Subcritical Debonding of Multilayer Interconnect Structures: Temperature and Humidity Effects
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ABSTRACT Thin film structures may fail by progressive or time-dependent debonding at stresses far below those required for catastrophic failure. Previous work has shown that progressive debonding in a typical interconnect structure occurs either along the TiN/SiO 2 interface or parallel to this interface in the SiO 2. Such subcritical debonding was found to span several orders of magnitude of debond growth rates and occur at significantly reduced driving forces. The presence of Si0 2 at the failure location indicates that the mechanisms which give rise to stress corrosion cracking in bulk glasses may also play a role in the subcritical debonding behavior of multilayer interconnect structures. Accordingly, this work focuses on the effects of temperature and humidity on subcritical debonding and rationalizes them in terms of the relevant chemical reactions taking place at the debond tip.
INTRODUCTION In order to fully understand the effects of environment on subcritical debonding, it is necessary to model the debonding process in terms of chemical reactions occurring at the debond tip. Several models have been proposed [1-3] and the one chosen for this analysis has been detailed elsewhere [2,3] with only a cursory review presented here. The fundamental basis of this model is that frequencies of bond rupture and healing in a reactive environment are modified by the magnitude of the energy release rate. The net frequency of bond rupture is represented by Maxwell-Boltzmann statistics as:
f= foexp{U
-jexp
j
(1)
where fo=kT/h is a characteristic attempt frequency, k is Boltzmann's constant, h is Planck's constant and T is the absolute temperature. The activation energies for debond advance and retreat are represented by the U% and the U*. terms which are modified by the mechanical energy release rate to promote macroscopic debond extension. The activation energies are themselves functions of surface and mechanical potentials. For a simple chemical reaction of the form: (2) xA + B-4 B where A is the reactive environmental species, B and B* represent the unbroken and activated complex state, the rate of surface potential change or the fracture resistance R, for an increment in crack area A, can be expressed as: dA where the J's represent the chemical potential of the reactants and products, N is the number of bonds per 2 unit area equal to 1/a where a is the bond separation and the last term represents modulation of the completed reaction resistance to include the possibility of incomplete reactions or debond healing processes. On integrating the above equation, the surface potential is obtained which is periodic in bond separation: U= u,o(NA) - -- cos(2NAT) where
(4)
U. =
(5)
B- -laB) -AxA, and
AR u1= 71tN
251 Mat. Res. Soc. Symp. Proc. Vol. 563 ©1999 Materials Research Society
(6)
The terms uo and ul are material quantities that are dependent on the atomistic structure which make them the most useful for interpreting experimental results. Specifically, the u, term represents the energy required to break bon
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