SI WAFER MANUFACTURING BY THERMAL SPRAY OF RECYCLED SI POWDERS

Abstract

The aim of the present work is to demonstrate processing of polycrystalline free-standing Si wafers and thin Si layers on various substrates using thermal spray of relevant Si powders, including recycled ones. Silicon powders were fabricated by milling from two different Si feedstocks: (i) Si kerf produced during sawing of Si wafers from the ingots, and (ii) Si powder from Si scrap (broken wafers). Two different Si powder based structures were produced by thermal spray: (i) free-standing Si-based wafers with thicknesses of about 300-400 micrometres and sizes up to 6 inches, which can be used as supporting parts for "Si wafer equivalent" structures, and, (ii) Si-based layers with thicknesses of about 50-100 micrometres deposited on different supporting substrates, such as aluminium, glass and highly conductive Si powder based sintered substrates. Raman spectroscopy, resistivity, and SEM analyses have been used for characterization of the Si based structures fabricated by thermal spray. It is observed that such layers are poly-crystalline with a low-fraction of the amorphous phase and can be considered as a low-cost alternative for Si PV, which are based on utilization of Si wafers and thin Si layers deposited by conventional methods such as CVD, PECVD, e-beam, etc.

Full text

SI WAFER MANUFACTURING BY THERMAL SPRAY OF RECYCLED SI POWDERS
M. Vardavoulias*1, A. S. Azar2, T. Halvorsen3 , M. Moen3, K. Mork3, P. A. Carvalho2, Ø. Dahl4, A. Ulyashin2
1 Pyrogenesis, Lavrion, Greece
2 SINTEF, Oslo, Norway
3 RESITEC, Kristiansand, Norway
4 SINTEF, Trondheim, Norway
*Corresponding author: mvardavoulias@pyrogenesis-sa.gr
ABSTRACT: The aim of the present work is to demonstrate processing of polycrystalline free-standing Si wafers and
thin Si layers on various substrates using thermal spray of relevant Si powders, including recycled ones. Silicon powders
were fabricated by milling from two different Si feedstocks: (i) Si kerf produced during sawing of Si wafers from the
ingots, and (ii) Si powder from Si scrap (broken wafers). Two different Si powder based structures were produced by
thermal spray: (i) free-standing Si-based wafers with thicknesses of about 300-400 micrometres and sizes up to 6 inches,
which can be used as supporting parts for "Si wafer equivalent" structures, and, (ii) Si-based layers with thicknesses of
about 50-100 micrometres deposited on different supporting substrates, such as aluminium, glass and highly conductive
Si powder based sintered substrates. Raman spectroscopy, resistivity, and SEM analyses have been used for
characterization of the Si based structures fabricated by thermal spray. It is observed that such layers are poly-crystalline
with a low-fraction of the amorphous phase and can be considered as a low-cost alternative for Si PV, which are based on
utilization of Si wafers and thin Si layers deposited by conventional methods such as CVD, PECVD, e-beam, etc.
Keywords: Thermal Spray, Silicon, c-Si, Si-Films, Thin Film, Substrates, Grain
1 INTRODUCTION
Thermal spraying technology has been employed for
many years to synthesize high performance coatings of a
wide range of materials, including Si [1-6]. In general,
the process of thermal spraying involves feeding a
powder based feedstock into a high-energy plasma or
flame, in which powder particles are heated, partly or
fully melted, and simultaneously accelerated, towards the
target surface of a substrate. Silicon particles cool and
then solidify onto the substrate, forming a re-crystallized
silicon layer. Two commonly used processes for various
applications are plasma spray (PS) and flame spray (FS)
[7]. It has been shown earlier, that in plasma sprayed
silicon conversion efficiencies of thin Si based solar cells
of 4.3% can be achieved [4]. However, it should be noted
that in flame spraying, relatively low temperatures can be
reached in the flame (~3000 °C) compared to common
plasma spraying ones, which are quite high (~15000 °C).
Therefore, FS has some advantages compared to plasma
spray and can be considered as an appropriate method to
produce Si coatings on various substrates. In particular,
as soon as Si particles are heated to lower temperatures
upon FS compared to PS, cooling of such heated/melted
particles can be done much faster in case of FS process
and a wider range of the supporting substrates for the
deposition of Si layers can be used. The aim of the
present work is to show the viability of flame spraying
for the fabrication of free-standing Si wafers and thin
layers on a wide range of supporting substrates, which
can be of interest for the low-cost Si based PV.
2 EXPERIMENTAL PROCEDURE
2.1 Si powders and relevant structures processed by
thermal spray
Silicon powders with particle sizes in the range of
about 10-50 µm were fabricated by conventional ball
milling from two different Si feedstocks: (i) Si kerf
produced during sawing of Si wafers from the ingots, and
(ii) Si powder from Si scrap (broken wafers). Two
different Si powder based structures were produced by
the flame based thermal spray: (i) free-standing Si wafers
with thicknesses of about 300-400 µm and sizes up to 6
inches, and, (ii) Si-based layers with thicknesses of about
50-100 µm deposited on different supporting substrates,
such as aluminium, glass, ceramics and highly conductive
sintered Si powder substrates.
2.1 Characterization
Raman spectroscopy, resistivity and SEM analyses
have been used for the characterization of Si powder
based free-standing wafers and thin Si layers deposited
on different substrates by flame based spray.
3 SILICON POWDER BASED WAFERS AND THIN
LAYERS DEPOSITED BY FLAME SPRAYING
3.1 Free standing wafers
Optimization of thermal spray parameters resulted in
reproducible fabrication of mechanically stable Si wafers
with dimensions 50x65 mm2 (Figure 1), 156x156mm2
(Figure 2) and thicknesses between 300 and 500
microns.
Figure 1: Free standing 50x65 mm2 Si powder based
wafers

Figure 2: Free standing 156x156 mm2 Si powder based
wafer
SEM images taken at different magnifications of the
surface of a thermal sprayed wafer are shown in Figure
3.
Figure 3: SEM images of the surface of a free standing
thermal sprayed wafer
From Figure 3 it can be concluded that surface of Si
powder based wafers consists of grains with different
shapes, which are assembled in a "naturally" textured Si
based structure.
Figure 4 shows results of Si line Raman mapping for the
surface area (100x100 µm2) of a free-standing Si powder
based substrate.
Figure 4: Si line Raman mapping: free standing Si
powder substrate, surface area 100x100 µm2.
From Figure 4 it can be concluded that Si wafer,
sintered by thermal spray, consists of mainly poly-Si
grains with small portion of the amorphous Si phase
inclusions.
As an option, resistivity of such wafers can be regulated
by the mixing of Al and/or B powders prior to thermal
spray. Table 1 shows results of the resistivity
measurements for Si wafers processed by thermal spray
of a mix of Al/Si powders with different proportions of
Al and Si.
Table 1: Resistivity of free standing
Al content in Si
powder
Resistivity, Ωcm
0%
10
1%
0.6
3%
0.47
5%
0.024
10%
0.018
15%
0.002
From Table 1 it can be seen that highly conductive Si
based wafers can be sintered by thermal spray of Al/Si
powders mixtures.
3.2 Si powder based layers deposited on Al substrates by
thermal spraying
Figure 5 shows SEM images taken at different
magnifications of the surface of a thermal sprayed pure Si
powder based layer on an Al substrate and Figure 6
shows results of the Si line Raman mapping for the
surface area (100x100 µm2) for the same structure.
From Figures 5 and 6 it can be concluded that similar to
the case of free-standing Si wafers, surface of Si powder
based wafers consists of grains with different shapes and
different degree of crystallinity. Mainly poly-Si phase is
dominating, with a small portion of the a-Si inclusions. It
should be noted that grains in this case are quite smooth
and grain boundaries are not as pronounced as in case of
free-standing Si powder based wafers processed by
thermal spraying. It can be speculated that presence of an
Al substrate provides specific conditions for the

crystallization of partly melted Si particles on the Al
surface. More detailed analysis is required to clarify the
origin of the observed effect.
Figure 5: SEM images of the surface of Si powder based
thin layer deposited on an Al substrate.
Figure 6: Si line Raman mapping: Si powder based layer
(~100 µm thick) deposited on an Al substrate, surface
area 100x100 µm2.
3.3 Si powder based layers deposited on glass substrates
by thermal spray
Figure 7 shows surface and cross section SEM
images of a thin Si layer deposited on a glass substrate by
thermal spray. Quite dense Si layer with a "naturally"
textured Si surface can be formed, as can be seen from
Figure 7. Several thermal spray scanning velocities were
tested in this case and crystallinity of Si layers obtained
at different conditions were monitored by micro-Raman.
Upon Raman measurements, laser beam was focused on a
central part of the largest grains, observed in an optical
microscope.
Figure 7: Surface and cross section SEM images of a
thin Si layer deposited on a glass substrate by thermal
spray.
Figure 8 shows results of Raman measurements for the
optimized thermal spray process (scanning velocity 5).
Raman spectra were taken from the centre of 2 different
grains and are indicated in Figure 8 as Si peak (1) and Si
peak (2).
480 500 520 540 560
0
8000
16000
24000
Si/glass, velocity 5
Si peak (1) 519.68 cm-1
Si peak (2) 520.25 cm-1
Intensity (a.u.)
Raman shift (cm-1)
Si reference, polished
Cz Si substrate
(red color),
Si peak 520.83cm-1
Figure 8: Raman spectra of Si lines taken from 2
different grains of Si powder based layer deposited on a
glass substrate with an optimized scanning velocity.
From Figure 8, it can be seen that Si grains are
crystalline, in contrast to similar Raman measurements
for Si powder layers deposited on a glass substrate with
the not optimized scanning velocity (velocity 3) upon the
thermal spray process (Figure 9).

480 500 520 540 560
0
8000
16000 Si/glass, velocit y 3
Si peak (1) 5 19.1 cm-1
Si peak (2a) 508.68 cm-1
Si peak (2b) 5 19.68 cm-1
Intensity (a.u.)
Raman shift (cm-1)
Si reference, polished
Cz Si substrate
(red color),
Si peak 520.83cm-1
Figure 9: Raman spectra of Si lines taken from 2
different grains of Si powder based layer deposited on a
glass substrate with the non-optimized scanning velocity
In this case, Si peak (1) (519.1cm-1) is shifted from the
standard peak position of crystalline Si (520 cm-1), and Si
peak (2) consists of 2 well pronounced lines – 508.68 cm-
1, which can be attributed to a poly-Si and 519.68 cm-1
line, which can be attributed to a stressed crystalline Si
grain. From Figures 8 and 9 it can be concluded that
scanning velocity in Si powder thermal spray process is
an important parameter, which should be taken into
account for the process optimization. In an optimized
case, substrate is heated by flame to the most suitable
temperatures, which can provide proper crystallization of
Si powders and relevant Si layers.
3.4 Si powder based layers deposited on Si powder
sintered substrates by thermal spray
It has been established, that sintering of Si powder
based substrates, including highly conductive ones, can
dramatically reduce the thermal budget required [8-11].
Such sintered wafers can be used as the low cost
supporting substrates, on which high quality Si layers
(solar cell base) can be deposited to fabricate "Si wafer
equivalent" [12]. Figure 10 shows results of the Si line
Raman mapping for the surface area (100x100 µm2) of a
thin Si layer deposited on top of the hot pressed and
sintered Si powder based substrate [9, 11].
Figure 10: Si line Raman mapping: Si powder based
layer (~100 µm thick) deposited on a sintered Si powder
based substrate.
From Figure 10 it can be seen, that only poly-Si and
crystalline Si phases are present in this case, without any
traces of amorphous silicon, which was observed in
previous cases (free-standing wafers, Al and glass
substrates). It can be concluded that the origin of a
supporting substrate plays an important role for the
resulting crystallinity of Si layers deposited on supporting
substrates by thermal spray of Si powders.
5 CONCLUSION AND OUTLOOK
It was demonstrated that Si powder based structures –
free-standing Si wafers or thin Si films can be processed
by the flame based thermal spray. Important to note that
thin Si layers can be deposited on several types of low-
cost substrates (Al, glass, Si) with the low temperature
melting points. It is shown, that thermal spray processing
conditions and the origin of the supporting substrates
play an important role for the crystallinity of the thermal
spray deposited Si layers.
It is established that fabricated at appropriate thermal
spray processing conditions, such layers are poly-
crystalline with a low fraction of the amorphous phase
and can be considered as a low-cost alternative for Si PV,
which is based on utilization of Si wafers and thin Si
layers deposited by conventional methods such as CVD,
PECVD, e-beam, etc.
ACKNOWLEDGEMENT
This project has received funding from the European
Union’s Horizon 2020 research and innovation
programme under grant agreement No. 641972
(CABRISS).
REFERENCES
[1] R. Suryanarayanan, G. Brun, M. Akani, Thin Solid
Films 119 (1984) 67–73.
[2] B. Kayali, M. Rodot, R. Suryanarayanan, L.
Quangnam, R. Mghaieth, R. Gauthier, P. Pinard,
Materials Science and Engineering B 5 (1989) 51–56.
[3] M. Akani, R. Suryanarayanan, G. Brun, Thin Solid
Films 151 (1987) 343–353.
[4] F. Tamura, Y. Okayasu, K. Kumagai, Solar Energy
Materials and Solar Cells 34 (1994) 263–270.
[5] B.D. Kharas, G. Wei, S. Sampath, H. Zhang, Surface
and Coatings Technology 201 (2006) 1454–1463.
[6] D.J. Varacalle, H. Herman Jr., G.A. Bancke, T.D.
Burchell, G.R. Romanoski, Surface and Coatings
Technology 49 (1991) 24–30.
[7] A. Nistal, E. García, C. García-Diego, M.I. Osendi,
P.Miranzo. Surface & Coatings Technology 270 (2015)
8–15.
[8] Peter, K., et al., Solar Energy Materials and Solar
Cells 74 (2002) 219-223
[9] G. Stokkan, A. Johanson, E. Øvrelid, A. Ciftja, J.
Hennicke, A. Ulyashin Proc. EUPVSEC 28, Paris,
2013, pp. 427-430.
[10] Bellanger, P., et al., Journal of Crystal Growth 359
(2012) 92-98.
[11] R. Buchwald, S. Würzner, H. J. Möller, A. Ciftja, G.
Stokkan, E. Øvrelid, A. Ulyashin Phys. Status Solidi
A 212, (2015) 25–29.
[12] Reber, S., et al., Solar Energy Materials and Solar
Cells 65 (2001) 409-416