A Tribochemical Study of Ceria-Silica Interactions for CMP

Abstract

Shallow trench isolation ͑STI͒ allows tighter device packing and reduced chip area for isolation. STI is critically dependent on the global planarity that is only possible using chemical mechanical polishing ͑CMP͒. Ceria-based slurries are considered the most promising candidates for STI CMP. Despite decades of use in glass polishing, the unique characteristics of ceria slurries are not well understood. In this study, we have conducted force measurements and tribological tests using an atomic force microscope ͑AFM͒ and a scanning electron microscope to investigate pH-dependent ceria-silica and silica-silica interactions that occur during CMP. Our studies confirm the effect of hydrolysis at high pH during silica-silica abrasion. An additional physicochemical contribution to ceria-silica polishing is identified and discussed. Furthermore, a strong correlation was observed between the AFM based studies and in situ friction force measurements during CMP.The increased packing density and ever shrinking dimensions of modern integrated circuits are direct consequences of what is commonly termed Moore's Law. 1 Shallow trench isolation ͑STI͒ is a novel manufacturing technique that facilitates the miniaturization of devices. In the STI process, trenches are etched into the Si 3 N 4 mask layer and the silicon substrate. Chemical vapor deposition ͑CVD͒ or spin-on-glass ͑SOG͒ techniques are used to fill the trenches with SiO 2 . CMP or a combination of reactive ion etching ͑RIE͒ and chemical mechanical planarization ͑CMP͒ then remove the overlying oxide. The planarized, trench-fill oxide isolates the device after removal of the Si 3 N 4 mask.STI is not possible without CMP. An ideal STI CMP process stops at the nitride layer, resulting in uniform removal of the trenchfill oxide. Naturally, STI CMP slurries must preferentially polish oxide with respect to the nitride layer, and should leave the surface globally planar and defect free. Over-polishing of the nitride or erosion of the oxide causes damage to the active region and degrades isolation performance. 2 Traditional interlayer dielectric consumables are not sufficient for the STI CMP process. 3 New slurry and pad chemistries with tunable properties are required to meet the stringent selectivity, planarity, removal rate, and surface finish requirements of the STI process. Perhaps the most important requirement is a high oxide to nitride polish rate. For STI CMP, ceria-based slurries outperform silica-based slurries. 4 Ceria slurries preferentially remove the oxide layer with minimal polishing of the underlying nitride. Despite the use of ceria abrasives in glass polishing for many decades, the ceria-silica polishing mechanism for STI CMP remains unclear.Ceria-based slurries are capable of silica removal rates ϳ5ϫ that of conventional silica slurries. This enhancement is even more pronounced when the silica removal rate is normalized to the respective solids loading ͑ϳ2% for CeO 2 slurries and ϳ10% for SiO 2 slurries͒. Cook attributed the enhanced polishing of glass by ceri...

Use of anionic surfactants for selective polishing of silicon dioxide over silicon nitride films using colloidal silica-based slurries

“…Similar oxide RRs were reported in our earlier publication [13,14]. The decrease in the oxide RRs from pH 2 to 4 is presumably due to the increase in the electrostatic repulsion between the negatively charged silica abrasives and the oxide surface [15] while the increase in the oxide RRs at higher basic pH values can be attributed to an increase in oxide solubility [16].…”

AFM-Based Study of the Interaction Forces between Ceria, Silicon Dioxide and Polyurethane Pad during Non-Prestonian Polishing of Silicon Dioxide Films

“…In contrast, force measurements between the ceriacoated silicon substrate and the silica particle in DI water resulted in an attractive/adhesive force at pH 4. Abiade et al 28 also showed similar results between a silica probe and ceria thin films at pH 4.5. This attractive force arises from the interaction of a silica surface consisting of negatively charged species with positively charged ceria particles.…”

The synthesis of silica and silica–ceria, core–shell nanoparticles in a water-in-oil (W/O) microemulsion composed of heptane and water with the binary surfactants AOT and NP-5

“…The oxygen storage capacity (OSC) and the catalytic activity of ceria are heavily deactivated when it is exposed to high temperatures over 900°C, which is mainly due to thermal sintering [17,18]. As a CMP abrasive, ceria has the inherent disadvantages of an irregular, non-spherical shape and a high specific gravity that cause serious defects (scratches) on the silicon surface [19,20].…”