Microcantilever structure was used to design and fabricate strain sensors. P-channel MOSFETs were designed and fabricated for strain sensors as hole mobility under uniaxial stress has higher mobility enhancement than that of electron mobility under either uniaxial stress or biaxial stress. For MOSFET sensors on microcantilevers, anchor area is the traditional position for sensors due to its highest stress. The aim of this research is to investigate the assumption of biaxial stress at the anchor area which makes the anchor location less sensitive. COMSOL software was employed to simulate stress profiles in silicon beams. Four-pointbending provides almost uniaxial stress at the surface, but cantilever bending generates biaxial stress at the anchor region but uniaxial stress away from anchor. The transverse stress at the anchor region increases with the bending and is depends on the longitudinal stress. In this research, the sensitivity of the sensor at the cantilever anchor was lower than that under uniaxial stress from four-point-bending measurement. It is suggested that the transverse stress at the anchor region will degrade the sensitivity compared to that under uniaxial stress. Although narrower cantilever provides higher longitudinal stress at the anchor, it brings higher transverse stress at the anchor as well. Possible solutions such as employing stressed layer, holes and slots were proposed to reduce the transverse stress without impact on the longitudinal stress, thus to increase the sensitivity. In comparison, employing slots is recommended as the transverse stress can be concentrated to nearly zero at the slot region with enhanced and elongated longitudinal stress.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:675436 |
| Date | January 2014 |
| Creators | Wang, Yao |
| Publisher | Queen's University Belfast |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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