Microfabrication methods

Becker H., Gartner C., Polymer microfabrication methods for microfluidic analytical applications, Electrophoresis 2000 21 12-26. 

H. Becker and C. Gartner, Polymer microfabrication methods for microfludic analytical applications. Electrophoresis 21, 12—26 (2000). 

Three-dimensional electrode arrays have been fabricated using two very different micromachining methods. One approach, named carbon MEMS or C-MEMS, is based on the pyrolysis of photoresists. The use of photoresist as the precursor material is a key consideration, since photolithography can be used to pattern these materials into appropriate structures. The second approach involves the micromachining of silicon molds that are then filled with electrode material. Construction of both anode and cathode electrode arrays has been demonstrated using these microfabrication methods. 

In the past decade, microfabrication methods developed in the microelectronic industry have led to new opportunities for device research and development involving chemically sensitive electronic structures. In 1980, this subject was reviewed in depth at a NATO Advanced Study Institute (1). Over the last five years, there have been three international conferences 2-k), devoted to sensors with a strong emphasis on chemical sensors as well as a number of national and specialized meetings on the subject (5,6). In this paper, some recent developments that will have long term consequences on the study of chemically sensitive electronic devices will be reviewed. To simplify the discussion, the topics are divided into the following categories ... 

As a matter of fact, microfabrication methods have to be introduced into chemical engineering in order to profit from the potential advantages of microreaction technology. Although this is a difficult hurdle, a few chemical companies have successfully started to utilize microreaction technology for commercial syntheses of fine and special chemicals. Nevertheless, much effort must still be spent to transfer further promising research results into commercial application and to... 

However, microfabrication methods that are usually unfamiliar to chemical engineers have to be introduced to profit comprehensively from microreaction technology. This transition from standard manufacturing methods of plant components to the development and production of microdevices is also inevitably connected with the application of special materials that are not yet proven in chemical engineering. In addition, novel design rules that have not existed until now should be implemented for the long term to speed up the development of novel devices. 

Chemical activities in the field of mass screening are often related to combinatorial chemistry [51,52]. One major goal, especially in the field of solid phase chemistry involving polymers like DNA or peptides, aims at the increase in the number of compounds per reactor volume and time. Commercially available microtiter plates are established as reactors in this case whereby robotic feed systems fit perfectly to their dimensions. A drastic reduction of reaction volume and increase in number of reaction vessels ( wells ) leads to the so-called nanotiter plates (e.g. with 3456 wells). Microfabrication methods such as the LIGA process are ideal means for the cost effective fabrication of nano-titer plates in polymeric materials by embossing or injection molding techniques so that inexpensive one-way tools are realized. 

None of the existing microfabrication techniques available cover the complete range of materials which might be of interest in the present case. The choice of the microfabrication method therefore selects the material to be applied and determines the range of temperature, pressure or solvents for the application. Material selection is therefore important when new applications of microreactors are envisaged. 

The industrially important nitration of aromatic compounds in a microreactor using two immiscible liquid phases was demonstrated in different studies using either parallel [220] or segmented flow [221]. In all studies, a PTFE capillary microchannel, connected to an inlet junction, was used in which either segmented or parallel flow can be created. The use of PTFE tubing is desirable as it is commercially available and no complicated microfabrication methods are involved. 

There has been a trend toward electrochemical reactions in lab-on-a-chip devices in the last few years.51,52 This is mainly because miniaturized electrodes can be fabricated using microfabrication methods and solutions can be transferred by microfluidics approaches.5354 Flow injection analysis and sequential injection analysis techniques were also employed for electrochemical enantioselective high-throughput screening of drugs.55... 

Popular microfabrication methods include lithographic, galvanoformung, abformtechnik (LIGA), wet and dry etching processes, micromachining, lamination, and soft lithography. An overview of these techniques is given here and specifics can be found in the references. 

Transmission electron microscopy (TEM) is probably the most powerful technique for obtaining structural information of supported nanoparticles [115-118], Complementary methods are STM, AFM, and SEM. Both the latter and TEM analysis provide more or less detailed size, shape, and morphology information, i.e., imaging in real space. TEM has the great additional advantage to provide information in Fourier transform space, i.e., diffraction information, which can be transformed to crystal structure information. From a practical point of view, considering the kinds of planar model catalysts discussed above, STM, AFM, and SEM are more easily applied for analysis than TEM, since the former three can be applied without additional sample preparation, once the model catalyst is made. In contrast, TEM usually requires one or more additional preparation steps. In this section, we concentrate on recent developments of microfabrication methods to prepare flat TEM membrane supports, or windows, by lithographic methods, which eliminate the requirement of postfabrication preparation of model catalysts for TEM analysis. For a more comprehensive treatment of other, more conventional, procedures to make flat TEM supports, and also similar microfabrication procedures as described here, we refer to previous reviews [118-120]. 

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. 

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