Silicon substrates, surface functionalization

The effect of constraints introduced by confining diblock copolymers between two solid surfaces was examined by Lambooy et al. (1994) and Russell et al. (1995). They studied a symmetric PS-PMMA diblock sandwiched between a silicon substrate, and silicon oxide evaporated onto the top (homopolymer PMMA) surface. Neutron reflectivity showed that lamellae formed parallel to the solid interfaces with PMMA at both surfaces. The period of the confined multilayers deviated from the bulk period in a cyclic manner as a function of the confined film thickness, as illustrated in Fig. 2.60. First-order transitions were observed at t d0 = (n + j)d0, where t is the film thickness and d0 is the bulk lamellar period, between expanded states with n layers and states with (n + 1) layers where d was contracted. Finally, the deviation from the bulk lamellar spacing was found to decrease with increasing film thickness (Lambooy et al. 1994 Russell et al. 1995). These experimental results are complemented by the phenomenologi-... 

Comyn [1] has pointed out that maximum bond strength and consequently greater adhesion between the substrate and polymer could be achieved with a monolayer of silane bound to both the adherend and adhesive. The current investigation was undertaken to evaluate the possibility of monolayer level depositions on silicon substrates by employing a few w -functionalized alkanoyl-substituted derivatives of APTES which will provide polar moieties as well. The interactions of these functionalized silanes covalently immobilized on silicon with octadecylamine and octadecanoic acid, used as models for basic and acidic polymeric adhesives, were also examined in this study. Characterization of the silanized surfaces as well as studies on their interactions with the above two chemical probes were carried out through ellipsometric and XPS measurements. 

As discussed in the introduction, a major motivation for the development of methods to controllably functionalize silicon surfaces is the opportunity to create novel hybrid organic/silicon devices. By integrating organic molecules with silicon substrates it should be possible to expand the functionality of conventional microelectronic devices. Possibilities include high-density molecular memory and logic as well as chemical and biochemical sensors. Realization of these opportunities requires not only the development of the attachment chemistries, as discussed in the previous sections, but also detailed studies of the electronic properties of the resulting surfaces. 

Functionalization studies have been carried out at both clean and hydrogen-passivated surfaces. The vast majority of studies on clean silicon substrates are performed under dry ultra-high vacuum conditions (UHV). On the other hand, reactions at hydride-terminated silicon commonly rely on wet chemical methods performed in solution. Regardless of the different environment and surface structure, common principles of the functionalization at semiconductor surfaces are emerging from these studies. 

The reactivity of the solid substrates is determined by the surface functional groups. Immobilization of the biomolecules to the substrates can be performed via several routes. Immobilization can be done by a direct attachment of the molecule to the functionalized or nonfunctionalized surface, or by the employment of a cross-linker (homobifunctional, heterobifunctional, or multifunctional) between the functionalized surface and the biomolecule. Generally, the selection of the substrate and the chemistry is crucial for the successful immobilization of biomolecules and application of that substrate. Here, the reaction between the biomolecule and the reactive surface groups is described for both types of substrates silicon oxide/glass and gold. 

If in the case of aluminized silicone we were able to evidence a drastic difference between sputtering and evaporation, it happens not to be the case for aluminized PET (13). Our preliminary results on this latter polymer indeed show no marked differences between the two deposition processes, both giving strong chemical interaction. By contrast we have also observed that with noble metals such as Au, no chemical interaction is taking place with silicone substrate with both deposition processes. This tells us that the nature of the polymer substrate and of the metal are most important for the interfacial and adhesive properties. The fundamental parameter seems to be the reactivity of both constituents of the interface. It has been confirmed by Pireaux et al. that the carboxylic function is one of the most reactive surface entity (14) and indeed for PET, the adsorption site for the Al atoms is found to be the carboxylic function (13). During this interaction, Al is oxidized and the diffusion of O into the Al film can occur. 

The prominent goal of these studies was the detection of interactions between substrate and silyl esters with different organic groups R. The model substrates used include various kinds of silica, alumina, titania in addition to typical sandstones and silicon wafers. With respect to a contribution in the first conference report [3] one example may suffice here to demonstrate the procedure and the importance of the result obtained. The example concerns the treatment of a silicon wafer surface with 3-aminopropyltriethoxysilane (APTES) aimed at the detection of covalent bonding and possible interaction of the functional aminopropyl group with reactive surface centers. The method of choice is a... 

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