Microfiltering Semiconductor Wastewater
Traditional membrane filtration techniques fail when applied to CMP (chemical-mechanical planarization) waste because of the small (sub-100-nm) size of particles in typical CMP wastewater effluent.
Even the newer cross-flow filtration techniques have problems with typical CMP flow rates and an intrinsic inability to remove dissolved contaminants.
Until now, the lack of effective treatment has not been a big concern because the effluent combined with the millions of gallons of water from other fab processes. Thus diluted below local discharge limits, CMP effluent went to the municipal water-treatment plant.
In light of the increasing contaminant concentrations and regulatory climate, semiconductor fabs are discovering that dilution is no longer a viable solution.
Using specially engineered polymers and membrane technology, a process called EnChem has been used to successfully remove solids and heavy metals from wastewater streams in such industries as mining, electroplating, textiles, and battery manufacturing [X].
The basis for the technology is specific contaminant removal through a two-fold process. First is the controlled chemical reaction using polymers to form non-tacky particles of an engineered size. Second is the removal of those particles from the waste stream by membrane filtration.
The technology
- permits a high degree of flexibility in the speciation of the polymer with respect to the contaminants involved;
- imposes few restrictions on the type of membranes employed; and
- tolerates broad changes in process conditions, allowing for a wide treatment window-- pH of 4 to 12; contaminant concentrations, > 1000 ppm; fluctuations in organics, > 25 ppm; and temperatures, > 75ºC.
Moreover, the stoichiometric relationship that drives the chemical reaction is predictable and repeatable for membrane separation. Last, the treated particles form a cake on the membrane filters, which maintains efficient filtering while allowing for low differential pressures and high flow rates
Application to the Semiconductor Industry
The parallels between mining experiences and challenges with semiconductor manufacturing suggested that specific features of the EnChem process--such as its producing non-tacky particles, high-flow, specific contaminant removals, and small footprint--would be ideal for fab wastewater facilities [Y].
A pilot system comprising a mixed-waste stream of oxide, metal, and back-grind became the first crossover-technology application for EnChem treatment. The results were compelling enough to warrant recycling tests and, ultimately, the designing and constructing of a large system for process-water treatment and water reuse. The system, which operated for several years in a semiconductor fab, produced water of sufficiently high quality to release directly to drain or reuse as feed water for the fab's ultra-pure-water system.
The pilot system proved the viability of the EnChem process for removal of silica and fluoride. The system used contaminant-specific instrumentation to drive the polymer-dosing rate and to ensure the elimination of silica and fluoride to acceptable levels. Closed-loop monitoring of contamination levels eliminated non-compliant discharges to drain by diverting out-of-spec filtrate for further processing. The system was high flow (> 150 GPM) and very consistent in its removal efficiency. Complete system automation using programmable logic controllers (PLC) ensured consistent and reliable treatment and minimal labor costs.
An EnChem organic polymer of low molecular weight effected metallic contaminant elimination. The process steps were as simple and easy as those for silica and fluoride treatment. In addition, the reaction sequences were flexible with respect to various contaminants. For example, many of the waste streams contained measurable concentrations of tungsten, alumina, and iron. These contaminants were removed at rates quite compatible with reuse of the treated water.
Conclusion
Leading semiconductor manufacturers recognize that environmental performance is now a competitive issue for the IC industry. Chipmakers are redefining Cost of Ownership (CoO) to include environmental issues. Fab managers are beginning to think of equipment as having an "environmental footprint", which is a measure of the degree of environmental impact of process tools. The environmental footprint of process tools for CMP, wafer cleaning and copper plating can be significantly reduced by finding solutions for wastewater treatment before the tools are delivered to the fab.
The EnChem process has successfully treated wastewater in the semiconductor and related industries, can reduce cost of ownership (CoO) of process tools. It does so by eliminating the need for expensive and unreliable treatment technologies and by providing water that is of sufficient quality for reuse or recycle.
The EnChem process can be scaled from small 10-GPM systems to systems of up to several thousand gallons per minute that handle an entire fab's wastewater treatment.
References
The National Technology Roadmap for Semiconductors, Semiconductor Industry Association, Third Edition, p. 155, 1997.
[X] Patent Pending: "Wastewater Treatment Process and Apparatus for High Flow Impurity Removal.
[Y] Patent Pending: "Wastewater Treatment Process and Apparatus for High Flow Silica Removal.