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On-site generated fluorine an alternative to greenhouse gases in PV manufacturing

22. August 2011 By:  Paul Stockman, Linde

Photovoltaics manufacturers are continually looking for ways to minimize their carbon footprint during the module production process. The use of on-site generated fluorine is a safe and reliable alternative to greenhouse gases in chemical vapor deposition (CVD) chamber cleaning, writes Linde's Paul Stockman, reducing the global warming potential (GWP) by tens of million tons of carbon dioxide equivalent per year.

On-site generated fluorine is currently used in the commercial production of thin film solar panels at many sites in both Asia and Europe. Production capacities ranging from one to over 100 tons per year have displaced high-pressure fluorine cylinder and bulk nitrogen trifluoride supplies.

On-site fluorine generators specifically designed to meet the requirements of the electronics industry have been supplying low pressure, high purity fluorine gas safely and reliably to CVD tools for more than ten years.

Delivering sustainable chamber cleaning

Fluorine is the highest performance cleaning gas available, which improves dramatically the productivity of CVD tools, and reduces energy consumption and the environmental impact of CVD cleaning, with zero GWP. It cuts the cleaning time by more than a half, while minimizing tool downtime and improving line throughput by up to 10 percent. High purity fluorine also slashes the mass of cleaning gas required by 50 percent and the power consumption of the plasma source by 60 percent.

These benefits stem from the simple mechanism and low energy required to create fluorine radicals compared with other fluorinated gases. Today, industrial fluorine is produced commercially at more than 20 locations for a total global production in excess of 50,000 metric tons.

Safety considerations

While industrial fluorine can be transported in large volumes either as a cryogenic liquid or as a compressed gas, safety and logistic concerns dictate that most fluorine is produced and consumed on-site and on-demand, at low or atmospheric pressure.

Concerns for safety and reliability have informed the design of on-site fluorine generators for use in the high-tech photovoltaic manufacturing. As with any chemical introduced into this market sector, safety risks need to be identified and mitigated.

Fluorine is the most electronegative element from the periodic table, and this extreme reactivity is the cause for risks to both health and equipment. Fluorine falls in the middle of the spectrum of the many toxic gas-phase chemicals commonly used in thin film device manufacturing.

Minimizing safety concerns

Although fluorine is highly reactive, which is its basis as a chamber cleaning agent, many common metal materials used in construction are compatible with fluorine at process temperatures. These include brass, copper, nickel, and many steel and nickel alloys, all of which form chemically inert, passivated metal fluoride layers when properly prepared.

All super-atmospheric sources of fluorine are contained by either ventilated enclosures or sealed and sectioned annular piping. Ventilated enclosures house all generation and compression equipment, purification and buffer vessels, and valve manifold boxes.

The extraction rates are determined to maintain safe external conditions in the event of a rapid release of the limited inventory. Extracted air is treated by either a dedicated or facility acid scrubber. Toxic gas detectors specific to fluorine are used to monitor all extracted volumes. Transport of fluorine is through double-layered piping.

Maintaining a minimal inventory is integral to the design and safe usage of fluorine. Because the fluorine is generated on-site, there is no need to compress it to high pressures in order to transport it in containers. And because fluorine is generated on-demand and proportionally to the current applied to the electrodes, the only vessels used are sized to buffer the generator against the periodic requirement for cleaning gas.

Maximizing reliability

As well as being safe, on-site generated fluorine must meet the very high industry standards for reliability. Design and operation are essential to achieving high uptime. Beyond proper material and component selection, moving parts are minimized in the system design, electrochemical duty is kept light, and preventative maintenance and monitoring leading indicators keep all performance parameters within controlled ranges.

Preventative maintenance and repairs can be performed quickly without compromising supply, and longer-term service can be done off-site. In addition to on-site operators, most fluorine plants for chamber cleaning take advantage of remote monitoring. This allows 24 hours a day coverage and diagnostics by fluorine plant experts located on three continents.

Meeting a surge in demand

On-site fluorine generation has been sized to meet the requirements of a number of chamber cleaning processes. With the advent of the high-volume manufacturing of photovoltaic modules, the demand for on-site generated fluorine has grown in scale.

In these applications, fluorine is used as a chamber cleaning agent for plasma-enhanced vapor deposition (PECVD) processes. Larger generators have been designed for supplying 100s of tons/year.

Although the process design remains the same, the form factor for these systems is grown to a separate on-site facility. This mirrors the track to other on-site supply schemes, such as nitrogen, oxygen and hydrogen, from which manufacturers benefit in cost of ownership as they scale their operations to achieve a critical capacity.

The supply of on-site generated fluorine requires different project considerations versus packaged material supplies to ensure proper footprint, utilities availability and expansion capability.

Planning for safety is integrated with the end user and community at an early stage, and because the on-site chemical inventory is substantially reduced, there are fewer licensing issues to deal with. Finally, on-site generated fluorine minimizes the risk of supply chain and pricing volatility.

In the last ten years, over 30 on-site fluorine generators have been installed and successfully operated in 12 countries.  These continue to operate without a single safety incident and provide a supply delivery reliability much greater than 99 percent.

Paul Stockman, Ph.D. has been Fluorine Technology Manager at Linde Electronics since 2008 with responsibility for commercializing on-site generated fluorine for CVD chamber cleaning in thin film semiconductor, display and photovoltaics manufacturing, as well as responsibility for fluorine intellectual property and R&D.  Prior to his current role, Stockman has held positions with increasing responsibility in Linde for electronics materials and systems development.