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Characterization of Ultra-Thin 3YSZ for High-Density Power Electronic Circuit Applications
Keywords: Power Module Substrate, Thermal Conductivity, Dielectric Properties
In this paper, new ultra-thin, 20m and 40m, 3 mol% Yttria-Stabilized Zirconia (3YSZ) ceramics are investigated for use as a substrate material in Wide Band Gap (WBG) Power Electronics Modules for applications ranging from 150V to 2,500V and >250˚C. The ceramic material is extensively characterized up to 250˚C and results discussed for various electrical and thermal properties. Several advanced power electronic circuit topologies are investigated that use the ceramic in 3D embedded structures, including ultra thick copper interconnects. These results will provide the power electronics circuit designers with a new approach to higher performance, high temperature circuits that use SiC and GaN semiconductors. As well-established Si technology is widely used in power electronics applications, such as transportation electrification, downhole drilling, and renewable energy, it is being supplanted with new wide band gap (WBG) power semiconductor devices (particularly SiC and GaN), which have high-voltage capabilities >10kV, and high-temperature operation (>250˚C), and high switching frequencies [ 1, 2, 3 ]. To fully take advantages of WBG power devices, different packaging components, including substrates, die attaches and encapsulations, are being developed and studied, as well as their reliability under high-frequency, high-voltage and high-temperature operation conditions [ 4, 5]. With relatively high thermal conductivity, low coefficient of thermal expansion (CTE), excellent heat-conductance and chemical resistance, Al2O3, Si3N4, and AlN direct-bonded-copper (DBC) have already been recognized as general substrates in power electronics packaging [6]. However, the firing temperature of these ceramic substrates is usually above 1700oC, and fabrication processes are complex [7]. Also, to get good blocking voltage capability, the thickness of the ceramic substrate in the DBC structure is relatively large. This creates a larger thermal mass and larger volume. To further improve the power density with smaller volume and weight, an alternative approach for a power module substrate is required. Thermal conductivity of ultra-thin 3YSZ was measured at different temperatures from 235K to 600K through the 3-omega method [ 8, 9]. Thermo-reflectance measurements [10] were also applied as a reference. Simulations were performed to verify testing results and showed good correlation of data. Laser Beam Deposition was applied to deposit Au patterns for measurements. The theoretical analysis and equipment set-up of the tests were also introduced. The results showed that the thermal conductivity of 3YSZ was around 3W/mK, and decreased slightly as temperature increased. Considering the thermal mass, the ultra-low thickness of 3YSZ can compensate for its low thermal conductivity. Thermal performance of different power modules with ultra-thin 3YSZ was compared to traditional DBC substrates through multi-physics simulations using COMSOL. The dielectric constant and blocking voltage of the ultra-thin 3YSZ was measured. Electron Beam Deposition was used to deposit 200 nm Titanium on opposite sides of 3YSZ samples to form electrodes for the testing. The dielectric constant was measured using an LCR meter, from 10 kHz to 10 MHz, which is typically the operation frequency range of power modules. Blocking voltage was measured at different temperatures from -65 oC to 250 oC through a high voltage test setup. The ultra-thin 3YSZ substrate provides a creative solution for high power density, high frequency and high temperature power electronics packaging.
Xin Zhao, PhD Student, Research Assistant
North Carolina State University
Raleigh, North Carolina

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