ARTICLE pubs.acs.org/JPCC
Nanoporous Aluminum Oxide Thin Films on Si Substrate: Structural Changes as a Function of Interfacial Stress Adriano F. Feil,*,† Pedro Migowski,‡ Jairton Dupont,‡ Lívio Amaral,† and Sergio R. Teixeira*,† †
Laboratory of Thin Films and Nanostructures Fabrication (L3Fnano), Institute of Physics, Universidade Federal do Rio Grande do Sul (UFRGS), Bento Gonc-alves Avenue 9500, P.O. Box 15051, 91501-970, Porto Alegre, Brazil ‡ Laboratory of Molecular Catalysis (LMC), Institute of Chemistry, Universidade Federal do Rio Grande do Sul (UFRGS), Bento Gonc-alves Avenue 9500, P.O. Box 15003, 91501-970, Porto Alegre, RS, Brazil
bS Supporting Information ABSTRACT: This paper reports the effect of Al2O3SiO2 interfacial stress on porous anodic alumina (PAA) structures formed by anodizing 150 nm thick aluminum thin films (ATF) deposited on silicon (Si) and glass substrates. The increase in the interfacial stress was achieved by growing thicker SiO2 layers by anodizing the substrate after complete ATF oxidation. Thicker SiO2 layers induced structural changes on the PAA. The nanopore diameter and interpore distance increased from 50 to 80 nm and 90 to 150 nm, respectively, and the nanopore density decreased from 60 to 43 nanopores μm2. The modifications of the PAA structure are related to the expansion of the SiO2 layer, which exerts a bottom-up pressure on the PAA film to minimize the stress generated on the Al2O3SiO2 interface.
’ INTRODUCTION The interest in porous anodic alumina (PAA) has grown significantly due to the high quality of the nanostructure compared to the relatively low manufacturing costs.1 Most of the developed methods for producing PAA generally yield highly ordered PAA on bulk Al foils.29 Until now, the fabrication of highly ordered thin layers of PAA on substrates (such as silicon or glass) has not been very well developed and still remains a major challenge from the scientific and technological points of view.10 The controlled growth of PAA on different substrates permits the use of these nanostructures as templates for growing a plethora of materials.1118 The specific case of PAA thin films on silicon has become highly interesting because it provides a direct attachment between the materials grown within the nanopores to the Si substrate.10,13,1926 One potential application for matrices of alumina is the manufacture of molds for ordered arrays of 2D nanomaterials. For example, nanowires,2730 nanotubes,3135 and nanocolumns36,37 with high aspect ratios were grown within PAA from Al bulk or aluminum thin films (ATF) deposited on a silicon substrate. The domain and reproducibility of the scale size (Si wafer scale) of PAA from thick ATFSi is already known. For example, it was reported that the fabrication of a near-perfect ordered PAA structure with a square and hexagonal lattice r 2011 American Chemical Society
configuration by using a 500 nm thick ATF on a silicon substrate over 4 in. wafer areas.10 Reports on ATF anodization were first published in the early 2000s with the objective of expanding the applications of PAA.10,17,1921,23,24,26,27,3842 It was verified in most of the studies that these nanostructures could be formed using the same anodization conditions as in bulk Al. The degree of organization of the nanoporous structure was shown to be lower than that in Al bulk, while the interpore distance (Dint) and nanopore diameter (Dp) were similar. The observed differences in the PAA films may be explained by three major differences that exist between the two distinct starting materials: • The average size of the aluminum (Al) grains: The size of the grains directly influences self-organization and the size of the nanopores.25,26 In the vast majority of cases, the films studied had thicknesses varying from 500 to 5000 nm (see details in Table 1). In these Al films the grain sizes are limited to the Al thickness and, thus, differ significantly from those of the Al bulk foils. • The number of interfaces: In Al bulk, the only interface is between Al2O3Al, but in the ATF system, interfaces exist between Al2O3Al-substrate. Received: January 19, 2011 Published: March 28, 2011 7621
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Table 1. Comparison of Anodized PAA Structures Formed by Different ATF Thicknesses Deposited Directly on Si Substrates substrate p-type Si(100)
a
ATF (nm) 30/500
potential (V)
DP (nm)
oxalic
50
not form/2040
sulfuric
20
1015/1530
electrolyte
p-type Si(100)
2000
sulfuric
40
6080
p-type Si(100)
500
sulfuric
20
16
Dint (nm)
ref 17, 20
100
38 39
p-type Si(111)
150
phosphoric
140
48125
90240
26
p-type Si(100)
440
sulfuric
40
25
50
40
oxalic
50
75 110
40
80 3070
100
n-type Si
500012000
phosphoric oxalic
n-type Si (100)
2000
oxalic
40
50
n-type Si (100)
1000
oxalic
60
60a
25 23 24
Estimated value.
• The formation of voids between Al2O3-substrate: The interface induces the reversal of the barrier layer.24,41 It was shown that this effect occurs with the increase of anodization time, in which small voids begin to appear between the barrier layer and the substrate as a consequence of the inversion of the Al2O3 barrier layer.24 This increases the stress at the interface (film substrate), reducing the adhesion of the PAA layer to the substrate. On the other hand, the manufacture of nanodot43 and nanorod44,45 arrays requires very thin PAA films (below 150 nm in thickness). The decrease in the thickness of alumina facilitates growth and increases the control of the diameter and shape of these nanostructures. However, reducing the thickness of ATF on the Si to less than 150 nm generates a series of changes in the PAA structure compared to a thickness of 500 nm of ATF (Table 1). For example, 30 nm of ATF does not show a PAA structure after anodization with an oxalic acid solution, while 500 nm of ATF presents a bulklike PAA structure after anodization.20 Also, it was reported that PAA films obtained from 150 nm of ATF on Si are directly related to the mean metallic aluminum grain size.26 Moreover, when the Al is completely oxidized, the anodic oxidation of the Si substrate begins. Consequently, a layer of amorphous SiO2 is formed below the alumina layer. This creates an additional stress at the interface generating structural changes in the PAA matrices because the Al2O3 barrier layer is inverted at the Al2O3SiO2 interface.24,41,46 In ATFs less than 150 nm thick, the interface stress generated by the substrate anodization process may induce more pronounced structural changes in the PAA films. Therefore, the effect of the stress in the Al2O3SiO2 interface obtained by ATF anodization remains unclear, especially in ATFs thinner than 150 nm. Herein, we report the effect of Al2O3SiO2 interfacial stress on anodized 150 nm thick ATFs deposited on Si substrates. The increase of the interfacial stress was achieved by growing thicker SiO2 layers below the PAA structure. The SiO2 was formed by anodizing the Si substrate after complete Al oxidation. The observed structural modifications of the PAA films were related to increasing the thickness of the SiO2 layer.
’ EXPERIMENTAL METHODS Thin Film Deposition. A 150 nm thick aluminum (Al) 99.99% thin film was deposited on a p-type Si(111) substrate (110 Ω 3 cm) and glass substrate by dc-magnetron sputtering equipment (Balzers, BAS-450) at a pressure of 3 101 Pa and a
deposition rate of 1.5 nm 3 s1. The Si substrate was dipped in by a diluted HF acid solution for 30 s prior to depositing the Al film to remove the native surface SiO2. Anodization. Anodization was performed in a conventional cell using a platinum sheet as a cathode. The electrical contact was made at the backside of the Si wafer, on which an aluminum Ohmic contact was formed after deposition. The samples were anodized in an acidic aqueous solution of 1.3 mM H3PO4 at a constant voltage of 140 V for 10180 min. The bottom of the anodization cell was cooled with a Peltier system to ensure a constant temperature of 20 °C. During anodization the electrolyte was vigorously stirred, and the values of voltage, current, and temperature were recorded via computer. Subsequently, the samples were dipped into a solution of 2.8 M H3PO4 at 20 °C for 1 min to widen and enlarge the pore sizes. After the anodization process, the samples were rinsed in deionized water and dried in N2 gas. ATF, PAA, and SiO2 Characterization. The thickness and composition of the Al, Al2O3, and SiO2 were measured by the Rutherford backscattering spectrometry (RBS) technique. The RBS characterization was obtained by the 3 MV tandem accelerator from the Ion Implantation Laboratory at UFRGS-Brazil. The incident ions were mainly 1000 keV 4Heþ ions. The scattered 4Heþ ions were detected by a solid-state particle detector placed at 165° in IBM geometry. The expansion of the SiO2 layer below the PAA was assessed qualitatively by approximation of the energy loss of alpha ion in the Al2O3SiO2 system.47,48 The PAA structure was observed using scanning electron microscopy (SEM) and was obtained with a JEOL 6060 microscope. For the cross section images, the samples were prepared in a dual-beam focused-ion-beam (FIB) facility at INMETRO, Rio de Janeiro, Brazil. Chemical analyses by line scans and by mapping were performed by probe energy-dispersive X-ray spectrometry (EDAX) and high-resolution scanning electron microscopy (HRSEM). Structural dimensions and mean pore size distributions were determined by counting more than 900 pores from SEM micrographs using the ImageJ code (image processing and analysis in Java). The reproducibility of the anodization process was evaluated from the preparation of successive samples. Thus, no deviations were observed in the mean values reported here.
’ RESULTS AND DISCUSSION PAA formation. Panels ae of Figure 1 show top surface SEM images of nanoporous structures obtained after depositing an 7622
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Figure 2. A typical currenttime transient curve for the anodization of an ATF deposited on a Si substrate using 1.3 mM H3PO4 as the electrolyte at 20 °C and 140 V.
Figure 1. Typical SEM images of PAA after anodization at 140 V with a 1.3 mM H3PO4 electrolyte at 20 °C for (a) 10 min, (b) 15 min, (c) 30 min, (d) 90 min, and (e) 180 min. (f) A schematic illustration of Dp and Dint and (g) Dp, Dint and pore density behaviors as a function of anodization time.
anodized Al thin film on a silicon substrate for 10, 15, 30, 90, and 180 min at 140 V in a H3PO4 electrolyte. All samples showed randomly distributed nanopores with irregular shapes and sizes. The DP and Dint are shown in Figure 1f, and the density of the nanopores as a function of anodizing time is shown in Figure 1g. The Dp and Dint increased from 50 to 80 nm and from 90 to 150 nm, respectively, with increasing anodization time, while the nanopore density decreased from 60 to 43 nanopores μm2. The nonuniform PAA morphology illustrated in images from panels ae of Figure 1 can be explained by the small grain sizes.7,19,26 On the other hand, the temporal changes observed in the DP and Dint of the PAA structures obtained from 150 nm of ATF differ from bulk and thick ATF behaviors.5,7,23 In bulk Al, a higher order is obtained when anodization is conducted for longer periods of time (i.e., the size domain structures should be a
function of anodization time). Therefore, the structural changes shown in PAA are likely governed by different phenomena than those observed in the PAA grown in bulk Al. PAA thin films can be considered 2D polycrystalline lattices with ordered pores forming individual domains separated by domain boundaries.25 The interface tension of a 2D structure could play an important role in the self-organization of 2D lattices by gradually forcing individual pores to move into their equilibrium positions at the oxidemetalsubstrate interface during oxide growth.25 Therefore, changes in the interface of the Al2O3Si substrate must play a crucial role in the DP, Dint, and nanopore changes as a function of anodization time (Figure 1g). It is important to point out that this effect occurs only for ATF samples deposited on silicon substrate. The anodization of 150 nm thick ATF deposited on glass substrate did not show the same structural changes observed herein; see details in Figure S1 in the Supporting Information. Growth of the SiO2 Layer under the PAA Structure. Figure 2 shows a typical currenttime transient curve for an anodized Al thin film on a Si substrate. The first interval I (0 to 2 min 42 s) corresponds to the complete anodization of the ATF.7,40,41 When the ATF is oxidized completely, the current increases abruptly from 4.1 to 5.85 mA 3 cm2 (region II) due to the formation of an insulating layer of SiO2 from the anodized Si substrate. As the anodization time increases, the density of the anodic current continues to grow, stabilizing around 10 mA 3 cm2. When the substrate is nonoxidizing, the current density growth observed in region II does not occur. See details in Figure S2 of the Supporting Information. Figure 3a shows a typical cross-section image after 10 min of ATF anodization. It is possible to see the formation of a 53-nm SiO2 layer under the 180 nm-thick PAA layer. This feature is confirmed by the EDAX qualitative analysis of the cross-section images and of the local Al, O, and Si concentrations on the surface, as can be seen in Figure 3b. The qualitative mapping of chemical analyses mainly shows the thin layer of SiO2 formed below the PAA structure. Figure 3a also displays small voids that were formed at the bottom of the barrier layer, where the electrolyte penetrated through to form SiO2 on the Si substrate.23,24,26,39,41,46,49,50 The composition and thickness of the ATF, PAA, and SiO2 samples were followed by qualitative RBS analysis, shown in panels a and b of Figure 4. When the aluminum is oxidized to alumina, the volume expands by roughly a factor of 2. This occurs because the atomic density of the Al in Al2O3 is a factor of 2 lower 7623
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Figure 3. (a) A typical cross section of a PAA structure after the anodization process. Shown directly below this in more detail is a typical overview of the PAA structure. (b) Qualitative chemical mapping for EDAX analysis of the surface and cross section of the anodizing sample for 10 min at 140 V, where (I and II), (III and IV), (V and VI), and (VII and VIII) images correspond to the EDAX mapping of the surface and cross-section concentrations of Al (blue), O (green), and Si (yellow), respectively. The scale bar is the same for all images.
than in metallic Al,4 as can be seen in the Al edge recoil of the anodized ATF samples. This effect confirms the expansion of 30% (180 nm, see Figure 3a) of the PAA layer in comparison with the as-deposited 150 nm thick ATF samples. After the ATF has been completely anodized, the growth of a SiO2 layer on the backside of the PAA structure can be observed. Also, phosphorus contaminations were observed in the RBS spectra of the anodized samples. The P atoms were introduced in the sample during anodization with the H3PO4 electrolyte. Figure 4a also shows the expansion of the SiO2 layer with increasing anodization time. The SiO2 thickness was estimated by using the energy loss of R particles47,48 accelerated at 1000 keV. The SiO2 layer increased linearly from 53 to 170 nm with increasing anodization time (see Figure 4b). Effect of the Expansion of the SiO2 Layer. Figure 5 shows the top surface HRSEM images in backscattering detection mode of the PAA samples anodized for different times. Note that the sample that was anodized for 180 min (Figure 5b) shows cracks and/or crevices (see red arrows) on the PAA surface, while the sample that was anodized for 10 min (Figure 5a) does not present such defects. Another important aspect observed is the difference in the contrast around the nanopores in the sample that was anodized for 180 min compared to the that was anodized for 10 min. The differences of contrast observed in the PAA surface layer in Figure 5b probably indicate variations in the alumina density. Regions with higher contrast (bright) indicate that the Al2O3 density is higher than in those regions with lower contrast (dark). This effect may be related to structural changes of the PAA layer during the expansion of the SiO2. When the SiO2 grows in the PAASi interface, a volumetric expansion occurs, generating stress forces in the PAA layer. To minimize this stored energy, the film starts to form cracks (arrows in Figure 5b) while closing the smallest nanopores and widening the largest. At certain points, the lateral movement of the nanopores generates changes in the alumina density, as can be seen in the differences in contrast in Figure 5b.
Figure 4. (a) RBS measurements of the as-deposited ATF anodized from 10 to 180 min. (b) Qualitative SiO2 expansion layer as a function of anodization time.
The excessive increase of the SiO2 layer leads to the delamination of the PAA layer (Figure 6ac). Figure 6a shows the freestanding transparent layer of PAA, whereas Figure 6b presents SEM images of this PAA layer. Figure 6c shows RBS analyses of the SiO2 layer with a 170 nm thickness formed by 240 min of anodization after the PAA layers have been pulled from the Si substrate. The inset of Figure 6c shows the SEM image of the SiO2 layer after rupture of the PAA layer. The RBS shows that no more Al atoms are present on the SiO2Si, confirming the complete Al2O3 delamination. The formation of free-standing PAA films is due to the excessive stress generated at the interface of Al2O3SiO2Si. When the SiO2 reaches this thickness, the structural reconstruction of the layer of PAA is not enough to minimize the generated stress, resulting in a disruption of the PAA film. This effect could be a new strategy to obtain freestanding PAA membranes. The observed effect reinforces our view that the expansion of SiO2 is responsible for the structural change in the PAA layer in order to minimize stress generated by the expansion of SiO2 during the anodization process. Mechanism. Figure 7 represents a qualitative model of the structural change of alumina nanopores with increasing anodization time. The anodization process of a 150 nm Al thin film 7624
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Figure 5. HRSEM images in backscattering mode of the structure of the PAA surface layer of samples anodized at (a) 10 min and (b) 180 min.
(Figure 7I) generates a layer of PAA with a heterogeneous distribution of DP and Dint (Figure 7II). With the increase of anodization time, the ATF is entirely consumed, and the process of Si oxidation begins (Figure 7III). The formation of the SiO2 layer leads to an expansion of the substrate layer under the PAA
Figure 6. (a) A digital photograph of the free-standing PAA layer. (b) SEM image of the free-standing PAA layer after rupture of the Si substrate. (c) RBS analysis of the SiO2 surface without the PAA layer. Inset is an SEM image of the SiO2 surface layer.
film due to differences in the density of the Si and SiO2. This expansion generates a bottom-up force, increasing the interfacial
Figure 7. Qualitative model of the structural change of alumina nanoporous structures, where the sequence of images represents the increase of the SiO2 expansion layer as a function of anodization time. The 3D model was developed from the original SEM of the surface of the PAA images at each corresponding stage of anodization time. 7625
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The Journal of Physical Chemistry C stress between the Al2O3 and substrate (black arrows in Figure 7IIIVII). The formation of voids is strong evidence of this mechanical stress on the interface.24 Increasing the anodization periods leads to the formation of thicker SiO2 layers, and a higher degree of interfacial stress will affect the PAA thin film (Figure 7IIIVII). To minimize the energy of the system and prevent the collapse of the PAA film, there is likely a horizontal movement of the PAA walls (red arrows). This horizontal reaction force may lead to the shrinkage of small nanopores, explaining the decrease in the nanopore density and the increase in the DP and Dint.
’ CONCLUSIONS In summary, we have demonstrated that the effect of Al2O3SiO2 interfacial stress on the PAA formed from 150 nm thick Al thin films deposited on a silicon substrate is related to the SiO2 expansion. With increase of the anodization time, structural changes in the alumina nanopores are observed, as well as an increase in the thickness of the SiO2 layer formed after the complete Al oxidation. The increase in SiO2 thickness induces changes in the DP, Dint, and nanopore density in the PAA structure. The reconstruction of the PAA film is a consequence of the stress generated at the Al2O3SiO2 interface. The interfacial stress occurs due to a volumetric expansion between the Si substrate and the SiO2 layer. These findings can be used to design new ATF devices, thus making available a variety of technological applications for nanostructured alumina thin films on Si substrates. ’ ASSOCIATED CONTENT
bS
Supporting Information. Details of SEM images of PAA on a glass substrate after ATF anodization at 140 V with a 1.3 mM H3PO4 electrolyte at 20 °C for different anodization times and currenttime transient curve for the anodization of an ATF deposited on a glass substrate. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]; aff
[email protected].
’ ACKNOWLEDGMENT This paper was supported in part by CNPq, CAPES, and FAPERGS Brazilian financial agencies. We acknowledge the Centro de Microscopia (CME) da UFRGS and Laboratorio de Implantac-~ao I^onica do IF-UFRGS. The authors acknowledge INMETRO-Brazil and Dra. Suzanna B. Peripolli for FIB and HRSEM images and EDAX analysis. ’ REFERENCES (1) Lee, W.; Ji, R.; Gosele, U.; Nielsch, K. Nat. Mater. 2006, 5, 741–747. (2) Masuda, H.; Fukuda, K. Science 1995, 268, 1466–1468. (3) Masuda, H.; Yamada, H.; Satoh, M.; Asoh, H.; Nakao, M.; Tamamura, T. Appl. Phys. Lett. 1997, 71, 2770–2772. (4) Jessensky, O.; Muller, F.; Gosele, U. Appl. Phys. Lett. 1998, 72, 1173–1175. (5) Li, A. P.; Muller, F.; Birner, A.; Nielsch, K.; Gosele, U. J. Appl. Phys. 1998, 84, 6023–6026.
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