Diamond is the hardest material on Earth. Nevertheless, polishing diamond is possible with a process that has remained unaltered for centuries and is still used for jewellery and coatings: the diamond is pressed against a rotating disc with embedded diamond grit. When polishing polycrystalline diamond, surface topographies become non-uniform because wear rates depend on crystal orientations. This anisotropy is not fully understood and impedes diamond's widespread use in applications that require planar polycrystalline films, ranging from cutting tools to confinement fusion. Here, we use molecular dynamics to show that polished diamond undergoes an sp(3)-sp(2) order-disorder transition resulting in an amorphous adlayer with a growth rate that strongly depends on surface orientation and sliding direction, in excellent correlation with experimental wear rates. This anisotropy originates in mechanically steered dissociation of individual crystal bonds. Similarly to other planarization processes, the diamond surface is chemically activated by mechanical means. Final removal of the amorphous interlayer proceeds either mechanically or through etching by ambient oxygen.

Anisotropic mechanical amorphization drives wear in diamond

Nanoceria is an exceptionally versatile, commercially valuable catalytic material whose properties vary dramatically from that of the bulk material. Nanoceria's redox properties can be tuned by choice of method of preparation, particle size, nature and level of dopant, particle shape and surface chemistry. The two oxidation states of the cerium element in the lattice make possible the formation of oxygen vacancies which are essential to the high reactivity of the material, its oxygen buffering capability and thus its ability to act as a catalyst for both oxidation and reduction reactions. Ceria has important commercial utility in the areas of chemical mechanical polishing and planarization, catalytic converters and diesel oxidation catalysts, intermediate temperature solid oxide fuel cells and sensors. Its potential future uses include chemical looping combustion, photolytic and thermolytic water splitting for hydrogen production and as a therapeutic agent for the treatment of certain human diseases. We have seen that the method of synthesis, particle size, stabilizing corona, and purity dictate where it is used commercially. Finally, in regards to the prescient words of Dr. Feynman, we note that while there is indeed “plenty of room at the bottom”, there quite possibly exists an optimal nanoceria size of between 2–3 nm that provides maximal reactivity and thermodynamic stability.

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Exploring the properties and applications of nanoceria: is there still plenty of room at the bottom?

Chemical mechanical polishing of thin film diamond

Cerium-based oxide compounds are known for their wide range of applications in catalysis, corrosion prevention, electrochemical cells, photocatalysis, UV absorbers, biomaterials, microelectronics, optical devices, thermal coatings, and glass abrasives. The technological applications of these materials are possible due to a combination of the electronic structure of Ce and the size effects at the nanoscale. In particular, reversible transformation between the Ce(III) and Ce(IV) oxidation states on the surface of cerium oxides is critical to the functionality and potential uses of the materials. In this paper, the main technological applications of cerium-based oxide coatings are reviewed based on the work done to date. Special interest is placed on the emerging trends.

Cerium-Based Oxide Coatings

Wear in self-mated tetrahedral amorphous carbon (ta-C) films is studied by molecular dynamics and near-edge X-ray absorption fine structure spectroscopy. Both theory and experiment demonstrate the formation of a soft amorphous carbon (a-C) layer with increased sp2 content, which grows faster than an a-C tribolayer found on self-mated diamond sliding under similar conditions. The faster $$\hbox{sp}^{3} \rightarrow\,\hbox{ sp}^{2}$$
 transition in ta-C is explained by easy breaking of prestressed bonds in a finite, nanoscale ta-C region, whereas diamond amorphization occurs at an atomically sharp interface. A detailed analysis of the underlying rehybridization mechanism reveals that the $$\hbox{sp}^{3}\, \rightarrow\hbox{ sp}^{2}$$
 transition is triggered by plasticity in the adjacent a-C. Rehybridization therefore occurs in a region that has not yet experienced plastic yield. The resulting soft a-C tribolayer is interpreted as a precursor to the experimentally observed wear.

Wear, Plasticity, and Rehybridization in Tetrahedral Amorphous Carbon

3. FEOL Applications: Device Level 180 3.1. Shallow Trench Isolation (STI) CMP 180 3.2. Replacement Metal Gate CMP 184 3.3. Poly-Si CMP for FinFET Devices 186 4. MOL Applications: Contact Level 187 4.1. Tungsten CMP 187 5. BEOL Applications: Multilevel Interconnects 188 5.1. Copper Interconnect Technology 188 5.2. CMP Challenges in Cu Interconnects 189 5.2.1. Low-k and Ultralow-k Material Challenges 189 5.2.2. Integration Challenges 190 5.2.3. CMP Process Challenges 191 5.3. Copper Planarizarion Process 191 5.4. Ta/TaN Liner CMP Process 193 6. CMP Process-Induced Defects 194 6.1. Defects in FEOL CMP 194 6.2. Defects in MOL Tungsten CMP 195 6.3. Defects in Cu BEOL CMP 195 6.3.1. Corrosion of Copper 196 6.3.2. Scratches 196 6.3.3. Dishing, Erosion, and Trenching 196 6.3.4. Mechanical Damage 197 6.3.5. Other Defects 197 7. Models of CMP Processes 198 7.1. Models Based on Contact Mechanics 198 7.2. CMP Process Models 199 8. Alternative CMP Processes 200 9. Concluding Remarks 201 10. Acknowledgments 201 11. References 201

Lee M. Cook

Papers

Chemical mechanical planarization: slurry chemistry, materials, and mechanisms.