Carbon material based microelectromechanical system (MEMS): fabrication and devices

Abstract

This PhD dissertation presents the exploration and development of two carbon materials, carbon nanotubes (CNTs) and carbon fiber (CF), as either key functional components or unconventional substrates for a variety of MEMS applications. Their performances in three different types of MEMS devices, namely, strain/stress sensors, vibration-powered generators and fiber solar cells, were evaluated and the working mechanisms of these two non-traditional materials in these systems were discussed. The work may potentially enable the development of new types of carbon-MEMS devices.

Firstly, a MEMS-assisted electrophoretic deposition (EPD) technique was developed, aiming to achieve controlled integration of CNT into both conventional and flexible MEMS systems. Selective deposition of electrically charged CNTs onto desired locations was realized in the EPD process through patterning of electric field lines created by the microelectrodes fabricated using MEMS techniques. A variety of 2-D and 3-D micropatterns of CNTs with controllable thickness and morphology have been successfully achieved in both rigid and elastic systems at room temperature with relatively high throughput. Studies also showed that high surface hydrophobicity of the non-conductive regions in microstructures was critical to accomplish well-defined selective micropatterning of CNTs through this strategy.

A patterned PDMS/CNT nanocomposite was then fabricated through the aforementioned approach, and was incorporated, investigated and validated in elastic force/strain microsensors. The gauge factor of the sensor exhibited a strong dependence on both the initial resistance of the device and the applied strain. Detailed analysis of the data suggests that the piezoresistive effect of this specially constructed bi-layer composite could be three folds, and the sensing mechanism may vary when physical properties of the CNT network embedded in the polymer matrix alter.

The feasibility of the PDSM/CNT nanocomposite serving as an elastic electret was further explored. The nanocomposite composed of these two non-traditional electret materials exhibited electret characteristics with reasonable charge storage stability. The power generation capacity of the corona-charged nanocomposite has been characterized and successfully demonstrated in both a ball drop experiment and the cyclic mechanical load experiments.

Lastly, in an effort to develop carbon-material-based substrates for MEMS applications, a carbon fiber-based poly-Si solar cell was designed, fabricated and investigated. This fiber-type photovoltaics (PV) takes advantage of the excellent thermal stability, electrical conductivity and spatial format of the CF, which allows CF to serve as both the building block and the electrode in the PV configuration. The photovoltaic effects of the fiber PV were demonstrated with an open-circuit voltage of 0.14 V, a short-circuit current density of 1.7 mA/cm2, and output power density of 0.059mW/cm2. The issues of this system were discussed as well.