High-speed UV laser scribing boosts blue LED industry
Blue LEDs consist of multiple GaN-based layers grown epitaxially on a silicon carbide or sapphire substrate. SiC wafers are typically diced using high-precision saws, while sapphire wafer die separation is a three-step process. First, the wafer is thinned to a thickness of about 100 µm. Then it is mechanically scribed with a diamond tool, and finally cleaved along the scribe lines by means of a fracturing machine. The extreme hardness of blue LED substrates and the small LED die size cause significant problems for both saw dicing and mechanical scribing, including low die yield, low throughput and high operating costs.
UV laser scribing is now emerging as an alternative method for separating blue LEDs. Compared with conventional separation methods, the UV laser scribing method dramatically increases die yields and wafer throughput, while at the same time significantly reducing the equipment operation and maintenance costs.
The physics of UV laser scribing
When short-duration UV laser light pulses are tightly focused onto the wafer surface, each pulse is absorbed into a sub-micron thick surface layer, which then vaporizes. The vaporized material carries away the energy of the interaction, minimizing heat transfer to the surrounding material. This process is known as photoablation. In order to produce deep cuts, hundreds of successive laser pulses are required.
Moving the wafer under a rapidly pulsed, focused laser beam produces an extremely narrow V-shaped cut, the depth of which is controlled by the scan speed. Typically, these cuts terminate 30-50% into the thickness of the wafer. After laser scribing, the wafer is fractured using standard cleaving equipment. The V-shaped laser cuts act as stress concentrators, inducing well controlled fracturing with excellent die yield.
Efficient photoablation is required for laser scribing and depends strongly on two properties of the UV laser light: wavelength and pulse duration. In general, photoablation benefits from a shorter laser wavelength and shorter pulse duration, for both optical and thermal reasons.
The key benefits achieved by short laser pulse duration are increased irradiance on target, and reduced heat transfer to the substrate due to the rapid absorption and ablation. For short laser wavelengths, the main benefits are improved optical absorption, reduced absorption depth, lower irradiance required for ablation and reduced cut width.
Shorter wavelengths impart more energy per photon. For SiC, optical wavelengths below 370 nm have photon energies that exceed the bandgap of the material, resulting in direct photon absorption. However, sapphire has a bandgap that is higher than the photon energy of any commercially available UV laser. Multi-photon absorption is therefore required to induce efficient optical absorption. Typically, the necessary irradiance (W/cm2) for multi-photon absorption is very high. The efficiency of multi-photon absorption in sapphire is strongly wavelength dependent. Shorter wavelengths are absorbed more completely in sapphire, resulting in less heat input to the bulk material. Figure 1 shows the optical absorptance of industrial-grade sapphire at 355 and 266 nm, over a range of irradiances. Note that the maximum absorption at 355 nm is about 96%, while the maximum absorption at 266 nm is greater than 99%.
The combination of short wavelength and short pulse duration provide complimentary benefits of improved absorption and ablation at lower irradiance, reduced heat transfer to the substrate, smaller cut width, and larger area coverage of the beam spot. These combined benefits serve to optimize cut speed and cut quality. Further, the smaller cut width helps to minimize surface debris.
