Reclamation of Water and Hazardous Materials in Silicon Processing
Krygier, Vivien; Berndt, Rolf
(Pall Coporation, East Hills, NY)
RECLAMATION OF WATER AND HAZARDOUS MATERIALS IN SILICON PROCESSING Vivien Krygier Ph.D. and Rolf Berndt Ph.D., Pall Corporation, East Hills, NY The semiconductor and PV industries not only consume an ever-increasing amount of water, solvents and other resources but, as a consequence, they also end up discharging large volumes of wastewater and potentially hazardous waste. In many places, expansion of production is restricted due to limited supply of water and by legal and local restraints. Among the processes requiring large quantities of water and producing significant amounts of wastewater or other contaminated fluids as well as silicon/silica fines are: • Pre-shaping (cutting, squaring, chamfering, surface grinding) of silicon ingots • Wafering (slicing of ingots) • Wafer cleaning • Chemical/Mechanical Polishing (CMP) • Silicon Backgrinding and Dicing Silicon/silica fines also appear in the exhaust gas of Czochralski pullers for mc-Si crystal growing. Despite some special features, these processes have a great deal in common. The particles (silicon, silica, silicon carbide) are typically generated or degraded by processes on the surface of workpieces. The majority of these particles are typically less than one micron in size, complicating separation by conventional methods (settling, centrifugation, cake filtration). Chemically enhanced coagulation or flocculation would result in additional contamination of water and adds cost to the reclamation process. A better way is to completely remove the solid particles by means of membranes or other filter media with pore sizes below the particle size. Special dynamic process modes – e.g. crossflow, backflushing/ blowback and air scrubbing – help to maintain the filtrate flow on a reasonable, stable level. The selection of filter media, filter elements, process mode and process parameters and the design of the entire separation system closely depend on the properties and behavior of the dispersion to be purified. Moreover, the architecture of the reclaim process is – aside from the feed specifications – strongly influenced by the technical and economic targets. The particle concentrate generated by the dynamic membrane filtration, for instance, may be further dewatered by cake filtration or centrifugation, sometimes followed by thermal drying, resulting in a powder ready for disposal or re-use. The clear filtrate of a membrane filtration may be used for raw water replacement in DI water plants, for direct reuse with/without polishing or for easier application, e.g. scrubbers. The paper analyses the influence of waste specifications (solids content, particle size, viscosity, temperature etc.) on the selection of technologies, gives a survey on applications, and illustrates details of process design and sizing on the basis of case studies in PV and semiconductor manufacturing.