Electrical and Structural Characteristics of Ohmic Contacts in Thinned SiC Based Diodes –

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1. 概要

To reduce the series resistance of devices, the new generation of SiC power devices require the use of ultra-thin substrates. This study focuses on the influence of thinning process on the formation of backside ohmic contact. Introduce different levels of crystal damage and surface roughness through three mechanical grinding processes, and prepare nickel silicide ohmic contacts on the backside of the wafer using excimer ultraviolet laser. By using atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), the characteristics of the reaction layer were systematically analyzed as a function of grinding parameters and laser annealing conditions. The ohmic contact performance was evaluated by measuring the thin layer resistance (Rs) of the silicide layer and the conduction voltage drop (Vf) of the 4H SiC Schottky diode. The results indicate that under fixed annealing conditions, the defects and surface roughness caused by thinning promote the silicification reaction, which in turn affects the morphology, structure, and electrical properties of the ohmic contact.

2. Sample Preparation

This study prepared Schottky barrier diode devices on 4H-SiC wafers with a diameter of 150 mm and a thickness of 180µm after backside grinding. Firstly, a 100 nm Ni layer was deposited on the backside of the wafer by sputtering in an Ar environment under a pressure of 1 × 10^-3 mbar, and the Ni layer was annealed by irradiation with an excimer UV laser with a wavelength of 308 nm and a pulse width of 160ns. Subsequently, three different mechanical thinning processes (rough grinding, fine grinding, and ultrafine grinding) were used to treat the substrate to investigate their effects on the formation of ohmic contacts under laser irradiation.

The sub surface damage and substrate roughness introduced by different thinning processes were analyzed using AFM and TEM. The phase structure and morphology of the nickel silicide contact layer were characterized by XRD, and the effects of thinning process and laser annealing conditions on the characteristics of the reaction layer were systematically evaluated by combining AFM and TEM results. In terms of electrical performance, the thin layer resistance was measured using the four point probe (FPP) to preliminarily evaluate the ohmic contact performance; And using a semiconductor parameter analyzer and a high-power curve tracker, measure the conduction voltage drop (Vf) of the SBD device at nominal current to comprehensively evaluate its electrical behavior.

Fig. 1 AFM surface morphology of SiC wafers after laser annealing: (a) ultrafine grinding, (b) fine grinding, (c) rough grinding

Fig. 2 TEM analysis of SiC sample cross-section after laser annealing: (a) ultrafine grinding, (b) fine grinding, (c) rough grinding, (d) reaction interface moving downward at the defect site

Fig. 3 Structural characteristics of annealed SiC based silicide layer

Fig. 4 Conducting voltage drop of SiC SBD after thinning and annealing treatment

3. Conclusion

This work systematically studied the process of preparing nickel silicide ohmic contacts by ultraviolet laser annealing, with a focus on analyzing the evolution of the reaction under different mechanical thinning conditions and laser annealing parameters. The results indicate that the increase in substrate roughness and crystal damage caused by mechanical thinning are significantly correlated with the microstructure, surface morphology, and electrical properties of the reaction layer. Under the condition of fixed annealing parameters, the sub-surface damage and higher surface roughness of SiC intensify the silicide reaction, thereby affecting the overall quality of ohmic contact formation.

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Reference：

Paolo Badalà,  Bongiorno, C. ,  Fiorenza, P. ,  Bellocchi, G. ,  Smecca, E. , &  Vivona, M. , et al. (2024). Electrical and structural properties of ohmic contacts of sic diodes fabricated on thin wafers. Solid State Phenomena,359.