Preparation of Atomically Smooth Aluminum Films: Characterization

Based on the TED patterns, Al films prepared at 20 °C consist of small crystals with ...... Morihide Higo , Katsuya Fujita , Masaru Mitsushio , Toshi...
0 downloads 0 Views 405KB Size
Download PDF
6176

Langmuir 1997, 13, 6176-6182

Preparation of Atomically Smooth Aluminum Films: Characterization by Transmission Electron Microscopy and Atomic Force Microscopy M. Higo* Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, Korimoto, Kagoshima 890, Japan

X. Lu, U. Mazur, and K. W. Hipps Materials Science Program and Department of Chemistry, Washington State University, Pullman, Washington 99164-4630 Received April 16, 1997. In Final Form: September 5, 1997X Atomically smooth aluminum films with thicknesses of about 200 nm were prepared by vacuum evaporation of Al on heated mica substrates at 250 and 350 °C. Characterization of the films by transmission electron microscopy and transmission electron diffraction showed that the films consist of single crystals about 300 nm in diameter with the (111) face. The crystals are oriented randomly along the [111] direction perpendicular to the substrate. Atomic force microscopy observation of the films gave the morphology and roughness of the film surfaces. It was found that the faces of the crystals of the films formed at 250 and 350 °C are atomically smooth and the root-mean-square roughness of the film surfaces is about 0.6 nm over an area of 1 µm2.

Introduction Atomic force microscopy (AFM) is a new technique for imaging surfaces with a high spatial resolution down to the nanometer and angstrom scales. The imaging is achieved by scanning the sample surfaces with a sharp tip attached to the end of a cantilever. The surface features of the sample cause the cantilever to deflect as the tip scans over the sample. The deflections are measured by the optical lever technique and give the surface topography of the sample. The principle, apparatus, and applications of AFM have been extensively reviewed in several books and articles.1-3 One of the most useful and successful applications of AFM is the surface characterization of metal thin films. The surface properties of thin Al and of surface-oxidized Al films influence properties such as coating,4 friction,5 wear,6 light scattering,7,8 hardness,9 and electronic device performance.10 The structure of polycrystalline thin films tends to be columnar and leads to microscopically rough surfaces.11 These microstructures also affect the thin film properties. The structure zone model, proposed by Movchan and Demchishin12 for evaporated metal and oxide * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Marti, O., Amrein, M., Eds. STM and SFM in Biology; Academic Press: New York, 1993. (2) Cohen, S. H., Bray, M. T., Lightbody, M. L., Eds. Atomic Force Microscopy/Scanning Tunneling Microscopy; Plenum Press: New York, 1994. (3) Martin, Y., Thompson, B. J., Eds. Selected Papers on Scanning Probe Microscopes; SPIE Optical Engineering Press: Bellingham, WA, 1995. (4) Synowicki, R.; Kubik, R. D.; Hale, J. S.; Peterkin, J.; Nafis, S.; Woollam, J. A.; Zaat, S. Thin Solid Films 1991, 206, 254. (5) Steinberg, S.; Ducker, W.; Vigil, G.; Hyukjin, C.; Frank, C.; Tseng, M. Z.; Clarke, D. R.; Israelachvili, J. N. Science 1993, 260, 656. (6) Gee, M. G.; Jennett, N. M. Wear 1995, 193, 133. (7) Weimer, J. J.; Kim, J.; Zukic, M.; Torr, D. G. J. Vac. Sci. Technol. 1995, A13, 1008. (8) Bennett, J. M.; Tehrani, M. M.; Jahanmir, J.; Podlesny, J. C.; Balter, T. L. Appl. Opt. 1995, 34, 209. (9) Wang, X.; Liu, X. H.; Zou, S. C.; Martin, P. J.; Bendavid, A. J. Appl. Phys. 1996, 80, 2658. (10) He, L.; Shi, Z. Q. J. Vac. Sci. Technol. 1996, A14, 704. (11) Westra, K. L.; Thomson, D. J. Thin Solid Films 1995, 257, 15.

S0743-7463(97)00395-8 CCC: $14.00

films and expanded to sputtered films by Thornton,13,14 is useful to explain the effect of the deposition conditions on the microstructure of deposited metal films. The film structure is strongly dominated by the substrate temperature. The epitaxial growth and bulk structures of evaporated Al films on various substrates have been investigated by transmission and reflection electron diffraction (TED/ RED) and transmission electron microscopy (TEM).15-19 The substrate temperature is the most important variable in controlling the film growth morphology. The surface structure of evaporated Al films also was studied by a replica technique.17,18 While electron diffraction and microscopy are useful for characterizing many features of thin metal films, it is difficult to obtain surface morphology and roughness by these traditional techniques.17,18,20 Mazur et al.21 have attempted to prepare atomically smooth thin Au, Au-Pd, and W films on mica and glass substrates at room temperature and below using several deposition techniques. These films were studied by scanning tunneling microscopy (STM). Mazur et al. found that, of the several deposition methods studied, ion beam deposition gave the flattest film surfaces. The Au-Pd films on mica had a root-mean-square roughness (Rms) of 0.29 ( 0.08 nm. Wang et al.9 also have investigated the surface morphology and mechanical properties of Al films on Si wafers prepared by ion-based deposition techniques using AFM. Their Al films were prepared by filtered arc (12) Movchan, B. A.; Demchishin, A. V. Phys. Met. Metallogr. 1969, 28, 83. (13) Thornton, J. A. Annu. Rev. Mater. Sci. 1977, 7, 239. (14) Thornton, J. A. J. Vac. Sci. Technol. 1986, A4, 3059. (15) Stary´, V. Czech. J. Phys. 1976, B26, 882. (16) Smola, B.; Stary´, V. Czech. J. Phys. 1977, B27, 332. (17) Barna, P. B.; Reicha, F. M.; Barcza, G.; Gosztola, L.; Koltai, F. Vacuum 1983, 33, 25. (18) Barna, P. B.; Bodo´, Z.; Gergely, G.; Szigethy, D. Vacuum 1983, 33, 93. (19) Thompson, C. V.; Floro, J.; Smith, H. I. J. Appl. Phys. 1990, 67, 4099. (20) Reimer, L. Transmission Electron Microscopy, 3rd ed.; SpringerVerlag: Berlin, 1993. (21) Mazur, U.; Fried, G.; Hipps, K. W. Surf. Sci. 1991, 243, 179.

© 1997 American Chemical Society

Atomically Smooth Aluminum Films

deposition (FAD) and exhibited columnar but dense surface morphology. The surface was exceptionally smooth with a Rms of less than 0.1 nm. All of these films had a domed surface morphology with no clear grains or boundaries and no evidence of well-defined atomic steps. Lu et al.22,23 have prepared atomically smooth Au films on mica at higher temperatures and obtained molecular images of cobalt and copper phthalocyanine adsorbed on these Au(111) surfaces by STM. Mica is easily cleaved, yielding an atomically smooth surface, and is stable up to about 700 °C. The large atomically smooth Au(111) terraces that result under these conditions suggest that mica may be a useful substrate for preparing many smooth metal films. An additional motivation for producing atomically smooth Al terraces is the desire to use them as model surfaces for the study of adsorption on alumina. Plasma oxidation of aluminum produces a dense self-limiting growth oxide film that is chemically similar to alumina preparations used in industry.24-28 The ability to create such a chemical surface in an atomically flat form opens the way for using modern surface science techniques to study chemisorption from submonolayer to multilayer coverage. In the present paper, we report the preparation of atomically smooth oxide covered aluminum films on mica. These films are characterized by TED, TEM, and AFM. The TED and TEM results provide information about the underlying metal films, while the AFM studies give the surface morphology and the roughness of the oxide surface. The correlation between surface properties of these oxidecoated Al films and their preparation conditions is studied. Experimental Section The Al films were prepared on mica by vacuum evaporation in a procedure similar to that used for Au films on mica.22,23 The evaporator was a liquid-nitrogen-trapped 4-in. diffusion-pumped glass bell jar (12 in. in diameter by 10 in. height) system having a base pressure on the order of 10-8 Torr. The details of the evaporator have been previously described.29 The mica sheets (8 × 8 mm, Ted Pella) were cleaved just before use with plastic tape. The substrate temperature was controlled by contact to a copper block heater. The temperature was monitored with an iron-constantan thermocouple and a millivolt meter giving an accuracy of about (2 °C. Aluminum (99.999%) was evaporated from a tungsten filament at a deposition rate of 1.2-3.0 nm/s to form films with a thickness of roughly 200 nm. For most of the samples, mica substrates were first heated to 550 °C for 2 h at a pressure of 10-7 Torr. The substrate was then cooled to either 250 or 350 °C and maintained there for the Al deposition. The pressure in the vacuum system prior to Al deposition was less than 2.0 × 10-7 Torr. The deposition rate was monitored with a thickness monitor (Sycon Instruments STM-100). The films were then cooled to below 90 °C in the vacuum, and the surfaces were oxidized in an oxygen ac glow discharge (50 mTorr, 3 min) to form a stable surface oxide. Low-temperature Al films were prepared at room temperature (about 20 °C) on unheated mica substrates, and the surfaces were also oxidized in the glow discharge. The TEM micrographs and TED patterns were taken with a 200 keV transmission electron microscope (Philips CM200) using (22) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (23) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (24) Hansma, P. K.; Hickson, D. A.; Schwarz, J. A. J. Catal. 1977, 48, 237. (25) Bowser, W. M.; Weinberg, W. H. Surf. Sci. 1977, 64, 377. (26) Hansma, P. K. Phys. Rep. 1977, C30, 145. (27) Evans, H. E.; Bowser, W. M.; Weinberg, W. H. Appl. Surf. Sci. 1980, 5, 258. (28) Kroeker, R. M.; Hansma, P. K. Catal. Rev. Sci. Eng. 1981, 23, 553. (29) Hipps, K. W.; Mazur, U. Rev. Sci. Instrum. 1984, 55, 1120.

Langmuir, Vol. 13, No. 23, 1997 6177 a double tilt holder. The Al films were peeled from the mica substrates with plastic tape, and then the film was sandwiched between two TEM grids. The TED patterns were taken with three different aperture settings, and the actual diameters of the observation areas were 0.3, 1.2, and 6.0 µm. The X-ray diffractions of the Al films (6 × 40 mm) on mica substrates were also taken with a Siemens Kristalloflex 810 using Co KR (1.788 97 Å) radiation. The AFM images (256 pixels wide) were taken with a Digital Instruments NanoScope III operating in tapping (cantilever length: 127 µm) and contact (200 µm, narrow) modes in air. The images of the Al films were independent of the mode. The images were automatically plane-fitted to account for sample tilt and then flattened using the standard NanoScope III software.

Results and Discussion Bulk Structure of Al Films. The bright-field and dark-field TEM micrographs of the Al film prepared at 20 °C are shown in Figure 1. The TED patterns of the film taken with apertures 1-3 are also shown in the figure. The diameters of the observation areas viewed with apertures 1-3 are 0.3, 1.2, and 6.0 µm, respectively. These diffraction patterns have six-fold symmetry spots and typical patterns produced by the (111) face of a facecentered-cubic lattice.20 The brightest spots in the diffraction pattern taken with aperture 1 result from {220} reflections, and the {111} and {200} spots are completely absent. As the size of the observation area increases, the diffraction spots become bigger but still have six-fold symmetry. Based on the TED patterns, Al films prepared at 20 °C consist of small crystals with the (111) face preferentially oriented parallel to the substrate. The bright-field TEM micrograph also shows that the film consists of relatively small crystals. The average diameter and the standard deviation of the crystals are calculated to be 130 ( 50 nm. From the bright part of the dark-field TEM micrograph, we can estimate that about 20% of the crystals are exactly oriented along the [111] direction perpendicular to the substrate, while the remaining crystals tilt from that direction. The bright-field and dark-field TEM micrographs of the Al film prepared at 350 °C are shown in Figure 2. The TED patterns of the film taken with apertures 1-3 are also shown. The diffraction pattern using aperture 1 (0.3 µm image diameter) has clear six-fold symmetry and is a typical pattern observed from the (111) face of a single crystal. The TED pattern shows that the film consists of relatively big crystals with (111) faces. As the size of the observation area increases, however, the pattern becomes continuous and finally becomes a ring. These diffraction patterns show that the film consists of relatively big crystals with (111) faces, but each crystal is oriented randomly along the [111] direction. The bright-field TEM micrograph shows that the film consists of crystals 270 ( 160 nm in diameter. From the bright-field TEM micrograph, the crystals are exactly oriented with the [111] direction perpendicular to the substrate, because all the crystals have almost the same contrast. The bulk structure of the Al film prepared at 250 °C also has been investigated by TED and TEM. The diffraction patterns and TEM micrographs are very similar to those of the films prepared at 350 °C. However, the average grain size of the films prepared at 250 °C is 250 ( 130 nm, slightly smaller than that of the film prepared at 350 °C. We can conclude that the Al films prepared at 250 and 350 °C are microcrystalline with (111) faces. The [111] direction is exactly perpendicular to the mica substrate, but the grains are oriented randomly. Thus, aluminum films prepared by vacuum evaporation of Al on mica at 250 and 350 °C are not epitaxial. The X-ray diffraction patterns of the Al films on mica prepared at 20, 250, and 350 °C have one sharp peak at

6178 Langmuir, Vol. 13, No. 23, 1997

Higo et al.

Figure 1. Bright-field (BF) and dark-field (DF) TEM micrographs of an Al film (thickness: 180 nm) prepared on a mica substrate at 20 °C. The magnification is 38 000×. The TED patterns of the film taken with apertures 1-3 are also shown.

45.0°. From the sharpness of the peak and the interplaner distance (2.338 Å) calculated from the angle and the wavelength of the X-ray, these films consist of crystals having their (111) face parallel to the mica surface, and this is consistent with the TED and TEM results. Stary´15 has studied the structure and epitaxial growth of evaporated Al films with thickness 50-100 nm on mica by TED, RED, and TEM. Both types of diffraction patterns of the film prepared at 20 °C have a continuous ring structure and indicate that the grains of the film are randomly oriented along the [111] direction. He has concluded that the film is not epitaxial. As the temperature increases up to 400 °C, a preferential orientation of the grains along the [111] direction appears, but these temperatures are not sufficient to result in epitaxial films. Heating the substrate to 500 °C during deposition ensures the growth of an epitaxial film.15 Our TED results from films prepared at 250 and 350 °C agree with those of Stary´. However, our TED patterns obtained from films prepared at 20 °C indicate preferential orientation of the grains along the [111] direction. Stary´’s films were prepared in a chamber having a base pressure of 10-5 Torr, about 2 orders of magnitude higher than in ours. Thus, the

amount of adsorbed gases, including water, should be much smaller on our mica surfaces. In fact, Stary´ has attributed at least some of the structure changes seen to the desorption of water which maximizes at 265 °C. Moreover, it is not perfectly clear that Stary´ actually preheated all his substrates to 500 °C or how long they were in the chamber before deposition and after achieving the desired temperature. Thus, the difference in our 20 °C results may be due to significantly different surface hydration. Thompson and Floro19 have reported epitaxial grain growth in thin (65 nm) metal films deposited on singlecrystal substrates in ultrahigh vacuum. The TED pattern of an Al film deposited on the vacuum-cleaved (001) surface of mica at room temperature indicates a significant population of grains with random in-plane orientations along the [111] direction. The pattern also indicates a very weak subpopulation of grains with epitaxial alignments. After in situ annealing at high temperatures (360500 °C), grain growth and evolution of grain orientation have occurred and resulted in preferred epitaxial orientations. Their results for Al on mica prepared at room temperature agree with those of our Al film prepared at

Atomically Smooth Aluminum Films

Langmuir, Vol. 13, No. 23, 1997 6179

Figure 2. Bright-field (BF) and dark-field (DF) TEM micrographs of an Al film (thickness: 180 nm) prepared on a heated mica substrate at 350 °C. The magnification is 38 000×. The TED patterns of the film taken with apertures 1-3 are also shown.

20 °C. Any differences in the two types of films are probably quantitative and not qualitative. Our 20 °C films have about 20% of the grains with epitaxial alignment while the percent of those of their film19 may be less. Surface Morphology of Al Films. The AFM surface plot and section analysis of an Al film prepared on mica at 20 °C are shown in Figure 3. The picture size is 1 µm wide and 20 nm high. The section analysis was carried out along the line from the right top corner to the left bottom corner. The surface plot shows that the surface is very rough. The section analysis along the line indicates that the standard deviation of the roughness (root-meansquare roughness: Rms) is about 2 nm and the maximum height between the highest point and lowest point (Rmax) is about 10 nm. These are small crystals about 100 nm in diameter, and their tops are rounded. The AFM surface plot and section analysis of an Al film prepared on mica at 350 °C are shown in Figure 4. The picture size and section analysis scale are the same as those of the film prepared at 20 °C (Figure 3). It is clear that the crystals become bigger and the surfaces are very smooth. The Rms and Rmax values are about 0.2 and about 1 nm, respectively. The left and right marked steps

in the section analysis are 4 and 3 atoms high, respectively. The average crystal size is about 300 nm, and their tops are atomically smooth. It should be possible to observe the images of a single molecule adsorbed on this surface. The AFM surface plot and section analysis of an Al film prepared on mica at 250 °C are very similar to those of the film prepared at 350 °C. The Rms and Rmax values of the 250 °C prepared film are about 0.5 and about 2 nm, respectively. The average crystal size is about 300 nm, and the surface is also atomically smooth. The surface morphology and roughness of the Al film prepared on an American Scientific Products cover glass (M6045-4) at 350 °C using the same procedures were also studied. The surface of the Al film on the glass slide was very rough. The Rms and Rmax values determined from an image of a 30 × 30 µm2 area were about 20 and about 100 nm, respectively. This roughness is probably caused by a surface deformation of the glass substrate during the preheating at 550 °C, because the annealing point of this glass is between 545 and 555 °C. The Al film prepared on an unheated mica substrate at 20 °C and then heated at 350 °C for 2 h in vacuum was also investigated. This AFM image showed that the film

6180 Langmuir, Vol. 13, No. 23, 1997

Higo et al.

Figure 3. AFM surface plot (top) and section analysis (bottom) of an Al film (thickness: 180 nm) prepared on a mica substrate at 20 °C. The section analysis is carried out along a line extending from the right top corner to the left bottom corner of the image.

was composed of many large round structures. The diameter and height of these domes was about 8 and about 0.4 µm, respectively. The film thickness was about 0.1 µm, so the peak height was 4 times bigger than the thickness. These structures were probably caused by bubbles of water vapor formed from molecular water initially adsorbed on the mica substrate. Since the mica surface is chemically inactive, water molecules adsorbed weakly on the surfaces are easily desorbed at 350 °C and the expanding gas creates bubbles between the substrate and the film. It is known in AFM that the finite size of a tip may distort the resultant profiles of finely structured surfaces.30-35 When a microstructure of the film is much sharper than the tip, the resulting image consists of an array of images of the apex of the tip. This imaging of the tip by a surface is called a tip artifact. The amount of distortion in the image depends on the relative sharpness (30) Keller, D. Surf. Sci. 1991, 253, 353. (31) Griffith, J. E.; Grigg, D. A.; Vasile, M. J.; Russell, P. E.; Fitzgerald, E. A. J. Vac. Sci. Technol. 1992, A10, 674. (32) Gru¨tter, P.; Zimmermann-Edling, W.; Brodbeck, D. Appl. Phys. Lett. 1992, 60, 2741. (33) Westra, K. L.; Mitchell, A. W.; Thomson, D. J. J. Appl. Phys. 1993, 74, 3608. (34) Westra, K. L.; Thomson, D. J. J. Vac. Sci. Technol. 1994, B12, 3176. (35) Westra, K. L.; Thomson, D. J. J. Vac. Sci. Technol. 1995, B13, 344.

of the tip and the surface feature. This tip artifact often occurs in AFM imaging of thin films with columnar structures, and it is not visually obvious. Thus, the AFM images of thin films must be carefully interpreted. Westra et al.33,34 have studied tip artifacts in the AFM images of thin films with columnar structure. The distortion due to the tip shape makes the columns broader than they actually are and the spaces between the columns become smaller. They have found that the ratio of the radius of curvature of the features in an AFM image to that of the tip (RAFM/Rtip) provides an effective measure of the degree of the tip artifact in the AFM images. If the radius of a feature in an image is greater than 10 times the tip radius, the surface is tracked accurately. The image of the surface is distorted but still contains useful information on the structure of the film when the ratio is larger than 2. The image is severely distorted and is not representative of the actual film surface when the ratio is less than 2. AFM is also used to measure surface characterization parameters, such as the surface roughness. Westra and Thomson35 have examined the effect of the finite tip radius on the measurement of surface roughness. They found that the apparent roughness is only weakly influenced by tip-induced distortion. In the case of typical columnar growth, the decrease in the roughness is less than 15% for RAFM/Rtip greater than 2.

Atomically Smooth Aluminum Films

Langmuir, Vol. 13, No. 23, 1997 6181

Figure 4. AFM surface plot (top) and section analysis (bottom) of an Al film (thickness: 180 nm) prepared on a heated mica substrate at 350 °C. The section analysis is carried out along a line extending from the right top corner to the left bottom corner of the image.

Though the radii of the tips used in the present study were not determined for each measurement, Digital Instruments literature indicates that the Si tapping mode tips should have radii on the order of 5-10 nm while the contact mode tips should fall in the broad range of 5-40 nm. Based on these values and tip measurements reported in the literature, we can safely assume that the radii were between 10 and 40 nm, with the lower end of the range being most appropriate for the tapping mode images.30-35 The section analysis in Figure 3 for the Al film prepared at 20 °C shows that the smallest structure has a radius of about 50 nm and the radii for most grains are on the order of 100 nm. We conclude, therefore, that the image of the Al film prepared at 20 °C is not severely distorted by the tip artifact, although the grain tops may be somewhat rounded by it. We also conclude that since the Al films prepared at 250 and 350 °C are atomically smooth, the images are relatively free from the effect. The Rms and Rmax values determined for the surfaces of several Al films prepared at 20, 250, and 350 °C are shown in Table 1. Images of 1 µm2 area were used for the roughness analysis. Preparation of Atomically Smooth Al Films. The TED and TEM of our Al film prepared at 20 °C show that the film consists of single crystals with an average grain size of 130 nm and about 20% of the grains have the [111] growth direction perpendicular to the mica substrate. The

Table 1. Grain Size (D) Calculated from the TEM Micrographs, and Roughness (Rms) and Maximum Height between the Highest and Lowest Points (Rmax) Obtained from the Roughness Analysis of the AFM Images (1 × 1 µm) of Al Films Deposited on Mica at the Indicated Temperatures (T) T/°C

D/nm

Rms/nm

Rmax/nm

20 250 350

130 ( 50 250 (130 270 ( 160

1.63 ( 0.47 0.60 ( 0.07 0.55 ( 0.18

12.2 ( 2.8 6.6 ( 2.3 5.4 ( 1.6

remaining grains lack this preferential orientation. However, it is difficult to tell if growth directions are random or if they have other preferential orientations. Another interesting result is that the grains with the [111] growth direction have a preferential orientation to the mica substrate. This is clearly indicated by six-fold symmetry spots in the diffraction pattern taken with the third aperture in Figure 1 (producing a 6 µm spot on the sample). The AFM observation of this type of Al film clarifies the surface morphology of the film. The film consists of microcrystals with diameters of about 100 nm. The tops of the crystals are round, and the surface is very rough. The roughness analysis of the images shows that Rms and Rmax are 1.63 ( 0.47 and 12.2 ( 2.8 nm, respectively. This film is not suitable for studying adsorbed species on the surface.

6182 Langmuir, Vol. 13, No. 23, 1997

The TED patterns taken from suitably large areas of the 250 and 350 °C prepared Al films consist of continuous rings from the (111) face and indicate a complete lack of epitaxy on mica at these temperatures. The dark-field TEM micrographs of these films show a crystal that has the exact orientation along the [111] direction perpendicular to the mica substrate. The grain size of the film prepared at 350 °C is slightly larger than that of the film prepared at 250 °C. The AFM observation of these Al films clearly shows that the films consist of relatively large crystals with diameters of about 300 nm and that the terrace surfaces are atomically smooth. The roughness analysis shows that Rms and Rmax are 0.55-0.60 and 5.4-6.6 nm, respectively. These values are due to steps of a few atoms high rather than to roughness in the terraces. The present procedure is shown to be most appropriate for preparing atomically smooth Al films. These films are clearly appropriate for observing the morphology of adsorbed species. Movchan and Demchishin12 proposed the structure zone model to describe the effect of deposition conditions on the microstructure of evaporated films. This model has been expanded to sputtered films by Thornton.13,14 Many evaporated and sputtered films have a columnar microstructure. The columns extend from the substrate to the surface of the film, and the structure is dominated by the substrate temperature (T). There are three structure zones with boundary temperatures T1 ) 0.3Tm and T2 ) 0.5Tm for evaporated films, where Tm (K) is the melting point of the film material. The films in zone 1 (below T1) are characterized by tapered columns separated by voids. The separated columns are caused by self-shadowing during the deposition, because the surface diffusion of adatoms is too small to diffuse into the shadowed regions at these temperatures. The films in zone 2 (between T1 and T2) have much larger columns with domed tops. At around T1, there occurs a coarsening of the internal structure and a gradual change to the clearly defined columnar structure. The width of the crystals increases with T, and the films have well-defined grain boundaries and the columnar orientation. This suggests that the surface diffusion of adatoms is sufficient and leads to a surface recrystallization in zone 2. The films in zone 3 (above T2) consist of polycrystalline grains. Westra and Thomson11 have studied the nanostructures of some evaporated and sputtered columnar thin films on Si wafers by AFM. The sputtered Al-Cu (4%) film prepared at 25 °C (T/Tm ) 0.32) has the rounded columns with distinct grain boundaries, while the columns in the films prepared at 300 °C (T/Tm ) 0.61) have flat and faceted faces and are larger in size. The flat and faceted faces are explained by the mobility of the adatoms and bulk diffusion. The surface structures of the sputtered Al-Cu

Higo et al.

(4%) films prepared at 25 and 300 °C are very similar to those of our evaporated Al films on mica prepared at 20 and 350 °C. The structure zone model well explains the microstructures of the Al films evaporated on mica at different temperatures. The Al film prepared at 20 °C (T/Tm ) 0.31) is a typical polycrystalline film in the boundary between zone 1 and zone 2. It shows columns with domed tops and clear grain boundaries. The films prepared at 250 and 350 °C (T/Tm ) 0.56 and 0.67) in zone 3 consist of larger grains with various orientations along the [111] direction perpendicular to the substrate. Apparently, surface diffusion of the adatoms is sufficient to overcome the self-shadowing effect, leading to the surface recrystallization at these temperatures. The surface of the grains that result when T > 250 °C is atomically smooth. Though the structural zone model is useful to explain the general trends in the variation of the film structure with deposition conditions, this model does not predict the morphology nor the roughness of the film surface. Since the microstructure of polycrystalline thin films tends to be columnar with microscopically and nanoscopically rough surfaces, it is difficult to observe the surfaces by traditional electron microscopy. On the other hand, AFM is suitable to observe the film surfaces and provides valuable information on the surface morphology and the roughness of the films. Conclusions We have reported the preparation and characterization of atomically smooth Al films on mica for the first time. The Al films were prepared by vacuum evaporation of Al on clean mica substrates at 250 and 350 °C. The bulk structures of these films were investigated by TEM and TED. The films consist of microcrystals with (111) faces, and the [111] direction is perpendicular to the substrate. The surface morphology and roughness were obtained by AFM. The root-mean-square roughness is about 0.6 nm, and the surfaces are atomically smooth. Acknowledgment. We acknowledge Prof. L. E. Thomas in the Department of Mechanical and Materials Engineering of Washington State University for his assistance with the measurements of TEM and TED and valuable discussions. We also thank Mr. S. Xu in the Department of Chemical Engineering for his help with X-ray diffraction measurements. M.H. thanks Prof. S. Kamata in the Faculty of Engineering of Kagoshima University for his encouragement. The present study was supported by the fellowship from the Ministry of Education of Japan. Instrumentation was provided, in part, through NSF Grant 9205197. LA9703959