Profile control of high aspect ratio trenches of silicon. II. Study of the mechanisms responsible for local bowing formation and elimination of this effect

Profile control of deep and narrow anisotropic trenches in silicon has been investigated in a low-pressure high-density plasma reactor working with a SF6/O2 plasma mixture. The aim is to achieve deep trenches with high aspect ratio (depth/width>10), high anisotropy, high selectivity (silicon/SiO2 mask>300:1), high etch rate (>5 μm/min) and with no local bowing. This study focuses on a cryogenic etching method. In the previous article [M. Boufnichel et al., J. Vac. Sci. Technol. B 20, 1508 (2002)] we presented a parametrical study of the evolution of local bowing as a function of the process conditions. Here we present a complete study of the mechanisms involved in the creation of local bowing at the top of deep narrow trenches, and in particular the effect (influence) of the characteristics (nature, thickness, and side slope) on local bowing. Explanations for the appearance of local bowing are given and targeted experiments are performed to eliminate it. The final performance shows that we are now able to etch a minimum of 60 μm in depth for a 4 μm trench aperture with no local bowing at a high etch rate of about 5.5 μm/min with a cryogenic method in which no cleaning step is necessary unlike for the Bosh process [M. J. Walker, Proc. SPIE 4407 (2001)].