Preferential Dissolution of Aluminum under the Tip of an Atomic Force

In each case, it is shown that the interaction of the microscope tip with the Al substrate causes the removal of Al from beneath the tip. This Al remo...
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Langmuir 1996, 12, 5818-5823

Preferential Dissolution of Aluminum under the Tip of an Atomic Force Microscope Lionel Roue´, Linlin Chen,† and Daniel Guay* INRS-E Ä nergie et Mate´ riaux, 1650 Boulevard Lionel-Boulet, C.P. 1020, Varennes, Que´ bec J3X 1S2, Canada Received March 24, 1995. In Final Form: August 30, 1996X The corrosion of aluminum in a chloride-containing solution was studied by atomic force microscopy. Two different Al substrates were used, namely thin Al films deposited on a glass substrate and polished commercial Al plates. In each case, it is shown that the interaction of the microscope tip with the Al substrate causes the removal of Al from beneath the tip. This Al removal does not occur when the Al substrates are scanned in air or in pure demineralized water. The rate of Al removal varies almost linearly with the load applied by the cantilever tip on the Al surface in the range between 0 and 80 × 10-9 N. At a given load, increasing the scanning rate causes an increase in the rate of Al removal up to a value of about 3 Hz. The rate of Al removal does not increase much for scanning rates above that value. The size of the smallest features that can be produced on a thin Al film is about 150 nm. The maximum resolution attainable seems to be limited more by the shape of the tip used to engrave the Al than by anything else.

Introduction Since their introduction in the last decade, the scanning tunneling and atomic force microscopes have evolved into powerful tools to characterize and manipulate matter on an extremely small scale. Moreover, both microscopes have the capability to operate with the sample immersed in an electrolytic environment,1 which makes them ideally suited for studying the surface of electrodes under real operating conditions. A number of in situ corrosion, passivation, and dissolution scanning tunneling microscopy studies have already appeared in the literature. Pickering et al.2 and Zhang and Stimming3 have studied Cu corrosion near and at open-circuit potential, respectively. Dissolution of an Ag-Au alloy was monitored by Oppenheim et al.,4 while corrosion of nickel and stainless steel was studied by Bard.5,6 Passivation of polycrystalline iron and corrosion of Al and an Al-Ta alloy have also been studied by Bockris.7,8 The imaging of Al with the scanning tunneling microscope is a particularly interesting case. As is well-known, a thin layer of native oxide is formed at the surface of Al. According to an earlier report, STM imaging of Al in air becomes difficult after this native oxide layer has reached a certain thickness (2.0-3.0 nm).9 While it seems that STM imaging of Al immersed in an aqueous solution is facilitated,8 the very presence of this oxide layer raises some concerns relating to the conduction mechanisms * To whom correspondence should be sent. Telephone: (514) 9298141. Fax: (514) 929-8102. E-mail: [email protected]. † Present address: Department of Chemical Engineering, University of California, Los Angeles, Los Angeles, CA 90095. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) Sonnenfeld, R.; Hansma, P. K. Science 1986, 232, 211. (2) Pickering, H. W.; Wu, Y. C.; Gregory, D. S.; Geh, S.; Sakurai, T. J. Vac. Sci. Technol., B 1991, 9, 976. (3) Zhang, X. G.; Stimming, U. Corros. Sci. 1990, 30, 951. (4) Oppenheim, I. C.; Trevor, D. J.; Chidsey, C. E. D.; Trevor, P. L.; Sieradzki, K. Science 1991, 254, 687. (5) Lev, O.; Fan, F.-R. F.; Bard, A. J. J. Electrochem. Soc. 1988, 135, 783. (6) Fan, F.-R. F.; Bard, A. J. J. Electrochem. Soc. 1989, 136, 166. (7) Bhardwaj, R. C.; Gonza´lez-Martı´n, A.; Bockris, J. O’M. J. Electrochem. Soc. 1991, 138, 1901. (8) Bhardwaj, R. C.; Gonza´lez-Martı´n, A.; Bockris, J. O’M. J. Electrochem. Soc. 1992, 139, 1050. (9) Szklarczyk, M.; Minevski, L.; Bockris, J. O’M. J. Electrochem. Soc. 1990, 289, 279.

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through this resistive layer and to the location of the interface that is actually imaged by the tunneling current.8 It is possible to alleviate these concerns by using an atomic force microscope. This strategy has already been used with success to make in situ observations of shape evolution during copper dissolution,10 to monitor corrosion near iron-rich inclusions in aluminum,11 and to follow the time evolution of microscopic topography on corroding aluminum surfaces during oxide film passivation.12 While studying the corrosion of aluminum immersed in various electrolytes, we found that under specific operating conditions the cantilever tip has a marked influence on the shape evolution of the Al film. In a brief report, we described how this effect could be used to selectively dissolve a portion of an Al thin film immersed in a 0.1 M NaCl aqueous solution by continuously scanning the atomic force microscope tip over a defined area.13 This paper presents a comprehensive study of the selective dissolution induced by the interaction of the AFM tip with an Al substrate. A number of previously unreported experiments have been performed which involved varying the type of Al substrates, the force applied by the cantilever on the Al surface, the time sequence of the scanning pattern, and the scanning rate. Results showing the ultimate pit resolution attainable are also presented. Various hypotheses are considered to explain the increase of the Al dissolution rate in the region of the film where the tip of the cantilever is scanned. This new capability is to be added to the list of micro- and nanofabrication techniques relying on the use of scanning tunneling and atomic force microscopes to manipulate matter on an extremely small scale. Experimental Section Most of the experiments to be described have been performed on thin Al films deposited on cleaned glass substrates. The glass substrates were first degreased with organic solvents and then soaked for 30 min in a sulfuric acid/chromic acid cleaning solution. They were then thoroughly rinsed with demineralized water and dried under a N2 stream. A thin layer of Al was then vacuum (10) Cruickshank, B. J.; Gewirth, A. A.; Rynders, R. M.; Alkire, R. C. J. Electrochem. Soc. 1992, 139, 2829. (11) Rynders, R. M.; Paik, C.-H.; Ke, R.; Alkire, R. C. J. Electrochem. Soc. 1994, 141, 1439. (12) Tak, Y.; Henderson, E. R.; Hebert, K. R. J. Electrochem. Soc. 1994, 141, 1446. (13) Chen, L.; Guay, D. J. Electrochem. Soc. 1994, 141, L43.

© 1996 American Chemical Society

Preferential Dissolution of Aluminum deposited on the substrate by Joule heating vaporization of pure Al particles (99.9%) (Alfa products). The base pressure of the vacuum system was 10-5 Pa. The film thickness was less than 100 nm. The Al thin layers were stored in air prior to experimentation. In some experiments, commercial Al plates were used as substrates. In these cases, the surface of the sample was manually polished with 1000, 1200, and 4000 emery paper. Three subsequent polishing steps were performed using a suspension of diamond particles of 6 and 1 µm diameter and a suspension of SiO2 particles of 0.05 µm diameter. The Al surface was then thoroughly rinsed with demineralized water and dried with nitrogen. The contact-mode AFM was a Nanoscope III (Digital Instruments, Inc., Santa Barbara, CA). All experiments were done in the constant-force mode with a scanning head having a maximum scanning range of 15 µm. The fluid cell supplied by the manufacturer was used to perform the imaging with the sample immersed in an electrolytic solution. Demineralized water with a specific resistivity larger than 18.0 MΩ cm was used throughout this study. NaCl was used directly from the bottle. The solution in the fluid cell could be changed without drying out the sample through a series of tubes connecting the cell to a reservoir containing the solution. Commercially available Si3N4 tips mounted on a cantilever were used throughout this study (nanoprobes supplied by Digital Instruments). In the previous report,13 the nominal spring constants of the cantilevers were used to calculate the load applied by the cantilever tip on the Al substrate. As it turns out, a large variation is observed between the nominal spring constants of the cantilevers and the measured ones. In the present study, the determination of the spring constants of the cantilevers was performed by measuring their resonant frequencies.14 As shown elsewhere, the spring constants of identical cantilevers do not vary much as long as they originate from the same wafer. In most experiments prolonged scanning was performed over the same area of the Al film. Special care was taken to minimize the lateral drift of the area under investigation. All the settings and the laser adjustment of the AFM were done with pure demineralized water in the liquid cell. Water was then replaced by allowing 15 mL of the electrolytic solution to run through the small volume (0.1 mL) of the fluid cell. The images shown herein are really 3-D pictures of the substrate surfaces, with the left to right (fast scanning direction) and the upper to lower (slow scanning direction) images corresponding to the X and Y axes of the sample, respectively. The gray scale displayed on the right-hand sides of the figures corresponds to the Z axis. The images have not been filtered nor processed, except for the substraction of a “polynomial plane” which removes the image bow and tilt. The polynomial plane is a surface whose cross section is a second-order polynomial in one axis and a horizontal line in the other axis. The height variation of the film under the AFM tip was followed by performing a roughness calculation. The Rq value of a surface is a measure of its roughness and is given by

Rq )

x∑

(Zi - Zave)2

i

N

where Zave is the average of the Z values, Zi is the current value, and N is the number of points within the given area. In some cases, the normalized loss of surface roughness, Norm Rq loss, defined as 100[Rq(t) - Rq(t)0)]/Rq(t)0), was used for comparative purposes.

Results Our initial aim was to study the dissolution of Al in a corrosive NaCl solution and to get information on the time evolution of the surface morphology as the film corrodes. To this end, multiple images were acquired sequentially by performing the continuous scanning of an Al surface immersed in a NaCl solution for over 2 h. Under these (14) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403.

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Figure 1. Line section analysis performed at the scanning time t ) 0 min (A) and t ) 60 min (B) over the same portion of an Al thin film immersed in a 0.1 M NaCl solution that was continuously scanned during 1 h (applied force ) 83 nN; scanning frequency ) 2 Hz).

conditions, preferential removal of Al from under the tip of the cantilever was observed.13 This paper is a followup of the previous one, where the phenomenon of preferential dissolution of Al was only briefly described. In this paper, the influence of various scanning parameters on the rate of Al removal is studied. There is gradual removal of Al from under the tip of the cantilever when a thin film of Al immersed in a 0.1 M NaCl solution is continuously scanned over a long enough period. Figure 1 shows a line section analysis that was performed on a given portion of such an Al thin film at two different times. It gives the variation of the height profile between the beginning of the experiment (t ) 0 min) and the fourteenth scan (t ) 60 min). There is a net decrease in the amplitude of the height profile over the scanning period. Using such a height profile, it is possible to get a more quantitative estimate of the height variation over the scanning period by calculating the line roughness. In the case of Figure 1, the line roughness is 3.10 and 1.94 nm for curves A and B, respectively. This corresponds to a Norm Rq loss value of 37.4%. This preferential removal of Al is not observed when the experiment is conducted with the film immersed in pure demineralized water. As shown in Figure 2, scanning of the microscope tip over the same area of the Al film under these conditions can be performed during 1 h without any noticeable change in the roughness value (to within 5%). Also, there is no noticeable variation of the film morphology when it is extensively scanned in air. Clearly, this indicates that the presence of aqueous NaCl favors Al removal from the substrate. In order to understand the mechanisms responsible for the observed phenomenon, it is important to determine if the removal of Al can continue by itself without any further interaction with the tip after its initiation by the first few contacts of the microscope tip with the surface of the film or if continuous contact of the tip with the surface is required for dissolution. Figure 3 displays the results of the experiment that was designed to answer

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A

B

Figure 2. AFM images of a 3 × 3 µm2 area of an Al thin film immersed in demineralized water: (A) before and (B) after a period of 1 h of continuous scanning at 2 Hz (applied load of about 80 nN).

Figure 3. Plot of the Rq value as a function of time for the complex scanning program that is schematically depicted at the bottom of the figure. The scanning periods and nonscanning periods are denoted by S and R, respectively. At the end of the program, the surface of the Al film was continuously scanned by the microscope tip.

this question. It shows the variation of the Rq value of a given area of an Al thin film as a function of time for the complex scanning pattern that is schematically displayed in the lower part of the figure. The first part of the scanning pattern is made of three cycles of alternating periods during which the microscope tip scans the surface of the film (S) or is at rest over it (R). As shown in Figure

Figure 4. Variation of the roughness loss as a function of the scanning frequency. All these experiments were performed on identical films with a load of 22 nN applied by the cantilever on the Al substrate. The experiments were conducted over a fixed period of time of 60 min.

3, the scanning periods lasted only a few minutes and are separated by resting periods of about 60 min. At the end of these three successive cycles, the film is scanned continuously (S). If one compares the Rq values at the end of any short scanning period with that at the beginning of the next one, it is clear from Figure 3 that the Rq value does not vary significantly during the period over which the area is not being scanned. However, when the film is continuously scanned, as it is at the end of the scanning pattern displayed in Figure 3, there is a steady decrease in the value of the surface roughness which reflects the fact that the film is slowly dissolving in the electrolyte. It is therefore clear that the preferential removal of Al is not a phenomenon that can proceed by itself once it is has been initiated by the first contact of the cantilever tip with the substrate. The continuous action of the cantilever tip is required. Figure 4 shows the variation of the roughness of the Al surface as a function of the scanning frequency. In these experiments, the load applied by the cantilever on the Al surface was kept constant at 22 nN, and the surface of the sample was scanned for a fixed period of time (60 min) at a predetermined scanning rate. Between each measurement, the sample was moved to expose a new portion of the film to the scanning tip. At the lower end of the scanning frequencies, the normalized roughness loss increases steadily up to about 3 Hz. Above this scanning rate, the increase of the Norm Rq loss value with the scanning frequency tends to level off. Figure 5 shows the variation of the Norm Rq loss value as the load applied by the tip of the cantilever is increased from 0 to about 80 × 10-9 N. There is a gradual increase of the Norm Rq loss value as the load is increased. The curve in Figure 5 (a polynomial of second order) was drawn just as a visual aid and does not carry any physical meaning. The size of the narrowest engraveable feature in an Al thin film was measured. To do this, we performed a 3 µm long line scan by disabling the Y slow scanning direction of the piezoelectric element. Figure 6A shows a 4 × 4 µm2 area image of the previously scanned region. The dark line visible in the center portion of the figure indicates areas where Al was removed. A section analysis performed along a line perpendicular to the elongated pit showed the trench to have an inverted triangle-like shape, with a width of 150 nm and a depth of ∼30 nm (Figure 6B). The cross-sectional view of the trench matched the commercial Si3N4 tip shape.

Preferential Dissolution of Aluminum

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A

Figure 5. Variation of the roughness of an Al thin film with the load of the cantilever. In all these experiments, the scanning frequency was kept constant at 2 Hz. The values of the load were calculated by first measuring the spring constant of the cantilever according to the procedure described in ref 14.

B

A

C

B

Figure 6. AFM image of a 4 × 4 µm2 area of an Al thin film that was previously scanned at 10 Hz for 50 min along a single line by disabling the Y slow scanning direction (A). The line section analysis at a direction perpendicular to the feature engraved in the Al film is also shown (B).

The removal of Al that occurs when a cantilever tip scans an Al surface with a sufficiently high load is not restricted to the Al-on-glass system. A similar phenomenon can be observed on commercial polished Al plates if they are imaged under the same experimental conditions. Figure 7 shows the image of a 3 × 3 µm2 area of an Al plate before (A) and after (B) extensive scanning (more than 6 h) in a 1 M NaCl solution (applied load ) 80 nN). In Figure 7C, a lower magnification view of this region is

Figure 7. AFM image of a commercial Al plate before (A) and after (B) more than 6 h of scanning by the cantilever tip in a 1 M NaCl solution. A lower magnification view of the same region of the film after the extensive scanning period is shown in part C.

shown. The removal of Al from the 3 × 3 µm2 area can be recognized from the darker appearance of the central portion of the image. The height difference between the region before and after scanning is about 6.4 nm. Unlike previous observations,13 the removal of Al from this sample increased the surface roughness by exposing numerous grains that remain undissolved. The fact that material can be removed from an Al plate indicates that the interaction between the cantilever tip and the Al substrate leading to the removal of material is not restricted to the Al-on-glass substrate system. It alleviates the concerns

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that might arise as to the possible artifactual nature of this particular system. Discussion We have shown that under certain conditions Al can be removed from a given area of an Al substrate if the cantilever tip scans the surface for sufficiently long. As far as we can tell, the occurrence of the phenomenon does not depend on the particular Al substrate used, it being observed on both Al thin films deposited on a glass substrate and on polished bulk Al particles. One of the first mechanisms that comes to mind to explain the removal of Al is a tip-induced wear of the scanned area. Such a mechanism has already been proposed to be responsible for the creation of patterns in MoO3 formed at the surface of a MoS2 substrate.15 This mechanistic argument was supported by the fact that the rate of structure formation (which is analogous to the rate of material removal) was proportional to the applied load and to the scan rate. Also, these authors observed that the cross section of the line they were making in the MoO3 matched the shape of the tip they were using. There are obviously some similarities between their experimental observations and ours, particularly on the effect of the applied load and the scanning rate (for values lower than about 3 Hz) on the rate of material removal. However, some other evidence leads us to believe that wear-induced removal of Al is not the only mechanism at play in the phenomenon described here. It is worth mentioning that the thinning of the Al film is not observed when the experiment is performed in air or in demineralized water. The fact that the occurrence of the phenomenon can vary so much according to whether or not the aqueous solution contains NaCl is hardly reconcilable with a mechanism based only on tip-induced wear effect. Indeed, this very fact suggests that the removal of Al proceeds at least partially through a dissolution process. The corrosion of Al in a chloride-containing solution is a complex multistep process. While there is still controversy as to whether one step of the process is more important than another, there is general agreement on the description of the overall multistep process that could account for the many experimental findings accumulated over the years concerning Al dissolution. According to general belief, dissolution of Al in a chloride-containing solution involves three steps: (i) adsorption of reactive chloride anion on the oxide-covered aluminum, (ii) chemical reaction of the adsorbed anion with the aluminum ion to form a soluble species, and (iii) thinning of the oxide film by dissolution.16 It is important here to stress the fact that all the experiments were performed in open-circuit conditions and, thence, that growth of the pits by the action of the AFM cantilever tip may occur according to quite different mechanisms compared to that encountered in potentiostatic growth conditions. One of the possible ways by which the scanning of the cantilever tip on the surface of the Al film may locally increase the Al dissolution rate is by locally enhancing the diffusion of either or both the chloride anions to the surface and of the soluble species away from it as a result of the tip motion over the surface of the film. However, our experimental findings are not consistent with such a mechanism. Firstly, one would not expect such an effect to lead to the creation of pits with very sharp edges, as increased diffusion outside the perimeter scanned by the tip should also help to remove Al. So, in Figure 6, instead of a (15) Kim, Y.; Lieber, C. M. Science 1992, 257, 375 (16) Foley, R. T. Corrosion 1986, 42, 277.

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rectilinear feature engraved in the Al film, one would expect it to have an ellipsoidal shape. Secondly, Figure 5 shows a marked variation of the Norm Rq loss value with the applied load at a fixed scanning rate. Remembering that all these data points were obtained by scanning the tip over a fixed period of time (1 h), clearly these data show that the rate of Al removal, expressed in this case as an increase in the Norm Rq loss value over a fixed scanning period, is changed even if the scanning rate is kept constant. Most importantly, Al removal can be stopped while the cantilever tip is still mechanically agitating the liquid boundary layer if the applied load is set to a near zero value. While diffusion of the various species from and to the Al surface is certainly an important parameter in the case of potentiodynamic growth of pits (see for example ref 17), it is not believed that this is the one reason responsible for the preferential dissolution of aluminum under the tip of the AFM. The other important parameter in the generally accepted multistep process leading to the dissolution of Al in a chloride-containing solution is the formation of a chemical complex between the chloride anion and the aluminum ion. There appears to be well-characterized aluminum-anion reaction products to support this description. Foroulis and Thubrikar18 have proposed that the formation of a soluble basic chloride salt (Al(OH)2Cl) was involved in the pitting of pure Al in neutral solutions. Stirrup et al.19 arrived at the same conclusion. Furthermore, compounds such as Al(OH)2Cl and Al(OH)Cl2 have been characterized by Turner and Ross20 in the study of the hydrolysis of aluminum chloride. There exists an activation energy barrier to the formation of this complex, which is of the order of 40 kJ/mol. If by any means some energy is locally deposited in a given area of the Al film, the formation of the soluble complex will locally be facilitated, leading therefore to a local enhancement of the dissolution rate and to the thinning of the Al film. In our previous study,13 it was suggested that the energy dissipation from frictional forces arising from scanning the tip over the surface of the film was responsible for a local heating of the substrate. This heating was thought to provide enough energy to overcome the activation energy barrier for the chemical reaction of the chloride anion with the aluminum ion, thereby accelerating its dissolution in the aqueous phase. A similar explanation was also used elsewhere.21 On one hand, one would expect the rate of Al dissolution to increase with the load applied by the cantilever tip on the surface of the substrate. This is what is observed in Figure 5. On the other hand, one would also expect that, at a given applied load, the rate of Al removal should increase with the scanning rate. This is not what is observed, as Figure 4 clearly shows that the rate of Al removal saturates above about 3 Hz. Please note that, in these experiments, varying the scanning rate is equivalent to varying the number of times the tip goes over the region of the film under investigation because the period over which the scanning of the surface is made is fixed. Indeed, the best way to explain the results is a two-step mechanism, where the first step is the removal or breaking of a thin passivating layer and the second step is the reaction of chloride ions with the underlying substrate to (17) Buzza, D. W.; Alkire, R. C. J. Electrochem. Soc. 1995, 142, 1104. (18) Foroulis, Z. A.; Thubrikar, M. J. J. Electrochem. Soc. 1975, 122, 1296. (19) Stirrup, B. N.; Hampson, N. A.; Midgley, I. S. J. Appl. Electrochem. 1975, 5, 229. (20) Turner, R. C.; Ross, G. J. Can. J. Chem. 1970, 48, 723. (21) Delamski, E.; Parkinson, B. A. J. Am. Chem. Soc. 1992, 114, 1611.

Preferential Dissolution of Aluminum

initiate corrosion. Such an explanation would be consistent with the observation that more Al can be removed from the substrate as the scanning rate (up to about 3 Hz) and the applied load of the cantilever are increased (Figures 4 and 5), as one expects the “effectiveness” of the removal or breaking of the passivating layer to increase with both parameters. In pure water or in air, this removal or breaking of the passivating layer by the cantilever tip has no effect, since chloride ions are not present and thus corrosion cannot be initiated. If the action of the cantilever tip on the Al surface is to disrupt the passivating layer and thereby allow the chloride ions to reach the underlying substrate, it is also clear that this passivating layer can be reformed and the corrosion process stopped. This is particularly evident from the results of Figure 3, where it was demonstrated that the removal of Al in the chloride solution cannot proceed by itself once the surface has been scanned a number of times by the cantilever tip. There is one last issue that needs explanation. This is the fact that the rate of Al removal does not keep increasing as the scanning rate of the cantilever tip exceeds about 3 Hz. In Figure 4, increasing the scanning rate causes a corresponding decrease in the time available for reforming the passivating layer at the surface of the Al substrate. So, as the scanning rate is increased, there may be a value over which the passivating layer is not fully reformed and thus corrosion cannot be stopped completely. Under these conditions, no further increase in the rate of Al removal would be expected above a given scanning rate value. In the present study this value occurs at about 3 Hz, which corresponds to about 165 s between successive passages of the tip at a given location. This would correspond to

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the period of time required for the formation of the passivating layer at the surface of the Al thin film. Finally, the maximum resolution attained in this series of experiments was about 150 nm. This resolution seems, however, to be mostly limited by the shape of the cantilever tip itself rather than by any other physical or chemical property of the system. In this respect, it would be very interesting to determine what is the influence of the tip aspect ratio on the maximum resolution. Conclusion It was shown that the tip of an AFM can be used to locally dissolve Al from a substrate immersed in a NaCl solution. This leads to the formation of cavities whose shape and depth can be precisely controlled through the displacement of the AFM tip. The ultimate resolution obtained in this work may in fact be limited by the shape and dimension of the tip used. In the future, the use of a narrower tip may be helpful in lowering this limit. Also, it might even be possible to shape the height profile of the cavity boundary to any desired form by developing a complex scanning trajectory for the microscope tip. This may turn out to be an extremely useful technique for micromachining. Extension to other types of materials may also be envisioned. Acknowledgment. This work was supported by the Natural Science and Engineering Research Council of Canada. LA950234+