Papers by Keyword: Epitaxy

Abstract: Silicon carbide (SiC) is one of the ideal electronic materials for producing high-temperature, high-frequency, and high-power electronic devices. In the past 20 years, with the continuous improvement of silicon carbide material processing technology, its application have been expanding. Unlike Si devices, SiC devices cannot be directly fabricated on crude wafers. Instead, epitaxial films need to be deposited and grown on SiC wafers, then the epitaxial films will be used to produce devices. The doping concentration performance of the epitaxial layer can determine the device performance, making it the most important indicator of the epitaxial layer quality. For a long time, nitrogen has been used as the dopant in the production of SiC epi-wafers. Due to the difficulty of nitrogen cracking and its adsorption in graphite, the concentration is prone to significant drift, resulting in a decrease in yield and low production efficiency. In this research a vertical epitaxial equipment was used to consecutively grow 10 8-inch SiC substrate with nitrogen and ammonia as dopant separately. The concentration and thickness of the grown epitaxial films were measured and studied. The results indicate that compared to nitrogen as a dopant, the results of ammonia doping are significantly better in terms of intra-wafer concentration uniformity and inter-wafer consistency. Using nitrogen as the dopant, the doping concentrations uniformity of epi-layer ranges from 1.31% to 2.18%, and the deviation is between ± 8.0%. As a comparison, using ammonia as the dopant, the doping concentration uniformity of epi-layer ranges from 0.65% to 0.89%, and the deviation is between ± 1.0%. Meanwhile, the thickness performance is at the same level. Therefore, ammonia as a dopant can solve the concentration drift problem that has long been a headache in large-scale production of SiC epitaxy, greatly improving production efficiency. Its advantages are obvious. This study analyzed the possible reasons for the superior performance of ammonia gas as a dopant for 4H SiC epitaxy compared to nitrogen.

Abstract: This study investigates the multifaceted relationships between key process parameters such as C/Si ratio, system pressure, temperature, and growth rate and their effects on nitrogen dopant incorporation in homoepitaxial layers on 4H-SiC substrates. We focus on understanding how these growth parameters influence the in situ nitrogen incorporation during chemical vapor deposition (CVD) of epitaxial layers on 150 mm commercially available SiC substrates. Through a carefully designed experimental framework, which explores the interactions between each parameter and the C/Si ratio, we have shed light on a refined approach for epitaxial growth. This approach may not only stabilize the nitrogen dopant concentration across the wafer but possibly also reduces the formation of epitaxial defects.

Abstract: Rapid progress in the growth of 4H-SiC epitaxial layers allow device scientists/engineers to tighten the specifications of doping and thickness uniformities of SiC epitaxial films. Further, reducing the cost of SiC epitaxial layers is a continuing goal. A compelling approach is to choose a multi-wafer warm-wall epi reactor which has been shown to have very high wafer throughput. The precursors decompose upon heating by passing over hot reactor components, however, the precursor molecules crack before reaching the substrate and can form parasitic SiC coatings. Such coatings change the emissivity of reactor parts, changing their temperatures. The allowed vapor pressure in the gas phase is also a function of the chemical composition of these deposits. Consequently, the effective Si/C ratio at the wafer varies the nitrogen incorporation efficiency on the SiC epitaxial wafer. In this paper, we have reported an approach on how to minimize the effect of changing Si/C ratio on absolute layer doping and thickness over the full campaign. We analyzed the data, identified the pattern, and have used it to make predictions or decisions to keep the deviation within control limits. The nitrogen incorporation was analyzed as a function of cumulative coating on the reactor parts. The derived models were used to make the decisions for predictive doping by adjusting the flow rates of nitrogen precursors during upcoming campaigns at specific cumulative thickness of reactor parts coating. The same approach was also used for the adjustment of growth time to obtain the targeted epi layer thickness as a function of cumulative coating. Consequently, the predictive doping control resulted in the improvement of doping Cpk from 0.37 to >1.67 and the predictive thickness control resulted in the improvement of thickness Cpk from 0.75 to 1.61. This implies that the process is six sigma qualified and expected overall nonconformance was 0.001% for doping. Moreover, the average 200 source contrast projected 5×5 mm² chip yield using a Lasertec system 88-HIT and the machine learning based PLDLZ recipe was >94% by considering the Particle, Bump, Micropipe, ComplexSF, Polytype Inclusion, Particle Inclusion, and ScratchTrace as device killer defects. The average BPDs were <25 on 150mm wafers using a 1µm thick buffer layer. Initial results on 200 mm wafers are also presented.

Abstract: The memory effect of Al doping in 3C-SiC prevents sharp interfaces between layers of different doping levels and can lead to unintentional doping of subsequent epilayers and even growth runs. Introducing HCl into the growth phase of 3C-SiC reduces the Al incorporation but has a significant impact on Al dopant decay rates and background levels within the chamber, resulting in far sharper doping profiles. The impact of relatively high flow rates of HCl is low within a chlorine-based growth system giving fine control over its influence on the growth process and memory effect.

Abstract: In this paper, the authors continue the experimental evaluation of bipolar degradation for different 1.2 kV SiC MOSFETs. All the devices are stressed by pulsed repetitive forward current through the body diode with current densities varying from 1000 A/cm² up to 5000A/cm². The 1.2 kV SiC MOSFETs are split into two major groups based on the differences in epitaxial material (Type A and Type B) that are subjected to the pulsed forward current stress through the body diode. Additionally, there is a third group with Type B epitaxial material, where p+ implantation process at different temperature is applied to evaluate potential impact on bipolar degradation. Devices are electrically characterized on the Keysight B1505A power device analyzer, both before and after stress testing to trace the drift in the electric parameters. Lastly, the drift in parameters observed in some of the devices, are additionally correlated by an electroluminescence (EL) and scanning acoustic tomography (SAT) analysis.

Abstract: We demonstrate quasi-vertical GaN MOSFETs fabricated on SiC substrates. The GaN epitaxial layers were grown via MOCVD on 100 mm 4H-SiC wafers, with the device structure consisting of a 2.5 μm drift layer and a Mg doped p-GaN body. The fabricated transistors exhibit normally-off characteristics, with low off-state leakage behavior and an on/off ratio of over . The specific on-resistance was measured to be which compares favorably to devices fabricated on other foreign substrates. Our results demonstrate an alternative substrate for realizing vertical GaN devices, which potentially offers better material quality and thermal properties compared with other foreign substrate choices.

Abstract: In this paper, the performance of a new CVD reactor (called PE1O8) designed by LPE and developed in the European project REACTION to process uniform 4H-SiC homoepitaxy on 200 mm substrate is reported. Its tunable multi-zone injection system and new gas delivery configuration ensure the uniform gas distribution throughout the substrate. Excellent thickness and doping uniformity on 200 mm substrates are achieved with run-to-run variation less than 1.4% and 5.6% respectively.

Abstract: Tantalum carbide (TaC) coating, produced in an ultrahigh temperature chemical vapor deposition (CVD) process, exhibited high thermal and chemical stabilities, low emissivity, and high purity. Low emissivity of 0.3~0.43 was measured on TaC coating at 1000°C and compared with the one of SiC coating. As revealed in both simulation and experiment, the low emissivity of TaC coatings not only improves temperature uniformity in the SiC PVT process, but also reduces power consumption in both SiC crystal growth and GaN epitaxial deposition. The results provide important guidance to process tuning when switching from a conventional graphite or SiC-coated component to its TaC-coated counterpart.

Abstract: This work details two approaches with multi-epitaxial growth to create a vertical superjunction structure made of alternating pillars. One approach is a chain of very high energy implants, the other uses a preferred implantation direction to achieve a channeled profile. The manufactured devices show a breakdown voltage of 1000 V for channeled, two-step epi with total 4.9 μm thickness. 800 V for regular high energy implants using three epi steps of total 3.7 μm thickness. The measured Rsp was 0.7 mOhm*cm² for dies with size 0.018 cm². UIS and temperature measurement show reliable performance. The channeled implant looks favorable to reduce the number of process steps needed to create an efficient superjunction structure.

Abstract: In an increasingly electrified technology driven world, power electronics is central to the entire clean energy manufacturing economy. Silicon (Si) power devices have dominated power electronics due to their low cost volume production, excellent starting material quality, ease of fabrication, and proven reliability. Although Si power devices continue to improve, they are approaching their operational limits primarily due to their relatively low bandgap, critical electric field, and thermal conductivity that result in high conduction and switching losses, and poor high temperature performance. Silicon Carbide’s (SiC) compelling efficiency and system benefits have led to significant development efforts over the last two decades and today planar and trench MOSFETs, and JFETs are commercially available from several vendors as discrete components or in high power modules in the of 650 V to 1700 V voltage range. High impact application opportunities, where SiC devices are displacing their incumbent Si counterparts, have emerged and include automotive and rail power electronics with reduced losses and reduced cooling requirements; novel data center topologies with reduced cooling loads and higher efficiencies; variable frequency drives for efficient high power electric motors at reduced overall system cost; more efficient, flexible, and reliable grid applications with reduced system footprint; and “more electric aerospace” with weight, volume, and cooling system reductions contributing to energy savings. In particular, SiC insertion in electric vehicles brings major competitive advantages and is a volume application opportunity that can spur manufacturing economies of scale and lower system costs. As SiC continues to grow, the industry is lifting the last barriers to mass commercialization that include higher than Si device cost, relative lack of wafer planarity, the presence of basal plane dislocations, reliability and ruggedness concerns, and the need for a workforce skilled in SiC power technology to keep up with the rising demand. It should be noted that in many applications, insertion of SiC reduces overall system cost compared to Si even though SiC devices can cost 2-3 more than their Si counterparts. This is due to the passive component and cooling system simplifications enabled by the efficient high frequency SiC operation. In this paper, we will review key aspects of SiC technology and discuss overcoming barriers to mass commercialization.