Development of 200mm SiC Technology. Epitaxial Thickness Uniformity Observation on Different 8 Inch 4H-SiC Substrates

. The enlargement of 4H-SiC seed size from 150 mm (6 inch) to 200 mm (8 inch) is currently underway and 8 inches SiC substrate is now facing the market to switch the actual 6-inch technology to 8-inch technology. The aim of this work is to evaluate the influence on the epitaxial layer (using an epi growth campaign made of 21 consecutive runs) using substrates coming from different vendors (3 different suppliers adopted). The same epitaxial process and same reactor were adopted to grow all the samples. After the growth campaign, a difference of thickness uniformity between the three substrate suppliers was observed while no difference of doping uniformity was detected.


Introduction
Wide bandgap (WBG) semiconductor devices based on both silicon carbide (SiC) and gallium nitride (GaN) can lead a revolution in power electronics through faster switching speeds, lower losses, and higher blocking voltages.The combination of electronic and physical properties that silicon carbide (SiC) exhibits make it an excellent semiconductor for electronic devices able to work in hostile environments, where conventional semiconductors like Si or GaAs cannot perform operations.The critical electric field of SiC is about one order higher than that of Si, which allows much higher doping and thinner drift region layers for a given blocking voltage, resulting in a lower specific onresistance.This implies that the die size of SiC devices can be trimmed by nearly one order compared with correspondingly rated Si devices [1].
4H-SiC-based device technology has made tremendous progress in recent years owing to the availability of large-diameter, high-quality silicon carbide boules [2].
For SiC wafers to be used in electronic applications, it is mandatory to periodically enlarge the SiC boule of a desired single polytype.The enlargement of 4H-SiC seed size from 150 mm to 200 mm is currently underway and 8 inches SiC substrate is now facing the market to switch the actual 6 in technology to 8-inch technology.Such milestone represents a challenge in terms of crystal quality improvement, tools availability and overall process flow.The seed expansion has to be monitored through careful metrology analysis in order to ensure the crystal quality keeping under control the defectiveness of the substrates that are generated along the ingot growth process.The substrate study is a crucial point for the technology development in the field of high-power SiC devices.
In this experiment, a 8 inch campaign was evaluated to investigate the impact of different substrate suppliers on the epilayer (3 different substrate suppliers were adopted to grow 21 consecutive epiruns).

Experimental Set-Up
In this work, different 8-inch substrates (three different suppliers) were compared with the same epitaxial growth process on top.For this experiment a commercial low-pressure, single wafer, hotwall Chemical Vapor Deposition (LP-CVD) reactor was adopted.The homo-epitaxial growth was carried out using the Trichlorosilane/Ethylene chemistry system for Si and C supply, respectively.A high-purity industrial grade H2 was used as a carrier gas as well as a reducing agent for the growth of epitaxial layers.Ammonium gas was added as a dopant for n-type layer.The n-SiC epitaxial layers with n-doping concentration of about 1e16 cm -3 were grown on the Si-face (0001) of 4H-SiC, n-type (~10 18 cm -3 ) 8-inch substrates with 4° off axis [3].Such epi-layer specification is suitable for medium/high voltage device technologies.

Results and Discussion
Figure 1a shows the thickness uniformity over 21 runs (to have sufficient statistics) performed on 8-inch substrates coming from 3 different suppliers under the same growth conditions (no recipe corrections were applied for thickness adjustment).The graph, obtained by InfraRed Fourier Transform (FT-IR) measurements (33 points map), shows the thickness uniformity, calculated as [max value -min value]/average [%]], where the different behavior of the supplier 1 is visible.Such difference is also visible in figure 1b right where the diameter of the 8-inch wafer is plotted.This uniformity and thickness shape wasn't observed for what concerns the doping analysis done by C-V meter tool (the three suppliers show the same uniformity trend and same diameter shape, fig.1c). Figure 2 shows the trend of the spreading analysis done on the epilayer growth on substrates supplier 1 and 2. The analysis was done to check the thickness on both layers and to validate what have seen by FT-IR.We adopt different ΔV values (10V and 50V) with 1°09' of lapping angle always obtaining the same results.The comparison of the analysis done on the center and the edge of the wafer shows a difference in thickness between the two points for supplier 2. This analysis confirms

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Growing and Forming of Semiconductor Layers what we have seen by FT-IT, showing different thickness uniformity between the two samples.This kind of analysis is under development for what concern the resistivity analysis because on SiC layer the measurement is not well defined and it seems depending on the lapping direction, but it shows a very good results for the thickness measurement that we confirmed by SIMS analysis made in EAG laboratories (figure 2b).Solid State Phenomena Vol.362 Figure 3 reports the thickness profile maps.The analysis shows a different shape of samples 1 and 2. The 3D profile (figure on the right) clearly shows different shapes between the samples.Sample 1 (the one with a lower value of uniformity) shows a concave shape while sample 2 shows a convex shape.To better understand the behavior of this phenomena the thickness map of the epilayer and the substrate resistivity map have been compared (figures not shown).The resistivity value was obtained by Kitec M-res tool for non-contact low resistivity measurement by Eddy current method.The comparison shows the same map shape between thickness of the epilayer and resistivity of the substrate of the low uniformity value sample (supplier 1).The resistivity of the substrate plays a fundamental role for the epitaxial growth as observed for the defectivity [4].

Summary and Conclusion
In this work, an 8 in.process growth campaign was done to evaluate different substrate suppliers with consecutive epi runs made on the same reactor with the same epi recipe.In our investigation, from the point of view of thickness uniformity (difference between the thickness of center and edge of the wafer) the substrate vendor plays a fundamental role.A low uniformity was observed for supplier 1 compared to the other three.The reason for such difference, after to be sure of our thickness measurement (confirmed by SIMS and SRP), probably could be ascribed to a difference of thickness shape between the different substrates and substrate resistivity that affect the epi growth on the wafer.

Fig. 1 .
Fig. 1. a) Uniformity [%] trend for different epitaxial runs on 8 inches substrate from different suppliers (three).b) 8 inches thickness diameter value comparison for the three substrate suppliers.The graph was obtained by InfraRed Fourier Transform (FT-IR) measurement.c) Doping Uniformity trend for the same samples obtained C-V measurement.

Fig. 2 .
Fig. 2. a) Spreading Resistance Profile (SRP) measurement performed on center and edge of the wafer for both supplier 1 and 2. The comparison shows the difference between center and edge for supplier 2 and no difference for sample 1. B) calibration sample with SIMS analysis made at EAG laboratories (www.eurofinsEAG.com).

Fig. 3 .
Fig. 3. Thickness profile of sample 1 and 2 without the epilayer in 2D map and 3D map (left and right part of the figure, respectively).