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Plasma Processing: The Essential Steps for High Performance GaN on SiC RF Devices
Dr Mark Dineen, Dr Harm Knoops Oxford Instruments Plasma Technology
GaN on SiC RF devices are becoming increasingly important to realise the efficient high frequency devices required for today’s communications. The high electron mobility of the AlGaN/GaN 2DEG enables fast switching times with low power loss. Building these devices requires precise control of plasma processing, without this knowledge device lifetime and performance will be compromised. Oxford Instruments Plasma Technology delivers the solutions through advanced technology and process know-how. GaN based RF devices offer many advantages over other materials such as switching speed and power density. The intrinsic material properties of GaN such as wide band gap, high electron mobility, high thermal conductivity and high breakdown voltage make it a good performance choice for high power high frequency devices. Native GaN substrates, however, are very expensive and are likely to remain so for the foreseeable future due to technological barriers. The substrate choices for GaN RF are currently between Si and SiC. Si is cheap and widely available upto 300mm wafer diameter but it has drawbacks in that the lattice mismatch and co-efficient of thermal expansion difference are large which creates defects in the GaN grown on top and high wafer stress unless complex stress relief structures are incorporated. While SiC has a closer lattice match it is still relatively expensive and wafers are limited to 150mm diameter. If considerations of grown GaN quality and substrate costs are balanced SiC is the substrate of choice for many applications. However, SiC as commonly used for substrates is semi-insulating which adds certain process requirements into the device fabrication chain, specifically a through wafer via to enable a backside contact to the active device. Figure 1 is a schematic of the finished device. It shows the features specific to the geometry of a GaN RF device grown on a SiC substrate. Many of these features require plasma processing to be realised. Oxford Instruments as a provider of advanced plasma processing tools enables these process steps. 2. Device Structure
Figure 2 is a typical process flow for making these devices with the plasma processing steps specifically highlighted.
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