Chemical Mechanical Polishing of Cobalt and Post-CMP Cleaning of Silica and Ceria Particles
Charith Ranaweera and Prof. S.V. Babu
Abstract
Chemical mechanical planarization (CMP) is the only process capable of achieving global planarization for both dielectric structures and metal interconnects that are integral parts of modern ICs. Recently, Co came into prominence as an alternative metallization for Cu interconnects in M1 and M2 levels. Suitability of ammonium persulfate (APS) and potassium oleate (PO) containing silica dispersions was investigated for polishing A slurry consisting of 3 wt% silica, 1 wt% APS and 0.2 mM PO produced a removal rate of ∼465 nm/min at pH 9, along with removal rate selectivity of >100:1 between Co and TiN. The same composition but without abrasives reduced the ΔEcorr between Co and TiN to ∼7 mV and Igc to ∼0.04 μAcm−2, indicating minimal galvanic corrosion. Although CMP brings many advantages to semiconductor processing, it also creates challenges such as particles contamination on post-polished wafer surfaces. PVA brush cleaning of contaminated Co and underlayer films, TiN, SiN, and SiO2 was studied. Silica dispersions without any additives readily contaminated the Co film, with the number of adsorbed silica particles decreasing as the pH increased from 3 to 12. However, the other three types of films remained uncontaminated. When a common complexing agent (50 mM citric acid) and a metal ion source (50 mM KNO3) used in Co CMP slurries were added to the silica dispersion, particle contamination on SiN and SiO2 films was increased.
We investigated the interactions among the ceria particles, PVA brush, film, and cleaning liquid from real-time video imaging of the brush cleaning by combining the brush setup with an evanescent wave (EW) microscopy system. It was found that purely hydrodynamic forces are unable to remove the ceria particles from the oxide film surface and direct contact between the solid regions of the brush nodule and film is crucial for particle detachment to occur. However, such a direct contact also causes the redeposition of some of the already dislodged particles. Preventing such redeposition is crucial for efficient surface cleaning. Also, the contact condition between PVA brush nodule surface and SiO2 substrate is investigated after the modification of the brush surface by removing the skin layer again using the same brush setup coupled with the EW microscopy system. When a brush nodule was rotated on the substrate at a speed of 50 rpm for 3 seconds, the total contact area of the skin layer brush surface was ~33% higher than the skin-free brush surface. Brush surface modification is one of the methods that can reduce cross-contamination and particle redeposition. The force required to move (FLM) a ceria particle adhered on the SiO2 surface was measured using lateral force microscopy. The force measurements were performed on ceria particles on polished and dipped contaminated films with and without proline as an additive in the ceria slurries. The adhesion of ceria particles on a polished surface is two times stronger than that of ceria particles on a dipped contaminated surface. In our study, the ceria particles modified with proline on a polished surface are the most challenging contaminant to remove, with a FLM of 176 nN. Once the binding between the ceria and oxide surface is broken, a particle can be easily moved during the subsequent manipulations by applying ~21 nN.
Location: MAE 151 conference room
Date: September 30th
Time: 11:00 am
PhD student: Charith Ranaweera
Advisor: Prof. S.V. Babu
Committee members: Prof. Silvana Andreescu
Prof. Sitaraman Krishnan
Prof. Jihoon Seo
Prof. Panart Khajornrungruang