W. Kulisch

Mechanisms of plasma polymerization of various silico-organic monomers

The plasma polymerization of hexamethyl disiloxane (HMDSO), divinyltetramethyl disiloxane (DVTMDSO), and octamethyl cyclotetrasiloxane (OMCATS) was investigated using either argon or argon/oxygen mixtures as reactants. With argon, different mechanisms of polymerization for each polymer were identified reflecting its respective nature. For HMDSO, formation of Si-CH2-Si bonds plays a decisive role, whereas in DVTMDSO Si-(CH2)4-Si bridges are formed. In both cases a second mechanism exists leading to the loss of SiCxHy groups. Finally, OMCATS polymerizes via the formation of polymethylsiloxane chains and rings. These various mechanisms result in differences in the properties of the polymer films. In contrast, almost inorganic films were obtained with oxygen irrespective of the monomer. An investigation of the deposition kinetics shows that the plasma power and the monomer evaporation temperature are the decisive parameters determining the polymer growth rates. A comparison of direct and remote plasma reaction reveals that in the cases of HMDSO and OMCATS direct plasma dissociation is necessary to promote the polymerization process, whereas DVTMDSO also polymerizes under remote conditions.

Recent results in cubic boron nitride deposition in light of the sputter model

Abstract In the field of c-BN deposition, ion-assisted processes are most common and best investigated. Understanding of the deposition mechanisms is the basic key to improve film quality and, therefore, to realize potential applications. Recently, Reinke et al. developed a sputter model for ion-assisted c-BN deposition in which material removal processes (sputtering and desorption) play major roles [1]. This model is used as a guideline in order to give an overview on c-BN deposition. The first part investigates the growth mechanisms of ion-assisted c-BN deposition. Therefore, a data collection which includes CVD and PVD deposition methods was performed. The resulting dependencies are discussed in light of the sputter model. By comparison with other models, it is found that most of the experimental facts can best be described in terms of the sputter model, whereas in the nucleation step compressive stress plays an additional role. In the second part, c-BN deposition is considered in a wider context. Current problems and some first approaches to overcome these problems are discussed. Furthermore boron nitride is compared with the sp 3 carbon modifications tetrahedral amorphous carbon (ta-C) and diamond. It is concluded that c-BN and ta-C rely on different growth mechanisms; the investigation of extended parameter ranges in the search for new ion-assisted carbon and BN modifications is proposed. Some approaches in c-BN deposition without ion assistance are discussed in accordance to diamond deposition. Nevertheless, the advances are small and no process yielding similar high-quality material as CVD diamond exists.

Nucleation of cubic boron nitride (c-BN) with ion-induced plasma-enhanced CVD

Abstract Nucleation and growth of c-BN films by means of the inductively coupled plasma technique was investigated. Two different sets of experiments were performed: deposition as a function of the bias voltage V B and experiments with varying deposition times. The films were characterized by Fourier transform IR spectroscopy, Auger electron spectroscopy and ellipsometry. Without substrate bias, crystalline h-BN was obtained, oriented with the c -axis normal to the surface. With sufficient bias voltage, a layer of vertically oriented h-BN was observed prior to the formation of c-BN, in agreement with transmission electron microscopy measurements published recently in the literature. We observed further a significant increase in the boron to nitrogen ratio prior to the formation of c-BN. Preferential sputtering of nitrogen with respect to boron with increasing ion bombardment seems to be the reason for this behaviour.

Mechanisms in ion induced c-BN growth

Abstract A literature survey on ion beam assisted deposition of boron nitride shows well defined ranges (domains) of microscopic parameters in which different phases of boron nitride occur. These are interpreted in terms of a growth model in which temperature dependent desorption of incoming material and selective sputtering of h-BN play dominating roles. This concept is transferred to chemical vapour deposition in which microscopic parameters cannot be quantified well. For this case a simple rule is developed which also shows the influence of sputtering during deposition. c-BN is compared with diamond-like carbon with respect to dominating deposition mechanisms.

Evaluating probes for “electrical” atomic force microscopy

The availability of very sharp, wear-proof, electrically conductive probes is one crucial issue for conductive atomic force microscopy (AFM) techniques such as scanning capacitance microscopy, scanning spreading resistance microscopy, and nanopotentiometry. The purpose of this systematic study is to give an overview of the existing probes and to evaluate their performance for the electrical techniques with emphasis on applications on Si at high contact forces. The suitability of the characterized probes has been demonstrated by applying conductive AFM techniques to test structures and state-of-the-art semiconductor devices. Two classes of probes were examined geometrically and electrically: Si sensors with a conductive coating and integrated pyramidal tips made of metal or diamond. Structural information about the conductive materials was obtained by electron microscopy and other analytical tools. Swift and nondestructive procedures to characterize the geometrical and electrical properties of the probes p...

Fabrication of integrated diamond cantilevers with tips for SPM applications

Abstract The fabrication of diamond cantilevers with diamond tips integrated on silicon wafers for scanning probe microscopy (SPM) applications is reported. Hot filament CVD diamond deposition and standard techniques of silicon micro-machining are employed. The deposition of well-developed tips depends critically on the pretreatment applied to enhance nucleation density. With an optimized process, well-shaped tips with a radius of curvature in the order of 30 nm are obtained. According to micro-Raman investigations they consist of high quality diamond. Another critical step is the definition of the cantilever area. It can be solved by proper process design. Preliminary performance tests show the cantilevers to possess high resonance frequencies.

Deposition of TiO2 thin films by plasma-enhanced decomposition of tetraisopropyltitanate

Abstract We investigated the suitability of tetraisopropyltitanate as a source gas for plasma-enhanced deposition of titanium dioxide thin films. Whereas TiO2 deposition using a standard planar reactor as well as a remote plasma process with O2 as reactant were found difficult to control, stoichiometric, carbon-free TiO2 can be deposited if the remote process with inert reactants such as argon or xenon is used. Deposition rates of 15 nm min-1 can be achieved. The refractive index, being a most important property in view of possible applications of TiO2 films as optical coatings, can be varied from 1.8 to 2.5 by suitable choice of deposition parameters. Film stoichiometry was found to be almost parameter independent whereas film density ϱ and, strongly correlated with ϱ, properties such as the refractive index and the chemical resistance are mainly influenced by processing temperatures, in situ during deposition as well as ex situ by post-deposition annealing.

Parameter spaces for the nucleation and the subsequent growth of cubic boron nitride films

Abstract The data existing in the literature about the deposition of cubic boron nitride thin films were reviewed critically in order to establish the parameter spaces of c-BN nucleation and growth. The ion energy Ei, the flux ratio F (=incoming ions/incoming boron atoms), the ion mass mi, (or the ratio Ar/N2, respectively), and the substrate temperature Ts, had already been identified as the decisive parameters which are, however, interdependent. Earlier data collections on c-BN deposition had shown that, irrespective of the deposition technique used, a well-defined c-BN region exists in the F/Ei parameter space, in which the deposition of c-BN is possible. Similar regions exist in the F/mi and F/Ts parameter spaces. The present collection extends these older diagrams considerably, especially to the low energy region. From this extention it can be concluded that the momentum transfer concepts proposed in the literature fail to explain the data. Furthermore, the older collections were considered valid for nucleation and growth likewise. However, in recent years data have been published showing that the boundaries of the c-BN regions are different for nucleation and growth. After successful nucleation, subsequent growth can occur either at reduced ion bombardment (either energy or flux ratio or ion mass) and also at reduced temperatures. The existing data for this parameter reduction have been collected in this paper. It will be shown that the growth depends in a similar way as the nucleation on the (interdependent) ion bombardment parameters but no longer on temperature. This means that the nucleation and growth of c-BN are based on different, although in both cases ion-induced, mechanisms.

Investigation of stress and adhesion of cubic boron nitride films

Abstract Cubic boron nitride (c-BN) films can be deposited by PVD and CVD methods. Massive ion bombardment of the growing film is required, leading to compressive stress and therefore poor adhesion. In a simple approach, stress is modelled by the balance of defect formation, due to ion bombardment, and thermal as well as ion-induced recombination processes. The model predicts stress to be reducible using high ion energies and high substrate temperatures, whereas the ion to atom flux ratio is of minor importance only. New results concerning c-BN nucleation show an orientational relationship between an initial textured h-BN layer and the first c-BN crystals. The influence of interlayers prior to c-BN deposition on the nucleation sequence and film adhesion is discussed.

FABRICATION OF SMALL DIAMOND TIPS FOR SCANNING PROBE MICROSCOPY APPLICATION

A process for the fabrication of diamond cantilevers integrated on a silicon wafer is presented. At one end the cantilevers possess a tip with a small radius of curvature thus allowing their use in scanning probe microscopy applications. The influence of various procedures to enhance diamond nucleation of the properties of the tips is investigated. Ultrasonic pretreatment with 1 μm diamond paste and subsequent hot-filament chemical-vapor deposition turned out to yield the best results. Micro-Raman measurements show the tips to consist of stress-free diamond up to their very apex.