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INDUSTRY GaN


sidewall spacer. Source-connected field plates are employed to increase gain and RF power density. Using a second field-plate also trims peak fields within the transistor, so this device can last longer. Thanks to the use of optimised field plates, the 1 mA/mm breakdown voltage of these HEMTs exceeds 120 V.


At the beginning of the Title III program, Cree’s engineers carried out a baseline assessment of the production cost, yield, cycle time and performance of their S-band and EW- band MMICs, which were formed by depositing an insulating GaN buffer and AlN and AlGaN cap layers on 100 mm semi- insulating SiC substrates by MOCVD. This initial study showed that Cree’s process exceeded the baseline key performance parameters set out by the title III program, with yields up to 75 percent above the benchmark, and cycle times 42 percent quicker than the initial standard.


During the programme, efforts have focused on driving yield higher, trimming cycle times and reducing manufacturing costs. This has included work to optimise gate metal electrode lift-off and reduce the number of damage sites; a switch to a new post-backside de-bond process that cuts front-side damage and drives down cycle time; and the qualification of a SiN passivation tool from a manually loaded, older PECVD platform to an automated version that diminishes particle count and reduces handling damage.


These efforts have increased yield, while cutting cycle time and cost. S-band and EW-band yields are 8 percent and 18 percent above the mid-point goals for the programme, while the MMIC cost and cycle time are 36 percent and 25 percent below the interim benchmark (see Figure 1).


Cost savings have been realised, thanks to cutting material costs, increasing automation and boosting throughput. “Higher factory loading is primarily due to the rapid adoption of our GaN products for high-volume markets, such as telecom base stations,” explains Fury. “These cost savings are helping us to further increase GaN penetration into the military and commercial markets.”


High reliability is another target within the Title III program. For continuous operation at a temperature of 125 °C for the back of the MMIC die – which equates to a maximum junction temperature of 225 °C – the mean-time-to-failure (MTTF) goal is a million hours. Cree’s MMICs are far more reliable than that, with MTTF values of 55 million hours for the G28V3 process and 150 million hours for the G28V4 process. “Our GaN process has the highest rated operational channel temperature of any currently on the market,” claims Fury.


It might seem that with goals appearing to be met so easily, these targets should have been higher. But that misses the point. “The goals were structured to ensure that Cree would be able to produce GaN products with the cost, performance and reliability needed to support a number of critical US defence programmes,” says Fury.


One key challenge remains for Cree in this Title III program: Passing an 8000-hour, RF high-temperature operating life test. This is designed to provide an additional assessment of the robustness of the process.


Cree is now involved in the final assessment phase of the programme. However, its processes have been independently assessed by two different external customers. They have judged


The Title III program will help to reduce the cost and increase the reliability of GaN MMICs that can be used for X-band radar systems. Examples of this type of system include the sea-based X-band radar, which can provide highly advanced ballistic missile detection to discriminate a hostile warhead from decoys and countermeasures. Credit: US Navy


32 www.compoundsemiconductor.net July 2013


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