Paper BI-ThP10
Fabrication and Chemical Surface Treatment of Solid State Nanopores in SiN Membranes

Thursday, October 23, 2008, 6:00 pm, Room Hall D

Solid state nanopores attracted broad attention in recent years as a tool to study biological molecules like DNA or proteins. In these experiments, the translocation of the molecule through the nanopore is detected by a blockade of the ionic current across the pore. Up to now solid state nanopores are mainly directly drilled into a freestanding silicon nitride membrane via an intense e-beam. Here we report a new pore fabrication technique. Single nanopores are processed in silicon nitride membranes by e-beam lithography and feedback-controlled wet chemical etching, followed by TEM induced shrinking. Moreover we present current noise data showing the influence of various chemical treatments of the pore surface. Starting with a (100) silicon chip of 200 µm thickness, which features 50 nm silicon nitride coatings on both sides, we use optical lithography to form an etch mask on the chip’s back side for anisotropic etching of the silicon with KOH. Subsequently we utilize e-beam lithography on the front side to open holes of 40 – 50 nm in the silicon nitride. In the next step the silicon is etched by KOH resulting in a pyramidal shaped undercut of the small holes on the chip front side. During a second KOH etching process from the backside only, we observe the time dependence of the electrical current across the silicon chip. The etching is stopped when a certain current threshold indicates the opening of the pyramid. In this way the pyramid is truncated in a controlled manner. This leads to a 5 x 5 µm freestanding silicon nitride membrane containing the pore. To get the desired pore size we shrink the pores using a TEM. Electrical noise analysis data is presented showing the influence of small membrane sizes resulting from feedback-controlled etching. In addition we studied the influence of the surface termination on wetting properties and electrical noise. In this context we applied both oxidizing (HF) as well as reducing agents (piranha, oxygen plasma) to change the surface properties of the nanopores. Our results show the benefits of the combination of feedback chemical etching and standard nanopatterning techniques on the electrical noise and indicate how current recordings can be obtained with low noise by a chemical treatment of the nanopore.