Waterjet-guided Laser Processing
Bernold RICHERZHAGEN*, Muneharu KUTSUNA**, Haruhiko OKADA*** and Takeshi IKEDA*****SYNOVA SA, Parc scientifique/EPFL 1015 Lausanne Switzerland
E-mail: richerzhagen@synova.ch**
Department of Material Processing Engineering, Nagoya University,Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan***
Itochu Mechatronics corp., 6-14, 3-chome Minamihon-machi, Chuo-ku, Osaka 541-0054, Japan****
Fine Process Co.Ltd., 1-36 Motoshio-cho, Minami-ku, Nagoya 457-0823, JapanThe waterjet-guided laser processing is a thermal cutting process in which the laser beam is used as
the machine tool for cutting and the fine waterjet plays a role as an optical waveguide. The laser beamis guided in the waterjet stream with the full reflection that takes place at the boundary between waterand air, in a manner similar to glass fibers. The waterjet also has the function of waveguiding and removing the cut products (see Fig. 1). The diameter of the waterjet can be controled between 50 and 200μm. The diameter of laser beam is completely utilised . The useful length of waterjet is usually approximately 50 to 100 mm. The type of laser used is a pulsed Nd:YAG laser. The advantages of waterjet-guided laser processing are (1) no Z-axis control, (2) narrower cut widths down to 50 μm, (3) verysmall heat affected zone, (4) a little oxidation of the cut edges, and (5) no assist gas for cutting. A reseach was carried out for the new industrial application which is unsuitable by conventional laser cutting.
Keywords: waterjet, waveguiding, coupling, working distance, Si wafer dicing

1. Coupling of water jet and laser beam
Waterjet-guided laser processing is an internationallypatented technique which has been developed by BernoldRicherzhagen[1]. It is based on guiding a laser beam insidea fine water jet. Because of the difference in the reflection
coefficient of water and air, the laser beam is fully reflected
at the water jet surface, similar to an optical fiber. The finewater jet can be called as "liquid fiber with variable
length"(see Fig. 2)[2].

Fig.2 Coupling of water jet and laser beam

Fig.1 Waterjet-guided laser processing

Although a high power laser beam normally vaporizes
small volumes of water, a special geometry of the couplingsystem avoids thermal coupling of laser with water. The
water jet at a pressure of about 100 bars (10 MPa), guides
the laser beam and cools the material during the processing,and remove the cut products.
The typical diameter of waterjet-guided laser beam is 100μm, which can be reduced to 50 μm. The useful length ofwater jet depends on the diameter and amounts to 100 mm
in the case of a 100 μm water jet.

1Another important characteristic of the technique is thefact that water jet is able to guide laser beam into the workpiece so that multilayered materials can be cut.
The process has some important advantages in comparison to conventional laser cutting, because of no focal pointof beam, the large working distance and cooling by the water jet.

the other hand, the laser processing is not subject to mechanical stress and wear, thus yielding constant cutting quality.
The diamond saw blade exerts extremely high forces on
the wafer during the cutting, which make it necessary to fix
the wafer firmly in place and can lead to chipping of the partat edges and corners and cracking of the wafer, which cansubsequently lead to circuit failure.

2. The Micro-waterjet laser machining system
The laser machining center consists of laser source with
an optical fiber for laser transmission, a hydraulic unit toproduce the required water pressure, a module for coupling
laser beam into water jet, a workpiece handling system andCAD programming unit.
A pulsed Nd-YAG laser with average output power of300-500 W and a peak power rating of 10 to 20 kW is used.
The maximum pulse frequency is 1000 Hz. CW lasers canalso be used for surface removal. The optical fiber used forlaser transmission to coupling unit has a core of 400 μm indiameter.
The hydraulic unit was developed especially for the “Micro-waterjet”. The reason for this effort was that although
the systems is available on the market cover low pressure
range up to 20 bar (2 MPa) and the high pressure range upward of 3000 bar (300 MPa), no system was found that wasadequate for the middle pressure levels required.
A new concept was developed for the workpiece handlingsystem, which is suitable to the demands of Micro-waterjetprocessing. Linear motors are integrated in the biaxial crosstable. These offer considerably greater dynamics and precision over conventional drive techniques.
The table speed of 3 m/s can be achieved with a maximum acceleration of 3 g. The motors are mounted on afinely ground granite bed on a welded steel frame.
The greatest breakthrough was in developing bundling
module which combines the two media of laser and water toform an extremely fine laser beam. The special geometry of
the coupling device allows a laser intensity of more than 50
MW/cm2 in a water jet with a diameter of 100 μm. The
compact laser processing head contains optical lenses thatfocus the laser beam in special water jet nozzles. A mechanized beam expander matches the laser beam into water jetof any desired diameter between 50 and 1000 μm. In orderto ensure flawless focusing, the beam is monitored withvideo camera and adjusted automatically.

Fig.3 Chipping by sawing

The most significant problem of abrasive sawing is chipping at the upper and lower edges of the cut wafer (see Fig.
3) – chipping across the line and into the die will inevitably
lead to rejection. Laser processing on the other hand, being
virtually force-free, so that chipping is not occurred. Theirregularities on the rear of wafer generally should not beless than 5 μm. The force on the wafer by waterjet is far
lower than that of the sawing. Indeed, it is even 10 times
more than the force caused by conventional laser cuttingwith assist gas. This is due to the very small cross-sectionof the waterjet. Fig. 4 shows the comparison between conventional laser and waterjet-guided laser processings.

(a) Conventional laser processing
Cutting width: 0.15mm

3. The Waterjet-guided laser processing for wafer cutting
The technology of wafer sawing using a saw with diamond tips has been developed in last 10 years
In the case of saw blade with diamond tips, the quality of
cut deteriorates with increasing cut length due to wear of thesaw blade. In other words, two wafers can never be diced tothe same degree of quality. In this context, it is up to themachine operator to decide when a tool change is due. On(b) Waterjet-guided laser processing
Cutting width: 0.085mm
Fig.4 Comparison between conventional laser and waterjet-guided
laser processing for cutting Si wafer

2

The saw can only cut in straight lines, with the geometryof cut being limited to one dimension. The laser processing
is punctiform and allows two-dimensional processing, thus
meaning that any contour imaginables can be cut.
Cutting Speed (mm/s)
140

Although somewhat more expensive to purchase the facility, the laser is characterized by extremely low runningcosts. The laser flashlamps have to be replaced after approx.
800 to 1000 hours, and the diamond waterjet nozzles afterapprox. 200 hours. This equates to considerably lower consumption costs for the laser. Fig. 7 shows an example ofcutting of IC devices for electric industry.

120
100
8060
4020Fig.7 Cutting of IC devices for electric industry

0500
1000
1500
Si Wafer Thickness (μm)

2000

Fig.5 Relationship between Si wafer thickness and cutting speed
by waterjet-guided laser processing

Fig.5 shows relationship between Si wafer thickness andcutting speed by waterjet-guided laser processing. The result that shows cutting speed is same or higher level compared with sawing with diamond tips.
This also renders the rotary axis required for orienting thewafer during sawing superfluous. A Z-axis is also no longer
needs, as the waterjet-laser need not be focussed.
As a result of the punctiform laser, both holes and slots
can now be drilled on one and the same machine – an application for which a particle jet with the associated compromises was hitherto used. A further advantage of the lowpressure laser cutting process is its ability to dice very thinwafers, as the wafer thickness has no lower limit. Fig.6
shows slots.

Fig.6 Slots made in Si wafer (20x)The running costs of sawing are very high on account of
the consumption of diamond edged saw blades. Furthermore, the manufacturing process has to be stopped for toolchange, and the actual change performed manually.

The waterjet-guided laser processing is also suitable forgrooving of wafer. The depth of groove is kept at constantdue to laser pulse energy.
The laser processing can be applied easily to materials
with different mechanical properties and hardness. However, the optical parameters such as reflectivity effects theprocess parameters. As a result, some materials can only becut to a limited extent.
Grass, for instance, is a material that is verytransparent to YAG laser. Metallic materials suchas gold and copper reflect the laser beam intensively,
therefore, only thin metal sheets can be cut.
However, the gold and copper vapour deposits of several
micrometers in thickness that are used in silicon technology
are not a factor in this context.
Ceramics such as Al2O3 can only be processed very slowly
when of high-grade purity, e.g. 99%.
The waterjet-guided laser has the energy used for cutting
the material. The waterjet is useful for cooling the workpiece, guiding the laser beam and removing cut products
from the cut. The actual cutting process is thus thermal one,
although the depth of thermal penetration is very low on
account of the waterjet. The edges of cut thus display amolten surface.
The resultant structure is very fine, but increases proportionally with a large wafer thickness. The roughness Ra is 3
μm for a wafer of 660 μm in thickness. Therefore, thesurface of edge can be comparable with that cut by a diamond edged blade.
In addition to high machine costs, a laser processing system requires wide floor space than that of the sawing, whichis merely driven via a spindle. A 300 W laser system requires between 10 and 20 m2.
Laser Dicing System, vacuum wafer chuck and LaserTape® (developed by Furukawa Japan) is developed lastyear.

34. Stencil cutting with waterjet-guided laser
Stencil is used solder paste mask for electric industry.
Conventionally stencil is made by chemical etching process.
Recently laser stencil cutting system is released. Debris is
made around pattern edge because laser cutting is thermalprocess. After laser stencil cutting, acid washing or grindingis necessary for removing debris.
Waterjet-guided laser is cooling pattern edge while laser
is cutting metal mask. Therefore acid washing and grinding
is not necessary. And cutting speed is same as conventionallaser stencil cutting. Fig.8 shows comparison laser and waterjet-guided laser.

ances, slight structural changes, slight burring and parallel
cutting edges.
Fig.10 shows microjet laser cutting of endoscopes for
medical technology. The demands of this application are nostructural changes, no drawing, clean cutting edges, narrow
beam diameter and slightest mechanical load during processing.
In these cases, conventional laser processing is not applicable, because of thermal damage to the workpiece. Only
the MicroJet laser processing could be applied to overcome
all the problems.

Fig.9 Cutting sheet for nuclear industry(a) Conventional laser processing

Fig.10 Cutting endoscopes for medical technology6.
(b) Waterjet-guided laser processing
Fig.8 Comparison between conventional laser and waterjet-guided
laser processing for stencil cutting (80x)5. Other applications
The applications of water jet-guided laser processing
have been considered to the metal sheet industry. The waterjet-guided laser processing is for the high precision machining and micromachining. This includes a broad spectrum of processing metal sheet.
Of the host of applications successfully performed todate, the following two examples are intended to illustrate
why waterjet-guided laser processing has been able to succeed where conventional laser technology has failed.
Fig.9 shows cutting sheet for nuclear industry. The demands of this application are slight drawing, close toler-Conclusion

Waterjet-guided laser processing has some important advantages in comparison to conventional laser cutting, because of no focal point of beam, the large working distance,and cooling by the water jet.
Now new machine is developing, which is used 120 W Qswitched YAG laser. The laser can irradiated very shortpulse (400 ns). Neat future other new applications will beproposed.
References[1] Bernold Richerzhagen: Device for Machining Material
with a Laser, International Patent WO 95/32834 Dec. 1995
[2] Bernold Richerzhagen: Water-guided laser processing,Industrial Laser Review Nov. 1997 84