# Influence of Thin Fluorine Resin Film on DUV LED Packaging Devices
## 3 Results and Discussion  
Figure 2 shows the structure and the absorption spectra of amorphous fluorine resin. The fluorine resin has a trifluoromethyl (-CF3) end, which has been shown to have a more stable performance in DUV devices [13]. To measure the absorption spectrum of amorphous fluorine resin, the fluorine resin film was fabricated on a 45 mm \(\times\) 20 mm quartz glass, and the thickness of the film was approximately 40 \(\upmu\)m. The absorption spectrum of the sample was compared with the quartz glass which had no fluorine resin film on it. As shown in Figure 2, the absorption spectra of the quartz glass with fluorine resin film were almost unchanged when the wavelength was between 200 nm and 500 nm, indicating that the fluorine resin has almost no absorption in this wavelength range. Chip A was used first for the study of the fluorine-resin-packaged device. The top view of the packaging device is shown in Figure 3a,b. When the fluorine resin solution was dripped on the device, hemispherical resin covered the surface of the chip and part of the submount. After curing, the fluorine resin formed a transparent thin film on the surface of the device, and both the chip and zener diode were coated by the fluorine resin film. Since the Tg of the fluorine resin in this study was 170 \({}^{\circ}\)C, the thermal stability of fluorine resin was tested by setting up an experimental point at intervals of 10 \({}^{\circ}\)C from 170 \({}^{\circ}\)C to 210 \({}^{\circ}\)C and then to 240 \({}^{\circ}\)C. No bubble was observed in the sample when the temperature was below 210 \({}^{\circ}\)C, which is shown in Figure 3c-g. However, as shown in Figure 3h, when the temperature came to 240 \({}^{\circ}\)C, the bubbles that existed between the film and submount could be observed. This indicated that the highest tolerance temperature of the fluorine-resin-packaged device was about 210 \({}^{\circ}\)C.  
In Figure 4, we present the cross-sectional SEM image of fluorine-resin-packaged devices that had been cured for 24 h with different dripping times. As shown in the image, fluorine resin covered both the top and side surfaces of the chip, forming a thin film overcoating the outside. We used the thickness of the film in the middle of the top surface to characterize the fluorine resin film, which is marked with d in Figure 4a. As shown in Figure 4e, the film thickness is only tens of micrometers, and with the increase in times the dripping was applied, the thickness of the film increased nearly linearly, from 36.33 \(\upmu\)m to 74.58 \(\upmu\)m. In addition, as can be seen in Figure 4a-d, the fluorine resin film was too thin  
Figure 2: Absorption spectra of amorphous fluorine resin between 200 nm and 500 nm. The resin’s structure is shown inside. to cover the chip completely when dripping only once, and the top edge of the chip was almost exposed to the air. When the dripping was performed more than two times, the chip was covered by fluorine resin completely and no obvious area could be found to be exposed outside. On the other hand, a crack between the film and the chip could be found at the bottom of the chip's side surface in all samples, as shown by the arrow. This may be due to the internal stress forming in the film and the poor adhesion of the fluorine resin material. With the increasing number of drip coating applications, the internal stress in the film also became larger, which resulted in the crack extending upward along the side surface. In the crack region, the surface of the chip was still in contact with the air; this may have affected the result of promoting light out of the chip. Figure 5 shows the characteristics of DUV LED packaging devices with different thicknesses of fluorine resin film at 40 mA.At the same time, the result of devices without fluorine resin is also shown in the figure, which we called the initial value.