Investigation Of Ions Accelerated Through Electrostatic Menisci In An Inductively Coupled Plasma

Plasmas are used in semiconductor fabrication as they allow for very precise control over processes such as etching and doping. This is achieved by extracting a beam of ions from the plasma to interact with and modify the surface of a silicon wafer. However, conventional fabrication methods are reaching spatial limitations as semiconductor features reach the atomic scale. Therefore, in order to better control the fabrication processes and facilitate the transition to three-dimensional architecture, a greater understanding of ion beam formation is needed. Ion beams are extracted at the boundary between the Debye sheath and an externally applied potential, which forms a unipolar sheath. This boundary, known as the plasma meniscus, is dependent on source parameters and acts as an electrostatic lens for ions that traverse it. This allows for control of ion beam properties through the adjustment of the source parameters that affect the meniscus. Presented here is an investigation into the plasma meniscus and the dependence of its topology on controllable source parameters. The plasma meniscus is formed by graphite extraction optics with a 5 mm square aperture for beam extraction. 12 mm from the aperture is an electrically isolated graphite wafer that is biased to different potentials. Laser induced fluorescence is employed to obtain ion velocity distribution functions: inside the inductively coupled plasma source, at the extraction aperture, and in the downstream ion beam. The use of the confocal telescope allows for first, non-perturbative measurements of ions inside an inductively coupled plasma source. The ion source power is varied (Pf = 1 kW, 2 kW, 3 kW, 4 kW) at different applied wafer bias voltages (Vb = 0 V, 1000 V, 2000 V, 3000 V). Ion temperature, velocity, and relative density are calculated from the ion velocity distribution functions. The ions\u27 speed increases as they travel through the source and form the beam. Additionally, there is a second population of ions that appears near the plasma meniscus. These ions form a beam halo, which hinders the creation of a uniform ion beam. This effect is mitigated at a sufficiently high bias voltage