Catalysis of dispersed silver particles on directional etching of silicon

Y.M. Yang a,b, Paul K. Chu a,*, Z.W. Wu a, S.H. Pu a, T.F. Hung a,K.F. Huo a, G.X. Qian a, W.J. Zhang a, X.L. Wu a,c

a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kongb Department of Physics, Southeast University, Nanjing 211189, China

c National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China

Received 28 July 2007; received in revised form 30 September 2007; accepted 23 October 2007

Available online 3 December 2007

Abstract

Silver particles are dispersed on silicon by magnetron sputtering and post-annealing to investigate the catalytic effects of individual silver

particles on wet etching of silicon surface. According to scanning electron microscopy, dispersed deep holes are present and the major etching

direction is vertical to the surface of a Si(1 0 0) wafer or inclined to that on a Si(1 1 1) wafer. Our experiments indicate that the effect of the

anisotropy of Si on directional etching is fundamental and the wafer resistivity and experimental process have important influence on the etching

results. In addition, aggregation of silver particles and random horizontal etching on the surface of the wafer are caused by the local imbalance

between the oxidant and HF. Our results enable better understanding of the catalytic effects of metal particles on silicon and are helpful to the

preparation new silicon nanostructures.

# 2007 Elsevier B.V. All rights reserved.

PACS : 81.65.Cf; 82.45.Jn; 81.05.Rm

Keywords: Metal catalysis; Directional etching; Porous silicon

1. Introduction

Chemical etching of Si is of scientific and practical interest

and various phenomena pertaining to nano/micro Si structures

as well as their applications in luminescence devices, photonic

crystals, chemical and biological sensors, and so on have been

reported [1–4]. Directional etching is especially important to

the fabrication of functional Si structures such as light filters [5]

and two-dimensional photonic crystals [1]. Deep and vertical

vias in the (1 0 0) [5–7], (1 1 1) [8], and occasionally (1 1 3)

directions [7] have been obtained by electrochemical methods.

The electrochemical etching processes are quite complicated

and these arrays may only be obtained in a small area [9].

Moreover, fabrication of photonic crystals with lattice

parameters below about 0.8 mm is very difficult at present.

A new chemical etching technique which utilizes the

catalytic effects of noble metal particles such as Au, Ag, Pt, Pd,

and Cu has recently been proposed to produce holes and other

Si microstructures [10–17]. It has been observed that Au and

Ag particles induce vertical vias in Si while Pt, Pd or Cu

particles induce irregular holes or only shallow pits [11,12]. It

is believed to be easier for the bonding electrons of the

surface Si atoms to transfer to the oxidant (such as H2O2, Ag+

and Fe3+) through the Si/metal interfaces than through the

bare wafer [13]. With Si is oxidized by the oxidant and SiO2 is

dissolved by HF simultaneously, pits and finally holes will

form at the location of the metal particles. If the metal

particles are connected, the formed holes will be connected

too, and as a result, Si nanowires can be fabricated [12–15]. Si

nanowire arrays in the (1 0 0) or (1 1 1) directions and with

controlled diameters, lengths, and densities have been

fabricated using silver films with prefabricated periodic

holes [16,17].

In spite of the interesting phenomenon, catalytic etching is

still not well understood, especially the origin of directional

etching. The old assumptions on catalysis of metal particles

and formation of holes cannot explain directional etching,

partly because anisotropy of Si has not been considered.

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Applied Surface Science 254 (2008) 3061–3066

* Corresponding author. Tel.: +852 27887724; fax: +852 27889549.

E-mail address: paul.chu@cityu.edu.hk (P.K. Chu).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.10.055

Well-dispersed holes catalyzed by dispersed small metal

particles have not yet been obtained [11,12]. It is still a problem

whether the etching direction induced by the individual metal

particle is in accordance with the connected metal particles,

e.g. on Si(1 1 1) wafer [11]. In the work reported here, it is

shown that without the interaction in the particle network,

catalysis of individual particles is easily influenced by the

wafer, solution, and experimental process. Our work focuses

on the catalysis of individual Ag particles and the related

mechanism is also discussed based on experimental results.

2. Experimental details

Silver films with thicknesses of 2, 5, and 20 nm were

fabricated on three types of Si wafers, P+ [(1 0 0), 0.005–

0.020 V cm], P� [(1 0 0), 10–20 V cm], and P� [(1 1 1), 1–

30 V cm], by direct current (DC) magnetron sputtering.

Dispersed Ag particles were formed by annealing the Ag films

in N2 at 450 8C for 30 min. Etching was performed in aqueous

HF (5 M) containing AgNO3 (0.01 M), Fe(NO3)3 (0.12 M) or

H2O2 (0.2 M) in sealed vessels at 45 8C for different periods of

time. After etching, the samples were rinsed with deionized

water and kept in heated ethanol for 20 min. X-ray diffraction

(XRD) was conducted on an X-ray diffractometer D500 and

scanning electronic microscopy (SEM) was performed on a

field emission SEM (JEOL JSM-6335F).

3. Results and discussion

A physical method which has recently been adopted to grow

epitaxial nanowires [18] is employed in our experiments to

prepare dispersed Ag particles on Si. Due to the large surface

energy, a very thin Ag film is unstable and will break and form

particles after annealing at above 400 8C. Fig. 1(a and b) show the

morphology of the particles on the 2 and 5 nm thick Ag films

annealed in nitrogen at 450 8C, respectively. The particles are

observed to disperse well but not spherical, especially those on

the thicker Ag film due to aggregation of the particles. The

particles in the annealed 2 nm thick Ag film have a narrow size

distribution (Fig. 1(c)) whereas those in the annealed 5 nm-thick

Ag film have a wide distribution with the particle density

decreasing by about 2/3. Since the eutectic temperature of the

Ag–Si binary alloy is as high as 840 8C, the particles are pure Ag.

The Ag particles have a face-centered cubic structure according

to the X-ray diffraction pattern (Fig. 1(d)) and it is noted that the

formation of Ag particles is not influenced by the wafer.

To compare the catalytic effects of the dispersed Ag particles

on etching of Si with those of connected Ag particles, we first

briefly discuss the formation of Si nanowires in the HF/AgNO3

solution. Fig. 2(a) shows the surface image of the P+ (1 0 0) Si

etched in the HF/AgNO3 for only 4 min. The Ag particle layer

on the surface of the sample has been removed by 10% HNO3

before SEM observation. As shown in the black regions of this

Fig. 1. SEM images of Ag particles from the annealed Ag films: (a) 2 nm and (b) 5 nm, (c) corresponding particle size distributions and (d) XRD spectrum of Ag

particles from the annealed 2 nm Ag film.

Y.M. Yang et al. / Applied Surface Science 254 (2008) 3061–30663062

image, Si nanowires with anomalous cross-sections are formed

in the space among the connected Ag particles. Around the

vertical Si nanowires, there are lots of horizontally etching

traces pointing to the nanowires. Such horizontal traces indicate

that some newly deposited Ag particles migrate horizontally on

the Si initially and then move down vertically after connecting

with other particles. Fig. 2(b) depicts the cross-section of the

wafer after 30 min etching under the same conditions and Si

nanowires aligned in the (1 0 0) direction.

It is quite difficult to analyze the catalytic effects of

individual Ag particles in the above case due to the interaction

among the connected particles. Immersion in HF/AgNO3 for a

short time can also produce dispersed Ag particles on Si

[11,12]. The further treatment is etching in HF containing

H2O2, Fe(NO3)3 or other oxidants. It is believed that when the

Si underneath the Ag particles is locally oxidized, H2O2 is

decomposed and Fe3+ ions become Fe2+ [11,12]. This

assumption is reasonable because we have noted that the

vessel wall near the sample turns pea green during etching that

is indicative of Fe2+. The HF/Fe(NO3)3 solution is colorless due

to the formation of metastable complex Fe2F6. Fig. 2(c and d)

display the surface images of P+ (1 0 0) Si etched in HF/

Fe(NO3)3 for 20 min and the Ag particles are from the annealed

2 and 5 nm thick Ag films, respectively. It can be observed that

the Si surface becomes rough after etching, and dense dispersed

holes with sizes close to those of Ag particles are formed.

Besides, a few horizontal etching traces in the shallow regions

can be seen. Fig. 2(e) shows the surface of Si after etching for a

short time and some large Ag particles can still be seen. Fig. 2(f)

depicts the cross-section of the sample whose top-view image is

shown in Fig. 2(c). Dense straight holes 3–5 micrometers deep

can be observed. The holes are vertical to the surface, i.e. in the

(1 0 0) direction. Etching of similar wafers in HF/H2O2 has also

been conducted and it is difficult to prepare dispersed deep

holes using aqueous 5 M HF containing 0.1–0.4 M H2O2.

Under the present conditions, most of the Ag particles have

aggregated (Fig. 2(g)) and only sparse shallow pits have formed

(see Fig. 2(h)).

Unlike connected Ag particles, it is noted that etching

catalyzed by dispersed Ag particles is easily influenced by the

experimental conditions such as the solution temperature,

solution flow, age of solution, light illumination, resistance of

the Si wafer, as well as the silicon crystal orientation. The

solution temperature and flow control the etching rate which

can be roughly estimated by the formation rate of the H2

bladder. Using solutions that have been made for several days,

especially HF/(FeNO3)3, evidently decreases the etching rate

and influences the etching results. Since the solution is sealed in

plastic bottles, the change of the solution is most likely related

to the change in Fe2F6 but its exact effect on etching is still

unclear at this moment. We have also examined etching on the

P+ (1 0 0) wafer using HF/Fe(NO3)3 in a dark room and found

no effective catalytic reactions arising from the Ag particles. In

this case, almost all Ag particles have aggregated while a loose

porous Si layer forms on the wafer. As aforementioned, the

dopant type and concentration in Si are key parameters

affecting electrochemical etching. Very different morphologies

have been observed on Pt-coated Si with different dopant

concentrations (etched in HF/H2O2) too [10]. As for the Ag

particles, the dopant type has been found to not noticeably

influence the morphology of the holes [11]. In our experiments,

it is found that the dopant concentration in Si has an obvious

effect on the formation of holes. Fig. 3(a and b) show the

surface of the P� (1 0 0) etched in HF/Fe(NO3)3 for 20 min and

in HF/H2O2 for 1 h, respectively. Their cross-sectional images

are exhibited in Fig. 3(c and d). Dense vertical and horizontal

holes can be observed in the shallow regions on the wafer

etched in HF/Fe(NO3)3. Moreover, each horizontal etching

trace ends at a vertical hole. The horizontal traces are random,

indicating that anisotropic etching has not happened on the

Fig. 2. Top-view (a, c–e, g) or cross-sectional view (b, f, h) SEM images of etched P+ (100) Si. Before etching, the wafers are bare (a–b) or with annealed Ag films (c–

h). (a) HF/AgNO3, 4 min, (b) HF/AgNO3, 30 min, (c, f) HF/Fe(NO3)3, 20 min, 2 nm Ag film, (d) HF/Fe(NO3)3, 20 min, 5 nm Ag film, (e) HF/Fe(NO3)3, 6 min, 20 nm

Ag film, (g, h) HF/H2O2, 60 min, 5 nm Ag film. The white arrows in (d) show the horizontal etching traces whereas that in (g) indicate the aggregated Ag particles.

Y.M. Yang et al. / Applied Surface Science 254 (2008) 3061–3066 3063

surface plane. In comparison, both small holes and aggregated

Ag particles can be seen on the wafer etched in HF/H2O2. In

addition, some aggregated particles are seen to induce large and

straight holes deeper than 20 mm. These characteristics are

quite different from those on the P+ Si (see Fig. 2). We have also

examined etching on P� (1 1 1) Si with the two solutions. In

both cases, only gradient holes are seen, as illustrated in Fig. 4,

and our results are consistent with those of Tsujino and

Matsumura [11] who did not show the related image but

revealed difference in the etching direction compared the case

involving the particle network [19].

The mechanism of metal-particle-assisted etching is still not

well understood. One of the mysteries is what drives the

observed horizontal etching on the Si surface observed in some

experiments. Furthermore, it is not certain how straight holes

are formed. The influence of the etching solutions, effects of the

Si resistivity, and the differences between the shallow layer and

deep layer have been mentioned too. Here, using our

experimental, we try to better understand the mechanism.

Firstly, it is believed that the horizontal etching traces of

individual Ag particles observed in our experiments (see

Fig. 2(d) or Fig. 3(a)) are formed when the local oxidizing rate

of Si is lower than the dissolving rate of SiOx. It is known that

Fe(NO3)3 is a mild oxidation agent and its oxidation on bare

wafer is very limited. Since the solution is not stirred in our

experiments, the produced Fe2+ will somewhat block contact

with Fe3+ ions or Fe2F6 with the Si surface and diffusing in the

holes. This is the reason that the vessel wall near the sample

shows a pea green color. On the contrary, we have rarely

observed horizontal etching traces of individual particles unless

the concentration of H2O2 is very low. In addition to the regions

under the metal particles, H2O2 oxidizes the bare regions of the

Fig. 3. Top-view (a–b) or cross-sectional view (c–d) SEM images of etched P� (1 0 0) Si with annealed 5 nm Ag film, (a, c) HF/Fe(NO3)3, 20 min and (b, d) HF/

H2O2, 60 min.

Y.M. Yang et al. / Applied Surface Science 254 (2008) 3061–30663064

wafer, and as a result, a large number of the Ag particles cannot

form stable pits on the wafer. This is the reason why

aggregation often happens when HF/H2O2 is used. The deep

holes in Fig. 3(d) may be an exception, but it should be noted

that such holes are sparse and mostly formed by aggregated Ag

particles.

The dopant type and concentration in the Si should influence

catalytic behavior of individual Ag particles. The influence of

the Si resistivity on etching is demonstrated in Figs. 3 and 2(c–

h). In the catalysis scheme proposed by Peng et al. [13], the

charge exchange and charge transport between Si and metal

particles depend on the quasi-Fermi level of the wafer, which is

directly related to the dopant type and concentration as well the

redox potential and metal materials. Hole injection into P� Si is

more difficult than into P+ Si through the Si/Ag interface. As a

result, the oxidizing rate of Si decreases leading to more

horizontal etching on the surface (see Fig. 3(a)). However, as in

the case of electrochemical etching, the detailed effects of the

Si resistivity require more work.

Metical-particle-induced directional etching arises funda-

mentally from the large anisotropy of crystal Si. Although the

origin of directional etching in electrochemical etching is still

not well understood, previous works have shown that well

aligned holes are always in some of the basic crystal directions

of Si such as (1 0 0), (1 1 1), and (1 1 3) [5–8]. For etching

catalyzed by a metal (Au and Ag) particle network, directional

etching in the (1 0 0) or (1 1 1) directions have been reported

[10,13,17]. With regard to etching catalyzed by dispersed Ag

particles, our experiments and those of Tsujino and Matsumura

has indicated that the major etching direction is (1 0 0) on

Si(1 0 0) and inclined to the surface on Si(1 1 1). Besides the

vertical holes, there are lots of horizontal holes which appear as

either holes or pits in the cross-section images obtained from

the Si(1 0 0) wafer (see Fig. 3d). Different from the Si(1 0 0)

wafer, we observe three basic etching directions on the Si(1 1 1)

wafer instead of horizontal holes (see Fig. 4). The three

directions appear othogonal to each other (see the inset of

Fig. 4) as predicted by the cubic crystals structure. Our analysis

cannot identify the crystal directions of the three basic etching

directions and more work is needed to understand the

mechanism in more details. Nonetheless, our experimental

results strongly support the fundamental effect of electrical and

mechanical anisotropy of Si on the directional etching. Besides

the crystal itself, some other factors such as the properties of the

solution are also important for anisotropic etching. For the

etching of Ge which has the same crystal structure of Si, it has

recently been observed that changing the composition of the

electrolyte from ‘‘aqueous’’ to ‘‘organic’’ induces a switch in

the major etching direction from h1 0 0i to h1 1 1i [20].

Finally, the particle shape and particle size can have a

limited effect on the formation of holes. In our experiments, the

holes induced by individual particles are mostly straight in the

deep regions, but there are also irregular holes and horizontal

holes. The irregular holes are mostly caused by large particles

composed of several small particles and their shape easily

changes during etching. The horizontal holes, which can be

seen often from Si(1 0 0) wafer, indicate changing of etching

direction from vertical to horizontal. Tsujino and Matsumura

have reported that the Ag particles with irregular shape need

change their shape to form deep straight holes, and the etching

direction may change during the shape change [11]. Based on

the X-ray diffraction analysis, we have not found structural

differences between the two kinds of Ag particles prepared by

annealing Ag nanofilms and by short-time electroless plating.

We also have not found any special orientation which the Ag

particles have turned to after 10 min etching for both Si(1 0 0)

and Si(1 1 1). This means that the orientation of the Ag

particles is not the key factor for the directional etching, maybe

due to the excellent conduction of Ag. As for the particle size, it

has no direct influence on the formation of holes except on the

diffusion rate of ions in the holes. From this point of view,

tortuous holes and horizontal holes in which the diffusion rate

of ions is relatively low are usually not long. Ordered holes can

be fabricated by combining the catalytic effects of metal

particles with lithography or templates. The Ag particles at the

bottom of the holes can be used to catalyze the growth of other

materials and act as antimicrobial agents in biosensors as well.

4. Conclusion

Ag particles are dispersed on Si by annealing silver

nanofilms to investigate the catalytic effects of individual

silver particles on chemical etching of Si. Unlike connected Ag

particles, it is noted that the catalytic effects of individual Ag

particles are easily influenced by many factors including the Si

resistivity, the type and concentration of the solution, and the

experimental process. In our present experiments, the major

Fig. 4. Coss-sectional view SEM images of P� (1 1 1) Si with annealed 5 nm

Ag film. The wafer is etched in HF/H2O2 for 20 min. The arrows show three

basic etching directions (D1, D2 and D3). The insert shows a position where the

three etching directions converge.

Y.M. Yang et al. / Applied Surface Science 254 (2008) 3061–3066 3065

etching direction is vertical to the surface of a Si(1 0 0) wafer or

inclined to that on a Si(1 1 1) wafer. Our results indicate that the

effect of the anisotropy of Si on the directional etching is

fundamental and that of the orientation of Ag particles is not

evident. Our comparative experiments with different oxidants

indicate that aggregation of Ag particles and random horizontal

etching on the surface layer of wafer are caused by the local

imbalance between the oxidant and HF. The effects of the Si

resistivity and dopant type are important too because they

contribute to electron transfer via the Si/Ag interface. Our

experiments provide better understanding of the catalytic

effects of metal particles and are helpful to the preparation of

new Si nanostructures.

Acknowledgment

The work was financially supported by City University of

Hong Kong Strategic Research Grant (SRG) No. 7002138.

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Erratum

Erratum to ‘‘Catalysis of dispersed silver particles on directional etchingof silicon’’ [Appl. Surf. Sci. 254(10) (2008) 3061–3066]

Y.M. Yang a,b, Paul K. Chu a,*, Z.W. Wu a, S.H. Pu a, T.F. Hung a, K.F. Huo a,G.X. Qian a, W.J. Zhang a, X.L. Wu a,c

a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, Chinab Department of Physics, Southeast University, Nanjing 211189, Chinac National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China

Available online 15 April 2008

The publisher regrets that during the publication of the above paper there was an error in the cited reference [14]. Please find theupdated reference reproduced below.

[14] T. Qiu, X.L. Wu, J.C. Shen, C.T. Peter Ha, Paul K. Chu, Nanotechnology 17 (2006) 5769.

Applied Surface Science 254 (2008) 5648

DOI of original article: 10.1016/j.apsusc.2007.10.055* Corresponding author. Tel.: +852 27887724; fax: +852 27889549.

E-mail address: paul.chu@cityu.edu.hk (P.K. Chu).

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