Next Article in Journal
Influence of Chitosan/Lycopene on Myoglobin and Meat Quality of Beef During Storage
Previous Article in Journal
Flow Field of Supersonic Oxygen Jet Generated by Various Wear Lengths at the Laval Nozzle Exit
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

On the Effect of Randomly Oriented Grain Growth on the Structure of Aluminum Thin Films Deposited via Magnetron Sputtering

by
Vagelis Karoutsos
1,*,
Nikoletta Florini
2,
Nikolaos C. Diamantopoulos
1,
Christina Balourda
1,
George P. Dimitrakopulos
2,
Nikolaos Bouropoulos
1,3 and
Panagiotis Poulopoulos
1,*
1
Materials Science Department, University of Patras, 26504 Patras, Greece
2
Physics Department, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
3
Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Chemical Processes, 26504 Patras, Greece
*
Authors to whom correspondence should be addressed.
Coatings 2024, 14(11), 1441; https://doi.org/10.3390/coatings14111441
Submission received: 30 October 2024 / Revised: 10 November 2024 / Accepted: 11 November 2024 / Published: 13 November 2024
(This article belongs to the Section Thin Films)

Abstract

:
The microstructure of aluminum thin films, including the grain morphology and surface roughness, are key parameters for improving the thermal or electrical properties and optical reflectance of films. The first step in optimizing these parameters is a thorough understanding of the grain growth mechanisms and film structure. To investigate these issues, thin aluminum films with thicknesses ranging from 25 to 280 nm were coated on SiOx/Si substrates at ambient temperature under high-vacuum conditions and a low argon pressure of 3 × 10−3 mbar (0.3 Pa) using the radio frequency magnetron sputtering method. Quantitative analyses of the surface roughness and nanograin characteristics were conducted using atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray diffraction. Changes in specular reflectance were measured using ultraviolet–visible and near-infrared spectroscopy. The low roughness values obtained from the AFM images resulted in high film reflectivity, even for thicker films. TEM and AFM results indicate monomodal, randomly oriented grain growth without a distinct columnar or spherical morphology. Using TEM cross-sectional images and the dependence of the grain size on the film thickness, we propose a grain growth mechanism based on the diffusion mobility of aluminum atoms through grain boundaries.

1. Introduction

Pure Al thin films exhibit excellent coating properties including low resistivity, high optical reflectance, good substrate surface adhesion and resistance to oxidation and corrosion [1,2]. That is why Al coatings find a wide variety of applications, such as in optics, microelectronics, and telecommunications. Furthermore, successful deposition on a broad variety of substrates, including stainless steel [2], titanium [3], silicon wafers [1], polycarbonates [4], and glass, makes aluminum thin films an attractive system for industrial and research applications. In order to tune thin film properties, physical and chemical deposition methods have been developed, with the former being preferable because they produce more adhesive films with lower substrate temperature and are environmentally friendly [5]. Among the physical deposition methods, magnetron sputtering produces Al films with a homogenous microstructure of lower porosity compared to similar methods such as thermal evaporation [3,6].
The two crucial factors affecting the properties and applications of thin film systems are the microstructure and surface roughness. As examples, film grain size and morphology, as well as failures and defects, control electron motion, which in turn affects thermal conductivity and resistivity. Surface roughness influences the film reflectivity and photolithography patterning processes. The first and most important step to control grain size and roughness during growth is a thorough understanding of the film structure, as well as the growth mechanism. In the case of normal grain growth, in which all grains develop at almost the same rate, previous theoretical and experimental studies have adopted two basic atomistic processes to explain the evolution of metal thin films: surface diffusion and grain boundary motion [7,8]. Existing literature regarding Al thin film microstructures shows that the dominant grain morphology is near-columnar or equiaxed, depending on deposition conditions such as substrate temperature, Ar discharge pressure, impurity content, and substrate morphology [8,9,10].
In this work, Al thin films with a thickness range of t = 25–280 nm were deposited on SiOx/Si at ambient temperature under high-purity conditions and low Ar pressure using the Radio Frequency (RF) magnetron sputtering method. Observations by cross-sectional transmission electron microscopy (TEM) revealed that the Al films were composed of randomly oriented crystals without a distinct geometric shape. The low surface roughness values indicated by atomic force microscopy (AFM) images, even for thicker films, have an impact on reflectivity. The reflectivity in the visible wavelength range is higher than that reported in other works in the literature. With the aid of high-resolution imaging of the cross-sectional grain morphology and the dependence of the grain diameter on the film thickness, we propose a grain growth mechanism based on the diffusion of Al atoms along grain boundaries, taking into account the extremely low mobility of Al atoms on the SiOx/Si substrate. The unusual randomly oriented crystals observed and the proposed grain growth mechanism are novel aspects of this work.

2. Materials and Methods

High-purity Al films were deposited on (001) SiOx/Si substrates by RF magnetron sputtering at room temperature. The experimental conditions are summarized and in Table 1. The Al target was fabricated by the machinery shop of the University of Patras from Al of series 1000. Six films were grown with a thickness range of 25–280 nm. More specifically, the thickness varied as follows: (a) sample ALM1, t = 25 nm; (b) sample ALM2, t = 37 nm; (c) sample ALM3, t = 65 nm; (d) sample ALM4, t = 97 nm; (e) sample ALM5, t = 197 nm; and (f) sample ALM6, t = 280 nm. The film crystallinity was determined using the X-ray powder diffraction (XRD) method with the aid of a Bruker D8 diffractometer (Bruker, D8-Advance, Karlsruhe, Germany) equipped with Ni-filtered CuKa1 radiation (λ = 0.154 nm). The microstructure was studied via atomic force microscopy (AFM), utilizing a Multimode Microscope with a Nanoscope IIIa controller and a 120 µm × 120 µm magnet-free scanner (Model AS-130 VMF) developed by Digital Instruments (Chapel Hill, NC, USA). The film thickness was evaluated using AFM image profiling of a narrow scratch etched by a razor blade on the film surface. The optical reflectance of the Al films was also investigated using a Perkin Elmer λ-35 spectrometer (PerkinElmer, Akron, OH, USA) equipped with an integrating sphere using a Spectralon® specimen as a standard material. The cross-sectional nanostructure of the films was characterized by TEM, high-resolution TEM (HRTEM), scanning TEM (STEM), and energy dispersive X-ray spectroscopy (EDS) (200 kV electron microscope model JEOL JEM F200 CFEG Akishima, Tokyo, Japan, equipped with an Oxford X-Max 65T EDS detector). Geometrical phase analysis (GPA) was employed to deduce grain morphologies and sizes by HRTEM.

3. Results

XRD was used to reveal the structure of the ALM6 sample. In Figure 1, an XRD pattern is presented showing the polycrystalline character of the 280 nm thick Al film deposited on SiOx/Si. The stronger X-ray reflection is that of Al(111) as compared to Al(200), Al(220), and Al(311). The reference Al XRD pattern (JCPDS file #2_787) recorded on the Al powder is polycrystalline with no texture. Since the intensity ratio of the two main peaks, I(111)/I(200), is the same for both patterns, we conclude that our film is also polycrystalline with no preferential growth orientation. Furthermore, no phase shift was detected between the film peak positions and the Al standard reference sample peaks, which is the first evidence of the absence of stress–strain phenomena in the Al film lattice or the Al/SiOx interface. The XRD patterns, combined with the EDS spectra, showed that the Al films deposited by the RF magnetron sputtering method consisted of pure aluminum free of any impurities. Figure 2 illustrates STEM/EDS maps of the heterostructure and interfacial region.
Figure 3 shows AFM surface images of six Al films deposited on SiOx/Si, with thicknesses ranging from t = 25 nm to t = 280 nm. To obtain the grain size distribution, grain size measurements were performed on each AFM image. In order to obtain an adequate statistical population, 200 grains were measured for each image. The grain size distribution histograms corresponding to the six AFM images in Figure 3 are depicted in Figure 4. The mean grain size, dg, was obtained by a Gaussian function fitted in each histogram:
f d = 1 σ 2 π e x p { 1 2 d d g σ }
where σ is the standard deviation of d. The parameters dg and σ obtained from the fitted curves in Figure 4 are presented in Table 2.

Film Surface Topography—Grain Morphology

By carefully inspecting the surface of the films in Figure 3, we observed that the grain shapes were spherical or ellipsoidal, and some of them were agglomerated, forming regions that were higher than the rest of the surface grains, indicated with white color in the AFM images. There were no “hillocks” or secondary grains with abnormal size on the film’s surface. The hillock morphology usually observed in Al thin films signifies film defects, film–substrate mismatch, or gas contamination during film deposition [11]. The absence of hillocks on the film surfaces associated with the XRD and EDS results mentioned in the previous section is strong evidence that the deposited films are free of impurities or contamination. The combination of AFM surface images and grain size distributions in Figure 3 and Figure 4 proves that as the film thickness increases, a normal monomodal growth of spherical or ellipsoid grains is observed until the top end of the film.
To quantitatively measure the grain height profile fluctuations, roughness measurements were conducted using AFM data. The root-mean-square roughness, Rrms, is the most commonly used measurement of surface roughness and corresponds to the standard deviation of the surface height profile from the mean height. It is given by the formula
R r m s = 1 N i = 1 N z i z 2 1 / 2
where N is the number of pixels in the image (or data points), zi is the height of the ith pixel, and z is the mean height for the entire image. Rrms values presented in Table 2 are the average values of five different regions for each Al sample surface. The size of each region is 1 × 1 μm2. The roughness values, Rrms, for six Al samples as a function of film thickness, t, are plotted in the log–log diagram of Figure 5. The error bars result from the averaging of five different scans. An increasing dependency of roughness on film thickness is observed following a power low of the form Rrms~tb. The linear fitting of the experimental points in Figure 5 gives b = 0.45 ± 0.08. This value is close to the values of 0.55 and 0.56 found by Lita et al. [12] and Fu et al. [13], studying sputter-deposited Al thin films on amorphous Si and Si(100) substrates, respectively. The deposition rates in these works were of 16 nm/s and 0.7 nm/s, which are higher than that of our experiment (0.13 nm/s) and this could be the reason for the rougher Al film surfaces. Apart from the film thickness, the roughness measurements are strongly dependent on the scale of the measured surface area [14,15]. While our scaling for AFM measurements was 1 × 1 μm2, which is a rather small area, the average roughness values derived from multiple surface regions enhance the reliability of our measurements.
The surface roughness and grain structure of thin films have a significant impact on their ability to reflect light. In this case, smoother films with lower roughness values have better reflectance properties. Reflectance measurements were carried out in the wavelength range of 400–1100 nm, focusing on how well the films reflect light across the visible and near-infrared spectrum. The results for the two thicker samples, ALM5 and ALM6, which are not transparent in the measured wavelength range, are presented in Figure 6, showing reflectance values greater than 90% in the visible range. The low surface roughness (see Table 2) contributes to this high reflectivity. The reflectance values obtained (>90%) were higher than those found in analogous studies on sputter-deposited [16] and thermally evaporated Al films [17] of similar thicknesses. This suggests that the deposition process used for our films produces high-quality surfaces with minimal defects, such as pores or voids, enhancing their optical reflectivity. The dip in the reflectance curve observed around the wavelength of 800 nm (Figure 6) is attributed to light absorption caused by electron transitions between energy bands at specific points in the Brillouin zone, namely the W and K points (which are specific wave vectors, k) [18]. The band gap for these transitions is approximately 1.5 eV, corresponding to a wavelength of 826 nm.
Apart from the AFM surface analysis, it is significant to closely examine the morphology of the grains beneath the surface of the deposited film. To this end, a comparison of cross-sectional TEM and AFM surface images of the same film (ALM5) is presented in Figure 7. Figure 7a illustrates a representative bright field TEM image of the heterostructure. The film exhibited a thickness of 180 ± 6 nm compared to the nominal thickness of 197 nm. The RMS roughness was consistent with the AFM measurement. A 3D AFM surface image of the same sample is presented in Figure 7b for comparison. Grains that appear darker in Figure 7a are close to a low-index zone axis orientation. The corresponding selected-area electron diffraction (SAED) patterns from the Si substrate and the film are given as insets. SAED was consistent with the polycrystalline structure of the Al film and indicated some partial alignment between the {111} planes of the film and the substrate. Figure 8 illustrates another region observed using bright field (Figure 8a) and dark field (Figure 8b,c) diffraction contrast. Dark field images were collected using various reflections corresponding to Al crystallites so that different grains are bright in each one. Most grains exhibited random geometrical shapes and a wide size dispersion with an average size of 22 ± 12 nm, which is consistent with the AFM measurement. The interfacial region appeared to comprise a zone of small-sized crystallites, as indicated by arrows in Figure 8b.
According to the literature [19,20,21], if the film thickness is greater than the grain size, normal grain growth occurs, which is our case, considering the AFM images and grain size distributions in Figure 3 and Figure 4, respectively. Hentzel et al. [22] studied the growth of mainly single-element metallic thin films including aluminum. They showed that the mean grain size for thin films strongly depends on the ratio of Tm/Ts, where Ts is the deposition temperature and Tm is the melting temperature. For the Al thin films and for Tm/Ts = 3.3, they found that the grain size values must be in the range of 5.5 nm–280 nm. Our experimental measurements, with the same ratio of Tm/Ts and a minimum–maximum grain size of dg = 7.5–48 nm (see Table 2), are in the range predicted by these authors.

4. Discussion—Film Growth Mechanism

As the TEM cross-sectional images show, most of the grains have random geometrical shapes and a few of them appear to have semi-spherical or near-columnar morphology. The basic structure zone model (SZM) for magnetron-sputtered thin films proposed by Thornton [23] predicts the microstructure of films as a function of discharge pressure, Pd, and homologous temperature, Ts/Tm. In our case, where Pd = 3 × 10−3 mbar and Ts/Tm = 0.3, the model anticipates a fibrous cone-column-like microstructure (Zone T), which does not agree with the grain morphology of our films. However, Hentzel et al. [22], in an experimental study including Al thin films, proposed a mixture of equiaxed and columnar-like grains for Zone T. For Ts/Tm < 0.3 (Zone I), they found that the grain morphology is fully equiaxed. It must be noted here that the boundaries of the zone are diffuse and blended, and transitions may occur over a wide range of Pd and Ts/Tm, which is dependent on the sputtering conditions [8,10]. Furthermore, according to a revised SZM concerning Al thin films [9], the grain morphology can be influenced by several factors beyond just the discharge pressure and substrate temperature. Impurity levels, substrate surface phases such as amorphous or crystalline, and sputtering discharge power play critical roles in nucleation and subsequent grain growth [6,24,25,26].
It is generally agreed that substrate surface adatom mobility and bulk grain boundary diffusion are the dominant atomic processes which affect the grain evolution during thin film growth. Levenson et al. [27] studied the Al adatom diffusion distance on amorphous SiOx surfaces. At ambient temperatures, the effective surface mobility of Al adatoms on SiOx is negligible, resulting in the rapid formation of Al nuclei. In our case, despite the relatively high sputtered atom velocity due to the low Ar discharge pressure (3 × 10−3 mbar), which could have enhanced Al adatom mobility, the low deposition energy (30 W) renders the Al diffusion distance on the SiOx layer even smaller. Many researchers have advocated that nuclei formed on amorphous substrates have restricted texture and no preferred in-plane orientation [19,28]. Thus, in the early first stage of deposition, the Al adatoms favor the formation of randomly oriented nuclei because of the amorphous SiOx layer. The SAED pattern rings of the polycrystalline Al layer shown in Figure 7 provide evidence that there is no preferred crystal orientation during film growth. Nucleation proceeds until the formed islands abut each other, and coalescence occurs. The HRTEM image in Figure 9 illustrates the coalesced and overlapping crystallites in the Al layer. Under conditions of low discharge power and low deposition temperature, while grain boundary mobility is indeed slow, the lack of impurities can facilitate some degree of diffusion [6]. This allows for a grain structure to develop, and therefore growth competition takes place among grains with different crystal orientations. Some grains that have planes with low surface energy, such as close-packed {111} planes, grow faster than others, which explains the crystallites’ wide size dispersion of 10–50 nm. But even for these energetically favorable grains, their {111} plane orientation is mostly random and not parallel to the Al/SiOx interface, as shown in the HRTEM image of Figure 9 and the SAED pattern of Figure 7. Therefore, their growth is obstructed and that is why there is no prevailing columnar or spherical grain evolution up to the film surface.
Summarizing the above discussion, we infer that the grains are randomly oriented and isotropic growth is the dominant process. Due to the low Al adatom mobility on the SiOx surface, grain boundary mobility is probably the prevailing growth mechanism. There is another way to explore the growth mechanism, and this is the dynamic dependence of the mean grain size, dg, and sample thickness, t, which, according to the relevant models [29,30], follows a power law form dg~tn, where the exponent, n, depends on the growth mechanism. The linear regression fitting of log(dg) as a function of log(t), derived by AFM analysis (Table 2) and presented in Figure 10, results in n = 0.53 ± 0.22. Very close to this value is the value of 0.45 obtained by Anderson et al. [30] in a Monte Carlo simulation for normal grain growth. Additionally, this value is close to the value of 0.38 obtained by Dulmaa et al. [29] in an experimental study concerning Al thin films deposited on Si wafers with a thickness range of 30–1000 nm and deposition rate of 0.88 nm/s, much higher than our case, where the rate was only 0.13 nm/s. Furthermore, studies on Ni/Pt multilayer on glass [31] and Au layer on SiOx/Si [32] found n = 0.45 and n = 0.41, respectively. Two research groups in a theoretical [33] and an experimental study [34] concerning the Au/SiOx/Si system deposited at ambient temperature found that, for normal grain growth, it should be n = 0.33. These researchers claimed that the dominant grain growth mechanism is the diffusion of mobile atoms on grain boundaries. On the other hand, Roufino et al. [35], studying the same system deposited at a higher temperature, found n = 0.25, and advocated that the dominant growth mechanism is the diffusion of mobile Au atoms on the SiOx substrate. Obviously, our exponent value is closer to the n = 0.33 obtained by the researchers in refs. [33,34]. Considering the low Al adatom mobility on the SiOx substrate discussed in the previous section, we believe that the diffusion of mobile Al atoms in the grain boundaries is the most likely process in the grain growth mechanism of the Al/SiOx/Si system.

5. Conclusions

In this study, high-purity Al thin films were deposited on SiOx/Si using RF magnetron sputtering. The low surface roughness observed in AFM images resulted in high reflectivity values, even for thicker films in the visible wavelength range. These results, correlated with the XRD and TEM-EDS analyses, confirm the high quality of the films.
The grain size distributions obtained from the AFM surface and TEM cross-sectional measurements showed normal grain growth with random geometric shapes and a wide size dispersion. Furthermore, HRTEM analysis revealed crystallites with randomly oriented planes that were not parallel to the amorphous SiOx substrate. These findings are unusual compared to similar studies and structural zone models, which typically predict columnar or equiaxed grain morphologies.
The low surface mobility of Al adatoms on the amorphous SiOx surface results in rapid nucleation, with no texture and no preferred in-plane orientation. Based on these findings and the grain size dependence on film thickness, we propose a grain growth mechanism driven by the diffusion of Al atoms along grain boundaries.

Author Contributions

Conceptualization, P.P.; methodology, P.P. and V.K.; validation, P.P. and V.K.; formal analysis, V.K., G.P.D. and N.F.; investigation, V.K., G.P.D., N.B., N.C.D. and C.B.; data curation, V.K., G.P.D., N.F., N.C.D. and C.B.; writing—original draft preparation, V.K.; writing—review and editing, P.P., G.P.D., N.F. and N.B.; visualization, V.K.; supervision, P.P. and G.P.D.; project administration, P.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Bordo, K.; Rubahn, H. Effect of deposition rate on structure and surface Morphology of thin evaporated Al films on dielectrics and semiconductors. Mater. Sci. 2012, 18, 313–317. [Google Scholar] [CrossRef]
  2. Esfahani, E.A.; Salimijazi, H.; Golozar, M.A.; Mostaghimi, J.; Pershin, L. Study of corrosion behaviour of Arc sprayed aluminium coating on mild steel. J. Therm. Spray Technol. 2012, 1195–1202. [Google Scholar] [CrossRef]
  3. Garbacz, H.; Wiecinski, P.; Adamczyk-Cieslak, B.; Mizera, J.; Kurzydlowski, K.J. Studies of aluminium coatings deposited by vacuum evaporation and magnetron sputtering. J. Microsc. 2009, 237, 475. [Google Scholar] [CrossRef] [PubMed]
  4. Arias, N.; Jaramillo, F. Highly reflective aluminum films on polycarbonate substrates by physical vapor deposition. Appl. Surf. Sci. 2020, 505, 144596. [Google Scholar] [CrossRef]
  5. Wallin, E. Alumina Thin Films: From Computer Calculations to Cutting Tools. Ph.D. Thesis, Linköping University, The Institute of Technology, Linköping Sweden, 2008. [Google Scholar]
  6. More-Chevalier, J.; Novotný, M.; Hruška, P.; Fekete, L.; Fitl, P.; Bulíř, J.; Pokorný, P.; Volfová, L.; Havlová, S.; Vondráček, M.; et al. Fabrication of black aluminium thin films by magnetron sputtering. RSC Adv. 2020, 10, 20765–20771. [Google Scholar] [CrossRef]
  7. Zöllner, D. Treating grain growth in thin films in three dimensions: A simulation Study. Comput. Mater. Sci. 2016, 125, 51–60. [Google Scholar] [CrossRef]
  8. Petrov, I.; Barna, P.B.; Hultman, L.; Greene, J.E. Microstructural evolution during film growth. J. Vac. Sci. Technol. A 2003, 21, S117–S128. [Google Scholar] [CrossRef]
  9. Gilmer, H. George; Huang, Hanchen; Diaz de la Rubia, Tomas; Jacques Dalla Frieder Baumann, Torre. Lattice Monte Carlo models of thin film deposition. Thin Solid Films 2000, 365, 189–200. [Google Scholar] [CrossRef]
  10. Mwema, F.M.; Oladijo, O.P.; Akinlabi, S.A.; Akinlabi, E.T. Properties of physically deposited thin aluminium film coatings: A review. J. Alloys Compd. 2018, 747, 306–323. [Google Scholar] [CrossRef]
  11. Cao Martin, B.; Tracy, C.J.; Mayer, J.W.; Hendrickson, L.E. A comparative study of hillock formation in aluminum films. Thin Solid Films 1995, 271, 64–68. [Google Scholar] [CrossRef]
  12. Lita, A.E.; Sanchez, J.E. Characterization of surface structure in sputtered Al films: Correlation to microstructure evolution. J. Appl. Phys. 1999, 85, 876–882. [Google Scholar] [CrossRef]
  13. Fu, T.; Shen, Y.G. Surface growth and anomalous scaling of sputter-deposited Al films. Phys. B Condens. Matter 2008, 403, 2306–2311. [Google Scholar] [CrossRef]
  14. Sharma, N.; Prabakar, K.; Dash, S.; Tyagi, A.K. Growth kinetics of ion beam sputtered Al-thin films by dynamic scaling theory. Thin Solid Films 2014, 573, 84–89. [Google Scholar] [CrossRef]
  15. Vazquez, L.; Salvarezza, R.C.; Herrasti, P.; Ocon, P.; Vara, J.M.; Arvia, A.J. Scale-dependent roughening kinetics in vapor deposited gold. Surf. Sci. 1996, 345, 17–26. [Google Scholar] [CrossRef]
  16. Schmitt, P.; Stempfhuber, S.; Felde, N.; Szeghalmi, A.V.; Kaiser, N.; Tünnermann, A.; Schwinde, S. Influence of seed layers on the reflectance of sputtered aluminum thin films. Opt. Express 2021, 29, 19472–19485. [Google Scholar] [CrossRef]
  17. Lugolole, R.; Obwoya, S.K. The effect of thickness of Aluminium films on optical Reflectance. J. Ceram. 2015, 2015, 213635. [Google Scholar] [CrossRef]
  18. Fox, M. Optical Properties of Solids, 2nd ed.; Oxford University Press: Oxford, UK, 2011. [Google Scholar]
  19. Thompson, C.V. Grain growth in thin films. Ann. Rev. Mater. Sci. 1990, 20, 245–268. [Google Scholar] [CrossRef]
  20. Thornton, J.A. High rate thick film growth. Ann. Rev. Mater. Sci. 1977, 7, 239–260. [Google Scholar] [CrossRef]
  21. Governor, C.R.M.; Hentzel, H.T.G.; Smith, D.A. The development of grain structure during growth of metallic films. Acta Metall. 1984, 32, 773–781. [Google Scholar]
  22. Hentzell, H.T.G.; Grovenor, C.R.M.; Smith, D.A. Grain structure variation with temperature for evaporated metal films. J. Vac. Sci. Technol. A 1984, 2, 218–219. [Google Scholar] [CrossRef]
  23. Thornton, J.A. Influence of apparatus geometry and deposition conditions on the structure and topography of thick sputtered coatings. J. Vac. Sci. Technol. 1974, 11, 666–670. [Google Scholar] [CrossRef]
  24. Roberts, S.; Dobson, P.J. The microstructure of aluminium thin films on amorphous SiO2. Thin Solid Films 1986, 135, 137–148. [Google Scholar] [CrossRef]
  25. Kaune, G.; Metwalli, E.; Meier, R.; Korstgens, V.; Schlage, K.; Couet, S.; Rohlasbeger, R.; Roth, S.; Muller, P. Growth and morphology of sputtered aluminum thin films on P3HT surface. Appl. Mater. Interfaces 2011, 3, 1055–1062. [Google Scholar] [CrossRef]
  26. Barajas-Valdes, U.; Suárez, O.M. Morphological and Structural Characterization of Magnetron-Sputtered Aluminum and Aluminum-Boron Thin Films. Crystals 2021, 11, 492. [Google Scholar] [CrossRef]
  27. Levenson, L.L.; Swartzlander, A.B.; Yahashi, A.; Usui, H.; Yamada, I. The Measurement of Aluminum Surface Diffusion on Si, SiO2, and Si3N4 by Scanning Auger Microscopy. Scanning Microsc. 1991, 5, 10. [Google Scholar]
  28. Barna, P.B.; Adamik, M. Fundamental structure forming phenomena of polycrystalline films and the structure zone models. Thin Solid Films 1998, 317, 27–33. [Google Scholar] [CrossRef]
  29. Dulmaa, A.; Cougnon, F.G.; Dedoncker, R.; Depla, D. On the grain size-thickness correlation for thin films. Acta Mater. 2021, 212, 116896. [Google Scholar] [CrossRef]
  30. Anderson, M.P.; Strolovitz, D.J.; Grest, G.S.; Sahini, P.S. Computer simulation of grain growth-I. Kinetics. Acta Metall. 1984, 32, 783–791. [Google Scholar] [CrossRef]
  31. Karoutsos, V.; Papasotiriou, P.; Poulopoulos, P.; Kapaklis, V.; Politis, C.; Angelakeris, M.; Kehagias, T.; Flevaris, N.K.; Papaioannou, E.T. Growth modes of polycrystalline Ni/Pt multilayers with deposition temperature. J. Appl. Phys. 2007, 102, 043525. [Google Scholar] [CrossRef]
  32. Karoutsos, V.; Toudas, M.; Delimitis, A.; Grammatikopoulos, S.; Poulopoulos, P. Microstructural evolution in nanostructured gold films. Thin Solid Films 2012, 520, 4074–4079. [Google Scholar] [CrossRef]
  33. Ruffino, F.; Grimaldi, M.G.; Bongiorno, C.; Giannazzo, F.; Roccaforte, F.; Raineri, V.; Spinella, C. Normal and abnormal grain growth in nanostructured gold film. J. Appl. Phys. 2009, 105, 054311. [Google Scholar] [CrossRef]
  34. Novikov, V.Y. Grain growth controlled by mobile particles on grain boundaries. Scr. Mater. 2006, 55, 243–246. [Google Scholar] [CrossRef]
  35. Ruffino, F.; Canino, A.; Grimaldi, M.G.; Giannazzo, F.; Bongiorno, C.; Roccaforte, F.; Raineri, V. Self-organization of gold nanoclusters on hexagonal SiC and SiO2 surfaces. J. Appl. Phys. 2007, 101, 064306. [Google Scholar] [CrossRef]
Figure 1. XRD pattern of the 280 nm thick Al film deposited on SiOx/Si.
Figure 1. XRD pattern of the 280 nm thick Al film deposited on SiOx/Si.
Coatings 14 01441 g001
Figure 2. (a) Cross-sectional high-angle annular dark-field (HAADF) STEM image of the Al/Si heterostructure. (b) Corresponding layered image of EDS maps. The inset illustrates the interfacial region with the oxygen signal due to the SiOx interlayer.
Figure 2. (a) Cross-sectional high-angle annular dark-field (HAADF) STEM image of the Al/Si heterostructure. (b) Corresponding layered image of EDS maps. The inset illustrates the interfacial region with the oxygen signal due to the SiOx interlayer.
Coatings 14 01441 g002
Figure 3. AFM images of the deposited Al films for the following samples: (a) ALM1, (b) ALM2, (c) ALM3, (d) ALM4, (e) ALM5, and (f) ALM6. All image dimensions are 1 × 1 μm2, except for image (a), whose dimensions are 500 × 500 nm2.
Figure 3. AFM images of the deposited Al films for the following samples: (a) ALM1, (b) ALM2, (c) ALM3, (d) ALM4, (e) ALM5, and (f) ALM6. All image dimensions are 1 × 1 μm2, except for image (a), whose dimensions are 500 × 500 nm2.
Coatings 14 01441 g003
Figure 4. Grain size distribution histograms corresponding to each AFM image in Figure 3; dg denotes the mean grain size obtained by the Gaussian function fitted to each histogram.
Figure 4. Grain size distribution histograms corresponding to each AFM image in Figure 3; dg denotes the mean grain size obtained by the Gaussian function fitted to each histogram.
Coatings 14 01441 g004
Figure 5. Measured average RMS roughness for the six film surfaces (Table 2) plotted as a function of film thickness.
Figure 5. Measured average RMS roughness for the six film surfaces (Table 2) plotted as a function of film thickness.
Coatings 14 01441 g005
Figure 6. Reflectance spectra of two Al thin films with different thicknesses deposited on glass substrate.
Figure 6. Reflectance spectra of two Al thin films with different thicknesses deposited on glass substrate.
Coatings 14 01441 g006
Figure 7. (a) Cross-sectional bright-field TEM image of a region of the Al/Si heterostructure obtained along the [1 1 ¯ 0]Si zone axis of the substrate. SAED patterns obtained from the substrate and the Al film are given as insets. Reflections from diffracting planes are denoted on the SAED patterns. In the case of the Al film, its polycrystalline character yields a ring-type SAED pattern. (b) The 3D AFM surface image of the same film.
Figure 7. (a) Cross-sectional bright-field TEM image of a region of the Al/Si heterostructure obtained along the [1 1 ¯ 0]Si zone axis of the substrate. SAED patterns obtained from the substrate and the Al film are given as insets. Reflections from diffracting planes are denoted on the SAED patterns. In the case of the Al film, its polycrystalline character yields a ring-type SAED pattern. (b) The 3D AFM surface image of the same film.
Coatings 14 01441 g007
Figure 8. (a) Cross-sectional bright field TEM image showing another region of the Al/Si heterostructure. (b,c) Corresponding dark field TEM images obtained with different reflections of the film, showing diffraction contrast from different crystallites. In (b), the arrows indicate smaller-sized crystallites near the heterointerface.
Figure 8. (a) Cross-sectional bright field TEM image showing another region of the Al/Si heterostructure. (b,c) Corresponding dark field TEM images obtained with different reflections of the film, showing diffraction contrast from different crystallites. In (b), the arrows indicate smaller-sized crystallites near the heterointerface.
Coatings 14 01441 g008
Figure 9. (a) HRTEM image along the [1 1 ¯ 0] zone axis of Si, showing in atomic resolution the polycrystalline Al epilayer grown on the Si substrate. Moiré fringes in the Al film are due to the overlap of grains along the projection direction. (b) GPA phase map illustrating the phase changes in the epilayer due to its polycrystalline structure. The inset is the corresponding diffractogram indicating the selected spatial periodicities close to 220 Si that were employed for creating the phase map.
Figure 9. (a) HRTEM image along the [1 1 ¯ 0] zone axis of Si, showing in atomic resolution the polycrystalline Al epilayer grown on the Si substrate. Moiré fringes in the Al film are due to the overlap of grains along the projection direction. (b) GPA phase map illustrating the phase changes in the epilayer due to its polycrystalline structure. The inset is the corresponding diffractogram indicating the selected spatial periodicities close to 220 Si that were employed for creating the phase map.
Coatings 14 01441 g009
Figure 10. Measured average grain diameter obtained by distribution histograms of Figure 3 plotted as a function of film thickness.
Figure 10. Measured average grain diameter obtained by distribution histograms of Figure 3 plotted as a function of film thickness.
Coatings 14 01441 g010
Table 1. Rf sputtering deposition conditions.
Table 1. Rf sputtering deposition conditions.
Deposition ParameterConditions
Substrate temperature300 K
Rf power30 W
Target to substrate distance7 cm
Deposition rate0.13 nm/s
Base pressure2 × 10−7 mbar
Sputtering pressure of Argon3 × 10−3 mbar
Table 2. Surface parameters obtained from the analysis of AFM measurements for Al thin films grown on SiOx/Si with various thicknesses.
Table 2. Surface parameters obtained from the analysis of AFM measurements for Al thin films grown on SiOx/Si with various thicknesses.
Sample NameThickness, t, (nm)AFM
Mean Grain Diameter, dg, (nm)
Std. Deviation, σ, (nm)Roughness Rrms
(nm)
δRrms (nm)
ALM1259.670.181.530.04
ALM23716.220.562.440.20
ALM36520.570.764.150.15
ALM49734.820.513.160.25
ALM519717.170.345.100.35
ALM628045.345.125.010.36
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Karoutsos, V.; Florini, N.; Diamantopoulos, N.C.; Balourda, C.; Dimitrakopulos, G.P.; Bouropoulos, N.; Poulopoulos, P. On the Effect of Randomly Oriented Grain Growth on the Structure of Aluminum Thin Films Deposited via Magnetron Sputtering. Coatings 2024, 14, 1441. https://doi.org/10.3390/coatings14111441

AMA Style

Karoutsos V, Florini N, Diamantopoulos NC, Balourda C, Dimitrakopulos GP, Bouropoulos N, Poulopoulos P. On the Effect of Randomly Oriented Grain Growth on the Structure of Aluminum Thin Films Deposited via Magnetron Sputtering. Coatings. 2024; 14(11):1441. https://doi.org/10.3390/coatings14111441

Chicago/Turabian Style

Karoutsos, Vagelis, Nikoletta Florini, Nikolaos C. Diamantopoulos, Christina Balourda, George P. Dimitrakopulos, Nikolaos Bouropoulos, and Panagiotis Poulopoulos. 2024. "On the Effect of Randomly Oriented Grain Growth on the Structure of Aluminum Thin Films Deposited via Magnetron Sputtering" Coatings 14, no. 11: 1441. https://doi.org/10.3390/coatings14111441

APA Style

Karoutsos, V., Florini, N., Diamantopoulos, N. C., Balourda, C., Dimitrakopulos, G. P., Bouropoulos, N., & Poulopoulos, P. (2024). On the Effect of Randomly Oriented Grain Growth on the Structure of Aluminum Thin Films Deposited via Magnetron Sputtering. Coatings, 14(11), 1441. https://doi.org/10.3390/coatings14111441

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop