The paper deals with a very high sensitive integrated humidity sensor compatible with CMOS technology. This sensor is a polysilicon Suspended Gate Thin Film Transistor (SGTFT), fabricated using a low temperature surface micromachining process. Microtechnology technics using sacrificial layer are used to fabricate polysilicon bridge which acts as the transistor gate. Transistors are characterized at various humidity rates and transfer characteristics show highly sensitive dependence with humidity. The small air-gap (0.5 μm) between the gate and the channel explains the amplifying effect of the sensitivity: threshold voltage shift is more than 17V when the humidity ratio varies from 20 to 70%.
Three-dimensional polysilicon microstructures, as cantilevers or bridges, give new opportunities to increase the application field of polycrystalline silicon. Polysilicon devices with new functions can be imagined. Air-gap thin film transistors can be built from polysilicon bridges for example. They use a highly doped polysilicon bridge as gate; the gate insulator is the air or any other ambience. Then, these devices can be used as ambience sensors including chemical or biological sensors. They are very interesting as they are deposited on any substrate. Moreover, their main interest is their full compatibility with the microelectronics technology on glass performed at temperature lower than 600°C and developed first for the flat panel displays addressing. Fully integrated sensing systems are then imaginable.
Here we present a low temperature process (< 600 °C) to built air-gap-polysilicon-thin-film-transistor with high mechanical and electrical performance. The transistor-active-layer is made of Low Pressure Chemical Vapor Deposition (LPCVD) undoped polysilicon while the suspended gate is made of LPCVD highly doped polysilicon. Deposition and annealing conditions of the doped polysilicon has first optimized to obtain high mechanical performance for the polysilicon micro-bridges. Hence, an optimal tensile stress to obtain longer bridges has been found. The mechanical stress has been determined from micro-Raman analysis.
Then, air-gap transistors with 500 nm thick air-gap as gate insulator have been designed and fabricated. The transistor channel is 7-μm long and 7-μm wide, with a 15-μm long and 10-μm wide bridge. Their subthreshold slope, threshold voltage, and field effect mobility are 0.22 V/dec, 2.4 V and 130 cm<sup>2</sup>/V.sec respectively. These parameters are comparable to that of the best polysilicon thin film transistors with thin silicon dioxide (100 nm) as gate insulator. Then, these transistors are interesting as electronic devices firstly. Moreover they can be involved in ambience sensors. Indeed, they are checked as oxygen sensor in a preliminary experiment. Transistor characteristics are measured in air ambience, then in oxygen and finally in air. The electrical parameters have shown a considerable dependence on the ambience with an excellent recovering after removing oxygen.