Encapsulants have several requirements including high transparency, high refractive index, chemical stability, high-temperature stability, and hermeticity. All encapsulants are based on polymers, several of which are shown in Fig. 11.8. A simple polymer molecule consisting of a hydrocarbon chain is shown in Fig. 11.8 (a). Branching and cross linking the polymer molecule results in rubber compounds as shown in Fig. 11.8 (b). Such rubber compounds lack transparency and cannot be used as LED encapsulants. However, it is well known that oxides are frequently transparent. In fact, all encapsulants used for LEDs contain oxygen.

A common encapsulant is epoxy resin (also called epoxy), which remains transparent and does not show degradation over many years for long-wavelength visible-spectrum and IR LEDs. However, it has been reported that epoxy resins lose transparency in LEDs emitting at shorter

wavelengths, i. e. in the blue, violet, and UV (Barton et al., 1998). Epoxy resins are chemically stable up to temperatures of about 120 °C. Prolonged exposure to temperatures greater than 120 °C leads to yellowing (loss of transparency).

HHHHHHHHHH (a) Polymer - С — С' — С — С’ — С — С — С’ — С — С — С

HHHHHHHHHH

Fig. 11.8. Chemical structures of polymers. Epoxy resins, silicone polymers, and poly methyl methacrylate (PMMA) are used as LED encapsulants. In the silicone structure, X and Y represent atoms or molecules such as H, CH3 (methyl), C6H5 (phenyl).

The epoxy group shown in Fig. 11.8 (c) contains an O atom attached to two C atoms already bonded to each other. Such a three-membered ring consisting of one oxygen and two carbon atoms is part of the epoxy resin structure shown in Fig. 11.8 (d). Solid epoxy resins are formed by stoichiometrically mixing two liquid compounds, an epoxide with another compound, a resin, having two hydroxyl groups. Resins are oil-like substances that frequently have phenol groups. The phenol group, - C6H4-OH, is derived from the phenyl group, - C6H5 by removing one H atom and replacing it with the hydroxyl group, -OH. The phenyl group is derived from benzene, the well-known six-atom carbon ring, C6H6, by removing one H atom. Under a thermal-setting process, the epoxide groups co-polymerize with hydroxyl groups of the resin.

Typical epoxy resins for encapsulation are thermally curable two-part liquid systems consisting of bisphenol-A based or cycloaliphatic epoxide and anhydride (Kumar et al., 2001). The formation of the epoxy resin requires a short high-temperature cure (120 °C). The two-part system has to be in stoichiometric proportions. Resin-rich compositions lead to lower glass transition temperatures while hardener-rich compositions may lead to discoloration of the

encapsulant. The refractive index of epoxy resin is near 1.6. Besides being transparent, epoxy resin is noted for its good mechanical properties and good thermal stability. However, prolonged exposure of the epoxy to temperatures exceeding 120 °C will lead to discoloration and loss of transparency. In addition to thermo-setting epoxy resins, UV-curable and microwave-curable epoxy resins have been reported (Kumar et al. 2001; Gorczyk, 2001; Flick, 1993).

To overcome the limited thermal stability of epoxies, silicone encapsulants have been used since the early 2000s. Silicone is thermally stable up to temperatures of about 190 °C, significantly higher than epoxies (Crivello, 2004). Furthermore silicone is flexible (and remains flexible for decades) thereby reducing the mechanical stress on the semiconductor chip. Silicone is a polymer whose basic structure is shown in Fig. 11.8 (e). Silicone contains Si and O thereby resembling SiO2 more so than epoxy resins. This resemblance suggests that silicone encapsulants are chemically and thermally stable and do not lose transparency as easily as epoxy resins. It may be desirable to develop encapsulants that are SiO2-like because SiO2 has excellent thermal and chemical stability and very high transparency (Crivello, 2004). On the other hand, silica lacks the flexibility that silicones offer.

Poly methyl methacrylate or briefly PMMA is a less common encapsulant used for LEDs. The chemical structure of methyl methacrylate, the elementary cell of PMMA, is shown in Fig. 11.8 (f). PMMA is also known under the name of acrylic glass and under the product name Plexiglas. The relatively low refractive index of PMMA (n = 1.49 in the wavelength range 500­650 nm) results in a limited extraction efficiency when used with high-index semiconductors.