NOVEL INDUSTRIAL ATMOSPHERIC PRESSURE

NOVEL INDUSTRIAL ATMOSPHERIC PRESSURE DRY TEXTURING PROCESS FOR SILICON SOLAR
CELL IMPROVEMENT
B. Dresler1, D. Köhler1, G. Mäder1, S. Kaskel1, E. Beyer1, L. Clochard2, E. Duffy2, B. Kafle3, M. Hofmann3, J. Rentsch3
1
Fraunhofer Institute for Material and Beam Technology (IWS), Winterbergstr. 28, D-01277 Dresden, Germany
2
Nines Photovoltaics, Synergy Centre, IT Tallaght, Dublin 24. Ireland
3
Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstr. 2, D-79110 Freiburg, Germany
Corresponding author: Gerrit Mäder, Fraunhofer IWS, email: [email protected]
ABSTRACT: The front texture of crystalline silicon solar cells plays a crucial role in order to effectively harvest light
and transform it into electricity. In this paper, a novel technology based on dry atmospheric pressure etching is
presented. It allows the single-sided inline etching of c-Si wafers using the global warming potential-free process gas
fluorine (F2). Vast improvements of light trapping are already achieved leading to weighted reflection values below
8% without dielectric anti-reflection coating. The passivation of the relatively rough surfaces can most effectively be
facilitated using ALD-Al2O3 films stacked with PECVD-SiNx layers. The paper presents SEM images of the surface
structures and the basic setup principle of the new production tool.
Keywords: Etching, texturisation, silicon solar cell, atmospheric pressure, dry etching, dry processing
1
INTRODUCTION
The current water usage in the photovoltaic (PV)
solar cell manufacturing industry is not sustainable in
some parts of the world. The PV solar industry as a
whole has been growing dramatically over the last 10
years [1]. PV solar is indeed recognised as the renewable
energy alternative that can meet our global energy needs
for the foreseeable future. Companies involved in this
industry have been growing significantly, answering the
demand, and scaling their production capacity
accordingly. Equipment and processes developed by cell
manufacturers have been for most adopted and scaled up
from semiconductor manufacturing. However, the valueadd and the cost structures for both sectors are vastly
different despite the manufacturing processing
technologies being similar. As the industry continues to
increase its capacity, very large footprint factories that
have heavy consumption of chemicals, water and
potential emissions of high Global Warming Potential
(GWP) gases become unsustainable. New regulations,
including the Kyoto agreement, will enforce strict control
on water management and emissions. The availability of
environmentally friendly production technologies that
can cope with emission regulations in Europe will be
crucial for the continuity of cell manufacturing in the EU.
At the same time EU regulations are expected to further
restrict the use of production technologies with high
GWP.
Silicon etching is a key technology in various
processing steps during the production of PV solar cells:
- Removal of sawing damage that has occurred
during the “wafering” process,
- Texturing of the Si wafer to reduce its reflection
prior to the formation of the emitter,
- The removal of the residual phosphorus silicate
glass (PSG) that grows during the hightemperature emitter-formation step.
Currently, most of the etching steps are carried out
using wet chemistry equipment; the wafers are moved
across large baths containing chemical liquid mixtures.
Large volume of water is also required for rinsing after
etch steps.
Dry processing is a generic term that indicates all
technologies that make little or no use of water. It has
been seen as a potential alternative to the wet benches in
the PV industry. However, despite being demonstrated in
the lab [2, 3], there has been no uptake of this dry
vacuum-based technology. This is mainly due to:
- Large price tag and cost of ownership (COO) of
the vacuum and plasma equipment
- Limited throughput
- High GWP gases required by the plasma etching
chemistry (NF3, SF6).
Within the EU SME project SOLNOWAT, 4 different
SMEs along with 3 research institutes try to overcome
these limitations by developing a novel dry etching
technology featuring
- use of 0 GWP gases (Fluorine)
- dry Atmospheric Process (AP) using thermal
activation for the etching steps
- non-contact ultra sound air bearing wafer
transport
- process monitoring solutions adapted to the use
of Fluorine.
This paper discusses first results of the SOLNOWAT
project on atmospheric pressure etching of crystalline
silicon using F2 gas.
2
EXPERIMENTAL SETUP
All experiments were carried out in a lab scale
atmospheric pressure reactor located at Fraunhofer IWS
in Dresden. The reactor exhibits a thermal etching zone
and a double waste gas extraction system, as etching gas
thermally activated diluted F2 is used. The working
principal of the reaction chamber is depicted in Figure 1.
The reactor can process wafers with size up to 156x156
mm². It allows for both static and continuous passthrough dynamic etching. The system is designed to
provide a dry chemical process that is single sided.
Based on the results and the experience of the labtype demonstrator, an automated process development
tool has been designed and built by Nines Photovoltaics
and will be shipped and installed at Fraunhofer ISE
(Figure 2) for full cell process integration.
Figure 1: Operating principle of the dry-chemical etching
technology at atmospheric pressure using climate neutral
etching gas used at Fraunhofer IWS.
3
OPTICAL PROPERTIES
3.1 Surface morphology
In order to remove the remaining saw damage from
the surface remaining from the wafer cutting, an etch
depth of around 4 to 5 µm per side is required. The first
studies concentrated on using the dry etching to produce
a low reflectivity surface texture from various multicrystalline silicon wafer starting surfaces. The dry AP
etching process is able to create various surface structures
(from more porous up to flattened surface morphologies)
depending upon the variation of the process parameters,
namely total gas flow, concentration, process temperature
and also hardware configuration of the way the gas is
delivered to the wafer. Figure 3 shows scanning electron
microscope (SEM) images of the different AP-etched mcSi starting surfaces produced for our first trials. Picture A
represents a typical wafer surface directly after sawing
and cleaning. Applying the AP etch with increasing etch
depth (B, C) first of all the silicon debris originating from
the cutting process are removed, while continuing the AP
etching the surface is roughened again and a
microstructure is formed. A Similar sequence can be
observed when starting from a wet chemically KOH
etched surface (picture D). The subsequent AP etching
leads to a strong surface enlargement forming feature
sizes in the sub-micrometer range (picture E). A more
porous surface structure results from an AP etch after
complete HF-HNO3-H2O iso-texturing (F), although the
final surface also depends very much on the process
parameters of the dry etch.
3.2 Surface reflectance
As it was shown in Figure 3, parameter settings as
well as wet chemical treatments prior to the AP etching
process may lead to very different surface morphologies.
However, reflectance measurements reveal nearly similar
optical properties over a broad wavelength range (see
Figure 4). All AP etched wafer surfaces reach weighted
reflectance values of around 7.5 % measured directly
after texturing. The corresponding wet isotextured wafer
surface reaches only values slightly above 30%. The
reflectance values were weighted with a typical IQE of a
multicrystalline Al-back surface field cell and with the
sun spectrum AM1.5g.
100
KOH + AP etch, Rw = 7.5 %
Isotexture 3µm + AP etch, Rw = 7.5 %
80
reflectance R [%]
Figure 2: Picture of the automated atmospheric pressure
inline dry etching tool (Nines PV) that will be set up at
Fraunhofer ISE.
Figure 3: SEM overview of dry textured surface
morphologies starting from various wafer surfaces: as-cut
(A-C), KOH saw damage etch (D,E), and wet isotexture
(F). References: as-cut (A) and KOH saw damage etch
(D).
A: as-cut wafer before AP etching
B,C: as-cut wafer with dry AP etching
D: wet saw-damage-etched (SDE) wafer (no AP etching)
E: SDE wafer with dry AP etching
F: wet isotexture with dry AP etching
Isotexture Std. + AP etch, Rw = 7.4 %
60
Isotexture reference, Rw = 30.3 %
40
20
0
400
600
800
1000
1200
wavelength  [nm]
Figure 4: Reflectivity measurements of different wafer
surfaces etched with F2. Weighing is performed with the
solar spectrum as well as the internal quantum efficiency
of a mc-Si solar cell between 300 and 1200 nm
4
ELECTRICAL PROPERTIES
Rear side
SiNx / SiOxNy
SiNx / SiOxNy
SiNx / SiOxNy
SiNx / SiO2
Front side (dry text.)
SiNx
Al2O3 / SiNx
SiOxNy / SiNx
SiO2 / SiNx
Plasma-enhanced chemical vapour deposition (PECVD)
was used to deposit the SiNx and SiOxNy layers
(Roth&Rau SiNA). The SiO2 layers were thermally
grown (Centrotherm). The Al2O3 layers were created by
atomic layer deposition (ALD) using an Oxford OPAL
system. The samples’ minority carrier lifetimes were
measured twice, before and after the firing process.
Figure 5 shows the process matrix at a glance.
Wet chemical pre-processing (SDE,
isotexture 3µm or standard isotexture)
AP dry etch both sides
Reflection measurement
Cleaning (wet)
PECVD SiOxNy (rear)
PECVD
SiNx
(front)
ALD
AlOx +
PECVD
SiNx
(front)
PECVD
SiOxNx
(front)
Therm.
SiO2 +
PECVD
SiNx
(front
and rear)
micron texture influence directly the carrier lifetime
results.
SiNx
eff. minority carrier lifetime [µs]
4.1 Carrier lifetime results
In order to investigate the influence of the AP
textured surfaces, float-zone (FZ) crystalline silicon
wafers (p-type, 1 Ohm cm, 250µm thick) were first wet
chemically etched to create different starting surface
morphologies (KOH saw damage etch, acidic
isotexturisation (only 3µm of silicon removed) or acidic
isotexturisation (standard amount of silicon removed)).
As a reference, standard isotextured surfaces were also
prepared, without a subsequent AP dry etch. All samples,
except the reference wafers, were subjected to the same
AP dry etch (Nines PV). After measuring the reflection
properties, a wet chemical cleaning was performed prior
to the deposition on both surfaces of a variety of
passivation layers. The samples were fabricated into the
following stacks:
100
FZ-Si as-cut 1 cm
SiOxNy
ALD-AlOx
SiO2+SiNx
10
1
saw damage isotexture isotexture isotexture
etch + AP 3µm + AP std. + AP no AP
Figure 6: Overview of QSSPC measured effective
minority carrier lifetime values of differently passivated
AP etched samples from different starting wafer surfaces.
Substrate material: FZ-c-Si, p-type, 1 Ohm cm, 250 µm,
(100) oriented.
4.2 SEM investigation
In order to further understand the carrier lifetime
results, SEM cross section investigations were carried out
for some of the passivated samples. The SEM images
show that for the texture produced in this first batch, the
conformality of the SiNx PECVD process is not
sufficient to effectively cover and passivate the entire
front surface of the textured wafer. Some AP etched
samples also show some relatively deep tunnel-like
openings making passivation quite challenging without a
process with excellent step coverage properties. From
that point of view, ALD provides a clear benefit over the
other deposition techniques. The ALD process is much
more conformal, and this is reflected in the lifetime
results.
PECVD SiNx (rear)
QSSPC lifetime measurement
Contact firing
QSSPC lifetime measurement
Figure 5: Process flow for the preparation of the
symmetrical wafer samples for QSSPC lifetime
measurements.
From the lifetime results, we can therefore conclude
that the passivation of a standard isotexturised surface is
easier than for the AP etched surface morphologies
produced for this first trial. As can be seen in Figure 6,
the minority carrier lifetimes of the AP etched samples
reached significantly lower values than the reference. For
the dry texture, passivation with an ALD-Al2O3 +
PECVD-SiNx stack layer system is clearly the most
efficient approach. The surface enlargement of the sub-
Figure 7: SEM cross section of AP etched and ALDAlOx/SiNx passivated wafer surface.
5
CONCLUSIONS
As an alternative to the current wet-chemical etching
steps used in crystalline solar silicon industry, a drychemical etching technology at atmospheric pressure was
developed using elemental fluorine as etching gas
(climate neutral etching gas). The novel cost effective
technology offers single side treatment of the wafers, inline vacuum-less solar cell processing, reduced wafer
breakage due to soft handling, decreased chemical waste
disposal and significant potential for costs saving in a
production environment.
First results for a texturing process show excellent
potential for the production of low reflectance surfaces.
The first textures produced were passivated using various
layers and deposition processes. The sub-micron structure
is more efficiently passivated using a conformal ALD
deposition of AlOx. This first batch represents a starting
point for this project. More texture variations with the AP
etching tool will be produced, aiming to reduce the aspect
ratio of the texture to allow for an easier passivation
while preserving the good optical properties. Beyond
texturing, other processes such as back side emitter
removal and surface flattening are also being developed
within this project.
The technology will enable solar cell producers to
dramatically lower their water consumption and enable
the processing of more advanced cell concepts, with the
high throughput required in the industry.
6
ACKNOWLEDGEMENT
The research leading to these results has received
funding from the European Union's Seventh Framework
Programme managed by REA under grant agreement n°
286658.
References:
[1] A. Jäger-Waldau, Quo vadis photovoltaics 2011,
European Physical Journal Photovoltaics 2, 20801
(2011)
[2] S. Schaefer, Plasmaätzen für die Photovoltaik, PhD
thesis, University of Konstanz, Konstanz (2000).
[3] G. Agostinelli et al., Dry Etching And Texturing
Processes For Crystalline Silicon Solar Cells, Proc.
19th EUPVSEC (2004), Paris

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