A study on polysilicon membrane photodetector embedded in silicon-based WLP LED for precision lighting

Abstract

The light-emitting diodes (LEDs) are rapidly replacing the existing lightings like incandescent lamp, fluores-cent lamp because of the high efficiency, long lifetime, non-toxic, eco-friendly characteristics. It also have capabilities of small size packaging and generating various color. The LEDs are usually used as indicator lamp and signal lamp before the invention of high-power, high brightness GaN-based LEDs. However, the LEDs are spreading the area to the general lighting solutions with the high-power, high-brightness LED. Unlike the indicator or signal lamp, it is required to retain a constant level of illuminance for the general lighting solu-tions. Therefore, there is a need for a package combining the LED with a photodetector capable of maintain-ing light output uniformly by conducting real time feedback control of the LED. A photodetector was integrated into a wafer-level packaging (WLP) light-emitting-diode (LED) for the feed-back of the illuminance level of a mounted LED chip. Because the silicon-based wafer-level packaging uses a silicon wafer as a substrate for the LED package, it has advantages in high thermal conductivity and low mismatch in coefficient of thermal expansion, and also a small and an accurate package can be achieved by using a highly improved CMOS and MEMS process technology. Based on the silicon-based WLP, a photode-tector was integrated into the LED package, and a thin polysilicon membrane structure also proposed for the accurate sensing the illuminance of the mounted LED chip. The proposed membrane structure allows the photodetector, which was fabricated on the membrane, to sense the light from the LED directly, therefore, it can provide highly accurate feedback information. We successfully demonstrated the fabrication of a pho-todetector-embedded WLP LED and the responsivity was evaluated using an external 150 W Xenon lamp. The MSM photodetector on the membrane showed high responsivities of 36.38 mA/W at 450 nm and 82.05 mA/W...