Papers by Keyword: Ohmic Contact

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: 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: This paper presents results from metal contact processing experiments towards the implementation of durable 500 °C high-frequency 4H-SiC bipolar junction transistors (BJTs). Specifically, p-type ohmic contacts have been demonstrated on a 0.25 μm-thick p-type homoepitaxial layer of doping 8 × 10¹⁸ ± 4 × 10¹⁸ cm^-3. Finally, preliminary current-voltage characteristics of fabricated BJTs are presented.

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: 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: 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: The Smart Cut^TM technology enables the combination of a high quality single crystal SiC layer onto a low resistivity handle wafer (<5mOhm.cm), allowing device optimization as well as the reduction of device’s conduction and switching losses. On this new SmartSiC^TM substrate, the sheet resistance of the back side contact after metal deposition, without anneal, is about 10x lower than the annealed back side contact on 4H-SiC. Schottky-barrier vertical structures thinned down to 250μm were prepared for power cycling tests (PCT) measurements. Up to 250 k cycles, the devices remained within the specifications of AQG324 for samples prepared from SmartSiC^TM substrates. We are demonstrating here that in addition to a higher current rating (up to 20%), the SmartSiC^TM substrate enables a device fabrication simplification by skipping the annealing of the back-side ohmic contact, without compromising either the back-side contact resistance or the assembly PC_sec reliability.

Abstract: The formation of ohmic contacts by laser annealing approach is of great importance for SiC power devices, since it allows their fabrication on thin substrates, that is of crucial significance to reduce power dissipation. Ni silicide reaction under UV laser irradiation has been studied in detail with particular focus on single pulse approach, in order to describe the early stage of reaction process. The use of a multi pulse approach, for the formation of Ni silicide-based ohmic contacts by means of excimer laser annealing, has been investigated in this work. The reaction process has been characterized, as a function of number of pulses, by means of X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) analysis. Laser process simulations, formulated in the framework of phase field theory, have been performed in order to predict the evolution of material during reaction under annealing. Simulations show that reaction moves to Si-reach phases with the increasing on pulses, with a co-existence of Ni₂Si and Ni₃Si₂ phases for the three pulses process. Moreover, simulations show critical differences, in terms of the uniformity of the distribution of the silicide phases along the film, between the single pulse and the multi pulses cases and the increasing of thickness of silicide phases with the pulse sequence. These predictions are in good agreement with the findings of XRD and TEM analyses. The electrical properties of the reacted layer have been evaluated on Schottky Barrier Diodes (SBD) devices, confirming the ohmic behaviour of multi pulse annealed samples.

Abstract: In this paper, the impact of the anode contact in SBDs, PiN, JBS and MPS diodes is analyzed through TCAD simulations. The focus of the investigation is the correct simulation of the Schottky barrier height on the different areas of the device to correctly simulate a JBS or MPS structure. It is found that the splitting of the anode contact and an accurate selection of the Schottky barrier height on pzone is necessary to allow the onset of the bipolar conduction in MPS devices. In this way, it is possible to correctly analyze the behavior of an MPS diode, including the snapback phenomenon.