Mixing laser and water

Dec. 1, 2003
The waterjet-guided laser is a hybrid technology allying laser and water. In this unique laser cutting technique a free laminar waterjet is used as an optical waveguide to guide a high-power laser onto the sample.

The waterjet-guided laser is a hybrid technology allying laser and water. In this unique laser cutting technique a free laminar waterjet is used as an optical waveguide to guide a high-power laser onto the sample. The main advantages of this method compared to conventional laser cutting are: (i) parallel sidewalls (even in thick mold compound layers), (ii) low thermal load of the sample due to the cooling of the sample between the laser pulses exactly at the place where it was heated before, and (iii) an efficient expulsion of the melted copper due to the high momentum of the waterjet. Compared to sawing, burr-free cuts of copper can be achieved and the mechanical force on the sample is much lower.

A sketch of the waterjet-guided laser cutting system is shown in Figure 1, where pure de-ionized and filtered water at 5 to 50 MPa is used for the waterjet. The nozzles are made out of sapphire or diamond in order to generate a long stable portion of the waterjet. The laser beam, coming from the fiber delivery of the laser, is collimated, passes a beam expander, and is then focused through a quartz window into the nozzle. The coupling unit is similar to a usual fiber-coupling unit, except that the intensity distribution of the light in the nozzle is flat-top and not Gaussian, due to the mode mixing in the fiber delivery of the laser and the imaging properties of the set-up. Once in the waterjet, the light is reflected at the air-water interface due to the refractive index step (see Figure 2).

Figure 1. Schematic of the waterjet-guided laser.
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Samples are fixed onto a CNC translation stage and moved under the waterjet-guided laser beam in one direction during the cutting process. The optical head moves in the perpendicular direction. The z-variation of the stage is only necessary to adapt to the different working distances of differently sized nozzles at different water pressures. It is not used during cutting.

For more than five years this tool has been applied to micromachining problems of various domains. During this time the Laser-Microjet (LMJ) waterjet-guided laser provided numerous advantages in industrial applications compared to classical laser cutting.

Figure 2. Detailed sketch of the coupling unit set-up.
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Usually, a solid-state Nd:YAG infrared (IR) (1064 nm, 50–200 W) laser is used with the LMJ technology. The IR waterjet-guided laser is efficient on silicon, ceramics, and hard metal, cubic boron nitride, ferrite cores, and thin metal foils. With this type of laser, the achievable cutting speed is up to eight times faster than with the abrasive saw. The LMJ is efficient on brittle and difficult-to-machine materials such as GaAs, GaN, and copper.

Figure 3. Seventy-five-micron thick GaAs wafer cut with the 200W green laser and a nozzle of 60 microns in diameter.
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These materials are insensitive to DI-water contact. All semiconductor products are produced using lithography and wet etching processes and are in frequent contact with DI-water and aqueous solutions. Water usage is tolerated during cutting of these materials.

Figure 4. Water absorption coefficient.
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Because the absorption coefficient of green light (532 nm) is slightly better than that of IR light, tests were performed with a 200W green laser to determine whether better results could be achieved in terms of speed, while aiming for the same cutting quality as that achieved with the IR laser. The results are positive; higher speeds are easily reached with the 200W green laser without any loss in quality (see Figure 3), making the LMJ even more attractive for the compound semiconductor industry. Practically, the advantages of the waterjet-guided laser technology translate to no chipping, no burrs, and no broken corners, even on wafers as thin as 75 microns.

Until now, the technology has only been used with infrared and green lasers, thus limiting the range of applications to materials with a sufficient absorption coefficient to these wavelengths. It was therefore difficult, or even impossible, to cut transparent materials (glass, diamond, sapphire, transparent polymers). The idea came about to adapt a UV laser, known for its better absorption coefficient in transparent materials, to the Microjet technology.

A set-up adapted to the UV wavelength has been built using quartz and CaF2 lenses.

The theory shows that useable wavelengths are limited to the range in which the absorption by water is low, which means with an absorption coefficient below 1/cm (see Figure 4). The UV is comprised in this window, but no practical tests at high intensity have been done so far.

Microjet machines are designed to be modular. The laser source is connected to the cutting head through an optical fiber. To prevent damage to the fiber, a core diameter of 100 microns is used for a waterjet of 50-microns diameter. The introduction of the UV laser should introduce new cutting possibilities regarding transparent material, as well as smaller waterjet diameters.

An infrared multimode laser requires the use of a very small nozzle for the waterjet. This requirement made it difficult because of the necessary size of the focus of the expanded laser beam into the waterjet.

The new capabilities regarding cutting or scribing of transparent materials such as polymers, glass, diamonds, and sapphire, are promising applications. A UV laser can cut silicon wafers for the semiconductor industry for example, covered by a layer of glass or diamond, a material commonly used for the production of fast opto-electronic components. The results may also be promising for the electronics industry, because the presence of glass or Kevlar fibers in PCB, and especially flexible ones, made it difficult to cut with the standard MicroJet.

Ochelio Sibailly ([email protected]) is communications manager and Bernold Richerzhagen ([email protected]) is the general manager of Synova SA, Ecublens, Switzerland. John Manley ([email protected]) is the U.S. sales manager for the company.

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