Department: NanoEngineering
Research Institute Affiliation: Graduate Program in Materials Science and Engineering
Faculty Advisor(s): Jan Talbot

Primary Student
Name: Neil A Brahma
Email: nbrahma@ucsd.edu
Phone: 858-534-5663
Grad Year: 2012

Chemical mechanical planarization (CMP) is a process used in integrated chip manufacturing to obtain uniformly planarized surfaces by utilizing both mechanical abrasion and chemical etching. During the process, the abrasive alumina particles may agglomerate into undesired larger particles that can cause scratches and other defects. By understanding the rate of agglomeration as a function of chemistry, new slurries can be developed that will ultimately reduce scratches and defects. Slurry chemistry, most notably the pH, has a significant effect on the colloidal behavior of the abrasive particles within the slurry. The pH of the slurry alters the zeta potential of the abrasives in the slurry and significantly impacts the aggregation of abrasive particles. The IEP of a material in solution refers to the pH where the zeta potential, the electrical potential of the dispersed medium and the stationary fluid layer surrounding the particles, is zero. In general, colloids are most stable if the magnitude of the zeta potential is high and will not aggregate appreciably while low zeta potential magnitudes have significant aggregation. This study focused on the effects of individual additives. Solutions of de-ionized water with 1 mM KNO3 (for constant ionic strength) were tested with copper and various additives, while the pH was adjusted using KOH and HNO3. Once the desired pH was reached, the selected additives were added. The additive tested were 1mM KNO3, 0.1M glycine (a complexing agent), 0.12mM Cu, 0.1 wt% H2O2, 0.01 wt% BTA (a corrosion inhibitor), and 0.1mM SDS( a surfactant). Once the additives were added, 0.05wt% of alumina, from a dispersion containing 40 wt% α-alumina in DI water manufactured by Cabot Corporation, was added to the solution. Solutions were stirred in a sonicator for 5 minutes. Particle size and zeta potential measurements were made using a ZetaPlus dynamic light scattering analyzer. Results from the measurements indicate that pH is the primary factor controlling aggregation. Altering the pH alone can stimulate or reduce aggregation. For suspensions tested with KNO3 and glycine only, aggregation occurs only at basic pH (10) where the zeta potential is measured to be almost 0 mV. BTA and H2O2 have negligible effects on aggregation and zeta potential while suspensions with SDS have significant aggregation occurring at acidic pH (3-6.5). Aggregation is most significant when the zeta potential is close to 0 mV (isoelectric point) and aggregation does not occur when the magnitude of the zeta potential is large.

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