Silver Film Surface Modification by Ion Bombardment Decreases

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Silver film surface modification by ion bombardment decreases surface plasmon resonance absorption David M. Fryauf, Juan Jose Diaz Leon, Andrew C Phillips, and Nobuhiko Paul Kobayashi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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ACS Applied Materials & Interfaces

Silver film surface modification by ion bombardment decreases surface plasmon resonance absorption

a

David M. Fryaufa, Juan J. Diaz Leona, Andrew C. Phillipsb, Nobuhiko P. Kobayashia Nanostructured Energy Conversion Technology and Research (NECTAR), University of California Santa Cruz, Santa Cruz, CA 95064, USA b University of California Observatories, Univ. of California, Santa Cruz, CA, USA 95064

Corresponding author email: [email protected]

ABSTRACT Silver thin films covered with dielectric films serving as protective coatings are desired for telescope mirrors, but durable coatings have proved elusive. As part of an effort to develop long-lived protected-silver mirrors, silver thin films were deposited by electron beam evaporation using a physical vapor deposition system at the University of California Observatories Astronomical Coatings Lab. The silver films were later covered with a stack of dielectric films utilizing silicon nitride and titanium dioxide deposited by ion assisted electron beam evaporation to fabricate protected mirrors. In-situ argon ion bombardment was introduced after silver deposition and prior to the deposition of dielectric films to assess its effects on the performance of the mirrors. We found that ion bombardment of the silver influenced surface morphology and reflectivity, and these effects correlated with time between silver deposition and ion bombardment. The overall reflectivity at wavelengths in the range of 350nm – 800nm was found to improve due to ion bombardment, which was qualitatively interpreted as a result of decreased surface plasmon resonance coupling. We suggest that the observed decrease in coupling is caused by silver grain boundary pinning due to ion bombardment suppressing silver surface diffusion, forming smoother silver-dielectric interfaces. Keywords: silver mirror, e-beam evaporation, ion bombardment, grain boundary pinning, surface plasmon resonance, barrier layers

1. INTRODUCTION Silver (Ag) thin films have been routinely studied, applied, and commercialized in a wide variety of industries. One of the older and more routine applications of Ag thin films is mirror making. Although vacuum-deposited aluminum has become the standard material for mirrors, astronomical observatory telescope mirrors can benefit from silver’s higher reflectivity and lower emissivity in the infrared compared to aluminum1. While Ag thin films are routinely deposited for various optical and electronic applications in which the surrounding materials naturally protect the Ag thin film, bare Ag quickly tarnishes due to oxidation with sulfur or formation of halide salts. Long-lasting Ag mirrors must be protected (i.e., Ag-based protected mirror) by barrier overlayers of transparent dielectrics to prevent tarnish and corrosion. The University of California Observatories has undertaken a program to develop and/or identify high-performance coatings useful for astronomical optics (Phillips et al.2-5). A strong motivation for our research is to develop durable Ag-based protected mirrors that meet the requirements of the Thirty-Meter Telescope (TMT) project, which requires high reflectivity in the spectrum 0.34 < ߣ < 28µm6. Protected Ag mirrors - mirrors made of Ag covered with dielectric films - suffer loss of reflectivity due to the finite coefficient of extinction of the dielectric films and destructive interference within them. In addition, surface plasmon resonance (SPR) is another source of loss of reflectivity. Absorption and scattering from surface plasmons exist in metals due to collective electron oscillations at the surface interface between the negative permittivity metal and the positive permittivity dielectric material. Resonance is established when electromagnetic radiation incident on the metal surface matches the surface plasmon frequency. SPR amplitude and frequency are dependent on the dielectric function of the Ag, the material adjacent to the Ag, and roughness at the Ag/dielectric interface7-9. Localized SPR absorption is widely employed in surface enhanced Raman spectroscopy (SERS) detection applications utilizing, for instance, nanostructured Ag and Ag nanoparticles10. Localized “hot spots” at sharp edges and corners of Ag nanostructures increase coupling efficiency between incident light and surface plasmons7. Relevant to thin films, the coupling strongly

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depends on the roughness of Ag/dielectric interfaces. For example, if an atomically smooth Ag film is covered with a dielectric film (and the atomically flat Ag/dielectric interface is preserved after dielectric deposition), then the coupling is expected to be negligible. If an Ag thin film is rough with grains that form “hot spot” features above the planar film surface, then the resulting Ag/dielectric interface is not atomically flat after the deposition of a dielectric film and coupling will be more efficient, resulting in increased SPR absorption and lower reflectivity. While many users and producers of Ag thin films, including TMT coating guidelines, recognize the undesired SPR absorption due to the presence of large Ag grains and resulting rough Ag surfaces6, key parameters such as chamber pressure, substrate temperature, and deposition rate in the deposition of Ag thin films have been identified as controlling factors of Ag thin film morphology11-13. Post-deposition argon ion (Ar+) bombardment is often applied to remove contamination on the surface of Ag thin films to condition surfaces (e.g., increase or decrease surface free energy) prior to a subsequent material coating ex-situ Ag deposition, such as for SERS detection applications14,15. Effects on surface morphology after post-deposition Ar+ bombardment have been studied for single crystal and polycrystalline Ag films16,17, and optical properties of Ag films have been studied in-situ during Ar+ bombardment of the Ag surface18. Effects of ion bombardment on film growth, surface modification, and interface engineering have been studied with extensive materials systems using different ion beam equipment19. However, to the best of the authors’ knowledge, postdeposition Ar+ bombardment has not been discussed in the application of improving optical properties of protected Ag by decreasing SPR absorption. In this work, we observe surface diffusion and grain growth of e-beam evaporated Ag thin films in vacuum at room temperature immediately following deposition. Reflectivity performance of Ag surfaces with/without Ar+ exposure are correlated with scanning electron microscopy (SEM) and atomic force microscopy (AFM) characterization of the respective thin film surfaces in order to analyze the possible effects of Ar+ exposure. Measured reflectivity of protected mirror stacks with dielectric materials on Ag films with larger grains show reflectivity loss as a result of SPR absorption coupling with rough features. We report improved Ag mirror reflectivity near the SPR by exposing Ag thin films to Ar+ bombardment in vacuum immediately after deposition and prior to being coated with protective dielectric overlayers.

2. SAMPLE PREPARATION AND EXPERIMENTAL DETAILS Three sets of samples were prepared in the UCO PVD coating chamber, using e-beam deposition. All samples were on glass microscope slide substrates. The UCO chamber allows for up to four groups of samples to be coated at one time, with three groups shielded behind baffles and the fourth exposed to the e-gun plume and ion source. Each set of samples was produced in the same vacuum pump-down. Each sample started with an adhesion layer of 22nm reactivelydeposited Y2O3; one set (samples 2A and 2B) had additional layers of YF3 and thin Al2O3 under the Ag, which appear to help the durability of protected-Ag coatings. Then a 120nm thick layer of Ag was deposited. Silver deposition was done at p150nm. These larger grains are high aspect ratio grains which are significantly wider in the x-y plane than the average Ag film thickness of 120nm. Such high-aspect ratio Ag grains are not expected to form in aligned parallel plane with the Ag thin film surface. Therefore, high-profile sharp edges and “hot spots” exist at the boundaries of these larger grains (observable in Fig. 1a). Note that the magnification of Fig. 1 SEM images is nearly double that of the AFM scans in Fig. 2 to better highlight the larger grain edge features.



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Fig. 1. SEM images of a) sample 1A with untreated Ag surface and b) sample 1B with Ag surface after 45 seconds of Ar+ exposure.

Representative AFM images collected from samples 1A and 1B, shown in Fig. 2 reveal substantial differences in surface morphology and roughness between the two samples in agreement with SEM images. The film with no ion bombardment exhibits a rougher surface than Ag exposed to Ar+ with features larger in both x-y and z dimensions. These more distinct z-dimensional features on the untreated Ag surface appear random without any periodicity, and the high density of sharp edges is expected to provide coupling sites of increased SPR absorption and scattering when an adjacent dielectric film is deposited.

Fig. 2. 10µm AFM scans of a) sample 1A with untreated Ag surface and b) sample 1B with Ag surface after 45 seconds of Ar+ exposure.


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Fig. 3. Average grain size histogram of AFM scans of sample 1A with untreated Ag surface (black) and sample 1B with Ag surface after 45 seconds of Ar+ exposure (red), plotted on a semi-log scale to emphasize the difference in large grain size distribution.

Comparing surface morphologies of the two bare Ag films suggests that 120nm Ag deposited by e-beam evaporation forms an inherently rough surface with a wider size distribution of larger grains, clearly indicating that exposure of the Ag film surface to Ar+ bombardment in vacuum alters the as-deposited Ag surface. On sample 1A, the untreated Ag film in Fig. 2a, the rms roughness ‫ݎ‬௥௠௦ was measured to be 2.82nm over a 10µm area scan, while on sample 1B, the Ag film exposed to Ar+ in Fig. 2b, ‫ݎ‬௥௠௦ was measured to be 1.47nm over an equivalent 10µm area. Three 10µm area AFM scans were taken from various points across each bare Ag sample. Similar roughness, morphology, and grain size distribution in Fig. 2 scans were also observed in other AFM scans on each sample. AFM analysis of roughness and grain size distribution, averaged using three 10µm area scans of each sample, is shown in the histogram in Fig. 3. Grains were identified and filtered using the watershed segmentation digital algorithm20, and high-frequency filtering was used eliminate identified grains with average radii less than 20nm. The semi-log scale of the histogram emphasizes the lower range of larger grains. Sample 1A has a much higher range of average grain size, up to 230nm radii, while Ar+ bombarded Sample 1B shows no grains with radius larger than 150nm. Both Ag surfaces show distributions weighted toward smaller grains, but Ar+ bombarded Ag has more grains with radii smaller than 80nm than untreated Ag while



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untreated Ag has more grains with radii larger than 80nm. Table 2 summarizes the analysis of the averaged AFM scans with ‫ݎ‬௥௠௦ , average total grains in each scan, average grain radius, and grain size areal distribution ratios characterized by grains with radii less than 80nm and more than 150nm.

Table 2: Average rms roughness, total grain count, grain radius, and grain size distribution ratios averaged from three 10µm scans on each sample.

Sample

Ar+ exposure

Rrms

# grains

Avg. grain radius

% area grains < 80nm radius

% area grains > 150nm radius

1A

no

2.61

8069

85.67

79.8

10.4

1B

yes

1.26

11026

72.67

49.4

0

With an average ~10% of untreated Ag surface occupied by grains with radius larger than 150nm, we expect a higher density of sharp Ag features caused by edges of large grains isolated amongst a more uniform background of smaller grains. Such sharp jagged features are expected to act as characteristic SPR absorption and scattering zones8-10. Combined with an overall higher average ‫ݎ‬௥௠௦ of 2.61nm, the distinct sharp surface features of untreated Ag are expected to cause significant reflectivity loss due to increased SPR coupling when coated with an adjacent protective dielectric layer. Samples 2A and 2B were fabricated to compare reflectivity between two complete mirror stacks when the Ag surface is exposed to Ar+ bombardment between Ag deposition and subsequent deposition of Si3N4.


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Fig. 4. Reflectivity of sample 2B with untreated Ag and sample 2A with Ag exposed to Ar+ bombardment plotted with emphasis on the UV/visible spectrum features. The dip at ~400nm is the core of the SPR. The inset shows the mirror stack schematically, with red arrows indicating the bare Ag surface which was exposed to Ar+ in vacuum.

Improvements to mirror reflectivity after Ar+ exposure to the Ag film surface of sample 2B are shown in Fig. 4. The overall reflectivity at wavelengths in the range of 330

Silver Film Surface Modification by Ion Bombardment Decreases

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