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

journal homepage: www.elsevier.com/locate/jmatprotec

Characterization of the drilling alumina ceramic using Nd:YAG pulsed laser E. Kacar a,b,∗ , M. Mutlu a,c , E. Akman a , A. Demir a,b , L. Candan a , T. Canel a,b , V. Gunay d , T. Sınmazcelik a,e a

University of Kocaeli, Laser Technologies Research and Application Centre, 41380 Umuttepe, Kocaeli, Turkey University of Kocaeli, Faculty of Science and Art, Department of Physics, 41380 Umuttepe, Kocaeli, Turkey c University of Kocaeli, Faculty of Education, 41380 Umuttepe, Kocaeli, Turkey d TUBITAK Marmara Research Centre, Materials Institute, 41470 Gebze, Kocaeli, Turkey e University of Kocaeli, Faculty of Engineering, Mechanical Engineering Department, 41040 Kocaeli, Turkey b

a r t i c l e

i n f o

a b s t r a c t

Article history:

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

Received 1 December 2007

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

Received in revised form

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

31 March 2008

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

Accepted 25 April 2008

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,

Keywords:

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

Laser drilling

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

Alumina ceramic Laser plasma interaction

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. © 2008 Elsevier B.V. All rights reserved.

1.

Introduction

Laser drilling has rapidly become an inexpensive and controllable alternative to conventional hole drilling methods such as punching, wire EDM, broaching or other popular destructive methods. Laser hole drilling in materials such as poly-

imide, ceramic, copper, nickel, brass, aluminium, borosilicate glass, quartz, rubber and composite materials offer highaccuracy, repeatability and reproducibility for the medical device industry, semiconductor manufacturing and nanotechnology support systems (Corcoran et al., 2002; Dhar et al., 2006).

∗ Corresponding author at: Kocaeli University, Department of Physics, Laser Technologies Research and Application Center, 41380 Umuttepe, Kocaeli, Turkey. Tel.: +90 262 3032915; fax: +90 262 3032003. E-mail addresses: [email protected], [email protected] (E. Kacar). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.04.049

2009

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

Laser drilling has many controlling parameters to obtain desired hole characteristics such as depth, entrance and exit diameters, circularities. Drilling characteristics are determined by exit hole diameter as a function of material thickness and pulse energy for the single pulse drilling of the material (Rodden et al., 2002). Rodden et al. also give information on the laser parameters required the holes of required dimensions with a single pulse, or, in the case of multiple pulses drilling, allows the number of pulses to drill a required thickness to be estimated. The machining of ceramics to their final dimensions by conventional methods is extremely laborious and timeconsuming. While laser machining is a non-contacting, abrasionless technique, which eliminates tool wear, machine-tool deflections, vibrations and cutting forces, reduce limitations to shape formation and inflicts less sub-surface damage. Therefore laser machining of ceramics is used extensively in the microelectronics industry for scribing and hole drilling (Lumpp and Allen, 1997). However, laser drilled holes are inherently associated with spatter deposition due to the incomplete expulsion of the ejected material from the drilling site, which subsequently resolidifies and adheres on the material surface around the hole periphery. The high hardness and brittleness lead to fracture (microcracks) of the ceramic material during laser machining. In order to prevent spatter and microcracks during the laser machining, many techniques based on either chemical or physical mechanisms have been developed (Guo et al., 2003; Orita, 1988). Also the influence of the temporal pulse train shaping is investigated on the material ejection (Low et al., 2001b). Low et al. (2001a) performed spatter-free laser percussion 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 three common methods used for laser hole drilling; single pulse, percussion and trepanning (or conventional laser cutting). Each method depends on depth requirement, hole diameter, number of holes, edge quality and production quantity. Mechanical hole drilling is difficult as the hole size decreases, furthermore laser drilling is limited because of the optical resolution and absorption of the wavelength to provide material ablation. In literature, different ceramic drilling studies have been done using different laser wavelengths. Alumina ceramic and green alumina ceramic sheets with approximately 1 mm thickness were drilled using 9.5 ␮m and 10.6 ␮m wavelengths (Imen and Allen, 1999). Excimer laser (KrF, 248 nm) was used to drill Aluminium nitride (AlN) ceramics with 635 ␮m thickness applying different design of the experiment providing with or without a metallization layer deposited on the hole walls (Lumpp and Allen, 1997). The main aim of the present study is drilling the alumina ceramic with a thickness of 10 mm, which is remarkably thicker than the previous studies as presented above. In order to drill thick alumina ceramic, percussion laser drilling method is used. Percussion laser drilling uses a “rapid-fire burst-of-pulses” micromachining method. Varying the laser pulse energy, duration, spot size, optics and beam characteristics in percussion laser drilling produces a high-quality hole with minimal residue and consistent edge quality from entry

Table 1 – Fundamental parameters of the JK 760 TR GSI Lumonics Nd:YAG Pulsed used in this work Wavelength (␮m) Average power (W) Pulse repetition rate (Hz) Pulse duration (ms) Focus diameter (␮m)

1064 600 500 0.3–50 480

Fig. 1 – Experimental setup.

to exit point. Percussion laser drilling (by using Nd:YAG laser) evaporates the machined substrates layer by layer without noticeable strata or striations, which enable us to drill highly thick alumina ceramic with a desired geometry and quality.

2.

Experimental method

From the large range of the solid-state lasers, the flash lamppumped Nd:YAG laser is used in this paper. Fundamental parameters of the pulsed Nd:YAG laser are tabulated in Table 1. Beam quality of the laser is 28 mm mrads. The experimental setup is shown in Fig. 1. The fibre optic cable is used to transfer the light around the 1.06-␮m range from the laser to the lens at the focus unit. Workpiece is infixed on the CNC table. The camera placed the top of the focus unit provide monitoring of the best focalization coupling with the CNC table. UV–vis spectrometer is used to record the visible light emitted from the plasma produced by laser during the drilling processes. In this study, 10-mm thick alumina ceramic block is drilled by using Nd:YAG laser system. The alumina which is used in this study is produced from Alcoa A16SG without any additives. Alumina content is over 99% and used as ballistic tiles or substrate in microelectronic industry. Surface can be tailored for fabricating microelectronic thin film circuitry. Absorption features of Al2 O3 are given in Table 2.

Table 2 – Absorption features of the alumina ceramic (Al2 O3 ) used in this work Irradiation wavelength,  (␮m) Fraction of deposited energy (%) Absorption coefficient, ˛ (cm−1 ) Absorption depth, ı (␮m)

1.064 98.7 4700 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 single laser shot as a function of the laser pulse duration. The laser peak power is fixed on 6.4 kW () and 11 kW ().

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 several tenths milliseconds are well-established tools for drilling of the metal alloys and composites. The major problem is the melt produced by using long pulsed lasers. Irregular and incomplete melt expulsion affects the shape of the hole, produces recast layers and may even partially close a hole during the drilling process that is open initially as discussed in (Abeln et al., 1999; Govorkov et al., 2001). In the present study, variation of the cladding diameter and hole shape is examined under different laser parameters. Drilling application of the 10-mm thick alumina ceramic block is performed at room atmosphere. The effects of the variation of the laser parameters such as peak power, pulse energy, pulse duration are examined on average hole diameter of the alumina ceramic. For this purpose, single pulse investigations are initially performed to obtain craters. Focal plane position is fixed on the material surface during these studies. The morphology of the drilled materials is analyzed using optical 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 and maximum diameter of the hole (or crater), respectively.

First of all, craters are produced by using single laser shot with different laser parameters. The average crater diameters are investigated by increasing pulse durations at the fixed peak power. The effects of the laser pulse duration at the average crater diameters are shown in Fig. 3. The laser peak power is fixed on 6.4 kW and 11 kW for this application. The laser pulse durations are changed between 1 ms and 7 ms. Average diameters of the craters increase with the laser pulse duration for the laser peak powers of 6.4 kW and 11 kW. Afterwards, the variations of the average diameters of the craters are investigated with increasing peak power at the constant laser pulse duration. The average crater diameters as a function of the laser peak power are shown in Fig. 4. The laser pulse duration is fixed on 2 ms for this application. The average diameters of these craters increase with the laser peak power at the laser pulse duration of 2 ms. In addition to single pulses, multiple pulse combinations are used to investigate the crater formations. A group of the laser pulses with a different pulse shape (triangular) are used: ramp-up and cool-down laser pulse shapes. Details of the triangular laser pulses are shown in Table 3. The used total pulse 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 a function of the peak power. Fig. 5 shows these variations for the rump-up and the cool down laser pulse shapes. The rumpup laser pulse shape gives the smaller average diameter than

Table 3 – Details of the triangular pulse parameters Pulse duration (ms)

Peak power

Pulse shapes Ramp-up pulses

Cool-down pulses

% Energy 2 2.5 3.5 4

5 4 2.9 2.5

40 27 6 4

50 37 16 10

60 47 26 18

71 57 36 28

– 67 46 36

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

% Energy – – 56 46

– – 66 56

– – – 62

76 73 72 72

66 63 62 62

57 53 52 52

47 43 42 42

– 33 32 32

– – 22 22

– – 12 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

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

Fig. 5 – The varying of the average crater diameters as a function of the laser peak power for the rump-up pulse shape () and the cool-down pulse shape (䊉). The used total pulse energy is fixed at 10 J for each triangular laser pulses.

the cool-down laser pulse shape at the same total pulse energy of 10 J. The studies of the crater formations (see Figs. 3–5) give the information of the effects of the laser parameters such as peak power and pulse duration on the alumina ceramic. After these pre-studies, 10-mm thick ceramic plates are drilled using multiple pulses with different laser parameters. Firstly, the laser peak power is changed between 5 kW and 10 kW. The laser pulse duration is fixed at 2 ms during the drilling. The images

2011

Fig. 7 – Variation of the average hole diameters a function of the laser pulse duration for the entrance of the hole (䊉) and the exit of the hole (). The peak power of the used laser is fixed at 6 kW.

of the holes obtained with optical microscope are shown in Fig. 6. In order to obtain detailed information of these holes, variation of the entrance and the exit hole diameters as a function of the pulse duration are examined. Fig. 7 shows these variations at the laser peak power of 6 kW. The exit hole diameter is equal the entrance hole diameter at the peak power of 6 kW and the pulse duration of 3 ms. The desired hole formation and the diameter for alumina ceramic can be obtained by the controlling of the peak power and the duration of the laser pulses. 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

D − D  en ex 2t

.

(2)

Here Den and Dex are entrance and exit diameters respectively; t is the thickness of the material. Fig. 8 shows the variation of the average taper angles as a function of the peak power for the holes shown in Fig. 6. The average taper angles decrease with the increasing peak power (Fig. 8). When the peak power is raised, the average taper angle has a negative value. This shows that the exit hole diameter is bigger than the entrance hole diameter. Melt erosion of the sidewalls of the hole caused by the high vapour

Fig. 6 – The hole formations are analyzed with optical microscope. First row includes the top image of the hole, second row includes the bottom image of the holes. The laser pulse duration is 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 of the peak power at the constant laser pulse duration of 2 ms.

pressure above the surface of the material during the laser irradiation of the surface will push the melt up and out of the hole (Tunnaa et al., 2005). These situations affect the geometry of the percussion laser drilled holes. The variation of the laser pulse duration is investigated to understand the reason of the negative values of the average taper angles. The peak power of the used laser pulses are fixed at the 6-kW peak power, and the pulse durations are changed between 1 ms and 8 ms. The images of the holes obtained with optical microscope are shown in Fig. 9. The variation of the calculated average taper angles as a function of the pulse duration is shown in Fig. 9. The average taper angles decrease with the increasing pulse duration. When the pulse duration is raised, the average taper angle has a negative value. This shows the exit hole diameter bigger than the enter hole diameter as shown in Fig. 9. There are negative taper values which are signifying that after certain peak power and pulse duration (see Figs. 8 and 10), resolidified ejected particles around top of the hole is dominant. This phenomenon is schematically represented in Fig. 11. In these drilling processes, a great number of the laser pulses sent to the alumina ceramic material to obtain the holes having different features. The sending pulses on the ceramic plates persist until to obtain the hole. Plasma generates on the target surface as a result of the interaction between the laser and the target during the drilling. This is called plasma plume in literature. The parameters of the plasma produced on the hole surface, such as electron tem-

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

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

perature, density and also the expansion of the plasma are important phenomena. Produced plasma on the target surface flows throughout the wall of the hole until the forming of the hole. To understand the effects of this phenomenon, the holes are cut longitudinally. The cross-section views of both sides (g1 –g2 ) of the holes are shown in Fig. 12.

Fig. 9 – The hole formations analyzed with optical microscope for different pulse durations. First row includes the top image of the hole, second row includes the bottom image of the hole. The 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

Fig. 12 – Cross-sections of the holes for different pulse duration at the constant peak power of 6 kW.

The entrance and the exit diameters of the holes give different characteristics. In order to understand better these characteristics and to evaluate the cross-sectional views of the holes shown in Fig. 12, we have to discuss the results of Figs. 6–11. The diameters of the craters show approximately linear proportion with the pulse duration and the peak power. The exit hole diameters proportionally change with the pulse duration and the peak power similar to crater diameters. On the other hand, the entrance hole diameters do not change proportionally with the pulse duration and the peak power as shown in Fig. 7. In general, the entrance diameter is larger than the exit diameter as indicated the positive taper values obtained by using Eq. (2). In this study, positive taper values are obtained, when the low peak power and low pulse duration are applied as shown in Figs. 8 and 10. The upper surface image of the hole entrance (at –ft ) and the bottom image of the hole exit (ab –fb ) are given in Fig. 6. When the peak power is ∼7.8 kW (see Fig. 8) and the pulse duration is ∼2.8 ms (see Fig. 10), the negative taper angles are observed. These values can be accepted the frontier values for formation of the negative and positive taper angles. Resolidification volume occurred around the hole entrance increase with increasing peak power and pulse duration as seen in Figs. 6 and 9. For this reason, the changing of the hole entrance diameter with the pulse duration seems constant (see Fig. 7). It can be seen the accumulation of the molten material from cross-sectional view of the hole (Fig. 12). Barelling of the hole can be evaluated by using the crosssections of A–A, B–B and C–C (Fig. 13) as explained by Yılbas (2002). The most significant effect on barrelling is due to the

Fig. 13 – Features of the laser drilled holes.

thickness of the material. When we take in to account the thickness of the target material is remarkably thicker than the previous studies in literature (Lumpp and Allen, 1997; Imen and Allen, 1999), not surprisingly the barrelling of the holes is clearly observed in Fig. 12. There are significant effects of the pressure and the energy belonging to the laser. Barrelling defines how parallel-sided is the hole. The target thickness and the laser focus settings are more effective the pressure and the energy for barrelling formation. This suggests that formation of a parallel-sided hole is related to the pressure rise inside the cavity before penetration rather than the mass

Fig. 14 – Spectra recorded by using home made UV–vis spectroscope during the laser focused on ceramic. Experimental conditions: the laser pulse energy is 0.31 J, the pulse duration 0.5 ms and the frequency is 20 Hz.

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

removal rate, i.e. the regulated mass removal rate improves barrelling more than the fast rate of mass removal. The first order interaction between the pressure and the focus setting shows that there is a coupling effect of these parameters on barrelling (Yılbas, 2002). Spectroscopic characterization of the laser drilling and welding processes is an important tool for applications such as determination of elemental atomic emission lines (Genc et al., 2007). Also, temperature and density measurements of the plasmas are based on the observation of the relative intensities and shapes of the emission peaks emitted from the induced plasmas (Goktas et al., 2007; Lacroix et al., 1997). The emission spectra from the laser-produced plasma during the drilling of the alumina ceramic plates are recorded by the UV-vis spectrometer developed in the Laser Technologies and Application Centre of Kocaeli University. Fig. 14 shows the emission spectrum from the plasma at the wavelength range between 350 nm and 600 nm.

4.

Conclusions

In this paper, the hole formation drilled on the 10-mm thick alumina ceramic are investigated by changing of the peak power and the pulse duration of the laser. This paper investigates these parameters using JK 760 TR GSI Lumonics Nd:YAG Pulsed laser with 600 ␮m optical fibre delivery system. The average hole diameters and the average taper angles are examined as a function of the laser pulse duration and the laser peak power. There are no remarkable varieties in the entrance hole diameters compare with the exit hole diameters during the drilling processes. The thickness of the alumina ceramic target material in the present study is remarkably thicker than the previous studies in literature. Barrelling of the holes is clearly observed for all the holes. The diameters of the craters show approximately linear proportion with the pulse duration and the peak power. The exit hole diameters also proportionally change with the pulse duration and the peak power similar to the crater diameters. On the other hand, the entrance hole diameters do not change proportionally with the pulse duration and the peak power. Main reason of this result can be explain by resolidified materials at the entrance of the hole. Longer pulse duration and higher peak power of the laser responsible for large amount of resolidified material and smaller entrance hole diameter. Also the thickness of the target material affects the amount of resolidified materials. Laser material interaction during the drilling processes takes longer time in thicker target materials like used in the present study. This is the main reason of getting negative taper angles of the holes.

Acknowledgements This work is supported by TUBITAK Carrier Project under contact 140T158. The authors would like to thank The Scientific and Technological Research Council of Turkey, Marmara Research Centre for permitting us to use their laboratories.

references

Abeln, T., Radtke, J., Dausinger, F., 1999. High precision drilling with short pulsed solid state lasers. In: Proceedings of the ICALEO’99, Orlando, FL, November 15–17. Laser Institute of America, Orlando, FL, pp. 195–203. Bandyopadhyay, S., Sarin Sundar, J.K., Sundararajan, G., Joshi, S.V., 2002. Geometrical features and metallurgical characteristics of Nd:YAG laser drilled holes in thick IN718 and Ti–6Al–4V sheets. Journal of Materials Processing Technology 127, 83–95. Corcoran, A., Sexton, L., Seaman, B., Ryan, P., Byrne, G., 2002. The laser drilling of multi-layer aerospace material systems. Journal of Materials Processing Technology 123, 100–106. Dhar, S., Saini, N., Purohit, R., 2006. A review on laser drilling and its techniques. In: Proceedings of the International Conference of Advances in Mechanical Engineering (AME 2006), Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Punjab, India, December 1–3. Genc, B., Kacar, E., Akman, E., Demir, A., 2007. Monitoring of laser material welding process using UV–visible Spectrometer. AIP Conference Proceedings 899, 319. Goktas, H., Demir, A., Kacar, E., Hegazy, H., Turan, R., Oke, G., Seyhan, A., 2007. Spectroscopic measurements of electron temperature and electron density in electron beam plasma generator based on collisional radiative model. Spectroscopy Letters 40 (1), 183. Govorkov, S.V., Scaggs, M., Theoharidis, H., 2001. High resolution microfabrication of hard materials with Diode-Pumped Solid State (DPSS) UV Laser (paper M603). Proceedings of the ICALEO 2001, October 15–18, Jacksonville, FL. Guo, D., Cai, K., Yang, J., Huang, T., 2003. Spatter-free laser drilling of alumina ceramics based on gelcasting technology. Journal of European Ceramic Society 23, 1263–1267. Imen, K., Allen, S.D., 1999. Pulse CO laser drilling of green alumina ceramic. IEEE Transactions on Advanced Packaging 22, 4. Lacroix, D., Jeandel, G., Boudot, C., 1997. Spectroscopic characterization of laser-induced plasma created during welding with a pulsed Nd:YAG laser. Journal of Applied Physics 81, 6599–6606. Low, D.K.Y., Li, L., Corfe, A.G., 2001a. Spatter-free laser percussion drilling of closely spaced arrayholes. International Journal of Machine Tools and Manufacture 41, 361–377. Low, D.K.Y., Li, L., Byrd, P.J., 2001b. The influence of temporal pulse train modulation during laser percussion drilling. Optics and Lasers in Engineering 35, 149–164. Lumpp, J.K., Allen, S.D., 1997. Excimer laser machining and metallization of vias in aluminium nitride. IEEE Transactions on Component Packaging and Manufacturing Technology Part B 20, 241–246. Orita, N., 1988. Laser cutting method for high chromium steel and a device to carry out that method. US Patent 4,774,392. Rodden, W.S.O., Kudesia, S.S., Hand, D.P., Jones, J.D.C., 2002. A comprehensive study of the long pulse Nd:YAG laser drilling of multi-layer carbon fibre composites. Optics Communication 210, 319–328. Sharp, C.M., Mueller, M.E., Murthy, J., 1997. A novel antispatter technique for laser drilling: applications to surface texturing. In: Proceedings of the Sixth International Congress on Applications of Lasers and Electro-Optics, San Diego, pp. 41–50. Tunnaa, L., O’Neilla, W., Khana, A., Sutcliffe, C., 2005. Analysis of laser micro drilled holes through aluminium for micro-manufacturing applications. Optics and Lasers in Engineering 43, 937–950. Yılbas, B.S., 2002. Parametric study for laser hole drilling of inconel 617 alloy. Lasers in Engineering 12, 1–16.