Photolithographic microfabrication

As the analytical, synthetic, and physical characterization techniques of the chemical sciences have advanced, the scale of material control moves to smaller sizes. Nanoscience is the examination of objects—particles, liquid droplets, crystals, fibers—with sizes that are larger than molecules but smaller than structures commonly prepared by photolithographic microfabrication. The definition of nanomaterials is neither sharp nor easy, nor need it be. Single molecules can be considered components of nanosystems (and are considered as such in fields such as molecular electronics and molecular motors). So can objects that have dimensions of >100 nm, even though such objects can be fabricated—albeit with substantial technical difficulty—by photolithography. We will define (somewhat arbitrarily) nanoscience as the study of the preparation, characterization, and use of substances having dimensions in the range of 1 to 100 nm. Many types of chemical systems, such as self-assembled monolayers (with only one dimension small) or carbon nanotubes (buckytubes) (with two dimensions small), are considered nanosystems. 

The earlier work on mica pores gave strong support to the hypothesis that the viscosity was a local property of liquids down to the molecular level. Supporting this result were some of Israelachvili s studies on shearing of layers several molecular distances thick. Because of the potential importance of microfluidic systems, several studies were initiated to examine fluid flow in micrometer scale ducts. Photolithographic microfabrication was used to make channels with D/, as small as 1 /zm. For these dimensions. Re is quite small and so is the volumetric flow. To put the point more finely, consider the case of a triangular microchannel with Z A = 10 /zm, L = 10 mm, and A/ = 10 kPa. Using equation (12.4) with o,he(,r = 13.30, the volume flow for water is of the order of 160 /u-1 day" ... 

Microfabrication involves multiple photolithographic and etch steps, a silicon fusion bond and an anodic bond (see especially [12] for a detailed description, but also [11]). A time-multiplexed inductively coupled plasma etch process was used for making the micro channels. The microstructured plate is covered with a Pyrex wafer by anodic bonding. 

Microfabrication technologies using photolithographic patterning processes (micromachining) were used first by Terry56,57 in 1975 for the integra-... 

The principal limitation of STM is that it cannot be used with insulating substrates. However, at the sort of distances where tunnelling currents occur, there is an attractive or repulsive force between atoms in the tip and the substrate, which is independent of the conducting or nonconducting nature of the substrate. In order to measure this, the tip is mounted on the end of a soft cantilever spring, the deflection of which is monitored optically by interferometry or beam deflection. These cantilever springs are microfabricated photolithographically from silicon, silica, or silicon nitride and have lateral dimensions of around 100 pm and thickness of 1 jum, to which tiny diamond tips are attached. 

Planar waveguides are also used for photometric transducers. Recently, there have been applications of photolithographic methods to create miniaturized waveguides, or other microfabrication techniques to produce thin films of organic materials for optical waveguides. For example, Hanken and Corn... 

Microfabrication and micromachining techniques have also been used in the manufacture of electrochemical sensors. This includes po and pco sensors. Zhou et al [9] describe an amperometric CO2 sensor using microfabricated microelectrodes. In this development, silicon-based microfabrication techniques are used, including photolithographic reduction, chemical etching, and thin-film metallization. In Zhou s study, the working electrodes are in the shape of a microdisk, 10 pm in diameter, and are connected in parallel. In recent years, silicon-based microfabrication techniques have been applied to the development of microelectrochemical sensors for blood gases, i.e. P02. Pcoj and pH measurements. 

Fig. 4.5. Microfabrication process, (a) Photolithographic photoresist patterning, (b) Photoresist development, (c) Protective layer etching, (d) Substrate etching, (e) Bonding to cover plate...

Microfabrication by SPMs is in part motivated by their potential to beat the diffraction limitations of optical methods for electronics applications. The spatial resolution of SPM is limited only by the tip size and STM has been used to manipulate individual atoms. However, the typical tip size in SFCM is ca. 1 pm and therefore a significant improvement over photolithographic resolution is not routinely obtained. Further, SPMs are rather slow since they are limited by the speed of the tip and this is especially so for SECM. Instead, SECM microfabrication is often more useful as a technique for patterning surfaces with chemical or biochemical functionality and fabricating microscopic structures with particular chemical properties. 

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