Effect of etch-clean delay time on post-etch residue removal for front-end-of-line applications

Abstract

The benefits of integrating wet clean with plasma dry etch processes have been investigated. The studied applications included shallow trench isolation (STI), hardmask-based poly-silicon (poly-Si) gate, and nickel silicide (NiSi) contact etch. In particular, the novel technology Confined Chemical Cleaning™ has been evaluated using diluted hydrofluoric acid or an ammonia hydroxide–hydrogen peroxide mixture at short and controlled exposure times on the order of seconds. It was observed that the ability to remove post-etch residues using the same wet clean process diminished with increasing delay time between etch and clean, in the timescale of hours. In addition, a detrimental effect on the electrical performance was observed for the contact application. As shown, applying stronger cleaning conditions is one solution to remove residues (STI and poly-Si gate) or to restore the electrical performance (contact). However, the more aggressive residue removal process resulted in a higher substrate loss. The mechanism of the delay effect for the poly-Si gate application has been investigated. Evaluation of the post-etch residues using thermodesorption mass spectrometry revealed that post-etch residues were primarily inorganic in nature. Interaction of the post-etch residue and/or substrate with water vapor from the ambient environment is at the origin of the observed delay effects. The mechanism proposed is a hydrolysis of oxychlorine bound in the top layer of the residue, in combination with the dissolution of SiO2 residue material into silicic acid, resulting in a strengthening of the SiO2 network structure and hence increased resistance to wet cleaning. For the contact application, the electrical degradation of the contact resistance under “no clean” or “delayed clean” conditions was correlated with the presence of an oxide layer at the metallic barrier/NiSi interface. This interfacial layer was not present for wafers with the integrated clean, which showed low contact resistances and high yield.