Study on Infrared Absorption Characteristics of Ti and TiN x Nanofilms. Mingquan Yuan, Xiaoxiong Zhou, Xiaomei Yu

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1 10.119/ The Electrochemical Society Study on Infrared Absorption Characteristics of Ti and TiN x Nanofilms Mingquan Yuan, Xiaoxiong Zhou, Xiaomei Yu National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing, , China Infrared detector has a wide range of applications. To fabricate an IR detector with good sensitivity, an efficient IR absorber is needed. In this paper, we theoretically and experimentally investigated the absorption characteristics of Ti, multi-layer structured Ti-SiO 2 and TiN x -SiN x nano films. The multi-layer structured TiN x -SiN x films with different nitrogen contents and Ti nano films with different thicknesses were deposited by e- beam sputtering. Their IR absorption characteristics were investigated by Fourier Transform Infrared Spectrometer. For single nano Ti film, 35% absorptivity was obtained at a wavelength of 1.67μm when the film thickness was 15.3nm. A better IR absorption characteristic was achieved for multi-layer structured Ti-SiO 2 nano film. In this structure, an absorption peak of appears at a wavelength of 9.5μm. Compared with the absorption behavior of single SiN x film, an additional nano TiN x film obviously improved IR absorption at a wider band. And a maximum IR absorptivity of 27% was obtained at the wavelength of 1μm. Introduction Infrared (IR) detector has a wide range of applications including night vision, environmental monitoring, security surveillance, remote sensing, biomedical diagnostics, and thermal probing of active microelectronic devices. It can be generally divided into two categories: photon and thermal detectors (1). When IR photon detectors are not widely used since they require huge and expensive cooling systems, MEMS based thermal detectors have gained increasing attention due to their low cost, low weight, low power, simple device structure and compatibility with microfabrication processes (2). Among them, uncooled bimaterial microcantilever IR detector is a typical one that has received considerable progress during the past years (3). To achieve good IR absorptivity and mechanical sensitivity simultaneously, a thin IR absorber that can convert IR radiation into heat efficiently is needed (). Thin metallic films and metallic compound can be used as IR absorbers because electrons in metallic films can interact with IR radiation but cannot move freely. In this way the energy of IR radiation is taken away and absorption occurs (5). For a thin film material, the maximum absorptivity occurs when its sheet resistance is 377Ω/ equaling to the impedance of free space. M. Almasri reported that IR absorptivity of 75% can be obtained in the long wavelength region using a.5nm thick NiCr layer as the absorber (6). A TiW layer with the thickness of 16nm and the sheet resistance of 50Ω/ was deposited for IR absorption. By using a resonance cavity, an IR absorptivity of 60% for TiW can be reached (7). In M. Hsieh s work, a strong absorption of about 90% at 7~13μm was achieved using a 6.2nm Ni film (8). 129

2 In this paper, Ti, multi-layer structured Ti-SiO 2 and TiN x -SiN x nano films were deposited as IR absorbers. And their IR absorption behaviors were studied. Theory of IR Absorption on Nano Metallic Films The absorptivity of a thin metallic film can easily be calculated from A=1-R-T [1] Where A is the absorptivity, R is the reflectivity, and T is the transmissivity. To decrease R and T is an effective way for increasing IR absorption, as shown in Figure 1. Infrared radiation Transmission (T) Reflection (R) lossless dielectric s Figure 1 A model of a thin metallic film in front a lossless dielectric Figure 2 shows the diagrammatic sketch that infrared radiates from left side through metal and dielectric films. In this model, media 1 and are vacuum medium, media 2 is thin metallic film and media 3 is dielectric film. The model is in free space, the first and fourth media have a refractive index equal to unity. 1 2 Metal 3 Dielectric E 12 E 22 E 32 E 11 E 21 E 31 E 1 x=0 x=s x=d+s Figure 2. Schematic diagram of metal film supported on a dielectric Using the equations set up by Hadley and Dennison, we can get the appropriate boundary equations through the theory of electromagnetic wave (9). We define absorptivity as A L when infrared radiates from the left-hand side and A R when infrared radiates from the right-hand side. 130

3 For the first condition, infrared radiates from left side through metal film and dielectric film. When x=0, E11 E12 E21 E22 [2] E E a ( E E ) [3] When x=s, E exp( iks) E exp( iks) E exp( iks) E exp( iks) [3] When x=d+s, E exp( iks) E exp( iks) a[ E exp( iks) E exp( iks)] [] E exp[ ik ( d s)] E exp[ ik ( d s)] E exp[ ik ( d s)] [5] E exp[ ik ( d s)] E exp[ ik ( d s)] a E exp[ ik ( d s)] [6] The expression of k 1, k 2, k 3, k are expressed as follows: k 1 =2π/λ, k 2 =2π(η+iκ)/λ, k 3 =2πn/λ, k =2π/λ. And a 1 =k 2 /k 1, a 2 =k 3 /k 2, a 3 =k /k 3. Where, s is thickness, η is refraction index and κ is extinction coefficient of metallic film. d is thickness and n is refraction index of the lossless dielectric film. The reflectivity R and transmissivity T can be calculated. R E E / E E [7] * * T E E / E E [8] * * A L and A R can then be calculated by formula [1] f(cos k3d sin k 2 3d) AL n [9] 2 f n ( 1) sin 2 k3d ( f 2) cos k3d n f AR 2 f [10] n ( 1) sin 2 k3d ( f 2) cos k3d n 1/2 Where f s( 0 / 0) is the ratio of free space impedance to sheet resistance of nano film, σ is conductivity, μ 0 is permeability, and ε 0 is dielectric constant. When sheet resistance of deposited metallic film equals to 377Ω/, the maximum absorptivity can be obtained. Thin metallic film can be deposited with a specific sheet resistance by changing its electrical resistivity ρ and the thickness s of the film. For a particular metallic film, ρ is a nature physical property. To decrease the IR reflection from the absorber surface, films with the thickness of a few nanometers should be designed to match the impedance of free space. 131

4 Experimental Procedure Experiments and Results Before depositing nano Ti film, a 100nm thick SiO 2 film was deposited on a double side polished silicon wafer by LPCVD at the temperature of 720 C and pressure of 250mtorr. Then, the nano Ti films were deposited via E-beam sputtering of Ti target at room temperature and base pressure of 1Pa. The electrical resistivity for bulk titanium (20 C) is 20 nω m, which means that the Ti film with a thickness of 1.1nm owns a sheet resistance of 377Ω/. Yet, thin metallic film deposited by PVD commonly has an electrical resistivity about 2~7 times larger than that of bulk material. Based on this, Nano Ti films with different thicknesses,.9nm, 6.8nm, 13.nm and 15.3nm, were deposited on 100nm thick SiO 2 layers. As a metallic compound, nano TiN x films can also be applied as IR absorbers. It is a transition metal nitride which has a structure combined with covalent bond, ionic bond and metal bond. TiN x films were sputtered in the N 2 atmosphere. In our design, the effect on IR absorption behaviors of an additional thin TiN x film deposited on 200nm thick SiN x films was investigated. Besides, we also tried to find out the relationship between IR absorptivity and the amount of nitrogen in TiN x. In order to obtain TiN x film with different proportions of titanium and nitrogen, different Ar-N 2 flow ratios were controlled during the sputtering process. In our design, TiN x films with the thicknesses of 9nm and 1.3nm were deposited on 200nm thick SiN x films with Ar-N 2 flow ratios of 1:1 and 1:2 respectively. Testing Results The reflectivity and the transmissivity were acquired using Nicolet Magna 750 Fourier Transform Infrared Spectrometer (FTIR). And the IR absorptivity was calculated by formula [1]. Figure 3 is the infrared absorption spectra of the Ti nano films at different thicknesses without SiO 2 layers in the wavelength region from 1.67μm to 12μm. Due to the limitation of IR source, the wavelength of 1.67μm is the threshold in our experiment. It can be seen that the 15.3nm thick Ti film exhibits the best IR absorption ability among the samples at short-, mid- and long-wave infrared bands. A maximum absorptivity of 35% is achieved at wavelength of 1. 67μm. And Absorptivity(%) nm 13.nm 6.8nm.9nm Wavelength(um) Figure 3. IR absorptivity of Ti films with different thicknesses without SiO 2 layer 132

5 the 15.3nm thick Ti film can also efficiently absorb about 30% IR radiation in midwave region. However, compared with absorption characteristics in short- and midwave band, an absorption bottom is shown at 9.5μm wavelength. Table I lists sheet resistance of Ti films, which was measured by four-point probe meter. The 6.8nm thick Ti film has a sheet resistance of 77 / that is about twice as much as the impedance of free space. Though sheet resistance of.9nm thick Ti film was not acquired exactly by four-point probe meter, it should be larger than that of 6.8nm thick Ti film due to its inverse relation with the impedance of free space. The sheet resistance of 15.3nm and 13.nm thick Ti films is relatively close to the impedance of free space, which explains the reason of high absorptivities of 13.nm and 15.3nm Ti films. TABLE I. Sheet resistance of Ti films. Thickness/nm Sheet Resistance/Ω The IR absorption spectra for multi-layer structured Ti-SiO 2 nano films were shown in Figure. The absorption peaks at the wavelength of 9.5μm were observed for the Ti films with four different thicknesses. And the improved IR absorptivities were exhibited in short-, mid- and long-wave infrared region for multi-layer structured films. Compared with the absorptivities of single Ti films shown in Figure 3, the absorptivities of multi-layer structured Ti-SiO 2 nano films have got an increase, especially in the long-wave infrared region. A maximum absorptivity of 35% is achieved at wavelength of 1.67μm. Besides, the IR absorptivity of a single nano SiO 2 film with the thickness of 100nm was also measured and was shown in Figure. And a strong absorption peak occurs at the wavelength of 9.5μm, which is beneficial to enhance the IR absorptivity of the multi-layer structured Ti-SiO 2 nano films. It also explains the phenomenon of two inverse absorption peaks shown in Figure 3 and Figure respectively. Absorptivity(%) nm Ti+ 100nm SiO2 13.nm Ti+ 100nm SiO2 6.8nm Ti+ 100nm SiO2.9nm Ti+ 100nm SiO2 100nm SiO Wavelength(um) Figure. IR absorptivity of Ti-SiO 2 films and SiO 2 film 133

6 IR absorption spectra of single 200nm SiN x film and multi-layer structured TiN x - SiN x film were measured from wavelength of 2.5μm to 25μm. The SiN x film, as is shown in Figure 5, has absorption peak at the wavelength of 12μm. The maximum absorptivity of 20% can be obtained. However, its absorptivity decreases rapidly in other bands. Compared with the absorptivity of single SiN x film, an additional TiN x film deposited obviously improves IR absorption at a wider band. In mid-wave infrared region, the absorptivity of 2% can be obtained at wavelength of 2.5μm. IR absorptivity of TiN x film deposited in the flow ratio of Ar/N 2 equaling to 9:1 was investigated by Y. Zheng (10). Compared with his testing results, much better IR absorptivities were obtained in this paper. In our work, nano TiN x films can efficiently absorb IR radiation at multi-wave bands. At the same time, a maximum absorptivity of 27% can be obtained in our work while it was only 20% in Y. Zheng s work (10). It is proven that an increased proportion of nitrogen in TiN x film can result in a better absorption behavior. This phenomenon can be explained as follows. When the content of nitrogen increases in TiN x, a rougher surface is formed. And the roughness of surface can decrease the reflectivity of infrared, which may result in an improvement of IR absorption nmTiNx+200nmSiNx(Ar:N=2:1) 9nmTiNx+200nmSiNx(Ar:N=1:1) 200nmSiNx Absorptivity(%) Wavelength(um) Figure 5. IR absorptivity of multi-layer structured TiN x -SiN x films and SiN x films Conclusions In this paper, we presented the theory, experiment and test results of IR absorption of Ti, TiN x, Ti-SiO 2 and TiN x -SiN x nano films. A good IR absorption characteristic of Ti nano film can be obtained with its sheet resistance close to 377Ω/. For multi-layer structure of 15.3nm Ti and 100nm SiO 2, IR absorptivity of 35% can be achieved at the wavelength of 1.67μm. Furthermore, an additional TiN x deposition on SiN x layer broadens the IR absorption spectra and results in a good absorptivity in mid-wave infrared band. Further researches in developing high efficient IR absorbers at multiwave are needed, especially on structure designing to target at certain frequency precisely. Acknowledgments This work was supported by Natural Science Foundation of China (founded No , and ). 13

7 References 1. X. Yu, Y. Yi, S. Ma, et al., J. Micromech. Microeng, (8pp), 18 (2008). 2. S. Eminoglu, M. Y. Tanrikulu and T. Akin, J. Microelectromech. Syst, 20, 17 (2008). 3. D. Grbovic, N. V. Lavrik and P. G. Datskos, Appl. Phys. Lett, 89, (2006).. M. Schossig, V. Norkus and G. Gerlach, IEEE Sens. J, 156, 10 (2010). 5. W. Lang, K. Kühl and H. Sandmaier, Sensor. Actuat. A-Phys, 23, 3 (1992). 6. M. Almasri, B. Xu and J. Castracane, IEEE Sens. J, 293, 6 (2006). 7. C. D. W. Jones, C. A. Bolle, R. Ryf, et al., Sensor. Actuat. A-Phys, 7, 155 (2009). 8. M. Hsieh, Y. Fang, P. Wu, et al., IEEE Sens. J, 360, 2 (2002) 9. L. N. Hadley and D. M. Dennison, J. Opt. Soc, 51, 37 (197). 10. Y. Zheng, X. Yu, M. Yuan, et al, IEEE NEMS, 25, Kaohsiung (2011). 135

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