Waterjet-guided laser addresses particle generation in semiconductor dicing
Coupled with a water film device, the inherent rinsing properties of a waterjet-guided laser significantly reduce particle contamination generated in most cutting processes
BY DELPHINE PERROTTET, AKOS SPIEGEL, FRANK WAGNER, ROY HOUSH and BERNOLD RICHERZHAGEN
Separating semiconductors has always been a challenge for laser-based cutting techniques. Until recently, the standard process for dicing semiconductor wafers was abrasive sawing, which has been optimized for this application over more than two decades. However, the demands of emerging types of semiconductor devices, like those based on thin wafers and on compound semiconductors, can no longer be fulfilled by abrasive sawing. New separation methods are needed. The waterjet-guided laser showed some years ago to be a good alternative for dicing and free-shape cutting of thin semiconductors, including gallium arsenide (GaAs), and has been recently qualified in terms of particle generation.
Indeed, particle generation and redeposition are important problems for all conventional cutting processes. Particles and dust (between 0.1 and 5 µm) are spread over the surface of the workpiece during the process, especially in the case of laser-based technologies. Small particles are difficult to clean as the adhesion forces (such as capillary, electrostatic and Van der Waals) are strong compared to the force that can be applied by a wiping or rinsing device. Additionally, in conventional laser cutting, droplets of molten material are "soldered" to the surface of the workpiece.
Because cleaning of these small particles is difficult and expensive (for example when employing laser cleaning) or even impossible, the common approach is to keep the wafer clean by preventing the particles from attaching to the surface during the cutting step. Today, the various singulation methods apply different implementations of this idea. The waterjet-guided laser features inherent rinsing by the waterjet and, thus, produces a much smaller amount of particles than any other laser process. Recently, particle-free dicing of wafers could be achieved by covering the whole workpiece with a well-controlled thin water film during the entire cutting process.
Conventional dicing techniques
In abrasive sawing of semiconductors a strong rinsing with DI-water is used to ensure that the generated particles are swept away before they can attach to the wafer surface. This method significantly increases the cost of ownership of the saws, as the DI-water consumption is very high.
In conventional laser cutting, particle contamination is extremely high. Even when cutting under light atmosphere or in raw vacuum, the required cleanliness of semiconductor samples can not be reached (see Figure 1).
Figure 1. The images show particle contamination in conventional laser cutting: in vacuum (left) and in air (right).
In general, if laser cutting is applied to semiconductor dicing, a sacrificial protection layer is needed on the wafer. In semiconductor applications, the protection layer is usually realized by a thick layer of photoresist. The particles thus deposit on the protection layer, which is removed by etching after the dicing step. This solution greatly increases the cost of ownership of the dicing process, as two supplementary process steps must be added.
As mentioned above, conventional laser cutting is not an alternative to abrasive sawing. Nevertheless, dicing applications increasingly require a force-free and particle-free cutting process featuring small street widths and, therefore, a minimization of the zone of damaged material (chipping upon sawing and heat-affected zone upon laser cutting). Dicing thin silicon and III-V wafers, both being at the base for devices with an increasing market share, is problematic with saws, and the semiconductor industry is looking for alternative technologies.
Waterjet-guided laser technology
The concept of the Laser Microjet (LMJ) is to couple a pulsed laser beam into a low-pressure waterjet in order to cut, scribe, drill holes and so on in any kind of materials. Its basic principle is to focus a laser beam into a nozzle while passing through a pressurized water chamber. The low-pressure waterjet emitted from the diamond nozzle guides the laser beam by means of total internal reflection at the water/air interface, in a manner similar to conventional glass fibers (see Figure 2).
Figure 2. Principle of the coupling unit: coupling the laser beam into the waterjet.
The waterjet acts as a stable fluid optical waveguide of variable length. It has three process-critical functions:
- Guiding the laser beam to the workpiece;
- Removing molten material;
- Cooling the workpiece.
The LMJ is a fast, efficient alternative for thin wafer dicing (thru-cut), scribing and edge grinding (where the outer 1-2 mm of the wafer is cut off to ensure a crack-free wafer edge). It can be applied to silicon as well as III-V semiconductors.
Particle contamination and LMJ
Compared to conventional laser-based technologies, the waterjet-guided laser produces a much smaller amount of particles, because the waterjet used to guide the laser beam onto the workpiece also efficiently removes most of the molten material. However, the development of a new cleaning device was necessary in order to reach a particle contamination level close to zero.
The aim in this development was to avoid the sacrificial layer approach. But simply implementing a strong rinsing is not possible with the waterjet-guided laser technique because of the sensitivity of the micron-sized waterjet, which has to be stable and cylindrical until it touches the workpiece.
During LMJ cutting, a part of the high-speed jet is deviated and builds up a thin water film on the sample surface. Once the cut goes through the sample a part of the water film will be sucked into the vacuum chuck. Usually a part of the water film remains in the middle of the chips. When cutting wafers, the remaining water may evaporate and the particles that are held in suspension in this water film can then attach to the sample. However, the particle contamination caused by this mechanism is negligible compared to the particle contamination caused by conventional laser techniques.
Water film device
While the waterjet-guided cutting process already has shown excellent results, the cleanliness of the processed wafers has been improved further with the development of a new device to complete the system. The "water film device" ensures that a continuous water layer of controlled thickness covers the wafer. The presence of the water layer prevents the particles in suspension from attaching to the wafer surface after evaporation of the water dispensed by the microjet. Removing the water layer after cutting in a controlled way then also removes the particles in suspension.
Usually the low-speed waterjet has a pressure of typically 0.5 to 2 Bar (relative to the atmospheric pressure) and a jet diameter of 0.5 to 4 mm. This waterjet impinges onto the surface of the workpiece close to the machining point (where the laser waterjet hits the surface) and creates a zone of intermediate water film thickness around it (see Figure 3).
Figure 3. Principle of the water film device (one water source).
The water film is composed of three zones:
- The thickness of the film in zone 1 is given by the parameters of the laser-guided high-speed jet, mostly its flow rate. Only the high-speed jet feeds zone 1.
- The thickness of the water film in zone 2 (0.1 to 0.5 mm) is mostly determined by the parameters of the low-speed jet, namely the angle of impingement of the low-speed jet (typically in the range of 45 to 90 degrees) and its input pressure. Both jets feed zone 2.
- The thickness of the water film in zone 3 (0.5 to 5 mm) is given by the geometry of the sample holder and the surface tension of the liquid.
The liquid dispensed by the low-speed jet can be adapted to the particular application and may contain buffer molecules in order to adjust the pH. The liquid film can thus be used to control the chemistry on the wafer and protect the wafer from electrostatic discharge (ESD) damage. This additional advantage is due to the fact that the water emerging from the water film device cannot penetrate zone 1 of the liquid film on the wafer.
At least one low-speed waterjet source is needed but more water sources can be added. This can be useful when the wafer presents many cut lines because, in that case, the vacuum that fixes the chips tends to absorb the water, drying the wafer by suction through the already finished cuts.
Figure 4. Inverted picture of the sample after preparation prior to cutting.
Quantitative particle reduction tests
The following tests have been generated using 100-µm thick bare silicon wafers that were stored under normal conditions for several weeks. Prior to the test, the wafers have been cleaned by wiping subsequently with acetone and isopropanol, but still many particles were visible under the optical microscope (see Figure 4).
Figure 5. Input picture (left) and processed picture (right) with the number of particles counted.
Particle counting was performed using LabView IMAQ Vision software. Figure 5 shows an example of an input image and the result. Figure 6 shows the situation after normal cutting without the water film device. The cutting parameters were: wavelength 532 nm, average power 24 W and pulse repetition 40 kHz for the laser; nozzle diameter 50 µm; waterjet pressure 350 bar; chip size 2x2 mm2.
Figure 6. Sample after cutting without water film device.
One can observe a line of changing particle density on Figure 6, which is indicated by the dotted line. This change in particle density is generated under the influence of the water suction through the finished cuts. Only a part of the water film dispensed during cutting was removed, generating a zone of few particles in the vicinity of the cuts except in a region right near the cut where water drops remain due to pinning effects.
Figure 7 shows the sample after the same cut but this time using the DI-water fed water film device. No clear line of changing particle density can be seen. As before, the cut is perfectly stable and the cut quality is unchanged compared to the "dry" cut.
Figure 7. Sample after cutting using the water film device with pure DI-water.
Compared to the initial state of the wafer, the number of particles is not enhanced. Comparing the dry cut with the water-film cut, and considering the fact that the samples were not completely clean prior to cutting, we can state that, with the water film device, the particle contamination has been reduced by at least 90 percent compared to the usual LMJ cut, which in turn is already much cleaner than any other laser cut. These are the first preliminary quantitative results. For a final analysis it will be necessary to use perfectly clean wafers and perform an analysis of large surfaces.
Results on clean samples
After removing the water layer and using a standard cleaning station, no contamination is observed under the optical microscope. The examples detailed below show the surface quality after a cutting process with the LMJ with three different types of material: silicon, GaAs and metal.
Figure 8. Blank silicon, thickness 100 µm.
Silicon—Silicon can be diced by through cutting or the scribe-and-break method. The quality and cleanliness of the material can be seen in Figure 8. It shows scribing of a 100-µm silicon wafer for a waterjet-guided Nd:YAG laser with a wavelength of 1064 nm, average power of 50 W, and pulse repetition rate of 50 kHz. The nozzle diameter was 40 µm, and the waterjet guiding the laser beam had a pressure of 400 bar. The scribing was done in one pass at 100 mm/s.
Gallium arsenide—(Conventional laser ablation of GaAs creates a significant amount of debris that is hard to remove and that can even damage nearby active components. Using the water film device keeps GaAs wafers clean and free of particles. The resulting level of chip contamination is as low as with a conventional saw, but the cut is much faster.
Figure 9. Dicing of a 100-µm thick GaAs wafer (kerf width 26 µm).
Figure 9 shows results of complete dicing of a thin GaAs wafer for a Nd:YAG laser, wavelength 1064 nm, average power 50 W, and pulse repetition rate 35 kHz. The waterjet diameter was 25 µm and the water pressure 400 Bar. The resulting cutting speed was 60 mm/s.
Metal—Metals can be cut with the LMJ as well. A potential application is drilling of stainless-steel stencils. The waterjet combined with the water film device avoids any deposition and there is no oxidation. The metal surface is perfectly clean. The backside is completely burr-free, without any post-treatment.
Figure 10. Hole (diameter 250 µm) in 50-µm thick stainless steel (stencil).
Figure 10 shows results of holes cut in a thin stainless-steel sheet (50 µm) for a pulsed Nd:YAG laser, wavelength 1064 nm, average power 55 W, and pulse repetition rate 700 Hz. The waterjet diameter was 40 µm and the water pressure 300 bar. The average resulting cutting speed was 20,000 holes/hour.
Contamination is a serious problem of any cutting process. The new device developed to complete the Laser Microjet inherent rinsing, using a thin water film to avoid redeposition of particles generated by the laser ablation, is efficient. The water film device consists of a continuous flow of water that provides a film of inert liquid on the workpiece in order to build a protection layer against the particles that might be generated in the machining process. The results for a wide range of different materials show that the particle number is strongly reduced. The processed workpieces are clean and practically free of particles. The resulting level of chip contamination is low.
- Klotzbach, U., Mälzer, S., Panzner, M., Kuntze, T., Sonntag, F. and Dötschel, M., Fraunhofer Institut for Material and Beam Technology (2004) "Influence of gas for structuring silicon," in Proceedings of SPIE (Photonics West), San Jose, USA, Vol. 5339.
Delphine Perrottet is the press contact, Akos Spiegel is R&D application engineer, Dr. Frank Wagner is R&D manager, Roy Housh is international sales manager and Dr. Bernold Richerzhagen is CEO and president all with Synova SA in Ecublens, Switzerland. For more information contact firstname.lastname@example.org or email@example.com.