- A Simpler Fabrication Technique for LEDs
The final step in fabricating an LED involves packaging the bare LED chip in an epoxy encapsulation. The epoxy not only protects the chip physically but also serves as a lens to focus the light emitted by the chip. Typically, this encapsulation step is performed by placing a mold around the chip, injecting an epoxy resin into the mold and curing the epoxy either at high temperature or with ultraviolet light.
Figure 1. Schematic depicting steps involved in the active packaging process for LED fabrication without a mold: (a) a bare LED chip, (b) coating (or immersion) of the bare LED chip with epoxy resin, (c) curing of the resin on the LED chip by emission from the LED itself, and (d) the final self-packaged LED after washing away any uncured resin.
Recently, professor Yong-Hoon Cho and his postdoctoral researcher Hao Wang, working at Chungbuk National University in Cheongju, and their collaborators at Chonbuk National University in Jeonju, both in South Korea, explored a simple alternate technique in which the bare LED chip is immersed in (or coated with) epoxy resin, and the resin is subsequently cured by emission from the LED itself (Figure 1).
The researchers discovered that the elimination of the mold significantly simplifies the fabrication process without sacrificing the ability to shape the epoxy into an effective lens. They used a mixture of commercially available resins whose spectral absorption region extended from about 460 nm to the ultraviolet. The InGaN-based LED chip emitted at 455 nm with a spectral width of 15 nm, and the spectral overlap was sufficient to cure the epoxy. Afterward, the uncured resin was washed off the encapsulated LED with an acetone bath.
Figure 2. Cross-sectional plots of the hardened epoxy along with the LED chips are shown in the form of red bars at the bottom. In the left image, current varying from 1 to 30 mA (PC1 to PC4) was applied for a total of 1 ms. In the right image, a constant current of 1 mA was applied for a period varying from 1 to 4 s (PT1 to PT4). Reprinted with permission of IEEE Photonics Technology Letters.
The shape of the cured epoxy shell around the LED depends both on the current applied to the LED during the curing process and on the length of time during which the current was applied (Figure 2). Not surprisingly, both higher currents and longer exposures result in a greater volume of hardened epoxy.
The scientists measured the angular beam profiles of the light emitted by the different encapsulated LEDs shown in Figure 2. They found that longer exposure time as well as higher currents resulted in greater focusing in the emitted light, and they concluded that a self-focusing effect was present in the curing process. During the curing process, as the beam became increasingly focused in a given direction, more resin was cured along that direction, leading to an enhancement of focusing in that direction (Figure 3).
Figure 3. Angular distribution of light emitted from various types of LEDs (corresponding to Figure 2) show an increase in focusing with an increase in current (above) or exposure time (below). Reprinted with permission of IEEE Photonics Technology Letters.
The surfaces of the self-cured LED were not as smooth as those of commercially available molded LEDs. The scientists believe that further refinements in the epoxy composition, in the curing process and in the postcuring acetone bath could result in smoother surfaces. However, rough surfaces are not necessarily bad and actually may be advantageous in applications requiring diffuse light.
IEEE Photonics Technology Letters, Jan. 15, 2008, pp. 87-89.
- Common name for a variety of adhesives used for lens bonding, fiber optic splicing and other photonics applications. The term is actually a prefix denoting the presence of an epoxide group in a molecule.
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