AVS 50th International Symposium
    Plasma Science and Technology Monday Sessions
       Session PS+MM-MoA

Paper PS+MM-MoA6
Etching of High Aspect Ratio Structures in Si using SF@sub 6@/O@sub 2@ Plasmas

Monday, November 3, 2003, 3:40 pm, Room 315

Session: MEMS Etching
Presenter: S. Gomez, University of California, Santa Barbara
Authors: S. Gomez, University of California, Santa Barbara
J. Belen, University of California, Santa Barbara
M.W. Kiehlbauch, Lam Research Corporation
E.S. Aydil, University of California, Santa Barbara
Correspondent: Click to Email
Plasma etching of high aspect ratio (depth-to-width) structures in Si is a crucial step in manufacturing trench capacitors for memory devices, and integrated components for microelectromechanical systems (MEMS). We have investigated etching of deep features (~10 µm) with high aspect ratios (~50) using plasmas maintained in mixtures of SF@sub 6@ and O@sub 2@ gases. The etching experiments were conducted in a low pressure (5-80 mTorr), high density, inductively coupled plasma etching reactor with a planar coil to maintain the discharge and with radio frequency (rf) biasing of the substrate electrode to achieve independent control of the ion flux and ion energies. Specifically, we have studied the effects of pressure, rf-bias voltage and SF@sub 6@-to-O@sub 2@ gas ratio on the etch rate, feature profile and selectivity using Si wafers patterned with 0.5-0.35 µm diameter holes in a SiO@sub 2@ mask. Visualization of the profiles using SEM is complimented by plasma diagnostics such as optical emission spectroscopy in conjunction with actinometry and mass spectrometry to understand the key factors that control the anisotropy, selectivity and etch rate. Oxygen ionization and dissociation products (O and O@super +@) oxidize the feature sidewalls and help achieve anisotropic etching through the sidewall passivation mechanism. F-to-ion flux ratio and F-to-O flux ratio are found to be the important internal plasma parameters that determine the etch rate and anisotropy. The mask undercut and the slope of the sidewalls is determined by the F-to-O ratio in the plasma. Increasing the SF@sub 6@-to-O@sub 2@ ratio in the feed gas increases F-to-O ratio and makes mask undercutting worse because passivation by O atoms cannot keep up with chemical etching by F atoms. As F-to-O ratio is decreased, effective sidewall passivation by O atoms results in nearly vertical sidewalls. Further reduction in F-to-O ratio results in sidewalls that slope inwards towards the bottom of the feature.