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Aluminum nitride thin film growth and applications for heat dissipation
Aluminum Nitride Thin Film Growth and Applications for Heat Dissipation Yingbin Bian, Moning Liu, Yigang Chen, Jim DiBattista, Eason Chan, Yimou Yang PII: DOI: Reference:
S0257-8972(14)01115-3 doi: 10.1016/j.surfcoat.2014.11.060 SCT 19921
To appear in:
Surface & Coatings Technology
Please cite this article as: Yingbin Bian, Moning Liu, Yigang Chen, Jim DiBattista, Eason Chan, Yimou Yang, Aluminum Nitride Thin Film Growth and Applications for Heat Dissipation, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.11.060
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Aluminum Nitride Thin Film Growth and Applications for Heat Dissipation Yingbin Bian1, Moning Liu1, Yigang Chen1, *, Jim DiBattista2, Eason Chan2, Yimou Yang2 1 Dept. of Electronic Information Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China 2 Darly Photonics Composite Materials (Shanghai) Corp. No.819, Songwei Bei Lu, Songjiang Industrial Zone, Shanghai 201613, China Abstract Aluminum nitride (AlN) thin film, due to its electrical and thermal properties, can be used as thermal interface material for flexible electronics. The relationship between thermal conductivity and microstructure of aluminum nitride film was studied on films grown on glass by DC magnetron reactive sputtering at room temperature. The crystal orientation, deposition rate and grain size of AlN films were affected by the deposition power. The crystallization quality and the effective thermal conductivity of the AlN films were strongly dependent on the film thickness at the optimum power of 600W. The bulk thermal conductivity of AlN films was found to be 15.4 W/(m·K) in this study.
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*Corresponding author. Phone: +86-21-66132807; fax: +86-21-66132807. Email: [email protected]
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1. Introduction Flexible electronics have become an important research topic in the past decades.1 As flexible substrates, films and fibers have advantages compared to classic semiconductor materials in terms of cost, large scale fabrication and biocompatibility.2,3 These diverse substrate materials are being investigated for displays, memory, circuitry, photovoltaic devices and MEMS sensors.2 However, the use of these flexible materials impose design limitations due to the heat dissipation and low thermal stability limits during device processing and operation.4 Aluminum nitride (AlN), one of the III-V compound semiconductors with a wurtzite crystalline structure, is promising for high-frequency surface acoustic wave (SAW) devices due to its good piezoelectric performance and as a heat dissipation layer for electronic devices due to its high thermal conductivity, wide energy band gap and high breakdown voltage.5-8 For flexible electronics AlN films combine high thermal conductivity, high optical transmittance, low expansion coefficient and low temperature preparation which are major factors favoring its use as a heat spreader material for these applications. For several decades the thermal conductivity of AlN films has been extensively investigated.9-11 Growth of AlN films with good crystal quality on flexible substrates is still a pending matter as the oriented growth of AlN requires specific surface conditions which are mainly achieved on metallic layers (Ir, Ru, Pt, W or Mo) or single crystal substrates such as sapphire or silicon.12 Due to temperature limitations, differences in thermal expansion coefficients and the amorphous state of the flexible substrates, it is a technological challenge to deposit high-quality AlN films on flexible substrates.1 In this study, glass is chosen as the substrate due to its amorphous state and relative temperature limitations similar to most flexible substrates. In this study AlN films are deposited at room temperature to simulate conditions that have the potential to be used to prevent damage to flexible substrate materials and is compatible with complementary metal oxide semiconductor (CMOS) technology.1 AlN films can be prepared by various techniques such as chemical vapor deposition (CVD), reactive magnetron sputtering, reactive evaporation, molecular beam epitaxy (MBE), ion beam-assisted deposition, laser and plasma assisted CVD and metal organic chemical vapor deposition (MOCVD).15-18 Magnetron sputtering has been widely used in the industrial process of thin-film deposited on glass over the last decades due to its high rate, low cost, low temperature and ease of scaling.18 In this paper the AlN films have been successfully deposited on amorphous glass substrates by DC magnetron reactive sputtering at room temperature. The structure, morphology and thermal conductivity of the AlN film have been investigated. There are a limited number of reports on the synthesis of AlN films on glass at room temperature to review for comparison.13,14
2. Experimental Section AlN films were deposited on commercial glass substrates (20 mm x 20 mm) using Al (99.99% purity) target (50 mm in diameter and 4 mm in thickness) by a DC planar magnetron sputtering system. The substrates were cleaned in a ultrasonic bath with acetone, ethanol and de-ionized water, respectively. In this study, the distance between the substrate and Al target was 65 mm, which was kept constant for all depositions. The
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chamber was initially evacuated to vacuum level of 5×10-3 Pa by using a turbo molecular pump followed by a mechanical pump and fixed as base pressure for coating. Ar (99.999%) and N2 (99.999%) mixed gas were used for AlN coating growth and the total pressure was 0.8 Pa. In order to remove the surface oxides of the target, pre-sputtering in Ar atmosphere was carried out for 10 min before AlN deposition with a pressure of 0.2 Pa. The DC power supplied was in a range of 100-700 W to meet the experimental demands. The deposition parameters of AlN films are summarized in Table 1.
Substrate
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Table 1. Deposition parameters of AlN films. Glass
99.99% 65
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Target to substrate distance (mm)
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Substrate temperature (°C)
RT
Power (W)
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Deposition time (h)
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N2 concentration (%)
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3. Results and Discussion
3.1 Deposition Rate Deposition rate of AlN film is an important factor to determine the adoption of this thin film method for industrial application. Figure 1 shows the relationship between the deposition rate of the AlN films and sputtering power. The film thickness is measured using a step profiler. From Figure 1 it is easily seen that the deposition rate of AlN films is linearly associated with power and the maximum rate of 3.3 μm/h at 600 W is higher than many other deposition methods13,19,20. Atul Vir Singh et al. also reported similar relationship between deposition rate and power in RF magnetron reactively sputtered AlN films.13 Figure 1. The relationship between the deposition rate of the AlN films and sputtering power. 3.2 XRD Figure 2(a) shows the XRD patterns of AlN films grown at different power at room temperature illustrating how the diffraction peaks of AlN films change with the power ranging from 100 to 700 W. For AlN film deposited at 100 W there is no visible diffraction peaks indicating that the deposited film is amorphous. When the power increases from 200 to 500 W the intensity of AlN (100) and (110) diffraction peaks (2θ=33.2° and 59.3°) increase (PDF 76-0702). Particularly at 500 W the intensity of AlN (100) peak is the highest and the value of FWHM (0.215°) is the lowest which indicates
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that the film was grown with a preferred orientation of AlN (100). When the power increases from 500 to 700 W the intensity of AlN (100) and (110) diffraction peaks recede significantly. AlN (002) diffraction peak (2θ=36°) emerges at 300W and the intensity of AlN (002) peak gradually increases with increasing power from 300 to 700 W. At the power of 700 W the intensity of AlN (002) is very high and the FWHM is 0.218°, which indicates that the film has a preferred orientation of AlN (002). It is shown from Figure 2(a) that the preferred orientation of AlN films grown on glass at room temperature has a turning point of power at 600 W. The atoms involved in the AlN (002) orientation require a higher kinetic energy to form a closely packed structure as compared to the (100) or (110) orientation.21 For the (002) orientation the c-axis is normal to the substrate and the plane parallel to the substrate is the closely packed basal plane with either all aluminum or nitrogen atoms.22 So the high kinetic energy and large mean free path of particles deposited on substrate caused by high power will grow c-axis AlN films. The AlN films deposited at 700W tend to brittle and prone to cracking from induced stress. Furthermore, the high-power deposition may increase the substrate temperature which may damage the flexible substrate.1,13 So in this study the optimized deposition power is 600W. Figure 2(b) shows the XRD patterns of AlN films grown on glass at the power of 600 W with different thickness. It can be seen that the intensity of AlN (002) diffraction peak increases significantly when the thickness increases from 1 μm to 10 μm, and the value of FWHM (0.312°) at 10 μm is lowest. As reported13, the oriented growth of AlN films requires specific surface condition and single crystal substrate is beneficial for preferred growth of AlN films. As the thickness increases, the crystal quality of the under layer improves, thus, affecting the preferred (002) growth of the AlN films.
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Figure 2 (a). The XRD patterns of AlN films grown at different power at RT; (b). The XRD patterns of the AlN films grown on the glass at the power of 600 W with different thickness.
3.3 AFM Figure 3 shows AFM images of AlN films (2.0×2.0 µm2) deposited on glass substrates at RT at different power levels. The mean surface roughness (Ra) and mean grain diameter (D) of deposited AlN films at different power are listed in Table 2 and the film thickness is approximately 3 µm. The AFM images show that the grain diameter increases by increasing the power, and the largest grain diameter reachs 250.8 nm at 600 W, while the value of the surface roughness (Ra) remains constant at around 15 nm. The grain size is determined by the competition between growth rate and nucleation rate. Usually, a low nucleation rate and a high growth rate lead to a larger grain size.16 During the deposition of AlN films, the high kinetic energy and high surface mobility caused by high power result in a high growth rate. Thus the grain size of the films becomes larger at high power. Figure 3. AFM images of AlN films grown at RT at different power: (a)300 W, (b)400 W, (c)500 W, (d) 600 W.
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Table 2. Mean square roughness (Ra) and the mean grain diameter (D) at RT at different power: (a)300 W; (b)400 W; (c)500 W; (d) 600 W. Sample Power(W) Ra (nm) D (nm) a 300 7.2 40.1 b 400 14.2 138.4 c 500 14.0 186.6 d 600 14.6 250.8
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3.4 SEM The microstructure and morphology of AlN films were observed by scanning electron microscopy (SEM). Figure 4(a) shows SEM image of AlN films grown at the power of 600 W at RT. The surface morphology features smooth and homogeneously uniform pebble-like grains formed on the substrate. Figure 4(b) shows a SEM cross section image of AlN films prepared at the power of 600W at RT. From the image a well-defined transition can be seen between the (002) crystalline AlN structure of columnar structure with a thickness of about 3 µm. and a disordered region about 400 nm thick deposited on the substrate.
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Figure 4 (a). SEM images of AlN films prepared at the power of 600 W at RT; (b). Cross section of AlN films prepared at the power of 600 W at RT.
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3.5 Thermal Conductivity Thermal conductivity of AlN films deposited at a power of 600 W at RT were characterized along the thickness direction using time-domain thermoreflectance (TDTR) method at Precision Measurements and Instruments Corporation. The specific heat capacity used was 0.76 J/(g·K) . In this method a Ti: Sapphire laser produces near-infrared radiation incident on the film surface and creates local heating. The same laser is used to simultaneously probe the surface by monitoring the changes in intensity of the reflected beam from changes in temperature of the film. Small changes in the intensity of the reflected probe beam are measured using lock-in detection. As is known, the thermal conductivity of thin films can be vastly different from that of the corresponding bulk materials. For the bulk material AlN has a theoretical thermal conductivity kTH of 320 W/(m·K), while the experimental value for AlN films has been reported to be much lower with a range from 0.4 to 170 W/(m·K).5,8,23-25 Thermal conduction of thin film is determined by the lattice vibration wave (phonon) and the mean free path of which is affected by the interface scattering, defects and impurity scattering as well as grains boundary scattering.23-26 The results of thermal conductivity tests have been shown in Table 3 for thickness values of 1, 5 and 10 μm, which are 7.5, 9.8 and 13.3 W/(m·K), respectively. It can be seen that the effective thermal conductivity of the AlN films is closely related to the thickness and this result agrees with that reported by Sun Rock Choi24 and K. Ait Aissa25. Table 3. Effective thermal conductivity of AlN films prepared on the glass with different thickness. Thickness (μm) Thermal Conductivity (W/(m·K)) 1 7.5
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9.8 13.3
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AlN films were examined by SEM and the cross section image of the film is shown in Figure 4(b). It is obvious that a disordered layer of AlN film with a thickness of 400 nm was grown initially on the glass substrate followed by the growth of highly oriented columnar AlN with a thickness of 3000 nm. Furthermore, the XRD patterns of AlN films are shown in Figure 2(b). It can be seen in the early growth stages that the AlN film grew poorly with (100) orientation that tends to form a disordered and amorphous layer. As the thickness of film increases crystalline AlN (002) planes begin to grow on the disordered layer and the intensity of AlN (002) diffraction peak becomes larger and matches the cross section image. G.W.Auner et al. observed a similar initial amorphous layer of AlN film grown on Si (111) substrate at 400 oC by plasma source molecular beam epitaxy.26 Furthermore, a similar observation has also been made by Yong et al. during the growth of AlN; H films on Si(100) substrates at RT by RF reactive sputtering.27 They suggest that the uniformly formed amorphous layer reduces the lattice mismatch between AlN film and substrate and the short range ordering in the amorphous layer provides nucleation sites for the growth of c-axis oriented crystallites perpendicular to the substrate. This study indicates that the quality of AlN films on the glass is affected by lattice mismatch and the energy of the adatoms. In the early growth stages of AlN the lattice mismatch between AlN film and substrate is large and the adatoms do not have the proper energy and mobility to align for preferred growth at RT. As the thickness of the disordered layer increases both the reduction of the lattice mismatch and the increase of the substrate temperature (caused by the continuous bombardment of sputtering ions during the sputtering process) lead to the crystalline AlN (002) planes which is roughly parallel to the substrate surface to grow on the disordered layer. As a result, with the increasing of the film thickness, AlN films prepared on the glass at RT at 600 W have a gradual change in orientation from disorder to ordered AlN (002). An intermediate amorphous layer which has poor thermal conductivity was formed between the glass substrate and the AlN film. The thickness of such an interfacial layer remains constant after the first stage of the deposition. It is difficult to directly evaluate the bulk thermal conductivity due to AlN/Glass interface and the thermal boundary resistance was not considered in Table 3. As reported8, effective thermal conductivity k appears as a thickness dependent property that can be related with the AlN thickness with relation (1) derived from a relationship: k (kb 1 RGlass / AlN
1 1
)
(1)
where RGlass / AlN is the thermal boundary resistance of the amorphous layer between glass substrate and AlN film and kb is the thermal conductivity in the bulk of the AlN film. Figure 5 shows the thermal resistance of the AlN films, k / according to the thickness. The line in figure 5 is obtained by linear regression. According to relation (1) and fitting line the bulk thermal conductivity and the boundary resistance are found to be equal to 15.4 W/(m·K) and 1 10-7 K·m2·W-1, respectively. As reported8,25,28 the thermal conductivity of AlN films can be strongly affected by impurities such as oxygen and will be researched in the future work. Compared to other materials used in IC technology, the
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Figure 5. Thermal resistance of AlN films prepared on glass with different thickness.
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4. Conclusion In this study AlN films are grown on glass substrate by DC magnetron reactive sputtering at room temperature. The optimum power is 600W according to the preferred orientation and temperature limitation of flexible electronics processing. The deposition rate is measured at a maximum of 3.3 μm/h with a grain size of 250.8 nm at 600 W. The crystalline quality and the effective thermal conductivity of the films are found to be improved with the increase of film thickness. Compared to other thermal materials used in IC technology, the bulk thermal conductivity of AlN films along the film thickness direction is 15.4 W/(m·K) is relatively high.
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Acknowledgement This study is primarily supported by Shanghai Pujiang Program under grant no. 11PJ1403400. The authors thank the support from the Instrumental Analysis and Research Center of Shanghai University and GE (China) Research and Development Center Co., Ltd..
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Refference [1] H. Jin, J. Zhou, S.R. Dong, B. Feng, J.K. Luo, D.M. Wang, W.I. Milne, and C.Y. Yang, “Deposition of c-axis orientation aluminum nitride films on flexible polymer substrates by reactive direct-current magnetron sputtering,” Thin Solid Films, 520, 4863–4870, 2012. [2] L. Chang, “Recent developments in polymer MEMS,” Advanced Materials, 19, 3783–3790, 2007. [3] S. Petroni, C.L. Tegola, G. Caretto, A. Campab, A. Passaseo, M. D. Vittorioa, and R. Cingolani, “Aluminum Nitride piezo-MEMS on polyimide flexible substrates,” Microelectronic Engineering, 88, 2372–2375, 2011. [4] D.A. van den Ende, R.H.L. Kusters, M. Cauwe, A. van der Waal, and J. van den Brand , “Large area flexible lighting foils using distributed bare LED dies on polyester substrates,” Microelectronics Reliability, 53, 1907–1915, 2013. [5] S. Shanmugan, D. Mutharasu, and A.H. Haslan, “A study on AlN thin film as thermal interface material for high power LED,” International Journal of Electronics and Computer Science Engineering, 2 (1), 296-300, 2012. [6] J. Petroski, “Thermal challenges facing new generation light emitting diodes (LEDs) for lighting applications,” International Symposium on Optical Science and Technology, pp. 215-222, 2002. [7] Y. Huaiyu, S. Koh, H. van Zeijl, A.W.J Gielen, and Z. Guoqi, “A review of passive thermal management of LED module J,” Journal of Semiconductors, 32 (1), 014008, 2011. [8] C. Duquenne, M.P. Besland, P.Y. Tessier, E. Gautron, Y. Scudeller, and D. Averty, “Thermal conductivity of aluminium nitride thin films prepared by reactive magnetron sputtering,” Journal of applied physics, 45 (1), 015301, 2012. [9] R.B. Dinwiddie, A.J. Whittaker, and D.G. Onn, “Thermal conductivity, heat capacity, and thermal diffusicity of selected commercial AlN substrate,” International Joural of Thermophysics, 10 (5), 1075-1084, 1989. [10] H. Kitagawa, Y. Shibutani, and S. Ogata, “Ab-initio simulation of thermal properties of AlN ceramics,” Modelling Simul.Mater.Sci.Eng, 3 (4), 521-531, 1995. [11] J. Bodzenta, B. Burak, A. Jagoda, and B. Stanczyk, “Thermal conductivity of AlN and AlN-GaN thin films deposited on Si and GaAs substrates,” Diamond and related materials, 14 (3), 1169-1174, 2005. [12] J. Olivares, J. Capilla, M. Clement, J. Sangrador, and E. Iborra, “Growth of AlN oriented films on insulating substrates,” International Ultrasonics Symposium Proceedings, 0428, 1716-1719, 2011. [13] A.V. Singh, S. Chandra, and G. Bose, “Deposition and characterization of c-axis oriented aluminum nitride films by radio frequency magnetron sputtering without external substrate heating,” Thin Solid Films, 519, 5846–5853, 2011. [14] S. Shanmugan, D. Mutharasu, P. Anithambigai, N. Teeba, and I. Abdul Razak, “Synthesis and structural properties of DC sputtered AlN thin films on different substrates,” Journal of Ceramic Processing Research, 14 (3), 385~390, 2013. [15] Q.X. Guo, M. Yoshitugu, T. Tanaka, M. Nishio, and H. Ogawa, “Microscopic investigations of aluminum nitride thin films grown by low-temperature reactive sputtering,” Thin Solid Films, 483 (1) 16–20, 2005
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[16] N. Setter, D. Damjanovic, L. Eng, G. Fox, S. Gevorgian, S. Hong, A. Kingon, H. Kohlstedt, N. Park, and G. Stephenson, “Ferroelectric thin films: review of materials, properties, and applications,” Journal of Applied Physics, 100 (5), 051606, 2006 [17] Vacuum science and technology: nitrides as seen by the technology, edited by T. Paskova and B. Monemar, Kerala, 2002. [18] P.J. Kelly and R.D. Arnell, “Magnetron sputtering: a review of recent developments and applications,” Vacuum, 56 (3), 159-172, 2000. [19] M. Alevli, C. Ozgit, I. Donmez, and N. Biyikli, “Structural properties of AlN films deposited by plasma-enhanced atomic layer deposition at different growth temperatures,” Physica Status Solidi, 209 (2), 266-271, 2012. [20] B. Potì, A. Campa, M.C. Larciprete, C. Sibilia, and A. Passaseo, “Growth and nonlinear characterization of AlN/GaN structures,” Journal of Optics A: Pure and Applied Optics, 8 (7), S524-S527, 2006. [21] H.L. Kao, P.J. Shih, and C.H. Lai, “The Study of Preferred Orientation Growth of Aluminum Nitride Thin Films on Ceramic and Glass Substrates,” Journal of Applied Physics, 38 (3R), 1526-1529, 1999. [22] W.L. Jung, and J.C. Jerome, “Aluminum nitride thin films on an LTCC substrate,” Journal of the American Ceramic Society. 88 (7), 1977-1980, 2005. [23] L.L. Spina, H. Schellevis, N. Nenadović, S. Milosavljević, W.H.A. Wien, A.W. van Herwaarden, and L.K. Nanver, “PVD aluminium nitride as heat spreader in IC technology,” STRESS. 407, 574, 2005. [24] C. Sun, K. Dongsik, C. Sung-Hoon, L. Sung-Hoon, and K. Jong-Kuk, “Thermal Conductivity of AlN and SiC Thin Films,” International Journal of Thermophysics, 27 (3), 896-905, 2006. [25] K. Ait Aissa, N. Semmar, D. De Sousa Meneses, L. Le Brizoual, M. Gaillard, A. Petit, P-Y. Jouan, C. Boulmer-Leborgne, and M.A.Djouadi, “Thermal conductivity measurement of AlN films by fast photothermal method,” 6th European Thermal Sciences Conference, 395, 012089, 2012. [26] G.W. Auner, F. Jin, V.M. Naik, and R. Naik, “Microstructure of low temperature grown AlN thin films on Si(111),” Journal of applied physics, 85 (11), 7879-7883, 1999. [27] Y.J. Yong, J.Y. Lee, H.S. Kim, and J.Y. Lee, “High resolution transmission electron microscopy study on the microstructures of aluminum nitride and hydrogenated aluminum nitride films prepared by radio frequency reactive sputtering,” Journal of applied physics. 71 (11), 1489-1491, 1997. [28] G.A. Slack, L.J. Schowalter, D. Morelli, and J.A. Freitas,” Some effects of oxygen impurities on AlN and GaN,” Journal of Crystal Growth, 246 (3), 287-298, 2002.
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Aluminum nitride thermal conductivity measured by TDTR method to be 13 W/mK Thermal conductivity is related to film thickness and crystal orientation Aluminum nitride film growth rate can reach 3.3 um/hour
S0257-8972(14)01115-3 doi: 10.1016/j.surfcoat.2014.11.060 SCT 19921
To appear in:
Surface & Coatings Technology
Please cite this article as: Yingbin Bian, Moning Liu, Yigang Chen, Jim DiBattista, Eason Chan, Yimou Yang, Aluminum Nitride Thin Film Growth and Applications for Heat Dissipation, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.11.060
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Aluminum Nitride Thin Film Growth and Applications for Heat Dissipation Yingbin Bian1, Moning Liu1, Yigang Chen1, *, Jim DiBattista2, Eason Chan2, Yimou Yang2 1 Dept. of Electronic Information Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China 2 Darly Photonics Composite Materials (Shanghai) Corp. No.819, Songwei Bei Lu, Songjiang Industrial Zone, Shanghai 201613, China Abstract Aluminum nitride (AlN) thin film, due to its electrical and thermal properties, can be used as thermal interface material for flexible electronics. The relationship between thermal conductivity and microstructure of aluminum nitride film was studied on films grown on glass by DC magnetron reactive sputtering at room temperature. The crystal orientation, deposition rate and grain size of AlN films were affected by the deposition power. The crystallization quality and the effective thermal conductivity of the AlN films were strongly dependent on the film thickness at the optimum power of 600W. The bulk thermal conductivity of AlN films was found to be 15.4 W/(m·K) in this study.
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*Corresponding author. Phone: +86-21-66132807; fax: +86-21-66132807. Email: [email protected]
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1. Introduction Flexible electronics have become an important research topic in the past decades.1 As flexible substrates, films and fibers have advantages compared to classic semiconductor materials in terms of cost, large scale fabrication and biocompatibility.2,3 These diverse substrate materials are being investigated for displays, memory, circuitry, photovoltaic devices and MEMS sensors.2 However, the use of these flexible materials impose design limitations due to the heat dissipation and low thermal stability limits during device processing and operation.4 Aluminum nitride (AlN), one of the III-V compound semiconductors with a wurtzite crystalline structure, is promising for high-frequency surface acoustic wave (SAW) devices due to its good piezoelectric performance and as a heat dissipation layer for electronic devices due to its high thermal conductivity, wide energy band gap and high breakdown voltage.5-8 For flexible electronics AlN films combine high thermal conductivity, high optical transmittance, low expansion coefficient and low temperature preparation which are major factors favoring its use as a heat spreader material for these applications. For several decades the thermal conductivity of AlN films has been extensively investigated.9-11 Growth of AlN films with good crystal quality on flexible substrates is still a pending matter as the oriented growth of AlN requires specific surface conditions which are mainly achieved on metallic layers (Ir, Ru, Pt, W or Mo) or single crystal substrates such as sapphire or silicon.12 Due to temperature limitations, differences in thermal expansion coefficients and the amorphous state of the flexible substrates, it is a technological challenge to deposit high-quality AlN films on flexible substrates.1 In this study, glass is chosen as the substrate due to its amorphous state and relative temperature limitations similar to most flexible substrates. In this study AlN films are deposited at room temperature to simulate conditions that have the potential to be used to prevent damage to flexible substrate materials and is compatible with complementary metal oxide semiconductor (CMOS) technology.1 AlN films can be prepared by various techniques such as chemical vapor deposition (CVD), reactive magnetron sputtering, reactive evaporation, molecular beam epitaxy (MBE), ion beam-assisted deposition, laser and plasma assisted CVD and metal organic chemical vapor deposition (MOCVD).15-18 Magnetron sputtering has been widely used in the industrial process of thin-film deposited on glass over the last decades due to its high rate, low cost, low temperature and ease of scaling.18 In this paper the AlN films have been successfully deposited on amorphous glass substrates by DC magnetron reactive sputtering at room temperature. The structure, morphology and thermal conductivity of the AlN film have been investigated. There are a limited number of reports on the synthesis of AlN films on glass at room temperature to review for comparison.13,14
2. Experimental Section AlN films were deposited on commercial glass substrates (20 mm x 20 mm) using Al (99.99% purity) target (50 mm in diameter and 4 mm in thickness) by a DC planar magnetron sputtering system. The substrates were cleaned in a ultrasonic bath with acetone, ethanol and de-ionized water, respectively. In this study, the distance between the substrate and Al target was 65 mm, which was kept constant for all depositions. The
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chamber was initially evacuated to vacuum level of 5×10-3 Pa by using a turbo molecular pump followed by a mechanical pump and fixed as base pressure for coating. Ar (99.999%) and N2 (99.999%) mixed gas were used for AlN coating growth and the total pressure was 0.8 Pa. In order to remove the surface oxides of the target, pre-sputtering in Ar atmosphere was carried out for 10 min before AlN deposition with a pressure of 0.2 Pa. The DC power supplied was in a range of 100-700 W to meet the experimental demands. The deposition parameters of AlN films are summarized in Table 1.
Substrate
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Table 1. Deposition parameters of AlN films. Glass
99.99% 65
Working pressure (Pa)
0.8
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Target to substrate distance (mm)
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Substrate temperature (°C)
RT
Power (W)
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3. Results and Discussion
3.1 Deposition Rate Deposition rate of AlN film is an important factor to determine the adoption of this thin film method for industrial application. Figure 1 shows the relationship between the deposition rate of the AlN films and sputtering power. The film thickness is measured using a step profiler. From Figure 1 it is easily seen that the deposition rate of AlN films is linearly associated with power and the maximum rate of 3.3 μm/h at 600 W is higher than many other deposition methods13,19,20. Atul Vir Singh et al. also reported similar relationship between deposition rate and power in RF magnetron reactively sputtered AlN films.13 Figure 1. The relationship between the deposition rate of the AlN films and sputtering power. 3.2 XRD Figure 2(a) shows the XRD patterns of AlN films grown at different power at room temperature illustrating how the diffraction peaks of AlN films change with the power ranging from 100 to 700 W. For AlN film deposited at 100 W there is no visible diffraction peaks indicating that the deposited film is amorphous. When the power increases from 200 to 500 W the intensity of AlN (100) and (110) diffraction peaks (2θ=33.2° and 59.3°) increase (PDF 76-0702). Particularly at 500 W the intensity of AlN (100) peak is the highest and the value of FWHM (0.215°) is the lowest which indicates
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that the film was grown with a preferred orientation of AlN (100). When the power increases from 500 to 700 W the intensity of AlN (100) and (110) diffraction peaks recede significantly. AlN (002) diffraction peak (2θ=36°) emerges at 300W and the intensity of AlN (002) peak gradually increases with increasing power from 300 to 700 W. At the power of 700 W the intensity of AlN (002) is very high and the FWHM is 0.218°, which indicates that the film has a preferred orientation of AlN (002). It is shown from Figure 2(a) that the preferred orientation of AlN films grown on glass at room temperature has a turning point of power at 600 W. The atoms involved in the AlN (002) orientation require a higher kinetic energy to form a closely packed structure as compared to the (100) or (110) orientation.21 For the (002) orientation the c-axis is normal to the substrate and the plane parallel to the substrate is the closely packed basal plane with either all aluminum or nitrogen atoms.22 So the high kinetic energy and large mean free path of particles deposited on substrate caused by high power will grow c-axis AlN films. The AlN films deposited at 700W tend to brittle and prone to cracking from induced stress. Furthermore, the high-power deposition may increase the substrate temperature which may damage the flexible substrate.1,13 So in this study the optimized deposition power is 600W. Figure 2(b) shows the XRD patterns of AlN films grown on glass at the power of 600 W with different thickness. It can be seen that the intensity of AlN (002) diffraction peak increases significantly when the thickness increases from 1 μm to 10 μm, and the value of FWHM (0.312°) at 10 μm is lowest. As reported13, the oriented growth of AlN films requires specific surface condition and single crystal substrate is beneficial for preferred growth of AlN films. As the thickness increases, the crystal quality of the under layer improves, thus, affecting the preferred (002) growth of the AlN films.
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Figure 2 (a). The XRD patterns of AlN films grown at different power at RT; (b). The XRD patterns of the AlN films grown on the glass at the power of 600 W with different thickness.
3.3 AFM Figure 3 shows AFM images of AlN films (2.0×2.0 µm2) deposited on glass substrates at RT at different power levels. The mean surface roughness (Ra) and mean grain diameter (D) of deposited AlN films at different power are listed in Table 2 and the film thickness is approximately 3 µm. The AFM images show that the grain diameter increases by increasing the power, and the largest grain diameter reachs 250.8 nm at 600 W, while the value of the surface roughness (Ra) remains constant at around 15 nm. The grain size is determined by the competition between growth rate and nucleation rate. Usually, a low nucleation rate and a high growth rate lead to a larger grain size.16 During the deposition of AlN films, the high kinetic energy and high surface mobility caused by high power result in a high growth rate. Thus the grain size of the films becomes larger at high power. Figure 3. AFM images of AlN films grown at RT at different power: (a)300 W, (b)400 W, (c)500 W, (d) 600 W.
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Table 2. Mean square roughness (Ra) and the mean grain diameter (D) at RT at different power: (a)300 W; (b)400 W; (c)500 W; (d) 600 W. Sample Power(W) Ra (nm) D (nm) a 300 7.2 40.1 b 400 14.2 138.4 c 500 14.0 186.6 d 600 14.6 250.8
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3.4 SEM The microstructure and morphology of AlN films were observed by scanning electron microscopy (SEM). Figure 4(a) shows SEM image of AlN films grown at the power of 600 W at RT. The surface morphology features smooth and homogeneously uniform pebble-like grains formed on the substrate. Figure 4(b) shows a SEM cross section image of AlN films prepared at the power of 600W at RT. From the image a well-defined transition can be seen between the (002) crystalline AlN structure of columnar structure with a thickness of about 3 µm. and a disordered region about 400 nm thick deposited on the substrate.
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Figure 4 (a). SEM images of AlN films prepared at the power of 600 W at RT; (b). Cross section of AlN films prepared at the power of 600 W at RT.
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3.5 Thermal Conductivity Thermal conductivity of AlN films deposited at a power of 600 W at RT were characterized along the thickness direction using time-domain thermoreflectance (TDTR) method at Precision Measurements and Instruments Corporation. The specific heat capacity used was 0.76 J/(g·K) . In this method a Ti: Sapphire laser produces near-infrared radiation incident on the film surface and creates local heating. The same laser is used to simultaneously probe the surface by monitoring the changes in intensity of the reflected beam from changes in temperature of the film. Small changes in the intensity of the reflected probe beam are measured using lock-in detection. As is known, the thermal conductivity of thin films can be vastly different from that of the corresponding bulk materials. For the bulk material AlN has a theoretical thermal conductivity kTH of 320 W/(m·K), while the experimental value for AlN films has been reported to be much lower with a range from 0.4 to 170 W/(m·K).5,8,23-25 Thermal conduction of thin film is determined by the lattice vibration wave (phonon) and the mean free path of which is affected by the interface scattering, defects and impurity scattering as well as grains boundary scattering.23-26 The results of thermal conductivity tests have been shown in Table 3 for thickness values of 1, 5 and 10 μm, which are 7.5, 9.8 and 13.3 W/(m·K), respectively. It can be seen that the effective thermal conductivity of the AlN films is closely related to the thickness and this result agrees with that reported by Sun Rock Choi24 and K. Ait Aissa25. Table 3. Effective thermal conductivity of AlN films prepared on the glass with different thickness. Thickness (μm) Thermal Conductivity (W/(m·K)) 1 7.5
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9.8 13.3
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AlN films were examined by SEM and the cross section image of the film is shown in Figure 4(b). It is obvious that a disordered layer of AlN film with a thickness of 400 nm was grown initially on the glass substrate followed by the growth of highly oriented columnar AlN with a thickness of 3000 nm. Furthermore, the XRD patterns of AlN films are shown in Figure 2(b). It can be seen in the early growth stages that the AlN film grew poorly with (100) orientation that tends to form a disordered and amorphous layer. As the thickness of film increases crystalline AlN (002) planes begin to grow on the disordered layer and the intensity of AlN (002) diffraction peak becomes larger and matches the cross section image. G.W.Auner et al. observed a similar initial amorphous layer of AlN film grown on Si (111) substrate at 400 oC by plasma source molecular beam epitaxy.26 Furthermore, a similar observation has also been made by Yong et al. during the growth of AlN; H films on Si(100) substrates at RT by RF reactive sputtering.27 They suggest that the uniformly formed amorphous layer reduces the lattice mismatch between AlN film and substrate and the short range ordering in the amorphous layer provides nucleation sites for the growth of c-axis oriented crystallites perpendicular to the substrate. This study indicates that the quality of AlN films on the glass is affected by lattice mismatch and the energy of the adatoms. In the early growth stages of AlN the lattice mismatch between AlN film and substrate is large and the adatoms do not have the proper energy and mobility to align for preferred growth at RT. As the thickness of the disordered layer increases both the reduction of the lattice mismatch and the increase of the substrate temperature (caused by the continuous bombardment of sputtering ions during the sputtering process) lead to the crystalline AlN (002) planes which is roughly parallel to the substrate surface to grow on the disordered layer. As a result, with the increasing of the film thickness, AlN films prepared on the glass at RT at 600 W have a gradual change in orientation from disorder to ordered AlN (002). An intermediate amorphous layer which has poor thermal conductivity was formed between the glass substrate and the AlN film. The thickness of such an interfacial layer remains constant after the first stage of the deposition. It is difficult to directly evaluate the bulk thermal conductivity due to AlN/Glass interface and the thermal boundary resistance was not considered in Table 3. As reported8, effective thermal conductivity k appears as a thickness dependent property that can be related with the AlN thickness with relation (1) derived from a relationship: k (kb 1 RGlass / AlN
1 1
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(1)
where RGlass / AlN is the thermal boundary resistance of the amorphous layer between glass substrate and AlN film and kb is the thermal conductivity in the bulk of the AlN film. Figure 5 shows the thermal resistance of the AlN films, k / according to the thickness. The line in figure 5 is obtained by linear regression. According to relation (1) and fitting line the bulk thermal conductivity and the boundary resistance are found to be equal to 15.4 W/(m·K) and 1 10-7 K·m2·W-1, respectively. As reported8,25,28 the thermal conductivity of AlN films can be strongly affected by impurities such as oxygen and will be researched in the future work. Compared to other materials used in IC technology, the
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Figure 5. Thermal resistance of AlN films prepared on glass with different thickness.
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4. Conclusion In this study AlN films are grown on glass substrate by DC magnetron reactive sputtering at room temperature. The optimum power is 600W according to the preferred orientation and temperature limitation of flexible electronics processing. The deposition rate is measured at a maximum of 3.3 μm/h with a grain size of 250.8 nm at 600 W. The crystalline quality and the effective thermal conductivity of the films are found to be improved with the increase of film thickness. Compared to other thermal materials used in IC technology, the bulk thermal conductivity of AlN films along the film thickness direction is 15.4 W/(m·K) is relatively high.
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Acknowledgement This study is primarily supported by Shanghai Pujiang Program under grant no. 11PJ1403400. The authors thank the support from the Instrumental Analysis and Research Center of Shanghai University and GE (China) Research and Development Center Co., Ltd..
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Refference [1] H. Jin, J. Zhou, S.R. Dong, B. Feng, J.K. Luo, D.M. Wang, W.I. Milne, and C.Y. Yang, “Deposition of c-axis orientation aluminum nitride films on flexible polymer substrates by reactive direct-current magnetron sputtering,” Thin Solid Films, 520, 4863–4870, 2012. [2] L. Chang, “Recent developments in polymer MEMS,” Advanced Materials, 19, 3783–3790, 2007. [3] S. Petroni, C.L. Tegola, G. Caretto, A. Campab, A. Passaseo, M. D. Vittorioa, and R. Cingolani, “Aluminum Nitride piezo-MEMS on polyimide flexible substrates,” Microelectronic Engineering, 88, 2372–2375, 2011. [4] D.A. van den Ende, R.H.L. Kusters, M. Cauwe, A. van der Waal, and J. van den Brand , “Large area flexible lighting foils using distributed bare LED dies on polyester substrates,” Microelectronics Reliability, 53, 1907–1915, 2013. [5] S. Shanmugan, D. Mutharasu, and A.H. Haslan, “A study on AlN thin film as thermal interface material for high power LED,” International Journal of Electronics and Computer Science Engineering, 2 (1), 296-300, 2012. [6] J. Petroski, “Thermal challenges facing new generation light emitting diodes (LEDs) for lighting applications,” International Symposium on Optical Science and Technology, pp. 215-222, 2002. [7] Y. Huaiyu, S. Koh, H. van Zeijl, A.W.J Gielen, and Z. Guoqi, “A review of passive thermal management of LED module J,” Journal of Semiconductors, 32 (1), 014008, 2011. [8] C. Duquenne, M.P. Besland, P.Y. Tessier, E. Gautron, Y. Scudeller, and D. Averty, “Thermal conductivity of aluminium nitride thin films prepared by reactive magnetron sputtering,” Journal of applied physics, 45 (1), 015301, 2012. [9] R.B. Dinwiddie, A.J. Whittaker, and D.G. Onn, “Thermal conductivity, heat capacity, and thermal diffusicity of selected commercial AlN substrate,” International Joural of Thermophysics, 10 (5), 1075-1084, 1989. [10] H. Kitagawa, Y. Shibutani, and S. Ogata, “Ab-initio simulation of thermal properties of AlN ceramics,” Modelling Simul.Mater.Sci.Eng, 3 (4), 521-531, 1995. [11] J. Bodzenta, B. Burak, A. Jagoda, and B. Stanczyk, “Thermal conductivity of AlN and AlN-GaN thin films deposited on Si and GaAs substrates,” Diamond and related materials, 14 (3), 1169-1174, 2005. [12] J. Olivares, J. Capilla, M. Clement, J. Sangrador, and E. Iborra, “Growth of AlN oriented films on insulating substrates,” International Ultrasonics Symposium Proceedings, 0428, 1716-1719, 2011. [13] A.V. Singh, S. Chandra, and G. Bose, “Deposition and characterization of c-axis oriented aluminum nitride films by radio frequency magnetron sputtering without external substrate heating,” Thin Solid Films, 519, 5846–5853, 2011. [14] S. Shanmugan, D. Mutharasu, P. Anithambigai, N. Teeba, and I. Abdul Razak, “Synthesis and structural properties of DC sputtered AlN thin films on different substrates,” Journal of Ceramic Processing Research, 14 (3), 385~390, 2013. [15] Q.X. Guo, M. Yoshitugu, T. Tanaka, M. Nishio, and H. Ogawa, “Microscopic investigations of aluminum nitride thin films grown by low-temperature reactive sputtering,” Thin Solid Films, 483 (1) 16–20, 2005
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Aluminum nitride thermal conductivity measured by TDTR method to be 13 W/mK Thermal conductivity is related to film thickness and crystal orientation Aluminum nitride film growth rate can reach 3.3 um/hour