industry LEDmanufacturing
used to remove the growth substrate and individual design differences.
Manufacturers of AlInGaP LEDs tend to carry out full-area wafer bonding prior to patterning, because this process does not introduce strain and the substrate can be removed by grinding and chemical removal. Making nitride LEDs is more complicated, however. When sapphire is separated from the epitaxial film by the widely employed process of laser lift-off, strain is induced at a die or in multiple die regions. This strain stems from decomposition of the interfacial GaN layer, which is triggered by high power ultraviolet irradiation. Cracking of the active layers can occur, impacting yield, but this can be addressed by confining the strain region via previous structurisation of the die.
Bonding options
In addition to high thermal conductivity, the bond interface in a vertical high-brightness LED must have excellent electrical conductivity. Fortunately, high thermal conductivity and high electrical conductivity tend to go hand-in-hand, and are found in germanium and metallic substrates. Both of these are popular, but silicon is emerging as a carrier material, featuring high heat dissipation and low thermal expansion. Using silicon also enables vertical LED producers to include a Zener diode directly into the carrier substrate, which serves for the electrostatic protection of the sensitive GaN LEDs.
Thermal expansion coefficients must also be considered when selecting a substrate material to bond to the epiwafer. All common wafer-to-wafer bonding processes require elevated temperatures and the bond process is fine tuned to accommodate thermal expansion mismatch between the substrates.
The bonding process must be based on metallic bonding layers because the technique used for wafer-to-wafer bonding must meet requirements for high thermal and high electrical conductivity. This limits the bonding process choices to solder (which includs eutectic and transient liquid phase) and thermo-compresion bonding. Both approaches are discussed in detail in the side panel “Attaching the wafers together”.
To decide which process is most appropriate, makers of vertical LEDs must consider the characteristics of both the growth and carrier substrates, and account for differences, such
Figure 3: The EVG560HBL, which is available in both fully automated and semi-automated operation, supports metal, adhesive and fusion bonds of various substrate types. EVG claims that it is well suited to high-volume LED manufacture, thanks to its combination of cassette-to-cassette operation, multi- substrate bonding capability and its modular design that features up to four swap-in process modules. According to the toolmaker, the combination of field- proven wafer bonding technology and a unique approach to low-temperature metal wafer bonding results in unprecedented throughput and yield
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www.compoundsemiconductor.net July 2011
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