University Oral Examination - Christopher S. Litteken (Nov. 21, 3PM)

Christopher S. Litteken litteken at stanford.edu
Fri Nov 14 14:29:26 PST 2003


University Oral Examination
Christopher S. Litteken
Department of Materials Science and Engineering

The Effects of Multi-Dimensional Constraint on the Adhesion of Thin-Films
and Patterned Structures

November 21, 2003, 3:00 PM (Refreshments will be served at 2:45 PM)
Cypress Auditorium, Paul Allen Center for Integrated Systems (CIS 101X)
Stanford University

	Successful integration of new materials, such as advanced low-k
dielectrics, into high-density interconnect structures within
microelectronic devices requires an understanding of the mechanisms that
control the adhesion of thin-film interfaces.  The intent of the present
study is to investigate the role of multi-dimensional elastic constraint on
the mechanical and interfacial properties of lithographically patterned
thin-film structures.  Accordingly, structures comprised of metal and low-k
films were fabricated to systematically vary in either one dimension
(blanket film thickness) or two dimensions (patterned feature aspect ratio).
Fracture mechanics techniques were employed to measure the critical
interfacial adhesion (Gc) at the relevant blanket or patterned film
interface.

	Results from blanket film structures indicate that the adhesive
characteristics of an interface can be dominated by plastic energy
dissipated within adjacent polymer or metal layers.  Due to the constraint
provided by the neighboring elastic layers or the relatively massive
substrate, the extent of plastic deformation was observed to be dependent of
the ductile film's thickness and residual stress.  In patterned structures
containing lithographically defined low-k polymer lines where the line width
varied between 2 and 12 microns, a 50% increase in Gc was observed for the
smallest feature width compared to a similar blanket polymer film interface.
This behavior is rationalized in terms of the effect of stress state on the
local fracture mode and resulting plastic deformation in the polymer line
similar to the plane stress to plane strain transition observed in bulk
materials.

	To investigate more technologically relevant patterned structures, arrays
of alternating Cu and low-k lines were fabricated with feature widths
varying between 0.2 and 3 microns.  A pronounced increase in Gc was observed
when the debond was propagated orthogonal, rather than parallel, to the
length of the line.  Morphological characterization of the fracture surface
revealed features with a pitch and orientation identical to the original
patterned structure resulting from variations in elastic properties adjacent
to the debonded interface.  Mechanics models detailing crack tip shielding
and energy dissipation through the frictional contact of these surface
asperities will be presented to explain the observed trends in adhesion.
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