Mariana A. Fraga

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).

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).

Fabrication and characterization of piezoresistive strain sensors for high temperature applications

In this work, we studied the fabrication and characterization of strain sensors based on semiconductor materials for high temperature applications: non-stoichometric amorphous silicon carbide (a-Si<inf>x</inf>C<inf>y</inf>) thin film and SOI (Silicon-On-Insulator) substrates. a-Si<inf>x</inf>C<inf>y</inf> were deposited onto thermally oxidized (100) Si wafers by plasma enhanced chemical vapor deposition (PECVD) technique using silane (SiH<inf>4</inf>) and methane (CH<inf>4</inf>) as precursor gases. The SOI wafer used had a sandwich structure of a 0.2 µm thick top p-type Si layer, 0.5 µm thick buried oxide (BOX) layer and 250µm Si substrate. The piezoresistive strain sensitivity element in the sensors is a a-Si<inf>x</inf>C<inf>y</inf> thin-film resistor or a Si p-type resistor formed on SOI substrate. Gauge factor (GF) measurements were done using the beam-bending method. One resistor of each type was bonded near the clamped edge of a stainless steel cantilever beam and on the free edge were applied different forces. The electrical resistance of each resistor was measured without applied load on the beam and during subsequent tensile load. The temperature coefficient of resistance (TCR) also was investigated from room temperature up to 250°C. The results indicate that the a-Si<inf>x</inf>C<inf>y</inf> thin-film resistor has a GF of 48 and a TCR of 35 ppm/°C whereas p-type Si on SOI substrate has gauge factor of 22 and TCR of 140 ppm/°C.

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.

Comparison between the Piezoresistive Properties of a-SiC Films Obtained by PECVD and Magnetron Sputtering

This work compares the piezoresistive properties of SiC thin films produced by two techniques enhanced by plasma, PECVD (plasma enhanced chemical vapor deposition) and RF magnetron sputtering. In order to study these properties, strain gauges based on SiC films produced were fabricated using photolithography techniques in conjunction with lift-off processes. The beam-bending method was used to characterize the SiC strain gauges fabricated.

Evaluation of piezoresistivity properties of sputtered ZnO thin films

Zinc oxide (ZnO) thin films were deposited by RF reactive magnetron sputtering on silicon (100) substrates under different experimental conditions. ZnO films were studied before and after annealing treatment at 600 °C. The crystallinity, electrical resistivity, stoichiometry, thickness, and elastic modulus of the films were investigated. ZnO piezoresistors were produced using microelectronics processes, such as photolithography, lift-off, and reactive ion etching (RIE). Cantilever method was used to determine the gauge factor, and measurements of Temperature Coefficient of Resistance (TCR) were performed on a hotplate. The optimization of the deposition conditions produced ZnO thin films with controlled stoichiometry (ZnO), crystalline microstructure (phase wurzite, 002), high elastic modulus (156 GPa), and low electrical resistivity (0.072 ohm.cm), which are good properties for application as piezoresistive pressure microsensor. In addition, the ZnO piezoresistors had a GF of 2.6 on the deformation in the plane (100) and TCR of –1610 ppm/K up to 250 °C.

Nano- and microcrystalline diamond deposition on pretreated WC–Co substrates: structural properties and adhesion

Many developments have been made to improve the quality and adherence of CVD diamond films onto WC–Co hard metal tools by the removing the cobalt from the substrate surface through substrate pretreatments. Here we compare the efficiency of three chemical pretreatments of WC–Co substrates for this purpose. First, the work was focused on a detailed study of the composition and structure of as-polished and pretreated substrate surfaces to characterize the effects of the substrate preparation. Considering this objective, a set of WC–9% Co substrates, before and after pretreatment, was analyzed by FEG-SEM, EDS and x-ray diffraction (XRD). The second stage of the work was devoted to the evaluation of the influence of seeding process, using 4 nm diamond nanoparticles, on the morphology and roughness of the pretreated substrates. The last and most important stage was to deposit diamond coatings with different crystallite sizes (nano and micro) by hot-filament CVD to understand fully the mechanism of growth and adhesion of CVD diamond films on pretreated WC–Co substrates. The transition from nano to microcrystalline diamond was achieved by controlling the CH4/H2 gas ratio. The nano and microcrystalline samples were grown under same time at different substrate temperatures 600 °C and 800 °C, respectively. The different substrate temperatures allowed the analysis of the cobalt diffusion from the bulk to the substrate surface during CVD film growth. Furthermore, it was possible to evaluate how the coating adhesion is affected by the diffusion. The diamond coatings were characterized by Raman spectroscopy, XRD, EDS, FEG-SEM, atomic force microscope and 1500 N Rockwell indentation to evaluate the adhesion.

Study on the Correlation between Film Composition and Piezoresistive Properties of PECVD SixCy Thin Films

The effects of carbon content on the piezoresistive properties of non-stoichiometric silicon carbide (SixCy) films deposited on thermally oxidized (100) Si substrates by Plasma Enhanced Chemical Vapor Deposition (PECVD) from silane (SiH4) and methane (CH4) gas mixtures have been investigated. Four different film compositions have been obtained by varying SiH4 flow ratios from 1.0 to 4.0 sccm, while the other parameters were kept constant. In order to evaluate the piezoresistive properties of the SixCy films, we have developed test structures consisting of SixCy thin-film resistors defined by reactive ion etching (RIE) with Ti/Au pads formed by lift-off process. The gauge factor (GF) and temperature coefficient of resistance (TCR) of each SixCy film were measured.