Chip singulation process with a water jet-guided laser
04/01/2001
SPECIAL SECTION: EUROPEAN TECHNOLOGY
Bernold Richerzhagen, SYNOVA SA, Lausanne, Switzerland
overview
A novel method for dicing wafers with a laser beam guided by total internal reflection in a water jet is described. A significant benefit is the reduction of chipping and imperfections at the edge of the singulated chips.
The laser has been used successfully as a cutting tool for almost 20 years now, primarily in the field of metal processing. The main advantages of laser cutting over mechanical cutting have been its great flexibility, high speed, small cutting width, and high precision. These advantages were expected to provide a solution for dicing silicon wafers in the semiconductor industry. Although silicon is a good candidate for cutting by a laser beam because it absorbs the radiation of the laser beam, a useful process for wafer dicing had not been developed previously because of crack formation, chipping, and deposits of silicon slag.
Attempts have been made to assist the laser in overcoming these problems by submerging the wafer in wafer, using very short pulses, or applying protective coatings to the wafer. Such efforts were not effective until a water jet was introduced into the process.
The new process
In 1993, Scientists at the Institute for Applied Optics at the Lausanne University of Technology in Switzerland succeeded in creating a laser light-guiding water jet, called by its inventors Microjet [1, 2]. The laser beam is focused in a nozzle while passing through a pressurized water chamber. The geometry of the chamber and nozzle are critical to coupling the energy of the laser beam to the water jet. The low-pressure water jet emitted from the nozzle guides the laser beam by means of total reflection at the transition zone between water and air, in a manner similar to conventional optical fibers (Fig. 1). The water jet can thus be described as a fluid optical waveguide of variable length.
To address the potential problem of heating in the silicon, a pulsed laser was used. The continuous water jet was able to re-cool the cut immediately after a laser pulse removed material, resulting in only a very slight depth of thermal penetration. The quality of the cut with this approach relative to a conventional laser-cutting technique is shown in Fig. 2.
Dicing performance
The cutting speed for this process essentially depends on the wafer thickness. More material requires a greater total amount of energy, and the energy of each pulse is increased for thicker wafers. Some data for cutting speed vs. wafer thickness is shown in the table. The cutting speed is not inversely proportional to the wafer thickness because the laser beam loses its ablation efficiency in the material.
 Figure 1. The mechanism for a water jet to guide a laser beam in a wafer-dicing process. |
The maximum cutting speed at a given wafer thickness depends only on the pulse refresh rate and the mean output of the laser, so there is great process latitude. The maximum pulse refresh rate of today's lasers is 1-4kHz, but there is no technical limit. In the future, lasers with a pulse rate of 10kHz and even more will be available.
The cutting width is typically about 50µm, but this can be reduced significantly. Water jets with a diameter of 30µm have been tested, and the nozzle diameter can probably be reduced to about 10µm. The laser beam diameter is a function of the beam quality, and the newest generation has a beam quality that allows focusing to a spot size of 25µm for a 200W laser or 12.5µm for a 100W laser.
One of the biggest problems of conventional saw cutting is the resulting chipping, which can lead to destruction of the die. Even a small defect acts as a stress concentration point that can lead to catastrophic failure from stress levels encountered by the IC. Chipping is almost totally eliminated with the new water jet-guided laser-dicing process, as shown in Fig. 3.
Another important parameter in the comparison between different dicing techniques is the fracture strength of the diced edges of the die. The thinner the wafer, the greater the importance of this factor. Mechanical deformation (bending) can cause a fracture of the die and its failure.
 Figure 2. Comparison between a) conventional laser cutting of silicon, and b) cutting with a water jet guiding a pulsed laser. |
A recent study made by a European chip manufacturer has shown that the water jet laser cut causes significantly less damage at the wafer edges than the conventional dicing saw. Two independent bending tests, with the wafer pressed against either a ball or a bar, have yielded similar results — an increase in fracture strength of 84% for the bar test and 51% for the ball test.
The reason for the poor fracture strength of the conventional laser is its excessive heating. The conventional laser induces heat into the material until a balance is created between the laser-induced heat and dissipated heat. The material reaches temperatures in the edge that lead to heat damage such as thermal microcracks. With the water jet-guided laser technique, the water jet takes out heat between each laser pulse, so the thermal microcracks associated with higher temperatures are avoided. In the case of the diamond blade, the microdefects caused by the blade limit the strength.
 Figure 3. Chipping at the edge of the die is virtually eliminated when dicing is performed with a water jet-guided laser. |
With the water jet-guided laser process, the edges of the cut display a molten surface. Therefore, the edge surface has a fine structure with no open pores, as shown in Fig. 4, resulting in higher strength. Although the edge surface is rougher than the edge of a sawn wafer, the laser cut produces less damage in the material than the saw.
Another advantage of the new process is geometric. A saw can only cut in a straight line, with the geometry of the cut being limited to one dimension. The laser, however, since it functions as a moving point rather than a straight line, allows 2-D processing, meaning that virtually any contour can be cut. As a result of this feature of the laser, both holes and slots can now be drilled with the same machine used for dicing. The need for a particle jet system — the equipment typically used for such applications — is eliminated by the flexible dicing process.
One potential challenge was the use of standard wafer tape to affix the wafer during the process. A special tape was developed, LaserTape, that is not cut by the laser and lets the water jet pass through it. This tape consists of a 200µm-thick porous, nonwoven polypropylene basis material with a 20µm-thick UV adhesive coating. Evaluations of this tape are continuing, but it is expected to meet all of the needs of the process.
 Figure 4. The edge of a die separated with the water jet-guided laser has no pores or visible defects that reduce its strength. |
Cost of ownership
The running costs of any saw are high because of the consumption of diamond-edged saw blades and DI water. Furthermore, the manufacturing process has to be stopped for a manual tool change. Although the initial investment might be 20-30% higher for the laser-cutting system, the cost of ownership is lower than conventional dicing equipment. The DI water consumption (0.05liters/min) is a fraction of that for a conventional saw. The only consumables are laser flash-lamps after 1000 hrs and water jet nozzles, which actually show essentially no wear but are typically replaced by the user after 1000 hrs. The remaining consumables, such as water filters and protection windows, make no difference. A laser source can have a lifetime of 20 years.
User evaluations
During a span of two years, chip manufacturers evaluated the suitability of the new dicing process for their needs. The first applications have been some in which conventional sawing does not work at all, such as the cutting of round die. Here, the new laser technique has proven that the process is reliable and the resulting quality and speed meet the requirements. Another similar application that has also been evaluated positively is the drilling of via holes or slots.
One European chip manufacturer has successfully tested the water jet-guided laser for dicing of thin wafers for applications such as smart cards. A customer study for laser dicing of GaAs wafers is also in progress. The results are promising, with a cutting speed and yield higher than a conventional saw.
Dicing of other materials
The water jet-guided laser is suited for any material that absorbs a laser beam at the wavelength of 1064nm. Si, GaAs, and Ge meet this requirement. Mechanical fragility is not important. The process also works for ceramic materials, such as aluminum oxide, silicon nitride, and silicon carbide. Nearly any metal is suitable for laser cutting, the only limitation being thickness. Highly reflective materials such as copper, gold, and silver can be cut up to 0.1mm. Transparent materials like glass and quartz cannot be cut because the absorption is too low, and sapphire can only be grooved. Material combinations like metal-coated wafers are not difficult for the laser as long as their thickness is not more than 0.1mm.
Current development work
The technique of the water jet-guided laser is under further development. One objective is to reduce the beam diameter to 25µm, which is technically feasible. Recently, a laser came on the market that allows focusing to such small diameters while maintaining sufficient average laser power. New smaller water jet nozzles are under evaluation as well.
Another possibility is the improvement of speed. Since there is essentially no physical limitation to the cutting speed, it will be possible in the future to increase significantly the dicing speed due to new laser sources with higher average power. A speed of 200mm/sec or higher is realistic.
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Finally, shorter wavelengths will be tested in combination with a smaller water jet. A frequency-doubled YAG laser with a wavelength of 532nm will allow the processing of polymer materials such as polyimide. In addition, with this new laser, copper should be cut much better than at 1064nm. This allows new potential applications, including singulation for package types such as chip-scale packages and ball grid arrays.
Conclusion
So far, the evaluations of the water jet-guided laser in wafer-dicing applications confirm that the new technology has distinct advantages over conventional cutting processes. The advantages include: 1) a higher fracture strength of the die because of the elimination of significant microcracks; 2) no chipping; 3) omnidirectional cutting; 4) drilling and marking capability on the machine with which dicing is performed; 5) excellent performance for thin wafers such as those used in smart cards; and 6) low operating costs in part due to no tool wear. The ultimate limitations in kerf width, cutting speed, and edge quality have not been encountered.
Acknowledgments
Microjet and LaserTape are registered trademarks of SYNOVA.
References
- B. Richerzhagen, "Development of a System for Transmission of Laser Energy," thesis work, EPFL, Switzerland, 1994.
- B. Richerzhagen, G. Delacrétaz, R.P. Salathé, "Complete Model to Simulate the Thermal Defocusing of a Laser Beam Focused in Water," Optical Engineering, 1996.
Bernold Richerzhagen is the director of SYNOVA SA, CP 117 PSE-A, CH-1015 Lausanne, Switzerland; ph 41/21 693 83 71, fax 41/21 693 83 70, e-mail [email protected].