Microfiltration membrane sieve with silicon micromachining for industrial and biomedical applications

With the use of silicon micromachining an inorganic membrane sieve for microfiltration is constructed, having a siliconnitride membrane layer with thickness typically 1 pm and perforations typically between 0.5 pm and 10 pm in diameter. As a support a -silicon wafer with openings of loo0 pm in diameter is used. The thin siliconnitride layer is deposited on an initial dense support by means of a suitable Chemical Vapour Deposition method (LPCVD). Perforations in the membrane layer are obtained through the use of standard microlithography and reactive ion etching (RIE). The flow rate behaviour and the pressure strength of the membrane sieve are calculated in a first approximation. A process for manufacturing is presented and some industrial and biomedical applications are discussed. Introduction Sieve Filters Sieve filters are characterized by thin membrane layers with uniformly sized pores and for most applications the membrane layer is sustained by a support. Until now lithographic techniques have not been used for the construction of micro filtration membrane layers made of inorganic materials as siliconnitride and silicon1. Inorganic membrane and in particular ceramic membranes* have a number of advantages above polymeric membranes like high temperature stability, relative inert to chemicals, applicable at high pressures, easy to sterilize and recyclable. However they have not been used extensively because of their high costs and relatively poor control in pore size distribution. Also the effective membrane layer is very thick in comparison to the mean pore size (typically 50 -loo0 times), which resiluts in a reduced flow rate. A composite filtrationmembrane having a relatively thin filtration or sieving layer with a high pore density and a narrow pore size distribution on a macroporous support will show good separation behaviour and a high flow rate. The support contributes to the mechanical strength of the total composite membrane. The openings in the support should be made as large and numerous as possible in Handbook of Industrial Membrane Technology, LPorter, C.Mark and H.Strathmann, 1990 Ceramic Membranes Growth Prospects and Opportunities, K.K. Chan and A.M. Brownstein, Ceramic Bulletin, vol70, 703-707, 1991 order to maintain the flow rate of the membrane layer and to reduce the interaction of the support with the fluid. An established use of inorganic membranes with very thin membrane layers, in particular microsieves with high flow rates, will result in an energyand cost-saving separation technology for present and future innovative applications, like micro liquid handling, modular fluidic systems or micro total analysis systems3. 'Track etched' membrane t 1 0 'Tortuous path' membrane .. .:. . Pore size Density Log scale 1.0 2.0 5.0 10 2 0 . Pore size in micrometer Figure I , Pore size distribution of various membrane f i l t ers . New Membrane Materials and Processes: A Survey of Work in The Netherlands, C.A. Smolders in 'Membranes' Oxford & IBH Publishing, 1992, ISBN 81-204-0686-9 83 0-7803-2503-6