Study of Steady and Transient Thermal Behavior of High Power Semiconductor Lasers

Study of Steady and Transient Thermal Behavior of High Power Semiconductor Lasers Zhenbang Yuan a, Jingwei Wang b, Di Wu c, Xu Chen a, Xingsheng Liu b,c a School of Chemical Engineering & Technology of Tianjin University, Tianjin 300072, China b State Key Laboratory of Transient Optics & Photonics, Xi an Institute of Optics & Precision Mechanics of CAS Xi an 710119, China c Xi an Focuslight Technologies Co., Ltd., Xi an 710119, China bj196@163.com Abstract High power semiconductor lasers have found increasing applications in many areas. The junction temperature rise may not only affect output power, slope efficiency, threshold current and lifetime, but also cause spectral broadening and wavelength shift, which makes thermal management one of the major obstacles of pump laser development. Therefore, developing optimized thermal design solution becomes especially important and critical. By means of numerical simulation method, the steady and transient thermal behavior of a single-bar CS-packaged 60W 808nm laser in continuouswave state has been investigated in this work. Thermal resistance and its compositions have been quantitatively analyzed. Guided by the numerical simulation and analytical results, a series of high power semiconductor lasers with good performances have been produced. Introduction High power semiconductor lasers have found increasing applications in many fields, including the pumping of solid state laser systems for industrial, military and medical applications as well as direct material processing applications. With the power increase of semiconductor lasers, heat generation increases correspondingly, and consequently causes the junction temperature to rise significantly. For a typical 808nm semiconductor laser, the wavelength shifts to the longer direction at a rate of ~0.27nm/ o C. Temperature rise in laser bar would not only cause wavelength shift, but may also lead to wavelength broadening [1]. Besides, it may directly affect the output power, slope efficiency, threshold current and lifetime, as well as spectral broadening and wavelength shift, which makes thermal management one of the major obstacles of pump laser development [2,3]. In actual manufacturing process, different packagings, including the packaging structure design and packaging materials selected, affects the junction temperature of a high power diode laser array. Therefore, developing optimized thermal design solution becomes especially important and critical. In this work, by using a single-bar CS-packaged 60W 808nm laser in continuous-wave state, the steady and transient thermal behavior of high power semiconductor laser bars/arrays has been studied, and strategies to optimize the thermal design and management have been discussed. The semiconductor laser die attach solder interface can significantly impact the thermal performance and reliability of a laser assembly. Fluxless soldering processes are required and solder voids should be controlled to the minimum as large solder voids would increase junction temperature and create hot spots. Solder voids near the front facet are especially detrimental as they can significantly raise the facet temperature and cause catastrophic optical mirror damage (COMD). Electromigration in metallic solders, especially in flip chip solder joints, can cause failure in electronic device and products, and has become a serious concern [3]. Laser Bar Heatsink Cathod Fig. 1. A sample of single-bar 808nm 60W continuouswave high power semiconductor laser Fig. 2. Schematic of semiconductor laser bars Both steady and transient thermal behavior of a single-bar CS-packaged 60W 808nm semiconductor laser made by Xi an Focuslight Technologies Co., Ltd. has been simulated using finite element method. The configuration of the semiconductor laser is shown in Fig. 1. A typical semiconductor laser consists of three parts: the laser bar (the chip), the cathode and the heat sink. The laser bar is schematically described in Fig. 2. The quantum well is the active region with 19 emitters in it. The Quantum well is cladded by the cladding layer and p-metal in turn. The semiconductor laser, whose chip is bonded by the Au wire, is packaged in the conduction cooled structure, with the bottom of the heat sink fixed with a TEC (Thermal-Electric Cooler). The bottom TEC surface is kept at 25 o C. 978-1-4244-4476-2/09/$25.00 2009 IEEE 831 2009 Electronic Components and Technology Conference

Discussion 2: Steady Thermal Behavior 1. Numerical In order to figure out ways to reduce the total thermal resistance, it is of great importance to understand how the thermal resistance of a high power pump laser composes. The steady thermal behavior of a CS-Packaged semiconductor laser in Continuous Wave (CW) mode was simulated using finite element method. The 19 emitters in the quantum well are heat producers of the device. The bottom side of the device is kept at 25 o C. As shown in Fig. 3, the transverse temperature profile of quantum wells in steady state is basically parabola-shaped distributed. At emitter regions, the heat flow is constrained in a small area and thus the temperature rises per unit length in these areas are higher than in the others. The local temperature rise in center emitters is greater than that in the side ones due to their longer heat transfer paths. Peak temperature of the device reaches to 51.8 o C. Thermal resistance of a semiconductor laser is defined by the formulation below: R th =ΔT/ΔQ (1) where R th is thermal resistance of the device; ΔT is temperature rise in the quantum well; ΔQ is thermal power of the device. For a typical 808nm semiconductor laser, the photoelectric conversion efficiency is about 50%. That is, the thermal power of a 60W (the optical power) 808nm semiconductor laser, ΔQ, is about 60W. Besides, according to the simulation result, the temperature rise in the device, ΔT, is 26.8 o C (51.8 o C minus 25 o C). Thus, the thermal resistance in the device is about 0.472K/W. where Q Q2 Q (3) 1 Q I V P (4) 1,2 o1,2 1,2 1,2 where I o1 and I o2 are different operation currents, λ 2, λ 1, V 1, V 2, P 1 and P 2 are wavelength, voltage and optical power at I o1 and I o2, respectively. One semiconductor laser sample was tested using a spectrum and PIV test system as shown in Fig. 4. The spectrum of the CS-packaged semiconductor laser was tested in both I op and 60% I op, as shown in Fig. 4. Obviously, the wavelength in Iop has shifted about 4.6nm towards the longer direction contrasted with 60% I op. Using the formulas above, thermal resistance of the device is about 0.656K/W, which is slightly larger than the simulation result. This is mainly because the simulation was conducted in perfect solder layer. In practice, there are always some solder voids which are inevitable in actual manufacturing processes. Additionally, nonlinear-in-the-materials, defects of the materials, as well as contact thermal resistance were also neglected in the simulation, which may cause the increase of the actual thermal resistance as well. Fig. 3 Transverse temperature profile of quantum wells at steady state 2. Experimental Actually, the temperature rise in a semiconductor, namely ΔT, cannot be measured directly due to the fairly small dimension of the quantum well. Instead, measuring the wavelength shift of a semiconductor laser sample is a practical method.. As it is well known, the wavelength of a typical 808nm GaAs semiconductor laser shifts to the longer direction at the rate of ~0.27nm/ o C linearly. Thus, ΔT can be calculated as below: In addition, ΔT = (λ 2 -λ 1 )/0.27 (2) Fig. 4 Testing spectrum of the sample Discussion 3 Transient Thermal Behavior Transient thermal behavior is also a crucial part of thermal behavior of a semiconductor laser. It reveals the heat propagation process inside the device. Transient thermal behavior helps to understand the role of each material layer in total thermal resistance, and also helps researchers to comprehend the failure mechanism of semiconductor lasers in quasi-continuous (QCW) work mode. 1. Numerical Simulation The transient thermal behavior of the CS-Packaged semiconductor is shown in Fig. 5. The boundary condition is the same as in the steady thermal analysis. As can be seen, it takes no less than 280ns for temperature disturbance to transmit to the upper side. Furthermore, it takes 900ns to transmit to the lower side of the solder layer. The simulation results suggest that the solder voids would not cause the nonuniformity of the temperature distribution in the quantum well, when the pulse width is shorter than 280ns. In the other words, a semiconductor working in pulse width 280ns or even lower should have pretty high reliability. 832 2009 Electronic Components and Technology Conference

heating time, and they are stabilized after 37μs. It means that the device has entered into the Formal Status of transient heat transmission [5]. The heat from the single emitter would not reach to the center of the space (in Fig. 6, the temperature between 0μm and 500μm is Zero). So the heat of this single emitter would not influence the neighbor one. Therefore, when the pulse is shorter than 37μs, even if one of the emitters does not work, or the heat of which could not be cooled, the heat of the single emitter would not impact the neighbors around it. In this way, this device would be much more reliable. Fig. 6 Lateral temperature profiles of a single emitter (150μm in width) Fig. 5 Curves of thermal resistance versus transient time of a semiconductor laser structure without voids in solder layer David Schleuning et al [4] have proved that for a typical semiconductor laser, its reliability is only good for controlled condition, for example, on short-pulse QCW operation or CW operation. When it works on long-pulse QCW operation, the reliability reduces rapidly. Our simulation gives this a reasonable explanation: when a semiconductor laser works on pretty short pulse QCW mode (pulse length τ<280ns), heat generated in the previous pulse dissipates completely before the next pulse, and there is not even enough time for the heat to transmit to the solder layer. In addition, when the device works in long pulse QCW mode (pulse length τ>900ns) or in CW mode, temperature distribution in the solder layer has already been steady within a single pulse. Even though the average temperature would be in a high level because of the solder voids, the temperature grads in the solder layer may not be so large. On the other hand, if the device woks on QCW mode and the pulse width is between 270ns and 900ns, the reliability of the assembly may be fairly low, because the heat generated in the previous pulse has already transmitted to the upper side of the solder layer before the next pulse, but not to the bottom yet. This, accordingly, would cause large temperature grads in the solder layer and finally lead to serious electromigration. So we can come to the conclusion that the reliability of a typical semiconductor laser is higher when works on short pulse QCW or long pulse QCW, or even CW mode, while Pulse width between 280ns and 900ns are not recommended due to low reliability. Fig. 6 gives the typical lateral temperature profiles of one of the 19 emitters. As can be seen, the profile of lateral temperature in the quantum well is elevated with increasing In addition, it can be seen from Fig. 6 that from the initial moment, the temperature rise is slowing down gradually. In the performance curve, within the same period, the difference between the two temperature curves is smaller. Fig. 7 Vertical temperature profiles of a single emitter during various heating time Fig. 7 shows that the heat generated by the quantum well in the device transmitted to the bottom of the heat sink 10ms after the device starts to function. This suggests that the TEC starts to work 10ms later than the device. 2. Experimental Test Based on the transient numerical simulation, the singlebar CS-packaged 60W 808nm semiconductor laser was tested in different pulses, as shown in Fig. 8. The thermal resistance is slightly larger than the results shown in Fig. 5. This is mainly because the simulation was conducted in perfect solder layer.. In practice, there are always some solder voids which are inevitable in actual manufacturing processes. Additionally, 833 2009 Electronic Components and Technology Conference

nonlinear-in-the-materials, defects of the materials, as well as contact thermal resistance are also neglected in simulation, which may cause the increase of the actual thermal resistance as well. Whatever, the trend of the curve obtained from the experimental work agrees well with that from the simulation result. Fig. 8 Curve of temperature versus transient time of a semiconductor laser structure Discussion 4 Thermal Management and Optimization Some voids filled with air may appear in the solder layer in the packaging process. As it is well known, the air has a pretty low thermal conductivity. The heat generated in the quantum well tends to accumulate in the area near the voids, and lead to local temperature rise. The uniformity of the heat distribution in the quantum well may be totally destroyed. As a result, the reliability and lifetime of semiconductor lasers can be reduced greatly. What s more, the spectrum may broaden seriously. Fig. 9 Lateral temperature profile of quantum wells with voids in solder layer beneath emitters in active regions Number the 19 emitters from left to right, in which the central emitter is numbered as the 10 th emitter. Select 10 of the 19 emitters, and set some voids of different dimension in the solder layer beneath these emitters. The voids are numbered in accordance with their top emitters. For instance, the void beneath the 3 rd emitter is called the 3 rd void. The lateral temperature distribution in the quantum well is shown in Fig. 9. As can be seen, the peak temperature of the device could reach up to 61 o C in the 11 th emitter. The thermal resistance of the device comes to 0.632K/W, which is around 34% higher than the perfect solder case. In addition, if there are voids in the solder layer, the temperature changes only in the emitter above, while the temperature remains the same in the rest area. The local temperature rise is listed in Table. 1. As can be clearly seen in the table, both void dimensions and location can affect local temperature rise..the larger the void is, the higher the local temperature rises and the wider the local temperature rise area. On the other hand, if the solder voids are in the same size, the void nearer to the center emitter would lead to higher local temperature rise. For example, No. 1 void has the same diameter (50μm) with No. 6 void, but because No. 6 void is nearer to the center void, local temperature rise around No. 6 void is 0.646 o C higher than that around No. 1 void. Similarly, the diameter of No. 11 void and No. 19 void are all 150μm, but the local temperature around No. 11 void is 0.039 o C higher than that around No. 19 void. Table. 1 Local temperature rise Void No. Void Local temperature diameter rise ( o C) (μm) 1 50 2.265 2 80 4.500 3 20 0.420 5 130 7.048 6 50 2.911 7 60 3.413 11 150 7.953 13 90 4.884 18 30 1.380 19 150 7.914 Next the influence of dimensions of the voids on local temperature are quantitatively discussed by ignoring the impact of the location of the voids. Fig. 10 gives the relationship of local temperature rise versus the diameter of solder voids. The local temperature rise increases linearly with void size if the location of the void is ignored, and the local temperature rise rate versus the diameter is 0.056 o C /μm. The vertical temperature distribution for conditions of perfect solder layer and solder with voids are compared in Fig. 11. As can seen in the figure, if the solder layer is perfect, the chip, the solder and the heatsink contribute 2.37%, 0.88% and 96.75 to the total thermal resistance, respectively. On the other hand, with some voids existing in the solder layer, the contributions of them change to 0.23%, 24.54% and 75.23%, respectively. These results suggests that the impact of solder voids on the thermal resistance is considerable. In addition, as 834 2009 Electronic Components and Technology Conference

the heat sink contributes most to the thermal resistance of the device, choosing heat sink with higher thermal conduction is an effective way to reduce the thermal resistance of the device. Additional calculation has proved that the thermal resistance of the device would reduce by 7.7% if the thermal conduction of the heat sink is 7.8% higher. device with voids in the solder layer has shifted to the longer position, compared with the device with perfect solder layer (see Fig. 4). A second peak appears. To make things worse, the operating and threshold current rise, while the slope efficiency and the photoelectric conversion efficiency drop significantly. The thermal resistance of the device climbs to 0.803, which is 22.4% higher than the device without voids in the solder layer. Fig.10 Relationship of local temperature rise versus the diameter of solder voids Fig.13 Performance of the sample with void in solder layer Fig.11 Vertical temperature profiles of the device at steady state Fig.12 Spectrum of the sample with void in solder layer A semiconductor laser with 80% voids in solder layer beneath the active regions was produced by means of infiltrative method, for the sake of validating the impact of solder voids to the temperature rise of the semiconductor laser. The spectrum and the LIV curves of the sample were both tested (see Fig. 12 and Fig. 13). Clearly, the spectrum of the Fig.14 One of test data of the products made under the guideline of theoretical simulation analysis Products Made Under The Guideline of Theoretical Simulation Analysis Guided by the numerical simulation and analytical results, a batch of high power semiconductor lasers with good performances were produced. Fig. 14 indicates one of the test data of this batch of products. The test condition is that the bottom temperature of the TEC is fixed at 25 o C.. The operating current is 66.63A, and the threshold current is only 13.66A. In addition, the photoelectric conversion efficiency and the slope efficiency are 50.03% and 1.14W/A, respectively. The center wavelength in I op (60A) is 809.25nm., which indicates that there is only a small wavelength shift. The FWHM (Full-Width Half-Maximum) is 1.61nm, while the FW90%E (Full Width 90% Energy) is only 2.47. That 835 2009 Electronic Components and Technology Conference

means the spectrum broadening of the device keeps at a very low level. The average performance parameters of the batch of products are shown in Table 2. As can be seen evidently, by optimizing of manufacturing processes, the semiconductor laser products show very good performance. Table 2 The average performance of semiconductor laser products by the optimization of manufacturing processes I op (A) 64.79 I th (A) 13.09 Slope Eff. (W/A) 1.17 Eff.@I op (%) 51.57 Centroid wavelength (nm) 809.55 FWHM (nm) 1.86 FW90% Energy (nm) 2.61 Conclusions 1. the simulation results demonstrate that the reliability of a CS-Packaged high power semiconductor laser can be very high in extremely short pulse (τ<280ns) or in short pulse (900ns<τ<37μs) QCW mode, or in CW mode. The device should avoid to work in pulse width between 280ns and 900ns. If the pulse width is longer than 37μs, the working condition is the same as in CW mode. 2. The temperature distribution of a CS-Packaged semiconductor laser comes to the steady state after it works for 1s, and the temperature-rise rate of the device is lower as the work time increases.. 3. If the solder layer is perfect, the active region, the solder layer and the sink contribute about 2.37%, 0.88% and 96.75%, respectively, to the total effective thermal resistance of the device. But when there are voids in solder layer beneath active regions, they become 0.38%, 30.64% and 64.98%, respectively. This suggests that the impacts of voids beneath active regions are considerable. 4. The voids in the solder layer would cause heat accumulation, and accordingly lead to the non-uniformity of the temperature distribution in the quantum well. With the solder voids located beneath the emitters, the local temperature rise could increase by 0.056 o C if the diameter of solder voids increases by 1μm. In addition, the nearer the voids from the center emitter, the higher the temperature rise is in the device. 5. Guided by the numerical simulation and analytical results, single bar CS-Packaged 808nm 60W semiconductor lasers with good performances were produced. References 1 Jingwei Wang, Xingsheng Liu, and Peiyong Wei. "Study of the mechanisms of spectral broadening in high power semiconductor laser arrays," Electronic Components and Technology Conference, IEEE, 2008: 1005-1010. 2 Xingsheng Liu, Martin H. Hu, Catherine G. Caneau, et al, "Thermal management stategies for high power semiconductor pump lasers," Inter Society Conference on Thermal Phenomena, IEEE, 2004: 493-500. 3 Xingsheng Liu, Ronald W. Davis, Lawrence C. Hughes, et al, "A study on the reliability of indium solder die bonding of high power semiconductor lasers," Journal of Applied Physics, 2006, 100(013104): 2006. 4 Schleuning David, Griffin Mike, James Phillip, et al, "Robust hard-solder packaging of conduction cooled laser diode bars," the International Society for Optical Engineering, Proceedings of SPIE, 2007: 645604(1)- 645604(11) 5 Shiming Yang, Wenquan Tao. "Heat Transfer," 3rd Edition. Higher Education Press, 1998: 63-100. Beijing, China 836 2009 Electronic Components and Technology Conference