A Review of Wet Chemical Etching of Glasses in Hydrofluoric Acid … · It can be readily dissolved...

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.

HNO31) Faster longitudinal etching rate

: Due to the strong oxidability of HNO3 which facilitates the reaction between HF and glass

H3PO4 Lower etching rate

H2SO41) 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


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


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


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


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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A Review of Wet Chemical Etching of Glasses in Hydrofluoric Acid … · It can be readily dissolved...

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