# Output Properties of Transparent Submount Packaged FlipChip Light-Emitting Diode Modules
## 2 Experimental and Simulation  
As shown in Figure 1, both the traditional and proposed FCLED mounting structures have been demonstrated. First, the TracePro software (Lambda Research Corporation, Littleton, MA, USA, 2015) is used to simulate the light output efficiency of our proposed FCLED mounting model with a transparent submount, and compares it with that of the traditional style of FCLED mounting on a non-transparent submount. Later, we measure the submount transparency and light output power of FCLED samples mounted on different substrates, respectively. Afterwards, these results are used to  
Figure 1: Traditional style of flip chip light-emitting diode (FCLED) on non-transparent submounts (**a**), and proposed transparent mounting of FCLED (**b**). Inset figures show the simulation flux results for the two mounting styles with silicon or glass, respectively. correlate with the experimental heat dissipation results measured using an infrared (IR) spectrometer (FLIR Systems, Wilson, OR, USA). The thermal results are also verified with the simulation results using ANSYS 14 (ANSYS, Inc., Canonsburg, PA, USA, 2015). The TracePro software is used to display the advantages of using the transparent submounts when compared to the traditional style of LEDs on non-transparent submounts, as shown in Figure 1.  
The above results shown in Figure 1 were simulated to show the corresponding number of light rays with a constant radius of the integrated sphere, and the corresponding lumen flux was calculated. During the simulation, we took three cases into consideration: the traditional model with the Si submount, and the proposed structure with glass and sapphire as submounts. As shown in Figure 2 (the simulation result), we can see that the proposed mounting LEDs yield high-output luminous flux as compared to the traditionally mounted LED. This is because the total number of emitting rays is larger for the transparent mounting system which has no rays blocked by the non-transparent traditional submount. In this experiment, three different submount materials are employed for the LEDs' flip chip package. They are glass, double side burnish (DSB) sapphire and silicon. All the samples were cleaned using Radio Corporation of America (RCA) cleaning processes, and followed by the metallization processes which are sputtered with 30 nm Cr and 120 nm Au. Finally, the photolithographic processes for contact electrode formation were carried out and the LED chips flip chip bonding are performed. LEDs adopted here had a size of 1 mm by 1 mm square and were from Epi-star Co. (Hsinchu, Taiwan) Those LEDs exhibit an epitaxial structure of a thin nucleation layer, a 2 nm Si-doped GaN layer, five periods of InGaN/GaN multi-quantum wells (MQWs) with a well thickness of 2.4 nm and a barrier thickness of 10 nm, a 20-nm-thick Mg-doped p-AlGaN layer and a 150-nm-thick Mg-doped p-GaN layer, respectively. Before the flip chip process, a Kulicke & Soffa 4522 series Wire Bonder (Kulicke & Soffa, Singapore) was used to make bumps for the electrode connection. Once the samples were ready, optical parameters of the corresponding LEDs were measured using Integrating sphere (AMA Optoelectronics) SLM-20T (AMA Optoelectronics Co., Ltd., Taiyuan, Taiwan). The light intensity (output lumens), color rendering index (CRI), correlated color temperature (CCT), and wavelengths at different bias currents, etc., were measured. Then a thermal imaging system, FLIR SC 620 (IR) (FLIR Systems, Wilson, OR, USA), was employed to take the corresponding thermal images. During measurements, these samples were kept on the specimen holder and current was applied with a constant current source. The thermal images were captured using a thermal video system which exhibits a recording computer. These images were later used to verify our experimental results with the simulation results from ANSYS. To avoid extra interference parameters such as the temperature dependence of the phosphors, all the chips adopted in this research were bare blue light LED dies.