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Fast speed pore formation via strong oxidizers
Electrochimica Acta 52 (2007) 6728–6733
Fast speed pore formation via strong oxidizers X.Q. Bao a,b , J.W. Jiao a,∗ , J. Zhou a , Y.L. Wang a a
State Key Laboratory of Transducer Technology, National Key Laboratory of Microsystem Technology, Shanghai Institute of Microsystem and Information, Chinese Academy of Sciences, Shanghai, China b Graduate School of Chinese Academy of Sciences, Beijing, China Received 15 March 2007; received in revised form 11 April 2007; accepted 21 April 2007 Available online 29 April 2007
Abstract It is difficult to produce nice macropores at high speed with strong oxidants via the breakdown mechanism on low doped n-Si. In this letter, HF-containing electrolytes were modified with strong oxidants; galvanostatically anodizing in the dark, macropores with depths upto 80 m and pore-diameters between 100 and 1000 nm were fast produced on low doped substrate. Two new phenomena were observed: pore tips could be tumefied simultaneously and macropores could be filled with a uniform microporous layer under dark conditions. Mild short-range ordering of pores was observed. The macropores are rather nice while the etching rate could reach the highest of its kind, 1800 m/h. The underlying mechanism was in the framework of the current-burst-model combined with avalanche breakdown; both pore pitch and pore-wall thickness could be significantly less than SCR (space charge region) width. Published by Elsevier Ltd. Keywords: Fast speed; Current-burst-model; Avalanche breakdown; Densely arrayed macropores; Nonlithographic photonic-crystal formation
1. Introduction Numerous reports concerning macropore formation were advanced with respect to SCR (space charge region) effects, electrochemical oxidation, hydrogen passivation, diffusionlimited random walk, surface instabilities, current bursts, etc. [1–29]. Based on the current-burst-model (CBM) [7] and via modulating the oxidizing power and conductivity of electrolytes (to mimic the conditions encountered in InP [26]), Frey et al. obtained 50–3000 nm macropores of nice quality with a growth speed up to 500 m/h on moderately doped n-Si [27,28], where a 15 wt.% organic and conductive HF electrolyte [based on a mixture of MeCN (acetonitrile) and TBAP (tetrabutylammonium perchlorate)] was employed. Their electrolytes were tuned on purpose to be weakly oxidizing in order to suppress oxide formation and hence enhance etching speed, since oxide dissolution is the most time-consuming part in a current burst. Low doped material was also avoided due to its high resistivity, thus making
∗
Corresponding author. Tel.: +86 21 62511070x5472; fax: +86 21 62131744. E-mail addresses: [email protected], [email protected] (J.W. Jiao). 0013-4686/$ – see front matter. Published by Elsevier Ltd. doi:10.1016/j.electacta.2007.04.088
it unsuitable for generating breakdown-currents at acceptable biases. However, even on low doped n-Si, it is still possible to produce nice macropores via strong oxidants with an etching speed comparable to or higher than the aforementioned 500 m/h. In our previous work [30], sparsely arrayed (i.e., with a pore spacing quite larger than pore-diameters) macropores and heavily branched mesopores were potentiostatically fabricated under low current densities (0–5 mA/cm2 ). Hence, it was found to be difficult to obtain both “nice” (i.e., straight, cylindrical and rather smooth) and densely arrayed (i.e., with a pore spacing comparable to pore-diameters) macropores with a low current density. Nevertheless, it is probable to galvanostatically obtain densely arrayed “nice” pores for a high HF concentration, if two basic conditions, a strong oxidizing electrolyte and a high current density (compared with a low one, 0–5 mA/cm2 , for an electrolyte of identical composition) could be fulfilled simultaneously. According to CBM [7], such combined conditions could promote hole-intensive oxidation and suppress direct dissolution but not necessarily etching speed: the correlation between etch rate and applied bias could be considerably positive for an electrolyte of identical composition as shown in this work. Thus, on low doped n-Si with electrolytes of strong oxidizing power [19], a high etching rate could be realized via an
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enhanced bias and/or HF concentration. Each of such enhanced factors can compensate for the loss of speeds induced by strong oxidizers. Generally, this work aims at fast growing densely arrayed nice pores with a strong oxidizier, which might potentially enable nonlithographic photonic-crystal formation [26–29]. The etching rate could be driven upto 1800 m/h. Not least, SCR effects and pore-wall passivation mechanisms were evaluated and the conclusions dissented quite a bit from the SCR model [1,8]. 2. Experiments N-type samples (3–8 cm, 450 m-thick, CZ-grown, [1 0 0]-oriented and polished) were used. A 500 nm-thick Au film was sputtered on the backside of the specimen to establish a good ohmic contact. Galvanostatic conditions of 15–600 mA/cm2 (corresponding to biases of 27–50 V) were applied from an Agilent power source with a tolerance down to 0.001 mA. The electrolytes were based on a mixture of 48 wt.% hydrofluoric acid and 30 wt.% H2 O2 solution; the HF concen-
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tration of electrolytes is expressed by the percentage of HF acid and ranges from 20 to 50% (by volume unless otherwise stated). All electrochemical etchings were conducted in the dark on an anodization platform in a decontamination chamber at 18 ± 1 ◦ C. The purely chemical silicon dissolution for all solutions was checked under open circuit conditions. The etching rate of chemical dissolution was found to be nearly three orders of magnitude slower than that of electrochemistry and thus can be neglected at 18 ± 1 ◦ C. Therefore, the silicon dissolution observed was electrochemical in nature. The pores produced were investigated with a field-emission scanning electron microscope (FESEM: Hitachi S-4700). Plan view micrographs show the (1 0 0) plane, whereas after cleaving the samples, cross-section view micrographs show the pores in the (1 1 0) cleavage planes. 3. Results First, for a given 50% HF concentration, the formation of macropores as a function of current density was investigated.
Fig. 1. SEM-micrographs: 10 ml HF in 10 ml 30 wt.% H2 O2 solution, 600 mA/cm2 for 2.5 min, minimum bias observed: 33 V. (a) Plan-view: the distance between macropores is approx. 1.5 m. Five examples of hexagonal close-packed lattice of pores are marked in five circles, but are not well developed. (b) Cross-section: macropores with depths upto 75 m are obtained; the macroporous layer is rather uniform. The pore tips are rather sharp and smooth (in the inlay). (c) Cross-section: stable macropores show a diameter around 1 m and an average wall thickness about 400 nm. The pores could be occasionally interconnected due to a local tumescence but not due to traditional branching effect, as marked in the picture.
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Fig. 2. SEM-micrographs: 50% HF (10 ml HF in 10 ml 30 wt.% H2 O2 solution), 500 mA/cm2 for 2.5 min, minimum bias observed: 27 V. (a) Plan-view: macropores with top diameters about 400 nm are densely arrayed. The distance between macropores is approx. 1 m (cf. the inlay and (b)). (b) Cross-section: stable pores show a diameter around 700 nm and an average wall thickness about 300 nm. The pores are straight, cylindrical and rather smooth.
Decreasing the current density from 600 mA/cm2 (Fig. 1) to 500 mA/cm2 (Fig. 2), the diameters of stable macropores deceased from approximately 1000 nm to about 700 nm, while the average pore pitches deceased from −1.5 to −1.1 m. In Fig. 1, macropores with depths upto 75 m are obtained in 2.5 min; the etching speed was as high as 1800 m/h. The macroporous layer is rather uniform and the pore tips are rather sharp and smooth. The macropores are generally nice, that is, straight, cylindrical and rather smooth (Figs. 1 and 2). However, the pores could be occasionally interconnected due to a local tumescence, i.e., a local enhancement of pore sizes. Note that such interconnections of pores only happen on an extremely limited part of the pore-walls, showing a distinct difference with the network-like interconnection via the traditional branching effect, often seen in classical n-mesopores [19]. In both cases (Figs. 1 and 2), the pores were densely arrayed in a self-organized manner; weak short-range ordering of pores, i.e.,
hexagonally close-packed lattice in limited space, was observed [28]. Then, the dependence of pore morphology on HF concentration was investigated in detail. In Fig. 3 (40% HF concentration, 200 mA/cm2 for 5 min), macropores with depths upto 65 m are obtained: the etching speed could reach 780 m/h. The pores are generally straight, cylindrical but not as smooth as in the cases of Figs. 1 and 2, since a thin microporous layer is omnipresent on the pore-walls in the unique case of Fig. 3. To our knowledge, this was the first time that a microporous layer in macropores was evidenced under “dark” conditions, that is, without illumination [19,21]. In addition, the pore tips are tumefied with a synchronized phase, i.e., simultaneously (cf. the inlay of Fig. 3(a)). By decreasing the HF concentration from 40 to 20% (Fig. 4, 15 mA/cm2 for 10 min), stable macropores with depths upto 60 m and diameters around 100–200 nm were obtained: the etch rate was upto 360 m/h. Compared with our
Fig. 3. SEM-micrographs: 40% HF (10 ml HF in 15 ml 30 wt.% H2 O2 solution), 200 mA/cm2 for 5 min, minimum bias observed: 35 V. (a) Cross-section: macropores with depths upto 65 m are shown. Largely, the pore tips are tumefied with a synchronized phase (cf. the inlay). (b) Cross-section: stable pores show a diameter of approx. 500 nm and an average wall thickness of about 500 nm. The pores are generally straight and cylindrical but a little rough; all pores are filled with a thin microporous layer.
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Fig. 4. SEM-micrographs: 20% HF (10 ml HF in 40 ml 30 wt.% H2 O2 solution), 15 mA/cm2 for 10 min, minimum bias observed: 46 V. Cross-section: stable macropores with depths upto 60 m and diameters around 100–200 nm (cf. the inlay) are shown. The pores are rather nice.
previous finding where weakly branched macropores occurred with an electrolyte of identical composition but under a constant bias [30], the macropores in Fig. 4 are unexpectedly nice (i.e., straight, cylindrical, smooth and without branches), but the pores are not so densely arrayed as the aforementioned cases (Figs. 1–3). Additionally, in some cases, branched pores [30] or macroetchpits [8] could occasionally occur in a limited area of the etched samples and were not shown in this work. 4. Discussion First, a fast etching speed (360–1800 m/h) was reached (Figs. 1–4); three factors could contribute to the fast etching. The first one is the high HF concentration (20–50%) adopted in this work, which, according to CBM [7], will accelerate the direct removal of silicon and the dissolution of the oxides (electrochemically generated by oxidants, the H2 O2 component for our case), compared with a lower HF concentration. The second factor is the rather high biases (27–50 V) observed in our experiment, which could also enhance the etch rate for a fixed HF concentration. In other words, if a lower bias (resulting in a lower current density at large) was applied in each case of Figs. 1–4, the etching speed could be obviously suppressed, since for an electrolyte of identical composition, the correlation between etch rate and applied bias could be highly positive, that is, etch rate could obviously increase with an enhanced bias. As shown in our previous work [30], where 120 m porous layer were obtained in 20 min under a 12 V voltage with the same electrolyte as that of Fig. 1, the etch rate was thus 360 m/h, significantly slower than in the case of Fig. 1 (1800 m/h), where the minimum bias was observed as 33 V. Note that similar results concerning the positive correlation between etch rate and bias were frequently obtained in our other experiments. The third element concerning fast etching is the conductivity of electrolytes. If the resistivity of the electrolyte is rather high, the ohmic and hence potential losses in the pores will be signif-
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icant, accordingly, resulting in a negative influence over fast etching. In this experiment, the conductivity of the HF/H2 O2 system is rather good (albeit not perfect). As shown in Figs. 1–4, the porous layers of considerable depths are rather uniform and no obvious puff pastry-like structures [27] were observed; this indicates that our selected electrolytes are of good conductivity. By adding some salts, the conductivity of the HF/H2 O2 system can be further enhanced. In short, according to CBM [7], the combination of three factors, i.e., a high HF concentration, a rather large bias and a good conductivity of electrolytes, contributed to fast pore etching for our cases, although the negative impact on speed induced by the strong oxidizer (H2 O2 ) could exist. The H2 O2 component does enhance the oxidation and thus the time of indirect dissolution in every single current burst, but the loss of speeds induced by strong oxidizers has been greatly compensated by the rate-enhancing factors as addressed above. For Figs. 1–3, the distance between pores varies from 1 to 1.5 m and the pore-wall thickness is about 300–500 nm, while the SCR width ranges from 3.6 m (corresponding to the minimum bias observed 27 V (Fig. 2) and a resistivity of 4 cm [31,32]) to 5 m (corresponding to the maximum bias 50 V and a resistivity of 4 cm [31,32]). According to the SCR model, the theoretical pore-wall thickness should equal 7.2–10 m (i.e., twice the SCR width), which is at least 1 order of magnitude (or more accurately, approx. 20 times for Figs. 1 and 2 and 16 times for Fig. 3) larger than the actual thickness of the pore-walls (300–500 nm). Even as a rule of thumb, the experiential pore pitch (7.2–10 m, equaling twice the SCR width) is about 5–10 times the actual pore pitch (1–1.5 m). Largely, this indicates that SCR effects could not affect both pore spacing and especially pore-wall thickness, thus dissenting from the SCR model. It is true at least when the current densities are relatively high, as shown in this work (compared with our previous work [30], where low current densities (0–5 mA/cm2 ) were applied for a 20 and 50% HF/H2 O2 electrolyte). What really passivates the pore-wall in our case must be the greatly biased distribution of field strength determined by three critical elements, i.e., the morphological, geometric and, probably, ohmic difference between pore-wall and pore tip. First, the morphology is different: pore tips are usually conic (or sometimes quasi-pyramidic in our experience) while a cylindrical pore may demonstrate any shape between a circle and a square. Even for the same radius of a hemispherical tip (usually as an approximation of a real pore tip [8]) as that of a circularly cylindrical wall (usually as an approximation of a real pore-wall [8]), the electric field strength of the pore tip is much greater than that of the pore-wall, if etching with an identical bias and electrolyte [8]. Secondly, the geometry is usually distinct. If considering a single pore, the radius of the pore tip is generally much smaller than that of the pore-wall (cf. the inlay of Fig. 1(b)). This further differentiates the field strength between a tip and a wall. Last but not least, the resistivity of thin walls (300–500 nm for our case) could probably be markedly higher than that of bulk silicon; however, this is more a “guess” than a fact to date. At least, for mesoporous silicon, this could be true as reported in existing works [33–35], the resistivity of mesoporous layer could be
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drastically enhanced by a factor upto more than 105 , compared with bulk silicon. No backside illumination was employed. The electronic holes needed must be generated by a breakdown mechanism. As mentioned in an existing work [8], the electric field strength, depending on both the pore tip radius and applied bias, could exert a decisive influence over the type of breakdown (either avalanche or tunneling). For our cases, the pore tip radii (usually quite smaller than pore radii [8], as indicated in the inset of Fig. 1(b)) range from 50 to 300 nm; accordingly, 14–25 V is enough to generate obvious avalanche multiplication around the pore tip (with a 50–300 nm radius as mentioned above) [8]. The actual bias applied was 27–50 V, significantly larger than the threshold bias (14–25 V) needed for avalanche breakdown, and therefore avalanche effects can dominate in all cases (Figs. 1–4). In the inset of Fig. 3(a), the pore tips were tumefied with a synchronized phase, i.e., simultaneously. This indicates that a localized electropolishing (EP) has occurred at the region of the pore tips due to a simultaneous decrease of both HF concentration and passivating species (H+ for our case) in the pores. This is possible if the diameters of all pores are rather small and quite uniform. Actually, this is the case as shown in Fig. 3(a) and (b). It is known that a lower HF concentration will lead to a lower critical current density jPS (indicating the lower limit of current density needed for EP), which makes possible a synchronized EP at the deep region of the porous layer (of 65 m depth for this case), where a constant and rather large current density (200 mA/cm2 for this case) was maintained [27]. Moreover, the increasing loss of passivating species over pore depth will facilitate the lateral tumescence (i.e., enlargement of diameters) of pore tips, since the hydrogen passivation of (1 1 1) facets and thus the preferential etching along the (1 0 0) direction will be accordingly weakened [23,24]. In short, due to the decrease of two factors (HF concentration and passivating species) over pore depth, the synchronized tumescence of pore tips can be understood. In Fig. 3(b), macropores filled with a thin uniform microporous layer (PSL) were also observed. To our knowledge, for n-Si to date, such macropores have only been observed under the conditions where illumination was used (never under dark conditions) [19,21]. This indicates that under optimized circumstances, the formation of macropores filled with a thin PSL is still possible in the dark, i.e., etching without illumination. The PSL can be removed by an anisotropic wet alkali etchant, e.g., diluted KOH solution. The formation of PSL is largely in the framework of the “quantum-size effect” model [36]. To the best of our knowledge, this is the first time that close-packed macropores of good quality were fast produced in the dark on low doped n-type silicon. Rather nice macropores with depths upto 80 m and pore-diameters between 100 and 1000 nm were produced in a self-organized manner. However, only weak short-range ordering of pores was observed (Figs. 1 and 2), as marked in the circles of Fig. 1(a): the hexagonal symmetry of close packing was severely frustrated by the fourfold symmetry of crystal for our (1 0 0) samples [27], where the rather high resistivity of low doped material could also exert a negative influence [26–29]. Nevertheless, via lithography, it
is highly likely that 100–1000 nm pores of hexagonal symmetry could be quickly fabricated by our method (as implied in Figs. 1–4). Even without lithography but on moderately doped (1 1 1) n-Si (which possesses both an ideal resistivity and a threefold symmetry of crystal [27]), it is still possible to rapidly fabricate 50–1000 nm pores of short or even long range ordering with HF/H2 O2 electrolytes, which might ultimately empower 2 or 3D nonlithographic PC (photonic-crystal) formation [26–29]. 5. Conclusion HF-containing electrolytes were modified with a strong oxidizer H2 O2 ; under dark conditions, densely arrayed macropores with depths upto 80 m and pore-diameters between 100 and 1000 nm were galvanostatically formed on low doped n-Si. Poretips could be tumefied with a synchronized phase; under dark conditions, macropores could be filled with a thin microporous layer. Weak short-range ordering of pores was evidenced. The etching rate could reach 1800 m/h. The current-burst-model combined with avalanche effects was employed to interpret the underlying mechanism; SCR width could not affect pore spacing and pore-wall thickness. Acknowledgements This work was supported by Chinese National “863” (No.: 2006AA04Z312) and“973” (No.: 2006CB300403) Project. Samsung Advanced Institute of Technology was also acknowledged. References [1] V. Lehmann, H. F¨oll, J. Electrochem. Soc. 137 (1990) 653. [2] V. Lehmann, J. Electrochem. Soc. 140 (1993) 2836. [3] M.H. Al Rifai, M. Christophersen, S. Ottow, J. Carstensen, H. F¨oll, J. Electrochem. Soc. 147 (2000) 627. [4] E.K. Propst, P.A. Kohl, J. Electrochem. Soc. 141 (1994) 1006. [5] E.A. Ponomarev, C. Levy-Clement, J. Electrochem. Soc. 1 (1998) 1002. [6] V. Lehmann, S. R¨onnebeck, J. Electrochem. Soc. 146 (8) (1999) 2968. [7] J. Carstensen, M. Christophersen, H. F¨oll, Mater. Sci. Eng. B 69–70 (2000) 23. [8] V. Lehmann, R. Stengl, A. Luigart, Mater. Sci. Eng. B 69–70 (2000) 11. [9] J.C. Claussen, J. Carstensen, M. Christophersen, S. Langa, H. Foll, Chaos 13 (1) (2003) 217. [10] M.J.J. Theunissen, J. Electrochem. Soc. 119 (1972) 351. [11] M.I.J. Beale, J.D. Benjamin, M.J. Uren, J. Cryst. Growth 73 (1985) 622. [12] R.L. Smith, S.F. Chuang, S.D. Collins, J. Electron. Mater. 17 (1988) 533. [13] P.C. Searson, J.M. Macaulay, S.M. Prokes, J. Electrochem. Soc. 139 (1992) 3373. [14] Y. Kang, J. Jorne, J. Electrochem. Soc. 140 (1993) 2258. [15] V.P. Parkhutik, in: Z.C. Feng, R. Tsu (Eds.), Morphology of Porous Silicon Layers, Porous SiliconWorld Scientific, 1994, p. 301. [16] A. Valance, Phys. Rev. B 52 (1995) 8323. [17] R.B. Wehrspohn, F. Ozanan, J.N. Chazalviel, J. Electrochem. Soc. 146 (9) (1999). [18] M. Christophersen, J. Carstensen, H. F¨oll, Physica Status Solidi(a) 182 (1) (2000) 45. [19] H. F¨oll, M. Christophersen, J. Carstensen, G. Hasse, Mater. Sci. Eng. R 39 (2002) 93. [20] S. Matthias, F. M¨uller, J. Schilling, U. G¨osele, Appl. Phys. A 80 (2005) 1391.
X.Q. Bao et al. / Electrochimica Acta 52 (2007) 6728–6733 [21] X.G. Zhang, Electrochemistry of Silicon and Its Oxides, Kluwer Academic, Horwell, MA, 2001. [22] X.G. Zhang, J. Electrochem. Soc. 151 (1) (2004) C69. [23] P. Allongue, V. Costa-Kieling, H. Gerischer, J. Electrochem. Soc. 140 (1993) 1009. [24] P. Allongue, C. Villeneuve, L. Pinsard, M. Bernard, Appl. Phys. Lett. 67 (7) (1995) 941. [25] P.C. Searson, J.M. Macaulay, F.M. Ross, J. Appl. Phys. 72 (1992) 253. [26] H. F¨oll, S. Langa, J. Carstensen, M. Christophersen, I.M. Tiginyanu, Adv. Mater. 15 (2003) 183. [27] S. Frey, M. Kemell, J. Carstensen, S. Langa, H. F¨oll, Phys. Stat. Sol. (a) 202 (8) (2005) 1369. [28] H. F¨oll, J. Carstensen, S. Frey, J. Nanomater. 2006 (2006) 1.
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[29] S. Langa, I.M. Tiginyanu, J. Carstensen, M. Christophersen, H. F¨oll, Appl. Phys. Lett. 82 (2) (2003) 278. [30] X.Q. Bao, J.W. Jiao, Y.L. Wang, K.W. Na, H. Choi, J. Electrochem. Soc. 154 (3) (2007) D175. [31] S.M. Sze, Physics of Semiconductor Devices, 2nd ed., Wiley, New York, 1981, p. 249. [32] X.Q. Bao, J.W. Jiao, Y.L. Wang, Y. Zhang, D.H. Ge, K.W. Na, H. Choi, Electrochem. Commun. 9 (2007) 1491. [33] M.I.J. Beale, N.G. Chew, M.J. Uren, A.G. Cullis, J.D. Benjamin, Appl. Phys. Lett. 46 (1) (1985) 86. [34] D. Bilenko, Sov. Phys. Semicond. 17 (1983) 1336. [35] T. Unagami, J. Electrochem. Soc. 127 (1980) 476. [36] V. Lehmann, U. G¨osele, Appl. Phys. Lett. 58 (1991) 856.
Fast speed pore formation via strong oxidizers X.Q. Bao a,b , J.W. Jiao a,∗ , J. Zhou a , Y.L. Wang a a
State Key Laboratory of Transducer Technology, National Key Laboratory of Microsystem Technology, Shanghai Institute of Microsystem and Information, Chinese Academy of Sciences, Shanghai, China b Graduate School of Chinese Academy of Sciences, Beijing, China Received 15 March 2007; received in revised form 11 April 2007; accepted 21 April 2007 Available online 29 April 2007
Abstract It is difficult to produce nice macropores at high speed with strong oxidants via the breakdown mechanism on low doped n-Si. In this letter, HF-containing electrolytes were modified with strong oxidants; galvanostatically anodizing in the dark, macropores with depths upto 80 m and pore-diameters between 100 and 1000 nm were fast produced on low doped substrate. Two new phenomena were observed: pore tips could be tumefied simultaneously and macropores could be filled with a uniform microporous layer under dark conditions. Mild short-range ordering of pores was observed. The macropores are rather nice while the etching rate could reach the highest of its kind, 1800 m/h. The underlying mechanism was in the framework of the current-burst-model combined with avalanche breakdown; both pore pitch and pore-wall thickness could be significantly less than SCR (space charge region) width. Published by Elsevier Ltd. Keywords: Fast speed; Current-burst-model; Avalanche breakdown; Densely arrayed macropores; Nonlithographic photonic-crystal formation
1. Introduction Numerous reports concerning macropore formation were advanced with respect to SCR (space charge region) effects, electrochemical oxidation, hydrogen passivation, diffusionlimited random walk, surface instabilities, current bursts, etc. [1–29]. Based on the current-burst-model (CBM) [7] and via modulating the oxidizing power and conductivity of electrolytes (to mimic the conditions encountered in InP [26]), Frey et al. obtained 50–3000 nm macropores of nice quality with a growth speed up to 500 m/h on moderately doped n-Si [27,28], where a 15 wt.% organic and conductive HF electrolyte [based on a mixture of MeCN (acetonitrile) and TBAP (tetrabutylammonium perchlorate)] was employed. Their electrolytes were tuned on purpose to be weakly oxidizing in order to suppress oxide formation and hence enhance etching speed, since oxide dissolution is the most time-consuming part in a current burst. Low doped material was also avoided due to its high resistivity, thus making
∗
Corresponding author. Tel.: +86 21 62511070x5472; fax: +86 21 62131744. E-mail addresses: [email protected], [email protected] (J.W. Jiao). 0013-4686/$ – see front matter. Published by Elsevier Ltd. doi:10.1016/j.electacta.2007.04.088
it unsuitable for generating breakdown-currents at acceptable biases. However, even on low doped n-Si, it is still possible to produce nice macropores via strong oxidants with an etching speed comparable to or higher than the aforementioned 500 m/h. In our previous work [30], sparsely arrayed (i.e., with a pore spacing quite larger than pore-diameters) macropores and heavily branched mesopores were potentiostatically fabricated under low current densities (0–5 mA/cm2 ). Hence, it was found to be difficult to obtain both “nice” (i.e., straight, cylindrical and rather smooth) and densely arrayed (i.e., with a pore spacing comparable to pore-diameters) macropores with a low current density. Nevertheless, it is probable to galvanostatically obtain densely arrayed “nice” pores for a high HF concentration, if two basic conditions, a strong oxidizing electrolyte and a high current density (compared with a low one, 0–5 mA/cm2 , for an electrolyte of identical composition) could be fulfilled simultaneously. According to CBM [7], such combined conditions could promote hole-intensive oxidation and suppress direct dissolution but not necessarily etching speed: the correlation between etch rate and applied bias could be considerably positive for an electrolyte of identical composition as shown in this work. Thus, on low doped n-Si with electrolytes of strong oxidizing power [19], a high etching rate could be realized via an
X.Q. Bao et al. / Electrochimica Acta 52 (2007) 6728–6733
enhanced bias and/or HF concentration. Each of such enhanced factors can compensate for the loss of speeds induced by strong oxidizers. Generally, this work aims at fast growing densely arrayed nice pores with a strong oxidizier, which might potentially enable nonlithographic photonic-crystal formation [26–29]. The etching rate could be driven upto 1800 m/h. Not least, SCR effects and pore-wall passivation mechanisms were evaluated and the conclusions dissented quite a bit from the SCR model [1,8]. 2. Experiments N-type samples (3–8 cm, 450 m-thick, CZ-grown, [1 0 0]-oriented and polished) were used. A 500 nm-thick Au film was sputtered on the backside of the specimen to establish a good ohmic contact. Galvanostatic conditions of 15–600 mA/cm2 (corresponding to biases of 27–50 V) were applied from an Agilent power source with a tolerance down to 0.001 mA. The electrolytes were based on a mixture of 48 wt.% hydrofluoric acid and 30 wt.% H2 O2 solution; the HF concen-
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tration of electrolytes is expressed by the percentage of HF acid and ranges from 20 to 50% (by volume unless otherwise stated). All electrochemical etchings were conducted in the dark on an anodization platform in a decontamination chamber at 18 ± 1 ◦ C. The purely chemical silicon dissolution for all solutions was checked under open circuit conditions. The etching rate of chemical dissolution was found to be nearly three orders of magnitude slower than that of electrochemistry and thus can be neglected at 18 ± 1 ◦ C. Therefore, the silicon dissolution observed was electrochemical in nature. The pores produced were investigated with a field-emission scanning electron microscope (FESEM: Hitachi S-4700). Plan view micrographs show the (1 0 0) plane, whereas after cleaving the samples, cross-section view micrographs show the pores in the (1 1 0) cleavage planes. 3. Results First, for a given 50% HF concentration, the formation of macropores as a function of current density was investigated.
Fig. 1. SEM-micrographs: 10 ml HF in 10 ml 30 wt.% H2 O2 solution, 600 mA/cm2 for 2.5 min, minimum bias observed: 33 V. (a) Plan-view: the distance between macropores is approx. 1.5 m. Five examples of hexagonal close-packed lattice of pores are marked in five circles, but are not well developed. (b) Cross-section: macropores with depths upto 75 m are obtained; the macroporous layer is rather uniform. The pore tips are rather sharp and smooth (in the inlay). (c) Cross-section: stable macropores show a diameter around 1 m and an average wall thickness about 400 nm. The pores could be occasionally interconnected due to a local tumescence but not due to traditional branching effect, as marked in the picture.
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Fig. 2. SEM-micrographs: 50% HF (10 ml HF in 10 ml 30 wt.% H2 O2 solution), 500 mA/cm2 for 2.5 min, minimum bias observed: 27 V. (a) Plan-view: macropores with top diameters about 400 nm are densely arrayed. The distance between macropores is approx. 1 m (cf. the inlay and (b)). (b) Cross-section: stable pores show a diameter around 700 nm and an average wall thickness about 300 nm. The pores are straight, cylindrical and rather smooth.
Decreasing the current density from 600 mA/cm2 (Fig. 1) to 500 mA/cm2 (Fig. 2), the diameters of stable macropores deceased from approximately 1000 nm to about 700 nm, while the average pore pitches deceased from −1.5 to −1.1 m. In Fig. 1, macropores with depths upto 75 m are obtained in 2.5 min; the etching speed was as high as 1800 m/h. The macroporous layer is rather uniform and the pore tips are rather sharp and smooth. The macropores are generally nice, that is, straight, cylindrical and rather smooth (Figs. 1 and 2). However, the pores could be occasionally interconnected due to a local tumescence, i.e., a local enhancement of pore sizes. Note that such interconnections of pores only happen on an extremely limited part of the pore-walls, showing a distinct difference with the network-like interconnection via the traditional branching effect, often seen in classical n-mesopores [19]. In both cases (Figs. 1 and 2), the pores were densely arrayed in a self-organized manner; weak short-range ordering of pores, i.e.,
hexagonally close-packed lattice in limited space, was observed [28]. Then, the dependence of pore morphology on HF concentration was investigated in detail. In Fig. 3 (40% HF concentration, 200 mA/cm2 for 5 min), macropores with depths upto 65 m are obtained: the etching speed could reach 780 m/h. The pores are generally straight, cylindrical but not as smooth as in the cases of Figs. 1 and 2, since a thin microporous layer is omnipresent on the pore-walls in the unique case of Fig. 3. To our knowledge, this was the first time that a microporous layer in macropores was evidenced under “dark” conditions, that is, without illumination [19,21]. In addition, the pore tips are tumefied with a synchronized phase, i.e., simultaneously (cf. the inlay of Fig. 3(a)). By decreasing the HF concentration from 40 to 20% (Fig. 4, 15 mA/cm2 for 10 min), stable macropores with depths upto 60 m and diameters around 100–200 nm were obtained: the etch rate was upto 360 m/h. Compared with our
Fig. 3. SEM-micrographs: 40% HF (10 ml HF in 15 ml 30 wt.% H2 O2 solution), 200 mA/cm2 for 5 min, minimum bias observed: 35 V. (a) Cross-section: macropores with depths upto 65 m are shown. Largely, the pore tips are tumefied with a synchronized phase (cf. the inlay). (b) Cross-section: stable pores show a diameter of approx. 500 nm and an average wall thickness of about 500 nm. The pores are generally straight and cylindrical but a little rough; all pores are filled with a thin microporous layer.
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Fig. 4. SEM-micrographs: 20% HF (10 ml HF in 40 ml 30 wt.% H2 O2 solution), 15 mA/cm2 for 10 min, minimum bias observed: 46 V. Cross-section: stable macropores with depths upto 60 m and diameters around 100–200 nm (cf. the inlay) are shown. The pores are rather nice.
previous finding where weakly branched macropores occurred with an electrolyte of identical composition but under a constant bias [30], the macropores in Fig. 4 are unexpectedly nice (i.e., straight, cylindrical, smooth and without branches), but the pores are not so densely arrayed as the aforementioned cases (Figs. 1–3). Additionally, in some cases, branched pores [30] or macroetchpits [8] could occasionally occur in a limited area of the etched samples and were not shown in this work. 4. Discussion First, a fast etching speed (360–1800 m/h) was reached (Figs. 1–4); three factors could contribute to the fast etching. The first one is the high HF concentration (20–50%) adopted in this work, which, according to CBM [7], will accelerate the direct removal of silicon and the dissolution of the oxides (electrochemically generated by oxidants, the H2 O2 component for our case), compared with a lower HF concentration. The second factor is the rather high biases (27–50 V) observed in our experiment, which could also enhance the etch rate for a fixed HF concentration. In other words, if a lower bias (resulting in a lower current density at large) was applied in each case of Figs. 1–4, the etching speed could be obviously suppressed, since for an electrolyte of identical composition, the correlation between etch rate and applied bias could be highly positive, that is, etch rate could obviously increase with an enhanced bias. As shown in our previous work [30], where 120 m porous layer were obtained in 20 min under a 12 V voltage with the same electrolyte as that of Fig. 1, the etch rate was thus 360 m/h, significantly slower than in the case of Fig. 1 (1800 m/h), where the minimum bias was observed as 33 V. Note that similar results concerning the positive correlation between etch rate and bias were frequently obtained in our other experiments. The third element concerning fast etching is the conductivity of electrolytes. If the resistivity of the electrolyte is rather high, the ohmic and hence potential losses in the pores will be signif-
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icant, accordingly, resulting in a negative influence over fast etching. In this experiment, the conductivity of the HF/H2 O2 system is rather good (albeit not perfect). As shown in Figs. 1–4, the porous layers of considerable depths are rather uniform and no obvious puff pastry-like structures [27] were observed; this indicates that our selected electrolytes are of good conductivity. By adding some salts, the conductivity of the HF/H2 O2 system can be further enhanced. In short, according to CBM [7], the combination of three factors, i.e., a high HF concentration, a rather large bias and a good conductivity of electrolytes, contributed to fast pore etching for our cases, although the negative impact on speed induced by the strong oxidizer (H2 O2 ) could exist. The H2 O2 component does enhance the oxidation and thus the time of indirect dissolution in every single current burst, but the loss of speeds induced by strong oxidizers has been greatly compensated by the rate-enhancing factors as addressed above. For Figs. 1–3, the distance between pores varies from 1 to 1.5 m and the pore-wall thickness is about 300–500 nm, while the SCR width ranges from 3.6 m (corresponding to the minimum bias observed 27 V (Fig. 2) and a resistivity of 4 cm [31,32]) to 5 m (corresponding to the maximum bias 50 V and a resistivity of 4 cm [31,32]). According to the SCR model, the theoretical pore-wall thickness should equal 7.2–10 m (i.e., twice the SCR width), which is at least 1 order of magnitude (or more accurately, approx. 20 times for Figs. 1 and 2 and 16 times for Fig. 3) larger than the actual thickness of the pore-walls (300–500 nm). Even as a rule of thumb, the experiential pore pitch (7.2–10 m, equaling twice the SCR width) is about 5–10 times the actual pore pitch (1–1.5 m). Largely, this indicates that SCR effects could not affect both pore spacing and especially pore-wall thickness, thus dissenting from the SCR model. It is true at least when the current densities are relatively high, as shown in this work (compared with our previous work [30], where low current densities (0–5 mA/cm2 ) were applied for a 20 and 50% HF/H2 O2 electrolyte). What really passivates the pore-wall in our case must be the greatly biased distribution of field strength determined by three critical elements, i.e., the morphological, geometric and, probably, ohmic difference between pore-wall and pore tip. First, the morphology is different: pore tips are usually conic (or sometimes quasi-pyramidic in our experience) while a cylindrical pore may demonstrate any shape between a circle and a square. Even for the same radius of a hemispherical tip (usually as an approximation of a real pore tip [8]) as that of a circularly cylindrical wall (usually as an approximation of a real pore-wall [8]), the electric field strength of the pore tip is much greater than that of the pore-wall, if etching with an identical bias and electrolyte [8]. Secondly, the geometry is usually distinct. If considering a single pore, the radius of the pore tip is generally much smaller than that of the pore-wall (cf. the inlay of Fig. 1(b)). This further differentiates the field strength between a tip and a wall. Last but not least, the resistivity of thin walls (300–500 nm for our case) could probably be markedly higher than that of bulk silicon; however, this is more a “guess” than a fact to date. At least, for mesoporous silicon, this could be true as reported in existing works [33–35], the resistivity of mesoporous layer could be
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drastically enhanced by a factor upto more than 105 , compared with bulk silicon. No backside illumination was employed. The electronic holes needed must be generated by a breakdown mechanism. As mentioned in an existing work [8], the electric field strength, depending on both the pore tip radius and applied bias, could exert a decisive influence over the type of breakdown (either avalanche or tunneling). For our cases, the pore tip radii (usually quite smaller than pore radii [8], as indicated in the inset of Fig. 1(b)) range from 50 to 300 nm; accordingly, 14–25 V is enough to generate obvious avalanche multiplication around the pore tip (with a 50–300 nm radius as mentioned above) [8]. The actual bias applied was 27–50 V, significantly larger than the threshold bias (14–25 V) needed for avalanche breakdown, and therefore avalanche effects can dominate in all cases (Figs. 1–4). In the inset of Fig. 3(a), the pore tips were tumefied with a synchronized phase, i.e., simultaneously. This indicates that a localized electropolishing (EP) has occurred at the region of the pore tips due to a simultaneous decrease of both HF concentration and passivating species (H+ for our case) in the pores. This is possible if the diameters of all pores are rather small and quite uniform. Actually, this is the case as shown in Fig. 3(a) and (b). It is known that a lower HF concentration will lead to a lower critical current density jPS (indicating the lower limit of current density needed for EP), which makes possible a synchronized EP at the deep region of the porous layer (of 65 m depth for this case), where a constant and rather large current density (200 mA/cm2 for this case) was maintained [27]. Moreover, the increasing loss of passivating species over pore depth will facilitate the lateral tumescence (i.e., enlargement of diameters) of pore tips, since the hydrogen passivation of (1 1 1) facets and thus the preferential etching along the (1 0 0) direction will be accordingly weakened [23,24]. In short, due to the decrease of two factors (HF concentration and passivating species) over pore depth, the synchronized tumescence of pore tips can be understood. In Fig. 3(b), macropores filled with a thin uniform microporous layer (PSL) were also observed. To our knowledge, for n-Si to date, such macropores have only been observed under the conditions where illumination was used (never under dark conditions) [19,21]. This indicates that under optimized circumstances, the formation of macropores filled with a thin PSL is still possible in the dark, i.e., etching without illumination. The PSL can be removed by an anisotropic wet alkali etchant, e.g., diluted KOH solution. The formation of PSL is largely in the framework of the “quantum-size effect” model [36]. To the best of our knowledge, this is the first time that close-packed macropores of good quality were fast produced in the dark on low doped n-type silicon. Rather nice macropores with depths upto 80 m and pore-diameters between 100 and 1000 nm were produced in a self-organized manner. However, only weak short-range ordering of pores was observed (Figs. 1 and 2), as marked in the circles of Fig. 1(a): the hexagonal symmetry of close packing was severely frustrated by the fourfold symmetry of crystal for our (1 0 0) samples [27], where the rather high resistivity of low doped material could also exert a negative influence [26–29]. Nevertheless, via lithography, it
is highly likely that 100–1000 nm pores of hexagonal symmetry could be quickly fabricated by our method (as implied in Figs. 1–4). Even without lithography but on moderately doped (1 1 1) n-Si (which possesses both an ideal resistivity and a threefold symmetry of crystal [27]), it is still possible to rapidly fabricate 50–1000 nm pores of short or even long range ordering with HF/H2 O2 electrolytes, which might ultimately empower 2 or 3D nonlithographic PC (photonic-crystal) formation [26–29]. 5. Conclusion HF-containing electrolytes were modified with a strong oxidizer H2 O2 ; under dark conditions, densely arrayed macropores with depths upto 80 m and pore-diameters between 100 and 1000 nm were galvanostatically formed on low doped n-Si. Poretips could be tumefied with a synchronized phase; under dark conditions, macropores could be filled with a thin microporous layer. Weak short-range ordering of pores was evidenced. The etching rate could reach 1800 m/h. The current-burst-model combined with avalanche effects was employed to interpret the underlying mechanism; SCR width could not affect pore spacing and pore-wall thickness. Acknowledgements This work was supported by Chinese National “863” (No.: 2006AA04Z312) and“973” (No.: 2006CB300403) Project. Samsung Advanced Institute of Technology was also acknowledged. References [1] V. Lehmann, H. F¨oll, J. Electrochem. Soc. 137 (1990) 653. [2] V. Lehmann, J. Electrochem. Soc. 140 (1993) 2836. [3] M.H. Al Rifai, M. Christophersen, S. Ottow, J. Carstensen, H. F¨oll, J. Electrochem. Soc. 147 (2000) 627. [4] E.K. Propst, P.A. Kohl, J. Electrochem. Soc. 141 (1994) 1006. [5] E.A. Ponomarev, C. Levy-Clement, J. Electrochem. Soc. 1 (1998) 1002. [6] V. Lehmann, S. R¨onnebeck, J. Electrochem. Soc. 146 (8) (1999) 2968. [7] J. Carstensen, M. Christophersen, H. F¨oll, Mater. Sci. Eng. B 69–70 (2000) 23. [8] V. Lehmann, R. Stengl, A. Luigart, Mater. Sci. Eng. B 69–70 (2000) 11. [9] J.C. Claussen, J. Carstensen, M. Christophersen, S. Langa, H. Foll, Chaos 13 (1) (2003) 217. [10] M.J.J. Theunissen, J. Electrochem. Soc. 119 (1972) 351. [11] M.I.J. Beale, J.D. Benjamin, M.J. Uren, J. Cryst. Growth 73 (1985) 622. [12] R.L. Smith, S.F. Chuang, S.D. Collins, J. Electron. Mater. 17 (1988) 533. [13] P.C. Searson, J.M. Macaulay, S.M. Prokes, J. Electrochem. Soc. 139 (1992) 3373. [14] Y. Kang, J. Jorne, J. Electrochem. Soc. 140 (1993) 2258. [15] V.P. Parkhutik, in: Z.C. Feng, R. Tsu (Eds.), Morphology of Porous Silicon Layers, Porous SiliconWorld Scientific, 1994, p. 301. [16] A. Valance, Phys. Rev. B 52 (1995) 8323. [17] R.B. Wehrspohn, F. Ozanan, J.N. Chazalviel, J. Electrochem. Soc. 146 (9) (1999). [18] M. Christophersen, J. Carstensen, H. F¨oll, Physica Status Solidi(a) 182 (1) (2000) 45. [19] H. F¨oll, M. Christophersen, J. Carstensen, G. Hasse, Mater. Sci. Eng. R 39 (2002) 93. [20] S. Matthias, F. M¨uller, J. Schilling, U. G¨osele, Appl. Phys. A 80 (2005) 1391.
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