Research on Au-In Flip-Chip Bonding Process of 2300 PPI Micro-LED Arrays

In this article, a 0.52-in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1056\times 586$ </tex-math></inline-formula> green micro-LED array with 2300 PPI and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10.8~\mu $ </tex-math></inline-formula>m pixel pitch was integrated with the substrate by Au-In flip-chip bonding. Through the analysis and comparison of the performance of the micro-LED devices at different bonding conditions, the temperature and pressure could significantly affect the optical performance of micro-LED and the strength of the bump connections. The experimental results indicate that the micro-LED device achieved the highest brightness and brightness uniformity at the bonding condition of 180 °C–6 kg, with an average 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">$6.7\times 10^{{4}}$ </tex-math></inline-formula> cd/m2 and a brightness uniformity of 59.3% at 3 V. Further analysis shows that the turn-on voltage of the micro-LED was about 2.7 V, and the ON-resistance was about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.99~\Omega $ </tex-math></inline-formula>. Moreover, the full width at half maximum (FWHM) spanned from 28.1 to 29.8 nm, accompanied by a peak wavelength fluctuation of 3.6 nm. Additionally, the shear force of the micro-LED devices and the intermetallic compound (IMC) composition of the fractured surface of the indium bumps at different bonding conditions were analyzed. The shear force increased with rising bonding temperature and was notably enhanced by increasing the pressure at lower bonding temperatures. Through SEM and energy dispersive X-ray spectroscopy (EDS) analysis, AuIn2 was the primary IMC formed in the bonding process, which further confirmed that the formation of IMC was closely related to the bonding strength.