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Low Temperature Homogeneous Epitaxy of 4H-SiC Using Novel Precursors

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: Develop Chemical Vapor Deposition (CVD) techniques using novel chemistries for epitaxial growth of 4H-SiC that allow lower than state-of-the-art growth temperatures while retaining state-of-the-art growth rates and improved material quality. 

DESCRIPTION: In recent years, silicon carbide has proven its worth in the power electronics industry. High performance diodes and transistors are commercially available and are being designed into power conditioning systems from a wide range of industries. Transistors can be purchased with voltage ratings up to 1700 V while diodes are available up to 8 kV. Discrete 1200 V diodes and transistors are available with current ratings over 100 A. The low on-resistance, fast switching, high current density, and temperature tolerance provide significant system benefits over silicon devices. These system benefits often outweigh the higher cost of the SiC devices as compared to silicon. While the cost per amp of SiC continues to drop, the cost of SiC bulk material and epitaxy will never be as low as silicon due to the high temperatures and manufacturing methods required to produce SiC. For instance, Si epitaxy is typically grown at temperatures ranging from 1000 to 1200 degrees C with growth rates of hundreds of µm/min. SiC epi, however, is grown from 1500 to 1800 degrees C and, while growth rates above 100 µm/hr are possible, they are challenging to the system and material quality [1]. Because of this, epi cost and throughput remains a cost driver even for 1200 V devices with relatively thin epilayers (~10 µm). For even higher voltage devices (10s of kV) epilayer thicknesses quickly grow to over 100 µm. The challenge of growing thick layers while keeping growth rates and material quality high increase the costs dramatically. The addition of HCl or chlorinated silicon precursors such as chlorosilanes (SiHCl3 and SiCl4) has mitigated some of the deleterious side effects of pushing growth rate such as the formation of silicon droplets on the wafer or other surfaces in the reactor [1]. In addition to providing Cl, chlorinated precursors also appear to provide beneficial surface kinetics that benefit material quality and allow for lower growth temperatures. Chlorinated carbon precursors, chloromethane (CH3Cl) in particular, have been shown to produce high quality films as at growth temperatures as low as 1300 degrees C [1][4]. Growth with chloromethane and chloromethane with HCl or chlorosilane has been shown to be more efficient than growth with hydrocarbons and HCl or chlorosilane alone [1]. Low temperature growth with CH3Cl and SiCl4 was also shown to produce very low donor concentrations (~3e14 1/cm³) in films as thin as 5 µm [2], and much higher than typical acceptor concentrations of over 2e20 1/cm³ [3]. Low temperature growth also has the benefit of allowing for selective area growth using an SiO2 mask [1]. Another area that needs to be investigated is the benefits of CH3Cl/SiCl4 growth on extremely low off-angle or on-axis substrates. Growth rates at 1300 degrees C using CH3Cl have not been shown to equal those of other precursors at much higher temperatures. However, the growth rate possible using CH3Cl/SiCl4 will increase with temperature and could possibly match the rate of other chemistries at still a much lower temperature. Even a more modest reduction in temperature of 100 degrees C could have a significant impact on tool life and throughput. The goal of this effort is to develop a production worthy 4H-SiC epitaxial growth process using chlorinated carbon precursors or other novel precursors with reduced growth temperature and improved material quality as compared to state-of-the-art. 

PHASE I: Exhibit homoepitaxial growth of 4H-SiC using precursors including chlorinated carbon sources or other novel gases with similar effect. Demonstrate growth rates ≥ 20 µm/hr with an ultimate goal of 50 µm/hr while maintaining good quality. Growth temperatures should be at least 100 degrees C less than state-of-the-art for similar growth rates. The n-type dopant concentration and uniformity as well as defect densities should be characterized through appropriate techniques. 

PHASE II: Further develop process to find the optimal balance between growth rate and temperature to maximize benefits to overall device production. Include considerations for thinner (up to 20 µm) and thick (> 100 µm) epi products, including heat up time, cool down time, growth time, material quality, and energy budget. Demonstrate process by growth of lightly doped 100 µm thick n-type epitaxy on whole wafers up to 100 mm in diameter. Perform industry benchmark characterization of the material including carrier lifetimes and defect densities. Also demonstrate p-type doping capability. 

PHASE III: DUAL USE COMMERCIALIZATION: Military Application: Power control and distribution of more-electric aircraft, hybrid electric ground vehicles, and directed- energy weapons. Commercial Application: Renewable energy harvesting, hybrid electric vehicles, commercial more electric aircraft, etc. 

REFERENCES: 

1. H. Pedersen, S. Leone, O. Kordina, A. Henry, S. Nishizawa, Y. Koshka, and E. Janzén, “Chloride-Based CVD Growth of Silicon Carbide for Electronic Applications,” Chemical Reviews 2012 112 (4), pp. 2434-2453.; 2. S. P. Kotamraju, B. Krishnan, F. Beyer, A. Henry, O. Kordina, E. Janzén, and Y. Koshka, "Electrical and Optical Properties of High-Purity Epilayers Grown by the Low-Temperature Chloro-Carbon Growth Method," Materials Science Forum, 2012 717-720, pp. 129-132.; 3. B. Krishnan, S. P. Kotamraju, G. Melnychuk, H. Das, J. N. Merrett, and Y. Koshka, “Heavily Aluminum-Doped Epitaxial Layers for Ohmic Contact Formation to p-Type 4H-SiC Produced by Low-Temperature Homoepitaxial Growth,” Journal of Elec Materi 2010 39 (1) pp. 34-38.; 4. Y. Koshka, “Method for Epitaxial Growth of Silicon Carbide,” US Pat No. 7,404,858 (July 29, 2008).

KEYWORDS: Silicon Carbide, Epitaxy, 4H-SiC, Semiconductor 

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