Polycrystalline SiC Thin Films Research Articles

Effects of thermal annealing on thermal conductivity of LPCVD silicon carbide thin films

The thermal conductivity (k) of polycrystalline SiC thin films is relevant for thermal management in emerging SiC applications like microelectromechanical and optoelectronic devices. In such films, k can be substantially reduced by microstructure features including grain boundaries, thin film surfaces, and porosity, although these microstructural effects can also be manipulated through thermal annealing. Here, we investigate these effects by using microfabricated suspended devices to measure the thermal conductivities of nine low-pressure chemical vapor deposition SiC films of varying thicknesses (from 120 to 300 nm) and annealing conditions (as-grown and annealed at 950 and 1100 °C for 2 h, and in one case 17 h). Fourier-transform infrared spectroscopy, x-ray diffraction spectra, and density measurements are also used to characterize the effects of annealing on the microstructure of selected samples. Compared to as-deposited films, annealing at 1100 °C typically increases the estimated grain size from 5.5 to 6.6 nm while decreasing the porosity from around 6.5% to practically fully dense. This corresponds to a 34% increase in the measured thin film thermal conductivity near room temperature from 5.8 to 7.8 W/m K. These thermal conductivity measurements show good agreement of better than 3% with fits using a simple theoretical model based on the kinetic theory combined with a Maxwell–Garnett porosity correction. Grain boundary scattering plays the dominant role in reducing the thermal conductivity of these films compared to bulk single-crystal values, while both grain size increase and porosity decrease play important roles in the partial k recovery of the films upon annealing. This work demonstrates the effects of modifying the microstructure and, thus, the thermal conductivity of SiC thin films by thermal annealing.

Crystal Growth of Epitaxial 3C-SiC Thin Film on Si Substrate by Chemical Vapor Deposition using Single Precursor of Vinylsilane

Cubic silicon carbide (3C-SiC) has attracted much attention since it has a wide bandgap, high dielectric breakdown strength, and high thermal stability. 3C-SiC promises various electronic applications such as power devices and MEMS. Also, 3C-SiC has an advantage for a lower cost among many SiC polytypes, as it can grow epitaxially on Si substrate. Generally, two types of precursor source materials, including Si and C, are necessary to grow 3C-SiC thin films for chemical vapor deposition (CVD) method. In addition, complex processes including carbonization with a temperature as high as about 1200 °C are required for conventional epitaxial growth of 3C-SiC [1]. We focus on vinylsilane as a single precursor material for SiC growth. The Si-C bond in vinylsilane is relatively stable due to the relationship of intramolecular bond energy, that is appreciable for SiC growth. The growth of SiC crystals using vinylsilane can be achieved at a low temperature of 1000 ºC or less and the deposition process will be simplified. Recently, we have achieved the formation of polycrystalline SiC thin films on metal substrate using CVD method only with vinylsilane [2]. In this study, we examine the epitaxial growth of 3C-SiC and also report phosphorous (P) doping for SiC thin films grown on Si substrate.SiC thin films were deposited on Si(001) and Si(111) substrates using CVD method with a vinylsilane precursor (manufactured by Japan Advanced Chemicals Co., Ltd.). The substrate temperature of the growth was ranging from 900 to 1000 °C, the growth pressure was 17 kPa, and the growth time was 60 min.The surface morphology of deposited film was observed using scanning electron microscopy (SEM). We can confirm that the uniform morphology of SiC film was formed on the Si substrate for a sample prepared with Si(111) substrate at 1000 ºC and also that there are no voids or defects at the interface.We also examined X-ray diffraction measurements to characterize the crystalline structure. Out-of-plane XRD measurement (2θ/ω method) shows a sufficiently large diffraction peak related to 3C-SiC(111) for the Si(111) sample prepared at 1000 ºC, which means the growth of a well (111)-oriented 3C-SiC thin film on Si(111) substrate. On the other hand, in the case of the grazing angle XRD measurement (2θ method) for the SiC/Si(001) samples prepared at both 900 and 1000 ºC, we observed many diffraction peaks meaning the formation of polycrystalline 3C-SiC. The analysis of peak intensity and full width at half maximum value (FWHM) revealed that the crystallinity of SiC film improved with increasing the growth temperature. Raman scattering spectroscopy measurement also revealed that the dominant bonding structure in the film is Si-C and not C-Hn or C-C for samples prepared at higher than 900 ºC, that means the formation of a high crystalline quality SiC thin film.In our presentation, we will discuss pros and cons of epitaxial growth of SiC films on Si(111) substrates and also reports P-doping into the SiC films will be discussed.

Fracture Analysis of a-SiC:H Membranes after Thermal Annealing

In this study, fracture analysis of hydrogenated amorphous silicon carbide (a-SiC:H) membranes deposited by inductively coupled plasma enhanced (ICP) CVD is performed using a bulge test approach. It is shown, that a thermal post-deposition annealing results in a significantly increased fracture strength of the membranes due to the formation of additional Si-C bonds in the material. The residual stress at burst is higher compared to similar thin films deposited by different techniques and even close to polycrystalline SiC thin films, demonstrating the outstanding potential of ICP-CVD deposited a-SiC:H thin films for the realization of robust MEMS devices.

Characterization of Thermal Expansion Coefficient of LPCVD Polycrystalline SiC Thin Films Using Two Section V-beam Actuators

In this paper we present the characterization of the coefficient of thermal expansion (CTE) of in-situ doped polycrystalline SiC thin films, obtained by low pressure chemical vapor deposition (LPCVD). The material is characterized using V-beam actuators on which the temperature coefficient of resistance (TCR) and the in-plane displacement versus current are measured. A CTE value of 4.3 ± 0.4 ppm/K is obtained in the temperature region of 20°C to 300°C. This value is used in a finite element modeling (FEM) simulation of vertical SiC-SiO2 bimorph beams. For an actuator length of 700 μm, width of 100 μm and layer thickness of 2 μm, a displacement up to 200 μm can be obtained.

Ambient condition laser writing of graphene structures on polycrystalline SiC thin film deposited on Si wafer

We report laser induced local conversion of polycrystalline SiC thin-films grown on Si wafers into multi-layer graphene, a process compatible with the Si based microelectronic technologies. The conversion can be achieved using a 532 nm CW laser with as little as 10 mW power, yielding ∼1 μm graphene discs without any mask. The conversion conditions are found to vary with the crystallinity of the film. More interestingly, the internal structure of the graphene disc, probed by Raman imaging, can be tuned with varying the film and illumination parameters, resembling either the fundamental or doughnut mode of a laser beam.

Low Stress Polycrystalline SiC Thin Films Suitable for MEMS Applications

This paper details the development of low residual stress and low stress gradient unintentionally doped polycrystalline SiC (poly-SiC) thin films. The films were deposited in a large-volume, low-pressure chemical vapor deposition (LPCVD) furnace on 100 mm-diameter silicon (Si) wafers using dichlorosilane (SiH2Cl2) and acetylene (C2H2) as precursors. We found that the flow rate of SiH2Cl2 could be used to control the residual film stress in the as-deposited films. Wafer curvature measurements for ∼2 μm-thick films indicated that tensile stress ranging from 4 to 55 MPa across a 25 wafer boat had been achieved. A variety of micromachined structures including lateral resonant structures, stress pointers and cantilevers were fabricated for characterization of the deposited SiC films. The average Young’s modulus was found to be 403 GPa. Residual stress measurements were consistent with those obtained using a wafer curvature technique. Interferometric measurements of cantilever beams indicated stress gradients with an upper bound of 52 MPa/μm for ∼2 μm-thick films with tensile stress less than 55 MPa.

Silicon carbide thin films as nuclear ceramics grown by laser ablation

Silicon carbide has been identified as a potential inert matrix candidate for advanced fuel. In this work, the growth of SiC thin films by pulsed laser deposition is reported. The stoicheometry and thickness of deposited films was investigated by non-Rutherford backscattering spectrometry. The influence of the deposition parameters, i.e. substrate temperature and laser fluence on the structure, morphology and optical properties of the deposited thin layers was studied. It was found that polycrystalline SiC thin films with uniform surface morphology were obtained at 873 K.

3C-SiC Films on Si for MEMS Applications: Mechanical Properties

Single crystal 3C-SiC films were grown on (100) and (111) Si substrate orientations in order to study the resulting mechanical properties of this material. In addition, poly-crystalline 3C-SiC was also grown on (100)Si so that a comparison with monocrystaline 3C-SiC, also grown on (100)Si, could be made. The mechanical properties of single crystal and polycrystalline 3C-SiC films grown on Si substrates were measured by means of nanoindentation using a Berkovich diamond tip. These results indicate that polycrystalline SiC thin films are attractive for MEMS applications when compared with the single crystal 3C-SiC, which is promising since growing single crystal 3C-SiC films is more challenging. MEMS cantilevers and membranes fabricated from a 2 µm thick single crystal 3C-SiC grown on (100)Si under similar conditions resulted in a small degree of bow with only 9 µm of deflection for a cantilever of 700 µm length with an estimated tensile film stress of 300 MPa. Single crystal 3C-SiC films on (111)Si substrates have the highest elastic and plastic properties, although due to high residual stress they tend to crack and delaminate.

Effects of Annealing on Residual Stress and Strain Gradient of Doped Polycrystalline SiC Thin Films

We report the effects of annealing at 925 and 1050°C in argon ambient on the resistivity, average residual stress, and strain gradient of highly doped polycrystalline 3C-SiC films deposited at 800°C and 170 mTorr from 1,3-disilabutane, dichlorosilane, and ammonia. Residual stress shifts from -569 to 274 MPa, compressive strain gradient shifts from 4 X 10 -4 to -0.018 μm -1 , and resistivity shifts from -0.037 to 0.030 Ω cm. Characterization of the SiC films reveals that oxygen impurity diffusion from the ambient during annealing and a change in bonding of nitrogen dopant atoms are sources of the shifts in these properties.

Formation of β-SiC nanowires by annealing SiC films in hydrogen atmosphere

The single crystalline β-SiC nanowires were grown through annealing polycrystalline SiC thin films in H 2 at 1150 °C. The SiC thin films were deposited on Si (1 1 1) substrate by radio frequency magnetron sputtering at room temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were employed to determine the structure, composition and surface morphology of the synthesized one-dimensional SiC nanostructures. SEM results show the diameters of SiC nanowires vary between 20 and 60 nm with length up to 50 μm. XRD and TEM confirm the grown SiC nanowires are single crystalline β-SiC.

Characterization of Polycrystalline SiC Thin Films for MEMS Applications using Surface Micromachined Devices

Characterization of Polycrystalline SiC Thin Films for MEMS Applications using Surface Micromachined Devices

Optical and structural properties of SiC layers grown by an electron cyclotron resonance CVD technique

Large area microcrystalline and polycrystalline SiC thin films have been grown by electron cyclotron resonance CVD over (100) silicon wafers. The samples have been characterized by X-ray diffractometry (XRD), transmission electron microscopy (TEM), micro-Raman and photo luminescence spectroscopy. Stoichiometric SiC films containing 3C–SiC crystals with orientation close to that of Si substrate and lateral grain dimension up to 1400 Å were obtained under suitable deposition conditions. They also exhibit blue photoluminescence at room temperature in the range of 400–450 nm with features dependent on their structure.

プラズマCVD法によるTMSを用いた多結晶SiC薄膜の生成

プラズマCVD法によるTMSを用いた多結晶SiC薄膜の生成

Ion implantation doping of polycrystalline SiC thin films prepared by PECVD

Ion implantation doping experiments have been performetion fine-grained, polycrystalline SiC films deposited onto insulating and optically transparent sapphire substrates. The efficiency of implantation doping has been found to be critically dependent on the actual implantation and annealing sequence. Using optim and implantation and annealing sequences efficient n-type doping has been obtained after implantation of N and P impurities. Implantation of Al under the same conditions leads to p-type doping. All films exhibit UV photoconductivity. Plasma hydrogenation has been found to reduce the density of grain boundary defects in the polycrystalline SiC films.

プラズマＣＶＤ法による多結晶ＳｉＣ薄膜

プラズマＣＶＤ法による多結晶ＳｉＣ薄膜