Perspective

Over the past several years, there has been an increasing trend toward reducing and mitigating such scratches by developing both calcined and colloidal ceria particles with controlled size and more spherical shape while maintaining a high SiO2 removal rate (RR).

Spherical ceria abrasives lead to less variability in frictional forces and less stick-slip events, 11 both of which are likely to reduce scratch generation without decreasing oxide RR and selectivity. The primary particles, pretreated with ethylene glycol before calcination, tend to form solid spheres driven by the minimization of the total energy of the system (Fig. 1a). 12 Spray drying has also been used to prepare fine spherical ceria particles before calcination. 21 These spherical calcined ceria particles (Fig. 1a) reduced polish scratch defects by about 10%. 11,12,21

Ceria-based slurries that can generate high oxide polish rates enable lower particle loading, effectively reducing polish scratch defects. It is well known that the Ce3+ species on the ceria surface are responsible for the high reactivity with SiO2 films. 22 Hence, several authors proposed achieving high SiO2 RR by preparing Ce3+-rich CeO2 particles through either doping with lanthanide (La, Sm, Gd, Nd, and Yb) 23 or transition metals. 24 Substitution of Ce4+ with the aliovalent dopants in the crystal lattice of CeO2 introduces O vacancies, which leaves two excess electrons localized on the Ce 4 f states that are then reduced to Ce3+. The purity and dopant concentration of ceria particles strongly influence their polishing characteristics. Recently, UV irradiation 25 and pulsed plasma treatment 26 have been implemented to increase the concentration of Ce3+ on the surface. The reactivity of ceria particles can also be controlled by changing the dissolved oxygen content of slurries at various pH levels up or down, 27 which can transform Ce3+ species into less reactive Ce4+-superoxo or a Ce3+-peroxo species.

Current thinking is that the use of smaller and smaller abrasive particles is the only path to move forward with obtaining defect-free surfaces. Attention has shifted towards the development of ceria-based CMP slurries containing colloidal ceria particles as indicated by the number of publications (Fig. 2). However, the challenge for using smaller colloidal ceria particles is the low SiO2 RR. One way to achieve higher oxide rates is to develop more chemically reactive colloidal ceria particles by controlling various synthesis parameters, doping with lanthanide ions, or coating particles with reactive layers (core/shell particles). 14,28–35 These are some of the methods used to prepare Ce3+-rich CeO2 particles mentioned above. There are several reports of using mixed abrasive particles such as colloidal ceria with different sized ceria, silica, and alumina as core particles. 32–34 One such example is the CeO2-based core/shell structures. CeO2 particles (∼60 nm) covered by 3–5 nm ceria particles or metal-doped ceria particles achieved almost two times higher SiO2 RR compared to uncoated ceria particles. 35 Colloidal silica, mesoporous silica, polystyrene, and polymethyl methacrylate were also used as a core material, and coated with small ceria or metal-doped ceria particles (Fig. 1b), 14,28–30 resulting in high oxide RR and low scratch due to the spring-like effect of the softer core-silica and their free-rolling on the films. However, broken particles and the peeling-off and brittle collapse of the shells from core–shell particles during polishing still remain challenging.

Figure 2. Overall publication trend for calcined and colloidal ceria particles for CMP applications during 2000–2021.

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As feature sizes shrink to 7 nm and smaller, smaller and smaller ceria particles (