Position Fluctuation of a Fine Particle Trapped with Laser Tweezers in Ar Plasma

High-precision nanofabrication based on plasma processing has been one of the main technology drivers of modern information society. Plasma etching of high aspect ratio holes in semiconductor manufacturing often suffers from fluctuation of feature profile evolution. Precise control of ion trajectory is the key to suppression if such fluctuation. The momentum of ions impinging to substrates is determined mainly by the sheath electric field. To obtain information on sheath electric field fluctuation, in this study, we measured position fluctuation of a fine particle trapped with optical tweezers in Ar plasma [1, 2].<br/>A plasma reaction vessel with a quartz window on the top and a sapphire window on the bottom was used in experiments. The vessel was set up in an epi-illumination microscope. A metal mesh grounded electrode was placed in the center of the vessel, and a ring electrode with an inner diameter of 15 mm and an outer diameter of 25 mm was placed on the bottom of the vessel. Gas pressure was 60 Pa. A high-frequency voltage of 13.56 MHz and Vpp of 100 V was applied between the electrodes to generate plasma. When an acrylic particle of 20 µm in diameter was introduced into the plasma, it was suspended near the plasma/sheath boundary above the powered ring electrode. The position fluctuation of a fine particle trapped with optical tweezers in plasma was recorded with a high-speed camera. The position of a fine particle was tracked by an image analysis software.<br/>A single particle was trapped in center of plasma with optical tweezers. The position of the fine particle fluctuates. First, we observed the position fluctuation for 5 s as a parameter of the laser power. The magnitude of position fluctuation decreases with increasing the laser power. The magnitude of position fluctuation (~0.2μm) for the laser power of 27.3 mW is about 1/6 the magnitude of one (~1.2μm) for the laser power of 4.8 mW. The force of the laser light exerted on the particle was deduced using a ray optical model [3]. Second, we moved the optical trapping fine particle horizontally until it was de-trapped. The magnitude of position fluctuation increases as the particle approaches the de-trapped point. The magnitude of position fluctuation in the same direction of particle movement is larger than ones in other directions. The center position of fluctuation shifts to the opposite direction to the direction of particle movement. It suggests that the horizontal electric field increases as the fine particle approaches the de-trapped point. Finally, in order to investigate fluctuation of electric field in plasma, we measured time evolution of levitation position of the fine particle in Ar plasma using amplitude modulation discharge voltage. The fine particle oscillates at the modulation frequency, showing the axial electric field fluctuates at the modulation frequency. The magnitude of levitation fluctuation decrease with increasing the modulation frequency from 1 Hz to 50 Hz. Details will be reported at the conference.<br/><b>Acknowledgements</b><br/>This work was partly supported by JSPF KAKENHI Grant No. JP19K03809 and JP20H00142.<br/>References<br/>[1] A. Ashkin, Biophys. J., 61 (1992) 569.<br/>[2] T. Ito, et al., J. Phys. Conf. Ser., 518 (2014) 012014.<br/>[3] P. H. Jones, et al., Optical Tweezers Principles and Applications, 22 (Cambridge university press, 2015).