Issue



Using Lasers to Dice Thin Silicon Wafers


04/01/2008







BY DELPHINE PERROTTET, Kali DUNNE, Gillian WALSH, AND Billy DIGGIN, XSIL LTD.

Mechanical saw dicing is the conventional technology used for dicing thick silicon wafers. However, below 100 μm, abrasive saws encounter issues — primarily low die strength and decreasing machining speed. Singulation of die mounted on die-attach film (DAF) also proves challenging for traditional methods. Laser-based technologies offer efficient solutions for thin wafers on DAF. With the increased percentage of thin wafers in high-volume applications and the decrease of wafer thickness (several manufacturers have roadmaps to reduce from 75 μm down to 25 μm and thinner), interest in laser solutions is growing significantly.

Laser-based Dicing System

For thin wafers, laser dicing demonstrates high throughput and provides higher yield than traditional methods. However, additional processes are essential to address die strength and contamination. The first step is to coat the wafer for protection. The wafer is then laser diced. After dicing, the wafer is cleaned to remove the coating and debris. The last step is an etching process, which increases die strength.

Coating

Coating provides effective protection against debris and contamination. This resist solution is dispensed onto the wafer surface then spread over the entire surface by virtue of a spinning process that rotates the wafer at high speed until the coating is dry.

As running cost is one of the main concerns of chip manufacturers, a low-cost product has been selected for coating. It is a non-ionic, water-soluble coating, which is dispensed and spin-coated inside the integrated wash station.

For a 300-mm thin silicon wafer, the volume of coating used will be 50 ??2ml, which corresponds to a coating thickness of 2.6 ??0.3μm. Coating uniformity is better than 5%.

Laser Dicing

Using lasers for dicing provides the ability to machine silicon and DAF with minimal damage and no chipping.

To optimize throughput, a combination of laser and a scanning galvanometer is used. Dicing is done area by area — each individual area is called a field-of-view (FOV). Within each FOV, the laser head and XY stage remain motionless, and dicing is executed using an ultra-fast galvanometer. An XY linear stage is used to move between FOVs. This step and scan approach results in a low-frequency step with high frequency scan, significantly increasing the machining speed and therefore the overall throughput of the system. The dicing speed scales upwards as wafers get thinner (Figure 1). For this reason, laser dicing is ideal for sub-100 μm silicon dicing.


Figure 1. Machining speed versus wafer thickness.
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This laser dicing process is compatible with all UV colorless/translucent dicing tapes and with most die-attach films.

Consumable costs can be reduced as DI water is not required for the dicing process and the frequency of laser maintenance is low. Prior to dicing, a low energy scribe step can be used to enhance the top surface dicing quality, for example in the case of low-k layers.

Cleaning

After dicing, a cleaning step removes the coating and any contamination suspended on it. The cleaning step takes place in the washing station, where coating was dispensed at the beginning of the process. This cleaning step is cost-effective, as it uses factory-supplied DI water. Wafers are subsequently spin-dried.

Etching

The final step is a unique etching process specifically developed to improve die strength after laser dicing. It performs a stress-relieving function on the silicon sidewall of each singulated die on the wafer. It is a low-cost, dry etching process, which does not use plasma or masks. The gas used is highly selective to silicon, and therefore does not attack the other materials present on the wafer (such as SiO2, SiC, Si3N4, Al, photoresist, etc). As a result of this process, a substantial die strength increase is obtained (Figure 2).


Figure 2. Increase in die strength after etching.
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A 2007 study showed that the side from which a wafer is laser diced displays lower die strength than the other side. For example, if a wafer is diced from the front side and no etch is applied, the die strength of the backside equals over two times the die strength of the front side (the front side contains the active layers and the backside is the side which has been background).

Etching stress relieves the entire die sidewall, increasing die strength on both sides of the die. The die strength of the same wafer described above will increase by almost ten times after etching. At this point the die strength of the backside is 50% lower than the front side as stresses incurred during wafer backgrinding, rather than laser-induced stresses, become the limiting factor.

Results

This laser-based system results in clean, strong die, produced with low running costs at high throughput. DAF characteristics are not altered by the dicing process. For example, a typical 50-μm thin silicon wafer will be diced at a machining speed between 125 and 280 mm/s. After etching, the kerf width will be typically 24 μm and the mean die strength will be above 1100 MPa. There is minimal chipping and delamination after laser dicing (Figure 3). The surface of the die and the sidewall are free from debris. Figure 4 shows a thin silicon die on DAF, after dicing. The sidewall quality is excellent and the DAF is also cleanly cut, without altering its properties.


Figure 3. SEM image of a 50 μm thin silicon die, after dicing, cleaning and etching.
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Figure 4. SEM image of a 75-μm thin silicon die on 20-μm DAF, after dicing and cleaning.
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Conclusion

For chip manufacturers, high yield, high throughput and low consumable costs are important. As wafers become thinner, achieving this becomes even more difficult. A laser-based approach, including specially developed, complementary processes, offers an interesting alternative, delivering high throughput and high yield, particularly through die strength enhancement.


DELPHINE PERROTTET, technical marketing manager; KALI DUNNE, dicing team leader; GILLIAN WALSH, business manager; and BILLY DIGGIN, VP business & development, may be contacted at XSil Ltd, Silverstone House, Ballymoss Road, Dublin 18, Ireland; +353 1 245 7500; E-mail: info@xsil.com.