Excimer Laser Technology || Marking

16.4 Marking 321

16.4 Marking

M. Wehner

Nearly all products in everyday life are labeled by identification codes andlot numbers. Expiry dates on food packages, batch identification of food andconsumer products and inscriptions on automotive parts and electronic de-vices are typical markings. In many cases, laser marking is even used for thedecoration of products, e.g. imprinting logos and providing functional infor-mation. Usually, Nd:YAG lasers emitting at 1.06 µm wavelength are employedfor laser engraving of metals or ceramics, but marking of plastics can also beperformed by laser-induced foaming which generally results in light markson dark surfaces. Further, frequency-doubled Nd:YAG lasers at 532nm aremainly affecting pigments by photo-thermal bleaching, leading to light markswithout engraving. High irradiance at both wavelengths results in removal ofmaterial which is often accomplished with carbonization of the polymer andthus darkening of light resins.

Excimer lasers find their application when least modification of a surfacelayer is desired, e. g. the extension of the heat affected zones has to be mini-mized and the formation of microcracks has to be strictly avoided. This canbe achieved when photochemical reactions or ablation of thin layers are usedto perform marking. In Table 16.4 examples of various markings printed withexcimer lasers are listed. Practically all materials can be marked in high qual-ity by engraving (ablation) or discoloration (photochemical process). Figure16.14 shows a product for home use, which is decorated by the typical excimerlaser marking process, resulting in grey marks on white material.

Table 16.4. Overview of excimer laser marking applications

Product Purpose Variety of Markings

Aircraft cables Identification label medium - high

Medical devices Product identification, Functionalinformation

medium

Consumer products(white goods)

Logo, Product identification, Functionalinformation

low - medium

Eye glasses Logo, Product identification low

Electronic devices Parts identification, Functionalinformation

medium

322 16 Material Modification

Fig. 16.14. Marking of products for home use, example electrical tooth brushes(with kind permission of Braun)

Table 16.5. Typical parameters for marking of different materials; by courtesy ofILT

Material Process Excimer Fluence[J/cm²]

Single Shot Area[mm²]

1)

Filledpolymers

Photochemicalreduction of TiO2

XeClXeF

0.5 30

Degradation ofpigments

XeCl, XeF 0.5 30

Metals Melting,recrystallization

KrF 1 15

Glass Ablation ArF 2 – 10 1.5 – 7.5

Ceramics Chemical changes,surface morphology

KrF, XeCl 1 – 2 7.5

Ablation KrF, XeCl 2 – 10 1.5 – 7.5

1) calculated for 300 mJ pulse energy and 50% transmission losses

The advantages of laser marking compared to hot stamping (Fig. 16.15),tampon printing or ink jet printing can be summarized as follows:

– marking of nearly any material by engraving or color change– no additional or extrinsic materials– contact-less marking– durable, indelible marks

In addition to the common features of laser marking, application of ex-cimer lasers provides extra technical advantages:

16.4 Marking 323

Fig. 16.15. Comparison of heat-affected zone when marking with excimer laser(left) or hot stamping technique (right)

– engraving of delicate or transparent materials with minimal thermal stress– microscopic marks of high precision (“hidden” marking)– simultaneous printing by mask projection (flash-on-the-fly)– photochemical marking of filled polymers

Depending on the size of the field to be marked, either mask printing ormask scanning can be applied. Small fields are marked with a single shottechnique, where the whole pattern is transferred simultaneously (flash-on-the-fly). Due to the short interaction time of typically 30ns, the motion of aproduct freezes and it can be marked with excellent edge definition while be-ing transferred on a feeding system. The laser pulse is triggered just when thesample is in the right position. A set of several different signs, e.g. characters,can be printed sequentially when a fast mask change dispenser is used. Alter-natively, scan marking is employed when the size of the pattern exceeds themaximum single shot area (cf. Table 16.5). Here, the beam is scanned acrossthe mask while the sample is stationary and marking is achieved by multiplepulsing. A synchronized motion between a moving product and a movingmask, similar to the technique described in Fig. 16.16, can also be appliedfor simple integration into an existing parts feeding system. Here the productcan be marked without interference with the transportation sequence.

Many polymer materials are pigmented to reveal a special coloration orthey are filled with ceramic particles to improve the wear resistance. Irradi-ating with fluences below the ablation threshold of the polymer can intro-duce structural or chemical changes in a near-surface layer which results inbleaching of dark pigments or darkening of light pigments. Examples of somematerials and the resulting color changes are given in Table 16.6. Excimerlaser marking of polymers results in a dark (grey or brown) mark on a light


substrate. The darkening of white material is related to the photochemicalreduction of TiO2 which is used as a filler. During laser irradiation, oxygendeficient TiO2 is generated, which induces coloration by increased absorptionin the infrared region near 1.1 µm overlapping in the visible [1].

Table 16.6. Marking of pigmented polymers by excimer laser radiation [2]

Trade Name Generic Material Original Color Color Change

Hostaform® POM red white

yellow gray

ivory black

white black

Hostacom® PP reinforced white black

Celanex® PBT white black

Excimer laser marking has been employed for identification codes on air-craft cables (see Fig. 16.16), where very strict regulations exist. For safetyreasons each cable has to be marked in regular intervals along its length withidentity codes. Cables wrapped with non-sticking fluorpolymers like PTFEtape or covered with FEP or PTFE emulsion cannot be marked by ink jetsystems but are susceptible to laser marking. In these polymers, titaniumdioxide is added which is one of the most commonly used pigments in theworld and an usual additive for many polymers to whiten the insulation andfor protective purposes. Studies on different pigmented cables revealed thatcontrast is optimized with a content of 4% TiO2 by weight and the integrityand electrical characteristics of cables were maintained after excimer lasermarking.

Compared to the previously used hot stamping technique, excimer lasermarking is regarded as much less aggressive and less potentially damaging.The typical marking depth is of about 10 − 20 µm [3] and the inner parts ofthe cables remain untouched. In case of hot stamping, the high heat inputand pressure leads to deformation and shrinking of the outer tubing as wellas of the inner cover (see Fig. 16.15). Now, excimer laser marking complieswith SAE 50881 as a proven and allowable marking technique for non-stickingPTFE cables. Laser radiation of 248nm, 308 nm and 351nm wavelength iswell suited for marking of TiO2 filled materials, whereby the highest contrastis realized using 308 nm radiation. Marking can be performed with fluencesabove 100mJ/cm2, with high contrast marks being achieved in the range

16.4 Marking 325

from 0.5 J/cm2 to 1 J/cm2. Further, the contrast of a mark can be increasedby applying several pulses, depending on the polymer and the concentrationof TiO2 particles. Commercially available cable marking systems equippedwith XeCl excimer lasers allow for a wire marking speed of up to 60m/min.[4]. A high-speed mask changer is used for printing different alpha-numericcharacters which can be marked on cable down to 0.85 mm diameter, barcodes are feasible on cable down to 0.60 mm diameter. Fig. 16.16 shows dif-ferent types of cables marked with 308 nm radiation. Even colored cables canbe printed with a high contrast.

Fig. 16.16. Marking of Teflonr©-coated wires with 308 nm radiation

Medical products like catheters have to be marked with identification la-bels and, for certain applications, with distance marks to give control overthe insertion length. If the tubings are made from TiO2 filled polymers, thesemarks can be easily applied with an excimer laser (see Fig. 16.17). Compared


to tampon or ink-jet printing, the durability of laser marking without the needfor any additional material results in higher product quality. Recently, themarking of single pills is discussed. Since upcoming regulations for identifica-tion of each pill by labeling are expected, techniques for marking capsules ortablets without destroying the protective cover are required. Here the photo-chemical reduction of TiO2 is also applicable, since many covers contain TiO2as a whitener. Even marking of packaged pharmaceuticals has been demon-strated. Long wavelength UV-light is able to penetrate the thin polymer filmof the blister package and thus influencing the TiO2 particles in the capsules[5, 6].

Fig. 16.17. Length marks near the distal end of catheters; by courtesy of ILT

Marking of small electronic components like surface mounted devices(SMDs) can be performed with XeF or XeCl lasers at fluences between1 J/cm2 to 2 J/cm2. Through irradiation with fluences equivalent to the ab-lation threshold, morphological and/or chemical changes are induced by thetransient heating of a thin layer. If the material consists of a mixture from dif-ferent oxides, modifications of the oxidation states and grain size could resultin coloration or topological changes of the surface, similar to the reductionof titanium oxide mentioned before. This technique has been proven for themarking of small capacitors for surface mounting (see Fig. 16.18) where en-graving would risk altering the electrical characteristics or would make thedevice susceptible to corrosion.

16.4 Marking 327

Fig. 16.18. Marking of surface mounted devices (SMDs) [7]

Laser ablation marking is accomplished by high fluences which leads tomelting and evaporation of a thin surface layer. In many cases, a single pulseresults in a clearly visible mark. When metals are irradiated, the surface ofthe molten metal layer is deformed through the action of the vapor pressureand freezes through rapid solidification in an irregular pattern. This leads toincreased surface roughness of the irradiated area which induces light scat-tering and, in that way, a visible mark is printed. The depth of the markscan be very small since a melting depth of about 1µm is obtained for a 30nspulse.

Marking of spectacles is performed with ArF laser radiation through abla-tion of a thin coating layer at typical fluence of 6J/cm2. Clean ablation withgood edge definition is a feature of this type of marking. Micro-cracks canbe definitely avoided and the fracture strength of products preserved. Due tothe very small ablation depth, light impinging normally onto the surface isnot affected, but under grazing incidence the mark becomes visible (cf. Fig.16.19). That has the advantage that the user’s vision is not perturbed. Forimproving the visibility of such markings, it is sometimes necessary to imposea superstructure consisting of edges and flanks to provide a sufficient degreeof light scattering. The example in Fig. 16.20 shows characters consisting ofmany small spots marked in glass. Here a mesh was included in the mask todivide large open areas into many small imprints.


Fig. 16.19. Logo-labeled eye glass, visible only under grazing incidence; by courtesyof Carl Zeiss [7]

Fig. 16.20. Improving visibility of glass markings by raster screen printing; bycourtesy of MicroLas [7]

Sometimes “hidden” identification marks are desired to identify the originof high value products or to distinguish them from fakes. These markingsshould not affect the visual appearance of the product. The mark itself hasto ensure high protection against faking up and therefore should be appliedby a special technique not available anywhere. The detection of identificationcodes may also require a special technique to read out, e. g. microscopicmarks only appearing under particular illumination.

16.4 Marking 329

Recently marking of diamonds has been investigated to certify quality andorigin since the quality of artificial diamonds has been improved dramaticallyand thus the differentiation between synthetic diamonds, treated diamondsand diamond simulants needs high expertise. Despite the fact that a highquality synthetic or treated diamond will have the same “practical value”,the imaginary value of such an emotional object as diamond jewellery willbe affected. Further, the term “conflict diamonds” [8] has been brought intopublic awareness since diamonds coming from suspicious sources had enteredthe market. Civil warfare has been financed by exploitation of diamonds andpeople were pressed to work under conditions normally not accepted. Simi-larly, child labor is not the appropriate background to enjoy luxury goods.

The precise ablation of diamond materials by excimer laser radiation canbe used to imprint “invisible” marks along the girdle of a polished diamond.The typical character size is between 30 µm and 150µm and the engravingdepth from sub-micron to about 10µm depth, typically 1 − 2 µm. Thermalstress has to be minimized to avoid initiation or enlargement of cracks, andbecause of the high absorption of the UV radiation, the thickness of heat-affected layers and therefore the total heat input is minimal. The visibilitydepends on the engraving depth and the process parameters used. The laserenergy is a main parameter influencing the darkness of marks: At low en-ergy, the marks are mostly transparent, and for higher energy, the marksare getting darker, which is attributed to the formation of carbon deposits.The pulse number per site as well as the spatial overlap are parameters con-trolling the engraving depth and the definition of lines, allowing for clearlyvisible marks (see Fig. 16.21), when viewed by a magnification lens, or veryshallow pits only visible under controlled illumination.

Fig. 16.21. Certification of diamonds by excimer laser scribing; by courtesy ofPhotoScribe Technologies Inc. [9]


In the last few years, frequency-tripled Nd:YAG lasers have become avail-able which emit up to 10W power in the UV. Thus vector marking by solid-state UV-lasers of 355 nm wavelength can also take advantage of the TiO2-process. These scribing systems use beam deflection by scanning mirrors andare very flexible because all patterns are generated under software control.Therefore the decision to use either an excimer laser or an UV solid-statelasers depends on the application and special requirements. Mask projectionor one-shot-marking by excimer lasers allows for high throughput on movingparts, but printing of many different patterns would require a complex andfast mask changer. Recently, the development of compact high-repetition-rateexcimer lasers [10, 11] of 1 kHz repetition rate provides high photon energyfor scribing applications.

The marking depth, in which the material is influenced by the laser ra-diation, correlates with the optical and thermal properties of the materials.When considering dielectric materials, highest precision can be obtained us-ing excimer lasers because heat conduction plays a minor role and the ab-sorption coefficient rises strongly going to deep UV. Therefore a superficialmarking in a depth ranging from sub-micron (ablation marking of eye glassesor diamonds) to about 10µm to 20 µm (photochemical marking of pigmentedpolymers) becomes feasible.

References

1. S.W. Williams, P.C. Morgan: “Excimer laser printing of aircraft cables”, LasersMaterials Processing, Proc. of the 7th International congress on Applicationsof Lasers and Electrooptics ICALEO ’88, Santa Clara, CA, USA, p. 148–156

2. Product information from TICONA, document pages 73–74, seehttp://www.ticonaeu.com/downl/hostaform/brochure/Product-broch-Hostaform.pdf

3. http://www.spectrum-technologies.co.uk/technology/marking.htm4. http://www.spectrum-technologies.co.uk/products/capris/c100 ts.htm5. Pharmaceutical & Medial Packaging News, November 2002, see

PMPN online, Nov 2002; http://www.devcelink.com/pmpn/archive/02/11/009.html6. Technical information Tri-star technologies, see

http://www.tri-star-technologies.com/m1001 pharm.htm7. Lambda Industrial Report No. 8, November 1994, Publication by Lambda

Physik8. http://www.debeersgroup.com/hotTopics/edrebellfunding.asp9. http://www.photoscribetech.com

10. http://www.potomac-laser.com http://www.tuilaser.com11. L. Herbst, I. Klaft, T. Wenzel, U. Rebhan: “High-repetition rate excimer laser

for micromachining”, Photonics West 2003, to be published, seehttp://www.lambdaphysik.com/pdf/pdf 208.pdf

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Excimer Laser Technology || Marking