• Kiihamäki, Jyrki

Abstract

The molecular-flow conductance of a high aspect ratio feature can limit the etching species arriving at the bottom of the feature and thus limit the etch rate. A simple conductance model can predict the etch rate of the time-domain multiplexed etchprocess with good results at moderate-aspect ratios (5–20) fortrenches, but at very high aspect ratios (>20) the conductance modelbreaks down. Other mechanisms are needed to explain the deceleration of etch rate and almost complete etch stop. In this article the reasons for etch stop at the bottom of deep features are discussed. Measurement results of deep silicon etching are presented. Very deep holes and trenches were etched into siliconto study the effect of process parameters. At moderate aspect ratiosthe bottom of the hole is nearly flat and the sidewalls are vertical. Athigh aspect ratio the sidewalls start to bow and the feature bottomturns into a sharp spearhead. The shape of the feature can have animpact on the step coverage of the passivation layer deposition during the passivation step and on passivation removal during the etch step, leading to excessive sidewall etching and reduced etch rate at the feature bottom. The nonzero sidewall reaction probability and flow conductance of tapered tubes were studied by Monte Carlo simulation. The main reason for deceleration of etching seems to be the loss of etchant species due to sidewall reactions combined with feature closure by ion-limited passivation polymer etching.

Topics

• Deposition
• impedance spectroscopy
• polymer
• simulation
• Silicon
• etching