Ultra-thin silver films grown by sputtering with a soft ion beam-treated intermediate layer

Silver thin films have wide-ranging applications in optical coatings and optoelectronic devices. However, their poor wettability to substrates such as glass often leads to an island growth mode, known as the Volmer–Weber mode. This study demonstrates a method that utilizes a low-energy ion beam (IB) treatment in conjunction with magnetron sputtering to fabricate continuous silver films as thin as 6 nm. A single-beam ion source generates low-energy soft ions to establish a nominal 1 nm seed silver layer, which significantly enhances the wettability of the subsequently deposited silver films, resulting in a continuous film of approximately 6 nm with a resistivity as low as 11.4 µΩ.cm. The transmittance spectra of these films were found to be comparable to simulated results, and the standard 100-grid tape test showed a marked improvement in adhesion to glass compared to silver films sputter-deposited without the IB treatment. High-resolution scanning electron microscopy images of the early growth stage indicate that the IB treatment promotes nucleation, while films without the IB treatment tend to form isolated islands. X-ray diffraction patterns indicate that the (111) crystallization is suppressed by the soft IB treatment, while growth of large crystals with (200) orientation is strengthened. This method is a promising approach for the fabrication of silver thin films with improved properties for use in optical coatings and optoelectronics.

Silver thin films have wide-ranging applications in optical coatings and optoelectronic devices. However, their poor wettability to substrates such as glass often leads to an island growth mode, known as the Volmer-Weber mode. This study demonstrates a method that utilizes a low-energy ion beam (IB) treatment in conjunction with magnetron sputtering to fabricate continuous silver films as thin as 6 nm. A single-beam ion source generates low-energy soft ions to establish a nominal 1 nm seed silver layer, which significantly enhances the wettability of the subsequently deposited silver films, resulting in a continuous film of approximately 6 nm with a resistivity as low as 11.4 µΩ.cm. The transmittance spectra of these films were found to be comparable to simulated results, and the standard 100-grid tape test showed a marked improvement in adhesion to glass compared to silver films sputter-deposited without the IB treatment. High-resolution scanning electron microscopy images of the early growth stage indicate that the IB treatment promotes nucleation, while films without the IB treatment tend to form isolated islands. X-ray diffraction patterns indicate that the (111) crystallization is suppressed by the soft IB treatment, while growth of large crystals with (200) orientation is strengthened. This method is a promising approach for the fabrication of silver thin films with improved properties for use in optical coatings and optoelectronics.

Supplementary material for this article is available online
Keywords: ultra-thin silver film, ion beam treatment, transmittance, resistivity (Some figures may appear in colour only in the online journal) * Author to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Introduction
Ultra-thin continuous silver films with thicknesses of less than 9 nm are attractive for low-E glass coatings and optoelectronic devices because of the high electrical conductivity, optical transmittance, and plasmonic figure of merit [1]. However, it is a challenge to produce ultra-thin and environmentally stable silver films. One of the limitations is the low wettability of silver on glass and many other surfaces. As a result, the initial growth stage of silver thin films follows the Volmer-Weber mode characterized by the formation of non-continuous islands with micro-voids [2,3]. The micro-porous silver films have poor adhesion to the substrate and are easily de-wetted in ambient air or at elevated temperatures, especially with the presence of reactive gases [4][5][6][7].
Various methods have been studied to enhance the wettability of silver to substrates and grow ultra-thin continuous and dense silver films. A primary method is to grow a wetting layer such as Ge [8][9][10], Cu [11,12], Ni [13], Al [14], oxygen-incorporated silver films (Ag (O) ) [15][16][17][18], and aluminum doped zinc oxide (AZO) [19]. Other methods such as using silver alloys were also reported [1,5]. However, there are several drawbacks to using a wetting layer-it requires additional processing steps and the added layer usually reduces the film transmittance. Furthermore, there is a large variation in the electrical conductivity of the silver films due to the effectiveness of different wetting layers. In addition, silver alloys generally result in increased resistivity due to reduced electron mobility [5,20].
Ion beam (IB) assisted deposition has been recognized as an effective approach to modulating thin-film growth. In IB assisted deposition, ions transfer energy to the atoms as they are deposited, leading to enhanced nucleation and lateral growth [21]. However, conventional ion sources (e.g. the anode layer ion source) compatible with thin-film growth usually create ions with energies over 100 eV. These energetic ions can intensively sputter the deposited silver atoms on the substrates due to the high sputtering yield, limiting the control of the silver film microstructure.
This work demonstrates the growth of ultra-thin silver films by using a soft IB treatment to enhance the wettability of silver films on glass substrates. The soft ions are generated by a proprietary single beam ion source that can emit ions with controllable energies below 60 eV. This study focuses on using the soft IB treatment to grow an initial silver seed layer of ca. 1 nm thickness and its effects on the structure of the ultimate films of 6-9 nm thickness. This growth scheme aims to mimic the in-line large-area coatings where an ion source combined with a sputtering magnetron would only treat the initially deposited film as the substrate passes in front of the plasma sources. The optical transmittance and electric resistivity of the silver films grown with and without the IB treated seed layer are compared and correlated with the film microstructure and morphology.

Experiment and method
Borosilicate glass was used as substrates. The substrates were cleaned in an ultrasonic bath using acetone and methanol followed by baking in the air at 100 • C for 30 min before the deposition. The sputtering system (Kurt J. Lesker Company ® PVD 75 PRO Line) had multiple sputtering magnetrons, each having a shutter for pre-sputtering. A single beam ion source (SPR-10, Scion Plasma LLC) was integrated into the sputtering system so that both ion gun and magnetron pointed to the substrate center from different directions at an angle of approximately 60 • (figure S1, supplementary data). The ion gun emitted argon ions with estimated peak energy of 60 eV and flux density of 1 × 10 20 m −2 .s −1 [21,22].
The vacuum chamber was pumped down to 1.3 × 10 −4 Pa before the deposition. The sputtering gas was ultra-high purity grade argon (99.999%) and the pressure was 0.4 Pa. Radio frequency (RF) sputtering was used to have better control over the film thickness (see figure S2, supplementary data, for the rate and power correlation) [23]. The ion source was excited by a DC voltage of 120 V with a discharge current of 0.8 A. This ion source operated in a low-voltage high-current regime, generating ions with relatively low energies below 60 eV that could restructure silver films without significant sputtering of the deposited film [21]. The substrate holder rotated at a constant speed of 10 rpm during the deposition. All the depositions were conducted at room temperature. A summary of the deposition conditions is described in table S1 in the supplementary data.
The film thickness was controlled by the deposition time assuming that the deposition rate was constant under specific process conditions. For each set of process parameters, a rate test was performed first by depositing a film for an extended period of time to achieve a thickness over 100 nm to ensure measurement accuracy. The film thickness was measured using a profilometer (DektakXT ® stylus, Bruker). Before deposition, an ink line was marked across the center of a cleaned substrate. After deposition, the ink mark was removed together with the silver film on top using acetone in an ultrasonic bath leaving behind a step profile for the profilometer measurement. Then, the deposition rates were determined from the film thickness and the deposition time (see supplementary data for the deposition rates of silver films).
Optical transmittance was measured using a spectrophotometer (F20, KLA Instruments). The sheet resistance was characterized in ambient air using a four-point probe sheet resistivity meter (SRM-232-1000, Guardian Manufacturing) having a range of 0-1000 Ω/□, resolution of 0.4 Ω/□, and accuracy of 0.7 Ω/□ at 100 Ω/□. The morphology of silver films was characterized using a scanning electron microscope (Auriga Dual Column Focus Ion Beam SEM, Carl Zeiss). Glancing angle xray diffraction (XRD) was performed at an incident angle of 1 • (SmartLab, Rigaku). The diffractometer used Cu Kα radiation having a wavelength of 1.54 Å.
The optical simulation was performed using the transfer matrix method [24,25]. The refractive indices of silver and glass were taken from Johnson and Christy [26] and SCHOTT Zemax catalog 2017-01-20b, respectively.

Characterizations
Scanning electron microscopy (SEM) images of the silver thin films of different nominal thicknesses are shown in figure 1. Although there are still small voids, the silver film of 5 nm thickness with the IB treatment is continuous and no isolated islands are observed. This is critical to achieving high electric conductivity. On the other hand, the silver film of 5 nm and 6 nm thicknesses without the IB treatment have isolated islands, resulting in poor conductivity. These islands become connected once the film reaches 8-9 nm. Hence, the IB treatment significantly reduces the percolation threshold for a continuous silver film.
The early growth stage of silver deposited on carbon grids with and without the IB treatment were examined to further investigate the effect of the IB treatment. As shown in figure 2, silver films thinner than 4 nm are still in islands with and without IB treatment. However, there are two distinctions between them. The first one is the island size in the IB treated film is bigger and a network between the islands has been formed. The second one is the islands in the IB treated film have irregular shapes other than round. These distinctions indicate that the IB treated silver has better wettability to the substrate than the untreated one.
The IB treatment could have several favorable effects to the growth of silver thin films. One was cleaning the substrate surface, which promoted the film wettability by increasing the substrate surface energy. The other was the ion bombardment that promoted the mobility of the deposited silver atoms and densified the film. It is worth noting that the single beam ion source discharge voltage was only 120 V, which led to a soft beam of ions with average energy below 60 eV [21]. This soft ion-surface interaction can effectively modulate the film microstructure without severe sputtering of the deposited atoms.
Glancing angle XRD could determine the crystal structures of ultra-thin silver thin films [27,28]. Figure 3 illustrates the glance angle XRD patterns of three silver films of 9 nm thickness: untreated silver film, film with 1 nm IB-treated seed layer, and film with 6 nm IB-treated seed layer. The 6 nm IBtreated layer is chosen for exaggerating the effects of IB treatment and examining the effects of simple sputtering deposition of the remaining 3 nm atop the treated layer. The XRD pattern of the untreated film shows (111) dominant crystal orientation. This result agrees with a previous report [29]. On the other hand, IB-treatment suppresses the (111) orientation and enhances the (200) growth as evidenced by the decreased intensity of (111) peak and increased intensity of (200) peak when the thickness of the IB-treated layer increases. Although the mechanism is still unknown, we assume it is because of suitable energy transferred to the deposited atoms, allowing them to organize into a thermodynamically stable structure on ITO surface and this is still needed to be studied further.
The IB treatment not only changes the crystal orientation, but also affects the crystallinity as evidenced by the full width at half maximum (FWHM) of the (200) peak. Scherrer equation is used to calculate the crystal size: T = Kλ β cos θ where T is the mean size of the oriented crystal, K is the shape factor and is given the value of 0.9 in this work for all films, λ is the x-ray wavelength of 0.154 nm in this work, β is the FWHM in radians, and θ is the Bragg angle. The crystal sizes of the 9 nm silver films without and with only 1 nm IB-treated seed layer are calculated to be ∼6 nm. The crystal size of the silver film with a 6 nm IB-treated seed layer is calculated to be ∼17 nm, much larger than the thickness of the film. Hence, the IB treatment greatly enhanced the lateral growth of the crystals oriented in (200) planes.
Previous computational results indicate that the surface energies of silver (200) and (111) planes are 0.810 and 0.773 J m −2 , respectively [30]. These results imply that (111) orientation would be a preferred growth direction if no additional energy is provided to the deposited atoms. In addition, Poletaev et al have also indicated that the activation energy for the migration for Ni atoms in (100) plane is multiple times higher than in (111) plane [31]. Both silver and nickel are transition metals and have FCC crystal structure. It is likely that the activation energy for silver atoms in (200) plane is higher than in (111) plane. Therefore, the IB treatment could provide significant energy to the silver atoms and enhances the growth of (200) orientation even at room temperature. This kind of microstructure modification could hardly be achieved even at elevated substrate temperatures.

Properties and performance
An immediate effect of the improved wettability with IB treatment was the increased silver film adhesion. This is confirmed by using a standard 100-grid tests on 100 nm silver films deposited on glass with and without IB treatment. The results show that the silver film with IB treatment had nearly no peeling off over the grids, whereas the majority of the grids were removed for the film deposited without IB treatment ( figure S3, supplementary data). The borosilicate glass substrate used has typical transmittance and reflectance with negligible absorption in the visible and nearinfrared wavelength range ( figure S4, supplementary data). Theoretically, an ultra-thin silver film (e.g. <10 nm thickness) has low absorption. The simulated transmittance and reflectance spectra of silver thin films of different thicknesses from 5 to 9 nm on glass substrates indicate that the thinner the film, the higher the transmittance, in the condition that the film is smooth and continuous (figure S5, supplementary data) Figure 1. SEM images of silver thin films with nominal thicknesses of 5-9 nm with and without IB-treated intermediate layer. IS: with ion source. The 5 nm without IS one was deposited on carbon grids so that the film can be conductive enough for SEM characterization. The scale bar is 300 nm. [24,25]. From the transmittance T and reflectance R, the absorptance A can be deduced (A = 100 − T − R). For a silver film of 6 nm, the absorptance is less than 5% in the visible and near-infrared range. Therefore, an ultra-thin silver film combined with appropriate anti-reflection coatings can be highly transparent.  Although an ultra-thin silver film is desirable to achieve attractive optical and electrical properties, it is challenging to produce continuous silver films of less than 9 nm thickness using conventional physical vapor deposition such as sputtering. Figure 4 shows the transmittance spectra of silver films of different thicknesses produced by RF magnetron sputtering. The transmittance of 9 nm thickness silver film has a similar trend as the simulated spectrum. However, the transmittance spectra of the 5-8 nm thickness films deviate from the simulation results and exhibit an obvious dip from 400 to 900 nm, which is due to the known plasmonic effect of non-continuous silver [32].
The single beam ion source was used to enhance the growth of silver thin films. Only the initial silver layer of ca. 1 nm was treated with the soft IB. This seed layer was not necessarily continuous yet. A subsequent silver layer was grown on top of this seed layer by magnetron sputtering without the IB treatment and the total film thickness included both layers (figure S6, supplementary data). Figure 5(a) shows the transmittance spectra of silver films of different thicknesses sputtered atop the 1 nm ion-beamtreated seed layer. Except the film of 5 nm thickness, the transmittance spectra of the other silver films of 6-9 nm thickness followed the same trend as the simulated results shown in figure 5(b). From SEM characterization, a continuous silver film of ∼6 nm was produced on glass with the assistance of the single beam ion source, whereas a thickness of about 9 nm was required to produce a continuous silver film without the IB treated seed layer. This indicates that the discontinuity is the reason for having concave shape in the transmittance spectrum at the wavelength region of 400 nm to 600 nm. Figure 6(a) shows the transmittance T, reflectance R, and T + R in one graph for a 6 nm silver film deposited on glass with the IB treated seed layer. A photograph is inserted to demonstrate the highly transparent film. The sum of transmittance and reflectance is higher than 95% in visible and the infrared ranges. Therefore, with an appropriate optical design, this ultra-thin silver film could lead to high reflectance in the infrared and high transmittance in visible light ranges, which is particularly attractive for low-E glass coatings [33]. Figure 6(b) shows a closer look of the transmittance of 6 nm and 7 nm compared to the simulation results using Johnson and Christy bulk silver refractive index [26]. The spectra are in good agreement in the long wavelength range and slightly off in the short wavelength range, likely due to the scattering caused by the voids.
In addition to achieving high transmittance, the IB treatment also resulted in significantly reduced resistivity of ultra-thin silver films, as shown in figure 7. For example, the IB treated silver thin film of 6 nm thickness had a   resistivity of ∼11.4 µΩ.cm, corresponding to a sheet resistance of ∼19 Ω/□, compared to ∼39 µΩ.cm of the untreated silver film of the same thickness. This is among the best performance of reported ultra-thin silver film (see table S2, supplementary data, for a collection of reported silver films and their resistivities). The film resistivity also matches the transmittance spectra presented above, confirming that the IB treatment results in continuous silver films.

Conclusions
This work studied the effects of IB treated seed silver layer on the structure, optical, electric, and mechanical properties of sputtering deposited ultra-thin silver films. An ion-beamtreated silver seed layer of ca. 1 nm greatly enhances the film wettability, leading to a continuous silver film as thin as 6 nm even though the subsequent 5 nm layer was deposited without the IB treatment. On the other hand, forming a continuous silver film requires a thickness of ∼9 nm by conventional magnetron sputtering. XRD study indicates that the IB treatment enhances the growth of (200) oriented planes, while conventional magnetron sputtering is in favor of the growth of (111) orientation, which has a lower surface energy than the (200) plane. The silver films deposited onto the IB treated 1 nm seed layer have high transmittance comparable to the simulation results and the resistivity of a 6 nm film is 11.4 µΩ.cm, which is among the lowest values reported.

Data availability statement
The data cannot be made publicly available upon publication because they contain commercially sensitive information. The data that support the findings of this study are available upon reasonable request from the authors.