Development of Measurement of Two-Dimensional Distribution of Strength of Electrical Field with High Spatial Resolution Using Optical Trapped Particle in Plasma

Introduction: High-precision nanofabrication based on plasma processing has been one of the main technology drivers of digital society. Development of highly sensitive diagnostic methods in process plasmas is imperative for understanding and controlling interactions between the materials and plasma. A diagnostic method using few dust particles in plasma is a possible solution of this problem. For faster development of ultra-precision nano-fabrication methods, effects of plasma fluctuations in reactive plasma must be taken into account, since such fluctuations affect the growth of nanostructures. Here we measured the strength and fluctuation of electric field in Ar plasmas using laser tweezers [1].<br/>Experimental: A plasma reaction vessel with a quartz window on the top and a sapphire window on the bottom was used in the experiments. It was set in an epi-illumination microscope. A perforated metal ground electrode was placed in the center of the vessel, and a ring-shaped electrode with an inner diameter of 15 mm and an outer diameter of 25 mm was placed on the bottom of the vessel. A high-frequency voltage of 13.56 MHz was applied between the electrodes to generate plasma in the vessel. When an acrylic particle of 20μm in diameter was introduced into plasma, it was suspended near the plasma/sheath boundary. A single particle was trapped with the laser tweezers and moved horizontally with the laser light until the particle was de-trapped.<br/>Results and Discussion: We measured the levitation positions of the laser-trapped fine particle in Ar plasma at 60 Pa for each laser power (10.4 ~ 27.3 mW). At the levitation position, the electrostatic force and the laser light force on the particle are balanced by the gravity. The force of the laser on the particle was obtained from an ray optical model [2], and a particle charge was deduced from Orbit Motion Limited (OML) model [3]. Therefore, we deduced vertical electric field strengths Ez from these derivations. We investigated 2D profile of Ez with a range of 3 ×10³ and 4 ×10³ [V/m] in plasma, deduced from the force balance of an optically trapped particle. Similarly, we derived horizontal electric field strength Er form the balance of forces. As the results, we investigated 2D profiles of electric field vector with high spatial resolution (micro meter order) using optical trapped particle in plasma.<br/>We will discuss details at the conference.<br/>Acknowledgements<br/>This work was partly supported by JSPS KAKENHI (Grant No. JP19K03809 and JP20H00142) and JSPS Core-to-Core Program (Grant No. JPJSCCA2019002).<br/>References<br/>[1] A. Ashkin, Biophys. J. 61 (1992) 569.<br/>[2] Philip H. Jones, et al., Optical Tweezers Principles and Applications, 22 (Cambridge university press, 2015).<br/>[3] G. Paeva, Sheath phenomena in dusty plasmas, (Technische Universiteit Eindhoven, 2005).