Page 6 - Read Online

Page 4 of 10 Zou et al. Soft Sci 2024;4:19 https://dx.doi.org/10.20517/ss.2024.13

integrating Micro-LEDs onto a flexible or curvy substrate, primarily because the conventional planar stamps
are difficult to be in conformal contact with the curvy/flexible surface. The issue can be partially alleviated
by micro-assembling Micro-LEDs onto a planar soft substrate fixed on a temporary hard substrate. The
temporary supporting substrate is then removed, enabling the integration of Micro-LEDs onto soft
substrates . Alternatively, several modified micro-assembly methods have been developed to address the
[6]
challenge of Micro-LED integration onto curved substrates directly [Figure 1D-F]. Rao et al. developed a
ballon-shaped stamp for printing micro-scale devices onto a curvy surface [Figure 1D]. Hu et al.
[60]
[11]
proposed using a roller stamp for flexible Micro-LED integration [Figure 1E]. Guo et al. developed a
photosensitive polymer adhesive stamp-based method [Figure 1F]. These approaches exhibit excellent
[17]
capability for micro-assembling micro-scale chips onto both planar and curvilinear surfaces due to the
improved conformity with the receiver and adhesion switchability. However, transfer speed, printing yield,
and placement accuracy are key factors that need continuous improvement, especially for the device
integration onto curvy substrates.

Mechanical structure design
The brittle nature of Micro-LEDs prevents the system (i.e., Micro-LEDs and integrated substrates) from
accommodating large strain induced by bending, especially in the localized areas close to Micro-LEDs. Extra
mechanical design can provide further room to enhance the system performance and stretchability.
Representative strategies toward enhanced deformation of Micro-LEDs include island-bridge [15,61,62] ,
buckling [16,63] , and kirigami/origami structures [9,60] . Stretchable Micro-LEDs with the island-bridge
configuration have rigid island arrays (where these LEDs are fixed) and meandering metallic interconnects
[61]
connected to individual islands [Figure 1G]. In this structure, the Micro-LED devices on the rigid islands
are hardly affected by external strain, whereas the meandering interconnects, which are commonly
patterned into serpentine or noncoplanar arc-shapes, can tolerate most of the strain and geometric
deformation. Stretchable Micro-LED devices can also be achieved by incorporating buckled structures
[16]
[Figure 1H], inspired by the fact that a flexible film on a pre-stretched soft substrate (e.g., PDMS) can turn
into periodically wrinkled structures once the pre-stressed substrate is relaxed. Such wrinkle structures can
be flattened if the substrate is re-stretched, which allows the flexible devices to be stretched without
mechanical failure. Micro-LED integrated onto a substrate with kirigami/origami structures is another
[9]
smart strategy for fabricating stretchable Micro-LEDs [Figure 1I]. The 2D stress exerted on the Micro-
LED device can be significantly reduced via 3D shape transformation induced by the cutting/folding lines,
thereby mitigating stress concentrations around the Micro-LED devices.

EMERGING APPLICATIONS
With the above innovations in material epitaxy, chip transfer, device fabrication, and mechanical designs,
great progress has been made toward developing advanced deformable Micro-LEDs with expanded
functionalities for applications such as advanced displays and healthcare.

Deformable displays
Currently, most display electronics take the rigid, planar shape, but in the future, foldable/flexible displays
may become the mainstream products. To this end, Micro-LEDs in deformable formats have undergone
intensive research recently due to their higher luminescent efficiency and longer lifetime than their organic
counterparts. Industrial research in this field has led to great progress in developing deformable Micro-LED
display prototypes with encouraging performances. For instance, AU Optronics successfully demonstrated a
9.4-inch 228-ppi flexible Micro-LED display integrated onto a flexible low-temperature polysilicon thin-film
transistor (LTPS-TFT) backplane based on fine-pitch flip-chip bonding, followed by LLO to take off the
temporary supporting carrier [Figure 2A]. The display features a high contrast ratio of > 1,000,000:1, a
[12]
brightness of up to 700 nits, good display uniformity, and minimized color shift at any off-axis viewing
angles, revealing its potential for high-resolution flexible automotive applications. Royole Co. demonstrated