TiO2 Abrasive for Dielectric CMP Application

Ceria (CeO₂) was demonstrated to have the highest polishing rate of glass than the other ceramic oxides by Cook [1] in 1990. Since then CeO₂ particle was increasingly applied as the abrasive in chemical mechanical planarization (CMP) of the SiO₂ and Si₃N₄ based dielectric materials. At present, the share of the CeO₂ slurry has grown to the second largest part in the CMP slurry market just next to the silica slurry. However, the price of precursor of CeO₂ synthesis, as one of the rare earth materials, rose significantly as long as the export quota was restricted for the political policy by the rare earth material exporting countries [2]. This rise in price would have direct influence on the cost of ownership and the profit margin of the semiconductor manufacturing companies. Therefore, an alternative is required to totally or partially replace the CeO₂ slurry in the present dielectric CMP application.

                  ZrO₂ and TiO₂ had the second and third high polishing rates in the Cook’s report. However, the extremely high mechanical hardness of ZrO₂ prohibited its potential application in the dielectric CMP due to the concern of producing scratches. TiO₂ has the moderate mechanical hardness and applied in large number of applications including food, cosmetic, catalyst, and paint [3]. Therefore, the annual production capacity and the price would be more stable than that of the rate earth materials. For this reason, we investigated the SiO₂ CMP using the TiO₂particle in the study.

                  Figure 1 shows the Transmission Electron Microscopy of the TiO₂ particle used in the study. We used commercially available TiO₂(P25, Degussa, Germany) nanoparticles. Degussa P25 consists of two crystal forms of rutile and anatase, and is widely used in photocatalytic degradation studies, because of its chemical stability, ready availability, reproducibility, and activity as a catalyst in oxidation processes [4].

                  Figure 2 indicates the polishing rate of SiO₂ film in the TiO₂ slurry as a function of the added polyacrylic acid (PAA) concentration. The polishing rate was below 200 Å/min in the TiO₂ slurry. However, it increased suddenly approximately to 700 Å/min as the PAA concentration increased to 0.025 wt%. Surprisingly, the polishing rate abruptly dropped below 100 Å/min as the PAA concentration increased by 0.005 wt% to 0.03 wt%. The dispersion stability of the TiO₂slurry was good as long as the PAA concentration was over 0.02 wt%. Therefore, the poor dispersion stability can explain the low polishing rate at the PAA concentration that was lower than that of the critical point. However, the dispersion stability was failed to explain the abrupt decrease in the polishing rate over 0.03 wt% of PAA.

                  Figure 3 shows the viscosity of the TiO₂ slurry as a function of the PAA concentration. Each slurry shows the shear thining behavior as the sear rate increases due to the structural rearrange of the adsorbed PAA on the TiO₂ particle. The magnitude of the viscosity shows a totally inverse trend as that of the polishing rate as a function of PAA concentration. The adsorbed PAA on the TiO₂particle probably undergoes a structural change as the PAA concentration over the critical concentration of 0.025 wt%. Therefore, the abruptly decreased polishing rate would be resulted from this structural change.

                  The TiO₂ slurry showed a moderate polishing rate of SiO₂ with PAA only in a very narrow concentration range. Therefore, we need more investigation to understand the CMP mechanism of the TiO₂ slurry and find out a more effective additive to increase the polishing rate of SiO₂ in order to satisfy at least the same polishing performance as the CeO₂ slurry.