by Robert P. Donovan
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Semiconductor facilities and manufacturing sites of other precision industries use large numbers of membrane filters, which consist of a continuous, solid sheet that is rendered porous by one of several methods.
Perhaps the easiest to visualize is the polycarbonate membrane filter. The polycarbonate membrane filter is formed by irradiating a solid sheet with high-energy, nuclear decay particles and then etching the irradiated sheet with a chemical solution that rapidly dissolves the damaged material left in the wake of the radioactive particle.
This procedure produces numerous randomly oriented and located cylindrical holes of uniform diameter that begin on the membrane surface and extend completely through the membrane. These narrow passageways serve as pores for fluid passage while the unetched membrane surface and volume become sites for particle capture. The flat surface of the polycarbonate membrane lends itself admirably to microscopic examination so that particles captured on the surface of the membrane can be easily analyzed.
Etch tracks are not the only technique for producing membrane filters—nor the most common. Solvent cast membrane filters are those formed from the evaporation of a volatile constituent in a liquid slurry. This process yields a network of connected arms and bridges, which forms a filter that microscopically resembles a fibrous filter but differs in that all the regions of the remaining, solidified solution are interconnected.
A common method for forming a membrane filter is to stretch a thin Teflon sheet to the point where it begins to tear and separate into threads but not beyond the point at which any part of the sheet becomes separated.
While the physical structure of a membrane filter differs from that of a fibrous filter, many of the dominant particle-capturing mechanisms are the same. Theoretical analyses of the performance of a fibrous filter have been successfully used to characterize the performance of membrane filters as well.1
However, the approach of using theoretical models of fibrous filters to describe and predict the performance of membrane filters may not be suitable for all membrane filters.2
Compared to fibrous filters, membrane filters offer higher particle-removal efficiencies and freedom from the particle shedding phenomena, as discussed in last month's column.
However, these advantages come at a cost. Membrane filters usher in significantly higher pressure drops than are typical for fibrous filters, making membrane filters unsuitable for filtering the air supplied to cleanrooms. Membrane filters are used to filter high-pressure gases and to collect particles from gases for counting and analyzing.
Overall, however, they are probably more widely used in liquid systems than in air or gaseous systems.
In liquids, the most common membrane filter configuration—certainly the configuration using the most area of filtration membrane—is that of a cross-flow filter.
A cross-flow filter separates the incoming fluid into two exhaust streams: 1.) the concentrate, which consists of the incoming stream passed down a long channel defined by a membrane filter boundary that is parallel to the direction of fluid flow; and 2.) the permeate, which is the purified fluid stream that, under pressure, has crossed through the membrane defining the channel.
A conventional filter has just one incoming stream and one exit stream, both perpendicular to the direction of fluid flow. Membrane filters are also used in this conventional configuration in liquids but the large filters used in semiconductor ultrapure water systems, for example, are primarily cross-flow filters that act more as fluid-particle separators than as static capture sites for particles.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM.
References
- Accomazzo, M. A., K. L. Rubow and B. Y. L. Liu, “Ultra-High Efficiency Membrane Filters for Semiconductor Process Gases,” Solid State Technology 27(3), March 1984, pp. 141-146.
- Emi, H., Y. Otani and J. Mori, “Collection Mechanisms of Membrane Gas Filters,” Kuki Sejo to Kontamineshyon Kontororu Kenkyu Yokoshu, pp. 241-244, 1990 (in Japanese; English abstract, JICST Accession Number 91A0392685).