Paper PS-ThA4
Ultra-low k Dielectric and Plasma Damage Control for Advanced Technology Nodes (10-nm and Below)

Thursday, October 31, 2013, 3:00 pm, Room 104 C

The continuous decrease of the critical dimension together with the introduction of new porous low-k materials (k-value lower than 2.5) make plasma etch more and more challenging. Besides the morphological aspect (profile of the structure or bottom roughness), the degradation of the dielectric properties of the low-k film is another important point that needs to be understood and well-controlled. In this work, we compare and analyze the damage (loss of Si-CH3 groups and moisture absorption) caused by different types of fluorocarbon-based chemistries and we propose a new damage-free chemistry to pattern advanced low-k materials identified for the most advanced technology nodes.

In the first part of the study, the low-k film is exposed to a selection of few conventional C4F8-based chemistries. In all cases, a significant level of damage is observed and is mainly attributed to the diffusion of fluorine radicals coming from the fluorocarbon polymer layer deposited on the low-k surface. As fluorine cannot be suppressed from the discharge, two options are considered to reduce its concentration in the passivation layer. First, a less polymerizing gas like CF4 is used to replace C4F8 then a carbon-free molecule like NF3 is considered to fully modify the nature of the passivation layer. Both approaches led to a very low level of damage. However, all CF4-based chemistries show very low etch rate and exhibit a poor selectivity towards masking layers like TiN. In contrast, a much higher etch rate and a greater selectivity is observed when NF3 is used to replace C4F8. Concerning the damage, an extremely thin (~1-nm) but very hydrophilic carbon depleted layer is formed at the low-k surface and a rough surface appears while the etch front progresses. We characterized and understood these issues using FTIR spectroscopy, Auger analysis and AFM and we fixed both instabilities together by slightly adapting the chemistry. The optimized chemistry leads to a very low level of water absorption within an extremely thin and smooth damaged layer. Finally, a comparative study including k-value, surface roughness and composition of the damage layer using TOF-SIMS is presented applying our best C4F8-, CF4- and NF3-based chemistries on two potential low-k candidates for the 10-nm technology node.