||People for long term exposed to an air-conditioned but highly humid environment|
are vulnerable to hyper-sensitivity or asthma triggered by fungi or dust mites. This
thesis aims to develop a high-linearity and low-hysteresis capacitive relative humidity
(RH) microsensor to more precisely accommodate the humidity of living spaces.
To reduce the hysteresis and enhance the linearity, this research uses not only one
polyimide (PI) thin film as a humidity sensing layer but also utilizes another PI thin
film as a protecting layer of the top electrodes. To improve further the RH sensitivity
and responding speed, interlacing out-of-plane electrodes are designed in the RH
microsensor. The main processing steps of the RH sensor developed in this study
involve at least five photolithographic and four thin film deposition processes. The
influences of sensing area, number of electrode pairs and testing temperature on the
sensitivity and sensing linearity of humidity microsensors were investigated.
Based on the measurement results, the sensitivity apparently increase as well as
the sensing area (2 mm × 2 mm: 0.12 pF/%RH, 3 mm × 3 mm: 0.48 pF/%RH, 5 mm ×
5 mm: 1.09 pF/%RH), and decrease with the number of electrode pairs (40 pairs: 0.51
pF/%RH, 20 pairs: 0.4 pF/%RH) and increase with the testing temperature. The thesis
has demonstrated that the capacitance of the RH sensor vary from the relative
humidity with a very linear relationship (linearity: 98.8%~99.99%) over the range of
30~70%RH. Finally, to increase effectively the surface area and to reduce further the
hysteresis, three-dimensional (3D) moisture entrances and exits were designed and a
very low hysteresis value (0.5%RH) can be achieved.