Thin-film transistors (TFTs) have been used successfully for several decades for the active control of large-area electro-optical systems such as displays and large-area sensors, e.g. x-ray sensors. More recent applications of these transistors relate to their use in micro-electromechanical systems (MEMS) and in RFID transponders (RFID tags).
Micro-electromechanical systems are being used increasingly in industry, for example, in acceleration sensors or electrically controllable micromirrors. There are basically two different types of MEMS fabrication processes: those in which the substrate (often a monocrystalline Si wafer) is used (or also used) to create the MEMS structures, and those in which all materials necessary to produce the MEMS structures are applied to a suitable substrate. This is usually done with typical processes used in thin-film technology, for example, by physical or chemical deposition from the vapour phase (PVD, CVD) or even by spin-on deposition. The structuring of micromechanical components is carried out mainly using established photolithographic processes used in the semiconductor industry.
The control of complex micro-electromechanical systems can be significantly simplified if active and possibly passive electronic and electrical components, such as transistors, diodes, resistors, capacitors etc., also are integrated on the same substrate in addition to the electromechanical components.
High-quality RFID transponders with high sensitivity require diodes with good rectification characteristics during a low voltage drop for rectification of the received electromagnetic alternating field.
In all these applications from thin-film transistors, the TFTs and the elements and/or additionally necessary components to be controlled by them have indeed until now been constructed on the same substrate, but successively, each using different structuring processes and stages.
Thus, for example, Lee, Czang-Ho and Vygranenko, Yuriy and Nathan, Arokia, “Process issues with Mo/a-Si:H Schottky diode and thin film transistors Integration for direct x-ray detection”, Journal of Vacuum Science & Technology A, 22, 2091-2095 (2004) discloses a method in which Schottky diodes and bottom-gate (inverted staggered) a-Si:H transistors are fabricated successively on a substrate and are switched to one another.
Furthermore, in Chow, Eugene M., et al. “High voltage thin film transistors integrated with MEMS.” Sensors and Actuators A: Physical 130, 297-301 (2006), a method is described in which the bottom-gate-a-Si:H transistors and MEMS cantilevers (bars) are fabricated successively on a substrate.
The known method therefore requires a large number of necessary process stages to fabricate the TFTs and the additional components, resulting in high manufacturing costs and a lower process yield.
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