ROLE OF Ag+ ION CONCENTRATION ON METAL-ASSISTED CHEMICAL ETCHING OF SILICON

O.V.Pyatilovaa,1, S.A. Gavrilov1, A.A. Dronov1, Ya.S. Grishina1 and A.N.Belov1 1 National Research University of Electronic Technology, Zelenograd, Moscow, 124498, Russia

ae-mail: 5ilova87@gmail.com

Keywords: silicon, chemical etching, silver, catalyst.

Abstract. Metal-assisted silicon etching in HF/H2O2/H2O solution with silver ions as catalyst was

investigated. It is found that the geometric parameters of silicon nanostructured layers are

determined by the silver-catalyst concentration. Spontaneous stop of the process during etching at

low Ag+ ion concentration is explained by insoluble Ag2SiO3 formation.

Introduction

Silicon based nanostructures are actively investigated due to its original properties and

applications in photovoltaics [1, 2], β-voltaics [3], solar cells, explosive devises etc. Chemical vapor

deposition to fabricate silicon nanowires via the "vapor-liquid-solid" growth mechanism [4] and

electro-chemical formation of the porous silicon [5, 6] are common methods for nanostructured

silicon. An alternative way to form both silicon nanowires and porous silicon is metal-assisted

chemical etching (MACE) [7]. MACE is a low temperature method, where an additional current

source, vacuum and dangerous gases are not needed for nanostructuring.

The MACE mechanism is explained in detail in Ref. 8, indicating holes (positive charges)

necessary for etching of porous silicon to be generated at a metal surface in contact with an

oxidative agent. Because of the insulating character of thin walls of porous silicon, the transport of

holes through this layer is not possible. Instead, it is found that the transport of holes proceeds

primarily by means of Ag/Ag+ redox pairs circulating in an electrolyte and diffusing through etched

pores in silicon. Morphology of silicon nanostructures depends on an etching solution composition

[7], etching process duration [9], reaction zone illumination, a silicon type and doping level [7]. In

addition, a metal deposition method [10], a type and amount of metal [11, 12], a size and shape of a

metal catalyst deposited on a silicon surface affect morphology of a formed nanostructured silicon

layer.

In the present work the role of Ag+ ion concentration in the MACE process of silicon is

investigated. The concentration of Ag-catalyst was varied by silver films with different thickness or

by addition of different amount of Ag+ ions into the etching solution. The influence of the silver-

catalyst amount on the etching rate, layers thickness and porosity formed by metal-assisted

chemical etching method was defined.

Experiment

MACE was performed in the mixture of hydrofluoric acid and hydrogen peroxide aqueous

solutions. Single-crystal p-Si wafers (100) with resistivity of 12 Ω·cm and 625 µm in thickness

were used as substrates. The samples were cut into 1x1 cm2 pieces. Samples were cleaned by a

sulfuric acid and hydrogen peroxide mixture (97% H2SO4/30% H2O2, volume ratio 1/1) during 10

min. Then they were rinsed in deionized water and dried by jet of isopropyl alcohol vapor. The

formation of nanostructured layers was performed in two ways, because of two sources of silver

were used. The analysis of the microstructure of the samples was performed by scanning electron

microscopy (SEM).

Solid State Phenomena Vol. 213 (2014) pp 103-108Online available since 2014/Mar/24 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.213.103

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The first method: silver thin films as a catalyst source

Silver films with 30 and 50 nm thickness were deposited on silicon wafers by the vacuum-

thermal evaporation method. Test samples were placed into the etchant for different time from 10 to

50 min. Silicon was etched in a solution contained 40% HF/35% H2O2/H2O, volume ratio 25/10/4.

Intensive gas evolving on the Si surface and color changing to light brown were observed. It is

evident that the silicon etching process occurs under the catalyst material only, as can be seen in

Fig.1. Then the samples were removed from the solution, rinsed in deionized water and, eventually,

dried in air.

Fig.1. The silicon etching process with and without silver coating

The second method: silver contained solution as a catalyst source

Samples were placed into the 40% HF/35% H2O2/H2O (volume ratio 25/10/4) etchant

containing silver ions. Silver ions were added by immersion of metallic silver into the solution for

certain time. Silver was weighed before and after immersion. Both silicon wafer surfaces changed

their color to light brown and gassed intensively after exposure to the solution for a few seconds.

Heating of the solution was observed. Samples were weighed before (m1) and after (m2) etching for

the porosity, etching rate and layer thickness calculations. Nanostructured silicon was then

dissolved in the 1 wt% NaOH aqueous solution at room temperature until gas evolution stopped.

After drying at room temperature the samples were weighed (m3). The calculation results are

presented in Table 1.

Table 1. Nanostructured silicon layers morphology dependence on the silver ion concentration in

solution N Sample

surface area,

[cm2]

Mass of

Ag,

dissolved

into

solution,

[g]

Ag ions number

in solution

Etching

duration

before

process

stop,

[min]

Nanostructured

silicon porosity*,

[%]

Silicon

etching

rate,

[g/min]

hpor-Si,

[µm]

1 6,21 0,00078 4,3·1018

6 15,3 0,0051 5

2 7,28 0,00142 7,8·1018

13 22 0,016 135

3 6,75 0,00180 10·1018

31 34,5 0,013 261

4 6,5 0,00247 13,7·1018

6 90 0,0835 263

5 6 0,00711 40·1018

8 100 (full dissolution) 0,0872 460

*Porosity (P) =(m1-m2)/(m1-m3)·100%.

104 Physics and Technology of Nanostructured Materials II

Results and discussion

We have carefully checked how metal catalyst amount affects the thickness of a nanostructured

silicon layer. Fig.2 and 3 show SEM-images of silicon structures formed by the first method with

50 and 30 nm silver films, respectively.

a b

Fig.2. SEM-images of structures formed after etching of p-Si coated by 50 nm silver films during

(a) 30 min, (b) 50 min

a b

Fig. 3. SEM-images of structures formed after etching of p-Si, coated by 30 nm silver films

during 10 (a), 20 min (b)

It is found that the duration of the etching process is defined by amount of Ag+

ions in the

reaction system. As it can be seen in Fig.2 and Fig.3 during the etching process from 30 to 50 min

the column highs are constant (~5 µm and ~2 µm) for 50 nm and 30 nm silver film thicknesses,

respectively. Thus, it is clear that the thickness (depth) of the nanostructured layer depends on the

silver film thickness. Moreover, it is found that continuous silver films are dissolved during the

etching process (Fig.4a). After the full process has stopped, insoluble particles are observed on the

nanostructured silicon surface (Fig. 4b).

Solid State Phenomena Vol. 213 105

a

b

Fig.4. (a) Dissolution of the silicon thin film during the silicon etching process; (b) Insoluble

particles on the silicon surface after etching has stopped.

In the case of the second method we have plotted dependences of nanostructured silicon layer

porosity and thickness (Fig.5a) as well as silicon etching rate (Fig.5b) with respect to Ag+ ion

concentration in the solution.

a b

Fig.5. Dependences of the nanostructured silicon layer (a) porosity and thickness, (b) etching

rate on the silver ions concentration in the solution

106 Physics and Technology of Nanostructured Materials II

According to Fig.5 morphology of the nanostructured silicon layers and etching rate directly

depend on the concentration of dissolved silver in the solution. Thus, the etching process can be

damped or can result in full silicon dissolution (the second method), while the less the silver amount

the sooner the stop of etching (the first method).

We suggest that the etching process is due to the silver ions presence in the solution. Our

assumption can be proved by the following reasons. First of all, the qualitative analysis on the silver

ions presence in the solution (formation of the white flakes) have been performed by adding

hydrochloric acid in the etchant before and after the process has stopped. Silver ions are not

observed after the stop of the process. Secondly, after the process stop the solution doesn’t etch

other silicon samples. It can be explained by depletion of such reagents as hydrofluoric acid,

hydrogen peroxide or silver ions. Adding of the hydrofluoric acid and hydrogen peroxide to the

solution doesn’t initiate the process. And finally, after the process has stopped the solution etches

other silicon samples coated with silver thin films. In accordance with these observations, silver

ions transform into an insoluble compound. This process is described by the following chemical

reactions:

1. Silver dissolution and Ag+ ions formation:

2Ag+H2O2+2H+→2Ag

++2H2O. (1)

2. Holes injection into silicon:

H2O2+2H+→2H2O+2h

+, (2)

where h+

is electronic hole.

3. Silicon dissolution:

Si+6HF+2h+→SiF6

2-+4H

++H2↑, (3)

or

Si+6HF+4h+→SiF6

2-+6H

+

3. Hydrolyze:

SiF62-

+3H2O→ SiO32-

+6HF. (4)

4. Precipitation:

SiO32-

+2Ag+→Ag2SiO3↓ (5)

In accordance with Eq.1-4 it can be assumed that the insoluble brown amorphous compound is

silver silicate (Ag2SiO3) [13], which is poorly studied. Nanostructured silicon surface contamination

by silver silicate does not cause any problem, since Ag2SiO3 can be dissolved in nitric acid.

Conclusions

In conclusion, morphology of nanostructured silicon surfaces is defined by the silver ion

concentration in the solution. It is independent of the catalyst injection way to the reaction mixture.

The etching process can be stopped due to the silver consumption and silver silicate formation. The

more the silver concentration, the longer and the faster the MACE process proceeds. Thus,

nanostructured silicon with certain surface morphology can be formed by the different catalyst

amount addition. Ag2SiO3 can be removed from the surface without nanostructured layer

degradation.

Acknowledgements

This work was supported by the European FP7 project PIRSES-GA-2011-295273-NANEL.

Solid State Phenomena Vol. 213 107

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Physics and Technology of Nanostructured Materials II 10.4028/www.scientific.net/SSP.213 Role of Ag+ Ion Concentration on Metal-Assisted Chemical Etching of Silicon 10.4028/www.scientific.net/SSP.213.103

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