By Tuan Anh Mai, Ph.D. and Bernhold Richerzhagen, Ph.D., Synova

February 20, 2008 — Inkjet printer heads hold one of the largest shares of the MEMS market. Printer manufacturers generally use two inkjet technologies. In thermal bubble jet technology, resistors create heat and vaporize ink to create a bubble. As the bubble expands, a small quantity of the ink is pushed out of a nozzle onto the paper. When the bubble collapses, a vacuum is created, pulling more ink into the print head from the cartridge. The piezoelectric jet technology uses piezo crystals located at the back of the ink reservoir of each nozzle. A piezo crystal vibrates when it receives a tiny electric charge. The inward vibration forces a minute amount of ink out of the nozzle, while the outward vibration pulls more ink into the reservoir to replace the dispersed ink.

A silicon chip is used as a barrier between the orifice plate, which contains hundreds of nozzles, and the ink reservoir. To let the ink pass through, slots are created directly in the 200-mm silicon wafer prior to dicing into chips. The wafer thickness varies between 600 and 700 &#181m. The size and geometrical shape of the slots vary depending on the technology used for the procedure (thermal or piezoelectric jet), as well as the size of the cartridge and the number of nozzles (dpi-resolution). The main targets of inkjet printer head manufacturers are low fabrication costs while improving production yield and product performance. Different technologies are being used for the manufacture of slots on patterned silicon wafers.

Conventional Manufacturing Processes
Silicon wafer slots with widths larger than 150 &#181m can be created by conventional sandblasting method. So far, these methods have not provided satisfactory results for narrow slots below 150 &#181m. Sandblasting limits the diameter of holes and their density, as the edges tend to be conical. Additionally, this process is incompatible with clean room conditions. Etching processes are slow, expensive, and require masks. Dry laser slotting creates heat affected zones (HAZ), slag, and micro cracks. Further drawbacks of conventional lasers include low throughput and additional processes. An alternative technology that combines quality and speed is needed for this particular application. Water jet-guided laser technology has been applied for this application with convincing results.

Water Jet-guided Laser
Initially, water jet-guided laser technology was developed for medical applications. It is also used for precision micro-machining in a wide range of industrial fields, such as semiconductors and electronics. The basic principle of this technology is to use an ultra-thin, low-pressure water jet to guide a laser beam to the workpiece. To achieve this, the laser beam is focused through a transparent window into a nozzle placed at the bottom of a water-filled chamber. The cylindrical hair-thin water jet generated below the nozzle guides the laser beam by means of total internal reflection at the water/air interface, similarly to conventional flexible glass fibers (Figure 1). The water jet is thus able to guide the light through the kerf down to the bottom of the cut; a very valuable property. The only losses are caused by the absorption in the liquid, depending on the applied wavelength, and Raman-scattering at high peak powers.

The jet length that can be used for light guiding is roughly 1000 times the nozzle diameter. Accordingly, a 50-&#181m jet can guide the laser beam for 50 mm. The water jet diameter is approximately 83% of the nozzle diameter because of sharp-edged nozzles and the consequential jet retraction effect (vena contracta). The laser source is typically a pulsed, all-solid-state laser at the fundamental wavelength of 1064 nm, or a frequency doubled (532 nm) or tripled (355 nm) laser. It is possible to use conventional lamp or diode-pumped lasers, or fiber lasers. A pressure intensifier pump delivers a constant water flow with pressures ranging from 2 to 50 MPa. Flow rates are typically only 5 to 75 ml/min, so recycling is not necessary. Today’s optimized water jet nozzles with diameters of 30 to 150 &#181m are made out of diamond or sapphire.

The capabilities and performances of this process are different from those of conventional dry lasers.1 First, because the water jet is cylindrical and the laser beam parallel, kerf walls are parallel. The working distance &#151 corresponding to the stable length of the jet &#151can be several centimeters long, depending on the jet diameter. Therefore, there is no need for focus control. Second, heat damage is nonexistent, since the water jet cools the edges between the laser pulses. The temperature of the cut edge rapidly decreases to the water temperature and heat generated by the laser is not conducted further into the material. The negative effects of heating, such as micro cracks, oxidation, structural changes or low fracture strength, do not appear.

Contamination is greatly reduced, as the water jet, whose pressure ranges from 50 to 500 bars, develops a high kinetic energy fully dedicated to the removal of the molten material. Additionally, a thin water film is generated on the wafer surface during the process, preventing particle deposition. Since the water jet is thin (diameter ranging from 20 to 100 &#181m), the mechanical force applied on the wafer is negligible (less than 0.1 N). As a result, the process does not generate chipping or micro-cracks.

Water jet-guided laser technology has been used for semiconductor micro machining (dicing and grooving). The process is especially efficient on thin wafers and brittle materials (such as GaAs) since it is a damage-free tool.2 High process rates are reached, and there is virtually no chipping on the wafer front and back side. Heat damage to the material is negligible and surface contamination can be avoided. Therefore, the technology has important potential for silicon slotting of inkjet-printer heads. For this specific application, additional advantages include straight slot walls, no transition region (vertical slot ends) and the possibility to program the slot width, as it does not depend on the nozzle diameter.

Damage-free Laser Slotting
The first tests of the water jet-guided laser for silicon-barrier slotting showed very promising results. Slots in 675-&#181m thick patterned silicon wafers were achieved in 10 seconds per slot (overall cutting speed: 1.2 mm/s). To protect the fragile structure on the wafer front side, slotting was executed from the backside and the process that was applied for the slotting ensures that the slot ends are always very steep. For this sample, a green Nd:YAG laser (wavelength 532 nm) was coupled with a 100-&#181m water jet nozzle.

However, some issues remained after these preliminary tests. It was observed that the exit edge was a bit chipped at different locations. The speed also needs to be improved. To reduce irregularities, a smaller nozzle (30 &#181m) was used to execute a “race-track” contour with removal of the inner part. The same green laser was applied for this test.

The long working distance of the water jet enables the manufacture of parallel walls on this thick silicon wafer (Figure 2). With this slotting strategy, the chipping on the laser exit side (the wafer front side) could be reduced to zero and the straightness of the slots could be improved further. The force of the water jet removed the remaining inner part automatically in all cases and no parts remained within the slot. The edge is free of recast and burrs. Taking into account that no protection coating has been used, the cleanliness is outstanding.

Further steps aim at improving speed by increasing axis acceleration. With the same laser and nozzle, an overall speed of 5 mm/s was achieved, reducing the cutting time by a factor of 2, e.g. only 5 seconds per slot. Even at this speed, the cut quality is maintained. Using a 35-&#181m nozzle, speed could be even increased: 9 mm/s, again reducing the cutting time by a factor of 2 &#151 only 2.6 seconds per slot. However, the edge is slightly rougher.

Conclusion
The water jet-guided laser technology has proved its capabilities of matching the requirements of silicon slotting for inkjet-printer heads. After a short phase of parameter optimization, the required quality and speed were not only reached but also surpassed. Additionally to produce through-slots, the process can also be used to create blind slots with very accurate control of the depth &#151 variations at the bottom of the slots can be kept to a minimum. The produced slots are free of any length or depth limitations and the quality remains constant over time. The processing speed is high due to efficient material removal and the use of a high laser power of up to 100 Watts.

REFERENCES
1. D. Perrottet, S. Amorosi and B. Richerzhagen, Which technology: conventional dry laser or water jet guided laser?, LIA Today, March/April 2005
2. D. Perrottet, Gentle wafer dicing, Industrial Laser Solutions, May 2005

Tuan Anh Mai, Ph.D. and Bernhold Richerzhagen, Ph.D., may be contacted at Synova SA, Ch. de la Dent-d’Oche, CH-1024 Ecublens, Switzerland; www.synova.ch

— From Advanced Packaging’s Semi-monthly e-newsletter