- Email: [email protected]
Studies on silicon etching using the confined etchant layer technique
PII:
Electrochimica Acta, Vol. 43, Nos 12±13, pp. 1683±1690, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(97)00301-0 0013±4686/98 $19.00 + 0.00
Studies on silicon etching using the con®ned etchant layer technique Yanbing Zu, Lie Xie, Bingwei Mao and Zhaowu Tian* State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Institute of Physical Chemistry, Xiamen University, Xiamen 361005, China (Received 25 June 1997) AbstractÐSilicon surface etching in HBr solutions using the con®ned etchant layer technique (CELT) as well as scanning electrochemical microscopy (SECM) has been carried out and comparison between the two methods has been made in terms of the etching resolution. It has been shown that the lateral diusion of the etchant in SECM con®guration can be suppressed in CELT by a homogeneous scavenging reaction and thus the etching resolution of surface, especially for those with slow etching rate such as Si can be improved. H3AsO3 was added to the solution containing HBr which reacts rapidly and homogeneously with the electrogenerated bromine, resulting in a very thin bromine diusion layer surrounding the tip. The size of the etching spot at the Si wafer surface obtained using the CELT matches that of the tip very well. # 1998 Elsevier Science Ltd. All rights reserved. Key words: silicon, etching, con®ned etchant layer technique (CELT), scanning electrochemical microscopy (SECM), high resolution.
INTRODUCTION Surface micro- and nano-fabrication techniques are of great importance in miniaturizing man-made systems. Intensive researches have been carried out in this ®eld for several decades. Although photo-lithography is still the dominant technique for the production of microstructures, eorts are also being made to develop better and more controllable wet etching approaches for their simplicity, low expense and chemical selectivity. The scanning electrochemical microscopy (SECM), for example, has been introduced as one of the very promising wet etching techniques in modifying metal and semiconductor surfaces with resolution down to the submicron scale [1±5]. The topographic imaging ability of the SECM is based on the principle that the magnitude of the diusion-limited faradic current of a microelectrode probe is sensitive to the distance between the tip and the substrate surface above which it is being scanned [1, 2, 6, 7]. The resolution of SECM achiev*Author to whom correspondence should be addressed. E-mail: [email protected]
able in surface analysis has recently been studied in detail by Bogwarth et al. [8]. It is found that the generator/collector mode depicts the concentration pro®le of the redoxactive species in the solution and the imaging resolution is inevitably restricted by the expanding diusion layer. The SECM with feedback mode yields images of surface properties, not of the solution properties, and the resolution is strongly dependent on the tip size. In addition to its imaging ability, the SECM can also be applied to surface modi®cation by deposition and etching. For etching of a speci®c surface conducted under the SECM con®guration with feedback mode, the etchant molecules generated at a microelectrode diuse to the sample and react therein with the surface species, resulting in a localized etching pattern. It has been shown that high resolution can be achieved using the SECM for surfaces with fast etching rate such as copper and GaAs [3±5], the sizes of the etching patterns on both materials match the diameters of the microelectrodes quite well. An etching spot as small as about 2 mm in diameter has been achieved on GaAs wafer with Pt ultramicroelectrode [4]. However, etching for other materials such as GaP, CdTe and Si which have
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relatively slower surface etching rate are not so promising and the high etching resolution is not guaranteed by SECM. The etching patterns obtained at the surfaces of these samples are much larger than the tip diameters [4, 9]. This is not surprising for the etchant not consumed by the surface right beneath the microelectrode has to diuse laterally along the surface which promotes further etching of the surface outside the tip area. This situation will be greatly changed if the remaining etchant can be destroyed or the etchant can be con®ned to within a thin layer surrounding the generating tip electrode. This requirement can be met with a new electrochemical wet etching technique, the con®ned etchant layer technique (CELT) proposed recently by Tian et al. [10±13] which allows the surface fabrication of complex threedimensional patterns with high resolution. The basic principle of the CELT is quite simple: the active etchant generated at a tip electrode can be rapidly consumed through a homogeneous scavenging reaction. Thus, the etchant will be con®ned within a very thin diusion layer surrounding the tip electrode surface. Etching of surface takes place only inside this thin diusion layer and etching patterns with high resolution are able to be achieved. This process can be outlined as: Tip generation reaction R4O ne Homogeneous scavenging reaction O S4R Y Heterogeneous surface etching reaction O M4R P,
1 2 3
where O is the eective etchant for the substrate M, S is the scavenger existing in solution which can react with O rapidly. Y and P are the products of the homogeneous scavenging reaction (2) and the heterogeneous surface etching reaction (3), respectively. It is noteworthy that the reactant R required in reaction (1) is regenerated during the homogeneous scavenging reaction (2) so that a positive feedback of R is built up which appears as the tip positive feedback current. Since the half-life of O is greatly shortened by the homogeneous scavenging reaction (2), O can only diuse away from the electrode surface in a very short distance. When the concentration of S is set to be much greater than that of O, it can be assumed that the concentration of S remains constant during the whole process and reaction (2) is of pseudo-®rst-order with rate constant ks. The thickness of the con®ned etchant layer (CEL) can be estimated by the speci®c thickness of the diusion layer (m), which is given by [10]: m D=ks 1=2 ;
4
where D is the diusion coecient of species O in solution.
In this paper, we report studies of silicon surface etching, using both the SECM and the CELT. The factors in¯uencing the etching resolution were examined with emphasis on the improvement of etching resolution by the CELT. EXPERIMENTAL Disk-shaped tip electrodes were made of 20 mm diameter carbon ®ber and 50, 60 or 100 mm diameter platinum wires sealed in glass capillaries, respectively. The insulator shield of each tip electrode is about three times the diameter of the conducting part, except for the case where the examination of the in¯uence of the insulator shield on the etching resolution was performed. The microelectrodes were polished with Al2O3 paste down to 0.05 mm grad to obtain the mirror surfaces, then ultrasonicated and thoroughly rinsed with doubly distilled water. Ntype Sih111i wafer was used as the substrate. It was cleaned with a 4:1 mixture of H2SO4 and H2O2, and the native oxide on the Si wafer surface was removed in 10% HF solution. Electrolyte solution was made from 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4. Various amounts of H3AsO3 were added to above solution as the scavengers for electro-generated bromine in the CELT experiments. H3AsO3 solution was obtained by adjusting the pH to 04 using H2SO4 after dissolving As2O3 in a basic solution. The tip electrode was mounted on a micropositioner and maintained at 1.2 V vs SCE with a PAR model 273 potentiostat throughout the experiments. The silicon wafer was left at open circuit, its actual potential value being governed by the redox couples in solution. A microcomputer unit was used to control the micropositioning system assembled in our laboratory. The tip/sample distance control was achieved by a step-motor with 2 mm per step in resolution, accompanied by a piezoelectric tube with the sensitivity of 060 nm Vÿ1. The microelectrode was brought slowly to the Si surface until a sudden rise of the tip current was detected due to the contact of the tip electrode with the Si surface, then withdrawn to a ®xed position by the retraction of the piezoelectric tube. The etching patterns were inspected later with AFM of Nanoscope IIIa from Digital Instruments. RESULTS AND DISCUSSION 1. Factors in¯uencing the SECM etching resolution Figure 1(a) and 1(b) are the surface plots of AFM images of typical etching spots on Si wafer surface obtained using SECM with a 20 mm diameter carbon ®ber tip electrode and a 60 mm diameter Pt tip electrode, respectively. The outer diameters of the etching patterns are 060 and 115 mm, much larger than those of the microelectrodes. A 100 mm di-
Studies on silicon etching
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Fig. 1. AFM surface plots of an n-type Si h111i surface after 10 min etching in a solution consisting of 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4 and 0.5 mol dmÿ3 HF with the tips of (a) 20 mm diameter carbon ®ber, (b) 60 mm diameter Pt wire.
ameter Pt tip was also used and the etching spot is too large to be imaged with AFM using our largest J scanner of 170 170 mm scan area. Therefore, an optical microscope was used to measure the spot diameter which is around 180 mm. These results indicate that the SECM resolution for the Si surface etching is not only dependent on the size of the tip. There are two factors that can lower the etching
resolution: First, the etchant generated at the tip will diuse radially in solution to a certain distance before joining in the etching reaction at the sample surface. Obviously, the etched area of the surface will be larger than the actual tip electrode diameter. Secondly, the remaining part of the etchant, due to the slow etching rate of the surface, will continue to diuse laterally along the surface and therefore con-
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tinue to etch the surface in an area wider than the tip diameter. Mandler et al. noticed that the size of the etching spot on the Si surface obtained using SECM is of the order of the microelectrode diameter (including surrounding insulator) [9]. However, the in¯uence of the size of the tip shield on the etching resolution has not been examined in detail. In our SECM etching experiments, three types of disk microelectrodes were prepared with the Pt wires of the same diameter (50 mm), but surrounded by various sizes of insulator shields. The shields were carefully ground and the ®nal sizes of the tips were around 500, 200 or 50 mm, respectively. However, the sizes of the etching spots obtained using these microelectrodes were very similar and all were around 100 mm. Little dierence in the etching resolution was observed. It should be noted that in the case that the rate of the etching reaction is fast enough, such as the etching of GaAs surface with Fe(phen)3+ 3 , a high etching resolution is achievable [5]. This is because the etchant generated at the microelectrode will be depleted by the fast etching reaction right beneath the microelectrode so that the lateral diusion of the etchant is almost absent. It is reasonable to expect that the resolution of the etching pattern under this condition is essentially the diameter of the electrode regardless of the size of the insulator in which the electrode is embedded. In contrast, for a slow surface etching reaction, the etchant electrogenerated is only partially consumed at the substrate surface right beneath the microelectrode. The
remaining etchant species will diuse outside and the etching reaction will take place in a larger scale. The insulator surrounding the microelectrode may in¯uence the distribution of the etchant and the etching resolution. To which extent this eect may be depends on the etching rate. For a very slow etching reaction, such as CdTe etched with Br2 (negative SECM feedback occurred during the etching process [4]), a thick shield may retard the escape of the etchant to bulk solution and the etchant species can only diuse along the lateral direction which is accompanied by further etching of the surface, resulting in a much larger etching pattern. While if the shield is very thin, there will be great amount of the etchant escaping to the bulk solution by backward diusion and the etching reaction in lateral direction will be suppressed to some extent. Therefore, a microelectrode with thin insulator shield may be suitable to achieve better etching resolution for a very slow etching reaction. Concerning silicon surface etched with bromine, although the reaction is slower than the etching of GaAs with Fe(phen)3+ 3 , it is fast enough in generating positive feedback current, which indicates that localization of the etchant can still be realized to some extent by the heterogeneous etching reaction. Therefore, the eect of shield thickness on the etching resolution is not obvious in this case. 2. Improvement of etching resolution using the CELT The above situation for slow surface etching can be greatly changed if the CELT is applied by introducing an additional homogeneous scavenging reac-
Fig. 2. Cyclic voltammograms of a 100 mm diameter Pt microelectrode with a scan rate of 50 mV sÿ1. The solution consists of 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4, 0.5 mol dmÿ3 HF and x mmol dmÿ3 H3AsO3. x = (1) 0, (2) 5, (3) 25 and (4) 50.
Studies on silicon etching tion which consumes the etchant rapidly and thus con®nes it to a thin layer around the microelectrode. According to equation (4), the speci®c thickness of the con®ned etchant diusion layer is determined by the rate constant reaction (2). Therefore, the etching resolution will dominantly depend on the rate of the etchant scavenging reaction. A large reaction rate constant (ks) is desirable
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for high lateral resolution of the etching pattern. In our experiment, H3AsO3 was used as the scavenger for bromine. The homogeneous bromine consuming reaction can be expressed as Br2 H3 AsO3 H2 O42Brÿ H3 AsO4 2H
5
Fig. 2 shows the dierent cyclic voltammograms of a 100 mm diameter Pt microelectrode in the etching
Fig. 3. (a) The cross-sections of the etching spots at an n-type Sih111i surface obtained with a 50 mm diameter platinum microelectrode in a solution consisting of 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4, 0.5 mol dmÿ3 HF and x mmol dmÿ3 H3AsO3. x = (1) 0, (2) 2.5, (3) 5, (4) 10, (5) 25 and (6) 50. (b) The relationship between the concentration ratio of H3AsO3 to Brÿ and the diameter of the etching spot (d ').
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solutions with and without H3AsO3. The reduction peak of electrogenerated Br2 was obvious when the homogeneous scavenging reaction is absent. However, when H3AsO3 was added to the solution, the reduction peak disappeared while the anodic current increased gradually in accordance of the concentration of the added H3AsO3. It is apparent that, with the addition of H3AsO3, the bromide species can be regenerated homogeneously via reaction (5) and resulting in the appearance of the
anodic current which is dependent on the H3AsO3 concentration. In the same course, the bromine molecules are so rapidly consumed following their generation that they become short-life species which are not detectable upon reverse cathodic scanning of the tip potential. The etching pro®les obtained using a 50 mm diameter Pt microelectrode with various amounts of H3AsO3 added to the solution are presented in Fig. 3. The sizes of the spots decrease gradually as
Fig. 4. AFM surface plot of an n-type Sih111i surface after 20 min etching in a solution consisting of 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4, 0.5 mol dmÿ3 HF and 50 mmol dmÿ3 H3AsO3. The tips are (a) 20 mm diameter carbon ®ber, (b) 60 mm diameter Pt.
Studies on silicon etching the concentration of H3AsO3 increases, suggesting that the lateral diusion layer of bromine was con®ned more and more near to the tip surface. When the concentration ratio of H3AsO3 to Brÿ was beyond 5:1, the diameters of the etching spots match that of the tip precisely. The high etching resolution is an evidence for the eective con®nement of the etchant diusion layer. The silicon surface which was not right beneath the tip survived the attack of the short-life bromine species. The 20 mm diameter carbon ®ber tip and 60 mm, 100 mm diameter Pt tips were also used to etch the Si wafer with 50 mM H3AsO3 added to the solution. All the etching patterns obtained were consistent in size with the tips. Figure 4 shows two typical AFM images of the spots. These results indicate that the CELT etching resolution is only determined by the tip size. In order to better facilitate the comparison between the SECM and CELT in terms of the etching resolution, the relationships between the diameters of the tips (d) and of the etching spots (d ') obtained with both methods are plotted in Fig. 5. It seems dicult to achieve an etching pattern of several micrometer on the Si surface using the SECM. However, due to its eective con®ning ability for the etchant, the CELT is prospective to be used as a submicro- or even nano-fabrication tool as long as the homogeneous scavenging reaction is fast enough. 3. Estimation of the eective con®nement of the etchant by CELT It should be noted that although the thickness of the CEL can be estimated by equation (4), the
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actual thickness of the layer may be larger. For example, the second-order rate constant of reaction (5) in sulfuric acid has been determined to be around 3.6 105 Mÿ1 sÿ1 [14]. For the arsenious acid concentration of 50 mmol dmÿ3, the pseudo®rst-order reaction rate constant ks is 01.8 104 sÿ1. Thus, the thickness of con®ned bromine layer can be calculated to be around 0.25 mm (taking the bromine diusion coecient of 1.2 10ÿ5 cm2 sÿ1 [15]). However, as the arsenious acid will be consumed during the etching process, and the substrate near the tip may block the diusion of arsenious acid from bulk solution to the tip surface, the eective con®nement of the bromine is degraded with the thickness of CEL larger than the above calculated one. A more convenient and straightforward way to qualitatively estimate the thickness of the etchant layer is the inspection of the deviation of the sizes of the etching spots from the real tip diameters. Figure 3 shows that the etchant layer can be best con®ned within less than 1 mm around the tip when the concentration ratio of H3AsO3 to Brÿ is beyond 5:1. It is interesting to note that the outline of an etching spot may not be a regular circle, as shown in Fig. 4(a). On examining the microelectrode surface under an optical microscope, it was found that the shape of the carbon ®ber top used to generate the etchant was irregular either, but very similar to that of the etching spot, which serves as an evidence that the CELT is able to replicate the pattern of the tip to the substrate surface, as has been pointed out in [10±12]. Without using the CELT, however, the diusion layer of the etchant was much thicker and the diusion pro®le in each direction overlapped gradually as the etchant diused into bulk solution. As a result, a more regular but much larger circleshaped pattern was obtained (see Fig. 1). In Fig. 4(b), because the top of the Pt microelectrode is a regular-shaped disk, the outline of the etching spot is also a very regular circle as a replica of the tip. CONCLUDING REMARKS
Fig. 5. The relationships between the diameters of the microelectrodes (d) and of the etching spots (d ') obtained using the SECM (a) and the CELT (b), respectively.
We have shown that the CELT is advantageous for high resolution etching of the surfaces, particularly for those whose etching rates are slow. Although the resolution of the surface etching using CELT is still limited in the present work because of the use of the bigger tip electrode, the etching pattern matches precisely the electrode size and retains the electrode shape. The improved resolution of silicon etching has been achieved by introducing a homogeneous scavenging reaction, instead of the SECM feedback con®ned to the etchant species. Therefore, even a tip electrode without insulator shield can be used to fabricate the surface based on the CELT principle and may give almost the same resolution.
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Resolution as high as nanometer scale is achievable if (1) a tip electrode or a ®ne pattern of nanometer scale is obtainable and used, (2) the etchant layer can be con®ned to that scale with a choice of suitable scavenger and (3) the substrate surface can be brought to within that region. Since the tip current dependence on the tip/sample distance is much more complicated in the CELT, an electrochemical scanning tunneling microscopy (ECSTM) system is desirable to de®ne the zero value of the tip position based on the tunneling current and tip/sample distance can be further controlled with respect to that zero value by the movement of a precise piezoelectric element. This will facilitate a tip electrode of nanometer scale to be used with controlled tip/ sample distance to achieve nanometer scale resolution in CELT. Work aiming at this goal is currently in progress.
ACKNOWLEDGEMENTS The support of this research by the National Nature Science Foundation of China (Fund numbers: 2938004 and 59605017) is gratefully acknowledged.
REFERENCES 1. A. J. Bard, F-R. F. Fan, D. T. Pierce, P. R. Unwin, D. O. Wipf and F. Zhou, Science 254, 68 (1992). 2. A. J. Bard, G. Denuault, C. Lee, D. Mandler and D. O. Wipf, Acc. Chem. Res. 23, 357 (1990). 3. D. Mandler and A. J. Bard, J. Electrochem. Soc. 136, 3143 (1989). 4. D. Mandler and A. J. Bard, J. Electrochem. Soc. 137, 2468 (1990). 5. D. Mandler and A. J. Bard, Langmuir 6, 1489 (1990). 6. A. J. Bard, F. Fan, J. Kwak and O. Lev, Anal. Chem. 61, 132 (1989). 7. J. Kwak and A. J. Bard, Anal. Chem. 61, 1221 (1989). 8. K. Borgwarth, C. Ricken, D. G. Ebling and J. Heinze, Fresenius J. Anal. Chem. 356, 288 (1996). 9. S. Meltzer and D. Mandler, J. Chem. Soc. Faraday Trans. 91, 1019 (1952). 10. Z. W. Tian, Z. Fen, Z. Q. Tian, X. Zhuo, J. Mu, C. Li, H. Lin, B. Ran, Z. X. Xie and W. Hu, Faraday Discuss. 94, 37 (1992). 11. Z. W. Tian, Proceedings of the Ninth International Conference on Photochemical Conversion and Storage of Solar Energy, (Edited by Z. W. Tian and Y. Cao) p. 2490, International Academic Publishers (1993). 12. Z. W. Tian, Z. Q. Tian, Z. H. Lin and Z. X. Xie, Chin. J. Sci. Instr. 17, 14 (1996). 13. L. Xie, L. Y. Su, S. F. Yang, J. Luo, Z. H. Lin and Z. W. Tian, ISE Extended Abstracts, 46th Annual Meeting, Xiamen, 7±98 (1995). 14. J. Luo, L. Y. Su, J. T. Wu, Z. H. Lin and Z. W. Tian, Electrochemistry 1, 270 (1995). 15. R. Parsons, Handbook of Electrochemistry Constants, Butterworths, London (1959).
Electrochimica Acta, Vol. 43, Nos 12±13, pp. 1683±1690, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(97)00301-0 0013±4686/98 $19.00 + 0.00
Studies on silicon etching using the con®ned etchant layer technique Yanbing Zu, Lie Xie, Bingwei Mao and Zhaowu Tian* State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Institute of Physical Chemistry, Xiamen University, Xiamen 361005, China (Received 25 June 1997) AbstractÐSilicon surface etching in HBr solutions using the con®ned etchant layer technique (CELT) as well as scanning electrochemical microscopy (SECM) has been carried out and comparison between the two methods has been made in terms of the etching resolution. It has been shown that the lateral diusion of the etchant in SECM con®guration can be suppressed in CELT by a homogeneous scavenging reaction and thus the etching resolution of surface, especially for those with slow etching rate such as Si can be improved. H3AsO3 was added to the solution containing HBr which reacts rapidly and homogeneously with the electrogenerated bromine, resulting in a very thin bromine diusion layer surrounding the tip. The size of the etching spot at the Si wafer surface obtained using the CELT matches that of the tip very well. # 1998 Elsevier Science Ltd. All rights reserved. Key words: silicon, etching, con®ned etchant layer technique (CELT), scanning electrochemical microscopy (SECM), high resolution.
INTRODUCTION Surface micro- and nano-fabrication techniques are of great importance in miniaturizing man-made systems. Intensive researches have been carried out in this ®eld for several decades. Although photo-lithography is still the dominant technique for the production of microstructures, eorts are also being made to develop better and more controllable wet etching approaches for their simplicity, low expense and chemical selectivity. The scanning electrochemical microscopy (SECM), for example, has been introduced as one of the very promising wet etching techniques in modifying metal and semiconductor surfaces with resolution down to the submicron scale [1±5]. The topographic imaging ability of the SECM is based on the principle that the magnitude of the diusion-limited faradic current of a microelectrode probe is sensitive to the distance between the tip and the substrate surface above which it is being scanned [1, 2, 6, 7]. The resolution of SECM achiev*Author to whom correspondence should be addressed. E-mail: [email protected]
able in surface analysis has recently been studied in detail by Bogwarth et al. [8]. It is found that the generator/collector mode depicts the concentration pro®le of the redoxactive species in the solution and the imaging resolution is inevitably restricted by the expanding diusion layer. The SECM with feedback mode yields images of surface properties, not of the solution properties, and the resolution is strongly dependent on the tip size. In addition to its imaging ability, the SECM can also be applied to surface modi®cation by deposition and etching. For etching of a speci®c surface conducted under the SECM con®guration with feedback mode, the etchant molecules generated at a microelectrode diuse to the sample and react therein with the surface species, resulting in a localized etching pattern. It has been shown that high resolution can be achieved using the SECM for surfaces with fast etching rate such as copper and GaAs [3±5], the sizes of the etching patterns on both materials match the diameters of the microelectrodes quite well. An etching spot as small as about 2 mm in diameter has been achieved on GaAs wafer with Pt ultramicroelectrode [4]. However, etching for other materials such as GaP, CdTe and Si which have
1683
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relatively slower surface etching rate are not so promising and the high etching resolution is not guaranteed by SECM. The etching patterns obtained at the surfaces of these samples are much larger than the tip diameters [4, 9]. This is not surprising for the etchant not consumed by the surface right beneath the microelectrode has to diuse laterally along the surface which promotes further etching of the surface outside the tip area. This situation will be greatly changed if the remaining etchant can be destroyed or the etchant can be con®ned to within a thin layer surrounding the generating tip electrode. This requirement can be met with a new electrochemical wet etching technique, the con®ned etchant layer technique (CELT) proposed recently by Tian et al. [10±13] which allows the surface fabrication of complex threedimensional patterns with high resolution. The basic principle of the CELT is quite simple: the active etchant generated at a tip electrode can be rapidly consumed through a homogeneous scavenging reaction. Thus, the etchant will be con®ned within a very thin diusion layer surrounding the tip electrode surface. Etching of surface takes place only inside this thin diusion layer and etching patterns with high resolution are able to be achieved. This process can be outlined as: Tip generation reaction R4O ne Homogeneous scavenging reaction O S4R Y Heterogeneous surface etching reaction O M4R P,
1 2 3
where O is the eective etchant for the substrate M, S is the scavenger existing in solution which can react with O rapidly. Y and P are the products of the homogeneous scavenging reaction (2) and the heterogeneous surface etching reaction (3), respectively. It is noteworthy that the reactant R required in reaction (1) is regenerated during the homogeneous scavenging reaction (2) so that a positive feedback of R is built up which appears as the tip positive feedback current. Since the half-life of O is greatly shortened by the homogeneous scavenging reaction (2), O can only diuse away from the electrode surface in a very short distance. When the concentration of S is set to be much greater than that of O, it can be assumed that the concentration of S remains constant during the whole process and reaction (2) is of pseudo-®rst-order with rate constant ks. The thickness of the con®ned etchant layer (CEL) can be estimated by the speci®c thickness of the diusion layer (m), which is given by [10]: m D=ks 1=2 ;
4
where D is the diusion coecient of species O in solution.
In this paper, we report studies of silicon surface etching, using both the SECM and the CELT. The factors in¯uencing the etching resolution were examined with emphasis on the improvement of etching resolution by the CELT. EXPERIMENTAL Disk-shaped tip electrodes were made of 20 mm diameter carbon ®ber and 50, 60 or 100 mm diameter platinum wires sealed in glass capillaries, respectively. The insulator shield of each tip electrode is about three times the diameter of the conducting part, except for the case where the examination of the in¯uence of the insulator shield on the etching resolution was performed. The microelectrodes were polished with Al2O3 paste down to 0.05 mm grad to obtain the mirror surfaces, then ultrasonicated and thoroughly rinsed with doubly distilled water. Ntype Sih111i wafer was used as the substrate. It was cleaned with a 4:1 mixture of H2SO4 and H2O2, and the native oxide on the Si wafer surface was removed in 10% HF solution. Electrolyte solution was made from 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4. Various amounts of H3AsO3 were added to above solution as the scavengers for electro-generated bromine in the CELT experiments. H3AsO3 solution was obtained by adjusting the pH to 04 using H2SO4 after dissolving As2O3 in a basic solution. The tip electrode was mounted on a micropositioner and maintained at 1.2 V vs SCE with a PAR model 273 potentiostat throughout the experiments. The silicon wafer was left at open circuit, its actual potential value being governed by the redox couples in solution. A microcomputer unit was used to control the micropositioning system assembled in our laboratory. The tip/sample distance control was achieved by a step-motor with 2 mm per step in resolution, accompanied by a piezoelectric tube with the sensitivity of 060 nm Vÿ1. The microelectrode was brought slowly to the Si surface until a sudden rise of the tip current was detected due to the contact of the tip electrode with the Si surface, then withdrawn to a ®xed position by the retraction of the piezoelectric tube. The etching patterns were inspected later with AFM of Nanoscope IIIa from Digital Instruments. RESULTS AND DISCUSSION 1. Factors in¯uencing the SECM etching resolution Figure 1(a) and 1(b) are the surface plots of AFM images of typical etching spots on Si wafer surface obtained using SECM with a 20 mm diameter carbon ®ber tip electrode and a 60 mm diameter Pt tip electrode, respectively. The outer diameters of the etching patterns are 060 and 115 mm, much larger than those of the microelectrodes. A 100 mm di-
Studies on silicon etching
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Fig. 1. AFM surface plots of an n-type Si h111i surface after 10 min etching in a solution consisting of 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4 and 0.5 mol dmÿ3 HF with the tips of (a) 20 mm diameter carbon ®ber, (b) 60 mm diameter Pt wire.
ameter Pt tip was also used and the etching spot is too large to be imaged with AFM using our largest J scanner of 170 170 mm scan area. Therefore, an optical microscope was used to measure the spot diameter which is around 180 mm. These results indicate that the SECM resolution for the Si surface etching is not only dependent on the size of the tip. There are two factors that can lower the etching
resolution: First, the etchant generated at the tip will diuse radially in solution to a certain distance before joining in the etching reaction at the sample surface. Obviously, the etched area of the surface will be larger than the actual tip electrode diameter. Secondly, the remaining part of the etchant, due to the slow etching rate of the surface, will continue to diuse laterally along the surface and therefore con-
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tinue to etch the surface in an area wider than the tip diameter. Mandler et al. noticed that the size of the etching spot on the Si surface obtained using SECM is of the order of the microelectrode diameter (including surrounding insulator) [9]. However, the in¯uence of the size of the tip shield on the etching resolution has not been examined in detail. In our SECM etching experiments, three types of disk microelectrodes were prepared with the Pt wires of the same diameter (50 mm), but surrounded by various sizes of insulator shields. The shields were carefully ground and the ®nal sizes of the tips were around 500, 200 or 50 mm, respectively. However, the sizes of the etching spots obtained using these microelectrodes were very similar and all were around 100 mm. Little dierence in the etching resolution was observed. It should be noted that in the case that the rate of the etching reaction is fast enough, such as the etching of GaAs surface with Fe(phen)3+ 3 , a high etching resolution is achievable [5]. This is because the etchant generated at the microelectrode will be depleted by the fast etching reaction right beneath the microelectrode so that the lateral diusion of the etchant is almost absent. It is reasonable to expect that the resolution of the etching pattern under this condition is essentially the diameter of the electrode regardless of the size of the insulator in which the electrode is embedded. In contrast, for a slow surface etching reaction, the etchant electrogenerated is only partially consumed at the substrate surface right beneath the microelectrode. The
remaining etchant species will diuse outside and the etching reaction will take place in a larger scale. The insulator surrounding the microelectrode may in¯uence the distribution of the etchant and the etching resolution. To which extent this eect may be depends on the etching rate. For a very slow etching reaction, such as CdTe etched with Br2 (negative SECM feedback occurred during the etching process [4]), a thick shield may retard the escape of the etchant to bulk solution and the etchant species can only diuse along the lateral direction which is accompanied by further etching of the surface, resulting in a much larger etching pattern. While if the shield is very thin, there will be great amount of the etchant escaping to the bulk solution by backward diusion and the etching reaction in lateral direction will be suppressed to some extent. Therefore, a microelectrode with thin insulator shield may be suitable to achieve better etching resolution for a very slow etching reaction. Concerning silicon surface etched with bromine, although the reaction is slower than the etching of GaAs with Fe(phen)3+ 3 , it is fast enough in generating positive feedback current, which indicates that localization of the etchant can still be realized to some extent by the heterogeneous etching reaction. Therefore, the eect of shield thickness on the etching resolution is not obvious in this case. 2. Improvement of etching resolution using the CELT The above situation for slow surface etching can be greatly changed if the CELT is applied by introducing an additional homogeneous scavenging reac-
Fig. 2. Cyclic voltammograms of a 100 mm diameter Pt microelectrode with a scan rate of 50 mV sÿ1. The solution consists of 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4, 0.5 mol dmÿ3 HF and x mmol dmÿ3 H3AsO3. x = (1) 0, (2) 5, (3) 25 and (4) 50.
Studies on silicon etching tion which consumes the etchant rapidly and thus con®nes it to a thin layer around the microelectrode. According to equation (4), the speci®c thickness of the con®ned etchant diusion layer is determined by the rate constant reaction (2). Therefore, the etching resolution will dominantly depend on the rate of the etchant scavenging reaction. A large reaction rate constant (ks) is desirable
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for high lateral resolution of the etching pattern. In our experiment, H3AsO3 was used as the scavenger for bromine. The homogeneous bromine consuming reaction can be expressed as Br2 H3 AsO3 H2 O42Brÿ H3 AsO4 2H
5
Fig. 2 shows the dierent cyclic voltammograms of a 100 mm diameter Pt microelectrode in the etching
Fig. 3. (a) The cross-sections of the etching spots at an n-type Sih111i surface obtained with a 50 mm diameter platinum microelectrode in a solution consisting of 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4, 0.5 mol dmÿ3 HF and x mmol dmÿ3 H3AsO3. x = (1) 0, (2) 2.5, (3) 5, (4) 10, (5) 25 and (6) 50. (b) The relationship between the concentration ratio of H3AsO3 to Brÿ and the diameter of the etching spot (d ').
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solutions with and without H3AsO3. The reduction peak of electrogenerated Br2 was obvious when the homogeneous scavenging reaction is absent. However, when H3AsO3 was added to the solution, the reduction peak disappeared while the anodic current increased gradually in accordance of the concentration of the added H3AsO3. It is apparent that, with the addition of H3AsO3, the bromide species can be regenerated homogeneously via reaction (5) and resulting in the appearance of the
anodic current which is dependent on the H3AsO3 concentration. In the same course, the bromine molecules are so rapidly consumed following their generation that they become short-life species which are not detectable upon reverse cathodic scanning of the tip potential. The etching pro®les obtained using a 50 mm diameter Pt microelectrode with various amounts of H3AsO3 added to the solution are presented in Fig. 3. The sizes of the spots decrease gradually as
Fig. 4. AFM surface plot of an n-type Sih111i surface after 20 min etching in a solution consisting of 5 mmol dmÿ3 HBr, 0.5 mol dmÿ3 H2SO4, 0.5 mol dmÿ3 HF and 50 mmol dmÿ3 H3AsO3. The tips are (a) 20 mm diameter carbon ®ber, (b) 60 mm diameter Pt.
Studies on silicon etching the concentration of H3AsO3 increases, suggesting that the lateral diusion layer of bromine was con®ned more and more near to the tip surface. When the concentration ratio of H3AsO3 to Brÿ was beyond 5:1, the diameters of the etching spots match that of the tip precisely. The high etching resolution is an evidence for the eective con®nement of the etchant diusion layer. The silicon surface which was not right beneath the tip survived the attack of the short-life bromine species. The 20 mm diameter carbon ®ber tip and 60 mm, 100 mm diameter Pt tips were also used to etch the Si wafer with 50 mM H3AsO3 added to the solution. All the etching patterns obtained were consistent in size with the tips. Figure 4 shows two typical AFM images of the spots. These results indicate that the CELT etching resolution is only determined by the tip size. In order to better facilitate the comparison between the SECM and CELT in terms of the etching resolution, the relationships between the diameters of the tips (d) and of the etching spots (d ') obtained with both methods are plotted in Fig. 5. It seems dicult to achieve an etching pattern of several micrometer on the Si surface using the SECM. However, due to its eective con®ning ability for the etchant, the CELT is prospective to be used as a submicro- or even nano-fabrication tool as long as the homogeneous scavenging reaction is fast enough. 3. Estimation of the eective con®nement of the etchant by CELT It should be noted that although the thickness of the CEL can be estimated by equation (4), the
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actual thickness of the layer may be larger. For example, the second-order rate constant of reaction (5) in sulfuric acid has been determined to be around 3.6 105 Mÿ1 sÿ1 [14]. For the arsenious acid concentration of 50 mmol dmÿ3, the pseudo®rst-order reaction rate constant ks is 01.8 104 sÿ1. Thus, the thickness of con®ned bromine layer can be calculated to be around 0.25 mm (taking the bromine diusion coecient of 1.2 10ÿ5 cm2 sÿ1 [15]). However, as the arsenious acid will be consumed during the etching process, and the substrate near the tip may block the diusion of arsenious acid from bulk solution to the tip surface, the eective con®nement of the bromine is degraded with the thickness of CEL larger than the above calculated one. A more convenient and straightforward way to qualitatively estimate the thickness of the etchant layer is the inspection of the deviation of the sizes of the etching spots from the real tip diameters. Figure 3 shows that the etchant layer can be best con®ned within less than 1 mm around the tip when the concentration ratio of H3AsO3 to Brÿ is beyond 5:1. It is interesting to note that the outline of an etching spot may not be a regular circle, as shown in Fig. 4(a). On examining the microelectrode surface under an optical microscope, it was found that the shape of the carbon ®ber top used to generate the etchant was irregular either, but very similar to that of the etching spot, which serves as an evidence that the CELT is able to replicate the pattern of the tip to the substrate surface, as has been pointed out in [10±12]. Without using the CELT, however, the diusion layer of the etchant was much thicker and the diusion pro®le in each direction overlapped gradually as the etchant diused into bulk solution. As a result, a more regular but much larger circleshaped pattern was obtained (see Fig. 1). In Fig. 4(b), because the top of the Pt microelectrode is a regular-shaped disk, the outline of the etching spot is also a very regular circle as a replica of the tip. CONCLUDING REMARKS
Fig. 5. The relationships between the diameters of the microelectrodes (d) and of the etching spots (d ') obtained using the SECM (a) and the CELT (b), respectively.
We have shown that the CELT is advantageous for high resolution etching of the surfaces, particularly for those whose etching rates are slow. Although the resolution of the surface etching using CELT is still limited in the present work because of the use of the bigger tip electrode, the etching pattern matches precisely the electrode size and retains the electrode shape. The improved resolution of silicon etching has been achieved by introducing a homogeneous scavenging reaction, instead of the SECM feedback con®ned to the etchant species. Therefore, even a tip electrode without insulator shield can be used to fabricate the surface based on the CELT principle and may give almost the same resolution.
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Resolution as high as nanometer scale is achievable if (1) a tip electrode or a ®ne pattern of nanometer scale is obtainable and used, (2) the etchant layer can be con®ned to that scale with a choice of suitable scavenger and (3) the substrate surface can be brought to within that region. Since the tip current dependence on the tip/sample distance is much more complicated in the CELT, an electrochemical scanning tunneling microscopy (ECSTM) system is desirable to de®ne the zero value of the tip position based on the tunneling current and tip/sample distance can be further controlled with respect to that zero value by the movement of a precise piezoelectric element. This will facilitate a tip electrode of nanometer scale to be used with controlled tip/ sample distance to achieve nanometer scale resolution in CELT. Work aiming at this goal is currently in progress.
ACKNOWLEDGEMENTS The support of this research by the National Nature Science Foundation of China (Fund numbers: 2938004 and 59605017) is gratefully acknowledged.
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