Microfabricated structures fabrication processes

Microfabrication processes have been used successfully to form micro-fuel cells on silicon wafers. Aspects of the design, materials, and forming of a micro-fabricated methanol fuel cell have been presented. The processes yielded reproducible, controlled structures that performed well for liquid feed, direct methanol/Oj saturated solution (1.4 mW cm ) and direct methanol/H O systems (8 mA cm" ). In addition to optimizing micro-fuel cell operating performance, there are many system-level issues to be considered when developing a complete micro power system. These issues include electro-deposition procedure, catalyst loading, channel depth, oxidants supply, and system integration. The micro-fabrication processes that have... 

Because the fabrication process is such an essential part of microfluidics, an overview of the principles underlying the microfabrication technology is presented. Pressure, flow, and temperature measurements are essential variables for characterizing fluid motion in any system. An important goal is the design and construction of self-contained microfluidic systems. Because of their small size, incorporation of pressure, flow, and temperature sensors directly on the microfluid system chip is highly desirable. There are relatively few examples where microfluidic systems have been constructed with these on-board sensors. There have been so many microsensor developments in recent years that it is only a matter of time before such systems will appear. Small-scale actuators to provide either open- or closed-loop control of the flow in microchannels are needed and these efforts are addressed. While experimental work on fluid flow itself in microscale structures is rather sparse, some results will be presented that emphasize the similarity and/or differences between macroscopic and microscopic flow of liquids. Although there are not many applications of... 

Microfabrication is a process used to generate physical devices onto substrates. These devices are formed by structures with dimensions from millimeter to nanometer range. Figure 3.1 shows a piece of silicon (Si) wafer with devices after the completion of the fabrication. Over the years, microfabrication has advanced significantly from the established semiconductor fabrication processes used for integrated circuits (ICs) to diverse materials and processes such as polymers, liquids, soft lithography, and liquid-based processes. 

In this section we will discuss three basic steps in microfabrication thin film deposition, photolithography, and etching. The whole fabrication process usually involves iterations of these steps so the device structure is built layer by layer until it is completed. 

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). 

A hybrid BCB-silicon neural implant with embedded microfluidic channels has been fabricated and tested in acute recordings [70]. A thin layer of silicon was used to add mechanical stiffness to the implant. The fabrication process is based on SOI technology, where the device layer of the wafer was the 2-, 5-, or 10-iim silicon backbone of the BCB structure. The microfluidic channels were made with a sacrificial photoresist layer. Cytotoxicity tests of BCB have demonstrated its biocompatibility in glial and fibroblast cell culture [71] and using brain slice culture [72]. A summary of several microfabricated thin-film electrodes is presented in Table 1. 

Figure 5 Kinetically controlled decal transfer printing, (a) Schematic drawing of the process used to transfer microfabricated structures from a donor substrate to a polydimethylsiloxane (PDMS) stamp, and then from the PDMS stamp to a receiver substrate, (b and c) scanning electron microscope (SEM) images oftwo-and three-dimensional structures fabricated by this process. Reproduced with permission from MeitI, M. A. Zhu, Z. T. Kumar, V. etal. Nat. Mater. 2006, 5,33-38. Copyright 2006, Nature Publishing Group.

EBL was used to fabricate uniform platinum nanoparticle arrays on Si02 (mean platinum particle diameter 30-1000 nm 52,53,106,107,398)), and evaporation techniques were used to prepare smaller particles and a continuous platinum film. The EBL microfabrication technique allows the production of model catalysts consisting of supported metal nanoparticles of uniform size, shape, and interparticle distance. Apart from allowing investigations of the effects of particle size, morphology, and surface structure (roughness) on catalytic activity and selectivity, these model catalysts are particularly well suited to examination of diffusion effects by systematic variations of the particle separation (interparticle distance) or particle size. The preparation process (see Fig. 1 in Reference 106)) is described only briefly here, and detailed descriptions can be found in References 53,106,399). 

Microfabrication has emerged from microelectronics manufacturing and is using its proven processes and process sequences. Additionally, specific methods have been developed to fabricate mechanical, electrical, optical, or sensor structures, which are characteristics of microfabrication. In order to stay within the scope of this book, only top-down methods, that is, the manufacture of smaller structures with higher functionality from larger structures by the use of subtractive methods, will be discussed. Bottom-up methods, which create larger structures by ordered arrangement of small units (molecules, nanoparticles), are still in their infancy and mainly employed for biosensors. 

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