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News Article

Laser-Microjet dicing – the only process to dice thin and thick wafers

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Scribing and through cutting is possible with the same process, without any damage to the dies. Delphine Perrottet and Bernold Richerzhagen of Synova report on progress with water-jet-guided laser technology.
With the growing popularity of new compound materials and thin wafers, abrasive dicing saws struggle to fulfil the requirements of the semiconductor devices. Laser processing offers advantages over saws, but two main drawbacks prevent dry lasers from being used for semiconductor cutting. First, dry lasers generate a large amount of hot particles that contaminate and damage the wafer surface. Second, dry lasers create a significant heat-affected zone, which results in chipping, micro-cracks and reduced die fracture strength.

A few years ago a new laser-based technology was developed, using a water jet to guide the laser beam. Originally designed for medical applications, the technology was soon adapted to the semiconductor field where it quickly proved to be very promising. Today it is integrated into the production of various electronic devices, for example for laser dicing of different kinds of wafers and substrates. Thanks to its unique capabilities, the water-jet-guided laser process is able to cut almost any thickness – from a scribe just a few microns deep to the dicing of full thickness-wafers. Even a thickness of 5mm can be cut through.

x-head: Water-jets

The water-jet-guided laser, also called Laser-Microjet, focuses a laser beam into a nozzle while passing through a pressurized water chamber. The water jet emitted from the nozzle guides the laser beam by total internal reflection at the water-air interface, similarly to conventional glass fibres. The water jet can thus be referred to as a fluid optical waveguide of variable length (see Figure 1).

Contrary to conventional lasers, the working distance – which corresponds to the length where the jet is cylindrical and constant – is very long (up to several centimetres, depending on the nozzle diameter). Therefore, there is no need for focus-distance control. As the water jet is perfectly cylindrical, the kerfs have a constant width and parallel walls.

Compared to dry lasers, the heat-affected zone is negligible. The material does not sustain heat damage as the water jet cools the cut edges between the laser pulses. The water jet also expels the molten material from the cut more efficiently than an assist gas. Contamination is prevented thanks to a thin water film that covers the wafer surface. This is a much cheaper solution than the protective layers that can be used with dry lasers.

In addition to preventing thermal damage, the water-jet-guided laser avoids mechanical damage, as the force applied by the water jet is negligible (less than 0.1N) due to the low pressure and small diameter. As narrow kerfs (down to 28µm) can be achieved very close to the active area, chip manufacturers can shrink the streets and thus increase the number of chips per wafer. The maintenance costs for the system are low, as there is no tool wear and the water consumption is negligible. According to the application, different laser sources can be used.

x-head: Through cutting

The first use of the water-jet-guided laser is through cutting – a process that can cut wafers up to almost any thickness (700µm and more) and is especially efficient with thin wafers, as it is a laser-based technique. A speed of up to 200mm/s can be reached on 50µm thick silicon. The advantage of the Laser-Microjet in this case is that it does not generate micro cracks in the material, which could lead to wafer breakage. Several studies have been conducted on the die fracture strength, proving that Laser-Microjet processing is particularly “gentle” compared to abrasive sawing [1, 2]. This essential particularity is also an asset for brittle materials such as GaAs, which can be diced with minimal chipping and no directional cracking even at high cutting speed.

The thin GaAs wafer shown in Figure 2 was diced with an infrared fiber laser (wavelength: 1070nm, average power: 35W) coupled in a 28μm water jet. Although GaAs is a very brittle material and although the wafer is very thin (100μm), the front side presents no chipping. Without burrs, the kerf is clean and regular. Contamination is trapped in the water and washed away. A speed of 40mm/s was reached to achieve this result.

As no gas is emitted during cutting, contrary to dry laser processing, any toxic by-product material is concentrated in the wastewater, which is filtered.

Another example of brittle, difficult-to-dice material is low-k wafers. Abrasive sawing, because it generates mechanical stress, cannot cleanly cut through low-k layers, as they tend to de-laminate during dicing. A solution using both laser and saw improves the cut quality but also greatly increases costs. The water-jet-guided laser is able to cut through both the top brittle layers and the lower bulk silicon without chipping and at high speed (for example, a 100µm thick low-k wafer can be processed at 50mm/s, for a 30µm wide kerf).

Figure 3 shows a thick silicon wafer as used in producing sensors. The entire thickness (650μm) was cut at an overall speed of 11mm/s. The kerf walls are perpendicular to the front face. In further tests, the kerf was placed closer to the device edges without damaging the active area. A green laser (wavelength 532nm, average power 26W) was coupled into a 45μm water jet.

As the water-jet-guided laser does not generate damage during dicing of thin wafers, it is also used for an operation called “edge grinding”, which cannot be achieved by any other cutting process without significant drawbacks. Edge grinding consists in removing the outer edge of thin wafers (0.5 to 2 mm) to eliminate the micro-cracks that are concentrated in the wafer edge after back grinding and cannot be removed with stress-release methods such as etching. Because propagation of these cracks can lead to wafer breakage during handling, this solution offers significant improvement for thin wafer production processes.

x-head: Scribing, grooving and trenching

In addition to through cutting, the same tool can be used for grooving at different depth levels, from surface scribing to deep trenching of up to 80% of the wafer thickness. Similar to dicing, grooving is achieved at high speed and without damage. This opens up new possibilities for water-jet-guided laser processing. One is edge grinding before back grinding. Grooving around the wafer, such as shown in Figure 4, removes the outer edge once the wafer becomes thin (after back grinding). The 725µm thick silicon wafer on the picture was grooved 1mm from the edge, at a speed of 50mm/s (80µm deep grooving).

Another example of the process versatility is wafer dicing on die attach film (DAF). As DAFs are bound to the wafer and stick to the die when picked up, the die can be fixed onto the die pad directly after pick-up. The dicing process has to cut through both wafer and DAF. Lasers are incompatible with DAF because of low absorption in the adhesive film and a sensitive thermal behaviour. Saws suffer quality problems with thin wafers as well as with brittle top layers (chipping and cracking). A combined solution in two steps would therefore offer many advantages: first, fast scribing of the brittle top layers in one pass using the Laser-Microjet technology; second, through dicing of bulk silicon and DAF in one pass using an abrasive saw. The wafer would benefit from clean, damage-free dicing at the top as well as efficient cutting of silicon and DAF.

x-head: Conclusions

The water-jet-guided laser is the first process able to achieve so many different tasks without damaging the material. For this reason, it is the first laser-based technology that has been accepted in the semiconductor industry. This success will expand in the coming years with the trends for thinner wafers, smaller street widths and increased use of compound semiconductors.

References

[1] D. Perrottet, J.-M. Buchilly, B. Richerzhagen, Water-jet-guided laser achieves highest die fracture strength, Future Fab International, January 2005, No. 18.

[2] W. Kröninger, D. Perrottet, J.-M. Buchilly, B. Richerzhagen, Stress release increases advantages of Laser-Microjet, Semiconductor International, April 2005.
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