Rajesh Kumar Malhan

Compact Double-Side Liquid-Impingement-Cooled Integrated Power Electronic Module

This paper presents a compact integrated power electronic module (IPEM) which seeks to overcome the volumetric power density limitations of conventional packaging technologies. A key innovation has been the development of a substrate sandwich structure which permits double side cooling of the embedded dies whilst controlling the mechanical stresses both within the module and at the heat exchanger interface. A 3-phase inverter module has been developed, integrating the sandwich structures with high efficiency impingement coolers, delink capacitance and gate drive units. Full details of the IPEM construction and electrical evaluation are given in the paper.

A New Integration Method for an Electric Vehicle Wireless Charging System Using LCC Compensation Topology: Analysis and Design

There is a need for charging electric vehicles (EVs) wirelessly since it provides a more convenient, reliable, and safer charging option for EV customers. A wireless charging system using a double-sided LCC compensation topology is proven to be highly efficient; however, the large volume induced by the compensation coils is a drawback. In order to make the system more compact, this paper proposes a new method to integrate the compensated coil into the main coil structure. With the proposed method, not only is the system more compact, but also the extra coupling effects resulting from the integration are either eliminated or minimized to a negligible level. Three-dimensional finite-element analysis tool ANSYS MAXWELL is employed to optimize the integrated coils, and detailed design procedures on improving system efficiency are also given in this paper. The wireless charging system with the proposed integration method is able to transfer 3.0 kW with 95.5% efficiency (overall dc to dc) at an air gap of 150 mm.

High performance cooling system for automotive inverters

A novel double-side cooled power module is presented which delivers superior cooling performance with the potential for improved robustness to thermal cycling. The semiconductor dies are sandwiched between conventional DBC substrates, the substrates being directly cooled rather than through a conventional heat spreader heat sink assembly. A theoretical analysis is presented illustrating that direct cooling can offer a lower total thermal resistance provided the heat transfer coefficient at the cooled surface is sufficiently high. Experimental results demonstrate the effectiveness of the selected impingement cooling technique when applied in both single- and double-side cooled formats. Measurements on the double-side cooled structure show a total thermal resistance (junction to ambient) that is less than 40% of the junction to case resistance of a conventional module. Similar improvements are observed in the transient thermal impedance (step response) curve indicating that thermal cycling ranges will be reduced under all operational conditions.

Structural pattern formation in titanium?nickel contacts on silicon carbide following high-temperature annealing

The composition and microstructure of compound contacts to 4H-SiC containing both titanium and nickel were investigated. Samples were prepared by metal evaporation on commercial 4H-SiC wafers followed by rapid thermal annealing (RTA). Contact structures with three different metal deposition sequences were investigated: (A) SiC/Ti(4 nm)/Ni(150 nm); (B) SiC/Ti(100 nm)/Ni(50 nm) and (C) SiC/Ti(4 nm)/Ni(50 nm)/Ti(100 nm). RTA was performed in a vacuum at 800, 925 and 1040 °C for a period of 800 s. X-ray diffraction, Auger electron spectroscopy and transmission electron microscopy were used for the characterization. A distinct spatial separation of nickel silicide and titanium carbide layers was observed in all samples. It was discovered that the final distribution of solid-state reaction products in B- and C-samples was independent of the order of the deposition of the initial metal films. In both samples, a two-phase TiC+C layer was found to be adjacent to the SiC substrate. Factors controlling phase formation and segregation are discussed. A two-stage reaction model is proposed to explain the reaction zone structure formed in the Ni–Ti–SiC system after high-temperature treatment.

SiC Migration Enhanced Embedded Epitaxial (ME3) Growth Technology

In this work, we have developed an innovative epitaxial growth process named the “Migration Enhanced Embedded Epitaxial” (ME3) growth process. It was found that at elevated growth temperatures, the epitaxial growth at the bottom of the trenches is greatly enhanced compared to growth on the sidewalls. This is attributed to the large surface diffusion length of reactant species mainly due to the higher growth temperature. In addition, it was found that this high temperature ME3 growth process is not influenced by the crystal-orientation. Similar growth behavior was observed for stripe-trench structures aligned either along the [11-20] or [1-100] directions. No difference was observed in the electrical performance of the pn diodes fabricated on either oriented stripe geometry. The ME3 process can also be used as an alternative to ion-implantation technology for selective doping process.

In Situ Nitrogen and Aluminum Doping in Migration Enhanced Embedded Epitaxial Growth of 4H-SiC

The in-situ doping of aluminum and nitrogen in migration enhanced embedded epitaxy (ME3) is investigated with the aim to apply it to the realization and fabrication of all-epitaxial, normally-off 4H-SiC JFET devices. This ME3 process consists of the epitaxial growth of an n-doped channel and a highly p-doped top gate in narrow trenches. We found that the nitrogen doping in the n-channel (a-face) is a factor 1.5 higher than layers grown with the same process on Si-face wafers. Due to the low C/Si ratio and the low silane flow rate used in the ME3 process, the growth of the p-doped top gate needs high flow rates of the aluminum precursor trimethylaluminum for several hours, which contaminates the CVD reactor and causes aluminum memory effects. These aluminum memory effects can be reduced by an extra high temperature bake-out run.

Compact Inverter Designed for High-Temperature Operation

This paper presents an integrated 10 kW inverter designed to operate in the demanding automotive environment. To achieve good compactness and high reliability to thermal cycling, several original approaches have been implemented together: the power modules are built with a sandwich structure, employing mechanical improvements compared to existing comparable structures; these sandwich modules are cooled directly (i.e without an intermediate base-plate layer), with the cooling fluid being sprayed to improve heat extraction; finally, 1200 V silicon carbide devices are used for improved performance. The specific requirements of the automotive environment are described as well as the chosen approaches. Experimental results are presented, giving details of the electrical, thermal and reliability performance.

Growth Mechanism and 2D Aluminum Dopant Distribution of Embedded Trench 4H-SiC Region

The migration enhanced embedded epitaxy (ME3) mechanism and 2D dopant distribution of the embedded trench region is investigated with the aim to realize the all-epitaxial, normally-off junction field effect transistor (JFET). We found that the embedded growth consists of two main components. First one is the direct supply without gas scattering and the other one is the surface migration supply via the trench opening edge, which dominate the ME3 process. An inhomogeneous 2D distribution of Aluminum (Al) concentration was revealed for the first time in the 4H-SiC embedded trench regions by the combined analysis of secondary ion mass spectrometry (SIMS) and scanning spreading resistance microscopy (SSRM) results. The maximum variation of Al concentration in the trench is estimated to be about 4-times, which suggests that the Al concentration is highest for the (0001) plane and lowest for the trench corner (1-10x) plane. Al concentration in the (1-100) plane, which determines the JFET p-gate doping level is 1.5-times lower than (0001) plane for trench region fabricated on Si-face wafers.

Homoepitaxy of 4H-SiC on Trenched (0001) Si Face Substrates by Chemical Vapor Deposition

The homoepitaxy of 4H-SiC on trenched substrates by chemical vapor deposition has been investigated. Two types of 4H-SiC (0001) substrates inclined 8° toward [110] or [100] were used. 4H-SiC growth near trenches perpendicular to the off-direction tended to be highly asymmetric due to the influence of step flow growth, and the (0001) facet was formed along the downstream side of trenches by step flow. In contrast, the growth near trenches parallel to the off-direction was symmetric. The strong influence of C/Si ratio on the growth behavior near trenches has been revealed. Although the growth rate on trench sidewalls is usually lower than that on the bottom of a trench and that on the top surface, this difference in epilayer thickness becomes smaller with decreasing C/Si ratio during growth. The use of a low C/Si ratio is effective in filling trenches. A smooth morphology was obtained on SiC substrates inclined toward the [110] direction, but triangular surface defects appeared on SiC substrates inclined toward the [100] direction.

Effect of C and B sequential implantation on the B acceptors in 4H–SiC

We have systematically investigated the effect of C and B sequential coimplantation on B-related acceptors and deep levels in 4H–SiC using thermal admittance spectroscopy. By increasing the concentration of coimplanted C, the density of deep levels decreased and was completely suppressed for a C and B ratio of 1:1. Moreover, the density and ionization energy of B acceptors increased and decreased, respectively, with increasing C concentration. However, we found that excess C content leads to the formation of a complex defect. Capacitance–voltage results also support the expected increase in the free hole concentration with increasing concentration of the coimplanted C atoms, which is followed by a decrease in the concentration under C-rich conditions. This is in reasonable agreement with the behavior of the B acceptors and deep defect levels. Therefore, the concentration of coimplanted C atoms is considered to be very sensitive to the formation of the B acceptor levels.