- Email: [email protected]
Qualification of GaN microwave transistors for the European Space Agency Biomass mission
Microelectronics Reliability 88–90 (2018) 378–384
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Qualification of GaN microwave transistors for the European Space Agency Biomass mission
T
⁎
A. Barnesa, , F. Helierea, P. Villara, H. Stuhldreierb, C. Beaurainc, D. Bouwc, M. Grunwaldb, E. Moessd, T. Muckd, C. Schildbachd, T. Aylese, A. Kramere, B. Bildnere a
European Space Agency, ESTEC, Noordwijk, the Netherlands United Monolithic Semiconductors GmbH, Ulm, Germany c United Monolithic Semiconductors SAS, Parc SILIC de Villebon-Courtabouef, France d Tesat-Spacecom GmbH, Backnang, Germany e Airbus Defence & Space, Friedrichshafen, Germany b
A B S T R A C T
This paper describes the methodology and results obtained from performing a rigorous space qualification of European microwave GaN technology for the European Space Agency (ESA) Biomass mission. This is the first time European GaN technology has been qualified for an ESA mission and represents a major breakthrough in the maturity and application readiness of this technology. The work has involved developing hermetic packaging solutions and performing extensive mechanical, assembly, endurance and space operating environmental tests to ensure the components are capable of satisfying the Biomass mission requirements.
1. Introduction The objective of the ESA Biomass mission [1] is to determine the distribution of above ground biomass in the world's forests and to measure annual changes in carbon stock over a 5 year mission period. Critical to the mission is a synthetic aperture radar (SAR), operating in P-band (≈435 MHz), that will be used to “probe” the earth's surface. Operation in P-band (70 cm wavelength) is necessary in order for the radar signal to penetrate through forest vegetation canopies, Fig. 1. The Biomass radar transmitter is a critical element of the SAR payload. Since the frequency is so low, a vacuum tube amplifier would be too heavy and bulky for the type of satellite envisaged. Therefore Biomass will deploy solid state power amplifiers (SSPAs) using the highpower semiconductor gallium nitride (GaN). 1.1. Choice of GaN technology GaN technology has the potential to provide an order of magnitude improvement in RF output power, is inherently radiation hard and tolerates higher bus-voltages and higher operational temperatures compared to GaAs or Si. The main challenge for space application has been to combine outstanding GaN performance figures with reliable operation. To address this situation, ESA launched its GaN REliability And Technology Transfer initiative (GREAT2, [2]) in 2008 with the aim
⁎
Corresponding author. E-mail address: [email protected] (A. Barnes).
https://doi.org/10.1016/j.microrel.2018.06.065 Received 24 May 2018; Received in revised form 14 June 2018; Accepted 25 June 2018
0026-2714/ © 2018 Elsevier Ltd. All rights reserved.
of establishing a European supply chain for manufacture of high reliability, space compatible, GaN based microwave transistors and integrated circuits. During the course of GREAT2, significant amounts of data concerning the space robustness of the United Monolithic Semiconductors (UMS) GH50-10 GaN process were obtained. This gave ESA confidence in using UMS transistor technology as the baseline for realising the Biomass SSPA. However, to allow the transistors to be used in space, a hermetic packaging solution was required and in 2014 an activity was initiated by ESA to develop a hermetic RF package and to qualify a 15 W and 80 W hermetically packaged transistor family. The qualification activity is led by UMS, who are responsible for wafer processing, with support from Tesat-Spacecom who are responsible for hermetic package development, transistor assembly and space qualification testing. The overall satellite development is the responsibility of Airbus Defence and Space. 1.2. Device technology and reliability The UMS GH50-10 process is based upon 0.5 μm gate length HEMT devices fabricated using an AlGaN/GaN heterostructure epitaxy on 75 mm diameter semi-insulating SiC substrates. The epitaxial structure has a semi-insulating (SI) GaN buffer optimised for high voltage operation, whereas the AlGaN Schottky barrier layer is optimised for RF
Microelectronics Reliability 88–90 (2018) 378–384
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Fig. 1. Biomass mission schematic.
illustrated in Fig. 3. The transistors are operated at a derated voltage of Vds = 45 V for space applications (derived from the absolute maximum rating value of Vds,amr = 60 V). Measured performance for the BIO80 device is given in Fig. 4, illustrating that > 80 W of RF output power is typically achieved over the −35 °C to +80 °C temperature range. Measured power added efficiency (PAE) is better than 65%.
performance, robustness and reliability aspects. A source terminated field plate is implemented in order to reduce the peak electric field and improve reliability. The GH50-10 process allows discrete microwave power transistors to be realised up to a maximum frequency of 5GHz, with an associated RF output power density of ≈5 W/mm of gate periphery. During the course of GREAT2, accelerated RF life tests were performed by Tesat to assess the reliability of the GH50-10 technology [2]. The devices were tested for up to 4400 h duration at channel temperatures up to 390 °C, under CW conditions, and with 4 to 5 dB RF gain compression. This data was complemented by DC and RF tests at UMS and ESA [7]. The life test results exhibited two dominant failure modes: RF output power degradation and gate current increase, both of which are associated with a thermal runaway mechanism [2]. For a mean time to failure requirement of ≥ 108 h (typical for space applications), derating to a channel temperature of 160 °C is recommended, as illustrated in Fig. 2. Based on this data, a channel temperature of 225 °C was adopted during qualification in order to demonstrate no failures, but at the same time provide a significant level of stress above the intended Biomass operating conditions. Channel temperature was determined using micro-Raman thermography in order to provide submicron resolution temperature information in the source–drain area of packaged devices [3]. For Biomass, two hermetically packaged transistor variants were developed, a 15 W driver stage (BIO15) and an 80 W output power stage (BIO80), the latter containing two separate 40 W transistor die, as
1.3. Hermetic package development The hermetic packages for Biomass were designed and simulated (RF and thermomechanical) by Tesat and fabricated using the HTCC ceramic production line of Schott Electronic Packaging. The housings consist of a copper molybdenum base plate with a pedestal for the placement of the power transistor die, multilayer HTCC alumina walls, Ni/Au galvanised metallisation and a laser sealable lid. Particular effort was made to achieve a high quality die attach assembly, with 100% Xray screening adopted for all parts. A more stringent die attach voiding specification was adopted compared to that proposed in Mil-Std-883 J TM 2012.9 [4] which specifies voiding criteria underneath the die, rather than focus on the active heat generating region. Fig. 5 shows a typical X-ray image used for die attachment solder void screening. The inner yellow rectangle defines the transistor active area, with pass/fail criteria determined by image recognition software. The BIO15 and BIO80 package variants were subjected to a separate evaluation test campaign prior to starting the space qualification program. This included mechanical tests (e.g. shock, vibration, thermal cycling, lead strength), assembly tests (e.g. soldering, hermeticity, constructional integrity) and endurance tests. All tests were passed successfully giving confidence to start the space qualification for Biomass. 2. Overview of project qualification flow The wafer acceptance, lot acceptance and hermetically packaged transistor project qualification flow was based around modified versions of ECSS Q-ST-60-12C [5] and ESCC 5010 standards [6] respectively, as shown schematically in Fig. 6. Adjustments were made to the standard ESA Lot Acceptance Test (e.g. inclusion of a 2 stage burn-in protocol (48 h HTRB, 240 h HTOL) and destructive power transistor breakdown tests. Adjustments were also made to the standard ESCC 5010 qualification flow to take account of the special characteristics needed for GaN (e.g. use of higher test temperatures, higher voltages) and inclusion of a 3000 h accelerated RF life test under CW operating conditions for worst case thermal stress.
Fig. 2. GH50-10 power cell life-time extrapolation [2]. 379
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A. Barnes et al.
Fig. 3. BIO15 (left) and BIO80 (right) devices, unsealed.
and with an RF output power of > 120 W. Multipaction (vacuum) and corona discharge (varying pressure) testing was also performed by ESA over the −35 °C to +65 °C temperature range with an RF output power level up to 50.5 dBm. No evidence of corona or multipaction phenomena over the Biomass operating range (temperature, RF power) was observed. 2.1. Wafer manufacturing The BIO15 15 W and BIO80 40 W transistors were manufactured using separate mask sets and thus on separate wafers. Two separate wafer batches were manufactured for delivery of the flight qualification parts, all of which passed wafer manufacturing screening tests (including PCM data, DC + RF on wafer probing, high and low temperature tests, die visual inspection and die bond pull/shear tests). Fig. 7 shows typical wafer homogeneity for the BIO15 driver transistor gate leakage current (Igs) taken from 2000 die measurements. A similar homogeneity was obtained for the BIO80 qualification wafers. As part of the lot acceptance screening tests, a 2000 hour RF life test (RFLT) was also included in order to give confidence that the qualification lot would meet the Biomass SAR electrical specification in Pband, and that good reliability performance could be obtained during qualification. Samples of BIO80 parts were screened and assembled into packages, subjected to burn-in and tested by UMS in a custom designed P-band amplifier test fixture. Fig. 8 shows the real time monitored RF output power for the P-band preliminary life test. The actual test duration was > 3000 h and showed no degradation. On this basis, the wafer lot acceptance test was deemed successful and naked die delivered to Tesat for assembly and qualification testing.
Fig. 4. Measured BIO80 RF output power over temperature.
2.2. Naked die assembly, pre-screening and burn-in Fig. 5. X-ray die attach screening of BIO80 device. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
After receipt of the naked die transistors, assembly into packages was performed using Tesat's hybrid assembly line with 100% pre-encapsulation inspection (precap) performed by ESA and Airbus for all assemblies. After precap and laser lid sealing, detailed screening steps were undertaken, as shown in Fig. 9. Four primary drift failure parameters were used; RF output power ( ± 1 dB), linear gain ( ± 1 dB), drain-source current ( ± 20%) and gate leakage (< ×10). After completion of the pre-screening and burn-in stages the qualification lot was quarantined and parts selected randomly for qualification tests.
The GH50-10 device technology was extensively tested in terms of radiation, corona discharge and multipaction robustness. The BIO15 and BIO80 devices withstand total ionising dose (TID) irradiation up to 1 Mrad and 35 MeV proton irradiation up to 1.5E12 protons/cm2 without any significant drift. To confirm the single event effect (SEE) robustness under heavy ions, BIO15 and BIO80 devices were tested under RF operation using Xe-ions with a surface linear energy transfer (LET) value for GaN (LET(GaN)) equivalent to 52.5 MeV/mg/cm2, which corresponds to a surface LET(Si) of 68.3 MeV/mg/cm2. All devices passed the RF SEE test campaign at nominal drain voltage conditions of Vds = 45 V up to the 4 dB RF gain compression point, at room temperature and at an elevated temperature of 65 °C. Furthermore, it was shown on a sample basis that the BIO80 devices have the capability to pass the RF SEE test with an increased drain voltage of Vds = 55 V
3. Project qualification of packaged parts The project qualification test flow was based around the ESA ESCC 5010 (issue 1) standard, as illustrated schematically in Fig. 10. A total of 31 parts were chosen randomly from both the BIO15 and BIO80 qualification lot. No failures are allowed during testing, otherwise the lot has to be scrapped and manufacturing re-started. The qualification 380
Microelectronics Reliability 88–90 (2018) 378–384
A. Barnes et al.
Fig. 6. Schematic overview of project qualification flow.
Fig. 7. Igs variation for 2000 BIO15 die (Vds = 50 V, Vg = −7 V, Wg = 4 mm). Fig. 8. Lot acceptance P-Band RFLT performed on BIO80 parts (inset shows example of test fixture).
flow is broken down into four sub-groups covering (1) mechanical tests, (2) assembly capability tests, (3) endurance tests and (4) special tests to determine amplitude and phase modulation (AM/PM, PM/PM) transfer characteristics for Biomass radar conditions. All subgroups (1 to 4) have been successfully completed, but only the results of the subgroup 3 endurance testing are described in detail here.
3.1. Endurance testing of BIO15 and BIO80 devices Accelerated RF life testing was performed using an Accel-RF AARTS test system (24 channels) at a test frequency of 1.1GHz. Twelve devices, instead of fifteen as required per ESCC 5010 [6], were used for BIO15 and BIO80 subgroup 3 endurance testing respectively since the test system could only populate a maximum of 12 components per device type. Custom designed input and output matching boards were designed to provide a representative RF load line in order to properly stress the devices in terms of RF current and voltage excursions, as
Step
Producon Test
14 15 16 17 18 19 20 21 22 23 24 25 26 27
Laser soldering lid to package Fine and gross leak seal test Laser marking with device SN X-ray inspecon Thermal cycling Parcle Impact Noise Detecon (PIND) test Pre burn in RF/DC measurement Burn-in (48 hr HTRB, 240hr HTOL) Post burn in RF/DC measurement Dri failure calculaon Fine and gross leak seal test Final external visual inspecon Check for lot failure (PDA calculaon) Release of flight model producon lot for qualificaon tesng
Fig. 9. Example screening steps used during production.
381
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Fig. 10. Project qualification flow.
Power transfer measurement results Test setup values SN 0247 0256 0270 0273 0508 0522 0538 0541 0595 0600 0608 0611
Fig. 11. Photograph of a BIO15 Accel-RF test fixture with custom designed input/output matching networks.
illustrated in Fig. 11. The accelerated RF life testing was done under CW operating conditions since this is the worst case thermal loading and operation at high average temperatures is a known reliability degradation factor. Each individual test channel was operated at Vds = 45 V, with a target device under test channel temperature of 225 °C. The RF input power for each test channel was individually adjusted such that Pout was set at the onset of a negative to positive gate current transition, as measured from Pin-Pout RF transfer characteristics. Heater baseplates were then individually adjusted to achieve a channel temperature of 225 °C, prior to the start of the life test. Fig. 12 shows the starting (t0) test setup values used for the BIO15 devices. A similar test setup was determined for the BIO80 parts.
Pout [dBm] 40.34 40.62 40.36 40.82 40.62 40.82 40.54 40.52 40.44 40.73 40.35 40.64
Min 40.34 Max 40.82
Gain PAE dB % 14.81 64.2 14.67 61.7 14.34 62.0 14.83 62.1 14.57 62.7 14.84 62.6 14.41 61.5 14.51 60.5 15.06 63.6 15.11 61.5 14.89 64.2 14.65 58.5
Ids mA 363 402 376 419 396 417 396 401 375 416 363 427
Igs mA 0.021 -0.086 0.043 -0.064 0.000 -0.060 0.018 -0.029 -0.022 -0.020 -0.019 -0.040
14.34 15.11
363 427
-0.086 5.84 0.043 7.96
58.5 64.2
Pdiss Pin Tcase W [dBm] °C 5.84 25.5 165 6.90 26.0 155 6.41 26.0 159 7.13 26.0 153 6.64 26.0 157 7.00 26.0 154 6.84 26.0 156 7.12 26.0 153 6.12 25.5 162 7.19 25.5 153 5.84 25.5 165 7.96 26.0 146 25.5 26
146 165
Rth [K/W] 10.34 10.11 10.22 10.06 10.17 10.09 10.12 10.06 10.28 10.05 10.35 9.89 9.89 10.35
Fig. 12. BIO15 RFLT starting (t0) setup conditions (Tj = 225 °C).
Using these predetermined starting conditions, gain compression sweeps were performed at the start (t0) and at the end of the 3000 h life test (t3000), with drift parameters calculated from these measurements using a baseplate temperature of 50 °C. Real time RF Pout monitoring data for the BIO15 and BIO80 devices is shown in Figs. 13 and 14 382
Microelectronics Reliability 88–90 (2018) 378–384
A. Barnes et al.
SN
Δ GL (dB)
Δ Pout (dB)
Δ Ids (%)
Δ Ig (%)
Status
BIO 15
Fig. 13. Real time monitored RF output power during BIO15 accelerated life test (Tj = 225 °C).
0247 0256 0270 0273 0508 0522 0538 0541 0595 0600 0608 0611
0.186 0.119 0.054 0.121 0.196 0.211 0.150 0.176 0.118 0.110 0.215 0.182
-0.011 0.063 -0.086 -0.042 -0.065 0.026 0.014 -0.070 -0.051 -0.099 -0.050 -0.147
1.333 -4.765 -5.891 -3.246 0.057 5.528 -2.571 -3.735 0.539 0.789 7.027 -11.424
14.3 -106.5 18.1 -101.5 36.8 179.8 49.9 89.7 127.2 -13.6 -251.6 -100.5
Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass
0135 0148 0158 0188 0193 0321 0328 0329 0369 0440 0442 0446 Dri Failure Criteria
0.155 0.071 0.137 0.025 0.270 -0.237 0.047 -0.044 0.162 0.123 0.043 -0.559 -1 to +1
BIO 80 -0.082 -0.054 -0.014 -0.055 -0.068 -0.075 0.003 -0.116 0.012 -0.019 -0.020 -0.077 -1 to +1
-1.468 0.578 0.242 -0.114 -1.259 -1.343 0.627 0.738 -0.642 0.239 0.017 -0.856 ± 20%
-31.4 2.8 496.2 10.2 -12.1 -27.9 85.9 -49.9 -54.6 9.6 -11.6 -42.5 one decade (>1000%)
Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass
Fig. 16. Summary of parameter drifts for BIO15 and BIO80.
sweep. The overall drift failure calculation for the BIO15 and BIO80 subgroup 3 testing is summarised in Fig. 16 showing that all devices passed the endurance test.
4. Conclusions
Fig. 14. Real time monitored RF output power during BIO80 accelerated life test (Tj = 225 °C).
This work has presented, for the first time, a rigorous project qualification methodology derived from existing ESCC (European Space Components Coordination) specifications used to space qualify a family of hermetically packaged 15 W and 80 W GaN microwave power transistors, manufactured in Europe, for the ESA Biomass mission. The devices have now successfully completed all qualification steps, including extensive radiation, corona and multipaction discharge tests, and are being integrated into the Biomass P-Band SAR SSPA flight
respectively, from which it can be seen that there is very little change in the measured RF output power. A small jump can be observed in six of the 80 W test channels due to a test system re-start. Fig. 15 shows the measured power transfer characteristics for a typical BIO15 and BIO80 device at t0 and t3000 hours, showing little or no drift in the linear and non-linear regions of the gain compression
Fig. 15. Power transfer characteristics for typical BIO15 and BIO80 device at the start (blue and purple lines) and end (green and red lines) of 3000 h RF life test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 383
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(GREAT2), (2014) Abstract to ESA/ESTEC contract 21.499/08/NL/PA. [3] R. Simms, et al., Channel temperature determination in high-power AlGaN/GaN HFETs using electrical methods and Raman spectroscopy, IEEE Trans. Electron Devices 55 (2) (2008). [4] Test Method Standards For Microcircuits – Mil-Std-883. [5] ECSS Q-ST-60-12C (Rev 2), Space product assurance, Electrical, electronic and electromechanical (EEE) components. [6] ESCC Generic Specification No. 5010 (Issue 1), Discrete Microwave Semiconductor Components. [7] A.R. Barnes, F. Vitobello, ESA perspective on the industrialization of European GaN technology for space application, European Microwave Conference, 2014.
model design activities. The work represents a major step forward for the European GaN development effort and will serve as a reference model for future ESA qualification activities and formalisation of new/ updated ESCC standards related to GaN. References [1] ESA, Report for Mission Selection: Biomass, ESA SP-1324/1, 3 volume series European Space Agency, The Netherlands, 2012. [2] K. Hirche, et al., GaN Reliability Enhancement and Technology Transfer Initiative
384
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Qualification of GaN microwave transistors for the European Space Agency Biomass mission
T
⁎
A. Barnesa, , F. Helierea, P. Villara, H. Stuhldreierb, C. Beaurainc, D. Bouwc, M. Grunwaldb, E. Moessd, T. Muckd, C. Schildbachd, T. Aylese, A. Kramere, B. Bildnere a
European Space Agency, ESTEC, Noordwijk, the Netherlands United Monolithic Semiconductors GmbH, Ulm, Germany c United Monolithic Semiconductors SAS, Parc SILIC de Villebon-Courtabouef, France d Tesat-Spacecom GmbH, Backnang, Germany e Airbus Defence & Space, Friedrichshafen, Germany b
A B S T R A C T
This paper describes the methodology and results obtained from performing a rigorous space qualification of European microwave GaN technology for the European Space Agency (ESA) Biomass mission. This is the first time European GaN technology has been qualified for an ESA mission and represents a major breakthrough in the maturity and application readiness of this technology. The work has involved developing hermetic packaging solutions and performing extensive mechanical, assembly, endurance and space operating environmental tests to ensure the components are capable of satisfying the Biomass mission requirements.
1. Introduction The objective of the ESA Biomass mission [1] is to determine the distribution of above ground biomass in the world's forests and to measure annual changes in carbon stock over a 5 year mission period. Critical to the mission is a synthetic aperture radar (SAR), operating in P-band (≈435 MHz), that will be used to “probe” the earth's surface. Operation in P-band (70 cm wavelength) is necessary in order for the radar signal to penetrate through forest vegetation canopies, Fig. 1. The Biomass radar transmitter is a critical element of the SAR payload. Since the frequency is so low, a vacuum tube amplifier would be too heavy and bulky for the type of satellite envisaged. Therefore Biomass will deploy solid state power amplifiers (SSPAs) using the highpower semiconductor gallium nitride (GaN). 1.1. Choice of GaN technology GaN technology has the potential to provide an order of magnitude improvement in RF output power, is inherently radiation hard and tolerates higher bus-voltages and higher operational temperatures compared to GaAs or Si. The main challenge for space application has been to combine outstanding GaN performance figures with reliable operation. To address this situation, ESA launched its GaN REliability And Technology Transfer initiative (GREAT2, [2]) in 2008 with the aim
⁎
Corresponding author. E-mail address: [email protected] (A. Barnes).
https://doi.org/10.1016/j.microrel.2018.06.065 Received 24 May 2018; Received in revised form 14 June 2018; Accepted 25 June 2018
0026-2714/ © 2018 Elsevier Ltd. All rights reserved.
of establishing a European supply chain for manufacture of high reliability, space compatible, GaN based microwave transistors and integrated circuits. During the course of GREAT2, significant amounts of data concerning the space robustness of the United Monolithic Semiconductors (UMS) GH50-10 GaN process were obtained. This gave ESA confidence in using UMS transistor technology as the baseline for realising the Biomass SSPA. However, to allow the transistors to be used in space, a hermetic packaging solution was required and in 2014 an activity was initiated by ESA to develop a hermetic RF package and to qualify a 15 W and 80 W hermetically packaged transistor family. The qualification activity is led by UMS, who are responsible for wafer processing, with support from Tesat-Spacecom who are responsible for hermetic package development, transistor assembly and space qualification testing. The overall satellite development is the responsibility of Airbus Defence and Space. 1.2. Device technology and reliability The UMS GH50-10 process is based upon 0.5 μm gate length HEMT devices fabricated using an AlGaN/GaN heterostructure epitaxy on 75 mm diameter semi-insulating SiC substrates. The epitaxial structure has a semi-insulating (SI) GaN buffer optimised for high voltage operation, whereas the AlGaN Schottky barrier layer is optimised for RF
Microelectronics Reliability 88–90 (2018) 378–384
A. Barnes et al.
Fig. 1. Biomass mission schematic.
illustrated in Fig. 3. The transistors are operated at a derated voltage of Vds = 45 V for space applications (derived from the absolute maximum rating value of Vds,amr = 60 V). Measured performance for the BIO80 device is given in Fig. 4, illustrating that > 80 W of RF output power is typically achieved over the −35 °C to +80 °C temperature range. Measured power added efficiency (PAE) is better than 65%.
performance, robustness and reliability aspects. A source terminated field plate is implemented in order to reduce the peak electric field and improve reliability. The GH50-10 process allows discrete microwave power transistors to be realised up to a maximum frequency of 5GHz, with an associated RF output power density of ≈5 W/mm of gate periphery. During the course of GREAT2, accelerated RF life tests were performed by Tesat to assess the reliability of the GH50-10 technology [2]. The devices were tested for up to 4400 h duration at channel temperatures up to 390 °C, under CW conditions, and with 4 to 5 dB RF gain compression. This data was complemented by DC and RF tests at UMS and ESA [7]. The life test results exhibited two dominant failure modes: RF output power degradation and gate current increase, both of which are associated with a thermal runaway mechanism [2]. For a mean time to failure requirement of ≥ 108 h (typical for space applications), derating to a channel temperature of 160 °C is recommended, as illustrated in Fig. 2. Based on this data, a channel temperature of 225 °C was adopted during qualification in order to demonstrate no failures, but at the same time provide a significant level of stress above the intended Biomass operating conditions. Channel temperature was determined using micro-Raman thermography in order to provide submicron resolution temperature information in the source–drain area of packaged devices [3]. For Biomass, two hermetically packaged transistor variants were developed, a 15 W driver stage (BIO15) and an 80 W output power stage (BIO80), the latter containing two separate 40 W transistor die, as
1.3. Hermetic package development The hermetic packages for Biomass were designed and simulated (RF and thermomechanical) by Tesat and fabricated using the HTCC ceramic production line of Schott Electronic Packaging. The housings consist of a copper molybdenum base plate with a pedestal for the placement of the power transistor die, multilayer HTCC alumina walls, Ni/Au galvanised metallisation and a laser sealable lid. Particular effort was made to achieve a high quality die attach assembly, with 100% Xray screening adopted for all parts. A more stringent die attach voiding specification was adopted compared to that proposed in Mil-Std-883 J TM 2012.9 [4] which specifies voiding criteria underneath the die, rather than focus on the active heat generating region. Fig. 5 shows a typical X-ray image used for die attachment solder void screening. The inner yellow rectangle defines the transistor active area, with pass/fail criteria determined by image recognition software. The BIO15 and BIO80 package variants were subjected to a separate evaluation test campaign prior to starting the space qualification program. This included mechanical tests (e.g. shock, vibration, thermal cycling, lead strength), assembly tests (e.g. soldering, hermeticity, constructional integrity) and endurance tests. All tests were passed successfully giving confidence to start the space qualification for Biomass. 2. Overview of project qualification flow The wafer acceptance, lot acceptance and hermetically packaged transistor project qualification flow was based around modified versions of ECSS Q-ST-60-12C [5] and ESCC 5010 standards [6] respectively, as shown schematically in Fig. 6. Adjustments were made to the standard ESA Lot Acceptance Test (e.g. inclusion of a 2 stage burn-in protocol (48 h HTRB, 240 h HTOL) and destructive power transistor breakdown tests. Adjustments were also made to the standard ESCC 5010 qualification flow to take account of the special characteristics needed for GaN (e.g. use of higher test temperatures, higher voltages) and inclusion of a 3000 h accelerated RF life test under CW operating conditions for worst case thermal stress.
Fig. 2. GH50-10 power cell life-time extrapolation [2]. 379
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Fig. 3. BIO15 (left) and BIO80 (right) devices, unsealed.
and with an RF output power of > 120 W. Multipaction (vacuum) and corona discharge (varying pressure) testing was also performed by ESA over the −35 °C to +65 °C temperature range with an RF output power level up to 50.5 dBm. No evidence of corona or multipaction phenomena over the Biomass operating range (temperature, RF power) was observed. 2.1. Wafer manufacturing The BIO15 15 W and BIO80 40 W transistors were manufactured using separate mask sets and thus on separate wafers. Two separate wafer batches were manufactured for delivery of the flight qualification parts, all of which passed wafer manufacturing screening tests (including PCM data, DC + RF on wafer probing, high and low temperature tests, die visual inspection and die bond pull/shear tests). Fig. 7 shows typical wafer homogeneity for the BIO15 driver transistor gate leakage current (Igs) taken from 2000 die measurements. A similar homogeneity was obtained for the BIO80 qualification wafers. As part of the lot acceptance screening tests, a 2000 hour RF life test (RFLT) was also included in order to give confidence that the qualification lot would meet the Biomass SAR electrical specification in Pband, and that good reliability performance could be obtained during qualification. Samples of BIO80 parts were screened and assembled into packages, subjected to burn-in and tested by UMS in a custom designed P-band amplifier test fixture. Fig. 8 shows the real time monitored RF output power for the P-band preliminary life test. The actual test duration was > 3000 h and showed no degradation. On this basis, the wafer lot acceptance test was deemed successful and naked die delivered to Tesat for assembly and qualification testing.
Fig. 4. Measured BIO80 RF output power over temperature.
2.2. Naked die assembly, pre-screening and burn-in Fig. 5. X-ray die attach screening of BIO80 device. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
After receipt of the naked die transistors, assembly into packages was performed using Tesat's hybrid assembly line with 100% pre-encapsulation inspection (precap) performed by ESA and Airbus for all assemblies. After precap and laser lid sealing, detailed screening steps were undertaken, as shown in Fig. 9. Four primary drift failure parameters were used; RF output power ( ± 1 dB), linear gain ( ± 1 dB), drain-source current ( ± 20%) and gate leakage (< ×10). After completion of the pre-screening and burn-in stages the qualification lot was quarantined and parts selected randomly for qualification tests.
The GH50-10 device technology was extensively tested in terms of radiation, corona discharge and multipaction robustness. The BIO15 and BIO80 devices withstand total ionising dose (TID) irradiation up to 1 Mrad and 35 MeV proton irradiation up to 1.5E12 protons/cm2 without any significant drift. To confirm the single event effect (SEE) robustness under heavy ions, BIO15 and BIO80 devices were tested under RF operation using Xe-ions with a surface linear energy transfer (LET) value for GaN (LET(GaN)) equivalent to 52.5 MeV/mg/cm2, which corresponds to a surface LET(Si) of 68.3 MeV/mg/cm2. All devices passed the RF SEE test campaign at nominal drain voltage conditions of Vds = 45 V up to the 4 dB RF gain compression point, at room temperature and at an elevated temperature of 65 °C. Furthermore, it was shown on a sample basis that the BIO80 devices have the capability to pass the RF SEE test with an increased drain voltage of Vds = 55 V
3. Project qualification of packaged parts The project qualification test flow was based around the ESA ESCC 5010 (issue 1) standard, as illustrated schematically in Fig. 10. A total of 31 parts were chosen randomly from both the BIO15 and BIO80 qualification lot. No failures are allowed during testing, otherwise the lot has to be scrapped and manufacturing re-started. The qualification 380
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Fig. 6. Schematic overview of project qualification flow.
Fig. 7. Igs variation for 2000 BIO15 die (Vds = 50 V, Vg = −7 V, Wg = 4 mm). Fig. 8. Lot acceptance P-Band RFLT performed on BIO80 parts (inset shows example of test fixture).
flow is broken down into four sub-groups covering (1) mechanical tests, (2) assembly capability tests, (3) endurance tests and (4) special tests to determine amplitude and phase modulation (AM/PM, PM/PM) transfer characteristics for Biomass radar conditions. All subgroups (1 to 4) have been successfully completed, but only the results of the subgroup 3 endurance testing are described in detail here.
3.1. Endurance testing of BIO15 and BIO80 devices Accelerated RF life testing was performed using an Accel-RF AARTS test system (24 channels) at a test frequency of 1.1GHz. Twelve devices, instead of fifteen as required per ESCC 5010 [6], were used for BIO15 and BIO80 subgroup 3 endurance testing respectively since the test system could only populate a maximum of 12 components per device type. Custom designed input and output matching boards were designed to provide a representative RF load line in order to properly stress the devices in terms of RF current and voltage excursions, as
Step
Producon Test
14 15 16 17 18 19 20 21 22 23 24 25 26 27
Laser soldering lid to package Fine and gross leak seal test Laser marking with device SN X-ray inspecon Thermal cycling Parcle Impact Noise Detecon (PIND) test Pre burn in RF/DC measurement Burn-in (48 hr HTRB, 240hr HTOL) Post burn in RF/DC measurement Dri failure calculaon Fine and gross leak seal test Final external visual inspecon Check for lot failure (PDA calculaon) Release of flight model producon lot for qualificaon tesng
Fig. 9. Example screening steps used during production.
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Fig. 10. Project qualification flow.
Power transfer measurement results Test setup values SN 0247 0256 0270 0273 0508 0522 0538 0541 0595 0600 0608 0611
Fig. 11. Photograph of a BIO15 Accel-RF test fixture with custom designed input/output matching networks.
illustrated in Fig. 11. The accelerated RF life testing was done under CW operating conditions since this is the worst case thermal loading and operation at high average temperatures is a known reliability degradation factor. Each individual test channel was operated at Vds = 45 V, with a target device under test channel temperature of 225 °C. The RF input power for each test channel was individually adjusted such that Pout was set at the onset of a negative to positive gate current transition, as measured from Pin-Pout RF transfer characteristics. Heater baseplates were then individually adjusted to achieve a channel temperature of 225 °C, prior to the start of the life test. Fig. 12 shows the starting (t0) test setup values used for the BIO15 devices. A similar test setup was determined for the BIO80 parts.
Pout [dBm] 40.34 40.62 40.36 40.82 40.62 40.82 40.54 40.52 40.44 40.73 40.35 40.64
Min 40.34 Max 40.82
Gain PAE dB % 14.81 64.2 14.67 61.7 14.34 62.0 14.83 62.1 14.57 62.7 14.84 62.6 14.41 61.5 14.51 60.5 15.06 63.6 15.11 61.5 14.89 64.2 14.65 58.5
Ids mA 363 402 376 419 396 417 396 401 375 416 363 427
Igs mA 0.021 -0.086 0.043 -0.064 0.000 -0.060 0.018 -0.029 -0.022 -0.020 -0.019 -0.040
14.34 15.11
363 427
-0.086 5.84 0.043 7.96
58.5 64.2
Pdiss Pin Tcase W [dBm] °C 5.84 25.5 165 6.90 26.0 155 6.41 26.0 159 7.13 26.0 153 6.64 26.0 157 7.00 26.0 154 6.84 26.0 156 7.12 26.0 153 6.12 25.5 162 7.19 25.5 153 5.84 25.5 165 7.96 26.0 146 25.5 26
146 165
Rth [K/W] 10.34 10.11 10.22 10.06 10.17 10.09 10.12 10.06 10.28 10.05 10.35 9.89 9.89 10.35
Fig. 12. BIO15 RFLT starting (t0) setup conditions (Tj = 225 °C).
Using these predetermined starting conditions, gain compression sweeps were performed at the start (t0) and at the end of the 3000 h life test (t3000), with drift parameters calculated from these measurements using a baseplate temperature of 50 °C. Real time RF Pout monitoring data for the BIO15 and BIO80 devices is shown in Figs. 13 and 14 382
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SN
Δ GL (dB)
Δ Pout (dB)
Δ Ids (%)
Δ Ig (%)
Status
BIO 15
Fig. 13. Real time monitored RF output power during BIO15 accelerated life test (Tj = 225 °C).
0247 0256 0270 0273 0508 0522 0538 0541 0595 0600 0608 0611
0.186 0.119 0.054 0.121 0.196 0.211 0.150 0.176 0.118 0.110 0.215 0.182
-0.011 0.063 -0.086 -0.042 -0.065 0.026 0.014 -0.070 -0.051 -0.099 -0.050 -0.147
1.333 -4.765 -5.891 -3.246 0.057 5.528 -2.571 -3.735 0.539 0.789 7.027 -11.424
14.3 -106.5 18.1 -101.5 36.8 179.8 49.9 89.7 127.2 -13.6 -251.6 -100.5
Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass
0135 0148 0158 0188 0193 0321 0328 0329 0369 0440 0442 0446 Dri Failure Criteria
0.155 0.071 0.137 0.025 0.270 -0.237 0.047 -0.044 0.162 0.123 0.043 -0.559 -1 to +1
BIO 80 -0.082 -0.054 -0.014 -0.055 -0.068 -0.075 0.003 -0.116 0.012 -0.019 -0.020 -0.077 -1 to +1
-1.468 0.578 0.242 -0.114 -1.259 -1.343 0.627 0.738 -0.642 0.239 0.017 -0.856 ± 20%
-31.4 2.8 496.2 10.2 -12.1 -27.9 85.9 -49.9 -54.6 9.6 -11.6 -42.5 one decade (>1000%)
Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass
Fig. 16. Summary of parameter drifts for BIO15 and BIO80.
sweep. The overall drift failure calculation for the BIO15 and BIO80 subgroup 3 testing is summarised in Fig. 16 showing that all devices passed the endurance test.
4. Conclusions
Fig. 14. Real time monitored RF output power during BIO80 accelerated life test (Tj = 225 °C).
This work has presented, for the first time, a rigorous project qualification methodology derived from existing ESCC (European Space Components Coordination) specifications used to space qualify a family of hermetically packaged 15 W and 80 W GaN microwave power transistors, manufactured in Europe, for the ESA Biomass mission. The devices have now successfully completed all qualification steps, including extensive radiation, corona and multipaction discharge tests, and are being integrated into the Biomass P-Band SAR SSPA flight
respectively, from which it can be seen that there is very little change in the measured RF output power. A small jump can be observed in six of the 80 W test channels due to a test system re-start. Fig. 15 shows the measured power transfer characteristics for a typical BIO15 and BIO80 device at t0 and t3000 hours, showing little or no drift in the linear and non-linear regions of the gain compression
Fig. 15. Power transfer characteristics for typical BIO15 and BIO80 device at the start (blue and purple lines) and end (green and red lines) of 3000 h RF life test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 383
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(GREAT2), (2014) Abstract to ESA/ESTEC contract 21.499/08/NL/PA. [3] R. Simms, et al., Channel temperature determination in high-power AlGaN/GaN HFETs using electrical methods and Raman spectroscopy, IEEE Trans. Electron Devices 55 (2) (2008). [4] Test Method Standards For Microcircuits – Mil-Std-883. [5] ECSS Q-ST-60-12C (Rev 2), Space product assurance, Electrical, electronic and electromechanical (EEE) components. [6] ESCC Generic Specification No. 5010 (Issue 1), Discrete Microwave Semiconductor Components. [7] A.R. Barnes, F. Vitobello, ESA perspective on the industrialization of European GaN technology for space application, European Microwave Conference, 2014.
model design activities. The work represents a major step forward for the European GaN development effort and will serve as a reference model for future ESA qualification activities and formalisation of new/ updated ESCC standards related to GaN. References [1] ESA, Report for Mission Selection: Biomass, ESA SP-1324/1, 3 volume series European Space Agency, The Netherlands, 2012. [2] K. Hirche, et al., GaN Reliability Enhancement and Technology Transfer Initiative
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