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Dual-beam focused ion beam (FIB): A prototyping tool for micro and nanofabrication
Microelectronic Engineering 84 (2007) 789–792 www.elsevier.com/locate/mee
Dual-beam focused ion beam (FIB): A prototyping tool for micro and nanofabrication Albert Romano-Rodrı´guez *, Francisco Herna´ndez-Ramı´rez EME, CeRMAE, Department of Electronics, Universitat de Barcelona, c/Martı´ i Franque`s, 1, E-08028 Barcelona, Spain IN2UB, Universitat de Barcelona, c/Martı´ i Franque`s, 1, E-08028 Barcelona, Spain Available online 1 February 2007
Abstract Focused ion beam (FIB) is a powerful and versatile tool for the maskless fabrication of structures and devices in the micro and nanometre scales. This can be performed by the milling and deposition capabilities of a focused ion beam, the latter being achieved by the ion beam-assisted decomposition of a metalorganic gas precursor of the specific material that has to be deposited. The combination of the FIB and a SEM in the same machine, giving rise to the so-called dual-beam or cross-beam machines, further expands the capabilities of the technique by the possibility of performing electron-beam assisted deposition and inspection, which is less harmful than using the ion beam. In this work three examples of the various capabilities of dual-beam systems for the fabrication of prototypes of different types of devices will be presented. The devices fabricated are a microinductor made in copper, the fine trimming of silicon mechanical resonators and the fabrication of nanocontacts to nanowires for the extraction of electrical parameters and for the fabrication of gas sensors from them. 2007 Elsevier B.V. All rights reserved. Keywords: Focused ion beam; Nanofabrication; Milling; Deposition
1. Introduction The development of fabrication processes of materials, structures and devices in the micro and nanometre scale is an important issue in the advancement of micro and nanotechnologies. Several techniques exist nowadays that can be used for this goal but, mostly, they have to be used in combination with photolithography processes, which makes this type of experiments relatively time consuming. Opposite to the presented scenario, focused ion beam (FIB) has the advantage that it can be used for processing directly, without the need of masks [1,2]. The technique, based on the use of accelerated and focused ion beams, is capable of milling virtually any material with a precision and resolution of few tens of nanometres. Additionally, when certain metalorganic compounds are introduced in the beam path, they can be dissociated and part of the com*
Corresponding author. Tel.: +34 93 403 91 56; fax: +34 93 402 11 48. E-mail address: [email protected] (A. Romano-Rodrı´guez).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.113
pound will be deposited on the surface of the sample, where the beam is scanned and the rest will be removed by the vacuum system, giving rise to the so-called ion-assisted deposition. The precision of this deposition process is in the range of 100 nm, and materials with completely different conductivities can be deposited on the sample’s surface. Additionally, more and more the actual FIB machines are equipped also with an electron beam (the so-called dualbeam or cross-beam machines), that allow the electron beam to be used both for imaging, like in conventional SEMs, and, similarly to the case of the ion beam, electron-assisted deposition can be performed when the metalorganic precursor is inserted in the electron beam path. Furthermore, when reactive gases are introduced in the chamber while the ion beam is scanned over the surface, enhanced milling occurs via a combined milling and reactive etching, which can be used in order to selectively remove certain materials. In this work examples of the use of dual-beam FIB for rapid prototyping of different devices will be presented,
790
A. Romano-Rodrı´guez, F. Herna´ndez-Ramı´rez / Microelectronic Engineering 84 (2007) 789–792
2. Experimental
Fig. 1. SEM image of the finalised microinductor fabricated completely inside the FIB. The helicoidal cut and the deposits of isolator and tungsten are clearly visible. The sample is tiled 52 in the image.
showing the capabilities of such a machine for both micro and nanotechnologies and using most of the described working modes.
All the experiments have been carried out in a FEI Strata DB235 dual-beam focused ion beam machine. The primary electron beam was accelerated at 5 kV and the primary Ga+ ion-beam was accelerated at 30 kV, the minimum beam current being 1pA, corresponding to a beam of 6 nm FWHM. For beam-assisted deposition of the different materials, a nozzle with a specific metalorganic precursor was employed for each deposition, namely WCO6 for tungsten, tetraethyl orthosilicate (TEOS) and water for the isolator, and trimethylcyclopentadienyl-platinum ((CH3)3CH3C5H4Pt)) for platinum. In all the experiments of deposition the current density was kept at about 5 pA/ lm2. 3. Results and discussion The result of the complete fabrication step of a microinductor is shown in the SEM image in Fig. 1, which has
Fig. 2. SEM images (a) of the photolithographically defined pads and contacts, (b) enlargement, showing that the three pads are connected, and (c) after cutting in the FIB using a 10pA current. The beam width is about 100 nm and the separation between to the other two electrodes, 80 nm. The sample is tiled 52 in all three images.
A. Romano-Rodrı´guez, F. Herna´ndez-Ramı´rez / Microelectronic Engineering 84 (2007) 789–792
4
Voltage (V)
3
2-probe
2
1
4-probe 0 100
200
300
Current (nA) 1.6
1.2
Zimg (Mohm)
been obtained with the sample tilted 52. The starting material was a Cu(1.5 lm)/Au(50 nm)/SiO2 (0.7 lm)/Si sample. First a square frame of 20 lm side and 1 lm width was cut down to the oxide in order to isolate the upper metals from the rest of the sample. Next the helicoidal cut (9 turns, 0.4 lm nominal separation) was performed with a current of 1 nA, again, down to the oxide. In order to access the centre of the inductor from the region from which it has been previously isolated, deposition of an isolating layer, with dimensions 10 · 3 · 0.5 lm, was carried out and, finally, on top of this isolating layer, a 12 · 1.25 · .25 lm tungsten line was also deposited. In both cases the deposition has been performed using the ion-beam assisted deposition. All the steps have been performed inside the FIB machine, without removing the sample from the vacuum chamber and using the electron beam for inspection. Another example is the fine trimming of the gap in a mechanical resonator. The basic idea is the fabrication of a suspended beam whose width is in the range of few tens of nanometres and whose distance to the exciting electrode(s) is as small as possible. If adequately excited, the beam presents a mechanical resonance frequency in the direction parallel to the shorter dimensions in the range of few hundreds of MHz and could reach up to GHz if the dimensions are the adequate ones [3]. For this the starting material is an SOI substrate, with an upper heavily doped silicon layer of 300 nm. Photolithographically structures of 3 contacts are defined and etching of the rest silicon is performed down to the buried oxide. As shown in Fig. 2a, several of the three contact structures have been fabricated. An enlargement of the area indicated in the image is presented in Fig. 2b, and it is possible to se that the three contacts are still connected in their upper part. Next fine milling around the central part of the figure is performed using a 10 pA ion current (nominal beam size about 10 nm), so that this central part is cut and separated from the two lateral ones, as shown in Fig. 2c. It has to be mentioned that, whole ensemble of metal lines is still sitting on top of the buried oxide. In this way, as can be seen in the figure, a fine beam of about 100 nm width is now free from the left and right sides by a gap of about 80 nm. After this, the processing microelectronic processing continues with the complete removal of the buried oxide underneath the beam by using wet chemical etching and the structure will be liberated. The experiments to remove the oxide and the measurement of the resonance frequency are ongoing. Here, it is important to mention that the FIB work should be performed before the underlying oxide is removed, because previous experiments have shown that, if first the buried oxide is removed, the resonator’s beam will bend left or right during cutting, because of the implantation of some Ga+ ions of the FIB during the trimming and could give rise to sticking of the beam to one of the two lateral contacts. This would irreversibly damage the structure.
791
SA 0.8
0.4
N2
CO
0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Zreal (Mohm) Fig. 3. (a) Monocrystalline SnO2 nanowire contacted by 4 Pt stripes electron-assisted deposited and their extension towards the pads by ionbeam assisted deposition. (b) 2- and 4-point probe resistance measurements, showing the important contribution of the resistance of the metallic stripes. (c) Nyquist plot of the impedance spectroscopy measurements, showing the variation of the impedance of the nanowire in the presence of different gases: synthetic air (SA), pure nitrogen (N2) and 1000 ppm of CO in pure nitrogen (CO).
792
A. Romano-Rodrı´guez, F. Herna´ndez-Ramı´rez / Microelectronic Engineering 84 (2007) 789–792
A third example of the capabilities of the dual-beam in nanotechnology is the fabrication of contacts to individual metal–oxide nanowires, whose dimensions are in the range of tens of nanometres, for the fabrication of nanodevices that, in this case, are nanosensors. The starting sample is a SiO2(0.7 lm)/Si substrate on top of which, photolithographically, metal electrodes (Ti/Ni/Au) have been defined by lift-off techniques and whose separation ranges from few hundreds of nanometres to some tens of micrometres. Next, a solution of propylene glycol containing monocrystalline SnO2 nanowires of lengths up to 10 lm and radii of around 25 nm [4] is dispersed on top of the sample, dried and loaded into the vacuum chamber. Next the sample is inspected using the SEM in the search for individual nanowires located between metallic contacts. Once found the procedure for contacting them in a 4-probe configuration starts with the electron-assisted deposition of metal stripes, in our case a platinum containing precursor (PtC9H16). In this way no exposure of the nanowire to the ion-beam occurs and possible ion-induced damage is avoided. After this the metal stripes are further extended to the photolithographically defined pads using ion-beam assisted deposition, which gives a much lower resistivity value of about 1000 times, but still 100 times more resistive than metallic platinum [5,6]. Reason for it is the high concentration of carbon in the deposited layer from the metallic precursor. A part of the resulting structure can be seen in Fig. 3a, where the nanowire, the four electron-beam and part of the four ion-beam deposited contacts can be identified. The measured resistance value for the 2- and 4-probe configuration of a similar nanowire are presented in Fig. 3b, proving the important contribution of the Pt contacts when performing electrical measurements [6]. The values extracted from this figure are a resistance of 12.5 MX for the 2 probe measurements and 1.93 MX for the 4 probe configuration. Although, the contact resistance is high, the nanowire still can be used for gas sensing applications [7]. Impedance spectroscopy measurements have been carried out on in a 2-probe configuration on these types of samples while the nanowire was exposed to different atmospheres, as can be seen in Fig. 3c. From this image it is clear that the impedance value as a function of the frequency changes with the presence of different gases. Modelling the behaviour of the nanowire by an equivalent RC parallel circuit has shown that the capacitance is a parasitic effect due to the SiO2/ Si substrate and that the value that changes in this case is just the resistance of the nanowire. A complete characterisation in the presence of different gases, operating temperatures and ambient humidity are ongoing.
4. Conclusions In this work the capabilities of dual-beam focused ion beam machines for the fabrication of structures and devices in micro- and nanotechnology has been presented. The possibility to perform in situ milling and deposition of different materials with some tens of nanometre lateral resolution without the need of masks and on virtually any material is one of the key aspects that make these machines especially interesting for this prototyping applications. In the particular case of dual-beam machines, as compared to conventional focused ion beam, the electron beam can be used both for the inspection of the structures as well as for the localised deposition. Three examples have been selected to illustrate the capabilities of the technique, namely the complete fabrication of microinductors, the fine trimming of mechanical resonators and the fabrication of nanocontacts to nanowires for their use as gas sensors. Acknowledgements Some of the results are taken from collaboration with the following colleagues: S. Barth and S. Mathur (INM, Saarbru¨cken, Germany), T.Y. Choi and D. Poulikakos (ETH, Zu¨rich, Switzerland), V. Callegari and P. Nellen (EMPA, Zu¨rich, Switzerland), J. Esteve (IMB-CNM, Bellatera, Spain), A. Vila`, A. Taranco´n, O. Casals and J.R. Morante (Universitat de Barcelona, Barcelona, Spain). Part of the work has been financed by the European Community – FP6, through the project NANOS4 and the Transnational Access to Research Infrastructure programme, and by the Spanish Ministry of Education and Science, through the projects CROMINA and MAGASENS, and through the fellowship of one of the authors (F.H.-R.). References [1] A.V. Stanishevsky, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, American Scientific Publishers, New York, 2004, p. 469. [2] S. Reyntjes, S. Puers, J. Micromech. Microeng. 11 (2001) 287. [3] A.N. Cleland, M.L. Roukes, Nature 392 (1998) 160. [4] S. Mathur, S. Barth, H. Shen, J.-C. Pyun, U. Werner, Small 1 (2005) 713. [5] P.G. Li, A.Z. Jin, W.H. Tang, Phys. Stat. Sol. A 203 (2006) 282. ´ rez, A. Taranco´n, O. Casals, J. Rodrı´guez, A. [6] F. Herna´ndez-Ramı Romano-Rodrı´guez, J.R. Morante, S. Barth, S. Mathur, T.Y. Choi, D. Poulikakos, V. Callegari, P.M. Nellen, Nanotechnology 17 (2006) 5577. ´ rez, J. Rodrı´guez, O. Casals, E. Russinyol, A. [7] F. Herna´ndez-Ramı Vila`, A. Romano-Rodrı´guez, J.R. Morante, M. Abid, Sens. Act. B 118 (2006) 198.
Dual-beam focused ion beam (FIB): A prototyping tool for micro and nanofabrication Albert Romano-Rodrı´guez *, Francisco Herna´ndez-Ramı´rez EME, CeRMAE, Department of Electronics, Universitat de Barcelona, c/Martı´ i Franque`s, 1, E-08028 Barcelona, Spain IN2UB, Universitat de Barcelona, c/Martı´ i Franque`s, 1, E-08028 Barcelona, Spain Available online 1 February 2007
Abstract Focused ion beam (FIB) is a powerful and versatile tool for the maskless fabrication of structures and devices in the micro and nanometre scales. This can be performed by the milling and deposition capabilities of a focused ion beam, the latter being achieved by the ion beam-assisted decomposition of a metalorganic gas precursor of the specific material that has to be deposited. The combination of the FIB and a SEM in the same machine, giving rise to the so-called dual-beam or cross-beam machines, further expands the capabilities of the technique by the possibility of performing electron-beam assisted deposition and inspection, which is less harmful than using the ion beam. In this work three examples of the various capabilities of dual-beam systems for the fabrication of prototypes of different types of devices will be presented. The devices fabricated are a microinductor made in copper, the fine trimming of silicon mechanical resonators and the fabrication of nanocontacts to nanowires for the extraction of electrical parameters and for the fabrication of gas sensors from them. 2007 Elsevier B.V. All rights reserved. Keywords: Focused ion beam; Nanofabrication; Milling; Deposition
1. Introduction The development of fabrication processes of materials, structures and devices in the micro and nanometre scale is an important issue in the advancement of micro and nanotechnologies. Several techniques exist nowadays that can be used for this goal but, mostly, they have to be used in combination with photolithography processes, which makes this type of experiments relatively time consuming. Opposite to the presented scenario, focused ion beam (FIB) has the advantage that it can be used for processing directly, without the need of masks [1,2]. The technique, based on the use of accelerated and focused ion beams, is capable of milling virtually any material with a precision and resolution of few tens of nanometres. Additionally, when certain metalorganic compounds are introduced in the beam path, they can be dissociated and part of the com*
Corresponding author. Tel.: +34 93 403 91 56; fax: +34 93 402 11 48. E-mail address: [email protected] (A. Romano-Rodrı´guez).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.113
pound will be deposited on the surface of the sample, where the beam is scanned and the rest will be removed by the vacuum system, giving rise to the so-called ion-assisted deposition. The precision of this deposition process is in the range of 100 nm, and materials with completely different conductivities can be deposited on the sample’s surface. Additionally, more and more the actual FIB machines are equipped also with an electron beam (the so-called dualbeam or cross-beam machines), that allow the electron beam to be used both for imaging, like in conventional SEMs, and, similarly to the case of the ion beam, electron-assisted deposition can be performed when the metalorganic precursor is inserted in the electron beam path. Furthermore, when reactive gases are introduced in the chamber while the ion beam is scanned over the surface, enhanced milling occurs via a combined milling and reactive etching, which can be used in order to selectively remove certain materials. In this work examples of the use of dual-beam FIB for rapid prototyping of different devices will be presented,
790
A. Romano-Rodrı´guez, F. Herna´ndez-Ramı´rez / Microelectronic Engineering 84 (2007) 789–792
2. Experimental
Fig. 1. SEM image of the finalised microinductor fabricated completely inside the FIB. The helicoidal cut and the deposits of isolator and tungsten are clearly visible. The sample is tiled 52 in the image.
showing the capabilities of such a machine for both micro and nanotechnologies and using most of the described working modes.
All the experiments have been carried out in a FEI Strata DB235 dual-beam focused ion beam machine. The primary electron beam was accelerated at 5 kV and the primary Ga+ ion-beam was accelerated at 30 kV, the minimum beam current being 1pA, corresponding to a beam of 6 nm FWHM. For beam-assisted deposition of the different materials, a nozzle with a specific metalorganic precursor was employed for each deposition, namely WCO6 for tungsten, tetraethyl orthosilicate (TEOS) and water for the isolator, and trimethylcyclopentadienyl-platinum ((CH3)3CH3C5H4Pt)) for platinum. In all the experiments of deposition the current density was kept at about 5 pA/ lm2. 3. Results and discussion The result of the complete fabrication step of a microinductor is shown in the SEM image in Fig. 1, which has
Fig. 2. SEM images (a) of the photolithographically defined pads and contacts, (b) enlargement, showing that the three pads are connected, and (c) after cutting in the FIB using a 10pA current. The beam width is about 100 nm and the separation between to the other two electrodes, 80 nm. The sample is tiled 52 in all three images.
A. Romano-Rodrı´guez, F. Herna´ndez-Ramı´rez / Microelectronic Engineering 84 (2007) 789–792
4
Voltage (V)
3
2-probe
2
1
4-probe 0 100
200
300
Current (nA) 1.6
1.2
Zimg (Mohm)
been obtained with the sample tilted 52. The starting material was a Cu(1.5 lm)/Au(50 nm)/SiO2 (0.7 lm)/Si sample. First a square frame of 20 lm side and 1 lm width was cut down to the oxide in order to isolate the upper metals from the rest of the sample. Next the helicoidal cut (9 turns, 0.4 lm nominal separation) was performed with a current of 1 nA, again, down to the oxide. In order to access the centre of the inductor from the region from which it has been previously isolated, deposition of an isolating layer, with dimensions 10 · 3 · 0.5 lm, was carried out and, finally, on top of this isolating layer, a 12 · 1.25 · .25 lm tungsten line was also deposited. In both cases the deposition has been performed using the ion-beam assisted deposition. All the steps have been performed inside the FIB machine, without removing the sample from the vacuum chamber and using the electron beam for inspection. Another example is the fine trimming of the gap in a mechanical resonator. The basic idea is the fabrication of a suspended beam whose width is in the range of few tens of nanometres and whose distance to the exciting electrode(s) is as small as possible. If adequately excited, the beam presents a mechanical resonance frequency in the direction parallel to the shorter dimensions in the range of few hundreds of MHz and could reach up to GHz if the dimensions are the adequate ones [3]. For this the starting material is an SOI substrate, with an upper heavily doped silicon layer of 300 nm. Photolithographically structures of 3 contacts are defined and etching of the rest silicon is performed down to the buried oxide. As shown in Fig. 2a, several of the three contact structures have been fabricated. An enlargement of the area indicated in the image is presented in Fig. 2b, and it is possible to se that the three contacts are still connected in their upper part. Next fine milling around the central part of the figure is performed using a 10 pA ion current (nominal beam size about 10 nm), so that this central part is cut and separated from the two lateral ones, as shown in Fig. 2c. It has to be mentioned that, whole ensemble of metal lines is still sitting on top of the buried oxide. In this way, as can be seen in the figure, a fine beam of about 100 nm width is now free from the left and right sides by a gap of about 80 nm. After this, the processing microelectronic processing continues with the complete removal of the buried oxide underneath the beam by using wet chemical etching and the structure will be liberated. The experiments to remove the oxide and the measurement of the resonance frequency are ongoing. Here, it is important to mention that the FIB work should be performed before the underlying oxide is removed, because previous experiments have shown that, if first the buried oxide is removed, the resonator’s beam will bend left or right during cutting, because of the implantation of some Ga+ ions of the FIB during the trimming and could give rise to sticking of the beam to one of the two lateral contacts. This would irreversibly damage the structure.
791
SA 0.8
0.4
N2
CO
0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Zreal (Mohm) Fig. 3. (a) Monocrystalline SnO2 nanowire contacted by 4 Pt stripes electron-assisted deposited and their extension towards the pads by ionbeam assisted deposition. (b) 2- and 4-point probe resistance measurements, showing the important contribution of the resistance of the metallic stripes. (c) Nyquist plot of the impedance spectroscopy measurements, showing the variation of the impedance of the nanowire in the presence of different gases: synthetic air (SA), pure nitrogen (N2) and 1000 ppm of CO in pure nitrogen (CO).
792
A. Romano-Rodrı´guez, F. Herna´ndez-Ramı´rez / Microelectronic Engineering 84 (2007) 789–792
A third example of the capabilities of the dual-beam in nanotechnology is the fabrication of contacts to individual metal–oxide nanowires, whose dimensions are in the range of tens of nanometres, for the fabrication of nanodevices that, in this case, are nanosensors. The starting sample is a SiO2(0.7 lm)/Si substrate on top of which, photolithographically, metal electrodes (Ti/Ni/Au) have been defined by lift-off techniques and whose separation ranges from few hundreds of nanometres to some tens of micrometres. Next, a solution of propylene glycol containing monocrystalline SnO2 nanowires of lengths up to 10 lm and radii of around 25 nm [4] is dispersed on top of the sample, dried and loaded into the vacuum chamber. Next the sample is inspected using the SEM in the search for individual nanowires located between metallic contacts. Once found the procedure for contacting them in a 4-probe configuration starts with the electron-assisted deposition of metal stripes, in our case a platinum containing precursor (PtC9H16). In this way no exposure of the nanowire to the ion-beam occurs and possible ion-induced damage is avoided. After this the metal stripes are further extended to the photolithographically defined pads using ion-beam assisted deposition, which gives a much lower resistivity value of about 1000 times, but still 100 times more resistive than metallic platinum [5,6]. Reason for it is the high concentration of carbon in the deposited layer from the metallic precursor. A part of the resulting structure can be seen in Fig. 3a, where the nanowire, the four electron-beam and part of the four ion-beam deposited contacts can be identified. The measured resistance value for the 2- and 4-probe configuration of a similar nanowire are presented in Fig. 3b, proving the important contribution of the Pt contacts when performing electrical measurements [6]. The values extracted from this figure are a resistance of 12.5 MX for the 2 probe measurements and 1.93 MX for the 4 probe configuration. Although, the contact resistance is high, the nanowire still can be used for gas sensing applications [7]. Impedance spectroscopy measurements have been carried out on in a 2-probe configuration on these types of samples while the nanowire was exposed to different atmospheres, as can be seen in Fig. 3c. From this image it is clear that the impedance value as a function of the frequency changes with the presence of different gases. Modelling the behaviour of the nanowire by an equivalent RC parallel circuit has shown that the capacitance is a parasitic effect due to the SiO2/ Si substrate and that the value that changes in this case is just the resistance of the nanowire. A complete characterisation in the presence of different gases, operating temperatures and ambient humidity are ongoing.
4. Conclusions In this work the capabilities of dual-beam focused ion beam machines for the fabrication of structures and devices in micro- and nanotechnology has been presented. The possibility to perform in situ milling and deposition of different materials with some tens of nanometre lateral resolution without the need of masks and on virtually any material is one of the key aspects that make these machines especially interesting for this prototyping applications. In the particular case of dual-beam machines, as compared to conventional focused ion beam, the electron beam can be used both for the inspection of the structures as well as for the localised deposition. Three examples have been selected to illustrate the capabilities of the technique, namely the complete fabrication of microinductors, the fine trimming of mechanical resonators and the fabrication of nanocontacts to nanowires for their use as gas sensors. Acknowledgements Some of the results are taken from collaboration with the following colleagues: S. Barth and S. Mathur (INM, Saarbru¨cken, Germany), T.Y. Choi and D. Poulikakos (ETH, Zu¨rich, Switzerland), V. Callegari and P. Nellen (EMPA, Zu¨rich, Switzerland), J. Esteve (IMB-CNM, Bellatera, Spain), A. Vila`, A. Taranco´n, O. Casals and J.R. Morante (Universitat de Barcelona, Barcelona, Spain). Part of the work has been financed by the European Community – FP6, through the project NANOS4 and the Transnational Access to Research Infrastructure programme, and by the Spanish Ministry of Education and Science, through the projects CROMINA and MAGASENS, and through the fellowship of one of the authors (F.H.-R.). References [1] A.V. Stanishevsky, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, American Scientific Publishers, New York, 2004, p. 469. [2] S. Reyntjes, S. Puers, J. Micromech. Microeng. 11 (2001) 287. [3] A.N. Cleland, M.L. Roukes, Nature 392 (1998) 160. [4] S. Mathur, S. Barth, H. Shen, J.-C. Pyun, U. Werner, Small 1 (2005) 713. [5] P.G. Li, A.Z. Jin, W.H. Tang, Phys. Stat. Sol. A 203 (2006) 282. ´ rez, A. Taranco´n, O. Casals, J. Rodrı´guez, A. [6] F. Herna´ndez-Ramı Romano-Rodrı´guez, J.R. Morante, S. Barth, S. Mathur, T.Y. Choi, D. Poulikakos, V. Callegari, P.M. Nellen, Nanotechnology 17 (2006) 5577. ´ rez, J. Rodrı´guez, O. Casals, E. Russinyol, A. [7] F. Herna´ndez-Ramı Vila`, A. Romano-Rodrı´guez, J.R. Morante, M. Abid, Sens. Act. B 118 (2006) 198.