High purity argon (greater than 99.999%) is used as a shield gas in the production of silicon ingots, to be fabricated into wafers for solar cells and micro-electronic devices. The argon is used to control the impurity levels present during the manufacturing process to an acceptable level. In response to rising costs, the trend in the solar industry has been to reduce the argon purge flows to a minimum, typically about 30slm; however, this cost reduction comes at the expense of wafer purity which can result in lower performance solar cells.
Ultra-High purity wafers for use in micro-electronic applications typically utilise two to three times as much argon as for the solar PV application, so rising argon costs are an even more critical issue in this sector.
The current trend in the Solar PV wafer market is to move to higher purity high performance and/or n-type doped wafers, to maximise the efficiency of the resulting solar cells and generate added value. This requires argon purge gas flows to increase up to about 70-90slm. The supply of high purity argon is primarily a by-product of the air separation process generating oxygen used in steel making.
This means, at best the, supply of high purity argon is essentially fixed and at worst is in shortening supply as air separation units are moth-balled in line with reducing steel manufacturing capacity. The laws of ‘supply and demand’ dictate that the argon price will increase. This is precisely what is being seen in the market, with argon prices increasing by as much as 400% in the last few years in some territories. For manufacturers faced with this ‘double whammy’ of increasing argon usage and cost, investing in ArgonØ argon recycling systems, which can reduce argon costs per wafer by more than 90%, has become more and more attractive!
Argon recycle strategies
A typical silicon furnace hall will have a forest of up to 100 silicon ingot furnaces, Figure 1, consuming upwards of 8 million litres of high purity argon gas per day. In order to recycle the argon, it first needs to be recovered from the exhaust lines of the vacuum pumps associated with the individual furnaces. Then purified and returned to the argon furnace purge supply line i.e. closed loop recycling. There are two strategies to the recycle of exhaust gases from process tools:
1. Point of Use recycle where the exhaust gas is diverted to a local purification system, close coupled to a limited number of tools, say <6.
2. Central processing where all the facility exhaust lines, say >100, are combined together and fed to a single, larger, purification system.