Inductive Coupled Plasma Etching of High Aspect Ratio Silicon Carbide Microchannels for Localized Cooling

High aspect ratio microchannels using high thermal conductivity materials such as silicon carbide (SiC) have recently been explored to locally cool micro-scale power electronics that are prone to on-chip hot spot generation. Analytical and finite element modeling shows that microchannels used for localized cooling should have high aspect ratio features (above 8:1) to enable the heat flux levels required to locally cool GaN transistors to temperatures below 100°C. This work presents experimental results of microfabricating high aspect ratio microchannels in a 4H-SiC substrate. Depths of 90 and 80 μm were achieved with a 5:1 and 12:1 aspect ratio, respectively. This microfabrication process used to create high aspect ratio features in SiC will enable the integration of microchannels (backside features) with highpower density devices such as GaN-on-SiC based electronics, as well as other SiC-based microfluidic applications. INTRODUCTION Gallium nitride (GaN) transistors used in high power and high frequency electronics generate power densities as high as 10 W/mm of channel width leading to temperature rises as high as 200°C which can decrease reliability [1–3]. Therefore, it is important to reduce the peak temperatures of these hot spots to increase operation lifetime. Silicon carbide (SiC) substrates are often used in the design of GaN-based electronics due to its high thermal conductivity (370 W/m-K) which can be leveraged to reduced device channel temperatures. However, the use of SiC alone is not sufficient to reduce the channel temperatures as the thermal conductivity of SiC drastically reduces with an increase in temperature [4]. Thus, the addition of microchannels and cooling fluid into the SiC substrate is one solution for decreasing device temperatures during operation. This approach allows the use of both convective and conductive heat transfer to reduce overall temperature of the system and increases the overall heat flux of the system and local cooling [5]. It is approximated that channel temperatures can remain below 110°C using this integrated cooling architecture [6]. Silicon based microchannel heat exchangers have been demonstrated in the past with various geometries to optimize thermal cooling [7–11]. However, Si thermal conductivity is low the growth of power devices on Si substrates results in increased leakage currents due to the generation of thin film defects. Therefore, SiC substrates, although costly, have been identified for the development of SiC-based and GaN-based power electronics devices. However, the plasma etching of SiC often results in slow etch rates (0.2 μm/min to 1 μm/min) and reduced selectivity in comparison to well established plasma etches for Si substrates. Previous work in plasma etching of SiC has primarily focused mainly on the manufacturing of large via holes and feature sizes greater than 50 μm [12]. However there are reports of 10 μm width trenches with depths of 110 μm, creating a 11:1 aspect ratio [13] and trench widths of 13 μm and 7.6:1 aspect ratio in 6H-SiC [14]. However, there is little reported on smaller opening widths and large depths (higher aspect ratios) in 4H-SiC. This paper presents analytical calculations of high aspect ratio SiC performance and experimental results in the fabrication of high aspect SiC microchannels. First the analytical