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Electrolytic thin layer scanning tunneling microscope: fabrication and first results
MICROELECTRONIC ENGINEERING ELSEVIER
Microelectronic Engineering 41/42 (l 998) 547-550
E l e c t r o l y t i c thin layer scanning tunneling m i c r o s c o p e : fabrication and first results P.-F. InderiniJhle", E. AInmann b, P. H~iringc, R. K6tz c, H. Siegenthaler b and N.F. de Rooij a Institute of Microtechnology, University of Neuchfitel, rue Jaquet-Droz 1, CH-2007 Neuchfitcl, Switzerland t' Department for Chemistry and Biochemistry', University of Bern. Freiestrasse 3, CH-3012 Bern. Switzerland Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
A novel scanning probe allowing STM measurements in electrolytic thin layer conditions was fabricated by achieving electrochemical metal growth in photostructurable glass. The concept of this new scanning probe is explained, then the fabrication process is described in detail and first results are presented. 1. INTRODUCTION The scanning tunneling microscope (STM), demonstrated for the first time in 1981 by G. Binnig and H. Rohrer [ 1, 2], very soon revealed itself to be a powerful tool for electrochemical studies at nanometer scale [3]. However, the problem of the access to the chemical information of the electrolyte near the scanning tip has not yet been solved in a satisfying way. The situation can be significantly improved if the scanning tip is located in a so called electrolytic thin layer [4,5,6]. in this paper, we describe the fabrication of a new scanning probe defining an electrolytic thin laver (ETL) around the tunneling tip. Furthermore, a ring electrode, which is integrated on the probe and around the tip, allows the control of the electrolyte compounds during the scan. First, we will briefly discuss the concept of the Electrolytic Thin Layer Scanning Tunneling Microscope (ETL-STM), then the fabrication process will be described in detail and finally first measurements will be presented and shortly commented. 2. THE ELECTROLYTIC ;CANNING TUNNELING ETL-STM)
probe able to control the electrolyte concentration of specific electroactive compounds near the scanning tip may be of great interest. Fig. 1 shows a schematic representation of such a device: a tip and a ring electrode for electrolytic compounds sensing and generation are fitted on a large cylinder (relatively to tip dimensions) made of cheinically inert and isolating material. The tip and the electrode are connected through the isolating material by metallic contacts. As the salient part of the tip is of a fcw tens
THIN LAYER MICROSCOPE
The control of tile chemical species implicated m the clectrolyte/electrodes interface reactions is of first importance for electrochemical scanning probe microscopy. Thus. the development of a scanning 0167-9317/98/$19.00 ~_')Elsevier Science B.V All rights reserved. •I1: SO 167-9317(98)00128-2
Electrolytic STM cdl Figure 1. Schematic illustration of electrolytic thin layer scanning tunneling microscope
548
P.-E Indermiihle et al./Microelectronic Engineering 41/42 (1998) 547-550
of micrometers only, the electrolyte layer between the cylindrical probe and the substrate forms an electrolytic thin layer. The whole probe is then mounted on a piezotube to be scanned over the sample. In this thin-layer STM concept, the ring is operated as a potential controlled electrode that can be used as an electrochemical generator or sensor of electrolyte compounds involved in electrochemical reactions at the substrate electrode. 3. STRUCTURE DESIGN AND FABRICATION 3.1. Design Its good chemical inertia, its high electrical resistance and its ability for high ratio etching made the photostructurable glass Foturan®[7, 8] the ideal candidate for the cylindrical body" of the probe. Cylinders with diameters varying between 250 and 1400 ~m and with 2 to 3 via holes with diameters varying between 30 and 160 p.m were designed to determine a compromise between dimensions of electrolytic thin layer and difficulty of probe approach. 3.2. Fabrication The key step of the fabrication process (Fig. 2) is the electrochemical growth of the metal contacts through the glass cylinders. This could be achieved in a parallel way by bonding the glass wafer with photoresist on an oxidized silicon wafer on which a seed layer was previously patterned. First, a 1 p,m thick aluminum layer was deposited and patterned on 1.1 mm thick Foturan® wafers to serve as an optical mask for UV light exposure (Fig.2.1 (b)). This step was necessary to avoid drift during the exposure time which is of several hours. Then the aluminum layer was wet etched and glass wafers were annealed following a ramp up to about 600 °C. During this step, the exposed glass was transformed into ceramic which could later be selectively etched in hydrofluoric acid (I-IF). Then, some wafers were polished and their thickness reduced to about 600 p.m. In parallel, silicon wafers were oxidized and a platinum seed layer was patterned on one face with the lift-off technique (Fig.2.1 (a)). A chemical vapor deposition (CVD) isolating oxide layer was then deposited and
patterned with standard lithography to avoid unwanted metal growth. The second step consisted in etching the transformed glass with a 10% ultrasonic HF solution (Fig.2.2 (b)). The lateral to vertical etch ratio was about 1 to 20, i.e. the glass transformed in ceramic
by 1 ) a)
b)
2) a)
b)
[
J I 4) ~
] -
1
~
- Monocryslalline t : , .! - CX:D oxide silicon [---~-] - Photostructurable ~ _ Photoresist glass ~ - Exposed photostr, l - Metals glass Figure 2. Fabrication sequence for electrolytic thin layer STM probe light exposure and annealing was etched about 20 times faster than the non exposed one. This allowed the microfabrication of high aspect ratio structures made of chemically inert material. During the third step (Fig.2.3), the glass and silicon wafers were aligned and glued together using a phototresist layer as bonding medium. After a gas phase priming, the Foturan® wafers were fixed in a commercial mask aligner (A1-6.2 by Electronic Vision). Then a thick layer of photoresist was spun on the silicon wafer and its edges removed with acetone to avoid unwanted sticking to occur on edges during alignment phase. The wafer was then inserted into the aligner without
P.-E lndermiihle et al./Microelectronic Engineering 41/42 (1998) 547-550
any baking, aligned and brought into close contact during one hour thanks to a piston and a vacuum system. A prebake sequence was then performed during half an hour at 85 °C. This degassing is required from the photoresist to allow the following UV light exposure and development. It was observed that this step increased the adhesion between glass and silicon wafer for polished glass wafers, while gas bubbles appeared between both wafers for unpolished Foturan® wafers. Then, photoresist was exposed to UV light and developed to be removed where metal was expected to be electrochemically grown.
549
To do this, the already defined structures were sunk in cyanoacrylate glue to avoid them to be torn away by lateral forces. Finally, a 6000 A thick silver layer with titanium anchorage was deposited on glass wafers to be patterned. Due to the high aspect ratio of Foturan® structures, the photoresist needed to be sprayed with an airbrush on the metal layer. It was then baked, exposed and developed in a standard way before silver was wet etched in an iron (lid nitrate solution [9] (Fig.2.5). Then the tip was introduced and glued manually with a microprobe stage: packaging and electrical contacts were also realized manually. As a very last step, a 5 p.m thick silver layer was electrochemically grown on the ring electrode to avoid its complete dissolution during electrolytic STM measurements. 4. FIRST RESULTS
Figure 3. Scanning probe in photostructurable glass with via holes for STM tip (centre) and for metal contact to ring electrode (left) Metal was then grown in the holes to serve as a contact for the ring electrode (Fig. 2.4). A first proto.type was realized with copper filling, using a 0.5 M Cu2+ / 0.1 M H2SO4 solution with a current density of 50 to 100 mA/cm2 and a potential of -470 mV versus mercury sulfate electrode (MSE). Following wafers were filled with gold, with a 14 g/1 potassium gold cyanide in Auro Dure 150 (Lea Ronal). Holes were filled up with copper to about 60 to 80% of their height, while they were slightly overfilled with gold. In both cases, separation of wafers could be done manually. Surfaces of glass wafcr wcre thcn equalized by mechanical polishing.
The good etch selectivib" of the glass transformed in ceramic towards the non exposed one allowed a 1.1 mm thick glass wafer to be etched through with an increase of hole diameter of only 70 ~m. For the polished wafers, lateral etching was about 50 pm for a thickness of about 600 p.m. Fig. 3 shows a micrograph of a 1,4 mm diameter glass cylinder with two holes still located in the unpolished supporting wafer. The cylinder can be released by' breaking the four sustaining arms. Holes with minimum diameter of about 80 ~tm have been etched through the whole 1.1 mm thick unpolished wafer. while holes with diameter as small as 60 gin were achieved through the about 600 p.m polished wafer. In both cases, holes had a circular shape. Tips were fitted in two 1.4 mm diameter probes on which the silver ring electrode x~ere then electrochemically thickened to 5 ~tm. One of the probe was then mounted on a commercial STM (Nanoscope by Digital Instrument). First lest measurements with this thin layer probe were performed in an electrolyte solution containing 0.01M perchloric acid and 0.005 M lead ions at a potential controlled Ag <111> substrate electrode with atomically flat terraces that were completely covered with a lead monolayer (starting conditions shown in Fig. 4 (a)). After releasing a finite atuount
550
P.-E lndermiihle et al. / Microelectronic Engineering 41/42 (1998) 547-550
of silver ions into the solution by the silver ring electrode, it is observed ( Fig. 4 Co)) that the shape and size of the terraces changed due to the diffusion and deposition of the generated silver ions. In this case, the thin layer electrolyte saturated with silver ions allowed the electrolytic STM imaging of an electrode interface with controlled reactive compounds, demonstrating thus the validity of the Electrolytic Thin Layer Scanning Tunneling Microscope (ETL-STM) concept.
4. E. Schmidt, H. Siegenthaler, Helv. Chim. Acta 52 (1969) 2245 5. E. Schmidt, H. Siegenthaler, Heir. Chim. Acta 53 (1970) 321 6. E. Schmidt, H. Siegenthaler, J. Electroanal. Chem. 150 (1983) 59 7. D. H~lsenberg, R. Bruntsch, K. Schmidt and F. Reinhold, Silikattechnik 41 (1990) 11,364 8. Schott/IMM information: FOTURAN- a material for microtechnology 9. M. Koudelka, Sensors & Actuators, 9 (1986) 249
5. CONCLUSION We demonstrated a novel scanning probe which enables reactive compounds control during electrolytic STM imaging. The geometry of this probe defines an electrolytic thin layer where the tip and an integrated ring electrode around it are sunk. This probe was realized thanks to a new microfabrication technique allowing metal growth in glass structures. First measurements realized with a probe prototype showed the validity of this concept, which is of special interest for applications like metal nnderpotential deposits and corrosion studies (halide-induced corrosion mechanism, influence of pH). 6. AKNOWLEDGEMENTS
This work was supported by the Swiss National Science Foundation and glass polishing was performed by the Guinchard S.A. company in Yverdon-les-Bains. We also would like to thank G. Mondin of Microsens for some metal depositions and the many persons at IMT who helped us in realizing this work. 7. REFERENCES
1. G. Binnig and H. Rohrer, Helv. Phys. Acta, 55 (1982) 726 2. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett, 49, (1982) 57 3. Scanning Tunneling Microscopy 1./11, (Edited by R. Wiesendanger and H.-J. Giintherodt), Springer Verlag, Berlin, 1992/1993
Figure 4. Electrolytic STM image of A g < l l l > electrode (covered with a Pb-monolayer) with monoatomic steps before generation of silver ions (a), and ca. 4 minutes after generation of a finite amount of silver ions in the thin layer (b). Electrolyte solution: 0.01 M HCIO~ + 0.005 M Pb"*; window size: 100 x 100 rim; greyscale range: 2.5 nm. The mark (+) designs a reference point on the substrate.
Microelectronic Engineering 41/42 (l 998) 547-550
E l e c t r o l y t i c thin layer scanning tunneling m i c r o s c o p e : fabrication and first results P.-F. InderiniJhle", E. AInmann b, P. H~iringc, R. K6tz c, H. Siegenthaler b and N.F. de Rooij a Institute of Microtechnology, University of Neuchfitel, rue Jaquet-Droz 1, CH-2007 Neuchfitcl, Switzerland t' Department for Chemistry and Biochemistry', University of Bern. Freiestrasse 3, CH-3012 Bern. Switzerland Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
A novel scanning probe allowing STM measurements in electrolytic thin layer conditions was fabricated by achieving electrochemical metal growth in photostructurable glass. The concept of this new scanning probe is explained, then the fabrication process is described in detail and first results are presented. 1. INTRODUCTION The scanning tunneling microscope (STM), demonstrated for the first time in 1981 by G. Binnig and H. Rohrer [ 1, 2], very soon revealed itself to be a powerful tool for electrochemical studies at nanometer scale [3]. However, the problem of the access to the chemical information of the electrolyte near the scanning tip has not yet been solved in a satisfying way. The situation can be significantly improved if the scanning tip is located in a so called electrolytic thin layer [4,5,6]. in this paper, we describe the fabrication of a new scanning probe defining an electrolytic thin laver (ETL) around the tunneling tip. Furthermore, a ring electrode, which is integrated on the probe and around the tip, allows the control of the electrolyte compounds during the scan. First, we will briefly discuss the concept of the Electrolytic Thin Layer Scanning Tunneling Microscope (ETL-STM), then the fabrication process will be described in detail and finally first measurements will be presented and shortly commented. 2. THE ELECTROLYTIC ;CANNING TUNNELING ETL-STM)
probe able to control the electrolyte concentration of specific electroactive compounds near the scanning tip may be of great interest. Fig. 1 shows a schematic representation of such a device: a tip and a ring electrode for electrolytic compounds sensing and generation are fitted on a large cylinder (relatively to tip dimensions) made of cheinically inert and isolating material. The tip and the electrode are connected through the isolating material by metallic contacts. As the salient part of the tip is of a fcw tens
THIN LAYER MICROSCOPE
The control of tile chemical species implicated m the clectrolyte/electrodes interface reactions is of first importance for electrochemical scanning probe microscopy. Thus. the development of a scanning 0167-9317/98/$19.00 ~_')Elsevier Science B.V All rights reserved. •I1: SO 167-9317(98)00128-2
Electrolytic STM cdl Figure 1. Schematic illustration of electrolytic thin layer scanning tunneling microscope
548
P.-E Indermiihle et al./Microelectronic Engineering 41/42 (1998) 547-550
of micrometers only, the electrolyte layer between the cylindrical probe and the substrate forms an electrolytic thin layer. The whole probe is then mounted on a piezotube to be scanned over the sample. In this thin-layer STM concept, the ring is operated as a potential controlled electrode that can be used as an electrochemical generator or sensor of electrolyte compounds involved in electrochemical reactions at the substrate electrode. 3. STRUCTURE DESIGN AND FABRICATION 3.1. Design Its good chemical inertia, its high electrical resistance and its ability for high ratio etching made the photostructurable glass Foturan®[7, 8] the ideal candidate for the cylindrical body" of the probe. Cylinders with diameters varying between 250 and 1400 ~m and with 2 to 3 via holes with diameters varying between 30 and 160 p.m were designed to determine a compromise between dimensions of electrolytic thin layer and difficulty of probe approach. 3.2. Fabrication The key step of the fabrication process (Fig. 2) is the electrochemical growth of the metal contacts through the glass cylinders. This could be achieved in a parallel way by bonding the glass wafer with photoresist on an oxidized silicon wafer on which a seed layer was previously patterned. First, a 1 p,m thick aluminum layer was deposited and patterned on 1.1 mm thick Foturan® wafers to serve as an optical mask for UV light exposure (Fig.2.1 (b)). This step was necessary to avoid drift during the exposure time which is of several hours. Then the aluminum layer was wet etched and glass wafers were annealed following a ramp up to about 600 °C. During this step, the exposed glass was transformed into ceramic which could later be selectively etched in hydrofluoric acid (I-IF). Then, some wafers were polished and their thickness reduced to about 600 p.m. In parallel, silicon wafers were oxidized and a platinum seed layer was patterned on one face with the lift-off technique (Fig.2.1 (a)). A chemical vapor deposition (CVD) isolating oxide layer was then deposited and
patterned with standard lithography to avoid unwanted metal growth. The second step consisted in etching the transformed glass with a 10% ultrasonic HF solution (Fig.2.2 (b)). The lateral to vertical etch ratio was about 1 to 20, i.e. the glass transformed in ceramic
by 1 ) a)
b)
2) a)
b)
[
J I 4) ~
] -
1
~
- Monocryslalline t : , .! - CX:D oxide silicon [---~-] - Photostructurable ~ _ Photoresist glass ~ - Exposed photostr, l - Metals glass Figure 2. Fabrication sequence for electrolytic thin layer STM probe light exposure and annealing was etched about 20 times faster than the non exposed one. This allowed the microfabrication of high aspect ratio structures made of chemically inert material. During the third step (Fig.2.3), the glass and silicon wafers were aligned and glued together using a phototresist layer as bonding medium. After a gas phase priming, the Foturan® wafers were fixed in a commercial mask aligner (A1-6.2 by Electronic Vision). Then a thick layer of photoresist was spun on the silicon wafer and its edges removed with acetone to avoid unwanted sticking to occur on edges during alignment phase. The wafer was then inserted into the aligner without
P.-E lndermiihle et al./Microelectronic Engineering 41/42 (1998) 547-550
any baking, aligned and brought into close contact during one hour thanks to a piston and a vacuum system. A prebake sequence was then performed during half an hour at 85 °C. This degassing is required from the photoresist to allow the following UV light exposure and development. It was observed that this step increased the adhesion between glass and silicon wafer for polished glass wafers, while gas bubbles appeared between both wafers for unpolished Foturan® wafers. Then, photoresist was exposed to UV light and developed to be removed where metal was expected to be electrochemically grown.
549
To do this, the already defined structures were sunk in cyanoacrylate glue to avoid them to be torn away by lateral forces. Finally, a 6000 A thick silver layer with titanium anchorage was deposited on glass wafers to be patterned. Due to the high aspect ratio of Foturan® structures, the photoresist needed to be sprayed with an airbrush on the metal layer. It was then baked, exposed and developed in a standard way before silver was wet etched in an iron (lid nitrate solution [9] (Fig.2.5). Then the tip was introduced and glued manually with a microprobe stage: packaging and electrical contacts were also realized manually. As a very last step, a 5 p.m thick silver layer was electrochemically grown on the ring electrode to avoid its complete dissolution during electrolytic STM measurements. 4. FIRST RESULTS
Figure 3. Scanning probe in photostructurable glass with via holes for STM tip (centre) and for metal contact to ring electrode (left) Metal was then grown in the holes to serve as a contact for the ring electrode (Fig. 2.4). A first proto.type was realized with copper filling, using a 0.5 M Cu2+ / 0.1 M H2SO4 solution with a current density of 50 to 100 mA/cm2 and a potential of -470 mV versus mercury sulfate electrode (MSE). Following wafers were filled with gold, with a 14 g/1 potassium gold cyanide in Auro Dure 150 (Lea Ronal). Holes were filled up with copper to about 60 to 80% of their height, while they were slightly overfilled with gold. In both cases, separation of wafers could be done manually. Surfaces of glass wafcr wcre thcn equalized by mechanical polishing.
The good etch selectivib" of the glass transformed in ceramic towards the non exposed one allowed a 1.1 mm thick glass wafer to be etched through with an increase of hole diameter of only 70 ~m. For the polished wafers, lateral etching was about 50 pm for a thickness of about 600 p.m. Fig. 3 shows a micrograph of a 1,4 mm diameter glass cylinder with two holes still located in the unpolished supporting wafer. The cylinder can be released by' breaking the four sustaining arms. Holes with minimum diameter of about 80 ~tm have been etched through the whole 1.1 mm thick unpolished wafer. while holes with diameter as small as 60 gin were achieved through the about 600 p.m polished wafer. In both cases, holes had a circular shape. Tips were fitted in two 1.4 mm diameter probes on which the silver ring electrode x~ere then electrochemically thickened to 5 ~tm. One of the probe was then mounted on a commercial STM (Nanoscope by Digital Instrument). First lest measurements with this thin layer probe were performed in an electrolyte solution containing 0.01M perchloric acid and 0.005 M lead ions at a potential controlled Ag <111> substrate electrode with atomically flat terraces that were completely covered with a lead monolayer (starting conditions shown in Fig. 4 (a)). After releasing a finite atuount
550
P.-E lndermiihle et al. / Microelectronic Engineering 41/42 (1998) 547-550
of silver ions into the solution by the silver ring electrode, it is observed ( Fig. 4 Co)) that the shape and size of the terraces changed due to the diffusion and deposition of the generated silver ions. In this case, the thin layer electrolyte saturated with silver ions allowed the electrolytic STM imaging of an electrode interface with controlled reactive compounds, demonstrating thus the validity of the Electrolytic Thin Layer Scanning Tunneling Microscope (ETL-STM) concept.
4. E. Schmidt, H. Siegenthaler, Helv. Chim. Acta 52 (1969) 2245 5. E. Schmidt, H. Siegenthaler, Heir. Chim. Acta 53 (1970) 321 6. E. Schmidt, H. Siegenthaler, J. Electroanal. Chem. 150 (1983) 59 7. D. H~lsenberg, R. Bruntsch, K. Schmidt and F. Reinhold, Silikattechnik 41 (1990) 11,364 8. Schott/IMM information: FOTURAN- a material for microtechnology 9. M. Koudelka, Sensors & Actuators, 9 (1986) 249
5. CONCLUSION We demonstrated a novel scanning probe which enables reactive compounds control during electrolytic STM imaging. The geometry of this probe defines an electrolytic thin layer where the tip and an integrated ring electrode around it are sunk. This probe was realized thanks to a new microfabrication technique allowing metal growth in glass structures. First measurements realized with a probe prototype showed the validity of this concept, which is of special interest for applications like metal nnderpotential deposits and corrosion studies (halide-induced corrosion mechanism, influence of pH). 6. AKNOWLEDGEMENTS
This work was supported by the Swiss National Science Foundation and glass polishing was performed by the Guinchard S.A. company in Yverdon-les-Bains. We also would like to thank G. Mondin of Microsens for some metal depositions and the many persons at IMT who helped us in realizing this work. 7. REFERENCES
1. G. Binnig and H. Rohrer, Helv. Phys. Acta, 55 (1982) 726 2. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett, 49, (1982) 57 3. Scanning Tunneling Microscopy 1./11, (Edited by R. Wiesendanger and H.-J. Giintherodt), Springer Verlag, Berlin, 1992/1993
Figure 4. Electrolytic STM image of A g < l l l > electrode (covered with a Pb-monolayer) with monoatomic steps before generation of silver ions (a), and ca. 4 minutes after generation of a finite amount of silver ions in the thin layer (b). Electrolyte solution: 0.01 M HCIO~ + 0.005 M Pb"*; window size: 100 x 100 rim; greyscale range: 2.5 nm. The mark (+) designs a reference point on the substrate.