The creation of 270 nm-wavelength deep ultraviolet-C (UVC) arrays of micro-LEDs for maskless proximity photolithography is reported by Chinese researchers [Feng Feng et al, nature photonics, published online 15 October 2024].
"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," remarks the team from Suzhou Institute of Nanotech and Nano-Bionics, Southern University of Science and Technology, and Hong Kong University of Science and Technology.
In contrast to mercury vapour lamps, UVC LEDs have historically been developed primarily for viral sterilisation applications because to their high efficiency, long lifetime, and no environmental effect.
A flip-chip UVC micro-LED system is shown in Figure 1. b. The shape of the 6μmx6μm UVC micro-LED array as seen by scanning electron microscopy, with a 5μmx5μm stand-alone array included as an inset. c. Micrography of standalone devices using electroluminescence (EL).
The UVC LED arrays were created by the researchers using commercial 2-inch aluminium gallium nitride (AlGaN) epitaxial wafers (Figure 1). "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," the team notes, referring to the difficulties caused by the wafers' more than 100μm bowing.
The strain effects brought on by the notable lattice and thermal expansion mismatch between the sapphire substrate and AlGaN layers are linked to this bending.
By employing tiny wafer portions that were singulated via laser dicing, the researchers were able to lessen the influence of the bending and achieve acceptable precision in array patterning down to 3μm mesa widths.
Ultrathin nickel/gold, which is almost transparent in the UVC wavelength region, made up the top p-contact.
Under reverse bias, the resultant device showed very low leakage currents, below the measuring equipment's 100fA detection limit. The team notes that this is because of the atomic layer deposition (ALD)-grown sidewall passivation and the decreased sidewall damage brought on by tetramethylammonium hydroxide (TMAH) treatment.
greater current density for a given bias was shown to be advantageous for smaller devices, leading to greater current uniformity throughout the LED.
"The improved surface-to-volume ratios and reduced current-crowding effect help to improve heat dissipation in smaller devices, reducing thermal degradation under high current injection," the team remarks.
As the forward bias rose from 3.95V to 4.2V, the devices' ideality factor decreased from 3.9 to 2.8. Non-radiative recombination resulting from the epitaxial wafers' suboptimal quality was credited with the high ideality.
According to the researchers, the sidewalls were a nearly insignificant source of non-radiative recombination centres because of the TMAH and passivation treatments they used. There was some indication, nonetheless, that "the passivation and TMAH treatments may not be entirely effective in suppressing non-radiative recombinations originating from defects caused by sidewall damage" in the smaller devices, down to 3μm.
As the device size shrinks from 100μm to 3μm, the peak external quantum efficiency (Figure 2) pushes towards greater current densities, going from 15A/cm2 to 70A/cm2. The EQEs were an order of magnitude lower than what could be obtained with green or blue passivated LEDs.
Figure 2 shows the peak EQE and EQE droop ratio for each device size (dots) together with trend lines relative to the peak value.
"The EQE droop decreases from 67.5% to 17.9% as the device size decreases," the team finds, demonstrating that smaller devices provide improved stability of light emission at higher current densities because of their superior heat dissipation.
Higher current-spreading uniformity and improved light extraction efficiency (LEE) are credited by the researchers for the rise in EQE for diameters less than 30 μm. "Smaller devices emit light closer to the sidewalls, resulting in more sidewall refraction and consequently higher LEE," the researchers say.
The devices' full-width at half maximum (FWHM) was less than 21 nm, and their peak wavelength was around 270 nm. At low currents, the 3μm device's peak wavelength blueshifted by 2 nm, while at higher currents (beyond 70A/cm2), it redshifted by 1 nm.
According to the scientists, this change is the result of band-filling effects and self-heating-induced bandgap shrinking competing with one another. The improved heat transfer route, which causes a slower rise in junction temperature, is responsible for the overall spectral shift across all current densities, which is only about 2 nm.
With a density of 43.6W/cm2, the light output power (LOP) of the 100μm LEDs was 4.5mW at 35mA. The maximal LOP density for the 3μm LEDs was 396W/cm2. "This may also be due to the waveguiding effect in AlGaN multi-layers, where larger devices experience increased power loss due to a longer optical path from the emissive multiple quantum wells to the air." The team notes that smaller devices, with better current-spreading uniformity and thermal stability, can sustain higher current densities, thereby achieving greater optical power densities.
Extreme junction temperatures caused by operating at maximum power point increase ageing and cause thermal deterioration.
The 3μm device's LOP density was 25.9W/cm2 at 100A/cm2. This has "excellent potential as a photolithography light source," according to the researchers.
Based on 6μm devices at 10μm pitch, the researchers were able to expand the size of UVC LED arrays from the 16x16 pixels that were previously documented in the scientific literature to 160x90 pixels (2540/inch). For improved rear-side light extraction through the thinner sapphire substrate, the arrays were coated with a highly UVC-reflective Al top surface.
With a forward bias of 12V and a current density of 20A/cm2, the array produced an optical output power of 16.6mW. At 8A/cm2, the EQE peaked at 4.1%.
According to the researchers, "The UVC micro-LED display surpasses the 25mW/cm2 calibration of the 365nm mercury lamp used in the Karl Suss MA-6 mask aligner to meet the photoresist exposure dose requirements by offering an adequate optical power density of up to 1.1W/cm2 for full-screen lighting."
To evaluate photolithography capabilities, a 320x140 UVC array with 9μm pixels at 12μm spacing was used (Figure 3). Indium bumps were used to flip-chip glue the array onto a CMOS driver chip. The i-line sensitive AZ MiR 703 in a proximity patterning setup served as the test's photoresist. Visible micro-LED displays, for instance, might be made using the photolithography approach.
Figure 3: UVC micro-LED display photolithography reveals the surface profile (right) and maskless photolithography pictures (left) on photoresist-coated wafers. For five seconds, exposure was at 80mA.
Although the structural resolution is not as good as that attained with contact exposure, the researchers remark that maskless photolithography might be much enhanced by similar lens and focussing methods. Such maskless photolithography methods might save the semiconductor industry a significant amount of time and money by doing away with the requirement for laser-writing masks, especially because narrower linewidths down to the pixel size of micro-display circuits show tremendous promise.
By improving the quality of the epitaxial wafer and attaining more precise alignment, the researchers want to go over the current restriction of 320x140 pixels and open the door for much higher-resolution UVC micro-LED displays with up to 8K pixels in each dimension, which is required for HD and UHD resolutions.
https://www.benweilight.com/lighting-tube-bulb/led-solar-street-light-outdoor.html
