Micross

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Manufacturing Copper/Silver Composite Substrates for High Power Laser Diode Packaging
Keywords: Copper/silver composite, High power laser diode packaging, Solid state bonding
Nearly all high power laser diode chips are bonded on packages or sub-mounts using Au20Sn eutectic solder. AuSn eutectic is chosen because, being a hard solder, it does not incur plastic deformation and does not have thermal fatigue problem. It is strong and reliable. Thus, regardless of ever rising Au price, the use of AuSn eutectic continues. A requirement of using hard solder is small chip size or good CTE match between the substrate and the chip. High power laser chips are quite large, with width of 5mm or more. Thus, CTE match is a must. The laser industry has used Cu-W alloy with CTE of 7 ppm/°C as sub-mounts. It is difficult to machine due to large hardness. The resulting Cu-W sub-mounts are expensive. Over the past 3 decades, the industry has not come up with a better solution. We hereby present the Cu/Ag composite substrate. Cu is the ideal substrate material because of high electrical and thermal conductivities, high strength, good rigidity, ease of forging and machining, and low cost. The challenge is its high CTE of 17 ppm/°C, much higher than that of all semiconductors, ranging from 2.7 ppm/°C for Si to 7 ppm/°C for GaAs. Ag is chosen due to its superior ductility. In manufacturing, Ag is either electroplated or laminated on Cu substrates. To verify bonding possibility, Si chips are bonded on Cu/Ag composite substrates at 260°C, a typical reflow temperature used in industries, without any use of flux or solder. The Ag layer serves as a strain buffer and can deform with adequate plastic strain to endure the CTE difference between the chip and the Cu substrate. Ag has the highest electrical and thermal conductivities among all metals. Its use over Cu substrates improves the thermal performance of the resulting laser diodes. We believe that the highly conductive Cu/Ag composite substrate can be an asset for high power laser diode industries.
Chu-Hsuan Sha, PhD Student
University of California, Irvine
Irvine, CA
USA


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