Angular distribution of particles sputtered from Si bottom in a CHF3 plasma

Abstract

The angular distribution (AD) of particles sputtered from a Si substrate in a CHF3 plasma at bias voltages between −200 and −400V was investigated using a specially designed experimental setup for controlling the angle of incident ions on the substrate. Particles were sputtered from a primary target substrate, which was placed in a Faraday cage and on the horizontal cathode plane of a plasma etcher, by bombardment with ions incident in a direction normal to the substrate. The sputtered particles were redeposited on the surfaces of SiO2 secondary targets, which were fixed in small pieces at different positions on the convave surface of a circular sample holder, which was positioned above the primary target. A line connecting the primary and secondary targets defined the sputtering angle of the etch products. The redeposition rate was estimated from the difference in the thickness of the secondary target in two independent experiments, with and without the primary target. The redeposition rate was plotted as a function of sputtering angle and, the AD was then obtained from the plot. X-ray photoelectron spectroscopy analysis of a Si primary target subjected to etching indicated that a steady-state CFx film with a thickness of about 50Å was formed on the Si at a bias voltage of −400V. This film was thicker than the ion-energy transfer depth (30Å), and, as a result, it can be concluded that particles redeposited on the secondary target were sputtered from a steady-state CFx film that had been formed on the Si primary target. The AD of particles sputtered from the Si primary target showed an over-cosine dependence on the sputtering angle (θ), corresponding to cos3–4θ, and the power of the over-cosine dependence increased with bias voltage. The characteristic changes in the AD support the view that particles contributing to the redeposition were generated largely by physical sputtering rather than by ion-enhanced chemical etching.