Evaluation of Glass Weldingby Ultrashort Pulse Laser

Supervisor: Professor Yoshiyuki UNO

Division of Mechanical and Systems Engineering(Mechanical Engineering)

Graduate School of Natural Science and Technology,Okayama University

(2008)

Student No. 43419040 Name Zazuli bin Mohid

Evaluation of Glass Welding by Ultra-short Pulse Laser

Zazuli bin Mohid

Division of Mechanical and Systems Engineering

(Mechanical Engineering),

Graduate School of Natural Science and Technology, Okayama University

It is considered that fusion welding should be the most promising technique in glass joining, since

joining can be accomplished without any intermediated layer and mechanical contact. High precision, small

heat-affected zone (HAZ) and small shock-affected zone (SAZ) makes ultra-short pulsed laser capable to

perform the process with minimal damages to the surrounding area. However, there are still a lot of things

to be clarified in this process, since glass materials are veiy sensitive to temperature gradients and available

in different thermal properties. In this study, molten zones created by high ultra-short pulsed laser in

borosilicate glass (Schott D263) and fused silica were evaluated. Laser irradiation was done inside the glass

some micrometers below the top surface under various processing conditions. Molten zone was observed

visually and its strength was measured using bending test. Actual fusion welding of two glass plates was

also successfully demonstrated and evaluated by strength measurement.

The influence of polarization, feed rate v, incident laser power Win and pulse repetition rate Rp to

molten zone strength and appearance were discussed. Main conclusions in this study are as follows:-

a) There was no obvious influence to the molten zone appearance by the difference of beam

polarizations.

b) Incident laser power played the most important role in controlling molten zone size. Molten zonesize increased with the increment of laser power.

c) Cracks and bubbles were effectively reduced by selecting the proper pulse repetition rate. Low

pulse repetition rate led to cracking at the surrounding area of molten zone and high pulse

repetition rate caused cracking in the bottom area of molten zone.

d) Measurement of mechanical strength had proven that low feed rate offered wider applicable pulse

repetition rate with no fatal strength decrement of molten zone by ultra-short pulse laser.

Keywords: Ultra-short pulse laser, borosilicate glass, fused silica, molten zones, heat effected zone (HAZ),

fusion welding, feed rate, polarization, pulse repetition rate.

CONTENTS

CHAPTER 1 Introduction 1

CHAPTER 2 Materials and experimental setup 2

2.1 Materials properties 2

2.2 Experimental equipment setup 4

2.3 Observing and measuring 5

CHAPTER 3 Influence of polarization differences 6

3.1 Experimental method 6

3.2 Experimental results 7

3.3 Conclusions

CHAPTER 4 Influence of feed rate and laser power 11

4.1 Experimental method 11

4.2 Experimental results 12

4.2.1 Molten zones appearance 12

4.2.2 Molten zones size 15

4.3 Conclusions 16

CHAPTER 5 Influence of pulse repetition rate 17

5.1 Experimental method 17

5.2 Experimental results 18

5.2.1 Molten zones appearance 18

5.2.2 Molten zone size 22

5.3 Conclusions 23

CHAPTER 6 Bubbles and cracks observation 24

6.1 Experimental method 24

6.2 Experimental results 25

6.3 Conclusions 28

CHAPTER 7 Mechanical strength evaluations of weld area 29

7.1 Bending test 29

7.1.1 Experimental method 29

7.1.2 Experiment results 31

7.1.3 Conclusions 35

7.2 Shearing test 36

7.2.1 Experimental method 36

7.2.2 Experiment results 37

7.2.3 Conclusions 41

CHAPTER 8 Conclusions 42

Acknowledgement 43

References 44

CHAPTER 1 Introduction

In the field of IT industries, glass materials are widely used as optical systems and

semiconductor parts manufacturing2-1. Parts miniaturization, performance improvement and

other demands had required precise joining technique of these materials. Conventionally, the

application of adhesive material is said to be the main joining technique in ceramics and glass

materials2). The performance of this technique is greatly influenced by the properties of

adhesive material and processing environment. Inconstant material shrinkage and quantity

applied highly influence the joint accuracy. In order to overcome these problems, many

attempts had been done in order to develop newly joining techniques without adhesive

material or interlayer.

It is considered that fusion welding should be the most promising techniques in glass

joining, since this joining can be accomplished without any intermediated layer and

mechanical contact. The development of ultra-short pulse laser makes it possible to apply

fusion welding method on glass materials. By using ultra short pulse laser, the heat affected

zone (HAZ) is able to be minimized. Thermal energy generated by laser beam in picoseconds

causes the material to melt selectively and re-solidify instantly without any fatal damages.

In this study, evaluations of molten zones created by high ultra-short pulse laser in

borosilicate glass (Schott D263) and fused silica were carried out. Two types of laser with

different pulse width were used in the study. Molten zones created in the specimens were

observed and measured using optical microscope and scanning electron microscope (SEM).

Optical microscope with 50x and 200x magnification lens was used to identify the specimen

internal condition while SEM was used to clarify the actual crack lines and bubbles shapes.

The specimens for observation using SEM were etched using 1% concentration hydrofluoric

acid for one hour to clean up their grinded surface. Then, conductive material was sputtered

on the grinded surface of glass materials. From the observed and measured results, the effect

of processing parameter to the molten zones characteristic were discussed. Evaluation on

molten zones strength was carried out based on maximum breaking load differences between

irradiated and non irradiated specimens. Actual fusion welding tests of two glass plates were

also successfully demonstrated and discussed in the last chapter.

1

CHAPTER 2 Materials and experimental set up

2.1 Material properties

Glass materials are highly potential in optics, MEMS, electronics and biomedical

applications because of its excellent optical, mechanical and chemical properties^.In this

study, two types of glass materials were chosen. One is borosilicate glass (Schott D 263),

which is most well known for very low coefficient of thermal expansion (~5 XLO"6 / °*c at

20°C),against thermal shock9). This glass is widely used in semiconductors parts

manufacturing due to its high thermo-stability. Some of them are used in optical systems due

to its high transparency2^ The other one is fused silica glass which is produced from quartz of

exceptional purity with Si02 content of 99.9%, and it is widely used in semi-conductor

industries.

Table 2.1 shows the thermal properties of both borosilicate and fused silica glass. The

melting temperature and thermal diffusivity of fused silica glass is much higher than

borosilicate glass. This indicates that fused silica requires higher laser energy to melt, and it

is cooled down faster compared to borosilicate glass. Therefore, molten zones of fused

silica tend to be created in smaller size with higher risk of damages due to large temperature

gradient under the same laser energy condition.

Glass materials tested are free from any artificial colorant and transparent to short

Table 2.1 Thermal properties of borosilicate and fused silica glass

Properties Borosilicate Fused

glass Silica

Density p (g/cnv) 2.51 2.2

Specific heat c (J/gK) 0.82 1.1

Thermal diffusivity a =A*/(c. p )(cm2/s) 0.003 0.008

Melting temperature 6 m (°C) 1051 2000

Thermal expansion coefficient (1/°C) 7.2 x 10"6 5.5 x 10"7

*1} Schott D263

2

2.2 Experimental equipment setup

In the recent year, development of ultra-short pulse laser is widely studied for its

possibility in micro machining. Many researches have been reported regarding the potential

of ultra short pulse laser in manufacturing and biomedical industries3). The pulse width of

this laser is thousands time shorter than conventional Q-switched laser pulse. High precision

and small heat-affected zone (HAZ) and small shock-affected zone (SAZ) make this laser

possible to perform micromachining with minimal damages to the surrounding area.

In this study, two types of ultra short pulse laser oscillator with different pulse duration

were used. Table 2.2 shows the specifications of both oscillators. These oscillators were

integrated to a motorized table controlled by PC and the laser beams were focused using

several types of lenses. The angel of sample surface was precisely aligned to keep the change

in the focus position from the top surface of glass sample within ±2̂ 111.

Table 2.2 Laser oscillator specifications

Items Type 1 Type 2

Maker/ Type Time Band Width - DUETTO IMRA - FCPA u Jewel

Power range >10W Average 400mW

Pulse width <12ps <lps

Wavelength 1064nm 1043nm

Pulse repetition rate <8MHz 0.2~5.0MHz

In order to investigate the molten zone characteristics, laser irradiation was done to a

single glass plate in a linear line under several processing parameters written in the next

chapters. Irradiations were done in the same direction and parallel to each other using

different processing parameters. In case of actual welding experiment, the specimens were

hold using a vacuum pressed jig illustrated in Figure 2.3 b. The specimens were hold and

pressed by 1mm thickness of holding glass. The pressing force is generated by vacuum pump

connected to the jig. Two pieces of specimens were stacked with 20mm x 20mm overlapped

surface and focus position of laser beam was adjusted by some micrometers below the top

surface of specimen.

4

CHAPTER 4 Influence of feed rate and laser average power

4.1. Experimental Method

In this experiment, ultra-short pulse of lOps laser beam (Time Band Width Laser-Duetto)

was used. Laser beam was focused using NA 0.55 lens to 0.170mm depth from the specimen

upper surface. Fused silica glass specimens of 1.1mm thickness were fixed on a motorized

fixture and irradiated linearly using parameters listed in Table 4.1. The schematic diagram of

experimental equipment setup is shown in chapter 3 (Figure 3.1). To observe and measure the

molten zones, they were cut and grinded using the same method as describe in chapter 2.

Table 4.1 Processing parameters.

Parameters Values

1 Pulse width tP lOps

2 Wavelength X 1064nin

3 Lens NA 0.55

4 Pulse repetition rate Rp 1.0MHz

5 Average power Win 2. 4. 6W

6 Feed rate v 10. 20. 50. 70. 100. 150. 200mm/s

11

Figure 4.7 shows the change of melted zone size changes with laser average power under

1MHz pulse repetition rate condition. Started from Win=lW, the melted zone length and width

increase linearly with the power increment. The size of molten zones by 6W laser average power

was twice bigger than that by 2W laser average power.

These results agree well with the theory that absorption rate is proportionate to pulse energy

increment and inversely proportionate to feed rate increment.

4.3. Conclusions

Main conclusions obtained in this chapter are as follows:-

1) Molten zones size was in proportion to laser power but in inverse proportion to feed

rate.

2) Laser average power was the most efficient parameter in molten zone size compared to

feed rate.

16

CHAPTER 5 Influence of pulse repetition rate

5.1. Experimental Methods

The same equipment written in chapter 4 was used in this experiment. In this chapter, laser

average power and feed rate were fixed at 6W and 20mm/s respectively. The experiment was

done using different pulse repetition rate increased gradually from 0.2MHz up to 6.0MHz. Table

5.1 shows the detail of parameters used in the experiment. To observe and measure the molten

zones, they were cut and grinded using the same method as described in chapter 2.

Table 5.1 Processing parameters

Pa rame te r s Values

1 Pulse width tP lOps

2 Wavelength X 1064nm

3 Lens NA 0.55

4 Pulse repeti t ion r a t e RP 0.2, 0.5, 0.7, 1, 2, 3, 4, 5, 6MHz

5 Average power Win 2, 4, 6W

6 Feed r a t e v 20mm/s

17

CHAPTER 5 Influence of pulse repetition rate

5.1. Experimental Methods

The same equipment written in chapter 4 was used in this experiment. In this chapter, laser

average power and feed rate were fixed at 6W and 20mm/s respectively. The experiment was

done using different pulse repetition rate increased gradually from 0.2MHz up to 6.0MHz. Table

5.1 shows the detail of parameters used in the experiment. To observe and measure the molten

zones, they were cut and grinded using the same method as described in chapter 2.

Table 5.1 Processing parameters

Pa rame te r s Values

1 Pulse width tP lOps

2 Wavelength X 1064nm

3 Lens NA 0.55

4 Pulse repeti t ion ra te RP 0.2, 0.5, 0.7, 1, 2, 3, 4, 5, 6MHz

5 Average power Win 2, 4, 6W

6 Feed ra te v 20mm/s

17

5.3 Conclusions

Conclusions obtained in the experiment under different pulse repetition rate are as follows:

1) The adjustment of pulse repetition rate was highly efficient tool to countermeasure

against bubble and crack without serious molten zone size modification.

2) Excessive higher pulse repetition rate increment was only brought adverse effect where

crack developed in the molten zone.

23

CHAPTER 6 Bubbles and cracks observation

6.1. Experiment Method

In some cases, when the parameters are not optimized, cracks and bubbles appeared in dark

and confusing images. To clarify their actual conditions, etching process were carried out and

the specimens were inspected using scanning electron microscope (SEM).

In this study, the difference from others chapters, is use of LUMERA laser oscillator for the

creation of molten zone. Since the pulse duration is the same lOps as others oscillator, it is

assumed the characteristic of molten zone created should be similar. The oscillator detail

specifications are shown in Table 6.1 and the processing parameters are shown in Table 6.2.

Table 6.1 Laser oscillator specification

Items Specification

Maker/ Type LUMERA (RAPID)

Average power >2.5W

Pulse width <15ps

Wavelength 1064nm

Pulse repetition rate ~ 0.5MHz

Table 6.2 Processing parameters

Parameters Values

1 Pulse width tP lOps

2 Wavelength X 1064nm

3 Lens NA 0.55

4 Pulse repetition ra te RP 0.1, 0.2, 0.3, 0.4, 0.5MHz

5 Average power Win 6W

6 Feed ra te v 50, 200mm/s

24

References

1. Y. Zhou: Microjoining and Nanojoining, Woodhead publishing, (2008), 356.

2. T. Yoshida: High Performance Glass and Nanoglass Advanced Technology, NTS, (2006), 6,

80-92.

3. I. Miyamoto: Novel Fusion Welding Technique of Glass Using Ultra Short Pulse Laser,

JSME, 11(2007), 853-855.. *

4. I. Miyamoto at el.: Fusion Welding of Fused Silica by Ultra Short Pulse Laser, Proc. of LPM

2008, (2008), 1-7.

5. William M. Steen: Laser Material Processing, (1998), 157-175.

6. D. G. Holloway (translated by Yoshihisa Oi at. el): The Physical Properties of Glass,

Kyouritsu Syuppann, (1977), 19, 143-185.

7. Heinrich Endert et al.: Novel Ultra Short Pulse Fiber Lasers for Micromachining

Applications, LPM2001, (2002), 23-27.

8. Adela Ben-Yakar: et al.: Femtosecond Laser Ablation Properties of Borosilicate Glass, J.

Appl. Phys., Vol. 96, No. 9, 5316-5322.

9. K. Hirao: Femtotechnology from Basis to Application, JCLS, (2006), 1-8, 35-39, 135-144.

10. T. Tamaki at el.: Direct Joining Between Glass and Copper by Using Localized Fusion of

Material with Femtosecond Laser Pulses, JLPS, 4(2008), 57-59.

11.K. Yamamoto at el: Influence of Glass Thickness in Laser Scribing of Glass, JLPS, 4(2008),

44-50.

12. I. Miyamoto at el.: Novel Fusion Technology of Glass Using Ultrashort Pulse Lasers, Proc.

of 25th Int. Congress on Applications of Lasers & Electro-Optics 2008, 112-121.

Conference participated.

1. Title: Temperature Distribution and Bending Characteristic in Plastic Laser Forming

Date and place of event: 21st. May 2008, Malaysia

Conference name: International conference on mechanical and manufacturing engineering

2008 (ICME2008)

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