Study of the Mechanism of "Smile" in High Power Diode Laser Arrays and Strategies in Improving Near-field Linearity

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1 Study of the Mechanism of "Smile" in High Power Diode Laser Arrays and Strategies in Improving Near-field Linearity Jingwei Wang 1, Zhenbang Yuan 3, Lijun Kang 2, Kai Yang 2, Yanxin Zhang 1, Xingsheng Liu 1,2 1. 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, , P.R. China 2. Xi an Focuslight Technologies Co., LTD No. 60 Xibu Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, , P.R. China 3. School of Chemical Engineering & Technology of Tianjin University No. 92 Weijin Road, Nankai District, Tianjin, , P. R. China Abstract High power diode lasers have found increased applications in pumping of solid state or fiber laser systems for industrial, military and medical applications as well as direct material processing applications. The non-linearity of the near-field of emitters (or the so called smile effect) in a laser diode array poses significant challenges in optical coupling and beam shaping and has become one of the major roadblocks in broader applications of laser arrays. Increasing the near-field linearity of a pumping laser diode array enables the laser system manufacturer to improve the laser system compactness, optical coupling efficiency, power, and beam quality while at the same time reducing manufacture cost in the laser system. Therefore, the near-field linearity of a laser bar is one of the key specifications of laser array products and improving the near field performance is especially important in order to increase production yield, reduce cost and gain competitiveness. In this paper, we will study the mechanism of smile in high power diode laser arrays and discuss the strategies and ways to achieve low smile. Introduction High power diode lasers (HPDLs) offer a variety of applications due to their higher electrical-optical conversion efficiencies, compact sizes and long life-times than the most prominent types of lasers by nearly an order of magnitude. High power semiconductor lasers, including single emitters, arrays, stacks, and two dimension area array stack have found increased applications in pumping of solid state laser systems for industrial, science and technology research, military, antiterrorism, entertainment display and medical applications as well as direct material processing applications such as welding, cutting, and surface treatment [ 1, 2, 3, 4 ]. With continuing improvement of the power, electrical-optical conversion efficiency, reliability, and manufacturability of high power semiconductor lasers, and decreasing manufacturing cost, many new applications of high power semiconductor lasers are being enabled [5]. The three key performance measures of high power semiconductor lasers are power, efficiency, and reliability. For diode laser arrays, the near field linearity is as important as the above three parameters as the near field linearity of a laser array as a pumping source significantly affects power, efficiency, compactness, beam quality and thermal management of a laser system, such as diode pumped solid state lasers and fiber lasers. If near field linearity of a laser array is poor, in other words, the smile is very large, the coupling efficiency of the laser array to a fiber array or micro-optics such as a fast axis collimation lens is very low. A typical 808 nm high power semiconductor laser array in a conduction cooled package has 19 to 49 individual emitters and the typical CW output power ranges from 30W to 80W. Figure 1 is a schematic diagram of a high power semiconductor laser array. To achieve high efficiency and high CW power, Cu is commonly used as the heatsink (anode block) for laser arrays. As it is known, copper has higher thermal conductivity in a variety of heatsink materials, such as Copper-Tungsten alloy, aluminum, etc. Effects of heatsink materials and packaging process on and the smile performance of laser diode array are discussed in more details later in this paper. Figure 1 A schematic diagram of a high power semiconductor laser array (laser bar) Near field non-linearity of laser diode array The individual emitters of high power diode lasers arrays (HPDLAs) are not perfectly aligned linearly because of the coefficient of thermal expansion (CTE) mismatch among the different layers of a bare bar, the packaging process and CTE mismatch between the laser bar and the bonding heat sink. Figure 2 shows a magnified smile image of a typical good diode-laser array. As can be seen from Figure 2, a typical good laser diode array is nearly linear. This is a better emitting light source for beam coupling. Figure 2 Enlarged smile image of a typical good laser diode array The individual emitters form a curvature, which can be concave or convex, corresponding to smile (Figure 3 (a)), /09/$ IEEE Electronic Components and Technology Conference

2 cry (Figure 3 (b)), or other types (Figure 3(c), (d)). Both smile, cry or others are commonly called smile due to historical reason. Due to the significant impact of smile on HPDLAs performance, reliability and cost, it is very important to reduce the smile [6]. (a) (b) (c) (d) Figure 3 Enlarged images of a diode-laser bar with various smile, (a) cry of approx. 2 µm, (b) smile of approx. 2.5 µm, or others (c),(d) Although the near field non-linearity is a common problem in manufacturing laser array products, the root cause or determining factors are not fully understood. As stated above, to achieve high efficiency and high CW power, Cu is commonly used as the anode block material for laser arrays due to its high thermal conductivity. However, Cu has large CTE mismatch with GaAs materials which is the base material for 808nm laser diode arrays. In order to reduce the thermal stress applied on the laser array due to CTE mismatch between the semiconductor laser bar and Cu anode, indium solder is primarily selected for die bonding. Most of the commercially available conduction cooled laser bar/array packages are in this format. The smile in HPLDAs can be originated from two aspects: wafer/bare bar related smile and packaging induced smile. Because the CTE of the copper heatsink is larger than the bare bar, a p-down bonded laser array would have the cry shape if there is no extra stress imposed on the laser arrays in the packaging. However,our study of various packaged laser array products from different suppliers showed that the nearfield of the laser array can be in convex shape or concave shape. This indicates that the packaging process and /or packaging structure design can play an important role in smile effect. For a high power semiconductor laser array, the mismatch between the semiconductor substrate and the metallization on the p-contact layer is the major reason that causes the smile in a wafer. This leads to the convex because the p-metal layer has a lager contraction than the GaAs substrate when wafer is cooled from a high temperature to room temperature during the wafer processing. The smile under this condition is negligibly low, so it will not be discussed here. In this work, finite element numerical analysis was conducted to study the smile of different laser array package structures. Meanwhile, conduction cooled 808nm CW laser array packages with different smile shapes were characterized using near field imaging, automated surface profiler to study the mechanism of smile in high power diode laser arrays. Wafer/bare bar related smile was analyzed and measured, and the smile evolution during the packaging process was studied experimentally and analyzed numerically. Furthermore, the evolution of the smile from a bare bar to the final packaged product was investigated. Numerical simulation and theoretical analysis In order to study the mechanism of the HPDLAs, a typical 808nm conduct cooled high power semiconductor laser manufactured by Xi an Focuslight Technologies Co., LTD was selected as the experimental sample. Figure 4 shows the PVI curve of a typical cooled high power semiconductor laser. Effects of important parameters like the die bonding process, different packaging structure, the thickness of the sub-mount heatsink, and different materials of pick-up tool for die bonding process, such as stainless steel and Tungsten, on smile of HPDL are discussed below. Finite element method was applied to simulate the different die bonding process, the thickness of heat sink, and different materials of pick-up tool for die bonding process. Figure 4 shows the PVI curve of a typical cooled high power semiconductor laser Electronic Components and Technology Conference

3 i. Effects of die bonding process on smile As stated earlier, indium solder is commonly selected for die bonding. However, with the improvement of new solder technology, several companies have adopted the hard solder (AuSn) as their key solder in die bonding process. For a typical 808 nm HPDLAs in conduction cooled package, die bonding process using common indium and hard solder is shown in Figure 5and Figure 6, respectively. N side GaAs based laser diode bar Mounting heatsink With Au finish (P side) T f is the freezing point of the solder, and T s is the temperature at which the stress is measured (operating temperature). For GaAs-based laser chip and copper heat sink, equation 2 describes the stress as follows, E E /( E E )( a a )( T T ) (2) Cu GaAs Cu GaAs Cu GaAs f s and the deformation of the laser diode bar/array is L ( a a )( T T ) L (3) Cu GaAs f s The copper- tungsten is alike; ECuW EGaAs /( ECuW EGaAs )( acuw agaas )( Tf Ts ) (4) and the deformation of the laser diode bar/array is L ( a a )( T T ) L (5) CuW GaAs f s According to the equation (3) and (5), numerical and simulation were conducted to study the deformation curve of laser array in vertical orientation. Figure 7 shows the deformation curve of laser array in vertical orientation as a function of lateral position. Figure 5 A schematic diagram of 808nm conduct cooled high power semiconductor laser with indium packaging process N side GaAs based laser diode bar AuSn solder CuW Mounting heatsink With Au finish (P side) Figure 6 A schematic diagram of 808nm conduct cooled high power semiconductor laser with hard solder (AuSn) packaging process For the indium packaging process, the laser diode bar/array is sandwiched between the cathode connector (nside) and the mounting heat sink with Au finish (p-side), with indium solder bonding them together. In contrast to the indium packaging process, the hard solder packaging process is of difference in bonding heat sink. The Copper-Tungsten alloy was used as heat sink with the chip bonded to it. Replacing indium with AuSn eliminates all problems associated with indium, but in the meantime introduces a new set of challenges to overcome. Foremost among them is the stress in the materials that arise from any differences in CTE between the diode and its sub mount. Equation 1 describes the stress in the region of the joint between two isotropic materials 1 and 2, as a function of CTE and temperature. EE 1 2 ( a1 a2)( Tf Ts ) E1 E (1) 2 Where E is the modulus of elasticity of the materials 1 and 2, α is the coefficient of thermal expansion of materials 1 and 2, Figure 7 The deformation curve of laser array in vertical orientation under indium and gold-tin From the Figure 7, the laser bar deformation using indium packaging process in vertical orientation is five times larger than that using harder solder. Although indium is a sort of very soft solder which can effectively release the stress to some extent, the very large CTE of indium (three times larger than laser bar/array) cause the chip to bend due to the extra thermal stress imposed on laser bar. In addition, Cu is commonly used as the sub mount heat sink material for laser arrays due to its high thermal conductivity. However, the large differences of CTE mismatch between GaAs-based laser diode bar/array and copper anode block make it easier to lead to very large deformation in the cooling process. Pure indium solder has a CTE of /K, however, gold-tin alloy has a lower CTE of /K than it[7]. In contrary to pure indium solder, it is a better mismatch solder material for GaAs-based laser diode bar/array. In addition, the heat sink material is copper tungsten alloy when using AuSn solder, its CTE is about m / and is very close to the laser bar s. The CuW submount serves as a thermal stress buffer layer between the copper heat sink and the laser bar. Therefore, the smile generated will be much lower than that using indium packaging process Electronic Components and Technology Conference

4 ii. Effects of the thickness of heat sink on smile Effects of the thickness of heat sink on smile are simulated using ANSYS tool. The deformations of two 808nm CS high power semiconductor laser arrays with different thickness (4mm and 8mm) of copper anode block were simulated. Assuming under the same condition, such as using indium packaging process, the deformation curve of laser bar in vertical orientation as a function of different thickness of copper heat sink is illustrated in Figure 8. The laser bar deformation in vertical orientation using thicker mounting copper heat sink is lower than that using thinner copper block. If the thickness of copper block heat sink is increased from 4mm to 8mm, the laser bar deformation in vertical position is reduced from 1.41µm to 1µm. The laser array deformation in vertical orientation reduces by 41%. 4mm 8mm Vertical Deformation(µm) Lateral Position (µm) Using Stainless Steel for Die Bonding Using Tungsten for Die Bonding Figure 9 The deformation curve of laser array in vertical orientation using different pick-up tool for die bonding process Experimental results In order to investigate the mechanism of smile effect the smile during each major packaging process step for both reflow oven process and die bonder process was measured for a typical bare bar/indium solder/cu block structure and a bare bar/ausn solder/cuw/ solder/cu block structure. Our study shows that the packaging process can significantly affect the smile of a laser diode array. Figure 8 The deformation curve of laser array in vertical orientation using copper heat sink with different thickness If the thickness of CS copper anode block is thicker, it can release the bulky thermal stress to some extent, caused by the large CTE mismatch between laser chip and CS mounting block heat sink. This makes the curvature smaller. If the thickness of CS copper anode block is thinner, it is very easy to generate warpage during the reflow process, bending together with the bar/array. In this way, a resultant force will be imposed on the bar/array. This leads to bar bending severely. With such thin and mechanically sensitive heat sinks the beam may be seriously degraded due to mechanical deformations which, in turn, lead to large smile. iii. Effects of different materials of pick-up tool for die bonding process on smile The laser bar deformation as a function of different pickup tool for die bonding process was simulated. Figure 9 shows the laser bar deformation in vertical orientation as a function of different materials of pick-up tool for die bonding process. Results from Figure 9 indicate that the laser bar deformation in vertical orientation is significantly different when using materials of pick-up tool. The laser array is concave if Tungsten is used during the die bonding process. The smile observed is less than 0.5µm. In contrary to the tungsten, stainless steel pick-up tool makes the smile larger than tungsten pick-up tool. Its smile is convex and exceeds 2.5µm. These results suggest that tungsten is a better option for the pick-up tool in die bonding process. Figure 10 The block diagram of smile test system Our experimental setup of smile test system is illustrated in Figure 10. The smiles were measured by using a cylindrical lens to image the slow axis onto a screen at a proper distance from the LDAs. In the smile test system, the rays from the LD Bars was fast-axis collimated first (after multiple tests, a fastaxis collimated lens with the smallest smile was selected as a calibrated sample). Then the collimated rays went through the Optical Imaging System, making the real bending shapes imaged onto the photo-sensitive surface of the CCD camera. The signals from the photo-sensitive surface through the image grabbing card were converted to data signals and delivered to the beam analyzing software. Images are captured on a digital camera for a low current. Our experiments exhibit there is more or less smile during packaging process. Figure 11shows an enlarged picture of a diode laser bar with a cry of approximate 2µm. According to the theoretical simulation, most of smiles are in this format due to the large mismatch between the GaAs laser chip and copper anode block. However, a variety of smile shapes, illustrated in the Figure 3 are observed during the packaging process. Study by Liu, et al [8] shows that thermal stress during the packaging process plays a great role on the smile for high Electronic Components and Technology Conference

5 power laser bar/array. Due to the CTE mismatch between the GaAs laser bar and the copper anode block, the extra thermal stress imposes on the chip and makes the chip bend to cry or smile. 3D Plot material. With a CTE difference of ~ / and a temperature difference of ~131 between the indium solder freezing temperature, which is treated as stress free point, and room temperature, there is a ~14 μm of contraction difference between the Cu heatsnik and laser array along the length of the laser array for a standard 10 mm long laser array. Figure 13 shows a simple model to describe the bending process during the cooling from indium solder freezing temperature to room temperature. Figure 13 A schematic diagram of smile formation Figure 11 Top surface profile of a sample with a cry of approx. 2 µm shown in Figure 3 (a). 3D Plot Figure 12 Top surface profile of a sample with a smile of approx. 2.5 µm shown in Figure 3 (b). Further work to test smile performance in details will be conducted in the future. Discussion According to the theoretical simulation, most of curvature of laser diode array on which are imposed extra thermal stress are cry, however, our experiments show that some curvatures of laser diode array are smile. Obviously, these smile curvature are closely related to packaging process. Therefore, packaging process plays an increasingly important role in the non-linearity of the near field of emitters in a semiconductor laser array. The CTE mismatch is a root cause of near field nonlinearity of laser diode array. Due to CTE mismatch, thermal stress is easier to form in the cooling procedure. Thermal stress is an inherent problem with the use of copper heatsink since copper has much larger coefficient of thermal expansion (CTE) than the laser array which is essentially made of GaAs Assuming CTE (a)>cte (b)>cte(c), the temperature in the chamber of reflow oven is dropped down during the reflow process, materials a, b and c are all contracted, but with different contaction magnitude. The material (a) has largest contraction, the material (b) has the second, and the material(c) has the smallest contraction. According to the mechanics theory, material (a) will impose a compress force on material (b), material(c) will impose a tensile force on the material (b), and thus, the upper and lower surfaces of the material (b) are subjected to an asymmetric force. The asymmetric force produces a bending moment on the material (b), as illustrated in the Figure 13. This bending moment leads to material (b) to bend and thus formation of smile. Work in a separate study [8] shows that the laser array was curved up after soldering which leads to thicker solder layer at two ends and thinner solder interface at the central region during the reflow process. To improve near-field linear of high-power semiconductor laser array,two factors need to be considered. 1) Control the CTE mismatch between packaging materials. A suitable heat sink material should be selected to match with the GaAs substrate, such as CuW alloys. It can ensure the mismatch is minimized between the chips and the heat sink. 2) Control the inhomogeneity of thermal stress distribution. First, minimize thermal stress concentration by improving the level of packaging technology to keep the thickness of die bonding layer consistency; Second, keep the consistency in manufacturing the chip and the heat sink with very flat and straight surfaces to avoid extra stress imposed on the laser bar/array. Conclusions Conduction cooled 808nm laser array packages with different packaging processes were characterized using high resolution surface profilers, and optical imaging techniques to study the smile of high power diode laser bar or array. It can be concluded that (1) ideally, the smile of a high power diode laser bar/array caused by indium solder packaging process is about less 1µm; and it is less 0.2µm if AuSn solder is used during the packaging process. (2) The smile of a laser bar/array caused by soldering on a thinner copper heatsink is Electronic Components and Technology Conference

6 larger than that caused by soldering on a thicker copper heatsink. (3) Selection of pick-up tool material of low CTE matched with laser diode array enables the low smile available in die bonding process. Study results suggest that tungsten is a better option for the pick-up tool in die bonding process. (4) The smile of a high power diode laser bar/array is significantly influenced by the packaging process. References [1] Uwe Brauch, Peter Loosen, and Hans Opower, High- Power Diode Lasers for Direct Applications, Springer- Verlag (Berlin Heidelberg ) 2000, pp.1-2. [ 2 ] Brian Faircloth, High-brightness high-power fiber coupled diode laser system for material processing and laser pumping, Proceedings of SPIE Vol. 4973, 2003, pp [3] J Dutta Majumdar and I Manna, Laser processing of materials, Sadhana( India),Vol. 28, Parts 3 & 4, 2003, pp [ 4 ] E. Rugi, P. Mueller, P. Lambelet, Scalable high brightness laser pump for aerospace applications, IEEE, 2003, pp81. [5] Michael A. Bolshov, Yuri A. Kuritsyn, Laser Analytical Spectroscope, Ullmann s Encyclopedia of Introdustrial Chemistry, Six Edition, Wiley (Weinheim, Germany), 2001, pp [6] H. Zhu, I. C. Ruset, and F. W. Hersman, Spectrally narrowed external-cavity high-power stack of laser diode arrays, OPTICS LETTERS, Vol. 30, No. 11, 2005, pp [7] Alan R. Mickelson, Nagesh R. Basavanhally and Yung- Cheng Lee, Optoelectronic packaging, John Wiley& Sons, Inc. (605 Third avenue, New York, NY) 1997,pp60. [8] Xingsheng Liu, Jingwei Wang, and Peiyong Wei, Study of the mechanisms of spectral broadening in high power semiconductor laser arrays, Electronic Components and Technology Conference, IEEE, 2008, pp Electronic Components and Technology Conference