Deposition of micrometric particles on a capacitive sensing area

We have built a setup for selective deposition of well-defined spherical particles in order to evaluate the performance of a microsensor for the capacitive detection of particulate matter. [1] [2]
The deposition setup consists of an atomizer (Topas ATM 221) in combination with a diffusion dryer (Topas DDU 570) which disperses PSL particles of defined size, shape, and dielectric constant in air. Said particles are then neutralized (TSI Aerosol Neutralizer 3077) in order to exclude charge effects. A tube with an inner diameter of 0.41 mm acts as a nozzle to accelerate the particles towards the sensor. The sensor is placed underneath the tube in a distance of a few millimetres where the particles are then deposited via impaction. A needle valve in parallel to the nozzle allows for steady control of the flow rate through the nozzle. We were able to assign the detected events of the sensor to deposited particles with a digital microscope (Keyence VHX and VHZ-250R), see fig. 1.
Figure 1. 1 μm PSL particles deposited on the investigated capacitive particle sensor.
Particles induce a sudden change in capacitance of interdigitated combs of microelectrodes when deposited. Width and distance between the electrodes are both 1 μm for the investigated sensor. The capacitive change depends on size, shape, dielectric constant, and the exact position of the particles and is typically on the order of hundreds of zF or a few aF for microparticles. The investigated sensor has a resolution of 35 zF. Since the dispersed particles are uniform within an experiment, the only influence left is the position of the particles relative to the surface structure of the sensor. Results see figure 2.
As shown in fig. 2 and fig. 3, both the experimentally evaluated and the simulated sensor response cover a range from approximately 0.6 aF up to several aF. This relatively broad range is due to the sensitivity of the sensor to the position of the particle as the capacitance is stronger affected by a particle in between two microelectrodes than by particles on top of a microelectrode. Particles are assumed to be equally distributed over the sensing area, which leads together with the fact that the microelectrodes protrude over the SiO2 to a more likely and more frequently observed deposition on top of the electrodes. Experimentally a small number of larger capacitance jumps than predicted by simulation are observed, which is most likely due to inhomogeneities of the sensors surface. Overall, the experimental results match the simulation reasonably well.
Figure 2. Histogram of capacitance jumps induced by 1 μm PSL particles. .
The experiment was also modelled using Comsol multiphysics, results see fig. 3.
Figure 3. Simulated sensor response to 1 μm PSL particles depending on the position.
As particles of larger diameters than the distance between two electrodes cannot fall between the electrodes, the sensor can operate in two regimes: large particles are detected at the surface, smaller ones which would induce a low capacitance jump on top of the electrodes are detected in between the electrodes. This finding can be utilized to redesign the capacitive microsensor e.g. with a focus on the detection of PM2.5.

This work was funded by FFG grant 86197, Fondazione Cariplo through the projects MINUTE (No. 2011-2118) and ESCHILO (No. 2013-1760), and ams AG.

[1] P.Ciccarella et al. (2016) IEEE Journal of Solid State Circuits, Vol. 51, NO. 11, 2545 – 2553
[2] M. Carminati et al. (2014) Sensors and Actuators A: Physical, Vol. 219, 80 - 87