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Thermal desorption of molecular monolayers grafted on silicon

par Rosso Michel - publié le , mis à jour le

Participants : P. Allongue, J. –N. Chazalviel, A.C. Gouget-Laemmel, C. Henry de Villeneuve, F. Ozanam

PhD students : D. Dusciac (2008), A. Faucheux (2005)

Functionalized organic monolayers have been considered for limiting silicon oxide formation during the deposition of high-k oxides on silicon at temperature above 300°C. The extremely low electronic-surface-state density of grafted silicon surfaces makes them usable in a field-effect transistor architecture.

Our investigations of the thermal stability of organic monolayers have been performed using in-situ IR spectroscopy in ATR geometry. The sample is inserted in a specially designed, home-built cell (Fig. 1) allowing for the monitoring of the monolayer evolution as a function of the temperature of a thermal treatment performed under controlled atmosphere or primary vacuum.

Figure 1 : Scheme of the IR cell.

The set of in-situ IR spectra of Fig. 2a shows that alkyl chains ­(CH2)n-CH3 (see page), start disappearing around 250°C (see the negative bands νCH around 2900 cm-1). A detailed analysis of the νCH band shows that desorption takes place in a single step, by breaking of the Si-C bond and formation of the corresponding alkene, which explains why the desorption temperature is independent of the chain length (Fig. 2b). However, a Si-CH3 monolayer (see page) is significantly more stable (up to 450° C). In this case, desorption takes place through a distinct reaction pathway, since the formation of a double C=C bond is impossible. In the case of monolayers functionalized with a terminal carboxylic group COOH (see page), desorption starts by anhydride formation. The obtained monolayers exhibit an increased stability due to the formation of organic bridges doubly anchored at the surface (Fig. 2c).

Alkoxy monolayers ­O(CH2)n-CH3 desorb according to another mechanism. Molecular chains are progressively fragmented by production of methane. The Si-O-C coupling to the surface then appears to exhibit an increased thermal stability as compared to the Si-C anchoring.

Figure 2. (a) Infrared spectra of a decyl layer as a function of the annealing temperature. Annealing is performed for 15 min and the reference spectrum is that recorded at 40°C prior to any annealing. (b) Fraction of remaining CHs as a function of the annealing temperature for different chain lengths. (c) First steps of thermal desorption in the case of an acid monolayer.

Publications :
[1] A. Faucheux, A. C. Gouget-Laemmel, P. Allongue, C. Henry de Villeneuve, F. Ozanam, and J. N. Chazalviel, "Mechanisms of Thermal Decomposition of Organic Monolayers Grafted on (111) Silicon," Langmuir 23 (3), 1326-1332 (2006).
[2] A. Faucheux, F. Yang, P. Allongue, C. Henry de Villeneuve, F. Ozanam, and J. N. Chazalviel, "Thermal decomposition of alkyl monolayers covalently grafted on (111) silicon," Appl. Phys. Lett. 88 (19), 193123 (2006).
[3] D. Dusciac, J. N. Chazalviel, F. Ozanam, P. Allongue, and C. H. de Villeneuve, "Thermal stability of alkoxy monolayers grafted on Si(111)," Surf. Sci. 601 (18), 3961-3964 (2007).
[4] D. Dusciac, C. Henry de Villeneuve, P. Allongue, F. Ozanam, and J. N. Chazalviel, "Thermal decomposition of alkoxy monolayers grafted on silicon : A mechanistic model," Surf. Sci. 609 (0), 230-235 (2013).