Published in Journal of Electronic Materials, 25 (10) 1585-1592 (1996) DOI: 10.1007/BF02655580 This version was prepared from the reviewed and accepted text and original artwork. Nanofabrication of a Quantum Dot Array: Atomic Force Microscopy of Electropolished Aluminum R.E. Ricker,* A.E. Miller,† D.-F. Yue,† G. Banerjee,† and S. Bandyopadhyay‡

*Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Technology Administration, U.S. Department of Commerce, Gaithersburg, MD 20899 †Department of Chemical Engineering, ‡Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556 ABSTRACT One step required for the fabrication of a quantum dot array on an aluminum substrate is the preparation of a flat aluminum surface. To enable the optimization of the electropolishing procedure, atomic force microscopy was used to examine the morphology of electropolished polycrystalline aluminum surfaces that were prepared under different electropolishing conditions. The electropolishing voltage, time, and temperature were varied. Two distinctly different surface morphologies were observed for different electropolishing conditions and transitional structures were observed for intermediate conditions. It was found that the type of surface morphology and the surface roughness could be controlled primarily with the electropolishing voltage while temperature and time had relatively little effect over the range examined in this study. Key words: Al, atomic force microscopy (AFM), chemical-mechanical polishing,

electropolishing, nanofabrication, quantum dots INTRODUCTION Recently, it was shown that it is possible to synthesize a two-dimensional array of quantum dots by electrodepositing metals into a porous aluminum oxide template.1, 2 This nanosynthesis technique takes advantage of the observations that porous oxide films grow on aluminum when it is anodized in certain electrolytes as illustrated in figure 1,3-7 that the size and spacing of these pores can be controlled by altering the electrolyte and anodizing conditions3-7 and that the pores can be closed or widened by subsequent chemical treatments.8 By assuming that an electrodeposit on the anodized surface will nucleate initially in the pores,3, 9 the quantum dots were electrodeposited into the pores as illustrated in figure 2 using a standard electrodeposition technique.10 This is not the first application of aluminum surfaces and oxide films to make functional materials and anodized aluminum has been used to produce functional materials such as filters,11, 12 catalysts,13-15 electrochromic media,16 magnetic recording media,17-19 and capacitors.20

Figure 3 is a flow chart of the quantum dot nanofabrication process.1, 2 From this diagram, it can be seen that the electropolishing step which proceeds the growth of the porous anodic oxide layer will be critical in determining the quality of the resulting array. Obviously, the flatness of the two-dimensional array will be dictated by this step, but more importantly, the pore size, spacing and uniformity could be dictated by the condition of the surface following these preparations. In spite of a lack of any direct physical evidence, numerous investigators have concluded from indirect observations and analysis that the pores in the anodic films initiate and grow from preexisting flaws in either the aluminum substrate, the surface film produced by the proceeding processing steps, the natural air formed film, or the film which grows in the

Figure 2 - Schematic illustration of the porous oxide film that grows on aluminum when it is anodized in certain electrolytes: (a) top view and (b) cross section.

Figure 1 – Schematic cross section of a "quantum dot" in a porous oxide film.

anodizing solution before the anodizing current is applied.5, 21-26 As a result, a study was undertaken to determine the influence of electropolishing conditions on the morphology of the samples as prepared for anodization and growth of the porous anodic layer, which is to serve as the template for deposition of the quantum dot array. EXPERIMENTAL Samples, 10 mm x 10 mm x 0.1 mm, were cut from 99.9 % pure aluminum sheet with a 100 µm grain size. The samples were degreased with trichloroethylene and cleaned with distilled water. The samples were electropolished in an electrolyte consisting of 62 mL perchloric acid, 700 mL ethanol, 100 mL butyl cellusolve, and 138 mL distilled water with a commercial electropolisher. The samples were electropolished at different cell voltages, temperatures, and polishing times as given in Table 1. The electropolishing cell voltage in this table and throughout the remainder of this paper is the voltage difference between the electrodes of the electropolisher. After electropolishing, the samples were thoroughly rinsed in deionized water and air-dried. The samples were then examined in a commercially available atomic force microscope (AFM) capable of atomic scale resolution. The surfaces of the samples were scanned in air with a 200 µm long, wide-leg, V-shaped Si3N4 cantilever. The data were recorded in height mode (constant force) and converted into pictures in three modes: top-view, surface, and cross-section. RESULTS AND DISCUSSION AFM examination of the samples prior to electropolishing revealed a rough surface where the range of the surface height measurements was over 50 nm for a 6 µm2 scan area. Electropolishing this surface dramatically reduced this height range for all of the electropolishing conditions examined and most height ranges were less than 5 nm after polishing. Figure 4 shows the top-view surface structure observed by AFM for the matrix of cell voltages and electropolishing times examined at 15 °C. In this figure, two distinctly different morphologies can be seen which, in these two-dimensional views, appear similar to the stripe and bubble patterns reported for modulated phases in two-dimensional films.27 Since the color density in these images is related to the displacement of the AFM probe and through this, the position of the surface, the stripe pattern is the result of a ridge and valley or wave surface morphology and the bubble like pattern is the result of spherically symmetric peaks rising above the surrounding surface like individual hills or mounds. As illustrated in figure 5, the wave morphology is the results of sinusoidal variations in the height of the surface in one direction along the surface while the hill or mound morphology is the result of sinusoidal variations in two directions across the surface.

Figure 3 - Flow chart for the nanofabrication technique

developed for the fabrication of quantum dot arrays on anodized

aluminum.

Table I - Electropolishing Conditions Examined in this Study

Cell Voltage (Volts)

Time (Seconds)

Temperature (°C)

30 10 15 30 20 15 30 30 15 40 10 15 40 20 15 40 20 4 40 20 -3 40 30 15 50 10 15 50 20 15 50 30 15 60 10 15 60 20 15 60 20 4 60 20 -3 60 30 15 70 10 15 70 20 15 70 30 15

Representative top-view AFM images of the dominant surface structure observed for each of the electropolishing voltages and times examined at 15 °C are presented in figure 4. By examining this figure, it can be seen that at the lowest electropolishing time and voltage, 10 seconds at 30 volts, insufficient polishing occurred and the surface was littered with relatively large scale debris. Increasing the polishing time to 20 seconds at this voltage resulted in a cleaner surface and the establishment of a shallow surface wave structure. However, the surface was still not clean enough to use. After 30 seconds at this voltage, the surface was much cleaner and the wave morphology was well established. Increasing the electropolishing voltage accelerated this process and 10 seconds at 40 volts resulted in a surface very similar to that obtained in 30 seconds at 30 volts. In this figure, it also appears that increasing the electropolishing voltage results in increasing the spacing and the height of the surface

Figure 5 – Schematic illustration of surface wave and mound morphologies which illustrate the one and two-dimensional nature of the height variations for these

two types of morphologies. Also, this figure illustrates the measurements, height range (h), and peak

spacing (l) used to quantify the surface morphology.

Figure 4 – One µm2 top view AFM images of electropolished aluminum for different electropolishing conditions at 15 °C.

wave peaks up until 10 seconds at 60 volts where the wave morphology starts to breakup. On increasing the electropolishing time to 20 seconds at 60 volts, a shallow mound morphology becomes the dominant surface structure and at 30 seconds, a well defined two dimensional array of mounds was observed on the surface. Increasing the electropolishing voltage further appears to increase the spacing of the peaks in this mound morphology in a manner similar to that observed for the peak spacing in the wave morphology at lower electropolishing voltages. In figure 4, both the surface wave period and amplitude appear to increase with increasing electropolishing voltage. To quantify these trends, the peak to peak amplitude or height range, h, and the peak spacing or wavelength of the height oscillations, λ, were measured as illustrated in figure 5. Figures 6-11 present the results of these measurements. In figure 6 the results of the peak spacing or wavelength measurements are presented against the voltage used to electropolish the samples for 10, 20, and 30 seconds of polishing. In this figure, the trend to increasing peak spacing with increasing voltage is obvious and the lines through the data points are computer generated linear regression fits for the spacing measurements. The numeric results of this regression analysis are given in Table II along with the regression results for Figure 7-11. By examining this table, it can be seen that all three of the regression lines in figure 6 have correlation coefficients, r2, which are greater than 0.88. Also included in this table is the observed significance probability for a t-test of the regression slope, P(t-test). This is the probability of measuring the observed slope when there is no influence of the independent variable on the dependent variable. That is, this is the probability that there is no influence of voltage on the spacing and the true slope of the relationship is zero.28, 29 By examining the table, it can be seen that the probability for this t-test is less than 1 % for all three polishing times. As a result, it can be concluded that the electropolishing voltage determines the spacing of the peaks. Also, since the linear relationship between peak spacing and electropolishing voltage is unaltered when morphology changes from wave-like to mound-like, it appears that this relationship is independent of the morphology. In fact, the transition in morphology with increasing electropolishing voltage may be result of the increasing wavelength.

Figure 6 - The influence of electropolishing voltage on the spacing of the wave or mound peaks after polishing (a) 10 s, (b) 20 s, and (c) 30 s. The lines in these figures are the regression lines for the data.

Table II – Regression Results for Figures 6-11 (a). Peak spacing as a function of electropolishing voltage (Figure 6) Polishing

Time Intercept (nm) Slope (nm/V) Corr.

Coef. Degrees of Freedom

Seconds Est. µ(int)* Est. µ(slope)* P(t-test) r2 DOF 10 -0.7 16.75 1.73 0.32 0.6 % 0.906 3 20 -18.1 16.62 2.39 0.32 0.3 % 0.949 3 30 22.8 14.93 1.36 0.29 0.9 % 0.882 3

(b). Peak spacing as a function of polishing time (Figure 7) Polishing Voltage

Intercept (nm) Slope (nm/s) Corr. Coef.

Degrees of Freedom

Volts Est. µ(int)* Est. µ(slope)* P(t-test) r2 DOF 30 48.27 7.73 0.38 0.36 24.1 % 0.530 1 40 65.40 22.08 0.57 1.02 33.8 % 0.237 1 50 76.67 39.91 0.60 1.85 40.0 % 0.095 1 60 113.33 12.47 0.00 0.58 50.0 % 0.000 1 70 134.00 43.03 -0.25 1.99 46.0 % 0.015 1

(c). Height range as a function of electropolishing voltage (Figure 8) Polishing

Time Intercept (nm) Slope (nm/V) Corr.

Coef. Degrees of Freedom

Seconds Est. µ(int)* Est. µ(slope)* P(t-test) r2 DOF 10 -3.382 0.968 0.13 0.018 0.3 % 0.946 3 20 -7.503 1.756 0.25 0.043 5.4 % 0.972 1 30 -0.107 0.801 0.08 0.019 7.9 % 0.939 1

(d). Height range as a function of polishing time (Figure 9) Polishing Voltage

Intercept (nm) Slope (nm/s) Corr. Coef.

Degrees of Freedom

Volts Est. µ(int)* Est. µ(slope)* P(t-test) r2 DOF 30 -0.86 1.10 0.09 0.051 16.7 % 0.750 1 40 2.23 1.09 0.02 0.051 36.7 % 0.166 1 50 3.50 2.50 0.03 0.116 42.8 % 0.050 1 60 3.22 4.53 -0.02 0.21 46.6 % 0.011 1 70 7.66 2.69 -0.21 0.13 17.3 % 0.734 1

(e). Peak spacing as a function of electropolishing temperature (Figure 10) Polishing Voltage

Intercept (nm) Slope (nm/ °C) Corr. Coef.

Degrees of Freedom

Volts Est. µ(int)* Est. µ(slope)* P(t-test) r2 DOF 40 53.95 2.75 0.64 0.301 14.0 % 0.819 1 60 92.62 7.53 1.61 0.825 15.1 % 0.792 1

(f). Height range as a function of polishing temperature (Figure 11) Polishing Voltage

Intercept (nm) Slope (nm/ °C) Corr. Coef.

Degrees of Freedom

Volts Est. µ(int)* Est. µ(slope)* P(t-test) r2 DOF 40 0.68 0.10 0.09 0.011 4.0 % 0.984 1 60 3.20 0.58 -0.21 0.063 9.4 % 0.916 1

* µ(i) is the standard uncertainty which is the calculated standard deviation for the parameter (i) estimate.

In contrast to electropolishing voltage, there appears to be little if any influence of electropolishing time on the spacing of the peaks as shown in figure 7. By examining the regression calculations in Table II for this figure, it can be seen that the probability of no relationship between peak spacing and electropolishing time is significant. Since the observed significance probabilities given in this table are based on only one side of the distribution (a one-tailed test), the worst possible observed significance probability is 50 %. For regression of peak spacing on electropolishing time, three of the curves had an observed significance probability of 40 % or higher and the best was 24.1 %. As a result, these data indicate that there is not an influence of electropolishing time on the spacing of the peaks. Of course, this conclusion must be limited to the range of conditions and electropolishing times examine in this study.

Figure 8 - The influence of electropolishing voltage on the height range of the surface measured in the AFM after polishing (a) 10 s, (b) 20 s, and (c) 30 s. The lines in these figures are the regression lines for the data.

The surface height range measurements followed a trend similar to that observed for the wavelength except that the height range does not appear to be independent of the surface morphology as shown in figure 8. In this figure, a very good regression line for peak height on the electropolishing voltage was obtained for 10 seconds of polishing where a wave morphology is observed for all electropolishing voltages. In Figure 4, It appears that the surface may be starting to transition to the mound morphology at 60 and 70 volts, but a well defined wave pattern is observed at all of the other voltages. For longer polishing times, there appears to be a discontinuity in the surface height range that coincides with the change in the surface morphology. For example, in figure 8(b), the first three data points where the morphology is

Figure 7 - The influence of electropolishing time on the spacing of the wave or mound peaks for different

electropolishing voltages.

wave-like appear to lie on a straight line while the next two measurements, where the morphology has changed, appear to be on a completely different line with a similar slope, but a much lower intercept. Similar behavior is observed for 30 seconds of electropolishing as shown in figure 8(c), but the surface height range does not appear to be on another line after the transition to the mound morphology. Regression analysis of the data for 10 seconds of electropolishing resulted in an observed significance probability of less than 1 % while higher values were observed for the other times where only three data points were used for each regression, Table II. Therefore, it is concluded that the peak to peak height range is determined by the electropolishing voltage and the type of surface morphology. The influence of electropolishing time on the height range is shown in figure 9. At the lowest electropolishing voltage, the peak height range increases with increasing electropolishing time, but the opposite trend is observed at the highest voltage. At intermediate voltages, intermediate slopes were determined for this relationship. Since the morphology changes from wave-like at the lower voltages to mound-like at the higher voltages, this change in the sign of the slope could be due to the change in morphology. However, examination of the regression statistics in Table II indicates that there is a significant probability that these observations are the result of measurement variability. As a result, additional measurements will be required to determine if a relationship exists between electropolishing time and peak height range for a given morphology or electropolishing voltage. The influence of electrolyte temperature on the surface morphology was examined by electropolishing samples for 20 seconds at 40 and 60 volts in the electrolyte at different temperatures (-3, 4, and 15 °C). At 40 volts, a wave-like morphology was found at all temperatures while a trend toward increasing wave character with decreasing temperature was noted for polishing at 60 volts. The influence of temperature on the peak spacing and the surface height range is shown in figures 10 and 11. By examining these figures, it appears that there is little effect of

Figure 9 – The influence of electropolishing time on the height range of the surface

measured in the AFM for different electropolishing voltages.

Figure 10 - The influence of temperature on the spacing of the wave and mound

peaks for electropolishing at cell voltages of 40 and 60 volts. The lines in

this figure are the regression lines for the data

temperature on the spacing of the peaks between -3 and 4 °C while there appears to be a slight increase in the spacing for both voltages between 4 and 15 °C. Fitting regression lines to the peak spacing data in figure 10 results in a positive slope for both voltages. However, examination of Table II shows that both lines have a significant probability (14 and 15 %) that the regression slopes are due to measurement variations and not due to an influence of temperature. For the surface height range measurements in Figure 11, the measurements indicated that the peak height increased with temperature for polishing at 40 volts while decreasing with temperature for polishing at 60 volts where the morphology is changing with temperature. Regression analysis of these data resulted in low, but not insignificant, probabilities that the regression slopes are due to measurement variability as shown in Table II. The purpose of this investigation was to identify the optimum surface preparation and electropolishing conditions for the preparation of aluminum for the anodic growth of porous templates to be used for the fabrication of quantum dot arrays. However, the results proved to be very interesting with respect to the influence of chemical and electrochemical conditions on the roughness of polished surfaces. It is unclear at this time whether or not these observations will prove to be common to all systems or restricted to this particular metal, electrolyte, and polishing system, but similar surface morphologies have been reported for anodically treated Zr.30 If one considers changes in voltage in an electropolishing solution to be similar to changes in the oxidizing potential of chemical polishing solution, then one would expect similar results for increasing the oxidizing potential of the solution as observed for increasing the cell potential. However, using electrode potential to control the oxidizing potential of the solution allows for changes over a much greater range and also allows for closed loop control. At this time, it is unclear why the surfaces develop the observed morphologies, but it has been postulated that these morphologies are due to either surface adsorption, mass transport through the diffusion layer, or to interfacial thermodynamics and all of these are being investigated. If mass transport through the diffusion layer is responsible for the evolution of these surface morphologies, then combined mechanical and electrochemical polishing will enable greater control of the resulting surface morphology. On the other hand, if surface adsorption and diffusion or interfacial thermodynamics are primarily responsible, then modifications in the chemistry of the electrolyte will enable the greatest improvements. The similarity of these morphologies to those observed for a wide variety of different types of systems is compelling and developing a better understanding of the these morphologies and why they develop will enable the production of surfaces to much smaller dimensions with improved reproducibility.

Figure 11 – The influence of the height range on the surface measured in the

AFM after electropolishing at cell voltages of 40 and 60 volts. The lines in

this figure are the regression lines for the data.

CONCLUSIONS Two distinctly different surface morphologies were observed for different electropolishing conditions: waves and mounds. These two morphologies appear to be the result of variations in the height of the surface in a sinusoidal manner in either one or two dimensions across the surface. In addition, morphologies that appear to be result of the surface making a transition from one of these morphologies to the other were observed for intermediate electropolishing conditions. The spacing of the peaks in these structures varied in a linear manner over the electropolishing voltages examined even when the surface morphology changed from waves to mounds. The surface height range varied in a linear manner with the electropolishing voltage for a single morphology, but deviated abruptly from this linear relationship when the surface morphology changed. These results demonstrate that developing a better understanding of the chemical and electrochemical conditions that determine surface roughness during polishing will enable better control and more reproducible production of surfaces that are flat on the sub-micrometer and nanometer levels. ACKNOWLEDGMENT This work is supported by contract of Midwest Superconductivity Consortium under a Department of Energy subgrant no. DE-FG02-90ER45427. The authors are also grateful to Dr. Wolf and G. X. Lee of the University of Notre Dame for use of their AFM facilities and for their assistance. REFERENCES 1. A. E. Miller, D. Yue, G. Banerjee, S. Bandyopadhyay, R. E. Ricker, S. Jones and J. A.

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