Pattern-induced ripple structures at silicon-oxide/silicon interface by excimer laser irradiation

Article abstract of DOI:10.1063/1.1496141

Ripple structures by KrF excimer laser irradiation have been observed on a silicon surface capped with a thin layer of patterned silicon oxide. The ripples are highly dependent on the patterns of the silicon oxide. They are centered and enhanced at the boundaries of the opened windows, forming a radial-wavelike structure. The formation of the ripples is attributed to the combined effect of surface stress, surface scattered wave and boundary effects.

Full text of DOI:10.1063/1.1496141

Pattern-induced ripple structures at silicon-oxide/silicon interface by excimer laser
irradiation
X. Y. Chen, Y. F. Lu, B. J. Cho, Y. P. Zeng, J. N. Zeng, and Y. H. Wu
Citation: Applied Physics Letters 81, 1344 (2002); doi: 10.1063/1.1496141
View online: http://dx.doi.org/10.1063/1.1496141
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 7
12 AUGUST 2002
Pattern-induced ripple structures at silicon-oxideÕsilicon interface
by excimer laser irradiation
X. Y. Chen, Y. F. Lu,a) B. J. Cho, Y. P. Zeng, J. N. Zeng, and Y. H. Wu
Laser Microprocessing Laboratory and Silicon Nano Device Laboratory, Department of Electrical and
Computer Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
͑
Received 26 February 2002; accepted for publication 30 May 2002͒
Ripple structures by KrF excimer laser irradiation have been observed on a silicon surface capped
with a thin layer of patterned silicon oxide. The ripples are highly dependent on the patterns of the
silicon oxide. They are centered and enhanced at the boundaries of the opened windows, forming a
radial-wavelike structure. The formation of the ripples is attributed to the combined effect of surface
stress, surface scattered wave and boundary effects. © 2002 American Institute of Physics.
͓
DOI: 10.1063/1.1496141͔
As the gate length of metal–oxide–silicon ͑MOS͒ tran-
sistors continues to shrink below 0.10 ␮m, the vertical and
lateral dimensions of p-n junction regions must be scaled
down as well to maintain device performance and control
short-channel effects. Conventional ion implantation and
thermal annealing is difficult to obtain abrupt, highly doped,
and ultrashallow junctions. Thus, new alternative processes
are being explored and developed. Among the new processes
for shallow junction fabrication, laser doping, and silicida-
tion technology is regarded as a revolutionary approach due
to its high local heating power and nanosecond short pulse
duration. It enables the formation of box-shaped junctions
Physik KrF excimer laser ͑LPX50͒ with a wavelength of 248
nm and a pulse duration of approximately 23 ns was used.
The unpolarized laser beam perpendicularly irradiated the
samples in air after passing through a homogenizer and being
focused by a quartz lens.
Figure 2 shows the optical microscope observation of
samples after a single pulse irradiation with a laser fluence of
2
430 mJ/cm . For comparison, observation was provided in
Fig. 3 for samples after a single pulse irradiation with a laser
2
fluence of 750 mJ/cm .
As shown in Fig. 2͑a͒, local ripples can be produced at
oxide-capped silicon area. The ripples exist only at
rectangle-window corners and high window density areas,
where surface stress is believed to be high. It is similar to the
formation of defect-induced ripple structures, in which the
defects become the surface wave center and produce local
ripples. When laser fluence increases, more ripples are pro-
duced around the opened windows, as illustrated in Fig. 3͑a͒.
The ripples, centered at the window boundaries, perpendicu-
larly run away from the boundaries. The amplitude of ripples
at rectangle-window corners and high window density areas
is also much higher. Since there is melting under laser irra-
diation, there is no more crystal orientation in Si substrate
1
with unequaled shallowness and low sheet resistance. A
large amount of effort has been made to the development of
2
–5
this technology. On the other hand, laser-induced damage
of solids has been an area of both theoretical and experimen-
tal interest ever since the development of the laser itself.
Ripples, i.e., gratinglike patterns or laser-induced periodic
surface structures, have been observed and studied on the
surface of various metals, semiconductors, dielectrics, and
polymers for more than three decades.6–9 The basis of the
currently accepted theory of ripples is the interference of the
incident lights and some form of ‘‘surface scattered wave.’’
The ripples appear as the result of nonlinear positive feed-
back growth process initiated by the light scattering due to
the random variation of any optically significant surface
8
property.
In laser doping and silicidation process, silicon oxide is
often used as a hard mask or cap layer. In this letter, we
report pattern-induced ripple structures at silicon-oxide/
silicon interface by excimer laser irradiation.
The samples used in this work are phosphorus-doped
n-type ͑100͒ silicon wafers with a resistivity of 6–9 ⍀ cm.
Silicon oxide with a thickness of 100 nm was deposited on
the silicon wafer by plasma-enhanced chemical vapor depo-
sition at 150 °C using silane ͓SiH /N ͑95%͔͒ and nitrous
4
2
oxide (N O). Then oxide patterns were generated by lithog-
2
raphy and etching process. Figure 1 shows the optical micro-
scope observation of the oxide-capped sample. A Lambda
a͒Author to whom correspondence should be addressed; electronic mail:
eleluyf@nus.edu.sg
FIG. 1. The optical microscope observation of the prepared samples before
irradiation.
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003-6951/2002/81(7)/1344/3/$19.00 1344 © 2002 American Institute of Physics
On: Sun, 30 Nov 2014 22:05:33
0

Appl. Phys. Lett., Vol. 81, No. 7, 12 August 2002
Chen et al.
1345
FIG. 2. The optical microscope observation of samples after a single pulse
2
irradiation with a laser fluence of 430 mJ/cm . The scale bar shows 20 ␮m
in ͑a͒ and 10 ␮m in ͑b͒ and ͑c͒.
FIG. 3. The optical microscope observation of samples after a single pulse
irradiation with a laser fluence of 750 mJ/cm . The scale bar shows 20 ␮m
2
in ͑a͒ and 10 ␮m in ͑b͒ and ͑c͒.
and the direction of heat flow is radially inward to bound-
aries. Therefore, it is believed that the ripples are not depen-
dent on laser polarization and crystal structure of the sub-
strate. Their alignment only depends on the local surface
morphology or the patterns, which differs greatly from the
previous observation of periodical ripples with no orientation
preference. Thus, we refer to such type of ripples as
pattern-induced ripples. They are the domain ripples near
boundaries while periodical ripples10 become dominant at
the distant area from boundaries. During KrF excimer laser
irradiation, the laser–material interaction occurs at the
silicon-oxide/silicon interface since silicon oxide is transpar-
ent to the laser light at the wavelength of 248 nm. Due to the
existence of silicon oxide, the light reflection is greatly
weakened at the silicon-oxide/silicon interface compared
with the light reflection at the air/silicon interface. The
equivalent reflectivity of the silicon-oxide/silicon system
eter. The reflectivity of the oxide-capped silicon and the bare
silicon surface is 46.8% and 63.0% respectively at 248 nm.
Therefore, the oxide-capped silicon will melt first under laser
irradiation. Then, the melting front advances to the bare sili-
con in the window. There is a temperature gradient perpen-
dicular to the window boundaries. Specifically, a sharp tem-
perature difference exists at the window boundaries, which
will induce a high stress. Moreover, the bare silicon surface
has a higher cooling coefficient as compared with the oxide-
capped silicon due to the existence of silicon oxide. Thus, the
bare silicon in the window will always cool faster, which
also generates a temperature gradient. Because of these
boundary effects, flow patterns with directions of heat flow
are recorded at the silicon-oxide/silicon interface after laser
pulse ends.
1
0
As shown in Fig. 2͑b͒, some circular ripples can be ob-
served around the dots with a diameter of 1.2 ␮m. The cir-
cular ripples centered at the dots are just identical with the
defect-induced ripple structures. Since these dots are small
͑air/silicon-oxide and silicon-oxide/silicon interfaces͒ will be
smaller than that of the bare silicon surface, which means
more light is absorbed in the oxide-capped silicon. It is con-
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firmed by measuring the reflectivity using a spectrophotom-
and comparable with local defects, they act as local defects
On: Sun, 30 Nov 2014 22:05:33

1346
Appl. Phys. Lett., Vol. 81, No. 7, 12 August 2002
Chen et al.
ripple formation is also studied in this work. It is found that
the threshold laser fluence on silicon capped with patterned
2
oxide ͑oxide with opened windows͒ is 380 mJ/cm , while
the threshold on silicon capped with whole layer oxide ͑ox-
2
ide without patterns͒ is 500 mJ/cm . Thus, the threshold en-
ergy is reduced due to the existence of oxide patterns.
Figure 4 shows the atomic force microscopy ͑AFM͒ im-
ages on the silicon surface. The oxide layer was removed by
diluted HF solution. It is found that the defined patterns com-
bined with fully developed ripples have been recorded at the
silicon surface. Two groups of ripples can be observed: ͑1͒
Pattern-induced ripples perpendicularly run across the win-
dows. From the cross-sectional topography, their average pe-
riod is around 1.6 ␮m and height around 80 nm. ͑2͒ Small
parallel ripples with a certain angle from the boundaries are
formed and confined in the window. Their average period is
around 50 nm, which is much smaller than that of the
pattern-induced ripples since the ripple periodicity is found
to increase linearly with increasing silicon-oxide thickness.11
Their height is identical with that of the pattern-induced
ripples, which indicates resonance of two groups of ripples
during their formations. The cross-sectional topography
along with the line B clearly shows a sudden surface mor-
phology change at the window boundary, which suggests an
abrupt change of surface melting process.
From our experiment, it is noticed that ripple structures
can be easily formed on the silicon surface within the fluence
range used for laser doping and silicidation. In MOS tech-
nology, the ripples in the contact windows can result in se-
rious problems in the following metallization processes. The
depth of p-n junctions formed under the rough silicon sur-
face will also not be uniform. Furthermore, the rough silicon
surface due to ripples can result in junction spiking.
In conclusion, pattern-induced ripple structures by KrF
excimer laser irradiation have been observed at the silicon-
oxide/silicon interface. The patterns introduce local stress
and abrupt changes of surface optical properties. During la-
ser irradiation, pattern-induced ripples can be formed at low
laser fluences due to the combined effect of surface scattered
waves, surface stress, and boundary effects.
FIG. 4. AFM images of the sample after a single pulse irradiation with a
laser fluence of 500 mJ/cm and silicon oxide removal.
2
to be the surface wave center and induce local ripples. The
same circular ripple structures are also found around small
dots with the diameter up to 1.8 ␮m. Thus, the existence of
surface scattered wave is confirmed.
As shown in Fig. 2͑c͒, circular ripples combined with
pattern-induced ripples are formed around the dots with a
diameter of 2 ␮m. Since the dots have a larger size than
those in Fig. 2͑b͒, the area and heat volume of bare silicon is
big enough to have an impact on the melting of the oxide-
capped silicon. Boundary effects will gradually become
dominant in the ripple formation. As illustrated in Fig. 3͑c͒,
fully developed pattern-induced ripples are formed in the dot
matrix at higher laser fluence, since the boundary effects are
enhanced at higher-energy density.
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From the observation in Figs. 2 and 3, it can be con-
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surface stress, and boundary effects. The threshold energy of
9
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