Materials Science Forum Vols. 740-742

Abstract: In this paper, we propose a new wafer slicing method for silicon carbide(SiC). SiC is well-known as a difficult-to-cut material, and a conventional slicing via multi-wire saw becomes more difficult with increasing ingot size. To solve this problem, the multi-wire electrical discharge slicing (EDS) method is applied to 100 mm-square SiC polycrystalline block. We successfully obtained the average thickness of 385 μm for nine sliced plates by ten-wires EDS. The thickness variation was measured to be less than 11.2 μm. This is the first demonstration of ten-wires EDS for 100 mm-square SiC material.

Abstract: A new method of electric discharge machining (EDM) is proposed for slicing a large silicon carbide (SiC) ingot in order to realize low kerf loss and fast cut. This principle is based on the rotating ingot, and it is called the rotating slicing method (RSM). It would be defecate the cutting chip effectively and one-point discharge. In this paper, we reported results of examinations of the RSM experimentally. Unstable discharge was not observed. Discharge damages on the wire surface were fewer than those of the conventional method. Net cutting speed was almost the same as the present method for the 2-inches ingot. The rotation axis of the ingot should be perpendicular to the feed direction of the wire, and it is important to fix the performance of the EDM such as the kerf loss. Roughness of the cutting surface was 3.4 µm of R_a

Abstract: In this study, we developed aA novel abrasive-free polishing method called the catalyst-referred etching (CARE) has been developed. CARE can chemically remove SiC chemically with using an etching agent activated by a catalyst. Platinum and hydrofluoric (HF) acid are used for the planarization of SiC substrates as a catalyst and etchant, respectively. CARE can produce an atomically flat surface of 4H–SiC (0001) with a root-mean-square roughness of less than <0.1 nm, regardless of the cut-off angle. However, the mechanism of CARE has hasis not yet been clarified to date. In this study, to clarify the mechanism, KF and NH4F are added to the etchant to clarify the mechanism. The An investigation of removal rate revealeds that the removal rate is proportional to [HF]×([F^- ]+[〖HF_2〗^- ]), and it is shown that both the HF molecule and fluorine ions (F− and HF2−) arethe reactive species of the CARE process are both HF molecule and fluorine ions (F- and HF2-).

Abstract: Pressureless silver sintering is an interesting die-attach technique that could overcome the reliability limitations of the power electronic devices caused by their packaging. In this paper, we study the manufacturing parameters that affect the die attach: atmosphere, drying time, heating ramp rate, sintering temperature and duration. It is found that sintering under air gives better results, but causes the substrates to oxidize. Sintering under nitrogen keeps the surfaces oxide-free, at the cost of a weaker attach.

Abstract: The multiple-zone junction termination extension (MJTE) is a widely used SiC edge termination technique that reduces sensitivity to implantation dose variations. It is typically implemented in multiple lithography and implantation events. To reduce process complexity, cycle time, and cost, a single photolithography/implantation (P/I) MJTE technique was developed and diodes with 3-zone and 120-zone JTEs were fabricated on the same wafer. Here, the process tolerance of the single (P/I) MJTE technique is evaluated by performing CCD monitored blocking voltage measurements on diodes from the same wafer with the 3-zone and 120-zone single (P/I) JTE. The 3-zone JTE diodes exhibited catastrophic localized avalanches at the interface between the 2nd and 3rd zones due to abrupt zone transitions. Diodes with the smooth transitioning 120-zone JTE exhibited no CCD detectable avalanches in their JTE regions up to the testing limit of 12 kV. Under thick dielectric (deposited for on-wafer diode interconnection), diodes with the single P/I 3-zone JTE failed due to significant loss of high-voltage capability, while their 120-zone JTE diode counterparts were minimally affected. Overall, the single (P/I) 120-zone JTE provides a process-tolerant and robust single P/I edge termination at no additional fabrication labor.

Abstract: Solar-to-hydrogen conversion efficiencies of water photolysis with epitaxially grown p-type 4H-, 6H- and 3C-SiC were estimated in the two electrode system. For all the polytypes, the efficiency with a Pt counter electrode in the two electrode system was very low compared with those in the three electrode system. However, when Ni was used as a counter electrode in the two electrode system, photocurrents were as large as the three electrode system. The estimated efficiencies seem to depend on the bandgap of SiC polytypes, and the highest solar-to-hydrogen conversion efficiency was 0.38% with 3C-SiC.

Abstract: Mesa-epitaxial 4H-SiC p⁺-p-n_o-n⁺-diodes were fabricated and their reverse recovery characteristics were measured in pulse regimes to be relevant to DSRD- and SOS-modes of operation [I.V. Grekhov, G.A. Mesyats, Physical basis for high-power semiconductor nanosecond opening switches, IEEE Transactions on Plasma Science 28 (2000) 1540-1544]. It has been found that after short pumping the diodes by forward current pulse (5-ns duration, 200-A/cm² peak current density) followed by applying the reverse voltage pulse (rise time 2 ns) the diodes are able to interrupt the reverse current density of 3.5 - 25 kA/cm² in a time less than 0.3 ns.

Abstract: Silicon carbide power devices are intended and to enter new application regimes in power electronics, in fact, they are enabling components mainly if higher switching frequencies in power electronics are considered. This trend can be clearly observed since power density can be increased and efforts towards passive components and other mechanical contributions to the system can be reduced. However, this trend imposes new challenges towards the surrounding of the chips in form of the package itself and the whole system around. Stray components like inductances and impedance elements become crucial elements in the whole circuit what results in the fact that a simple exchange of silicon chips by silicon carbide in a given package can be ruled out. In addition different considerations regarding the thermal design especially in power modules arise when SiC chips are considered, triggered by the fact that the cost balance between assembly and chip is shifted compared to silicon based solutions. Thus, different optimization criteria can be used, leading to new design approaches for power modules. The following paper will give a first inside how those boundary conditions can be implemented in innovative solutions using SiC components.

Abstract: This work presents experimental results on surge current capability of SiC Schottky diodes performed on free floating press-pack encapsulation technologies. For the tests, we used a home-made workbench. The aim of our studies is to improve the current robustness and power density of Silicon Carbide (SiC) devices able to operate at temperatures over 300C°. Various technological approaches have to be considered, mainly on the interconnection technique and metallization layers in order to improve the power density and temperature operation of the diodes. Our investigation showed a strong improvement of electro-thermal performances, and especially the surge current capability that is almost doubled by using press-pack encapsulation.

Abstract: High-voltage 4H-SiC Schottky Barrier Diodes (SBDs) and Junction Barrier Schottky (JBS) diodes have been fabricated and evaluated. Current-voltage (I-V) characteristics were measured in a wide temperature range. All diodes fabricated showed nearly ideal forward behavior. For SBDs with Schottky Barrier Height (SBH) of 1.12 eV, the reverse I–V characteristics are described well by the thermionic emission model (at voltages varying from several mV to 2 kV and temperatures ranging from 361 to 470 K) if barrier lowering with increasing band bending is taken into account. For SBDs with SBH of 1.53 eV, no thermionic current was detected in reverse direction at temperatures below ~500 K. The leakage currents appeared only at high reverse voltages and elevated temperatures. The analysis of reverse I-V characteristics allowed to propose dislocation related mechanism of current flow due to the local injection of electrons from metal to semiconductor. It is shown that defect related leakage currents can be significantly reduced by JBS-structure.