PRACTICAL APPLICATION OF A FRACTIONAL FACTOR EXPERIMENT FOR ASSESSING THE INFLUENCE OF FACTORS ON THE SENSITIVITY OF A CAPACITIVE HUMIDITY SENSOR OF A TWO-LAYER STRUCTURE

Abstract

The work uses a fractional factorial experiment in order to reduce the number of experiments. Capacitive humidity sensors of a two-layer structure made on a sitall substrate served as experimental samples. Hygroscopic salt NaCl was used to create a moisture-sensitive layer that would perform the function of a dielectric. Solutions of this salt with concentrations of 0,89 mol/l and 5,33 mol/l were applied by spraying on the surface of capacitive humidity sensors with thicknesses of 5,0 μm and 10.0 μm. In order to prevent the drop of the dew point, a protective layer was created − it is a moisture-absorbing polymethyl methacrylate film with thicknesses of 40 μm and 80 μm. According to the Cochrane’s test, it is proved that the fractional factorial experiment is reproducible. Using regression analysis, a quantitative assessment of the influence of factors on the sensitivity of the capacitive humidity sensor of the two-layer structure was carried out. Based on the developed regression equation, it was established that the concentration of the NaCl salt solution significantly affects the sensitivity of the capacitive humidity sensor of the two-layer structure. Significant coefficients of the regression equation were determined using the Student's test. Adequacy of the adjusted regression equation to the results of the factorial experiment was proved using Fisher's test. Based on the regression equation on the scale of real factors, which contributes to the optimization of the humidity sensor manufacturing parameters, it was established that the sensitivity of the capacitive humidity sensor of the two-layer structure depends on such factors as the thickness of the protective layer and the concentration of the NaCl salt solution. However, the sensitivity of the capacitive humidity sensor of the two-layer structure is significantly affected by the concentration of the hygroscopic NaCl salt solution. The highest sensitivity of 2,46 nF/% is provided with the following optimal parameters of the manufacturing process of the capacitive humidity sensor of the two-layer structure: the concentration of the hygroscopic NaCl salt solution is 5,33 mol/l, the thickness of the protective layer is 80 μm. Considering that the thickness of the moisture-sensitive layer does not significantly affect the sensitivity of the capacitive humidity sensor, the thickness of this layer can be taken in the range from 5,0 μm to 10,0 μm.