Abstract
Displays in which arrays of microscopic ‘particles’, or chiplets, of inorganic light-emitting diodes (LEDs) constitute the pixels, termed MicroLED displays, have received considerable attention1,2 because they can potentially outperform commercially available displays based on organic LEDs3,4 in terms of power consumption, colour saturation, brightness and stability and without image burn-in issues1,2,5,6,7. To manufacture these displays, LED chiplets must be epitaxially grown on separate wafers for maximum device performance and then transferred onto the display substrate. Given that the number of LEDs needed for transfer is tremendous—for example, more than 24 million chiplets smaller than 100 μm are required for a 50-inch, ultra-high-definition display—a technique capable of assembling tens of millions of individual LEDs at low cost and high throughput is needed to commercialize MicroLED displays. Here we demonstrate a MicroLED lighting panel consisting of more than 19,000 disk-shaped GaN chiplets, 45 μm in diameter and 5 μm in thickness, assembled in 60 s by a simple agitation-based, surface-tension-driven fluidic self-assembly (FSA) technique with a yield of 99.88%. The creation of this level of large-scale, high-yield FSA of sub-100-μm chiplets was considered a significant challenge because of the low inertia of the chiplets. Our key finding in overcoming this difficulty is that the addition of a small amount of poloxamer to the assembly solution increases its viscosity which, in turn, increases liquid-to-chiplet momentum transfer. Our results represent significant progress towards the ultimate goal of low-cost, high-throughput manufacture of full-colour MicroLED displays by FSA.
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Data availability
The data that support the findings of this study are available from the corresponding authors on request.
Code availability
The code used to determine the alignment errors of the assembled chiplets is available from the corresponding authors on request.
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Acknowledgements
This research was supported by LG Electronics Research Project no. 0534-20140020, Global Research Development Center Program no. 2015K1A4A3047345, grants from the National Research Foundation of Korea funded by the Ministry of Science and ICT (no. NRF-2020R1A3B3079653) and the Ministry of Education (nos. NRF-2019R1I1A1A01059219 and 2020R1I1A1A01075042), the BK21 Plus Program (Creative Research Engineer Development for IT, Seoul National University, 2019–2020) and the BK21 FOUR Program (Education and Research Program for Future ICT Pioneers, Seoul National University, 2021–2023).
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Contributions
S.K. conceived the original idea. D.L., S.C. and Cheolheon Park performed all experiments except for the fabrication of LED chiplets and characterization of MicroLED panels. Changseo Park, K.J., H.Y., W.C., C.H.L., T.K., Y.H.M., M.J., Y.-H.C., J.S.L. and K.R.P. manufactured LED chiplets and performed electrode fabrication. J.L. performed characterization of MicroLED panels. D.L., S.C., Cheolheon Park and K.R.P. fabricated the chiplets used in the study of the assembly mechanism. K.A. and J.N. provided help with viscosity measurements and analysed fluid and chiplet dynamics. D.L., S.C., Cheolheon Park, C.K. and S.K. analysed experimental results. C.K. and S.K. supervised the project. D.L., S.C., Cheolheon Park, J.L., S.K. and C.K. wrote the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Reduction of assembly yield due to the reduced mass of GaN chiplets in conventional agitation-based FSA.
a, The width (WCh) and depth (DCh) of the chiplets are 20 and 40 μm, respectively. The binding sites with a width (WB) of 20 μm and a depth (DB) of 40 μm are arranged with horizontal (PH) and vertical (PV) periods of 120 and 360 μm, respectively. b, WCh = 30 μm, DCh = 30 μm, WB = 30 μm, DB = 30 μm, PH = 120 μm, PV = 360 μm. c, WCh = 20 μm, DCh = 40 μm, WB = 30 μm, DB = 30 μm, PH = 80 μm, PV = 80 μm. d, WCh = 30 μm, DCh = 30 μm, WB = 30 μm, DB = 30 μm, PH = 80 μm, PV = 80 μm. In all cases, the thickness of the chiplets is 5 μm, and the assembly yields are smaller than 5%; scale bars in a and b, 500 μm; scale bars in c and d, 300 μm.
Extended Data Fig. 2 Dependence of the assembly yield on the concentration of high molecular weight poloxamer in the assembly solution.
The uses of rectangular-cuboid-shaped (width, 20 μm; depth, 40 μm; height, 5 μm) GaN chiplets and square shaped (30 × 30 μm2) binding sites resulted in random rotational orientations, which was not of concern because these experiments were performed only to find the optimized concentration of the poloxamer. a, poloxamer concentration: 0.2 wt%, assembly yield < 40%. b, poloxamer concentration: 1 wt%, assembly yield < 80%. c, poloxamer concentration: 2 wt%, assembly yield > 97%. d, poloxamer concentration: 4 wt%, assembly yield < 15%; scale bars, 400 μm.
Extended Data Fig. 3 Yield versus viscosity characteristics.
To investigate the dependence of the assembly yield on viscosity while ruling out other chemical effects from poloxamer, we used a water/glycerol mixture (red) as the assembly solution, where the viscosity was varied by adjusting the glycerol concentration. The yield versus viscosity curve for this system is found to be similar to that for the water/poloxamer system (green).
Extended Data Fig. 4 Three major forces exerted on chiplets.
For model descriptions see Methods, ‘Mechanical analysis of a single chiplet travelling in a vigorously agitated liquid’.
Extended Data Fig. 5 Effect of the size of the binding sites on the assembly quality.
a, Three or more Si chiplets (width, 150 μm; depth, 150 μm; height: 150 μm) were assembled at a single binding site (200 × 200 μm2). b, Two or more GaN chiplets (width, 30 μm; depth, 30 μm; height, 5 μm) were assembled at a single binding site (30 × 30 μm2). In both experiments, the entire bottom surface of the chiplets were coated with Au; scale bars, 100 μm.
Extended Data Fig. 6 Schematic of MicroLED chiplet fabrication.
a, 5-μm-thick multilayer for InGaN/GaN-based blue LEDs epitaxially grown on a sapphire substrate is patterned by photolithography followed by inductively coupled plasma etch. b, Au layer deposited using an electron-beam evaporator is patterned by a lift-off process. c, SiO2 deposited by plasma-enhanced chemical vapor deposition is patterned by photolithography followed by dry etch. d, Photoresist layer, which later serves as a sacrificial release layer, is patterned by photolithography, onto which an adhesive film is attached. e, Chiplets are separated from the sapphire wafer by laser lift-off (LLO). f, Ti and SiO2 layers are sequentially deposited. g, Chiplets are released from the adhesive film by immersion into a solvent.
Extended Data Fig. 7 Images of the elements of a MicroLED array panel assembled by using an assembly solution with high molecular weight poloxamers (2 wt%).
a, Optical microscope image of the assembled MicroLED lighting panel; scale bar, 500 μm. b, Scanning electron microscope (SEM) image of disk-shaped GaN chiplets (diameter: 45 μm, thickness: 5 μm) before assembly; scale bar, 80 μm. c, Cross sectional SEM image of an assembled chiplet; scale bar, 10 μm.
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Lee, D., Cho, S., Park, C. et al. Fluidic self-assembly for MicroLED displays by controlled viscosity. Nature 619, 755–760 (2023). https://doi.org/10.1038/s41586-023-06167-5
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DOI: https://doi.org/10.1038/s41586-023-06167-5
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