Paper PS1-TuA7
Plasma-induced Damage Reduction in Porous SiOCH Dielectrics by Replacement of H₂ and N₂ by CH₂F₂ and Ar in Fluorocarbon Based Plasmas

Tuesday, November 1, 2011, 4:00 pm, Room 202

With the scaling of devices and related interconnects, the integration of dielectric materials with a k-value lower than 2.5 is greatly required. In particular hybrid porous SiOCH low-k materials are considered as promising candidates. However, such low-k dielectrics generally have a poor resistance to plasma damage due to their high connected porosity. It is therefore important to develop less damaging etching chemistries as well as more resistant low-k materials. During the discharge, ion bombardment, active radicals and VUV radiations are responsible for the degradation of the low-k. Mainly, -CH₃ groups are removed from the film resulting in its hydrophilization and consequent moisture uptake which has a severe impact on the final k-value after patterning.

In order to limit the damage during plasma etch, fluorocarbon-based chemistries allow a thin protective etch polymer film to be deposited on the surface of the low-k film during plasma etch and therefore effectively prevents possible damage by penetration of active radicals. We have evaluated the etching of a SiOCH dielectric with a k value of 2.0 using CF₄ in combination with either C₂H₄ or CH₂F₂ and using H₂, N₂ or Ar as tuning gas. The plasma damage has been evaluated by measuring the loss of –CH₃ groups and –OH uptake by FTIR and by measuring the k-value variation by forming MIS planar capacitor. Starting from a CF₄/H₂/N₂ plasma, it has been found that the damage is reduced by substituting H₂ by N₂. H is able to diffuse deep into the low-k film and probably remove –CH₃ groups by forming CH₄. Switching to a CF₄/C₂H₄/N₂ plasma further allows to decrease the damage, most probably through the formation of a thicker passivation layer on the low-k. Plasma damage can be further reduced by replacing N₂ by Ar, leading to a CF₄/C₂H₄/Ar plasma. Possibly, N₂ is causing damage by extracting C from the low-k film by forming HCN by-product. However the replacement of N₂ by Ar resulted in a reduction of the etch rate which could be recovered by finally replacing C₂H₄ by CH₂F₂ while maintaining a low damage level.

Finally, 90nm half pitch trenches have been patterned into the SiOCH dielectric film using a dual hard mask approach (spin-on glass and spin-on carbon films). The influence of the hard mask on the plasma composition required optimization of the etch conditions in order to obtain straight profiles. Integrated k-value after complete processing will be discussed.

In conclusion, we have showed that H₂ and N₂ were responsible for the low-k damage during etch in flurocarbon based plasmas. Switching to a CF₄/CH₂F₂/Ar plasma allows to reduce damage while maintaining good patterning capability.