Advances in sophisticated electronic systems have driven the development of large-scale, programmable assembly techniques for integrating ultrasmall components onto arbitrary substrates. However, conventional assembly techniques based on mechanical grippers and vacuum nozzles are difficult for manipulating ultrasmall objects, because the surface capillary force and electrostatic force of a small object are dominant over the gravity force. Furthermore, these techniques cannot meet the stringent requirement for fast manipulation of large-quantity devices, due to the fact that only one device or a few devices can be manipulated a time. For these reasons, it is highly desirable to develop fast programmable assembly techniques for parallel manipulation of tiny objects with good uniformity, large area, excellent printing accuracy, and high printing yield.

Fig.1 Working principle of the photo-triggered selective micro-assembly technique based on a photosensitive polymer adhesive.

 To address these challenges, a team led by Prof. Zheng Gong at the Institute of Semiconductors, Guangdong Academy of Sciences have developed a light-based micro-assembly technique based on a photosensitive polymer adhesive, which allows fast, large-area programmable assembly of microscale objects onto arbitrary substrates (Fig.1). The developed polymer adhesive exhibits fast optical response, undulated surface topography, and tunable adhesion upon UV light irradiation. Large-scale micro-assembly is enabled by flood exposure of the photo-sensitive polymer, which results in the polymer switching from the strong adhesion state to the weak adhesion state. Masked UV exposure, on the other hand, provided an additional means to modulate the polymer surface topography. In the latter case, the indented regions of the polymer surface switch to the non-contact state because of the gap formed between the inks and the polymer. With this light-mediated surface topography, micro-objects can be assembled in a programmable format based on users’ needs.

The fidelity of this technique has been verified by assembling a wide range of materials and devices, such as  indium tin oxide (ITO), gallium nitride (GaN), gold membranes, perovskite QDs, with wafer size up to 4-inch (Fig.2). Regardless of the different inks, very high transfer printing yield and placement accuracy have been achieved in most cases.

Fig.2 Transfer printed inks made from different materials. (a) ITO patterns transferred to a 2-inch glass substrate. (b) Ultrathin gold patterns transferred to a 4-inch PET substrate. (c) Transfer printed perovskite QDs. (d) Selectively transfer-printed GaN inks using a photomask.

 The proposed technique has also been further applied in assembling Micro-LEDs for display purposes (Fig.3). Micro-LEDs are miniaturized LEDs with typical dimensions of a few tens of microns, which have received intense research interest for developing high-resolution displays, because of their excellent characteristics such as short response time, low power consumption, high brightness, high stability, and long lifetime. However, the lack of mature Micro-LED mass transfer techniques has hindered the path for commercializing Micro-LED display technology. Although a variety of Micro-LED assembly techniques have been proposed, they commonly show compromised performance in terms of the transfer speed, placement accuracy, and transfer yield. The light-triggered micro-assembly proposed in this work may shed light on overcoming these challenges. High selectively printed Micro-LEDs with high yield, large-area, and high placement accuracy are demonstrated in Fig.3, revealing its potential for developing high-resolution Micro-LED display panels.

The proposed technique overcomes some limits of currently available transfer printing techniques. First, the stamp adhesion switchability is simply tuned by external ultraviolet irradiation which is widely used in modern semiconductor industry such as curing, bonding, and photolithography. The ultraviolet stimulus has minimized damage to inks to be transferred since it can be remotely delivered to the target object. Second, the current method facilitates the formation of a patterned stamp through simple masked irradiation without the need for accessing expensive lithography and nano-imprinting tools, which would be otherwise required in other techniques. Most importantly, the current technique has the ability for fast, large-area assembly (up to 4-inch) of ultrasmall, and delicate functional components in a programmable manner into spatially organized arrangements with arbitrary layouts. 

Fig.3 Selectively transfer printed Micro-LEDs for displays. (a) “Arrow” shaped Micro-LED arrays. (b) Micro-LEDs transferred to a curvy surface. (c) “LED” shaped Micro-LED arrays. (d) Printed 20x25 Micro-LED arrays for micro-displays.

Article info:

Chan Guo, Zhangxu Pan, Changhao Li, Shenghan Zou, Chao Pang, Jiantai Wang, Jinhua Hu, Zheng Gong*, npj Flexible Electronics, 6, 44 (2022).

Link: https://www.nature.com/articles/s41528-022-00180-w.