Monolithic nanofluid sieving structures for DNA manipulation

A new technique for fabricating two-dimensional artificial gels for DNA electrophoresis is presented. The technique differs from previous approaches in that the entire device is fabricated as a monolithic unit using exclusively planar processing techniques adapted from semiconductor electronics fabrication. The height of the fluid gap between the dielectric floor and ceiling is determined by the thickness of a sacrificial layer which is removed by a wet chemical etch. This allows precise control and excellent uniformity of the gap over an entire silicon wafer. Floor-to-ceiling height control better than 5 nm has been demonstrated over a 1.5 cm device. Electron beam lithography is used to define a square array of 100 nm obstructions in the sacrificial layer. Chemical vapor deposition silicon nitride is applied over the sacrificial layer. Reactive ion etching (RIE) is used to create access holes in the nitride layer, so that the sacrificial layer can be removed with a wet chemical etch. After the wet etch, the access holes are resealed with very low temperature oxide (VLTO) silicon dioxide. Finally, loading widows 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 electric field can be observed. The DNA molecules are labeled with a fluorescent dye and observed through the dielectric top layers with an optical microscope. The electrophoretic mobility is measured for two different DNA chain lengths, 43 and 7.2 kbase. The velocity for both DNA lengths is reported for an applied potential between 2 and 20 V over the 15 mm device. At some voltages the velocities differed by nearly a factor of 2.