Improving thermal management in high power LEDs through fabricating nano-twinned copper substrates

Abstract

The epitaxial growth of GaN-based epilayers on sapphire substrates still remains as the main stream of manufacturing LED chips. The thermal load in high power and ultra-high power LEDs keeps being a big issue in engineering practices. Self-heating effects will result in efficiency droop, degradation of lighting qualities, activation of defects, etc. As an unavoidable problem accompanying LED's low external quantum efficiency, thermal management attracts great interests both in academic and industry areas. The laser lift-off technique was developed to decrease the thermal resistance of a LED chip. However, the process of laser lifting-off sapphire substrates will not only increase the cost, but also raise reliability concerns. Efforts paid on managing thermal issues in high power LEDs are never stopped. The present work intends to ease the thermal load in LEDs through adding heat dissipation channels, i.e. electro-depositing special copper films on the back of sapphire substrates. The special copper substrate is expected to be constructed by micro-/nano- dual sale crystals with highly (111) textures and naturally formed high density nano-twins. Such microstructures will endow the copper film characteristics of high strength, high thermal conductivity and high heat capacity. The highly (111) texture also can help to control the solder joint microstructures. In this paper, process validation and device fabrication are done to verify the applicability to ease thermal load through electrodepositing the special copper film on the sapphire substrate. Experimentally, copper films with a degree of 95% (111) texture are successfully obtained. Microstructure analyses show that the copper grains are constructed by high density nano-twins. In other words, the copper grains present a micro-/nano- dual scale structure, which has been confirmed to have high strength, high thermal conductivity and high electrical conductivity in literatures. Based on the electroplated highly (111) textured copper films, high power LEDs with a thermal resistance below 3.0°C/W are achieved experimentally.