Guest columnist Greg Shuttleworth, product manager at Linde looks at options for eliminating greenhouse gases from the manufacturing process of TFT-LCD displays.
Almost all manufacturers claim that their TFT-LCD TVs and other displays are environmentally friendly. To date though, focus has been mostly on specific consumer benefits – lower device power consumption, for example. There is only so far these benefits can continue to appeal to an increasingly environmentally-aware consumer base.
TFT-LCD manufacture is a complex process, carried out on a huge and increasing scale and thus an ever increasing consumption of chemicals, gases, water and power. To increase sustainability, the industry is studying the overall supply chain, using Life Cycle Analysis to identify the environmental impact of creation, use and disposal of the wide range of materials used.
One area of manufacturing with a potentially high environmental impact is building the thin-film transistors on the back-plane of the display. Silicon films deposited using a chemical vapour deposition (CVD) process must be etched to create the required patterns.
That CVD process also deposits quantities of silicon on the inside of the process chamber, which must be periodically cleaned to maintain process cleanliness and production efficiency. Both these etching and cleaning processes use fluorine based gases, and given that process chambers need to accommodate glass substrates with an area of more than five square metres, the amount of gas required is very large.
Why is this important? The gases used have very high global warming potential (GWP). Sulphur hexafluoride (SF6) used for etching, has a GWP 23,900 times that of CO2, while nitrogen trifluoride (NF3) used for cleaning, has a GWP of 17,200.
So with a typical Gen8 facility using upwards of 300 tonnes of such gases per year, the potential environmental impact must be considered.
To minimise process emissions, most manufacturers have installed high performance scrubbing systems. However, there exists the risk of emissions during the whole life cycle of the material.
Measurements by the Scripps Institute have shown a rapid growth in the amount of NF3 present in the atmosphere, which correlates to a figure as high as 16% of NF3 produced ultimately escaping. So there remains an incentive to consider materials with lower GWP to minimise the impact of emissions due to the manufacturing, transport and disposal of the materials, areas outside the control of TFT-LCD manufacturers. One strong contender is fluorine gas (F2).
The science goes like this. NF3 or SF6 gas is activated by a plasma to release fluorine atoms which then etch or clean the silicon films. The alternative, F2 gas, is the simplest molecule containing fluorine atoms and has the lowest bond energy. These properties of F2 provide significant benefits to the cleaning and etching processes; faster etching or cleaning, less tool down-time, reduced gas use, and less electrical power consumed by the plasma to break down the simpler F2 molecules.
Environmentally speaking, the big win is the zero GWP of F2. To highlight the impact, the table below shows the potential CO2 equivalent savings from replacing NF3 with F2 in a typical large scale TFT-LCD fab chamber-cleaning process.
Typical Gen8 fab
NF3 consumption per year (tons) >300
CO2 eq emissions (tons) =300 x 17,200 x 16% (ref 1 Scripps data) >825,600
CO2 eq emissions (tons) =300 x 17,200 x 10% (conservative figure) >516,000
It is thus likely that the replacement of NF3 and SF6 in the display manufacturing process can have a larger overall environmental benefit for TFT-LCD manufacturers than is achievable by any other programme.
So, while a reduction in power consumption for the TV on your wall at home will help lower your electricity bills and your own carbon footprint, changes to the way they are manufactured can have a much more significant effect on their total environmental impact.
The Linde Group