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Micromachining of silicon with a proton microbeam
Nuclear Instruments and Methods in Physics Research B 158 (1999) 173±178
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Micromachining of silicon with a proton microbeam P. Polesello a, C. Manfredotti a,b, F. Fizzotti a,b, R. Lu b, E. Vittone a,b,*, G. Lerondel c, A.M. Rossi c, G. Amato c, L. Boarino c, S. Galassini d, M. Jaksic e, Z. Pastuovic e b d
a INFM, UdR Torino Universit a, via Pietro Giuria 1, 10125 Torino, Italy Dipartimento di Fisica Sperimentale, Universita di Torino, via Pietro Giuria 1, 10125 Torino, Italy c IEN ``Galileo Ferraris'', Strada delle Cacce 91 Torino, Italy Facolt a di Scienze, Universit a di Verona, Strada Le Grazie, C a Vignal, Borgo Roma, Verona, Italy e Rudjer Boskovic Institute, Laboratory for Nuclear Microanalysis, Zagreb, Croatia
Abstract In the recent years the fabrication of sensors and actuator devices on a microscopic scale and their integration with electronic devices and micro-electromechanical systems (MEMS) has become an area of considerable commercial and technological interest, with huge development potentialities. High energy ion microbeam is a suitable tool for such purposes. In this paper we present an alternative way to exploit the lithographic properties of micro ion beams based on the selective damage of silicon to produce porous silicon microstructures. We used a 2 MeV proton microbeam to irradiate de®nite areas of silicon samples in order to produce damaged layers localised at the end of the proton trajectories. By performing an electrochemical etching in a suitable HF solution, a porous silicon pattern, complementary to the irradiated one, is always formed. The main eect of the damage on the porous silicon formation is to reduce the velocity of formation. To interpret this, such dead layers can be seen to be more or less opaque to the migration of free holes. Consequently the patterned region can be more or less revealed according to the formation time. The procedure allows for the production of microstructures of porous silicon whose unique properties are of great interest for applications. Preliminary results obtained on silicon samples, with dierent doping levels (p+, p, n+) and irradiating regions with dierent areas (from 200 200 lm2 to 25 25 lm2 ) are presented in order to evaluate the most suitable range of exposure and aspect-ratio of the microstructures. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 41.75.Ak; 81.20.Wk; 61.80.Jh; 82.80.Fk Keywords: Porous silicon; Radiation damage micromachining
1. Introduction Machines, sensors, actuator miniaturisation and the micromechanical components integration * Corresponding author. Tel.: +39-0116707317; fax: +390116691104; e-mail: [email protected]
with electronic devices and micro-electromechanical systems (MEMS) is considered a high technology area with high development potential. As far as the silicon industry is concerned, there is a large experience in UV photolithography for two-dimensional structures. In order to obtain three-dimensional structures, a process has been
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 8 2 - 1
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P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
set up recently in which deep X-ray lithography is combined with galvanoplastic and moulding techniques [1]. This process, named LIGA, has become an important technique for MEMS fabrication. In this method synchrotron light is used to expose a high density polymethylmethacrylate (PMMA) structure with the help of a mask. Deep Ion Beam Lithography (DIBL) is a technique similar to LIGA in which a high energy ion beam (protons at several MeV energy) is used instead of synchrotron radiation. This technique presents two main advantages: the ®rst one lies in the wellde®ned penetration path of the ion beam in the material; the second one lies in the absence of masks during the process. These features allow to produce complex three-dimensional microstructures through a fast direct writing process, in which the resist is modi®ed along the whole path of the beam, and in particular at the end of the ion range, so that a chemical etching could give rise to deep structures. Preliminary DIBL studies on PMMA have been made by the ``Nuclear Microscopy Group'' at the National University of Singapore [2]. Using a 2 MeV proton high resolution microbeam, which penetrates into the resist material up to 63 lm, it is possible to obtain nearly perfectly vertical structures with 80 nC/mm2 doses. Until now very little work has been carried out on silicon; actually, porous silicon (PS) micromachining has been recently proposed [3] as an interesting alternative to bulk and surface micromachining, because it oers a better compatibility with C-MOS microelectronic processes, shorter processing times, and, thanks to the etching mechanisms, minor area consumption. In fact, latest studies on PS have indicated a way to obtain tailor-made structures into silicon substrates by means of an easy and suitable electrochemical etching process: this consists in the dissolution of a silicon anode in an electrochemical cell containing an HF-based solution. The Si anode is positively polarised with respect to a counterelectrode, so that holes are drifted towards the Si/HF interface, thus allowing for the chemical dissolution of Si atoms and creation of pores [4]. In this work we investigate the possibility to selectively irradiate a pattern into a silicon substrate before PS formation. The aim is to create
zones modi®ed by the proton beam at a de®nite depth in order to have dierent etching rates and porosities: in this way, microstructures with different light emission properties can be built in a silicon substrate with a micrometric resolution. 2. Experimental Irradiation by proton microbeam has been performed at the microbeam facility of the Ruder Boskovic Institut in Zagreb (Croatia). The proton energy was 2 MeV. Fig. 1 shows the vacancy pro®les created by 2 MeV protons in Si, as evaluated by TRIM97 simulations [5]. Selected squares with dierent sizes ranging from 25 to 200 lm have been irradiated with the same amount of charge (4 nC) in order to study the eect of the dose on the structure formation. Table 1 reports the side patterns, the amounts of delivered charge and the corresponding charge densities. In order to
Fig. 1. TRIM97 simulation of vacancy pro®les created by 2 MeV protons. Table 1 Square sizes and charge ¯uxes; proton energy 2 MeV, Total charge 4 nC Size (lm)
Area (lm2 )
Charge ¯ux (nC/mm2 )
24 48 96 192
576 2304 9216 36864
6944 1736 434 108
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
study the dependence of structure formation on the substrates, n+ and p+ silicon samples have been used.
175
After selective irradiation, PS has been formed by standard electrochemical etching in an HF solution in water and ethanol [4]. Table 2 sum-
Table 2 PS formation parameters Sample
Resistivity (mX cm)
HF concentration (%)
Current density (mA/cm2 )
Etching time (s)
P+ N+
10±20 15±20
25 10
111 10
180 1st step 3000 2nd step 1700 3rd step 1000
Fig. 2. Optical micrographs of p+ sample irradiated with a 2 MeV proton microbeam: (a) whole pattern; (b) 25 lm side structure.
176
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
marises the dierent conditions (HF concentration, current density, process time) used for each kind of sample. The PS was removed by dipping the sample in a KOH solution in order to leave just the bulk silicon and get a better insight into the remaining structures from the combined ion irradiation-electrochemical etching-PS removal process. Characterisation has been performed by means of optical and electron microscopy (Leica Stereoscan 420). Thickness pro®les have been collected by means of a stylus pro®lometer (Tencor P10). 3. Results and discussion Optical micrographs of the p+ sample, irradiated with a 2 MeV proton microbeam, are shown in Figs. 2(a) and (b). In this case, PS has been etched down to 18 lm and then removed in a KOH solution. Columnar structures have been left corresponding to the irradiation pattern, both in shape and in size. Looking more closely at the smallest square Fig. 2(b), a lined pattern can be recognised reproducing the microbeam scanning. This perfect agreement between the scanning irradiation pattern and the resulting structures is con®rmed by the pro®lometer observations Figs. 3(a) and (b). Actually, a pro®le with better resolution of the 50 lm sized square shows on top of the column a regular undulated structure Fig. 3(b); as the groove-to-groove distance is 6 lm (the scanning microbeam step) we can surely ascribe the undulated feature to a sort of ``damage pro®le'' introduced by the proton beam. This shows the extremely high sensitivity of PS formation to proton damage in a p+-doped sample. Besides, in the pro®le shown in Fig. 3(a) a clear dependence of the height of the column on the side of the irradiated squares can be observed: as the amount of delivered charge on each square has always been the same (4 nC), we can try to establish a relationship between the column heights and the corresponding delivered charge density. This kind of analysis is carried out and shown in Fig. 4, where the etch rate is plotted versus the dose for each observed pro®le. Etch rate is de®ned as the (grown PS depth)/(formation
Fig. 3. Pro®lometer observation of p+ sample: (a) whole pattern; (b) 50 lm side structure as indicated by the dash line in (a).
Fig. 4. Etch rate as a function of dose for p+ sample. The line represents a best ®t of experimental data.
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
time) ratio, whereas the dose is de®ned as the (delivered charge)/(Irradiated area) ratio. The grown PS depth has been evaluated by measuring the height of the structures on the pro®le and subtracting it from the depth of the layer as measured from the wafer surface to the unirradiated zone. The experimental data can be well ®tted by a linear function, showing a linear dependence between etch rate and delivered dose. A similar situation can be observed on the n+ sample: Fig. 5(a) and (b) show SEM micrographs taken after a 60 lm deep PS layer has been formed and then removed by KOH. In order to be able to observe the lateral column structure, the sample has been tilted by 60° under the electron beam. Fig. 5(a) shows the whole pattern, reproducing exactly the irradiation one; Fig. 5(b) is the SEM micrograph of the smallest square at a higher magni®cation. In this case, the columnar structure looks as displaying a ``roof '' consisting of the irradiated square: this is a clear sign of an underetching phenomenon taking place in the most irradiated zone. In order to monitor the actual behaviour of the sample during PS etching, several PS formation and removal cycles have
177
been performed: the ®rst one after 30.3 lm, the second one down to 48.9 lm, the last one down to 60 lm PS formation. Fig. 6(a)±(c) show profilometer observations, where the dependence of the height of each column on the square sides is clearly evident. The discussed experimental results allow us to ascribe to proton damaged layers an eective masking action on PS etching: the created vacancies aect the hole drift action, thus limiting the etch rate. When the PS layer is formed deeper than the proton penetration depth, an underetching phenomenon under the vacancies dead layer may be observed on the most damaged zone Fig. 5(b): this phenomenon may be ascribed to the behaviour of holes during electrochemical etching and must be studied further considering dierent doping levels and etching conditions. 4. Conclusions PS has demonstrated to be a promising approach to Si macromachining, due to the easy production and selective removal. When applied to
Fig. 5. SEM micrographs of n+ sample: (a) whole pattern; (b) 25 lm side square.
178
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
HF, which makes structuring by an appropriate mask uncomfortable. The present paper shows how proton microbeam implantation can be a solution. These results are of interest for a better understanding of the physics and chemistry of porous silicon production. Moreover, the proton microbeam technology is shown to oer several degrees of freedom for the design of new devices.
Acknowledgements This work was partially supported by Croatian Ministry of Science and Technology and by the Istituto Nazionale di Fisica della Materia (INFM).
References
Fig. 6. Pro®lometer observations of n+ sample at dierent PS etch and removal steps: (a) at 30.3 lm depth; (b) at 48.9 lm depth; (c) at 60 lm depth.
Si bulk micromachining, however, drawbacks arise from the long exposure to an aggressive acid like
[1] W. Ehrfeld, H. Lehr, Radiat. Phys. Chem. 45 (1995) 349. [2] S.V. Springham, T. Osipowicz, J.L. Sanchez, L.H. Gan, F. Watt, Nucl. Instr. and Meth. B 130 (1997) 155. [3] W. Lang, in: G. Amato, C. Delerue, H.J. Bardeleben (Eds.), Optical and Structural Properties of Porous Silicon Nanostructures, Gordon and Breach, Newark, 1997, pp. 597± 620. [4] G. Amato, N. Brunetto, Defect and Diusion Forum 134/ 135 (1996) 15. [5] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985.
www.elsevier.nl/locate/nimb
Micromachining of silicon with a proton microbeam P. Polesello a, C. Manfredotti a,b, F. Fizzotti a,b, R. Lu b, E. Vittone a,b,*, G. Lerondel c, A.M. Rossi c, G. Amato c, L. Boarino c, S. Galassini d, M. Jaksic e, Z. Pastuovic e b d
a INFM, UdR Torino Universit a, via Pietro Giuria 1, 10125 Torino, Italy Dipartimento di Fisica Sperimentale, Universita di Torino, via Pietro Giuria 1, 10125 Torino, Italy c IEN ``Galileo Ferraris'', Strada delle Cacce 91 Torino, Italy Facolt a di Scienze, Universit a di Verona, Strada Le Grazie, C a Vignal, Borgo Roma, Verona, Italy e Rudjer Boskovic Institute, Laboratory for Nuclear Microanalysis, Zagreb, Croatia
Abstract In the recent years the fabrication of sensors and actuator devices on a microscopic scale and their integration with electronic devices and micro-electromechanical systems (MEMS) has become an area of considerable commercial and technological interest, with huge development potentialities. High energy ion microbeam is a suitable tool for such purposes. In this paper we present an alternative way to exploit the lithographic properties of micro ion beams based on the selective damage of silicon to produce porous silicon microstructures. We used a 2 MeV proton microbeam to irradiate de®nite areas of silicon samples in order to produce damaged layers localised at the end of the proton trajectories. By performing an electrochemical etching in a suitable HF solution, a porous silicon pattern, complementary to the irradiated one, is always formed. The main eect of the damage on the porous silicon formation is to reduce the velocity of formation. To interpret this, such dead layers can be seen to be more or less opaque to the migration of free holes. Consequently the patterned region can be more or less revealed according to the formation time. The procedure allows for the production of microstructures of porous silicon whose unique properties are of great interest for applications. Preliminary results obtained on silicon samples, with dierent doping levels (p+, p, n+) and irradiating regions with dierent areas (from 200 200 lm2 to 25 25 lm2 ) are presented in order to evaluate the most suitable range of exposure and aspect-ratio of the microstructures. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 41.75.Ak; 81.20.Wk; 61.80.Jh; 82.80.Fk Keywords: Porous silicon; Radiation damage micromachining
1. Introduction Machines, sensors, actuator miniaturisation and the micromechanical components integration * Corresponding author. Tel.: +39-0116707317; fax: +390116691104; e-mail: [email protected]
with electronic devices and micro-electromechanical systems (MEMS) is considered a high technology area with high development potential. As far as the silicon industry is concerned, there is a large experience in UV photolithography for two-dimensional structures. In order to obtain three-dimensional structures, a process has been
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 8 2 - 1
174
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
set up recently in which deep X-ray lithography is combined with galvanoplastic and moulding techniques [1]. This process, named LIGA, has become an important technique for MEMS fabrication. In this method synchrotron light is used to expose a high density polymethylmethacrylate (PMMA) structure with the help of a mask. Deep Ion Beam Lithography (DIBL) is a technique similar to LIGA in which a high energy ion beam (protons at several MeV energy) is used instead of synchrotron radiation. This technique presents two main advantages: the ®rst one lies in the wellde®ned penetration path of the ion beam in the material; the second one lies in the absence of masks during the process. These features allow to produce complex three-dimensional microstructures through a fast direct writing process, in which the resist is modi®ed along the whole path of the beam, and in particular at the end of the ion range, so that a chemical etching could give rise to deep structures. Preliminary DIBL studies on PMMA have been made by the ``Nuclear Microscopy Group'' at the National University of Singapore [2]. Using a 2 MeV proton high resolution microbeam, which penetrates into the resist material up to 63 lm, it is possible to obtain nearly perfectly vertical structures with 80 nC/mm2 doses. Until now very little work has been carried out on silicon; actually, porous silicon (PS) micromachining has been recently proposed [3] as an interesting alternative to bulk and surface micromachining, because it oers a better compatibility with C-MOS microelectronic processes, shorter processing times, and, thanks to the etching mechanisms, minor area consumption. In fact, latest studies on PS have indicated a way to obtain tailor-made structures into silicon substrates by means of an easy and suitable electrochemical etching process: this consists in the dissolution of a silicon anode in an electrochemical cell containing an HF-based solution. The Si anode is positively polarised with respect to a counterelectrode, so that holes are drifted towards the Si/HF interface, thus allowing for the chemical dissolution of Si atoms and creation of pores [4]. In this work we investigate the possibility to selectively irradiate a pattern into a silicon substrate before PS formation. The aim is to create
zones modi®ed by the proton beam at a de®nite depth in order to have dierent etching rates and porosities: in this way, microstructures with different light emission properties can be built in a silicon substrate with a micrometric resolution. 2. Experimental Irradiation by proton microbeam has been performed at the microbeam facility of the Ruder Boskovic Institut in Zagreb (Croatia). The proton energy was 2 MeV. Fig. 1 shows the vacancy pro®les created by 2 MeV protons in Si, as evaluated by TRIM97 simulations [5]. Selected squares with dierent sizes ranging from 25 to 200 lm have been irradiated with the same amount of charge (4 nC) in order to study the eect of the dose on the structure formation. Table 1 reports the side patterns, the amounts of delivered charge and the corresponding charge densities. In order to
Fig. 1. TRIM97 simulation of vacancy pro®les created by 2 MeV protons. Table 1 Square sizes and charge ¯uxes; proton energy 2 MeV, Total charge 4 nC Size (lm)
Area (lm2 )
Charge ¯ux (nC/mm2 )
24 48 96 192
576 2304 9216 36864
6944 1736 434 108
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
study the dependence of structure formation on the substrates, n+ and p+ silicon samples have been used.
175
After selective irradiation, PS has been formed by standard electrochemical etching in an HF solution in water and ethanol [4]. Table 2 sum-
Table 2 PS formation parameters Sample
Resistivity (mX cm)
HF concentration (%)
Current density (mA/cm2 )
Etching time (s)
P+ N+
10±20 15±20
25 10
111 10
180 1st step 3000 2nd step 1700 3rd step 1000
Fig. 2. Optical micrographs of p+ sample irradiated with a 2 MeV proton microbeam: (a) whole pattern; (b) 25 lm side structure.
176
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
marises the dierent conditions (HF concentration, current density, process time) used for each kind of sample. The PS was removed by dipping the sample in a KOH solution in order to leave just the bulk silicon and get a better insight into the remaining structures from the combined ion irradiation-electrochemical etching-PS removal process. Characterisation has been performed by means of optical and electron microscopy (Leica Stereoscan 420). Thickness pro®les have been collected by means of a stylus pro®lometer (Tencor P10). 3. Results and discussion Optical micrographs of the p+ sample, irradiated with a 2 MeV proton microbeam, are shown in Figs. 2(a) and (b). In this case, PS has been etched down to 18 lm and then removed in a KOH solution. Columnar structures have been left corresponding to the irradiation pattern, both in shape and in size. Looking more closely at the smallest square Fig. 2(b), a lined pattern can be recognised reproducing the microbeam scanning. This perfect agreement between the scanning irradiation pattern and the resulting structures is con®rmed by the pro®lometer observations Figs. 3(a) and (b). Actually, a pro®le with better resolution of the 50 lm sized square shows on top of the column a regular undulated structure Fig. 3(b); as the groove-to-groove distance is 6 lm (the scanning microbeam step) we can surely ascribe the undulated feature to a sort of ``damage pro®le'' introduced by the proton beam. This shows the extremely high sensitivity of PS formation to proton damage in a p+-doped sample. Besides, in the pro®le shown in Fig. 3(a) a clear dependence of the height of the column on the side of the irradiated squares can be observed: as the amount of delivered charge on each square has always been the same (4 nC), we can try to establish a relationship between the column heights and the corresponding delivered charge density. This kind of analysis is carried out and shown in Fig. 4, where the etch rate is plotted versus the dose for each observed pro®le. Etch rate is de®ned as the (grown PS depth)/(formation
Fig. 3. Pro®lometer observation of p+ sample: (a) whole pattern; (b) 50 lm side structure as indicated by the dash line in (a).
Fig. 4. Etch rate as a function of dose for p+ sample. The line represents a best ®t of experimental data.
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
time) ratio, whereas the dose is de®ned as the (delivered charge)/(Irradiated area) ratio. The grown PS depth has been evaluated by measuring the height of the structures on the pro®le and subtracting it from the depth of the layer as measured from the wafer surface to the unirradiated zone. The experimental data can be well ®tted by a linear function, showing a linear dependence between etch rate and delivered dose. A similar situation can be observed on the n+ sample: Fig. 5(a) and (b) show SEM micrographs taken after a 60 lm deep PS layer has been formed and then removed by KOH. In order to be able to observe the lateral column structure, the sample has been tilted by 60° under the electron beam. Fig. 5(a) shows the whole pattern, reproducing exactly the irradiation one; Fig. 5(b) is the SEM micrograph of the smallest square at a higher magni®cation. In this case, the columnar structure looks as displaying a ``roof '' consisting of the irradiated square: this is a clear sign of an underetching phenomenon taking place in the most irradiated zone. In order to monitor the actual behaviour of the sample during PS etching, several PS formation and removal cycles have
177
been performed: the ®rst one after 30.3 lm, the second one down to 48.9 lm, the last one down to 60 lm PS formation. Fig. 6(a)±(c) show profilometer observations, where the dependence of the height of each column on the square sides is clearly evident. The discussed experimental results allow us to ascribe to proton damaged layers an eective masking action on PS etching: the created vacancies aect the hole drift action, thus limiting the etch rate. When the PS layer is formed deeper than the proton penetration depth, an underetching phenomenon under the vacancies dead layer may be observed on the most damaged zone Fig. 5(b): this phenomenon may be ascribed to the behaviour of holes during electrochemical etching and must be studied further considering dierent doping levels and etching conditions. 4. Conclusions PS has demonstrated to be a promising approach to Si macromachining, due to the easy production and selective removal. When applied to
Fig. 5. SEM micrographs of n+ sample: (a) whole pattern; (b) 25 lm side square.
178
P. Polesello et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 173±178
HF, which makes structuring by an appropriate mask uncomfortable. The present paper shows how proton microbeam implantation can be a solution. These results are of interest for a better understanding of the physics and chemistry of porous silicon production. Moreover, the proton microbeam technology is shown to oer several degrees of freedom for the design of new devices.
Acknowledgements This work was partially supported by Croatian Ministry of Science and Technology and by the Istituto Nazionale di Fisica della Materia (INFM).
References
Fig. 6. Pro®lometer observations of n+ sample at dierent PS etch and removal steps: (a) at 30.3 lm depth; (b) at 48.9 lm depth; (c) at 60 lm depth.
Si bulk micromachining, however, drawbacks arise from the long exposure to an aggressive acid like
[1] W. Ehrfeld, H. Lehr, Radiat. Phys. Chem. 45 (1995) 349. [2] S.V. Springham, T. Osipowicz, J.L. Sanchez, L.H. Gan, F. Watt, Nucl. Instr. and Meth. B 130 (1997) 155. [3] W. Lang, in: G. Amato, C. Delerue, H.J. Bardeleben (Eds.), Optical and Structural Properties of Porous Silicon Nanostructures, Gordon and Breach, Newark, 1997, pp. 597± 620. [4] G. Amato, N. Brunetto, Defect and Diusion Forum 134/ 135 (1996) 15. [5] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985.