Filtration and Purification Solutions for the Total Photovoltaic Value Chain
Product: PV Blowback Filtration Units
Filtration for Process Consistency
A manufacturing process consists of many steps, each fraught with potentially costly problems. A dirty fluid stream can decrease productivity and lead to high rejection rates. Pall filtration devices are designed to be deployed in strategic locations in the process stream in order to maximize productivity and profitability. A filtration device, for example, can protect a critical orifice (i.e. a cleaning nozzle) so that the openings do not become clogged and cause downtime. If the fluid in question is re-circulating, bath lifetime can be maximized by placing a filtration device in the line. Removing haze or dust are other reasons for using filtration. This is exemplified by the increase in tool up-times that can be achieved with proper filtration to protect vacuum pumps.
Finally, since gases are fluids, the removal of aerosols or mists can be achieved with high efficiency elements known as coalescers. Vapors can be removed with activated carbon filters and molecular contamination can be removed by purification technology. Cleaner gases lead to reduced defects within thin film layers.
Pall’s Value Chain for Photovoltaics
Pall has designed specific filtration technologies for almost all process steps from polysilicon manufacturing, ingot growing and wafering to cell processing. Pall also addresses waste treatment for slicing slurry, water/silicon reclamation in pre-shaping operations and puller exhaust gas dust abatement. There are many steps in thin film processing, however Pall provides filtration, purification and separation solutions for substrate cleaning, chemical bath deposition, thin film deposition and reclaim applications. The following pages show these diverse products. Pall, through Total Fluid ManagementSM (TFM) filters literally all process fluids. Ask us about our solutions for ultrapure water, process cooling water, bulk gases and bulk chemical filtration.
Purification and refining of metallurgical silicon
The process of producing polysilicon (PS) begins with refining quartz or sand into metallurgical grade silicon, which is then purified in a series of chemical reactions. Next, during the Müller Rochow process, purified metallurgical grade silicon reacts with hydrogen chloride at elevated temperatures in the presence of a catalyst, to produce trichlorosilane (TCS).
The TCS gas is cooled and liquified. Impurities with higher or lower boiling points than the TCS liquid are then removed using distillation. The refined TCS is now ready for use in reactors that make PS. The TCS liquid is vaporized, mixed with hydrogen gas and reduced in either a Chemical Vapor Deposition (CVD) furnace resulting in PS rods; or, Fluid Bed Reactor (FBR) to produce PS granules. Exhaust gases which are released during this process are cooled, liquified and distilled, yielding the co-products silicon tetrachloride (STC) and TCS, which is recycled.
Hydrogen recycle is an important unit operation in PS production. Exhaust gases consisting of hydrogen and chlorosilane from the reduction reaction are recovered. The hydrogen goes through a clean up process and is then recycled back to the reduction plant in a continuous recovery loop.
Applications for Pall Filtration Products
• Catalyst fines must be separated from the product stream during the Müller Rochow process. This can be accomplished with Blowback elements made of either ceramic or a special alloy of PSS® filter media.
• Newly produced or recycled liquid as well as gas phase TCS, are filtered before the reduction reaction. The level of filtration will depend on each site’s requirements. Rigimesh® metal filters are utilized for coarse applications, with all-fluoropolymer Emflon® PF cartridge filters for submicron requirements.
• Feed gases to the reactor are filtered to submicron levels to protect critical control valves and maintain purity within the reactor. Reactor vent applications capture valuable solid product using cleanable PSS metal filters, with Blowback technology as an option (refer to page 5). Recovering product from vent streams increases process yields, reduces loading to pollution control systems and protects downstream equipment from PS build-up. High capacity filtration systems allow for complete production campaigns.
• Where a recycle hydrogen compressor is used, filtration with Pall metal filters and polishing membrane cartridges protects the compressor from PS deposits and reduces expensive compressor maintenance and associated process downtime.
• Caustic baths followed by water rinsing are used to clean metal filter elements by dissolving off the PS particulate. These baths are filtered with Profile® II and Profile UP filters to recover the PS and keep it from contaminating waste streams.
• Depending on the final product, there may be an etching step on the final ingots, rods or granules using either hydrofluoric and/or nitric acid. These etch baths as well as downstream water rinses are filtered to submicron levels using Emflon PF cartridges.
• Pall provides filtration products and systems for varying grades of water, including ultrapure and wash water, as well as wastewater where there may be a requirement to remove or recover PS fines.
Protection of Vacuum
Pumps with PV Blowback Filtration Units The exhaust gases out of Czochralski pullers used for monocrystalline ingot growing contain dust particles that form by condensation above the silicon melt. This dust needs to be filtered to protect people, the environment and the vacuum pumps. Fine silicon dust is highly reactive and immediately oxidizes to SiO and SiO2, an exothermic reaction. Depending on the dopant this dust can be very hazardous. Pall has extensive experience in this application. Pall PSS stainless steel filter elements and housings in combination with our Blowback technology are the best long-term proven solution in the industry.
Traditional filter technology:
• Dust removal with disposable filters requires opening of the filter housing during filter change-out which exposes operators to dust and contaminates the pumps environment
• Oxidation of Si/SiO to SiO2 is exothermic and traditional filter media typically cannot repeatedly withstand the high temperatures, leading to loss of filter integrity. This results in passage of the abrasive silicon potentially damaging the pump.
• Frequent filter change-outs result in long process downtimes and high cost of ownership.
PSS Filter Media with Blowback Technology:
• Pall has the most efficient, proven long-term solution on the market
• The filter housing does not need to be opened for dust removal
• Cleaning is quick and automated after each cycle using the Blowback technology
• Oxidation of the Si/SiO to SiO2 occurs in the filter vessel under well controlled conditions
• Extremely robust PSS filter elements are resistant to high temperatures and provide optimum retention of the silicon dust
• These filtration units are available semi or fully automated
• Blowback units are typically in service for several years without a filter change-out
• Cost of ownership model shows return of investment (RoI) in one to two years
• No leakage, long pump life, low cost of ownership are the hallmarks of the Blowback Units
Recover Your Waste
Shaping of silicon ingots prior to wafering comprises several cutting and grinding operations. Large quantities of water must be used to cool tools and silicon surfaces, to act as a lubricant and to remove the resultant silicon fines. The spent process water leaves the tools highly contaminated by silicon particles. Pall Corporation introduces a new generation of fully automated separation systems for the reclamation of water and silicon sludge from the spent grinding/sawing water. The systems enable PV wafer manufacturers to:
• Re-use up to 95 % of spent process water as high quality water
• Reduce wastewater discharge volumes • Improve sawing/grinding performance by controlling process water specifications
• Reduce machine fouling and uncontrolled sedimentation in pipes, tanks and sinks
• Reduce chemical hazards due to silicon-water interaction by controlled pre-treatment of spent process water
• Collect de-watered silicon debris having very little chemical contamination, as feedstock for silicon re-use. The heart of such a reclaim system (see diagram below) is a dynamic membrane filtration (MF) unit combined with physico-chemical treatment of the concentrated silicon.
The systems clarify the contaminated process water. Typically 90 to 95 % of the contaminated process water is perfectly clarified ready for re-use. The remaining concentrate may be discharged, mixed with other wastewater streams, or subjected to additional treatment to meet discharge regulations or to de-water the silicon debris. Other options are available on request. Pall has over 20 systems, treating 3 to 33 m³/h silicon containing wastewater, operating successfully throughout the world. The photograph below shows a dynamic filtration unit designed for 30 m³/h (7925 usg/h) silicon grinding/sawing wastewater. It provides 2 x 75 % architecture for safe process water supply.