Investigating the Performance and Reliability of Au-Sn Bonded Flip-Chip Micro-LEDs
Luqiao Yin, Haojie Zhou, Xiaoxiao Ji, Jianxin Li, Kefeng Wang, Chunya Li, Jianhua Zhang
Abstract
In this article, a flip-chip <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$15\times30$ </tex-math></inline-formula> blue micro-LED array with a pixel size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20\times 22\,\,\mu \text{m}$ </tex-math></inline-formula> and a pixel pitch of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$84.6~\mu \text{m}$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.9\times 10^{{6}}$ </tex-math></inline-formula> cd/m2 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~\mu \text{A}$ </tex-math></inline-formula> 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 °C, it presents a uniformly high luminance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.51\times 10^{{5}}$ </tex-math></inline-formula> cd/m2) and a large shear force (2.83 g). With the further increase in temperature (320 °C), despite the increase in shear force (3.36 g), the luminous efficiency of micro-LED decreases significantly ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5.7\times 10^{{4}}$ </tex-math></inline-formula> cd/m2). 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.