# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 8 Packaging for DUV-LEDs  
Packaging loss is often neglected in scientific research; however, even a simple mount such as a TO-5 stem causes packaging loss. This is mainly because of the low reflectivity of metals in the DUV region. AlN ceramic heatsink contains impurities because AlN ceramic is fabricated by sintering, reducing the transparency to DUV. Accordingly, the measurement of EQEs is unavoidably accompanied by packaging loss. Even though a high EQE is obtained, transverse or backward EL emission to the normal vector of the sapphire light extraction plane opposite to the device layers is not useful. It would be preferable to collect this EL closer to the light extraction side of devices. Practically, an Al reflector around the die is effective to reduce the waste of EL. An Al coating on the surface of the submount will also reduce the waste.  
From the industrial viewpoint, packaging loss is a serious problem for DUV-LEDs. Today's regular DUV-LEDs are mounted on ceramic container-type package, which enables applying established mass-production machines and attaching the quartz window, because it is well known that most polymers are decomposed by DUV light. In addition, heat dissipation is important to enable the injection of a larger current. High EQE does not necessarily lead to highly useful devices, and a high absolute output power or irradiation intensity is often required. Robust InGaN LEDs are often driven at a high current density over 50 A/cm\({}^{2}\) (500-700 mA per 1 \(\times\) 1 mm\({}^{2}\) die).  
Currently, WPEs of DUV-LEDs are less than 10% and the injected energy to DUV-LEDs is mainly converted into heat. Since AlGaN, as well as InGaN, material is robust, to increase usable EL outputs, heat dissipation of the packaging is the key to injecting a larger current. Sintered AlN ceramic has a thermal conductivity coefficient of more than 170 Wm\({}^{-1}\)K\({}^{-1}\) and is durable against DUV light. One of the current problems in AlN packaging originates from the reflection between the quartz window and the atmosphere in DUV region. The loss at the quartz window is 8% or greater. In addition, the coefficients of thermal expansion differ for AlN and quartz window. To attach the packaged DUV-LEDs on the patterned circuit board (PCB), solder reflow process is desired to be used, which needs to increase the furnace temperature up to 260 \({}^{\circ}\)C. Despite the problems of reflow temperature and the reflection loss at the window, many manufacturers currently use precise ceramic packaging with a quartz window. To attach the quartz window, AlN multilayer sintering technique was applied to make the height of inner walls greater than the die thickness. However, the inner walls cause serious packaging loss of the EL flux oriented to transverse to the axis of light extraction direction, which is not expected to be highly reflected from the inner walls. The total packaging loss is estimated to be 40%, resulting EL output of 45 mW in the case of packaging when the output of the corresponding bare die is more than 70 mW [127]. To reduce the loss of the quartz window and low reflection from the inner walls, encapsulation using a fluoro-polymer is considered to be useful [14; 15].