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GaN  microelectronics


secured €8.6 million of funding and there are plans in place for a follow-on phase that will concentrate on validating and qualifying the manufacturing process for the special requirements of the space environment. Seven academic and industrial partners from four European countries are involved in the programme, and each has a specific responsibility (see Figure 1 for details).


The German firm Tesat-Spacecom (Tesat) is coordinating and managing the project, which is divided into five primary work packages (see figure 2). The aim of the largest of these, WP2000, involves undertaking process development trials to establish a space-compatible foundry process. Performance validation is being undertaken using L-band and X-band technology evaluation structures.


Figure 2: Activities within the €8.6 million GREAT2


project


have been divided into several work packages


national agencies, with efforts supported by innovative component development activities at the European Space Agency (ESA) and EU defence initiatives, such as the Korrigan project. Although these activities produced a portfolio of devices with very impressive performance figures that were reported in the 2002-2006 timeframe, European commercial suppliers of GaN microwave devices still had to overcome reliability-related issues before establishing themselves on a commercial basis.


Reliability requirements


If GaN devices are to be deployed in space applications, they must deliver high levels of reliability without compromising performance. Typical satellite operating lifetimes are eight-to-nine years for Earth Observation missions and upwards of 18 years for telecommunication satellites with little room for failure. To address this, in 2008 the ESA launched its GaN REliability And Technology Transfer initiative (GREAT2


), with the aim of establishing


a European supply chain for the manufacture of high reliability, space compatible, GaN-based microwave transistors and integrated circuits that would be free from any ITAR or end-user licence restrictions. The first phase of the project, which focuses on device reliability, has


This work package is led for ESA by United Monolithic Semiconductors (UMS). It is responsible for establishing the final commercial foundry process. Additional support for this effort comes from: IAF and FBH, who are undertaking specific processing trials under the guidance of UMS to help improve the reliability performance of the foundry process; Tesat, which is responsible for device packaging and reliability assessment in RF packages; and the University of Rome, which is providing support for simulating and understanding the physics of GaN HEMT failure. Two types of foundry process are being produced by UMS in this programme: a 0.5 µm gate length process (GH50) for fabrication of discrete GaN HEMTs for operation up to 6 GHz, and a full MMIC process using 0.25 µm gate length technology (GH25) for operation to about 20 GHz.


Two of the other work packages, WP3000 and WP4000, are focusing on optimising device processing and epitaxial growth on silicon and SiC substrates. One of the primary aims of this work is to confirm whether device passivation produces better, i.e. more reliable, devices when this process is carried out in-situ, rather than ex- situ. The in-situ nitride devices are being fabricated at imec, and the ex-situ nitride devices fabricated at Fraunhofer IAF. In the WP3000 programme, accurate measurements of channel temperature, which are needed to determine the mean-time-to-failure in accelerated lifetime tests, are being obtained through Raman micrography measurements at the University of Bristol. The GaN microwave devices made during the project are being assessed under space environmental operating conditions by Tesat as part of the work package WP5000.


Figure 3. Engineers at Fraunhofer IAF in Germany are fabricating GaN-on-SiC epiwafers with an MOCVD multi-wafer reactor


32 www.compoundsemiconductor.net October 2011


By the end of the first phase of the project, which is scheduled for completion in November 2012, more than 170 wafers will have been fabricated and assessed for reliability and suitability in a space operating environment. The successes of these efforts will be judged against performance-related milestones, which feature incremental increases in performance and reliability over time, and were drawn up by the ESA. Meeting the interim milestones M3 and M5 requires device operation in excess of 1000 hours and 10,000


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