Langmuir 1996,11,211-213
211
Writing and Reading at Nanoscale with a Scanning Tunneling Microscope Rui Yang,*9+D. F. Evans,* and W. A. Hendricksong 3M Company, Electronic Product Division, Austin, Texas 78759,Department of Chemical Engineering, University of Minnesota, Minneapolis, Minnesota 55455,and AVEIKA, Inc., 2045 Wooddale Drive, Woodbury, Minnesota 55125 Received May 11, 1994. I n Final Form: August 5, 1994@
The scanning tunneling microscope (STM) has grown and evolved at a n incredible rate. Now STM not only is used as a powerful imaging tool to provide atomic resolution surface topography but also quickly becomes a nanoscale manipulator after slight modification. In this paper, we describe a nanoscale lithography technique, which was developed by combining STM with an electrochemical cell. We have demonstrated the feasibility of this technique, which allows us to write, read, and erase letters at a nanoscale level. At present, there still exist some difficulties but we believe that the ability to write, edit, and archive symbols and patterns on a nanoscale level constitutes a significant advance and a n enabling concept in nanolithography.
Introduction We have developed a nanoscale lithography technique which permits writing, reading, a n d erasing dots as small as 1nm. This method employed an STM electrochemical cell filled with a solution containing a monomer of a conducting polymer such as pyrrole (Figure 1). By applying a potential between a coated tip a n d a gold surface, we can deposit a nanoscale dot of conducting polymer. By control of t h e position of the tip, patterns or letters can be formed a n d assembled into words (Figure 2). After deposition, the polymer dots can be read using the STM in the imaging mode. Each dot can be changed from a conductive to nonconductive state by a controlled reduction of the polymer. Alternately a dot can be removed by positioning the tip above the dot a n d applying a large enough potential to decompose it (Figure 3). In addition, lines a n d circles can be drawn (Figure 4)which provides a way to create nanoscale patterns. Deposited patterns are stable for months (our present length of observation) a n d can be imaged after storage in the dry state.
M tetraethylammoniumtetrafluoroborate. AU the chemicals are from Aldrich and are reagent grade. Depositions were carried out in air; no precautions were taken to exclude oxygen. The deposition process involved the followng steps. (1)The substrate was placed in the electrochemical cell, solution was added, and the gold surface was imaged in the STM imaging mode. (2) The tip was disconnected from the STM feedback system, switched to a deposition mode, and a high voltage pulse was applied between the tip and substrate to deposit polymer. The pulse voltages used in the experiments are 3-7 V and pulse time length is 1-10 s, which depend on the deposited feature and amount of polymer deposited onto surface. (3) The tip was moved to the next location and the deposition process repeated. (4) Step 3 was repeated as many times as needed to produce the desired pattern. (5) The resulting pattern was imaged by switching to STM mode. In order to produce a line or a circle, the tip was moved during the deposition process. The polymer is in the conducting state as deposited onto surface. To reduce the deposited polymer to a nonconducting state, the STM tip was positioned over a specific feature and the voltage bias reversed for reduction process. To erase a deposited polymer dot, a higher voltage was applied which obliterated the polymer structure.
Experimental Section
Results and Discussion
A Nanoscope I11 (Digital Instrument) equipped with an electrochemical cell was used to deposit polymer patterns and to read the resulting images. Figure 1 shows a schematic to illustrate the basic concept of the deposition process. The insulated tip serves as the counter electrode (C.E.) and the substrate serves as the working electrode (W.E.) in the electrochemical cell, which contains monomer solution. The electrochemical polymerization process will occur when an electrical potential pulse is applied between the electrodes. Consequently, a small amount of polymer will be directly deposited onto the substrate surface. The STh4 tips were formed by cutting Pt/Ir (80/20)wire and were coated with an insulating material by dipping them into molten Apiezon (Digital Instrument) and withdrawing them. Each coated tip was examined under an optical microscope in order to detect coating defects and to determine the size of the noninsulated portion of the tip. The exposed tip feature and size determine the size of the dot formed during deposition. The gold surfaces were prepared by heating a gold wire to produce a gold ball (about 3 mm in diameter) containing (111) facets into which the polymers were deposited. The aqueous plating solution contained 0.05-0.04 M pyrrole and 0.05-0.02
Previous efforts to use STM a n d AFM for nanoscale lithography or device fabrication reported in the literat~rel-~ have involved direct tip-to-surface contact to punch holes6 or cut lines' in a substrate, leave some tip
@
Abstract published inAdvanceACSAbstracts, October 1,1994. 0'743-7463/95/2411-0211$09.00/0
(1)(a) Dobisz, E. A.; Marrian, C. R. K. Appl. Phys. Lett. 1991,58, 2526. (b) Marrian, C. R. R;Dobisz, E. A.; Colton, R. J. J. Vac. Sci. Technol. 1991,B9, 1367. (c) Chang, - T. H. P.; et al. ZBM J. Res. Dew. 1988,32,462. (21(a) Ehrichs. E.E.: Silver. R. M.: DeLozanne, A. L. J. Vac. Sci. Tech& 1988,A6,540.' (b) McCord, M. A.; Hern, D. P.; Chang, T. H. P. J. Vac. Sci. Technol. 1988,B6,1877. (3)Dagata, J.A.; Schneir,J.;Harary, H. H.; Bennett, J.;Tseng, W. J. Vac.Sci. Technol. 1991,B9, 1384. !4)(a)Whitman, L. J.;Stroscio, R. A.; Dragoset, J. A,; Celotta, R. J. Science 1991,251,1206.(b) Stroscio,J.A.; Eigler, D. M. Science 1991, 254,1319. (c)Lyo, I. W.; Avouris, P. Science 1991,253,173. (5)(a)Abraham, D. W.; Marti, H. J.;Ganz, E.; Clarke, J.ZBM J.Res. Deu. 1986,30,492.(b) Jacklevic, R.C.; Elie, L. Phys. Rev. Lett. 1988, 60,120.(c) McCord, M. A.; Pease, R. F. W. Appl. Phys. Lett. 1987,50 569. (d) Rabe, J. P.; Buchholz, S. Appl. Phys. Lett. 1991,58,702. (6)(a) Kim, Y.;Lieber, C. M. Science 1992,257,375.(b) Aono, M. Science 1992,258,586. (7)(a)Mamin, H. J.,Guether, P. H.; Rugar, D. Phys. Rev. Lett. 1990, 65,2418.(b) Mamin, H. J.; Chiang, S.; Birk, H.; Guether, P. H.; Rugar, D. J. Sci. Technol. 1991,B9, 1398.
0 1995 American Chemical Society
212 Langmuir, Vol. 11, No. 1, 1995
Yang et al. ATING MONOMER SOLUTION
DEPOSITED. POLYMER
Figure 1. A schematic to illustrate the basic concept of the deposition process. The insulated tip serves as the counter electrode (C.E.)and the substrate serves as the working electrode (W.E.) in the electrochemical cell. A small amount of polymer will be directly polymerized onto the substrate following a single high voltage pulse.
Figure 2. A n STM image of the word “science”,which was written by polymerizing polypyrrole directly onto gold surface in an electrochemical cell using STM. The size of the polymer dots are 5-10 nm across. material behind,* or etch off surface material^.^ Most of these processes are irreversible. Alternately, the STM has been used to maneuver adsorbed molecules and atoms such as COgCbenzene, and xenon10 on the substrate; however, these procedures do not yield stable configurations amenable to room temperature storage and reuse. We illustrate formation of a pattern with a specific example. We first deposited a n array of seven polymer dots on the substrate to form the pattern shown in Figure 3a. We then added a dot to the top line (Figure 3b), erased a dot, and deposited another one on the middle line (Figure 3c) to form a pattern with two dots on the top line and three on the middle and bottom lines. During the erasing processes, a high voltage should not be used at first. This is to avoid blasting a hole, which will be irreversibly formed in the surface. A low voltage is suggested to start with the gradually increase the blasting voltage. Figure 2 shows a STM image of the wood “science”, which was written by polymerizing polypyrrole directly
onto gold surface in an electrochemical cell using STM. The sizes of the polymer dots are 5-10 nm across. The size of the deposited polymer dots are sometimes not uniform. This is mainly caused by the following reasons: (1)The electrochemical cell used here only holds a tiny amount of liquid solution and is open to air during operation, which leads to the concentration of the cell changing with the time due to fast thermal evaporation. (2) The variation of local surface roughness requires a varied tip travel distance at different locations in order to keep a constant deposition separation. These uncontrolled varying factors make the precise control of the amount of polymer deposited difficult, because these factors greatly affect the amount of polymer deposition onto the surface since the same deposition potential and tip travel distance are usually used in any one pattern deposition. Figure 4 shows a few STM images revealing the ability to deposit different shape features such as dots, lines, and circles. Four polymer dots about 1 nm across were deposited with a regular spacing (Figure 4a). Polymer lines about 10 nm wide were deposited in a vertical form (Figure 4b). The line length depends on the voltage pulse time applied. The line gap shown here was due to the gap between the two different length pulses used. A small polymer circle with an inside diameter of 6 nm was deposited, as seen in Figure 4c. The process described here provides considerably more flexibility in controlling the deposition process. However, there are at present a number of technological limitations, some of which arise in any nanoscale process and others imposed by the present capabilities of the STM. In our initial experiments, we deposited one dot at a time, a rather
figure 3. A series of STM images show the ability of depositing polymer dots onto or removing the dots from substrate surface: (a, left) seven polymer dots were deposited onto the gold surface with three at the bottom and two at the top and middle lines; (b, center) one more dot was added at the top line forming an eight-dot array; (c, right) one more dot was added to the middle line and the previous added dot was removed from the top line forming a different eight-dot array.
Nanoscale Writing and Reading
Langmuir, Vol. 11, No. 1, 1995 213
I
Figure 4. A few STM images show the ability to deposit different shape features: (a, left) Four polymer dots of about 1nm across were deposited with regular spacing, (b, center) Polymer lines about 10 nm wide were deposited in a vertical form. The line length depends on the voltage pulse time applied. The line gap shown here was due to the gap between two different length pulse used to the scanning tip. (c, right) A small polymer circle with an inside diameter of 6 nm was deposited.
laborious process requiring a n hour to produce a letter. Subsequently, we wrote a computer program which enabled the STM to deposit one dot after another thereby permitting a letter to be formed in a few seconds. However, the complexity of the pattern that can be produced is presently limited by (1)thermal drift which is always a concern in any nanoscale process and (2) hysteresis associated with the STM piezocrystal used to maneuver the tip. While the latter factor is not a major limitation (8)(a)Delawski, Ed; Parkinson, B. A. J . Am. Chem. Soc. 1992,114, 1661. (b) Ehrichs, E. E.; dehzanne, A. L. J . Vac.Sci. Technol. 1990, A8,571. (c)Li, Y .Z.;Vazquez, L.; Piner,R.;Andres, R. P.;Reifenberger, P. Appl. Phys. Lett. 1989,54, 1424. (9)(a) Eigler, D.M.;Schweizer, E. K. Nature 1990,344, 524. (b) Schweizer, E. K.; Eigler, D. M. Science 1991,254,1319. (c)Zeppenfeld; et al. Ultramicroscopy 1992,42, 128. (10)Eigler, D. M.,Lutz, C. P. and Rudge, W. E., Nature 1991,352, 600.
when the STM is used in a n imaging mode, it can become a major constraint in the writing mode because of a lack of feedback control of the tip position. The effects of thermal drift can be decreased by isolating the STM, and those associated with hysteresis can be overcome by moving the sample using nanoscale controlled x-y stages during the deposition process rather than moving the tip. In spite of these technological limitations, we believe that the ability to write, edit, and archive symbols and patterns on a nanoscale level constitutes a significant advance and a n enabling concept in nanolithography.
Acknowledgment. Support by the Center for Interficial Engineering, a National Science Foundation Engineering Research Center, and the 3M Company, a CIE sponsor, is gratefully acknowledged. LA940386P
