The Effects of Friction and Temperature in the Chemical-Mechanical Planarization Process

Chemical-mechanical planarization (CMP) represents the preferred technology in which both chemical and mechanical interactions are combined to achieve global planarization/polishing of wafer surfaces (wafer patterns from metal with a selective layer, in this paper). CMP is a complex process of material removal process by friction, which interferes with numerous mechanical and chemical parameters. Compared with chemical parameters, mechanical parameters have a greater influence on the material removal rate (MRR). The mechanical parameters manifest by friction force (F_f) and heat generated by friction in the CMP process. The F_f can be estimated by its monitoring in the CMP process, and process temperature is obtained with help of an infrared rays (IR) sensor. Both the F_f and the MRR increase by introducing colloidal silica (SiO₂) as an abrasive into the selective layer CMP slurry. The calculated wafer non-uniformity (WNU) was correlated with the friction coefficient (COF). The control of F_f and of the slurry stability is important to maintain a good quality of planarization with optimal results, because F_f participates in mechanical abrasion, and large F_f may generate defects on the wafer surface. Additionally, the temperature generated by the F_f increases as the SiO₂ concentration increases. The MRR of the selective layer into the CMP slurry showed a non-linear (Prestonian) behavior, useful not only to improve the planarization level but to improve its non-uniformity due to the various pressure distributions. The evaluation of the F_f allowed the calculation of the friction energy (E_f) to highlight the chemical contribution in selective-layer CMP, from which it derived an empirical model for the material removal amount (MRA) and validated by the CMP results. With the addition of abrasive nanoparticles into the CMP slurry, their concentration increased and the MRA of the selective layer improved; F_f and MRR can be increased due to the number of chemisorbed active abrasive nanoparticles by the selective layer. Therefore, a single abrasive was considered to better understand the effect of SiO₂ concentration as an abrasive and of the MRR features depending on abrasive nanoparticle concentration. This paper highlights the correlation between friction and temperature of the SiO₂ slurry with CMP results, useful to examine the temperature distribution. All the MRRs depending on E_f after planarization with various SiO₂ concentrations had a non-linear characteristic. The obtained results can help in developing a CMP process more effectively.