Nano And Microengineered Membrane Technology

Micro- and nanoengineering refers to the technology and practice of making three-dimensional structures and devices with dimensions on the order of micrometers, respectively nanometers. When looking at such small devices, a number of physical effects have different significance on the micrometer scale compared to macroscopic scales. This includes such topics as micromechanics, which deals with the moving parts of micro/nanoengineered devices, and micro/nanofluidics, which deals with the fluidic properties of such small devices.

The two constructional technologies of microengineering are microelectronics and micromachining. Microelectronics, producing electronic circuitry on silicon chips, is a very well developed technology. Micromachining is the name for the techniques used to shape the structures and moving parts of microengineered devices. In recent years microma-chining and microengineering have become synonyms, also because of the advent of nanoengineering techniques (i.e., techniques capable of shaping structures below the 100 nm scale). See Figure 16.

The formation of micro- and nanopores in a thin but strong membrane structure and its potential applications is the key element in engineered membrane technology. Up to now relatively large pores (ยป1 micrometer) were made in thin foils or membranes with conventional microperforation methods (e.g., laser drilling, electroforming). With microengineering techniques originating from the semiconductor industry, it is relatively easy to downscale and form submicrometer pores (down to 500 nm) using conventional photolithographic methods, with, for example, contact masks and wafer steppers.

Many applications, especially filtration applications, prefer pores in a membrane with a low flow resistance (i.e., the length of the pore should be as small as possible). Therefore

Figure 16. Micromachining: Deep trench etching in silicon. Reprinted with permission from [47], H. V. Jansen, Plasma Etching in Micro-technology, Thesis, 1996.

van Rijn and Elwenspoek [48] introduced the microsieve, a very thin membrane with a specially designed macroperfo-rated support structure to strengthen the thin membrane. See Figures 17 and 18.

Using supported microsieve structures and submicrome-ter lithography techniques from the semiconductor industry it is relatively straightforward to produce microsieves with submicrometer features. The microengineered pores, which are well defined by photolithographic methods and anisotropic etching, allow, for example, accurate separation of particles by size. The membrane thickness is usually smaller than the pore size in order to keep the flow resistance small (one to three orders of magnitude smaller than other types of filtration membranes). See Figure 19.

Various lithographic process flows will now be discussed for economical production of microsieves with a high effective use of the membrane area. Also new nanopatterning techniques like laser interference lithography, shadow mask techniques (nanostencilling), and micro/nanocontact printing will be presented.

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