We have investigated the etching of high aspect ratio holes (similar to 4 mu m deep, similar to 0.2 mu m diameter) in silicon using plasmas maintained in mixtures of SF6, O-2, and HBr or Cl-2 gases. The etching experiments were conducted in a low pressure (25 mTorr), high density, inductively coupled plasma etching reactor with a planar coil. Visualization of the profiles with scanning electron microscopy is used in conjunction with plasma diagnostics such as optical emission and mass spectroscopies to understand the key factors that control the feature profile shape and etch rate. HBr addition to SF6/O-2 mixture reduces the F-to-O ratio, increases sidewall passivation and reduces mask undercut. Addition Of Cl-2 to SF6/O-2 discharge also decreases the F-to-O ratio, but Cl-enhanced F chemical etching of silicon significantly increases the mask undercut and lateral etching. In both SF6/O-2/HBr and SF6/O-2/Cl-2 mixtures, reduction of O-2 flow rate and subsequent increase of the halogen-to-O ratio eventually results in significant lateral etching because of the lack of oxygen required to form a siliconoxyhalide passivating film on the sidewalls. (c) 2005 American Vacuum Society.