Effects of Packaging on the Performances of High Brightness 9xx nm CW Mini-bar Diode Lasers Xiaoning Li 1a,b,c, Jingwei Wang b, Feifei Feng a, Yalong Liu b, Dongshan Yu b, Pu Zhang a, Xingsheng Liu a,b a State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, No. 17 Xinxi Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, 71119, P.R. China b Focuslight Technologies Co., LTD, No. 6 Xibu Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, 71119, P.R. China c Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique, Xi an Jiaotong University, No.28, Xianning West Road, Xi'an, Shaanxi, 7149, P.R. China ABSTRACT 9xx nm CW mini-bar diode lasers and stacks with high brightness and reliability are desired for pumping fiber lasers and direct fiber coupling applications. For the traditional cm-bar with 1mm-2mm cavity, it can provide CW output power up to 8W-1W and high reliability, whereas the brightness is relatively low. In comparison, mini-bar based diode lasers with 4mm cavity offer a superior performance balance between power, brightness, and reliability. However, the long cavity and large footprint of mini-bar diode laser renders its sensitivity towards thermal stress formed in packaging process, which directly affects the performances of high bright mini-bar diode lasers. In this work, the thermal stress correlating with package structure and packaging process are compared and analyzed. Based on the experiment and analysis results, an optimized package structure of CW 6W 976 nm mini-bar diode lasers is designed and developed which relieves thermal stress. Key words: diode laser, Mini-bar, brightness, thermal stress, packaging process 1. INTRODUCTION With the increasing applications of high power semiconductor lasers in industry, advanced manufacturing, aerospace, medical systems, display, entertainment, etc., semiconductor lasers with high power, high brightness and high reliability are required. The higher the source brightness, the easier an emitting laser can be concentrated into a small focused area [1]. 9xx nm CW mini-bar diode lasers and stacks with high brightness and reliability are desired for pumping fiber lasers and direct fiber coupling applications [2-4]. For the traditional 9xx nm cm-bar with 1mm-2mm cavity and 19 emitters, it can provide CW output power up to 8W-1W and high reliability, whereas the brightness is relatively low, only 4W to 5W per emitter. The beam quality in slow axis is very poor with a beam parameter product (BPP) of 525mm mrad. In 1 smto@opt.ac.cn, Phone: (86)-29-8888786; Fax: (86)-29-8888775. Components and Packaging for Laser Systems, edited by Alexei L. Glebov, Paul O. Leisher, Proc. of SPIE Vol. 9346, 9346C 215 SPIE CCC code: 277-786X/15/$18 doi: 1.1117/12.2848 Proc. of SPIE Vol. 9346 9346C-1
order to increase the brightness of laser diodes, longer cavities is designed accordingly to reduce the carrier densities and to lower the junction temperature. Mini-bar based diode lasers with 3.5mm to 5mm width, 4mm cavity length and 5 emitters which can achieve 8W to 1W per emitter offer a superior combination of power, brightness, and reliability. The beam quality in slow axis is improved with a beam parameter product (BPP) of 175mm mrad. However, long cavity length and large footprint of mini-bar diode laser renders its sensitivity towards thermal stress formed in packaging process, which directly affects the performances of high bright mini-bar diode lasers [5]. Thermal stress in 9xx nm mini-bar diode laser arrays is widely concerned because it may reduce the device life time and affect the performance including the spectra characteristics and the polarization state. The performance of mini-bar diode laser is greatly affected by packaging structure and process. In this paper, the device structure and packaging process of an all AuSn CS-packaged diode laser array are designed and optimized. A high brightness 976 nm CW Mini-bar Diode Laser is successfully fabricated. Through comparative analysis, device performances including output power, thermal behavior, polarization and lifetime influenced by different package structures are studied. 2. STRUCTURE DESIGN AND FABRICATION 2.1 Packaging structure design and optimization Package structure of a semiconductor laser influences the major characteristics, such as thermal behavior, output power and polarization [6-7]. A typical package structure in which a diode laser chip is mounted on a conduction heat sink is known as conduction cooled semiconductor laser bar (CS). Internal structure of the CS package is illustrated in Fig. 1.. The laser chip is epi-down mounted on the copper heat sink. For the indium packaging process, the laser diode bar/array is bonded to the mounting heat sink with indium solder and an N-side connection is made. N-metal (Cathode) Diode Laser Bar Indium solder N-metal (Cathode) Diode Laser Bar AuSn Sokier CTE- matched submount Indium solder N -metal Cathode Heatsink (Anode) Heatsink (Anode) Heatsink (Anode) (c) Fig. 1 Schematic of a high power diode laser array packaged on CS heatsink using different technology. The indium structure; The AuSn/indium structure; (c) The all AuSn structure In order to relieve the thermal stress, a hard solder packaging structure is designed as shown in Fig. 1.. The diode chip is bonded to a CTE-matched submount material, and then the resulting subassembly is soldered to a CS copper heatsink using indium solder. In the case of extreme on-off power cycling as well as environmental thermal cycling conditions, devices fabricated by Indium solder might be problematic. To overcome the limitation of harsh environments, a new packaging technology for high power diode laser arrays which uses AuSn solder at both the die bonding interface and the submount attachment is presented, as shown in Fig. 1. (c). Proc. of SPIE Vol. 9346 9346C-2
2.2 Fabrication process Thermal stress occurring at the packaging and operating processes influences the performance and reliability of high power semiconductor lasers [8]. The stress is mainly caused by the mismatch of coefficient of the thermal expansion (CTE) between the heat sink and laser bar. For all AuSn CS-packaged diode laser array, although the diode laser bar is bonded on a CTE matched submount, the contraction of CS copper heatsink still imposed significant stress on the diode laser bar through the submount. Therefore, the die attach process should be improved so that it induces less residual stress. As there are two soldering interfaces, voids are more likely to be formed in the AuSn solder layer during packaging process; therefore, advanced fabrication process has been carried out, including AuSn film deposition and void-free die bonding process etc. The reflow profile procedures are also optimized to reduce the thermal stress, which is also validated by different batches. Solder Contact copper sheet "It, Mar, Gold wires isw. gifts alikz-,-,.--- Cathode block - linitilimlit.v. gab no. illft, (c) Fig. 2 Fabrication process of an all AuSn single bar CS-packaged high power diode laser, soldering chip and submount on the heatsink; wire bonding; (c) installing the cathode block Three steps are used to fabricate an all AuSn CS-package high power diode laser arrays. First, a diode laser bar, a CTE-matched submount and heatsink are bonded together by AuSn solder. Second, n-side metallization of the diode laser bar and contact copper sheet are connected through wire bonding process. Finally, the copper block served as the cathode is installed on the copper sheet by screws, as shown in Fig. 2. 3. PERFORMANCE CHARACTERIZATION According to the different packaging structures and procedures, a series of CW 6W mini-bar diode laser arrays using hard soldering technologies are fabricated using mini-bar (5 emitters with 4 mm cavity length), as shown in Fig. 3. The performances of the all AuSn solder CS-packaged mini-bar laser in CW mode, including output power, spectral width, polarization and smile, are characterized compared with the typical indium CS-packaged mini-bar laser. Fig. 3 CW 6W mini-bar CS-packaged high power diode laser, The indium structure; The all AuSn structure Proc. of SPIE Vol. 9346 9346C-3
3.1 PVI test The PVI and spectrum properties of CW 6W 976nm laser array with 4-mm cavity length using all AuSn packaging technology are characterized and compared with the typical indium CS-packaged mini-bar laser. The maximum electro-optical conversion efficiency could reach 62.62% in CW mode, as shown in Fig. 4. The full width of half maximum (FWHM) and full width of 9% energy (FW9%E) is 2.99nm and 4.26nm, respectively. It can be seen that the device with different structure shows similar PVI properties, whereas the spectrum of indium structure laser is not as good as that of all AuSn structure. i- PVI characteristics Pr^^- volt.tr 9 7_ ib 1 1 w aÿ Pop(R) lop(,) 1k(3) Slope Ef(WA) EfaIop(411 Max 65(1 Rx(mOhon) V $1op() ic 6. 6118 4.5 1.7 611 63A SAO 1.63 23 PVI characteristics /2 Pow*. -Muse.. CRoentil 2 8 6 t 491?. g N Po9R 6. Iop(A) 6156 Ith(A) 393 Sope Eft(NVA) 1.6 Efrvoixo.) 59.62 51axEff() 62.62 Rs(mOblas) 5.1 V19() 1.64 TC 23. ItOP' 44 IL 14.2 VOA S7 MA 488 erntla S11 S74 4 64 124 1.2 NA 32 NA al OLI 674 64 NOMA Spectrum 1 to r ten n7] Peakwaieterglh(m4 977.1 Certroid wae@rgth(rml 976.76 pnmlyrer) 344 WUBVA Energ(rm) 7. Spectrum 1 8 lot mityl Peak vavelength(nrri 97165 Centroid vovelengtlynnj 97119 FIVHM(nrn) 2.99 FV.19.Energy(nni 4.26 9 94 97 98 W...e.ma.r. 99 1 9 98 9 8 wavoienefo. sun Fig. 4 Tested results of a typical CW 6W packaged high power diode laser The indium structure; The all AuSn structure 3.2 Thermal resistance The performance of semiconductor laser is greatly dependent on junction temperature [9]. High device junction temperature will result in high threshold current, low conversion efficiency and the reduced output power. The junction temperature of diode laser device can be calculated by: where Th represents the heat sink temperature, Rth T = Th + Rth( IV P) (1) represents the device thermal resistance, I, V and P are driving current, voltage and output power, respectively. The formula (1) shows that the junction temperature of the laser s is mainly determined by the heat sink temperature and the thermal resistance. The heat sink temperature is dependent on the operating conditions; therefore, thermal resistance is the primary limitation of junction temperature. The thermal resistance can be calculated by: R th = ΔT ΔQ = ( Δλ ΔT ) 1 ( Δλ ΔQ) (2) Proc. of SPIE Vol. 9346 9346C-4
where Q is generated heat, Δλ ΔT Δλ ΔQ is the wavelength temperature coefficient and is the wavelength heat coefficient. Typically, for GaAs-based 976nm laser devices, the wavelength increase at the rate of.35nm/k. The heat generation of a laser package can be calculated by: Q = IV P (3) 978.-. 977 E C 976 m 975 O9 974 973 O 972 FL435 Linear Fit of Datai FL435 A =.27574Q+968.62 L 977 976 975 C d Ñ 974 > N 973 FL412 Linear Fit of Datai B A =.28886Q+969.98961 C) 971 972 6 1 12 14 16 16 2 22 24 26 1 15 2 25 3 Q(W) Q(W) Fig. 5 The wavelength varies with the heat power. The indium structure; The all AuSn structure It can be conclude that the wavelength heat coefficient of the indium CS-packaged structure diode laser is.28 nm/w, while that of the all AuSn CS-packaged structure laser is.29 nm/w. From equations (2) and (3), the thermal resistance of the indium CS-packaged structure diode laser can be calculated as.79 K/W, and that of the all AuSn CS-packaged structure laser is.83 K/W. This result confirms that the heat dissipation capacity is close between these two structures. 3.3 Smile The smile in a diode laser bar poses significant challenges in optical coupling and beam shaping and has become one of the major obstacles in increasing the brightness of laser arrays [1]. Improving the smile of pump diode laser bar enables the laser system manufacturer to improve the laser system compactness, optical coupling efficiency, power, and beam quality while at the same time reducing optical coupling cost in the laser system [11]. The smile effect of indium and all AuSn bonding CS-packaged high power diode laser arrays are characterized, as shown in Fig. 6. From left to right the smile values are.359,.382,.145, and.118. The result shows that the smile value of high power diode laser arrays using all AuSn bonding technology is relatively lower than that of using indium bonding technology. Fig. 6 Enlarged smile image. The indium structure; The all AuSn structure Proc. of SPIE Vol. 9346 9346C-5
3.4 Polarization In order to meet the brightness and power requirements, the beam of several laser bars need to be combined and coupled into an optical fiber. Polarization coupling technology is one of the most cmmmon ways to achieve this goal [12]. Thus, the excellent polarization state of the diode laser bar is critical to improve the brightness of semiconductor lasers. For 9xx nm CW mini-bar diode lasers, poor polarization could limit its application. The polarization behaviors are related to the bar itself as well as the stress induced in the packaging process. From Fig. 7, it can be seen that the average degree of polarization of mini-bar diode laser packaged using the optimized structure are 97.6%, which is about 5% higher than that of using the conventional structure. It is also found that the degree of polarization correlated with the spectrum and smile in our experiments. The better the spectrum shape is, the higher the degree of polarization will be. The lower degree of polarization and poor spectrum and smile can be attributed to the thermal stress of mini-bar diode laser. 1 95 9.) W S- 98.2 92.47 85 8 75 +comenrtoral structure (optimized structure 7 65 6 2 3 4 5 6 7 8 9 SN Fig. 7 Comparison of degree of polarization for different mini-bar package structure 3.5Lifetime Compared with cm-bar laser arrays, the mini-bar laser arrays suffer from shorter lifetimes and are more susceptible to degradation and premature failure. This is mainly due to the excessive localized heating and substantial thermal cycling of the laser active regions. The lifetime test of two types CS-package high power diode laser arrays is conducted using the lifetime test system. These results show the robustness of the device through lifetime under the harsh conditions of current cycling and therefore thermal cycling of the solder joint to the junction temperature. The lifetime test shows that the indium CS-packaged high power diode laser arrays failed after few hundred hours. However, the all AuSn CS-packaged high power diode laser arrays are still stable after 1728 hours operation, which indicates an acceptable reliability. 5 <4 4 :f 3 Lifetime data of indium CS- packaged diode laser 7 78M 6 f 315 -ihm- 61N dq 676N 2 -LF 6786 1 --6816 o t 736 6 13 335 461 785 82 96 Tilne/H 1211 Proc. of SPIE Vol. 9346 9346C-6
7 6 5 d 4 p 3 2 1 lifetime data of A..So- packaged diode lacer 24 768 1128 1728 Time/13 t17 t55 t53-3f41-3if23 t5 Fig. 8 Lifetime of mini-bar CS-packaged devices. The indium structure; The all AuSn structure 4. CONCLUSION In summary, based on the experiment and analysis results, an optimized package structure of mini-bar diode lasers and process relieving the thermal stress is designed and developed. A high brightness 976 nm CW Mini-bar diode laser is successfully fabricated and characterized and compared with the typical indium CS-packaged mini-bar laser. Diode laser in all AuSn packaging structure has similar PVI properties as that of indium packaging structure, whereas the spectrum, smile and polarization of indium structure laser is quite undesired due to thermal stress. The lifetime test shows that the indium CS-packaged high power diode laser arrays failed after few hundred hours. However, the all AuSn CS-packaged high power diode laser arrays are still stable after 1728 hours operation, which indicates an acceptable reliability. 5. ACKNOWLEDGEMENTS We would like to acknowledge support from the Natural Science Foundation of China under Grant 613341 and 6144172. REFERENCES [1] Xingsheng Liu and Wei Zhao, Technology Trend and Challenges in High Power Semiconductor Laser Packaging, Proceedings of 59th Electronic Components and Technology Conference (ECTC), 216-2113(29). [2] Brian Faircloth, High-brightness high-power fiber coupled diode laser system for material processing and laser pumping, Proceedings of SPIE 4973, 34-41(23). [3] Yongkun Sin, Nathan Presser, Brendan Foran, Neil Ives and Steven C. Moss, "Catastrophic Facet and Bulk Degradation in High Power Multi-Mode InGaAs Strained Quantum Well Single Emitters," Proceedings of SPIE 7198, 719818(29). [4, 12] M. Dogan,R, Pathak,S, Ellison,H, Eppich,G, et al, "Record-Brightness Laser-Diode Bars for Fiber Coupling," Proc of SPIE, Vol. 8241, 8241G-1(212). [5] Xingsheng Liu, Jingwei Wang, and Peiyong Wei, "Study of the Mechanisms of Spectral Broadening in High Power Semiconductor Laser Arrays, " Proceedings of 58th Electronic Components and Technology Conference (ECTC), Proc. of SPIE Vol. 9346 9346C-7
pp.15-11(28). [6] X.S Liu, K.C. Song, R.W. Davis, L.C. Hughes, M.H. Hu, and C.E. Zah, A Metallization Scheme for Junction-Down Bonding of High Power Semiconductor Lasers, IEEE Transactions on Advanced Packaging, Vol. 29, No. 3, 533-541(26). [7] X.S. Liu, J.W. Wang, and P.Y. Wei, Study of the Mechanisms of Spectral Broadening in High Power Semiconductor Laser Arrays, Proceedings of 58th Electronic Components and Technology Conference (ECTC), 15-11(28) [8] K. Shigihara, Y. Nagai, S. Karakida, M. Aiga, M. Otsubo, K. Lkeda. Estimation of strain arising from the assembling process and influence of assembling materials on performance of laser diodes, J. Appl. Phys. Vol. 78, 1419-1424 (1995). [9] Xiaoning Li, Yanxin Zhang, Jingwei Wang, Xingsheng Liu, et al, Influence of package structure on the performance of the single emitter diode laser, IEEE transactions on components, packaging and manufacturing technology, VOL.2, 1592-1599 (212). [1] Jingwei Wang, Zhenbang Yuan, Lijun Kang, Kai Yang, Yanxin Zhang, Xingsheng Liul, Study of the mechanism of "smile" in high power semiconductor laser arrays and strategies in improving near-field linearity, IEEE Proceedings of 59th Electronic Components and Technology Conference (ECTC), 837-842 (29). [11] M. Dogan, et al, Record-Brightness Laser-Diode Bars for Fiber Coupling, Proc. of SPIE, Proc. of SPIE, Vol. 8241, 241G-1(212). Proc. of SPIE Vol. 9346 9346C-8