Solid State Phenomena Vol. 359

Abstract: In this work, a nitrogen plasma treatment process was employed on n-type 4H-SiC. Both the Si- and C-face were studied and treated with N₂ plasma. The surface concentration of nitrogen increased from 5×10¹⁸ cm^-3 to 5×10²¹ cm^-3 in both the Si-face and C-face as analyzed by secondary ion mass spectroscopy (SIMS). This shows that a simple plasma treatment process was able to incorporate very high concentration of nitrogen dopants otherwise done using high temperature implanters. Titanium-based Ohmic contacts were formed at ~800 °C thanks to the presence of high concentration nitrogen dopants. Specific contact resistance of (ρ_c ~1.5 × 10^-6 Ω.cm² and ~1.9 × 10^-6 Ω.cm²) was obtained on Si-face and C-face, respectively.

Abstract: Nowadays, the growing worldwide electrification requires new materials for power management. SiC currently dominates the market thanks to excellent energy efficiency and broad operating capabilities. The present paper proposes an experimental study of the Ni-SiC backside ohmic contact formation using 308 nm nanosecond laser annealing (NLA). After Nickel (80 nm) sputtering over 4H-SiC wafers, various laser conditions are investigated, with energy density (ED) ranging from 2.4 to 5.4 J/cm², pulse number from 1 to 20 and chuck temperature from 25 °C (RT) to 400 °C. For all series, a common scenario is noticed as the ED increases, with first solid-state reactions, then local melt and, finally, complete top layer melt and de-wetting at high ED. An in-depth understanding of the impact of laser conditions on these stages is achieved, based on electrical data, Raman spectroscopy, optical microscopy, Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM). Results reveal that both high pulse numbers and the use of a hot chuck enable to significantly reduce the ED needed to form low resistance contacts. In addition, sheet resistances and contact resistivities are linked to the microstructure evolution upon NLA exposure. As a proof-of-concept, an acceptable process point yields a contact resistivity around 5×10^-5 Ω cm² when the wafer is processed at 25 °C and a value as low as 10^-5 Ω cm² for 400 °C processing. The mechanisms involved and discussed in the present work may very likely pave the way for other contact formation with limited thermal budget.

Abstract: With the rising need for power devices suitable for harsh environment conditions like high temperature applications, contact materials and packaging of the devices have become critical factors in device fabrication [1, 2]. Therefore, a contact metal stack containing silver and titanium nitride which can be used at elevated temperatures under oxygen atmosphere was investigated. For patterning of the approx. 2 µm thick sputter-deposited metal stack on the wafer front side, a lift-off process using a negative photoresist was established. Characterization of the photoresist sidewall shape was performed by cross-sectional views prepared with SEM and top view images taken on a microscope. It was found that for a successful lift-off, a distinct undercut is needed so no metal is deposited at the downside of the undercut, ensuring a metal-free surface for the solvent to reach the photoresist. To obtain this, most influencing factors are exposure dose and development time, which were optimized considering the undercut shape as well as pattern fidelity. Lift-off with acetone proved to be good for the fabricated 4H-SiC MOSFET devices.

Abstract: To achieve low on-resistance in any vertical 4H-SiC semiconductor power device, it is essential to create a suitable ohmic contact on the corresponding n-doped SiC substrate. In particular after wafer thinning, a common technology to reduce substrate resistivity, laser annealing for ohmic contact formation on the wafer backside is the only option due to temperature sensitive materials (such as Titanium or Aluminum) on the partially or fully processed wafer frontside. In this work, to solve adhesion issues of the backside metallization, plasma treatments, as easy to integrate process steps, were examined. By stripping obstructive carbon layers, formed after ohmic contact laser annealing, and without damaging the wafer frontside, an enhanced adhesion of following metallization layers was achieved. Both O₂- and H₂-plasma processes were investigated and demonstrated significant improvements to the adhesion of metallization stacks on the wafer backside compared to untreated surfaces and without drawbacks in the ohmic contact quality.

Abstract: N atoms were doped into SiN_x/4H-SiC substrates by KrF laser irradiation while the substrates were heated. The diffusion depth of nitrogen increased above the solubility limit when the sample heated to 600°C was irradiated by the laser compared to the sample at room temperature. In addition, a clear 4H-SiC pattern was observed in the cross-sectional TEM diffraction image, thereby suggesting that sufficient crystal recovery was achieved even under melt-solidification conditions owing to the effect of substrate heating.

Abstract: Laser Thermal Annealing (LTA) is a key process step to improve the 4H-SiC devices by reducing their on-state resistance. In this study, we investigate the electrical, structural and morphological properties of nickel contact fabricated by LTA. A contact formed by a classical Rapid Thermal Annealing (RTA) was also fabricated as reference. Based on structural analysis, the phases formed by LTA do not match with RTA sample ones that has better ohmic properties. Nevertheless, the LTA contacts reach a specific contact resistance of 2.4×10^-5 Ω.cm² for an annealing at 4.75 J.cm^‑2, which represents a significant improvement in comparison with our previous contacts fabricated with the same experimental protocol using titanium.

Abstract: New generations of SiC power devices require to be fabricated on very thin substrates, in order to significantly reduce the series resistance of the device. The role of thinning process on the formation of backside ohmic contact has been investigated in this work. Three different mechanical grinding processes have been adopted, resulting in different amounts of defectivity and surface roughness values. An excimer UV laser has been used to form a Ni-silicide based ohmic contact on the backside of the wafers. The reacted layer has been studied by means of Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD) analyses, as a function of grinding process parameters and laser annealing conditions. The ohmic contact has been evaluated by measuring the Sheet Resistance (R_s) of silicided layers and the V_f at nominal current of Schottky Barrier Diode (SBD) devices, fabricated on 150 mm-diameter 4H-SiC wafers. A strong relationship has been found between the crystal damage, induced by thinning process, and the structural, morphological and electrical properties of silicided ohmic contact, formed by UV laser annealing, revealing that the silicide reaction is moved forward, at fixed annealing conditions, by the increasing of crystal defectivity and surface roughness of SiC.

Abstract: In this work, an empirical model of structural and material composition of low-ohmic nickel silicide contact formation on n-type 4H-SiC by laser annealing as well as by RTA is presented. For this purpose, systematic studies with different annealing parameters were performed. The development of the empirical model is based on results from characterization of the nickel silicide by FIB-SEM, TEM, XRD analysis as well as electrical characteristics received from 4-point-measurements.

Abstract: In this work, the influence of different surface roughness and surface treatments on the minimum energy density required to form low-ohmic nickel contacts on n-type 4H-SiC by laser annealing was investigated. The annealing was performed by a frequency-tripled Nd:YVO₄ laser with a pulse duration of 50 ns. To evaluate the effects, the grinded or polished C-side of 4H-SiC wafers with surface roughness between 0.3 and 70 nm was sputter-deposited with nickel and subsequent laser annealed. Sheet resistance measurements showed that the minimum energy density required to achieve a low-resistance contact depends significantly on the surface roughness. The rougher the surface, the lower the minimum energy density to form a low-ohmic contact.

Abstract: In order to make SiC devices more accessible for high-temperature applications, reliable ohmic contacts and metallization systems which can also withstand extended operation at high temperatures are needed. In this work, metal layer stacks containing Ag, Ti, TiN, Ni and NiAl, where NiAl refers to a mixture of 97,4 wt% Ni and 2,6 wt% Al, were deposited on Si and SiC samples and consecutively thermally aged at 400 °C for 100 h in air. Mesa structures were found to be challenging for keeping oxygen from diffusing through the metal stack to the substrate. On flat surfaces, diffusion barriers were successfully used to protect the ohmic contact on 4H-SiC samples from oxidizing. Diffusion barriers made of TiN were found to show pore formation after the thermal treatment. The reason for the pores is thought to be gas formation, which is believed to be the result of the TiN layers containing too much nitrogen. The exact chemical composition of TiN layers therefore seems to be of vital importance for high-temperature applications.