Other Microfabrication Techniques

Background. The term microsensor denotes a transducer that, in some fashion, exploits advanced miniaturization technology, whether an adaptation of integrated circuit technology, or some other microfabrication technique. Within the past decade, a myriad of microsensors have been developed, with capabilities for measurement of temperature, pressure, flow, position, force, acceleration, chemical reactions, and the concentrations of chemical species. The latter measurements, of chemical species, are intrinsically more difficult than the measurement of mechanical variables because in addition to requirements of accuracy, stability, and sensitivity, there is a requirement for specificity. 

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... 

Laser ablation has become a versatile machining technique, particularly in machining of microstructures. Although there are other microfabrication techniques, such as lithography, wet chemical and reactive ion etching, laser ablation provides a direct and fast structuring method. For some materials, laser ablation seems to be the only... 

The packaging (i.e., electrical insulation for operation in electrolytes) is more difficult with SAWs due to their rectangular geometry. SAWs are easier to fabricate with lithographic microfabrication techniques and therefore are more suitable for use in an array (Ricco et al., 1998). The choice of electrode materials is critical for QCM, where acoustic impedance mismatch can result in substantial lowering of the Q factor of the device. On the other hand, it does not play any role in the SAW devices. The energy losses to the condensed medium are higher in SAWs and this fact makes them even less suitable for operation in liquids. Nevertheless, SAW biosensors have been reported (Marx, 2003). 

To build an efficient, high-quality microscale fuel cell, microfabrication techniques need to be combined with appropriate materials such as Nation based membrane electrode assemblies (MEAs). These techniques must be able to produce three-dimensional structures, allow reactant and product flow into and out of the device, process appropriate materials, and should be of low cost. Fortimately, traditional thin film techniques can be modified for microscale fuel cell fabrication, while maintaining their advantages of surface preparation, sensor integration, and finishing or packaging. In addition, other techniques are also available and are discussed in the following sections. 

A first interest of MS when used in combination with microfabricated structures, or at the outlet of microfluidic devices, is the match in the volume of liquid handled. A typical MS analysis requires less than 1 pL of liquid, for ESI-MS as well as for MALDI-MS techniques. When working with a continuous flow of liquid and ESI-MS, the MS performance is even more enhanced for flow rates down to 50-100 nL min-1 the lower the flow rate, the better the MS analysis. This flow-rate range corresponds to flow-rate values observed in microfluidic devices. Consequently, the technique of MS is easily scalable and exhibits an enhanced response when the sample size is decreased. This is not the case for instance for other detection techniques, such as UV absorbance or amperometry these two techniques require large detection area or volume, which is the opposite of the quest of microfluidics. This first advantage of MS compared to other technique goes together with its high sensitivity. 

Other sectors have been reached since then by the combined use of microfabrication and MS techniques, such as forensics and homeland safety. For instance, microfabrication techniques are now also used in view of the miniaturization of the mass spectrometer itself and its implementation on a microchip to kill the current paradox of combining tiny devices for sample preparation to bulky and almost room-sized instrumentation. From such ongoing development, one can expect soon the appearance of fully integrated and portable devices for on-site analysis, with both the implementation of the microfluidic-based sample preparation step and the MS analysis on a single device of a few inches in size. 

For a review of these early microfabrication techniques, see Stokes and Palmer and others... 

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