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Non-destructive inspection methods for LEDs using real-time displaying Optical Coherence Tomography.

Cho NH, Jung U, Kim S, Kim J - Sensors (Basel) (2012)

Bottom Line: The SD-OCT and SS-OCT images were compared with each other in the same sample to study their advantages.In addition, the volume of the fluorophore space was calculated from the OCT images.We expect this method can improve the inspection efficacy over traditional inspection methods such as Charged Coupled Device (CCD) camera or X-ray instruments.

View Article: PubMed Central - PubMed

Affiliation: School of Electrical Engineering and Computer Science, Kyungpook National University, 1370, Sankyuk-dong, Buk-gu, Daegu 702-701, Korea. nhcho@knu.ac.kr

ABSTRACT
In this study, we report the applicability of two different Optical Coherence Tomography (OCT) technologies for inspecting Light Emitting Diode (LED) structures. Sectional images of a LED were captured using a Spectral Domain OCT (SD-OCT) system and a Swept Source OCT (SS-OCT) system. Their center wavelengths are 850 and 1,310 nm, respectively. We acquired cross-sectional two dimensional (2D) images of a normal LED and extracted sectional profiles to inspect possible wire disconnection that may be present in the LED manufacturing process. The SD-OCT and SS-OCT images were compared with each other in the same sample to study their advantages. The distribution of fluorescence material was observed more clearly with the SD-OCT of 850 nm wavelength, whereas the status of wire connection was clearer in the SS-OCT images with 1,310 nm wavelength. In addition, the volume of the fluorophore space was calculated from the OCT images. This is the first report that a nondestructive optical imaging modality such as OCT can be applied to finding screen defects in LED. We expect this method can improve the inspection efficacy over traditional inspection methods such as Charged Coupled Device (CCD) camera or X-ray instruments.

No MeSH data available.


Related in: MedlinePlus

LED structure. (a) cylindrical LED; (b) chip LED.
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f1-sensors-12-10395: LED structure. (a) cylindrical LED; (b) chip LED.

Mentions: A LED is a semiconductor device consisting of a p-n junction diode that emits light when it is biased in a forward direction, called electroluminescence. By the recombination of electrons in the n-type layer and holes at the p-type layer, an LED emits energy corresponding to the energy gap between the conduction band and the valance band. The energy dissipates in forms of a thermal increase or light emission, and the LED corresponds to the latter case [16]. In general, sapphire (Al2O3) or SiC are used for commercial LED substrates and LED chip are manufactured on those substrates by epitaxial growth. Improving the quantum yield inside the LED active layer is based on control of the growth parameters such as a growth temperature, rate, pressure, and materials used. Crystal defects such as pits which are generated at the early growth phase are diffused to the surface of a completed surface. Those defects are the main reason for decreases in the optical and electrical characteristics of LED devices [19,20]. The wire disconnections during the growth process are hard to find with traditional visual or electrical inspection methods, so if we use the OCT for defect inspection, these defects can be found with this nondestructive method. Figure 1 is an illustration of two LED structures. Figure 1(a) shows the sectional structure of a cylindrical LED. The LED consists of an anode and cathode, which can bias the voltage to the LED; a light-emitting LED chip; a gold bonding wire; fluorophor on the LED chip, which acts as a reflector; synthetic resins filling the LED inside; and a convex lens for the light divergence. This type of LED is less expensive and bigger than the chip type LED, and its driving circuit is easy to make, so it is widely used for general illumination purposes. Figure 1(b) shows the sectional structure of a chip type LED. The chip type is smaller than the cylindrical LED and can be used on a printed circuit board (PCB). It is beginning to make a mark as an alternative LCD backlight, and many other applications are being studied. However, its fabrication and inspection processes are complicated, so reliability of the products is affected by these drawbacks. In this paper, we performed experiments focused on the chip type LED as shown in Figure 1(b).


Non-destructive inspection methods for LEDs using real-time displaying Optical Coherence Tomography.

Cho NH, Jung U, Kim S, Kim J - Sensors (Basel) (2012)

LED structure. (a) cylindrical LED; (b) chip LED.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3472834&req=5

f1-sensors-12-10395: LED structure. (a) cylindrical LED; (b) chip LED.
Mentions: A LED is a semiconductor device consisting of a p-n junction diode that emits light when it is biased in a forward direction, called electroluminescence. By the recombination of electrons in the n-type layer and holes at the p-type layer, an LED emits energy corresponding to the energy gap between the conduction band and the valance band. The energy dissipates in forms of a thermal increase or light emission, and the LED corresponds to the latter case [16]. In general, sapphire (Al2O3) or SiC are used for commercial LED substrates and LED chip are manufactured on those substrates by epitaxial growth. Improving the quantum yield inside the LED active layer is based on control of the growth parameters such as a growth temperature, rate, pressure, and materials used. Crystal defects such as pits which are generated at the early growth phase are diffused to the surface of a completed surface. Those defects are the main reason for decreases in the optical and electrical characteristics of LED devices [19,20]. The wire disconnections during the growth process are hard to find with traditional visual or electrical inspection methods, so if we use the OCT for defect inspection, these defects can be found with this nondestructive method. Figure 1 is an illustration of two LED structures. Figure 1(a) shows the sectional structure of a cylindrical LED. The LED consists of an anode and cathode, which can bias the voltage to the LED; a light-emitting LED chip; a gold bonding wire; fluorophor on the LED chip, which acts as a reflector; synthetic resins filling the LED inside; and a convex lens for the light divergence. This type of LED is less expensive and bigger than the chip type LED, and its driving circuit is easy to make, so it is widely used for general illumination purposes. Figure 1(b) shows the sectional structure of a chip type LED. The chip type is smaller than the cylindrical LED and can be used on a printed circuit board (PCB). It is beginning to make a mark as an alternative LCD backlight, and many other applications are being studied. However, its fabrication and inspection processes are complicated, so reliability of the products is affected by these drawbacks. In this paper, we performed experiments focused on the chip type LED as shown in Figure 1(b).

Bottom Line: The SD-OCT and SS-OCT images were compared with each other in the same sample to study their advantages.In addition, the volume of the fluorophore space was calculated from the OCT images.We expect this method can improve the inspection efficacy over traditional inspection methods such as Charged Coupled Device (CCD) camera or X-ray instruments.

View Article: PubMed Central - PubMed

Affiliation: School of Electrical Engineering and Computer Science, Kyungpook National University, 1370, Sankyuk-dong, Buk-gu, Daegu 702-701, Korea. nhcho@knu.ac.kr

ABSTRACT
In this study, we report the applicability of two different Optical Coherence Tomography (OCT) technologies for inspecting Light Emitting Diode (LED) structures. Sectional images of a LED were captured using a Spectral Domain OCT (SD-OCT) system and a Swept Source OCT (SS-OCT) system. Their center wavelengths are 850 and 1,310 nm, respectively. We acquired cross-sectional two dimensional (2D) images of a normal LED and extracted sectional profiles to inspect possible wire disconnection that may be present in the LED manufacturing process. The SD-OCT and SS-OCT images were compared with each other in the same sample to study their advantages. The distribution of fluorescence material was observed more clearly with the SD-OCT of 850 nm wavelength, whereas the status of wire connection was clearer in the SS-OCT images with 1,310 nm wavelength. In addition, the volume of the fluorophore space was calculated from the OCT images. This is the first report that a nondestructive optical imaging modality such as OCT can be applied to finding screen defects in LED. We expect this method can improve the inspection efficacy over traditional inspection methods such as Charged Coupled Device (CCD) camera or X-ray instruments.

No MeSH data available.


Related in: MedlinePlus