TB, AR, JMM/219320, 4/04/2006
INSTITUTE OF PHYSICS PUBLISHING
JOURNAL OF MICROMECHANICS AND MICROENGINEERING
J. Micromech. Microeng. 16 (2006) 1–6
UNCORRECTED PROOF
A femtosecond laser-induced periodical
surface structure on crystalline silicon
B Tan1 and K Venkatakrishnan2
Q1
1
Department of Aerospace Engineering
Department of Mechanical and Industrial Engineering, Ryerson University,
350 Victoria Street, Toronto, M5B 2K3, Canada
2
E-mail: [email protected]
Received 21 February 2006, in final form 22 February 2006
Published DD MMM 2006
Online at stacks.iop.org/JMM/16/1
Abstract
A laser-induced periodic surface structure (LIPSS) has attracted research
interest for its promising potential in micromachining for microelectronics
and microelectromechanical systems. A femtosecond laser-induced
periodical surface structure was investigated for polished crystalline silicon.
The observed structure is similar to the classical ripples that are
characterized by long, nearly parallel lines extending over the entire
irradiated area on the metal and silicon surface after continuous or pulsed
laser irradiation. The spacing of the ripples nearly equals the irradiation
wavelength. The depth of these ripples increases nonlinearly with the
fluence of irradiation. The orientation of these periodic structures is
perpendicular to the vector of electric field of the laser beam. It seemed that
pattern formed by a femtosecond laser complies well with conventional
models. Unlike the patterns formed by a continuous or nanosecond pulsed
laser, however, the spacing of the ripple formed by femtosecond pulses is
not influenced by the incident angle of the laser beam. The formula used to
predict the ripple spacing in the conventional model does not apply to the
femtosecond laser induced ripple structure. A plausible explanation to this
phenomenon is proposed. The effect of the pulse repetition rate was studied
and it was found that a femtosecond laser oscillator generates the same
periodic structure as the amplified laser system does.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
A laser-induced periodical surface structure (LIPSS) has been
investigated for nearly four decades. Laser or pulsed laser
beams of low fluence (below damage threshold) have been
used to produce spatially periodic structures on surfaces of
dielectric materials and semiconductors [1, 2]. A wide range of
laser sources have been investigated, including the nanosecond
laser [3, 4] and the femtosecond laser [5]. Researchers
attributed the ripple structure formation to interference
between the incident beam and scattered beam parallel to
the substrate [6], induced polarization charge on defect
boundaries [7], holographic recording of surface polarization
due to surface defects, surface waves due to surface
roughness and inhomogeneity, freezing of capillary waves,
0960-1317/06/000001+06$30.00
generation transient periodic heating pattern during laser
irradiation [4].
More recently, LIPSS attracted research interest for its
promising potential in micromachining for microelectronics
and microelectromechanical systems (MEMS) [8, 9]. Ripple
formation may be applied to fabricate gratings, shallow
junctions of metal-oxide-silicon transistors and liquid-crystal
display and to texture magnetic recording media. A ripple
structure would also be used to roughen the surface of
MEMS components so as to enhance the surface adhesion
and improve the performance or lift time of micro-devices.
Recently, a femtosecond pulsed laser has been investigated for
surface modification of dielectric materials [10], metals [11]
and semiconductor [9]. Periodical structures produced by
femtosecond laser pulses are very similar to those classical
© 2006 IOP Publishing Ltd Printed in the UK
1
B Tan and K Venkatakrishnan
Mechanical Quarter
waveplate
shutter
Ti:sapphire
femtosecond
laser oscillator
(775 nm, 50 fs,
80MHz)
Pulse
stretcher
Pulse
compressor
(775 nm, 150
fs, 1KHz)
mirror
Converging lens
Half
waveplate
Nd: YAG
Pumping laser
Silicon substrate
Power amplifier
x-y translational
stage
Personal computer
Figure 1. Schematic of experimental setup.
ripples created by continuous wave or nanosecond pulse.
However, they also demonstrated some unique characteristics
which cannot be explained by interfering model. In this
paper, we report the observation of orientated periodic
structures on polished crystalline silicon after exposure to low
fluence femtosecond pulses. The effects of laser parameters
on ripple spacing, orientation and depth were studied in
depth.
rim
center
2. Experimental works
Experiments were carried out in atmosphere without the use
of any ambient gas or vacuum. The experimental setup was
an amplified kHz Ti:sapphire femtosecond laser system, which
consists of a laser oscillator, a pulse stretcher, a power amplifier
and a pulse compressor. The laser generates pulses of 150 fs
at full width half maximum (FWHM) with linearly polarized
light at a central wavelength of about 775 nm in the near
infrared. The laser beam has transverse electromagnetic mode
TEM00 and pulse repletion rate of 1 kHz. The pulse energy
was attenuated by rotating a half waveplate placed just before
the pulse compressor, as illustrated in figure 1. The laser
beam was directed onto the target at right angle and was
focused into an almost circular spot of 150 µm diameter by
a converging lens with a focal length of 75 mm. The tested
sample was a polished silicon with a crystal orientation of
(1 1 1). A computer controlled two-axis translational stage
was used to position the laser spot at a desired location on
the silicon substrate. The number of pulses reached the target
surface was controlled by a mechanical shutter placed just
before the converging lens. The shutter was synchronized
with the pocket cell, which determines the pulse repetition
rate. The polarization state of the laser beam was rotated by
a quarter waveplate placed after the mechanical shutter. The
modified surface was investigated and measured by an atomic
force microscope (AFM) and a scanning electronic microscope
(SEM). The laser fluence was measured by a power-meter
placed just before the sample surface. The pulse energy used
in this experiment was set far below the ablation threshold
value. At each machining location, 1000 pulses were fed onto
the sample surface.
2
Figure 2. Pattern formed by the p-polarized laser beam at a pulse
energy of 80 nJ.
3. Results and discussions
Figure 2 gives the SEM images of morphology on the silicon
surface after exposing to the p-polarized laser beam at a pulse
energy of 80 nJ, implying a laser fluence of 0.453 mJ cm−2.
This is far lower than the damage threshold of 0.4 J cm−2
observed in our previous study [12]. Figure 3 presents the
magnified images of rim and central part of the machining spot.
The patterns are evidently distinguished into two regimes. In
the outer rim, the ripples are similar to the classical ripple
pattern, which are characterized by long, nearly parallel lines
extending over the entire area. Figure 4 gives the cross-section
profile of the ripples obtained from AFM measurements.
The spacing of the ripples was measured as in the range of
720–750 nm, which is slightly smaller than the irradiation
wavelength. In the center, the classical ripples, same as those
observed in the rim, co-exist with another type of periodic
pattern which is formed by short crooked lines aligned by many
small and deep holes. These short lines are perpendicular to the
orientation of the ripples. The spacing of this periodic pattern
is not as uniform as that of the ripple structure. It is about
2–3 µm near the outer rim and increase to 5–6 µm at
the intensity peak of the laser spot. These characteristics
reveal that this structure is of the second type LIPSS, whose
spatial period is significantly longer than the laser wavelength
and depends more on the irradiation fluence than on the
wavelength. The uniformly spaced ripple structure is of
A femtosecond laser-induced periodical surface structure on crystalline silicon
100
90
depth (nm)
80
70
60
50
40
30
60
70
80
90
100
110
pulse energy (nJ)
Figure 5. Ripple depth versus pulse energy (s-polarization).
(a)
(a)
(b)
Figure 3. (a) Ripples in the outer rim. (b) Two types of structures in
the center of the machined spot.
(b)
Figure 4. Cross-section profile of ripples produced by the
s-polarized laser beam at a pulse energy of 100 nJ.
great research interest because of potential applications in
MEMS. Therefore, our study focused on the ripple structures
observed in the rim of the machined spot. The effects of
laser parameters, such as influences, polarization and incident
angle, on the ripple formation were studied in depth.
Firstly, pulse energies of 100 nJ, 90 nJ, 80 nJ and 70 nJ
were used to irradiate the sample surface. It is found that
the fluence of the laser spot influences the depth but not the
spacing. The depth of the ripples was analysed through the
AFM cross-section analysis and was plotted in figure 5. It
shows that the depth of the ripples increases nonlinearly with
the pulse energy.
(c)
Figure 6. Ripple pattern formed by laser beam at a pulse energy of
100 nJ. (a) p-polarized beam, (b) s-polarized beam and (c) circularpolarized beam.
Secondly, laser beams of different polarizations were used
to irradiate the target surface. The morphologies of the rim are
given in figure 6. With linear polarization, the orientation of
3
B Tan and K Venkatakrishnan
in ripples. The period of the ripples depends on the
wavelength, the angle of incidence and the polarization. The
orientation of ripples is determined by the polarization of
the laser beam. With metals and semiconductors, the ripples
are mainly orientated perpendicularly to the electric vector of
the incident laser beam and the period can be predicted by
λ
1 ± sin θi
λ
≈
cos θi
≈
(a)
(b)
(c)
Figure 7. Cross-section profile of ripples formed by laser beam of
100 nJ pulse energy. (a) s-polarization, (b) circular polarization and
(c) p-polarization.
the ripples is perpendicular to the polarization. With circular
polarization, the ripples are at 45◦ to the horizontal line. It
is found that polarization has no obvious effect on the period
of the ripples. The spacing of the ripples remains around
750 nm regardless of the polarization. However, polarization
does have appreciable effect on the depth of the ripples.
Figure 7 gives the cross-section profile of the ripples formed at
a pulse energy of 100 nJ with s-, p- and circular polarizations.
The deepest lines were generated by the beam of s-polarization,
40% deeper than those generated by p-polarization. This result
agrees well with our previous observations [12].
In the classical model [13–17], it is understood that ripples
originate from the interference of incident laser light with the
scattered or diffracted light parallel to the surface. Scattering
of the incident light may be caused by the microscopic
roughness of the surface, defects, spatial variations in the
dielectric constant, etc. The interference of different waves
leads to an inhomogeneous energy distribution, which results
4
for s-polarized light,
(1)
for p-polarized light.
(2)
According to equation (2), if the laser beam irradiates the
surface at an angle of 45◦ , the spacing of the ripples will be
around 1130 nm. When we irradiated the sample surface at
an incident angle of 45◦ , however, no appreciable variation
in the spacing was measured. We repeated the attempts at
incident angles of 30◦ and 60◦ ; the spacing remained constant
regardless of the incident angle. The mechanisms described
by Young [17] may provide a plausible explanation to the
phenomena associated with the femtosecond laser induced
periodical surface structure. Young studied LIPSS with a
nanosecond laser and characterized the development of LIPSS
into four regimes based on the irradiation fluence. At the
lowest fluence, regime A, material melts locally and forms
periodic concave meniscus. This type of localized melting
is not influenced by an incident angle or polarization. After
irradiation these meniscuses resolidify, leaving steady-state
morphology on the material surface. Structures obtained at
higher fluence do not attain such an invariant form. We
suppose that the mechanism of the femtosecond laser-induced
ripple structure is similar to that in the regime A, characterized
by the extremely localized melting. On the other hand, the
pattern around the peak intensity demonstrates a connection
between intensity and ripple spacing. The ripple spacing
increases as it moves closer to the intensity peak. This
type of LIPSS resembles more of the second type of LISPP
frequently observed for nanosecond laser illumination. A
recent study of femtosecond micromachining suggested that
commercial femtosecond laser systems produce energetic
pedestal containing approximately same amount of energy as
femtosecond component of the pulse [18]. This nanosecond
pedestal possibly explains the co-existence of the second type
of LIPSS and the intensity invariant ripples at the center of the
laser spot.
4. Ripple structure formed by high repetition
rate femtosecond pulses
To investigate the influence of the pulse repetition rate on the
morphology, the experimental work described in section 2 was
repeated with the mode-locked Ti:sapphire oscillator, which
produces laser pulses of 50 fs at the repetition rate of 80 MHz
(the seed laser of the amplifier). At the machining spot, the
pulse energy was measured at about 1.6 nJ. At each irradiation
spot, 80–1600 million pulses were used. As given in figure 8,
the morphologies reveal that the formed ripple pattern and
the period of the ripples are the same as those obtained from
exposure to amplified pulses. The orientation of the ripples
is perpendicular to the polarization of the laser beams. The
A femtosecond laser-induced periodical surface structure on crystalline silicon
(a)
(b)
(c)
(d )
Figure 8. Surface modification using high repetition rate femtosecond laser beam. (a) The entire spot. (b) Ripples produced by the
p-polarized laser beam, 800 M pulses. (c) Ripples produced by the circular-polarized laser beam, 1600 M pulses. (d ) Ripples produced by
the s-polarized laser beam, 800 M pulses.
result shows that the pulse repetition rate has no influence on
the ripple formation on crystalline silicon. A high repetition
rate laser can be used for the same purpose, with better pulseto-pulse stability, lower cost and much simpler configuration.
With the recent advancement of a solid-state diode-pumping
technique, the output power of the high repetition rate ultrafast
lasers has been improved significantly. Femtosecond lasers
with power up to 20 W at mega hertz pulse repetition rate
are commercially available. Thus, it is feasible to apply
femtosecond laser for surface modifications at high production
volume.
5. Conclusions
In this research work, surface modification of silicon using the
femtosecond pulsed laser was investigated. The morphologies
reveal that periodical ripples are formed on the silicon surface
after the irradiation. It is found that these periodical ripples
induced by the femtosecond pulsed laser agree partially with
the classical model. When the laser beam irradiates the
surface normally, the periodicity of the ripples approximately
equals the wavelength. The orientation of the structures is
perpendicular to the polarization of the laser beam. Neither
incident angle nor polarization affects the period of the ripple,
which is inconsistent with the classical model. The study also
reveals that high repetition rate pulse emitting directly from
an oscillator will generate the same type of ripple structures
on crystalline silicon. Since Gaussian beams were used for
this study, uniformed ripple structures were obtained only at
the rim of the focused laser spot. At the center of the laser
spot, LIPSS of second type was induced by the high intensity
and destructed the ripple pattern. If laser intensity profile can
be modulated into a hat-top, together with the employment of
a high-powered femtosecond laser oscillator, large range of
machining area would be achieved. A top-hat beam profile
could be obtained by the transmitting laser beam through
a homogeneizer, which is generally used with an excimer
laser for lithographic processing. This research work will
be conducted soon and the result will be reported soon after.
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