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Applied Surface Science 273 (2013) 107– 110

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

etal-assisted homogeneous etching of single crystal silicon: A novelpproach to obtain an ultra-thin silicon wafer

an Baia,b, Meicheng Lib,c,∗, Dandan Songb, Hang Yub, Bing Jiangb, Yingfeng Lib

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, ChinaState Key Laboratory for Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, ChinaSu Zhou Institute, North China Electric Power University, Suzhou 215123, China

r t i c l e i n f o

rticle history:eceived 7 September 2012eceived in revised form 20 January 2013ccepted 28 January 2013

a b s t r a c t

Homogeneous etching of silicon is achieved through one-step metal-assisted chemical etching (MACE),which offers a simple route to obtain the ultra-thin silicon wafer with thickness below 50 �m. The sur-face of the ultra-thin silicon wafer obtained by this method is smooth at the nanometer scale, and itssurface roughness is around 10 nm. The homogenous etching mechanism is discussed in terms of the

vailable online 4 February 2013

eywords:hin silicon waferomogeneous etchingolesACE

hole injection principle. It’s found that the introduction of a high concentration of H2O2 facilitates theuniform distribution of the holes injected on the silicon surface, causing the homogeneous etching of thesilicon. Meanwhile, the thinning is uniform across a large wafer area, and ultra thin silicon wafers up to4 in. in diameter were obtained. Furthermore, any thickness of silicon wafer within 30–180 �m can beobtained by modulating the etching process accurately.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

The metal-assisted chemical etching (MACE) method hasmerged as a simple method to fashion materials at nanometerength scales [1–3]. The common MACE process has a preferentialtching orientation, such as, the preferential etching direction islong 〈1 0 0〉 orientation for single crystal silicon substrate coveredith noble metal nanoparticles [4]. Just because of the selective

tching, MACE can be used to prepare porous silicon [5,6], andanowire array [7,8]. However, if the homogeneous etching cane achieved on the silicon substrate, the MACE method will getew fields of applications. First of all, this homogeneous etchingechnique can be used to perform the thinning of silicon wafer.t present, the thin silicon wafer with a thickness under 50 �m

s a promising building block for a range of microelectronics andicrosystems, such as 3D integrated circuits [9], ultra-thin chips

10], thin silicon solar cell [11] and flexible electronics [12].The work described herein uses the simple one-step MACE

ethod to realize the homogeneous etching of silicon. The ultra-hin silicon with a thickness of 30 �m is obtained, while theurface roughness is around 10 nm. Meanwhile, the mechanise that

∗ Corresponding author at: State Key Laboratory for Alternate Electrical Powerystem with Renewable Energy Sources, School of Renewable Energy, North Chinalectric Power University, Beijing 102206, China. Tel.: +86 10 61772951;ax: +86 10 61772951.

E-mail address: mcli@ncepu.edu.cn (M. Li).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.01.196

explains the homogenous etching is proposed. Furthermore, anythickness of silicon wafer within 30–180 �m can be obtained bymodulating the etching process (etching rate, reaction time, andet al.). The facile, practical method to achieve ultra thin siliconshould be of particular interest, due to its low cost, and absenceof a residual mechanically damaged layer after general etching.

2. Experimental

P-type monocrystalline silicon (1 0 0) samples with a thicknessof 525 �m were used as the starting samples. Generally, we usedsmall silicon sample with the size of 1.5 cm × 1.5 cm, for the largescale investigation, we used 4 in. Si wafer directly. The Si sampleswere cleaned using acetone, absolute ethyl alcohol and deionizedwater in the ultrasonic condition, respectively. The cleaned Si sam-ples were dipped into dilute HF solution to remove native oxide.Following the pretreated step, the Si samples were immediatelyplaced into the etching solution for an appropriate duration. Here,the etching solution contained HF, AgNO3, and H2O2 with suitableratio. The reaction temperature was in the range of 10–50 ◦C.

Surface morphology and transverse thickness of the etchedSi samples were characterized by scanning electron microscope

(SEM) with FEI Quanta 200F. Surface roughness of the etched Sisamples was measured by atomic force microscope (AFM) withVeeco Dimension 3100V. Ag information in thin silicon wafer wasdetected by X-ray fluorescence spectroscopy with Thermo FisherK-Alpha.

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. Results and discussion

Using one-step MACE method, which combines the metal cata-yst deposition with the silicon dissolution in the mixture solutiont the same time, we got the homogeneous etching of the sili-on, as shown in the Fig. 1. The surface morphology of the etchedilicon is shown in Fig. 1a using 3D AFM image. The correspond-ng average roughness is around 13 nm, which is smooth at theanometer scale, rather than smooth at macroscopic level [13]. Toividly illustrate the surface smoothness of the etched silicon, theeflectance comparison of the silicon wafers before and after etch-ng is shown in Fig. 1b. Their reflectance values have very littleariation in the wavelength range of 300–1000 nm, suggesting thathe surface of the etched silicon is as smooth as the original wafer,hich is absolutely different with the nanopore structure obtained

y silver-catalyzed etching of silicon in HF/H2O2 solution [5]. Theurface roughness of the etched silicon wafer is varying as a func-ion of etch conditions; there is still a large space to improve byurther optimizing the process parameters.

In addition, the ultra-thin silicon sample with a thickness below0 �m can be achieved simply. In Fig. 1d, it can be seen that theverage thickness of the silicon wafer decreased from 525 �m to0 �m after 22 min etching reaction. And this method is suitable for

arge scale thinning application. A 4 in. silicon wafer with a thick-ess of 98 �m, obtained by this method, is ready for large-scale use,s depicted in Fig. 1c.

What’s the mechanism that explains the observation of theomogeneous etching of silicon with MACE method? In MACErocess, as catalyst agents, noble metal nanoparticles catalyzehe production of holes from chemical oxidants, commonly H2O2,hich are then injected into the valence band of the silicon, result-

ng in the anisotropic dissolution of the silicon [1,14,15]. Therefore,he diffusion of the injected holes around the silicon underneathhe noble metal particles is the key factor to the dissolution of theilicon. To enable homogeneous etching of silicon in the MACE pro-ess, a homogeneous distribution of the injected holes on the siliconurface is essential. To achieve this goal, in the present work, both

xcessive H2O2 and a suitable concentration AgNO3 are introducednto HF solution, which facilitate the generation and the diffusionf holes, and densify the silver nanoparticles, finally leading to theomogeneous etching of the silicon.

ig. 1. (a) 3D AFM image of the silicon surface etched for 25 min at 30 ◦C; (b) The reflectilicon wafer thinned in the solution for 20 min at 50 ◦C; (d) The comparison of the originatching the silicon sample for 22 min at 30 ◦C; The inset in Fig. 1b is the photos of the originnd concentration, containing 4.6 M HF, 0.02 M AgNO3, and 5 M H2O2.

ence 273 (2013) 107– 110

In details, the basic processes of the silicon selective etchingby MACE mainly includes the oxidation of silicon by injected holesand the dissolution in the HF solutions by reaction (1). At the siliconareas with metal covered, the hole injection from metal into the sil-icon can perform with the presence of oxidizing agents by reaction(2). However, at the silicon areas without metal covered, the directinjection of hole from solution is impossible. Therefore, usually thedistribution of holes on the surface is un-uniform, causing selectiveetching of silicon. However, at the silicon areas without metal cov-ered, the slightly etching layer (several nanometers in thickness)was observed [1,16–18]. And the etching layer became obviouswith increasing the content of oxidizing agents in the etchant,which is related to the diffusion of the injected holes around thesilicon underneath the metal [1]. Hence, to get the homogeneousetching of silicon, a uniform distribution of holes on the silicon sur-face is crucial, requiring fast diffusion of injected holes to the siliconsurface there without metal covered. In view of this point, a densedistribution of Ag particles, and Ag nanoparticles with small size areable to meet this requirement. These small and dense Ag nanoparti-cles can provide much more sites of hole injection and also shortenthe diffusion distance of the injected holes in the vicinage of AgNPs, which facilitate the homogeneous distribution of holes on thesilicon surface.

Si + 6HF + 4h+ → H2SiF6 + 4H+ (1)

H2O2 + 2H+ → 2H2O + 2h+ (2)

To make sure these analyses, the etchings of silicon wafers as afunction of H2O2 concentration were investigated. Fig. 2 shows SEMimages of the etched silicon samples in the presence of H2O2 withdifferent concentrations. When the concentration of H2O2 is low, ascan be seen from Fig. 1a, the surface of resultant silicon is texturedby silicon nanowires arrays. With the increasing of H2O2 concentra-tion, the unordered silicon pillars form on the silicon substrate (asshown in Fig. 2b). It is noted that the porous layer is observed on thesurface of these pillars (as displayed in Fig. 2b inset). When the con-centration of H2O2 increases to 1 M in the etching bath, the silicon

surface appears quite flat, as shown in Fig. 2c. These results indicatethat the etching of the single crystal silicon can vary from selectiveetching to homogeneous etching with the increase of H2O2 con-centration. In theory, the addition of massive H2O2 into HF/AgNO3

ance comparison of the silicon wafer before and after etching; (c) A picture of 4 in.l silicon wafer and the thin silicon wafer, which obtained through homogeneouslyal silicon wafer and the thin silicon wafer. All etchant solution has same composite

F. Bai et al. / Applied Surface Science 273 (2013) 107– 110 109

F ing 4.6 M HF, 0.02 M AgNO3 and H2O2 with different concentration at 30 ◦C. (a) 0 M; (b)0

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ig. 2. Surface morphologies of the silicon samples etched in the solution contain.5 M; (c) 5 M. Inset of (b) is the magnified image of silicon pillars.

olution can avoid the formation of silver dendrites or larger silverarticles. These larger silver particles can be dissolved into Ag+ ionsue to the larger redox potential of H2O2 (1.72 eV) compared to thatf Ag (0.79 eV) by reaction (3) in this case [19]. And the dissolvedg+ ions can re-form small Ag NPs on the silicon surface by reac-

ion (4) due to a continuous chemical reduction reaction in thistchant system. Hence, the number of small Ag NPs is increasedelatively, which facilitates their dense deposition on the siliconurface. Meanwhile, the existence of massive H2O2 can promotehe generation and accumulation of the injected holes at the Ag/Sinterface. And, a number of holes accumulated can diffuse to theicinity of Ag nanoparticles on the silicon surface. Therefore, withhe assistance of H2O2, a homogeneous distribution of holes onhe silicon surface can be realized, and results in the homogeneoustching of silicon observed in this work.

Ag + H2O2 + 2H+ → 2Ag+ + 2H2O (3)

g+ + e → Ag (4)

Moreover, the one-step MACE etching method can be a control-able approach through adjusting the etching conditions, including

2O2 concentration, reaction temperature, and etching time, ando on. H2O2 concentration also influences the etching rate. Whenhe concentration of H2O2 was over 1 M, the etching rate linearlyncreased with the increasing of H2O2 concentration and finallypproached to a saturation value of 18.4 �m/min in the case of 5 M2O2, as shown in Fig. 3a. Also, the etching rate increases rapidlyith increasing reaction temperature (as depicted in Fig. 3b). Fur-

hermore, it is noted that the etching rate can change with time,esulting from the consumption of the etchant during the reaction

rocess. Fig. 3c gives the relation between the etching rate andhe reaction time at 40 ◦C. The etching rate firstly increases withncreasing reaction time, reaches to a peak value of 35.8 �m/min,nd finally decreases rapidly. The decrease of the etching rate

ig. 3. Variations of etching rate with (a) H2O2 concentration, (b) reaction temperature an.02 M, and 5 M, respectively.

Fig. 4. X-ray fluorescence spectroscopy images of thin silicon wafer before and afterHNO3 treatment.

originates from the consumption of the etchant while the increaseof the etching rate is due to the increased solution temperatureresulting from strongly exothermic reactions [20]. The effects ofreaction temperature and reaction time on the surface morphol-ogy of etched silicon have also been studied, and these effects arefound to be quite weak. These results suggest that high concentra-tion of H2O2 (5 M) and high temperature (30–50 ◦C) can promotethe thinning of silicon wafer. Hence, the thin silicon samples withpredictable thickness can be achieved through adjusting etching

rate and reaction time in the thinning process.

Moreover, majority of Ag NPs are retained on silicon surfaceafter thinning silicon wafer. And the presence of Ag NPs in thin

d (c) time, respectively. The concentration of HF, AgNO3 and H2O2 is fixed at 4.6 M,

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ilicon can degrade the performance of the silicon based device.n this case, thin silicon wafers are cleaned using concentratedNO3 to remove Ag contaminants on the thin silicon wafer. And

he information of Ag in the thin silicon wafer was detected by-ray fluorescence spectroscopy. From Fig. 4, it can be seen that,efore and after HNO3 treatment, the atom ratio of Ag to Si on thehin silicon wafers dramatically decreases from 34.53% to 0.14% bymmerging it into concentration HNO3 just for 1 min. It is believedhat Ag contaminants in thin silicon can be removed absolutely by

ultiple cleaning or prolonging cleaning durations. Therefore, thebtained thin silicon wafers by the silver-assisted homogeneoustching has less metal contaminants, and can be applied to producelectronic device.

We anticipate that the facile, predictable and controllable homo-eneous etching process presented will accelerate the developmentnd application of ultra-thin silicon wafer, including flexible elec-ronics, ultra-thin chips, and thin silicon solar cell with mechanicalexible property.

. Conclusions

In conclusion, the MACE technique is improved to homoge-eously remove silicon materials, and realize the thinning of theingle crystal silicon wafer. In the case of massive H2O2, the homo-eneous etching of silicon was observed, which could be explainedy the homogeneous distribution of injected hole on the surfacef the silicon. And the suitable concentration of AgNO3 is also aey factor to realize the homogeneous etching, which promotesetting the small and dense Ag nanoparticle during the reactionrocessing. Using this electroless method, the large scale ultra-hin (∼30 �m) silicon is obtained, while the surface roughnessf the etched silicon is 13 nm. Meanwhile the thickness can be

odulated by etching rate and reaction time. This simple method

rovides a novel approach to get ultra-thin (<50 �m) silicon,hich opens applications in the field of microelectronics and solar

ells.

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ence 273 (2013) 107– 110

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant No. 51172069), and Ph.D. ProgramsFoundation of Ministry of Education of China (20110036110006),and the Fundamental Research Funds for the Central Universities(Key project 11ZG02).

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