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Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry
G Model
ARTICLE IN PRESS
APSUSC-34044; No. of Pages 5
Applied Surface Science xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
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Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry M. Magnozzi a,∗ , F. Bisio a,b , M. Canepa a a b
OPTMAT Lab, Dipartimento di Fisica, Universitá di Genova, via Dodecaneso 33, 16143 Genova, Italy CNR-SPIN, corso Perrone 24, 16152 Genova, Italy
a r t i c l e
i n f o
Article history: Received 31 July 2016 Received in revised form 21 September 2016 Accepted 22 September 2016 Available online xxx Keywords: Ellipsometry Solid-state dewetting Thin Au films Plasmonics
a b s t r a c t We report the design and testing of a small, high vacuum chamber that allows real-time, in situ spectroscopic ellipsometry (SE) measurements; the chamber was designed to be easily inserted within the arms of a commercial ellipsometer. As a test application, we investigated the temperature-induced solid-state dewetting of thin (20 to 8 nm) Au layers on Si wafers. In situ SE measurements acquired in real time during the heating of the samples reveal features that can be related to the birth of a localized surface plasmon resonance (LSPR), and demonstrate the presence of a temperature threshold for the solid-state dewetting. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Spectroscopic ellipsometry (SE) is extensively used for realtime, in situ monitoring of many processes in materials science and related fields, including thin film and nanoparticles growth [1–3], molecular adsorption [4–6], biomolecular interactions [7,8], and surface modification [9,10]. SE is well suited for the task, since it is non-destructive, applicable in many – even aggressive – ambients, and relatively fast [11]. In situ and real-time spectroscopic ellipsometry can follow two different experimental approaches. In the most common option, the ellipsometer arms are mounted on two windows of the experimental chamber [12,13], practically dedicating the ellipsometer to that setup. Alternatively, a roll-on/roll-off setup can be devised, where the chamber fits between the arms of the ellipsometer, without any modification to the instrument; in this way, the chamber can be easily mounted and removed, thus becoming an add-on of the ellipsometer. In this work, we report the realization and testing of a high vacuum chamber, small enough to fit within the arms of a commercial ellipsometer. After describing the characteristics of the chamber and the typical experimental procedures, we will present – as an example of application – real time, in situ SE measurements
∗ Corresponding author. E-mail address: magnozzi@fisica.unige.it (M. Magnozzi).
acquired during the solid-state dewetting of ultra thin Au films deposited on Si wafers. Biocompatibility, good chemical stability, affinity to thiol or selenide molecules [14], and strong plasmonic effects, make thin gold films a popular topic in nanoscience and nanotechnology. In order to reliably produce nanostructured films, a real-time, in situ diagnostic tool is especially useful; for this reason, the dewetting of thin films represents an ideal example to illustrate the capabilities of our chamber. Moreover, since thin Au films are well described in the literature [15,16], they constitute a sensible choice to test our chamber. In our SE measurements we clearly observe the evolution of the dewetting process in real time as a function of the initial Au-layer thickness. In principle, the chamber can be used to study not only the dewetting, but also other processes of clear scientific interest, such as films deposition and nanoparticles alloying [17]. 2. Experimental setup The chamber is based on a cylindrical tube with two KF100 flanges at both ends (see Fig. 1, left). The bottom KF100 flange (a) supports the sample holder, fitted with a heater and two thermocouples; the main body (b) houses several flanges welded at appropriate angles and meant for SE measurements, electrical connections and pumping; the top KF100 flange (c), easily removable for accessing the inner chamber, hosts four flanges that can accommodate a vacuum gauge, gas inlets and sources for thin films
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Please cite this article in press as: M. Magnozzi, et al., Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.09.115
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Fig. 1. Left: photo of the experimental setup, with the chamber (a = bottom flange; b = body; c = top flange) inserted within the arms of the M2000 ellipsometer (d). In the foreground is placed the turbomolecular pump (e). Right: section of the high vacuum chamber.
deposition, according to the needs of the user. Thus, by changing the elements attached to the flanges (windows, gas inlets, sources) it is possible to modify the applications of the chamber (SE at various angle of incidence, controlled atmosphere, thin films deposition). A cross section of the chamber is reported in Fig. 1, right. The ellipsometry measurements can be performed thanks to three pairs of flanges equipped with fused silica windows, that allow SE measurements at 66◦ , 70◦ and 90◦ (transmission) incidence. The chamber was designed to fit within the arms of a Woollam M2000 rotating compensator ellipsometer (d), without any modification to the instrument, similar to what is usually done for in-liquid measurements cells [18]. The design of our chamber is in principle compatible with any ellipsometer with horizontal setup. A base pressure in the 10−8 mbar range can be reached after some hours of pumping without bakeout. A ceramic button heater (HeatWaveLabs) allows to heat the sample holder; in this work, we annealed the samples up to ∼350 ◦ C. The chamber is fixed to the ellipsometer sample stage via a custom-designed, removable support; when mounted, the chamber can be tilted in the horizontal plane, moved in the vertical direction, and shifted in the xy horizontal plane. The sample holder provides independent sample tilting in the xy plane and a limited z travel. The alignment of the chamber with respect to the ellipsometer can be achieved by adjusting the position of the chamber and the sample, so that the light beam forms a 90◦ angle with respect to both input and output windows. During the heating, the sample holder undergoes a small deformation that determines a slight change in the alignment of the light beam. Preliminary tests on Si wafers, conducted to assess this effect, revealed that the misalignment caused only a slight variation in the intensity of the detected light beam; this does not represent a problem because SE measures the change in the polarization state of the light reflected from the sample. The spurious contribution to and due to the possible windows birefringence can be compensated by the procedure recommended by the ellipsometer supplier [19,20]. Once implemented, the difference between the ex situ and in situ spectra are less than 0.1◦ and 0.6◦ for and , respectively, over the whole spectrum.
Fig. 2. Above: (left) and (right) measured in real time and in situ on Sample 1. Below: Difference spectra for (left) and (right).
intensity depend on several factors, including the size, shape and spacing of the nanoparticles, and the substrate [26,27]. For this study, we considered three samples characterized by a different Au thickness: 20 nm for Sample 1, 11 nm for Sample 2, and 8 nm for Sample 3. The Au films were deposited via molecular beam epitaxy and analyzed with AFM to check their as-grown morphology. Then, they were inserted into the chamber and heated to 350 ◦ C with a heating rate of 5 ◦ C/min, while performing SE measurements. The acquisition of a full SE spectrum (245–1700 nm) requires about 0.3 s. The pressure inside the chamber remained below 5 × 10−6 mbar during the whole process. Real-time, in situ SE measurements were acquired at 66◦ of incidence, with a spot size of 8 mm, and at 12.5 ◦ C temperature intervals. Finally, after each sample was cooled and removed from the chamber, it was analyzed again with AFM to investigate the modifications due to the dewetting. SE measurements for Au thickness of 20, 11 and 8 nm are presented in Figs. 2–4. The real time, in situ and spectra are reported as a function of wavelength and temperature in the top panels of each figure. In order to facilitate the inspection of the temperature-induced changes, difference spectra are reported in the bottom panels of Figs. 2–4 as colour scales. The difference spectra were calculated by subtracting the as-grown, room temperature spectrum from the measured spectra at all given temperatures; thus, a negative value in the difference spectra imply a decrease with respect to the room temperature, and vice versa. We also reported the spectra measured at room temperature before and
3. Results and discussion In order to test the chamber, we studied the real-time evolution of the optical response of thin Au films on Si substrates during annealing. The substrates consist of crystalline Si covered by a native oxide layer. The characteristics of thin Au films are well described in the literature; in particular, it is known that, below a critical thickness, Au undergoes a temperature-driven morphological modification (solid-state dewetting) [21–24]. When gold nanoparticles are created as a result of the dewetting, they exhibit a plasmonic resonance in the visible range [25], whose position and
Fig. 3. Above: (left) and (right) measured in real time and in situ on Sample 2. Below: Difference spectra for (left) and (right).
Please cite this article in press as: M. Magnozzi, et al., Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.09.115
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Fig. 4. Above: (left) and (right) measured in real time and in situ on Sample 3. Below: Difference spectra for (left) and (right).
after the annealing, to allow a direct comparison between the three samples (Fig. 5, top). Representative AFM images of Sample 1, 2 and 3 are reported in Fig. 6 (top to bottom), before (left) and after (right) the annealing. In order to better explain and discuss the characteristics of each sample, they will be presented in the following order: Sample 1, Sample 3, and Sample 2. Before the annealing, Sample 1 appears as a grainy layer of gold, with little or no substrate exposed (Fig. 6, top left), a morphology consistent with previous studies [16]. Real time SE spectra reported in Fig. 2 reveal only minor changes of the optical response during the heating; this finding indicates that no dewetting occurred
Fig. 5. Top: In-situ spectra for the three samples before (left) and after (right) the thermal annealing. The red curve is the for bulk Au. Bottom: SE spectra calculated using the Maxwell–Garnett EMA. Inset: 1 and 2 calculated using the Maxwell–Garnett EMA.
Fig. 6. Top: AFM images of Sample 1 before (left) and after (right) the heating. Middle: AFM images of Sample 2 before (left) and after (right) the heating. Bottom: AFM images of Sample 3 before (left) and after (right) the heating.
for the sample under investigation. Such behaviour is expected, because of the combination of initial Au thickness and annealing temperature [28]. The increase in the spectra can be related to a reduction in the roughness caused by thermal annealing [29]. Indeed, a representative AFM image (see Fig. 6, top right) shows that the grains coalesced and, as a result, the surface after the annealing became smoother (surface RMS decreased from 1.5 to 1.0 nm). This roughness reduction has been observed in Ref. [15] for samples with nominal Au thickness ≥14 nm (there defined as “Type II films”), and represents the first step towards the solid-state dewetting of such systems. A representative AFM image of the thinnest sample (Sample 3, 8 nm Au) before the annealing is reported in Fig. 6, bottom left. The morphology is different with respect to the 20-nm thick film, as in this case the Au forms percolated structures and a significant fraction of the substrate is left uncovered. The thermal annealing induced major modifications of its ellipsometric spectra, as seen in Fig. 4, top left: the most relevant feature is the birth of a peak in the around 570 nm for T > 200 ◦ C, accompanied by a decrease of the curve in the IR. In the spectra (Fig. 4, top right) a general decrease is observed starting approximately above 200 ◦ C. For Sample 2, featuring a 11-nm thick Au film, a representative AFM image is reported in Fig. 6, middle left. This image is strongly resemblant of the 8-nm case, with however less substrate exposed. The spectrum before the annealing (Fig. 5, top left) lays between those of Sample 1 and Sample 3. The evolution of and spectra (Fig. 3) shows some similarities to the thinnest case, save a different behaviour of in the IR. Here, we see that increases in the IR to a maximum around 200 ◦ C, and then decreases again for higher temperatures, finally remaining a few degrees below the starting values. Moreover, the decrease of in the IR is accompanied by the formation of a maximum around 570 nm. As for the previous samples, a quick evaluation of the effects of the annealing can be made by comparing the graphs in Fig. 5, top. We now discuss the temperature-driven dynamics of Sample 2 and 3. For 8-nm thick Au films, a thermal dewetting – characterized
Please cite this article in press as: M. Magnozzi, et al., Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.09.115
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by the progressive transition to isolated particles – is expected [28], leading to the birth of a localized surface plasmon resonance (LSPR) [30]. From the observation of real-time, in situ SE measurements during the heating, we observe that the dewetting process has a threshold at about 200 ◦ C (Fig. 4, bottom panels). According to Ref. [15], the initial thickness of Sample 3 implies that no smoothing is expected before the dewetting (“Type I film”). The peak in the around 570 nm has already been associated with a LSPR of Au nanoparticles on SiO2 [31], and can be reproduced by relatively simple models of the system. To this end we created a model composed by three layers: the substrate, the oxide layer and the gold layer. For the first two materials we used library models; the thickness of the SiO2 layer was set to 2.2 nm, a value obtained by fitting the SE spectrum of the substrate before the Au deposition. For the Au film, due to its grainy morphology, we applied a Maxwell–Garnett Effective Medium Approximation (MG-EMA) [32,33], with 24% volume occupied by Au, 76% volume of void, and thickness of 34.0 nm. These parameters were optimized to fit the data, and to preserve the nominal Au thickness (8 nm). The dielectric constants of Au were parametrized according to Etchegoin et al. [34]. As mentioned before, the and curves calculated with this model (Fig. 5, bottom) reproduce the main features observed experimentally at room temperature after the heating; the 1 and 2 of Au calculated with the model show a sharp resonance around 570 nm, a clear sign of the LSPR (Fig. 5, inset). We note that our model represents a LSPR peak in the which is smaller than the one observed experimentally; this is due to the simplifications of the model, and to the size distribution of the gold nanoparticles. To conclude the discussion of Sample 3, we note that the dewetting of the Au film, so far deduced only by the observation of the SE measurements, is confirmed by the AFM image after the annealing (see Fig. 6, bottom right), where isolated gold particles with different shapes are observed (typical size around 100 nm); about 25% of the area is bare substrate, exposed as a result of the Au dewetting. Drawing from the analysis of Sample 1 and 3, we can identify different trends in the evolution of the optical response of Sample 2, and formulate a reasonable hypothesis about its final morphology. First, the increases in the IR starting from 150 ◦ C, suggesting a smoothing of Au; then, from ≈175 ◦ C on, a peak around 570 nm appears; finally, from 200 ◦ C on, the curve decreases in the IR. We note that around 200 ◦ C also the shows significant changes, very similar to those observed for Sample 3; thus, a threshold in the evolution of the Au layer is present also in Sample 2. The two main features of (peak around 570 nm and decrease in the IR) can be attributed to the dewetting process, at least for part of the film; however, the relatively high values of in the IR (see also Fig. 5, top right) suggest that the process was not concluded by the end of the thermal annealing. This guess, based solely on the analysis of the in situ SE spectra, is confirmed by the AFM measurement ex situ (see Fig. 6, middle right): in fact, we see that the Au film has dewetted only partially, thus creating a percolated-like structure and exposing about 15% of the substrate.
4. Conclusions We created a portable, high vacuum chamber for real-time, in situ SE measurements. As a first test of the chamber, we used it to monitor the thermal annealing of three Si wafers covered by Au layers of different thickness (20, 11 and 8 nm). The real time, in situ SE measurements showed that our experimental setup can detect relevant variations in the optical response of the samples. Such variations can be related to the onset of plasmonic resonances and to morphological changes (smoothing, dewetting). Moreover, the real time, in situ SE easily detects any temperature threshold;
this greatly simplifies the manufacturing of samples where a thermal process is involved. The detection of the threshold is made easier by the observation of both and spectra, thus adding information with respect to any reflectivity measurement, where only one quantity is acquired. It is noteworthy that the real-time, in situ SE allows the user to evaluate the key features of the samples while the process is under way, thus enabling effective control over the final result. The possibility to assess the state of dewetting in real time has practical value in the manufacturing of the samples; for example, our qualitative analysis of the SE spectra for Sample 2 allowed us to determine that the Au film did not dewet completely, even before removing the sample from the chamber. From this knowledge, immediate actions can be undertaken: for example, it is possible to raise the temperature in order to complete the dewetting process. Finally, the interpretation of the SE data, which includes an EMA-based optical model for Sample 3, was fully consistent with the ex situ AFM measurements. Acknowledgments We thank Prof. Ornella Cavalleri for support in AFM measurements, Ennio Vigo for technical support and Dr. Marco Palombo for the design of the chamber. Financial support from FIRB (RBAP11ETKA 005) and the Fondazione San Paolo (Project PANLAB) is acknowledged. References [1] E.A. Irene, In situ real-time characterization of surfaces and film growth processes via ellipsometry, in: O. Auciello, A.R. Krauss (Eds.), In Situ Real-time Characterization of Thin Films, John Wiley & Sons, Inc., 2001, pp. 57–104. [2] H.T. Beyene, J.W. Weber, M.A. Verheijen, M.C.M. van de Sanden, M. Creatore, Real time in situ spectroscopic ellipsometry of the growth and plasmonic properties of Au nanoparticles on SiO2 , Nano Res. 5 (8) (2012) 513–520. [3] S.A. Henck, In situ real-time ellipsometry for film thickness measurement and control, J. Vac. Sci. Technol. A 10 (4) (1992). [4] S. Lousinian, S. Logothetidis, In-situ and real-time protein adsorption study by spectroscopic ellipsometry, Thin Solid Films 516 (22) (2008) 8002–8008. [5] S.G. Thakurta, H.J. Viljoen, A. Subramanian, Evaluation of the real-time protein adsorption kinetics on albumin-binding surfaces by dynamic in situ spectroscopic ellipsometry, Thin Solid Films 520 (6) (2012) 2200–2207. [6] R. Synowicki, C.D. Garcia, M.M. Mora, J.L. Wehmeyer, Investigating protein adsorption via spectroscopic ellipsometry, in: D.A. Puleo, R. Bizios (Eds.), Biological Interactions on Materials Surfaces Understanding and Controlling Protein, Cell, and Tissue Responses, Springer, 2009, pp. 19–41. [7] Y.S. Sun, X. Zhu, An ellipsometry-based biosensor for label-free, real-time, and in-situ detection of DNA–DNA and DNA–protein interactions, Chin. J. Phys. 52 (4) (2014) 1398–1406. [8] C. Toccafondi, M. Prato, G. Maidecchi, A. Penco, F. Bisio, O. Cavalleri, M. Canepa, Optical properties of yeast cytochrome c monolayer on gold: an in situ spectroscopic ellipsometry investigation, J. Colloid Interface Sci. 364 (1) (2011) 125–132. [9] F. Weilnboeck, N. Kumar, G.S. Oehrlein, T.Y. Chung, D. Graves, M. Li, E.A. Hudson, E.C. Benck, Real-time measurements of plasma photoresist modifications: the role of plasma vacuum ultraviolet radiation and ions, J. Vac. Sci. Technol. B 30 (3) (2012). [10] S. Munteanu, N. Garraud, J.P. Roger, F. Amiot, J. Shi, Y. Chen, C. Combellas, F. Kanoufi, In situ, real time monitoring of surface transformation: ellipsometric microscopy imaging of electrografting at microstructured gold surfaces, Anal. Chem. 85 (4) (2013) 1965–1971. [11] M. Losurdo, et al., Spectroscopic ellipsometry and polarimetry for materials and systems analysis at the nanometer scale: state-of-the-art, potential, and perspectives, J. Nanoparticle Res. 11 (7) (2009) 1521–1554. [12] D. Yokoyama, C. Adachi, In situ real-time spectroscopic ellipsometry measurement for the investigation of molecular orientation in organic amorphous multilayer structures, J. Appl. Phys. 107 (12) (2010). [13] I.S. Nerbo, S. Le Roy, M. Kildemo, E. Sondergard, Real-time in situ spectroscopic ellipsometry of GaSb nanostructures during sputtering, Appl. Phys. Lett. 94 (21) (2009) 213105. [14] M. Canepa, G. Maidecchi, C. Toccafondi, O. Cavalleri, M. Prato, V. Chaudhari, V.A. Esaulov, Spectroscopic ellipsometry of self assembled monolayers: interface effects. The case of phenyl selenide SAMs on gold, Phys. Chem. Chem. Phys. 15 (27) (2013) 11559–11565. [15] A.B. Tesler, B.M. Maoz, Y. Feldman, A. Vaskevich, I. Rubinstein, Solid-state thermal dewetting of just-percolated gold films evaporated on glass: development of the morphology and optical properties, J. Phys. Chem. C 117 (21) (2013) 11337–11346.
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ARTICLE IN PRESS
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry M. Magnozzi a,∗ , F. Bisio a,b , M. Canepa a a b
OPTMAT Lab, Dipartimento di Fisica, Universitá di Genova, via Dodecaneso 33, 16143 Genova, Italy CNR-SPIN, corso Perrone 24, 16152 Genova, Italy
a r t i c l e
i n f o
Article history: Received 31 July 2016 Received in revised form 21 September 2016 Accepted 22 September 2016 Available online xxx Keywords: Ellipsometry Solid-state dewetting Thin Au films Plasmonics
a b s t r a c t We report the design and testing of a small, high vacuum chamber that allows real-time, in situ spectroscopic ellipsometry (SE) measurements; the chamber was designed to be easily inserted within the arms of a commercial ellipsometer. As a test application, we investigated the temperature-induced solid-state dewetting of thin (20 to 8 nm) Au layers on Si wafers. In situ SE measurements acquired in real time during the heating of the samples reveal features that can be related to the birth of a localized surface plasmon resonance (LSPR), and demonstrate the presence of a temperature threshold for the solid-state dewetting. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Spectroscopic ellipsometry (SE) is extensively used for realtime, in situ monitoring of many processes in materials science and related fields, including thin film and nanoparticles growth [1–3], molecular adsorption [4–6], biomolecular interactions [7,8], and surface modification [9,10]. SE is well suited for the task, since it is non-destructive, applicable in many – even aggressive – ambients, and relatively fast [11]. In situ and real-time spectroscopic ellipsometry can follow two different experimental approaches. In the most common option, the ellipsometer arms are mounted on two windows of the experimental chamber [12,13], practically dedicating the ellipsometer to that setup. Alternatively, a roll-on/roll-off setup can be devised, where the chamber fits between the arms of the ellipsometer, without any modification to the instrument; in this way, the chamber can be easily mounted and removed, thus becoming an add-on of the ellipsometer. In this work, we report the realization and testing of a high vacuum chamber, small enough to fit within the arms of a commercial ellipsometer. After describing the characteristics of the chamber and the typical experimental procedures, we will present – as an example of application – real time, in situ SE measurements
∗ Corresponding author. E-mail address: magnozzi@fisica.unige.it (M. Magnozzi).
acquired during the solid-state dewetting of ultra thin Au films deposited on Si wafers. Biocompatibility, good chemical stability, affinity to thiol or selenide molecules [14], and strong plasmonic effects, make thin gold films a popular topic in nanoscience and nanotechnology. In order to reliably produce nanostructured films, a real-time, in situ diagnostic tool is especially useful; for this reason, the dewetting of thin films represents an ideal example to illustrate the capabilities of our chamber. Moreover, since thin Au films are well described in the literature [15,16], they constitute a sensible choice to test our chamber. In our SE measurements we clearly observe the evolution of the dewetting process in real time as a function of the initial Au-layer thickness. In principle, the chamber can be used to study not only the dewetting, but also other processes of clear scientific interest, such as films deposition and nanoparticles alloying [17]. 2. Experimental setup The chamber is based on a cylindrical tube with two KF100 flanges at both ends (see Fig. 1, left). The bottom KF100 flange (a) supports the sample holder, fitted with a heater and two thermocouples; the main body (b) houses several flanges welded at appropriate angles and meant for SE measurements, electrical connections and pumping; the top KF100 flange (c), easily removable for accessing the inner chamber, hosts four flanges that can accommodate a vacuum gauge, gas inlets and sources for thin films
http://dx.doi.org/10.1016/j.apsusc.2016.09.115 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Magnozzi, et al., Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.09.115
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Fig. 1. Left: photo of the experimental setup, with the chamber (a = bottom flange; b = body; c = top flange) inserted within the arms of the M2000 ellipsometer (d). In the foreground is placed the turbomolecular pump (e). Right: section of the high vacuum chamber.
deposition, according to the needs of the user. Thus, by changing the elements attached to the flanges (windows, gas inlets, sources) it is possible to modify the applications of the chamber (SE at various angle of incidence, controlled atmosphere, thin films deposition). A cross section of the chamber is reported in Fig. 1, right. The ellipsometry measurements can be performed thanks to three pairs of flanges equipped with fused silica windows, that allow SE measurements at 66◦ , 70◦ and 90◦ (transmission) incidence. The chamber was designed to fit within the arms of a Woollam M2000 rotating compensator ellipsometer (d), without any modification to the instrument, similar to what is usually done for in-liquid measurements cells [18]. The design of our chamber is in principle compatible with any ellipsometer with horizontal setup. A base pressure in the 10−8 mbar range can be reached after some hours of pumping without bakeout. A ceramic button heater (HeatWaveLabs) allows to heat the sample holder; in this work, we annealed the samples up to ∼350 ◦ C. The chamber is fixed to the ellipsometer sample stage via a custom-designed, removable support; when mounted, the chamber can be tilted in the horizontal plane, moved in the vertical direction, and shifted in the xy horizontal plane. The sample holder provides independent sample tilting in the xy plane and a limited z travel. The alignment of the chamber with respect to the ellipsometer can be achieved by adjusting the position of the chamber and the sample, so that the light beam forms a 90◦ angle with respect to both input and output windows. During the heating, the sample holder undergoes a small deformation that determines a slight change in the alignment of the light beam. Preliminary tests on Si wafers, conducted to assess this effect, revealed that the misalignment caused only a slight variation in the intensity of the detected light beam; this does not represent a problem because SE measures the change in the polarization state of the light reflected from the sample. The spurious contribution to and due to the possible windows birefringence can be compensated by the procedure recommended by the ellipsometer supplier [19,20]. Once implemented, the difference between the ex situ and in situ spectra are less than 0.1◦ and 0.6◦ for and , respectively, over the whole spectrum.
Fig. 2. Above: (left) and (right) measured in real time and in situ on Sample 1. Below: Difference spectra for (left) and (right).
intensity depend on several factors, including the size, shape and spacing of the nanoparticles, and the substrate [26,27]. For this study, we considered three samples characterized by a different Au thickness: 20 nm for Sample 1, 11 nm for Sample 2, and 8 nm for Sample 3. The Au films were deposited via molecular beam epitaxy and analyzed with AFM to check their as-grown morphology. Then, they were inserted into the chamber and heated to 350 ◦ C with a heating rate of 5 ◦ C/min, while performing SE measurements. The acquisition of a full SE spectrum (245–1700 nm) requires about 0.3 s. The pressure inside the chamber remained below 5 × 10−6 mbar during the whole process. Real-time, in situ SE measurements were acquired at 66◦ of incidence, with a spot size of 8 mm, and at 12.5 ◦ C temperature intervals. Finally, after each sample was cooled and removed from the chamber, it was analyzed again with AFM to investigate the modifications due to the dewetting. SE measurements for Au thickness of 20, 11 and 8 nm are presented in Figs. 2–4. The real time, in situ and spectra are reported as a function of wavelength and temperature in the top panels of each figure. In order to facilitate the inspection of the temperature-induced changes, difference spectra are reported in the bottom panels of Figs. 2–4 as colour scales. The difference spectra were calculated by subtracting the as-grown, room temperature spectrum from the measured spectra at all given temperatures; thus, a negative value in the difference spectra imply a decrease with respect to the room temperature, and vice versa. We also reported the spectra measured at room temperature before and
3. Results and discussion In order to test the chamber, we studied the real-time evolution of the optical response of thin Au films on Si substrates during annealing. The substrates consist of crystalline Si covered by a native oxide layer. The characteristics of thin Au films are well described in the literature; in particular, it is known that, below a critical thickness, Au undergoes a temperature-driven morphological modification (solid-state dewetting) [21–24]. When gold nanoparticles are created as a result of the dewetting, they exhibit a plasmonic resonance in the visible range [25], whose position and
Fig. 3. Above: (left) and (right) measured in real time and in situ on Sample 2. Below: Difference spectra for (left) and (right).
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Fig. 4. Above: (left) and (right) measured in real time and in situ on Sample 3. Below: Difference spectra for (left) and (right).
after the annealing, to allow a direct comparison between the three samples (Fig. 5, top). Representative AFM images of Sample 1, 2 and 3 are reported in Fig. 6 (top to bottom), before (left) and after (right) the annealing. In order to better explain and discuss the characteristics of each sample, they will be presented in the following order: Sample 1, Sample 3, and Sample 2. Before the annealing, Sample 1 appears as a grainy layer of gold, with little or no substrate exposed (Fig. 6, top left), a morphology consistent with previous studies [16]. Real time SE spectra reported in Fig. 2 reveal only minor changes of the optical response during the heating; this finding indicates that no dewetting occurred
Fig. 5. Top: In-situ spectra for the three samples before (left) and after (right) the thermal annealing. The red curve is the for bulk Au. Bottom: SE spectra calculated using the Maxwell–Garnett EMA. Inset: 1 and 2 calculated using the Maxwell–Garnett EMA.
Fig. 6. Top: AFM images of Sample 1 before (left) and after (right) the heating. Middle: AFM images of Sample 2 before (left) and after (right) the heating. Bottom: AFM images of Sample 3 before (left) and after (right) the heating.
for the sample under investigation. Such behaviour is expected, because of the combination of initial Au thickness and annealing temperature [28]. The increase in the spectra can be related to a reduction in the roughness caused by thermal annealing [29]. Indeed, a representative AFM image (see Fig. 6, top right) shows that the grains coalesced and, as a result, the surface after the annealing became smoother (surface RMS decreased from 1.5 to 1.0 nm). This roughness reduction has been observed in Ref. [15] for samples with nominal Au thickness ≥14 nm (there defined as “Type II films”), and represents the first step towards the solid-state dewetting of such systems. A representative AFM image of the thinnest sample (Sample 3, 8 nm Au) before the annealing is reported in Fig. 6, bottom left. The morphology is different with respect to the 20-nm thick film, as in this case the Au forms percolated structures and a significant fraction of the substrate is left uncovered. The thermal annealing induced major modifications of its ellipsometric spectra, as seen in Fig. 4, top left: the most relevant feature is the birth of a peak in the around 570 nm for T > 200 ◦ C, accompanied by a decrease of the curve in the IR. In the spectra (Fig. 4, top right) a general decrease is observed starting approximately above 200 ◦ C. For Sample 2, featuring a 11-nm thick Au film, a representative AFM image is reported in Fig. 6, middle left. This image is strongly resemblant of the 8-nm case, with however less substrate exposed. The spectrum before the annealing (Fig. 5, top left) lays between those of Sample 1 and Sample 3. The evolution of and spectra (Fig. 3) shows some similarities to the thinnest case, save a different behaviour of in the IR. Here, we see that increases in the IR to a maximum around 200 ◦ C, and then decreases again for higher temperatures, finally remaining a few degrees below the starting values. Moreover, the decrease of in the IR is accompanied by the formation of a maximum around 570 nm. As for the previous samples, a quick evaluation of the effects of the annealing can be made by comparing the graphs in Fig. 5, top. We now discuss the temperature-driven dynamics of Sample 2 and 3. For 8-nm thick Au films, a thermal dewetting – characterized
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by the progressive transition to isolated particles – is expected [28], leading to the birth of a localized surface plasmon resonance (LSPR) [30]. From the observation of real-time, in situ SE measurements during the heating, we observe that the dewetting process has a threshold at about 200 ◦ C (Fig. 4, bottom panels). According to Ref. [15], the initial thickness of Sample 3 implies that no smoothing is expected before the dewetting (“Type I film”). The peak in the around 570 nm has already been associated with a LSPR of Au nanoparticles on SiO2 [31], and can be reproduced by relatively simple models of the system. To this end we created a model composed by three layers: the substrate, the oxide layer and the gold layer. For the first two materials we used library models; the thickness of the SiO2 layer was set to 2.2 nm, a value obtained by fitting the SE spectrum of the substrate before the Au deposition. For the Au film, due to its grainy morphology, we applied a Maxwell–Garnett Effective Medium Approximation (MG-EMA) [32,33], with 24% volume occupied by Au, 76% volume of void, and thickness of 34.0 nm. These parameters were optimized to fit the data, and to preserve the nominal Au thickness (8 nm). The dielectric constants of Au were parametrized according to Etchegoin et al. [34]. As mentioned before, the and curves calculated with this model (Fig. 5, bottom) reproduce the main features observed experimentally at room temperature after the heating; the 1 and 2 of Au calculated with the model show a sharp resonance around 570 nm, a clear sign of the LSPR (Fig. 5, inset). We note that our model represents a LSPR peak in the which is smaller than the one observed experimentally; this is due to the simplifications of the model, and to the size distribution of the gold nanoparticles. To conclude the discussion of Sample 3, we note that the dewetting of the Au film, so far deduced only by the observation of the SE measurements, is confirmed by the AFM image after the annealing (see Fig. 6, bottom right), where isolated gold particles with different shapes are observed (typical size around 100 nm); about 25% of the area is bare substrate, exposed as a result of the Au dewetting. Drawing from the analysis of Sample 1 and 3, we can identify different trends in the evolution of the optical response of Sample 2, and formulate a reasonable hypothesis about its final morphology. First, the increases in the IR starting from 150 ◦ C, suggesting a smoothing of Au; then, from ≈175 ◦ C on, a peak around 570 nm appears; finally, from 200 ◦ C on, the curve decreases in the IR. We note that around 200 ◦ C also the shows significant changes, very similar to those observed for Sample 3; thus, a threshold in the evolution of the Au layer is present also in Sample 2. The two main features of (peak around 570 nm and decrease in the IR) can be attributed to the dewetting process, at least for part of the film; however, the relatively high values of in the IR (see also Fig. 5, top right) suggest that the process was not concluded by the end of the thermal annealing. This guess, based solely on the analysis of the in situ SE spectra, is confirmed by the AFM measurement ex situ (see Fig. 6, middle right): in fact, we see that the Au film has dewetted only partially, thus creating a percolated-like structure and exposing about 15% of the substrate.
4. Conclusions We created a portable, high vacuum chamber for real-time, in situ SE measurements. As a first test of the chamber, we used it to monitor the thermal annealing of three Si wafers covered by Au layers of different thickness (20, 11 and 8 nm). The real time, in situ SE measurements showed that our experimental setup can detect relevant variations in the optical response of the samples. Such variations can be related to the onset of plasmonic resonances and to morphological changes (smoothing, dewetting). Moreover, the real time, in situ SE easily detects any temperature threshold;
this greatly simplifies the manufacturing of samples where a thermal process is involved. The detection of the threshold is made easier by the observation of both and spectra, thus adding information with respect to any reflectivity measurement, where only one quantity is acquired. It is noteworthy that the real-time, in situ SE allows the user to evaluate the key features of the samples while the process is under way, thus enabling effective control over the final result. The possibility to assess the state of dewetting in real time has practical value in the manufacturing of the samples; for example, our qualitative analysis of the SE spectra for Sample 2 allowed us to determine that the Au film did not dewet completely, even before removing the sample from the chamber. From this knowledge, immediate actions can be undertaken: for example, it is possible to raise the temperature in order to complete the dewetting process. Finally, the interpretation of the SE data, which includes an EMA-based optical model for Sample 3, was fully consistent with the ex situ AFM measurements. Acknowledgments We thank Prof. Ornella Cavalleri for support in AFM measurements, Ennio Vigo for technical support and Dr. Marco Palombo for the design of the chamber. Financial support from FIRB (RBAP11ETKA 005) and the Fondazione San Paolo (Project PANLAB) is acknowledged. References [1] E.A. Irene, In situ real-time characterization of surfaces and film growth processes via ellipsometry, in: O. Auciello, A.R. Krauss (Eds.), In Situ Real-time Characterization of Thin Films, John Wiley & Sons, Inc., 2001, pp. 57–104. [2] H.T. Beyene, J.W. Weber, M.A. Verheijen, M.C.M. van de Sanden, M. Creatore, Real time in situ spectroscopic ellipsometry of the growth and plasmonic properties of Au nanoparticles on SiO2 , Nano Res. 5 (8) (2012) 513–520. [3] S.A. Henck, In situ real-time ellipsometry for film thickness measurement and control, J. Vac. Sci. Technol. A 10 (4) (1992). [4] S. Lousinian, S. Logothetidis, In-situ and real-time protein adsorption study by spectroscopic ellipsometry, Thin Solid Films 516 (22) (2008) 8002–8008. [5] S.G. Thakurta, H.J. Viljoen, A. Subramanian, Evaluation of the real-time protein adsorption kinetics on albumin-binding surfaces by dynamic in situ spectroscopic ellipsometry, Thin Solid Films 520 (6) (2012) 2200–2207. [6] R. Synowicki, C.D. Garcia, M.M. Mora, J.L. Wehmeyer, Investigating protein adsorption via spectroscopic ellipsometry, in: D.A. Puleo, R. Bizios (Eds.), Biological Interactions on Materials Surfaces Understanding and Controlling Protein, Cell, and Tissue Responses, Springer, 2009, pp. 19–41. [7] Y.S. Sun, X. Zhu, An ellipsometry-based biosensor for label-free, real-time, and in-situ detection of DNA–DNA and DNA–protein interactions, Chin. J. Phys. 52 (4) (2014) 1398–1406. [8] C. Toccafondi, M. Prato, G. Maidecchi, A. Penco, F. Bisio, O. Cavalleri, M. Canepa, Optical properties of yeast cytochrome c monolayer on gold: an in situ spectroscopic ellipsometry investigation, J. Colloid Interface Sci. 364 (1) (2011) 125–132. [9] F. Weilnboeck, N. Kumar, G.S. Oehrlein, T.Y. Chung, D. Graves, M. Li, E.A. Hudson, E.C. Benck, Real-time measurements of plasma photoresist modifications: the role of plasma vacuum ultraviolet radiation and ions, J. Vac. Sci. Technol. B 30 (3) (2012). [10] S. Munteanu, N. Garraud, J.P. Roger, F. Amiot, J. Shi, Y. Chen, C. Combellas, F. Kanoufi, In situ, real time monitoring of surface transformation: ellipsometric microscopy imaging of electrografting at microstructured gold surfaces, Anal. Chem. 85 (4) (2013) 1965–1971. [11] M. Losurdo, et al., Spectroscopic ellipsometry and polarimetry for materials and systems analysis at the nanometer scale: state-of-the-art, potential, and perspectives, J. Nanoparticle Res. 11 (7) (2009) 1521–1554. [12] D. Yokoyama, C. Adachi, In situ real-time spectroscopic ellipsometry measurement for the investigation of molecular orientation in organic amorphous multilayer structures, J. Appl. Phys. 107 (12) (2010). [13] I.S. Nerbo, S. Le Roy, M. Kildemo, E. Sondergard, Real-time in situ spectroscopic ellipsometry of GaSb nanostructures during sputtering, Appl. Phys. Lett. 94 (21) (2009) 213105. [14] M. Canepa, G. Maidecchi, C. Toccafondi, O. Cavalleri, M. Prato, V. Chaudhari, V.A. Esaulov, Spectroscopic ellipsometry of self assembled monolayers: interface effects. The case of phenyl selenide SAMs on gold, Phys. Chem. Chem. Phys. 15 (27) (2013) 11559–11565. [15] A.B. Tesler, B.M. Maoz, Y. Feldman, A. Vaskevich, I. Rubinstein, Solid-state thermal dewetting of just-percolated gold films evaporated on glass: development of the morphology and optical properties, J. Phys. Chem. C 117 (21) (2013) 11337–11346.
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Please cite this article in press as: M. Magnozzi, et al., Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.09.115