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1. Introduction
Glasses, due to their unique properties, are widely used in the
optical devices, MEMS devices, and solar cells. The optical
transparent and the ability to withstand much light scattering
influences can be considered as the most remarkable properties
in this respect. Glass is a combination of the relative ease with
which the material can give the desired shape due to its unique
the light trapping properties of the solar cell.
The thin film silicon solar cell is a great potential as photovol-
taic devices. The production on a large scale in a fully automated
manner allows glass substrate and low material usage, low cost
per watt is better than crystalline Si solar cells.1,2) In thin film
silicon solar cells, hydrogenated amorphous silicon (a-Si:H) or
microcrystalline silicon (μc-Si:H) used as an absorption layer.
To have an effectively light absorption to the solar cell, light
management in the textured surface is necessary for the high-
efficiency solar cell. Surface texturing can result in enhanced
scattering from transmitted light and then low reflection on the
surface of the incoming light.3)
Textured surface increases the optical path length, which is
improved the photocurrent and quantum efficiency as well. To
make the surface morphology of the light trapping, it obtained
by etching process like wet chemical, dry and direct pattern.4-6)
Textured surface allowed more light scattering and low reflection
at rough internal interfaces, leading to more efficient light trapping
and subsequent increase of light absorption in the solar cell.
Textured surface regarding light trapping carried out on the
transparent conductive oxide layer (TCO) of thin film silicon solar
cells. Chemical etched AZO films shows better light trapping
properties, it is only efficient at shorter wavelengths, but with
relatively decreases at longer wavelengths. It must be obtained
to reduce the loss for better light scattering and internal reflection.
It can be readily dissolved by hydrofluoric acid or other HF
containing aqueous solutions at room temperature.7) Controlled
dissolution in HF-based etchants can be applied to remove
material using related glass application. Wet chemical etching
of glasses in aqueous HF solutions is a subject that has studied
for many years. Scheele et al. reported about the discovery of HF
in 17718) and then is being studied more extensively, due to the
solar cell application for the light trapping as the substrate.
For achievement of better light trapping using etching tech-
nique, it considered for scattering effect having a small-sized
structure in the short wavelength and large sized structure in the
long wavelength region, respectively. Recently, there are several
reports related to the etching by random, periodic using photo-
lithography pattern for light trapping; most research is rarely
systematic study for the substrate of thin film silicon solar cells.
In this study, the reaction mechanism, etch rate as well as the
surface morphology reviewed, and the effect of the glass types,
Current Photovoltaic Research 5(3) 75-82 (2017) pISSN 2288-3274
DOI:https://doi.org/10.21218/CPR.2017.5.3.075 eISSN 2508-125X
A Review of Wet Chemical Etching of Glasses in Hydrofluoric
Acid based Solution for Thin Film Silicon Solar Cell ApplicationHyeongsik Park1,3) ․ Jae Hyun Cho1) ․ Jun Hee Jung2) ․ Pham Phong Duy1) ․ Anh Huy Tuan Le1) ․ Junsin Yi1)*
1)College of Information and Communication Engineering, Sungkyunkwan University, Suwon 16419, Korea2)Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea
3)Electronic Convergence Materials & Device Research Center, Korea Electronics Technology Institute, Seongnam 13509, Korea
ABSTRACT: High efficiency thin film solar cells require an absorber layer with high absorption and low defect, a transparent conductive
oxide (TCO) film with high transmittance of over 80% and a high conductivity. Furthermore, light can be captured through the glass
substrate and sent to the light absorbing layer to improve the efficiency. In this paper, morphology formation on the surface of glass
substrate was investigated by using HF, mainly classified as random etching and periodic etching. We discussed about the etch
mechanism, etch rate and hard mask materials, and periodic light trapping structure.
Key words: Light trapping, Wet etching, Chemical, Hydrofluoric acid, Thin film, Solar cell
*Corresponding author: [email protected]
Received June 8, 2017; Revised June 14, 2017;
Accepted August 15, 2017
ⓒ 2017 by Korea Photovoltaic Society
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License
(http://creativecommons.org/licenses/by-nc/3.0)
which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
75
H.S. Park et al. / Current Photovoltaic Research 5(3) 75-82 (2017)76
etchant types and hard mask layers discussed. Finally, application
and conclusion described.
2. The etch mechanism, etch rate, and glass
types regarding random etching of glass
by wet chemical
2.1 The etching reaction; HF, HF/HCl, and HF/H2SO4
(mechanism)
For the etching of glasses, only hydrofluoric acid or other HF
containing aqueous solutions used. The chemical reaction
regarded as the following9):
SiO2 + 6HF → 2H2O + H2SiF6 (1)
This equation is a simplification of the reactions during the
heterogeneous SiO2 dissolution as shown in Fig. 1 (a). HF is
made up of the corrosive hydrogen ion (H+) and the toxic
fluoride (F-), thereby acting in two ways (see picture above).
The acid corrodes the glass surface and thus allows the toxic
fluoride ion to penetrate the glass. Once in the glass, the fluoride
ion binds themselves among others calcium and thus disturb the
other chemical components of the glass10,11). Glassy SiO2 consists
of tetragonal SiO4 units connected at all four corners to four
other SiO4 units by covalent siloxane bonds12). It is necessary to
break these bridging oxygen bonds to break down the network
and release silicon from the glass. HF, dissolved in water, is a
weak acid and its solutions containing, H+, F- and HF2- ions and
un-dissociated HF molecules13). The dissolution of glassy SiO2
is a heterogeneous reaction, which makes it difficult to study the
mechanism governing the dissolution process. However, the
breaking of all the chemical bonds, which results in eq. (1), will
require several reaction steps. At first, glass reacts with HF
forming silicon tetra-fluoride and water and SiF4 reacts with HF
to form H2SiF6, which is not soluble in HF solutions.
By adding to buffer acid agent such as HCl, HNO3, H3PO4
and H2SO4 to HF solution, the concentration of the more
reactive HF ion in the etchant is lowered, following equation (2)
and (3)14).
KHF
H・F (2)
KHF
HF ・F (3)
K1 = 6.74×10-4 mol/l, K2 = 0.26 mol/l
(a)
(b)
Fig. 1. A schematic of glass etch mechanism based on (a) HF, and (b) HF/H2SO4
H.S. Park et al. / Current Photovoltaic Research 5(3) 75-82 (2017) 77
Only in etchants containing both HF and concentrated H2SO4
were the etch rates higher than for HF/HCl or HF/HNO3 etchants,
an effect which described to the formation of active fluorine-
containing HSO3F acid. For further detailed information, the
role of buffer acid etchant from HCl to H2SO4 presented as
shown in Table 1.
A higher HF concentration generates the by-product H2SiF6
in the first reaction, and then the regeneration of the HF occurs
after the second reaction with the H2SO4 during the etching
process. The HF concentration continually maintained along
with the sulfuric-acid changes, and this then continues with the
increasing of the activation energy. The addition of H2SO4 is,
therefore, effective for the increasing of the etch rate and the
surface roughness. The HF-H2SO4 etching systems, an effect
that ascribed to the formation of the strong fluorine-containing
HSO3F acid15). From the etching of the surface, these surfaces
are less smooth than the mechanically polished ones, meaning
that it is possible to transform ground surfaces into optically
transparent surfaces as shown in Fig. 1 (b).
As mentioned above, the etch mechanism of HCl to HF is
slightly different. Initially, the HF solution etches the glass and
insoluble byproduct deposited on top of the glass surface. As the
time goes on progressed, the size of insoluble byproducts
becomes larger and accumulates to interfere with etching on the
glass. However, HCl (other oxide series) releases impurities in
HF-HCl mixed solutions [ref 16, JNN 2016 Park et al.]. Thus, it
can be seen that the similar shape as the crater surface appears as
shown in Fig. 2 (b).
The etch rate of agitation of the solution is related to the
adsorption or chemisorption on the siloxane bonds at the glass
surface dominating the dissolution process. The etching mechanism,
in particular, the role of the various fluorine-containing species,
has studied in more detailed by many researchers for the etching
rate8,15,17-19). Fluorine absorption complexes have observed at
hydrated SiO2 surfaces in gaseous HF. It changed into a surface
group such Si-F or Si-O-SiF3. The absorption of HF and HF2-
increases the electronic density on the bridging oxygen more
Table 1. The role of buffer acid etchant
Etchant The role of buffer acid etchant
HF
1) Involved in the etch rate of glass: fluorine containing species
2) Insoluble products generation as masking layers on glass substrate during etching: textured surface becomes rough and
non-uniformities
HCl
1) Significantly reduced roughness of side wall
: preventing the formation of alkaline earth fluorides with low solubility on the surface, the etching rate of HF+HCl etchant was not
sufficiently stable & can transform insoluble products to become soluble.
HNO3
1) Faster longitudinal etching rate
: Due to the strong oxidability of HNO3 which facilitates the reaction between HF and glass
H3PO4 Lower etching rate
H2SO4
1) HF+H2SO4 etchants: the formation of the strong fluorine-containing HSO3F acid. By etching the surface, these surfaces are less
smooth than mechanically polished ones; it is possible to transform ground surfaces into optically transparent surfaces.
Fig. 2. A SEM images of etched surface as a function of etching
solution (a) HF, (b) HF+HCl, and (c) HF+H2SO4
H.S. Park et al. / Current Photovoltaic Research 5(3) 75-82 (2017)78
basic, so more H+ ions are adsorbed, which leads to more
siloxane bonds being broken per time units, for example a kind
of catalytic effect. Determination of the etching rate is then the
destruction of the siloxane bond by the plus action of the
adsorbed species.
The catalytic action of H+ ions on breaking siloxane bonds
also occurs in the dissolution of glasses in acidic and weakly
alkaline solution18). Methods for the decision of the etch rate of
glasses are several ways as follows: The weight loss of dispersed
powders, partially masking the surface of a glass body, e.g. a
disk, by photoresist or wax. The etch rate for the glass is measured
by monitoring the depth of the recessed etched region after
etching of glass. It is slightly affected by agitation of the etchant
from the rotation, stirring or ultrasonic.19)
2.2 The surface morphology, etch rate, and the glass
types for etching of glass
Glass etching process not only removes surface products, but
it also changed the surface morphology of the glass depending
on the etching solution. When polished, the surface has slightly
etched the surface roughens and verge-like structures. The
experiment carried out to confirm the light trapping structure by
varying the type of glass as shown in Fig. 2. Here, HF solution
of ~50% was used as the glass etch solution and etch time is 10
min. The Corning Eagle XG glass showed a smooth surface, while
super-white glass had a severely rough surface and observed the
texturing size of the tens to hundreds of μm. Although the same
HF solution used, it showed the surface properties depending on
the type of glass and related to the chemical composition of the
glass.
We investigated three kinds of glasses such as Corning Eagle
XG, soda lime, super white to understand for the etching of glass.
This table shows the chemical composition, optical properties
and thermal resistance of glass as a representative three kinds of
glasses. Silicon dioxide (SiO2) constitute more than fifty percent
of total glass composition as shown in Table 1 and it included in
calcium oxide (CaO) or sodium dioxide (Na2O) or aluminum
oxide (Al2O3) according to the kinds of glasses20). In the case
of optical properties, the transmittance is around 92% at the
wavelength of 500 nm and around 8% for the reflectance.
Corning Eagle XG glass is superior to the different things in
quality for the thermal resistance of more 600oC. However,
Corning Eagle XG glass cost per units was more expensive than
other glass because of less dopant component in the glass.
It shows that this formation is related to the presence of some
products when etching of glass. Etched shape related size or
depth is presenting after etching to large cracks formation by the
products. Fig. 3 shows that the growth of product increased and
this surface morphology is gradually changed into a verge as the
etching process. This process indicates a size-up of fluorine
(a)
(b)
(c)
Fig. 3. Top view images of product size formed on a glass
substrate by the etching treatment in, (a) 30 min, (b) 60
min and (c) 90 min, respectively
H.S. Park et al. / Current Photovoltaic Research 5(3) 75-82 (2017) 79
products on the glass surface and a diffusion of fluorine species,
which is related to the etching rate. Therefore, we have a
conclusion that fluorine products play a crucial role in the
macrocrack formation with random-etching without masking
layers.
Based on the facts mentioned above, it affects the glass
texturing depending on the type of glass, composition ratio of
the etching solution, temperature, and agitation. Fig. 4 shows
that etch rates investigated according to various glass types, and
HF 25% (volume ratio of DI:HF = 1:1) was fixed. The graph
shows that the etch rate of the SiO2-based glass (Fused natural
silica) substrate having a low impurity content lowered even
though it is the same solution. It contained less impurities of the
glass substrate, which reported in previous research groups21,22).
3. Investigation of periodic textured glass
morphology
3.1 Glass etching and hard mask layer types
So far, we have briefly discussed the random etching surface
using wet chemical etchant. To solve this problem as per the
previous subchapter, we are going to talk about wet etching with
periodical pattern mask. It is a well-constructed periodic etched
shape as referred in23). The Periodically patterned shape has
various light trapping structures such as high aspect ratio, square
type, honeycomb, hemisphere as shown in Fig. 7. We should
take into consideration such as masking materials, adhesion
between substrate and masking layers, masking layer height,
and size, spacing, and shape of the pattern to have an optimized
light trapping structure. It is related to make the light trapping
structure on a glass substrate having mask pattern and etching
solution.
Fig. 5 presents the glass etching step (before etching ~ etch-4)
for isotropic etching. This is possible for making a high aspect
ratio structure according to specific etching step. It is crucial
factor such as the concentration, time, and solution ratio of the
etching for high aspect ratio structure, but they also depend on
the characteristics of the mask layer material. We investigated
regarding the effect on the hard mask material as shown in Fig. 6
to improve the light trapping structure with the periodic
pattern17). The photoresist (PR) is used as the most commonly
used masking material in wet chemical etching. When the
concentration of HF increases or deeper etch performed, it is Fig. 4. Etching rate comparison of various glass types in HF
etchant solutions
Fig. 5. A glass etching step using periodic pattern hard mask (a) before the etching, (b) etch-1, (c) etch-2, (d) etch-3, and (e) etch-4
Fig. 6. A microscope images of micro etched surface based on various hard mask material
H.S. Park et al. / Current Photovoltaic Research 5(3) 75-82 (2017)80
easy to be penetrated by HF acid between the glass and PR.
Metal is widely used to serve as the masking layer. For the
Comparison to a photoresist, the HF molecules get absorbed
inside the inherent pinholes and cause the enhanced defects on
the glass surface. The Al–glass adhesion was still too weak to
prevent the lateral HF penetration for long-time etch. The silicon-
based thin film is another well-known inert material to HF, it
shows excellent adhesion with the glass substrate, and more
importantly, the surface of the silicon-based material is hydro-
phobic14) which highly prevents the formation of surface pin
holes and notching effect during the HF etch. This property
implies that this masking material could be a good candidate for
deeply fused silica etches.
One of the process steps is checking for hardening the
photoresist (PR)/HMDS solution, and the other is maintaining
the etchant temperature during the etching. Masking layers are
not only PR/HMDS but also different materials such as hydro-
genated intrinsic amorphous silicon layer (i-a-Si:H) or silicon
nitride (SiNx) by PECVD, metal mask like Al or Cr by the
thermal evaporator (or sputtering), a thermal oxide (SiOx). All
of masking materials excepting for PR/HMDS layers need to
remove Metal or (SiNx or i-a-Si: H) with RIE or wet etching
right after hard baking. However, and PR/sub-layers on the
glass have excellent etching characteristics like a high aspect
ratio, isotropic etching, selectivity and fast etching without a
new process (RIE process).
3.2 Wet etching parameters and light trapping structure
Fig. 8 shows the light trapping structure for Etch-3 at
different mask pattern from (2×2) to (15×2), which are based on
wet etching.
It consists of 2 types such as randomly textured surface
without a pattern and periodically textured surface with the
pattern. The textured surface used to be captured the light are
based on the crystalline silicon (c-Si) solar cell as a substrate and
thin film silicon solar cell as superstrate or substrate as well. In
Fig. 7. Investigation of etching effect using hard mask materials (Al, PR, a-Si, and SiNx)
Fig. 8. A SEM images of light trapping structure (etch-3) and
haze value with periodic mask pattern variation
H.S. Park et al. / Current Photovoltaic Research 5(3) 75-82 (2017) 81
the mask pattern, the spacing is fixed at 2 μm, and the size
gradually increases. As the size increases, the etching height
gradually increases. The increase in the etching height serves to
capture the incident light and increase the light scattering effect.
In this way, the haze ratio can be determined using a haze meter
or integral sphere to determine the light scattering effect.
The haze ratio value according to the mask pattern change is
shown in Fig. 8. As shown in the figure, the mask pattern of
(10×2) or more maintained an average haze ratio of 50% or more
to the long wavelength region. However, using the same
structure as in the figure, the haze ratio is more than 60%. This
requires careful examination of various parameters such as
height, spacing, and shape of the structure.
4. Application and Conclusion
Based on HF containing a solution, we used as a device
application such as MEMS, biotechnology and photovoltaic
based on HF glass etching to better the life. In this section, we
have discussed the brief applications and concluded in our work.
Glass etching process regarding the purpose of thin film solar
cell applied satisfactorily when the textured glass surface is free
from contaminants. The etching key point necessary to clean a
glass surface depends strongly on the etching solution and
concentration for the light trapping structure. This changed by
alternating short time etching of glass substrate with HF/H2SO4
without etching mask. Transforming smooth surface into optical
light trapping morphology is possible.
Although the reaction mechanism did not understood at this
moment, we are available for the use of an industry if having
satisfactory experiment results. HF solutions considered dangerous
together with insoluble products. Therefore, it is of great im-
portance to reduce the quality of the etchants used during the
work and to develop methods to apply for the solar cell devices.
For this work, it is replaced by dry etching on behalf of the wet
etching to reduce the insoluble products. However, strong points
of wet etching process are still better than that of dry etching
because of a simple process, cost efficient and well-controlled
the surface structures. Therefore, we will be interesting to see
how the wet etching glass will be resolved.
Acknowledgment
This work was supported by the ‘New & Renewable Energy
Core Technology Program’ of the Korea Institute of Energy
Technology Evaluation and Planning (KETEP) granted
financial resource from the Ministry of Trade, Industry &
Energy, Republic of Korea (No. 20153010012090)
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