Copper Substrate Electroplating For Vertical-structure LEDs

Ga N-based light emitting diodes(LEDs) have been widely used in various fields such as lightings, displays, and communications due to their outstanding merits of long life time, high reliability, low energy cost, etc. However, as the LEDs are used in highend industry, which requires the high-power and high-efficiency LEDs. Therefore, the development of high-power and high-efficiency LEDs has been attracted considerable interests. Among various technologies in preparation of high-power and high-efficiency LEDs, the vertical structure technology seems to be a suitable one and has been attracting more and more attentions due to the fact that the vertical structure LEDs are beneficial to the heat dissipation and current distribution of LED devices. When fabricating LEDs with vertical structure, substrate transferring technique which can greatly improve the light extraction efficiency of LEDs is the emphasis of research. However, the commonly used technology, wafer bonding, is cost-consumed and requires for rigorous experimental conditions. In contrast, low-cost electroplating technology has lower demands on experimental conditions, and enhances the contact performance between LED chips and new substrates. In this regard, adopting electroplating technology instead of traditional bonding technology to fabricate vertical-structure LED devices is of significance to the industrialization of high-power and high-efficiency LEDs.In this paper, electroplating technology is adopted to obtain high-quality Cu substrates on Ga N-based LED wafers. The problems of various structures for LEDs and the processes of substrate transferring are introduced in detail, and, therefore, an optimal technology roadmap for electroplating Cu substrates is proposed. The influences of electrode positions, current density, electroplating time, Cu SO4 and additives contents, on the qualities of as-fabricated Cu substrates are systematically studies. The composition, thickness uniformity and surface morphology of Cu substrates are characterized by x-ray diffraction, scanning electron microscope, and atomic force microscope.It is found that the optimal current density is 30 m A/cm2, under which Cu substrates with no copper scraps attaching can be obtained, and its RMS surface roughness is measured to be 13.6 nm. The thickness of Cu substrates increases gradually with the increase of electroplating time. However, when it exceeds 3 hours, the surface of Cu substrates becomes rough, with many large particles attached. In view of the surface roughness and deposition rate, the best electroplating time is considered to be 3 hours. Both the current density and electroplating time have significant impacts on the surface roughness and thickness of as-fabricated Cu substrates.As for the influence of Cu SO4 electroplating solution, it is revealed that when the Cu SO4 content is less than 65 g/L, the Cu substrate surface becomes rough and visibly dim, while when it exceeds 95 g/L, the shining area decreases and the deposition become insufficient. It indicates that Cu SO4 content is the key to successful electroplating. Through carefully analyzing the thickness homogeneity, surface morphology of as-fabricated Cu substrates, the optimal Cu SO4 content is obtained to be 85g/L. When it comes to additives, it is revealed that the concentration and ratio of brightening agent, leveling agent and cylinder opening agent are critical to the qualities of as-fabricated Cu substrates, i.e. an inappropriate proportion would cause serious fragility and cracks. The influence of different additives contents is comprehensively studied. When the contents of these three additives are all 5 m L/L, high-quality Cu substrates with smooth surface can be obtained, and its RMS is greatly reduced to 7.17 nm.In conclusion, high-quality electroplated Cu substrates ready for vertical-structure LEDs have been obtained. The adhesion strength between Cu substrates and LED wafers is strong, and the thickness homogeneity is more than 90%. They exhibit excellent surface morphology without any copper particles, confirmed by the reduced RMS roughness at around 7.17 nm. This thesis presents an electroplating technique that is convenient to implement with high quality products at low cost, which has great perspectives in the vertical LEDs.