Detection of Micro/Nano Droplet By Galvanic-Coupled Arrays
Authors: Kawakita Jin , Chikyow Toyohiro 
Published: February 2020

When moisture such as water vapor, mist and fog is condensed on the surface of a metal in the atmosphere, it can cause atmospheric corrosion of the metal. Although sensors capable of detecting condensation of water are commercially available, they are intended to detect visible condensation electrically because the gap between their electrodes to detect droplets is generally more than 100 micrometers and most of the sensors are said to be a passive type that requires the signal input. On the other hand, atmospheric corrosion monitoring (ACM) sensor can evaluate the degree of corrosion environment regarding iron exposed in atmosphere, in more accurate words, the condensation state of droplet on the sensor, and furthermore it is classified into an active type so that it can provide a voluntary signal output on the basis of the galvanic action. The ACM sensor authorized by Japan Society of Corrosion Engineering has the gap with 20 micrometers between the dissimilar electrodes, which can be regarded as the detectable size of droplets condensed on the sensor. It is preferable to detect small size of the droplets in order to find initiation of corrosion. In this study, for the purpose of detection of condensation water droplets at a particle size less than 20 micrometers, a sensor was made, which had the gaps in micro/nano size between arrays of dissimilar metal and could obtain the galvanic current when water droplet is in contact on the gap as it bridged the arrays.
For the experiment, a silicon wafer of 3-inch diameter as the substrate, after providing an insulating layer on the substrate surface by thermal oxidation, was treated by combining photolithography and sputtering processes, to fabricate arrays, which had a structure of the comb-shaped gold and copper metals facing each other. The wafer processed was cut into chips with 5 mm in square. The gap between the adjacent arrays on one chip was set from 0.5 to 20 micrometer. The number of each metal arrays was from 50 to 200. The width of the array was 1 micrometer. When the water droplet was put on the arrays of the chip, the current obtained from the chip was measured by a semiconductor evaluation instrument and by a device which amplified an analog signal from chip, converted to digital one and collected as the time-current data. In addition, the surface of the chip was observed by using an optical microscope.
The experimental results showed that the current started to increase within 0.5 sec when the water droplet was contacted on the arrays of the silicon chip, suggesting the accurate monitoring of condensation initiation. This response time was remarkably shorter than the value around 10 sec observed with the commercial hygrometer of an electrical type. This is due to the detection method based on the galvanic action, which was caused at the moment the droplet was touched on the adjacent arrays like a bridge. On the other hand, the hygrometer needs a period for absorption of water in the reactive material to obtain the electric signal. In addition, the S/N ratio of response current was four orders of magnitude or more, indicating the sufficiently high sensitivity. This is because the background current should be theoretically zero in a dry state and current enhancement was easily possible by utilizing the scale merit which is well known in the semiconductor fabrication technology. In fact, the current per unit width of the arrays was improved with the decreasing of the gap size. Furthermore, on the arrays of the chip on which the current response was observed, for instance on the arrays with 0.5 micrometer in gap size, the optically microscopic image showed that the water droplets bridged the arrays, confirming the formation of galvanic cell in a micro scale. On the other hand, on the arrays with a gap of 10 micrometers, on which no response current was obtained, water droplets were also observed while no bridging droplet between the arrays was found. These facts suggest the arrays with different gap sizes can used to determine the size of droplet.
As a conclusion, a sensor was made, which detected the small water droplet with a particle size down to 0.5 micrometer at a high-speed response within 0.5 sec in a high sensitivity with the four orders magnitude of S/N ratio. This sensor can be used for detection of condensation of small droplet which might initiate corrosion of metals.

Figure 1