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Temperature and Current Dependences of Reliability Degradation of Buried Heterostructure Semiconductor Lasers Jia-Sheng Huang
Abstract—Failure times of semiconductor lasers are usually lengthy under normal aging conditions. Determination of failure times typically involves extrapolation using a sublinear or linear model. It becomes increasingly difficult to experimentally determine activation energy and current exponent since data based on lower temperatures and lower stress currents are required. In this paper, the temperature and current dependences of 1310-nm buried heterostructure (BH) InP lasers were studied. We show that the activation energy of 1310-nm BH lasers based on life test data at 70 C–100 C is higher than the value of 0.4 eV suggested by Telcordia. The activation energies estimated by sublinear and linear models were 0.87 and 0.55 eV, respectively. We also show that the current exponents are 1.4 and 1.0, respectively, for sublinear and linear models. We discuss the implications of the reliability results in field reliability predictions. Index Terms—Activation energy, buried heterostructure, current exponent, life test, reliability extrapolation, semiconductor laser.
I. INTRODUCTION
D
ESPITE nearly 40 years of field service, the reliability of photonics devices is still an unresolved issue for the fiber optic data communications industry. Since the invention of the semiconductor laser in 1962, photonics devices have evolved into indispensable components in virtually every data communication market including long-haul terrestrial, submarine, and metro applications [1]. As public acceptance and use of optical data communications increases, end users have come to expect a very high level of network availability. This expectation translates into equally stringent requirements for the long-term reliability of individual photonics devices. For example, at present the typical reliability requirement for optical transmission sysdevice hours) or a median tems is 100 FIT (100 failures in time to failure (MTTF) in excess of 10 000 000 h under full rated power. Due to the time required to gather lifetime data from operational devices, we must rely instead on laboratory data for qualification of new devices and technologies. Laboratory reliability data is usually obtained from accelerated life tests where bias currents and junctions temperatures in excess of those expected during normal operation are applied to shorten times to device failure. Reliability assessment based on such high stress tests requires extrapolation back to
Manuscript received April 30, 2004; revised October 14, 2004. The author is with Ortel, Division of Emcore, Alhambra, CA 91803 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/TDMR.2005.843834
normal operating conditions using the modified electromigration stress acceleration model that includes a current density exponent and Arrhenius temperature acceleration factors [2]. Although field data provide the best direct confirmation of product reliability, it usually takes many years to compile statistically meaningful field data in order to establish the correlation. Longterm mildly accelerated life tests have been attempted as an interim reliability evaluation means [3]. For example, Nortel evaluated 980-nm pump lasers from 1990 through the end of 1998. However, such mildly accelerated life tests may pose problems such as noise resulting from tester instabilities or relocation. Such prolonged tests also appear impractical for smaller companies due to high costs and dynamic employee turnover. Accuracy in reliability extrapolation from accelerated life tests is critical. Overly pessimistic reliability estimates reduce the margin for laser chip design in device speed and/or bias current. On the other hand, overly optimistic reliability predictions may lead to early field failure resulting in high customer returns and erosion of confidence in the technology. Knowledge of the actual activation energy and current acceleration factors is important for accurate reliability assessment. Many companies, however, defer to the values suggested by Telcordia [4] for reliability extrapolation since experimental values are difficult to obtain. The difficulty lies mainly in the length of time required for life tests and the lack of actual failures for intrinsically long-lived devices. Ridge lasers, for example, typically do not show failure before 5000–10 000 h of aging under accelerated conditions. The testing time is much longer compared to Si integrated circuit (IC) devices where actual failures can often be obtained within 10–1000 h. In this paper, we report the temperature and current dependencies of reliability degradation in BH lasers. We show that the activation energy in these devices is higher than the Telcordia suggested value. The fitting methodology, current acceleration factor, and reliability implications will also be discussed. II. EXPERIMENTAL Fig. 1 shows the schematic cross section of the laser device. The process methodology used to fabricate the buried heterostructure InGaAsP/InP lasers with lasing wavelengths at 1310-nm to generate the data presented in this paper [5] is as follows. An active epitaxial layer consisting of multiquantum well structures and grating layers was grown on Sn-doped n-type InP substrate using metal–organic chemical vapor deposition (MOCVD). The composition of the active region was
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Fig. 1. Schematic cross section of the laser device.
InGaAsP. The active region was then wet-etched to form a mesa structure. Subsequently, p-InP and n-InP were grown to form current blocking layers. After removing a mask layer, p-InP and p-InGaAs layers were grown to form the BH structure. Au/Zn/Au/Cr/Au was chosen for the p-metallization. The remaining steps of the laser fabrication process such as cleaving and facet coating are typical for edge emitting devices. The resulting laser chips were life tested under constant current and constant temperature. Ambient temperatures of 100 C, 85 C, and 70 C were used to determine the activation energy of the dominant failure mechanism. Bias currents ranging from 90 to 150 mA were used to determine the current density exponent. Room temperature (25 C) light versus current (LI) curves were taken at every 100 h interval during life test aging. Threshold current (Ith) was derived from the LI curve. Since initial threshold currents were below 20 mA, the failure criterion for life test was defined as 50% increase in the initial threshold current [4]. III. RESULTS Device failure times usually can be described by the following equation [2]: (1) where MTTF is the time for 50% cumulative failure, is a conis stant, is the current density, is the current exponent, the activation energy, and is the junction temperature. Depending on the design of the tested devices, the junction temperature may be approximated by the ambient stress temperature. Theoretically, two temperatures and two currents can determine the activation energy and the current exponent as shown in (2). In practice, estimates based on three temperatures and three currents are more reliable since the confidence level is higher. One has to ensure that no new failure mechanism is introduced at the elevated temperature or the increased current density.
(2)
The activation energy and current exponent are difficult to determine experimentally since data based on lower stress
Fig. 2. Relative threshold current change as a function of aging time. The laser diode was stressed with a current of 150 mA at 85 C. Experimental data (shown by solid square) and fitting results (shown by line) are compared. Sublinear model (m = 0:6) provides a better fit than linear model (m = 1).
temperatures and currents require more accumulation time to reach failure. In order to shorten the time for the analysis and to improve the signal-to-noise ratio, the Au/Zn/Au metallization system was chosen for this study. The Au/Zn/Au was shown to exhibit faster laser degradation compared to the Ti/Pt/Au system [6]. It was suggested that the degradation was related to defect formation resulting from migration of Au from the p-contact [7]. We showed previously that the alloy spike primarily consisted of Au-rich compounds [8], [9]. In order to complete the study of the temperature and current matrix in a reasonable time frame, chips from each group were stressed for 1000 h. Fig. 2 shows one example of the life test curve where the chip was stressed with a current of 150 mA at 85 C. The life test curve was fitted and extrapolated to determine the failure time since there was no failure during the aging period. Several models have been developed to describe the degradation of threshold current over aging time. The Sim model [10] was based on a polynomial function while the Chuang [11] and Lam [12] models were based on exponential functions. The models based on exponential functions described the early stage of degradation well. The exponential term was derived from the rate equation assuming that the degradation rate was proportional to the defect density. On the other hand, the polynomial expression described the late stage of degradation well. For the sake of simplicity and practicality, we used the polynomial expression recommended by Telcordia for the analysis in the paper. Sublinear and linear models based on polynomial function were used for the curve fitting. In the sublinear model, the experimental curve was where is the normalized fitted by the equation threshold current change, is a constant, is the aging time is an exponent. The exponent was varied until the and best correlation was found. The constant was then used to determine the failure time. In this case, was 0.60, and was 0.002 059, yielding a failure time of 9449.7 h. In the was equal to 1. The linear linear model, the exponent model yielded a more conservative failure time: the predicted value was 3382.1 h. The sublinear model provided a better fit to the experimental life test curve than the linear model. The correlation factors for sublinear and linear models were 0.995 and 0.895, respectively.
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Fig. 3. MTTF as a function of stress temperature where the sublinear model was used for the estimate.
Fig. 4. MTTF as a function of stress current where the sublinear model was used for the estimate.
For the sublinear model, the fitting results based on longer aging data are similar to those based on the 1000-h data. We have shown in a separate paper that extrapolation based on 1000-h experimental data is consistent with that based on 5000-h data [13]. For the linear model, the fitting results are strongly dependent upon the accumulation time. For consistency, the analyses presented in the following are all based on the 1000-h aging data. A. Sublinear Model Fig. 3 shows the MTTF versus stress temperature where results were obtained by sublinear fits to the aging data. Chips from two different wafers were stressed at three different temperatures. The MTTF for each temperature was determined based on a population of 8–16. Since some variation between wafers was expected, each wafer was analyzed separately. Based on the plot, the activation energies for wafer A and wafer B were 0.88 and 0.86 eV, respectively. These experimental values were significantly higher than the 0.4 eV suggested by Telcordia for wear-out failure. To determine the current acceleration factor, chips from two wafers were stressed with three different currents. Fig. 4 shows the MTTF versus stress current where results were obtained by sublinear fits to the aging data. The MTTF for each stress current was based on a sample size of 8–16. Again wafers were segregated for the analysis. Since all the chips had the same physical size, current instead of current density was plotted. The
Fig. 5. MTTF as a function of stress temperature where the linear model was used for the estimate.
Fig. 6. MTTF as a function of stress current where the linear model was used for the estimate.
current exponents for wafer A and wafer B were 1.5 and 1.3, respectively. B. Linear Model For the worst case situation, failure times based on linear models were also determined. Fig. 5 shows the MTTF versus stress temperature where results were obtained by linear fits to the 1000-h aging data. The activation energies for wafer A and wafer B in the linear model were 0.52 and 0.58 eV, respectively. Fig. 6 shows the MTTF versus stress current where results were obtained by linear fits to the 1000-h aging data. The current exponents for wafer A and wafer B were both 1.0. IV. DISCUSSIONS The higher activation energies in our BH lasers imply that the field failure times at normal operating conditions would likely be superior to those predicted based on the Telcordia value. The higher activation energy leads to a larger temperature extrapolation factor from stress condition to use condition. This would allow laser chip designers to more aggressively design the maximum rated power and/or device speed. We note that life tests at 70 C and below are recommended by Telcordia to estimate the activation energy. Stress temperatures of 70 C and above were chosen in this study to enhance the measurement confidence and to shorten the stress time. The value of the activation energy derived from the life test at 70 C–100 C is assumed to be extendable to a lower temperature regime.
HUANG: TEMPERATURE AND CURRENT DEPENDENCES OF RELIABILITY DEGRADATION OF BH SEMICONDUCTOR LASERS
The current acceleration factor depends upon the ratio of stress current and operating current, and it would provide further reliability performance margin. Normally lasers are operated over a range of operating current; hence a precise estimate of current acceleration is difficult. One conservative way is to use the maximum rated operating current in the calculation. The derived results are more pessimistic, but ensure a greater reliability margin. The other aspect of bias current is Joule heating, which affects the junction temperature. The junction temperature is critical in the device design since it determines the device lifetime. Knowledge of junction temperature is also necessary for specification of device parametric performance. However, it is not as readily measurable as the ambient temperature. The junction temperature is higher or equivalent to the ambient temperature, depending on the Joule heating. Typical ways of characterizing junction temperature include electrical and optical methods [14]. There are some uncertainties in activation energy and current exponent, arising from uncertainties in MTTF. The sources of the variation include accumulated aging time, sample size and fitting parameter. The confidence level increases with increasing aging time and sample size. One needs to compromise between economy and accuracy when deciding the optimized experimental design. Care needs to be taken when choosing the matrix of stress conditions to determine the activation energy and current exponent. The optimized stress conditions depend upon the nature of lasers. The laser degradation rate is sensitive to processes such as mesa etch, p-metallization and MOCVD overgrowth [15]. Overstress may induce new failure mechanisms that are irrelevant to the primary failure mechanism of interest. Careful interpretation of reliability data is necessary for the high stress regime. On the other hand, when the stress condition is too mild, the long aging time may become impractical and additionally the noise may become significant compared to the signal. Unlike IC devices where dedicated test structures are used for accelerated test [16]–[22], optical devices are usually stressed at the device level. As a consequence, the experimentally determined activation energy is a convoluted result of all process parameters. Variation in any single parameter such as p-metallization is likely to alter the activation energy. The complexity associated with the ensemble system also makes it difficult to correlate the measured activation energy with a specific physical failure mechanism. V. CONCLUSION We have studied the temperature and current dependencies of BH InP lasers. We have found that the activation energy determined by the life test data at 70 C–100 C was significantly higher than the 0.4-eV value suggested by Telcordia. The activation energies estimated by sublinear and linear models were 0.87 and 0.55 eV, respectively. The higher activation energy suggested that the extrapolated device lifetime at use conditions is much longer than expected. The field reliability performance was likely to be superior to that predicted based on the activation energy of 0.4 eV. We also found that the current exponents
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were 1.4 and 1.0 for sublinear and linear models, respectively. This would provide further extrapolation margin compared to the case where no current acceleration was considered. ACKNOWLEDGMENT The author thanks I. Aeby for technical comments, J. Krogen for support of the work, C. Helms, C. Tsai, and T. Schrans for helpful discussions, and M. Geva and P. C. Chen for review of the paper. REFERENCES [1] R. V. Steele, “Review and forecast of the laser markets, part II: diode lasers,” Laser Focus World, vol. 39, no. 2, pp. 63–76, Feb. 2003. [2] J. R. Black, “Electromigration failure modes in aluminum metallization for semiconductor devices,” Proc. IEEE, vol. 57, no. 9, pp. 1587–1594, Sep. 1969. [3] H. Pfeiffer, S. Arlt, M. Jacob, C. S. Harder, I. D. Jung, F. Wilson, T. Oldroyd, and T. Hext, “Reliability of 980 nm pump lasers for submarine, long-haul terrestrial, and low cost metro applications,” in Tech. Dig. OFC 2002, paper no. ThN4. [4] “Generic reliability assurance requirements for optoelectronic devices used in telecommunications equipment,” Bellcore, GR-468-CORE, 1998. [5] T. R. Chen, P. C. Chen, J. Ungar, and N. Bar-Chaim, “High power operation of multiquantum well DFB lasers at 1.3 m,” Electron. Lett., vol. 31, p. 1344, 1995. [6] M. Fukuda, “Laser and LED reliability update,” J. Lightw. Technol., vol. 6, no. 10, pp. 1488–1495, Oct. 1988. [7] A. K. Chin, C. L. Zipfel, M. Geva, I. Gamlibel, D. Skeath, and B. K. Chin, “Direct evidence for the role of gold migration in the formation of dark-spot defects in 1.3 m InP/InGaAsP light emitting diodes,” Appl. Phys. Lett., vol. 45, no. 1, pp. 37–39, 1984. [8] J. S. Huang and C. B. Vartuli, “Scanning transmission electron microscopy study of Au/Zn/Au/Cr/Au and Au/Ti/Pt/Au/Cr/Au contacts to p-type InGaAs/InP,” J. Appl. Phys., vol. 93, pp. 5196–5200, 2003. [9] J. S. Huang and C. B. Vartuli, “The effect of Cr barrier on interfacial reaction of Au/Zn/Au/Cr/Au contacts to p-type InGaAs/InP,” Thin Solid Films, vol. 446, no. 1, pp. 132–137, 2004. [10] S. P. Sim, “A review of the reliability of III-V opto-electronic components,” in Proc. Semiconductor Device Reliability: Advanced Workshop II, NATO International Scientific Exchange Program, Crete, Greece, Jun. 1989, p. 301. [11] S. K. K. Lam, R. E. Mallard, and D. T. Cassidy, “Analytical model for saturable aging in semiconductor lasers,” J. Appl. Phys., vol. 94, no. 3, pp. 1803–1809, 2003. [12] S. L. Chuang, N. Nakayama, A. Ishibashi, S. Taniguchi, and K. Nakano, “Degradation of II-VI blue-green semiconductor lasers,” IEEE J. Quantum Electron., vol. 34, no. 5, pp. 851–857, May 1998. [13] J. S. Huang, “Reliability extrapolation methodology for determination of activation energy of buried heterostructure semiconductor lasers,” in Proc. Compound Semiconductor Manufacturing Expo, Monterey, CA, Oct. 2004, pp. 52–54. [14] B. Siegal, “Measurements of junction temperature confirms package thermal design,” Laser Focus World, vol. 39, no. 11, pp. S-12–S-14, 2003. [15] M. Fukuda, Optical Semiconductor Devices. New York: Wiley, 1999, ch. 7. [16] P. S. Ho and T. Kwok, “Electromigration in metals,” Rep. Prog. Phys., vol. 52, pp. 301–348, 1989. [17] A. S. Oates, F. Nlkansah, and S. Chittipeddi, “Electromigration-induced drift failure of via contacts in multilevel metallization,” J. Appl. Phys., vol. 72, no. 6, pp. 2227–2231, 1992. [18] J. S. Huang, A. S. Oates, Y. S. Obeng, and W. L. Brown, “Asymmetrical critical current density and its influence on electromigration of two-level W-plug interconnection,” J. Electrochem. Soc., vol. 147, no. 10, pp. 3840–3844, 2000. [19] J. Proost, K. Maex, and L. Delaey, “Electromigration-induced drift in damascene and plasma-etched Al(Cu). II. Mass transport mechanisms in bamboo interconnects,” J. Appl. Phys., vol. 87, pp. 99–109, 2000.
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[20] J. C. Lee, I. C. Chen, and C. Hu, “Modeling and characterization of gate oxide reliability,” IEEE Trans. Electron Devices, vol. 35, no. 12, pp. 2268–2278, Dec. 1988. [21] M. A. Alam, R. K. Smith, B. E. Weir, and P. J. Silverman, “Thin dielectric films: uncorrelated breakdown of integrated circuits,” Nature, vol. 420, p. 378, 2002. [22] K. P. Cheung and C. P. Chang, “Plasma-charging damage: a physical model,” J. Appl. Phys., vol. 75, pp. 4415–4426, 1994.
Jia Sheng Huang received the B.S. degree in physics from National Taiwan University, Taiwan, R.O.C., in 1992, and the M.S. and Ph.D. degrees in materials science from The University of California at Los Angeles in 1996 and 1997, respectively. During 1992–1993, he was a Research Assistant at the Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan, studying surface physics of gallium ion beam in ultrahigh vacuum. From 1997 to 2000, he was a Member of Technical Staff at Lucent Technologies, Bell Labs, Orlando, FL, working on electromigration and stress migration of 0.3, 0.25, 0.2, and 0.16 m CMOS, ASIC, and FPGA devices. In October 2000, he joined Lucent Optical Access Division (formerly Ortel), Alhambra, CA, working on process and failure analysis of 1310-nm and 1550-nm analog and digital InP lasers. Currently, he is a Scientist at Ortel, a Division of Emcore, working on device reliability and failure analysis of analog and digital Fabry–Perot (FP) and distributed feedback (DFB) laser diodes and modules. He has two U.S. patents (more pending) and over 45 publications in international journals and conferences.
