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NATO ASI Series Advanced Science Institutes Series
A Series presenting the results of activities sponsored by the NA TO Science Committee, which aims at the dissemination of advanced scientific and technological knowledge, with a view to strengthening links between scientific communities.
The Series is published by an international board of publishers in conjunction with the NATO Scientific Affairs Division
A Life Sciences B Physics
C Mathematical and Physical Sciences
o Behavioural and Social Sciences E Applied Sciences
F Computer and Systems Sciences G Ecological Sciences H Cell Biology
Series E: Applied Sciences - Vol. 175
Plenum Publishing Corporation London and New York
Kluwer Academic Publishers Dordrecht, Boston and London
Springer-Verlag Berlin, Heidelberg, New York, London, Paris and Tokyo
Semiconductor Device Reliability edited by
A. Christou Surface Physics Branch, Naval Research Laboratory, Washington, D.C., U.S.A.
B. A. Unger Bell Communications Research, Red Bank, N.J., U.S.A.
Kluwer Academic Publishers
Dordrecht / Boston / London
Proceedings of the NATO Advanced Research Workshop on Semiconductor Device Reliability Heraklio, Crete, Greece June 4-9, 1989
Library of Congress Cataloging In Publication Data NATO Advanced Research Workshop on Semiconductor Device
Reliability (1989 : Herakleion, Greece) Semiconductor device reliability.
(NATO ASI series. Series E: Applied sciences ; vol. 175)
"Published in cooperation with NATO Scientific Affairs Division."
1. Semiconductors--Reliability--Congress. I. Christou A. II. Unger, B. A. III. Title. IV. Series: NATO A.'SI series. Series E, Applied sciences no. 175. TK7871.85.N3758 1989 621.381'52 89-24589
ISBN-13: 978-94-010-7620-3 e-ISBN-13: 978-94-009-2482-6 DOl: 10.1007/978-94-009-2482-6
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1.1 The Influence of Temperature and Use Conditions on the Degradation of LED Parameters
R. Goarin. J.P. Defars. M. Robinet. P. Durand and B. Bauduin (CNET. France)
An Historical Perspective of GaAs MESFET Reliability Work at Plessey
James Turner and R Conlon (plessey Research Caswell Ltd .• U.K.)
Screening and Bum-In: Application to Optoelectronic Device Selection for High-Reliability S280 Optical Submarine Repeaters
M. Gucguen. J.L. Boussois. J.L. Goudlard and S. Sauvage (Alcatel CIT. France)
Assuring the Reliability of Lasers Intended for the Uncontrolled Environment
J.L. Spcncer (Bellcore. U.S.A.)
Component Bum-In: The Changing Attitude F. Jensen (The Engineering Academy of Denmark)
Statistical Models for Device Reliability; An Overview J. M¢1toft (The Engineering Academy of Denmark)
Computer-Aided Analysis of Integrated Circuit Reliability P. Mauri (SGS-Thomson Microelectronics. Italy)
Reliability Asscssment of CMOS ASIC Designs M.S. Davies (University of Leeds. U.K.) and P.D.T. O'Connor (Britisch Aerospace DynamiCS Group, U.K.)
Models Used in Undersea Fibre Optic Systems Reliability Prediction
RH. Murphy (STC Submarine Systems. U.K.)
3.l. Failure Analysis: The Challenge RG. Taylor and I.A. Hughes (British Telecom, U.K.)
Gate Metallisation Systems for High Reliability GaAs MESFET Transistors
D.V. Morgan (College of Cardiff, U.K.) and J. Wood (University of York, U.K.)
Reliability Limitations of Metal Electrodes on GaAs H.L. Hartnagel (Institut fur Hochfrequenztechnik, F.R.G.)
Failure Mechanisms of GaAs MESFETs and Low-Noise HEMTs
F. Magistrali (Telettra S.p.A., Italy), C. Tedesco and E. Zanoni (Univcrsita' di Padova, Italy)
Metal Contact Degradation on III-V Compound Semiconductors G. Kiriakidis (Research Center of Crete/FORTH, Greece) W.T. Anderson (Navel Research Laboratory, U.S.A.) z. Hatzopoulos, C. Michelakis (Research Center of Crete/ FORTH, Greece) and D.V. Morgan (University of Wales, College of Cardiff, U.K.)
Nuclear Methods in the Characterization of Semiconductor Reliability
J.C. Soares (Centro de Ffsica Nuclear da Universidade de Lisboa, Portugal)
A Review of the Reliability of III-V Opto-electronic Components S.P. Sim (British Telecom Research Laboratories, U.K.)
Considerations on the Degradation of DFB Lasers T. Ikegami, M. Fukuda and M. Suzuki (NTT Opto-electronics Laboratories, Japan)
InP-Based 4 x4 Optical Switch Package Qualification and Reliability
K. Mizuishi, T. Kato, H. Inoue and H. Ishida (Hitachi Ltd., Japan)
Modelling the Effects of Degradation on the Spectral Stability of Distributed Feedback Lasers
A.R. Goodwin, J.E.A. Whiteaway (STC Technology Ltd., U.K.) and R.H. Murphy (STC Submarine Systems Ltd., U.K.)
Optoelectronic Component Reliability and Failure Analysis P. Montangero (CSELT, Italy)
Temperature Cycling Tests of Laser Modules P. Su and B.A. Unger (Bellcore, U.S.A.)
An Experimental and Theoretical Investigation of Degradation in Semiconductor Lasers Resulting from Electrostatic Discharge
L.F. Dechiaro, C.D. Brick-Rodriguez and R.G. Chemelli (Bell Communications Research, U.S.A.) J.W. Krupsky (South Central Bell, U.S.A.)
Reliability Testing of Planar InGaAs Avalanche Photodiodes M. Kobayashi and T. Kaneda (Fujitsu Ltd., Japan)
Status of Compound Semiconductor Device Reliability W.T. Anderson and A. Christou (Naval Research Laboratory, U.S.A.)
Investigation into Molecular Beam Epitaxy-Grown FETs andHEMTs
S. Mottet and J.M. Dumas (Centre National d'Etudes des Telecomunications, France)
Reliability of GaAs MESFETs B. Ricco (University of Bologna, Italy), F. Fantini (S.S.S.U.P.S. Anna, Italy), F. Magistrali and P. Brambila (Teletlra Spa, Italy)
Hydrogen Effects on Reliability of GaAs MMICs W.O. Camp, Jr., R. Lasater, V. Genova and R. Hume (IBM Systems Integration Division, U.S.A.)
Temperature Distribution on GaAs MESFETs: Thennal Modeling and Experimental Results
G. Clerico Titinet and P.M. Scalafiolli (CSELT, Italy)
7.1 High Speed IC Reliability: Concerns and Advances A.A. Iliadis (EO.R.T.H./University of Maryland, U.S.A.)
Reliability of short channel silicon SOl VLSI Devices and Circuits
D.E. Ioannou (University of Maryland, U.S.A.)
Special Reliability Issues and Radiation Effects of High Speed ICs
G.I. Papaioannou (University of Athens, Greece)
Reliability of High Speed HEMT Integrated Circuits and Multi-2DEG Structures
A. Christou (Foundation of Research and Technology­ Hellas, Greece)
AlGaAs as a Dielectric on GaAs for Digital IC'S: Problems and Solutions
W.T. Masselink (IBM T.J. Watson Research Center, U.S.A.)
APPENDIX A. RELIABILITY STRESS SCREENING F. Jensen (Leader), W.E. Camp, R. Murphy and R. Goarin
APPENDIX B. LIFETIME EXTRAPOLATION AND STANDARDIZATION OF TESTS 571 A. Christou (Leader), J. Mfi)ltoft, P.D.T. O'Connor, W.T. Anderson and P. Mauri
This publication is a compilation of papers presented at the Semiconductor Device Reliabi­ lity Workshop sponsored by the NATO International Scientific Exchange Program. The Workshop was held in Crete, Greece from June 4 to June 9, 1989. The objective of the Workshop was to review and to further explore advances in the field of semiconductor reliability through invited paper presentations and discussions. The technical emphasis was on quality assurance and reliability of optoelectronic and high speed semiconductor devices.
The primary support for the meeting was provided by the Scientific Affairs Division of NATO. We are indebted to NATO for their support and to Dr. Craig Sinclair, who admin­ isters this program.
The chapters of this book follow the format and order of the sessions of the meeting. Thirty-six papers were presented and discussed during the five-day Workshop. In addi­ tion, two panel sessions were held, with audience participation, where the particularly controversial topics of bum-in and reliability modeling and prediction methods were dis­ cussed. A brief review of these sessions is presented in this book.
The success of any conference, but particularly one with a small attendance, depends not only on the technical content and preparation of each paper and presentation, but also on the willingness of each participant to share and socialize data and experiences and to contribute to the technical discussions. In this regard, the Semiconductor Device Reliability Workshop was a stellar example with each participant contributing freely and profession­ ally and presenting papers of considerable merit. The co-directors wish to acknowledge this and thank the attendees for contributing to a splendid week of technical exchange.
It is also a pleasure to acknowledge the Organizing Committee consisting of Professor J. Mf/lltoft, Dr. G. Kiriakidis and the co-directors. This Committee planned the Workshop: set the format, the program, and the activities of the Workshop. We are also indebted to Dr. G. Kiriakidis for taking care of all the conference and attendee hotel arrangements as well as handling all the operational details during the meeting. He was ably assisted at the meeting by Ms. Lia Papadoulau and Ms. Georgia Papadaki. We also acknowledge the secretarial help of Mrs. Mary Daley, who did a splendid job of maintaining order during the planning phase of the meeting and assisting in the preparation of this publication.
And finally a word about the conference. Reliability and quality have become buzz words in our society. The Japanese emphasis on R&Q and demonstrated performance in this area, with the attendant economic benefits, have raised the importance of reliability and quality in all areas of technology. R&Q attributes have become an important part of any sales program, commanding considerable emphasis in sales literature. Indeed, we believe that, in general, product R&Q has improved, even as the products, particularly electronic products, have become more complex. This conference focused on the R&Q of devices to be incorporated in the next generation of electronic and communications products. It was one of the few platforms solely dedicated to the discussion of R&Q of optoelectronic and GaAs circuitry. R&Q emphasis on new emerging technologies at meetings such as this will help to continue this trend toward improved reliability and quality of the next generation of solid state electronics.
Dr. A. Christou Dr. B.A. Unger
R. GOARIN, J.P. DEFARS, M. ROBINET, P. DURAND, B. BAUDUIN C.N.E.T. Departement lABjIFEjCOD BP40 22300 Lannion France
ABSTRACT. This paper is intended to illustrate the quality and reliability of optoelectronic devices. Limited exemples to LED indicate how laboratory tests can be used for preparing component specifications.
The usefulness of burn-in and screening procedures is indicated based on real experience. The importance of technology and manufacturer is presented. The quality and reliability of devices based on failure analysis are often more related to external causes than due to intrinsic reliability of the semiconductor.
1. Introduction
A long experience has been obtained on LED components tested in CNET laboratories showing the contribution of temperature and current on the degradation of optical power. A good correlation between the variations at different conditions has been observed. The Paper gives the results of the evolution of diodes over more than 3 years duration. Those results were very useful to decide on the choice of components for the broadband network installed by FRANCE TELECOM. Several ten thousands of LEDs are now under operation corresponding to excellent reliability results compared to results obtained from lasers.
The observation of individual variations for devices from different manufacturers can be used to evaluate the degradation and extrapolate the behaviour for a long term duration. A position concerning burn-in and screening can be derived, showing that no general philosophy or strategy can be the rule, due to improvement of technology and based on different failure mechanisms.
Investigation on available components and also previous experience on the evolution of optocouplers was a good base to undertake laboratory tests.
The development in France of optical broadband network using optical fibers at a reasonnable cost was only possible if cheap and reliable optical components were available which was not the case in the Biarritz experience.
Due to the fact that a subscriber uses less than 1 kilometer of optical fiber, LED for emission was the best solution combined with multimode fibers. In order to share the fiber using wavelength
A. Christou and B. A. Unger (eds.), Semiconductor Device Reliability, 1-28. © 1990 Kluwer Academic Publishers.
multiplexers two values of length 850 nm and 1300 nm were choosen. The first subscribers are installed with a 850 nm diode, then when the number of subscribers exceeds the number of available fibers the second length is necessary.
The cost for those new subscribers is based on the fact that the same fiber is used allowing an extra cost for the multiplexer and the 1300 nm LED more expensive than the 850 nm. It was also necessary to make evaluation on those components.
Depending on the position of the subscriber and the length of needed fiber, the power through the LED is adjusted and so the intended life. The system budget was based on the worst case evaluation so that any subscriber even at a distance of 1 Km should be in a confortable position.
2. Components for the 850 nm window
A. For the length 850 nm a GaAIAs/GaAs diode, double heterostructure, Burrus structure with active region on the solder side was tested. The lens is attached on the top of the die with silicone glue. After a wafer selection all diodes are screened after 100 ·C burn-in during 21 days at 100 mAo Those devices are encapsulated in a hermetic package with a glass window without pigtail and integrated in an active duplexer(bidirectional integrated coupler) by combination of LED, PIN and optical connector.
The structure is described in figure 1
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B. Another type of double heterostructure using a dome diode was investigated due to the high optical power available. (fig. 2)
Figure 2
C. A third type of double heterostructure, buried localisation layer, mounted junction side up was tested without lens on the die. (fig. 3)
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D. A single heterostructure diode, buried localisation layer, mounted junction side up was also tested.
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The two main parameters interferring with reliability are temperature and current conditions.
Temperature and related activation energies is commonly used to accelerate degradation, that is why 4 values were chosen 20, 70,100 and 125° C (ambiant temperature).
Two sets of diodes were put on test at 50 mA or 100 mA (aging conditions). Measurements at typical use condition (100 mA) and low current condition (2 mA) were made on each device in order to reveal more sensible variations.
Based on thermal resistance it is possible to know the junction temperature which corresponds to an increase of temperature of 22° C at 50 mA and 45° C at 100 mA for instance for the first manufacturer.
Measurements on each device during the test were performed on different batches having specific test conditions based on a combination of temperature and aging current. The main parameter for those devices is the radiant power measured from time to time at room temperature using an integrater sphere.
Two categories of presentation have been used:
- the variation of optical power with time for each individual device (the dotted lines are measurements for a current of 2 mA, the full lines at 100 mA),
- for an individual diode and at the measurement times, the variation of optical power with forward current and voltage and also reverse characteristics.
It can be seen from the curves that both temperature and direct aging current have an effect on the degradation of the radiant power. The evolution is gradual, showing a more sensitive drift on measurements at low current level (2 rnA) compared to typical current (100 rnA).
For the same aging current 100 rnA the degradation is accelerated by temperature (see fig. 5, 6, 7) and at a fixed temperature 100°C the degradation is accelerated by the aging current, seen by comparison between fig 7 and 8.
For the same junction temperature the degradation is higher for a higher aging current by comparison between fig 7 and 9 (same junction temperature 145°C).
From the individual variation we can see that a group degradation is observed plus some devices having a higher degradation detectable after an aging time which can be greater than several thousand hours. This indicates the effectiveness of a burn-in for screening purpose, the most sensitive parameter beeing the measure of radiation power at 2 rnA.
An "acceleration factor" of about 10 on aging time gives a good correlation between observation of radiant power drift measured at low current at time T compared to the drift on normal use conditions at time 10xT.
When considering the results observed on the diodes from the manufacturer A more details can be seen on the degradation through observation of other parameters (fig 10) on individual diodes.
As an example iP = f(Vf) do not indicate any variation while iP = f(If) and If = f(Vf) indicate a non radiative current increase due to active interface defects affecting more the characteristics at low current level (fig. 11). The same amplified phenomena can be observed on diodes submitted to higher temperature stresses (lOO°C compared to 20°C) (fig. 12). This is the mechanism affecting the global population.
A failure analysis (see photos) was made on those diodes having an erratic behaviour. It can be observed that corrosion under the lens explains the degradation of parameters. The same analysis made on "better" diodes shows the start of the same phenomena. The analysis of the silicone glue used to attach the lens revealed that acetic anhydrid is present in the case of diodes and dissapears with aging time.
The failure mechanism seems to come from acetic acid in the silicone glue used during the process of manufacturing.
The same kind of aging test was conducted on another manufacturer B giving quite stable variations for several thousands hours at "normal" use conditions (SO rnA, 20°C) (fig 13).
At higher temperature (70°C) (fig. 14) for the same aging current the variations show more disturbances due to high stress temperatures, even if 70°C is not so high compared to 100°C and 125°C for the first manufacturer. The activation energy for those diodes is higher. The influence of current is put on evidence on fig. 15 to be compared to fig 13.
A remark can be made at this step. If a standard has to be prepared in order to evaluate the behaviour of the diodes and a possible qualification through accelerated test, the habit (rule, fashion ... ) used in many standards is to choice a 125°C test (why 125°C ? because it is often found in standards and reciprocally !!)
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The choice for the best manufacturer based on accelerated test should certainly be A even if at "normal" temperature 20·C the behaviours of A and B are quite similar.
Observation of aged devices indicated a degradation due to well known phenomena for those devices which is the dark line defect affecting the efficiency of diodes. (see photos).
The variation between A and B is due to different failure mechanisms corresponding to different physical origin. A conclusion at this step is that we cannot use the same accelerated conditions (temperature and aging current) to predict the real behaviour in use conditions. Another manufacturer C was tested at 100 rnA aging current and two different temperatures 70°C and 125°C. Compared to the manufacturer A for the same test condition those diodes look more stable, and the influence of temperature is less (fig. 17 compared to fig. 16). Even at this high stress (100 rnA - 125°C) the variations are lower than observed on manufacturer B (see fig. 14).
In fact different batches from different wafers were put on test and we can see a significant variation from one (fig 16) to the other (fig. 18) corresponding to the same test conditions.
That means, even for a technology and a manufacturer, that the wafer selection is important. The quality level for a manufacturer will be related to his criteria used to decide about the rejection of a wafer if a portion of diodes taken as a sample will exceed a specific requirement on the power drift during a burn-in test. Observation on diodes from this manufacturer showed some dark lines on the die, but without affecting the optical power. (see photos)
The photo technology D was also tested at different conditions. In order to reveal the influence of the lens, tests on components with or without lens were conducted at different test conditions.
Without lens, the influence of current at normal temperature cannot be detected. The "improving" of diodes is the same. Only one failure (catastrophic failure) occured after 2 thousands hours, the reaction would be to prove the influence of current on reliability, it is better not to reach this step.
The non influence of the lens is seen by comparison between fig. 19 and fig. 21 corresponding to the same aging conditions. On this manufacturer we can see a good stabilisation of optical power with or without lens in opposition to the first manufacturer where the main cause of degradation was due not directly to the lens but due to the glue used to attach the lens. Is it to conclude that the reliability is dependent more on the glue than on the semiconductor?
It seems when comparing fig. 21 and fig 22 that temperature has some influence on the behaviour of diodes. Problems we had on those diodes are in fact not detectable on curves, those diodes were rejected for bad encapsulation!
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The technology for different tested eomponents is quite similar, double heterostructure on InP, active region on the solder side. (fig. 23)
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Four manufacturers were tested.
For the manufacturer A, for quite severe conditions, we can see that the optical power is quite stable, even if important variations of the power measured at low current (2 rnA) indicate a possible degradation in the future (see figure 24). The measurement at low current is an amplification of what can be expected for high current, with an acceleration factor higher than observed on 850 nm diode (manufacturer A). Another test was also eonducted at less severe conditions (fig. 25) indicating a more stable optical power at 100 rnA and by the same way the influence of stress eonditions. But the erratic behaviour of diodes measured at 2 rnA make difficult any extrapolation nicely seen on fig. 24. Details on a specific diode can be seen on fig. 26.
The failure mechanism which was revealed is gold electrodiffusion and accumulation in the active region. (see photos)
For the manufacturer B we have also stable optical power (see fig. 27).
The influence of temperature (fig. 28 to compare to fig. 27) is not very important. It can be noticed than failures occur after a very long duration (more than one year), which indicate the limitation of burn-in possibilities (or the beginning of wear out !)
At 4500 hours a sudden variation (- 20 %) of optical power was observed. The failure was due to the glue used to mount the die in the case. (see photos)
In fact this glue caused a short circuit on the device causing this failure which was recovered by eliminating the extra glue. This is again, a failure which has nothing to do directly with the structure of the diode.
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For the manufacturer C we can see a very stable power on the fig. 29.
For the manufacturer D even if the variations are more important, (fig. 30) they stay in a range giving good expectation for the future, even if the time duration for those devices do not reach experience already accumulated on 850 nm diodes. Some diodes from this manufacturer were put on test without lens (fig. 31) in order to try to characterize the semiconductor itself (intrinsic reliability).
In general the behaviour of 1300 nm diodes compared to 850 nm seems better and we can expect to have better reliability results.
When the costs will reduce (due to quantities) they will be well adaptated to our broadband networks.
4. Conclusion
A general conclusion is that... it is impossible to have general conclusion on requirements to be put in a general specification.
The same accelerated conditions cannot be applied to different components, due to different activation energies and due to different mechanisms of degradation.
For a specific component (technology and manufacture) an accelerated test can be a way to select good wafers. The burn-in can be useful to reject abnormal components but it can be costly due to the fact that a long duration is often needed to reveal the weak components (sometimes several thousand hours).
The goal of laboratory tests is to evaluate the quality and reliability of components (not to produce nice curves obtained from mathematics), to reveal the weak point(s) in a technology and to find the basis for improvement.
The need for burn-in has to be based on the behaviour of a manufacturing process for a specific manufacturer. The efficiency of such a burn-in is determined on real experience, it can be costly and needs a long time to reveal weak components.
Variations from lot to lot, from a manufacturer to another make that a permanent view on the quality is necessary. General extrapolation is hazardous as well as reliability predictions based on accelerated laboratory test.
Experience and investigation can give the possibility for a user to make the best choice.
Those results are only a part of all experience accumulated in CNET on LED. Preparing aging positions, measuring devices and interpretation of results as well as failure analysis required the assistance from more people than referred in the author list. Thanks have also to given to J. MARTRET, M. OLLIVIER, M. DONTENWILLE, C. BOIS ROBERT, C. PATRAC, D. RIVIERE ... and those manufacturers who provided the facility to have a better knowledge of technology and so help in the interpretation of degradation mechanisms.
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Fig. 31 MANUF. D
James Turner and Rodney Conlon Plessey Research Caswell Limited, Allen Clark Research Centre, Caswell, Towcester, Northants. England.
ABSTRACT. Gallium Arsenide MESFET fabrication technology has become more and more sophisticated and this has led to a marked improvement in device reliability. This paper charts the progress of MESFET life tests at Plessey Research Caswell Limited and shows how by adopting new fabrication techniques the MTTF of the device has been improved by four orders of magnitude.
This paper reviews some of the life test programmes carried out at Plessey Research since 1975 and shows how the results of the tests nave influenced the meta11isations and geometries used by Plessey for both ohmic and rectifying contacts to small signal and high power GaAs MESFETs.
The life testing programmes reviewed here are representative of many performed at Plessey:
(a) Small signal 1.0 micron gate length MESFETs (1975) (b) Power 1.0 micron gate length MESFETs (1978) (c) Electron beam processed 0.3 micron gate length MESFETs (1984) (d) Ion implanted 0.7 micron gate length MESFETs (1988).
In this programme a number of X-band devices with two different ohmic metallisation systems were stress tested:
(1) indium/gold/germanium (In/Au/Ge); (2) nickel/gold/germanium (Ni/Au/Ge).
The devices had a passivation layer of silicon monoxide covering the channel area and parts of the drain and source contacts.
The tests performed included reverse biasing of the gate diode at an elevated ambient temperature, thermal cycling from -65 to +15QoC, AC modulation of the drain current by application of gate volts to
.1. Christou and B. A. Unger (eds.), Semiconductor Device Reliabilily, 29-42. <l 1990 Kluwer Academic Publishers.
• turn the device on and off, and high-temperature DC and RF tests. The major failure mode identified was ohmic contact migration.
Metal migration, the current induced transport of material, appears to have been first correctly identified as a potential cause of failures in microcircui t interconnection around 1965 [I]. Since then there has been a lot of work done on identi fying the causes of migration and evaluating preventive measures. Without question, at some elevated temperature metal migration effects can be observed in all semiconductor devices - what is important is to determine the conditions under which this occurs in the GaAs MESFET. These results show that metal migration does indeed occur but only noticeably at channel temperatures around 250°C and so does not represent a serious hazard to high reliability usage.
In order to accelerate the migration and shorten the time to failure, the GaAs MESFETs were subjected to DC biasing at an ambient temperature of 200°C (i.e. approximately 250°C junction temperature). The devices were run at +5V drain bias with the gate grounded, giving a current density of about 105 A/cm 2 . Under these conditions migra­ tion effects were observed on all devices tested within 1000 h. Following the initial observation of metal migration, further experi­ ments were carried out to assess the effect of contact shape on the time to failure.
With In/Au/Ge ohmic contacts the combination of time, temperature, and current caused material to accumulate at the edge of the source contact adjacent to the etched channel, and to deplete the drain contact at a similar place. Fig. 1, with the source on the right and drain on the left, illustrates this effect. Microprobe analysis confirmed that the accumulated material was that of the ohmic contact. It is not believed, though, that the material crosses the gate region,
but that it moves across the contact from t.he source bond wire towards the edge of the source contact, and from the edge of the drain contact to the drain bond wire. This direction of material transport is the same as that of electron flow, and thus agrees with present explana­ tions of migration [2]. Nodules of mate rial have been seen on the drain bond wire which were not present when the devices were put on test (Fig. 2).
(a) (b)
lal Initial state of bond wire
IbI Nodule formation after life test
For the Nil Au/Ge system the primary effect under high temperature stress was the "balling" of the metallisation causing increased contact resistance. Heime et a1 [3] observed a similar effect when alloying the contact, but found that evaporation of a thin nickel layer on top of the Au/Ge before alloying could reduce the balling. In the devices tested there was no evidence of balling after alloying; this only appeared after high temperature testing. The increased contact resistance during the life test n!duced the drain current and transconductance and led to a general deterioration in microwave performance.
2.1. Effect of ohmic contact shape
Three ohmic contact shapes were examined - T-shaped, triangular and rectangular - to evaluate their effect on the time to failure. It was found that the accumulation rate, depletion rate, and the positions of accumulation and depletion were all affected by contact shape. Thermal profiling with a nematic liquid crystal [4] showed that these differences were caused by localised hot spots due to current crowding in the contact. The positions of these hot spots were also affected by the "heat-sinking" effect of the bonds. The migration rate was slowest for the rectangular and fastest for the T-shaped contact.
Migration occurred mainly at the narrow ends of the T-shaped contacts, at the apex of the triangular contacts, and was distributed evenly along the edge of the rectangular contacts.
2.2. Conclusions and Recommendations
In small-signal GaAs MESFETs produced at this time the dominant failure mode was metal migration of the ohmic contact. Reduction of this high temperature effect can be accomplished in several ways. The current densi ty can be reduced by making the metallisation thicker, but due to limits imposed by the float-off process used to define the metal areas, plating up of the contacts is necessary to achieve maxi­ mum effect. The contact shape, also influences the migration. Rectangular contacts give the most uniform distribution of current and therefore are the most promising. As the migration is dependent on surface temperature, improvements in mounting techniques to reduce the thermal impedance could aiso give improvements in reliability. To obtain the optimum noise performance from the GaAs MESFET, it is necessary to bias them at low drain currents. This is beneficial to the device lifetime as it reduces both the current density and channel temperature.
These life test resul ts on small-signal MESFETs with In/ Au/Ge ohmic contacts gave a room temperature mean time to failure in excess of 107 h. This compares with the only other published data for GaAs MESFETs at the time of 106 h [S]. This MTTF is in excess of that required for most high reliability applications; devices fabricated in this way have already undergone successful qualification for space flight use [6].
These recommendations were implemented by Plessey and all devices in the following life test programmes incorporated.
(a) Rectangular contacts with rounded corners. (b) Plated up source and drain contact areas to a thickness of 3
microns. (c) Improved thermal impedance.
In this programme both step stress and constant temperature stress life tests were carried out on 84 devices biased at a drain source vol tage of 9V wi th drain currents ranging from 100-200 rnA and at ambient temperatures between 130°C and 18Soc. In the tests the devices had In/Au/Ge ohmic contacts and an aluminium gate electrode. The effect of gold and aluminium bond wires to the gate bond pad were investigated. Using a nematic liquid crystal technique the channel temperature of the devices under test was set at 4SoC above ambient by controlling the drain current of each transistor. The tests were conducted in a dry nitrogen environment but the devices were not passivated.
3.1. Constant Stress Tests
Three batches of gold gate wired devices and two batches of Al gate wired devices each containing twelve transistors were mounted in alumina microstrip test fixtures and were constant stress tested at 9V under the temperature conditions shown in Table 1.
In all cases the devices failed due to a greater than 10% reduc­ tion in transconductance caused by the formation of voids in one or more of the gate 'fingers'. The presence of the voids prevented that section of the channel current from -being modulated by the applied gate voltage. A secondary effect was that of drain ohmic contact migration but this was relatively minor and is believed to have had no effect on the DC degradation observed. The gate voids observed are shown in Figure 3.
Prior to the commencement of the tests it was expected that the devices with gold wire bonds to the gate would fail much more quickly due to the enhancing effect of gold/aluminium intermetallic alloy formation at the bond Wire/gate metallisation interface. This proved not to be the case and Arrhenius plots of the failures gives MTTFs of 105 and 2 x 105 at 65°C channel temperature for gold and aluminium bonded devices respectively. Independent measurements of gate current on similar devices showed that values as high as 300~ could be reached for the highest temperature constant stress test. This corresponds to a current density of 8 x 10 3 A cm- 2 in the 0.3 micron thick, '1 micron long gate metallisation. This current is sufficiently high to cause 'straightforward' electromigration of the gate metalli­ sation. Examination of the grain structure of the aluminium gate metal showed it to be particularly small and therefore more prone to
electromigration effects. The activation energy for the voiding process was determined from Arrhenius plots to be 0.67 eV.
3.2. Conclusions and recommendations
This series of tests left many questions unanswered such as;
(a) the extent to which electromigration is enhanced by the gold concentration in the Au/AI interaction.
(b) the distance over which the Au/AI interaction can influence electromigration of AI.
(c) the effect of GaAs substrates, grain structure and geometry on the electromigration of AI.
It also prompted Plessey to modify its gate metallurgy and to incor­ porate a silicon nitride dielectric passivation layer in the channel region of the FET. The combination of these two modifications led to a greatly reduced gate leakage current. These changes were incorpora­ ted in the life tests reported in the following section (section 4).
In this test small signal devices with gate lengths of 0.3 micron were mounted in microstrip test fixtures and subjected to elevated tempera­ ture life tests. During the test both DC and RF performance was monitored. The devices had titanium/aluminium gate metallisation and In/Au/Ge ohmic contacts. The ohmic contact areas had additional gold based metallisation in order to locally reduce the current density in the ohmic contacts to minimise the susceptibility to metal migration. As in previous tests some samples had aluminium bond wires to the gate, others gold wires. Tests were carried out at junction tempera­ tures of 165°C, 192°C and 220°C and, apart from one device (out of a total of 30) which showed some sign of metal migration of the ohmic contact, there was no visible sign of failure. In these tests failure was defined as a 10% change in DC characteristics or a 1 dB change in noise figure or gain at 14 GHz.
There was no apparent correlation between changes in DC current and RF insertion gain, 15211 but there is a strong correlation between changes in transconductance particularly at high gate bias (close to pinch off) and 15211. It was therefore concluded that the observed changes in both DC and RF performance were due to a change in the electrical parameters of the channel. This change in channel properties was examined further by examining the change in transcon­ ductance at zero gate volts and close to pinch off. The results indicate a greater change at zero vol ts than at large negative gate voltages perhaps indicating a general degradation at the surface (Figure 4).
From these tests an activation energy of 1.0 eV was deduced leading to an MTTF of 3.5 x 105 hrs at 70°C.
iii 0
" .. c: ~ ~
- 1.8 - 1.4 - 1.0 - 0.6 - 0.2 0 Gate Voltage (volts)
4.1. Conclusions and recommendations
Whilst there was no obvious difference in life between the aluminium and gold gate wire bonded devices it was realised that the practice of using aluminium for gates and gold for source and drain bonding was not cost effective and could eventually lead to reliability problems. For the life tests described in Section '5 the gate metallisation was changed to Ti/Pt/Au, a metallisation system used by Plessey in its MMIC process since 1982. All of the other previously recommended geometry and process changes were also incorporated into the life tested devices.
The latest set of life test data has heen obtained as part of the ESPRIT 1270 programme. In these tests devices with Ti/Pt/Au gates and In/Au/Ge ohmic contact fabricated in the Plessey GaAs IC Foundry were subjected to temperature accelerated tests at 220°C and 250°C ambient under DC bias. The devices tested were part of the process control monitor chip and had gate lengths of 1 and 5 microns.
3000 4000
The general trend in these characteristics for both l~m and 5~m
devices was of decreasing current with time being most pronounced for the shorter gate length devices. These changes were confirmed by the periodic measurement of all devices at room temperature. For both the 250°C and 220°C tests the mean change in I DSS has been plotted in Figure 5. Accompanying these changes were reductions in pinch off voltage and drain-gate breakdown voltage. Only small changes in contact resistance were observed for both tests (see Figure 6) although there was a greater change at the higher temperature as might De expecteu.
u 40 a: .E
30 G> c:n c: 20 ca o 250°C .c: ... _220°C
~ 10 • 0
2000 3000 4000 Time (hrs)
Although detailed failure analysis has not yet been carried out on degraded devices, other workers have attributed such parameter changes to gate metal interdiffusion (gate-sinking) causing shrinkage of the effective channel thickness.
Referring to Figure 5 and applying a failure criterion of 20% reduction in I DSS then from the 220°C test the mean time to failure is approximately 4000 hours for both the l~m and 5~m device. Assuming an acti vation energy of 1. 6 eV, a value generally accepted for the above failure mechanism, results in a predicted median lifetime of 1 x 107 hours at 150°C operating temperature or 5 K 109 hours at 100°C.
5.1. Channel Temperature Estimation
The above reliability testing was carried out with the baseplate temperature controlled to 220°C or 250"C. To evaluate the true channel temperature under such conditions and establish the detailed temperature distribution over the MESFET surface, a small programme of work has been undertaken with the University of Birmingham, UK. Based on a combination of infra-red thermal imaging and numerical simulation, the channel temperature of the l~m and 5~m test FETs has been estimated. An example of the surface temperature distribution for the 250°C baseplate condition is seen in Figure 7 which predicts a peak channel temperature of 289°C for the l~m FET and 274°C for the 5~m device. These therefore are the true channel temperatures applicable for the life test described above.
This paper has followed the progress of a number of Plessey reliability programmes since 1975. It has shown how detailed failure analysis has led to a continual improvement in life expectancy of the transistors by removing the obvious short comings in geometry and process technology exposed by the life tests.
Table 2 summarises all these results and shows how the MTTF of the Plessey device has been increased by 3.5 orders of magnitude since the testing began in 1975. Table 3 shows the technology changes that improved device lifetime.
Progress has been such that gate voiding and ohmic contact migra­ tion are now no longer major failure modes but that device life is limited by degradation of the electrical properties in the channel. Failure analysis on the recently tested ion implanted devices coupled with the fabrication of special test structures could well help to suggest ways of reduc Lng this ef fec t thereby further improving the reliability of the GaAs MESFET.
This paper is a compilation of the efforts of many Plessey workers over the past 14 years and the authors would like to acknowledge the help gained from reviewing their past reports and published papers. Support for the reliability programmes came from a number of sources - the European Space Agency, the Procurement Executive Ministry of Defence (Directorate of Components, Valves and Devices), the sponsor-
ship and technical direction of INTELSAT and the European Commission (ESPRIT 1270). The support of the Plessey Company pIc is also acknow­ ledged.
l. Blech, I.A. et aI, R.A.D.C., Griffiss A.F.B., NY, Tech. Rept. TR-66-31 (Dec. 1965).
2. Black, J. R., "'Electromigrat.ion - A brief survey and some recent results"' IEEE Trans. ED, Vol. ED-16, pp.338-347, 1969.
3. Heime, K. et aI, Solid State Electronics, Vol. 17, 835-847, 1974.
4. Stephens, C.E. and Sinnadurai, E.N., Journal of Physics E: Scientific Instruments, Vol. 7, 1974.
5. Wireless World, June 1975, p.271. 6. James, D.S. et aI, Proceedings of 19"75 European Microwave Conf-,
Montreux Switzerland. 7. Roesch, W.J. and Peters, M.F., Proceedings of the GaAs IC
Symposium, p.27-30, 1987.
Screening and burn-in: application to optoelectronic device selection for high-reliability S 280 optical submarine repeaters
M. GUEGUEN, J. L. SOUSSOIS, J. L. GOUDARD and D. SAUVAGE Alcatel CIT, Centre de Villarceaux, Route de Vil/ejust-Nozay BP 6, 91620 LA VILLE-OU-BOIS, FRANCE
After reiterating the reliability objectives of such equipment in a special link such as EMOS and the respective allowances made for transmit and receive components, and a overview of the consolidated reliability data obtained on these components, the emphasis will be put on the resulting selection procedures that have been chosen after an optimisation and validation phase. The practical application of these procedures in supplies for the EMOS link shall be described in particular detail.
In conclusion, even if a few uncertainties remain with regard to demonstration, the ATe reliability assurance programme for submarine optoelectronic components shows that these sensitive components satisfy the reliability requirements of the ori~linal Submarcom S 280 system.
A. Christou and B. A. Unger (eds.), Semiconductor Device Reliability, 43-73. © 1990 Kluwer Academic Publishers.
I. Introduction
The rapid commercial success of optical fibre submarine systems with links already brought into seIVice such as Alcatel Submarcom S 280 French mainland-Corsica III, Rome-Sardinia and TAT 8, has called for a special effort at every level during the development stage to meet the stringent reliability objectives that are required.
After reiterating the reliability objectives of such equipment as the future EMOS link that is in the manufacturing stage and the respective allowances made for the various components such as optoelectronic devices, we shall present the reliability data obtained on these particular components. This will enable us to consolidate reliability objectives that are, moreover, associated with the concept of complete qUality. On the basis of a number oftests conducted for their final qualification, the emphasis will be put on the resulting selection procedures that have been chosen. These procedures were defined using basic principles and pretested methods to select components used in analogue submarine equipment. Modifications were made to them in orderto take into account the special character of each component. The particular example of transmit components, laser diode emitter and receive components, PIN photodiode is illustrated, paying particular attention to the consistency of the total quality assurance system implemented right up to the final selection of the submersible components.
Needless to say, the selection procedures that have been chosen are the result of continuous optimization, validation, field experiments throughout the component manufacturing, inspection and utilization cycle.
The practical application of the procedures involves: • Process quality assurance with line inspection to control the crucial technological stages • The finished product quality assurance corresponding to the final selection destructive tests in order to verify the reliability level measured during qualification • Unitary selection to eliminate marginal parts using non-reliability, nonconfonnity and non­ homogeneity criteria for each testable, sensitive basic component and fibred finished product that is specially developed for the EMOS contract.
II. Reminder of reliability objectives
The reliability requirement for a submarine optical system is generally a minimum lifetime during which a mean number of ship repairs is allowed due to component failures (this number is based upon a mathematical prediction). The link parameters that have a crucial influence on reliability perfonnance are the number of repeaters and the mean temperature.
From a reliability standpoint, S 280 repeater components can be classified into the following groups: • Components where the failure mechanism leads to a log nonnal distribution of the lifetime (estimators are mean time MTTF and standard deviation 0). In this group only the laser behaviour is predicted. • A group showing a constant failure rate behaviour (exponential law) which includes the photode­ tectors, integrated circuits, SAW filters and the other passive components. • The switching devices for redundancy operation where we have to consider the probability of non­ operation.
2.1.1 Brief description The EMOS system is a submarine cable system linking Palenno (Italy) with Lechaina (Greece), Mannaris (Turkey) and Tel Aviv (Israel). The system includes three subsystems, each consisting of one fibre pair providing 2 x 139 264 kbit/s digital line sections: • One subsystem between Palenno and Lechaina • One subsystem between Palenno and Mannaris • One subsystem between Palenno and Tel Aviv
This configuration is shown in the diagram below (Figure 1). The geographical length of the submerged plant is approximately 2760 km.
D Cable station
Branching [!J unit Figure 1 - Schematic diagram of the EMOS-1 system
The EMOS 1 system is made with the Alcatel Submarcom S 280 system. It includes 46 repeaters as follows: • 12 repeaters housing three double regenerators (3S) • 13 repeaters housing two double regenerators (2S) ·21 repeaters housing one double regenerator (1S) and two branching units with a passive transmission path.
Each double regenerator is defined as the equipment required for signal regeneration in both transmission directions of a fibre pair and for its supervision.
The mean temperatures associated with each type of repeater are 32.2 °C (3S), 26.3 °C (2S) and 20.4 °C (IS) respectively.
2.1.2 EMOS system reliability objectives Reliability requirements for the EMOS system are as follows: • The system lifetime shall be at least 25 years • During this period, reliability shall be such that the expected number of mean ship repairs due to system component failures shall not be more than three.
In order to meet reliability requirements for EMOS, the MTBF of each double regenerator shall be greater than 700 years, meaning a FIT value ofless than 165 FITs.
Compared with a terrestrial system consisting of two single regenerators for example, this objective figure is more than fifty times greater, highlighting the reliability effort being made for this type of application.
In order to assign a separate reliability budget to each type of component, components are classified in the following categories: • Laser diodes • Transmitter packages • Optoelectronic receivers • "Critical" integrated circuits • SA W filters • Optical switches and dc motors • All other "critical" components such as resistors, capacitors or protection devices "Critical" components are those for which a failure causes a hard transmission fault if no redundancy is provided.
The failure of one supervisory device does not impair the operation of the whole line supervisory system for the other regenerators. Moreover, in this case fault location redundancy is provided by loopback and laser switchover operations. Supervisory devices are therefore not considered as critical components, though they have the same reliability level.
We shall reserve our comments to active optoelectronic component allocations. The allocations for the other categories will be summarized in Table I on the following page showing the reasonable objectives given to various components.
Laser diodes For this type of component, wear out phenomena are considered more frequent than random failures. These ageing mechanisms are temperature accelerable which has made it possible to characterize the component and establish basic data on lifetime duration. Results already obtained from tests carried out during characterization have enabled us to use the following values for the EMOS link that were standardized at 20°C, considered as a reasonable objective and obtained after component selection: • Mortality law: log normal distribution • Median lifetime MTTF (at 20°C/5 mW): 7.6 x 1()6 hours • Standard deviation a: 1.35 This results in 30 FITs in 25 years. • Activation energy: 0.9 eV
a = 1.35 (30 FITs in 25 years at 20°C)
10 FITs
0.63 FITs
5 FITs
Non-operating probability
0.015 FITs
Transmitter package In addition to contributions derived from the reliability of the laser alone, experience already obtained leads us to say that two types of failure can be encountered in an optical transmitter package: • Rogue failures comprising standard sudden failures well known in semiconductors and failures' specific to the optical output such as fibre breakage and sudden misaligmnent between the fibre and the laser . • Gradual failure mechanisms associated with a gradual change in coupling efficiency. They can be treated as a drift failure mode that can be compared with the mode characterizing the laser wear out. However, according to the gradual change law with time and temperature, which exhibits a marked sublinearity, individual long-term ageing is expected in order to carry out very accurate selection and to eliminate all parts which show a drift incompatible with the allocated margin. We have therefore considered it reasonable only to use rogue failures to characterize the reliability of the emitter module with an allocation objective of 10 FITs.
Optoelectronic receiver In view of the highly reliable planar technology used with the receiver that closely resembles the silicon planar technology used in our analogue submarine transistors, we are very confident about having a similar rogue failure rate of less than 1 FIT. The tests that have been carried out and the results obtained showing that this hypothesis is valid are described below.
Optical switch The reliability figure must be estimated on the basis oftest results.
An operating test has been conducted on a representative 60 sample lot showing a 0.5 dB mean insertion loss without any loss variation above 0.05 dB after 200 operations. This corresponds to 1.2 x 104 operations without failure and a subsequent probability of non-operation of less than 10-4.
A fibre breakage phenomenon similar to the one in the optical transmitter can be encountered. Therefore a random failure rate of 10 FITs is allocated to the switch.
Applying these figures to EMOS repeaters fitted with a duplicated laser transmitter package, one mean ship repair is estimated for the submerged plant. This value is consistent with the contract and is on average three times better than the objective and consequently reveals the potential reliability margin taken on the link even if it is customary, without being contractual, to use the upper confidence level of 90% (see Figure 2).
III. Consolidated reliability data obtained with optoelectronic components
This data results from successive characterization and qualification tests carried out on these components since 1984 and that today are being completed with final product certification.
The test programme consists of short robustness tests (step stress technique) and long lifetime tests (isothermal tests) for characterization and standardized tests for qualification. This programme has been conducted for the two types of optoelectronic components in the following way using the
T yp
e o
f N
o o
f N
o o
separate technological variable method and by testing the finished component: • Studying the crystal, basic active component on test vehicle, submount support or in a hermetic package representative of the final configuration and enabling accelerated tests. This is the case with PIN IIJJV photodiode crystal (KDOl), the Ge 800 Ilm monitoring photodiode (ceramic on Kovar support transferred to a T08 package) and lasersubmount (Cu) using nitrogen chamber for reliability tests. • Studying the various assembly supports and techniques for a total quality plan analysing the materials and processes and validating every point of their construction and implementation. Moreover, this plan has made it possible to fix the sequence of manufacturing operations and to implement inspection and tests and a quality assurance process. • Studying the finished fibred component: - The laser emitter module subassembly comprising the laser on submount (BH 1.3 Ilm on Cu/ln), the monitoring photodiode on its ceramic/Kovar support and the metallized fibre for the direct coupling function with the laser (front holding point) and the optical output with regard to the package (rear holding point in the fibre feed-through tube and output pigtail structure). - The receiver module corresponding to the assembly of the detector in a hermetic KDOI package and the fibre soldered in its fibre holder, itself a testable item.
The purpose here is notto describe all the different test stages oTto provide detailed results and failure mechanisms encountered, but rather to provide a synthesis of basic reliability data that has been extracted and that have made it possible to position the reliability level obtained for these two sensitive components in relation to the objectives set.
Moreover, this basic data resulting from accelerated tests has been consolidated by the results of practical experience acquired when producing components used in commercial links that are or will soon be in service and whose budget will also be given.
3.2.1 Requirements The EMOS 1 system requires a pigtailed source including a laser diode capable of delivering an optical signal from an electrical modulation of its operating drive current at 280 Mbit/s, the specifications for the laser source and the pigtail being summarized in Figure 3.
3.2.2 Choice of technology The laser and pigtailed transmitter have already been described in detail [ref 1].
By way of a reminder, the laser diode is an InGaAsP/lnP buried heterostructure (BH), the monitoring photodetector a large germanium photodiode (800 Jlffi dia.) used in photovoltaic mode and the fibre pigtail a tapered and metallized single-mode fibre.
The BH laser has a very suitable structure for submarine systems and offers the following major advantages: • Its highly directive optical beam enables high coupling cfficiency • Thanks to a low threshold current, it operates on low electrical power and at high temperatures These characteristics have the following advantages in terms of reliability: - In normal operating conditions, the temperature is low and the associated lifetime is high
Laser source Wavelength in the full temperature range
Linear output power Half spectral width at one sigma Threshold current in the temperature range Optical noise level
Operating temperature range
Pigtailed laser Mean optical power Bias current for maximum output power allocated by the system design Modulation current Photodiode current at maximum output power Dark current for monitoring diode Output power variation within the temperature range 10 to 40·C at constant photodiode current Reliability allocation: Wear out failure rate (log normal distribution)
Random failure rate (exponential law)
Figure 3 • Laser source and pigtail requirements
1310±20 nm
> S mW (laser) < 1.2 nm <SOmA < 30 dB up to 296 MHz and 20 dB above 10/40 ·C
Q< 10%
MTTF (20 ·C, 5 mW) s7.6106 h 30 FITs after 25 years
0= 1.35
• Including the threshold current drift allocation for ageing (50% drift) and the modulation current at maximum optical output power.
-For screening, tests can be perfonned at high temperatures that increase the confidence level insofar as weeding out marginal and flawed parts is concerned • The BH laser chip can be soldered "p side up" preventing all catastrophic failure modes due to possible whisker growth
3.2.3 Consolidated emitter reliability data
a) Wear out failure contribution Two drift failure mechanisms that can be modelled and predicted and that can be classified as wear out failures, have been identified and characterized as follows: • Gradual degradation of the laser chip only evident through the increase in polarization current required for a given power on the front side • Gradual change in coupling efficiency characterizing the laser coupled to the glass fibre at the output of the package It shall be seen that with ageing, many thousands of testing hours are required in order to extrapolate in time and to distinguish between extrapolatable drifts and transient and measurement errors. Moreover, it can be noted that the monitoring photodetector did not show up any defects representative of wear out mechanisms. The latter have a negligible contribution since with a pessimistic activation energy of 0.6 e V, a nonnallog distribution with a high standard deviation «J = 2) and zero faults during reverse bias type tests at 125 °Cf}. V carried out on 60 parts during 5000 hours, the instantaneous failure rate after 25 years would be < 0.1 FIT (20°C).
Laser evolution data First lifetests have been perfonned on about 340 laser chips on submount of which 95 were screened according to the standard procedure (reference DI: "autoselection" level) and 245 were raw and unscreened and realized at the beginning of 1984 when the characterization stage started. They consist oftwo long-tenn isothennal tests (30°C and 60°C) at constant optical power output (3 m W). The main results obtained from data after a time lapse of 22000 operating hours ('" 2.4 years) are given in the table in Figure 4.
The table shows the main characteristics of log-nonnal distributions of lifetimes obtained for two ageing temperatures and three groups of lasers classified in several sub-populations compiled in December 1986. The lifetime is defined as the time when the drive current for each laser is increased to 1.5 times its initial value.
We may recall that the law describing the degradation kinetics in its useful part in relation to the wear out mechanism is of the tm type where m (med) = 0.7, ie. between 0.5 (favourable case) and 1.5.
Experimental results lead us to the following conclusions: • The minimum value obtained of 0.9 e V is taken as the activation energy value • A relation is established for the influence of power at least up to levels of 8 m W. A conservative factor of 0.8 has been used when going from 3 to 5 m W (useful power level).
The projected MTTF (mean time to failure) associated with an end of life criterion .-'11 .... /1_ of 50% at 20°C (standard temperature) and 5 m W of power are given in the same table in Figure 4 (last three columns). The accumulated failure rate corresponding to the various populations selected are shown in Figure 5. This figure clearly shows the effect of the screening severity level on the failure rate
Results Lifetest temperature Sub-population Batch
& type of lasers definition size Median time for MTTF
."" Standard 3mW dnlatlon
Name variation (hours)
Component hours (hour.)
60 DC All 130 7.510 3 2 5.510 4 A1
unscreenoo 6 Lasers with more 104 1.610 4 1.2 3.310 4
210 than 1600 hours A2
30 DC All 113 2 10 5 2.6 5.810 6 B1
unscreened 6 lasers with more 75 7.510 5 2.910 6 3.410 than 4.310 4 hours B2 1.65
All 95 3.210 4 5
01 2 2.310
60 DC 84 4 1.910 5 screened Levell 02 5.410 1.6
72 h1150 mAl80DC 74 , 5
& P (60 DC) over 4 mW Level 2 03 7.610 1.45 2.110
Level 3 80 6.810 ' 1.5 2.110 5 0'
6 58 1.0510 5 2.6 10 5 2.510 Level 4 05 1.35
Figure 4 • Laser lIfetest results
A (FITs)
A =f(t) ~
2O°C/5mW 25 years
2.410 6 530 1.1510 6
1.910 7 920 6.510 5
9.510 6 370 2.410 6
7 6 1.710 480 2.310
1.410 7 200 3.910 6
7 6 1.610 90 5.510
1.510 7 120 4.910 6
1.910 7
30 6
108 Time (h)
Figure 5 • Laser failure rate at 20 °C corresponding to different selection levels
Screened (72 hl150 mAl80 °C P max (60°C) > 4 mW)
Llfetest temperature
Sub-population definition
A2 - P max (60°C) > 3 mW + lifetime> 1600 h
81 - P max (30°C) > 5 mW
81 - P max (30°C) > 5 mW + lifetime> 4.3104 h
01 - all
02 - .!lIth (150 mAl72 hl80 0c) < 20 mA P max (60°C) > 5 mW
.!ll/If (168 hl60 °C/3 mW) < 2%
03 - .!lIth (150 mAl72 h/80 0c) < 20 mA P max (60°C) > 7 mW
.!ll/If (168 hl60 °C/3 mW) < 2%
04 - Lllth (150 mAl72 h/80 °C) < 20 mA P max (60°C) > 5 mW
.!llf Ilf (336 h/60 °C/3 mW) < 2%
05 - .!lIth (150 mAl72 h/80 °C) < 5 mA Pmax (60°C) > 7 mW
.!ll/If (336 h/60°C/3 mW) < 2%
versus time at 20°C for the various populations defined according to the specific criteria (Fig.6).
The following two scenarios have been chosen for comparison insofar as they correspond to the reliability objectives of two specific links, and have been used for the tests consolidating the initial results given in Table 2.
Median (20°C, 5 mW) = 5.5 x 106 hours
(1 = 1.45
I ~ 90 FITs in 25 years
I ~ MTTF = 1.6 x 107 hours , or A. (25 years) = 0.9% ~ (accumulated failure rate)
Median (20°C, 5 mW) = 7.6 x 106 hours
(1 = 1.35
~ 30 FITs in 25 years ~ MTTF = 1.84 x 107 hours or A. (25 years) = 0.3% (accumulated failure rate)
Table 2 - Initial test results
Selection level D3 "TAT 8"
dlth (80 °Cf150 mN72 h) < 15 rnA dl/I, (3 mWf60 °Cf168 h) < 2%
Ith < 50 rnA (20°C) P max (60°C) > 5 mW
Selection level D5 (level 4) "EMOS"
dlth (80 °Cf150 mN72 h) < 5 rnA dl/I, (3 mWf60 °Cf336 h) < 2%
Ith < 50 rnA (20°C) P max (60°C) > 7 mW
It can be underlined that the first level of selection corresponding to the ageing test in mode 150 mAl 80 °Cn2 h can be used to position lasers nOimally in the ageing mode associated with the wear out mechanism (after a certain drift effect on the wafers associated with the saturation of the leakage channels). These initial results have already becn confirmed (see Figure 7): • During the laser qualification stage on 38 parts (from four wafers) • For laser quality assurance tests performed on 84 lasers from 21 wafers during commercial selection of TAT 8/Sardinia-Sicily • On the lasers in a lot of 48 emitter modules allocated to test 50 °C/3 m W with a predetermined D3- type laser selection level and totalling 12000 testing hours to date
These results can be compared with those of a similar D3-type laser population which proves that emitter module integration does not modify the distribution noted on the laser.
The selection procedure to be used for EMOS with a laser selection even more severe than a strict D3 and completed by an emitter module laser ageing of2500 hours at 40 °C (equivalent to 600 hours at 60°C) enables us to be more or less compatible with the equivalent selection reference D5 and to confirm the reliability figures allocated to the laser wear out mechanism.
Coupling evolution data The long-term tests on emitter modules have also had the purpose of determining changes in transmitter power in conditions close to those encountered with real usage (rear side adjustment) in order to predict their system lifetime. Determining a change law for the front side power on the basis of accelerated tests comes up against several difficulties: • A change is caused by several different factors that may develop independently or even compensate
Test Noot Laser Results parts tested selection level Median time to failure
at test temperature
60 °C/3 mW/20 000 h 58 05 1.0510'h (I = 1.35
Laser qualification 60 °C/3 mW/12 000 h
38 03 810' h (I =1.1
Laser quality assurance 60 °C/3 mW/5 000 h 42 03 (32 parts) 8.6 10' h (03 parts only)
80°C 1150 mA 15000 h 42 2.610'h (I = 1.8
Laser module qualification 48 03 2.810' h (I = 1.6 50°C 13 mW 112000 h (-10' h 60°C)
Figure 7 - Confirmed laser reliability data
themselves as follows: - Laser/fibre coupling (adaptation and stability of optical accesses) - Laser front side/rear side ratio - Coupling with rear side photodetector - Photodetector sensitivity The first of these is generally considered to be more important and crucial . • Front coupling changes are caused by microdisplacement (some hundredths of A) of the fibre in front of the transmission field of the laser .
. It may be noted that it is not easy to identify the exact Origin of the fibre displacement: creep and assembly stress relaxation are typical causes. This makes modelling all the more complex from the physical point of view. AnotJrr point is that the three dimensional nature of the problem and the shape of the transmission field of the laser show that there is no straightforward relation between the apparent displacement of the fibre with regard tho lhe laser and the reSUlting coupling variation. The same fibre displacement may lead to either a drop, an increase or even an appllrent stability in coupling depending on the initial position of the fibre. It shall be noted that technologicillly our emitter .corresponds experimentally to positive drifts which poses a problem when applying the system end ofHfe criterion normally associated with a 35% drop in the transmitter power. It should be mentioned that an arbitrary decision was made to fix the same end oflife cri terion of 35 % for both positive and negative drifts which is an unfavourable situation.
• The order of magnitude of the drifts observed remains low'" 5% for the test at 30°C that to date has accumulated more than 20000 hours. All these considerations lead us to the conclusion that a too complex. unusable model should be avoided. Analysing coupling changes over long durations (> 10' hours) and at temperatures between 30 and 50°C has made it possible to show the following: • That most parts demonstrated positive coupling changes • That the coupling drift can reasonably be modelled with a change law in relation to time tD where n median = 0.35 over the model validation period between 1000 and 20000 hours. This law provides good confirmation of the considerable sublinear effect expected for coupling changes. • That the activation energy for the lifetime (and not for related physical phenomena) is 1 e V between the two test temperatures. This model with t" is confirmed by the analysis of drifts during qualification on 48 emitter modules each having to date more than 12000 ageing hours at 50°C/3 m W (see Figure 8). In these observation conditions. the coupling changes related to a drift mechanism with a log normal distribution for defects on the criterion of a maximum of one drift of35% by extrapolation using the model, lead us to the following results: • Median (50°C) = 1.7 x lOs hours • a = 1.2 (max value)
Figure 8 " Typical coupling evolution during long IIfetest
I . No:CIT363555 Temb: 50 'C-Tboit: 50 'C-Puis: 3 mW-TIROIR: 17 -VOlE: 4 A % Pav/Par: .59 Initial ~/9 PENTE Ma 4 DUREE DE VIE A 35%' 11E+004H S~oIl.CllB.-T
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----vc=-r :=tr =±=::!--= - I- I--H=-+-=+-=-:-i11: ~~_ H-~-!--';'-!' ~tf1=-~-~-+----+--~:H-HH-+-HI-+-I-----~-I--_=_,t_-+I-++-++-HI-'-+i-H
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The selection principle chosen aims, for a subsequent operating (ageing) duration, for an individual guarantee for each part to satisfy the objective of 35% maximum drift over 25 years at the average repeater operating temperature. This principle will be described in section IV.
b) Random failure contribution The statistical analysis of random failures requires, to have a sufficient confidence level, tests of a large number of components over significant testing periods so that the maximum component hours is accumulated in the face of an objective that is all too often inaccessible.
In order to bypass this difficulty, the strategy to adapt is related to the concept of complete quality that has already been the subject of a publication [ref 2) and to which we shall return.
An estimation of the importance of random failures can, however, be made using an accumulated number of component hours during various tests and also using selection tests on all the commer­ cially manufactured equipment to date. At this stage we are in the process of selecting the emitters for EMOS and have accumulated a total for this type of component of 1.8 x 107 component hours at 20°C with zero faults (see Figure 9). This leads to a failure rate of 56 FITs.
Of course, this is not sufficient to prove failure rates as low as 10 FITs, for example. However, it is on the basis of such tests with equivalent volume that we have always successfully included new components in submerged repeaters in the past.
Ageing Device hours Acceleration Effective temperature factor* device hours
75°C 8 10' 43 3.4 106
65°C 6.4 10' 23 1.5 106
50°C 106 9.1 9.3 106
40°C 8 105 4.6 2.1 106
30°C 8 105 2.2 2 106
Figure 9 - Laser package device: hour summary
• Ea = 0.6 eV Total: 1.8 107 h.c: zero faults
[n any case, we consider that these results have to be completed by continuous long duration tests as we have already done with previous technology.
Therefore in order to increase our confidence level continuously with a view to eliminate specific random failures completely, we have intensified the process quality assurance, the precap inspec­ tions, the severity of the mechanical screening tests and the duration of ageing to 2500 hours/40 °C for EMOS selection.
3.3.1 Requirements The photodetectors must detect very faint optical pulses at a wavelength of 1.3 J.l.m and a bit rate of 280 Mbit/s and convert them into electrical pulses. The association of the photodetector and its amplifier constitutes the receiver which performs the optical-to-electrical conversion.
The main characteristics of the photodetector are designed to ensure the required level of perform­ ance for the receiver (see Figure 10).
3.3.2 Technological description The back illuminated planar PIN InGaAs/lnP structure coupled to a fibre pigtail (single-mode fibre) in a hermetic KDOI package, has been chosen. This technology has already been described in detail I ref 1].
Responsivity Rise and fall time Leakage current CapaCitance Breakdown voltage Forward voltage Noise voltage
Receiver failure rate
< 70 nA (6 V) < 0.6 pF (0.25 pF chip)
> 30 V < 0.65 V (1 mAl
< 500 nV-,J Hz (300 Hz - 10 V)
< 1 FIT
3.3.3 Consolidated photodlode reliability data Crystal The crystal characterization stage has not shown any wear out degradation mechanism (log normal distribution, for example) or any modelable drift so that there is no activation energy [ref 4). As the failure rate connected with the wear out mechanism has to be calculated from a description of failure distribution by a log normal law , in the absence of this type of failure we shall admit in the worst case that they are likely to be distributed according to such a law with a minimum median lifetime of 8000 testing hours at 200 °e (less than 50% at 8000 hours) and a standard deviation of 1.2, which is a reasonable value for a selected population.
With a pessimistic activation energy of 0.6 eV compared with the 1.2 eV announced by AT&T, for example, this leads to an instantaneous failure rate over 25 years of 0.01 FITs (median lifetime at 20 °e equal to 7 x 107 hours). The wear out mechanisms, therefore, have a negligible effect during the useful life of the component.
It has been possible to confirm these results by the final qualification programme on the detectors on submount carried out on 200 parts spread over several test groups. Reliability was calculated on the basis of results obtained during an HTRB test (high temperature reverse bias) covering 80 parts of which 40 had already been burnt in that revealed seven defects (of which six on non-selected parts).
It can be noted that the distribution of these seven defects cannot be satisfactorily interpreted using a log normal law that leads to the presence of aberrations and unrealistic values for the standard deviation (0 = 3.8). On the other hand, it can be described satisfactorily by a Weibulllaw whose parameters are as follows (with median ranks approximated):
"" = 2.5 x lOS hours "~= 0.6 The value of ~ indicates that we are going through a phase where the failure rate is decreasing with time (infant mortality period). By using an activation energy ofO.6 e V, the failure rate after selection should be 0.22% for a service life of 25 years at 20 °e, giving an average instantaneous failure rate of 11 FITs. Taking only the burnt in parts, this rate is reduced to about 3 FITs.
However, it will benotcd thatthe description of the phenomenon as given above is a pessimistic case since this description only considers serious defects such as short circuits that are eliminated during bum in or ageing. In fact parts of reduced reliability can be eliminated by selection process based on parametric criteria and distribution (parameters and drifts), especially on high Idnk and low frequency noise en parameters before and after bum in.
In order to describe crystal reliability completely, the random failures should be added to this failure rate connected with "infant mortality defects".
The total number of component hours at 200 °e is 6 x lOs hours without random failure. With an activation energy of 0.6 e V, this gives an instantaneous failure rate of 0.5 FITs at 90% confidence level.
Accumulating "infant mortality defects", wear out failures and random failures, the maximum failure rate over 25 years for selected parts is 3 FITs for crystal. The apparent efficiency of selection
and doubts concerning the nature of at least some defects that might be connected with electrostatic discharge, make it reasonable to suppose that this rate is lower than the one indicated, all the more as the activation energy value used corresponds to the worst case envisaged by AT&T (the most probable being 1.1 eV).
Fibred detector Tests on fibred single-mode photodiodes have shown that behaviour during selection is very satisfactory on the whole with average displacements ofless than 2 or 3 !!lIl (analysis made on parts deliberately fibred at the edge of the plateau).
The standard reference qualification tests: thermal shocks (- 40/+ 90°C - 100 cycles) have shown that they can cause displacements likely to attain + 10 !!lIl. The reverse bias tests at 80°C are likely to cause a displacement of less than 5 J.Lm with a phenomenon of stability after 500 hours of ageing.
This behaviour is a priori associated with microdisplacements of the same nature as that observed on the multimode fibred photodiodes but with a far greater dfect on single-mode parts insofar as this variation on a marginal part positioned at the limit with regard to the plateau causes a more sensitive variation of about 25% for 5 !!lIl.
'This has led to the fabrication process being optimized in order to guarantee that the fibre is centred on the active zone during fibre coupling operations. This gives important margins (> ± 20 J.Lm) for acceptable microdisplacements without losing sensitivity. This operation that forms part of the complete quality programme, constitutes the key to the success of our components; by taking the selection procedure to 1000 hours at 50°C for ageing of the detector with fibre coupling any residual risk of abnormal components should be detected.
Except for the mean time to failure of the lasers which can be determined, other failure rate objectives as defined in previous sections, will require an excessive amount of components to be reasonably proved. Fortunately, reliability requirements can be reached thanks to a reliability assurance programme based upon a general philosophy called compkte reliability which has been successfully implemented with submarine components over the last twenty years in analogue systems. The concept of complete reliability aims at minimizing drastically the risks of failure. The main principles which have provided the guidelines for our assurance strategy and programmes are as follows: • To manufacture all critical key components so as to keep all reliability aspects under tight control within the company (in particular optoelectronic devices and ICs) • To know all extreme operating conditions (electrical, optical, mechanical and environmental) so as to determine the required level of ruggedness for each device • To use the best technological process so as to reach an intrinsic lifetime (the lifetime which depends only on the norm:iI wear out mechanisms) compatible with the required failure rate • To set up monitoring procedures including all quality control methods to be applied throughout the manufacturing cycle so as to make certain that each step has been processed according to specified conditions • To apply selection procedures including all teSts and measurements so as to be sure that each lot reaches the expected robustn