- Email: [email protected]
Fabrication of tips with controlled geometry for scanning tunnelling microscopy
Fabrication of tips with controlled geometry for scanning tunnelling microscopy I. H. M u s s e l m a n , P. A. Peterson and P. E. Russell* Electrochemical etching techniques have been developed to fabricate tungsten and platinum/iridium tips with controlled geometry for scanning tunnelling microscopy ( S T M ) . These tips, which have a high aspect ratio and a small radius of curvature, are particularly useful for the imaging and metrology of precision engineered surfaces. Keywords: scanning tunnelling microscopy, electrochemical etching, instrument tips, tungsten, platinum / iridium
Scanning tunnelling microscopy ( S T M ) is a high resolution surface imaging technique which has a wide range of potential applications to the imaging and metrology of precision engineered samples. Recently published STM studies of technical surfaces have demonstrated the usefulness of STM for the analysis of diamond-turned samples 1 4 ruled grating replicas 2, x-ray reflecting optics 5, and optical discs, s The tip is an important component of the STM instrument and its shape dramatically affects the information content of STM images. Many precision surfaces have features ranging from 10 to 1000 nm in height and less than 2/~m in width (ie optical discs, non-linear optical waveguides, grating replicas, photolithographic samples, and x-ray masks) which can not be resolved by conventional profilometry. 4 For such samples, symmetric, controlled geometry tips with small radii of curvature and high aspect ratios are needed to probe crevices so that STM images which accurately represent the surface topography can be acquired. The need for specially shaped tips for imaging samples with large features is illustrated schematically in Fig 1. In this figure, the effect of tip geometry (aspect ratio and radius of curvature) on the measured profile of grooves 1 /~m wide and 1 Fm deep can be observed. Fig 1 (a) shows the measured profile of the groove using a low aspect ratio tip approximately 150 nm in radius. As this tip is scanned across the top of the sample (point 1 ), electron tunnelling occurs from the very end of the tip. However, when the tip descends into the groove (point 2), tunnelling takes place between its left side and the left lip of the groove. After the tip has reached the groove bottom (point 3), tunnelling once again occurs from the end of the tip. When the tip approaches the right lip of the groove (point 4), * Department of Materials Science and Engineering and Precision Engineering Center, North Carolina State University, Raleigh, North Carofina 27695, USA
PRECISION ENGINEERING
iI
I/ a
3
~\
'--/"'1
"+~
V b Fig 1 Effect of tip geometry on measured S T M profile of grooves 1 i~m wide and 1 I~m deep using ( a ) tip with 150 nm radius of curvature and small aspect ratio; ( b ) tip with 50 nm radius of curvature and large aspect ratio electron tunnelling occurs from the right side of the tip as the feedback pulls the tip out of the groove. The profile in Fig 1 (a) demonstrates that this tip is too broad to resolve the square bottomed sample feature. A tip with larger dimensions than that shown in Fig 1 (a) would never reach the groove bottom because electron tunnelling in the groove would only occur between the sides of the tip and the walls of the groove. The resulting profile would therefore reflect an inaccurate measure of the groove depth. Fig 1 ( b ) shows the measured profile of the same size groove using a high aspect ratio tip approximately 50 nm in radius. Electron tunnelling
0141 6359/90/010003-04 © 1990 Butterworth & Co (Publishers) Ltd
3
Musselman et al--tip fabrication for scanning tunnelling microscopy occurs between this tip and the groove sample in the same manner as described above yet the dimensions of this tip are such that the tip can more accurately follow the topography. It is clear that proper tip geometry is crucial to obtaining an accurate STM image of the sample surface because the shape of the tip is always convoluted into the resulting image. Tips for scanning tunnelling microscopy have been made from a variety of materials using several fabrication techniques. To date, electrochemical etching has been the most widely employed method of tip prepatation. Many electrochemical polishing methods, solutions, and conditions appropriate for tip specimen preparation in field ion microscopy are also applicable for the preparation of STM tips. 7 In STM, tungsten tips, which fulfil the instrument requirement of being stiff, have been used to a great extent to image specimens, a historic trend carried over from field ion microscopy. Tungsten tips for STM have been etched in a 1M sodium hydroxide solution using a dc potential of 12 V (see Ref 8). In addition, sharp tungsten tips (5 nm diameters) with low-aspect ratio shanks to minimize vibrations, have been prepared 9 in a 3 % - 5 % potassium hydroxide solution at a potential of 20 V ac. It is often difficult to image with electrochemically etched W tips because the electrons must tunnel through a thick aqueous oxide layer which is formed during the etching process. This problem can be partially solved by evaporating the oxide in an ultra-high vacuum system just prior to imaging in vacuum or, if the tips are to be used in air, the aqueous oxide can be removed by annealing prior to imaging. With time, however, a native oxide will regrow on the surface of the W tip. Platinum, although a softer metal, is a preferable tip material to tungsten because it is inert to oxidation. In addition, platinum/iridium alloys can be used to fabricate chemically inert tips with sufficient stiffness due to the iridium content. Tips of Pt/Ir, prepared for atomic resolution imaging in redox solutions, have been electrochemically etched in a solution of saturated CaCI 2 (60% by volume), H20 (36%), and HCI (4%) against a carbon electrode at 25 V ac (see Ref 10). Platinum/iridium tips have also been fabricated 11 for in situ electrode characterization processes by electrochemical etching in a solution of 6M NaCN and 2M KOH. Other unconventional STM tip preparation methods have been explored. Ion sputter thinning has been used for final stage preparation of electrochemically etched tungsten tips to improve their imaging reliability. 12'~3 Elaborate field-ion microscopy techniques have been used to create stable mono-atomic STM tips from single crystal tungsten. TMTips capable of achieving atomic resolution have also been fabricated from pencil lead (graphite), ~5 by coating tungsten tips with colloidal graphite, ~5 and by simply cutting Pt/Ir wire. 16 The cut tips, although variable in shape, appear to produce good quality images from samples with atomic level topography, eg graphite and thin metal films. Cut Pt/Ir tips have been used to measure the surface roughness of microareas of thin Pt films deposited on silicon and highly orientated pyrolytic
4
graphite, 17 as well as Pt-coated non-linear optical polymer films, le For samples with large topography, however, etched tips with controlled geometry provide more reproducible images. In our laboratory, we have therefore developed electrochemical etching techniques to fabricate W and Pt/Ir tips having small radii of curvature and high aspect ratios, important tip characteristics for imaging precision surfaces by STM. Unlike tip preparation by field ion microscopy or ion milling, the method described herein is simple, fast, and inexpensive and is therefore readily available to the general STM community.
Experimental method and results Preparation o f tungsten tips by electrochemical etching and heating A schematic of the set-up for etching W tips is shown in Fig 2. A 1.25 cm long piece of 0,2 mm tungsten wire is secured with set screws in a notch in a T-shaped stainless steel fixture. The fixture is placed on a levelled copper gasket which is attached with a clamp to a ring stand. The etchant solution, consisting of 5 % by weight NaOH, is held in a square Plexiglass vessel and placed on a laboratory jack. A piece of carbon rod is used as the counter electrode. The etchant solution is raised with the jack such that tungsten wire is approximately 1.5 mm into the solution as can be viewed with a low-power telescope. An ac potential ranging from 15 to 5 V is used to etch the W tips. Initially 15 V is applied between the W wire and the carbon electrode to remove the region which had been cold work damaged by the wire cutters. The potential is then decreased to 12 V ac until the wire in solution is observed to be rapidly thinning. Etching is continued at 7 V ac until the wire starts to decrease in length. Final etching is conducted at 5 V ac until just before the wire etches to the surface. A push-button switch has been incorporated into the etching circuit to apply a momentary potential to the tip in order to control better the tip shape during the last few seconds of etch. Etching to the surface has the tendency to dull the tips. With this method, the radii of curvature of the W tips range from less than 50 nm up to 200 nm. Scanning electron micrographs, at low and
Copper gasket T-shaped stainless steel fixture Wire Telescope
~,
Counter electrode
Fig 2 Schematic of etch set-up for W and Pt/Ir. Etch for W uses 5% by weight NaOH and a range of potential from 15 to 5 V ac. Etch for Pt/Ir uses by volume 60% saturated CaCI2, 36% H20, and 4% HCI and a potential of 25 V ac
JANUARY 1990 VOL 12 NO 1
Musselman et al--tip fabrication for scanning tunnelling microscopy
Fig 4 Scanning electron micrographs of Pt/Ir tip after initial etching; (a) 80 x ; (b) 10000 x
Fig 3 Scanning electron micrographs of etched W tip. Radius of curvature < 5 0 nm (a) 80 x ; (b) 20000 x
high magnification, of a tungsten tip with a high aspect ratio and a radius of curvature below 50 nm, are presented in Fig 3. Auger analysis of these W tips after electrochemical etching has revealed the presence of a surface layer of carbon and oxygen approximately 2 to 4 nm thick. 19 The oxygen and carbon Auger signal intensities were reduced to approximately half after heating the tips for 15 s in vacuum at 1400"C. This heat treatment was a necessary preparation step prior to imaging in order to achieve a low noise tunnelling current.
is continued until only a few bubbles emerge from the tip. Fig 4 provides low and high magnification SEM images of the Pt/Ir tips after the first etching step. The most important characteristic of the Pt/Ir tips etched at 25 V ac is the long slender region just below the tip end (Fig 4 ( a ) ) . This feature facilitates the production of a tip with a high aspect ratio as will be explained below. The second step involves precision micropolishing of the end of the tip in a thin film of etchant 2°'21 held in a gold wire loop. With micropolishing, observable surface roughness is removed and the geometry (aspect ratio and radius of curvature) is tailored to meet the desired specifications. A schematic of the micropolishing set-up is shown in Fig 5. The tip is again secured in the stainless steel fixutre which is supported by a copper gasket. A loop 2 mm in diameter made from 0.2 mm gold wire (Ernest F. Fullam, Inc), containing a thin film of the etchant used in the first step, is attached to a micropositioner (Line Tool Co). The wire loop is placed under the tip. An ac potential of 2 V is applied between the tip and the gold wire loop. While watching through a stereomicroscope, the loop is raised and lowered with the micropositioner so that the etchant film makes contact with the tip. The micropolishing method can be used to thin the long slender region at the very T-shaped stainless steel fixture
Preparation of p l a t i n u m / iridium tips by electrochemical etching and micropolishing The Pt/Ir tips are formed in a two step process whereby the wire is first etched in bulk solution to obtain the basic tip shape. The same set-up used for etching W (Fig 2) is employed to initially etch 0.2 mm 80:20 Pt/Ir wire (Ernest F. Fullam, Inc). The etchant solution consists of saturated CaCI 2 ( 6 0 % by volume), H20 (36%), and HCI (4%) (see Ref 10). A 1.25 cm long piece of Pt/Ir wire, which is secured in the stainless steel fixture and submerged 1.5 mm into solution, is etched against a carbon electrode at 25 V ac. Electrochemical etching
PRECISION ENGINEERING
0
.
Tip ~
2V ac
/ Micropositioner q ,i
~/
~:Z::)
/
/
Solution:60% CaCl2 Fig 5 tips
Gold wire loop ~2mm in diameter
36% H20
4% HCI
Schematic of micropolishing set-up for Pt/ Ir
5
Musselman et al--tip fabrication for scanning tunnelling microscopy References 1
Gehrtz, M., Strecker, H. and Grimm, H. Scanning tunnelling microscopy of machined surfacesJ Vac Sci Technol, 1988, A6 (2) 432-435
2
Dragoset, R. A., Yound, R. D., Layer, H. P., Mielczarek, S. R,, Teague, E. C. and Celotta, R. J.
Scanning tunnelling microscopy applied to optical surfaces Opt Let, 1988, 11 (9) 560-562
3 Grigg, D. A. Mechanical design of a scanning tunnelling microscope for the observation of machined surfaces, Masters Thesis Dept Mechanical and Aerospece Engineering, North Carolina State University, Raleigh, NC, 1989
4 Grigg, D. A. Observation of machined surfaces using the scanning tunnelling microscope Fifth Int Prec Engng 5 6
Fig 6 S c a n n i n g electron m i c r o g r a p h s o f same P t / I r ' tip s h o w n in Fig 4 after m i c r o p o l i s h i n g . R a d i u s o f curvature < 5 0 n m ( a ) 8 0 x ; ( b ) 2 0 0 0 0 x
end of tips by moving the tip end through the film or to sharpen the tips by making brief contact with the film. At various intervals throughout the micropolishing process, the tip is observed w i t h a light microscope at 1000X to assess the progress of tip formation. Fig 6 shows l o w and high magnification SEM micrographs of the same P t / I r tip shown in Fig 4 after micropolishing. Note the small radius of curvature, < 50 nm, high aspect ratio, and smooth, featureless surface. The radii of curvature of these tips has been observed by SEM to be as small as 5 nm. If micropolishing is continued for too long thereby totally etching away the slender region at the end of the tip, tips with radii as large as 200 nm can be formed. Although the P t / I r tips are somewhat more difficult to etch than W ones, they have a distinct advantage in that an oxide layer does not form at their surface. With the use of an appropriate etchant, the micropolishing technique described above can be applied to other metals, including W, for the preparation of tips with high aspect ratios and small radii of curvature. 22
Conclusions Electrochemical etching methods have been developed to fabricate W and Pt/Ir tips with controlled geometry for scanning tunnelling microscopy. By combining bulk etching and micropolishing techniques, Pt/Ir tips which are inert to oxidation can be formed having a high aspect ratio and a small radius of curvature. These tips are especially useful for the imaging and metrology of precision engineered surfaces. Using the n e w l y etched P t / I r tips, the study of surface roughness in our laboratory is currently being extended to microareas in crevices previously inaccessible with cut tips.
tunnelling microscopy study of groove structures in polycarbonate optical discs J Mat Sci, 1989, 24, 1739-1747 7 MLiller, E. W. and Tsong, T. T. Field ion microscopy, principles and applications American Elsevier Publishing 8
9
6
Company, Inc. 1969, New York, 119-127 Nicolaides, R., Liang, Y., Packard, W. E., Fu, Z. W., Blackstead, H. A., Chin, K. K., Dow, J. D., Furdyna, J. K., Hu, W. M., Jaklevic, R. C., Kaiser, W. J., Pelton, A. R., Zeller, M. V. and Bellina, Jr. J. Scanning tunnelling microscopy tip structuresJ Vac Sci Technol, 1988, A6 (2), 445-447 Bryant, P. J., Kim, H. S., Zheng, Y. C. and Yang, R.
Technique for shaping scanning tunnelling microscope tips Rev Sci Instrum, 1987, 58 (6), 1115 10 Gewirth, A. A., Craston, D. H, and Bard, A. J. Fabrication and characterization of microtips for in situ scanning tunnelling microscopy J Electroanalytical Chem, submitted September 1988 11 Heben, M. J., Dovek, M. M., Lewis, N. S., Penner, R. M. and Quate, C. F. Preparation of STM tips for in-situ characterization of electrode surfacesJ Microsc, 1988, 152 (3), 651-661 12 Biegelsen, D. K., Ponce, F. A. and Tramontana, J. C. Simple ion milling preparation of<~111 ~ tungsten tips. Appl Phys Lett, 1989, 54 (13), 1223 1225 13 Biegelsen, D. K., Ponce, F. A., Tramontana, J. C. and Koch, S. M. Ion milled tips for scanning tunnelling microscopy Appl Phys Lett, 1987, 50 (11), 696-698
14
Fink, H. W. Mono-atomic tips for scanning tunnelling microscopy IBM J Res Develop, 1986, 30 (5), 460 465
15
Colton, R. J., Baker, S. M., Baldeschwieler, J. D. and
Kaiser, W. J. 'Oxide-free' tip for scanning tunnelling microscopy Appl Phys Lett, 1987, 51 (5), 305-307 16 Digital Instruments, Inc. Goletta, CA 17 Musselman, I. H. and Russell, P. E. Platinum thin film roughness measurementsby scanning tunnelling microscopy Microbeam Analysis - 1989, Russell, P. E. ( Ed. ) San 18
Francisco Press, San Francisco, to be published July 1989 Musselman, I. H., Chen R. T. and Russell, P. E.
Roughness measurementsof nonlinear optical polymers by scanning tunnelling microscopy Electron Microscopy Society 19
of America 1989 San Francisco Press, San Francisco, to be published August 1989 Musselman, I. H., Peterson, P. A., Day, R. J. and
Russell, P. E. Preparation and surface analysis of tungsten tips for scanning tunnelling microscopy Poster Presentation at Seventh Annual Symposium on Advances in Microscopy Sponsored by the Duke Microscopy and Microbeam Analysis, Pine Knoll Shores, North Carolina, September 24, 1988 20 Niemeck, F. W. and Ruppin, D. Elektrolytischeatzung
yon Kathodenspitzen fL~rdas FeldelektronenmikroskopZ
Acknowledgements The authors wish to acknowledge NSF Grant # D M R - 8 6 5 7 8 1 3 and the NCSU Precision Engineering Center for the support of this work.
Seminar/ASPE annual meeting 1989 Green, M., Richter, i . , Kortright, J., Barbee, T., Cart, R. and Lindau, I. Scanning tunnelling microscopy of x-ray optics J Vac Sci Technol, 1988, A6 (2), 428-431 Baro, A. M., Vazquez, L., Bartolame, A., Gomez, J., Garcia, N., Goldberg, H. A., Sawyer, L. C., Chen, R. T., Kohn, R. S. and Reifenberger, R. A scanning
Angew Phys, 1954, 6, 1 3
21
Uelmed, A. J. Helium field-ion microscopy of hexagonal close-packed metals. Surf ScL 1967, 8, 191-205 22 Musselman, I. H., Scattergood, T. R. and Russell, P. E. Manuscript in preparation.
J A N U A R Y 1990 VOL 12 NO 1
Scanning tunnelling microscopy ( S T M ) is a high resolution surface imaging technique which has a wide range of potential applications to the imaging and metrology of precision engineered samples. Recently published STM studies of technical surfaces have demonstrated the usefulness of STM for the analysis of diamond-turned samples 1 4 ruled grating replicas 2, x-ray reflecting optics 5, and optical discs, s The tip is an important component of the STM instrument and its shape dramatically affects the information content of STM images. Many precision surfaces have features ranging from 10 to 1000 nm in height and less than 2/~m in width (ie optical discs, non-linear optical waveguides, grating replicas, photolithographic samples, and x-ray masks) which can not be resolved by conventional profilometry. 4 For such samples, symmetric, controlled geometry tips with small radii of curvature and high aspect ratios are needed to probe crevices so that STM images which accurately represent the surface topography can be acquired. The need for specially shaped tips for imaging samples with large features is illustrated schematically in Fig 1. In this figure, the effect of tip geometry (aspect ratio and radius of curvature) on the measured profile of grooves 1 /~m wide and 1 Fm deep can be observed. Fig 1 (a) shows the measured profile of the groove using a low aspect ratio tip approximately 150 nm in radius. As this tip is scanned across the top of the sample (point 1 ), electron tunnelling occurs from the very end of the tip. However, when the tip descends into the groove (point 2), tunnelling takes place between its left side and the left lip of the groove. After the tip has reached the groove bottom (point 3), tunnelling once again occurs from the end of the tip. When the tip approaches the right lip of the groove (point 4), * Department of Materials Science and Engineering and Precision Engineering Center, North Carolina State University, Raleigh, North Carofina 27695, USA
PRECISION ENGINEERING
iI
I/ a
3
~\
'--/"'1
"+~
V b Fig 1 Effect of tip geometry on measured S T M profile of grooves 1 i~m wide and 1 I~m deep using ( a ) tip with 150 nm radius of curvature and small aspect ratio; ( b ) tip with 50 nm radius of curvature and large aspect ratio electron tunnelling occurs from the right side of the tip as the feedback pulls the tip out of the groove. The profile in Fig 1 (a) demonstrates that this tip is too broad to resolve the square bottomed sample feature. A tip with larger dimensions than that shown in Fig 1 (a) would never reach the groove bottom because electron tunnelling in the groove would only occur between the sides of the tip and the walls of the groove. The resulting profile would therefore reflect an inaccurate measure of the groove depth. Fig 1 ( b ) shows the measured profile of the same size groove using a high aspect ratio tip approximately 50 nm in radius. Electron tunnelling
0141 6359/90/010003-04 © 1990 Butterworth & Co (Publishers) Ltd
3
Musselman et al--tip fabrication for scanning tunnelling microscopy occurs between this tip and the groove sample in the same manner as described above yet the dimensions of this tip are such that the tip can more accurately follow the topography. It is clear that proper tip geometry is crucial to obtaining an accurate STM image of the sample surface because the shape of the tip is always convoluted into the resulting image. Tips for scanning tunnelling microscopy have been made from a variety of materials using several fabrication techniques. To date, electrochemical etching has been the most widely employed method of tip prepatation. Many electrochemical polishing methods, solutions, and conditions appropriate for tip specimen preparation in field ion microscopy are also applicable for the preparation of STM tips. 7 In STM, tungsten tips, which fulfil the instrument requirement of being stiff, have been used to a great extent to image specimens, a historic trend carried over from field ion microscopy. Tungsten tips for STM have been etched in a 1M sodium hydroxide solution using a dc potential of 12 V (see Ref 8). In addition, sharp tungsten tips (5 nm diameters) with low-aspect ratio shanks to minimize vibrations, have been prepared 9 in a 3 % - 5 % potassium hydroxide solution at a potential of 20 V ac. It is often difficult to image with electrochemically etched W tips because the electrons must tunnel through a thick aqueous oxide layer which is formed during the etching process. This problem can be partially solved by evaporating the oxide in an ultra-high vacuum system just prior to imaging in vacuum or, if the tips are to be used in air, the aqueous oxide can be removed by annealing prior to imaging. With time, however, a native oxide will regrow on the surface of the W tip. Platinum, although a softer metal, is a preferable tip material to tungsten because it is inert to oxidation. In addition, platinum/iridium alloys can be used to fabricate chemically inert tips with sufficient stiffness due to the iridium content. Tips of Pt/Ir, prepared for atomic resolution imaging in redox solutions, have been electrochemically etched in a solution of saturated CaCI 2 (60% by volume), H20 (36%), and HCI (4%) against a carbon electrode at 25 V ac (see Ref 10). Platinum/iridium tips have also been fabricated 11 for in situ electrode characterization processes by electrochemical etching in a solution of 6M NaCN and 2M KOH. Other unconventional STM tip preparation methods have been explored. Ion sputter thinning has been used for final stage preparation of electrochemically etched tungsten tips to improve their imaging reliability. 12'~3 Elaborate field-ion microscopy techniques have been used to create stable mono-atomic STM tips from single crystal tungsten. TMTips capable of achieving atomic resolution have also been fabricated from pencil lead (graphite), ~5 by coating tungsten tips with colloidal graphite, ~5 and by simply cutting Pt/Ir wire. 16 The cut tips, although variable in shape, appear to produce good quality images from samples with atomic level topography, eg graphite and thin metal films. Cut Pt/Ir tips have been used to measure the surface roughness of microareas of thin Pt films deposited on silicon and highly orientated pyrolytic
4
graphite, 17 as well as Pt-coated non-linear optical polymer films, le For samples with large topography, however, etched tips with controlled geometry provide more reproducible images. In our laboratory, we have therefore developed electrochemical etching techniques to fabricate W and Pt/Ir tips having small radii of curvature and high aspect ratios, important tip characteristics for imaging precision surfaces by STM. Unlike tip preparation by field ion microscopy or ion milling, the method described herein is simple, fast, and inexpensive and is therefore readily available to the general STM community.
Experimental method and results Preparation o f tungsten tips by electrochemical etching and heating A schematic of the set-up for etching W tips is shown in Fig 2. A 1.25 cm long piece of 0,2 mm tungsten wire is secured with set screws in a notch in a T-shaped stainless steel fixture. The fixture is placed on a levelled copper gasket which is attached with a clamp to a ring stand. The etchant solution, consisting of 5 % by weight NaOH, is held in a square Plexiglass vessel and placed on a laboratory jack. A piece of carbon rod is used as the counter electrode. The etchant solution is raised with the jack such that tungsten wire is approximately 1.5 mm into the solution as can be viewed with a low-power telescope. An ac potential ranging from 15 to 5 V is used to etch the W tips. Initially 15 V is applied between the W wire and the carbon electrode to remove the region which had been cold work damaged by the wire cutters. The potential is then decreased to 12 V ac until the wire in solution is observed to be rapidly thinning. Etching is continued at 7 V ac until the wire starts to decrease in length. Final etching is conducted at 5 V ac until just before the wire etches to the surface. A push-button switch has been incorporated into the etching circuit to apply a momentary potential to the tip in order to control better the tip shape during the last few seconds of etch. Etching to the surface has the tendency to dull the tips. With this method, the radii of curvature of the W tips range from less than 50 nm up to 200 nm. Scanning electron micrographs, at low and
Copper gasket T-shaped stainless steel fixture Wire Telescope
~,
Counter electrode
Fig 2 Schematic of etch set-up for W and Pt/Ir. Etch for W uses 5% by weight NaOH and a range of potential from 15 to 5 V ac. Etch for Pt/Ir uses by volume 60% saturated CaCI2, 36% H20, and 4% HCI and a potential of 25 V ac
JANUARY 1990 VOL 12 NO 1
Musselman et al--tip fabrication for scanning tunnelling microscopy
Fig 4 Scanning electron micrographs of Pt/Ir tip after initial etching; (a) 80 x ; (b) 10000 x
Fig 3 Scanning electron micrographs of etched W tip. Radius of curvature < 5 0 nm (a) 80 x ; (b) 20000 x
high magnification, of a tungsten tip with a high aspect ratio and a radius of curvature below 50 nm, are presented in Fig 3. Auger analysis of these W tips after electrochemical etching has revealed the presence of a surface layer of carbon and oxygen approximately 2 to 4 nm thick. 19 The oxygen and carbon Auger signal intensities were reduced to approximately half after heating the tips for 15 s in vacuum at 1400"C. This heat treatment was a necessary preparation step prior to imaging in order to achieve a low noise tunnelling current.
is continued until only a few bubbles emerge from the tip. Fig 4 provides low and high magnification SEM images of the Pt/Ir tips after the first etching step. The most important characteristic of the Pt/Ir tips etched at 25 V ac is the long slender region just below the tip end (Fig 4 ( a ) ) . This feature facilitates the production of a tip with a high aspect ratio as will be explained below. The second step involves precision micropolishing of the end of the tip in a thin film of etchant 2°'21 held in a gold wire loop. With micropolishing, observable surface roughness is removed and the geometry (aspect ratio and radius of curvature) is tailored to meet the desired specifications. A schematic of the micropolishing set-up is shown in Fig 5. The tip is again secured in the stainless steel fixutre which is supported by a copper gasket. A loop 2 mm in diameter made from 0.2 mm gold wire (Ernest F. Fullam, Inc), containing a thin film of the etchant used in the first step, is attached to a micropositioner (Line Tool Co). The wire loop is placed under the tip. An ac potential of 2 V is applied between the tip and the gold wire loop. While watching through a stereomicroscope, the loop is raised and lowered with the micropositioner so that the etchant film makes contact with the tip. The micropolishing method can be used to thin the long slender region at the very T-shaped stainless steel fixture
Preparation of p l a t i n u m / iridium tips by electrochemical etching and micropolishing The Pt/Ir tips are formed in a two step process whereby the wire is first etched in bulk solution to obtain the basic tip shape. The same set-up used for etching W (Fig 2) is employed to initially etch 0.2 mm 80:20 Pt/Ir wire (Ernest F. Fullam, Inc). The etchant solution consists of saturated CaCI 2 ( 6 0 % by volume), H20 (36%), and HCI (4%) (see Ref 10). A 1.25 cm long piece of Pt/Ir wire, which is secured in the stainless steel fixture and submerged 1.5 mm into solution, is etched against a carbon electrode at 25 V ac. Electrochemical etching
PRECISION ENGINEERING
0
.
Tip ~
2V ac
/ Micropositioner q ,i
~/
~:Z::)
/
/
Solution:60% CaCl2 Fig 5 tips
Gold wire loop ~2mm in diameter
36% H20
4% HCI
Schematic of micropolishing set-up for Pt/ Ir
5
Musselman et al--tip fabrication for scanning tunnelling microscopy References 1
Gehrtz, M., Strecker, H. and Grimm, H. Scanning tunnelling microscopy of machined surfacesJ Vac Sci Technol, 1988, A6 (2) 432-435
2
Dragoset, R. A., Yound, R. D., Layer, H. P., Mielczarek, S. R,, Teague, E. C. and Celotta, R. J.
Scanning tunnelling microscopy applied to optical surfaces Opt Let, 1988, 11 (9) 560-562
3 Grigg, D. A. Mechanical design of a scanning tunnelling microscope for the observation of machined surfaces, Masters Thesis Dept Mechanical and Aerospece Engineering, North Carolina State University, Raleigh, NC, 1989
4 Grigg, D. A. Observation of machined surfaces using the scanning tunnelling microscope Fifth Int Prec Engng 5 6
Fig 6 S c a n n i n g electron m i c r o g r a p h s o f same P t / I r ' tip s h o w n in Fig 4 after m i c r o p o l i s h i n g . R a d i u s o f curvature < 5 0 n m ( a ) 8 0 x ; ( b ) 2 0 0 0 0 x
end of tips by moving the tip end through the film or to sharpen the tips by making brief contact with the film. At various intervals throughout the micropolishing process, the tip is observed w i t h a light microscope at 1000X to assess the progress of tip formation. Fig 6 shows l o w and high magnification SEM micrographs of the same P t / I r tip shown in Fig 4 after micropolishing. Note the small radius of curvature, < 50 nm, high aspect ratio, and smooth, featureless surface. The radii of curvature of these tips has been observed by SEM to be as small as 5 nm. If micropolishing is continued for too long thereby totally etching away the slender region at the end of the tip, tips with radii as large as 200 nm can be formed. Although the P t / I r tips are somewhat more difficult to etch than W ones, they have a distinct advantage in that an oxide layer does not form at their surface. With the use of an appropriate etchant, the micropolishing technique described above can be applied to other metals, including W, for the preparation of tips with high aspect ratios and small radii of curvature. 22
Conclusions Electrochemical etching methods have been developed to fabricate W and Pt/Ir tips with controlled geometry for scanning tunnelling microscopy. By combining bulk etching and micropolishing techniques, Pt/Ir tips which are inert to oxidation can be formed having a high aspect ratio and a small radius of curvature. These tips are especially useful for the imaging and metrology of precision engineered surfaces. Using the n e w l y etched P t / I r tips, the study of surface roughness in our laboratory is currently being extended to microareas in crevices previously inaccessible with cut tips.
tunnelling microscopy study of groove structures in polycarbonate optical discs J Mat Sci, 1989, 24, 1739-1747 7 MLiller, E. W. and Tsong, T. T. Field ion microscopy, principles and applications American Elsevier Publishing 8
9
6
Company, Inc. 1969, New York, 119-127 Nicolaides, R., Liang, Y., Packard, W. E., Fu, Z. W., Blackstead, H. A., Chin, K. K., Dow, J. D., Furdyna, J. K., Hu, W. M., Jaklevic, R. C., Kaiser, W. J., Pelton, A. R., Zeller, M. V. and Bellina, Jr. J. Scanning tunnelling microscopy tip structuresJ Vac Sci Technol, 1988, A6 (2), 445-447 Bryant, P. J., Kim, H. S., Zheng, Y. C. and Yang, R.
Technique for shaping scanning tunnelling microscope tips Rev Sci Instrum, 1987, 58 (6), 1115 10 Gewirth, A. A., Craston, D. H, and Bard, A. J. Fabrication and characterization of microtips for in situ scanning tunnelling microscopy J Electroanalytical Chem, submitted September 1988 11 Heben, M. J., Dovek, M. M., Lewis, N. S., Penner, R. M. and Quate, C. F. Preparation of STM tips for in-situ characterization of electrode surfacesJ Microsc, 1988, 152 (3), 651-661 12 Biegelsen, D. K., Ponce, F. A. and Tramontana, J. C. Simple ion milling preparation of<~111 ~ tungsten tips. Appl Phys Lett, 1989, 54 (13), 1223 1225 13 Biegelsen, D. K., Ponce, F. A., Tramontana, J. C. and Koch, S. M. Ion milled tips for scanning tunnelling microscopy Appl Phys Lett, 1987, 50 (11), 696-698
14
Fink, H. W. Mono-atomic tips for scanning tunnelling microscopy IBM J Res Develop, 1986, 30 (5), 460 465
15
Colton, R. J., Baker, S. M., Baldeschwieler, J. D. and
Kaiser, W. J. 'Oxide-free' tip for scanning tunnelling microscopy Appl Phys Lett, 1987, 51 (5), 305-307 16 Digital Instruments, Inc. Goletta, CA 17 Musselman, I. H. and Russell, P. E. Platinum thin film roughness measurementsby scanning tunnelling microscopy Microbeam Analysis - 1989, Russell, P. E. ( Ed. ) San 18
Francisco Press, San Francisco, to be published July 1989 Musselman, I. H., Chen R. T. and Russell, P. E.
Roughness measurementsof nonlinear optical polymers by scanning tunnelling microscopy Electron Microscopy Society 19
of America 1989 San Francisco Press, San Francisco, to be published August 1989 Musselman, I. H., Peterson, P. A., Day, R. J. and
Russell, P. E. Preparation and surface analysis of tungsten tips for scanning tunnelling microscopy Poster Presentation at Seventh Annual Symposium on Advances in Microscopy Sponsored by the Duke Microscopy and Microbeam Analysis, Pine Knoll Shores, North Carolina, September 24, 1988 20 Niemeck, F. W. and Ruppin, D. Elektrolytischeatzung
yon Kathodenspitzen fL~rdas FeldelektronenmikroskopZ
Acknowledgements The authors wish to acknowledge NSF Grant # D M R - 8 6 5 7 8 1 3 and the NCSU Precision Engineering Center for the support of this work.
Seminar/ASPE annual meeting 1989 Green, M., Richter, i . , Kortright, J., Barbee, T., Cart, R. and Lindau, I. Scanning tunnelling microscopy of x-ray optics J Vac Sci Technol, 1988, A6 (2), 428-431 Baro, A. M., Vazquez, L., Bartolame, A., Gomez, J., Garcia, N., Goldberg, H. A., Sawyer, L. C., Chen, R. T., Kohn, R. S. and Reifenberger, R. A scanning
Angew Phys, 1954, 6, 1 3
21
Uelmed, A. J. Helium field-ion microscopy of hexagonal close-packed metals. Surf ScL 1967, 8, 191-205 22 Musselman, I. H., Scattergood, T. R. and Russell, P. E. Manuscript in preparation.
J A N U A R Y 1990 VOL 12 NO 1