Characterization of amorphous silicon carbide and silicon carbonitride thin films synthesized by polymer-source chemical vapor deposition mechanical structural and metal-interface properties
Awad, Yousef Odeh
Amorphous silicon carbide (a-SiC) and silicon carbonitride thin films have been deposited onto a variety of substrates by Polymer-Source Chemical Vapor Deposition (PS-CVD). The interfacial interaction between the a-SiC films and several substrates including silicon, SiO[subscript 2], Si[subscript 3]N[subscript 4], Cr, Ti and refractory metal-coated silicon has been studied. The effect of thermal annealing on the structural and mechanical properties of the prepared films has been discussed in detail. The composition and bonding states are uniquely characterized with respect to the nitrogen atomic percentage introduced into the a-SiCN:H films. Capacitance-voltage (C-V) measurements were systematically used to evaluate the impurity level of the deposited a-SiC films. The chemical bonding of the films was systematically examined by means of Fourier transform infrared spectroscopy (FTIR). In addition, elastic recoil detection (ERD) and X-ray photoelectron spectroscopy (XPS) techniques were used to determine the elemental composition of the films and of their interface with substrates, while X-ray reflectivity measurements (XRR) were used to account for the film density. Spectral deconvolution was used to extract the individual components of the FTIR and XPS spectra. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were also employed to characterize the surface morphology of the films. In addition, their mechanical properties [(hardness (H) and Young's modulus (E)] were investigated by using the nanoindentation technique. The impurity levels of the a-SiC films were found to be clearly correlated with the nature of the underlying substrates. The Pt-Rh and TiN-coated Si substrates were shown to lead to the lowest impurity level (~ 1×10[superscript 13] cm[superscript -3]) in the PS-CVD grown a-SiC films, while Cr and Ti-coated Si substrates induced much higher impurity concentrations. Such high impurity levels were shown to be a consequence of a strong metallic diffusion of the metallic species (Cr or Ti). In contrast, no diffusion was observed at the interface of a-SiC with either Pt-Rh or TiN. Our results pinpointed TiN-coated Si as the electrode material of choice that satisfied best all criteria required for the integration of a-SiC into opto-electronic devices. FTIR measurements revealed that not only the intensity of a-SiC absorption band linearly increased, but also its position is found to shift to a higher wave number as a result of annealing. In addition, the bond density of Si-C is found to increase from (101.6-224.5)×10[superscript 21] bond[dot]cm[superscript -3] accompanied by a decrease of Si-H bond density from (2.58-0.46)× 10[superscript 21] bond[dot]cm[superscript -3] as a result of increasing the annealing temperature (T[subscript a]) from 750 to 1200 [degrees]C. Annealing-induced film densification is confirmed by the XRR measurements, as the a-SiC film density is found to increase from 2.36 to ~ 2.75 g/cm[superscript -3] when T[subscript a] is raised from 750 to 1200 [degrees]C. In addition, as annealing temperature T[subscript a] is increased from 750 to 1200 [degrees]C, both hardness and Young's modulus are found to increase from 15.5 to 17.6 GPa and 155 to 178 GPa, respectively. On the microstructural level, the increase incorporation of N in the a-SiCN:H films is found not only to lead to C atoms substitution by N atoms in the local Si-C-N environment but also to the formation of a complex structure between Si, C and N. For instance, the FTIR spectra show a remarkable drop in the intensity of Si-C vibration accompanied by the formation of further bonds including Si-N, C-N, C=N, C[identical to]N and N-H with increasing NH[subscript 3]/Ar ratio. Moreover, the XPS spectra showed the existence of different chemical bonds in the a-SiCN:H films such as Si-C, Si-N, C-N, C=N and C=C. Both FTIR and XPS data demonstrate that the chemical bonding in the amorphous matrix is more complicated than a collection of single Si-C, Si-N, or Si-H bonds. Furthermore, the increase incorporation of N in the a-SiCN:H films resulted in an increase of the average R[subcsript rms] surface roughness from 4 to 12 nm. Moreover, the films became porous with pore size and density increase as a result of increasing N at.%. Ultimately, both H and E of the a-SiCN:H films were found to be sensitive to their N content, as they decreased (from ~17 GPa and 160 GPa to ~13 GPa and 136 GPa, respectively) when the N content was increased from 0 to 27 at.%. The formation of Si-N, Si-H, and N-H bonds at the detriment of the more stiffer Si-C bonds are thought to account for the observed lowering of the mechanical properties of the a-SiCN:H films such as their N content increased. Our results confirmed the previously-established constant-plus-linear correlation between the mechanical properties of the a-SiC films and their bond densities.
- Génie – Thèses