The study on the interface adhesion comparison of the MgF 2, Al 2 O 3, SiO 2 and Ag thin films

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1 Applied Surface Science 245 (2005) Short communication The study on the interface adhesion comparison of the MgF 2, Al 2 O 3, SiO 2 and Ag thin films Xueke Xu*, Zhaosheng Tang, Jianda Shao, Zhengxiu Fan Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai , China Received 6 July 2004; accepted 9 October 2004 Available online 14 November Abstract Adhesion between the interface of pure silver thin film and three kinds of low refractive index coatings MgF 2,Al 2 O 3, SiO 2 were compared in this article. The results indicated that the adhesion of Al 2 O 3 and Ag was evidently superior to that of MgF 2 and Ag, and the adhesion of MgF 2 and Ag was evidently superior to that of SiO 2 and Ag. Reasons were analyzed accordingly. On the other hand, we compared the effect on the optical characteristic of Ag film when these three kinds of films were used as protective coatings and enhanced coatings. Considering the difference of the adhesion between Ag and MgF 2,Al 2 O 3, SiO 2, suited uses are given for each other. # 2004 Elsevier B.V. All rights reserved. Keywords: Ag films; Adhesion; Optical performance 1. Introduction Owing to its high reflectance over a wide spectrum and angular acceptance, silver films are extensively employed in various optical applications. However, in order to gain more excellent performance, protective coatings and enhanced coatings must be added to the surface of Ag film due to the soft texture, low chemical stability and low environment adaptability (such as weak humidity durability). * Corresponding author. Tel.: ; fax: address: xuxk@opfilm.com (X. Xu). In order to take little effect on the high reflectance over wide angular acceptance and broad spectral band width. Almost all the materials used as protective coatings and enhanced coatings (the layer deposited on the metal film, which can enhance the reflectance of the metal) need low refractive index (such as SiO 2, Al 2 O 3, MgF 2 and so on). The adhesion between these low refractive index coatings and Ag film is important for the main performance of the coating system, In this article, we compared the adhesion between the interface of SiO 2,Al 2 O 3, MgF 2 film and pure Ag film on the basis of lots of experiments, we analyzed the interface character of them. Advantage and disadvantage of each of them were drawn on the basis of theoretically analysis. It is important for us to /$ see front matter # 2004 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 12 X. Xu et al. / Applied Surface Science 245 (2005) Table 1 Experimental parameters Material Evaporation way Evaporation condition Thickness control Vacuum (Pa) Ag Ta boat Current (110 A) Minimax method SiO 2 Electron beam Beam current (60 ma) Minimax method MgF 2 Electron Beam current (20 ma) Minimax method Al 2 O 3 Electron Beam current (100 ma) Minimax method select the protective and enhanced coating material of Ag film, and it is also of great help to find the intermediate layer between Ag film and other materials. 2. Experiment 2.1. Experiment condition Samples were prepared by resistance evaporation and electron beam evaporation techniques in ZZSX- 800 coating machine (the manufacturer is BeiYi Innovation Vacuum Technology Co. Ltd., China). Monitoring wavelength is 500 nm and the substrate temperature is 100 8C. Regardless of optical properties, the optical thickness of all the dielectric layer is 2L (L: quarter-wavelength optical thickness). The detailed experimental parameters were described in Table 1. In order to avoid the peeling of Ag film from the glass substrate (BK7, which is a common type glass and easy to get), a thin Cr film was deposited on the glass as an intermediate layer before depositing Ag film [1]. And then the above three kinds of material were deposited on Ag film to get three different film systems (Fig. 1), the difference of them only exist in the outer layer. In this experiment all film layer were deposited in the same vacuum, which means after we deposited the intermediate layer and Ag film system we did not open the chamber door, and then we deposited the MgF 2,Al 2 O 3 and SiO 2 film at the same condition. We defined the sample numbers as followings: 1. SiO 2 /ag/intermediate layer/glass. 2. MgF 2 /ag/intermediate layer/glass. 3. Al 2 O 3 /ag/intermediate layer/glass Adhesion test Fig. 1. The structure of film system. To compare the adhesive power of Ag film and SiO 2, MgF 2 and Al 2 O 3 layer, we adopted common stripping method and humidity testing method [2]. And then we analyzed surface morphology of the three kinds film system after tested, which were observed by optical microscope. All the adhesion tests ignored the change of the optical property in the experiment. First, we used special adhesion test pen to stripping the film system surface, average pressure is 5 N, from the damage degree of the morphology (Fig. 2) we can Fig. 2. The morphology after striking of the three film systems.

3 X. Xu et al. / Applied Surface Science 245 (2005) Fig. 3. The morphology of the three samples after immersed in water for 3 h. find the adhesion between Ag and MgF 2 is larger than between Ag and SiO 2, and the adhesion between Ag and Al 2 O 3 is the largest one (all the experiment was carried in a moisture-free environment). Second, we placed the three samples in water, after 3 h, SiO 2 peeled from Ag layer markedly, on the contrary samples 2 and 3 have little damage (Fig. 3). After 36 h later, sample 1 almost all peeled off, and there are lots of patches in the sample 2 surface, but sample 3 still has little damage (Fig. 4). On the other hand, in order to prove the results is effective, we repeated the above experiments more than three times, and we found the experiment result can be reproducible. 3. Discussion Due to the mismatch of the lattice and the coefficient of thermal expansion between metal and dielectric, coupling with impurity, dislocation and stresses, there will be transition layer at the interface, whose properties play key roles in adhesion and stabilization [3]. From the view of structure of crystal, monatomic Ag is close-over hexagon atomic crystal, while Ai 2 O 3 is a-ai 2 O 3 when substrate is 100 8C, and structure is corundum close-over hexagon, MgF 2 is fluoritic closeover cubic structure, and SiO 2 is ruleless meshwork when it deposited on Ag. In the theory of coalescent of molecules, the adhesion is stronger between two materials when their structure is sameness or resemble. So, the sequence of adhesion between these three materials and Ag is a-al 2 O 3 > MgF 2 > SiO 2 in theory, and this is same to experiment. From the adhesion, we know, there are mainly physical or chemical adsorption between two films. SiO 2 is ruleless meshwork structure with covalent bond, and its adsorption is mainly physical, so whether their thermal inflate index is match is key to their fastness while the thermal expand index of SiO 2 is /8C, Ag is /8C, there are big difference, so, when bring them to air, there will generate big stress, and then generate small crack accompany with the fall of temperature [4]. To a-al 2 O 3 and MgF 2, their ion crystal, so, there are chemical adsorption besides physical adsorption, so their adhesion to Ag are far lager than SiO 2. Fig. 4. The morphology of samples 2 and 3 after immersed in water for 36 h.

4 14 X. Xu et al. / Applied Surface Science 245 (2005) Table 2 The adherence contrast of Ag film and these three kinds of film Character Result Adherence Al 2 O 3 > MgF 2 > SiO 2 Humidity durability Al 2 O 3 > MgF 2 > SiO 2 And what is more, MgF 2 is susceptible to hygroscopic, so when it is placed in water, the MgF 2 surface will give birth to pieces of spot resulting from water corrosion. On the other hand we give the X-ray data of the three samples as follows: we can easily find that the MgF 2, SiO 2,Al 2 O 3 are almost amorphous, so the character position of XRD diffraction spectrum of the film system is the same as pure Ag. But we can easily find the grain size of Ag after deposited with dielectric layer has the order: SiO 2 > MgF 2 > Al 2 O 3. It is coherent with the adhesion of the three samples: SiO 2 <MgF 2 <Al 2 O 3. And we can draw a conclusion the larger size of the Ag, the smaller adhesion between the layers Ag and the dielectric film. Through analysis above, we can draw the following conclusion: when argentums bond with following material, the adhesion: Al 2 O 3 > MgF 2 > SiO 2,Itis important for us to select the protective and enhanced coating material of Ag film (Table 2). The transmittance of Ag film with protective coating and enhanced coating are given in Fig. 5, the protective material is SiO 2 or Al 2 O 3 or MgF 2, and the enhanced coating structure is Ag/LH (where L and H mean quarter wavelengths, the material of L is SiO 2 or Al 2 O 3 or MgF 2, n H = 2.2). Through the contrasted research, we can draw the following conclusion: When protective coating is needed for Ag film, we should select MgF 2, if humidity durability is especially needed we can choice the Al 2 O 3, and the SiO 2 cannot be used as protective coating. When we need enhancing coating (which can enhance the reflectance of the Ag film) the for Ag film, the best low refractive material is MgF 2. Fig. 5. XRD structure of the three samples: (a) sample 1, (b) sample 2 and (c) sample 3.

5 X. Xu et al. / Applied Surface Science 245 (2005) In order to enhance the adherence the Ag film and the other coating or substrate, Al 2 O 3 must be one of the best intermediate layer in common materials. 4. Conclusion In this article, we compared the adherence of Ag films and Al 2 O 3, SiO 2 and MgF 2 on the basis of lots of experiment results and theory analysis. The conclusion can be drawn that a-al 2 O 3 > MgF 2 > SiO 2 the conclusion can be explained by the difference of the three samples XRD diffraction spectrum. When protective coating is needed for Ag film, we should select MgF 2, if humidity durability is especially needed we can choice the Al 2 O 3, and the SiO 2 cannot be used as protective coating. When we need enhancing coating for Ag film, the best low refractive material is MgF 2. In order to enhance the adherence the Ag film and the other coating or substrate, Al 2 O 3 must be one of the best intermediate layer. References [1] X. Xu, et al. Effect of chromium intermediate layer on properties of silver coatings, Opt. Eng. 43 (3) (2004). [2] J. Sun, et al. Study on adhesion and stress of Hg sensitize photo- CVD Sio 2 film on silicon substrate, Microelectron. Comput. 2 (1988) (in Chinese). [3] J. Tang, Solid Thin Film in the Optics, Science Press House, Beijing, 1987, p. 142 (in Chinese). [4] Z. Zhou, Glass Technology, Northwest Institute of Light Industry Publishing Company, 1992.