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In order to integrate porous dielectric materials into the next generation of Cu/low-k interconnect, the porous material has to be sealed against metal barrier precursor. We have reported pore sealants which forms ultra-thin (< 3 nm-thick) layer on top of the surface of porous low-k film while the pore sealant does not diffuse into pores. In this study, it was investigated how pore seal layer is formed on the surface of porous material and how pore mouths are sealed by pore seal layer. It was found that 1) thickness of the pore seal layer is well-controlled in the range < 5 nm, by varying spin rate and concentration of solid, 2) minimal thicknesses of the pore seal layer needed to achieve an efficient sealing for porous low-k films whose pore radius is 1.5 nm was 2.6 nm. 3) Larger pores, whose pore radius is 4.2 nm, were sealed completely with an expansion of our technology.
Pore sealing has become a critical issue for the implementation of porous low-k dielectrics and for realizing acceptable reliability performance of the interconnect. This study focuses on fabrication of ultra-thin, conformal and plasma resistant pore seal layer and on understanding parameters playing a role in sealing the surfaces of porous low-k films. It was found that 2.5 nm-thick pore seal layer shows a perfect toluene seal property for the porous low-k film whose pore radius is 1.48 nm. The pore seal layer still show a good toluene seal property after irradiation of He plasma at 250°C for 10 sec. The increments of dielectric constant by applying the pore seal layer and by the He plasma irradiation for 10 sec are 0.04 and 0.03, respectively. Interestingly, all of toluene seal property, refractive index of the bottom part of the film and dielectric constant started to deteriorate after irradiation of He plasma for 20 sec. It was suggested that when toluene seal property degrades, plasma would start diffusing into pores and both refractive index of the bottom part of the film and k value start to increase.
It was found for the first time that the control of the size of pore sealant is important to prevent diffusions of pore sealant into pores of porous low-k films and to achieve a good toluene seal property. Two pore sealants (PS-A, B) were prepared and the seal property and porous structure were studied using toluene based ellipsometric porosimetry (EP) measurements. It was revealed that small pore sealant (PS-B) diffuses into pores of porous low-k (PLK) films and did not show any seal property, while large pore sealant (PS-A) does not diffuse into pores of porous low-k films and shows a good toluene seal property. Ellipsometry shows that PS-A forms conformal layer only on the vicinity of surface of porous low-k films, but porous structure of porous low-k films at the bottom part is kept, according the fact that the refractive index did not increase.
Furthermore, we developed a new pore seal material (PS-C) to form ultra-thin conformal layer by a single pass, which shows a good toluene seal property. The dielectric constant increased from 2.10 to 2.25 by covering with PS-C. The obtained layer also shows the effect as the protect layer of porous low-k films from plasma damages.
We focused on detailed evaluations of properties of the ultra-thin pore-seal layer (< 3 nm-thick), such as Cu diffusion barrier property and thermal stability. Cu diffusion into dense thermal silica and porous silica low-k which are covered with the pore seal layer was evaluated using metal-insulator-semiconductor (MIS) capacitors under bias thermal stress (BTS). Triangular voltage sweep (TVS) measurement shows that the ultra-thin layer on dense thermal silica suppresses the drift of Cu ions. The Time-Dependent Dielectric Breakdown (TDDB) lifetime of porous silica low-k covered with the ultra-thin pore seal layer results in a drastic increase of the capacitor lifetime with respect to the no-pore-seal control system (stable at 125 °C at least for 10000 s). Thermal decomposition of bulk material of the pore sealant was measured by thermal gravity (TG) test in nitrogen. Bulk material did not decompose through around 350 °C. The amount of ultra-thin pore seal layer fabricated on silicon wafer after thermal cycle stress in vacuum was measured by x-ray photoelectron spectroscopy (XPS). Amount of pore sealant did not decrease even after 2 cycles of 20 min, at 250 °C. Those results show that the ultra-thin layer, which we propose here, has a potential as a pore seal layer for porous low-k films.
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