Interdigitated capacitance sensors in the mm scale with sub-femtoFarad resolution suitable for monitoring processes in liquid films

Abstract

We propose and analyze a high resolution capacitive sensor appropriate for monitoring physical or chemical processes in liquid films. The proposed sensor is based on a planar interdigitated capacitor with planar electrodes of dimensions in the milimeter scale. The electric field between electrodes extend above the plane of the electrodes up to a few milimeters and thus permits placing a dielectric container with a liquid film on top and still be sensitive to changes in the liquid film. First, we present numerical calculations of the capacitance of an interdigitated sensor as a function of the thickness and dielectric constant of a film placed on top of it using a finite element method. Then, we describe a suitable electronic system to sense with very high-resolution capacitance variations of the sensor by measuring the phase lag and amplitude change of a sinusoidal current signal passing through it. This is accomplished by subtracting a stable sinusoidal current of the same frequency going through a reference device. Initially the system is balanced by adjusting the reference current to cancel out the net output current. In this way, we compensate parasitic capacitances due to electronics, wiring and system hardware as well. When the capacitance of the sensor element varies the system gets unbalanced and a net current appears. The resulting current is measured with a locking-amplifier. To illustrate the sensitivity and resolution achieved by the sensing system, we present results of monitoring the capacitance of the sensor during the evaporation of liquid solvent films and discuss the time-evolution of the registered signals. The floor noise throughout the measurements was in the order of 50 atto-Farads while the signal varied in the range of several femto-Farads.