Investigating the Performance and Reliability of Au-Sn Bonded Flip-Chip Micro-LEDs

In this article, a flip-chip 15 $\times$ 30 blue micro-LED array with a pixel size of 20 $\times$ 22 $\mu $ m and a pixel pitch of 84.6 $\mu$ m was prepared. The micro-LEDs connected with Si substrate containing Sn bumps by flip-chip bonding and sapphire substrate was removed by wet etching. Prior to bonding, the single micro-LED exhibits a leakage current of 2.91 pA at $-$ 5 V and a threshold voltage of 2.5 V. After bonding, the leakage current slightly rises to 3.97 pA while the threshold voltage stays constant. The micro-LED array has a high brightness of 2.9 $\times$ 10 $^{\text{6}}$ cd/m $^{\text{2}}$ at 1 mA, as well as the peak wavelength exhibits a blue shift of 6.3 nm. The FWHM increased from 14.5 to 23.2 nm within the range of 10 $\mu $ A to 1 mA. Moreover, the electrical, optical, shear force, and Au-Sn intermetallic compound (IMC) composition of the fracture surfaces of the micro-LEDs under different bonding conditions were also analyzed. At 280 $^{\circ}$ C, it presents a uniformly high luminance (1.51 $\times$ 10 $^{\text{5}}$ cd/m $^{\text{2}}$ ) and a large shear force (2.83 g). With the further increase in temperature (320 $^{\circ}$ C), despite the increase in shear force (3.36 g), the luminous efficiency of micro-LED decreases significantly (5.7 $\times$ 10 $^{\text{4}}$ cd/m $^{\text{2}}$ ). The essential steps and bonding parameters for preparing high-performance micro-LED arrays are discussed in this study, which provides useful guidance for the application of micro-LEDs in the field of display technology.