AVS 45th International Symposium
    The Science of Micro-Electro-Mechanical Systems Topical Conference Monday Sessions
       Session MM+PS-MoM

Paper MM+PS-MoM11
Pattern Shape Effects and Artefacts in Deep Silicon Etching

Monday, November 2, 1998, 11:40 am, Room 324/325

Session: MEMS Processing and Deep Si Etch Technology
Presenter: J. Kiihamaki, VTT Electronics, Finland
Authors: J. Kiihamaki, VTT Electronics, Finland
S. Franssila, VTT Electronics, Finland
Correspondent: Click to Email
Deep silicon etching in inductively coupled plasma (ICP) reactor offers high etch rate, nearly vertical profile, and good selectivity against most common masking materials. Crystal orientation independent ICP etching can replace area consuming KOH wet etch in many micromechanical applications. We have etched various test structures with patterns of different sizes and shapes, using different etch chemistries and etch times. The widths of etched patterns range from submicron to over 100 µm, the etched depths were up to 500 µm. Long narrow features are etched faster than wide short features, indicating three dimensional nature of RIE-lag. Aspect ratio dependent etch rate is also present, further complicating design rule - process interactions. The difference in etch rate of a rectangular hole with respect to a trench of same width increases with aspect ratio and can be up to 20%. Typically narrow trenches have positive angled sidewalls and as trenches get wider the profile turns into retrograde, which implies serious limitations to device design. Positive or vertical profiles can be achieved if etch rate is lowered to 1-2 µm/min. Amount of etchable area also affects profile. Coalescence of closely spaced trenches into a larger feature (due to retrograde profile and/or undercutting) causes both etch rate and profile to change. Large area structures connected to narrow trenches assist the etching of the narrow trenches over considerable distances. To fully utilise the benefits of ICP etching, the design rules must be tailored for each application, because of various pattern shape effects.