ИСТИНА

Nowadays, porous organo-silicate low-k dielectrics with permittivity lower than 2.5 are under intense investigation for future interconnects. Their integration in back-end-of-line (BEOL) interconnect technological flow suffers from many issues, amongst them plasma damage caused by etching process. Several approaches for reducing the plasma damage were proposed, the most recent are Post-Porosity Plasma Protection/P4 (pores stuffing) and cryogenic etching. An alternative solution is based on pore walls passivation by selective deposition or grafting of permanent polymer filler, which suppress the need to remove it afterwards. This layer of polymer should be sufficiently thin to avoid too high increase of the intrinsic dielectric constant, but also provide enough protection against plasma reactive radicals. In this work the polymer grafting of pore sidewalls is studied as a protecting agent against processing damage. Polymethyl-methacrylate (PMMA), an improved polystyrene (PS-pro), and a tailored Plasma Damage Management polymer (PDM SHIELDTM UT002) are considered as potential candidates. PMMA and PS-pro show non-homogeneous grafting properties, while PDM coats the pore walls uniformly through the bulk of the porous low-k film. A k ~ 2.2 porous spin-on glass is used as vehicle for processing damage study. Around a monolayer is grafted on the pore walls, leading to a k-value increase up to △k ~ 0.2. Using grafted PDM, the porous low-k chemical stability, in 0.5% diluted HF, is significantly improved. Concerning plasma damage, at constant etch depth, methyl depletion is also significantly decreased after etching in TCP discharge (Ar, O2, CF4 plasmas), and mainly in CCP discharge (Ar/CF4 plasma which is typical for SiO2 – like materials etching) showing high polymerizing character, leading to similar damage depth as found for a reference OSG 2.55 low-k. Moisture uptake is however not improved, leading to significant drift of dielectric constant. To get better understanding of PDM protection during the plasma etching the specially designed downstream ICP set up was used. It is well known that the main damage of low-k materials comes from fluorine radicals and photons in vacuum ultraviolet range. PDM protection from F radicals only, VUV photons only and their joint effect was studied. Absolute values of photons and radicals coming on the samples surface were measured so that not only qualitative but also a quantitative analysis of PDM protection was performed.