Materials Science Forum Vols. 615-617

Abstract: Diamond saw is generally used to make the silicon carbide (SiC) wafers from ingots, but it takes long time for cutting. We have used the electric discharge machining (EDM) to cut SiC. The cutting speed of EDM for SiC is almost 10 times faster than the diamond saw, and the surface roughness is 1/10 for the diamond saw. EDM cut SiC by the plasma produced between the wire and SiC material. The emissions from EDM plasma may involve much information for EDM cutting. We monitored the total light intensity by a photodiode, and observed the spectrum of the emission from EDM plasma by a visible spectroscopy. The discharge gas used helium and argon. In both discharges, the light emission was observed when the current was not zero. Also, many lines were observed Si I, Si II and C I from the SiC sample, and Cu I and Zn I from the wire. And, the electron temperature of EDM plasma was estimated to be under several eV because the observed lines were almost the emission from atoms. Also, the scars, which show the copper-alloy wire was hurt by discharge, were observed from the wire.

Abstract: SiC power devices have reached a high market penetration, especially for high-voltage applications like switch mode power supplies. In the past, however, the superior material properties like, e.g., good thermal conductivity, have often not been put to full use due to the limitations of current packaging techniques. Especially the inferior thermal conductivity of current die attach materials have been an obstacle to realise the full potential of SiC technologies. In this paper, we describe in detail the use of diffusion solder for the die attach of SiC chips. Replacing the conventional solder layer by a thin metal stack for diffusion soldering, the thermal conductivity of the device is significantly improved. In addition, we show the positive impact of diffusion soldering on the assembly process and on the device reliability. These results are interesting for, both, SiC diodes and switches.

Abstract: The fabrication of freestanding SiC microstructures on Silicon-On-Insulator (SOI) and semi-insulating Silicon substrates is reported. SiC layers were grown on SOI and semi-insulating Si by chemical vapour deposition (CVD) and to avoid the instability currently obtained in SOI structures, the growth process parameters have been optimized. Isotropic wet chemical etching of the Si sacrificial layer released the electrostatic SiC microstructures patterned by dry etching. Moreover a new concept for reducing the gap between resonators and electrodes by the uses of bistable mobile electrodes is introduced.

Abstract: The adjustment of the properties of 3C-SiC based MEMS devices, i.e. the quality factor and resonant frequency, was achieved by changing the residual stress and the 3C-SiC material quality of the SiC-layers grown on Si(111) by manipulating the nucleation conditions and growth conditions with Ge deposition prior to the carbonization and epitaxial growth. Previous Raman analysis of the SiC-layers and measured resonant frequencies and quality factors of the processed MEMS show a dependence on the Ge amount at the interface of the Si/SiC heterostructure, which allows to adjust the MEMS properties to the requirements needed for certain applications.

Abstract: Cantilever resonators have been fabricated from two types of materials, single crystal and polycrystalline 3C-SiC films. The films have been grown in a hot-wall chemical vapor deposition reactor on 100 mm diameter p-type boron-doped (100) Si wafer without rotation of the wafer. The crystal structure of the films have been accessed with X-ray diffraction. The cantilever devices have been fabricated using a one-step etch and release process; the beam length has been varied between 50 and 200 µm. Resonant frequencies in the range 110 KHz – 1.5 MHz and 50 – 750 KHz have been obtained for single crystal and polycrystalline SiC devices, respectively. Furthermore, the experimental resonance frequencies have been used to calculate the Young’s Modulus E for the two different types of SiC. The single crystal SiC, possessing a very high Young’s Modulus (446 GPa), should be an optimal material for RF-MEMS applications.

Abstract: The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication. This approach puts together the standard silicon bulk microfabrication methodologies with the robust mechanical properties of 3C-SiC. Using this approach we were able to fabricate SiC cantilevers for a new class of pressure sensor. In the present research, chemical vapour deposition (CVD) in the low pressure regime of 3C–SiC on silicon substrates was carried out using silane (SiH4), propane (C3 H8) and hydrogen (H2) as the silicon supply, carbon supply and gas carrier, respectively. The resulting bow in the MEMS structures was evaluated optically and the residual stress in the films calculated using the modified stoney equation and determined to be approximately 300 MPa.

Abstract: 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.

Abstract: We report on constructive methods providing a large range of high purity porous SiC products. All methods are based on modified sol-gel processes combined with carbothermal re¬duction. We obtain monodisperse regular pores of well defined diameters by using carbon sphere templates which are removed after SiC infiltration. A different way is a sol-gel based conversion of graphite bodies into SiC, which transfers the porosity from the graphite matrix into the final SiC product. Thus a large variety of porosity features are available, originating either from natural poro¬sity of graphite or from priorly created nano-/ microstructures in the carbonaceous base material. Whereas all our pristine porous sol-gel derived silicon carbide products are semi-insulating, doping is possible, during the growth to modifiy the electrical and optical properties.

Abstract: The Ti/4H-SiC Schottky barrier diodes with a field limiting ring (FLR) structure are fabricated. Two types of SBDs are prepared; one (SBD-A) is covered and another (SBD-B) isn’t covered with a carbon cap during high temperature annealing after ion implantation. The breakdown voltage at room temperature for SBD-A and SBD-B are 1400 V and 1000 V, respectively. The breakdown for both SBDs occurs due to an avalanche breakdown. The light emission images are obtained at the breakdown voltage by photo emission microscope (PEM). The light emission is observed along an FLR of the SBD-A as designed. On the other hand, the spot of light emission is observed on a FLR structure of the SBD-B. This light emission spot indicates that leakage current is concentrated because an electrical field concentration is generated at this one for the SBD-B. The root-mean-square roughness of the Al-implanted region on the FLR structure calculated from the atomic force microscopy (AFM) images for the SBD-A and the SBD-B are 0.697 nm and 5.58 nm, respectively. Therefore it is considered that large surface roughness on the FLR decreases breakdown voltage of SBD because an electrical field concentration is generated at a spot.

Abstract: We have studied different Schottky and ohmic contacts on 4H-SiC with the aim to obtain Schottky barrier diodes (SBDs) capable to operate at high temperatures, frequencies and power densities for long periods of time, and showing low power losses. The control of the Schottky barrier plays an important role in minimizing the power loss of a SBD, and the metal-semiconductor interface properties strongly affect the overall performances of such a device. Schottky contacts were deposited using Ni, Ti, Ti/Al, Mo and Mo/Al layers, and the annealing treatments have been performed up to 600 °C using a rapid thermal annealing process (RTA). Ohmic contacts have been deposited on the wafer backside using Ti/Al or Ti/Ni/Ag layers. The Schottky diodes have been characterized by means of standard current-voltage (I-V) and capacitance-voltage (C-V) techniques. Schottky diodes with Mo and Mo/Al barriers show a lower barrier height and better overall performances in forward polarization when compared to the Ti- and Ni-based contacts.