Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film
Introduction
The development of smooth, thin diamond films using plasma-assisted chemical vapor deposition (CVD) techniques depends greatly on the surface condition. Surface pretreatment methods to enhance diamond nucleation and growth include seeding or activating the substrate surface by mechanical abrasion and chemomechanical polishing techniques which use diamond powders (or pastes) and hard abrasive particles [1], [2], [3], [4]. These surface treatments result in the deposition of residual diamond grits or carbonaceous precursors and/or the generation of high surface free energy nucleation sites through the removal of surface contaminants and the formation of surface steps, kinks and imperfections. However, such surface preparation methods are not appropriate for applications requiring undamaged surfaces such as optical windows, lenses and smooth wear-resistant coatings. Pretreatment methods yielding high diamond nucleation densities without altering the original surface topography are therefore highly desirable.
The most common approaches for achieving high nucleation densities without inducing permanent surface damage involve the deposition of different carbonaceous precursors such as carbon fibers [5], fullerene clusters [6], [7], and thin films of graphitic carbon [8], diamond-like carbon [9] and hard amorphous carbon (a-C) [10], [11]. The formation of an amorphous silicon carbide interfacial layer in bias-enhanced microwave plasma CVD has been reported to yield high diamond nucleation densities on unscratched and polished silicon and refractory metal surfaces, depending on the time of exposure to a methane-rich hydrogen plasma and the magnitude of the negative bias voltage applied to the substrate during the pretreatment [12], [13], [14], [15]. It was argued that the biasing process actually produces diamond nuclei, as opposed to just creating diamond nucleation sites, in addition to removing and/or suppressing the formation of a surface oxide [12].
Diamond nucleation densities in the range of 104–1011 cm−2 have been observed, depending on the thickness and microstructure of the predeposited carbon film [7], [11] and methane concentration, chamber pressure, and exposure time to a low temperature, methane-rich hydrogen plasma [16]. The highest diamond nucleation densities (∼1010 cm−2) and better quality diamond films were obtained with hard a-C films synthesized on smooth silicon surfaces by a vacuum arc plasma deposition technique (also known as cathodic arc) using a pulsed bias voltage of −100 to −200 V [17], [18]. Scanning electron microscopy and Raman spectroscopy studies revealed that diamond nucleation occurred from submicron residual clusters generated during the pretreatment [10]. The high diamond nucleation densities obtained were attributed to the large number of nanoparticles produced during the pretreatment which provided numerous high surface free energy nucleation sites, and the high resistance of the nanoparticles to etching by atomic hydrogen resulting from the significant percentage of tetrahedral (sp3) atomic carbon configurations [7], [10], [16], [17].
Although the previous studies have demonstrated that enhancement of the diamond nucleation density on smooth silicon surfaces subjected to a methane-rich hydrogen plasma is due to the formation of high density, etch-resistant residual nanoparticles (or clusters), detailed information about the structure of these residual clusters and the associated mechanism of diamond nucleation was not obtained, mainly due to the resolution limits of the microscopy and Raman spectroscopy techniques used in these investigations. The main objective of the present work, therefore, was to further investigate the nucleation and initial growth of diamond on pretreated smooth silicon substrates coated with a hard, ultrathin a-C film produced by cathodic arc deposition. High-resolution TEM was used to characterize the submicron residual clusters generated during the pretreatment and to observe their structural evolution during the nucleation and initial growth stages of the diamond films.
Section snippets
Experimental procedures
Commercially available smooth wafers of p-type Si(100) were used to cut 6.35-cm-diameter disks. The silicon disks (substrates) were coated with a hard a-C film using a filtered vacuum arc plasma source and film deposition system [19]. The cathodic arc films were synthesized while subjecting the substrate to a bias voltage of −100 V. Surface topography analysis performed with a mechanical stylus profilometer (Dektak IIA) revealed that the a-C films possessed a thickness of ∼80 nm and a surface
Characterization of pretreated specimens
The hard a-C films prepared by the cathodic arc technique were found to be featureless under TEM and as smooth as the silicon substrate. For a substrate bias voltage of −100 V, the films contained very small amounts of hydrogen (<1%) and exhibited the highest content of tetrahedral carbon bonds (∼85% sp3) and maximum hardness (35–40 GPa) [18]. Previous work has shown that the substrate bias voltage has a profound effect on the quality of cathodic arc a-C films and the diamond nucleation density
Conclusions
Diamond nucleation and growth on smooth Si(100) substrates coated with a hard a-C film having a high sp3 carbon content was investigated using a plan-view TEM technique. It was shown that a low-temperature pretreatment with methane-rich hydrogen plasma produces a carbon-saturated surface pervaded by submicron, carbon-rich residual clusters. TEM characterization demonstrated that the residual clusters consist of diamond nanocrystallites randomly distributed in an a-C phase possessing a
Acknowledgements
This research was supported by the Surface Engineering and Tribology Program of the National Science Foundation under Grant No. CMS-9734907. The authors gratefully acknowledge Z. Feng for experimental assistance, I. G. Brown from the Lawrence Berkeley National Laboratory for the use of the MPECVD facility, and Professor R. Gronsky from the Department of Materials Science and Mineral Engineering, University of California at Berkeley, for informative discussions on the TEM work.
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