Silane adhesion promoters are commonly used to improve the adhesion, durability, and corrosion resistance of polymer-oxide interfaces. The current study investigates a model interface consisting of the natural oxide of 〈100〉 Si and an epoxy cured from diglycidyl ether of bisphenol A (DGEBA) and triethylenetetraamine (TETA). The thickness of (3-glycidoxypropyl)trimethoxysilane (GPS) films placed between the two materials provided the structural variable. Five surface treatments were investigated: a bare interface, a rough monolayer film, a smooth monolayer film, a 5 nm thick film, and a 10 nm thick film. Previous neutron reflection experiments revealed large extension ratios (>2) when the 5 and 10 nm thick GPS films were exposed to deuterated nitrobenzene vapor. Despite the larger extension ratio for the 5 nm thick film, the epoxy/Si fracture energy (Gc) was equal to that of the 10 nm thick film under ambient conditions. Even the smooth monolayer exhibited the same Gc. Only when the monolayer included a significant number of agglomerates did the Gc drop to levels closer to that of the bare interface. When immersed in water at room temperature for 1 week, the threshold energy release rate (Gth) was nearly equal to Gc for the smooth monolayer, 5 nm thick film, and 10 nm thick film. While the Gth for all three films decreased with increasing water temperature, the Gth of the smooth monolayer decreased more rapidly. The bare interface was similarly sensitive to temperature; however, the Gth of the rough monolayer did not change significantly as the temperature was raised. Despite the influence of pH on hydrolysis, the Gth was insensitive to the pH of the water for all surface treatments.

Thin films of organosilanes have great technological importance for adhesion promotion, durability, and corrosion resistance. Although the cross-link density profile within such films is likely to strongly affect their performance, no reliable assay has been available to characterize this distribution. In this work we use solvent swelling combined with neutron and X-ray reflection to study the cross-link density distribution within ultrathin films (50−100 Å) of (3-glycidoxypropyl)trimethoxysilane (GPS) and bis(triethoxysilyl)ethane (BTSE) on silicon. The films are swelled with vapors of the good solvent nitrobenzene (NB). For GPS films, the solvent concentration profile has a strong maximum in the central region of the film. We conclude from this that the GPS films possess a high cross-link density near the silicon surface and a much lower cross-link density in the bulk of the film. The decreased solvent concentration at the air surface is more difficult to interpret. It appears to also indicate an elevated cross-link density, but may also involve contributions from other effects such as the difference in surface tension between GPS and NB. The degree of swelling reaches ∼50−60% in the bulk of the films. Good reproducibility is obtained for several swelling/deswelling cycles, indicating that swelling with d-NB does not irreversibly change the structure of the network. In contrast to GPS, highly cross-linked films of BTSE do not swell when exposed to d-NB, even though NB is also a good solvent for the BTSE monomers.

The effect of the density and in-plane distribution of interfacial interactions on crack initiation in an epoxy-silicon joint was studied in nominally pure shear loading. Well-defined combinations of strong (specific) and weak (nonspecific) interactions were created using self-assembling monolayers. The in-plane distribution of strong and weak interactions was varied by employing two deposition methods: depositing mixtures of molecules with different terminal groups resulting in a nominally random distribution, and depositing methyl-terminated molecules in domains defined lithographically with the remaining area interacting through strong acid-base interactions. The two distributions lead to very different fracture behavior. For the case of the methyl-terminated domains (50 m on a side) fabricated lithographically, the joint shear strength varies almost linearly with the area fraction of strongly interacting sites. From this we infer that cracks nucleate on or near the interface over nearly the entire range of bonded area fraction and do so at nearly the same value of local stress (load/bonded area). We postulate that the imposed heterogeneity in interfacial interactions results in heterogeneous stress and strain fields within the epoxy in close proximity to the interface. Simply, the bonded areas carry load while the methyl terminated domains carry negligible load. Stress is amplified adjacent to the well-bonded regions (and reduced adjacent to the poorly bonded regions), and this leads to crack initiation by plastic deformation and chain scission within the epoxy near the interface. For the case of mixed monolayers, the dependence is entirely different. At low areal density of strongly interacting sites, the joint shear strength is below the detection limit of our transducer for a significant range of mixed monolayer composition. With increasing density of strongly interacting sites, a sharp increase in joint shear strength occurs at a methyl terminated area fraction of roughly 0.90. We postulate that this coincides with the onset of yielding in the epoxy. For methyl-terminated area fractions less than 0.85, the joint shear strength becomes independent of the interfacial interactions. This indicates that fracture no longer initiates on the interface but away from the interface by a competing mechanism, likely plastic deformation and chain scission within the bulk epoxy. The data demonstrate that the in-plane distribution of interaction sites alone can affect the location of crack nucleation and the far-field stress required.

No abstract

As electronic and optical components reach the micro-and nanoscales, efficient assembly and packaging require the use of adhesive bonds. This work focuses on resolving several fundamental issues in the transition from macro-to micro-to nanobonding. A primary issue is that, as bondline thicknesses decrease, knowledge of the stability and dewetting dynamics of thin adhesive films is important to obtain robust, void-free adhesive bonds. While researchers have studied dewetting dynamics of thin films of model, non-polar polymers, little experimental work has been done regarding dewetting dynamics of thin adhesive films, which exhibit much more complex behaviors. In this work, the areas of dispensing small volumes of viscous materials, capillary fluid flow, surface energetics, and wetting have all been investigated. By resolving these adhesive-bonding issues, we are allowing significantly smaller devices to be designed and fabricated. Simultaneously, we are increasing the manufacturability and reliability of these devices.4