MANUFACTURING I MATERIALS
T
he drive to grid parity for the PV industry is forcing manufacturers to continually seek innovative and sustainable solutions to reduce manufacturing costs. Savings can come from lowering raw material costs, improving efficiency or increasing the available throughput of existing production lines. The use of on-site fluorine to clean Plasma Enhanced Chemical Vapour Deposition (PECVD) process chambers offers these advantages to the manufacturer, without any detrimental impact to product quality or process yield. In addition, there is a environmental benefit providing the environmental sustainability which most PV manufacturers are striving to achieve.
In silicon thin-film fabrication both particulate and molecular cleanliness are essential for high process yields. Fluorine radicals, produced by thermal or plasma activation from fluorine
(F2) or F-gases such as NF3 or SF6, are the preferred dry cleaning agents for Chemical Vapour Deposition (CVD) process chambers. Fluorine is the most electronegative element in the periodic table and forms strong bonds with most other
atoms but is weakly bound to itself. Compared with NF3 or SF6, fluorine is a more effective cleaning agent as it can be broken down into the reactive radicals with less energy [See table 1.]
The chemical simplicity of the fluorine molecule also means there is only one possible reaction pathway and gas-phase recombination is kinetically forbidden, thus almost 100% of fluorine radicals created are available for cleaning. So not only does it require less energy to create the radicals in the first place from fluorine, more of them are available for cleaning thus lowering overall material
costs.Therefore, to minise costs and maximise efficiency, on-site generated fluorine is the most viable solution for chamber cleaning.
Using commercially available process tools from all the leading OEMs, the cleaning performance of nitrogen trifluoride, sulfur hexafluoride and fluorine has been compared with both in-situ and remote plasma activation. The results show benefits in both mass and energy efficiency when using fluorine. For the same plasma source an increase in cleaning speed, or a reduction in cleaning time, can be achieved with fluorine. This is due to the limitations of the power available for activation, so three to five times as many fluorine radicals can be produced using fluorine instead of nitrogen trifluoride. and there is a directly proportional reduction in cleaning time. The larger process window advantage of fluorine over nitrogen trifluoride can be taken advantage of in different ways. The cleaning time could be shortened by more than 66% or the tool design could use a smaller RPS, or do away with it entirely in favour of in-situ cleaning gas activation. Additional savings come from a reduction in the mass of gas used and energy consumed.
High volume manufacturing
On-site fluorine generation equipment has been sized to meet the requirements of a number of chamber cleaning processes for PV, semiconductor and TFT-LCD manufacturing. With the advent of the high volume manufacturing of PV modules, the demand for on site generated fluorine has grown in scale.
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