Less is More: Improved Thermal Stability and Plasmonic Response in

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Surfaces, Interfaces, and Applications

Less is more: improved thermal stability and plasmonic response in Au films via the use of sub-nanometer Ti adhesion layers William M Abbott, Christopher P Murray, Chuan Zhong, Christopher Smith, Cormac McGuinness, Ehsan Rezvani, Clive Downing, Dermot Daly, Amanda Petford-Long, Frank Bello, David McCloskey, and John F Donegan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21193 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Applied Materials & Interfaces

Less is more: improved thermal stability and plasmonic response in Au films via the use of subnanometer Ti adhesion layers William M. Abbott†, Christopher P. Murray†, Chuan Zhong†, Christopher Smith†, Cormac McGuinness†, Ehsan Rezvani†, Clive Downing†, Dermot Daly†, Amanda K. PetfordLong‡ ‖, Frank Bello†, David McCloskey†, and John F. Donegan†*

†School

of Physics, CRANN, & AMBER, Trinity College Dublin, Dublin 2, Ireland

‡Materials

Science Division, Argonne National Laboratory, Lemont, IL 60439, USA

‖Department

of Materials Science & Engineering, Northwestern University, Evanston, IL

60208, USA

*Corresponding author: [email protected]

KEYWORDS: anti-dewetting, heat-assisted magnetic recording, gold thin films, plasmonics, diffusion

ACS Paragon Plus Environment

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ABSTRACT: The use of a metallic adhesion layer is known to increase the thermomechanical stability of Au thin films against solid-state dewetting, but in turn results in damping of the plasmonic response, reducing their utility in applications such as heatassisted magnetic recording (HAMR). In this work, 50 nm Au films with Ti adhesion layers ranging in thickness from 0 – 5 nm were fabricated, and their thermal stability, electrical resistivity and plasmonic response were measured. Sub-nanometer adhesion layers are demonstrated to significantly increase the stability of the thin films against dewetting at elevated temperatures (>200oC), compared to more commonly-used adhesion layer thicknesses that are in the range of 2 - 5 nm. For adhesion layers thicker than 1 nm, the diffusion of excess Ti through Au grain boundaries and subsequent oxidation was determined to result in degradation of the film. This mechanism was confirmed using transmission electron microscopy and X-ray photoelectron spectroscopy on annealed 0.5 nm and 5 nm adhesion layer samples. The superiority of sub-nanometer adhesion layers was further demonstrated through measurements of the surface-plasmon polariton resonance; those with thinner adhesion layers possessed both a stronger and spectrally


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sharper resonance. These results have relevance beyond HAMR to all Ti/Au systems operating at elevated temperatures.

INTRODUCTION

Au is a highly prized material in industrial applications, due to its combination of high electrical conductivity, strong plasmonic response1, and chemical stability2. Its resistance to oxidation, however, has the undesirable effect of making Au bond poorly with oxide substrates3. The use of thin oxidizing metal adhesion layers is a common practice to improve the bonding of Au thin films to oxide substrates4–7, but results in a damped plasmonic response8,9. For applications that require both thermal stability and optimized plasmonic properties, such as the developing field of thermo-plasmonics10 and in the commercialization of heat-assisted magnetic recording (HAMR)11, this is highly limiting. The level of damping is reduced when using thinner layers8,12, but it has been shown that sputtered Ti does not form a continuous film until it reaches a thickness of 1 - 2 nm13. Recent work on organic monolayers, such as (3-mercaptopropyl)trimethoxysilane8, have


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shown that it is possible to improve Au/SiO2 adhesion without sacrificing plasmonic response, but these organic materials will degrade at elevated temperatures14, limiting their utility for high temperature plasmonic applications.

In this work, we study the thermal stability and plasmonic performance of Ti (x) nm/Au 50 nm thin films, where 0 ≤ x ≤ 5 nm. Samples were fabricated using DC magnetron sputtering on quartz and SiO2 (500 nm)/Si wafers. Deposition rates as low as 0.08 Å s-1 allowed for timed depositions of Ti layers to a nominal accuracy of +/- 0.1 Å. The target thicknesses in this work are based on deposition rates calculated for thicker films, not actual measurements, and should be regarded as nominal. Previous work has shown that the system can reliably produce sub-nanometer film thicknesses critical in the production of magnetic tunnel junction devices15.

The resistance of the films against solid-state dewetting was investigated by measuring the back-reflected signal from a 488 nm cw-laser heat-source. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), energy dispersive X-rays (EDX) and tape tests were used to


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characterize the Ti/Au films and to elucidate the mechanisms of adhesion for the different adhesion layer thicknesses. Finally, the surface-plasmon polaritons (SPPs) of the films were measured and compared pre- and post-annealing using a Kretschmann configuration, in order to confirm the improved response gained from using thinner adhesion layers.

RESULTS AND DISCUSSION

Solid-state dewetting is a thermally activated process where films act to minimize surface energy by material diffusion16. Its dynamics depend on parameters such as interfacial surface energy3, film thickness17,18, and temperature19, and it is often studied using either ex-situ statistical processing of images20, or using a separate probe laser measuring changes in film transmittance as a function of annealing time19. The control of solid-state dewetting is essential to bring HAMR to commercial realization. We have


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developed an optical technique which allows in-situ measurement of dewetting dynamics of sputtered thin films. A schematic of the back-reflected laser signal technique is displayed in Figure 1a. The use of a localized micron-scale heat source, namely a focused 488 nm CW-laser, closely represents the expected heat sources and resultant thermal gradients in applications such as HAMR. The focused laser spot was used to locally induce solid-state dewetting (which would in turn cause a local reduction in the film reflectivity, as depicted in Figure 1b), and the resultant reduction in the back-reflected laser signal over time was used to show the progress of solid-state dewetting17.

Figure 1: (a) schematic diagram detailing the back-reflected laser signal technique used


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to evaluate film stability against solid-state dewetting. (b) schematic diagram depicting the progress of solid-state dewetting in the laser-annealed Au film, and how the reflectivity of the film will be affected in turn.

The results obtained from the dewetting measurements are shown in Figure 2. Figure 2a shows the averaged normalized back-reflected laser signals obtained for an absorbed power of 25 mW. As expected, the Au film without any Ti adhesion layer suffered the greatest damage, its reflectivity dropping very quickly as the film dewetted. Counterintuitively, the back-reflected laser signal for Au films with standard 2 and 5 nm Ti adhesion layers decrease far quicker than those whose adhesion layers are ≤ 1 nm. To allow for a quantitative comparison between the different samples, a characteristic dewetting time, t1/2, is extracted from the data by measuring the time at which the reflectivity reached 50% of its initial value. Figure 2b shows the t1/2 values measured for 50 nm Au films with different Ti adhesion layer thicknesses at absorbed powers of 20 mW, 25 mW, and 30 mW, resulting in maximum temperature increases of 195 K, 244 K,


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and 292 K, respectively. The maximum temperature increase in the film induced for each absorbed power was calculated using;

(1)

where Pabs is the absorbed power, w0 is the beam waist, and 𝜅 = 𝜅𝑠 +2

( )𝜅 ℎ𝑓

𝑤0

𝑓

is the

effective thermal conductivity of the system, taking into account the thickness of the film (hf) relative to w0, and the thermal conductivities of the film and substrate (κf and κs, respectively)17. The t1/2 values increase as the adhesion layer thickness decreases down to 0.5 nm, below which the t1/2 values decrease again. This result is highly unexpected, as it shows that the standard thicknesses (2 - 5 nm) for Ti adhesion layers are far from optimal in terms of maximizing the long-term stability of Au films. The decrease in stability for adhesion layer thicknesses below 0.5 nm can likely be attributed to incomplete coverage of Ti on the SiO2 surface.


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Figure 2: Measurements of dewetting times for different Ti/Au systems. (a) degradation curves obtained for 50 nm Au films with varying thickness of Ti adhesion layer, at 25 mW absorbed power; (b) plot of dewetting half-life (t1/2) against Ti adhesion layer thickness for the above films.


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SEM images showing a comparison of the irradiated areas for the Au films with 0 nm to 5 nm adhesion layers can be observed in Figure 3. The extent of dewetting damage for each area is consistent with the previous results; adhesion layers of thickness  0.5 nm result in smaller dewetted areas for a given absorbed power. For the Ti 5 nm/Au 50 nm film, the grain boundary contrast is stronger in areas close to the irradiation centre. This is indicative of grain boundary grooving, caused by the onset of dewetting19. Also, a higher density of hillocks (areas of increased film height) can be seen, suggesting a greater degree of compressive stress after annealing21. AFM measurements of the films showed that the surface roughness of the Ti 5 nm/Au 50 nm had increased by a factor of 2 after annealing (Figure S1).


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Figure 3: SEM images of 50 nm Au films with varying thickness of Ti adhesion layer, pre- and post-laser irradiation. In addition to larger dewetted areas, films with thicker Ti adhesion layers show evidence of grain-boundary grooving and hillock-formation.

Further evidence for the stabilization effects of sub-nanometer Ti adhesion layers was found via electrical resistivity measurements on the Au films, as shown in Figure 4. 50 nm Au films with varying thickness of adhesion layer were annealed at 250oC for up to 10 hours in air, and their film resistivity was measured using a 4-point probe technique at different times during the annealing process. Both the 50nm Au and the Ti 0.5 nm/Au 50nm films showed a c.10% reduction in film resistivity over the annealing period, most likely due to the annealing out of defects. In contrast, the Ti 5 nm/Au 50 nm film showed a steady increase in resistivity: after 10 hrs of annealing, the resistivity had increased by ~40%. To understand the difference in behavior between the samples, their morphology was investigated using SEM.


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Figure 4: Resistivity measurements of Ti/Au films, annealed at 250oC for 10 hrs.


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Figure 5. SEM images of 50 nm Au, 0.5 nm Ti/50 nm Au, and 5 nm Ti/50 nm Au films pre-anneal (a - c) and post-anneal at 250oC for 10 hours in air (d - f), respectively.

Figure 5 shows the SEM images of Ti (x)/ Au 50 nm films where x = 0, 0.5 & 5 nm. Prior to annealing, the grain size for each film appears roughly the same at c. 40 nm diameter (Figure 5a, 5b & 5c). Upon annealing, the Au grain size increases substantially for the sample with no adhesion layer. Grain growth in the Ti (0.5 nm and 5 nm)/Au 50 nm films is comparatively minor upon annealing, but the thicker Ti 5 nm/Au 50 nm film appears to have begun to undergo solid-state dewetting, with dark (i.e. lower atomic number) intergrain features becoming apparent. Even so, both annealed Ti (0.5 nm & 5 nm)/Au 50 nm


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samples survived an ASTM Standard D3359-09 tape test with 0% film removal. In contrast, the annealed Au 50 nm film without Ti adhesion layer was completely removed from the substrate (Figure S2).

To understand the nature of the features in Figure 5, electron transparent lamellae were cut from the annealed samples shown in Fig. 5e and 5f. The TEM analysis of these lamellae are shown in Figure 6. For the Ti 0.5 nm/Au 50 nm system, it can be seen that after 10 hours of annealing at 250°C in air, there are no signs of dewetting or changes in film quality (Figure 6a and 6b). The composition map of the Ti EDX signal in Figure 6c shows that the Ti remains confined to the adhesion layer region to within a thickness of 2 nm. In stark contrast, significant changes occur in the case of the Ti 5 nm/Au 50 nm system. TEM analysis shows that the titanium layer is not confined to a flat layer and has, in places, diffused up into the Au grain boundaries (Figure 6d). The presence of many features with different contrast throughout the thickness of the Au layer can be seen in the high-angle annular dark-field STEM (HAADF-STEM) image in Figure 6e. An EELS map of this area, seen in Figure 6f, shows that such features contain a significant amount


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of Ti, confirming that Ti has diffused through the Au grain boundaries all the way to the surface of the Au layer in some places. Close scrutiny of the SiO2/Ti interface in these images reveals an interesting deformation of the SiO2 substrate where Ti has diffused above it. In particular, the Si EELS map in Figure 6g shows that the SiO2 substrate has been pulled upwards. This is not due to silicide formation as there is no overlap with the Ti EELS signal in Figure 6f. It is known that oxidation of Ti results in significant volume expansion, leading to compressive stress in Ti/Pt films21. This stress could account for the substrate deformation observed in this sample. Further EELS analysis suggests almost all of the 5nm Ti layer has been converted to TiOx, even far from Au grain boundaries (Figure S3). This is expected given the exposure of the lamella to air. This analysis also indicates that the small crystalline particles observed in the Figure 6d are remnants of the metallic Ti adhesion layer.


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Figure 6: Cross-sectional TEM analysis of 50 nm Au films annealed at 250°C with nominal 0.5 nm and 5 nm Ti adhesion layers. (a) high-resolution TEM image of Ti 0.5 nm/Au 50 nm sample cross-section; (b) HAADF-STEM cross-sectional image of Ti 0.5 nm/Au 50 nm sample. (c) EDX mapping of Ti signal in an area in (b), with shade-bar indicating Ti signal; (d) highresolution TEM image of Ti 5 nm/Au 50 nm sample cross-section; (e) HAADF-STEM crosssectional image of Ti 5nm/Au 50 nm sample; (f) & (g) show EELS maps of Ti & Si respectively; (h) shows an EDX map of Au signal. Note that some deformation of the SiO2 substrate has occurred where the Ti has diffused through the Au grain boundary.


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In Figure 7, XPS data for Au films annealed at 250oC for 10 hours with different Ti adhesion layer thicknesses are shown. The Ti 5 nm/Au 50 nm film shows a Ti 2p3/2 peak at 458.9 eV (Figure 7a), which conversely, is not observable for the Ti 0.5 nm/Au 50 nm film. Fitting the Ti 2p3/2/2p1/2 core levels with Gaussian-Lorentzian line-shapes (Figure 7b) gives a value for the Ti 2p3/2 binding energy of 458.9±0.1 eV, with a position and spinorbit splitting of 5.64 eV consistent with Ti(IV)O222, and at a ratio of 1:2 with the O 1s component observed at 531.0 eV binding energy, again consistent with oxygen in Ti(IV)O222. This is further confirmation that Ti has diffused through the Au grain boundaries, and has oxidized as TiO2 when observed upon the surface.

Figure 7c shows the angle-resolved XPS (ARXPS) measurements for the Ti 5 nm/Au 50 nm sample. ARXPS measurements allow for measuring integrated XPS signals from a depth upwards to the surface, where the sampling depth is determined by the photoelectron take-off angle (TOA) and by the inelastic mean free path (IMFP) of the outgoing photoelectrons in the material. In normal photoemission geometry, i.e. a TOA of 90° to the surface, the sampled depth is at a maximum, while for lower TOA the integrated


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XPS signal is from a shallower maximal depth and hence is a more surface sensitive measurement. Firstly in Figure 7c, the Ti 2p peak does not decrease as the TOA decreases (where ARXPS becomes more surface sensitive), again clearly indicating that Ti has diffused to the surface of the Au film and is more prevalent there than in the more bulk sensitive measurement. Plots of the relative intensities of Ti, O, and Au peaks as a function of TOA (Figure 7d), show that at the lowest angles, the areal intensities of the Ti 2p and TiO2 O 1s peaks have increased but have done so while consistently exhibiting a 1:2±0.1 Ti:O stoichiometry, conclusively demonstrating the presence of TiO2 at the film surface and in the near-surface regions. At the lowest TOA of 40°, the increased ratio of TiO2 signal to Au signal clearly indicates a greater proportion of TiO2 either on the surface or in the sampled near surface layer as compared to the higher TOA spectra of 90°. The variation of the ARXPS via a slow non-uniform decrease of TiO2:Au ratio on going to higher TOA is inconsistent with a distribution of TiO2 as if there were only an ultrathin layer on the surface. Further, the constancy of the observed Ti:O stoichiometry ratio implies even at normal emission geometry (TOA=90°), the greatest depth to which the (AR)XPS samples, that it is still TiO2 at this furthest sampling depth from the surface


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region. This maximal sampling depth may be estimated from the IMFP, for O 1s or Ti 2p photoelectrons in Au photoionised with Al Kα excitation which are 1.33 or 1.40 nm respectively23–25, where 86% of photoelectrons being emitted are obtained from within a depth less than twice the IMFP, if these elements were uniformly distributed with depth throughout the sampling volume. In contrast, the shallowest TOA (40°), the depth from which 86% of such photoelectrons are sampled from is a maximal depth of 1.70 nm. Thus, at least to the depths and subsurface region sampled by the ARXPS, the data is consistent with the proposition that TiO2 throughout the volume of the Au film and is present away from the surface as it coats the walls of the fissures observed in the microstructure of the de-wetted Ti 5 nm/Au 50 nm film, these fissures being directly imaged via the cross-sectional TEM (see Figure 6f). Unlike the cross-sectional TEM of the bottom adhesion layer (Figure S3), no metallic Ti 2p signal is observable in the XPS, either at normal incidence, potentially viewing any part of the exposed adhesion layers, or at any other angle in the ARXPS data.


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Figure 7: XPS measurements of Ti/Au films annealed for 10 hours at 250oC in air. (a) Survey XPS spectra for 50 nm Au films with Ti adhesion layer thicknesses of 0.5nm (red) and 5nm (black). C 1s peak and majority of O 1s peak is due to presence of organic contaminants on the film surface. (b) fitting of Ti 2p peaks observed in Ti 5nm/Au 50nm sample. (c) ARXPS spectra of the Ti 5nm/Au 50nm film (d) plot of relative peak percentages for Au 4d, Ti 2p, and TiO2 O 1s (531.0 eV) peaks seen in (c), as a function of takeoff angle.

Finally, to evaluate the plasmonic performance of varying adhesion layer thickness, the surface plasmon polariton (SPP) response was measured for the Ti/Au films through


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a Kretschmann configuration. In Figure 8a, the reflectivity data as a function of the input angle at an excitation wavelength of 633 nm for as-deposited 50 nm Au films with varying Ti adhesion layer thickness is shown. The reflectivity was taken as the ratio of the amplitudes of the p-polarized (Rp) and s-polarized (Rs) component of the input light. It is advantageous to use this technique rather than measuring the reflectivity Rp directly in photometry, as any fluctuations in the light source intensity or walk-off from the sample are removed through normalization. As the lines are clearly asymmetric, we use the halfwidth-half-maximum (HWHM) on the broader side of the line as a measure of the plasmon resonance. The deepest reflectivity dip is for the pure Au film, as expected. Next comes the 0.5 nm and 1 nm adhesion layer samples, indicating a stronger excitation of the plasmonic mode than for thicker adhesion layers. Once the films have been annealed at 250oC for 10 hours in air, the stability enhancement of the 0.5 nm adhesion layer becomes more apparent, the reflectivity depth decreases significantly for the Ti 5 nm/Au 50 nm, whereas it increases slightly for the Ti 0.5 nm/Au 50 nm film. This improved plasmonic performance after annealing for the 0.5 nm Ti adhesion layer is likely due to defect reduction in the Au, which will reduce the number of scattering points within the film

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As the adhesion layer thickness decreases, so too does the HWHM of the resonance, from a value of 0.72° for the 5 nm adhesion layer, to a value of 0.53° for the 0.5 nm adhesion layer (Figure 7b). This result is consistent with previous studies on varying the adhesion layer thickness in plasmonic systems8,27. Additionally, after annealing, the Ti 0.5 nm/Au 50 nm film is the only system to exhibit a narrowing of the HWHM, which decreases to a value of 0.43°. The HWHM broadened for each of the other films investigated, due to the Ti diffusion through the grain boundaries.


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Figure 8: (a) reflection of surface plasmon polariton as a function of input angles at an excitation light wavelength of 633 nm for 50 nm Au films with Ti adhesion layers ranging from 0 nm to 5 nm in thickness. (b) reflection of surface plasmon polariton as a function of input angles at an excitation light wavelength of 633 nm for 50 nm Au films with 0.5 nm and 5 nm Ti adhesion layers, pre- and post-annealing at 250oC for 10 hours. (c) plot of the SPP HWHM as a function of Ti adhesion layer thickness, pre- and post-anneal.

It has previously been established that Ti adhesion layers oxidize partially even when deposited in high vacuum conditions, with oxygen originating from both the deposition chamber walls and the substrate surface4. Despite this, the partially oxidized Ti layer maintains good chemical reactivity with both substrate and Au layer. It has long been observed that the adhesion of oxygen-active metals to oxide surfaces is better if the deposition is started in a poor vacuum. It is a delicate balance, however, as total oxidation passivates the Ti surface leading to poor adhesion28,29. While controlling the extent of Ti layer oxidation at the nano-scale is beyond the scope of this work, it is not unreasonable to speculate that the Ti layers at the substrate interface are more oxidized, and therefore


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less mobile when thermally activated, than those at the Au interface, especially for thicker Ti layers13.

Diffusion of Ti through Au at elevated temperatures has been studied for Ti layers (10 200 nm)

5,6,30.

At temperatures above 175oC, Ti will diffuse into Au thin films6, mainly

through the grain boundaries5 due to the activation energy for grain boundary diffusion being lower than that for volume diffusion31. Eventually this Ti will reach the surface and oxidize to form TiO232. The resulting volume expansion causes compressive stress within the film33, which increases the tendency towards dewetting34. Stresses within thin films have previously been shown to induce dewetting, even at room temperature35. From our observations we conclude that this mechanism is also responsible for the poor dewetting performance of our Ti 5 nm / Au 50 nm films at elevated temperatures. However, when the thickness of the Ti adhesion layer is reduced to a nominal thickness of 0.5 nm, there is evidently sufficient material to allow for effective wetting and adhesion of the Au layer. As the Ti layer is probably substantially oxidized upon deposition, little or no excess Ti remains to diffuse through the Au grain boundaries resulting in optimal dewetting stability.


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When Ti < 0.5 nm layers are used, dewetting stability again decreases. This is probably due to incomplete surface coverage and ineffective wetting of the Au layer.

CONCLUSIONS

In this work, we have demonstrated that an improved thermal stability can be obtained in Au thin films via the use of sub-nanometer Ti adhesion layers. This increase in stability is due to less Ti being available to diffuse through the Au grain boundaries. For the thicker Ti adhesion layers, diffusion of Ti through the Au grain boundaries causes the Ti to reach the surface and oxidize. This leads to volume expansion and compressive stress within the Au film, which enhances Au dewetting when thermal energy is provided to the system. The optimal Ti layer thickness is nominally 0.5 nm, with thinner layers being insufficient to form a continuous film. The 0.5 – 1nm nm adhesion layer samples also gave the best SPP response. In fact the 0.5 nm adhesion layer even improved upon annealing. This work demonstrates that for high-temperature applications in plasmonics and electronics, one need not choose between thermal stability and performance, as both can be achieved via the use of sub-nanometer Ti adhesion layers. The results will have an important


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impact on the realization of HAMR as a commercial technology, and also for applications that require Ti/Au films to retain their properties at elevated temperatures.

EXPERIMENTAL METHODS

Film deposition and thickness characterisation: Films were deposited using a 6-source SHAMROCK 19608 DC sputtering system with a base pressure of 5 x 10-7 Torr. Si/SiO2 and quartz substrates were cleaned using acetone and IPA, followed by oxygen plasma clean prior to deposition. Samples rotate on a planetary stage above the source at 44 rpm. Timed calibration depositions were undertaken for Ti and Au, and resulting film thicknesses were measured using X-ray reflectivity (Phillips X’pert Pro XRD with Cu k radiation). Using these results to calculate deposition rates, films were deposited at ambient temperature in mTorr Ar partial pressures producing stable deposition rates ranging from 0.08-0.36 Å s-1.


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Laser-induced dewetting measurements: Absorption spectra for the films were obtained using a Perkin Elmer UV-Vis Spectrophotometer. For each film, the reflectivity and transmission was measured, which then allowed the absorption to be calculated from the relation A = 100 – R – T. This ensured that the absorbed power was consistent across all samples.

Dewetting curves were obtained using a customized microscope system17. A COHERENT INNOVA 90C Ar+ laser operating at 488 nm was directed into the back of an Olympus Plan 0.4NA microscope objective in order to focus the laser (w0 = 1.8 ± 0.1 μm) onto the sample. As the sample is irradiated, the film will undergo solid-state dewetting which will result in a decrease in the local reflectivity of the film. The back-reflected laser signal, after moving back through the objective, passes through a beamsplitter (Thorlabs) which directs it onto a photodiode (Thorlabs), allowing the local change in reflectivity to be monitored. For each sample, 5 measurements for a given absorbed power were taken and then averaged. Standard error was calculated using OriginLab. A Point Grey GXFW-2885M-C CCD-camera was used to position the laser spot and to measure w0.


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Scanning electron microscopy: SEM images were obtained using a Zeiss ULTRA scanning electron microscope equipped with a GEMINI FESEM column capable of 1 nm resolution at 15 kV, using the SE2 detector. The beam voltage was 5 kV for all images.

4-point probe: Film resistivity measurements were performed using an Alessi Industries 4-point probe stage connected to a Keithley 2401 Sourcemeter, typically supplying 50 mA in 4-wire mode.

X-ray photoelectron spectroscopy: XPS was carried out in ultra-high vacuum (

Less is More: Improved Thermal Stability and Plasmonic Response in

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