Adhesion and Subcritical Debonding of Polymer Interfaces for Microelectronic Packaging

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power-law dependence of crack growth rate on crack driving force for soda-lime glasses in moist air under static loads. This can be expressed by the empirical relationship:

S=

da dt

r• )m(1) C

(

where da/dt is the crack velocity, GEA is the environment-assisted driving force, and C and m are fitting parameters. Weiderhorn also identified three distinct regimes of power-law dependence associated with different micromechanisms of cracking growth. Williams determined the existence of a similar stress corrosion process in bulk polymeric materials [5], and identified a power law exponent for Region I of m=20. Other investigators have subsequently described similar stress corrosion effects in bulk polymers [6,7], however, there is little information about stress corrosion processes occurring in polymer interfaces. EXPERIMENT BCB/Silicon interface Sandwich structures were fabricated with a benzocyclobutene-type polymer (CYCLOTENETM 5021, Dow Chemical Co.) constrained between two silicon substrates. BCB is a polymer typically used as a dielectric material in microelectronic applications. A 1.5 jim film of BCB was spin-coated onto a silicon wafer, soft-cured, then placed face to face with a similar wafer. The two wafers were then hard cured for I h. at 300'C under high purity N2. In order to evaluate the efficacy of an adhesion promoting layer on the strength of the interface, one set of wafers had, in addition, an aminopropyltriethoxysilane (APS) adhesion promoter between the silicon/BCB interfaces. After bonding, the samples were machined into four-point bend bars with dimensions 42 mm x 3 mm x 1.35 mm and a notch was cut into one of the interfaces to facilitate debonding along the interface of interest. Critical adhesion energies were measured by loading the bars in four point bending until a debond formed at the silicon/BCB interface. The adhesion energy of the interface is related to the load, P, necessary to cause steady state growth of the debond by the equation [8]: = 21(1 v2)p212 23

16Eb h

(2)

where b is the bar width, h is the half thickness, I is the spacing between the inner and outer loading lines, E is the Young's modulus and v is the Poisson's ratio. This geometry was chosen because Gc is insensitive to crack length, eliminating the need to measure crack lengths in situ. Note also that Eqn. 2 neglects the very small contributions of the compliance of thin polymer layer to the measured adhesion energy, Gc. Epoxy Underfill/Polyimide/Silicon interface system Double cantilever beam (DCB) specimens were fabricated by first spinning an adhesion promoting layer [Hitachi Coupler-3] and then a 6 micron polyimide film [Hitachi PIQ 3200-7H] onto silicon substrates. The wafers were cured to remove thermal stresses and subsequently diced into rectangular sections. A gold film (0.1 pim thick) was deposited onto the polyimide over the first 10 mm of the substrates to aid in the formation of an initial debond. The substrates were then placed face-to-face, separated by a gap of 75 jim of Kapton tape. Epoxy underfill