Thin-film technologies microfabrication

Microwave power transmission is an off-board source of power such that the weight of target system can be ultimately reduced by eliminating the need of a battery. The thin-film technology developed for semiconductor microfabrication enables the fabrication of rectennas in a miniaturized scale and at low cost. The use of high frequency microwaves, such as K-band (30 GHz) and W-band (90 GHz), allows a dense population rectenna array that can increase the efficiency of the receiving electrical power. 

Polycrystalline silicon thin film transistors have also been employed for the detection of DNA hybridization [16]. A mixed self-assembled monolayer of thiolated DNA probes and mercapto-hexanol was immobilized onto the gold gate of an extended gate poly-Si TFT. A shift of the I-V characteristics on the order of 300 mV was obtained upon hybridization of the immobilized probe with a fully complementary strand. The shift is independent of electrode area, so microarrays can be constructed where a known DNA probe is immobilized on each FET. The inherent miniaturization and compatibility with microfabrication technologies make the technique highly promising for the development of low-cost portable devices. 

Microfabrication has been the topic of a recent review in which thin-film (<1 pm, based on vacuum evaporation, sputtering or chemical vapor deposition) and thick-film (>10pm, based on screen printing or lamination) technologies are described for the mass production of potentiometric sensors and sensor arrays [80]. Current challenges include the cost of fabrication, especially for thin-film devices, the control of physical dimensions of the sensing elements, the incorporation of liquid reservoirs, and the stability of the integrated reference electrodes. 

Low-aspect-ratio (ratio of the height of the microelectrode divided by its lateral dimension is larger than 0.2) microelectrode arrays are fabricated using conventional silicon-based microfabrication technologies, such as hthographic techniques to yield arrays of thin films of metallic or carbon electrodes on a silicon substrate. These arrays often have limited stability and life times as a result of defects in the metal layer poor resistance to corrosion is... 

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. 

Table 1 Comparison of several microfabricated thin-film microelectrode technologies... 

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