Measurements of strength and fluctuation of 2D electric fields in plasmas using a fine particle trapped with laser tweezers*

There is a strong need for processing of materials at nanometer scale to allow the continuous miniaturization of devices. Plasma processing is mainly used to manufacture these devices. The measurement of sheath electric field in a microscopic space is an important issue for optimizing plasma processing of materials. For example, small changes and fluctuations in the electric field have a significant impact on etching and deposition into high aspect ratio micro-/nano-structure. However, few reports have been published on sensitive measurements of electric field distributions with high spatial resolution on the micrometer scale. Therefore, in this study, we have investigated such sensitive measurements of strength and fluctuation of electric field in plasma using optically trapped fine particles by a laser-tweezer technique [1]. 

To generate Ar plasmas, a rf voltage with a frequency 13.56MHz was applied between a powered ring electrode and ground planar electrode. A fine particle (a diameter of 20 µm) was injected into the plasmas. The laser trapped fine particle was suspended at plasma/sheath boundary region. At the levitation position of a fine particle, the electrostatic force and the laser light force on the particle were balanced by the gravity. The force of the laser on the particle was obtained from a ray optical model [2], and a particle charge was deduced from Orbit Motion Limited model with ion collision [3]. Because the trapped particle was negatively charged, it can be a probe of electric fields. Therefore, we deduced vertical electric field strengths Ez from these derivations. Moreover, strengths and fluctuations of horizontal electric fields Er were deduced by deriving horizontal force balance. As the results, we investigated 2D profiles of electric field vector with high spatial resolution in micrometer scale. Details are shown in the conference.