Silicon Micro-Fabrication Technologies for Micro-Filters

Silicon micro-fabrication techniques allow for the development of microfluidic systems with very accurate control of size and uniformity of structures. In this paper we report on the silicon fabrication process of micro-filters for versatile application in fluidics systems. Micro-filters are composed of an ordered array of pillars and supply channels. Depending on pillar pitch, they can be used for, e.g., electrophoresis, chromatography and purification of biological mixtures. In this paper we focus on high performance liquid chromatography. The process that we have developed for micropillar fabrication consists of defining first 1μm diameter pillars with an inter-pillar distance of 1μm or less in an oxide hard mask with a DUV stepper, stitching is used to form few cm long patterns across the 200mm wafers. Second, the supply channels are defined with 1× alignment lithography. After definition of supply channels, deep reactive ion etching of silicon is performed with an optimised recipe to etch submicron pillars and supply channels of 100μm wide at the same time. The simultaneous etch of both structures avoids complex lithography steps otherwise necessary to protect the pillars while etching the supply channels or vice versa as would be done conventionally. Wafers are then anodically bonded to 200mm Pyrex wafers in order to seal the channels. Pyrex wafer also allows the use of optical detection system. Feed through holes for accessing the supply channels are etched on the backside of Si wafer. Filter characterization has been performed: a plate height of 1μm was measured and successful separation of 3 coumarin dyes is achieved.