INDUSTRY GaN
There are 10 manufacturing readiness levels associated with the Title III program
Raytheon’s refinements Raytheon’s engineers started work on their Title III program back in 2009, and have now realised the primary goal, demonstrating and validating manufacturing readiness level 8 and qualifying this process for insertion into DoD production systems.
This project involved three major tasks: processing 24 wafers with the baseline production process to evaluate yield and reliability; reducing variations through identification of root causes, using a minimum of 150 wafers; and processing 24 wafers with the improved process, to demonstrate that the requirements for manufacturing readiness level 8 have been satisfied.
For the first part of this project, 24 wafers were processed with a mask set that included FETs, small MMICs for reliability evaluation and large periphery X-band MMICs for yield evaluation. Reasonable yields were obtained for the latter device, but they were less than those for comparably sized GaAs MMICs. Meanwhile, reliability assessments based on three-temperature, DC Arhenius testing fulfilled the programme
objective for MTTF, but a small proportion of the devices under test exhibited early degradation at 380 °C. This initial study shaped the primary goals for the next phase of the project: To improve X-band MMIC yield to a level comparable to that for GaAs MMICs, and to eliminate early degradation observed in a small proportion of devices at elevated temperatures.
Success resulted from increasing the yield of capacitors and improving the gate process, which largely addressed the early degradation issue. Engineers also investigated more aggressive DC and RF screening tests. More stringent screening reduced yield at the wafer level, but was beneficial elsewhere, leading to dramatic improvements in the predictability of reliability data and next-level assembly yield.
An assessment of the RF operating life of the devices was made at this point in the project, with the performance of seven X-band MMICs scrutinised during a 5,000-hour test, where they were driven in continuous-wave operation. No discernable change in power could be seen during this evaluation.
By the end of the project, GaN process yield more than tripled to a level that is comparable with Raytheon’s GaAs process. This gain contributed to a fall of greater than 75 percent in the cost-per-Watt associated with GaN, making it three times cheaper than GaAs, according to calculations that include substrate costs.
Reliability assessments at the end of the project show a MTTF at 150 °C of greater than a billion hours, more than three orders of magnitude higher than the goal set out in the programme. Even the time to 1 percent failure is more than 10 million hours.
Figure 1: Through process improvements, such as refinements to the gate metal lift-off procedure and the post-backside de-bond process, Cree’s engineers have reduced the cycle time of their manufacturing process and increased its yield
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www.compoundsemiconductor.net July 2013
Himes is pleased with the progress made by Raytheon, and also that accomplished by Cree and TriQuint: “Each of our GaN projects has demonstrated tremendous success in establishing mature and capable production processes.” He attributes this success to outstanding industry-government partnerships involving talented, committed industry teams that communicate bi-weekly with their government partners.
© 2013 Angel Business Communications. Permission required.
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