Effects of Ti Transition Layer on Stability of Silver/Titanium Dioxide Multilayered Structure ⁎ Zhenguo Wang A, , Xun Cai A, Qiulong Chen A, Paul K

Thin Solid Films 515 (2007) 3146–3150 www.elsevier.com/locate/tsf

Effects of Ti transition layer on stability of silver/titanium dioxide multilayered structure ⁎ Zhenguo Wang a, , Xun Cai a, Qiulong Chen a, Paul K. Chu b

a Key Laboratory of the Ministry of Education for High Temperature Materials and Testing, Shanghai Jiao Tong University, Shanghai 200030, China b Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China Received 24 August 2005; received in revised form 14 March 2006; accepted 30 August 2006 Available online 12 October 2006

Abstract

Thermal stability is critically important for silver-based multilayered structures such as Ag/TiO2. Al and NiCr are reported to be applied as interlayers between Ag and TiO2 to improve the thermal stability but both decrease the transparency in visible region. In this work, a Ti interlayer can effectively improve the thermal stability of the Ag film but does not adversely affect the optical transmission through the multilayer from the near ultraviolet to near infrared spectral region. Atomic force microscopy indicates that the titanium interlayer improves the surface quality and agglomeration resistance of the Ag film. Depth profiles obtained by Auger electron spectroscopy show that the L3M23V line of titanium shifts to a higher binding energy after annealing, indicating that the titanium layer is oxidized during annealing at 573 K in air for 30 min. X-ray diffraction results indicate that the transition layer is oxidized to Ti2O3 during deposition and the Ag (111) orientation preferentially emerges after the thermal treatment. The Ag layer remains chemically stable but some diffusion is observed through the top dielectric layer after annealing. About 10% degradation in the visible light transmission and 20% in near IR reflection are observed from the samples without the interlayers after annealing. In comparison, less than 10% decrease in the near IR reflectivity is observed from the samples with the interlayers. The transition layer is found to stabilize the silver film by improving the wettability, agglomeration resistance, and diffusion resistance. © 2006 Elsevier B.V. All rights reserved.

Keywords: Titanium; Transition layer; AES; Thermal stability; UV; Multilayers

1. Introduction crystals. Consequently, the thermal stability of the multilayered structure is of great importance. Moreover, silver films are Silver-based multilayers are widely used in low emission prone to agglomeration even at a relatively low temperature in coatings [1,2], transparent conductor coatings [3,4],and air due to silver diffusion [6]. The agglomeration is generally electromagnetic inference shielding filters [5] due to their low driven by the reduction of the interfacial free energy [7]. electrical resistance, flexible bandpass tuneability from near Therefore, the thermal stability of the multilayers is closely ultraviolet (UV) to visible, and higher infrared (IR) reflectivity related to the silver growth mechanism on different dielectric than transparent conducting oxides. Silver films can be inserted materials. Silver has poor wettability on most metal oxides and between the high refractive index dielectrics or transparent it grows via a 3D Volmer–Weber mechanism [8]. One of the conducting oxides to cater to different applications. In trans- solutions is to deposit an interlayer to improve the wettability. parent electrodes used in flat panel displays, the electrodes have Al and Ni–Cr interlayers are effective to a certain extent, but to undergo a heat treatment at about 573 K to orient the liquid both materials degrade the transparency of the multilayers in the visible spectral range [9,10]. It is thus of scientific and practical importance to produce an interlayer that improves the thermal stability but does not affect the optical properties. Here, a ⁎ Corresponding author. Present address is Komag USA (Malaysia) Sdn. titanium interlayer is proposed for the Ag/TiO2 multilayered Phase III, FIZ Bayan Lepas 11900, Penang Malaysia. Tel.: +6 046397678. structure and the thermal stability and optical properties are E-mail address: [email protected] (Z. Wang). subsequently investigated.

2. Experimental details Elmer Company's Lambda 900 ultraviolet/visible/near infrared spectrophotometer. The surface morphology and roughness were The films were deposited in a DMDE450 vacuum evapora- determined employing a Digital instrument Nanoscape III A tion system manufactured by Beijing Instrument Factory for atomic force microscope (AFM) under the tapping mode with a optical coatings. Silicon wafers with a 5 nm thick native oxide silicon cantilever. The force on the cantilever was 48 N/m, were used in the ellipsometry (SE) and atomic force microscopy resonant frequency was 315 kHz, and the scanner was 3859 E. (AFM) analyses. For the X-ray diffraction (XRD), transparency The elemental depth profiles were acquired by sputtering and Auger electron spectroscopy (AES) analyses, glass was used Auger electron spectroscopy on a PHI 550 ESCA/SAM. The as the substrate. The substrates were first cleaned by common base pressure was 1.33×10− 8 Pa and the partial pressure in the chemical detergents ultrasonically. Silver (99.9% pure) was Ar ion gun was 2.66×10− 5 Pa. The primary electron beam heated in a Mo boat. Titanium (99.9%) and titanium dioxide energy and current were 3 keV and 1 μA respectively. A Ta2O5 (99.9%) were put in tantalum crucibles and heated by an electron standard was employed as the sputtering rate reference and the gun. The multilayered structure consisting of 25 nm TiO2/18 nm sputtering rate was 2–3 nm/min. X-ray diffraction (XRD) was Ag/28 nm TiO2 (S1) and 25 nm TiO2/3 nm Ti/18 nm Ag/28 nm performed on a Rigaku B/max-2550 VB X-ray diffractometer TiO2 (S2) were deposited on different substrates at room with Cu Kα radiation. temperature, and the base pressure in the vacuum chamber was 6.5×10− 3 Pa. The films' thickness was measured in real time by 3. Results and discussion a quartz crystal monitor (LTC-2 model, Beijing Instrument Factory) and calibrated by SE. The two sets of samples were 3.1. Surface roughness subsequently annealed in air at 573 K for 30 min. The thickness and optical constants of the films were A very thin titanium film can enhance the opto-thermal determined by the V-VASE32TM type SE made by J.A. Woollam stability of silver films on polyethylene terephthalate [11,12] and Co. The transmittance of the films was measured on a Perkin– act as an interlayer for silver and alumina to improve the

Fig. 1. AFM micrographs of as-deposited and annealed silver films on titanium dioxide with and without titanium interlayer. 3148 Z. Wang et al. / Thin Solid Films 515 (2007) 3146–3150

hand, the structure with the titanium interlayer maintains good continuity.

3.2. Optical properties and thermal stability

Silver-based multilayers have been investigated as a heat mirror for many years due to their high transmittance in the visible spectrum and high reflectivity in the IR region. We have in fact adopted this structure for UV curing heat isolation [15]. The wavelength for commercial UV curing is in the range of 350–450 nm, while most light sources do not emit real cool light but radiation extending into the IR instead. In order to improve the life time, the multilayers should have good opto-thermal stablility. The extinction coefficient (k) of a metal film at a particular wavelength is proportional to its thickness. A2–3nm thick titanium film has a small k value and the real part of the 2 2 dielectric constant is ε1 =n −k N0. Hence, its effects on the optical properties of the multilayers should be negligible. As shown in Fig. 2, very little differences can be observed in the transmission spectra of the multilayers by inserting a 3 nm thick titanium layer. To investigate it in more details, the two samples: 25 nm TiO2/18 nm Ag/28 nm TiO2 and 25 nm TiO2/ 3 nm Ti/18 nm Ag/28 nm TiO2 were annealed in air at 573 K for 30 min. Comparing the transmission spectra from the near UV to near IR (Fig. 2) acquired from the specimens before and after the heat treatment, the transparency of the film without the Ti interlayer decreases by about 10% between near UV and visible Fig. 2. Effect of a 3 nm-titanium layer on the transmittance of as-deposited and and the near IR reflectivity decreases by more than 20% after annealed multilayers deposited on glass. annealing. However, the as-deposited film with the Ti interlayer exhibits almost the same transparency from near UV to visible wettability [13]. Titanium has a better affinity to oxygen than and the near IR reflectivity diminishes by only about 10%. It silver and consequently, it has higher adhesion strength on suggests that the effective thickness of the silver layer may be titania than silver. It has been shown that noble metals have poor reduced by thermal diffusion and/or agglomeration. It should be affinity to oxygen and therefore, it should not wet the oxide [7]. noted that the transmittance peak of the sample with a Ti From the perspective of film growth, titanium as an interlayer interlayer shifts towards a longer wavelength after annealing. can improve the wettability of noble metals. Silver atoms have This can be attributed to the phase shift caused by oxidation of been shown to preferentially nucleate on the Ti4+ sites by XPS the titanium interlay during the heat treatment. The conclusion and low-energy ion scattering spectroscopy [14]. Metal–metal is corroborated by the Auger depth profiles and XRD data to be bonds are generally stronger than those of metal–oxygen, and so presented in the next section. a titanium interlayer will enhance silver nucleation by preferential occupation of the O2− sites where silver cannot nucleate. Accordingly, the interlayer can even alter the growth mechanism of the silver films. In our experiments, AFM is utilized to assess the surface roughness of the silver films with and without the titanium interlayer. The two-dimensional AFM images acquired from the silver films on different interlayers are displayed in Fig. 1. It can be observed that the TiO2/Ti/Ag sample has better surface quality. The mean surface roughness (Ra) values of the as-deposited and annealed TiO2/Ag and the as- deposited and annealed TiO2/Ti/Ag are 3.0, 9.8, 2.7 and 5.0 nm, respectively. The quality of the silver films critically affects the optical and electrical properties. The reduced roughness improves the metal reflectivity and optical transmission through the multilayers due to decreased absorption. The sheet resistance also diminishes due to the better continuity of the metal film.

After annealing, the TiO2/Ag structure agglomerates due to the Fig. 3. AES depth profile of the as-deposited and annealed 25 nmTiO2/3 nmTi/ poor wettablity of silver on titanium dioxide [8], but on the other 18 nm Ag/28 nmTiO2 multilayers. Z. Wang et al. / Thin Solid Films 515 (2007) 3146–3150 3149

3.3. Ti interlayer chemical properties and silver films orientation analysis

In order to analyze the thermal stability of the multilayered samples with and without the titanium interlayer, AES depth profiles and XRD spectra were acquired. The results are also used to determine the elemental distributions, bonding infor- mation, and texture orientation before and after annealing. The AES profiles of the 25 nm TiO2/3 nm Ti/18 nm Ag/28 nm TiO2 multilayered structure are shown in Fig. 3. Comparing the as-deposited multilayered structure with the annealed one in Fig. 3, it can be observed that some silver atoms diffuse into the top and bottom titanium dioxide layers after annealing. About 15% of the Ag atoms diffuse through the top layer to the surface, whereas the amount of Ag diffusing to the dielectric layer underneath is smaller due to the interlayer resistance. The diffusion process reduces the effective silver film thickness and consequently the IR reflectivity degrades. It is believed to be the primary reason for the degradation of the optical properties of the multilayers without the titanium interlayer after annealing. Similar results have been reported on the photo-excitation of silver films [16]. In addition, the ratio of titanium to oxygen remains the same. Titanium dioxide is an easily reducible oxide and Ti in different valence states shows different Auger peaks. It is believed that the Ti L23M23V line shape is the most sensitive one that can be employed to determine the stoichiometry of titanium dioxide [17]. The 410 eV and 415 eV peaks correspond to the Ti L3M23V and L2M23V lines, respectively. As shown in Fig. 4a, the low energy Fig. 5. X-ray diffraction patterns for TiO2/Ag/TiO2 and TiO2/Ti/Ag/TiO2 before and after annealing at air for 30 min at 573 K.

peak shifts to higher energy after annealing due to oxidation of the titanium interlayer. It can be attributed to the fact that L3M23V is responsible for the interatomic process between Ti and O [18]. Silver is not oxidized during annealing, as indicated by the Ag M45N45N45 line shape and position that do not change after annealing as shown in Fig. 4b. The Ag M4N45N45 and M5N45N45 lines are too close to separate and the combined peak position is located at 349 eV. This is consistent with the Auger spectrum obtained from pure silver published in the literature [19]. Pure titanium peaks cannot be found in the XRD spectra even from the as-deposited structure, suggesting that titanium has been oxidized to Ti2O3 during deposition (Fig. 5). This confirms the aforementioned assumption and is consistent with the AES results. The titanium interlayer has little influence on the transmittance of the TiO2/Ag/TiO2 multilayered structure because Ti2O3 has an optical constant similar to that of TiO2. The as-deposited titanium dioxide produced without substrate heating is amorphous and this explains the absence of the diffraction peaks. The temperature at which titanium dioxide transforms from amorphous to anatase is higher than 623 K [20]. Therefore, titanium dioxide remains amorphous even after annealing at 573 K. However, new products such as Ti2O3 can

Fig. 4. AES line shapes of the Ti L23M23V and Ag M45N45N45 peaks of the as- form crystalline structures even during deposition and after deposited and annealed 25 nmTiO2/3 nmTi/18 nm Ag/28 nmTiO2 multilayers. annealing, and the diffraction intensity is greater than that of the 3150 Z. Wang et al. / Thin Solid Films 515 (2007) 3146–3150 as-deposited one. Another interesting result is that the Ag (111) Acknowledgements texture of the structure with the Ti interlayer is enhanced after the heat treatment but the opposite trend is observed from the This work was supported by the Shanghai Nano-Tech Fund sample without the interlayer. This can be explained by that Ag Grant No. 0452nm067 and the City University of Hong Kong (111) has a close-packed crystal structure with minimum surface Direct Allocation Grant No. 9360110. energy and so enhancement of Ag (111) can improve the stability of the silver film. References

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