High-power AlGaN deep-ultraviolet micro-light-emitting diode displays for maskless photolithography | Nature Photonics

Nature Photonics (2024)Cite this article

Metrics details

Developing aluminium gallium nitride deep-ultraviolet (UVC) micro-light-emitting diodes (micro-LEDs) with sufficient power has been a challenge, which particularly limits these devices to various applications. However, advanced fabrication processes have been developed to enable the demonstration of highly efficient 270 nm UVC micro-LEDs and large-format UVC micro-LED displays with high resolution for maskless photolithography. Optical and electrical characterizations were performed on UVC micro-LEDs with sizes ranging from 3 µm to 100 μm to evaluate these emerging devices. The 3 μm device achieved a record-high peak external quantum efficiency of 5.7% and a maximum brightness of 396 W cm–2. Moreover, 2,540 pixels per inch parallel-connected UVC micro-LED arrays featuring rear-side reflection layers exhibited emission uniformity and collimation. UVC micro-LED displays, with a resolution of 320 × 140, were explicitly designed for maskless photolithography applications utilizing a customized integrated circuit driver for optimal performance. The UVC micro-LEDs and UVC micro-displays provide sufficient doses to fully expose the photoresist film within seconds, owing to their enhanced current spreading uniformity, improved heat dispersion and superior light extraction efficiency. This work may open a path to maskless photolithography, potentially leading to revolutionary developments in the semiconductor industry.

Gallium nitride (GaN)-based light-emitting diodes (LEDs) have played a pivotal role in reshaping the landscape of lighting technology since their groundbreaking debut in 1993 (ref. 1), revolutionizing solid-state lighting due to their remarkable efficiency and versatility. The potential of GaN-based optoelectronics extends far beyond lighting, fostering innovation across various applications, including displays, visible-light communications, sensors and photolithography2,3,4,5. Historically, blue GaN-based LEDs have been dominant. However, the escalating demand for ultraviolet (UV) technology in sterilization combatting viruses6 has fuelled interest in AlGaN-based UV LEDs (aluminium nitride alloyed with GaN) spanning the entire UV spectrum (210–400 nm). These UV LEDs are gaining ground due to their superior efficiency, extended lifespans and reduced environmental impact7.

UV photolithography, critical in semiconductor and microelectronic devices manufacturing8, is shifting from conventional mercury-vapour lamps to UV-LED lamps or arrays9. Simultaneously, there is growing interest in maskless photolithography, as it promises pattern flexibility, customization options and real-time adjustments by harnessing smaller spot sizes and precise exposure control. UV micro-scale LEDs have become instrumental in this context, where size impacts resolution and light output power (LOP). Although smaller sizes offer higher resolution, they often come with lower LOP, spurring research in UV micro-scale LEDs and maskless photolithography.

Numerous research groups have explored maskless photolithography using UV micro-scale LEDs, with particular emphasis on the shortest deep-UV (UVC) wavelengths (210–280 nm) due to their potential for higher resolution, following Rayleigh’s equation10. For instance, a circular array of a UVC micro-light-emitting diode (micro-LED) emitting at 262 nm was pioneered, generating 196 μW of optical power from a 566 μm2 emission area11. In particular, slanted sidewalls in the UVC micro-LEDs have been shown to enhance the external quantum efficiency (EQE), primarily through improved light extraction efficiency (LEE), particularly in devices down to 5 µm (ref. 12). In another development, a 280 nm UVC parallel-arrayed planar 16 × 16 UVC array achieved a wall-plug efficiency of 4.7% and an LOP of 83.5 mW, benefiting from an aluminium mirror reflection at the mesa surface13. However, a significant challenge in maskless UVC photolithography remains protracted exposure times due to insufficient optical power14. Despite recent reports of large-sized (>200 μm) UVC LEDs with EQEs exceeding 10% (ref. 15), challenges related to power and efficiency persist in the realm of current UVC micro-LEDs (≤100 μm), particularly under demanding conditions16,17.

This study presents a comprehensive analysis of the size-dependent optical and electrical characteristics of UVC micro-LEDs ranging in size from 3 μm to 100 μm. Additionally, we provide an in-depth examination of uniformity and light distribution analysis in 2,540 pixels per inch (ppi) UVC micro-LED arrays. In particular, our smallest device, 3 μm, achieves a remarkable peak EQE of 5.7% and an impressive LOP density of 396 W cm–2. Furthermore, a 320 × 140-resolution UVC micro-display is fabricated using a flip-chip bond with a customized complementary metal–oxide–semiconductor (CMOS) integrated circuit driver. To highlight the practical application, we demonstrated maskless photolithography on positive photoresists using the developed UVC LEDs and UVC micro-display, paving the way for short-time exposure in UV photolithography applications.

AlGaN-based UVC micro-LEDs, ranging in size from 3 µm to 100 µm, are fabricated using a commercial epitaxial wafer (TES). Figure 1a presents the schematic of a stand-alone UVC micro-LED studied in this research. Owing to the significant compressive strain, extensive lattice and thermal coefficient mismatch between the AlGaN/AlN epitaxial layers and sapphire substrate, the two-inch wafer displays a considerable bowing height exceeding 100 μm (Supplementary Fig. 1a). This pronounced bowing effect poses a major obstacle in achieving large-format UVC micro-LED displays, as it causes substantial alignment gaps during fabrication processes such as electrode patterning, hole etching and flip-chip bonding. By using an optimized alignment method using laser-diced small UVC wafer pieces, the reduced height difference between the epitaxial wafer and photomask ensures an acceptable exposure pattern. UVC micro-LED arrays and stand-alone devices (Fig. 1b) are fabricated with mesa lengths as short as 3 μm. An ultrathin annealed Ni/Au metal stack is chosen for the p-contact layer, as this nearly transparent alloy in the UV region minimizes the absorbance of top-emitted UVC light. Figure 1c displays the electroluminescence (EL) images of various-sized devices under different injected current densities, demonstrating that devices of all sizes function well at operational current densities, with smaller devices requiring higher current density for similar brightness in EL images.

a, Schematic of a flip-chip UVC micro-LED. b, Scanning electron microscopy morphology of a fabricated 6 × 6 μm2 UVC micro-LED array, with the inset showing a stand-alone 5 × 5 μm2 UVC micro-LED. c, EL micrography of stand-alone devices.

The current density–voltage (J–V) characteristics of stand-alone UVC micro-LEDs with varying sizes are displayed in Fig. 2a. Leakage currents remain below the instrument’s minimum detection limit (under 100 fA) at −5 V reverse bias for devices of all sizes (Supplementary Fig. 2a). This can be attributed to the reduced sidewall damage resulting from tetramethylammonium hydroxide (TMAH) treatment and the atomic layer deposition (ALD)-grown sidewall passivation18. As the device size decreases, micro-LEDs support higher current densities under identical forward-bias conditions due to the reduced lateral spreading length and improved current spreading uniformity19. Furthermore, the alleviated current crowding effect and increased surface-to-volume ratios contribute to enhanced heat dissipation in smaller devices, mitigating thermal degradation under high current injection20.

a, J–V characteristics of UVC micro-LEDs in different device sizes in semi-log scales. b, Size-dependent EQE at various injected current densities. c, Peak EQE and EQE droop ratio in relationship to each device size point (shown as dots) with fitted trendlines. d, Normalized EL spectra of 3 μm UVC micro-LED. e, Size-dependent optical power density at various injected current densities.

The size-dependent ideality factors (n) are determined using derivative methods from the current–voltage (I–V) characteristics (Supplementary Fig. 2b). According to the Shockley model, n > 2 is attributed to deep-level traps or high-resistance ohmic contact in GaN-based LEDs21. The calculated ideality factors for UVC micro-LEDs range from 3.9 to 2.8, indicating that non-radiative recombination processes dominate across all devices. Each device features an ALD-Al2O3 passivation layer and TMAH treatment, which can help suppress sidewall damage caused by dry etching22,23. Therefore, non-radiative recombination centres at the sidewall surfaces can be considered negligible. We speculate that the primary source of non-radiative recombination centres stems from the unoptimized quality of epitaxial wafers, necessitating significant improvements in epitaxial wafer quality. Intriguingly, the n value decreases from 3.9 to 2.8, and the voltages corresponding to the lowest n value as a function of the increase in current density from 3.95 V to 4.2 V with decreasing device size. This trend might be explained by improved current spreading uniformity in smaller devices. However, identifying the precise physical reasons requires further investigation, which is beyond the scope of this study.

Figure 2b displays the EQE as a function of current density for various device sizes, with the corresponding wall-plug efficiency shown in Supplementary Fig. 3c. The current density (Jpeak) at the peak EQE rises from 15 A cm–2 to 70 A cm–2 as the device size decreases from 100 μm to 3 μm. This increase in Jpeak in smaller devices, coupled with the relatively modest EQEs (below 6%) compared with blue or green passivated micro-LEDs24,25, suggests that passivation and TMAH treatments may not be entirely effective in suppressing non-radiative recombinations originating from defects caused by sidewall damage. Therefore, the higher Jpeak value in smaller devices may be attributed to remaining defects, even with passivation and TMAH treatments.

Figure 2c presents the size-dependent peak EQE and EQE droop ratio, with the EQE droop defined as the ratio of the EQE at a current density of 500 A cm–2 (a typical current density for micro-LEDs used in optical communication26) to the peak EQE value. As the device size decreases, the EQE droop diminishes from 67.5% to 17.9%, indicating that smaller devices offer higher stability of light emission at elevated current densities due to their superior heat dissipation.

The 3 μm UVC micro-LED reaches a record-high EQE of 5.7% at a current density of 70 A cm–2, one of the highest values achieved for UVC micro-LEDs emitting at 270 nm. The EQE comprises the internal quantum efficiency (IQE) and LEE, two mechanisms potentially counteracting each other. As the size decreases, non-radiative recombinations at the sidewall may reduce the IQE, leading to higher Jpeak. Conversely, smaller devices emit light closer to the sidewalls, resulting in more sidewall refraction and consequently higher LEE. These effects scale with the device size. In the range of 100 μm to 30 μm, the changing trends of IQE and LEE remain relatively stable or comparable, resulting in a consistent EQE of about 4.8%. However, for sizes smaller than 30 μm, LEE enhancement dominates over potential IQE reduction12,27. Additionally, smaller devices exhibit improved current spreading uniformity. Consequently, the increased EQE from 30 μm to 3 μm can be attributed to enhanced current spreading uniformity and higher LEE in these smaller devices.

Figure 2d shows the EL spectra of the 3 μm device at various injection current densities. The spectra exhibit a strong emission at the peak wavelength (λp) of ~270 nm. The full-width at half-maximum ranges from 16.3 nm to 20.8 nm, corresponding to 0.277 eV to 0.348 eV. The spectral shift is dominated by blueshift (from 272 nm to 270 nm) at low current densities (up to 70 A cm–2) and then transitions to redshift (from 270 nm to 271 nm) at higher current densities (Supplementary Fig. 3a). This shift is due to the competition between bandgap shrinkage caused by self-heating and band-filling effects28,29,30,31. However, the total spectral shift across all the current densities is only about 2 nm, which can be attributed to the enhanced heat transfer path, leading to a slower rise in junction temperature28.

The LOP is calculated by multiplying the absolute spectral response with the emitting area. Optical power increases with higher current injection for devices of all sizes until performance degradation occurs due to extreme junction temperatures at high-brightness operation, resulting from thermal management issues and related ageing effects. The largest 100 μm UVC micro-LED device provides the highest LOP of 4.5 mW at a current injection of 35 mA (Supplementary Fig. 3b). Conversely, the smallest 3 μm device delivers a maximum LOP density of 396 W cm–2, approximately nine times higher than the 100 μm device (43.6 W cm–2) (Fig. 2e). Smaller devices, with better current spreading uniformity and thermal stability, can sustain higher current densities, thereby achieving greater optical power densities. This may also be due to the waveguiding effect in AlGaN multilayers27, where larger devices experience increased power loss due to a longer optical path from the emissive multiple quantum wells to the air. Additionally, it is worth noting that smaller devices yield slightly higher optical power densities at the same injection current density. At a current density of 100 A cm–2, commonly used in UVC LED illumination, the LOP density reaches 25.9 W cm–2 when the device size is reduced to 3 μm, demonstrating its excellent potential as a photolithography light source.

UVC micro-LED arrays are increasingly valued in photolithography and photochemistry as tools for generating arbitrary image patterns and transferring them onto light-sensitive materials like photoresists, eliminating the need for costly photomasks. Previously, demonstrations were limited to small arrays, such as 16 × 16 (ref. 13). The main obstacle in scaling up to large-array format has been the emission non-uniformity across devices, primarily due to variations in crystalline growth and fabrication processes32. With recent advancements in AlGaN strain modulation and advanced fabrication techniques, we have successfully developed a uniform 160 × 90 UVC micro-LED array. This array features a pixel size of 6 μm and a pitch of 10 μm (Fig. 3a). Furthermore, a highly reflective Al-based layer within the UVC wavelength range mounted on top of the entire array enables parallel connections. This setup enhances the collection of rear-side light, predominantly sourced from the mesa sidewall extractions, to boost the front screen emission intensity. Each pitch of the 2,540 ppi screen comprises a 6 μm emission area and a 4 μm space gap between pixels (Fig. 3b). Figure 3c,d displays favourable uniformity of the forward voltage (VF) (at J = 10 A cm–2) and the normalized EL intensity (at VF = 5 V) extracted from the I–V curves and spectra of periodically selected single pixels within the same array (Supplementary Fig. 4a,b). The optical emission power of 16.6 mW (Supplementary Fig. 5a) was achieved at a current density of 20 A cm–2 and a forward bias of 12 V (Fig. 3e). The EQE of the screen peaks at 4.1% under 8 A cm–2 current injection.

a, Micrography of the whole screen at the top view after pad connection. b, Front-view EL micrography of UVC micro-LED array. The inset shows the appearance of the whole screen. c,d, Periodically selected pixel behaviour of a UVC micro-LED screen: I–V curves in the semi-log scale (c) and the EL emission intensity spectra (d). e, EQE and LOP versus injection current density, with I–V characteristics shown in the inset. f, Far-field luminance distribution.

Figure 3f presents a three-dimensional plot generated from angle-dependent light distribution measurements of the UVC micro-LED array (Supplementary Fig. 5b), demonstrating a narrow viewing angle of 48.9°. This characteristic results in superior light collimation compared with a typical blue or green screen with a viewing angle of around 120° (ref. 33). A narrower viewing angle may be attributed to the whole rear-side reflection layer enhancing front light emission. The UVC micro-LED display offers an adequate optical power density of up to 1.1 W cm–2 for full-screen lighting, surpassing the 25 mW cm–2 calibration of the 365 nm mercury lamp used in the Karl Suss MA-6 mask aligner to meet the photoresist exposure dose requirements.

In maskless lithography applications, a UVC light source and optical image projection component should be integrated with circuit boards to generate and project digital UV patterns onto the photoresist layer. For testing maskless photolithography, 320 × 140-resolution UVC micro-displays are fabricated with a pixel size of 9 μm and a pitch of 12 μm. Indium bumps (Supplementary Fig. 6a) are utilized to connect the devices and CMOS driver, which is controlled by a circuit board (Supplementary Fig. 6b). To display various images and videos (Supplementary Videos 1 and 2), a field-programmable gate array motherboard is connected using a flexible cable. Since UVC micro-LEDs typically require a relatively high VF, the full-load voltage provided by the CMOS driver restricts the display brightness under controllable operation, resulting in unclear images (Supplementary Fig. 6c). To achieve a high injection driving condition, custom carrier substrates (see the ‘Flip-chip process of the UVC micro-display chip’ section) are designed with a patterned metal layer deposited on Si wafers. Figure 4a demonstrates that carrier-controlled UVC micro-display can provide excellent uniformity and considerable LOP by applying sufficient injected currents. Table 1 compares the performance of UVC micro-LED micro-displays reported in this work with representative references. The advanced fabrication process for ultrafine-pitch UVC micro-LEDs enables the successful demonstration of a lower forward voltage, higher peak EQE and LOP density, and large-format micro-displays.

a, 320 × 140 UVC micro-LED displays controlled by carriers. b,c, Optical microscopy images of the photoresist pattern exposed by stand-alone 100 × 100 μm2 (b) and 10 × 10 μm2 (c) UVC micro-LEDs. d,e, Surface profiles of the photoresist pattern fully exposed by 100 × 100 μm2 (d) and 10 × 10 μm2 (e) UVC micro-LED. f,g, Corresponding maskless photolithography images (f) and surface profile (g) revealed on photoresist-coated wafers by UVC micro-LED display photolithography.

Finally, maskless photolithography is carried out using high-performance stand-alone UVC micro-LEDs in single-point exposure. The proximity approach is used, eliminating the need for projective microscope objectives for demagnification. A commonly used positive photoresist, AZ MiR 703, is used for a test. After post-exposure development in FHD-5 for 60 s, circular patterns emerge on the photoresist film. Figure 4b illustrates the photoresist being exposed for 1, 2 and 3 s using a 100 μm device with an injected current density of 100 A cm–2. For a 10 μm device emission at 1 kA cm–2 injection, overexposure takes slightly longer at 30 s due to the three-dimensional diffuser of light (Fig. 4c). Supplementary Fig. 7a confirms that even the smallest 3 μm UVC micro-LED emits enough optical power to partially expose a wafer, generating a 0.5-μm-height step pattern (Supplementary Fig. 7b) on the photoresist film after 5 min exposure. Although Newton’s rings appear in this process because of a thin air layer trapped between the surfaces, the lithography process can be further optimized by minimizing the gap between the UVC micro-LED and the wafer using a vacuum chuck. A stylus profilometer measures the surface profiles of fully exposed patterns, yielding step heights of 1.1 μm (Fig. 4d,e). The thickness consistency with the coating manual validates the full exposure results for 100 μm and 10 μm UVC micro-LEDs.

Pattern transfer from UVC micro-displays to the photoresist film is also investigated. With an 80 mA current injection, the UVC micro-display provides a sufficient dose for maskless photolithography. After a 5 s exposure on the photoresist-coated wafer, a mirror-written structure develops on the wafer surface (Supplementary Fig. 7c). Figure 4f displays the revealed pattern micrography on the exposed photoresist-coated Si wafer (Supplementary Fig. 7d (scales are shown)). The surface profile of the exposed and unexposed areas reveals a photoresist thickness of 1.1 μm (Fig. 4g). These results bring the practical use potential of maskless photolithography using UVC micro-LED displays into semiconductor manufacturing, including the fabrication of micro-LED displays (Supplementary Fig. 8).

Although the structural resolution is not as high as that achieved with contact exposure, related lens and focusing systems could significantly improve maskless photolithography. As smaller linewidths down to the pixel size of micro-display chips show great potential, such maskless photolithography systems could provide considerable time and cost savings for the semiconductor industry by eliminating the need for laser-writing masks.

Advanced fabrication methods have been used to fabricate highly efficient UVC micro-LEDs, with sizes ranging from 3 μm to 100 μm. The smallest device size of 3 μm has an impressive peak EQE of 5.7% and a maximum LOP density of 396 W cm–2, owing to improvements in light extraction, heat dispersion and partial strain relief. The stand-alone UVC micro-LEDs and 320 × 140 UVC micro-display have successfully demonstrated the feasibility of maskless photolithography by offering sufficient LOP density for photoresist film exposure within seconds.

These findings significantly affect the development of UVC micro-LED display sources for maskless photolithography. One major constraint is the quality of epitaxial layers, which is the primary factor affecting the performance of UVC micro-LED devices. Therefore, research is urgently needed to improve the epitaxial growth processes of AlGaN multiple quantum wells and electron blocking layers in conjunction with micro-LED fabrication. Additionally, the intense bowing effect of UVC epitaxial wafers can cause mask alignment deviation, presenting a fundamental limitation for reducing device sizes to a few micrometres or a few hundred nanometres. Although our advanced fabrication process enables the creation of a UVC micro-LED array with a resolution of 320 × 140, this falls short compared with the 1K or 2K resolution of GaN-based blue and green micro-LED arrays or displays34. To overcome this limitation, enhancing the epitaxial wafer quality and achieving more accurate alignment could pave the way for developing much-higher-resolution UVC micro-LED displays such as 2K to 8K.

The mesa area of each pixel was patterned by photolithography with pre-deposited SiO2 as a hard mask. A hybrid etching scheme combining inductively coupled plasma and chemical (TMAH) treatment was utilized for precise mesa definition to enhance radiative recombination39. The mesa was treated with 3.5 wt% TMAH solution in a 40 °C water bath for a total of 30 min, resulting in slightly sloped sidewalls that facilitated light extraction40. Metal combinations of Ti/Al/Ni/Au were electron-beam evaporated on the n-AlGaN surface and annealed at 950 °C in N2 as electrodes. Although indium tin oxide is typically used for p contacts on GaN-based blue or green micro-LEDs, it gets rapidly absorbed in the UVC region. Thus, an ultrathin Ni/Au layer was deposited and annealed at 550 °C in an N2 and O2 ambiance to perform ohmic contacts to p-GaN. Subsequently, a 50 nm Al2O3 passivation layer was applied using ALD to passivate the exposed sidewall dangling bonds. During the thermal ALD synthesis, trimethylaluminum (Al(CH3)3) was exposed to the micro-LED surface, reacting with H2O vapour at 300 °C. This reaction produced methane (CH4) as a byproduct and formed a hydroxylated Al2O3 layer, enhancing the device’s overall performance. The passivation layer was patterned by inductively coupled plasma etching to open the contact holes. Last, Ti/Al-based metal stacks with a 500-nm-thick aluminium reflection layer were mounted on top as connection pads, providing connections for wire bonding onto a printed circuit board driving board after the wafer was thinned and polished by grinding.

A Si3N4 passivation layer with plasma-enhanced chemical vapour deposition was further grown on the thin ALD-Al2O3 layer to guarantee complete coverage by the dielectric process and enough protection for the devices. The contact holes were patterned by a high-temperature overcoated photoresist and etched with inductively coupled plasma in an SF6/CHF3 atmosphere. A short buffered oxide etch dip followed the dry etching to fully uncover the electrodes for the following metal contact. After a 5 μm indium layer was deposited and lifted off, the wafer was grinded and polished on the rear side. The reflow process was then performed in an ATV SRO-700 vacuum oven at 220 °C in a formic acid gas atmosphere, turning the flat pads into spherical bumps. The indium bumps were aggregated right in the centre of the contact holes with a height of 3.5 μm. Applying 15 kg pressure and thermal treatment in an ACCμRA-100 bonder, the UVC micro-LED screens were finally flip chipped to a customized two-transistor one-capacitor CMOS integrated circuit driver, which is wire connected to a printed circuit board with metal core backplanes. Indium bumps fabricated carriers as separated metal pads for p and n connections. Each carrier was designed to display a different image through parallel-connected electrodes. By flip chipping to different carriers, the UVC micro-LED chips display various patterns.

The electrical characteristics were analysed using a SEMISHARE probe station with a Keysight B1500A semiconductor analyser, and the optical behaviours were measured using an Ocean Optics USB 2000+ spectrometer with QP600-1 UV VIS fibre optics. For output light collection, an Ocean Optics FOIS-1 integrating sphere with Spectralon light-diffusing coating that reduces cavity fluorescence41 was utilized to measure the indistinctive optical power of single microdevices and micro-LED screens. The absolute spectral response system was calibrated using a radiometrically light source, DH-3P-BAL-CAL (Ocean Optics). Considering the insensitive detection of UVC emission to the spectrometer and the small LOP values at low injections, an integrating time of up to 5 s was applied for the ultrafine-pitched stand-alone devices to achieve an accurate accumulation. All the optical measurements are processed in a black-shield box in a cleanroom area (class 1,000) to minimize outside disturbance and organic contamination.

The angle-dependent light distribution was measured using a SITAN-LDM2 system, in which the d.c. power source (Keithley 2400) provided a continuous injected current to the micro-LED screen and the optical probe (Konica Minolta CA-P410) collected emission light from the screen. The brightness value (Lv) with the corresponding horizontal rotating angle (in the φ direction) and vertical rotating angle (in the θ direction) were transferred to the computer for post-processing. By horizontally rotating this screen from a viewing angle φ (−0° to 180° in a step of 10°) at each vertical rotating angle θ (−90° to 90° in a step of 10°), the far-field luminance distribution was measured and transferred by

onto a three-dimensional plot.

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Nakamura, S., Senoh, M. & Mukai, T. High‐power InGaN/GaN double‐heterostructure violet light emitting diodes. Appl. Phys. Lett. 62, 2390–2392 (1993).

Article ADS Google Scholar

Ferreira, R. X. G. et al. High bandwidth GaN-based micro-LEDs for multi-Gb/s visible light communications. IEEE Photon. Technol. Lett. 28, 2023–2026 (2016).

Article ADS Google Scholar

Feng, F. et al. AlGaN multiple quantum well deep‐ultraviolet micro‐light‐emitting diodes for high color conversion efficiency quantum dots display. J. Soc. Inf. Disp. 30, 556–566 (2022).

Article Google Scholar

Liu, Y. et al. R/G/B micro-LEDs for in-pixel integrated arrays and temperature sensing. ACS Appl. Electron. Mater. 3, 3–10 (2021).

Article Google Scholar

Hölz, K., Lietard, J. & Somoza, M. M. High-power 365 nm UV LED mercury arc lamp replacement for photochemistry and chemical photolithography. ACS Sustain. Chem. Eng. 5, 828–834 (2017).

Article Google Scholar

Raeiszadeh, M. & Adeli, B. A critical review on ultraviolet disinfection systems against COVID-19 outbreak: applicability, validation and safety considerations. ACS Photonics 7, 2941–2951 (2020).

Article Google Scholar

Schindler, K. et al. High intensity UV-LED mask aligner for applications in industrial research. In MikroSystemTechnik 2017; Congress 1–4 (VDE, 2017).

del Barrio, J. & Sánchez‐Somolinos, C. Light to shape the future: from photolithography to 4D printing. Adv. Opt. Mater. 7, 1900598 (2019).

Zheng, L. et al. UV-LED projection photolithography for high-resolution functional photonic components. Microsyst. Nanoeng. 7, 64 (2021).

Article ADS Google Scholar

Okazaki, S. Resolution limits of optical lithography. J. Vac. Sci. Technol. B 9, 2829–2833 (1991).

Article Google Scholar

He, X. et al. 1 Gbps free-space deep-ultraviolet communications based on III-nitride micro-LEDs emitting at 262 nm. Photon. Res. 7, B41–B47 (2019).

Rass, J. et al. Enhanced light extraction efficiency of far-ultraviolet-C LEDs by micro-LED array design. Appl. Phys. Lett. 122, 263508 (2023).

Li, D. et al. Deep-ultraviolet micro-LEDs exhibiting high output power and high modulation bandwidth simultaneously. Adv. Mater. 34, e2109765 (2022).

Article Google Scholar

Wu, M.-C. & Chen, I. T. High‐resolution 960 × 540 and 1920 × 1080 UV micro light‐emitting diode displays with the application of maskless photolithography. Adv. Photon. Res. 2, 2100064 (2021).

Takano, T. et al. Deep-ultraviolet light-emitting diodes with external quantum efficiency higher than 20% at 275 nm achieved by improving light-extraction efficiency. Appl. Phys. Express 10, 031002 (2017).

Tian, P. et al. AlGaN ultraviolet micro-LEDs. IEEE J. Quantum Electron. 58, 3300214 (2022).

Kneissl, M., Seong, T.-Y., Han, J. & Amano, H. The emergence and prospects of deep-ultraviolet light-emitting diode technologies. Nat. Photon. 13, 233–244 (2019).

Article ADS Google Scholar

Kim, K. et al. Efficiency enhancement of InGaN/GaN blue light-emitting diodes with top surface deposition of AlN/Al2O3. Nano Energy 43, 259–269 (2018).

Article Google Scholar

Malyutenko, V., Bolgov, S. & Podoltsev, A. Current crowding effect on the ideality factor and efficiency droop in blue lateral InGaN/GaN light emitting diodes. Appl. Phys. Lett. 97, 251110 (2010).

Article ADS Google Scholar

Luo, X., Hu, R., Liu, S. & Wang, K. Heat and fluid flow in high-power LED packaging and applications. Prog. Energy Combust. Sci. 56, 1–32 (2016).

Article Google Scholar

Shah, J. M., Li, Y.-L., Gessmann, T. & Schubert, E. F. Experimental analysis and theoretical model for anomalously high ideality factors (n ≫ 2.0) in AlGaN/GaN p-n junction diodes. J. Appl. Phys. 94, 2627–2630 (2003).

Article ADS Google Scholar

Tang, B. et al. Enhanced light extraction of flip-chip mini-LEDs with prism-structured sidewall. Nanomaterials 9, 319 (2019).

Article Google Scholar

Wong, M. S. et al. Size-independent peak efficiency of III-nitride micro-light-emitting-diodes using chemical treatment and sidewall passivation. Appl. Phys. Express 12, 097004 (2019).

Article ADS Google Scholar

Sheen, M. et al. Highly efficient blue InGaN nanoscale light-emitting diodes. Nature 608, 56–61 (2022).

Article ADS Google Scholar

Liu, Y. et al. Analysis of size dependence and the behavior under ultrahigh current density injection condition of GaN-based micro-LEDs with pixel size down to 3 μm. J. Phys. D: Appl. Phys. 55, 315107 (2022).

Li, Z. et al. Bandwidth analysis of high-speed InGaN micro-LEDs by an equivalent circuit model. IEEE Electron Device Lett. 44, 785–788 (2023).

Article ADS Google Scholar

Ley, R. T. et al. Revealing the importance of light extraction efficiency in InGaN/GaN microLEDs via chemical treatment and dielectric passivation. Appl. Phys. Lett. 116, 251104 (2020).

Gong, Z. et al. Size-dependent light output, spectral shift and self-heating of 400?nm InGaN light-emitting diodes. J. Appl. Phys. 107, 013103 (2010).

Article ADS Google Scholar

Takeuchi, T. et al. Quantum-confined Stark effect due to piezoelectric fields in GaInN strained quantum wells. Jpn. J. Appl. Phys. 36, L382–L385 (1997).

Article Google Scholar

Pong, B.-J. et al. Abnormal blue shift of InGaN micro-size light emitting diodes. Solid State Electron. 50, 1588–1594 (2006).

Article ADS Google Scholar

Yang, L., Wei-Choon, N., Choquette, K. D. & Hess, K. Numerical investigation of self-heating effects of oxide-confined vertical-cavity surface-emitting lasers. IEEE J. Quantum Electron. 41, 15–25 (2005).

Article ADS Google Scholar

Kojima, K. et al. Carrier localization structure combined with current micropaths in AlGaN quantum wells grown on an AlN template with macrosteps. Appl. Phys. Lett. 114, 011102 (2019).

Article ADS Google Scholar

Gou, F. et al. Angular color shift of micro-LED displays. Opt. Express 27, A746–A757 (2019).

Article Google Scholar

Wu, M. C., Chung, M. C. & Wu, C. Y. 3200 ppi matrix-addressable blue MicroLED display. Micromachines 13, 1350 (2022).

Qian, Z. et al. Analysis of the efficiency improvement of 273 nm AlGaN UV-C micro-LEDs. J. Phys. D: Appl. Phys. 55, 195104 (2022).

Floyd, R. et al. Enhanced light extraction efficiency of micropixel geometry AlGaN DUV light-emitting diodes. Appl. Phys. Express 14, 084002 (2021).

Yu, H. et al. AlGaN-based deep ultraviolet micro-LED emitting at 275 nm. Opt. Lett. 46, 3271–3274 (2021).

Article ADS Google Scholar

Yao, Y. et al. Size dependent characteristics of AlGaN-based deep ultraviolet micro-light-emitting-diodes. Appl. Phys. Express 15, 064003 (2022).

Feng, F. et al. Enhancing the optical and electrical properties of AlGaN ultraviolet-C micro-LED via a hybrid scheme of plasma and chemical treatment. Appl. Phys. Lett. 121, 221104 (2022).

Tian, M. et al. Enhanced light extraction of the deep-ultraviolet micro-LED via rational design of chip sidewall. Opt. Lett. 46, 4809–4812 (2021).

Article ADS Google Scholar

Janecek, M. Reflectivity spectra for commonly used reflectors. IEEE Trans. Nucl. Sci. 59, 490–497 (2012).

Article ADS Google Scholar

Download references

We thank L. Samuelson for discussions. This work was supported in part by the National Key R&D Program of China under grant no. 2023YFB2806800, Fundamental and Applied Fundamental Research Fund of Guangdong Province 2021B1515130001 grant, and Shenzhen Science and Technology Program (grant no. JCYJ20220818100603007). We acknowledge Nanosystem Fabrication Facility (NFF (CWB)), Materials Characterization and Preparation Facility (MCPF (CWB)) and Electronic Packaging Laboratory (EPACK Lab) at the Hong Kong University of Science and Technology; Core Research Facilities at Southern University of Science and Technology; and Shenzhen Sitan Technology for precious technical support.

State Key Laboratory on Advanced Displays and Optoelectronics Technologies, The Hong Kong University of Science and Technology, Hong Kong, People’s Republic of China

Feng Feng, Yibo Liu, Ke Zhang & Hoi-Sing Kwok

Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, People’s Republic of China

Hang Yang, Byung-Ryool Hyun & Zhaojun Liu

Department of Platform for Characterization & Test, Suzhou Institute of Nanotech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, People’s Republic of China

Ke Xu

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

F.F. and Z.L. conceived the core strategy and designed the experiment. Z.L. and H.-S.K. supervised this project. F.F., Y.L. and K.Z. designed the layout. F.F. performed the fabrication and characterizations with the help of Y.L. and H.Y. F.F., K.Z., K.X., H.-S.K. and Z.L. provided the materials and analysis instruments. F.F., Y.L. and Z.L. discussed and interpreted the experimental results. F.F. and B.-R.H. prepared the paper. All authors reviewed and commented on the paper.

Correspondence to Zhaojun Liu.

The authors declare no competing interests.

Nature Photonics thanks Peter Parbrook and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Figs. 1–8.

Moving images of geometries.

Moving images of logos.

Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.

Reprints and permissions

Feng, F., Liu, Y., Zhang, K. et al. High-power AlGaN deep-ultraviolet micro-light-emitting diode displays for maskless photolithography. Nat. Photon. (2024). https://doi.org/10.1038/s41566-024-01551-7

Download citation

Received: 18 April 2023

Accepted: 12 September 2024

Published: 15 October 2024

DOI: https://doi.org/10.1038/s41566-024-01551-7

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative