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MAXIMIZING THE CHEMICAL REMOVAL OF CERIA ABRASIVES IN CMP FOR SILICON OXIDE AND METAL POLISHING
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2020-08
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Cerium oxide or ceria has garnered a wide range of applications due to its redox active nature.
This redox activity is due to oxygen vacancies on the surface of the ceria creating a layer of
mixed oxide with the unstable oxide Ce2O3 (Ce[superscript 3+]) present at the same time as the bulk oxide
CeO2 (Ce[superscript 4+]). Possible applications for ceria include water splitting, oxidation of carbon
monoxide, oxidation of reactive oxygen species and polishing of glass films. In recent years,
ceria nanoparticles have been used for polishing thermal silicon oxide during the early steps of
semiconductor fabrication in a process referred to as chemical mechanical planarization (CMP).
The advantage of these particles is their ability to abrade an oxide surface chemically using the
aforementioned redox properties, as well as mechanically. To meet the needs of manufacturing,
mainly removal rate and surface roughness, the particles used must have well controlled physical
properties such as size and shape for mechanical removal and ratio of cerium oxidation state for
chemical removal. This study encompasses three parts following the design of ceria slurries,
their implementation in the existing silicon oxide polish and applying these findings to create
novel slurries for polishing metals.
To design ceria slurry, the ratio of Ce[superscript 3+]/Ce[superscript 4+] on the surface of abrasive was maximized by
altering the slurries’ chemical environment. Maximizing this ratio increases the proportion of
active Ce[superscript 3+] sites which participate in removal reactions. The effect of chemical environment on
the Ce[superscript 3+]/Ce[superscript 4+] ratio was determined through XPS analysis of the Ce 3d spectrum. The knowledge
gained in this first section informed the design of ceria slurries for the following two parts to
maximize their effectiveness. The second part of this thesis applies this knowledge to create ceria iv
slurries that polished thermal oxide with higher material removal rate (MRR) and lower postpolish
roughness than slurries that are currently being used in industry. The basis of ceria
polishing is known as the tooth-comb model. In this model oxygen at Ce[superscript 3+] sites will undergo a
condensation reaction with oxygen on the surface to be polished. As the particle leaves this will
rip material off of the wafer surface. While the tooth-comb model was proposed for polishing
silica, the final part of this thesis seeks to generalize it to encompass polishing any oxide given
the correct conditions. To demonstrate this, I created ceria slurries to polish metals relevant to
the semiconductor industry (copper, tungsten and ruthenium) with polishing metrics that equal or
exceed those of industry standard slurries.