Summary

Spin-on mesoporous silica thin films were prepared by self-assembled templation. The density and the dielectric properties of the film were controlled by changing the surfactant/TEOS molar ratio of a silica precursor. The as-calcined mesoporous silica thin films have pore surfaces that are rich in surface silanol groups and easily adsorb water in the ambient atmosphere. Adding TMCS to the sol-gel precursor during gelation or exposing the

mesoporous thin film to HDMS vapor after calcination stabilizes the electrical properties and the hydrophobicity of the mesoporous silica film. HMDS-treated mesoporous silica thin films had an ultralow k value of 2.1. Combining HMDS vapor and low-power reductive H2 plasma treatment further improved the dielectric properties of the mesoporous silica film. However, H2 plasma treatment is suggested to be performed after HMDS vapor treatment to ensure the completeness of trimethylsilylation.

Chapter 5

Mechanical Strength of Surfactant-Templated Mesoporous Silica Thin Films

Silica dielectrics are typically prepared with high porosity to yield k<2.0. Given a lack of complete chemical bonding support, more porous films have less mechanical strength, allowing breakage within the porous layer during chemical mechanical polishing. The controllable porosity and uniform pore size distribution are such that molecularly templated mesoporous silica films can provide better mechanical and dielectric properties than many other porous low-k dielectrics. However, the mechanical strength and dielectric stability of molecularly templated mesoporous films are still far inferior to those of conventional silicon oxide ILD and the former are liable to take up water. Before being implemented at sub-65 nm IC technology, mesoporous silica dielectrics must overcome the integration challenges. As stated in the preceding chapter, the hydrophobicity of a calcined silica film can be effectively improved by in-situ TMCS silylation in the sol precursor solution, and/or by HMDS vapor post-treatment. However, adding TMCS to the sol solution may disturb the self-assembly of template molecules and suppress the condensation reaction, resulting in the formation of mesoporous silica films with a less ordered pore structure and random micropores in the silica solid matrix. These changes mechanically weaken the TMCS-derivatized mesoporous films.

In this work, the relationship between the microstructure and the mechanical properties of the mesoporous silica films was studied by Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy and nanoindentation test. Hardness and elastic modulus depend strongly on the methods of preparation and modification. They are effectively enhanced by

trimethylsilylation of HMDS.

5.1 Microstucture of Mesoporous Silica Thin Films

The thickness of the thin mesoporous silica films prepared as described above can be well controlled within the range of 250-450 nm, as measured by cross-sectional SEM. Figure 5.1 shows the cross-sectional SEM image of an as-calcined mesoporous silica thin film with no film modification. The SEM image clearly shows that the mesoporous silica thin film has a smooth surface and a uniform thickness. The average surface roughness was estimated to be less than 10 Å (5 x 5 μm²) by AFM (Fig. 5.2). A smooth surface and a uniform thickness are essential to an accurate nanoindentation measurement for thin-film samples. According to the krypton adsorption/desorption isotherm, the total porosity of the as-calcined silica thin films ranges from 53% to 72% with an adjustable pore size from 43 to 80 Å, depending on the

Figure 5.1 Cross-sectional SEM image of as-calcined mesoporous silica thin film. The thickness is estimated to be about 300 nm.

Figure 5.2 AFM image of the as-calcined mesoporous silica thin film. The image size is 5 x 5 μm².

preparation of the precursor solution [16]. Self-assembled mesoporous silica thin films templated with P123 are known to have a hexagonal pore structure [104]. According to the XRD spectra shown in Fig. 5.3, the strong <100> diffraction signal reflects that the spin-coated mesoporous silica film without TMCS derivatization has an ordered pore-to-pore spacing of 61 Å after calcination. The single <100> peak without any other discernible diffraction signal reveals that the pore channel array is lying parallel to the silicon substrate surface. Compared with the sample set without TMCS modification, the TMCS derivatized mesoporous silica films have a weaker and broader <100> diffraction peak indicating a less ordered packing of the mesopores in the TMCS derivatized silica film. This suggests that the addition of TMCS in the sol solution significantly degrades the hexagonal pore structure of the mesoporous films. The <100> diffraction peak shifts to lower diffraction angles as the TMCS concentration in the sol precursor solution increases. When the TMCS molar ratio increases from 5 to 10%, the diffraction angle 2θ shifts from 1.42 to 1.29, corresponding to an increase in the pore-to-pore spacing from 64 to 70 Å. While the pore space increases with the TMCS concentration, there is no significant difference in the intensity and the full width at

Figure 5.3 XRD spectra of the as-calcined mesoporous silica thin films; (a) without TMCS derivatization, (b) with 10% molar ratio TMCS, (c) with 15% TMCS, and (d) 25% TMCS.

For clarity, the XRD spectrum of the mesporous film with 5% TMCS is not shown. The (100) diffraction peak of the silica film with 5% TMCS is situated at 2θ = 1.42, and has an intensity and a fwhm comparable to the mesoporous film with 10% TMCS.

half-maximum (fwhm) of the <100> diffraction peaks between the 5 and 10% TMCS derivatized mesoporous films. The diffraction peak of mesoporous silica films with a TMCS molar ratio over 15%, has a 2θ angle close to that of the film with 10% TMCS, but becomes very broad and almost vanishes. The presence of TMCS in the sol precursor not only hampers condensation reactions but also perturbs self-assembly of the surfactant micelles, and thus leads to the formation of mesoporous silica thin films with a less ordered microstructure for the pore network and the silica matrix. It seems that, under the film preparation condition used in the study, there is a critical TMCS concentration in the sol precursor solution, above which the formation of an ordered self-assembled pore channel array is seriously retarded. Moreover, the surface roughness increases abruptly when the TMCS/TEOS molar ratio is higher than 0.15. Some islands of 1 μm round base and 5-9 nm thickness were found on the silica film

surface as shown in Fig. 5.4. These islands are composed of SiO2 and the formation is closely related to the viscosity of precursor solutions [15].

FTIR was used to study the chemical structure of the mesoporous films. Figure 4.4 shows the FTIR spectra of the as-deposited, HMDS treated, and TMCS derivatized mesoporous silica thin films. As mentioned in Chap. 4, the as-calcined TMCS derivatized sample has a less extent of OH absorption (~3600 cm^-1) as compared with the as-calcined

Figure 5.4 SEM images of the as-calcined mesoporous silica thin films with 15% molar ratio TMCS derivatization. The magnification is (a) 500x and (b) 3000x.

(a)

(b)

mesoporous film. After treatment with HMDS vapor, both mesoporous silica thin films with and without TMCS modification are effectively trimethylsilylated as indicated by the two strong absorption peaks at 1258 and 2965 cm^-1, which are due to Si-CH3 and C-H3 stretching vibration modes, respectively. The broad absorption band in the range of 1080-1280 cm^-1 is due to asymmetric stretching of the intertetrahedral oxygen in the SiO2 network, and is usually assigned to an overlap of two asymmetric stretching (AS) modes. The absorption band has been widely studied for various silica materials, and, generally, is assigned to be the overlap of two pairs of transverse optical (TO) and longitudinal optical (LO) modes [167-169].

Figure 5.5 shows that the absorption band of the as-calcined mesoporous silica film without TMCS modification can be well resolved into four peaks by curve fitting, assuming a Gaussian shape for the absorption peaks. The low wavenumber peak at ~1069 cm^-1 is assigned to be a transverse optical mode (TO3), and is due to the stretching motion of oxygen atoms moving back and forth with respect to the adjacent silicon atoms and in phase with neighboring oxygen atoms. Paired with the TO3 absorption is the longitudinal optical mode (LO3) centered at ~1223 cm^-1. According to previous studies [167], another pair of TO-LO vibration modes (denoted by TO4-LO4 in the following text) is responsible for the overlapping absorption signal between the TO3 and LO3 peaks. The TO4-LO4 pair mode is due to the vibration in which oxygen atoms execute AS motion 180^o out of phase with neighboring oxygen atoms. Figure 5.6 illustrates these two AS motion of the TO3 and TO4 modes. The TO4

peak is estimated to center around 1177 cm^-1 and the LO4 is around 1125 cm^-1 from Fig. 5.5.

Compared with the as-calcined samples without TMCS modification, the TMCS derivatized mesoporous films have a higher absorbance for LO4 and TO4 vibration peaks as shown in Fig.

5.5.

Increasing TMCS concentration in the sol precursor results in a signal increase of the LO4-TO4 pair. Kirk has shown that disorder-induced mode coupling may result in

Figure 5.5 Curve fitting for the absorption band of the Si-O-Si asymmetric stretching modes of as-calcined mesoporous silica film without TMCS modification, assuming a Gaussian shape for the resolved peaks. For comparison, the absorption band of the TMCS (5%) derivatized film is also shown in the figure (dashed line). A broader and symmetric absorption feature at the low wavenumber side can be clearly seen for the TMCS derivatized film. The broader feature is ascribed to the presence of cyclosiloxane like rings as explained in the discussion.

enhancement of absorption of the TO4 and LO4 modes [168]. The absorption strength of the TO4 mode of bulk silica was estimated to be three to five times that of the same mode in quartz, indicating that microstructure disorder may effectively enhance the absorption strength

Figure 5.6 Asymmetric stretching motion of the (a) TO3 and (b) TO4 modes of the intertetrahedral oxygen in the SiO2 network.

of the out-of-phase AS mode [169]. Therefore, the observation of a larger absorbance of the TO4-LO4 pair for TMCS derivatized mesoporous silica films suggests that the addition of TMCS in the sol solution results in a less ordered microstructure in the silica matrix of the mesoporous network. In addition, the TO3 peak becomes broader and shows less symmetric at the low wavenumber side for the TMCS derivatized samples. This may be ascribed to a poorly ordered microstructure and stress effects due to the presence of terminal methyl groups inside the silica matrix [170]. Besides the two pairs of TO-LO modes, one more peak at the low wavenumber side (~1030 cm^-1) is needed to obtain a rational curve fit for the TMCS derivatized films. It is possible that random and small cyclic structures in the silica backbone, which are created due to incomplete condensation reactions and may be associated with cyclosiloxane like rings [171], are responsible for the broader and asymmetric absorption band. By analyzing the intensity variation of the absorption band of the Si-O-Si stretching modes, one can determine the relative orderliness of microstructure of the silica matrix in different mesoporous silica films. The FTIR spectra in the range of 975-1300 cm^-1 for the mesoporous silica thin films with and without TMCS derivatization are shown in Fig. 5.7.

Comparison of curves c and d in Fig. 5.7 shows that mesoporous film with 10% TMCS has a slightly higher TO4-LO4 absorption signal than the film with 5% TMCS indicating a less ordered microstructure for the former. Also shown in Fig. 5.7 is the absorption band of the HMDS treated mesoporous silica film (curve b). Compared with the as-calcined film (curve a), curve b shows little difference in peak shape and position except an additional peak at 1258 cm^-1 due to the Si-CH3 stretching. This suggests that trimethylsilylation by the HMDS vapor treatment makes little change in the chemical structure of the silica matrix of the mesoporous films except the increase in the surface density of terminal methyl groups.

Figure 5.7 FTIR spectra of mesoporous silica thin films in the range of the Si-O-Si asymmetric stretching modes; (a) the as-calcined film, (b) the as-calcined film after the HMDS treatment for 30 min, (c) the as-calcined film with 5% TMCS, (d) the as-calcined film with 10% TMCS. The small peak at 1258 cm^-1 is due to the Si-CH3 stretching mode.

5.2 Effect of Methylsilylation on the Mechanical Properties

Elastic modulus is an important property characterizing the ability of porous materials to withstand stress-induced deformations. The elastic modulus and hardness of the mesoporous silica thin films are shown in Fig. 5.8. The TMCS derivatized mesoporous thin films have a much smaller elastic modulus and a slightly lower hardness than mesoporous films without TMCS modification. This is expected from a microstructural view point for the mesoporous thin films with an ordered pore structure. The introduction of TMCS in the precursor solution decreases the degree of the cross-linkage of the silica network and suppresses the amount of silanol groups on the silica species [15]. The less ordered hexagonal pore structure and a broad pore size distribution, as revealed by the XRD results, deteriorate the mechanical strength of the mesoporous films. In addition, micropores were also present in the silica matrix of the TMCS derivatized mesoporous films according to our previous study [16]. For

Figure 5.8 Young’s modulus and hardness of the mesoporous silica thin films; (a) the as-calcined film, (b) the as-calcined film after the HMDS treatment for 30 min, (c) the as-calcined TMCS (5% molar ratio) derivatized film, (d) the TMCS (5% molar ratio) derivatized film after the HMDS treatment for 30 min, (e) the as-calcined TMCS (10% molar ratio) derivatized film, (f) the TMCS (10% molar ratio) derivatized film after the HMDS treatment for 30 min.

porous materials, while the k value decreases linearly with increasing porosity, the mechanical properties change as a power law with density or porosity [172]. The presence of micropores in the silica matrix not only supplies an additional porosity to the mesoporous film leading to a lower k value, but also deceases the density of the silica matrix. The molecularly templated mesoporous silica thin films prepared in this work can be considered as cellular material with a thick cell wall. For cellular materials, a reduction in the density of the cell wall certainly decreases the elastic modulus and other mechanical properties [173].

Nanoindentation measurements show that the mechanical strength of the mesoporous films can be greatly improved by trimethylsilylation. As shown in Fig. 5.8, the mesoporous silica films, both with and without the TMCS derivatization, gain an increase in hardness by a

factor of more than 75% after HMDS vapor treatment for 30 min. An increase of <20% in Young’s modulus was observed. The improvement of mechanical strength can be attributed to the presence of trimethylsilyl groups on the pore surfaces. The mechanical strength of the 5%

TMCS derivatized mesoporous film is somewhat weaker than that of the 10% TMCS derivatized film. As discussed previously, a higher TMCS content in the sol-gel precursor solution leads to the formation of a less ordered mesoporous silica network, resulting in weaker mechanical strength. The observation of the opposite trend is ascribed to that more terminal trimethylsilyl groups in the 10% TMCS derivatized film may enhance the mechanical strength due to a spring-back effect, and thereby compensate for the loss of the mechanical strength caused by the less ordered structure. Prakash et al. [174] and Smith et al.

[172] have reported a “spring-back” feature for aerogel and xerogel films during the drying stage. The thickness of the xerogel film or the volume of the silica gel which receives methylation treatments can recover to a certain extent after the gel reaches its greatest compaction. On the contrary, shrinkage of the gel is completely irreversible for those nonmodified mesoporous silica thin films. The spring-back feature is attributed to the presence of terminal organosilane groups that cannot participate in condensation reactions.

The electrostatic repulsion interaction between the crowded and bulky trimethylsilane groups on pore surfaces may effectively increase the resistance of the silica network to deformation under an applied load.

Table 5.1 lists some reported Young’s modulus of low-k materials. Most silica based low-k materials have better modulus than polymer dielectrics. However, it shows a corresponding variety on the modulus and k values, and silica low-k material with high porosity often contains a modulus value below 10. The mesoporous silica thin film with methylsilylation in this work shows outstanding characters on both mechanical strength and dielectric constant by comparison, and, therefore, it has more integration advantages on the

Table 5.1 Elastic modulus for low-k materials.

development of sub-65 nm IC technology nodes.

5.3 Summary

Microstructural and mechanical properties of organic surfactant templated mesoporous thin silica films have been studied by XRD, FTIR and nanoindentation. Compared with many other porous low-k dielectrics, the self-assembled molecularly templated mesoporous silica films demonstrate better mechanical properties. This is ascribed to the presence of a well-ordered pore channel structure in the mesoporous silica films. When the sol precursor solution is mixed with TMCS, the resulting mesoporous films have a weaker mechanical

strength. The pore channel structure of the mesoporous silica film becomes less ordered for the TMCS derivatized mesoporous films. In addition, FTIR analysis reveals that the chemical structure in the solid matrix of the porous network of the TMCS derivatized films is more disordered than those without TMCS modification. Trimethylsilylation by the HMDS vapor treatment can significantly improve the mechanical strength of the mesoporous silica thin films. The nanoindentation measurement results can be explained in terms of the pore microstructure of the mesoporous silica network and the spring-back effect due to the presence of trimethylsilyl groups in the nanopores.

Chapter 6

Effect of Trimethylsilylation on the Film Stress of Mesoporous Silica Ultralow-k Film Stacks

Mesoporous silica dielectrics have much poorer mechanical properties than their condensed counterparts and attention must be paid to their mechanical problems that arise upon integration into the Cu interconnect structure. One of the mechanical difficulties is the maintenance of satisfactory adhesion with adjacent layers and resistance against delamination or cracking during such processes as thermal and CMP. Amorphous hydrogenated silicon carbide (α-SiC:H) has many of the dielectric properties that are similar to those of silicon nitride used in the barrier/etch stop layer in the dual damascene structure, but it has a much lower dielectric constant (k<5) than that of PECVD silicon nitride [60]. α-SiC:H thin films are typically deposited by PECVD, and the properties of the film, including the microstructure, the chemical stability and the thermal stability, strongly depend on various process variables [175-185]. Properly choosing deposition parameters such as, in particular, the deposition temperature is, therefore, critical to the application of α-SiC:H in Cu interconnect technology.

Mesoporous silica thin films prepared by the sol-gel method have a large tensile stress after drying because of film shrinkage, and plasma-assisted α-SiC:H films typically have a compressive stress, depending on the deposition temperature. When these two dielectric materials are placed together in the dual damascene structure, the concern regarding the compatibility between their mechanical characteristics demands close examination of the film stress that develops in a film stack that has ultralow-k mesoporous silica and barrier/etch stop

layers. The chapter discusses the effect of methylsilylation on the stress behavior of the mesoporous silica thin films and its film stack with an α-SiC:H upper layer.

Trimethylsilylation relieved much of the tensile stress in the mesoporous silica thin film, and the compressive stress that developed in the α-SiC:H layer deposited by HDP-CVD could compensate for the intrinsic tensile stress of the mesoporous silica thin film, making the combination of these two dielectrics effective in the dual damascene process.

6.1 Volume Expansion by Spring-back Effect

The as-calcined mesoporous silica thin film has a very smooth film surface with a root-mean-squared roughness <1 nm according to atomic force microscopy (AFM) study.

The as-calcined mesoporous silica thin film has a very smooth film surface with a root-mean-squared roughness <1 nm according to atomic force microscopy (AFM) study.