Microfabrication processes

GP 8[ [R 7[ The structure of the rhodium catalyst changed during operation. Owing to the microfabrication process (thin-wire pEDM), the surface of the micro channels was rough before catalytic use [3]. After extended operational use, small crystallites are formed, especially in oxygen-rich zones such as the micro channels inlet. Thereby, the surface area is enlarged by a factor of 1-1.5. 

Clearly this approach is not suitable for preparing large quantities of products, its main purpose being to permit the greatest amount of information to be obtained concerning the reactivity of a new material. If the coreactant is expensive and/or difficult to prepare then this procedure is invaluable. However, it is important to consider that the quantities of derivatized polymer obtained in this approach [ 10-6 mole based on -N=C repeat unit] might well represent sufficient material if such a process were to be used for the direct preparation of chemically modified electrodes (12), or incorporated into a planar microfabrication process, with which it would appear to be compatible. 

Fig. 4.14. Schematic ofthe microfabrication process for the high-temperature microhotplate with Pt temperature sensor...

CE chips are mainly obtained using various glass substrates, from inexpensive soda-lime glass to high-quality quartz.Various polymer materials are also used. The choice of a particular material depends on its surface properties, ease of fabrication, which can be quite different according to the material origin, disposability, and price. Microfabrication processes were recently reviewed and the reader is thus referred to dedicated literature for additional useful information on microfluidic device fabrication. ... 

FIGURE 18.2 Scanning electron micrographs of silicon microneedles, (a) Silicon microneedles micro-fabricated using a modified form of the BOSCH deep reactive ion etching process. The microfabrication process was accomplished at CCLRC Rutherford Appleton Laboratory (Chilton, Didcot, Oxon, UK). The wafer was prepared at the Cardiff School of Engineering, Cardiff University, UK. Bar = 100 pm (b-d) platinum-coated silicon microneedles prepared using a wet-etch microfabrication process performed at the Tyndall National Institute, Cork, Ireland. Bar = 1 mm (b), 100 pm (c,d). 

Since it will take several years to realize such an integral software toolbox, individual approaches with separate steps have to be applied to meet gradually the requirements of microreactor design. Standard software for computational fluid dynamics is directly applicable in this context, and there are also powerful software tools for the simulation of special steps in microfabrication processes. However, there has been rather little experience with materials for microreactors, optimization of microreactor design, and, in particular, the treatment of interdependent effects. Consequently, a profound knowledge of the basic properties and phenomena of microreaction technology just described is absolutely essential for the successful design of microreaction devices. 

Besides such basic aspects concerning the shape of and materials for microreaction devices, costs play a major role in the selection of a microfabrication process. In this respect, the number of pieces and the precision that is really required, as well as aspects like availability and manufacturing experience, must be taken into account. In contrast to the situation some years ago, the prerequisites for cost-effective mass fabrication as well as small-scale production or rapid prototyping have essentially changed. Modem commercial equipment for the production of microdevices is available that allows unreliable and uneconomic laboratory-scale manufacturing devices to be replaced. 

The actual device consists of two parallel rows of five mixing elements each [125, 126], The lateral displacement and recombination of the sub-channels are done via 90° angles. In contrast, the variation of channel depth and width is done in a continuous fashion, yielding a curved horizontal and vertical structure. In particular, the curved floor structure is demanding, since a real 3-D and not just a multi-level microfabrication process is needed. 

The actual retina contact structure incorporates 12 or 24 independent electrodes, respectively. The electrodes were arranged concentrically to minimize the electrical stray field during stimulation. We established the microfabrication process for double metallization layers needed to obtain concentric microelectrodes. In a temper step, the electrodes were formed into a convex shape according to the curvature of the eye. The generation of convex shapes was possible since the stimulator was designed in concentric rings interconnected by s-shaped bridges (Fig. 26). 

Microreactor technology has developed to such an extent that a wide variety of microreactor components, e.g. micropumps, mixers, reaction chambers, heat exchangers, separators and complete integrated microreaction systems with process control units have been fabricated using the appropriate microfabrication process and materials that are suitable for specific applications. 

FIGURE 2.25 Schematic of the main parts generated by the single toner layer (STL) microfabrication process I, perforated PET polyester cover II, printed polyester base (a, toner layer) III, cover and base laminated together IV, the final device (b, liquid reservoirs) [238]. Reprinted with permission from the American Chemical Society. 

In a later report, inhibition of PCR in Si-glass PCR chips was found to be mainly caused by the adsorption of the Taq polymerase, rather than by the adsorption of DNA [299]. It was also found by XPS analysis that the primary PCR inhibitor in a glass PCR chamber was Cr, which was involved in the microfabrication process. The highest Cr concentration that could be tolerated was found to be 0.1 M [932]. 

To follow this approach subsystem layout and functions as well as microfabrication processes have to be selected, which are apt to generate and integrate the various subsystems of a mass spectrometer into one substrate in one batch process, i.e., all the components are defined in their geometry and orientation among each on one photo mask, which is then transferred into features in the substrate. Thus, submicron accuracy of device features and their orientation and adjustment can reproduc-ibly be guaranteed, i.e., many identical devices can be fabricated simultaneously in one run. This, however, also means that all the subsystems are fabricated in the same material within a narrow range of the necessarily different dimensions. 

Self-assembly of beads into cavities. All cavities are the same and their generation is a parallel, well-established microfabrication process. No miniaturized placement of individual beads to particular positions is necessary. 

In order to keep the polishing rate constant during the long CMP steps, in situ conditioning of the polishing pad using diamond wheels is often a characteristic of a microfabrication process. To control the process more precisely, an end-point detection system that can measure the remaining layer thickness is extremely useful. 

Microfabrication processes consist of many different procedures, and four major processes will be discussed here ... 

As in all microfabrication processes, the cleanliness of the substrate is very critical for the silicon oxidation process. As discussed above, the crystalline orientation will influence the oxidation rate of silicon dioxide. There are other operational parameters that will affect the oxidation rate of silicon. This includes the dopants in silicon, the trace amount of water, the concentration of Cl-bearing species, the temperature control, and its profile. Extensive assessment of each parameter on the growth rate and the quality of the Si02 layer on silicon can be found in microfabrication related literature elsewhere. ... 

In conclusion, silicon-based microfabrication processes prove to be effective in producing miniature size. 

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