Polymerization Techniques and New Polymer Materials
Professor Shipp and his group's study of the synthesis and characterization of polymer-silicate nanocomposites has continued, with the successful production of a number of materials that have well-defined molecular weights, polydispersities typically less than 1.5, and chain-end functionality. The nanocomposite materials offer improved tensile strength, reduced flammability, and improved gas permeability properties. Their research has focused on in situ polymerization, initiated from within the silicate layers of clays, using living radical polymerization techniques such as atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addition-fragmentation chain transfer (RAFT) polymerization. Their goal is to produce novel polymers with a variety of compositions (e.g. block copolymers) and architects (e.g. star polymers), and to examine the polymer properties.
Polymer-silicate nanocomposites - incorporating montmorillonite clay and polymers such as polystyrene, poly(methyl methacrylate), poly(styrene-block-methyl methacrylate), and poly(styrene-block-butyl acrylate) - have been made. A range of analytical methods (TEM, XRD, DSC) have shown that the regularity in the silicate layers in these nanocomposites have been significantly disrupted, leading to a mixture of exfoliation and intercalation. The research group is now embarking on further physical characterization of these materials, and exploring their potential applications.
Another, more fundamental, advancement that has been made is in the analysis of ATRP kinetics. Careful reaction design and procedures, along with kinetic modeling, have allowed the determination of radical-radical termination rate coefficients. Such data are invaluable in predicting polymerization behavior, yet have proved exceedingly difficult to measure with accuracy in the past. Experimental results indicate that the technique they developed is robust enough to enable reliable termination rate coefficients to be measured. However, special consideration also has to be made of various side reactions that also occur during ATRP, such as chain transfer and chain-end elimination reactions.
Professor S.V. Babu's research group is continuing the investigation of various aspects of chemical-mechanical planarization (CMP) of metal and dielectric films. Recent emphasis has been on mixed abrasives and 'engineered' particles in different chemical environments and defect mitigation. It was discovered that some of the problems associated with the use of a single abrasive slurry, such as poor polish selectivity, surface defects and slurry instability, can be overcome by combining two or more different abrasives. It was shown that by using different particle sizes and taking advantage of differing surface charges on the abrasives, both selectivity and polished surface roughness can be improved systematically. These results were presented at several conferences and in journal papers.
Several new results have also been obtained with high selectivity ceria-based mixed slurries for STI planarization, including reduced scratching and other defects. Several patents have been filed to cover these discoveries. A large number of polishing experiments have also been performed using fixed abrasive pad systems for achieving planarization of STI and similar related structures. The pattern density has a very large effect on removal rates and it was also shown that different patterns' densities from different parts of the wafer are coupled in their role in pad "activation" and the associated particle generation. These results are very useful in determining the planarization end point and controlling dishing and erosion.
The effects of abrasive shape, size and morphology in CMP are being investigated in collaboration with Professor Matijevi_ and supported by Intel through the Semiconductor Research Corporation (SRC). Well-defined dispersions using monodispersed spherical silica particles, ellipsoidal hematite particles of different anisometries coated with silica, and silica particles coated with ceria have been prepared and evaluated as abrasives for CMP as a function of particle size and shape.
A new set of experiments are underway, also with SRC/Intel
support, to explore at a fundamental level the relationship between particle/surface
interactions and removal rate as well as contamination using a column
technique well known for investigating particle adhesion, defects, and
delamination phenomena. Professor Matijevic''s group has already done
extensive work with this technique. The slurry is fed into a small vertical
column, packed with beads of glass, copper, etc., that are large enough
both to avoid filtration of the slurry particles and to simulate the appropriate
wafer surface. Since the flow occurs under essentially hydrostatic conditions,
it represents the dynamic situation in a polishing tool at low applied
pressures. The smaller slurry particles pass through the packed column,
unless attached to the larger glass or copper collector beads. Hence,
analysis of the time dependent composition of the effluent can provide
valuable information about abrasive-film surface interactions. In addition,
subsequent rinsing of the loaded column with solutes at different pH and/or
with additives will permit evaluation of particle removal. These measurements
can be performed in different chemical environments. Indeed, it was already
observed that adhesion between silica abrasives and copper is strongly
influenced by H2O2 concentration in the slurry,
with a peak in silica particle retention observed around 0.5 to 1% H2O2
in the slurry. This corresponds with the peak in the removal rate of copper.
By altering the pH and other conditions, and by subjecting the column
to an external sonication energy source, it is also possible to identify
conditions that will facilitate particle removal from the film surface.
Professor Babu and his group also plan to investigate the behavior of
copper particles coated with polymeric films.