J. of Supercritical Fluids 90 (2014) 134–143 Contents lists available at ScienceDirect The Journal of Supercritical Fluids j o ur na l ho me page: www.elsevier.com/locate/supflu In situ FTIR experimental results in the silylation of low-k films with hexamethyldisilazane dissolved in supercritical carbon dioxide Eduardo Vyhmeister a,b,c,∗ , Lorenzo Reyes-Bozo d , Héctor Valdés-González e , José-Luis Salazar e , Anthony Muscat a , L. Antonio Estévez b , David Suleiman b a University of Arizona, Department of Chemical & Environmental Engineering, 1133 E. North Campus Drive, Tucson, AZ 85721, USA b University of Puerto Rico, Mayagüez Campus, Department of Chemical Engineering PO box 9046, Mayagüez, 00681-9046, Puerto Rico c Universidad de Santiago de Chile, Departamento de Ingeniería Química, Casilla 10233, Santiago, Chile d Universidad Andres Bello, Facultad de Ingeniería, Departamento de Ciencias de la Ingeniería, Sazié 2315, Santiago, Chile e Universidad Andres Bello, Facultad de Ingeniería, Escuela de Industrias, Sazié 2325, Santiago, Chile a r t i c l e i n f o Article history: Received 6 August 2013 Received in revised form 30 January 2014 Accepted 31 January 2014 Available online 13 February 2014 Keywords: HMDS In situ FTIR Silylation scCO2 Heterogeneous reaction Low-k a b s t r a c t In situ Fourier Transform Infrared Spectroscopy measurements were performed using an innovative equipment to study the surface modification reaction between a functionalized porous MSQ-film and hexamethyldisilazane (HMDS) dissolved in CO 2 at supercritical conditions (scCO 2 ). scCO 2 was used in the heterogeneous reaction due to enhancing properties, ideal for porous materials. Different infrared signatures, from the gas and solid phases, were observed and identified, implying gas–gas and solid–gas phase reactions. Among the different component signatures observed in the gas phase, carbonic acid was observed as a possible silylating gas phase nucleophilic component, while in the solid phase the predominant reaction mechanism proceeded by forming Si O Si bonds and Trimethylaminosilane (as gas phase product). © 2014 Published by Elsevier B.V. 1. Introduction The technological importance of silicon has enabled and driven an extensive body of research into the chemical functionalization of silicon surfaces for its applications to silicon-based devices, such as biochips, optoelectronic, cantilever sensors, (bio)chemical sensors, and photovoltaic and photoelectrochemical cells [1]. Functional- ization of oxygen free surfaces targeting Si N or Si C linkages via alkylation, hydrosilylation, or cyclocondensation, among other reactions, are traditionally used for the functionalization of oxygen- free surfaces [1,2]. Surface modification can also be performed via silylation, which has extensively been used as a derivatizating technique. This reaction involves the replacement of an acidic hydrogen on the compound with an alkylsilyl group. The choice of a silylating reagent depends on the desire functional group, as necessary for the intended application, and the chemical state and nature of the surface being modified [3–11]. Different works have focused on the application of silylating reagents to silicon based surfaces which ∗ Corresponding author. Tel.: +56 9 63656304. E-mail address: eduardo.vyhmeister@unab.cl (E. Vyhmeister). have shown a strong dependence on the surface modification capa- bilities based on the radicals included on the silylating reagent, and the phase state used for the derivatization (liquid, gas, supercritical) [12–21]. Hexamethyldisalazene (HMDS) is another reagent that has been successfully applied to modify silicon based surfaces, and has the advantage of being less toxic that its counterparts. Additionally, HMDS cannot generate self-condensation as chlorosilanes do [21]. HMDS effectively removes hydrophilic sites by the mechanism showed by Eqs. (1)–(3) [15,17,22,23]. (CH 3 ) 3 Si - NH - Si(CH 3 ) 3 + Si s - OH ⇔ (CH 3 ) 3 Si - NH 2 + Si s - O - Si(CH 3 ) 3 (1) (CH 3 ) 3 Si - NH 2 + Si s - OH ⇔ NH 3 + Si s - O - Si(CH 3 ) 3 (2) (CH 3 ) 3 Si - NH - Si(CH 3 ) 3 + 2Si s - OH ⇔ NH 3 + Si s - O - Si(CH 3 ) 3 (3) HMDS can readily react with hydroxyl groups found in the sur- face following a S E i mechanism, further described in the work of http://dx.doi.org/10.1016/j.supflu.2014.01.019 0896-8446/© 2014 Published by Elsevier B.V.