather than allowing it to escape into space. The global warming potential of a gas — measured in 100-year CO2 equivalents — reflects both its IR absorption and the rate at which it will break down through atmospheric processes or be reabsorbed at the Earth’s surface. PFCs are problematic on both counts. They absorb strongly in the IR range and, being stable molecules that are almost entirely generated by human activity, there are few processes that will break them down or extract them from the atmosphere. (In contrast, CO2 is naturally absorbed from the atmosphere by plants.)
Because of their stability, PFCs often have relatively low utilization rates in fab processes. Researchers at Chia Nan University in Taiwan estimated that as much as 60% to 70% of the CF4 and C2F6 used in CVD reactor cleaning failed to break down and passed through to the chamber exhaust. For the same reason, thermal abatement is extremely challenging, requiring temperatures in excess of 1000 ºC.
In 1999, the World Semiconductor Council set voluntary targets for PFC emissions, setting its goal at 90% of 1995 emissions by 2010, and an additional 30% reduction relative to 2010 by 2020.
In the aluminum industry, fluorine is used to help reduce oxygen from bauxite ore, with CF4 as a by-product.
Replace or abate?
However, as Czerniak pointed out, CF4 is more difficult to replace in etch processes, where carbon “polymer” helps control the etch profile. Nor, as the table shows, is NF3 a panacea. Though not a PFC, it too has high IR absorption and high atmospheric persistence, giving a 100-year CO2 equivalent global warming impact of 8,000. On the positive side, NF3 does break down more easily under process conditions, increasing utilization and improving abatement efficiency.
With the “easy” greenhouse gas reductions accomplished, efforts to meet the WSC’s 2020 target are focusing on abatement.
Once the easy solutions have been implemented, continuing to reduce emissions requires the same level of innovation in waste reduction and waste management that engineers have been applying to process improvement for decades.
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3 Comments
Tomi Engdahl says:
More Than Just Carbon Dioxide
Part 4: The quest to limit greenhouse gases.
http://semiengineering.com/more-than-just-carbon-dioxide/
lifecycle analyses of greenhouse gas emissions consider both direct and indirect sources. Indirect CO2 emissions, attributed to electricity and other forms of energy purchased by the fab, are the semiconductor industry’s single largest environmental impact. Of those emissions, a large fraction are attributable to plasma-based etch and deposition steps, the most energy-intensive processes in the fab.
Those same deposition and etch processes are also major contributors to direct, Scope 1, fab emissions. Up until 1999, perfluorocarbons (PFCs) such as CF4 and C2F6 were routinely used in dry etch and post-deposition chamber cleaning steps. With good stability and relatively low toxicity, they gave fab managers a versatile library of potential etch and cleaning chemistries.
Trace amounts, big impact
Unfortunately, PFCs also have substantially greater global warming potential than CO2. As the name suggests, greenhouse gases in the upper atmosphere absorb infrared radiation, trapping it near the Earth’s surface rather than allowing it to escape into space. The global warming potential of a gas — measured in 100-year CO2 equivalents — reflects both its IR absorption and the rate at which it will break down through atmospheric processes or be reabsorbed at the Earth’s surface. PFCs are problematic on both counts. They absorb strongly in the IR range and, being stable molecules that are almost entirely generated by human activity, there are few processes that will break them down or extract them from the atmosphere. (In contrast, CO2 is naturally absorbed from the atmosphere by plants.)
Because of their stability, PFCs often have relatively low utilization rates in fab processes. Researchers at Chia Nan University in Taiwan estimated that as much as 60% to 70% of the CF4 and C2F6 used in CVD reactor cleaning failed to break down and passed through to the chamber exhaust. For the same reason, thermal abatement is extremely challenging, requiring temperatures in excess of 1000 ºC.
In 1999, the World Semiconductor Council set voluntary targets for PFC emissions, setting its goal at 90% of 1995 emissions by 2010, and an additional 30% reduction relative to 2010 by 2020.
In the aluminum industry, fluorine is used to help reduce oxygen from bauxite ore, with CF4 as a by-product.
Replace or abate?
However, as Czerniak pointed out, CF4 is more difficult to replace in etch processes, where carbon “polymer” helps control the etch profile. Nor, as the table shows, is NF3 a panacea. Though not a PFC, it too has high IR absorption and high atmospheric persistence, giving a 100-year CO2 equivalent global warming impact of 8,000. On the positive side, NF3 does break down more easily under process conditions, increasing utilization and improving abatement efficiency.
With the “easy” greenhouse gas reductions accomplished, efforts to meet the WSC’s 2020 target are focusing on abatement.
Once the easy solutions have been implemented, continuing to reduce emissions requires the same level of innovation in waste reduction and waste management that engineers have been applying to process improvement for decades.
Tomi Engdahl says:
Scientists Sound the Alarm: CO2 Levels Race Past Point of No Return
http://www.ecowatch.com/noaa-carbon-dioxide-levels-2321635970.html
Tomi Engdahl says:
Humans Have Pushed Earth To A Terrifying New Milestone, And It Could Have Deadly Effects On Our Health
http://www.iflscience.com/health-and-medicine/humans-have-pushed-earth-to-a-terrifying-new-milestone-and-it-could-have-deadly-effects-on-our-health/
Monthly average atmospheric carbon dioxide levels have topped 410 ppm for the first time in more than 800,000 years, according to recent research.
There’s good reason to think this will have disastrous effects on human health.