Th. Welzel

Formation of c-BN thin films under reduced ion impact

A special multistep r.f. magnetron sputtering process with an h-BN target was investigated. After proper pretreatment a very thin c-BN film was grown under optimum conditions. For further film deposition the gas atmosphere was changed to pure nitrogen and the substrate bias voltage was reduced. Using Langmuir probe measurements we analyzed the ion flux towards the growing film and found that the strength of the ion impact sufficient to maintain c-BN growth is much smaller than that necessary to initiate the growth of c-BN. This has been demonstrated in terms of the total incorporated momentum per deposited atom as well as considering ion energy and ion-to-atom flux ratio. We conclude that continuation of c-BN growth after the formation of a certain initial c-BN layer is governed by an epitaxial mechanism.

Physics and phenomena in pulsed magnetrons: an overview

This paper reviews the contribution made to the observation and understanding of the basic physical processes occurring in an important type of magnetized low-pressure plasma discharge, the pulsed magnetron.In industry, these plasma sources are operated typically in reactive mode where a cathode is sputtered in the presence of both chemically reactive and noble gases typically with the power modulated in the mid-frequency (5?350?kHz) range. In this review, we concentrate mostly, however, on physics-based studies carried out on magnetron systems operated in argon. This simplifies the physical?chemical processes occurring and makes interpretation of the observations somewhat easier.Since their first recorded use in 1993 there have been more than 300 peer-reviewed paper publications concerned with pulsed magnetrons, dealing wholly or in part with fundamental observations and basic studies. The fundamentals of these plasmas and the relationship between the plasma parameters and thin film quality regularly have whole sessions at international conferences devoted to them; however, since many different types of magnetron geometries have been used worldwide with different operating parameters the important results are often difficult to tease out. For example, we find the detailed observations of the plasma parameter (particle density and temperature) evolution from experiment to experiment are at best difficult to compare and at worst contradictory.We review in turn five major areas of studies which are addressed in the literature and try to draw out the major results. These areas are: fast electron generation, bulk plasma heating, short and long-term plasma parameter rise and decay rates, plasma potential modulation and transient phenomena. The influence of these phenomena on the ion energy and ion energy flux at the substrate is discussed. This review, although not exhaustive, will serve as a useful guide for more in-depth investigations using the referenced literature and also hopefully as an inspiration for future studies.

Optical properties of nitrogen-rich carbon films deposited by d.c. magnetron sputtering

Abstract Nitrogen-rich carbon films were grown by d.c. magnetron sputtering from a graphite target in various nitrogen/argon gas mixtures using two different substrate temperatures. Infrared spectroscopical investigations were carried out in order to determine differences in the film composition and structure caused by the variation of the deposition conditions. The spectra are interpreted in terms of two bands which are typically found in the Raman spectra of CN α films. While the band at 1280 cm −1 only slightly increases in absorption intensity with increasing nitrogen incorporation, the band at 1520 cm −1 becomes much more pronounced. Both bands shift to higher wavenumbers with increasing nitrogen content. Additionally, the films show a decrease in the absorption above 2500 cm −1 . The existence of different functional groups absorbing at 2130 cm −1 can be suppressed by substrate heating up to 400°C during preparation, CN groups absorbing above 2200 cm −1 , however, remain in the samples. In order to support the IR results additional ellipsometric measurements, investigations of film density and stoichiometry were performed. Heating the substrates during deposition leads to a reduced nitrogen incorporation.

Process diagnostics during the deposition of cubic boron nitride

Abstract Using spatially resolved optical emission spectroscopy and Langmuir double probe technique, the magnetron deposition process of cubic boron nitride thin films has been investigated. The ion current to the r.f-biased susbstrate electrode was estimated by means of Bohms sheath criterion. In order to deposit the cubic boron nitride phase, a much higher ion energy is required in the d.c. magnetron in comparison to the r.f. sputtering magnetron mode at usually applied target power. Furthermore, there is a significant phase inhomogeneity across the substrate holder. Both facts have been explained in terms of the total momentum per deposited boron atom. The plasma excitation degree (vibrational and excitation temperatures) determined by emission spectroscopy was found to be higher in the r.f. sputtering mode. It has been shown that both in situ techniques applied can supply reliable information on the reactive magnetron deposition process.

Spectroscopic studies of a magnetron sputtering discharge for boron nitride deposition

Abstract Magnetron sputtering discharges for the deposition of cubic boron nitride have been investigated using optical emission spectroscopy and Langmuir double probes. Spatially resolved measurements revealed the inhomogeneous discharge structure with respect to the distribution of excited species, electron density and electron temperature. The optical emission spectra were analysed by means of the corona model. The vibrational excitation of the nitrogen molecule was examined. The ratio of selected Ar and N 2 emission intensities provided a qualitative measure of the electron temperature, which was determined by probe measurements. However, the intensity ratios of Ar and Ar + lines did not correspond to changes in electron temperature. Varying the electron density in a wide range, it could be demonstrated that the excitation mechanisms do not remain constant, mainly due to multistep excitation. The vibrational temperature of N 2 (C 3 IIΠ u ) has been found to be much higher in the case of r.f. sputtering than in the d.c. mode.

A time-resolved Langmuir double-probe method for the investigation of pulsed magnetron discharges

Langmuir probes are important means for the characterization of plasma discharges. For measurements in plasmas used for the deposition of thin films, the Langmuir double probe is especially suited. With the increasing popularity of pulsed deposition discharges, there is also an increasing need for time-resolved characterization methods. For Langmuir probes, several single-probe approaches to time-resolved measurements are reported but very few for the double probe. We present a time-resolved Langmuir double-probe technique, which is applied to a pulsed magnetron discharge at several 100 kHz used for MgO deposition. The investigations show that a proper treatment of the current measurement is necessary to obtain reliable results. In doing so, a characteristic time dependence of the charge-carrier density during the “pulse on” time containing maximum values of almost 2∙1011cm−3 was found. This characteristic time dependence varies with the pulse frequency and the duty cycle. A similar time dependence of the...

Determination of radical densities by optical emission spectroscopy during the ECR plasma deposition of Si-C-N : H films using TMS as a precursor

The ECR plasma enhanced chemical vapour deposition has been studied by optical emission spectroscopy in the substrate region. Tetramethylsilane (TMS) was used as precursor for the deposition of Si-C-N:H thin films in Ar-N2 and Ar-N2-H2 gas mixtures. Using absolute electron-impact cross sections available in the literature absolute densities of H, CH, and Si have been calculated from the emission intensities of these species according to the corona model with respect to electron temperatures obtained by Langmuir probes. The contribution of dissociative excitation out of H2 and TMS to the emission of the particles is accounted for. The investigations were carried out varying the precursor flow for different microwave powers, substrate potentials, and gas mixtures. By comparison of the radical number densities in the gas phase and the atomic deposition rate, conclusions are drawn concerning the deposition mechanisms of the Si-C-N:H films.

Comparison of the radio frequency hollow cathode to the microwave antenna discharge for plasma processing

Two nonconventional systems, the radio frequency hollow cathode discharge (RHCD) and the microwave antenna discharge (MWAD), with almost identical geometry of electrodes generating a nitrogen plasma at power level up to 60 W were compared. Both systems were used for deposition of nitride films at similar experimental parameters. The Al–N films were deposited in the RHCD system by reactive physical vapor deposition (PVD) using an Al radio frequency hollow cathode and the CNx films were deposited in the MWAD system by plasma activated chemical vapor deposition (PACVD) from N2+1% (alternatively 0.5% or 0.4%) C2H2 gas mixtures. The vibrational temperatures of nitrogen molecules in both systems were compared as functions of experimental parameters and discussed with respect to the film growth rates in the particular systems. It was found that irrespective of frequency difference of two orders of magnitude the vibrational temperatures of nitrogen molecules were similar, between 3000 and 4600 K, in both systems ...

Influence of aluminum additions on phase formation in boron nitride films deposited by magnetron sputtering

Boron nitride (BN) thin films have been deposited by magnetron sputtering of an hexagonal BN (h-BN) target with an additional flux of aluminum provided by a special Al sputtering target. Up to an aluminum percentage of about 1.0 at. % the formation of the cubic BN (c-BN) phase was not influenced by the aluminum addition. An Al content of 1.3 at. % was connected with a remarkable delay in the transition from h-BN to c-BN growth as well as a certain deterioration in the crystal quality. An aluminum flux causing a concentration of 1.5 at. % did prevent the growth of the cubic BN phase completely. The dynamics of impurity incorporation were investigated using secondary ion mass spectrometry measurements. We found that the aluminum electrode plays an important role in the incorporation of hydrogen in the films. In connection with other results it can be concluded that for both h-BN and c-BN strong evidence for substitutional incorporation of aluminum on boron sites exists. Cross-section transmission electron m...

Characterisation of a BN magnetron sputtering process with an additional aluminium sputter electrode

Abstract The deposition of boron nitride (BN) films by r.f. magnetron sputtering of a hexagonal BN target with intended additional aluminium incorporation has been studied using various in situ plasma diagnostic techniques. The aluminium flux towards the substrates was supplied by an additional ring-shaped aluminium electrode mounted all around the circular magnetron. Langmuir double probe investigations reveal only little influence of the electrode position and electrostatic potential on the magnetron discharge. The most appropriate electrode position was found to be levelled with the anode of the magnetron. The aluminium density in the gas phase was investigated by laser-induced fluorescence and optical emission spectroscopy (OES) yielding an increase with the square of the electrode power. Moreover, a strong dependence on the Ar–N 2 gas mixture was observed which is explained by nitride formation on the surface. OES measurements also imply a significant hydrogen and boron contamination of the electrode during its power-off phases.