Etching of high aspect ratio features in Si using SF ₆/O ₂/HBr and SF ₆/O ₂/Cl ₂ plasma

We have investigated the etching of high aspect ratio holes (∼4 μm deep, ∼0.2 μm diameter) in silicon using plasmas maintained in mixtures of S F6, O2, and HBr or Cl2 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 S F6 O2 mixture reduces the F-to-O ratio, increases sidewall passivation and reduces mask undercut. Addition of Cl2 to S F6 O2 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 S F6 O2 HBr and S F6 O2 Cl2 mixtures, reduction of O2 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.