Reliability investigations Lifetime measurements and degradation mechanisms

Transcription

1 Reliability investigations Lifetime measurements and degradation mechanisms Bernd Sumpf, Ute Zeimer, Karl Häusler, Andreas Klehr Ferdinand-Braun-Institut für Höchstfrequenztechnik Gustav-Kirchhoff-Straße 4, D Berlin, Germany Jens W. Tomm Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie Berlin Max-Born-Str. 2 A, D Berlin, Germany...translating ideas into innovation

2 Outline I. Aging tests for high power diode lasers 1. Degradation measurements: tasks and objectives 2. Statistical basics, acceleration factors 3. Experimental set-up 4. Aging test selection of samples, accompanying measurements 5. Statistical analysis of results II. Analytical work for understanding gradual degradation mechanisms: Strain measurement 1. Approach and Definitions 2. Theory 3. Experimental methods and results 3.1 Micro-Photoluminescence (µpl) 3.2 Photocurrent spectroscopy (PCS) 3.3 Degree-of-polarization PL or R (DoP-PL) 4. Summary

3 Aging tests for high power diode lasers 1. Degradation measurements: tasks and objectives 2. Statistical basics, acceleration factors 3. Experimental set-up 4. Aging test selection of sample, accompanying measurements 5. Statistical analysis of results

4 Degradation measurements: tasks and objectives Degradation Measurements Defined aging of the diode laser Accompanying diagnostic measurements to understand aging Aim of the measurement Qualification of laser diode structure Long term stability of - Epitaxial structure - Surfaces - Contacts - Facets Estimation of life time of the laser diode Suggestions for the improvement of the structure

6 Statistics: reliability definitions Reliability = Probability of a reliable operation until time t. R(t) Failure probability: = Probability for a device failure until time t. F( t) = 1 R( t) Mean time to failure (MTTF) = Expectation of failure time. MTTF = 0 df( t) dt t dt

7 Statistics: assumption exponential distribution Reliability: Failure probability: λ: Failure rate = const. ( hazard failure rate ) Mean time to failure: R( t) = exp(-λ t) F( t) = 1- exp(-λ t) 1 top MTTF = = λ - ln( R) Example: Demanded reliability R = 0.98 over an operating time t op = 4 years (35040 h) MTTF = h MIL-HDBK-217F, 6.13 Optoelectronics, Laser Diode, 6-21 (1991)

8 Design of experiments unknown failure time Determination of the failure probability of samples: n number of test samples in the lot r number of samples with failure within the test F failure probability unknown value Solution: Beta-distribution B( F) = ( n + 1)! ( n r )! r! F 0 x r (1- x) n-r dx

9 Design of experiments unknown failure time Assuming a certain confidence level (1 - α) Determination of F ( n + 1)! r n-r B( F) = = α x (1- x) dx 1 ( n r )! r! 0 Beta-distribution for n = 5 and r = 1 i.e.: 5 samples in test, 1 failure B(F, r + 1, n r + 1) = 0.6 F = B 0.6 (r +1, n r +1) e.g. F = B 0.6 (2;5) = B(F, r+1, n r+1) 1.0 n = r = Confidence level 1 α = F

10 Design of experiments unknown failure time Calculation of the MTTF: MTTF 1 α t 1-α aging ( r + 1, n r + 1)] = ln[1- B Aging time of the experiment: t aging = h (417 days) h MTTF 1 α = = h ln[ ] 60% probability that the MTTF MTTF 0.60 Under nominal conditions requested MTTF not verifiable! Accelerated lifetime tests

11 Reasons for degradation Point defects, Dislocations Absorb the laser light and convert it into heat Move during operation and accumulate in the active region recombination enhanced defect motion REDM-process Facet degradation Band gap deformation within the cleaved facets Absorption and non-radiative recombination COD Local stress Local changes in the band gap absorption and heating Point defects - recombination enhanced defect reaction REDR Processes accelerated at high power and high temperature

12 Acceleration factors Overstressing of devices to proof longer lifetimes by Higher output power, i.e. higher current Higher temperature Failure rate for accelerated lifetime test: λ aging = λ op π T π π I P op: operational conditions aging: conditions of the aging test π T : π I : π P : acceleration factor caused by temperature acceleration factor caused by higher current acceleration factor caused by higher output power MIL-HDBK-217F, 6.13 Optoelectronics, Laser Diode, 6-21 (1991) and Bellcore GR-468-CORE Iss. 1 (1998)

13 Acceleration factor temperature Temperature: Activation energy E a = ev empirical value, different values from literature π T = - E exp k a T 1 aging 1 T op Heating during operation e.g. at R th = 10 K/W; P opt = 10 W; η C = 0.5 temperature increase of about 100 K 1 T = Rth P 1 ηc MIL-HDBK-217F, 6.13 Optoelectronics, Laser Diode, 6-21 (1991) and Bellcore GR-468-CORE Iss. 1 (1998)

14 Acceleration factors power and current Power: Derating exponent β = 2 6; P optical power π P = P P aging op β Current Coefficient x = 0 2 π = I I I aging op x MIL-HDBK-217F, 6.13 Optoelectronics, Laser Diode, 6-21 (1991) and Bellcore GR-468-CORE Iss. 1 (1998)

15 Example accelerated lifetime test Target MTTF 1.7 Mio h Assumption: E A = 0.5 ev, β = 2.3, x = 0; 1 α= 0.6; n = 5; t aging = h Conditions of the accelerated test: P aging = 2 x P op and T aging = 65 C; T op = 25 C π T x π P = 66, i.e. 66 times increase of the failure rate MTTF 0.6 πp π = T - ln[1- B h ( r + 1, 6 - r ] Number of failures r = 0, 1 or 2: To detect the target MTTF only one diode out of five is allowed to fail! r MTTF x 10 6 h x 10 6 h x 10 6 h

17 Experimental set-up: Requirements Measurement of degradation rates below 10-5 h -1 Accuracy better than 1% within 1000 h - Assumption 1 A current, 10 ma changes in 1000 h - in 24 h changes of 0.24 ma - 10 bit resolution of 1 A: ma - 12 bit resolution of 1 A: ma - 16 bit resolution of 1 A: ma Aging tests at temperatures 15 C T 80 C for more than 1000 h - Temperature stability better 0.1 K No degradation of the set-up

18 Experimental set-up: Test Environment Test chambers with stabilized temperature Measurement of power between 100 mw 15 W with high accuracy Challenge: attenuation of power - RW laser diodes: emission power up to P = 500 mw, - High-power BA laser diodes up to P = 15 W - Laser bars P 100 W Computer controlled current supply and measuring system Selection criteria for devices: Power-voltage-current characteristics Facet inspection e.g. longitudinal mode analysis Failure analysis e.g. cathodoluminescence

19 Experimental set-up Geometrical attenuation of light in test chamber Current supply e.g. Thorlabs - Profile LDC A Laser diode Photodiode LiTHERM Temperature Controller 15 C T 75 C Test Chamber

20 Experimental set-up Test chamber for five laser diodes

21 Experimental set-up Temperature controller Test chamber

22 Experimental set-up Attenuation of light with an integrating sphere travelling detector GPIB Current and motion controller, e.g. Newport Power meter Current supply Motion Controller 40 laser diodes at 8 banks

23 Experimental set-up Aging box with integrating sphere inside Holder for 5 laser diodes Electronics

24 Experimental set-up Current supply e.g. delta electronica SM35-45 Laser diode Measurement of optical power with a detector capable to measure 100 W e.g. Gentec Power meter Temperature Controller 15 C T 75 C Test Chamber

25 Experimental set-up 19" Rack system Test chamber Holder for detector Heat sinks

27 Selection of samples Power-current-characteristics: Selection of devices Definition of operational current Example: 9 devices; λ = 650 nm Geometry: 100 µm x 750 µm T = 15 C Measurement before h of aging test Output power P / mw Current I / A before aging

28 Selection of samples Measurement of near field (optional) Detection of facet failures Example: 650 nm device 100 µm x 750 µm; T = 15 C, P = 500 mw Near Field: Top-hat shape; W 1/e2 = 100 µm relative intensity / arb. units position x / µm

29 Selection of samples Measurement of near field (optional) Detection of facet failures Example: 730 nm tapered laser 2.75 mm long T = 25 C, P = 2 W Near Field: W 1/e2 = 160 µm rel. intensity / arb. units Facet failure position x / µm

30 Selection of samples Visual inspection of the facet: Example 650 nm BA-laser 60 µm x 750 µm 60 µm Electroluminescence image: Current below threshold Optical microscope 60 µm

31 Selection of samples Visual inspection of the facet: Example 650 nm BA-laser 60 µm x 750 µm 60 µm Electroluminescence image: Current below threshold Optical microscope 60 µm Failure in the IR Image

32 Selection of samples Analysis of the longitudinal mode spectrum Spacing of longitudinal modes λ depends on Wavelength λ Laser length L Refractive index n 0 z d 2 λ λ = 2 L n g L z Defect change local refractive index Modulation of the envelope of the mode spectrum Crystal defect Formation of subcavities Frontfacet Rearfacet Measurement of spectrum below threshold Fourier-Transformation delivers position of defect

33 Selection of samples Analysis of the longitudinal mode spectrum simulation Mode-Spectrum of a laser diode with L = 1 mm Defects at 125 µm and 350 µm measured from front facet Fourier-Transformation of Mode spectrum Amplitude / arb. units Power P / arb. units 8 R = Wavelength λ / µm µm 128.5µm 348.6µm = 3.06µm Position inside the cavity / arb. units.

34 Aging test nm DQW broad area lasers 100 µm x 750 µm Reliable operation at T = 15 C mw (4 mw/µm): 9(9) - t = 1000 h mw 500 mw (5 mw/µm): 7(9) - t = h 0.8 MTTF h 400 mw No COD failures 0.6 Lifetime sufficient for medical applications (1000 h) and close to the demands for 0.0 display application ( > h) Current I / A Aging time t / h

35 Measurements after aging test Power-current-characteristics: Test of devices Comparison with aging result Example: Nine 650 nm devices Geometry: 100 µm x 750 µm T = 15 C Measurement after h of aging test All devices still operational. No COD-Failures Output power P / mw Current I / A before aging after aging

36 Measurements after aging test Device without degradation before h aging test after h aging test

37 Aging test nm DQW broad area lasers 100 µm x 750 µm Reliable operation at T = 15 C 600 mw (6 mw/µm): 7(7) - t 1100 h Lifetime sufficient for medical applications (1000 h) Current I / A mw at 15 C Aging Time t / h

38 Measurements after aging test Device with degradation before aging test after aging test Defects

39 Cathodoluminescence (CL): Principle Electron beam, U B : 5-35 kv Monochromator Detector Penetration depth of electron beam about 3 µm Generation of electron-hole-pairs Sample, T = 80 K Lateral resolution 1 µm Spectral resolution 0.5 nm Electronics, Monitor Radiative recombination At defects non-radiative recombination Measurements: Spectra Images

40 Preparation of samples for CL Au Substrate Epi-Layers Unsoldering from mount Grinding of the n-side metallization CuW Wet chemical removal of GaAs substrate CuW Thickness of sample about 3 4 µm CuW

41 CL-measurement: facet failure 930 nm,10 W, 25 C, 3700 h, 100 µm stripe width SE-image 6x10 5 Intensity / arb. u. 5x10 5 4x10 5 3x10 5 2x10 5 1x10 5 outside stripe front facet centre rear facet 0 λ = ± 1 nm λ / nm

42 CL-measurements: Internal defects I Analysis of the longitudinal mode spectrum comparison with CL power P / a.u T = 25 C λ max = nm L = 4000µm power p / a.u wavelenghtλ / nm amplitude / a.u µm 1063µm 2928µm wavelength λ / nm position inside cavity / µm 100 µm 100 µm

43 CL-measurements: Internal defects II 980 nm, 8 W, 25 C, 2410 h, 100 µm stripe width 100 µm EL-image facet

44 CL-measurements: Internal defects III 930 nm, 8 W, 25 C, 1400 h, 100 µm stripe width No facet damage, electroluminescence image without failure [110] DLDs (dark line defects) [110] 15 - branching in [112] direction Confined to stripe No DSDs (dark spot defects)

46 Statistical analysis Input: n devices per lot r failures are observed Assumption: exponential distribution of failure function Total test time t Failure time for the failed diode t i α-quantile of χ 2 -function χ 2 α : maximum MTTF 1 = = λ likelihood ( n r ) t : + r r j = 1 t j 1 α confidence level : MTTF 1 α 2 α r j = 1 ( n r ) t + t = χ (2r + 2) / 2 j Ref: Reliability and Degradation of Semiconductor Lasers and LEDs,M. Fukuda, Artec House, 1991

47 Aging test nm DQW broad area lasers 100 µm x 750 µm Reliable operation at T = 15 C mw (4 mw/µm): 9(9) - t = 1000 h mw 500 mw (5 mw/µm): 7(9) - t = h 0.8 MTTF h 400 mw No COD failures 0.6 Lifetime sufficient for medical applications and close to the demands for 0.0 display application Current I / A Aging time t / h

48 Statistical analysis Maximum likelihood estimation: Aging experiment 500 mw, total test time t = h 9 devices per lot; 2 failures are observed Assumption: exponential distribution of failure function Failure time for the failed diode t j : 3703 h, 9041 h MTTF = r 1 ( n r ) t + j = 1 = λ r t j MTTF ( 9 2 ) = h + ( h h) = h Ref: Reliability and Degradation of Semiconductor Lasers and LEDs,M. Fukuda, Artec House, 1991

49 Statistical analysis 1 α confidence level: Aging experiment 500 mw, total test time t = h 9 devices per lot; 2 failures are observed Assumption: exponential distribution of failure function Failure time for the failed diode t j : 3703 h, 9041 h MTTF 1 α = ( n r ) t + 2 α χ (2r r j = 1 t + 2) / 2 j MTTF 0.6 = 2 [( 9 2 ) h h h] 6.2 = h Ref: Reliability and Degradation of Semiconductor Lasers and LEDs,M. Fukuda, Artec House, 1991

50 Aging test nm DQW broad area lasers 100 µm x 750 µm Reliable operation at T = 15 C 600 mw (6 mw/µm): 7(7) - t 1100 h Lifetime sufficient for medical applications Current I / A mw at 15 C Aging Time t / h

51 Statistical analysis Maximum likelihood estimation: Aging experiment 600 mw, 2377 h: 7 devices per lot; 7 failures are observed Assumption: exponential distribution of failure function Failure time for the failed diode t i : 1175 h, 1328 h, 1800 h, 1822 h, 1822 h, 2309 h, 2377 h MTTF = r 1 ( n r ) t + j = 1 = λ r t j MTTF (1175 = ) h = 1805 h Ref: Reliability and Degradation of Semiconductor Lasers and LEDs,M. Fukuda, Artec House, 1991

52 Statistical analysis Maximum likelihood estimation: Aging experiment 600 mw, 2377 h: 7 devices per lot; 7 failures are observed Assumption: exponential distribution of failure function Failure time for the failed diode t i : 1175 h, 1328 h, 1800 h, 1822 h, 1822 h, 2309 h, 2377 h MTTF 1 α = ( n r ) t + 2 α χ (2r r j = 1 t + 2) / 2 j MTTF 0.6 = 2 [ ] 16.8 h = 1506 h Ref: Reliability and Degradation of Semiconductor Lasers and LEDs,M. Fukuda, Artec House, 1991

53 Summary Lifetime measurements for laser diodes and bars Deliver: Information on quality of materials, processes, mounting, etc. Hints for the improvement of technology Data for reliable lifetime of devices, MTTF, reliability Require: Enough experimental data Statistical analysis of data Highly stable measurement set-up Accompanying measurements Non-destructive failure analysis Destructive failure analysis