Micromachining Techniques

Currently, corrugated metal foils fabricated by rolling are apphed in the field of automotive exhaust gas systems [96]. The metallic monoHths fabricated in this way actually have channel dimensions in the sub-miUimeter range and can be truly termed microtechnical devices.  

Laser ablation is a frequently apphed fabrication technique of proven industrial suitabihty [41, 42]. However, fabrication of microchaimels of several hundred micrometers depth, as typically required for many apphcations using microstructured reactors, will take too long and therefore the method is not cost competitive. For smaller chaimel dimensions, laser ablation is a viable option, especially for apphcations on the smaUer scale. 

---

RECENT ADVANCES AND FUTURE PERSPECTIVES 4.1 Micromachining Techniques... 

With the advancing automatization and computerization of CE instruments, the application of micromachining techniques, and the improvement of the devices for coupling CE with CL detection, it is hoped that both techniques may be incorporated in the future as suitable methodology in routine laboratories, being complementary to classical techniques such as HPLC and offering new alternatives to the analytical chemist. 

PR Tsai, I.C. Chen, and C.J. Ho. Ultralow power carbon monoxide microsensor by micromachining techniques , Sensors and Actuators B76 (2001), 380-387. 

Initially, the reactor was to be built using silicon wafers, 92 but more recent efforts have focused on a stainless steel reactor. The reformer, 7.5 x 4.5 x 11.0 cm (371 cm ), houses up to 15 stainless steel plates (0.5 mm thick) with chemically etched microchannels and heating cartridges. Conventional and laser micromachining techniques were used to fabricate the reformer body. The microchannel dimensions are 0.05 x 0.035 x 5.0 cm . The reactor inlet was carefully designed to allow uniform flow conditions. ... 

Usually, microfabrication techniques are used to prepare cantilevers with integrated tips of various shapes, mass and spring constants [197,198]. Depending on the cantilever geometry and material used to construct the cantilever [52], the frequency of commercial cantilevers typically varies from 15 kHz to more than 500 kHz, and the spring constants range from 0.01 to 100 N/m. Micromachining techniques can be used to prepare special probes such as meander-type cantilevers for bidirectional force microscopy [199]. 

Hollow planar waveguides have been fabricated by several techniques, including physical vapor deposition and CVD of silver and dielectric layers on metallic substrates. Nevertheless, better results can be obtained by taking advantage of silicon micromachining techniques. Perhaps the most important advantage of silicon hollow waveguides over other hollow structures is the... 

Two- and three-dimensional shapes and patterns can be reproduced in silicon with high precision using bulk and surface micromachining techniques [46,54]... 

Campana, A.M.G., Baeyens, W.R.G., Aboul-Enein, H.Y., Zhang, X., Miniaturization of capillary electrophoresis system using micromachining techniques. J. Microcolumn Separations 1998, 10(4), 339-355. 

Murakami, Y., Takeuchi, T., Yokoyama, K., Tomiya, E., Karube, I., Integration of enzyme-immobilized column with electrochemical flow cell using micromachining techniques fora glucose detection system. Anal. Chem. 1993, 65, 2731-2735. 

The characteristics of Lamb waves have been derived and explored by a number of authors [61,62,63,64]. Here we will focus on the aspects of the waves that are most relevant for sensing and actuating in devices made by micromachining techniques. These techniques include many of those customarily employed in making silicon integrated circuits. Examples are processes for depositing thin... 

A. M. Rossi, G. Amato, L. Boarino, and C. Novero, Realization of membranes for atomic beam collimator by macropore micromachining technique (MMT), Mater. Sci. Eng. B69-70, 66, 2000. 

Bulk micromachining relies on several etching techniques and creates projections of planar photolithographic masks in 3 dimensions. Surface micromachining relies on sacrificial layer and wafer bonding techniques. It creates true 3D structures as a stack of 2D patterned layers. Hence, it is more correct to refer to both micromachining techniques as two and a half dimensional (2% D). 

Micromachined techniques enable batch production (silicon wafer processing). 

The functional principle is again in line with Fig. 7.2.1. Oscillating masses and acceleration sensors are produced in the same plane and consist (depending on the micromachining technique) for example of 2-10 pm thick epitactical deposited polycrystalline silicon. 

The advent and development of micromachining techniques, which enabled microflow based systems, has opened a potentially very important new area for development. The small size and potential for low cost means that new chemical production concepts such as point of use manufacturing could be realized. Microfabrication techniques also offer the potential to imbed analytical measurement within the flow path for close coupled control of the production process. Finally, new chemistries, which are difficult or just dangerous to practice in conventional equipment, are much more easily controlled in microscale equipment. However, given the conservative nature of the chemical processing industry this technology, like all new innovations, will have a slow introduction into mainstream manufacturing. 

Figure 14.6 A schematic view of structures defined by micromachining techniques [71]. The Sn02 layer is applied in a spin coating process. 

Micromachined and microfabricated electrochemical sensors have been used either per se, or as part of a sensor system, in many practical applications. This includes various biosensors and chemical sensors reported in research literature. An example of a practical electrochemical sensor is the yttria-stabilized zirconium dioxide potentiometric oxygen sensor used for fuel-air control in the automotive industry. Thick-film metallization is used in the manufacture of this sensor. Even though the sensor is not microsize, this solid electrolyte oxygen sensor has proven to be reliable in a relatively hostile environment. It is reasonable to anticipate that a smaller sensor based on the same potentiometric or the voltammetric principle can be developed using advanced microfabrication and micromachining techniques. 

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. 

---