j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2008–2014

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Characterization of the drilling alumina ceramic usingNd:YAG pulsed laser

E. Kacara,b,∗, M. Mutlua,c, E. Akmana, A. Demira,b, L. Candana,T. Canela,b, V. Gunayd, T. Sınmazcelika,e

a University of Kocaeli, Laser Technologies Research and Application Centre, 41380 Umuttepe, Kocaeli, Turkeyb University of Kocaeli, Faculty of Science and Art, Department of Physics, 41380 Umuttepe, Kocaeli, Turkeyc University of Kocaeli, Faculty of Education, 41380 Umuttepe, Kocaeli, Turkeyd TUBITAK Marmara Research Centre, Materials Institute, 41470 Gebze, Kocaeli, Turkeye University of Kocaeli, Faculty of Engineering, Mechanical Engineering Department, 41040 Kocaeli, Turkey

a r t i c l e i n f o

Article history:

Received 1 December 2007

Received in revised form

31 March 2008

Accepted 25 April 2008

Keywords:

Laser drilling

Alumina ceramic

Laser plasma interaction

a b s t r a c t

Laser micromachining can replace mechanical removal methods in many industrial applica-

tions, particularly in the processing of difficult-to-machine materials such as hardened met-

als, ceramics, and composites. It is being applied across many industries like semiconductor,

electronics, medical, automotive, aerospace, instrumentation and communications. Laser

machining is a thermal process. The effectiveness of this process depends on thermal and

optical properties of the material. Therefore, laser machining is suitable for materials that

exhibit a high degree of brittleness, or hardness, and have favourable thermal properties,

such as low thermal diffusivity and conductivity. Ceramics which have the mentioned prop-

erties are used extensively in the microelectronics industry for scribing and hole drilling.

Rapid improvement of laser technology in recent years gave us facility to control laser

parameters such as wavelength, pulse duration, energy and frequency of laser. In this study,

Nd:YAG pulsed laser (with minimum pulse duration of 0.5 ms) is used in order to determine

the effects of the peak power and the pulse duration on the holes of the alumina ceramic

plates. The thicknesses of the alumina ceramic plates drilled by laser are 10 mm. Average

hole diameters are measured between 500 �m and 1000 �m at different drilling parameters.

The morphologies of the drilled materials are analyzed using optical microscope. Effects of

the laser pulse duration and the peak power on the average taper angles of the holes are

investigated.

accuracy, repeatability and reproducibility for the medical

1. Introduction

Laser drilling has rapidly become an inexpensive and control-

lable alternative to conventional hole drilling methods such aspunching, wire EDM, broaching or other popular destructivemethods. Laser hole drilling in materials such as poly-

∗ Corresponding author at: Kocaeli University, Department of Physics, Ltepe, Kocaeli, Turkey. Tel.: +90 262 3032915; fax: +90 262 3032003.

E-mail addresses: ekacar2001@yahoo.com, elifk@kocaeli.edu.tr (E. K0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2008.04.049

© 2008 Elsevier B.V. All rights reserved.

imide, ceramic, copper, nickel, brass, aluminium, borosilicateglass, quartz, rubber and composite materials offer high-

aser Technologies Research and Application Center, 41380 Umut-

acar).

device industry, semiconductor manufacturing and nanotech-nology support systems (Corcoran et al., 2002; Dhar et al.,2006).

t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2008–2014 2009

demtmtdpt

bcsdtTt(

serhfmti2tac(

cpEtMfoa

dat(dao(

mtombpiw

Table 1 – Fundamental parameters of the JK 760 TR GSILumonics Nd:YAG Pulsed used in this work

Wavelength (�m) 1064Average power (W) 600Pulse repetition rate (Hz) 500Pulse duration (ms) 0.3–50Focus diameter (�m) 480

tives. Alumina content is over 99% and used as ballistic tiles orsubstrate in microelectronic industry. Surface can be tailoredfor fabricating microelectronic thin film circuitry. Absorptionfeatures of Al2O3 are given in Table 2.

Table 2 – Absorption features of the alumina ceramic(Al2O3) used in this work

j o u r n a l o f m a t e r i a l s p r o c e s s i n g

Laser drilling has many controlling parameters to obtainesired hole characteristics such as depth, entrance andxit diameters, circularities. Drilling characteristics are deter-ined by exit hole diameter as a function of material

hickness and pulse energy for the single pulse drilling of theaterial (Rodden et al., 2002). Rodden et al. also give informa-

ion on the laser parameters required the holes of requiredimensions with a single pulse, or, in the case of multipleulses drilling, allows the number of pulses to drill a requiredhickness to be estimated.

The machining of ceramics to their final dimensionsy conventional methods is extremely laborious and time-onsuming. While laser machining is a non-contacting, abra-ionless technique, which eliminates tool wear, machine-tooleflections, vibrations and cutting forces, reduce limitationso shape formation and inflicts less sub-surface damage.herefore laser machining of ceramics is used extensively in

he microelectronics industry for scribing and hole drillingLumpp and Allen, 1997).

However, laser drilled holes are inherently associated withpatter deposition due to the incomplete expulsion of thejected material from the drilling site, which subsequentlyesolidifies and adheres on the material surface around theole periphery. The high hardness and brittleness lead to

racture (microcracks) of the ceramic material during laserachining. In order to prevent spatter and microcracks during

he laser machining, many techniques based on either chem-cal or physical mechanisms have been developed (Guo et al.,003; Orita, 1988). Also the influence of the temporal pulserain shaping is investigated on the material ejection (Low etl., 2001b). Low et al. (2001a) performed spatter-free laser per-ussion drilling closely spaced array holes. Also Sharp et al.1997) applied another antispatter technique for laser drilling.

Depending on the laser drilling application there are threeommon methods used for laser hole drilling; single pulse,ercussion and trepanning (or conventional laser cutting).ach method depends on depth requirement, hole diame-er, number of holes, edge quality and production quantity.echanical hole drilling is difficult as the hole size decreases,

urthermore laser drilling is limited because of the optical res-lution and absorption of the wavelength to provide materialblation.

In literature, different ceramic drilling studies have beenone using different laser wavelengths. Alumina ceramicnd green alumina ceramic sheets with approximately 1 mmhickness were drilled using 9.5 �m and 10.6 �m wavelengthsImen and Allen, 1999). Excimer laser (KrF, 248 nm) was used torill Aluminium nitride (AlN) ceramics with 635 �m thicknesspplying different design of the experiment providing withr without a metallization layer deposited on the hole walls

Lumpp and Allen, 1997).The main aim of the present study is drilling the alu-

ina ceramic with a thickness of 10 mm, which is remarkablyhicker than the previous studies as presented above. Inrder to drill thick alumina ceramic, percussion laser drillingethod is used. Percussion laser drilling uses a “rapid-fire

urst-of-pulses” micromachining method. Varying the laserulse energy, duration, spot size, optics and beam character-

stics in percussion laser drilling produces a high-quality holeith minimal residue and consistent edge quality from entry

Fig. 1 – Experimental setup.

to exit point. Percussion laser drilling (by using Nd:YAG laser)evaporates the machined substrates layer by layer withoutnoticeable strata or striations, which enable us to drill highlythick alumina ceramic with a desired geometry and quality.

2. Experimental method

From the large range of the solid-state lasers, the flash lamp-pumped Nd:YAG laser is used in this paper. Fundamentalparameters of the pulsed Nd:YAG laser are tabulated in Table 1.Beam quality of the laser is 28 mm mrads. The experimentalsetup is shown in Fig. 1. The fibre optic cable is used to transferthe light around the 1.06-�m range from the laser to the lensat the focus unit. Workpiece is infixed on the CNC table. Thecamera placed the top of the focus unit provide monitoringof the best focalization coupling with the CNC table. UV–visspectrometer is used to record the visible light emitted fromthe plasma produced by laser during the drilling processes.

In this study, 10-mm thick alumina ceramic block is drilledby using Nd:YAG laser system. The alumina which is used inthis study is produced from Alcoa A16SG without any addi-

Irradiation wavelength, � (�m) 1.064Fraction of deposited energy (%) 98.7Absorption coefficient, ˛ (cm−1) 4700Absorption depth, ı (�m) 2.1

2010 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2008–2014

Fig. 3 – The average crater diameter produced by a singlelaser shot as a function of the laser pulse duration. The

Fig. 2 – The minimum (Dmin) and maximum (Dmax) hole

diameter of the given hole in Eq. (1).

3. Results and discussions

Lamp-pumped Nd:YAG lasers with pulse length of severaltenths milliseconds are well-established tools for drilling ofthe metal alloys and composites. The major problem is themelt produced by using long pulsed lasers. Irregular andincomplete melt expulsion affects the shape of the hole, pro-duces recast layers and may even partially close a hole duringthe drilling process that is open initially as discussed in (Abelnet al., 1999; Govorkov et al., 2001). In the present study, vari-ation of the cladding diameter and hole shape is examinedunder different laser parameters.

Drilling application of the 10-mm thick alumina ceramicblock is performed at room atmosphere. The effects of thevariation of the laser parameters such as peak power, pulseenergy, pulse duration are examined on average hole diameterof the alumina ceramic. For this purpose, single pulse inves-tigations are initially performed to obtain craters. Focal planeposition is fixed on the material surface during these stud-ies. The morphology of the drilled materials is analyzed usingoptical microscope.

The average hole (or crater) diameter is determined by

Dav = Dmin + Dmax

2. (1)

Here, Dmin and Dmax as shown in Fig. 2 are the minimum andmaximum diameter of the hole (or crater), respectively.

Table 3 – Details of the triangular pulse parameters

Pulse duration (ms) Peak power

Ramp-up puls

% Energy

2 5 40 50 60 71 –2.5 4 27 37 47 57 673.5 2.9 6 16 26 36 464 2.5 4 10 18 28 36

The used total pulse energy is fixed at 10 J for each triangular laser pulses.

laser peak power is fixed on 6.4 kW (�) and 11 kW (�).

First of all, craters are produced by using single laser shotwith different laser parameters. The average crater diametersare investigated by increasing pulse durations at the fixed peakpower. The effects of the laser pulse duration at the averagecrater diameters are shown in Fig. 3. The laser peak power isfixed on 6.4 kW and 11 kW for this application. The laser pulsedurations are changed between 1 ms and 7 ms. Average diam-eters of the craters increase with the laser pulse duration forthe laser peak powers of 6.4 kW and 11 kW. Afterwards, thevariations of the average diameters of the craters are investi-gated with increasing peak power at the constant laser pulseduration. The average crater diameters as a function of thelaser peak power are shown in Fig. 4. The laser pulse durationis fixed on 2 ms for this application. The average diameters ofthese craters increase with the laser peak power at the laserpulse duration of 2 ms.

In addition to single pulses, multiple pulse combinationsare used to investigate the crater formations. A group of thelaser pulses with a different pulse shape (triangular) are used:ramp-up and cool-down laser pulse shapes. Details of thetriangular laser pulses are shown in Table 3. The used totalpulse energy is fixed at 10 J for each triangular laser pulses.As a result of these triangular laser pulse applications, the

variations of the average crater diameters are obtained as afunction of the peak power. Fig. 5 shows these variations forthe rump-up and the cool down laser pulse shapes. The rump-up laser pulse shape gives the smaller average diameter than

Pulse shapes

es Cool-down pulses

% Energy

– – – 76 66 57 47 – – – –– – – 73 63 53 43 33 – – –

56 66 – 72 62 52 42 32 22 12 –46 56 62 72 62 52 42 32 22 12 2

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2008–2014 2011

Fig. 4 – The average crater diameter produced by a singlelaser shot as a function of the laser peak power. The laserpulse duration is fixed at 2 ms.

Fig. 5 – The varying of the average crater diameters as afunction of the laser peak power for the rump-up pulsesp

to

ipptpp

Fig. 7 – Variation of the average hole diameters a functionof the laser pulse duration for the entrance of the hole (�)

Fi

hape (�) and the cool-down pulse shape (�). The used totalulse energy is fixed at 10 J for each triangular laser pulses.

he cool-down laser pulse shape at the same total pulse energyf 10 J.

The studies of the crater formations (see Figs. 3–5) give thenformation of the effects of the laser parameters such as peakower and pulse duration on the alumina ceramic. After these

re-studies, 10-mm thick ceramic plates are drilled using mul-iple pulses with different laser parameters. Firstly, the lasereak power is changed between 5 kW and 10 kW. The laserulse duration is fixed at 2 ms during the drilling. The images

ig. 6 – The hole formations are analyzed with optical microscopncludes the bottom image of the holes. The laser pulse duration

and the exit of the hole (�). The peak power of the usedlaser is fixed at 6 kW.

of the holes obtained with optical microscope are shown inFig. 6.

In order to obtain detailed information of these holes, vari-ation of the entrance and the exit hole diameters as a functionof the pulse duration are examined. Fig. 7 shows these varia-tions at the laser peak power of 6 kW. The exit hole diameteris equal the entrance hole diameter at the peak power of 6 kWand the pulse duration of 3 ms. The desired hole formationand the diameter for alumina ceramic can be obtained by thecontrolling of the peak power and the duration of the laserpulses.

Ideal hole is characterized if it is in cylindrical form.Cylindricality degree of a hole is given by taper angle(Bandyopadhyay et al., 2002). Taper angle, �, is calculated by

� = tg−1[

Den − Dex

2t

]. (2)

Here Den and Dex are entrance and exit diameters respectively;t is the thickness of the material. Fig. 8 shows the variation ofthe average taper angles as a function of the peak power forthe holes shown in Fig. 6.

The average taper angles decrease with the increasing peak

power (Fig. 8). When the peak power is raised, the averagetaper angle has a negative value. This shows that the exit holediameter is bigger than the entrance hole diameter. Melt ero-sion of the sidewalls of the hole caused by the high vapour

e. First row includes the top image of the hole, second rowis fixed at 2 ms.

2012 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2008–2014

Fig. 8 – Variation of the average taper angles as a function Fig. 10 – Variation of the average taper angles as a functionof the pulse duration. The peak power of the used laser isfixed at 6 kW.

of the peak power at the constant laser pulse duration of2 ms.

pressure above the surface of the material during the laserirradiation of the surface will push the melt up and out of thehole (Tunnaa et al., 2005). These situations affect the geometryof the percussion laser drilled holes.

The variation of the laser pulse duration is investigatedto understand the reason of the negative values of the aver-age taper angles. The peak power of the used laser pulsesare fixed at the 6-kW peak power, and the pulse durationsare changed between 1 ms and 8 ms. The images of the holesobtained with optical microscope are shown in Fig. 9. The vari-ation of the calculated average taper angles as a function ofthe pulse duration is shown in Fig. 9. The average taper anglesdecrease with the increasing pulse duration. When the pulseduration is raised, the average taper angle has a negative value.This shows the exit hole diameter bigger than the enter holediameter as shown in Fig. 9.

There are negative taper values which are signifying thatafter certain peak power and pulse duration (see Figs. 8 and 10),resolidified ejected particles around top of the hole is dom-inant. This phenomenon is schematically represented inFig. 11.

In these drilling processes, a great number of the laserpulses sent to the alumina ceramic material to obtain theholes having different features. The sending pulses on theceramic plates persist until to obtain the hole. Plasma gen-

erates on the target surface as a result of the interactionbetween the laser and the target during the drilling. Thisis called plasma plume in literature. The parameters of theplasma produced on the hole surface, such as electron tem-

Fig. 9 – The hole formations analyzed with optical microscope foof the hole, second row includes the bottom image of the hole. T

Fig. 11 – Schematically represent for ejected particlesresolidified around the laser drilled hole.

perature, density and also the expansion of the plasma areimportant phenomena. Produced plasma on the target sur-

face flows throughout the wall of the hole until the forming ofthe hole. To understand the effects of this phenomenon, theholes are cut longitudinally. The cross-section views of bothsides (g1–g2) of the holes are shown in Fig. 12.

r different pulse durations. First row includes the top imagehe peak power of the laser pulses is fixed at 6 kW.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2008–2014 2013

pulse duration at the constant peak power of 6 kW.

dctFlTdtpatooat(Fatthdocm

s(

Fc

Fig. 12 – Cross-sections of the holes for different

The entrance and the exit diameters of the holes giveifferent characteristics. In order to understand better theseharacteristics and to evaluate the cross-sectional views ofhe holes shown in Fig. 12, we have to discuss the results ofigs. 6–11. The diameters of the craters show approximatelyinear proportion with the pulse duration and the peak power.he exit hole diameters proportionally change with the pulseuration and the peak power similar to crater diameters. Onhe other hand, the entrance hole diameters do not changeroportionally with the pulse duration and the peak powers shown in Fig. 7. In general, the entrance diameter is largerhan the exit diameter as indicated the positive taper valuesbtained by using Eq. (2). In this study, positive taper values arebtained, when the low peak power and low pulse duration arepplied as shown in Figs. 8 and 10. The upper surface image ofhe hole entrance (at–ft) and the bottom image of the hole exitab–fb) are given in Fig. 6. When the peak power is ∼7.8 kW (seeig. 8) and the pulse duration is ∼2.8 ms (see Fig. 10), the neg-tive taper angles are observed. These values can be acceptedhe frontier values for formation of the negative and posi-ive taper angles. Resolidification volume occurred around theole entrance increase with increasing peak power and pulseuration as seen in Figs. 6 and 9. For this reason, the changingf the hole entrance diameter with the pulse duration seemsonstant (see Fig. 7). It can be seen the accumulation of the

olten material from cross-sectional view of the hole (Fig. 12).Barelling of the hole can be evaluated by using the cross-

ections of A–A, B–B and C–C (Fig. 13) as explained by Yılbas2002). The most significant effect on barrelling is due to the

ig. 14 – Spectra recorded by using home made UV–vis spectrosconditions: the laser pulse energy is 0.31 J, the pulse duration 0.5

Fig. 13 – Features of the laser drilled holes.

thickness of the material. When we take in to account thethickness of the target material is remarkably thicker than theprevious studies in literature (Lumpp and Allen, 1997; Imenand Allen, 1999), not surprisingly the barrelling of the holesis clearly observed in Fig. 12. There are significant effects ofthe pressure and the energy belonging to the laser. Barrellingdefines how parallel-sided is the hole. The target thickness

and the laser focus settings are more effective the pressureand the energy for barrelling formation. This suggests thatformation of a parallel-sided hole is related to the pressurerise inside the cavity before penetration rather than the mass

ope during the laser focused on ceramic. Experimentalms and the frequency is 20 Hz.

g t e

r

2014 j o u r n a l o f m a t e r i a l s p r o c e s s i n

removal rate, i.e. the regulated mass removal rate improvesbarrelling more than the fast rate of mass removal. The firstorder interaction between the pressure and the focus settingshows that there is a coupling effect of these parameters onbarrelling (Yılbas, 2002).

Spectroscopic characterization of the laser drilling andwelding processes is an important tool for applications suchas determination of elemental atomic emission lines (Gencet al., 2007). Also, temperature and density measurementsof the plasmas are based on the observation of the relativeintensities and shapes of the emission peaks emitted fromthe induced plasmas (Goktas et al., 2007; Lacroix et al., 1997).The emission spectra from the laser-produced plasma dur-ing the drilling of the alumina ceramic plates are recorded bythe UV-vis spectrometer developed in the Laser Technologiesand Application Centre of Kocaeli University. Fig. 14 shows theemission spectrum from the plasma at the wavelength rangebetween 350 nm and 600 nm.

4. Conclusions

In this paper, the hole formation drilled on the 10-mm thickalumina ceramic are investigated by changing of the peakpower and the pulse duration of the laser. This paper investi-gates these parameters using JK 760 TR GSI Lumonics Nd:YAGPulsed laser with 600 �m optical fibre delivery system.

The average hole diameters and the average taper anglesare examined as a function of the laser pulse duration andthe laser peak power. There are no remarkable varieties in theentrance hole diameters compare with the exit hole diametersduring the drilling processes.

The thickness of the alumina ceramic target material in thepresent study is remarkably thicker than the previous studiesin literature. Barrelling of the holes is clearly observed for allthe holes.

The diameters of the craters show approximately linearproportion with the pulse duration and the peak power. Theexit hole diameters also proportionally change with the pulseduration and the peak power similar to the crater diameters.On the other hand, the entrance hole diameters do not changeproportionally with the pulse duration and the peak power.Main reason of this result can be explain by resolidified mate-rials at the entrance of the hole. Longer pulse duration andhigher peak power of the laser responsible for large amountof resolidified material and smaller entrance hole diameter.Also the thickness of the target material affects the amountof resolidified materials. Laser material interaction during thedrilling processes takes longer time in thicker target materi-als like used in the present study. This is the main reason ofgetting negative taper angles of the holes.

Acknowledgements

This work is supported by TUBITAK Carrier Project undercontact 140T158. The authors would like to thank The Scien-tific and Technological Research Council of Turkey, MarmaraResearch Centre for permitting us to use their laboratories.

c h n o l o g y 2 0 9 ( 2 0 0 9 ) 2008–2014

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