Nanoscale coating for microchannel cooler protection in high powered laser diodes

High powered laser diodes are used in many applications that require stable optical output, such as materials processing, medical and military applications, and solid-state laser pumping [1,2]. Due to the low electrical to optical efficiencies of 25-35%, significant waste heat is generated which increases the junction temperature and alters the wavelength of the emitted beam. The industry standard for rejecting this waste heat is single-phase, copper microchannel cooler (MCC) using high-purity de-ionized water (DIW) coolant. However, the high-purity DIW coolant is pumped through the copper MCCs in presence of leakage current at high velocity, which causes erosion-corrosion damage to the micron sized heat transfer surface areas, and reduces the thermal performance of the cooler. Since the corrosion of copper is highly dependent on the pH and dissolved oxygen (DO) content of the DIW coolant, strict control on water conditioning system is employed. This strict control scheme leads to additional equipment, costs and reliability concerns. Extension of the cooler lifetime by protecting the MCCs against erosion-corrosion can be achieved by applying a uniform, pin-hole free and conformal coating to the internal features of the MCCs. However, many coating and plating techniques, such as nickel and gold plating, cannot meet these requirements. Thus, a vapor phase deposition technique that uniformly applies a nanometer thin, conformal, inert, hard, coating to the high-aspect ratio internal features of the MCC was developed to protect the copper MCC against erosioncorrosion in high powered laser diode applications. Corrosion rate measurements of baseline uncoated and coated copper samples exhibited a one to two order of magnitude reduction in corrosion rate when exposed to DIW with a pH of 6.0 - 9.0 and a DO concentration ranging from 0.5 ppm to 10.0 ppm. This study shows that the strict controls required to maintain the pH and DO can be severely relaxed by applying the coating resulting in reduced operational costs and increased reliability. Furthermore, evaluations of the coating thickness deposited throughout the microchannel region of the MCC demonstrate the uniform and conformal application of the coating in the high aspect ratio features. Lastly, thermal and hydraulic performance evaluations of coated MCCs revealed that the application does not impede the thermal or hydraulic performance of the MCC, thereby enabling lifetime extension without adding pumping po wer requirements.