IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 7, NO. 2, JUNE 2007 363

Theoretical and Experimental Thermal Analysis ofInP Ridge Lasers on Submounts and TO Packages

Jia-Sheng Huang, Rongsheng Miao, Hanh Lu, Member, IEEE, Charlie Wang,Chris Helms, and Hamdi Demirci

Abstract—We have determined the junction temperatures oflaser diodes mounted on ceramic submounts and hermetictransistor-outline (TO) packages using data from wavelength-shift measurement as well as finite-element modeling (FEM). Wefound that the difference in the junction temperature betweenchip/submount and chip/submount/TO measured at the free-standing room-temperature condition cannot be used as the ref-erence data for the design of accelerated life-test condition athot oven. For the free-standing condition, the heat spread fromthe active region of the chip to the chip substrate, submount,and TO. Hence, there was an additional thermal gradient in thechip/submount/TO arising from the thermal impedance of the TOheader. For the oven condition, the heat dissipation via forcedconvection from the outer surface of the TO was important. Hence,the thermal gradient of the TO resulting from heat conductionbecame smaller. Using the FEM, we showed that the simulatedjunction temperature was in close agreement with the experi-mental value. The heating difference between the chip/submountand chip/submount/TO was smaller at the oven condition than atthe free-standing condition.

Index Terms—Finite-element modelings (FEMs), semiconduc-tor devices, semiconductor lasers, simulation, thermoresistivity.

I. INTRODUCTION

THERMAL management is important for the design of theparametric and reliability performance of semiconductor

devices [1]–[4]. As device feature size shrinks, the joule heatingis expected to increase for a given bias current. For semicon-ductor lasers, the device heating arises from semiconductor andpackage. Typically, the package is a ceramic submount attachedto a butterfly dual-in-line module or transistor-outline (TO) can[2]. The optical package could generate a significant thermalgradient depending on the material, geometry, and experimentalsetup. Hence, the device junction temperature could be signif-icantly higher than the ambient temperature when the thermalgradient of the package is large.

One of the common methods to characterize the device junc-tion temperature is by wavelength shift. However, the wave-length shift is usually conducted at a free-standing condition,

Manuscript received June 13, 2006; revised April 17, 2007.J.-S. Huang, R. Miao, and H. Lu are with the Ortel Division, EMCORE

Corporation, Alhambra, CA 91803 USA (e-mail: jshuang@emcore.com).C. Wang was with the Fiber Optics Division, EMCORE Corporation,

Albuquerque, NM 87123 USA. He is now with EMCORE China, Beijing100088, China.

C. Helms and H. Demirci are with the Fiber Optics Division, EMCORECorporation, Albuquerque, NM 87123 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TDMR.2007.901637

and it may not be applicable to the life-test oven condition.In this paper, we study the device heating at free-standingand oven conditions. The junction temperatures of laser diodeson AlN submounts and on TO packages that were derivedfrom wavelength shift were compared with the life-test data.We show that the temperature rises based on wavelength es-timates at the free-standing condition are different from thoseat the oven condition. We explain the discrepancy using finite-element modeling (FEM) results involving heat conduction,natural convection, and forced convection.

II. EXPERIMENTAL

A. Sample Preparation

Distributed-feedback (DFB) InP laser diodes with lasingwavelengths at 1310 nm were used in this paper [5]–[9]. AnInGaAlAs active epitaxial layer consisting of grating and mul-tiquantum well structures was grown on n-type InP substrateusing metal–organic chemical vapor deposition (MOCVD).Subsequently, a p-type InP cladding layer and the InGaAscontact layer were grown. Finally, a ridge structure was formedby wet etch. Au–Ti–Pt–Au was chosen for the p-metalization.The remaining steps of the laser fabrication process, suchas cleaving and facet coating, were typical for edge-emittingdevices. The laser chips were mounted on ceramic submounts(blocks) or TO packages.

B. Wavelength-Shift Measurement and Life Test

For wavelength-shift measurement, chip/submount or chip/submount/TO was loaded on a test station that was maintainedat 25 ◦C. To establish the correlation of temperature rise ofcertain laser chip mount configuration and bias current, thelaser devices were subjected to the current ramp ranging from30 to 160 mA.

For life-test aging, lasers were loaded in the oven and sub-jected to a constant stress current of 160 mA during life test.For the chip/submount, the samples were loaded in a carrierand probed by an array of pogo pins. Oven temperatures at105 ◦C and 120 ◦C were used. For the chip/submount/TO, theexternal leads were loaded into the sockets of the test boards.Oven temperature at 95 ◦C was used for the TOs.

When applying a bias current to the laser diode, the heat isproduced at the semiconductor junction and flows to the back-side of the diode to the mounting surface [1]. Fig. 1(a) and (b)illustrates the schematics of a laser diode mounted on a ceramic

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364 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 7, NO. 2, JUNE 2007

Fig. 1. Schematics of (a) laser diode on a ceramic submount and (b) laser diode on a hermetic TO package at the free-standing condition. The thermal path ofeach component is indicated.

submount and on a hermetic TO package, respectively. Thesemiconductor junction is close to the top surface of the laserdiode, and the temperature of the junction is denoted as TJ .The bottom of the diode is cooler due to the thermal dissipationassisted by the package, and the temperature of the mountinginterface between the laser diode and submount is denoted asTM . For the AlN submount, the temperature of the bottomof the submount is close to the ambient temperature TA. Forthe TO package, there is an additional thermal gradient in theTO header base. Hence, the temperatures of the submount/TOinterface and the bottom of TO header are denoted as TC andTA, respectively.

C. FEM

Due to the difference in the feature size and geometry, wesimulated the laser diode and the package separately usingAlgor simulation software [10]. There were two conditions thatwere studied in the simulation: 1) free-standing condition and2) life-test oven condition.1) Free-Standing Room-Temperature Condition: The free-

standing condition simulated the condition at which thewavelength-shift measurement was performed. The measure-ment was conducted outside the oven and, thus, exposed to theair at room temperature. For the chip/submount, there were twothermal resistances: 1) the laser diode thermal resistance θJM

and 2) the AlN submount thermal resistance θMA [Fig. 1(a)].The bottom surface of the submount block was set to con-duction at 25 ◦C. All other surfaces of the block were set tonatural convection. For the chip/submount/TO, there were threethermal resistances: 1) the laser diode thermal resistance θJM;2) the AlN submount thermal resistance θMC; and 3) the TOheader base thermal resistance θCA [Fig. 1(b)]. The boundaryconditions were the same except that the external leads of theTO were set to conduction at 25 ◦C. The thermal conductivitiesof the thermal components that were used as the simulationinput parameters are summarized in Table I.

For the laser diode, the heat spreads from the active region tothe substrate, as shown in Fig. 2. The forward voltage was about

TABLE ITHERMAL CONDUCTIVITIES OF THE THERMAL

COMPONENTS USED IN THE FEM

Fig. 2. Schematics of the laser diode structure. The heat transfer is from theactive region to the substrate.

1.9 V for a bias current of 160 mA, leading to a heat generationof 304 mW.2) Life-Test Oven Condition: The life-test oven condition

simulated the condition at which the life test was performed.For the chip/submount, the bottom surface of the block wasset to conduction at 105 ◦C to mimic our experimental life-testcondition. All other surfaces of the submount block were setto forced convection. For the chip/submount/TO, the boundaryconditions were the same except that the external leads of theTO were set to conduction at 95 ◦C to mimic our experimentallife-test condition.

HUANG et al.: THERMAL ANALYSIS OF InP RIDGE LASERS ON SUBMOUNTS AND TO PACKAGES 365

Fig. 3. (a) Measured lasing wavelength and (b) derived temperature rise as afunction of bias current for chip/submount and chip/submount/TO at the free-standing condition.

The thermal gradient of the laser diode at the oven conditionwas the same as that at the free-standing condition. The heatgeneration of the laser stripe was 304 mW for a bias current of160 mA.

III. RESULTS AND DISCUSSION

Fig. 3(a) shows the measured lasing wavelength as a functionof bias current for chip/submount and chip/submount/TO atthe free-standing room-temperature condition. The solid andopen circles represented the experimental wavelength shifts ofsubmount and TO, respectively. The lines were the regressionlines fitting to the experimental data. The diode was a DFBlaser grown and processed based on InP material. The biascurrent ranged from 30 to 160 mA. From the temperatureramp experiment ranging from 25 ◦C to 95 ◦C, the rate ofwavelength shift was determined to be 0.09 nm/◦C. Once thewavelength-shift rate was determined, we were able to derivethe temperature rise versus bias current plots, as shown inFig. 3(b).

The data suggested that there was significant heating at highbias current in both chip/submount and chip/submount/TO. Inaddition, the temperature rise in the TO package was apprecia-bly higher than that in the submount. For example, at a biascurrent of 160 mA, the temperature rises in the submount and

Fig. 4. Life-test data comparison of (a) 120 ◦C submount versus 95 ◦CTO package and (b) 105 ◦C versus 95 ◦C TO package. The stress currentwas 160 mA.

TO package were estimated to be 30 ◦C and 55 ◦C, respectively.The difference in temperature rise between the chip/submountand chip/submount/TO was 25 ◦C.

Comparative life-test studies of laser diodes mounted onsubmount and TO packages were also performed. Threshold-current shift was used as an indirect measure of laser junctiontemperature. Since threshold-current shift followed Arrheniusequation [6], the laser junction temperature of the two packageswas the same when the threshold-current shift was equal.Fig. 4(a) shows the change in threshold current Ith based on120 ◦C submount and 95 ◦C TO package, where stress currentwas 160 mA for both cases. The oven temperatures were origi-nally chosen based on the wavelength-shift data, where temper-ature rises of 30 ◦C and 55 ◦C were predicted for chip/submountand chip/submount/TO, respectively. However, the data showedthat Ith degraded faster in the 120 ◦C submount than in the95 ◦C TO package, suggesting that the junction temperatureof the laser on the submount was higher than that on the TOpackage. Fig. 4(b) shows the change in threshold current Ith

based on 105 ◦C submount and 95 ◦C TO package, where stresscurrent was 160 mA for both cases. The Ith degradation wassimilar for the two packages, suggesting that the laser junction

366 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 7, NO. 2, JUNE 2007

Fig. 5. FEM results of the thermal gradient at the laser diode. The bias currentwas 160 mA.

temperature was the same when the ambient temperature offsetwas 10 ◦C.

In the following, we attempt to explain the discrepancybetween wavelength-shift data and life-test data. For thewavelength-shift measurement setup, both chip/submount andchip/submount/TO were exposed to the air at room temperature.For the chip/submount, there were two thermal resistances:1) the laser diode thermal resistance θJM and 2) the AlNsubmount thermal resistance θMA [Fig. 1(a)]. For the chip/submount/TO, there were three thermal resistances: 1) thelaser diode thermal resistance θJM; 2) the AlN submount ther-mal resistance θMC; and 3) the TO header base thermal resis-tance θCA [Fig. 1(a)]. The heat transfer occurred predominantlyvia heat conduction. The heat transfer via natural convectionin both chip/submount and chip/submount/TO was negligible.Fig. 5 shows the simulated temperature contour of the laserdiode at a bias current of 160 mA. As a reference point, thebottom surface of the laser diode was set to conduction at25 ◦C. As shown by the contour plot, the maximum temperatureis 55.2 ◦C, showing that the temperature gradient across thelaser diode was 30.2 ◦C. Fig. 6(a) and (b) shows the simulatedtemperature contours of the submount and TO package at25 ◦C, respectively. The bottom surface of the submount blockor the TO leads was set to 25 ◦C as a reference temperature.The temperature of the maximum scale showed the temperatureat the mounting interface between the laser diode and thesubmount. The mounting interface temperatures TM for theblock and TO were 28.8 ◦C and 52.8 ◦C, respectively. It wassuggested that the temperature rises of the block and TO were3.8 ◦C and 27.8 ◦C, respectively. The FEM data also indicatedthat at a bias current of 160 mA, the heating difference betweenthe chip-on-board and TO was 24.0 ◦C. Table II summarizesthe simulated temperature rises of the laser diode and thepackage and the experimental values based on the free-standingcondition with a bias current of 160 mA. The FEM results werein close agreement with the wavelength-shift data.

For the life-test experimental setup, the heat transfer occurredvia two parallel modes: 1) heat conduction and 2) forcedconvection. For the chip/submount, the surface area of thesubmount was significantly smaller, so the heat dissipation viaforced convection was negligible. Hence, the thermal gradient

Fig. 6. FEM results of the thermal gradients at (a) chip/submount at 25 ◦C and(b) chip/submount/TO at 25 ◦C for the case where the samples were outside theoven and, thus, exposed to the air. The bias current was 160 mA.

TABLE IIFEM-SIMULATED TEMPERATURE RISES OF LASER DIODE AND PACKAGE

AT THE FREE-STANDING ROOM-TEMPERATURE CONDITION, WHERE

THE BIAS CURRENT WAS 160 mA. EXPERIMENTAL VALUES

WERE SHOWN FOR COMPARISON

of package in the chip/submount was expected to be similar forfree-standing and oven conditions. For the chip/submount/TO,the heat dissipation via forced convection was large due to thelarge total outer surface of the TO. Hence, there was a smallerthermal gradient via heat conduction.

Fig. 7(a) and (b) shows the simulated temperature contoursof the submount and TO package at the life-test oven condition,respectively. The reference temperatures were based on theoven temperatures of 105 ◦C and 95 ◦C that were used in thelife test. The mounting interface temperatures of the block and

HUANG et al.: THERMAL ANALYSIS OF InP RIDGE LASERS ON SUBMOUNTS AND TO PACKAGES 367

Fig. 7. FEM results of the thermal gradients at (a) chip/submount at the oventemperature of 105 ◦C and (b) chip/submount/TO at the oven temperature of95 ◦C for the case where the samples were inside the oven. The bias currentwas 160 mA.

TABLE IIIFEM-SIMULATED TEMPERATURE RISES OF LASER DIODE AND PACKAGE

AT THE OVEN CONDITION, WHERE THE BIAS CURRENT WAS 160 mA.EXPERIMENTAL VALUES WERE SHOWN FOR COMPARISON

TO were 108.7 ◦C and 108.2 ◦C, respectively. It was suggestedthat the temperature rises of the block and TO were 3.7 ◦C and13.2 ◦C, respectively. At life-test condition, the heating differ-ence between the chip/submount and chip/submount/TO wasabout 9.5 ◦C. Table III summarizes the simulated temperaturerises of the laser diode and the package and the experimentalvalues based on the oven condition with a bias current of160 mA. The FEM results were again in close agreement withthe wavelength-shift data.

It was noted that the simulated junction temperatures basedon the FEM were generally higher than the measured val-ues. The differences between the simulation and experimentalresults were about 11% and 4% for the chip/submount andchip/submount/TO, respectively. Such variations could be at-tributed to the uncertainty in wavelength and voltage measure-ment and the selection of mesh size in the FEM solid model.In addition, we did not consider the laser output power in oursimulation that would lead to an overestimate of the net heating.

Our experimental and simulation results are applicable toother laser structures and packages. When applying the modelto other structures, it may be needed to modify the thermalconductivities of the components and the boundary condition.For the latter, the thermal transport mechanism depends on theambient and experimental setup.

IV. CONCLUSION

We have investigated the junction temperatures of InP DFBlaser diodes mounted on submounts and TO packages usingwavelength shift and FEM. Based on the wavelength-shift data,we showed that there was significant heating at high bias currentat both chip/submount and chip/submount/TO. The heatingwas dramatically higher at the TO than at the submount dueto the heat impedance of the TO. The heat transfer occurredpredominantly via heat conduction since natural convectionwas negligible. At the free-standing condition, the heatingdifference was about 25.0 ◦C at a bias current of 160 mA.We also showed that the temperature rise strongly dependedon the experimental testing setup. In our life-test configuration,the heat transfer via forced convection was important. The heatdissipation via forced convection from the outer surface of theTO was large due to its large surface area. Therefore, therewere two parallel heat transfer modes, resulting in a smallerthermal gradient across the TO from heat conduction. At theoven condition at around 100 ◦C, both life test and FEM showeda 10 ◦C heating difference between the chip/submount andchip/submount/TO at a bias current of 160 mA.

ACKNOWLEDGMENT

The authors would like to thank H. Hou for the review of thispaper and J. Krogen for the MOCVD growth.

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368 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 7, NO. 2, JUNE 2007

[6] J. S. Huang, “Temperature and current dependences of reliability degrada-tion of buried heterostructure semiconductor lasers,” IEEE Trans. DeviceMater. Rel., vol. 5, no. 1, pp. 150–154, Mar. 2005.

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[10] Algor Professional Solution Manual, Jan. 2004. Version 1.00.

Jia-Sheng Huang received the B.S. degree inphysics from National Taiwan University, Taipei,Taiwan, R.O.C., in 1992 and the M.S. and Ph.D.degrees in materials science from the University ofCalifornia, Los Angles (UCLA), in 1996 and 1997,respectively.

During 1992–1993, he was a Research Assistantin the Institute of Atomic and Molecular Sciences,Academia Sinica, Taipei, where he studied surfacephysics of gallium ion beam in ultrahigh vacuum.From 1997 to 2000, he was a member of Technical

Staff at Lucent Technologies, Bell Laboratories, Orlando, FL, where he workedon electromigration, stress migration, and failure analysis of 0.3-, 0.25-, 0.2-,and 0.16-µm ASIC and FPGA devices using CMOS technology. In 2000,he joined Lucent Optical Access Division (Ortel), Alhambra, CA, workingon process and failure analysis of 1310- and 1550-nm analog and digitalInP lasers. He is currently a Technical Manager of Laser R&D in the OrtelDivision, EMCORE Corporation, Alhambra, CA, where he works on devicedesign/process/characterization, reliability, electrostatic discharge, and failureanalysis of analog and digital Fabry–Pérot and distributed-feedback laserdiodes and modules for cooled and uncooled applications. He is the authoror coauthor of more than 50 papers published in international journals andconference proceedings. He is the holder of two U.S. patents.

Dr. Huang serves as a Technical Committee Member in the CompoundSemiconductor Manufacturing Expo Conference, Bristol, U.K. He is an activereviewer for the IEEE ELECTRON DEVICE LETTERS, IEEE TRANSACTIONS

ON ELECTRON DEVICES, Microelectronics Reliability, Thin Solid Films, Ap-plied Surface Science, and Optics & Laser Technology. He is also listed in the2006–2008 publications of Marquis Who’s Who in America, Who’s Who inthe World, and Who’s Who in Science and Engineering. He was a recipient ofthe Outstanding Ph.D. Award and the Dissertation Year Fellowship Awardfrom the Henry Samueli School of Engineering, UCLA.

Rongsheng Miao received the D.Eng. degree fromTianjin University, Tianjin, China, in 1990, for hiswork on the fundamental mechanism of the interfa-cial heat and mass transfer between various industrialmaterials using laser techniques, i.e., the holographicinterferometer and laser Doppler anemometer, andthe Ph.D. degree in mechanical engineering from theUniversity of Illinois, Chicago, in 1996.

He is a Senior Principal Design Engineer in theOrtel Division, EMCORE Corporation, Alhambra,CA, where he works on the packaging design and

thermal analysis of lasers and photodiode receivers. He previously worked forseveral other optoelectronic companies as a Packaging/Mechanical Engineerin the areas of acoustic optic modulators, photodiode detectors, lasers, andelectronic systems. His primary interests for product development span a rangeof areas, including mechanical, material, and thermal management and analysisof optoelectronics packaging at both the chip level and the component level.

Hanh Lu (M’88) was born in Vietnam. He received the Ph.D. degree in phys-ical electronics engineering from the Tokyo Institute of Technology, Tokyo,Japan, in 1978.

Over the last 30 years, he has held various positions in telecom and CATVindustries in North America, such as in Corning-Lasertron and BNR-Nortel,in managing, designing, and fabricating optoelectronic devices, semiconductorlasers, DFB lasers, EMLs, LEDs, amplifiers, modulators, detectors, and inte-grated devices. He is currently with the Ortel Division, EMCORE Corporation,Alhambra, CA.

Dr. Lu is a member of the IEEE Lasers and Electro-Optics Society.

Charlie Wang received the Ph.D. degree from the University of New Mexico,Albuquerque, in 1996.

He joined EMCORE Corporation, Albuquerque, in 1998, where he served asthe Director of Research and Development in the Fiber Optics Division. He hasbeen appointed as the General Manager of EMCORE China, Beijing, China.His area of interests includes optoelectronics device packaging and testing,high-speed fiber-optics communication, physics of semiconductor laser and PDdevices, and reliability of optoelectronic devices.

Chris Helms received the M.S.E.E. degree fromArizona State University, Tempe, in 1989.

During 1990–1998, he was a Member of Tech-nical Staff at Sandia National Laboratories, wherehe produced SPICE models and conducted opto-electronic characterization and life tests. He was anIntern at Sanyo Electric Company Research Center,Hirakata, Japan, in 1998–1999, where he producedSPICE models for OLEDs. Since 2000, he has beena Reliability Engineer at EMCORE Corporation,Albuquerque, NM, where he conducts life tests and

produces reliability reports for VCSELs, DFBs, and PDs.

Hamdi Demirci received the B.S. degree in mechanical engineering fromBosphorus University, Istanbul, Turkey, and the M.S. and Ph.D. degrees inmechanical engineering from Lehigh University, Bethlehem, PA.

After receiving his Ph.D. degree, he joined AMP, Inc., Harrisburg, PA, as anR&D Engineer, where he specialized in injection molding processes. He joinedEMCORE Corporation, Albuquerque, NM, in 2001 as a Process Engineer.He has been a Reliability Manager in the Fiber Optics Division, EMCORECorporation, since 2003.