# Three-Dimensional Residue-Free Volume Removal inside Sapphire by High-Temperature Etching after Irradiation of Femtosecond Laser Pulses

Authors: Shigeki Matsuo; Kensuke Tokumi; Takuro Tomita; Shuichi Hashimoto
Journal: Laser Chemistry
 (2008)

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## Abstract

We applied the femtosecond laser-assisted etching technique, that is, irradiation of focused femtosecond
laser pulses followed by selective chemical etching, to volume removal inside sapphire.
At room temperature, volume etching only slightly advanced while residue remained inside the
volume. By increasing the etching temperature, complete volume etching without residue was
achieved. Complete etching was, however, accompanied by undesirable phenomena of surface pits
or cracks, which are expected to be excluded through further improvement of processing.


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## Body

## 1. Introduction

Despite recent
progress in micro/nanotechnology, material processing with three-dimensional
(3D) shape flexibility remains a challenging task in the micrometer domain.
Sacrificial layer etching [1] is an established technique in the field of
microelectromechanical systems (MEMSs) for fabricating suspended structures.
Nevertheless, it requires large-scale semiconductor equipment. Femtosecond (fs)
laser processing of photopolymers [2, 3] is a widely used technique for 3D submicrometer
processing, but the target materials of that technique are limited to polymers.
For that reason, the technique is inapplicable to solid materials.Femtosecond laser-assisted etching offers the
possibility of removal processing with 3D flexibility in the micrometer domain.
The technique consists of two steps: irradiation of focused femtosecond pulses
along a predesigned pattern and subsequent chemical etching. The modified region
will be etched out selectively; a 3D empty space is fabricated if the
irradiated region is more soluble to the etchant. In fact, 3D microstructuring
by femtosecond laser-assisted etching was first reported in 1999 with a
photosensitive glass [4]. Subsequently, similar 3D microstructuring was
reported with a nonphotosensitive material, silica glass [5], and other materials
[6–9].Sapphire (Al2O3 crystal) is a hard and inert material that
plays important roles in optics and electronics. For example, it is used as a
substrate material of a GaN light-emitting diode. Several techniques have been
reported for micropatterning of sapphire and Ti:sapphire, such as a combination
of ion implantation and wet chemical etching [10], reactive ion etching [11], and argon ion-beam etching
[12]. Especially, a
combination of ion implantation and wet chemical etching [10] enabled us to underetch the
surface layer, but this technique requires very large and expensive equipment.
Femtosecond laser-assisted etching is a promising technique for 3D
microstructuring of sapphire; microchannels were fabricated by this technique
[8, 13]. However, we have reported that complete removal of
the volume region is difficult. Mesh-like residue remains after etching
[14]. Volume etching
is necessary for applications that require a complete hollow space, such as
embedded optical components [15, 16], nanoaquariums for dynamic observation of living
cells [17], and fluid
mixing by optical rotators [18]. A new technique is necessary to achieve volume
etching for sapphire. In all previous reports of femtosecond laser-assisted
etching, wet etching was carried out at room temperature. In the present study,
we carried out etching at high temperatures aiming at complete residue-free
removal of the volume region inside sapphire.


## 2. Experimental

The sample used was sapphire (0001) substrate
(Shinkosha Co., Ltd., Yokohama, Japan) grown using the
Kyropoulos method. The typical FWHM of its X-ray rocking curve is 4 seconds.
The sample was set on an inverted microscope (IX-70; Olympus Corp.,
Tokyo, Japan), and
femtosecond laser pulses (800 nm, 130 femtoseconds) were focused by an
objective lens (UPlanApo100; Olympus Corp.) and irradiated to the sample.
Irradiation of a single femtosecond pulse with a typical pulse energy of about
40 nJ produced a micrometer-scale modified spot (amorphous region surrounded by
a strain field [8])
inside the sapphire without cracking. Hereinafter, this spot is referred to as
voxel.The whole modified region is a square cuboid10μm below the surface, with four paths which
connect the square cuboid and the surface. The size of the square cuboid was x×y×z=10×10×10μm3,
where the z-axis is the direction perpendicular to the substrate surface. In
the square cuboid region, voxels are arranged in a simple tetragonal lattice.
The period on the x-axis and y-axis, px,
was typically 0.2μm.
The period on the z-axis, pz,
was 1–4 μm;
here the results with pz=1μm  and 2μm are reported. After irradiation, the sample
was observed using optical microscopy.The etchant was a 10% aqueous solution of
hydrofluoric acid. Etching was carried out in a teflon-coated high-pressure
cell. The cell, containing etchant and sample, was put in an electric oven at
80–150C°. After etching,
the sample was inspected using optical microscopy and scanning electron
microscopy (SEM). SEM observation was carried out on the surface and the subsurface
irradiated region. For SEM observation of the subsurface irradiated region, the
sample was mechanically polished so that the region was observable directly.
The direct SEM observation of the irradiated region is important because
nondestructive optical observation alone might be insufficient to reveal the
residue [14].


## 3. Results and Discussion

Through these
experiments, we discovered that both irradiation and etching conditions can
improve removal capability, although each has its own disadvantages.First, the results withpz=2μm are described. Figure 1(a) depicts an SEM
micrograph inside after etching at room
temperature for 72 hours. Volume etching only slightly advanced; a mesh-like
residue was observed. Figure 1(b) presents an SEM micrograph of the interior
after etching at an elevated temperature of 100C° for 24 hours. Volume
etching was somewhat improved. A single layer was removed, but further advances
in depth (in the z direction) did not take place. When etching temperatures
were further elevated to 120C°, much advance took place in volume etching; residue was lost and complete
removal was achieved, as shown in Figure 1(c). However, another problem appeared.
Figure 1(d) shows an optical micrograph of sapphire after etching. In addition
to the subsurface etched regions (dark squares), surface pits (small bright
spots) can be seen. The pits were absent before etching but they appeared after
etching for a long period. Reportedly, pits are generated on the surface of
sapphire during high-temperature etching; the origin of the pits is related to
dislocations [19]. The
pits observed here have a hexagonal shape and are aligned to the same
direction, which suggests that the shape is related to the crystalline nature
of sapphire. In the present experiments, the generation of surface pits became
a problem when the etching temperature was elevated. The pits on the surface
scatter light to unintended directions. For that reason, they are undesirable
for optical applications. We have performed examination with several etching
temperatures and etching periods, but we were unable to find a
temperature-period window in which complete volume etching can be achieved
without surface pits. The surface pits can probably be removed by polishing
subsequent to the etching, but it is a time-consuming process. A complete
etching method that leaves no surface pits is desired. It is noteworthy that
the roughness of the surface in the pit-free region did not deteriorate by
etching (Ra≲1 nm; Ra is the average of the roughness profile, as
evaluated using atomic force microscopy).(a)–(c) SEM
micrographs inside sapphire with a laser irradiation condition ofpx=0.2μm and pz=2.0μm,
and after etching (a) at room temperature for 72 hours, (b) at 100C°
for 24 hours, and (c) at 120C° for 24 hours. (d)
Optical micrograph of sapphire after etching at 120C°
for 48 hours. In this micrograph, both the surface and subsurface are
visible.
(a)(b)(c)(d)Forpz=1.0μm,
etching advanced better than in the case of pz=2.0μm.
Figure 2(a) depicts an SEM micrograph inside after
etching at 100C° for 24 hours. From comparison with
Figure 1(b),
volume etching appears to be much more advanced with pz=1.0μm.
However, another problem, cracking, appeared with pz=1.0μm.
Figure 2(b) portrays optical micrographs inside sapphire after laser
irradiation with px=0.2μm and pz=1.0μm before and after etching. As shown there,
cracks are visible from the corner of the square cuboid to the outside before
etching (b1). After etching (b2), the laser-irradiated region darkened; the
cracks are still observable. Cracks reduce the precision of fabrication and
enable fluid to penetrate to unexpected regions. For that reason, generation of
cracks should be avoided.(a) SEM
micrographs inside sapphire with a laser irradiation condition ofpx=0.2μm and pz=1.0μm after etching at 100C°
for 24 hours. The inset presents a close-up oblique view tilted at 20°. (b) Optical micrographs of the same sample as (a), (b1) before
etching, and (b2) after.
(a)(b)As described above, cracks did not appear by
irradiation of a single femtosecond pulse with the present irradiation
condition. After irradiation of the cuboid region with a smallpz of 1.0μm,
cracks appeared at the outside of the corner of the cuboid region. These
results suggest that cracks are generated by superposition of stress of
multiple voxels. Consequently, the decrease in the period of voxels enhanced
the generation of cracks.Figure3
schematically summarizes the results. The horizontal axis indicates the laser
irradiation condition: the right side corresponds to the higher density
(smaller period of voxels) and higher pulse energy. The vertical axis indicates
etching conditions: “up” corresponds to the higher temperature and longer
period. Three thick lines mark the borders of the issues to be overcome.
Corresponding double arrows mark the desirable side. The three double arrows
point to the different directions. Therefore, the overlapping region of the
three is small even if it exists. We have not yet found such an overlapping
region. Three borderlines cross at a point in Figure 3, but the relative position
of the lines has not been clarified yet. The existence of the overlapping
region remains as an open question.Figure 3
Schematic
diagram of removal property of sapphire by femtosecond laser-assisted etching.
Thick broken lines are borderlines of three issues; corresponding double arrows
show the desirable region.Another remaining problem is the roughness at the interface
between etched and unetched regions. As shown in Figure1(c), where complete
volume etching was achieved with a generation of surface pits, the bottom of
the etched cuboid is rough. At present, this roughness cannot be controlled
during processing. A new processing strategy is necessary for applications in
which roughness hinders its use, as in optical applications.


## 4. Conclusion

Complete
removal of the volume region by femtosecond laser-assisted etching inside
sapphire was achieved. The increased etching temperature and increased density
of laser irradiation points aided the progress of etching. Changes in the
experimental parameters, however, caused undesirable phenomena of surface pits
and cracks. The compatible region of experimental parameters, where complete
etching is achieved without such problems, is expected to be small. For that
reason, refinement of the experimental parameters or establishment of a new
processing strategy is required.



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*Source: 892721-2008-09-30.xml*