Measurement of Negative Carbon Ions near a Plasma Deposited Carbon Thin Film by Laser Photodetachment
Plasmas to prepare functional carbon films contain negative carbon ions which should modify the sheath structure on the formed films. The density distributions of carbon negative ions in the sheath region around the carbon film must critically determine the property of the film. A DC photodetachment method was utilized to measure negative carbon ion density in the plasma formed by discharge between a carbon hollow cathode and a hollow anode.
A DC glow discharge was maintained between carbon electrodes of cylindrical 19 mm inner diameter, 23 mm outer diameter and 45 mm long. These electrodes were placed 14 mm apart facing each other. The chamber was made of electrically insulating borocilicate glass. A Langmuir probe is inserted into the chamber in the direction perpendicular to the carbon electrode tubes. The position of the probe tip can be adjusted in the direction along the probe. A glass substrate for a thin film deposition was set near the carbon cathode. By using a 808 nm (1.53 eV) semiconductor laser exceeding the electron affinity of negative carbon ions (1.27 eV), a signal corresponding to the local negative ions density was measured to construct the spatial distribution of negative ions. The laser passed through the axis of the carbon cylindrical electrodes. The photodetachment signal induced onto the discharge current was detected with a two-phase lock-in amplifier synchronized to the amplitude modulation of the laser power. The effect on the formed film due to the presence of negative ions in the sheath region is investigated through X-ray diffraction and scanning electron microscope.