Study on Diffusion Behavior of Transition Metals in SiC Wafer

Study on Diffusion Behavior of Transition Metals in SiC Wafer

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Silicon carbide (SiC) is an important material for developing high-power, high-temperature resistant, and high-frequency switching devices due to its superior performance. In order to achieve high-performance devices, it is necessary to reduce defects in silicon carbide crystals, such as stacking faults, dislocations, and point defects. Therefore, the deep energy levels generated by point defects (internal defects and impurities) in SiC are increasingly becoming a research hotspot.

Element impurities, especially transition metals, in silicon wafers can form deep energy levels and have adverse effects on device performance. If the minority carrier lifetime is reduced, the reliability of the gate oxide layer is changed, and the leakage current of the PN junction is increased. In the process of device manufacturing, the rapid diffusion of metal impurities will transfer from the contaminated area to the device area. Although many doped elements (except for boron) have much lower diffusion coefficients in silicon carbide than in silicon, and research on the diffusion coefficients of transition metals is very limited, they are highly likely to have higher diffusion coefficients than doped elements (similar to silicon). If transition metals become movable in SiC wafers, once they reach the device area, their performance will be significantly reduced. According to reports, Sc, Ti, V, Cr, and Fe elements all form deep energy levels within the bandgap of silicon carbide, affecting the electrical performance of the device.

Researchers select N-type heavily doped silicon carbide substrate and deposits 2-6um homogeneous epitaxial layers through horizontal hot wall CVD at 1500 ℃. Transition metals Ti, Cr, Fe, and Ni are introduced into the SiC epitaxial and substrate layers through ion implantation. The basic concentration of these four metals is about 1×1017cm-3. The implanted depth of Ti, Cr, and Ni is 0.4 um. Fe implanted depth of 0.2 um. To enhance the metal diffusion ability, the sample was annealed at 1780 ℃ in an inert argon atmosphere for 30 minutes. Then, a dynamic secondary ion mass spectrometer (D-SIMS) was used to study the concentration distribution of metal elements in depth profiles before and after annealing.

1. SIMS Study of Depth Profile Distribution of Metal Ions in 4H-SiC Epitaxial Layers

It can be seen that from Fig. 1 the concentration near Cr injection decreases after annealing. The Cr concentration in SiC rpitaxial layer (0.5~2.3um) is increased from 1014cm-3 to 1015cm-3. This indicates that the annealing process has caused Cr diffusion. At the interface layer between the epitaxial and substrate layers, the Cr concentration significantly increases, which is likely due to the capture of defects at the interface during the migration from the ion implantation region to the substrate. The concentration at Cr ion implantation site (0.3~0.4um) did not significantly decrease, which may be related to lattice damage caused by ion implantation process.

Fig. 1 SIMS determination of depth profile distribution of Cr ions in 4H-SiC epitaxial layers before and after annealing

Fig. 1 SIMS determination of depth profile distribution of Cr ions in 4H-SiC epitaxial layers before and after annealing

 

From Fig. 2, it’s shown that the majority of Fe diffuses from the injection zone to deeper positions after annealing. However, the increase in Fe concentration at the interface between epitaxial layer and substrate is much smaller than that of Cr.

Fig. 2 SIMS determination of depth profile distribution of Fe ions in 4H SiC epitaxial layers before and after annealing

Fig. 2 SIMS determination of depth profile distribution of Fe ions in 4H SiC epitaxial layers before and after annealing

Similar to Fe, Ni has a stronger diffusion ability after annealing (Fig. 3).

Fig. 3 SIMS determination of Ni ion depth profile distribution in 4H SiC epitaxial layer before and after annealing

Fig. 3 SIMS determination of Ni ion depth profile distribution in 4H SiC epitaxial layer before and after annealing

This research compares the diffusion ability of metal ions at different annealing temperatures and finds that significant diffusion occurs when the annealing temperature is above 1500 ℃. The performance of Ti is different from other elements, and there is no significant diffusion at 1780 ℃. This is consistent with the conclusion reported in previous literature that Ti does not diffuse after annealing at 1700 ℃ after ion implantation using a deep level transient spectrometer.

2. SIMS Determination of Depth Profile Distribution of Various Ions in 4HSiC Substrate

To compare the diffusion ability of each element more intuitively, researcher chooses to perform ion implantation on an N-type SiC substrate and observe their respective diffusion after annealing at 1780 ℃ (Fig. 4). Ti, Cr, and Fe are still at the ion implantation site (Ti and Cr are 0.4um; Fe is 0.2um) maintained a high concentration (platform profile), while the concentration of Ni was very low at the ion implantation site. Among them, Cr and Fe did not show the same diffusion trend as in the SiC epitaxial layer, which is likely related to the presence of more crystal defect trapping in the substrate.

Fig. 4 SIMS determination of the depth profile distribution of various ions after annealing in 4H-SiC substrate

Fig. 4 SIMS determination of the depth profile distribution of various ions after annealing in 4H-SiC substrate

Based on the above analysis, it can conclude that the diffusion ability of the four transition metal ions in SiC epitaxial layer and substrate is as follows: Ni>Fe>Cr>Ti. This is consistent with the diffusion ability between them in silicon wafers. More interestingly, the diffusion ability of the four metals is consistent with the atomic number arrangement (Ni: 28>Fe: 26>Cr: 24>Ti: 22).

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