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GaN  microelectronics


GH50 transistors and IAF GaN25 MMIC devices passed accelerated RF life tests carried out by Tesat in December 2010. Confirmation of reliability to 1000 hours of operation came from independent, internal testing at ESA.


In fact, tests on 11 packaged L-Band test vehicles show that, in general, the RF output power changes very little over 1400 hours of operation at a peak channel temperature in excess of 230°C. Ten devices had a maximum drift in RF output power of less than 0.5dB – just one showed rapid degradation at the start of the test. However, this particular device had been subjected to failure analysis to identify the physical changes occurring within the device and to better understand any underlying failure mechanisms. No burn-in or screening other than electrical performance and assembly integrity test has been performed before life testing.


Figure 6: As part of the assessment for the M3 milestone,researchers studied the change in RF output power for 11 packaged L-Band test vehicles with a peak channel temperature of more than 230°C.Some of these devices were operating at 4 dB of gain compression (Tesat), and others at 6 dB of RF gain compression (ESA).Both devices were also assessed at two different baseplate temperatures – 125°C and 150°C.No burn-in or screening,other than electrical performance and assembly integrity test,has been performed before life testing


for these epiwafers, which featured intentional variations in AlGaN composition and GaN buffer structure and were fabricated with both in-situ and ex-situ nitride passivation. UMS, a firm with considerable manufacturing experience, provides feedback concerning the optimum choice of epitaxial structures and processing recipes to be considered.


By the end of 2010, two wafer-batch-processing trials had been successfully completed at FBH and imec, plus three wafer batch trials by Fraunhofer IAF. In addition, UMS had finished processing 30 wafers focusing on ‘pre-process freeze’ engineering variations of its GH50_10 production process and development trials on the GH25_10 production process dedicated to GREAT2


.


In total, over 74 wafers had been fabricated and assessed, providing enough information to understand the impact of the epitaxial design on the reliability of the structures.


Initial processing batches were screened for DC, pulsed DC and RF performance, plus on-wafer reliability. Wafers delivering the most promising performance were diced and used to form packaged test samples that were evaluated by Tesat. Both the L-band HEMT and the X-band MMIC passed M3, the first critical milestone (see Figures 4 and 5 for details).


To assess the RF reliability criteria within the M3 milestone, the consortium assembled 68 L-Band and 67 X-band parts in hermetic packages and a further 64 test cells that were housed in DIL24 ceramic packages, specifically for performing radiation and hydrogen poisoning tests. UMS


34 www.compoundsemiconductor.net October 2011


The radiation tests that have been performed have scrutinized the impact on transistor performance following exposure from gamma radiation, total ionising dose, proton radiation (displacement damage) and single event burnout effects under heavy ions. In assessments made against the M3 milestone, which were carried out using the GREAT2


p/cm2 (35 MeV proton


radiation test cell (see figure 7), no devices from the batch underwent any appreciable DC parameter drift for a total ionising dose of 1Mrad and for a proton fluence of 1.7x1012


energy). However, under heavy ion excitation (Xenon: LETGaN


=52.93 MeV/mg/cm2 ) the open channel current


and threshold voltage for UMS GH50 devices drifted by about 10 percent. This variation is still comfortably below the target drift specifications of less than 15 percent. Initial tests on devices fabricated with the UMS GH50 process revealed that DC static burnout occurred at 200 V, while under heavy ion excitation single event burnout typically occurred at 150 V. To verify these findings, the consortium will conduct further tests on multiple wafer batches. Further radiation test campaigns, planned for M5 and M7 milestones, will also scrutinize any changes to larger devices under both DC and RF operating conditions.


The first wafer batches have also been subjected to 24- hour hydrogen poisoning tests, which involve subjecting unbiased devices at 250°C to a mixture of 5 percent hydrogen gas and 95 percent nitrogen gas. These initial measurements indicate that performance is not particularly sensitive to this effect and will be further validated by incresing the hydrogen test exposure time to several hundred hours.


By December 2010, evaluation devices had passed all the tests related to the M3 milestone, an encouraging achievement that suggests there should be no major showstoppers for using GaN technology in space. That is not to say that lessons have not been learnt from this exercise. This programme of work has shown that controlling the strain in the epitaxial layers, appropriate tailoring of the electric field distribution close to the gate, appropriate choice of surface pre-treatments and


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