Pyrolysis of self-assembled organic monolayers on oxide substrates
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Pyrolysis of self-assembled organic monolayers on oxide substrates Hyunjung Shin,a) Yuhu Wang,b) Uma Sampathkumaran, Mark R. De Guire, and Arthur H. Heuer Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
Chaim N. Sukenik Department of Chemistry, Bar Ilan University, Ramat Gan, 52100, Israel (Received 3 August 1998; accepted 14 January 1999)
The pyrolysis of siloxy-anchored, organic self-assembled monolayers (SAMs) on oxide substrates [titanium dioxide powder; hydrolyzed silicon dioxide on (100) silicon] was studied using x-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and mass spectroscopy (MS). Pyrolysis in air began on heating at 200 ±C and was complete by 400 ±C for both octadecyltrichlorosilane (OTS) and C16 -thioacetate (TA) SAMs, as observed in TGA of SAM-coated TiO2 powders, and in XPS studies of TA-SAM-coated TiO2 powders and Si wafers after various heat treatments. In low-oxygen environments, pyrolysis of SAMs began at higher temperatures: between 250 and 400 ±C for heating in ultrahigh vacuum (1028 Torr) as observed in XPS studies of TA-SAMs on Si, and between 300 and 400 ±C in nitrogen, as observed in TEM analysis of sulfonate SAMs under a TiO2 thin film on Si substrates.
I. INTRODUCTION
The ability of organic self-assembled monolayers (SAMs)1 to enhance the deposition of adherent oxide thin films at low temperatures has been previously reported.2–8 During heat treatments of these films, the pyrolysis of the SAM is an important issue, mainly because of its potential for disruption of the adherence, continuity, or uniformity of the oxide films. In work previously reported by our laboratory,3 cross-sectional TEM analysis of samples comprised of TiO2 layers on sulfonate-terminated SAMs on silicon wafers showed that heating of such multilayer samples in air did not cause any delamination or disruption within the ceramicSAM-wafer interface at temperatures up to 600 ±C. The thermal stability of SAMs on the surfaces of inorganic substrates and their fate upon thermolysis are important in a number of potential engineering applications, e.g., in microelectronics.9 As temperature increases from room temperature to about 200 ±C, siloxy-anchored SAMs (such as those studied here) become progressively more disordered.10–12 Monolayers formed from octadecyltrichlorosilane (OTS) were shown to be stable to heating to 140 ±C (and recooling). In contrast, films which (unlike the siloxy-anchored SAMs studied in the present work) were not covalently attached showed irreversible breakdown between 110 and 125 ±C. In general, the heating and recooling of monolayer films shows several effects. At lower temperatures, some variations in a)
Now at Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea. b) Now at Ceramem Corporation, Waltham, Massachusetts 02453. 2116
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 5, May 1999
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