Solid State Nanopores for Selective Sensing of DNA - Nanopores: Sensing and Fundamental Biological Interactions - page 109

Similar efforts have been made by Iqbal and co-workers. Their process involved
a series of recursive and continuous sub-processes as follows:

l Dry oxidation of Silicon (Si) wafers
l Conventional photolithography
l Wet etching of SiO 2 using buffered hydrofluoric acid (BHF)
l Tetramethylammonium hydroxide (TMAH) anisotropic wet etching of Silicon
l Reactive ion etching (RIE)
l Focused ion beam (FIB) Drilling
l Scanning electron microscope (SEM) or Transmission electron microscope
(TEM) pore shrinking and characterization.

The fabrication process started with the oxidation of Si wafers. The whole
process is explained with the help of the process flow diagram in Fig. 5.1 . The
oxidation process depends on the furnace temperature and the diffusion rate
coefficient. After oxidation, the oxidized wafers were coated with a thin layer
of positive photoresist (PR) by using a spinner. After soft baking, the wafer was
ready for photolithography, where a contact mask aligner was used for exposure.
After development of the PR, small squares hundreds of micrometers on a side
were opened on one side of the wafer.
Wet etching was then performed using buffered hydrofluoric acid at room
temperature to etch the silicon dioxide (SiO 2 ) while the other side was protected
against buffered hydrofluoric acid (BHF) etching by application of PR. At this
point, the wafer was ready to go into a tetramethylammoniumhydroxide (TMAH)
etch solution at 90 C. TMAH is an anisotropic etchant; it etched the Si while the

Fig. 5.1 The process flow for fabrication of a membrane which is used for FIB SSN drilling. The
process starts with a bare silicon wafer which is oxidized at a temperature

900 C. PR is then
applied at one side of the wafer. Photolithography, BHF etching, Acetone Wash, and TMAH
etching (90 C and 20% concentration) completes the process flow

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