# Micro-LEDs, a Manufacturing Perspective
## 3 Assembly Technologies
### Monolithic Fabrication of Micro-Displays  
Simply, the monolithic methods can be divided into two categories, i.e., fully monolithic integration and monolithic hybrid. In conventional display technologies, such as LCDs or OLEDs, the processing temperatures for the emission elements are typically below 400 \({}^{\circ}\)C, which can be withstood by the CMOS-based driver ICs. Therefore, fully monolithic integration via direct process of emission elements on the driver ICs is not problematic. However, the growth temperatures of  
Figure 4: Schematic illustration of the generic process flow for elastomer stamp micro-assembly. (**1**) Prepare donor substrate; apply rubber stamp; (**2**) quickly peel-back stamp; grab objects off of donor; (**3**) apply inked stamp to receiving substrate; and (**4**) slowly peel-back stamp; print objects onto receiver. Reprinted/adapted by permission from Springer Nature: Nature materials, reference [41]. Copyright (2009). III-nitrides are typically above 900 \({}^{\circ}\)C. It is impossible to grow and process LEDs directly on top of backplane with conventional ICs. Additionally, some new fully monolithic methods have been proposed and investigated. Since III-nitrides are excellent materials for both LEDs and high-frequency and high-power transistors, fully monolithic active matrix micro-displays can be realized by the integration of individual GaN-based LED driven by a GaN-based transistor for each pixel. [PERSON] et al. have reported monolithic integration of InGaN/GaN light emitting diodes (LEDs) with AlGaN/GaN heterojunction field effect transistors (HFETs) [44] or vertical metal-oxide-semiconductor field effect transistor (VMOSFET) drivers [45], by selective growth of HFET or VMOSFET structures on the LED epilayers, as shown in Figure 6. Alternatively, lateral integration via selective removal of the HFET structure followed by selective growth of the LED structure on top of the exposed GaN buffer layer [46] or selective removal of the LED structure followed by selective growth of the metal-oxide-semiconductor field effect transistor (MOSFET) on top of the exposed n-GaN layer [47] have also been reported. Although it has been demonstrated that the performance of the integrated single LED-transistor chip is comparable to discrete LEDs and transistors [44] fabrication of micro-LED displays using this approach has not yet been reported. Si-based TFT, instead of III-nitride-based transistor, has also been investigated for on-chip driving III-nitride LEDs. [PERSON] et al. have reported monolithic integration of a Si n-MOSFET and a GaN  
Figure 5: Schematics of (**a**) roll-to-roll nanoimprint lithography (R2 RNIL) and (**b**) roll-to-roll nanoimprint lithography (R2 PNIL) process. (**c**) Photograph of a six-inch-capable R2R/R2 PNIL apparatus [38]. Copyright 2009 American Chemical Society. Figure 6: Optical micrograph of the monolithically integrated VMOSFET-LED device. Reprinted from reference [45], with the permission of AIP Publishing. micro-LED [48; 49]. To achieve this, a Si layer from a silicon-on-insulator (SOI) substrate was bonded to a GaN-based LED structure with a top SiO\({}_{2}\) layer via surface-activated bonding [44]. Si-MOSFETs were then fabricated on the Si/SiO\({}_{2}\)/GaN-LED structure by using a CMOS process line. The interconnects were realized via the connection of the cathode of a 30 \(\times\) 30 \(\upmu\)m\({}^{2}\) sized GaN-LED to the drain of a Si n-MOSFET with a gate width of 100 \(\upmu\)m and gate length of 10 \(\upmu\)m. It was found that thermal degradation could be avoided by optimizing the thickness of the top-Si layer coupled with processing below 900 \({}^{\circ}\)C. It is stated that electrical and optical properties of the integrated Si n-MOSFET and GaN micro-LED devices are comparable to those of the standard devices. Due to the difficulties and incompatibility between the micro-LED and driver IC processes in fully monolithic integration, the most common approach adopted by micro-LED display developers is monolithic hybrid.