Using 355nm Lasers to Scribe LED Sapphire Wafers

Light-emitting diodes (LEDs) are in high demand due to the ever increasing need for energy-efficient lighting.

LEDs are commonly used in the following areas:

  • Traffic control
  • Automotive headlights
  • Photosensors
  • Mobile devices
  • Consumer electronics
  • Transportation
  • Display technology
  • General illumination applications.

Today they are extensively used in optoelectronic products.

Types of LED Devices

Indium gallium nitride-based LED devices are generally manufactured on a single crystal sapphire (Al2O3) substrate which has superior thermal conductivity. A basic two-inch LED wafer comprises numerous LED devices, and the street width between LED active devices is narrow ranging between 20 and 50mm.

Conventional mechanical and diamond saws used for die separation are very wide (50–250µm kerf width) and tend to create unwanted effects such as microcracking, chipping, and delamination, thereby causing the die yield and throughput to be negatively impacted.

Laser Scribing Technology

After several testing, laser scribing technology is considered to be an efficient method for LED die separation providing advantages such as ease of use, low cost, increased throughput, and high yields.

Figure 1. Edge-view cross-sectional picture of scribe depths corresponding to various focal planes

Figure 2. Example scribes at the processing focal plane (left) Top View of Laser Scribe: Clean scribe with little heat damage to surrounding material and circuitry. (right) Cross-sectional View Along the Scribe: Smooth, consistent laser scribing translates to clean cleaving of Sapphire for higher yield and higher brightness LEDs.

The frequently used lasers for LED scribing are the frequency tripled (355nm) and frequency quadrupled (266nm) diode pumped solid state (DPSS) Q-switched lasers. The laser is basically focused on the wafer substrate to ablate material and develop a thin scribe line between the active devices.

Spectra-Physics’s Lasers

Spectra-Physics provides a wide range of lasers designed with various wavelengths, pulse widths, and power levels so as to address LED wafer sapphire scribing and laser lift off applications.

Studies conducted in the Spectra-Physics Industrial Laser Applications Lab have enabled the characterization of the effect of 355nm DPSS Q-switched laser pulse width and repetition rates on sapphire scribe depth. It was revealed that the sapphire material removal thresholds were dependent upon pulse durations. Another revelation was that shorter pulse width lasers (<30ns) were inclined to have lower material removal thresholds than longer pulse width lasers (>40ns).

Spectra-Physics’ lasers have a shorter pulse width, thus allowing material to be removed at lower fluence with an insignificant HAZ and limited heating of the wafer and adjacent circuitry. Likewise, the studies revealed that deeper scribes could be realized by operating lasers at higher repetition rates.

Figure 3. Plot of scribe depth versus fluence, showing the advantage of short pulse widths

Figure 4. Plot illustrating efficiency gain by beam splitting for the higher power Pulseo 355-10 laser

Most of the current laser products provide better power levels e.g., short pulse width, 355nm wavelength lasers. The Spectra-Physics lab has studied methods to improve process efficiency and at the same time balancing of sapphire scribe quality using various lasers and beam delivery systems.

Another observation made was that with the lower power Tristar™ 355-3 laser system process efficiency could be increased when it was operated at low fluence and high repetition rates (up to 150kHz).

Spectra-Physics Products

The Spectra-Physics Tristar 355nm DPSS Q-switched laser provides short pulse width, high peak power, and high-quality manufacturing. The Tristar laser is suitable for challenging UV laser applications which need accurate positioning, highly focused spots, and very high throughput.

The higher power Pulseo® 355-10 laser system provides a potential gain of up to 75% in processing speed by beam splitting. In the case of 25µm-deep scribes, a scribing speed of up to 188mm/sec can be attained.

The Spectra-Physics Tristar and Pulseo lasers’ short-pulse and 355nm wavelength are capable of superior scribing quality with limited heat damage to adjacent material which is important for higher yield and higher brightness LEDs.

The Spectra-Physics 355nm DPSS Q-switched lasers, 25mm deep scribes with high quality can be realized in sapphire wafer at speeds in the range of 60–200mm/sec by correct laser selection and optical beam delivery design.

Conclusion

The Spectra-Physics’ group of Pulseo laser (Table 1) maintain the same standards of the high power, high repetition rate Q-switched DPSS laser products. Pulseo lasers provide customers with better peak power, short pulse width, and high-quality manufacturing, which are essential for demanding industrial applications that need a lot of precision.

Table 1. Specifications of Spectra-Physics’ Pulseo Lasers

Model Wavelength Peak Power Average Power Pulse Width Repetition Rate (nominal)
Tristar 355-3 355 nm 0.9 kW >2 W <25 ns 90 kHz
Pulseo 355-10 355 nm ~5 kW >10 W <23 ns 90 kHz

This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.

For more information on this source, please visit Spectra-Physics.

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