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info@WITec.de, www.WITec.de

APPLICATION NOTE

Raman, PL & AFM
measurements onlithographically-createdstructures

Raman, PL & AFM measurements onlithographically-created structures
Silicon (Si), being the second most common element in the earth’s crust, is thematerial most widely used for integrated
circuits as well as solar cells. Modern
integrated circuit devices work with structures down to 14 nm in size[1] and while
the latest developments use a variety ofdifferent technologies, traditionally thestructures in semiconductor devices areproduced using lithography. In order toensure the highest quality in the devices,the production steps need to minimize
stress and stress induced artifacts. Other
structures in which layers with different
lattice constants are grown on top ofeach other require the strain for lattice
matching. It has been shown [2,3] that Raman measurements can effectively probe
the strain states of crystalline materials
and provide an effective, nondestructive
way of developing the most appropriate
production processes. From solar cell research it is additionally known that stressand crystallinity in Si has a strong effect on
the photoluminescence (PL) signal of theSi [4].
In this application note we report onstructures in crystalline Si created by laser
lithography which were in turn examinedon the same instrument using confocal
Raman imaging, confocal PL imaging andAFM.
Experimental
The experiments were performed using analpha300 RA confocal Raman-AFM. The XY
positioning of the sample for scanning andlithography was achieved using a piezoelectric scanning stage. A stepper motorwas used for the focus control (10 nm stepFig. 1: White-light image of the lithographically-created structure in Si.

size). With this system all experiments
could be performed as follows:a) Lithography
The system was equipped with a pulsed 532 nm laser (typically employed for
fluorescence lifetime imaging microscopy– FLIM) which was used for laser writing in
combination with the DaVinci Lithography
Package. The CW power of the laser was 15mW with a repetition rate of 20 MHz and apulse duration of 20 ms. The scripting function of the lithography package allowed
not only the arbitrary movement of thesample in XY but also the upward movement of the microscope (to defocus thelaser beam) when a movement withoutlaser lithography is desired. Alternatively,
an electronic shutter could have been used
to achieve this.
b) Confocal Raman Imaging
An additional frequency-doubled Nd:YAGlaser emitting 532 nm (CW - continuouswave) was used for confocal Raman imaging at 20 mW. The 100x NA 0.9 objective used allowed the laser to focus to adiffraction-limited spot with a diameter of355 nm. The signal was collected using a 25μm core diameter optical multimode fiber
which acted as a pinhole to achieve thebest depth resolution. A UHTS 300 spectrometer in combination with a 1800 g/mm
grating and a back-illuminated CCD camera were used for detection. The integration

time per spectrum was 12.2 ms. Two scans
were performed on the laser lithographed
area: A planar scan in XY (50 x 50 μm2 with
200 x 200 points) an a depth scan in theXZ plane (20 x 5 μm2 with 200 x 50 points).
c) Confocal PL Imaging
For confocal PL imaging the same laser asfor Raman imaging was used. In this case a
specially selected 50x NA 0.8 objective waschosen which allowed the best throughput for the PL signal (in the range of 1000- 1250 nm for Si) while showing minimal
chromatic aberration between the excitation wavelength (532 nm) and the PL peak.
A 100 μm core diameter multimode fiber
was used to collect the signal and deliver
it to a SpectraPro 2300i mirror-based spectrometer using a 150 g/mm grating and an
InGaAs CCD camera 1024 pixels in width. A
scan of 50 x 50μm2 with 100 x 100 points
and 0.21 s integration time per spectrum
was performed.
d) Atomic Force Microscopy
The measurements were performed in AC
mode using a force-modulation cantilever(Nanoworld) with a resonance frequencyof 87 kHz. The first overview scan was performed on an area of 50 x 50 μm with 512
x 512 points. A second scan was performed
with 14 x 14 μm2 and 256 x 256 points.

Results
The lithography process for creating thestructure took approximately 2 minutes atthe specified laser power.
Following this, the white-light image asshown in Figure 1 could be seen. The image
was recorded with the WITec Control software using the built-in video camera.
Following the planar confocal Raman
image scan, Figure 2a could be extracted
from the integrated intensity of the firstorder Si band near 520 rel. cm-1.
Figure 2b shows the intensity profile along
the blue cross section shown in Figure 2a.
The image as well as the cross section
show an increased intensity of the Si signal close to the laser written structure aswell as a strong decrease exactly along it.
The cross section reveals a FWHM of theintensity profile of 680 nm. As the(a)system is diffraction-limited, the laser
for writing the structures was as narrowas 355 nm full width at half maximum(FHWM) (0.6xl NA = 0.6 x 532 nm/0.9 = 355nm). When scanning the same structures
again with diffraction-limited resolution
the Gaussian profile of the laser during theRaman measurement is convoluted withthe geometry of the structure itself. Since
the scribing is a thermal process, it is not
surprising that the detected structures areslightly larger than diffraction-limited. The
brighter and darker areas in Figure 2a suggests that this could be caused by differences in the height of the sample. In such acase the brighter areas could be lying in an
optimal focal plane and the dark areas inplanes which are not well in focus.
In order to check this, a depth scan was(b)performed along the line marked ingreen in Figure 2a. The result can be seenin Figure 3a where the integrated intensity of the first order Si peak is shown.
It can clearly be seen that the intensity
originates from approximately the sameheight and that only very little signal isoriginating from the positions where thelaser-written structures are. It can additionally be noted that brighter areas arelocated right next to the structures. This
is in agreement with the results as seenin Figure 2a where higher intensity is detected right next to the structures. Figure
3b shows the intensity profile extracted
along the turquoise line. The sharpness of
this profile is partly due to the high depth
resolution achievable with the systemand partly due to the limited penetration
depth of the laser light into Si.

680 nmFig. 2: Confocal Raman image of the intensity of the first order Si line [a] and the intensity profile along the cross section marked in blue [b]. The green
line in [a] indicates where the depth scan was performed.
(a)(b)
880 nmFig. 3: Confocal Raman image of the intensity of the first order Si line along a depth scan [a] and the intensity profile along the cross section marked inturquoise [b].

In order to probe the strain state of the Siaround this structure, the data acquired in
the planar as well as in the depth scan wasanalyzed in terms of peak shift of the firstorder Si peak. For the analysis, the advanced fitting option in the WITec Project Plus
software was used and a Lorentzian curve
was fitted to the spectra. From the results
the exact position of the peaks could be
extracted and this is plotted for the planarscan in Figure 4a and for the depth scan in
Figure 4c. Figure 4b shows two representative, averaged spectra with the corresponding fitted Lorentzian curves. The red curveis representative of the red regions and theblue curve of the blue-purple regions. The
peak shift can clearly be seen from thisand it is also obvious that the Lorentzian
fit follows the data points extremely well,(a)which also demonstrates the imaging
quality of the UHTS 300 spectrometer (seealso [5]). The shift to lower wavenumbers
at the edges of the structures is obvious inboth scans. This is indicative of compressive stress in the crystalline structure atthese positions.
In order to further examine the laser-written scribed structures, a confocal PL scan
was performed as well. Figure 5a shows
the integrated intensity of the Si PL signal
and in Figure 5b the PL spectrum itself isshown. This spectrum was acquired with a
210 ms integration time right in the centerof the structure and only the camera background signal was subtracted (no smoothing or averaging was applied).
(b)

(c)

(a)

The second order peak of the excitation
wavelength at 1064 nm can also be seen inthis spectrum. As Si is an indirect semiconductor, the PL efficiency is extremely
low compared to i.e. GaAs. Therefore, only
a highly effective detection beam pathallows the measurement of the PL signal
from a diffraction-limited point with integration times similar to those necessary for
Raman imaging.
Similar to the previously seen confocal
Raman image (Figure 2a), the intensity ofthe signal is stronger close to the structures. The signal was also examined for peakshifts, but did not show a significant change. It has been shown, however, that moreelaborate fitting models also allow theidentification of the stress in Si structuresfrom the PL signal [4].

Fig. 4: Confocal Raman image of the positionof the first order Si line in the planar scan [a]and in the depth scan [c]. Two representative
spectra showing the shift are shown in [b].
(b)Second Order
excitation peakFig. 5: Confocal PL image of the intensity of
the PL signal of Si in a planar scan [a] and a
representative PL spectrum which originatesfrom the center of the structure [b].

The PL and the Raman scans clearly showthat where the laser writing took place,the structures emitted weaker Raman andPL signals. Looking at the depth scan, however, it seemed puzzling that inside thestructures no signal was detected. If thelaser writing had generated grooves then
one would have expected the signal at alower depth if the grooves were not extremely narrow and deep. In order to clarify
this question, AFM scans were performedon the structure. Figure 6a shows an AFM
overview scan and Figure 6b a zoomed-in
scan focusing on one of the arrows.
Both AFM images clearly show higher
topography along the structure and from
Figure 6b it can clearly be seen that dot-like structures are present along the written(a)structure. At a distance of more than 10
μm from the structure, none of these features were visible. The extent of the elevated structures range from about 1 to 2 μm
FWHM and the dots approximately range
in diameter from 300-600 nm. The AFM
images indicate that significant structural changes occurred in the Si where thelaser writing took place. The significantly
lowered Si signal near 520 rel cm-1 in
combination with the AFM images could
indicate that the crystalline structure of
the Si was destroyed. In such a case one
could expect to see traces of amorphous
Si along the structures. In order to checkthis, the average spectrum far away fromthe structure and the average spectrum
inside the grooves were calculated fromthe planar Raman data set and are shownin Figure 7.
The spectra clearly show the strong
decrease in the intensity of the first order
Si peak, but also an increase in the signal
background. It is striking that the background is higher on the low-wavenumber
side of the Si peak compared to the highwavenumber side of it. Since amorphous
Si shows a broad peak in this region, one
could postulate that the material generated through the laser writing is in someways related to amorphous Si.

(a)

(b)

(b)
Fig. 6: AFM scan of thestructure. Overview [a] andzoomed-in scan on one of
the structures [b].

Fig. 7: Raman spectra well outside the structure(red) and inside the grooves (blue).

Conclusion

References

In a single microscopy system laser lithography, confocal Raman imaging, confocal
PL imaging and AFM were performed on acrystalline Si sample. All experiments werecontrolled through the same electronics(alphaControl) and the same software(WITec Control). The written structures areclearly detectable with all measurement
techniques. The Raman as well as the PL
signal originating from crystalline Si isstrongly reduced at the laser written
structures and a broad enhancement of
the signal background of the Raman Si signal can be detected. AFM measurementsallowed the exclusion of narrow grooves
being the origin of the reduced Raman andPL signal and showed that the structuresconsist of dot-like elevations.

[1] http://www.intel.com/content/www/
us/en/silicon-innovations/silicon-technologies-general.html[2] Gigler A.M., Huber A.J., Bauer M., Ziegler
A., Hillenbrand R. and Stark R.W. (2009),
Nanoscale residual stress-field mapping
around nanoindents in SiC by IR s-SNOMand confocal Raman microscopy.
Optics Express, 17, 22351-22357.
[3] Wermelinger T., Borgia C., Solenthaler C.
and Spolenak R. (2007), 3-D Raman spectroscopy measurements of the symmetryof residual stress fields in plastically deformed sapphire crystals. Acta Materialia, 55,4657-4665.
[4] Gundel P., Schubert M.C. and Warta W.
(2009), Simultaneous stress and defectluminescence study on silicon. Phys. Status
Solidi A, 207, 436-441.
[5] Dieing T. and Hollricher O. (2008),
Highresolution, high-speed confocal
Raman imaging. Vibrational Spectroscopy
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