LEDmanufacturing industry
as those in thermal expansion and surface properties. For AlInGaP LEDs, which usually have a highly flat surface thanks to their lattice match to the GaAs substrate, thermo-compression bonding between two gold surfaces is frequently used.
For InGaN LEDs, however, surface roughness and defect density are considerably larger and eutectic or transient liquid phase bonding is preferred. This tends to result in a high yielding bonding process (see the example of Au:Sn bonding in Figure 2).
Further decisions to be made by any vertical LED manufacturer include the thickness of the bonding metals and how they are deposited, and also the selection of adhesion and barrier layers. These layers – typically made from platinum, aluminium and gold, or stacks combining these metals – are often needed to ensure sufficient adhesion and prevent migration of bonding metals into either of the substrates.
Yet another choice facing any vertical LED manufacturer is the choice of tool that will use for the bonding process, which will include alignment of the wafers prior to bonding. We believe that our EVG 560 HBL is worthy of consideration, thanks in part to its combination of high throughput and versatility – it supports metal, adhesive and fusion bonds of various substrate types. It also delivers high yields, so it is capable of helping to drive far greater adoption of LEDs in general lighting, by helping to make these devices deliver more light more efficiently.
© 2011 Angel Business Communications. Permission required.
Further reading E.F. Pabo, V. Dragoi, “Wafer Bonding Process Selection”, MEMS Industry Group
Attaching the wafers together
The two approaches to bond one wafer to another during the manufacture of high brightness LEDs are solder bonding (also known as eutectic or diffusion bonding) and thermo- compression bonding.
Solder bonding is a general term for a metal bond formed by liquid metal, which could be a pure metal, but is typically a binary alloy and in some cases a ternary one. A eutectic wafer bonding alloy is formed at the bonding interface in a process which goes through a liquid phase: for this reason, eutectic bonding is less sensitive to surface flatness irregularities, scratches, and particle contamination, compared to the direct wafer bonding methods.
A successful eutectic bonding process requires bonding equipment that combines good temperature control with temperature uniformity across the entire wafer. The temperature ramp for heating and cooling processes are important. Selection of the details of this process should depend on the particular substrate materials employed to avoid thermal shock for dissimilar materials, and should also be governed by device requirements. For example, process engineers must consider whether the device will be impaired by heating or cooling cycles.
The liquid melt formed during the bond process allows the embedding of interfacial particles in the melt without creating defects. Good wetting is achieved even on very rough surfaces, which are typical for InGaN-based LEDs. This contributes to enhanced device yield and performance.
For some high-brightness LED manufacturing process flows, the material should be kept below the bonding
temperatures for the most usual eutectic alloys (300°C - 400°C). In such situations an alternative process can be used – diffusion soldering or transient liquid phase (TLP) bonding, which results in an inter-metallic compound bonding layer. This technique uses one thin metal layer –
typically 1-10 µm thick – which inter-diffuses with its bonding partner during a thermal process to yield an inter- metallic compound layer with a re-melting temperature higher than the bonding temperature. Cu-Sn and Au-Sn are the most popular TLP systems. Like eutectic wafer bonding, diffusion soldering bonding is an attractive option for high-brightness LED manufacturing, because it can planarize over surface defects or particles resulting from prior processes due to surface wetting by the molten metal.
The alternative process, thermo-compression bonding, involves adhesion of two surfaces to one another through diffusion of the metal molecules, such as gold, copper and aluminium, across the bond interface. The diffusion rate is a function of: the metal; the diffusion barriers on the surface, such as oxides; the pressure; the temperature; and the surface roughness. Cranking up the pressure increases the fundamental diffusion rate and also enhances diffusion. The latter results from deformation of the two surfaces in contact, which leads to disruption of any intervening surface films and enables increased metal-to-metal contact. As this diffusion continues, grain growth occurs across the bond interface. Heating the metal increases diffusion and slightly softens the metals, increasing deformation at any given pressure. Excellent bonding yield results when a high force capability combines with pressure uniformity across the bonding area.
July 2011
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