Materials Science Forum, ISSN: 1662-9752, Vol. 1158, pp 81-86

doi: 10.4028/p-PnS5D9

© 2025 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2024-09-19

Revised:
2025-08-20

Accepted:
2025-08-21

Online:
2025-09-08

Improved Angle Tolerance in -SiC Trench Filling Epitaxy Using Chlorinated Chemistry

The original version of this paper is available on https://www.scientific.net/MSF.1158.81.pdf

Rydhem Grover , Kelly Turner , Gerard Colston , Arne B. Renz , Marina Antoniou , Peter Gammon , Philip. A. Mawby , Vishal A. Shah

School of Engineering, University of Warwick, Coventry, CV4 7AL, United Kingdom

rydhem.grover@warwick.ac.uk, kelly.turner@warwick.ac.uk, g.colston.1@warwick.ac.uk, p.a.mawby@warwick.ac.uk, vishal.shah@warwick.ac.uk

Keywords: trench filling epitaxy, 4H-SiC, superjunction, chemical vapor deposition

Abstract

Trench filling epitaxy on using trichlorosilane ( ) and hydrogen chloride has shown to improve the tolerance to trench angle misalignment relative to the [11 ] substrate direction in deeper trenches than previously reported. Extraction of growth rates from cross-sectional SEM shows that epilayer growth on the mesa corner facet is the most sensitive to trench misalignment, suggesting that HCl may mediate the facet growth rate within from to maintain symmetric growth.

Introduction

The original version of this paper is available on https://www.scientific.net/MSF.1158.81.pdf

Trench filling epitaxy (TFE) is a practicable approach to produce high-voltage, energy efficient superjunction power electronic devices from silicon carbide [1,2]. For superjunction fabrication, TFE involves etching uniformly spaced trenches into an n -doped substrate and homoepitaxially refilling the trenches with p-doped material to produce a lateral stack of p-n junctions with a charge-balanced depletion region. To achieve this balance, trenches must be filled without the formation of voids to ensure an even doping distribution [3]. However, typical TFE by chemical vapor deposition (CVD) from silane and propane under hydrogen dilution, leads to faster SiC deposition at mesa tops than inside the trenches, which causes trenches to seal before filling can be completed [4-6]. The direction of epilayer growth near the mesa top is also shown to be sensitive to slight misalignment of the trench angle to the direction in (Fig. 1), which also can lead to premature trench closure by inclination of the growth angle [7,8].

Fig. 1. Schematic diagram of inclined epilayer growth on mesas due to trench/ substrate misalignment with and without HCl during CVD.

Fig. 1. Schematic diagram of inclined epilayer growth on mesas due to trench/ substrate misalignment with and without HCl during CVD.

Efforts to mediate the growth rate at the mesa top and improve trench filling rate and quality have focused on the use of halogenated agents, typically HCl or chlorinated precursors [9-12]. It is known that chloride compounds can improve the SiC growth rate on planar substrates, which is suggested to be caused by removal of condensed Si in the boundary layer and by the formation of surface-active intermediates, such as and . In trench filling, HCl also has the benefit of acting as a spatially selective etchant, favoring etching near the mesa tops rather than inside the trenches, which can prevent trench-sealing overgrowth. Recently, it is also shown that HCl can improve the processing window for TFE by increasing the tolerance for trench angle misalignment to the substrate [15]. In this report, the relationship between misalignment angle, epilayer growth rate and the role of HCl are considered. Although improved misalignment tolerance improves trench filling, voids remain present in this work due to the influence of other factors affecting trench filling, such as trench aspect ratio, sidewall angle and tuning of the CVD conditions and chemistry. The relationship between void size and misalignment tolerance is not directly extracted and is therefore not discussed.

Experimental

The original version of this paper is available on https://www.scientific.net/MSF.1158.81.pdf

Commercial doped substrates off-cut by in were coated with a 500 nm layer of by low-pressure chemical vapor deposition (LP-CVD) at via tetraethyl orthosilicate (TEOS). The trench pattern was then applied to the wafer by photolithography using AZ5214E photoresist and subsequent sputtering of a 900 nm thick Ni layer at from an Ar plasma at 140 W RF power with a 30 sccm Ar injection rate. After the photoresist and excess Ni was removed by solvent lift-off in acetone/ isopropanol, trenches were etched to and depth by ICP-RIE in an / Ar plasma at respective RF and LF powers of 60 W and 1000 W . To prepare the trenches for epitaxy, the remaining Ni and oxide layers were removed by a full RCA clean.

Trench epitaxy was performed using an LPE ACiS-M8 reduced-pressure (RP)-CVD at and 100 mBar with the susceptor rotating at 60 rpm . Facets at mesa corners were introduced by ramping the temperature from to the growth temperature under flow prior to growth. Source gases were used at fixed flow rates of and with dilution to grow a epilayer at . Growth was completed both with and without HCl , which was added at a flow rate of 500 sccm . In all cases, doping markers were added for every of growth by 30 s bursts of . The samples are characterized by cross-sectional SEM recorded on the ( ) plane by cleaving through the trench patterns along the direction.

Results and Discussion

The original version of this paper is available on https://www.scientific.net/MSF.1158.81.pdf

Trench Refill Misaligned to the Direction. It is previously shown that using a chlorinated growth process in trench filling epitaxy can significantly improve the tolerance for trench misalignment to the direction [14]. This effect has been replicated here for trenches of both and depth ( pitch), shown by a plot of trench misalignment angle ( ) versus epilayer growth angle ( in Fig. 2a. Using doping markers recorded by cross-sectional SEM of refilled trenches, the angle at which adjacent growing surfaces fuse over time, denoted as shows how far the epilayer growth deviates from vertical as the trench direction is varied about (Fig. 2c). Similar to previously reported data, using and in the growth process allows vertical epilayer growth ( ) even when the trench angle is misaligned at , indicated by the green shaded section in Fig. 2a. A so far unreported trend emerges when the misalignment is in magnitude, where growth begins to straighten by from a local maximum at . This is visualized in Fig. 2c by arrows marked on cross-sectional SEM images of deep filled trenches. Notably, misalignment at negative angles seems to cause a further deviated growth angle of approximately relative to positive misalignment angles.

Fig. 2. a) Graph of trench misalignment angle versus epilayer growth angle for and deep trenches refilled under 500 sccm of HCl . b) Graph of difference in facet growth rate on each side of the trench (left axis) and the magnitude of the epilayer growth angle (right axis) versus trench misalignment angle. c) Cross-sectional SEM images recorded on the ( ) plane of deep trenches from to misalignment, showing inclined growth marked by the arrow direction.

Fig. 2. a) Graph of trench misalignment angle versus epilayer growth angle for and deep trenches refilled under 500 sccm of HCl . b) Graph of difference in facet growth rate on each side of the trench (left axis) and the magnitude of the epilayer growth angle (right axis) versus trench misalignment angle. c) Cross-sectional SEM images recorded on the ( ) plane of deep trenches from to misalignment, showing inclined growth marked by the arrow direction.

Initial growth rates can be extracted by measuring the thickness of the epilayer at the first doping marker on each trench surface (top, bottom, sidewall and facet shown in Fig. 2c). For the deep trenches, the growth rate on the mesa top and trench bottom are unaffected by , remaining at and , respectively. However, the sidewall growth rate varies from to and can be asymmetric at each side of the trench, leading to a discrepancy of up to at . Most significantly, the growth rate on the facet (top corner of the mesa) is dependent on , varying from to with a difference between left and right facet growth of up to at . The difference in facet growth rate for each misalignment angle is plotted in Fig 1b (left axis) with the magnitude of (right axis) showing a clear correlation between asymmetric facet growth and deviated overall epilayer growth. Since HCl has been shown to etch the SiC epilayer near the mesa top and on the facet, it is possible that HCl improves trench misalignment tolerance through controlling growth at the facet by crystal plane dependent etching [8, 9]. As misalignment will expose difference facet and sidewall surfaces, their activity for both growth and etching will likely be asymmetric, accounting for non-vertical epilayer growth.

Fig. 3. a) Plot of step angle versus trench misalignment angle for deep, pitch trenches filled under 500 sccm HCl . Step angle refers to the surface step direction in the epilayer overgrowth relative to the trench direction. b) Top-down SEM images of the epilayer overgrowth for trenches filled under 500 sccm HCl , showing surface step angles.

Fig. 3. a) Plot of step angle versus trench misalignment angle for deep, pitch trenches filled under 500 sccm HCl . Step angle refers to the surface step direction in the epilayer overgrowth relative to the trench direction. b) Top-down SEM images of the epilayer overgrowth for trenches filled under 500 sccm HCl , showing surface step angles.

Further evidence for HCl affecting growth on corner facets is observed by top-down SEM of the epilayer surface. Fig. 4 shows the epilayer surface of deep trenches filled without HCl at varied misalignment angles. The direction of surface steps is observed, which shows a single step direction for each misalignment angle, that decreases from from the trench direction at and at . Notably, the step direction for trenches aligned to the direction is , rather than , which are present in the substrate and would be expected to be inherited by the epilayer for step-flow growth on a planar 4 H -SiC substrate. This may indicate that facet or sidewall growth can cause a twisting of the overall growth direction, since both facets and sidewalls will exhibit different surface structures based on the trench misalignment angle.

Comparing the epilayer step directions for trenches filled without HCl (Fig. 4) and with HCl (Fig. 3) shows a clear difference, especially at low ( ) misalignment angles. Surface steps at show two symmetric directions at and from the trench direction, compared with the single step at for trenches filled without HCl .

Fig. 4. Top-down SEM images of epilayer overgrowth on refilled deep, pitch trenches filled without HCl . Images are recorded with the stage tilted away from the viewing angle. Annotations mark the surface step angle relative to the trench direction.

Fig. 4. Top-down SEM images of epilayer overgrowth on refilled deep, pitch trenches filled without HCl . Images are recorded with the stage tilted away from the viewing angle. Annotations mark the surface step angle relative to the trench direction.

According a plot of step angle versus trench misalignment angle shown in Fig. 3a, which was extracted from top-down SEM images partly presented in Fig. 3b, the step angle abruptly increases from to at and decreases to 70 at . This twist is followed by an approximately linear deviation in step angle from misalignment angles of to , with a larger deviation observed at . This trend reflects that of the growth angle extracted from cross-sectional SEM and, therefore, suggests that an appearance of two symmetric step directions indicates vertical growth. While the mechanism of this can only be conjectured from these empirical data, it can be suggested that the step angle may be controlled by the relative epilayer growth rates on the trench sidewall, facet and mesa top, causing a twist in step angle. The difference in step angles between trenches filled with and without HCl may, therefore, by rationalized by HCl acting as a mediator for the facet growth rate for trench directions from , which may expose surface structures more susceptible to etching by HCl compared with those exposed at higher degrees of misalignment.

Summary

The original version of this paper is available on https://www.scientific.net/MSF.1158.81.pdf

Using a chlorinated gas mixture in trench filling CVD on can help to minimize epilayer growth inclination caused by misalignment of the trench direction to the substrate direction. Using HCl in refill epitaxy has broadened the processing window for photolithography processing to allow a error in mask alignment, and this effect seems independent of the trench depth. Based on the relationship between mesa corner facet growth rate and trench misalignment angle, asymmetric facet growth accounts for epilayer deviation, indicating that HCl most likely controls growth on the facets and its activity depends on the stability of the exposed facet.

References

The original version of this paper is available on https://www.scientific.net/MSF.1158.81.pdf

T. Kimoto, High-Voltage SiC Power Devices for Improved Energy Efficiency, Proc. Jpn. Acad. Ser. B Phys. Biol. Sci., 98 (2022) 161-189

V. Veliadis, SiC Mass Commercialization: Present Status and Barrier to Overcome, Mater. Sci. Forum, 1062 (2022) 125-130

F. Udrea, G. Deboy and T. Fujihira, Superjunction Power Devices, History, Development, and Future Prospects, IEEE Transactions on Electron Devices, 64 (2017) 713-727

R. Kosugi, Y. Sakuma et al, Development of SiC Super-Junction (SJ) Device by Deep TrenchFilling Epitaxial Growth., Mater. Sci. Forum, 740-742 (2013) 785-788.

S. Ji, K. Kojima, R. Kosugi et al, Influence of Growth Pressure on Filling 4H-SiC Trenches by CVD Method, Jpn. J. Appl. Phys., 55 (2015) 01AC04

S. Ji, R. Kosugi et al, A Study of CVD Growth Parameters to Fill Deep 4H-SiC Trenches, Mater. Sci. Forum, 963 (2019) 131-135

Y. Ishida, Proposal of the Mechanism for Inclination Growth on a Mesa Top During 4H-SiC Trench Filling Epitaxy, Jpn. J. Appl. Phys., 56 (2017) 070307

R. Kosugi, J. Shiyang, K. Mochizuki et al, Strong Impact of Slight Trench Direction Misalignment from [11-20] on Deep Trench-Filling Epitaxy for SiC Super-junction Devices, Jpn. J. Appl. Phys., 56 (2017), 04CR05

S. Ji, R. Kosugi et al, An Empirical Growth Window Concerning the Input Ratio of HCl/ SiH4 Gases in Filling 4H-SiC Trench by CVD, Appl. Phys. Express, 10 (2017), 055505

S. Ji, R. Kosugi et al, CVD Filling of Narrow Deep 4H-SiC Trenches in a Quasi-Selective Epitaxial Growth Mode, Mater. Sci. Forum, 924 (2018) 116-119

Z. Zhao, Y. Li et al, 4H-Trench Filling by Chemical Vapor Deposition Using Trichlorosilane as Si-Species Precursor, J. Cryst. Growth, 607 (2023) 127104

K. Turner, G. Colston et al, Effect of Mesa Sidewall Angle on 4H-Silicon Carbide Trench Filling Epitaxy Using Trichlorosilane and Hydrogen Chloride. Adv. Mater. Interfaces, (2024) 2400466

Ö. Danielsson, Understanding the Chemistry in Silicon Carbide Chemical Vapor Deposition, Mater. Sci. Forum, 924 (2018) 100-103

P. Sukkaew, E. Kalered et al, Growth Mechanism of SiC Chemical Vapor Deposition: Adsorption and Surface Reactions of Active Si Species, J. Phys. Chem. C, 122 (2017) 648-661

G. Colston, K. Turner et al, Epitaxial Trench Refill of 4H-SiC by Chlorinated Chemistry, Appl. Phys. Lett., 124 (2024) 192102

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