Thermal Analysis of High Power Pulse Laser Module

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1 Thermal Analysis of High Power Pulse Laser Module JinHan Ju PerkinElmer Optoelectronics Salem MA Abstract Thermal management is very critical in laser diode packaging, especially for a high power laser module, because the excessive heat generated in the laser diode can result in thermally induced optical failures, and mechanical failures like micro crack in tunnel junction due to high thermal stress, and also lead to long term reliability problems due to laser chip degradation and joint fatigue. In this paper, thermal analysis has been conducted on a 3-D finite element model of a high power pulse laser module to evaluate the thermal performance of different chip bonding media, different carrier and main substrate materials, for both steady state of the module and transient analysis of pulses of nanoseconds. Thermal stress is also calculated for different bonding media for comparison. A program using APDL of ANSYS has been developed for the analysis that helps the laser module packaging material selection and process evaluation. Introduction Laser diode packages have been widely used in various applications including medical and biomedical instruments, telecommunications, consumer products, military and space industries. The performance and reliability of the laser diode module is highly dependent on how it is packaged. Thermal management is very critical in laser diode packaging, especially for high power laser module, because the excessive heat generated in the laser diode can result in thermally induced optical failures, and mechanical failures like micro crack in tunnel junction due to high thermal stress, and also lead to long term reliability problems due to laser chip degradation and joint fatigue. The semiconductor Gallium Arsenide (GaAs) laser chip is bonded onto a ceramic substrate (carrier). The Chip on Carrier (CoC) is attached onto a main ceramic board and then using thermally conductive epoxy glued to a heat sink. The heat sink is eventually mounted on the Kovar package body. The laser chip bonding material can be hard solder, soft solder or thermal and electrical conductive epoxy in different applications. Hard solder results in high thermal stress because of its high melting temperature, but it yields high bonding strength and long term joint reliability; soft solder has lower melting temperature, but it incurs time and temperature dependent creep deformation at low stress level; while epoxy has easier manufacturing process, but it has much lower thermal conductivity and also other electrical and material stability issues. In this paper, thermal analysis is conducted on a 3-D finite element model of high power pulse laser module to evaluate the thermal performance of different chip bonding materials, different carrier materials and main board materials. A set of materials is defined for the laser module, based on the steady state tunnel junction temperature from a designed experiment matrix. Thermal transient analysis of pulses of nanoseconds is calculated, as well as the thermal stress in the laser chip. A program using APDL of ANSYS has been developed for the analysis that helps the laser module packaging material selection and process evaluation. The bonding material and substrate material with higher thermal conductivity are preferable to achieve lower laser chip working temperature and the bonding material with lower melting temperature is preferable to achieve lower thermal stress in the laser chip. To determine an optimum set of material and an optimum laser module operating temperature, not only should the relationship of laser performance and life span with respect to temperature, but also the thermal stress and bonding strength be considered and evaluated according to different laser chips and their different applications.

2 Analysis The FEA model of the laser module is built in ANSYS as shown in Figure 1, which is half of the laser module due to its structural symmetry. Laser chip (100 microns thick) material is Gallium Arsenide (GaAs) and its Quantum Well and active Tunnel Junction (5 microns thick) are AlGaAs and InGaAs. The carrier and main board are ceramic substrates made of Aluminium Nitride (AlN) or Alumina (Al 2 O 3 ). The heat sink is either the same as the package body material Kovar, or some other material, for example Aluminium alloy, or copper. Figure 1. FEA Model of Half Laser Module In this study, Au80Sn20 is selected as hard solder, Sn60Pb40 as soft solder to be the laser chip bonding materials, and AlN or Al 2 O 3 as carrier and main board, Kovar or Aluminium alloy as heat sink for laser packaging material evaluation. Semiconductor material and ceramic material have strong temperature dependent thermal and mechanical properties, like GaAs in the laser operating range, its thermal conductivity K= T^(-1.23) W/m- K being used in the simulation. Here in Table 1 gives the thermal conductivity K and Coefficient of Thermal Expansion (CTE) at temperature of 25 C [1] [2] [3]. Orthotropic material properties are used for laser chip and others are isotropic. Table 1. Some Material Properties Material GaAs Junction AlN Al2O3 Kovar Al AuSn SnPb Epoxy K (W/m/K) CTE (ppm)

3 First of all, the tunnel junction temperature Tj at steady state is taken as the target and a lower laser chip working temperature is preferred, therefore, a 2-Level 4-Factor Design of Experiment matrix [4] as shown in Table 2 is worked out to evaluate the sensitivity of each variable. Table 2. Design of Experiment Matrix Run Chip Bond Carrier Main Board Heat Sink Tj ( C) 1 SnPb Al 2 O 3 Al 2 O 3 Kovar SnPb Al 2 O 3 AlN Al SnPb AlN Al 2 O 3 Al SnPb AlN AlN Kovar AuSn Al 2 O 3 Al 2 O 3 Al AuSn Al 2 O 3 AlN Kovar AuSn AlN Al 2 O 3 Kovar AuSn AlN AlN Al The base is controlled at testing environment temperature 25 C. The power applied on the active Tunnel Junction is pulse of 1760W for 6.5 nanoseconds at the frequency of 22KHz, so for steady state analysis, the average power Pa is applied on the tunnel junction, where Pa = 1760*6.5E-9*22E3 = 0.25 W. The transient analysis is done for 100 seconds using the average power and the first 5 pulses using pulse power input, followed by the thermal stress analysis on the laser chip based on its working temperature. Analysis Results & Discussion Steady State Thermal Analysis The steady state thermal analysis is conducted at testing temperature of 25 C for the DOE matrix and the tunnel junction temperatures calculated are shown in Table 2 whereby we can get the average Tj for each material candidate from all the combinations and then the sensitivity of the junction temperature to component material selection, or the impact of the material selection from Tj difference of its change, as listed in Table 3. So, the biggest impact on the junction temperature is the carrier material, C and followed by the heat sink material, 8.88 C. Table 3. Sensitivity of Junction Temperature to Component Material Component Material Average Tj ( C) Tj Difference ( C) Chip Bond SnPb AuSn Carrier Al 2 O AlN Main Board Al2O AlN Heat Sink Kovar Al 52.74

4 Of course, there are other factors that we have to take into account when we determine the material set, besides the thermal effect, such as the material cost of AlN being higher than Al 2 O 3, and AuSn solder having higher reflow temperature to induce higher thermal stress in the chip on carrier, and other manufacturing process related factors. Therefore, for example here in this study we would choose SnPb for chip bonding material, AlN for carrier, Al 2 O 3 for main board, and Al for heat sink as the selected material set for the laser chip packaging. Transient Thermal Analysis The transient of first 100 seconds are calculated for the laser chip module by using the average power, as shown in Figure 2. The tunnel junction temperature reaches C, which is about 99.5% of the total temperature rise from testing temperature 25 C to steady state temperature C. Figure 2. Laser Chip Transient in First 100 Seconds Thermal transient of the laser module by applying pulse power is also simulated and the first 5 pulses are shown in Figure 3 that the laser module temperature is climbing up pulse after pulse, and only the area near the junction follows the pulse temperature fluctuation. The tunnel junction temperature at pulse peak is about 10 C higher than chip bottom.

5 Figure 3. First 5 Pulse Cycles of Laser Chip Thermal Stress Analysis Thermal stress is calculated at steady state by converting thermal element to structural element and transferring thermal results to structural analysis, for both AuSn and SnPb solder materials, based on the assumption of stress free at solder solidifying temperature. The feature of element birth and death is used in this study to simulate the laser module packaging process flow at different temperatures, from chip bonding at solder melting temperature to CoC attachment to main board and the main board attachment to package body at epoxy curing temperature. Table 4 gives the maximum von Mises stress of the laser chip operating at 25 C. The stress contour of the module using SnPb solder is shown in Figure 4.

Figure 4. Stress Distribution of Laser Module Table 4. Laser Chip Stress for Different Solders Material AuSn SnPb Laser Chip 106.66 30.12 Tunnel Junction 21.38 4.

6 Figure 4. Stress Distribution of Laser Module Table 4. Laser Chip Stress for Different Solders Material AuSn SnPb Laser Chip Tunnel Junction Some laser modules operate at very low temperature, down to minus 40 C or minus 55 C. It should be noted that the thermal stress in the chip and solder joint would increase accordingly, as the operating temperature decreasing. For example, the chip stresses are as listed below in Table 5 for operating temperature of minus 40 C. Table 5. Stress in Laser Chip Operating at - 40 C Material AuSn SnPb Laser Chip Tunnel Junction Here in this study, the analysis is only focused on the laser chip, and further analysis is needed for the bonding material layer on its stress and strength, time and temperature related stability, according to laser module design, packaging materials and application. Conclusion The thermal analysis is conducted on the high power pulse laser module for different laser chip packaging materials, which is very helpful to evaluate various situations to eliminate numerous expensive prototypes and difficult measurements. The packaging material set should be evaluated to cost-effectively achieve

7 lower laser chip working temperature. To determine an optimum laser module operating temperature, not only should the relationship of laser performance and life span with temperature, but also the thermal stress in the chip and joints be considered and evaluated according to different laser chips and their different applications. References 1. R.O.Carlson, G.A.Slack and S.J.Silverman, Thermal Conductivity of GaAs and GaAsP Laser Semiconductors, Journal of Applied Physics, Vol. 36 No.2 pp , Feb Thermal of Gallium Arsenide (GaAs), (Current Oct 1, 2003) 3. F.G.Yost, M.M.Karnowasky, W.D.Drotning and J.H.Gieske, Thermal Expansion and Elastic Properties of High Gold-Tin Alloys, Metallurgical Transactions A, 21A, pp , July Stephen R. Schmidt, Robert G.Launsby, Understanding Industrial Designed Experiments, 4 th Edition, Air Academy Press & Associates, 2000, p.2-28

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