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Brittle materials are widely used in semiconductor and photoelectron components, such as glass, ceramic, silicon wafer, sapphire and etc. Laser processing of brittle materials is an important technique including micro-drilling, scribing, cutting and microstructures. However, the traditional processing for material removal with mechanical processing or chemical wet etching that usually accompany many problems in the processing procedure consists of long time for and high cost of manufacturing equipments. In this research, the second harmonic generator (SHG) and third harmonic generator (THG) Nd:YAG laser microvia is utilized on silicon wafer. Moreover, optical microscope (OM), scanning electron microscope (SEM) and surface finish and form measurement have been used for micro-drilling characterization measurement such as morphological of drilling profile, taper and melted residual of the hole. Finally, the proportion of diameters and depths for various parameters including laser power, pulse repetition rate and machining time are determine.
brittle material, micro-drilling, surface profile, residual
Recenly, laser micro technologies are developing and widely used in industries. Laser materials processing is composed of laser cutting, drilling, marking, scribing, laser modification of material surface, laser annealing and laser trimming with regard to the brittle materials including silicon wafer, quartz, sapphire substrate, and glass structure. Silicon wafer is the most common material for Micro-Electro-Mechanical Systems (MEMS), photonics and semiconductor manufacturing industries.
Conventional brittle material machining method uses a diamond wheel grinding or cubic boron nitride (CBN) tools cutting to finish the material surface. It’s a difficult machining for brittle material. Because the brittle material has a micro groove crack and a chip on machining surface.
Therefore, the contact processing that easily induces the residual stress and heat on the material surface will influence the following step processing.
Light, thin, short, small, high reliability and high efficiency components are important key points of the substrates can play an important role for computer communications and solar PV semiconductor.
There are many kinds of micro and macro-hole drilling processing of nontraditional machining such as (a) chemical wet etching process of semiconductor processing, (b) electrical-discharge machining (EDM), electrochemical machining (ECM), electro-chemical drilling (ECD), shaped tube electrolytic machining
(STEM), capillary drilling (CD), electro-stream (ESD), jet electrolytic drilling (JED) and etc. [1] However, the above methods are time-consuming, high cost for the fabrication.
Therefore, utilizing the laser microvia of silicon plate can improve time-consuming and cost of the manufacturing equipments. The traditional wet etching forming technology of semiconductor processing is used as particular etching solution and different etching rate to cause different depth etching process. The advantages of mechanism for laser material processes are including:
(1) Non-contact processing;
(2) No traditional tool wear; advanced manufacturing technologies. The smaller components will be made on silicon wafer so that production of the IC device volume has also be increased.
For the integrated circuit wafers production of micro structure and multi-layer stacking circuit, the limited space can create more features. Therefore, the design of the circuit is tend to 2D and 3D development of the microelectronics circuit elements. The silicon substrate based that can be applied to MEMS, system on chip (SOC) and other related products. The three dimensional integrate circuits is made by independence construct 2D circuit pattern. Afterward, using wafer to wafer, die to
(3) High speed and high accuracy;
(4) A little absence of residual thermal effect and residual stress;
(5) Using trepanning method that can drill a large hole;
The literature reviews relative to laser material processing are shown in the following: Liming He et al. [2] proposed the paper that discussion a short pulsed
Nd:YAG laser from infrared to ultraviolet spectra by using an nonlinear optical crystals, such as KDP, KTP,
BBO, CBO, LBO, YCBO and etc.. Fundamental harmonic of Nd:YAG laser with wavelength 1064nm can be converted into the second, third and fourth harmonic wafer or die to die joint method can accomplish the vertical integrated circuits. Furthermore, the silicon with wavelength 532nm, 355nm and 266nm, respectively.

S.C. Tam et al. [3] investigated the pulsed Nd:YAG laser drilling of inconel alloys by the Taguchi method. In the proposed research, a 25mm thick substrate was used assistance gas – oxygen to carry out laser drilling. Besides, the different parameters of the laser material processing including laser power, machining time, laser mode, focusing length of focus lens and pressure of assist gas were influence and efficiency of laser drilling. In 2002, S.
Bandyopadhyay et al. [4] used the 400W Nd:YAG laser manufacturing the aeronautics materials. Ghoreishi et al.
[5] discussed that the six controlled parameters of laser processing constant can involve in peck power of laser generator, pulse width, pulse frequency, number of pulses, assist gas pressure and focal plane. The processing performance such as hole taper and hole entrance roundness was determined by using analysis of variance
(ANOVA) method. S. Nikumb et al. [6] proposed a short pulsed solid-state laser with pulse duration from nanosecond to femtosecond range to machine the different types of glass materials. Edge quality, roundness, aspect ratio and machining rate were discussed. A.H.
Khan and S. Celotto et al. [7] investigated different sizes of supersonic micro gas jets percussion drilling of 200μm thick 316L stainless steel plates using 355nm wavelength nanosecond laser. S. Kim et al. [8] proposed that using laser drilling method had higher rates than conventional dry etching of SiC substrate.
In this research, we will develop the second harmonic generator laser processing system with wavelength 532nm and third harmonic generator laser processing system with wavelength 355nm. And then, we utilize laser microvia on silicon wafer. The scanning electron microscope, optical microscope and surface profile have been used for measuring properties of the laser micro-drilling. Finally, the drilling effects on morphological parameters which consist of laser power, pulse repetition rate and machining time are discussed.
2.1. Absorption and reflection of material
Laser material removal is a process of ablation in fusion, vaporization and chemical degradation. Owing to laser beams focusing on surface of material, one part of the irradiate energy is reflected and the other is absorbed by materials that will cause photothermal phenomenon.
Figure 1 shows the relationship of the reflectivity versus wavelength for several kinds of metal materials. [9] In figure 1, in order to decrease reflectivity, the short wavelength light must be adopted for material processing.
Figure 1. Reflectivity dependence on wavelength for different material
Furthermore, material absorption is an important factor. Figure 2 shows the material absorption versus different wavelength. [10]
Figure 2. Absorption versus wavelength of various materials
Reflectivity of the material can be used Fresnel equation that considers laser beams in a direction normal to the surface of material. The surface reflectivity of the material is given by [11] :
R

( n
R

1 )
2

1 ) 2

 n
I
2 n
I
2
(1)
( n
R
Where n
R is real-part reflectivity of the material, n
I
is imaginary-part reflectivity of the material. The absorption is given by:
A

1

R (2)
2.2. Laser beam diameter
According to diffraction limit, theoretically the TEM
00 mode laser minimum diameter D f
is given by Eq. (3) [12] :
D f

2 .
44

F d
B
( 2 M

1 ) 2
1
(3) where λ is a laser wavelength, d
B is a laser beam spot diameter, F is the focal length of optics, M is the laser beam quality can be calculated by Eq. (4)
M

p
 l

(4) where p and l are laser mode of circular symmetry TEM pl
.
3.1. Experimental material
Recently, wafer is an important material.
Semiconductor materials have electrical properties between the conductors and insulators, such as the resistivities between 10 -3 Ω-cm to 10 8 Ω-cm. Thereby, semiconductor material- silicon wafer has become the most important material for semiconductor, photoelectric

and photovoltaic industries to fabricate integrated circuit, devices, micro-structure. The common types of the silicon wafer can be classified as shown in Figure 3. [13] In this research, the (1 1 1) p-type silicon wafer is used for laser micro drilling.
Figure 3. Orientation of single crystal wafer of silicon
Firstly, six-inch p-type silicon wafer with thickness
600μm is adopted. The bottom of the wafer is fixed using blue tape, and then sawed by diamond wheel. Automatic dicing machine called DS-1611, are produced by Uni-tek system, Inc. Figure 4 illustrates the dicing saw processing.
Before the laser drilling, the specimens are cleaned on ultrasonic bath with clean liquid and dried by high pressure gas.
Figure 5. Schematic diagram of SHG Nd:YAG laser processing system
The specification of the SHG laser processing system is wavelength 532nm, average power 0.05W, focal length of the focusing lens 75mm and the spot size 50μm.
Because of the scan head assembled in this system, the principal processing method is direct drilling.
3.1.2. DPSS Laser Processing System
The main part of this experimental processing system contains a laser source, path of optical system, pc-based controller, dual axes linear motor with drivers, feeding stage, and man-made granite. Moreover, with high-speed scanning process or trepanning drilling method, we add a telecentric scan lens into the optical paths. The processing speed cannot be restricted with changing machining time by the feeding speed. Figure 6 shows the DPSS laser processing system.
Figure 4. Schematic diagram of wafer dicing saw processing
3.1. Equipment
3.1.1 Second harmonic generator Nd:YAG Laser
Processing System
Figure 5 shows a schematic diagram of the second harmonic generator (SHG) Nd:YAG laser processing system. This system mainly consists of Nd:YAG laser source, a beam expander, delivery optics, focusing lens, and a movable XY table. An Nd:YAG laser oscillator includes a couple of reflector mirror, a pump module excited by flash lamp generated near infrared light with a wavelength of 1064nm, and then the light wavelength
532nm is generated by SHG crystal.
Figure 6. DPSS laser processing system
Diode Pumped Solid State (DPSS) laser system with a diode-pumped laser of the third harmonic frequency can operate wavelength 355nm. The laser mode of this system is TEM
00
. The maximum average power of the laser source is among 0.05 W to 3.3 W. The range of pulse repetition rates is 1 kHz to 10 kHz. Complete specification of the DPSS laser processing system is shown in Table 1.

Table 1. Specification of DPSS laser processing system
Wavelength, (nm) 355
Output power, @ 6 kHz (Watt) > 3
Spatial Mode, TEM
00
Beam quality, (M 2 ) 1.5
Pulse repetition frequency, (kHz) 1 ~ 10
Pulse width @ 6 kHz, (ns) 35
Beam Diameter,
±10%, (mm) 0.3
Moving stage types Linear motor
Manufacturing type Direct
Maskless
Hole diameter, (μm) > 5
3.2 Laser drilling procedure
There are several types of laser drilling methods [13] as shown in Figure 7. specimens are moved by the Z-axis movable table driven by a servo motor and ballscrew mechanism so that it can achieve positive and negative focusing position, respectively.
In this paper, an experimental equipment for laser drilling of silicon wafer used by SHG Nd:YAG laser and
DPSS laser processing system is presented, respectively.
Optical microscope, scanning electron microscope and surface finish and form measurement have been used for properties of the laser microvia such as morphological of drilling profile, melted residual and taper of the laser drilling silicon wafer material. In this research, we utilize optical microscope (OLYMPS BX60M), scanning electro microscope (JEOL JSM-6360), field emission scanning electron microscope (FE-SEM Hitachi S-4300), and surface finish and form measurement (Taylor hobson,
Talyscan 150), respectively.
According to the above laser drilling methods including single pulse, percussion, trepanning and helical drilling method, we adopt the several ways as the major operation in the experiments.
4.1 Research for case
Ⅰ
For the SHG laser processing system, holes were drilled on the silicon wafer by using single pulse drilling method. Figure 8 shows the results of laser drilling 5×5 matrix and an obvious single hole on the surface of the silicon wafer front and backside, respectively.
Figure 7. Several methods of laser drilling
3.2.1. Single-pulse drilling method
Single-pulse drilling method is displayed in Figure
7(a). The laser beam is delivered by the beam expander, an adjustable aperture, and mirrors of the optical system.
Finally, focused laser beams reach on the work-piece with the focusing lens to perform the single-pulse hole drilling.
For the various diameters of drilling holes, we can change different kinds of the focal lens.
3.2.2. Percussion drilling method
Figure 7(b) shows the percussion drilling method. The diameter of the hole is controlled by the focusing lens.
This method is used with the short pulse laser. Owing to the short pulse width, less thermal diffusion in the transverse surface of the work-piece can control the diameter and shape of the hole. This method is usually applied to singular hole drilling.
3.2.3. Trepanning drilling method
Fig. 7(c) is a trepanning irradiation method of the laser drilling. Processing path for diameters and profiles of holes are controlled by the programs of the PC-Based controller to drill on surface of the work-piece. By this method, the hole diameter can be increased and the profile without a mask in this processing can be executed.
3.2.4. Helical drilling method
Fig. 7(d) is another laser drilling method – helical drilling method. Firstly, single pulsed laser drilling on the surface of the silicon wafer causes fusion, vaporization effect, and the laser beams focusing position is not the same at the next step drilling position. In this case, the
(a) 5x5 matrix and an obvious single hole on the surface of the silicon wafer front
(b) 5x5 matrix and an obvious single hole on the surface of the silicon wafer backside
Figure 8. SEM images of a SHG laser drilling on the silicon wafer
In this case, we utilized SHG laser processing system with wavelength 532nm, average power with 0.05W, single pulse drilling method and XY movable stages to achieve 5x5 matrix drilling. The repetition rate is 30Hz and the feeding speed of XY tables is 10mm/s. The specimens were measured by FE-SEM. The diameter of single hole drilling is about 50μm. Figure 8 displays the

splatter particle residual around drilling holes.
Furthermore, the roundness of holes with laser drilling silicon wafer is not circular. Because of drilling without mask in the optical path, it caused non-circular drilling holes.
4.2 Research for case II
For the case II, we utilized DPSS laser processing system with wavelength 355nm in which the average power at operation frequency 6kHz was variable from
0.07 W to 3.33W. Average output power is a major factor that affects the result of the microvia. The varied output power of this system is changed by the input current and the frequency. The measured data of average power as shown in Fig. 9.
Figure 9. Average output power versus operation current
Figure 10 shows results of the single-pulse laser drilling method on the silicon wafer. The average power of a laser is 0.07W, 0.46W, 1.40W, 2.20W, 2.80W, 3.23W,
3.12W, 3.26W, 3.28W and 3.33W, respectively. The spot size of laser beams is about 20μm. The repetition rate of a laser is 6kHz.
(i) 6 kHz, 3.28W (j) 6 kHz, 3.33W
Figure 10. SEM images of different DPSS laser average power for drilling on the silicon wafer
After drilling processing on the silicon wafer, Figure
10 reveals the recasting layer, surface debris, heat affected zones, melt zones and non-circular holes.
4.3 Research for case
Ⅲ
We took use of trepanning drilling method in the case
Ⅲ
. The diameter and profile with this method can be defined by galvanometer scanning path on the surface of the work-piece. Figure 11 shows the results of different laser pulse repetition rates which are 2kHz, 4kHz, 6kHz and 8kHz, respectively. The drilling hole diameter is almost 30μm with the pulse duration 1000ms and the average power 2.20W.
(a) 2kHz @ 2.20W (b) 4kHz @2.20W
(a) 6 kHz, 0.07W (b) 6 kHz, 0.46W
(c) 6 kHz, 1.40W (d) 6 kHz, 2.20W
(c) 6kHz @ 2.20W (d) 8kHz @ 2.20W
Figure 11. SEM images of different pulse repetition rates for drilling on the silicon wafer (pulse duration: 1000ms)
Figure 12 displays the results of different pulse repetition rates at 2kHz, 4kHz, 6kHz, and 8kHz. The drilling hole diameter is 30μm with the pulse duration
3000ms and the average power 2.20W.
(a) 2kHz @ 2.2W (b) 4kHz @2.2W
(e) 6 kHz, 2.80W (f) 6 kHz, 3.23W
(g) 6 kHz, 3.12W (h) 6 kHz, 3.26W
(c) 6kHz @ 2.2W (d) 8kHz @ 2.2W
Figure 12. SEM images of different pulse repetition rates for drilling on the silicon wafer (pulse duration: 3000ms)

Figure 13 shows a image for TalyScan 150 scanning system of the laser drilling and the cross-section measurement which can analyze the taper hole dimension.
µm Alpha = 171° Beta = 29°
48.9 µm
45
40
35
30
25
20
1.83 mm
187 µm
15
10
5
0
(a) 3D reconstruction for laser drilling
Length = 1.97 mm Pt = 56.1 µm Scale = 100 µm µm
30
20
10
0
-10
-20
-30
-40
-50
-60
0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1 1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9 mm
(b) Cross-section profile
Figure 13. 3D images and profile of laser drilling on the silicon wafer
In this research, we have developed two types including Nd:YAG laser processing system and varied drilling methods for the silicon wafer. In the experiments, we adopted no mask and single-pulse laser drilling method practically so that non-circular holes are occurred.
In addition, the roundness with trepanning method is batter than the one with single-pulsed drilling method.
After laser drilling, the particle recasting layer, the poor roundness and the surface debris around the drilling holes are appeared obviously. Therefore, we can improve above conditions as shown in the following:
(1) Specimens have to be placed in the vacuum chamber.
(2) Reduce oxidation with assist gas.
(3) Utilize underwater and water assisted method of laser processing to reduce recasting phenomenon.
(4) Using a cleaning process in HF and KOH etch.
(5) Improve the roundness of drilling holes with trepanning method.
(6) Modify Gaussian mode with beam shaping method.
The authors would like to thank the National Science
Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC
96-2221-E-018-014.
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Macro - to Micro-Hole Drilling Processes, International
Journal of Machine Tools & Manufacture, 45,
2005:137-152
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Wavelength Optimization for Machining Metals with the
Harmonic Generations of a Short Pulsed Nd:YAG Laser,
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[3] S.C. Tam, C.Y. Yeo, S. Jana, W.S. Lau, E.N. Lin, L.J.
Yang, Optimization of Laser Deep-Hole Drilling of
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[4] S. Bandyopadhyay, J. K. Sarin Sundar, G.
Sundararajan, and S. V. Joshi, Geometrical Features and
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Takahashi, and Osamu Noda, Diode-Pumped Solid-State
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Heavy industries Ltd. Technical Review, 40 (2), 2003:1-5
[10] Sid Clouser and Masaru Sakamoto, Copper Foil
Treatment for CO
2
Laser Direct Drilling, www.gouldelectronics.com/paper
[11] Eugene Hecht, Optics, Addison Wesley, San
Francisco, 2002:115-131
[12] John C. Ion. Laser Processing of Engineering
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2005:104-105
[13] Rainer Paetzel, Comparison Excimer Laser - Solid
State Laser, Lambda Physok AG, 2002:1-10
W.T.Hsiao (Wen-Tse Hsiao)
Ph.D. student;
Department of Mechatronics Engineering
National Changhua University of Education
Changhua 50058, Taiwan
Tel:/Fax: +886 4 7232105 ext. 7283 / +886 4 7211149.
E-mail: d94631003@mail.ncue.edu.tw
Website:http://www.me.ncue.edu.tw/teachers/chenmf/me mbers.htm