Monolithic nanofluid sieving structures for DNA manipulation

A new technique for fabricating two-dimensional artificial gels for DNA ele ctrophoresis is presented. The technique differs from previous approaches i n that the entire device is fabricated as a monolithic unit using exclusive ly planar processing techniques adapted from semiconductor electronics fabr ication. The height of the fluid gap between the dielectric floor and ceili ng is determined by the thickness of a sacrificial layer which is removed b y a wet chemical etch. This allows precise control and excellent uniformity of the gap over an entire silicon wafer. Floor-to-ceiling height control b etter than 5 nm has been demonstrated over a 1.5 cm device. Electron beam l ithography is used to define a square array of 100 nm obstructions in the s acrificial layer. Chemical vapor deposition silicon nitride is applied over the sacrificial layer. Reactive ion etching (RTE) is used to create access holes in the nitride layer, so that the sacrificial layer can be removed w ith a wet chemical etch. After the wet etch, the access holes are resealed with very low temperature oxide (VLTO) silicon dioxide. Finally, loading wi dows are opened with RIE at both ends of the device so that DNA in aqueous solution can be introduced and its motion under the influence of an electri c field can be observed. The DNA molecules are labeled with a fluorescent d ye and observed through the dielectric top layers with an optical microscop e. The electrophoretic mobility is measured for two different DNA chain len gths, 43 and 7.2 kbase. The velocity for both DNA lengths is reported for a n applied potential between 2 and 20 V over the 15 mm device. At some volta ges the velocities differed by nearly a factor of 2. (C) 1998 American Vacu um Society. [S0734-211X(98)15106-5].