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Stackable SiC Embedded Ceramic Packages for High Voltage and High Temperature Power Electronics Applications
Keywords: module concept, ceramic embedding, high temperature packaging
SiC semiconductor devices are used in power electronics mainly for switching high electrical currents (up to several 100 A) and high voltages (≥ 1 kV) in automotive drives and energy converters. Due to its larger band gap (3.2 eV for SiC-4H), SiC is superior to Si in terms of high temperature capability (junction temperature of the power devices), high power density as well as high switching speed [1]. However, the approximately three times higher cost of SiC devices (per chip area) is a not negligible issue for the industry [2, 3]. In order to compensate the higher device fabrication costs, a cost optimization at the system level including all related relevant components is needed. Therefore, reducing the chip dimensions can be a cost effective solution. As a result, the power density and the power losses will be increased and rise the operating temperature of the chip significantly up to 300 °C or above. Basicly, the properties of SiC devices allow them to withstand this high temperature range. However, most of the current packaging techniques are limited to operating temperatures of Tmax < 175 °C due to the insufficient thermal stability of the complete power module [1, 4]. Especially solder joints, bond wires, organic potting compounds and housings are critical in terms of high thermal cycle capability. Therefore, this work focuses on a novel packaging concept with lesser vulnerabilities. The approach is based on the embedding of power devices into DBC packages. These packages are designed to carry out the maximal performance of SiC and future WBG devices and are a promising solution for applications in harsh ambient environment such as aerospace and turbine, geothermal well logging, down hole-well oil & gas. By sealing the chip into high thermal conductive ceramics (Al2O3, AlN or Si3N4) with thick copper metallizations, high current carrying, high temperature and high corrosion capability can be achieved. Furthermore, conventional plastic housings and bond wires can be eliminated by stacking the packages and using electrical vias to manage the current paths. The advantages and methods of producing vias in DBC multilayer packages have been shown in the previous work [5]. The concept and the ceramic embedding process steps (laser structuring, chip assembling and package sealing) have been described in [6]. This study continues the work by investigating the properties of the DBC embedded package at high voltages ≥ 12 kV. This is especially advantageous for the upcoming SiC device generations which are expected to possess higher voltage blocking classes (> 3.3 kV). Thus, the number of devices connected in series in high voltage converters (e.g. modular multilevel converter) can be reduced. Various demonstrators have been produced in the scope of this study. Vertical power devices such as high-blocking SiC junction barrier schottky diodes (3300 V) were successfully embedded in DBC substrates (20 mm x 20 mm). First, a package with a single embedded SiC diode inside has been produced. After the chip assembly process, the interior of the DBC substrate was filled with high temperature capable silicone gel (≤ 250 °C). Afterwards, the electrical properties of the embedded SiC diode were tested. The result showed that the current and blocking characteristics correspond to the values from the data sheet. The complete package was able to withstand blocking voltage up to 3.3 kV or higher. In the next step, an additional demonstrator was built by stacking four single diode packages together via soldering. The SiC power devices were connected in series in this multilevel DBC package. Here, high blocking voltages of up to 12.5 kV could be measured at a leakage current of approx. 4 µA. Therefore, the electrical functionality and the suitability of the embedded package for SiC power devices could be successfully demonstrated. In the final paper, the built demonstrators and the assembly process will be described in detail with respect to the current challenges. Furthermore, the aspects regarding high temperature operation and reliability will be discussed. Optimizations for this packaging concept are ongoing. The main focus is on the operation of the complete power module at 300 °C and above. In order to achieve this goal, organic elements in the power module such as molding compound must be excluded completely. One possible solution could be the hermetic sealing of the ceramic packages under vacuum atmosphere. Therefore, reliable joining techniques such as high temperature soldering, silver paste and copper paste sintering are needed and will also be investigated in the scope of this work.
Hoang Linh Bach, Scientistic Worker
Fraunhofer Institute for Integrated Systems and Device Technology IISB
Erlangen, Bayern

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