Origin, control and elimination of undercut in silicon deep plasma etching in the cryogenic process

The aim of this work is to demonstrate the ability of our system to etch deep high aspect ratio trenches (HART's) with a high etch rate (>5 μm/min), high selectivity, no local bowing [M. Boufnichel, Gravure profonde cryogénique du silicium dans un réacteur ICP utilisant une chimie SF6/O2. Application pour la mise au point d'un procédé d'isolation électrique sur plaquette SOI, Thesis of the University of Orléans in France, defended on December 2002; M. Boufnichel, S. Aachboun, F. Grangeon, P. Lefaucheux, P. Ranson, J. Vac. Sci. Technol. B 20 (4) (2002) 1508-1512; M. Boufnichel, S. Aachboun, P. Lefaucheux, P. Ranson, J. Vac. Sci. Technol. B 21 (1) (2003) 267-273] and with a perfect mask pattern transfer on silicon, which means no undercut. The process of plasma cryogenic dry etching can be considered as the key technology for micro- and nano-engineering as in the case of, e.g., trench capacitors, and trench isolation for vertical transistors. The most important motivation of this work is advancement of the trench cryogenic technology. The basic aspects of pattern transfer of the prepared mask-patterns by plasma etching into the substrate, considering plasma chemistry, gas flow, substrate temperature, pressure, bias voltage, RF source power, mask nature and thickness will be presented. Special attention will be given to discuss the physical and/or chemical phenomena, which are involved in the generation of undercut in features with high aspect ratios. Sidewall etching of ntype silicon in ICP SF6/O2 discharges is completely suppressed by cooling the sample to about 100 °C during process and using an over-passivation step at the beginning of the process.