Developing an Empirical Model for Tunable Porosity in Porous Nanocrystalline Silicon Membranes

In this study, a new process for the fabrication of porous nanocrystalline silicon (pnc-Si) membranes is proposed, and the early stages in the development of an empirical, stressbased model for tunable porosity are revealed. Pnc-Si membranes that were fabricated by the proposed process showed substantial improvements in membrane morphology, i.e., porosity and pore uniformity across the wafer. These improvements were assumed to be related to stress, as suggested by the addition of materials that differ in Young’s Modulus and in coefficients of thermal expansion. In order to explore this assumption, stress measurements were conducted via stylus trace on a Tencor P2 Profilometer. Extensive average stress (dynes/cm2) measurements were performed by varying thermal and RF Magnetron sputtered Si02 thickness on bare, 400 μm thick, \u3c100\u3e orientation, double-sided polished silicon wafers. It was found that the stress-profile differences between the standard and proposed processes were significant as the change in the wafer bow (due to film stress), across the wafer (relative to the bare substrate), were plotted for varied oxide thickness during the three major stages of pnc-Si development. The resulting plots were 2nd order polynomials (best-fit RMS ≅ .970- .999), which agree with the form given by Stoney’s equation for macro stress acting in a coating deposited on a thick substrate. These differences have been consistent in multiple experiments and are currently being evaluated for their role in membrane porosity

Ultrathin Silicon Membranes for Wearable Dialysis

The development of wearable or implantable technologies that replace center-based hemodialysis (HD) hold promise to improve outcomes and quality of life for patients with ESRD. A prerequisite for these technologies is the development of highly efficient membranes that can achieve high toxin clearance in small-device formats. Here we examine the application of the porous nanocrystalline silicon (pnc-Si) to HD. pnc-Si is a molecularly thin nanoporous membrane material that is orders of magnitude more permeable than conventional HD membranes. Material developments have allowed us to dramatically increase the amount of active membrane available for dialysis on pnc-Si chips. By controlling pore sizes during manufacturing, pnc-Si membranes can be engineered to pass middle-molecular-weight protein toxins while retaining albumin, mimicking the healthy kidney. A microfluidic dialysis device developed with pnc-Si achieves urea clearance rates that confirm that the membrane offers no resistance to urea passage. Finally, surface modifications with thin hydrophilic coatings are shown to block cell and protein adhesion

The Journal of Microelectronic Research 2008

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