IR and UV laser-induced morphological

phys. stat. sol. (c) 2, No. 10, 3798 – 3801 (2005) / DOI 10.1002/pssc.200461773
IR and UV laser-induced morphological changes
in silicon surface under oxygen atmosphere
J. Jiménez-Jarquín*, M. Fernández-Guasti, E. Haro-Poniatowski,
and J. L. Hernández-Pozos
Laboratorio de Optica Cuántica, Departamento de Física, Universidad Autónoma MetropolitanaIztapalapa, Av. San Rafael Atlixco No. 186, Col. Vicentina, C.P. 09340, México D.F., Mexico
Received 11 October 2004, revised 11 May 2005, accepted 3 June 2005
Published online 29 July 2005
PACS 61.80.Ba, 61.82.Fk, 68.37.Hk, 79.20.Ds
We irradiated silicon (100) wafers with IR (1064 nm) and UV (355 nm) nanosecond laser pulses with energy densities within the ablation regime and used scanning electron microscopy to analyze the morphological changes induced on the Si surface. The changes in the wafer morphology depend both on the incident radiation wavelength and the environmental atmosphere. We have patterned Si surfaces with a single
focused laser spot and, in doing the experiments with IR or UV this reveals significant differences in the
initial surface cracking and pattern formation, however if the experiment is carried out in O2 the final result is an array of microcones. We also employed a random scanning technique to irradiate the silicon wafer over large areas, in this case the microstructure patterns consist of a “semi-ordered” array of micronsized cones.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
When laser radiation is absorbed by a solid surface, the electromagnetic energy is converted to electronic, thermal, chemical and mechanical energy. The ejected material may include neutral atoms and
molecules, positive and negative ions, clusters, micron-sized solid particles, molten globules and electrons. The generated plasmas may have electron temperatures of thousands of degrees [1].
Laser pulses rarely remove material in a clean, orderly, layer-by-layer fashion. Instead, laser-irradiated
surfaces become altered both physically and chemically. Morphological changes take the form of periodic structures such as ripples, ridges, and the most intriguing features of all, cones [2].
Conical structures have been observed in metals, ceramics, polymers, and semiconductors as a result
of repetitive pulsed-laser irradiation, at laser fluences in the range of 1.0 to 10.0 J cm–2 (ablation regime)
[2–4].
In this paper we report the growth of the conical structures on the surface of silicon (100) upon cumulative pulsed-laser irradiation both at the Nd:YAG fundamental (1064 nm) and third harmonic (355 nm)
wavelengths in presence of an oxygen atmosphere. Additionally to irradiate samples by focusing the
laser beam on a fixed point of the target, we employ a random scanning technique to irradiate silicon
surfaces over large areas. We show the differences between these laser-etching processes at its initial
stages and also once the microstructures have became fully growth under both IR and UV irradiation.
*
Corresponding author: e-mail: [email protected], Phone: +52 55 5804 4614, Fax: +52 55 5804 4611
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
phys. stat. sol. (c) 2, No. 10 (2005) / www.pss-c.com
3799
2 Experimental
We carried out our experiments on n-type (arsenic-doped) Si (100) wafers with resistivity less than
7 x 10–5 Ω m. The samples are ultrasonically rinsed in methanol in the first place, then in acetone and
finally in 5% HF(aq) solution. The wafer is mounted in a vacuum chamber with a base pressure of less
than 10–5 Torr. The chamber is backfilled with (normally) 100-Torr of O2.
The laser used for irradiation is a Nd:YAG (Lumonics HY1200) with a maximum output of 1.2 J per
pulse at 10 Hz repetition rate in the fundamental wavelength (1064 nm) and 10 ns pulse width. All
the samples have been irradiated at normal incidence. The laser beam is focused with a quartz lens
(f = 80 cm) in order to obtain the desired energy density (≈6.0 J cm–2). We do not necessarily work at the
focus of the lens, given an energy per pulse of the laser beam, we work at a distance from the focusing
lens such that we obtain the mentioned energy density. Unless otherwise stated, the area of the spot is
2.1 x 10–2 cm2 at 1064 nm and 3.2 x 10–3 cm2 at 355 nm. Due to some damage to the optical components
in our laser, the spatial profile of the beam is not Gaussian as it can be appreciated in some of the pictures we show below.
We use a random scanning technique to irradiate silicon surfaces over large areas. In these instances
the laser beam is steered using two computer-controlled mirrors such that we can guide the beam at will.
With this method we select an area (i.e 1 cm2) of the wafer and randomly scan the beam within it until
each point of the selected region receives on average the same dose as it would in a single spot experiment.
3 Results
3.1 IR (1064 nm) pulsed-laser irradiation in O2
With an ambient pressure of 100 Torr of oxygen and after the first 60 laser shots, fractures begin to develop in the irradiated surface as show in Fig. 1a and Fig. 1b. For (100)-oriented wafers, two sets of
fracture lines intersect at 90° forming a grid that divides the surface into rectangular blocks, these blocks
eventually form the bases of the cones.
The fractures open up with increasing number of laser pulses, and deep grooves and craters develop at
the fracture walls. Apparently, these grooves trap and concentrate the laser radiation and act as emitters
of ablated material.
In Fig. 1c we can see the laser-induce damage after 2000 laser shots and Fig. 1d shows a close up of a
region of Fig. 1c viewed at an angle of 45° from the surface normal. Note that Fig. 1c reflects the variation of the spatial profile of the laser beam.
a)
b)
200 µm
c)
50 µm
d)
900 µm
30 µm
Fig. 1 SEM images of a Si(100) specimen irradiated at a laser fluence of 6 J cm–2 (λ = 1064 nm) in an ambient
pressure of 100 Torr of oxygen after (a) 60 laser pulses, (b) higher magnification of (a), (c) 2000 pulses and (d)
higher magnification of (c), the sample is viewed at 45° to the normal. Note that the region depicted in Fig. 1a (60
shots) close up of the lower left spot in Fig. 1c (2000 shot).
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3800
J. Jiménez-Jarquín et al.: Morphological changes in silicon surface under oxygen atmosphere
When irradiating larger areas with the method described at the end of Section 2 the resulting structures
are shown in Fig. 2. Figures 2c and 2d show fully growth cones, and as we can appreciate in 2c, the
cones form a “mesh” with their bases originated at the rectangular blocks that can be observed in Figs.
1b (for the case of a single laser spot), 2a and 2b. When the irradiation is performed with a single laser
spot previous work and our own results show that the high of the cones follow the spatial profile of the
laser beam, i.e the higher cones are at the centre of the laser spot (for a Guassian beam profile) and the
high decreases towards the edges of the laser spot. In this case, as we used the random scanning technique, roughly all the cones have the same height (Fig. 2d).
a)
b)
90 µm
c)
20 µm
d)
150 µm
20 µm
Fig. 2 SEM images (normal incidence, except d) of a Si(100) specimen randomly irradiated (area ≈ 1 cm2) at a
laser fluence of 6 J cm–2 (λ = 1064 nm), in a pressure of 100 Torr of O2 (a) early stages (800 laser pulses for the full
1 cm2), (b) higher magnification of (a), (c) after 30,000 laser shots and (d) cleaved image.
3.2
UV (355 nm) pulsed-laser irradiation in O2
Following the same sequence, in Figs. 3a and 3b we can observe the superficial morphology in the first
stages, in which we found that the irradiated surface is not formed by rectangular blocks, but by structures that appear to be capillary waves.
After irradiate the sample with 2000 pulses, the surface shows several regions with different damages.
This reflects the influence of the spatial profile of our laser (and the differences in local fluence within
the laser spot) rather than any particular effect of irradiating the Fig. 3c.
When we performed random irradiation with 1000 pulses the silicon surface increases its roughness,
however we have not found fractures. And is only after 30000 pulses that fractures begin to develop in
the irradiated surface similar to the first stages of an IR-laser irradiated surface.
a)
b)
200 µm
c)
60 µm
Cones
300 µm
d)
30 µm
Fig. 3 SEM images (normal incidence, except d) of a Si(100) Si specimen irradiated at a laser fluence of 6 J cm–2,
(λ = 355 nm) in an ambient background pressure of 100 Torr of oxygen after (a) 60 laser pulses, (b) higher magnification of (a), (c) 2000 pulses, and (d) higher magnification of (c), the sample is viewed at 45° to the normal.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
phys. stat. sol. (c) 2, No. 10 (2005) / www.pss-c.com
3801
4 Discussion
4.1
IR (1064 nm) pulsed-laser irradiation in O2
Fractures similar to the ones observed in Fig. 1a and Fig. 1b were already reported by Lowndes et al. [4]
they found them after irradiate a Si(001) surface with KrF excimer laser (248 nm, 25 ns). In our case,
these fractures form a pattern which follows the symmetry of the Si wafer, ie, lines intercecting at 90
degrees for Si(100). In this process, the structures formed on the surfaces show a characteristic period
that is smaller than the spot size of the laser and larger than the wavelength of the radiation used [5].
Despite previous results [6] where excimer laser irradiation produced microcolumn patterns, in this
work we report that a conical microstructure array is always obtained when the sample is irradiated by IR
laser pulses at 1064 nm in presence of O2.
4.2
UV (355 nm) pulsed-laser irradiation in O2
The Si(100) surface morphology irradiated with more energetic photons (355 nm) presents significant
differences in respect to surface irradiated with IR-photons (1064 nm). The structures that appear to be
capillar waves have been previously reported when the experiment carried out with a copper vapour laser
(wavelength of 510.6 nm, pulse duration of 20 ns) [7]. Systematically, when the irradiation is performed
with UV photons the micron-sized structures are mountain-like structures, whereas for IR photons usually we obtained conical structures.
5 Conclusions
In conclusion, when a Si wafer is irradiated with a high power Nd:YAG laser the morphology of the
surface changes, these changes begin with cracks on the surface (for λ = 1064 nm), capillary waves (for
λ = 355 nm) and finally micron-sized structures, being these sharp conical structures (IR light) or mountain-like structures (with UV light). This points towards that the mechanism of growing is likely to be
different depending on the wavelength of the irradiation and the ambient atmosphere [8]. Currently, we
are further investigating these processes.
On the other hand, the random irradiation procedure is a novel method that creates a semi-ordered
array of cones, which exhibits the symmetry of the substrate (Si(100) in this work) contrary to other
studies in which this effect of the substrate has not been observed [9].
Acknowledgements J. J. J. acknowledges CONACYT for the scholarship awarded.
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© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim