Application of magnetic neutral loop discharge plasma in deep silica etching

The electron temperature Te is one of the key parameters for process plasma because the decomposition of most reactive gases depends on the kinetic energy of electrons in the plasma. Pressure is another important parameter in the etching process for microelectromechanical systems (MEMS). Low pressure can avoid etch product substrate redepositing by reducing the collision between neutral particles and etch products in the gas phase. Also, low pressure may reduce the scattering of incident ions in the sheath that may reduce the negative taper angle for trench etching. Therefore, this study is focused on low pressure (<0.67 Pa), low Te plasma production for optical MEMS etching processes. To reduce the Te and keep the high density of the plasma, use of a parallel turn antenna was proposed and it was applied in magnetic neutral loop discharge plasma, where the Te is desirably reduced to about 2.5 eV while the density is about 1.2×1011 cm−3 at pressure of 0.2 Pa. With this improvement in plasma production, fused quartz and chemical vapor deposition SiO2 were successfully etched in a trench 5–40 μm deep at a high etch rate of over 500 nm/min. The vertical angles are about 90° and the surface roughness is less than 50 nm as evaluated by a scanning electron microscope picture, where Cr, WSi and Si were used as hard masks of SiO2 in order to achieve the selectivity required.