A novel CMOS sensor for measuring thermal diffusivity of liquids

This study aims at designing and developing a CMOS sensor chip for measuring thermal diffusivity of liquids. The CMOS sensor chip is realized by VIS 0.5 μm 2P3M CMOS process with maskless post-CMOS micromachinings. The procedure used to measure thermal diffusivity of liquids is to replace conventional methods that require a great deal of specimen and take a long measuring time. The CMOS chip consists of a thin film of polysilicon covering a cavity of the substrate. The thin film is equipped with a rectangular centered heater and four temperature sensors that are located at different locations from the heater. Once a known voltage is applied to the heater, the thermal diffusivity of the liquid drop that spreads over the heater and the temperature sensors can be determined from the measured temperature responses of the temperature sensors. Different kinds of tested liquids result in different temperature variations. The temperature variation corresponds to the resistance variation of temperature sensors. Three kinds of liquids were measured in this study. The conclusions drawn from the experimental results are as follows: high thermal diffusivity of a liquid causes large heat flux in vertical direction and small heat flux in horizontal direction, thus there is small resistance variation of the temperature sensor. In addition, liquid with high thermal diffusivity gives the experiment system a large time constant. Thermal diffusivities of other liquids can be measured based on a relation between time constant and thermal diffusivity derived from the tested liquids.