Jiho Joo

Perovskite quantum dots (PQDs) are promising color‐converting materials for full‐color micro‐light‐emitting diode (micro‐LED) displays; however, PQDs have intrinsic poor stability which hinders practical applications. In addition, micro‐LEDs that incorporate color‐converting layers with reliable operation remain limited. Herein, we report a highly‐stable thixotropic PQD‐based color converting resin (thixo‐PQD resin) by precise control of rheological properties and robust matrix encapsulation. Fumed silica nanoparticles (FSNs) are incorporated in thixo‐PQD resin that serve as thixotropic and scattering agents, thereby forming microlens thixo‐PQD arrays (ML thixo‐PQD arrays). The resulting ML thixo‐PQD arrays retain high photoluminescent quantum yield (PLQY ∼ 78%) in ambient air, under 85°C/85% RH, and under continuous blue‐light irradiation for > 2400 h. This stability of the ML thixo‐PQD array arises from synergistic effects of (i) effective physical blocking of water and oxygen by the FSN network and encapsulating matrix and (ii) chemical interactions between the PQD surface ligands and the encapsulating matrix. To confirm the feasibility as a color‐converting layer on micro‐LED, we print ML thixo‐PQD arrays directly onto blue‐emitting micro‐LED backplanes; the devices achieve a 59.2% increase in luminous flux and reliability under 85°C/85% RH for > 60 d. These thixo‐PQDs have potential application in high‐resolution, full‐color micro‐LED displays.

As the trend in advanced semiconductor packaging continues toward finer pitch and higher integration, bonding technologies are increasingly expected to meet stricter thermal and mechanical performance requirements. However, conventional methods such as Thermo-Compression Bonding (TCB) often lead to problems including thermal stress, long processing times and limitations of fine-pitch bonding. To overcome these challenges, this study introduces Room-Temperature Laser-Assisted Bonding with Compression (LABC), a next-generation bonding technique that enables localized heating and rapid cooling, minimizing thermal stress while improving alignment accuracy and process efficiency. To further enhance the electrical and mechanical reliability of LABC, we developed two types of Laser Non-Conductive Paste (LNCP), designated LNCP-(A) and LNCP-(B). These eco-friendly materials, flux-free and solvent-free, not only prevent void formation and fume generation but also eliminate the need for post-bond cleaning and underfill processes. The bonding experiments were conducted on 30 µm-pitch daisy-chain bump structures formed on 11 mm × 7 mm silicon chips, simulating High Bandwidth Memory (HBM). The glass transition temperatures (Tg) of LNCP-(A) and LNCP-(B) were measured to be 36.27 °C and 51.23 °C, respectively, via Differential Scanning Calorimetry (DSC). Following the bonding process, electrical resistance measurements, cross-sectional microstructural analysis, and temperature cycling (TC) tests were performed. LNCP-(B), with its higher Tg, exhibited improved thermal stability and lower resistance variation compared to LNCP-(A). Furthermore, the LABC process effectively suppressed intermetallic compound (IMC) growth, resulting in consistently thinner IMC thickness compared to those formed by TCB. In addition, shear strength testing confirmed the mechanical robustness of the bonded joints. These results demonstrate the effectiveness of LABC with optimized LNCP materials as a promising solution for high-reliability, fine-pitch interconnections in next-generation semiconductor packaging.

A simultaneous transfer and bonding (SITRAB) process using areal laser irradiation is introduced for high‐yield and cost‐effective production of mini‐ or micro‐light‐emitting diode (LED) display panels. SITRAB materials are special epoxy‐based solvent‐free pastes. Three types of pot life are studied to obtain a convenient SITRAB process: Room temperature pot life (RPL), stage pot life (SPL), and laser pot life (LPL). In this study, the RPL was found to be 1.2 times the starting viscosity at 25°C, and the SPL was defined as the time the solder can be wetted by the SITRAB paste at given stage temperatures of 80°C, 90°C, and 100°C. The LPL, on the other hand, was referred to as the number of areal laser irradiations for the tiling process for red, green, and blue LEDs at the given stage temperatures. The process windows of SPL and LPL were identified based on their critical time and conversion requirements for good solder wetting. The measured RPL and SPL at the stage temperature of 80°C were 6 days and 8 h, respectively, and the LPL was more than six at these stage temperatures.

The aim of this study was to develop a flexible package technology using laser-assisted bonding (LAB) technology and an anisotropic solder paste (ASP) material ultimately to reduce the bonding temperature and enhance the flexibility and reliability of flexible devices. The heat transfer phenomena during the LAB process, mechanical deformation, and the flexibility of a flexible package were analyzed by experimental and numerical simulation methods. The flexible package was fabricated with a silicon chip and a polyimide (PI) substrate. When the laser beam was irradiated onto the flexible package, the temperatures of the solder increased very rapidly to 220 °C, high enough to melt the ASP solder, within 2.4 s. After the completion of irradiation, the temperature of the flexible package decreased quickly. It was found that the solder powder in ASP was completely melted and formed stable interconnections between the silicon chip and the copper pads, without thermal damage to the PI substrate. After the LAB process, the flexible package showed warpage of 80 μm, which was very small compared to the size of the flexible package. The stress of each component in the flexible package generated during the LAB process was also found to be very low. The flexible device was bent up to 7 mm without failure, and the flexibility can be improved further by reducing the thickness of the silicon chip. The bonding strength and environmental reliability tests also showed the excellent mechanical endurance of the flexible package.

Anisotropic conductive paste (ACP) or anisotropic conductive film (ACF) consisting of a polymer matrix and a metal-coated polymer bead are introduced as interconnection materials in mini-LED display panel packaging through a thermal compression bonding process. However, this packaging solution has been leading to important problems such as alignment difficulties due to high bonding temperature and pressure, and its repair issues for bad LED devices after the bonding process. Recently, laser-assisted bonding (LAB) process for mini-LED display packaging has been introduced to solve these problems[Ref. 1-2]. This study introduces, an anisotropic solder paste (ASP) using conductive solder particles and applies it to the mini-LED packaging through the LAB process.

An epoxy-based solder paste (ESP) is a promising alternative to conventional solder pastes to improve the reliability of fine-pitch electrical joining because the epoxy encapsulates the solder joint. However, development of an appropriate epoxy formulation and investigation of its reaction mechanism with solder powder is challenging. In this study, we demonstrate a newly designed ESP consisting of diglycidyl ether of bisphenol F (DGEBF) resin, Sn-3.0 Ag-0.5 Cu (SAC305) solder powder, and L-glutamic acid (Glu), which is a proteinogenic amino acid for biosynthesis of proteins in living systems. The mechanism of the thermochemical reaction was explored and tentatively proposed, which reveals that the products of the reaction between SAC305 and Glu function as catalysts for the etherification of epoxides and alcohols produced by chemical bonding between DGEBF and Glu, consequently leading to highly crosslinked polymeric networks and an enhancement of impact resistance. Our findings provide further insight into the mechanism of the reaction between various formulations comprising an epoxy, amino acid, and solder powder, and their potential use as ESPs for electrical joining.

We present a silicon nitride (SixNy) arrayed waveguide grating (AWG). Crosstalk in this AWG was reduced by decreasing the phase error in a multimode arrayed waveguide (WG) at center wavelength of 838 nm. 8-channel SixNy AWGs with double-etched inverse-tapered WG structure and cross-sectional area of 1.5 ×1.5 µm2 were fabricated from arrayed WGs. The AWG exhibited less than.29.6 dB crosstalk with 0.43 dB insertion loss for transverse electric (TE) polarization.

This brief presents a clocked pluggable optics suitable for high-density data center interconnections. The proposed architecture performs a SERDES function at the module side by exploiting a forwarded clock from the ASIC. Due to the relaxed channel loss of the ASIC-to-module interface, the use of power-hungry equalizers can be avoided. Based on an 850-nm multi-mode fiber interface, a 25-Gb/s link operation is demonstrated. A vertical-cavity surface-emitting laser-based transmitter outputs an optical modulation power of 0.6 mW. The optical receiver sensitivity is measured to be -7.5 dBm at 21.2 Gb/s with an optical excitation, and 120 μApp at 25 Gb/s with an electrical excitation. The jitter tracking capability of the implemented clock and data recovery is evaluated in the presence of ±100ppm frequency offsets and the measured jitter tolerance complies with the 100 GbE specification well. The optical transceiver is implemented in 65-nm CMOS technology and consumes 281 mW at 25 Gb/s, corresponding to the energy efficiency of 11.2 pJ/b.