In-plasma, vacuum ultraviolet photon-assisted etching of silicon*

In-plasma photo-assisted etching (PAE) of p-type Si (100) in a Cl₂/Ar inductively-coupled plasma (ICP) was investigated. Experiments were carried out in a tandem reactor that allowed predominantly 104 and 106 nm Ar emission from the upper He/Ar ICP to enter the lower, main Cl₂/Ar ICP through a mesh that prevented plasma transmission. With ion energies below the ion-assisted etching threshold in the Faraday-shielded main ICP, VUV photons are found to be remarkably efficient in promoting etching, with measured yields of over 100 Si atoms etched per photon. Masked samples were found to etch with no undercutting and smooth sidewalls with angles of 125^o, corresponding to (111) planes. It was found that PAE in the main ICP was enhanced by up to 50% by simultaneous operation of both the upper and lower ICPs. When only the upper ICP VUV source was operated, irradiating the substrate in the lower chamber in the presence of Cl₂, no etching occurred. Energetic ion bombardment obtained by rf-biasing the substrate in the main ICP suppressed PAE, as did the addition of trace O₂. The PAE mechanism is believed to be due to the formation of hot holes (h) that photo-catalyze etching due to the hole-weakened Si-Si bonds. Ion bombardment creates e-h recombination centers and loss of holes, while O suppresses the formation of volatile products.