Silicon nitride and oxynitride films deposited from organosilicon plasmas: ToF–SIMS characterization with multivariate analysis
Introduction
Silicon nitride-like films (SiNx) are dense, hard, refractory materials which find several applications in optoelectronics as well as in the field of wear and corrosion protection. PECVD processes for SiNx deposition are generally based on SiH4 feeds mixed with NH3 and/or N2 [1], [2], [3], [4], [5], [6]. SiOxNy films are emerging materials especially for micro-electro-opto-mechanical systems (MEOMS) because of the possibility of tuning the refractive index and the dielectric constant as a function of the nitride-to-oxide character ratio. Mixtures of SiH4 and N2O (sometimes containing also NH3 or N2) or mixtures of SiH4–N2–O2 are generally used for depositing SiOxNy films [7], [8], [9], [10], [11]. Since SiH4 is a toxic and explosive gas, nowadays many organosilicon compounds less hazardous than SiH4 and widely used for plasma deposition of SiO2-like films, are under study as feed sources in PECVD of SiNx and SiOxNy coatings. Unfortunately low quality films, i.e. with high hydrogen and organic carbon content, are generally obtained with organosilicons [12], [13], [14].
Silicon nitride-like films at low substrate temperature and without reactive gas were lately deposited from inductively coupled discharges fed with bis(dimethylamino)dimethylsilane (BDMADMS) (Fig. 1) in mixture with Ar. In particular, it was observed that input power is a crucial parameter for reducing the amount of both organic groups and total carbon and therefore to pass from organic silicon nitride-like to fairly inorganic silicon nitride-like films [15]. Furthermore, it has been found that a tailored addition of oxygen to BDMADMS-Ar mixtures at high power allows the deposition of fairly inorganic silicon oxynitride-like films. Such composition is obtained in a narrow operating window (O2/monomer ratio = 0.5–1) since the nitride → oxide transition is highly activated by oxygen [16].
Static time-of-flight secondary ion mass spectrometry (ToF–SIMS) is extremely useful to investigate the composition and chemical structure of materials surface thanks to its high surface sensitivity (10–20 Å), analytical sensitivity, and direct relation between the chemical composition/structure of surfaces and the fragmentation patterns of ToF–SIMS spectra [17]. The use of static SIMS for characterizing plasma-deposited thin films has been reported in the literature; most of these works concerns with organic/polymeric matrixes [18], [19], [20], [21]. Studies on silicon-based coatings with static SIMS are quite rare. Generally, conventional silicon nitride and silicon oxynitride films, obtained from inorganic precursors, are analyzed by dynamic SIMS to determine the N, O, and H concentration at interfaces [22], [23], [24]. Few studies have been published on the use of static SIMS to characterize films obtained from PECVD of organosilicon compounds, such as hexamethyldisiloxane (HMDSO) [25], [26] or hexamethyldisilane (HMDS) [27], [28]. These investigations are limited to highly organic films and mainly to qualitative purposes.
The power of multivariate analysis of ToF–SIMS data for interpreting spectra and maximizing information from SIMS analysis of complex surfaces has been demonstrated and published [29], [30]. Among the multivariate methods, partial least squares (PLS) and principal component analysis (PCA) have been successfully applied to SIMS data for characterizing organic plasma polymer films [31], [32]. PCA transforms a number of related variables to a smaller set of uncorrelated variables. The new variables, called principal components (PCs) are linear combinations of all of the original variables and, therefore, capture more information than any one of the original variables. Consequently, a small set of PCs is required to capture all the important information in a data set [33].
In this work, thin silicon nitride-like and oxynitride-like films have been deposited on silicon substrates from inductively coupled (IC) plasmas fed with BDMADMS-Ar and BDMADMS-Ar-O2 mixtures. The objective of this study is to investigate the effect of deposition conditions, such as input power, for silicon nitride-like films, and O2/BDMADMS ratio, for silicon oxynitride-like films, on the films chemical composition using ToF–SIMS and PCA. The detected variations in surface composition as a function of input power and O2/BDMADMS ratio have been analyzed taking into consideration data obtained from infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS), in order to thoroughly characterize both silicon nitride-like coatings with a different organic character and inorganic silicon oxynitride-like films with a different nitride/oxide character.
Section snippets
Materials and sample preparation
The utilized inductive coupled plasma reactor consists of a RF (13.56 MHz) powered planar coil (two turns) placed on the top of a quartz plate covering a cylindrical stainless steel plasma chamber. The coil is surrounded by a high magnetic permeability ferrite which concentrates the magnetic field below the quartz plate [34]. The feed gas is admitted through a shower ring positioned close to this plate. The gas flow rates are controlled by MKS mass flow controllers, while the BDMADMS vapour
ToF–SIMS spectra of silicon nitride-like films
The positive ion ToF–SIMS spectrum of the film deposited from BDMADMS-Ar fed plasma at input power of 100 W is shown in Fig. 2a. The most intense peak is the Si+peak at 27.98 m/z. The peaks at 15.02 m/z, 43.01 m/z, and 73.05 m/z assigned to the CH3+, CH3Si+, and C3H9Si+ion fragments, respectively, are significantly intense as well. In addition, the spectrum is characterized by peaks attributed to the species CxHySi+and CxHyN+. However, only one CxHySiN+ peak, weakly intense, has been observed
Conclusions
Thin silicon nitride-like and oxynitride-like films were deposited from plasmas fed with BDMADMS. ToF–SIMS and PCA were used to investigate the effect of input power (silicon nitride-like) and O2/BDMADMS ratio (oxynitride-like) on the films surface chemistry.
PCA has been demonstrated to be an important tool for maximizing the information obtained from the ToF–SIMS spectra of silicon nitride-like and silicon oxynitride-like thin films. PCA of these complex spectra has revealed the differences
Acknowledgments
Financial support from the MPS Foundation, MiUR (PRIN 2005 — contract number 2005030003-003), and the European Community VI Framework Program (MATECO — contract number NMP3-CT-2003-505928) is gratefully acknowledged. F.M. Piras acknowledges MiUR for financial support through the “Incentivazione alla mobilità di studiosi stranieri e italiani residenti all'estero” Program. Dr. L. Bracchini is acknowledged for helpful discussions on PCA. JRC-Institute for Health and Consumer Protection is
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