Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
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Development of Microfabrication Technologyusing DUV Laser

YUJI IMAMIYA*1

SATORU AKAMA*1

YOSHIHITO FUJITA*2

HARUHIKO NIITANI*3

Laser machining technology has advanced significantly in recent years and has been appliedin various different fields for industrial use due to its convenience and the great variety ofapplicable materials. In laser micromachining represented by drilling, high-quality andhigh-precision machining is possible by utilizing a short-pulse laser. However, a higher level ofprecision is desired. Mitsubishi Heavy Industries Machine Tool Co., Ltd. has developed an opticalsystem utilizing a Deep Ultraviolet (DUV) laser in which further improvement is expected in themachining resolution, and has successfully identified the machining characteristics of individualmaterials. Furthermore, a drilling test was conducted with a laser micromachining unit equippedwith a newly-developed optical system, which, as a result, achieved ultrafine drilling to make ahole of a minimum of 10μm in diameter and an aspect ratio of 10.

|1. Introduction
As electronic devices have become increasingly smaller in recent years, various components
including electronic parts and printed circuit boards that used in such devices have beenminiaturized and have become increasingly sophisticated. Consequently, there is growing demand
for further miniaturization and increased quality of ultraprecision machining including drilling. Asa means to satisfy such demand, a machining method that utilizes a short-pulse, short-wavelength
laser has attracted attention. Utilizing a short-pulse laser such as a picosecond pulse laser enablesablation with no thermal effects, achieving high-precision machining with superior quality. Inaddition, using a short-wavelength laser can make the condensing spot diameter of a laser beamsmaller, which enables finer machining. Currently, as shown in Figure 1, popular short-pulse lasers
for micromachining include the Nd:YAG laser (Wavelength: 1064nm), followed by the green laser
by the second harmonic generation of the Nd: YAG laser (Wavelength: 532nm) and the UV laser
by the third harmonic generation of the Nd:YAG laser (Wavelength: 355nm). On the other hand,
the Deep Ultraviolet (DUV) laser by the fourth harmonic generation (Wavelength: 266nm) has notbeen widely used due to its difficulty in handling, having made very little actual achievement as a
micromachining system(1). However, the short wavelength can make the focused laser spot
diameter small at the irradiation point, from which finer hole-drilling can be expected. In addition,
the DUV laser has high absorbance through various materials and high light energy, which isexpected to be applicable to materials that have been difficult to machine using conventional
methods.
Mitsubishi Heavy Industries Machine Tool Co., Ltd. (MAT) has already brought the“ABLASER” Laser Micromachining System equipped with a picosecond green laser to market(2).
Subsequently, the company developed an optical system and machining technology utilizing apicosecond DUV laser for the purpose of achieving finer hole-drilling with higher quality. This
paper will clarify the basic machining characteristics of picosecond DUV lasers, and will introduce
actual ultrafine drilling performed by ABLASER utilizing the DUV laser.
*1*2*3

Advanced Manufacturing System Development Center, Mitsubishi Heavy Industries Machine Tool Co., Ltd.
Chief Staff Manager, Advanced Manufacturing System Development Center, Mitsubishi Heavy Industries Machine Tool Co., Ltd.
General Manager, Advanced Manufacturing System Development Center, Mitsubishi Heavy Industries Machine Tool Co., Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)50

Figure 1 Various types of laser beams depending on the wavelength|2. Structure of DUV laser optical system and its machiningcharacteristics
2.1

Optical laser focusing system for DUV laser

In laser micromachining, a laser beam is focused and irradiated on the surface of the target
workpiece from which the debris produced by machining is removed by the light energy. At thetime of irradiation, when the focused laser spot diameter is smaller, the area from which the debris
needs to be removed also gets smaller, which increases the quality of micromachining. In order toachieve a smaller focused laser spot diameter, there are multiple means, such as utilizing ashort-wavelength laser, enlarging the spot diameter before focusing and shortening the focal length.
However, when the wavelength of the laser light reaches the ultraviolet range, the laser’s
absorbance through glass materials increases. Therefore, it is necessary to ease the strain on opticalelement materials such as the lens. In other methods, the convergence angle is large, which is likely
to cause a gap in size of the hole between its entrance and exit and to reduce the focal depth,
limiting the shape of the hole created by drilling. The newly-developed optical laser focusing
system adopts the DUV laser, which offers a small diameter of the condensed light. Therefore, thedurability of the lens was ensured by testing the durability against the DUV laser beam of the glass
used for the optical element beforehand, while selecting the optimal glass material and setting anappropriate laser irradiation intensity. Furthermore, by optimizing the focal length and lens form,
the convergence angle is minimized and a long focal depth is achieved, while the focused laser spot
diameter remains very small.

2.2

Assessment of focused laser spot diameter of DUV laser

Figure 2Structure of the focused laser spot diameter measuring system of DUV laser

Based on the study described in 2.1 above, a prototype optical laser focusing system for
DUV laser was created in order to confirm the expected level of micromachining by measuring thefocused laser spot diameter and energy distribution. Figure 2 shows the structure of the spotMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
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diameter measuring system. The beam emitted from a DUV laser oscillator is transmitted throughthe total reflection mirror which simulates the optical transmission path of a laser machining device
to be installed and condensed through a focusing lens.
Figure 3 shows the energy distribution at the focal point measured by a CCD camera-stylemeasuring instrument. The energy distribution is mostly consistent with the expected Gaussiandistribution, which is ideal, and the measured spot diameter was 5.7μm. On the other hand, thedesigned diameter thereof calculated in accordance with the theoretical formula is 5.5μm, which ismostly consistent with the measured value. Compared with the spot diameter by ABLASER
utilizing MAT’s conventional green laser, the diameter of a DUV laser beam is about a half of thatof the green laser, from which finer hole-drilling can be anticipated. In addition, the ellipticity at
the focal point is 0.998, which confirms that hole-drilling with high accuracy can be expected.

Figure 3

Energy distribution at the focal point

The focused DUV laser spot diameter is 5.7μm and the energy distribution
results in ideal curve in agreement with Gaussian distribution.

2.3

Examination of fabrication rate of DUV laser beam with various materials

Fabrication rates of a DUV laser beam and green laser beam (wavelength: 515 nm) withvarious different materials are obtained in order to compare and evaluate their characteristics. In an
optical system structure similar to the focused spot diameter measuring system, individualmaterials are placed at the focal position (Figure 2). The materials listed in Table 1 wereindividually irradiated with a focused DUV or green laser beam by 1 pulse only in order to measure
the depth of the hole made by drilling with a 3D laser scanning confocal microscope(VK-X150/160) manufactured by Keyence corporation. As the cross section of the hole after
drilling is cratered, the measurement was carried out in terms of the maximum depth at the centerof the hole. Fabrication rates for individual materials were obtained by conducting machining andmeasurement with different levels of pulse energy.
Table 1 Materials used for 1-pulse irradiation test with
DUV and green laser beam
Metal

Stainless steel

Semiconductor

Single-crystal silicon carbide wafer

Resin

Polyethylene terephthalate

Glass

Borosilicate glass

The measurement results are shown in Figure 4. The vertical axis of the chart indicates thefabrication depth, while the horizontal axis represents the energy intensity of the irradiated laser
beam. As shown in Figure 3, the energy distribution is uneven within the irradiation range.
Therefore, the intensity is calculated using an energy value near the center.
Fabrication rates were compared between the DUV laser beam and the green laser beam
where a noticeable difference was confirmed in every material. Figure 5 shows fabrication marks
and cross-sectional views of holes produced by both lasers. A single-crystal silicon carbide wafer(SiC), a compound semiconductive material, has a high degree of hardness and chemical stability
where precision machining and fabrication based on chemical actions are extremely difficult.

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Therefore, laser-based precision machining is highly expected to be capable of handling thismaterial. However, due to the large bandgap, a short-wavelength laser beam which has a higher
light energy than other types of laser beams is considered suitable for machining. In thisexperiment result, the fabrication rates of the green laser beam were fairly low and materials melted
within the laser irradiation range. On the other hand, the DUV laser beam, which has a high lightenergy, achieved relatively high fabrication rates and the cross-sections of the fabrication marks
were similar to the Gaussian distribution. Therefore, precise machining can be expected in ultrafinehole-drilling as well. The DUV laser beam successfully formed clear fabrication marks onborosilicate glass. The green laser beam mostly penetrates materials, which makes the fabrication
itself extremely hard, and the fabrication rates obtained are very low. With the DUV laser,fabrication marks were confirmed to have very little melting on the surface of the workpiece wherereattachment of debris removed was hardly observed. Accordingly, it is assumed that ideal ablation
phenomena are induced, from which high-quality ultrafine hole-drilling can be expected. The DUVlaser beam achieved higher fabrication rates for stainless steel than the green laser beam,demonstrating its superiority. Further, the green laser beam created bubbles in the fabrication marks
on polyethylene terephthalate, a high-polymer material, which indicated melting of the materials.
On the contrary, an ablation activity with less thermal effects and sufficient fabrication rates wereconfirmed with the DUV laser beam.
Accordingly, the superiority of the DUV laser to the green laser beam was verified withmultiple materials, and it is also possible to estimate the optimum pulse output in order to obtainthe fabrication depth required for individual materials. Thus, improved machining quality andoptimized performance are possible.

Figure 4 Fabrication depth of individual materials corresponding to the energy intensity
of DUV and green laser
Normalized values are utilized for the vertical and horizontal axes. All charts utilize a unified scale.

Figure 5 Images of fabrication marks by 1-pulse irradiation on individualmaterials and their cross-sectional views
A unified scale is utilized for all images and cross-sections.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)53|3. Actual machining conducted with ABLASER utilizing DUV laser
A drilling test was conducted utilizing an "ABLASER" laser micromachining systemequipped with a DUV laser optical system. ABLASER has already been offered on the market by
MAT. In order to achieve high-quality hole-drilling, ABLASER adopts the helical drilling method
in which the path of the laser beam can be rotated and adjusted with high accuracy as shown inFigure 6. In addition, an optical head, one of our unique developments, allows the control of theincidence angle as well as the rotation diameter of the laser beam as shown in Figure 7, which canachieve any hole diameter and cross-sectional shape.

Figure 6

Helical drilling method

The helical drilling is carried out while rotating the laser
beam at a high speed with an arbitrarily-adjusted rotation
diameter and irradiating with a pulse laser.

Figure 7 Control of the laser incidence angle by ABLASER
Drilling can achieve arbitrary cross-sections such as a straight hole, a tapered hole and a reverse-tapered
hole by controlling the incidence angle of a laser beam.

According to the machining characteristics obtained from the 1-pulse irradiation test
described above, laser irradiation conditions were determined, based on which hole-drilling wasperformed on stainless steel and a single-crystal SiC wafer. A hole of 20μm in diameter was
created through the stainless plate (Thickness: 0.2mm). The roundness of the hole both on the frontand back of the plate was excellent, forming smooth edge portions with no chip. A hole of 10μm in
diameter was made through a SiC wafer (Thickness: 0.1mm). Figure 8 shows a hole-drillingsample thereon. An intended hole of 10μm in diameter was successfully created on SiC, which is
generally hard to machine due to its bandgap. Further, the quality of the edge portion after drillingis exceptionally high, which indicates that ablation achieved machining with reduced thermaleffects.
As described above, due to utilization of the DUV laser beam, optimization of thelight-collecting system and precise control of the beam trajectory, high-quality hole-drilling ofMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)54

20μm or 10μm in diameter and an aspect ratio of 10, which conventionally had been very hard, wassuccessfully achieved. In the future, hole-drilling, cutting and slit-curving will be performed withother dielectric materials, semiconductors and metal materials in order to clarify the superior
applicability of the DUV laser by optimizing the machining conditions.

Figure 8 SEM images of hole on a single-crystal siliconcarbide wafer drilled with DUV laser
Material: single-crystal SiC wafer, Thickness: 0.1mm, Drilled hole diameter:10μm|4. Conclusion
In order to satisfy the need for finer laser micromachining, MAT has developed an opticalsystem utilizing a short-pulse DUV laser with which MAT confirmed that a micro focused laser
spot diameter can be achieved while maintaining the required focal depth. Furthermore, machining
characteristics with various different materials were identified by utilizing this new optical system.
In addition, MAT installed this optical system in an “ABLASER” laser micromachining systemmanufactured and sold by MAT, and achieved ultrafine hole-drilling of a minimum of 10μm in
diameter and an aspect ratio of a maximum of 10. An ABLASER equipped with the short-pulse
DUV laser that is introduced in this paper will be exhibited and sold at the 28th Japan InternationalMachine Tool Fair (JIMTOF 2016). MAT will strive to continue technical development in thefuture so that we will be able to offer optimal solutions to satisfy the need for finer micromachining
with higher quality in various industrial fields.

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Ikeda, N. et al., Diode-Pumped Solid-State Ultraviolet Laser Micro Processing System, Mitsubishi HeavyIndustries, Ltd. Technical Review Vol. 40 Extra No. 2 (2003)
Nakagawa, K. et al., Microdrilling Technology using Short Pulsed-laser, Mitsubishi Heavy Industries
Technical Review Vol. 52 No. 3 (2015)