Effect of Particle Size During Tungsten Chemical Mechanical Polishing
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ABSTRACT Abrasive particle size plays a critical role in controlling the polishing rate and the surface roughness during chemical mechanical polishing (CMP) of interconnect materials during semiconductor processing. Earlier reports on the effect of particle size on polishing of silica show contradictory conclusions. We have conducted controlled measurements to determine the effect of alumina particle size during polishing of tungsten. Alumina particles of similar phase and shape with size varying from 0.1 tun to 10 pm diameter have been used in these experiments. The polishing experiments showed that the local roughness of the polished tungsten surfaces was insensitive to alumina particle size. The tungsten removal rate was found to increase with decreasing particle size and increased solids loading. These results suggest that the removal rate mechanism is not a scratching type process, but may be related to the contact surface area between particles and polished surface controlling the reaction rate. The concept developed in our work showing that the removal rate is controlled by the contact surface area between particles and polished surface is in agreement with the different explanations for tungsten removal. INTRODUCTION Chemical mechanical polishing (CMP) has been widely accepted for oxide and metal layer planarization to eliminate step coverage concerns and improve lithographic resolution. Tungsten (W) CMP is used to remove the excess W deposited by nonselective chemical vapor deposition (CVD) for the formation of vias in IC multilevel interconnects. Regarding the mechanism of Tungsten CMP, Kaufman et al. [I] proposed a model based on the sequential chemical formation of the passivating layer and its mechanical abrasive removal. Recently Stein et al. [2] reported results based on electrochemical measurements during polishing which suggest that the removal mechanism of W during CMP does not require a blanket passive film or the oxidation of all the removed tungsten, which contradicts the previous model. According to the empirical Preston equation [3] described as follows RR = KpPV
[1]
the polish rate varies linearly with pressure (P) and linear speed at the wafer-to-pad interface (V). However, the theoretical value of the Preston coefficient, described by Brown et al. [4] as Kp = 1/2E where E is the Young's modulus of the polished surface, does not explain the polish rate variation with other important process variables such as pad properties, slurry chemicals and slurry abrasive. 103
2000 Materials Research Society Mat. Res. Soc. Symp. Proc. Vol. 566 ©
The effect of slurry abrasive size on polish rate is not very clear. Different results have been reported for oxide polishing showing contradicting conclusions. Results obtained by Jairath et al. [5] suggested that the oxide polish rate increases with both abrasive particle size and concentration. However, other reports found that glass polish rate is constant with abrasive size [61, [7] or even decreases with abrasive size [8]. To the best of our knowledg
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