MEMS microfabrication techniques

The term micromachining refers to the mechanical aspect of fabrication processes. MEMS microfabrication techniques, while based on conventional IC fabrication technology, also include more specialized and refined processes which permit the formation of mechanical structures. The key for both MEMS and IC fabrication is photolithography, which permits high volume, batch production of devices with microscale dimensions. In photolithography, a thin, photosensitive polymer film ( photoresist ) is selectively exposed to UV light using a photomask (Fig. 1). 

Microfabrication techniques used for the production of MEMS (micro electro-mechanical systems) have been successfully used to produce highly efficient micro fluidic systems. 

Standard commercial RTDs (i.e., IPRTs) are typically at least several millimeters in size, which is much larger than would be desired for many applications in MEMS and microfluidics. For these applications, there is a clear need for integrated temperature sensors that are smaller than commercial sensors. Microfabricated metallic RTDs are the most common solution to this challenge, although microfabricated thermocouples and thermistors (e.g., doped silicon) have also been used. Micron-sized metallic RTDs are easily patterned using standard microfabrication techniques such as wet or dry etching, liftoff, or... 

Given the birth of MEMS from the IC industry, the dominant material used in the early devices was silicon. The use of silicon as a substrate and structural material, and the use of polysilicon as a thin film structural material, has continued to the present day for several reasons. The microfabrication techniques for silicon are highly developed and flexible, the microfabrication equipment has been designed for silicon, the properties of silicon are very well known and can be tightly controlled, silicon has excellent mechanical properties, and silicon has an insulating native oxide that can be used as a sacrificial layer. Other thin film materials that are commonly used in MEMS include silicon nitride, metals, and conventional polymers, such as polyimide. 

One of the technological ways to miniaturize fuel cells is to have recourse to standard microfabrication techniques mainly used in microelectronics and more especially the fabrication of micro- and nano-electro-mechanical systems (MEMS/NEMS). Actually more and more papers show the interest in developing MEMS-based fuel eells, either directly with silicon substrates (fig. 2), or adapting the methods to other substrates such as metals or polymers. These techniques enable notably mass fabrication at low cost (very large number of devices on a very small area) and then eould lead to the reduetion of the global cost of the miniature fuel cells. 

Historically microfabrication techniques have first been developed to meet the requirements of microelectronics, but they also have allowed the emergence and the development of a new research field where mechanical elementscan be manufactured and actuated with electrical signals at a micro- and even nanometer scale. To describe this emerging research field, R. T. Howe and others proposed the expression Micro Electro Mechanical Systems or MEMS in the late 1980 s [14]. MEMS is not the only term used to describe this field which is also called as micromachines, for instance in Japan, or more broadly referenced as microsystem technology (MST) in Europe. 

This section will briefly introduce the reader to the microfabrication techniques that will be more generally used for applications in miiuature fuel cells, i.e. photolithography, etching, deposition, and sealing. These techitiques mainly concern silicon as the basic substrate, but are also fitted to other materials such as some polymers or foil materials. For these materials, the specific techniques will be described in the following overview on microfabricated fuel cells (section 4.). More informations on techniques and MEMS in general can be found in [18-21],... 

As the basic material for MEMS technologies, silicon (Si) is also the most common material encountered in MEMS-based FCs. Its properties and the microfabrication techniques associated to it and previously described are now well-known and mastered. Another advantage of Si-based FCs may also be to facilitate the possible integration of the FCs with other electronic devices on the same chip. 

Using microfabrication techniques is advantageous it is possible to readily achieve micron-scale, or even smaller, lateral feature dimensions as well as excellent reproducibility of the structures and their behavior, since they are batch fabricated and not individually hand-made. Batch fabrication also makes the devices potentially low cost. The techniques described above are compatible with those used to make other MEMS devices, so conjugated polymers can be combined with them to form complex systems. As mentioned above, because of their low operating voltages, conjugated polymers can also be interfaced with standard integrated circuitry [7]. 

In the 1990s photolithography and micromachining in silicon were the most popular microfabrication techniques, particularly for integration of microelectronics circuits and MEMS, especially due to silicon s well-known properties. Additionally, glass has also been widely reported because of its biocompatibility, resistance to some... 

Instead of merely exploiting microscale physical phenomena using simple straight-channel patterns with constant cross section, it is certainly feasible to make use of well-developed microfabrication methods for introducing flow control functionality into individual components. Although various MEMS approaches have been utilized for this purpose (as reviewed by Oh and Ahn ), many of these techniques require complex fabrication and will not be discussed here. This section focuses on more practical passive components that can be fabricated in PDMS or PDMS-glass hybrid devices, requiring minimal fabrication complexity in addition to the typical microchannel assembly methods. 

The microfabrication of electrode arrays built with silicon micromachining techniques illustrates an positive approach towards future Cl electrode array development in respect to the traditional manufacturing method used now days. Also lithography and MEMS technology facilitates the addition of enhanced functionality to the microelectrode arrays. There is, however, still a long way to go until these devices can be used in real Cochlear Implants. The fabrication possibilities and characterization of different CMOS compatible metals (Ti, TiN and Al) provides a strong base to go ahead with further research in this direction. In our electrical tests done we conclude that TiN is able to withstand a high current density 2.8, while aluminium failed... 

These submillimeter devices are machined using specific techniques globally called microfabrication technology. This definition also includes microelectronic devices, but in addition to electronic parts, MEMS also features mechanical parts like holes, cavity, channels, cantilevers, or membranes. This particularity has a direct impact on their manufacturing processes which need to be adapted for thick layer deposition, deep etching and to introduce special steps to free the mechanical structures. Moreover, many MEMS are now not only based on silicon but are also manufaetured with polymer, glass, quartz or thin metal films. 

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