Rodrigo S. Pessoa

Longitudinal magnetic field effect on the electrical breakdown in low pressure gases

The electrical breakdown has been investigated for low-pressure argon and nitrogen discharges under the influence of an external longitudinal magnetic field. Plane-parallel aluminum electrodes (5 cm diameter) separated by a variable distance d (4.0 cm < d < 11.0 cm) were sustained with a dc voltage (0 < V < 1 kV). A Helmholtz coil was used to produce an uniform magnetic field(B) parallel to the discharge axis. Paschen curves were obtained and the secondary electron emission coefficient (g), the first Townsend ionization coefficient (a) and the ionization efficiency(h), were plotted with respect to the variation of the reduced field (E/P). To observe the effect of the magnetic field these curves were plotted for fixed values of B=0 and B=350 Gauss. As consequence of the longitudinal magnetic field, the free paths of the electrons in the Townsend discharge are lengthened and their lateral diffusion is reduced, thus reducing electron losses to the walls. The data presented in this paper give a quantitative description of the B-field effect on the Townsends coefficients and overall it is concluded that the DC electrical breakdown of the gases is facilitated if a longitudinal magnetic field is applied along the discharge axis.

Study of SF6 and SF6/O2 plasmas in a hollow cathode reactive ion etching reactor using Langmuir probe and optical emission spectroscopy techniques

In this work, electrical and optical studies of SF6 and SF6/O2 plasmas generated in a hollow cathode reactive ion etching reactor were performed using the Langmuir probe and optical emission spectroscopy techniques, respectively. We carried out an investigation aimed at understanding the influence of radio-frequency power, gas pressure and O2 gas mixing ratio on plasma parameters, namely electron temperature, electron density and electronegativity, and also atomic fluorine density. The results indicate an increase of up to one order of magnitude in electron density and atomic fluorine in the overall gas volume when compared with a conventional reactive ion etching plasma generated under the same operation conditions.

Applications of SiC-Based Thin Films in Electronic and MEMS Devices

The great development of thin film growth techniques has stimulated the industrial and aca‐ demic researches about design, fabrication and test of thin film based devices. The replace‐ ment of the conventional bulk materials by thin films allows the fabrication of devices with smaller volume and weight, higher flexibility besides lower cost and good performance. It has been shown that the efficiency of thin film based devices is strongly dependent on their structural, electrical, mechanical and optical properties (Fraga, 2011a; Fraga 2012). At the same time that there is a trend in the miniaturization of electronic and electromechanical de‐ vices, there is also a considerable interest in the study of wide bandgap materials to replace the silicon as base material in these devices for harsh applications such as high temperatures and high levels of radiation (Fraga, 2012, Yeung, 2007).

Hollow cathode magnetron deposition of AlN thin films: crystalline structure and morphology

A new dc hollow cathode plasma source has been assembled whith a conventional planar magnetron cathode used together with another plane cathode plate to form a hollow cathode cavity. The system comprises two cathode plates of aluminium separated by a distance d, one of them acting as target of the magnetron cathode, the other being an ordinary plate. The discharge anode is a metallic flange of the vacuum chamber. This leads to enhanced ionization in the cathode cavity region and enables the discharge to operate at significantly lower pressures than for a typical planar magnetron configuration. As a consequence, sputtered atoms can reach a substrate with minimum energy loss due to collisions with filling gas atoms. The discharge gas was a mixture of argon and nitrogen. AlN thin films were grown on silicon substrates, at ambient temperature, and characterized with respect to the structure and morphology by XRD and AFM analyses respectively. The structure and roughness of the AlN films were studied as a function of the deposition parameters.

Recent Developments on Silicon Carbide Thin Films for Piezoresistive Sensors Applications

The increasing demand for microelectromechanical systems (MEMS) as, for example, piezoresistive sensors with capabilities of operating at high temperatures, mainly for automotive, petrochemical and aerospace applications, has stimulated the research of alternative materials to silicon in the fabrication of these devices. It is known that the high temperature operating limit for silicon-based MEMS sensors is about 150oC (Fraga, 2009). Silicon carbide (SiC) has shown to be a good alternative to silicon in the development of MEMS sensors for harsh environments due to its excellent electrical characteristics as wide band-gap (3 eV), high breakdown field strength (10 times higher than Si) and low intrinsic carrier concentration which allow stable electronic properties under harsh environments (Cimalla et al., 2007; Wright & Horsfall, 2007; Rajab et al., 2006). In addition, SiC exhibits high elastic modulus at high temperatures which combined with the excellent electronic properties make it very attractive for piezoresistive sensors applications (Kulikovsky et al., 2008). Silicon carbide can be obtained in bulk or film forms. In recent years, great progress has been made in the field of the growth of SiC bulk. Currently there are 6H-SiC, 4H-SiC and 3C-SiC wafers commercially available. However, these wafers are still very expensive (Hobgood et al., 2004; Camassel & Juillaguet, 2007), so encouraging studies on crystalline and amorphous SiC films deposited on silicon or SOI (Silicon-On-Insulator) substrates using appropriate techniques. The use of SiC films besides being less expensive has another advantage which is the well known processing techniques for silicon micromachining. The challenge is to obtain SiC films with mechanical, electrical and piezoresistive properties as good as the bulk form. Nowadays, some research groups have studied the synthesis and characterization of SiC films obtained by different techniques namely, plasma enhanced chemical vapour deposition (PECVD), molecular beam epitaxy (MBE), sputtering, among others, aiming MEMS sensors applications (Chaudhuri et al., 2000; Fissel et al., 1995; Rajagopalan et al., 2003; Lattemann et al., 2003).

Influence of process parameters on the growth of pure-phase anatase and rutile TiO2 thin films deposited by low temperature reactive magnetron sputtering

In this work is investigated the optimal conditions for deposition of pure- phase anatase and rutile thin films prepared at low temperatures (less than 150oC) by reactive dc magnetron sputtering onto well- cleaned p- type Si substrates. For this, the variation of deposition plasma parameters as substrate- to- target distance, total gas pressure, oxygen concentration, and substrate bias were studied and correlated with the characteristics of the deposited films. The XRD analysis indicates the formation of pure rutile phase when the substrate is biased at voltages between - 200 and - 300 V. Pure anatase phase is only attained when the total pressure is higher than 0.7 Pa. Moreover, its noticeable a strong dependence of surface roughness with parameters studied.

Relationships among growth mechanism, structure and morphology of PEALD TiO2 films: the influence of O2 plasma power, precursor chemistry and plasma exposure mode.

Titanium dioxide (TiO2) thin films have generated considerable interest over recent years, because they are functional materials suitable for a wide range of applications. The efficient use of the outstanding functional properties of these films relies strongly on their basic characteristics, such as structure and morphology, which are affected by deposition parameters. Here, we report on the influence of plasma power and precursor chemistry on the growth kinetics, structure and morphology of TiO2 thin films grown on Si(100) by plasma-enhanced atomic layer deposition (PEALD). For this, remote capacitively coupled 13.56 MHz oxygen plasma was used to act as a co-reactant during the ALD process using two different metal precursors: titanium tetrachloride (TiCl4) and titanium tetraisopropoxide (TTIP). Furthermore, we investigate the effect of direct plasma exposure during the co-reactant pulse on the aforementioned material properties. The extensive characterization of TiO2 films using Rutherford backscattering spectroscopy, ellipsometry, x-ray diffraction (XRD), field-emission scanning electron microscopy, and atomic force microscopy (AFM) have revealed how the investigated process parameters affect their growth per cycle (GPC), crystallization and morphology. The GPC tends to increase with plasma power for both precursors, however, for the TTIP precursor, it starts decreasing when the plasma power is greater than 100 W. From XRD analysis, we found a good correlation between film crystallinity and GPC behavior, mainly for the TTIP process. The AFM images indicated the formation of films with grain size higher than film thickness (grain size/film thickness ratio ≈20) for both precursors, and plasma power analysis allows us to infer that this phenomenon can be directly related to the increase of the flux of energetic oxygen species on the substrate/growing film surface. Finally, the effect of direct plasma exposure on film structure and morphology was evidenced showing that the grid removal causes a drastic reduction in the grain size, particularly for TiO2 synthesized using TiCl4.

Influence of the Al2O3 partial-monolayer number on the crystallization mechanism of TiO2 in ALD TiO2/Al2O3 nanolaminates and its impact on the material properties

TiO2/Al2O3 nanolaminates are being investigated to obtain unique materials with chemical, physical, optical, electrical and mechanical properties for a broad range of applications that include electronic and energy storage devices. Here, we discuss the properties of TiO2/Al2O3 nanolaminate structures constructed on silicon (1 0 0) and glass substrates using atomic layer deposition (ALD) by alternatively depositing a TiO2 sublayer and Al2O3 partial-monolayer using TTIP–H2O and TMA–H2O precursors, respectively. The Al2O3 is formed by a single TMA–H2O cycle, so it is a partial-monolayer because of steric hindrance of the precursors, while the TiO2 sublayer is formed by several TTIP–H2O cycles. Overall, each nanolaminate incorporates a certain number of Al2O3 partial-monolayers with this number varying from 10–90 in the TiO2/Al2O3 nanolaminate grown during 2700 total reaction cycles of TiO2 at a temperature of 250 °C. The fundamental properties of the TiO2/Al2O3 nanolaminates, namely film thickness, chemical composition, microstructure and morphology were examined in order to better understand the influence of the number of Al2O3 partial-monolayers on the crystallization mechanism of TiO2. In addition, some optical, electrical and mechanical properties were determined and correlated with fundamental characteristics. The results show clearly the effect of Al2O3 partial-monolayers as an internal barrier, which promotes structural inhomogeneity in the film and influences the fundamental properties of the nanolaminate. These properties are correlated with gas phase analysis that evidenced the poisoning effect of trimethylaluminum (TMA) pulse during the TiO2 layer growth, perturbing the growth per cycle and consequently the overall film thickness. It was shown that the changes in the fundamental properties of TiO2/Al2O3 nanolaminates had little influence on optical properties such as band gap and transmittance. However, in contrast, electrical properties as resistivity and mechanical properties as hardness and elastic modulus were shown to be very dependent. From these analyses, several applications could be suggested for different kinds of nanolaminates obtained in this work.

SixCy Thin Films Deposited at Low Temperature by DC Dual Magnetron Sputtering: Effect of Power Supplied to Si and C Cathode Targets on Film Physicochemical Properties

A DC dual magnetron sputtering system with graphite (C) and silicon (Si) targets was used to grow stoichiometric and non-stoichiometric silicon carbide (SixCy) thin films at low temperature. Two independently DC power sources were used to enable the total discharge power be shared, under certain proportions, between the Si and C magnetron cathodes. The motivation was to control the sputtering rate of each target so as to vary the stoichiometric ratio x/y of the deposited films. The species content, thickness and chemical bonds of as-deposited SixCy films were studied by Rutherford backscattering spectroscopy (RBS), profilometry analysis and Fourier transform infrared absorption (FTIR), respectively. Overall, the present work reveals a new reliable plasma sputtering technique for low temperature growth of amorphous SixCy thin films with the capability of tuning the degree of formation of a-SiC, a-Si and a-C bonds in the film bulk.

Flexible camphor diamond-like carbon coating on polyurethane to prevent Candida albicans biofilm growth

Camphor was incorporated in diamond-like carbon (DLC) films to prevent the Candida albicans yeasts fouling on polyurethane substrates, which is a material commonly used for catheter manufacturing. The camphor:DLC and DLC film for this investigation was produced by plasma enhanced chemical vapor deposition (PECVD), using an apparatus based on the flash evaporation of organic liquid (hexane) containing diluted camphor for camphor:DLC and hexane/methane, mixture for DLC films. The film was deposited at a low temperature of less than 25°C. We obtained very adherent camphor:DLC and DLC films that accompanied the substrate flexibility without delamination. The adherence of camphor:DLC and DLC films on polyurethane segments were evaluated by scratching test and bending polyurethane segments at 180°. The polyurethane samples, with and without camphor:DLC and DLC films were characterized by Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and optical profilometry. Candida albicans biofilm formation on polyurethane, with and without camphor:DLC and DLC, was assessed. The camphor:DLC and DLC films reduced the biofilm growth by 99.0% and 91.0% of Candida albicans, respectively, compared to bare polyurethane. These results open the doors to studies of functionalized DLC coatings with biofilm inhibition properties used in the production of catheters or other biomedical applications.