Femtosecond laser micromachining of SiC ceramic structures

GAI Xiaochen1,a, DONG Zhiwei2,b, ZHAO Qingliang1,c, LIU Hongbin1,d

1Center for Precision Engineering, Harbin Institute of Technology, Harbin 150001, China

2National Key Laboratory of Science and Technology on Tunable Laser,

Harbin Institute of Technology, Harbin 150001, China

agaixiaochen4@163.com

Keywords: Femtosecond laser, SiC ceramic, Parameter optimization, Micromachining

Abstract. Femtosecond laser micromachining technology shows abroad application background in

the field of micro manufacturing due to its unique advantages, especially for micromachining of

ultrahard materials such as Silicon carbide (SiC) ceramic. The femtosecond laser micromachining

system was set up, by using the system, effects of scanning velocity and laser pulse energy on quality

of micromachined features were evaluated. The optimized technological parameter was obtained as

8mW, 1mm/s with 1kHz repetition frequency respectively on the basis of the morphological

characteristics and microstructure accuracy. Besides, V-shaped cavity of 300µm depth and 120°angle

was generated with layer-by-layer scan machining. Thus femtosecond laser micromachining

technology is an effective method for hard and brittle materials precision processing.

Introduction

SiC ceramic has recently gained much attention in the field of micro mold manufacturing due to its

exceptional physical properties, such as high stiffness, mechanical strength, extreme chemical

inertness. However, the excellent physical properties also lead to the issue of hard machinability. So

far, the ultraprecision grinding is the most effective method[1], but there are still some problems

existed. First, though micron-sized surface accuracy and nanoscale roughness can be obtained

through grinding[2], the grinding wheel is easily wear owing to material hardness, which has great

impact on the processing efficiency and machining accuracy[3]. In addition, this method has great

difficulty in machining the pointed angle of composed faces, especially the inner pointed angle.

Therefore, it has great significance of developing the precision machining technique for ultrahard

material.

Much attention has been paid to femtosecond laser micromachining in recent years. Femtosecond

laser micromachining has the advantages of machining extremely tiny features in almost any

materials in the absence of thermal damage, with accuracy ablation threshold and avoiding plasma

shielding[4]. It display the superiority more when micromachining ultrahard materials such as SiC

due to the ability to machine almost any materials. For this reason, femtosecond laser micromachining

technique is a promising technique for micrmachining of ultrahard materials. In this work, we present

the results of a study of the effects of pulse energy and scanning velocity on quality of machined

features in SiC samples using femtosecond pulsed laser; then machining parameters were optimized

according to the results; V-shaped cavity in SiC sample was manufactured with the optimized

parameters finally.

Experimental

Fig. 1 shows a schematic of the experimental setup. The Ti: sapphire femtosecond laser produced

50-fs pulses at a given pulse repetition frequency(f ) of 1kHz and a wavelength(λ) of 800 nm. The

beam approximated to a Gaussian shaped intensity profile. The number of pulses striking the target

was controlled by means of a computer controlled fast optical shutter, which had a rise and fall time of

5 ms and resolution of 0.1ms. The pulse energy was controlled using a Glan-Laser polarizer and was

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measured using a pyroelectric detector placed just before the objective lens. The beam passes through

a pinhole of 3 mm in diameter in order to achieve a more circular spot. To achieve a smaller focused

spot, the beam was expanded to three times its size by passing it through a telescope. A waist of 7µm

can be achieved at the focal plane by a focusing objective (50mm planoconvex lens). For fabrication

of 3D microstructures, samples were translated by a computer controlled three-axis(XYZ) motion

stage(PI, Inc.) with a resolution of 0.1µm. A CCD camera allowed to control the position of the

focused spot relative to the targets using Z-axis translation stage as well as monitoring the process in

situ. Compressed air was used to purge across the ablated region to remove the debris.

Fig.1 Schematic of femtosecond laser micromachining system

The SiC samples had dimensions of 10mm×10mm×3mm. The main characteristics of the

materials used for all the experiments are resumed in Table 1. The samples were cleaned

ultrasonically with alcohol before the experiments. The samples were mounted on the stage using a

fixture.

Table 1 Characteristics of SiC

Parameters Values

Density, ρ 3.15 kg·dm3

Hardness,HK 2800 kg·mm2

Surface Roughness, Ra 23 nm

Grain Size, r 4~10µm

Porosity, Vs <0.2%

Two types of experiments were conducted: (1) Parameter optimization experiments: the main

parameters influencing femtosecond laser micromachining are laser pulse energy(P), repetition

frequency and scanning velocity(v) when the laser wavelength and pulse width are fixed. Machining

effects depend on fluence, which is up to pulse energy and pulse overlap. Scanning velocity and

repetition frequency decide the pulse overlap. To simplify the experimental process, we change the

pulse overlap by changing scanning velocity at f =1kHz. Therefore, line structures were machined by

combining different scanning velocities and pulse energies. The corresponding parameters were P=

8µJ, 15µJ, 20µJ, 40µJ, 80µJ, 120µJ and v=0.01mm/s, 0.05mm/s, 0.1mm/s, 0.5mm/s, 1 mm/s, 2 mm/s.

Optimized parameters were obtained in this set of experiments. (2) Micromachining experiments:

V-shaped cavity of 300µm depth and 120°angle was produced using optimized parameters acquired

in the previous experiments, layer by layer scan machining was adopted with small ∆z steps. The

samples were then ultrasonically cleaned with alcohol. All the machined samples were examined in

scanning electron microscope, surface profilometer and optical microscopy.

Results and discussion

Parameter optimization experiments. Fig.1 shows morphologies of microstructural evolution of

SiC samples with scanning velocity changing from 2mm/s to 0.01mm/s at P=40µJ. It shows that the

line width increase with the scanning velocity decreasing, and the increasing is no long obvious when

v≤0.05mm/s. In Fig. 1a, which shows the structure at a overly fast scanning velocity, the morphology

shows that there was no ablation after machining, only left thermal vestige. Fig. 1b shows a

reasonably well-defined profile, the surface appears to be quite smooth with no evidence of melt

22 Advances in Materials Manufacturing Science and Technology XV

formation, micro-cracks or other thermal damages, that proves the SiC sample was machining at

proper parameters. In Fig. 1c, the structure exhibites a rough surface morphology of significant heat

conduction and thermal damage due to high pulse energy. A mount of material was removed that lead

to a deep, low precision structure. Bubbles were generated beside the walls when scanning v≤

0.05mm/s, the reason is that a large amount of the vaporizable material or plasma were removed from

the base metal due to strong ablation, some of them condensed before escaping from the base metal

and formed bubbles under the action of the nether vaporization.

Fig.2 Lines with varying velocities at P=40µJ

1. v=2mm/s 2. v=1mm/s 3. v=0.5mm/s 4. v=0.1mm/s 5. v=0.05mm/s 6. v=0.01mm/s

Fig.3 shows the images at the pulse energy of P=15µJ, P=120µJ with v=0.5mm/s. Evolution of

morphologies caused by pulse energy increasing is similar to that of scanning velocity decreasing.

Increasing pulse energy and reducing scanning velocity can both result in the fluence increasing.

Fig.3 Lines with varying pulse energies at v=0.5mm/s (a)P=15µJ (b)P=120µJ

Therefore, ablation can not occur when the fluence is lower than the ablation threshold; when the

fluence is lower but above the ablation threshold, laser ablation is a kind of non-thermal gentle

ablation that has the ability to precisely remove the material in nanoscale through Coulomb explosion

mechanism,in which the laser pulse rapidly evacuates the electrons from the surface region leading to

the formation of positively charged ions in the near-surface layers that in turn electrostatically repel

each other causing ejection of material[5]. Gentle ablation usually results in excellent surface quality

with no evidence of melt zone. Strong ablation occurs when the fluence is high, large amount of

material removal through thermal vaporization leads to rough surface. So well-defined profile is

gained at or near the threshold.

There is a reduction in the ablation threshold fluence with increasing incident laser shot number

until up to 500[6].Change of pulse overlap is equivalent to a change of the number of pulses, so

different scanning velocities correspond different optimal pulse energies respectively. That indicates

that well-defined profile can be obtained by different combines of scanning velocities and pulse

energies. Parameters corresponding good morphologies were picked up as preliminarily optimized

parameters, they were further optimized by measuring the width and depth of the structures and chose

P=8µJ，v=1mm/s which machined the best precision as optimal parameter. The corresponding width

and depth were 16µm and 15µm respectively. The optimal parameter was used in micromachining

experiments.

Micromachining experiments. The V-shaped cavity was divided into 20 layers based on the

optimal parameter, the focus in Z direction moved down 15µm after each layer. Fig.4 shows there

Materials Science Forum Vol. 770 23

were scan direction and step direction in every scanning layer, there should be the same overlap in the

two directions, so the stepover was worked out at 1µm by v=1mm/s and f=1kHz. In he first layer the

femtosecond laser scanned 520 times with 26 times reduction layer by layer.

Fig.4 Schematic of laser scanning

Fig. 5 shows the profile of the V-shaped cavity. A compeletely V-shape with well-defined features

was processed. There was good edge definiton, smooth sidewalls and subfaces was generated without

melt formation, micro-cracks and debris. Therefore, femtosecond laser is efficient for microstructures

machining of ultrahard materials, microstrucures can be preliminarily machined by femtosecond laser

to acquire definite shape, sizes and precision, then shape precision and surface roughness is improved

further by grinding or polishing, the grinding wheel wear can be greatly reduced.

Fig5. V-shaped cavity machined by femtosecond laser

Summary

Ablation can not occur with fluence lower than the ablation threshold while high fluence result in

rough surface due to strong ablation. Excellent surface quality is gained at or near the threshold. Pulse

energy increase and scanning velocity decrease can both lead to the fluence increasing. Well-defined

profile can be acquired by different combines of scanning velocities and pulse energies. Optimized

parameter is P=8µJ, v=1mm/s at 50-fs, f=1kHz, λ=800nm as good surface quality and highest

machining precision. Microstructures with sharp edges and smooth srufaces can be produced by the

optimized parameter. In conclusion femtosecond laser micromachining technology is an effective

method for hard and brittle materials precision processing.

References

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(2009), p. 311

[3] Launch-micro Micro-technologies for Re-launching. European Machine Manufacturing SMEs,

2010, p. 12

[4] Chicbkov B N, Momma C, Nolte S, et al. Applied Physics A Vol. 63 (1996), p. 109

[5] Pal Molian, Ben Pecholt, Saurabh Gupta. Applied Surface Science Vol. 255 (2009), p. 4515

[6] DONG YY, MOLIAN P. Appl. Phys. Lett. Vol. 84(2004), p. 10

24 Advances in Materials Manufacturing Science and Technology XV

Advances in Materials Manufacturing Science and Technology XV 10.4028/www.scientific.net/MSF.770 Femtosecond Laser Micromachining of SiC Ceramic Structures 10.4028/www.scientific.net/MSF.770.21