Abstract
1. Introduction
With the rapid evolution of semiconductor laser technology, the optical power of laser diodes (LDs) keeps increasing, and the joule heat induced by series resistance increases. The joule heat needs to be dissipated effectively otherwise will adversely affect LD performance and reliability[
As the material connecting the LD chips and the heat sink, solder plays an important role in the thermal packaging system. Au80Sn20 alloy has good thermal and electrical conductivity. As a hard solder with high melting point, Au80Sn20 alloy has good creep resistance and mechanical properties, and thus it can be used in LDs, power electronics, MEMS sensors, and other applications[
In this paper, the thermal resistance of GaN-based blue laser diodes packaged in TO56 cans were measured by the forward voltage method. The microstructures of Au80Sn20 solder were then investigated to understand the reason for the difference in thermal resistance. It was found that the microstructure with higher content of Au-rich phase in the center of the solder and lower content of (Au,Ni)Sn phase at the interface of the solder/heat sink resulted in lower thermal resistance.
2. Experimental details
The packaged LD samples studied here are from the same epitaxial wafer and chip processing but different packaging processing, and the chip size of both is 200 × 400 μm2. The results shown in this paper are from typical samples of each batch named E09 and Y00. These two samples have a similar structure, and the schematic diagram of the structure is shown in Fig. 1. The Au80Sn20 solder bonds the LD chip and heat sink, and there are two solder layer interfaces.
Figure 1.The schematic diagram of structure of packaged laser diodes.
We first measured the thermal resistance of TO56 packaged GaN-based LDs using the forward voltage method, which was reported by us previously[
where T is the junction temperature of the LD, and A and B are the fitting parameters. We first measured the value of the temperature-sensitive parameter A. Next, by changing the injection current from an operation current to a very low current at which joule heat is negligible, we measured the voltage variation (ΔV) of the LD caused by the variation of the junction temperature (ΔT) using a MDO4104-3 mixed domain oscilloscope. The ΔT can be calculated by the following equation:
where Vft is the forward voltage of the LD at high injected current. As the joule heat dissipates rapidly, the decrease of junction temperature will cause the forward voltage to increase[
where IH is the injection current, while VH and Popt are the voltage and optical output power of the LD under the corresponding injection current IH.
For IMCs analysis, the scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) have been carried out. We selected FEI quanta FEG 250 to perform SEM, which has EDS subassembly named EDXX Apollo XP. In order to observe the IMCs in the solder joint clearly, the packaged LDs were polished into cross-sectional samples.
3. Results and discussions
Fig. 2 shows the power–current (P–I) curves of two LD samples. E09 has a lower threshold current and a higher slope efficiency than Y00. It is suggested that the difference of optical power between these two LD samples is caused by the different thermal resistance.
Figure 2.(Color online) The
In order to verify the effect of thermal resistance on optical power, we then measured the thermal resistance of the two samples. From the I–V curves under different measured temperatures, the relation between the junction temperature and the forward voltage was fitted with Eq. (1), and the temperature-sensitive parameter A was thus calculated to be 4.33 mV/K for E09, 2.66 mV/K for Y00. We then measured the time-resolved voltage variation of these two samples. In the working state, after the LD samples reached a steady-state, we measured the working current IH, voltage VH and optical output power Popt. In the measuring state, we measured the variation of forward voltage from the very beginning of the measuring state to room temperature. According to our previous simulation and experimental work[
Figure 3.(Color online) The time-resolved voltage variation during the measurement.
The voltage variation of the solder layer (Vf0–Vft) is 85.37 mV for E09 and 59.05 mV for Y00. Table 1 shows all the parameters of thermal resistance calculation. According to Eqs. (2) and (3), the thermal resistance was 41.95 K/W for E09 and 65.29 K/W for Y00. The total thermal resistance of a LD is determined by chip structure, bonding materials and microstructure, and heat sink. In our study, the chip structure, the heat sink, and the bonding materials are the same, and therefore we focused on the microstructure of the solder layer, as follows.
The gold-rich portion of the Au–Sn phase diagram is shown in Fig. 4[
Figure 4.Gold-rich portion of the Au–Sn phase diagram.
Fig. 5 shows the cross-sectional SEM images of the solder layer in these two samples. The EDS results and the possible phase of the marked points are shown in Table 2. The thickness of solder layer is roughly 4 μm. The a1, a2, b1, b2 points are at the LD chip/solder interface region. From the EDS results, it can be found that the δ phase is in the same region. Because Sn-rich phase such as δ has a lower surface tension than Au-rich phase, δ phase tends to coalesce at the surface[
Figure 5.Cross-sectional SEM of the two samples. The red crosses are the EDS measure points.
It is suggested that the main reason for the higher thermal resistance in sample Y00 is the lower content of the Au-rich phase. Au-rich phases in the center are the primary phase ζ’ and metastable phase. The Au-rich phase not only has a good mechanical property, but also has a lower thermal resistance. As the content of Au-rich phase in E09 is much higher than that in Y00, E09 has a lower thermal resistance. In addition, the interface also has a great effect on thermal resistance. In this case, the LD/solder interfaces of the two samples are analogous to some extent. However, the content of Ni in the solder/heat sink interface has a remarkable difference. It is believed that Ni comes from the heat sink by thermal diffusion. Since (Au,Ni)Sn is harmful to thermal resistance improvement, the decrease of (Au,Ni)Sn phase in E09 ensures a good thermal contact[
4. Conclusion
In summary, the thermal resistance of GaN-based blue laser diodes packaged in TO56 cans were measured by the forward voltage method. The microstructures of Au80Sn20 solder were then investigated to understand the reason for the difference in thermal resistance. It was found that the microstructure with higher content of Au-rich phase in the center of solder and lower content of (Au,Ni)Sn phase at the interface of the solder/heat sink resulted in lower thermal resistance. This finding will help improve the packaging processing in the future.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0401803, 2017YFE0131500, 2017YFB0405000), National Natural Science Foundation of China (Grant Nos. 61834008, 61574160, 61804164, and 61704184), Natural Science Foundation of Jiangsu province (BK20180254), China Postdoctoral Science Foundation (2018M630619).
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