Furong Chen4,1,2, Jing Bian4,1,2,3, Jinlong Hu1,2, Ningning Sun1,2, Biao Yang1,2, Hong Ling1,2, Haiyang Yu1,2, Kaixin Wang1,2, Mengxin Gai1,2, Yuhang Ma1,2 and YongAn Huang5,1,2Hide full author list
Published 14 November 2022 • © 2022 The Author(s). Published by IOP Publishing Ltd on behalf of the IMMT
International Journal of Extreme Manufacturing, Volume 4, Number 4
Citation Furong Chen et al 2022 Int. J. Extrem. Manuf. 4 042005
DOI 10.1088/2631-7990/ac92ee
Abstract
Inorganic-based microlight-emitting diodes (microLEDs) offer more fascinating properties and unique demands in next-generation displays. However, the small size of the microLED chip (1–100 µm) makes it extremely challenging for high efficiency and low cost to accurately, and selectively, integrate millions of microLED chips. Recent impressive technological advances have overcome the drawbacks of traditional pick-and-place techniques when they were utilized in the assembly of microLED displays, including the most broadly recognized laser lift-off technique, contact micro-transfer printing (µTP) technique, laser non-contact µTP technique, and self-assembly technique. Herein, we first review the key developments in mass transfer technique and highlight their potential value, covering both the state-of-the-art devices and requirements for mass transfer in the assembly of the ultra-large-area display and virtual reality glasses. We begin with the significant challenges and the brief history of the mass transfer technique and expand that the mass transfer technique comprises two major techniques, namely, the epitaxial Lift-off technique and the pick-and-place technique. The basic concept and transfer effects for each representative epitaxial Lift-off and pick-and-place technique in mass transfer are then overviewed separately. Finally, the potential challenges and future research directions of mass transfer are discussed.
Keywords: mass transfer, microLED displays, transfer printing, interfacial adhesion
Introduction
After liquid crystal display (LCD) and organic lightemitting diode (OLED) display, micro light-emitting diode (microLED) displays are recognized as the next-generation display technology in terms of their superior characteristics, such as ultrahigh brightness (∼107 cd m−2, compared with 1500 cd m−2 of OLED [1, 2]), nanosecond response time (∼104 and ∼107 times shorter than those of OLED and LCD [3], respectively), low power consumption (∼1% of LCD and 40% of OLED [4]), and long lifetime (>10 years, ∼8 years of LCD, ∼4 years of OLED [5]), high color reproduction of 140% (75% of LCD and 100% of OLED [6]) and wide view angle (Max.180◦ , Max. 89◦ of LCD and OLED [7]), etc. Currently, many fascinating displays, which cannot be realized with traditional display technologies, become reality with microLED, examples vary from ultra-large displays (e.g. 146 inches ‘The Wall’ developed by Samsung) to ultrahigh-resolution (>5000 PPI [8]) virtual reality (VR) glasses and head-up displays. In addition, microLED-based flexible electronics can be served as medical sensors [9–11] to monitor physical/psychological conditions or treat disease [12], and microLED-based visible light communication (VLC) systems with high data rate (⩾10 Gbps [13–15]) and modulation bandwidth (⩾100 MHz [16, 17]) make deep-sea communication possible [18, 19]. These excellent features provide a strong incentive to develop high-efficiency and low-cost assembly concepts and processes for large-scale and highdensity microLED arrays. High-resolution microLED displays rely on millions of polychromatic self-emissive elements that consist of red, green and blue (RGB) microLEDs [20]. Figure 1 shows the simplified fabrication process of microLED displays (center part in figure 1) and representative applications that have been described above. Generally, limited by the growth techniques, it is difficult to simultaneously grow RGB microLEDs on a epitaxial wafer (only ∼8 inches [21, 22]). Thus, deterministic assembly of microLED chips from different growth/- donor substrates is required. However, since the feature size of microLED chips are <100 µm, there is a trade-off between tiny feature size and fabrication feasibility. There are three enormous challenges. (a) The extreme transfer efficiency (∼tens of millions h−1 ) is vital because of their vast numbers. For an 8 K displays, more than ∼100 million chips need to be transfered onto the receiver substrate, which will take several weeks for traditional pick-and-place techniques (e.g. assembly throughput ∼8000 chips per hour [23–25] for current flipchip bonding equipments [23, 26]). (b) The extreme placement accuracy is critical for microscale chips (∼5% of the microLED chip size [20, 27]). For example, the transfer error of a 10 µm-sized microLED chip should be smaller than 0.5 µm, which is far beyond the accuracy of traditional transfer techniques (e.g. only ∼30 µm [28]). (c) The extreme reliability (∼99.9999% [21]) is also essential. Otherwise, a 0.01% failure rate could result in thousands of dead pixels [29]. In short, traditional assembly methods are not practical for MicroLEDs. As a revolutionary technique specified by industry and mass transfer, which can release massive microscale chips (e.g. ∼millions h−1 ) from the donor/growth substrate an efficient rate and move them to the backplane/receiving substrate with high precision and reliability, it has proven to be a promising solution. These techniques typically introduce physical/chemical interactions (e.g. Van der Waals (VDW) force [33, 34], fluid tension [35], electrostatic force [36, 37], laser ablation [38, 39], selective etching [40, 41], etc) to switch the adhesion/de-adhesion state of the interface between the donor/transfer medium (e.g. elastomer stamp, fluid) and microLED chips in a highly controlled, scalable, and accurate way. For instance, the micro-transfer printing (µTP) utilized an elastomer stamp with thousands of tiny posts (∼10 800 posts [42]) to control the interface adhesion by tuning the peeling speed. Thus, microscale and even nanoscale structures can be selectively transferred in parallel. X-Celeprint demonstrated that µTP can achieve the transfer reliability (chip size of 8 × 15 µm2 ) of ∼99% [43], with the maximum transfer efficiency of >6.5 million h−1 [44] and the placement accuracy of ∼1.5 µm [45]. Recently, different kinds of lasers have been introduced to enable digital and parallel transfer processes with highly enhanced throughput, cost-efficiency, and process flexibility. Especially, extremely high assembly rates (>100 million h−1 ) can be achieved by arrays of laser beams [39, 46]. Because of recent significant progress, these mass transfer techniques could become candidates for industrialized manufacturing of microLED displays. Recently, many review papers have merely focused on microLED displays [5, 7, 47–49], which mainly concluded the technological path of microLED chips or the solution of full-color displays. Here, we attentively present an in-depth analysis of the latest developments of mass transfer techniques. In section 2, we first discuss the general assembly process (i.e. epitaxial lift-off and pick-and-place process) of microLED displays and highlight key challenges of mass transfer techniques. Then, various state-of-the-art mass transfer strategies and principles adopted in different production steps of microLED displays are described in the subsequent two sections. Finally, we discuss the future opportunities and challenges in this field.
Source of information: IOP SCIENCE
Disclaimer: The views and opinions expressed in this article are those of the original authors and do not necessarily reflect the official policy or position of MiniMicroLED Insights . While we strive to ensure the accuracy and reliability of the information provided, the content on this website may include translations, re-edited versions of second-hand information, or information derived from unverifiable sources. MiniMicroLED Insights makes no representations or warranties, express or implied, regarding the completeness, accuracy, or timeliness of such content. The information in this article is for informational purposes only and should not be construed as professional advice. Any reliance you place on such information is strictly at your own risk. To the fullest extent permitted by law, MiniMicroLED Insights disclaims all liability for any direct, indirect, incidental, consequential, or punitive damages arising out of your use of, or reliance on, the information contained in this article.
Copyright Notice: This article may include translated and re-edited content derived from various online sources, including websites and social media platforms. While we strive to credit the original authors and sources to the best of our ability, we may not always be able to verify the original source of the content. All rights to the original content remain with the original author or source publication. Where applicable, this content is reproduced for educational and informational purposes under the fair use doctrine. If you believe any content on this site infringes upon your intellectual property rights, or if you are the copyright owner and believe we have not credited you correctly, please contact us at minimicroled.business@gmail.com. We will investigate and take corrective action, including removing or properly crediting the content if necessary.
Content sourced and adapted by MiniMicroLED Insights (Doris).
