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Investigating the impact of etching time on 4H-SiC defects

Posted: 31 Mar 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Silicon carbide? SiC? etching time? stacking faults? photoluminescence?

KLA-Tencor Candela CS920 is an inspection system allowing a surface defect detection and photoluminescence (PL) technology in a single inspection platform. It was employed for the surface defects (micro-pits, carrots, comets and triangles) and stacking faults detection. Macro e micro defect detection and auto classification is obtained through a cross correlation between channel having different specification (wavelength of the laser, angle with the surface, amplitude of the scattered light).

Hg-probe Capacitance Voltage (Hg-CV) measurements was adopted for the doping concentration evaluation (17 points diameter from the primary flat to the top position). The doping concentration was fixed at 1.

Nanometrics Stratus Fourier Transform Infra-Red spectroscopy (FT-IR) was adopted for the thickness evaluation of the samples. For the surface analysis, AFM measurement was performed using a Dimension 3100 AFM. The microscope was operated in contact mode and equipped with a single crystal Silicon tip. In order to scan a large area, the scan size was 90x90?m2 with the scan rate of 1.0Hz.

Results and discussion
The in-grown stacking faults (SFs) are commonly observed in the layers, generally nucleated at the beginning of epitaxial growth, and can result in a larger forward voltage drop of bipolar devices [11]. Basal Plane Dislocations (BPDs) act as nuclei of Shockley stacking faults, which expand during bipolar operation, and cause the degradation of forward characteristics of bipolar devices [12,13]. In order to decrease the substrate roughness after the Chemical Mechanical Polishing (CMP), the H2 surface etching is a fundamental step of the growth process but it could enlarge the dislocation of the substrate [14]. The trend of the Stacking Faults density [%] as a function of the H2 surface etching time performed by PL analysis is shown in figure 1. The figure shows the increase in Stacking Faults density as surface etching time increases, from 0.6 to 0.9 % density for x0.5 and x3 of etching time, with respect to the reference time.

Figure 1: Stacking Faults density as a function of the etching time.

Also for the surface defects density (carrots, comets, micro-pits and triangles), the same trend from the previous chart has been observed. Figure 2 clearly shows an increase in surface defects with the increase in H2 etching time. The substrate surface etching causes a dislocation to enlarge on the substrate, as a consequence more surface defects and SFs appear on the epi-layer.

Figure 2: Surface density (carrots, comets, micro-pits and triangles) as a function of the etching time.

The hydrogen etching process was found to improve the surface morphology [8], but at the same time it has also been observed to introduce a different type of micro and macro step bunching on the epi-layer [15]. AFM surface roughness analysis show the surface of the sample affected by step bunching. Both samples show a step bunching with a height of about 6 nm and width of about 1 micron. Figure 3 shows a clear difference between the samples x0.5 (left) and x3 (right) in terms of density.

Figure 3: Samples x0.5 (left) and x3 (right) in terms of density.

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