J. of Supercritical Fluids 32 (2004) 265–273 Spectroscopic ellipsometry of grafted poly(dimethylsiloxane) brushes in carbon dioxide S.M. Sirard, H. Castellanos, P.F. Green ∗ , K.P. Johnston ∗ Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712, USA Received in revised form 12 January 2004; accepted 12 January 2004 Abstract Spectroscopic ellipsometry was used to characterize the chain extension and optical properties of end-grafted deuterated poly(dimethylsil- oxane) (d-PDMS) brushes on SiO x wafers exposed to liquid and supercritical carbon dioxide (CO 2 ). The brush properties were manipulated by tuning the CO 2 solvent quality over a large range from ideal gas conditions (non-solvent) to a near- solvent by varying temperature and CO 2 density. The chain extension determined by ellipsometry, for an average value of the pure component refractive index of CO 2 in the film (average n CO 2 model), is in good agreement with that determined from neutron reflectivity. The d-PDMS chains extend into the CO 2 and the brush refractive index decreases as the solvent density is increased, especially at densities above the upper critical solution density for bulk PDMS in CO 2 . © 2004 Elsevier B.V. All rights reserved. Keywords: Supercritical carbon dioxide; Grafted brush; PDMS; Solvent quality; Ellipsometry 1. Introduction Liquid and supercritical carbon dioxide (CO 2 ) have gained considerable attention in recent years as viable al- ternatives to organic solvents [1,2]. CO 2 has mild critical points: T c = 31 ◦ C and P c = 73.8 bar [1] and is considered a “green solvent” since it is non-toxic and non-flammable. Compressed CO 2 has liquid-like densities and gas-like vis- cosities and diffusivities, and near the critical point, small changes in temperature and/or pressure cause large changes in the density, viscosity, and optical properties of CO 2 . Thus, CO 2 is a tunable solvent with unique properties that can be used advantageously in advanced materials pro- cessing involving microelectronics, nanotechnology, and biomaterials. CO 2 has been used successfully in the synthesis, pro- cessing, and separation of colloidal dispersions, such as water-in-CO 2 emulsions [3] and microemulsions [4], poly- mer latexes [5], and dispersions of metal oxides [6] and metal nanoparticles [7]. The modification of polymeric materials ∗ Corresponding authors. Tel.: +1-512-471-4617. E-mail addresses: green@che.utexas.edu (P.F. Green), kpj@che.utexas.edu (K.P. Johnston). via impregnations, extractions, and foaming has also been demonstrated with CO 2 due to its effective ability to plasti- cize polymers [8,9]. Furthermore, CO 2 is being explored as a potential solvent in microelectronics, as shrinking device dimensions present new processing demands. For example, CO 2 has been used as a solvent in coating [10], developing [11], and stripping photoresist films [1]. Supercritical CO 2 cleaning [1] and drying [12,13] processes are also being de- veloped for resist systems in order to prevent image col- lapse due to the destructive capillary forces normally present when drying liquid-rinsed systems. The direct patterning of low-k dielectric films with CO 2 [14,15] as well as the depo- sition of device-quality copper films into high-aspect-ratio features (sub-100 nm) from supercritical CO 2 [16] have also been demonstrated. Many of the above mentioned CO 2 -based applications require the use of polymeric and low molecular weight stabilizers at interfaces. CO 2 has no permanent dipole moment and has weak van der Waals interactions, due to its low polarizability per volume. Consequently, polymers with low cohesive energy densities, such as fluoroacrylates, fluoroethers, and siloxanes are most effective as stabilizers at moderate pressures in CO 2 [17,18]. In addition, various hydrocarbon-based stabilizers have been explored including 0896-8446/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2004.01.001