Microfabrication with silicon

Electrochemical experiments have been carried out on materials deposited by PVD on silicon microfabricated arrays of Au pad electrodes [Guerin et al., 2006a]. The substrate is made up of a square silicon wafer capped with silicon nitride (31.8 mm x 31.8 mm), which has an array of 100 individually addressable Au pad electrodes. These electrodes make up a square matrix on the wafer, which can be masked when placed in a PVD chamber, allowing deposition of thin films on the Au electrodes. Figure 16.3 is a schematic drawing of the configuration. Small electrical contact pads in Au for the individual addressing of electrodes (0.8 mm x 0.8 mm) are placed on the boundaries. 

A new alternative to solve this problem is atomic force microscopy (AFM) which is an emerging surface characterization tool in a wide variety of materials science fields. The method is relatively easy and offers a subnanometer or atomic resolution with little sample preparation required. The basic principle involved is to utilize a cantilever with a spring constant weaker than the equivalent spring between atoms. This way the sharp tip of the cantilever, which is microfabricated from silicon, silicon oxide or silicon nitride using photolithography, mechanically scans over a sample surface to image its topography. Typical lateral dimensions of the cantilever are on the order of 100 pm and the thickness on the order of 1 pm. Cantilever deflections on the order of 0.01 nm can be measured in modem atomic force microscopes. 

However, to realize a practical and cost-effective system for biomedical applications, a microvalve system that will process human whole blood is essential. To date, most microvalve systems have been microfabricated from silicon, although valves using plastic membranes have also been developed. Chip-based microvalve systems have been classified as either active microvalves (with an actuator) or passive (check) microvalves (without an actuator). The miniaturization of the active microvalve systems is restricted by the size of the actuator. 

The development of microfluidic devices with MALDI-MS detection has focused on interfaces that enable off-chip or on-chip MALDI-MS analysis and the microfabrication of silicon/polymeric MALDI target plates. 

Additional effort by Chou et al. applied the above methodology to microfabricate of silicon array. With this device, it was possible to separate a mixture of Xho -cvX X DNA fragments (15 and 33.5 kbp) in the vertical electric field of 1.4 V/cm. They calculated that the DNA fragments moved in trajectories at different angels through the array. Interestingly, molecules that diffuse very slowly... 

Titanium is an important metal, but it is difficult to microfabricate by LIGA. Titanium and its alloys possess the best strength-to-weight ratio and corrosion resistance. In contrast with silicon, Ti and its alloys have much higher fracture toughness, better electrical conductivity, and greater biocompatibility and are very attractive for MEMS applications. 

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... 

As far as microfabrication is concerned, especially with silicon-based processes, microneedle design can be classified as out-of-plane or inplane depending on the orientation of the needle relative to that of the substrate material. Out-ofplane design refers to the microneedle configuration that protmdes perpendicularly from the plane of the substrate material (e.g., a silicon wafer), while in-plane refers to the design where the resulting needle lies lengthwise in the substrate plane. Examples of various microneedles (solid and hollow, out-of-plane and in-plane) are shown in Fig. la-d. 

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. 

Foraker et al. 2003). In the paper, Foraker et al. used microfabricated porous silicon particles to enhance insulin permeabihty across Caco-2 cell monolayer, which is commonly used in vitro model of the human small intestinal mucosa to predict the absorption of orally administrated drugs. They observed that the flux of insulin across the cell monolayer was approximately 50-fold compared with liquid formulations and nearly 10-fold higher compared with liquid formulations with permeation enhancer, if insulin was loaded in PSi. 

As with silicon, polyimide has also been used in IC fabrication processes. Polyimide is used as a dielectric and encapsulant because it provides good planarization, electrical insulation, and resistance to solvents. Polyimide thin films are patterned using standard microfabrication processes such as photolithography and reactive ion etching both photosensitive and nonphotosensitive formulations are available. 

Microfabrication of the silicon part of the device is done by processing a silicon wafer with LPCVD and other thin-film techniques, standard photohfhography, dry... 

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. 

J. Yeom, G. Z. Mozsgai, B. R. Flachsbart, et al. Microfabrication and characterization of a silicon-based millimeter scale, PEM fuel cell operating with hydrogen, methanol, or formic acid. Sensors Actuators B 107 (2005) 882-891. 

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