# Influence of Thin Fluorine Resin Film on DUV LED Packaging Devices
## 1 Introduction  
Over the last three years, the demand for deep-ultraviolet (DUV) light sources with wavelengths between 250 nm and 300 nm increased rapidly due to the spread of the COVID-19 pandemic. Due to their advantages of being mercury-free, having long lifetimes, being small in size and being easily integrated, nitride-based DUV light-emitting diodes (LEDs) have been attracting attention, which are considered to be an alternative to mercury lamps [1; 2; 3]. However, compared with blue LEDs, the external quantum efficiency (EQE) of DUV LEDs remains low and degrades significantly with a decreasing wavelength due to immature epitaxial growth and process technology [4]. For blue LEDs (around 455 nm), an EQE of over 80% has been achieved [5]. For DUV LEDs (wavelengths between 250 nm and 300 nm), EQE values are below 10% [6]. As an important factor of EQE, the light-extraction efficiency (LEE) is as low as 9% without any assisted light extraction [7; 8]. The main reason for low LEE is the refractive index contrast between the chip and the environment [9; 10]. Light propagating outside the escaping cone will be reflected back to the chip and absorbed by active layers, p-type layers and electrodes that have low reflectivity in the DUV region. Many works have been carried out to increase the LEE: for example, pattern sapphire substrates were used, n- and p-reflective electrodes were proposed and new materials and designs were used for encapsulation [11; 12].  
Resin materials are commonly used in blue LED packaging, since their refractive index is located between the chip and the air, which can increase the critical angle of an escaping cone and promote the light propagating out of the chip to increase the LEE.  
Si-O-based polymers exhibit absorption when the wavelength is below 300 nm, causing the materials to have low transmittance in the ultraviolet region, and so cannot be used in DUV LED packaging devices. Amorphous fluorine resins, which have almost no absorption in the DUV region, can be used for the encapsulation of DUV LEDs as promising new materials [13,14].In previous studies, [PERSON] et al. discovered that polymerized perfluoro-4-vinyloxy-1-butene (BVE), having a five-membered ring with a single oxygen, was suitable from the industrial viewpoint [14], and amorphous fluorine resin with the end functional group of -CF3 achieved high UV transparency and excellent UV durability [13]. [PERSON] et al. found that a 50% light output power enhancement could be achieved by forming a hemispherical encapsulation layer [15]. [PERSON] et al. also found that a 16.5% light output power enhancement could be achieved by doping AlN nanoparticles into fluorine resin [16]. However, in all studies, at least 10% decay was observed after aging when the fluorine resin was used. On the other hand, the cost of fluorine resin is very expensive, and therefore we want to determine the applicable scope of fluorine resin and a method of increasing the light output power by using fewer materials.  
In this study, we fabricated DUV LEDs packaged with thin amorphous fluorine resin film by using the drip-coating method. Different thicknesses of fluorine resin coating and different output powers of DUV LED packaging devices were prepared. The influence of characteristics and reliability were studied. The results reveal that when using thin amorphous fluorine resin film for DUV LEDs' packaging, a ~10% increase in the light output power was achieved, and no influence during the aging time was found in the low-power device.