A cryogenic etching method with SF6/O-2 chemistry plasma in an Alcatel inductively coupled plasma reactor is used to achieve deep trenches with high aspect ratio (depth/width > 10) and high anisotropy. The mean etch rate in 2 mum wide and 100 mum deep trenches is about 3.5 mum/min. The slope of the trenches can be adjusted from 88degrees to 90degrees by selecting appropriate process conditions and selectivity (silicon/SiO2 mask) is higher than 400:1. However, profiles still present a defect: they need to be improved, mainly by reducing the local bowing effect. Local bowing is lateral etching located on the sidewalls and resulting in profiles destruction. This article deals with the study of the local bowing and profile improvement by changing process parameters. We investigated the effect of the process parameters (pressure, bias voltage, temperature, gas flow rates, etc.), and the mechanisms responsible for local bowing are discussed and evaluated. The final aim is to find the cause of the appearance of local bowing and thus the means to reduce or eliminate it. We especially showed that local bowing depends on the efficiency of the trench passivation mechanism and on ion energy and density: ions seem to be responsible of the destruction of the passivation layer.