Imaging of an Electret Film Fabricated on a Micro-Machined Energy Harvester by a Kelvin Probe Force Microscope

An imaging method for a highly charged material using a Kelvin probe force microscope (KFM) was developed. Energy harvesters generate electric energy from the surrounding energy sources for a stand-alone power supply for Internet-of-Things devices. An electret film is a key component in an electrostatic induction-type energy harvester. However, the evaluation method of the electret film remains a problem for microscale measurements. KFM can provide a microscopic surface potential image corresponding to charge distribution. KFM is generally used to measure low potential distributions below a few volts because the electrostatic force between the cantilever that holds the tip and the sample is negligible. A 50-
$\mu \text{m}$
-long tip probe was fabricated, which has a longer tip than any commercially available probes. The surface potential difference of 50 V with a step distribution was measured using the developed KFM with a long tip probe. A potassium ion electret film formed on the sidewall of a comb electrode in an energy harvester fabricated by microelectromechanical systems’ technology was measured using the developed KFM. The measured charging potential distribution was from −30.1 to −28.9 V in a measured area, which was close to the mean charging potential measured using a conventional method of −29.2 V. The developed KFM can contribute to the evaluation of highly charged micro-sized samples.