# Micro-LEDs, a Manufacturing Perspective
## 3 Assembly Technologies
### Mass Transfer Technologies  
Traditional pick and place equipment via mechanical manipulation fails to transfer millions of LED dies in a cost-effective manner. In recent years, many companies have devoted considerable effort  
Figure 2: Effect of size-reduction on the maximum EQE of GaN-based micro-LEDs. Data compiled from reference [31]. to overcome this problem by exploring a number of approaches involving of laser, electrostatic, or electromagnetic, van der Waals forces, or fluidic forces, as summarized in Table 1.  
Laser-induced forward transfer is a mass transfer process which uses a laser beam to induce the separation of the micro-LEDs from their carrier substrate and then transfer them to a receive substrate, as shown in Figure 3[35]. The irradiation with a laser beam results in the light-matter interaction at the interface between the carrier substrate and the dies, with the result that the dies are detached from the substrate and, at the same time, generates a local mechanical force to push the dies toward the receive substrate. The interfacial interaction can be the same as that used in laser lift-off process of GaN LEDs from the sapphire substrate, in which a thin GaN layer (-10 nm) is ablated at the substrate epi interface and decomposed into nitrogen gas and liquid Ga. It may also be implemented via using a temporary substrate with a polymer adhesive serving as the interfacial layer, which decomposes when irradiated with a laser beam. It has been reported that the Laser-induced forward transfer technology enables some 1000 chips to be transferred per laser shot. Optovate, a British optic tech company, has demonstrated its patterned laser lift-off (p-LLO) transfer technology to transfer blue micro-LEDs from a sapphire wafer to a receive substrate. [PERSON] has reported that a transfer rate above 100 million per hour can be achieved using the massively parallel laser-enabled transfer (MPLET) technology [36]. Fluidic assembly via gravity and capillary forces to drive and capture the micro-LEDs on to the array of the driver ICs has also been investigated for mass transfer of LED chips [37; 38]. The self-assembly is conducted in a fluid, such as isopropanol, acetone or distilled water. The fluid suspends and lets the LEDs move across the surface of the receive substrate till they reach the open receptor sites where they are captured. Once positioned, the anode and cathode electrodes of the micro-LEDs can then be bonded with the driver ICs for electrical connections. eLux, a Foxconn-owned company, has been developing its fluidic assembly methods for massive parallel assembly of micro-LEDs [39]. It was reported that a fill (transfer) rate over 50 million devices per hour can be achieved. \begin{table}
\begin{tabular}{c c c c} \hline
**Methods** & **Company/Institute** & **Force/means** & **Transfer rate** \\ \hline Laser lift-off & Optovate/Uniqrta & Laser & 1000 per shot/100 million per hour \\ Fluidic assembly & Foxconn(eLux) & Gravity and capillary & 50 million per hour \\ Electrostatic array & Apple (LuxVue) & Electrostatic & (Not found) \\ elastomer stamp & X-Celeprint & van der Waals & 1 million per hour \\ Roll-to-roll & KIMM & Roll stamp & 10,000 per sec \\ \hline \end{tabular}
\end{table} 
Table 1: Some mass transfer technologies. Figure 3: Schematic of laser-induced forward transfer (LIFT) process. Adapted from reference [35], Copyright 2016, with permission from Elsevier. LuxVue, an Apple Co.-owned company, has developed a transfer technology that operates in accordance with the electrostatic principle, which uses the attraction of opposite charges to pick up micro-LEDs [40].