Improved durability of Ag thin films under high humidity environment by deposition of surface Al nanolayer

Applied Surface Science 506 (2020) 144929

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Improved durability of Ag thin films under high humidity environment by deposition of surface Al nanolayer

T

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Yuya Sasakia, Midori Kawamuraa, , Takayuki Kibaa, Yoshio Abea, Kyung Ho Kima, Hiroshi Murotanib a b

School of Earth, Energy and Environmental Eng., Kitami Institute of Technology, 165 Kitami, Hokkaido 090-8507, Japan Faculty of Eng., Tokai University, Hiratsuka, Kanagawa 259-1292, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Silver Surface layer Aluminum Environmental test Reflectance

Silver exhibits the highest reflectance from the visible to the infrared region however, its low durability is also well known. In this study, Al nanolayers were deposited on Ag thin films (Al/Ag) by vacuum evaporation, where a 1 nm thick or 3 nm thick Al layer was deposited on Ag film. The durability of the Al/Ag films under high humidity was investigated by holding the samples at 55 °C, 90% relative humidity (RH) for 6 h. After the test, remarkable agglomeration occurred in the Ag single film, but not in Al/Ag films. Therefore, the passivation effect of the Al oxide nanolayer, which was formed by natural oxidation of the Al nanolayer, was confirmed. This effect also meant that the optical reflectance was kept high, even after the test. Without the Al surface layer, diffuse reflectance caused by roughening of the film was significantly large. The humidity test was carried out in air, so that sulfide was formed at the surface of the Ag single film, but not formed on the Al oxide surface layer. As a result, a several nanometer thick passivation layer was determined to improve the durability of the Ag layer and maintain high reflectance.

1. Introduction Silver (Ag) is known for its excellent electrical and optical properties; Ag has the lowest electrical resistivity, highest optical reflectance, and the lowest emissivity among the metals [1]. However, one drawback is that Ag thin films readily agglomerate under heat treatment due to its low adhesion to glass (oxide) substrates and the ease with which Ag atoms migrate, which leads to a deterioration of the Ag film properties. Besides, deterioration of the Ag films under high humidity in air [2] or in pollutants [3] is also reported. For optical applications such as mirrors, protective coatings consisting of metal oxides, nitrides, or oxynitrides have been proposed [3–8]. It is complicated to compare their performance because the thickness of the layers and conditions of the environmental tests are different. But generally thicker layers are considered to offer better protection effect, and about 100 nm thick layers are commonly used [5–8]. On the other hand, it is recently reported that 2 nm thick Ti surface nanolayer worked against plasma oxidation of Ag films when an appropriate interlayer was used [9]. Also, Cr nanolayer is reported that it could stabilize metal oxide films in high humidity [10]. But so far, few attempts have been done to investigate the ability of nanolayer as a protective coating.

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We have reported that thermally stable Ag film where Al nanolayer was deposited at the surface and interface with the substrate. Here 1 nm thick Al nanolayer was oxidized to be 3 nm thick Al oxide nanolayer, which was confirmed by ellipsometry measurements [11]. This measurement result was comparable to the result that an approximately 2.5 nm thick porous oxide layer on Al nanoparticles was confirmed by high-resolution transmission electron microscopy measurements [12]. We have also found the optical reflectance of the Al nanolayer deposited Ag films is as high as that of Ag single film [13] because Al oxide formed at the surface of Ag film is optically transparent due to its wide energy bandgap of ca. 8.4 eV [14]. In the present paper, we investigate and discuss the passivation effect resulting from nanolayers at high humidity for the potential practical applications of this technology. It is worthwhile to investigate the usefulness of very thin surface layers to prevent the degradation of Ag films under severe conditions. Here, we investigate the durability of Ag films with 1 or 3 nm thick deposited Al surface layers under high humidity, which is defined as the International Organization for Standardization (ISO) test for optical coatings [15].

Corresponding author. E-mail address: [email protected] (M. Kawamura).

https://doi.org/10.1016/j.apsusc.2019.144929 Received 23 January 2019; Received in revised form 4 November 2019; Accepted 2 December 2019 Available online 06 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Optical microscope images of Ag single film (a) before the humidity test, (b) and (c) after the test in different magnifications. (d) Size distribution and number of agglomerates formed in an 8 mm2 deposition area of Ag single film.

a)

2 μm

b)

c)

2 μm

20 μm

a)

b)

50 μm

50 μm

c)

d)

Fig. 2. SEM images of Ag single film (a) before the humidity test, (b) and (c) after the test in a different magnification.

a)

2 μm

c)

2 μm

50 μm

50 μm

b)

2 μm

d)

2 μm

Fig. 4. SEM images of (a) Al (1 nm)/Ag film, (b) Al (3 nm)/Ag film before the humidity test, (c) Al (1 nm)/Ag film, (d) Al (3 nm)/Ag film after the test.

Fig. 3. Optical microscope images of (a) Al (1 nm)/Ag film, (b) Al (3 nm)/Ag film before the humidity test, (c) Al (1 nm)/Ag film, (d) Al (3 nm)/Ag film after the test.

(99.99%) and Al wire (99.99%) successively without breaking the vacuum after evacuation to less than 2.2 × 10−4 Pa. The deposition rates of Ag and Al were ca. 0.8 nm/s and 0.05 nm/s, respectively. Environmental tests under high humidity were conducted in an environmental test chamber where the temperature and relative humidity (RH) were set to 55 °C and 90%RH for 6 h in air. The test condition is based on the ISO 9211-3 standard for the environmental durability of optical coatings. The surface of the films was observed using an optical microscope

2. Experimental Multilayers were prepared by depositing an Al nanolayer (1 or 3 nm thick) on 150 nm thick Ag thin films on glass substrates (Corning EAGLE XG) by vacuum evaporation. An Ag single layer was also prepared as a reference. Deposition was performed by heating Ag wire 2

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Fig. 5. AFM images of (a) Al (1 nm)/Ag film, and (b) Al (3 nm)/Ag film after the humidity test. The scanning area is 1 × 1 μm2.

a) Ag (111)

Ag (200)

* b)

Fig. 6. XRD patterns for the samples. (a) the pattern for Ag single film before the test. Asterisk shows Kβ diffraction of Ag (1 1 1). (b) shows the patterns of Ag (1 1 1) diffraction peaks obtained in small step size for Ag film, Al (1 nm)/Ag film, Al (3 nm)/Ag film before and after the test.

(Keyence Co., VHX-5000). Ag films are highly reflective; therefore, a halation-eliminating function was used to obtain the images. A scanning electron microscope (JEOL JSM-6510A) was also used to observe the surface of the samples. The surface morphology was then observed using atomic force microscopy (AFM; Hitachi Co., N5100) in dynamic mode to investigate the surface roughness in detail. The electrical resistivity of the prepared films was measured using the four-point probe

method. The crystal orientation was measured and crystallite size in the Ag film was estimated from the full width at half maximum (FWHM) of the Ag (1 1 1) diffraction peak using X-ray diffraction (XRD; Rigaku Co., Ultima IV). Optical reflectance spectra were measured using a spectrophotometer (Jasco Co., V-670) equipped either with an absolute reflectance measurement unit or an integrated sphere. X-ray photoelectron spectroscopy (XPS; Ulvac Phi Co., PHI5000 VersaProbe) was 3

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humidity. The surface morphology of the Al (1 nm)/Ag and Al (3 nm)/Ag films after the test was further investigated using AFM. Fig. 5(a) and (b) show AFM 2D images of surface images of the Al (1 nm)/Ag and Al (3 nm)/Ag films, respectively. The scan area was 1 × 1 μm2, and the z-axis range was 15 nm. The root mean square (rms) roughness was 1.9 nm for the Al (1 nm)/Ag film and 1.5 nm for the Al (3 nm)/Ag film, which remained unchanged by the humidity test. In either film, the outermost surface is considered to be the Al oxide layer. Fig. 6 shows XRD patterns for Ag single film and Al (1 nm)/Ag and Al (3 nm)/Ag films. Fig. 6(a) shows a wide scan pattern of Ag single film before the test. It is clearly shown that the film has a strong preferential orientation of (1 1 1) which is the close-packed plane of face-centered cubic structure. Due to the high diffraction intensity, a small kβ reflection was also recognized. To determine the crystallite size based on Ag (1 1 1) peak, narrow step scanned patterns were also obtained for all the samples before and after the humidity test, and the peaks are shown in Fig. 6(b). The peak position is unchanged in all the samples; d-spacing of Ag (1 1 1) is 0.360 nm which is accordant with JCPDS card [16]. Table 1 shows the crystallite size of Ag in the films based on the FWHM of the Ag (1 1 1) diffraction peaks (shown in Fig. 6(b)) estimated using the Scherrer equation [17]. All the samples exhibited grain growth of ca. 10 nm by the humidity test. These values are not necessarily the same as those determined from AFM 2D images, because the estimated grain size is in the vertical (z-axis direction) and also this represents the sizes of primary grains and not apparent grains. As a result, it is considered that the conditions of the humidity test had an annealing effect on the films. Though the outer most surface is capped with Al oxide layer, heat conduction from the bottom (substrate) was influenced. The electrical sheet resistance of the samples was correspondingly decreased by approximately 3–10% from that in the as-deposited state. Fig. 7 shows XPS spectra for the Ag single film and Al (1 and 3 nm)/ Ag films measured at the outermost surface after the humidity test. All the spectra were calibrated based on C 1s peak obtained at the outermost surface of each sample. In the Ag single film, the formation of Ag2S at the surface was determined by the S 2p peak. It is difficult to distinguish Ag2S (368.0 ± 0.3 eV) [18] from Ag metal (368.2 ± 0.1 eV) in the Ag 3d5/2 peak due to overlap. However, it is considered that Ag atoms are primarily in the metallic state and partially in the sulfide state because the intensity of the S peak is not strong. In the Ag single film, surface roughening as a result of the humidity test was confirmed. It is possible that roughening of the surface could accelerate sulfide formation by increased contact of the surface

Table 1 Crystallite size of the samples before and after humidity test.

Ag single film Al(1 nm)/Ag film Al(3 nm)/Ag film

The crystallite Before test

size (nm) After test

69 65 63

77 77 76

used to investigate the chemical bonding state of the elements in the films. 3. Results and discussion Fig. 1 shows optical microscope images of Ag single films before and after the environmental test under high humidity. Fig. 1(a) is an image before the test. Here, halation is eliminated from the image. Therefore, the dark surface means the entire film is reflective. Fig. 1(b) and (c) show the film surface after the test in different magnifications and significant agglomeration are seen in the images. Here, agglomerates appear as white dots in the images, while the relatively flat parts of the film appear dark. To obtain information on surface roughness, we have observed the same samples using SEM. Fig. 2(a) shows an image of Ag single film before the test and the surface seems to be flat which corresponds with highly reflective features as shown in Fig. 1(a). Fig. 2(b) and (c) show the sample surface after the test. In the images, there are rough parts in round shape, which appeared in white circles in Fig. 2(b) and (c). There are small hillocks inside of the circles. On the other hand, outside of the circles is found to be relatively smooth. Consequently, it is confirmed that round agglomerates formed in Ag single films after the test. The optical microscope image shown in Fig. 1(b) was analyzed using an image analysis program, ImageJ, and the typical size of an agglomerate was determined to be ca. 30 μm in diameter, as shown in Fig. 1(d). The area of agglomeration on the Ag single film was up to 54%. Then we examined the surface of Al (1 nm)/Ag and Al (3 nm)/Ag films. Consequently, as shown in Fig. 3, both Al (1 nm)/Ag and Al (3 nm)/Ag films before and after the test show reflective surface which suggest Al deposited Ag films can maintain smooth surface morphology even when the surface layer is extremely thin. Similarly, SEM images of them are all smooth as shown in Fig. 4. They are all in a low contrast, and there is any difference among the samples before and after the test. Therefore, it is found that the deposition of a 1 nm thick Al layer was useful to maintain the smooth and reflective surface under the high

Samples

Ag 3d

O 1s

Intensity (arb. unit)

368.2

80

530.7

S 2p 161.5

Ag

Al 2p 74.0

75

531.2 367.4

Al(1) /Ag

367.0

Al(3) /Ag

70 376 374 372 370 368 366 540 535 530 525 170 165 160 155

Binding Energy (eV) Fig. 7. XPS spectra of Ag single film, and Al (1 and 3 nm)/Ag films after the humidity test. Spectra were obtained at the outermost surface. 4

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At the surface of the Al (1 and 3 nm)/Ag films, small peaks assigned to Ag oxides (367.7 ± 0.4 eV) were observed in the Ag 3d spectra. Here we investigated as-deposited Al(1 nm)/Ag film by angle-resolved XPS spectra where angle to the detector was varied from 45 to 5°. As a result, the Ag 3d5/2 spectra show Ag atoms in metallic state at any depth position shown in Fig. 8. Therefore, we conclude that Ag oxide formation occurred during the humidity test. In the O 1s spectra, peaks due to Al oxide appeared around 531.2 eV, while that of adsorbed O2 appeared around 530.7 eV. Peaks due to Ag oxide seem to be apparent at lower binding energy around 529.8 eV; however, these are not recognizable. No peaks appeared in the S 2p spectra of the Al (1 and 3 nm)/Ag films, which indicates that sulfide was not formed at the surface. The reason for only oxide formation and not sulfide formation on the Al (1 and 3 nm)/Ag films has yet to be clarified. However, no sulfide formation indicates the complete coverage of Ag film by Al oxide nanolayer. We consider that Ag oxide was formed by nanoscale Kirkendall effect [19] as reported for instance in the case of Ti nanolayer on Ag films [9]. It is possible that oxygen diffuses into the Al oxide layer and generates vacancies which then induce migration of Ag atoms at the interface with the Ag film surface. Fig. 9 shows the optical reflectance of the films before and after the humidity tests. In the Ag single film, the specular reflectance around 400 nm was considerably decreased, as shown in Fig. 9(a). Instead, diffuse reflectance measured with an integrated sphere was increased up to 16%, which was caused by surface roughening by the humidity test. In contrast, the Ag films with the surface Al nanolayer (Fig. 9(b) and (c)) showed almost the same specular reflectance before and after the humidity test. The maximum diffuse reflectance is less than one third that of the Ag single film, which corresponds to the smooth film surface after the humidity test. As a result, the Ag film with the surface Al nanolayer can retain high optical properties due to the passivation effect of Al oxide. The degradation of Ag films or Ag nanostructures under high humidity have been reported and the mechanism has also been discussed. Glover et al. explained that Ag ion diffuse in a chemisorbed water layer and precipitate to form a different morphology from the original [20]. Xang reported that no agglomeration occurred when held under dry argon [2]. The state of the water layer was also reported to change depending on the humidity [21], i.e., icy at low humidity and in the liquid state at high humidity. We consider that the liquid state was dominant in the water layer formed on the Ag films under the conditions used in the present work. Al nanolayer deposition without breaking the vacuum is believed to prevent contact of the Ag film with water vapor in the air. This is considered to be the key factor to improve the durability of the Ag film by the use of a surface nanolayer. It should be noted that such a nanoscale surface layer functioning as a significant passivation material is unexpected. However, the result shows the role of nanolayers has been confirmed to be more than expected.

Intensity (arb. unit)

Ag 3d5/2

(deg) 45 30 10 5

371

370

369

368

367

366

Binding Energy (eV)

30

a) 80

Before test

60

After test

20

40

10

20 0

200

500

Wavelength (nm)

100

800

0 30

b)

80

Before test

60

20

After test

40

10

20 0

200

Wavelength (nm)

800

0 30

c)

80

Before test

60

After test

20

40

10

20 0

200

500

Wavelength (nm)

800

0

Diffuse reflectance (%)

100

500

Diffuse reflectance (%)

Specular reflectance (%)

100

Diffuse reflectance (%)

Specular reflectance (%) Specular reflectance (%)

Fig. 8. The Ag3d5/2 spectra of as-deposited Al (1 nm)/Ag film obtained by angle-resolved XPS.

4. Conclusion The deposition of 1 or 3 nm thick Al surface nanolayers onto Ag thin films was shown to be an effective method for protection under high humidity, whereby smooth surface morphology and high specular reflectance were maintained. Although the thickness of the passivation layer for protection is extremely thin, the layer plays a significant role to improve the durability of Ag thin films by preventing direct contact of Ag film with water vapor. It is expected that Ag films with such nanolayers could be adopted for practical use.

Fig. 9. Specular reflection and diffuse reflection spectra for (a) Ag single film, (b) Al (1 nm)/Ag film, and (c) Al (3 nm)/Ag film before and after the humidity test.

Acknowledgments

with the air. However, it becomes clear that degradation of the Ag single film under the current humidity test condition, where the test period is short as 6 h, is found to be governed by roughening due to agglomeration.

The authors would like to thank Mr. M. Yamane for XPS mearuements of the samples. This work was partly supported by JSPS KAKENHI grant number JP16H04503. 5

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