68 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 21, NO. 1, FEBRUARY 2012 Micromachined Microsieves With High Aspect Ratio Features Kathleen M. Vaeth Abstract—Micromachined microsieves fabricated entirely by dry etch processes with pore diameters ranging from 3 to 5 μm are reported. Two microsieve architectures are explored: 2.1 and 5.3 μm thin dielectric layer membranes supported by high aspect ratio rib features (10 : 1), and 50 μm thick silicon membranes from silicon-on-insulator wafers with no supporting ribs and high as- pect ratio pore features (16 : 1). The flow throughput of each design is evaluated experimentally and theoretically, and the expected relative robustness is assessed and compared to typical microsieve structures reported in the literature. The experimental and the- oretical work suggests that both structures have the potential for higher robustness than the typical micromachined microsieve architectures, with reduced but still reasonable flow throughputs in the range of 10 5 to 10 6 L/m 2 -hr-bar, depending on the mi- crosieve porosity. Both microsieve architectures are shown to block monodisperse polymer beads 3 μm in diameter. [2011-0127] Index Terms—Microfiltration, microfluidics, micromachining. I. I NTRODUCTION F ILTERS with pore sizes on the order of 0.1 to 20 μm are of interest for management of air and waterborne particles in biological and industrial applications. In many instances, it is desirable for the filter to have a sharp cutoff for the excluded particle size. Microsieves, or filters that consist of uniform, well-defined holes drilled through a membrane material, exhibit such properties. Compared to fibrous and mesh filter materials, which remove particles based on settling in a tortuous path within the membrane, microsieves have the potential to be periodically regenerated more easily during operation, since the majority of the particles sit on the surface of the filter. Mi- crosieve structures that have been demonstrated include poly- mer track etched membranes, micromolded [1] and templated [2] polymer membranes, and micromachined inorganic mem- branes fabricated with thin-film deposition and etch techniques commonly used for the construction of microelectromechani- cal systems (MEMS) [3]–[8]. Micromachined microsieves are particularly attractive due to the excellent control over feature size, design, and placement, and as well as the ease of handling, the potential for a high degree of cleanliness, and the ability to tailor the surface properties of the membrane with additional coatings. Manuscript received April 28, 2011; revised August 29, 2011; accepted September 18, 2011. Date of publication November 7, 2011; date of current version February 3, 2012. Subject Editor C. Liu. The author is with Corporate Research and Engineering, Eastman Kodak Company, Rochester, NY 14650 USA (e-mail: Kathleen.vaeth@kodak.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2011.2171324 One of the challenges with microsieves is that the flow throughput for a given applied pressure tends to be low com- pared to fibrous and mesh filters rated for a similar particle size. This is because microsieves directly exclude the particles of interest, with pores sizes the order of the rated particle di- mension. To mitigate this issue, the micromachined membrane is typically made to be very thin—generally from dielectric layers 1 μm in thickness [3], [4]. To prevent breakage, these thin membranes are supported periodically with ribs formed in the substrate. Incorporation of the ribs does reduce the area available for active pores, and therefore, an optimization must be made between the mechanical robustness and flow through- put of the microsieve by adjusting the thickness and open area of the suspended membrane [8]. It is worth noting that the maximum packing density of the suspended membranes is a function of the substrate thickness and slope of the supporting rib wall, the latter of which is determined by the etch method used. Ribs spaced 1 mm apart formed by an anisotropic wet etch of silicon with KOH, which produces a 54.7 ◦ sloped wall, have been most commonly reported [3], [4], but thin, more closely spaced ribs (150 μm) have also been demonstrated using a two- step process consisting of a shallow dry silicon etch from the membrane side of the wafer, followed by a wet KOH etch from the backside of the wafer to define larger, more largely spaced support ribs [8]. In this paper, we describe two alternative designs for MEMS microsieves that leverage recent advances in Bosch deep reac- tive ion etch processes that enable fabrication of high aspect ra- tio features (10 : 1 or greater) with sidewall angles greater than 75 ◦ [9]. In the first design, thin dielectric layer membranes are combined with supportive ribs fabricated entirely with a Bosch etch, which streamlines the fabrication and allows for closer rib spacings (40 μm) and smaller suspended membrane areas than reported previously. In the second design, microsieve pores are defined by a Bosch etch in the device layer of a silicon- on-insulator (SOI) wafer (50 μm). The choice silicon for the membrane provides access to thicker layers than typically avail- able with thin dielectric materials such as silicon nitride (SiN) (typically limited to < 5 μm due to cracking from residual stress), eliminating the need for closely spaced supportive ribs. Use of an SOI wafer also eliminates the need for a timed etch to create the suspended silicon membrane reported previously [7] and excellent control over the membrane thickness. Both designs presented here have the potential for greater robustness to backpressure due to the smaller suspended membrane area, in the case of the thin dielectric membrane, or thicker membrane layer, in the case of the thick silicon membrane design, with some penalty in flow throughput due to reduced porosity or 1057-7157/$26.00 © 2011 IEEE