Etching high aspect ratio structures in silicon using sulfur hexafluoride/oxygen plasma
Plasma etching of high aspect ratio structures in Si is an important step in manufacturing capacitors for memory devices and integrated components of microelectromechanical systems. In these applications, the goal is to etch deep features anisotropically with high etch rates and selectivities to the mask while maintaining good uniformity and reproducibility. This study investigates the etching of deep sub-half-micron diameter holes in Si using SF6/O 2 plasma. Etching experiments and plasma diagnostics are combined with modeling to gain a fundamental understanding of the etching and passivation kinetics and mechanism necessary in developing and scaling-up processes.
Etching experiments are conducted in an inductively coupled plasma reactor with a planar coil. The substrate electrode is biased with a separate rf power supply to achieve independent control of the ion flux and energy. The effects of pressure, rf-bias and SF6-to-O2 ratio in the feed gas on the etch rate, selectivity and feature profile shape are studied using Si wafers patterned with 0.35 μm-diameter holes in a SiO2 mask. Visualization of profiles using scanning electron microscopy is complemented by plasma diagnostics such as mass spectrometry and actinometry.
Simultaneous with experiments, reactor-scale and feature-scale models are developed to quantify the etching and passivation kinetics and identify the important kinetic parameters that affect feature profile evolution. Information from plasma diagnostics and previously published data are used to reduce the degrees of freedom in the model. Experiments are designed to directly measure kinetic parameters such as the chemical etch rate constant and the incidence angle dependence of the etching yield. Experimentally inaccessible parameters such as the sticking coefficients, etching yield and ion scattering parameters are determined through feature profile simulation.
The key internal plasma parameters that affect profile evolution are the F-to-O and F-to-ion flux ratios and the ion energy and angle distributions. Ion-neutral synergy results in a maximum etch rate with pressure. Increasing the applied rf-bias voltage increases the average ion energy, which increases the etch rate and improves feature anisotropy. The SF6-to-O 2 ratio in the feed gas determines the balance between etching and passivation, which controls the etch rate and feature profile shape.
0794: Materials science