Christian Förster

Low Temperature Chemical Vapor Deposition of 3C-SiC on Si Substrates

In the present work an UHVCVD method was developed which allows the epitaxial growth of 3C-SiC on Si substrates at temperatures below 1000°C. The developed method enable the growth of low stress or nearly stress free single crystalline 3C-SiC layers on Si. The influence of hydrogen on the growth process are be discussed. The structural properties of the 3C-SiC(100) layers were studied with reflection high-energy diffraction, atomic force microscopy, X-ray diffraction and the layer thickness were measured by reflectometry as well as visible ellipsometry. The tensile strain reduction at optimized growth temperature, Si/C ratio in the gas phase and deposition rate are demonstrated by the observation of freestanding SiC cantilevers.

Etching of SiC with Fluorine ECR Plasma

Electron cyclotron resonance (ECR) dry etching of 3C-SiC with different fluorinated gases, namely, sulfurhexafluoride (SF6) and tetrafluoromethane (CF4), was carried out. The influence of the gas flow, the etch gases and the applied bias voltages on the etch rate was studied. The maximum etch rates in the case of SF6 achieved were 1570 Å/min and 260 Å/min for Si and 3CSiC, respectively. In the case of CF4 the 260 Å/min (Si) and 160 Å/min (3C-SiC) were obtained. Furthermore, we investigate the selectivity of this dry etching process of SiC against Si. The residue free surface conditions were determined with Auger electron spectroscopy.

High-Resolution XRD Investigations of the Strain Reduction in 3C-SiC Thin Films Grown on Si (111) Substrates

In this work the biaxial stress of 3C-SiC thin films epitaxia lly grown on Si(111) substrates has been investigated by using x-ray diffraction methods. The influence of the resulting strain on the electrical properties of SiC/Si heterojunctions was an lyzed. Different methods to prepare the surface prior to the SiC deposition were compared: (i) ex situ carbonization, (ii) interface modification by deposition of Ge prior to epitaxial growth and (iii) annealing of the silicon surface. The x-ray measurements revealed the lowest strain in ex situ carbonized samples, showing a transition from tensile to compressive strain when off-axis substrates were used. The highest strain appeared in SiC layers grown on a thin Ge intermediate layer whi ch was deposited prior to SiC growth without an additional annealing step of the substrate. The strai n in the SiC layer is directly correlated with the reverse current through the heterojunction. Introduction Epitaxially grown mismatched semiconductor heterostructures are of increasing importance for microand optoelectronic devices or circuits. Lattice mismatched layers can be elastically strained by pseudomorphic growth on the substrate. Alternatively the strain can be relieved by relaxation of the epilayer due to formation of misfit dislocations resulting in a n in-plane lattice parameter of the epitaxial film close to that of the bulk material. If epitaxia l l yers of 3C-SiC are grown on Si substrates the large mismatch in the lattice constants and the thermal expansion coefficients lead to a substantial residual tensile strain. A significant part of the 20% mismatch in lattice constants can be released by the formation of a dislocation network. However, the mis match in thermal expansion coefficients of SiC and Si introduces an additional strain into the s yst m during the cooling down process after growth. This strain results in a strong degradation of the layer properties and a wafer warpage, limiting the use of SiC/Si hetrostructures for device a pplications and as pseudo substrate for the deposition of group III-nitrides. In this work we analyze the e ffect of different techniques to minimize the residual strain of the SiC layers and to improve the structural and electrical properties of the grown heterostructures. Experimental The 3C-SiC thin films (thickness ~120 nm) were grown by solid-source mol cular-beam epitaxy (MBE) on (111)-oriented onand off-axis Si crystal wafers at a substrate temperature of 1000°C with a growth rate around 1 nm/min. Prior to epitaxial growth the Si( 111) substrates were prepared by different methods. The first method uses an ex vacuo carbonization process at 1280°C in a propane-hydrogen atmosphere inside a rapid thermal processing (RTP) sy stem [1]. The MBE Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 233-236 doi:10.4028/www.scientific.net/MSF.433-436.233

FTIR Ellipsometry Analysis of the Internal Stress in SiC/Si MEMS

The resonant frequencies and quality factors of MEMS and NEMS depend critically on the layer quality and the residual stress in the SiC/Si heterostructure. It is demonstrated, that FTIRellipsometry is a suitable technique for monitoring the inhomogeneous residual stress inside SiC/Si heterostructures containing thin layers and their variation with during processing.

Morphology and Stress Control in UHVCVD of 3C-SiC(100) on Si

The influence of the growth conditions on the 3C-SiC layer quality in terms of crystallinity, morphology and residual strain was investigated. In dependence on the chosen growth conditions the stress state can be varied between inhomogeneous and homogeneous strain. For the reduction of the residual strain an alternative route for the improvement of the epitaxial growth of 3CSiC( 100) on Si(100) was developed. It consists in covering the silicon wafers with germanium prior to the carbonization step. The achieved improvement in the residual strain and crystalline quality of the grown 3C-SiC layers is comparable to SOI substrates. These beneficial effects were reached by using a Ge coverage in the range of 0.5 to 1 monolayer with respect to the silicon surface.

Low Energy Ion Modification of 3C-SiC Surfaces

The effects of argon and nitrogen bombardment of 3C-SiC surfaces at acceleration voltages below 2 keV were studied by stylus profilometry, reflectometry, reflection high energy electron diffraction and Auger electron spectroscopy (AES). The erosion rate of the SiC surface was determined. It was found that the sputtering rate for argon was three times higher compared to nitrogen. AES measurements revealed argon and nitrogen incorporation at a depth of a few nanometers as well as stoichiometric changes at the same depth scale independent of the acceleration voltage. In the case of the interaction of nitrogen ions with the 3C-SiC surface the formation of a SiCNalloy was detected.

Micromachining of novel SiC on Si structures for device and sensor applications

In this paper the multifariousness of SiC/Si heterostructures for device and sensor applications will be demonstrated. 3C-SiC based microelectromechanical resonator beams (MEMS) with different geometries actuated by the magnetomotive effect operating under ambient conditions were fabricated. The resonant frequency reaches values up to 2 MHz. The applications of these resonators are the measurement of the viscosity of liquids or mass detection. Furthermore, photonic devices in the form of SiC/Si infrared gratings for wavelength and polarization filters in infrared spectra are processed. SiC wear protection for a dosing system with the possibility to dose nano- or picoliter droplets of water based liquids as well as SiC nanomasking for catalytic agent nanostructures are demonstrated.

Infrared Gratings Based on SiC/Si-Heterostructures

The fabrication process and the spectral properties of gratings for the infrared wavelength region on the basis of 3C-SiC layers grown by CVD on (100) oriented Si substrates are demonstrated. The formed 3C-SiC gratings on Si support two phonon polaritons as a function of the geometrical properties excited between 10.3 and 11.4 µm. They appear as a dip in the transmission spectrum. A third minimum in the transmission spectrum is caused by the substrate – grating interaction. The obtained resonances were polarization sensitive, i.e. they appeared only under TMpolarized illumination.

Buckling Stabilization and Stress Reduction in SiC on Si by i-FLASiC Processing

One of the main challenging tasks in the prospective technology is the buckling suppression of the 3C-SiC film due to the melting and solidification process and the stress relief as a consequence of the short time Si melting during the Flash Lamp Annealing. To overcome this effect and to stabilize a flat surface morphology an alternative i-FlASiC process was developed. This work refers to the influence of the layer stack modifications by doping and meltstop formation by ion implantation on the wafer buckling. The samples were studied by transmission electron microscopy, high resolution x-ray diffraction and infrared ellipsometry. The aim was to optimize the doping and flash lamp annealing conditions in relation to the i-FLASiC layer stack modification.

Growth Acceleration in FLASiC Assisted Short Time Liquid Phase Epitaxy by Melt Modification

Flash lamp annealing of multilayer stack of the type SiC/Silicon overlayer(SOL)/SiC reduces the defect densities in the 3C-SiC/Si heteroepitaxial structure. Ge and C additions to the SOL lead to a substantial increase of the mass transfer from the upper layer to the lower SiC layer. If the Ge content of the SOL and the flash lamp annealing conditions are properly chosen a homogeneous layer with a 3C-SiC thickness between 150 and 200 nm can be achieved corresponding to a growth rate between 7.5 and 10.0 +m/s. The thickening of the lower layer depends on the SOL composition. Ge and/or C incorporation into the SOL and therefore into the Si melt enhances the mass transport from the upper SiC layer to the lower one.