M.F. Plass

Optical, electrical and mechanical properties of nitrogen-rich carbon nitride films deposited by inductively coupled plasma chemical vapor deposition

An inductively coupled plasma utilizing chemical transport reactions has been used to deposit thin carbon nitride films with a high nitrogen content [N/(C+N) of approximately 0.5 or higher]. We report on the characterization of the application relevant properties of these films, especially the optical (refractive index, transmission), electrical (dielectric constant, resistivity) and mechanical characteristics (stress, hardness, wear resistance). The refractive index is in the order of 1.5–1.8 depending on the deposition conditions; furthermore, the films are highly transmitting for wavelengths above 600 nm. C–V curves indicate the insulating character of the CNx films which was confirmed by I–V measurements yielding resistivities up to 1011 Ω cm at room temperature. The layers possess marginal stress as measured by the bending method on silicon cantilevers. The hardness is in the range of 1 GPa, and a friction coefficient of 0.6 was determined by ball-on-disc tests against stainless steel balls. The investigations showed that these films may be suited especially for optical and electrical applications. Finally, we correlate the films characteristics with the composition and structure of the coatings.

Stress of c-BN thin films: a parameter investigation

Abstract Boron nitride thin films were ion-beam-assisted deposited on bare silicon substrates or silicon cantilever structures. The resulting film stress was studied by determination of the resulting bending of the beam using either a profilometer or optical microscopy, respectively. Employing the latter method, the reliability of the stress measurement on the BN-coated cantilever structure could be increased. The theoretically predicted E I O N - 2 / 3 dependence of the resulting film stress on the ion energy was verified. Generally, the stress of the coatings increased with increasing content of cubic boron nitride (c-BN) in the sample. However, no correlation between the c-BN infrared peak position and the film stress could be observed. In addition, the development of stress — i.e., the distribution of the total film stress — was studied. Therefore, an as-deposited coating containing c-BN was subsequently resputtered and examined after each sputter step. The total stress of the non-cubic base layer increased gradually with increasing thickness. After the onset of c-BN growth, the film stress exhibited only a marginal increase. So far, any reduction of film stress after c-BN nucleation has not been observed.

A concept for the deposition of adherent cubic boron nitride films

Abstract Measures to reduce the high compressive stress of c-BN films and to improve their adhesion are discussed. A simple model of stress formation in c-BN films reveals that high ion energies and low Ar/N2 ratios should be used for stress reduction. These predictions are experimentally confirmed by means of depth resolved stress measurements using back-etching techniques with films deposited on cantilever substrates for accurate stress measurements. By suitable choice of ion energy and Ar/N2 ratio, the stress can be reduced by a factor of four to values below 5 GPa. The adhesion of c-BN films, on the other hand, can be improved by engineering the interface; controlled surface roughening and the deposition of gradient interlayer B→BN both lead to better adhesion. Based on these results, a multi-step process is proposed for the deposition of thick, well-adherent c-BN films.

Low-Pressure CVD of Carbon Nitride Using Triazine-Containing Precursors†

Carbon nitride (CxNy) layers with N/C ratios of 0.76–1.26 have been grown under low pressure from two organic chemical systems, namely 2,4-dichloro-6-bis(trimethylsilyl)imido-1,3,5-triazine [C3N3Cl2N(SiMe3)2] and tetramethylguanidine (C5H13N3) + 2,4,6-trichloro-1,3,5-triazine (C3Cl3N3). The films synthesized are transparent, well-adhered on silicon, homogeneous in morphology, and uniform in thickness. In addition, a technology has been developed for the deposition of carbon nitride powders with a N/C ratio of 1.3 as starting materials for further thermal treatment attempts in order to synthesize crystalline C3N4. At deposition temperatures between 400 and 800 °C and high supersaturation of reagents in the reactor, the powders obtained exhibit nuclei of crystalline phases.

Correlation between stress values of cubic boron nitride thin films and intrinsic film properties or the infrared peak position

Boron nitride thin films were ion beam assisted deposited on silicon substrates. The c-BN content was investigated with IR spectroscopy. The intrinsic film stress, studied by measuring the resulting bending of the substrate using a profilometer, increases generally with increasing c-BN content. In addition, the IR peak position of the c-BN reststrahlen band was determined for adherent coatings and after the films completely peeled off the substrate. This stress relief due to the peeling-off induces a shift of the IR peak position to smaller wavenumbers. However, no correlation between the total stress obtained by the bending experiments and the IR peak position for adherent films could be found. In contrast, the value of the shift of the c-BN peak position to smaller wavenumbers after peeling-off shows a good correlation with the stress values from the bending method. Several mechanisms which may influence the peak position, such as stress, coalescence and film thickness, will be discussed. Finally, we conclude that an estimate of c-BN film stress from IR data is only possible on the basis of the peak shift after peeling-off, which on the other hand means that stress measurement using the IR peak position has to be a destroying technique.

Plasma chemical vapor deposition of thin carbon nitride films utilizing transport reactions

Abstract Inductively coupled plasma chemical vapor deposition (ICP-CVD) has been used for the preparation of thin carbon nitride films from a solid carbon source (at floating potential) and nitrogen. Atomic nitrogen obtained as a result of the r.f. plasma activation interacts with the carbon source to form volatile carbon–nitrogen species. The latter are transported to the substrate where carbon nitride films are deposited in excess of atomic nitrogen. The supposed process mechanism was verified by microscopic observations of the carbon source before and after the process and by in situ mass spectrometric studies of the gas phase. The main advantage of the process is the possibility to obtain carbon nitride films with rates between 2 and 10 nm/min at rather low r.f. powers. The basic deposition parameters varied were the r.f. power (up to 100 W) and the working pressure in the reactor (up to 100 Pa). The surface topography of the films was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM); the deposited layers were found to be very smooth and uniform. The N/C ratio in the films was close to 1 as detected by Auger electron spectroscopy (AES) and elastic recoil detection (ERD) analysis. Infrared absorption and X-ray photoelectron spectra showed the presence of different carbon–nitrogen bonds in the layers.

The effect of d.c. substrate bias on the properties of nitrogen-rich CNx films

The influence of d.c. substrate bias on the properties of nitrogen-rich CNx films deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD) utilizing transport reactions has been investigated. The film forming species are CN and/or (CN)2 generated by the interaction of the atomic nitrogen from the ICP with a solid pure carbon mesh; they are deposited on the substrate in the presence of nitrogen species from the plasma. A study of the surface topography of the coatings by atomic force microscopy (AFM) shows that the average roughness slightly increases from below 1 nm without bias to 1.6 nm at −300 V. The deposition rate decreases by a factor of 1.3–1.5 (depending on the working pressure) with increasing the bias up to −300 V, mainly as a result of desorption of CN species from the substrate enhanced by the ion bombardment. The CNx films deposited with bias exhibit nitrogen atomic fraction N/(C+N) in the range of 50–60%, as revealed by surface and bulk techniques. The chemical bonding structure of the layers investigated by Fourier transform infrared (FTIR) spectroscopy showed only a marginal influence of the d.c. substrate bias. The increase of the refractive index n from 1.6 to 1.8 is probably due to slight densification of the films deposited with substrate biasing as a result of reduction of voids.

Depth resolved stress investigations of c-BN thin films

Abstract Cubic boron nitride thin films were deposited by ion beam assisted deposition (IBAD) on (100)-oriented silicon cantilever structures prepared by standard micro-machining processes. This enables an accurate determination of the stress-induced bending of the beam by optical microscopy. Depth resolved characterisation of the coatings was achieved by subsequent back-etching and examination of the film stress and the IR data after each sputtering step. Cubic BN containing films exhibit a three layer sequence: non-cubic baselayer/transition h-BN→c-BN/c-BN toplayer. This layered sequence was verified by the evolution of the IR data as well as the stress distribution. These investigations confirm the existence of a transition region between the h-BN baselayer and the c-BN toplayer. Furthermore, the dependence of the depth-resolved c-BN stress σc-BN on the main deposition parameters was investigated. A stress reduction can be achieved by reducing the Ar/N2 ratio and/or by increasing the ion energy. As this stress relief is correlated with an increase of the sp2 bonded material within the c-BN toplayer, it can be concluded that stress relaxation occurs at the sp2 bonded grain boundary material. Finally, the influence of the stress on the nucleation and the growth of c-BN containing films will be discussed.