Micromachined Membrane Particle Filters

We report here several particle membrane filters (8 × 8 mm) with circular, hexagonal and rectangular through holes. By varying hole dimensions from 6 to 12 μm, opening factors from 4 to 45 % are achieved. In order to improve the filter robustness, a composite silicon nitride/Parylene membrane technology is developed. More importantly, fluid dynamic performance of the filters is also studied by both experiments and numerical simulations. It is found that the gaseous flow through the filters depends strongly on opening factors, and the measured pressure drops are much lower than that from numerical simulation using the Navier-Stokes equation. Interestingly, surface velocity slip can only account for a minor part of the discrepancy. This suggests that a very interesting topic for micro fluid mechanics research is identified. INTRODUCTION Filtration and collection of particles is an important process in airborne particle sampling. This work focuses on airborne particles in the range of 1 to 10 μm [1], and micromachined membranes with perforations are ideal candidates for such filters. Although several MEMS filters [2,3,4] have been reported in the past, a comprehensive study of their strength and fluid dynamic performance is not available. For MEMS membrane filters to be effective, various requirements must be met. They must be mechanically robust so as to stand a potentially high pressure drop. The filter opening factor must be high to allow for a large amount of air flow. Finally, the pressure drop, and hence the power loss, has to be low. As a result, factors including hole dimension, shape, membrane thickness and the opening area factor (β = area of holes/total area) decide the main performance of the filters. Large holes and a large opening factor will decrease the pressure drop and increase the flow rate, but decrease the strength of the membrane. Different hole shapes will not only change the flow rate and pressure drop, but also the stress concentration level in the membrane, and thus the strength of the filter as well. More interestingly, our work has also identified the importance of micro fluid dynamics factors that micro gas flow in small holes, although not yet fully understood, should also be considered. The goal of this project is to develop new structures and fabrication processes to enhance the strength of the filters and to establish a guideline to design an optimal membrane filter by studying the fluid dynamic performance experimentally and numerically. FABRICATION The filters are fabricated by using the process shown in Fig. 1. First, a layer of 1 μm thick LPCVD silicon (d) Si Etching Si Substrate SiN Parylene C (a) SiN Deposition