BoroGaN

It is recognised that the performances of electronic devices, circuits and related subsystems are drastically impacted by thermal issues, and by the associated reliability issues. The most classical solution today is to let the heat spreading into the component structure; the heat is then removed on a larger area through the packaging stack. The efficiency of the thermal cooling solutions increases as they are located closer to the heat sources. The BoroGaN project proposes to study the impact of a self-aligned Boron Nitride (BN) thin film that will be deposited on top of a Gallium Nitride-based (GaN) transistor, in the closest vicinity of the active zone, where the heat is generated. This unique aligned film has its basal planes oriented perpendicularly to the substrate. This nano-structured material thus has a much improved thermal conductivity in the z-axis compared with its nominal counterparts. This approach should improve drastically the efficiency of dissipated power removal. The High Electron Mobility Transistors (HEMTs) are subject to intense current densities and high electric fields (several MV/cm). During operation, the estimated temperature of the channel increases significantly. A reduction of the operating temperature by 35°C would lead to more than tenfold device lifetime improvement. A new technological building block based on high thermal conductivity BN will be developed, which will be compatible with microelectronics processes. The BN deposition technique will allow room temperature deposition, i.e. will be compatible with the other technological steps. Moreover the BN shows suitable physical properties for heat spreader with a high thermal conductivity (>450 W.m-1.K-1), a high breakdown electric field (>5 MV/cm), a high electrical resistivity (1E12 Ohm.cm). The last parameter is important to give rise to low microwave losses up to 60 GHz. The work programme is broke down in the following way. The first work package is devoted to the growth, optimization and characterization of BN thin films on various substrates. 3-5 Lab will fabricate specific elementary devices as MIM structures for electrical BN characterisation. The second work package is dedicated to the application of BN thin films for the GaN HEMT heat-spreader. 3-5 Lab will elaborate GaN-based heterostructures and microwave transistors to allow the growth of BN on top of devices. These transistors will be characterised through static, dynamic and load-pull measurements. Finally, the devices will be characterised thermally during operation with the thermoreflectance technique. This non-contact optical method shows excellent spatial resolutions of the order of 200–500 nm. The thermal resistance values for the structures with and without BN heatspreader will be measured and compared. In terms of economic impact, communications and power electronics are the main markets where GaN devices are expected to achieve the most significant commercial impact. Indeed, their high power density at high frequencies make them choice candidates for a large set of electronic applications. The complementary expertise of the BoroGaN partners (CINTRA/NTU, 3-5 Lab and CNRS-neurophotonique), will enable the demonstration of innovative BN top heat spreaders for the improvement of GaN HEMTs. Furthermore the low thermal budget will let the use of this technology to other devices using for instance silicon carbide or gallium arsenide semiconductors. The project responds to the Materials, Nanotechnologies and Nanosystems themes of the joint ANR/NRF Grant Call 2016. The project responds to ANR Challenge 3 "Stimualte Industrial Renewal" / Axis 3 "Materials and Processes" and French National Research Strategy Orientation 14 "Conception of New Materials". The project will focus on the development of room-temperature nano-crystalline BN materials applied to the micro- and nano-meter size fabrication and characterisation of Gallium Nitride HEMT.