I

Ka

b

a

ARRAA

KCGLTT

1

tdlsTs˛

aigetc[

fisldf

0d

Precision Engineering 34 (2010) 70–75

Contents lists available at ScienceDirect

Precision Engineering

journa l homepage: www.e lsev ier .com/ locate /prec is ion

nfluence of thermal expansion coefficient in laser scribing of glass

oji Yamamoto a,∗, Noboru Hasaka b, Hideki Morita b, Etsuji Ohmura a

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, JapanMitsuboshi Diamond Industrial Co., Ltd., 1-4-37 Minami-kaneden, Suita, Osaka, Japan

r t i c l e i n f o

rticle history:eceived 18 August 2008eceived in revised form 20 February 2009ccepted 11 March 2009vailable online 27 June 2009

a b s t r a c t

Aluminosilicate glass, which has a relatively low thermal expansion coefficient, is used as a glass substratein liquid crystal displays, whereas soda-lime glass is generally used for such items as windows. The aimof this study was to clarify the influence of the thermal expansion coefficient on the glass substrateduring laser scribing with crack propagation produced by laser heating and followed by quick quenching.The laser scribe conditions with aluminosilicate glass and fused silica were obtained in laser irradiation

eywords:O2 laserlassaser scribing,hermal stresshermal expansion coefficient

experiments to compare the difference of glass materials. Two-dimensional thermal elasticity analysiswas then conducted with a finite element method based on the experimental results. The laser scribablecondition of aluminosilicate glass can be estimated by combining the upper limit of the maximum surfacetemperature and the lower limit of the maximum tensile stress in the cooling area. The tensile stressgenerated in the cooling area decreases as the thermal expansion coefficient decreases. Therefore, fusedsilica, whose thermal expansion is much lower than aluminosilicate glass, is rarely applicable in laser

scribing for separation.

. Introduction

Today the dimensions of glass substrates are increasing in ordero increase panel yield in the production process of liquid crystalisplays (LCD) used in flat-screen televisions. However such prob-

ems as malpositioning in lamination are observed when the glassubstrate expands during the heating process in LCD production.herefore aluminosilicate glass, which has a lower thermal expan-ion coefficient than soda-lime glass (thermal expansion coefficient= 8.7 × 10−6 K−1) is used.

A scribe method exists as a way of separating glass by quenchingfter CO2 laser irradiation that advances a fissure in the quench-ng area. This method is called laser scribing. After scribing, thelass is separated by loading along the scribed line. This is anffective method of processing glass, as microcracks are less likelyo be generated and the glass edge strength is improved [1,2]ompared with the use of a mechanical glass separation method3,4].

The authors [5,6] conducted thermal elasticity analysis with anite element method based on a laser scribe experiment using

oda-lime glass with thickness of 0.7 mm, and proposed the fol-owing laser scribe mechanism. Fig. 1(a) illustrates the temperatureistribution on the glass surface. In laser scribing, the heat is trans-

erred from the surface to the interior of the glass, and the surface is

∗ Corresponding author. Tel.: +81 6 6378 3843; fax: +81 6 6378 3550.E-mail address: kyamamoto@mitsuboshi-dia.co.jp (K. Yamamoto).

141-6359/$ – see front matter © 2009 Elsevier Inc. All rights reserved.oi:10.1016/j.precisioneng.2009.03.005

© 2009 Elsevier Inc. All rights reserved.

quenched by a water jet immediately after the laser heat as shownin Fig. 1(b). In this way, tensile stress induced in the surface layer andthe laser scribe crack advances. Since the laser scribe method usesthermal stress, it is assumed to be easily affected by the thermalexpansion coefficient.

The laser irradiation experiment in this study was executedusing soda-lime glass, aluminosilicate glass and fused silica, eachof which has a different thermal expansion coefficient. Next, two-dimensional thermal elasticity analysis was conducted with a finiteelement method based on the experimental results in order to clar-ify the influence of the thermal expansion coefficient during laserscribing.

2. Experimental method and results

2.1. Experimental method

The same apparatus as in a previous study [5,6] was used. Theglass substrate was fixed on vacuum suction stage. An initial crackwas notched at the glass edge with a glass cutting wheel as the ori-gin point for scribing, and a CO2 laser formed a beam that madean elliptic shape on the glass surface. The laser beam was directedonto the glass surface to heat the planned scribe line and a rela-

tive velocity was produced between the laser beam and the glasssubstrate. The distal end of the laser beam was quenched by waterjet. With this method, a median crack was formed and moved inthe scribe direction, and the laser scribe line was produced. Fig. 2shows the positional relationship between the area heated by the

K. Yamamoto et al. / Precision En

Fig. 1. Schematic of laser scribe mechanism. (a) Top view: temperature distributionon glass surface. (b) Section view (at A-A): tensile stress over compressive stressfield.

Fr

lr

wf1Aactoczbitpd

TT

vP22d22

ig. 2. Definitions and variables of geometry used for heating area, cooling area andespective distances.

aser beam and the cooling area quenched by a water jet, and theespective distances.

The specimens used were aluminosilicate glass (Corning 1737)ith a thickness of 0.7 mm and size of 360 mm × 460 mm, and

used silica (Asahi Glass AQ) with a thickness of 0.6 mm and size of50 mm × 150 mm, in comparison to soda-lime glass (Asahi GlassS) with a thickness of 0.7 mm and size of 300 mm × 400 mmpplied in the previous study [5,6]. Table 1 shows the laser scribeonditions. Typical values are shown for the scribe velocity andhe laser power. The size of the heating area is shown using 1/e2

n the major axis and 1/e2 on the minor axis, and the size of theooling area is indicated by the diameter determined by the noz-le diameter of the water jet, the dispersing angle and the distance

etween the nozzle and the glass surface. The beam shape, the cool-

ng condition and the cooling point distance d were fixed, whilehe glass materials were changed. Thus, scribing was defined to beossible when a laser scribe line was produced after the laser irra-iation experiment, and was defined to be not possible when the

able 1ypical conditions for experiment.

Scribe velocity 150 mm/sLaser power 58.7 W

x0 Minor axis of heating area 2.1 mmy0 Major axis of heating area 22.0 mm

Cooling point distance 10 mmxc Minor axis of cooling area 2.0 mmyc Major axis of cooling area 3.0 mm

gineering 34 (2010) 70–75 71

crack progress was arrested. Since the glass edge strength decreasesif thermal damage remains in the glass surface, such cases werejudged to be unscribable. After scribing, the glass substrate was sep-arated manually and crack depth Dc was measured with an opticalmicroscope.

2.2. Experimental results

Scribable velocity with respect to laser power was obtained asthe laser scribable condition for amuminosilicate glass. The resultsand the crack depth Dc in each condition are shown in Fig. 3(a).Soda-lime glass experimental results from a previous study [5,6]are shown in Fig. 3(b) again to compare difference in the thermalexpansion coefficient. In Fig. 3(a) (i) and (b) (i), the “×” marks on thehigher velocity side show the conditions in which the crack progresswas arrested, and the “×” marks on the lower velocity side showthe conditions in which there was residual thermal damage. For thecrack depth Dc related to each laser power in Fig. 3(a) (ii) and (b)(ii), deeper crack depth corresponds to lower velocity and shallowercrack depth corresponds to higher velocity. Nonetheless, fused silicacould not be scribed under the conditions in which aluminosilicateglass was scribable.

Even for aluminosilicate glass, scribable velocity tends to behigher as laser power increases. As compared with soda-lime glassin Fig. 3(b), the both the scribe velocity at which thermal dam-age was generated (“×” marks on the lower velocity side) and thescribe velocity at which the crack progress was arrested (“×” marksof higher velocity side) were lower.

3. Thermal stress analysis

Based on the scribe conditions obtained in the experiment, thefollowing two-dimensional thermal elasticity analysis was con-ducted with a finite element method (FEM). The laser scribablecondition and the influence of the thermal expansion coefficientin laser scribing are discussed in the next chapter.

The x–y coordinates were set on the laser irradiated surface,with scribe direction on the y-axis and glass thickness on the z-axis. Fig. 4 shows the element division using FEM analysis withaluminosilicate glass having a thickness of 0.7 mm as an example.By considering symmetry, the target for analysis was focused inthe range of 0.7 mm × 30 mm. The minimum division value in thebeam width direction (x-axis direction) was set as 3.7 �m, and theminimum value of division in the thickness direction (z-axis direc-tion) was 2.7 �m. The total number of nodes was 769 and the totalnumber of elements 721. The values in Table 2 were used as phys-ical properties of each specimen. The time step was 0.25 mm/v (s),which is the time when 0.25 mm was divided by the scribe velocityv. The values shown in Table 1 were used for the size of the heatingarea and the cooling area, and these intensities were assumed tohave Gaussian distributions.

In two-dimensional heat conduction analysis on the x–z plane,the center of the laser beam was scanned from the position ofy = −15 mm in the y-axis direction (farther to the front on thepaper), and both the heating and cooling conditions were changedwith time. Since the optical attenuation rate was calculated to be0.9793%, the heat absorption was set as 0.798P (W) considering thatthe measured reflection ratio was 18.5% in Table 2. The heat trans-fer coefficient ˛0 at the collision point [15] was calculated usingthe water jet flow rate (0.8 ml/min in this case). The water tem-

perature was controlled at 20 ◦C. ˛0 was always set at 105 W/m2 K,since the water jet flow rate was kept stable even while laser powerand scribe velocity were changed in the experiment.

Next, two-dimensional thermal stress analysis was conductedon the x–z plane for the issue of plane stress, which was assumed

72 K. Yamamoto et al. / Precision Engineering 34 (2010) 70–75

F glassg r. (b) Pv velocl ser he

tTa

is

4

4

saFpduaT

TP

DSTEYPSCFR

ig. 3. The domain of laser scribable conditions and crack depth of aluminosilicatelass. (i) Scribable velocity versus laser power and (ii) crack depth versus laser poweersus laser power and (ii) crack depth versus laser power. (i) “×” marks at higherower velocities represent conditions in which the glass surface was damaged by la

o be �yy = �yx = �yz = 0 using the obtained temperature distribution.he laser irradiation side-end was restrained in the x-axis directionnd the other side-end in the x-axis and z-axis direction.

As in a previous study [5,6], maximum surface temperature dur-ng the laser scribing was denoted as Tmax, and maximum tensiletress of �xx in the cooling area was denoted as �tmax.

. Result and discussion

.1. Laser scribable condition

A previous study [5,6] has shown that the scribable condition foroda-lime glass can be estimated by the maximum surface temper-ture Tmax and the maximum tensile stress �tmax in the cooling area.ig. 5(a) shows again the soda-lime glass analysis results to com-

are the scribable conditions by the thermal expansion coefficientifference. Similarly, a thermal elasticity analysis was conductednder the experimental conditions (Fig. 3 (a) (i)) for laser powernd scribe velocity at which aluminosilicate glass was scribable.he result is shown in Fig. 5(b). The upper side of the plots in these

able 2hysical properties of glass substrates.

Soda-lime Ref.

ensity 2520 kg/m3 [7]pecific heat 800 J/kg K [7]hermal conductivity 1.03 W/m K [7]xpansion coefficient 8.7 × 10−6 K−1 [8]oung’s modulus 71.6 GPa [9]oisson’s ratio 0.23 [9]oftening temperature 720–730 ◦C [9]ritical stress intensity factors (N2 300 K) 0.76 MPa m1/2 [10]racture surface energy (N2 300 K) 3.9 J/m2 [10]eflectance 18.5%

and soda-lime glass. (a) Plots of scribe conditions for substrate of aluminosilicatelots of scribe conditions for substrate of soda-lime glass [5,6]. (i) Scribable velocityities represent conditions in which crack progress was arrested and “×” marks atating. (ii) Deeper crack depth corresponds to lower velocity conditions.

figures shows the maximum surface temperature Tmax (right verti-cal axis), and lower side shows the maximum tensile stress �tmax

(left vertical axis). The “×” marks on the higher velocity side of eachplot correspond to the conditions under which crack progress wasarrested, and the “×” marks on the lower velocity side correspond tothe conditions determined to be unscribable as a result of the ther-mal damage generation in Fig. 3(a) (i) and (b) (i). Fig. 5(c) shows theanalysis results under conditions in which fused silica could not bescribed.

For the aluminosilicate glass in Fig. 5(b), the upper limit of Tmax

was nearly stable independent of scribe velocity, and was approx-imately 700 ◦C at every laser power. This value is greater than theupper limit of Tmax of soda-lime glass. This is considered to bebecause the softening point of aluminosilicate glass is as high as975 ◦C, considerably higher than that of soda-lime glass at around

720–730 ◦C, shown in Table 2. In either case, thermal damage is notgenerated on aluminosilicate glass surface below this upper limitof Tmax.

On the other hand, focusing on �tmax in Fig. 5 (b), �tmax decreasesas scribe velocity becomes higher when laser power is stable. This

Aluminosilicate Ref. Fused silica Ref.

2540 kg/m3 [11] 2200 kg/m3 [12]707.5 J/kg K [11] 730 J/kg K [13]0.9085 W/m K [11] 1.3 W/m K [13]4.2 × 10−6 K−1 [11] 0.6 × 10−6 K−1 [12]70.9 GPa [11] 73.4 GPa [12]0.23 [11] 0.17 [12]975 ◦C [11] 1600 ◦C [12]0.91 MPa m1/2 [10] 0.79 MPa m1/2 [10]4.7 J/m2 [10] 4.4 J/m2 [10]12.7% 12.83% [14]

K. Yamamoto et al. / Precision Engineering 34 (2010) 70–75 73

iv�blctll

fctcbaa

Td

4s

etld�(wa

tbdth�c

Fig. 5. Analysis results of maximum surface temperature Tmax and maximum tensilestress �tmax. (a) Soda-lime glass [5], (b) aluminosilicate glass, and (c) fused silica. The

Fig. 4. Mesh geometry for FEM analysis.

s the reason that the crack depth becomes shallower on the higherelocity side in Fig. 3(a) (ii). At every laser power, the lower limit oftmax is nearly stable and independent of scribe velocity. The valueecomes approximately 60 MPa, which is almost equal to the lower

imit of �tmax of soda-lime glass (approximately 65 MPa). This isonsidered to be because there is no large difference in the frac-ure surface energy in each specimen in Table 2. In either case, theaser scribe crack progresses when �tmax is higher than this lowerimit.

Thus, upper limit of Tmax and lower limit of �tmax also existor aluminosilicate glass, which has a smaller thermal expansionoefficient (˛ = 4.2 × 10−6 K−1) than soda-lime glass. If thermal elas-icity analysis is conducted using these values, the laser scribableonditions obtainable experimentally can be predicted from a com-ination of the upper limit of maximum surface temperature Tmax

nd the lower limit of maximum tensile stress �tmax in the coolingrea.

It is also seen from Fig. 5(c) that, even on fused silica, bothmax and �tmax tend to increase when laser power is constant atecreasing scribe velocities.

.2. Influence of thermal expansion coefficient during lasercribing

To investigate the influence on laser scribing when the thermalxpansion coefficient decreased, the time variations of tempera-ure and stress of �xx were analyzed under the scribe conditions ofaser power P = 58.7 W and scribe velocity v = 150 mm/s. The stressistribution of �xx along the z-axis was analyzed at the time whentmax was generated. The result is shown in Fig. 6. In Fig. 6(a) and

b), the time t = 100 ms corresponds to the center of the laser beam,hile the time t = 167 ms corresponds to the center of the cooling

rea.In Fig. 6(a), although the difference in the maximum surface

emperature Tmax is approximately 100 ◦C, the temperature distri-ution profiles are almost the same. This temperature difference

epends on the physical property of each specimen. The stress dis-ributions in Fig. 6(b) and (c) are almost the same. Stress generation,owever, shows a great difference. The maximum tensile stresstmax in the cooling area sharply drops as the thermal expansionoefficient decreases. Here, the lower limit value of �tmax of fused

“×” marks at higher velocity correspond to the conditions in which laser scribe crackprogress was arrested, and “×” marks at lower velocity correspond to the conditionswith which there was residual thermal damage. (a) and (b) correspond to Fig. 3(a)and (b) (i), respectively.

silica is assumed to be approximately 60 MPa from the fact thatthere is no significant difference in the fracture surface energy ofeach specimen in Table 2. Compared with �tmax of soda-lime glasswith 103 MPa, �tmax of fused silica dropped as low as 7 MPa, whichis lower than the lower limit of assumed �tmax. This may be thereason that fused silica could not be scribed under the conditionsin which aluminosilicate glass was scribable.

Even in fused silica, which has a small thermal expansion coeffi-cient and is less likely to generate maximum tensile stress �tmax inthe cooling area, �tmax can be increased by decreasing the scribevelocity and increasing laser power from that in Fig. 5(c). Themaximum surface temperature T and the maximum tensile

max

stress �tmax were then investigated when the scribe velocity v, wasdecreased to 150, 100, 50, and 10 mm/s, and the laser power P, wasincreased to 58.7, 80.5, and 120 W. The results are shown in Fig. 7.In Fig. 7, the assumed lower limit of approximately 60 MPa and the

74 K. Yamamoto et al. / Precision Engineering 34 (2010) 70–75

Fig. 6. Temperature distributions and stress distributions of �xx for soda-lime glass,aluminosilicate glass, and fused silica (P = 58.7 W, v = 150 mm/s). (a) Time variationsof glass surface temperature, (b) Time variations of surface stress �xx , and (c) Stressdistributions along the z-axis at the time �tmax is generated.

Fig. 7. Analysis results of maximum tensile stress �tmax and maximum surface tem-perature Tmax of fused silica with lower scribe velocity and higher power comparedwith Fig. 5(c).

Fig. 8. An example of a plane of alumina ceramics (Al2O3) scribed by laser.

softening point of 1600 ◦C of fused silica are shown. Under the con-ditions of P = 120 W and v = 10 mm/s, �tmax increased to as muchas 23 MPa. This value is, however, far smaller than the lower limitof approximately 60 MPa of the assumed �tmax. Additionally, underthese conditions Tmax is approximately 3900 ◦C and is heated higherthan the fused silica softening point of 1600 ◦C. Therefore, the crackdoes not proceed and it is predicted that the thermal damage hasoccurred on the glass substrate surface. It is probably true that forthis reason fused silica cannot normally be laser scribed.

Reasoning from these results, it would seem that laser scribing ispossible with the method shown in Fig. 2 in glass for which the ther-mal expansion coefficient is more than that of aluminosilicate glassand the softening point is equal to or more than that of aluminosil-icate glass, or in brittle materials that have higher thermostabletemperatures.

A laser irradiation experiment was then executed using an alu-mina ceramics substrate (Kyocera A476T [16]) that had a thermalexpansion coefficient ˛ = approximately 7 × 10−6 K−1, and thick-ness 0.635 mm. The results demonstrated that it is possible toperform laser scribing even on alumina ceramics. Fig. 8 shows opti-cal microscopic photographs of a scribed plane that was manuallyseparated. The laser scribed crack Dc can be confirmed (Fig. 8(b))and no thermal damage is seen (Fig. 8(a)). The thermostable tem-perature of alumina ceramics is more than 1500 ◦C, higher than thesoftening point of aluminosilicate glass. This experimental result isan example proving the previous assumption.

5. Conclusion

Two-dimensional thermal elasticity analysis with a finite ele-ment method was conducted under the laser scribe conditions forglass materials with different thermal expansion coefficient. Theconclusions from this study are as follows.

The upper limit of the maximum surface temperature Tmax andthe lower limit of the maximum tensile stress �tmax in the coolingarea are nearly stable and independent of laser power and scribevelocity for aluminosilicate glass, whose thermal expansion coef-ficient is smaller than that for soda-lime glass. Therefore, the laserscribable condition for aluminosilicate glass can be predicted froma combination of the upper limit of maximum surface temperatureTmax and the lower limit of maximum tensile stress �tmax in thecooling area following thermal elasticity analysis based on experi-mental results.

Laser scribing is basically impossible for fused silica, whose ther-mal expansion coefficient is as small as ˛ = 0.6 × 10−6 K−1.

Reasoning from these results, it seems that laser scribing is pos-

sible when the thermal expansion coefficient is ˛ = 4.2 × 10−6 K−1

and the softening point and thermostable temperature are morethan approximately 975 ◦C. It was also found that alumina ceram-ics with a thermal expansion coefficient of ˛ = approximately

on En

7c

R[

[[[[14] Kitamura R, Pilon L, Jonasz M. Optical constants of Silica glass from extreme

K. Yamamoto et al. / Precisi

× 10−6 K−1 and a thermostable temperature of more than 1500 ◦Could be scribed.

eferences

[1] Miyake Y. Separation technology for FPD glass. J Jpn Soc Abrasive Tech2001;45(7):342–7 [in Japanese].

[2] Hermanns C. Laser separation of flat glass. In: Proceedings 63rd Laser Mater.Process. Conference, Jpn. Laser Process. Soc. 2005. p. 105–10.

[3] Ono T, Teng O, Pai G. Breakless cutting EagleXGTM using standard scoring wheel.

In: Proceedings 14th inter. display workshops, FMC2-2. 2007.

[4] Wang S-C, Yeh L-Y, Lin C-C, Chen MS, Gan FY. Glass-strength dependence ofcutting conditions in thin laminated TFT-LCD. In: Proceedings 14th inter. displayworkshops, FMCp-1. 2007.

[5] Yamamoto K, Hasaka N, Morita H, Ohmura E. Thermal stress analysis on laserscribing of glass. J Laser Appl 2008;20(4):193–200.

[

[

gineering 34 (2010) 70–75 75

[6] Yamamoto K, Hasaka N, Morita H, Ohmura E. Three-dimensional thermal stressanalysis on laser scribing of glass. Precision Eng 2008;32:301–8.

[7] JSME Data Book, Heat transfer, 4th ed.; 1984 [in Japanese].[8] Shand EB. Glass engineering handbook. 2nd ed. McGraw-Hill; 1958.[9] Watanabe N. Glass engineering handbook, Asakura Shoten; 1999 [in Japanese].10] Wiederborn SM. Fracture surface energy of glass. J Am Ceram Soc

1969;52:99–105.11] Corning® 1737 AMLCD Glass Substrate Material Information; 2004.12] http://www.agc.co.jp/quartz/sq4.html.13] http://www.roymech.co.uk/Useful Table/Prop Solids.htm.

ultraviolet to far infrared at near room temperature. J Appl Optics, Optical SocAm 2007;46:8118–33.

15] Yamamoto A. On the heat transfer properties of cutting fluids (Part 2). J Jpn SocPrecision Eng 1960;26:17–25 [in Japanese].

16] http://www.kyocera.co.jp/prdct/fc/product/pdf/material.pdf.