Chapter 7 Thermal Stability of Trimethylsilylated Mesoporous Silica Thin Film as an
7.1 Trimethylsilylation of the Mesoporous Silica Thin Film
Figure 7.1 shows thermal desorption spectra of CH4 for the as-calcined and the
HMDS-treated mesoporous silica thin films. CH4 (m/e = 16, solid line) molecules, which can be ascribed to be an associative desorption product of methyl groups, began to desorb from the mesoporous silica thin film at ~375oC, but significant desorption did not occur until the sample temperature exceeded 400oC, and the desorption reached its maximum around 580oC.
This suggests that trimethylsilyl surface groups in the HMDS-treated mesoporous film are thermally stable up to 400oC.
The FTIR study also indicated that notable decomposition of the trimethylsilyl surface groups took place at temperatures larger than 450oC. Figure 7.2(a) shows FTIR spectra of the as-calcined, the HMDS-treated, and the annealed mesoporous silica thin films. Figures 7.2(b) and 7.2(c) are the enlarged absorption spectrum windows for the C-H stretching and Si-O-Si stretching modes, respectively. The as-calcined mesoporous silica films [Fig. 7.2(a)(i)] are rich in residual OH groups (~3600 cm-1), and thus are liable to adsorb a large amount of water molecules (~3300 cm-1). As shown in Fig. 7.2(a)(ii), an absorption peak developed at
Figure 7.1 Thermal desorption spectra of CH4 (m/e = 16) for the as-calcined (dash line) and the HMDS treated (solid line) mesoporous silica thin films.
Figure 7.2 (a) FTIR spectra of (i) the as-calcined film, (ii) the HMDS treated film, and the HMDS treated film annealed at (iii) 300oC, (iv) 400oC, (v) 500oC, and (vi) 600oC. The enlarged absorption spectrum windows for the C-H stretching and the Si-O-Si asymmetric stretching modes are shown in (b) and (c), respectively.
1258 cm-1, which is attributed to Si-CH3 stretching vibration in the trimethylsilyl groups, and the methyl group-related C-H stretching peaks appeared around 2965 cm-1 after the HMDS vapor treatment, indicating that the mesoporous silica thin film was effectively methylsilylated. In addition, the disappearance of the OH absorption peak and the broad water absorption band in the FTIR spectrum suggests that the mesoporous dielectric became hydrophobic after the HMDS treatment. When the HMDS-treated mesoporous silica film was thermally annealed, the intensities of the CHx and the Si-CH3 absorption peaks did not show discernable decrease until the annealing temperature was higher than 400oC as revealed by Figs. 7.2(a)(iii) and 7.2(a)(v). The methylsilyl related absorption peaks shown in Figs.
7.2(b)(v) and 7.2(c)(v), which are for samples annealed at 500oC, exhibit a slight intensity decrease, as compared with Figs. 7.2(b)(ii) and 7.2(c)(ii), implying that decomposition of trimethylsilyl terminal groups on the pore surface did take place at 500oC, but the decomposition was rather moderate. On the other hand, the mesoporous silica annealed at 600oC lost most of the intensity of the two CH3 absorption peaks, and the Si-CH3 peak had obvious changes in the peak’s shape and position. According to Fig. 7.2(c)(vi), the peak at 1258 cm-1 for the sample annealed at 600oC became vague, and two new absorption peaks emerged at 1268 cm-1 and 1278 cm-1, which can be attributed to the Si-CH3 stretching mode as well. It has been reported that a change in the number of the CH3 moiety on the terminal Si-(CH3)3 group of the trimethylsilylated silica will cause slight shifts in the peak position of the Si-CH3 stretching mode [191, 192]. The absorption peak centered at ~1278 cm-1 is attributed to the Si-CH3 stretching of the monomethylsilyl group, and the absorption peak of the dimethylsilyl group is situated between the monomethylsilyl and trimethylsilyl peaks, around 1268 cm-1. The obvious blueshift of the Si-CH3 stretching absorption peak suggests that severe decomposition of terminal Si-(CH3)3 groups on the pore surface of the mesoporous silica thin film occurs at 600oC. In addition, the significant intensity loss and dramatic change
in the peak shape of the asymmetric CH3-Si rocking mode, which is situated in the 700–900 cm-1 range [158], also indicates the decomposition of terminal Si-(CH3)3 groups at the high annealing temperature. In addition to the intensity variation of the absorption peaks associated with the Si-CH3 group, the asymmetric Si-O-Si stretching absorption band situated between 1000 and 1300 cm-1 also reveals that the chemical structure of the silica matrix became less ordered as the annealing temperature increased. The asymmetric Si-O-Si stretching absorption band is, in general, assigned to be the overlap of two pairs of transverse optical (TO) and longitudinal optical (LO) modes [167-169]. One of the two pairs, denoted as TO3-LO3, is due to the asymmetric stretching motion of oxygen atoms moving in phase with neighboring oxygen atoms, and has the associated TO and LO peaks situated at ~1069 cm-1 and ~1223 cm-1, respectively. The other pair, denoted as TO4-LO4, is due to the Si-O-Si vibration in which oxygen atoms execute stretching motion 180o out of phase with neighboring oxygen atoms, and has the TO4 peak situated around 1177 cm-1 and the LO4 peak around 1125 cm-1. Previous studies suggested that the TO4 and LO4 modes in the SiO2 matrix with a less ordered chemical structure have a greater absorption [168, 169]. By analyzing the intensity variation of the absorption band of the Si-O-Si stretching modes, one can evaluate the microstructure ordering of the silica skeleton in the mesoporous silica thin film (as referred to Chap. 5). From Fig. 7.2(c), the Si-O-Si stretching absorption band of the HMDS-treated mesoporous silica has an almost identical peak position and shape as the as-calcined one, except the additional peak at 1258 cm-1 due to the Si-(CH3)3 stretching. This implied that trimethylsilylation by the HMDS treatment introduces little change in the chemical structure of the silica matrix of the mesoporous films. On the other hand, as shown in Figs. 7.2(c)(v) and 7.2(c)(vi), the LO4 peak of the mesoporous film is apparently enhanced after the anneal at 500oC, and a substantial change in the absorption shape of the Si-O-Si absorption band of the sample annealed at 600oC. Combining previous discussion on the absorption peaks associated with the Si-(CH3)3
surface group, the chemical structure of the mesoporous silica thin film becomes less ordered upon the anneal at temperatures >450oC due to the decomposition of terminal trimethylsilyl groups. The extensive decomposition of the trimethylsilyl surface group at 600oC will have a destructive impact on the mechanical strength of the mesoporous silica thin films.
Table 7.1 lists the dielectric constant and the leakage current density of the HMDS-treated mesoporous silica film after annealed at 400oC. For mesoporous silica films without the HMDS treatment, a rational capacitance-voltage (C-V) curve could not be obtained because the leakage current was enormously large. On the other hand, the mesoporous silica thin film receiving the HMDS treatment and 400oC anneal has a dielectric constant as low as 2.14 and a leakage current density smaller than 3.1 x 10-8 A/cm2 at 2 MV/cm. The significant decrease in the k value is ascribed to extensive elimination of water molecules physisorbed in the mesoporous silica film after the trimethylsilylation treatment.
The trimethylsilylated mesoporous silica film is electrically reliable as shown by the dielectric property of the sample which was stored in cleanroom ambient for over 50 days. The trimethylsilylated mesoporous silica film shows little change in the dielectric constant and the leakage current density after the 50 shelf days, suggesting that bulky trimethylsilyl groups can effectively block further moisture uptake and, thus, the mesoporous silica thin film retains its hydrophobicity.
Table 7.1 Dielectric properties of the as-calcined and HMDS treated mesoporous silica thin films after 400oC anneal.