METAL-ASSISTED CHEMICAL ETCHING OF SILICON:

PREPARATION OF SILICON NANOWIRE ARRAYS

Elisabeth Galopin, Gaëlle Piret, Yannick Coffinier, Sabine Szunerits and Rabah Boukherroub

Institut de Recherche Interdisciplinaire (IRI) USR-3078

Institut d’Electronique de Microélectronique et de Nanotechnologie (IEMN), CNRS-8520

Villeneuve d’Ascq, France.

Rabah.boukherroub@iemn.univ-lille1.fr

Topic : Integration and synthesis of new materials

The paper reports on the preparation of SiNW

arrays and patterns using the chemical etching

method of crystalline silicon substrate. Electroless

deposition of silver nanoparticles onto the silicon

nanowires was further exploited for the

preparation of highly sensitive Surface Enhanced

Raman Spectroscopy (SERS) substrates.

I. INTRODUCTION

In recent years, a great deal of effort has been made to

the fabrication of one-dimensional nanostructured materials

owing to their submicron ultimate feature size and

dimensionally dependent physical properties for future use

in nanodevices [1]. Silicon is the basic material in

microelectronics, and silicon nanowires (SiNW) and

nanowire arrays have attracted much attention for their

potential applications in the field of silicon nanoelectronics

and use the SiNWs as nanocomponents to build nano

circuits and nanobiosensors [2].

II. SILICON NANOWIRE FABRICATION

A. Metal Assisted Ckemical Etching

Various methods have been developed to prepare one-

dimensional silicon nanostructures [1, 2]. Even though the

techniques offer a good control over the nanostructures

dimensions, most of the methods require high temperatures

or a high vacuum, templates and complex equipment.

Metal-assisted chemical etching of silicon has been used

in the past for the preparation of porous silicon substrates

[3]. The technique has successfully been applied for the

fabrication of large-area aligned SiNW arrays on single

crystal silicon wafers [4-6].

Silicon wafers with different doping levels were used in

the study. The surface was first degreased in acetone and

isopropanol, rinsed with Milli-Q water and then cleaned in

a piranha solution (3:1 concentrated H2SO4/30% H2O2) for

15 min at 80°C followed by copious rinsing with Milli-Q

water. The SiNW arrays were prepared by chemical

etching of the clean substrate in HF/AgNO3 (5.25./0.02 M)

solution at 50°C for a given time. Fig. 1 displays a SEM

image of the resulting surface. It consists of SiNW arrays

wrapped by dendrites. The energy dispersive X-ray

spectroscopy shows that these dendrites are composed of

silver (Fig. 2).

Figure 1. SEM image of branched silver dendrites formed on Si sample

after etching in HF/AgNO3 aqueous solution at 50°C for 10 min.

Figure 2. Energy dispersive X-ray image of silver dendrites formed on

Si sample by etching in HF/AgNO3 aqueous solution at 50° C for 10 min.

The electroless deposition of Ag and silicon etching in

the fluoride solution containing Ag+ occurs according to

the following equations:

Si + 4h+ + 4HF SiF4 + 4H+

SiF4 + 2HF H2SiF6

Ag+ + 1e- Ag

Chemical removal of the silver deposits using a mixture

of HCl/HNO3/H2O (1/1/1) at room temperature led to the

formation of well-aligned SiNW arrays (Fig. 3).

Figure 3. Cross-sectional SEM image of the silicon nanowire array

formed by chemical etching of Si(100) in HF/AgNO3 aqueous solution at

50°C for 10 min.

By combining optical lithography and chemical etching,

we have successfully prepared SiNW patterns in a

controllable fashion. The choice of the resist that is stable

in HF/AgNO3 aqueous solution is the key step in this

process.

B. Silver nanoparticles deposition

We then have used the electroless technique for the

deposition of silver nanoparticles onto the silicon nanowire

arrays. The nanoparticles were obtained by dipping the

silicon nanowires 1min in HF/AgNO3 (0.26/5.10-4 M) at

room temperature (Fig. 4).

Figure 4. Electroless technique for the deposition of silver nanoparticles

onto the silicon nanowire arrays.

III. CONCLUSION

We successfully prepared silicon nanowire arrays using

the chemical etching method for crystalline sililcon

substrate. These surfaces were then covered by silver

nanoparticles by electroless deposition. Finally, we

investigated the surface-enhanced Raman Scattering

(SERS) performances of the resulting structure. The

performance of the substrate was demonstrated for Rh6G

concentrations down to 1x10-9 M.

ACKNOWLEDGMENT

The authors would like to acknowledge FEDER (Fonds Européen de DEveloppement Régional) who supported this work by equipment contribution.

REFERENCES

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[6] Piret, G.; Coffinier, Y.; Roux, C.; Melnyk, O.; Boukherroub, R. Langmuir 2008, 24, 1670-1672.