Thermophysical properties of aluminum oxide and molybdenum layered films

Thin Solid Films 518 (2010) 3119–3121

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Thermophysical properties of aluminum oxide and molybdenum layered films Nobuto Oka a, Ryo Arisawa a, Amica Miyamura a, Yasushi Sato a, Takashi Yagi b, Naoyuki Taketoshi b, Tetsuya Baba b, Yuzo Shigesato a,⁎ a b

Graduate School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba central 3, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan

a r t i c l e

i n f o

Available online 22 October 2009 Keywords: Thermal diffusivity Thermal boundary resistance Thermoreflectance system Aluminum oxide Molybdenum

a b s t r a c t The thermal diffusivity of aluminum oxide (Al2O3) films and the thermal boundary resistance between Al2O3 and molybdenum (Mo) films were investigated using ‘rear heating/front detection (RF) type’ picosecond and nanosecond thermoreflectance systems. Amorphous Al2O3 films sandwiched between Mo films (Mo/Al2O3/Mo) were prepared on fused silica substrates by RF magnetron sputtering using Al2O3 and Mo targets. The thicknesses of the Al2O3 and Mo layers were 0.5–100 nm and 70 nm, respectively. The thermal diffusivity of the amorphous Al2O3 films was found to be 9.5 × 10− 7 m2/s. The thermal boundary resistance between Al2O3 and Mo was 1.5 × 10− 9 m2K/W, corresponding to the thermal resistance of a 4.2 nm thick Al2O3 film or a 77 nm thick Mo film. However, the thermal diffusivity of the amorphous Al2O3 film is approximately one twelfth that of bulk polycrystalline Al2O3. This difference was attributed to the smaller mean free path of phonons in amorphous Al2O3 due to its disordered structure. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Insulating oxide thin films, such as aluminum oxide (Al2O3), have been widely used in various semiconductor devices because they possess high chemical stability and electrical insulating characteristics. Many semiconductor devices are composed of different types of layers, some of them insulating, which can have many interfaces between them. The thermal design of such devices has received a lot of attention recently since the heat diffusion characteristics of layers and interfaces are complex, and excessive heat can damage the device. Thermophysical properties, especially thermal diffusivity and thermal boundary resistance, are essential parameters for effective thermal design. To date, however, there have been few detailed studies on the thermophysical properties of Al2O3 thin films (see, for example, Stoner et al. [1] and Bai et al. [2]), even though they are key elements in semiconductor devices. In this study, the thermophysical properties of Al2O3 films sandwiched between Molybdenum (Mo) films (Mo/Al2O3/Mo) were investigated. Mo films have been also used in semiconductor devices. The thickness of the Al2O3 was varied from 0.5 nm to 100 nm to correspond to practical device designs. To characterize the thermal diffusivity of the Al2O3 layer and the thermal boundary resistance between the Al2O3 and Mo layers, ‘rear heating/front detection type’ picosecond and nanosecond thermoreflectance systems, developed by the National Metrology Institute of Japan (NMIJ) / AIST [3–6], were employed. The wavelengths of the pulse lasers used in the thermo⁎ Corresponding author. E-mail address: [email protected] (Y. Shigesato). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.180

reflectance systems were 780 nm for the picosecond system and 785 nm/1064 nm for the nanosecond system. Although Al2O3 films are transparent at these wavelengths, Mo can act as a reflective layer for the laser pulses. Using this setup, a detailed analysis was performed on the heat propagation through the Mo/Al2O3/Mo layered structures. 2. Experimental Mo/Al2O3/Mo layered films were prepared on unheated fused silica glass substrates by RF magnetron sputtering with powers of 100 W and 50 W, using Al2O3 target (99.99%, Furuuchi Chemical Corp., Japan) and Mo metal target (99.95%, Furuuchi Chemical Corp., Japan), respectively. Total gas pressure was maintained at 0.5 Pa (Al2O3)/ 1.0 Pa (Mo) of 100% Ar. The substrate temperature during deposition was confirmed to be below 50 °C by a thermo-label. The Mo/Al2O3/Mo layered structure was fabricated with no exposure to atmosphere between each deposition. The thicknesses of the Al2O3 and Mo layers were 0.5–100 nm and 70 nm, respectively. The thermoreflectance systems operate under the following principles to measure heat propagation. A pump laser pulse is focused on the rear side of the Mo/Al2O3/Mo specimen, and a fraction of its energy is absorbed into the skin depth of the bottom Mo layer and converted into heat. This heat then diffuses one dimensionally towards the front side of the specimen. A probe laser pulse is used to detect the temperature change at the front side as a change in reflectivity. The normalized temperature rise, i.e. the thermoreflectance signal, is recorded as a function of the delay time relative to the pump laser pulse. To derive the thermal diffusivity of the Al2O3 film and the thermal boundary resistance between the Al2O3 and Mo

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layers, the thermoreflectance signals are analyzed on the basis of an analytical solution of the one-dimensional heat flow across the threelayered film [4,6]. In order to characterize their structural properties, Mo and Al2O3 monolayer films and Al2O3/Mo layered films were also deposited directly on alkali free glass substrates. The infrared spectra of the films were obtained by means of a Fourier transform infrared spectrometer (FTIR, IRPrestige-21, Shimadzu). The crystallinity was analyzed by X-ray diffraction (XRD) carried out using 40 kV, 20 mA CuKα1 radiation (XRD6000, Shimadzu). The total thickness of monolayer or layered films was measured using a surface profiler (Dektak-III, Sloan Tech.). Concerning Mo/Al2O3/Mo layered films, the thickness of Al2O3 layer (≤5 nm) and Mo layer was estimated from deposition rate and time whereas that of Al2O3 layer (≥5 nm) was obtained by subtracting the thickness of Mo layers from the total thickness. 3. Results 3.1. Structure of Al2O3 films Fig. 1 shows the FTIR spectra of Mo film and Al2O3/Mo layered films. The peaks of “1100–500 cm− 1” and “2000–1250 cm− 1 and 700– 400 cm− 1” represent the vibration of Al2O3 and water, respectively [7]. Figs. 2 and 3 show the XRD patterns of Al2O3 films and Mo/Al2O3/Mo structures, respectively, with various thicknesses of Al2O3. The Al2O3 in both the single films and three-layer structures was found to be amorphous whereas the Mo layers had a polycrystalline structure, as evidenced by the appearance of XRD peaks from (110) planes. 3.2. Thermal diffusivity of Al2O3 films Fig. 4 shows typical thermoreflectance signals from the Mo/Al2O3/ Mo structure. It should be noted that that the intensity of the thermoreflectance signal increases linearly with temperature [8]. The areal heat diffusion time is defined as the area bounded by the history curve, a horizontal line corresponding to a thermoreflectance value of 1, and a vertical line corresponding to a delay time of 0, and can also be derived analytically from the following equation [4,6]: 

CAl2 O3 dAl2 O3 + 43 CMo dMo

A=



d2Mo κMo

 +

2 CMo d2Mo CAl O dAl O 2 3

2 3

 + 16 CAl2O3 dAl2 O3 +CMo dMo

Fig. 2. X-ray diffraction (XRD) patterns from Al2O3 monolayers with various thicknesses deposited on alkali free glass substrates. The halo pattern observed around 2θ = 20–25° was attributed to the glass substrate.

where A is a real heat diffusion time, C is heat capacity per unit volume, d is film thickness, κ is thermal diffusivity, the subscript identifies whether the layer is Al2O3 or Mo, and Rbd is thermal boundary resistance. In this analysis, an Mo film thickness (dMo) of 70 nm, a thermal diffusivity (κMo) for Mo of 2.1 × 10− 5 m2/s, a heat capacity per unit volume (CMo) for Mo of 2.53 × 106 J/m3K [6], and a heat capacity per unit volume (CAl2O3) for Al2O3 of 3.08 × 106 J/m3K, derived from the heat capacity [9] and the density [10], were used.

d2Al O 2 3 κAl O 2 3

CAl2 O3 dAl2 O3 + 2CMo dMo +

2CMo dMo ðCAl2 O3 dAl2 O3 + CMo dMo Þ CAl2 O3 dAl2 O3 + 2CMo dMo

Rbd ;

ð1Þ

Fig. 1. Infrared transmission spectra of Mo monolayer and Al2O3/Mo layered films deposited on alkali free glass substrates.

Fig. 3. X-ray diffraction (XRD) patterns of Mo/Al2O3/Mo structures with various thicknesses of Al2O3 deposited on fused silica glass substrates.

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4. Discussion The mechanism of thermal transport in Al2O3 films must be discussed in terms of a phonon contribution since the thermal diffusivity of the amorphous Al2O3 film is small compared to that of the bulk sintered Al2O3 which should have a polycrystalline structure. The thermal conductivity due to phonons, λph, for the Al2O3 layer can be estimated as 2.9 W/mK from the measured thermal diffusivity. On the other hand, λph for bulk sintered Al2O3 is calculated to be 33 W/ mK [11]. Consequently, phonon propagation in amorphous Al2O3 films clearly differs from that in bulk sintered Al2O3. It is important to estimate the mean free path of phonons, lph, in terms of phonon propagation in Al2O3. λph is described by the following equation [14,15]: 1 Cvlph 3   E 12 v= ρd λph =

Fig. 4. Thermoreflectance signals from the Mo/Al2O3/Mo structure obtained by picosecond and nanosecond thermoreflectance measurements.

Fig. 5 shows the results of the curve fitting with Eq. (1) for the areal heat diffusion times. The thermal diffusivity of Al2O3 layers with a thickness of more than 5 nm was 9.5 × 10− 7 m2/s, which is almost the same as that Bai et al. reported [2], the Al2O3 thicknesses of which are 330 nm, 370 nm and 1000 nm. However, the value is approximately one twelfth of the value for bulk sintered Al2O3 (1.11 × 10− 5 m2/s) [11]. The thermal boundary resistance between Al2O3 and Mo was 1.5 × 10− 9 m2K/W, corresponding to the thermal resistance of a 4.2 nm thick Al2O3 film or a 77 nm thick Mo film, which is much smaller than that between Al (1800 nm) and Al2O3 (330 nm, 370 nm and 1000 nm) reported by Bai et al. (2.6 × 10− 8 m2K/W) [2]. Possible explanations on this discrepancy should be the difference in possible substrate damage and interfacial imperfection during depositions [12] or in the measurement techniques. The areal heat diffusion time of films less than 5 nm thickness, however, was smaller than the theoretical value, as shown in Fig. 5. This difference might be due to a direct contact occurring between the two Mo layers (thermal diffusivity of Mo: 2.1 × 10− 5 m2/s) because the Al2O3 layer has an island structure at the very early stages of film growth which should be expected to be generally a three-dimensional (Volmer–Weber type) growth in case of sputter depositions [13].

ð2Þ ð3Þ

where C is the heat capacity per unit volume, and v is the average phonon velocity. Assuming that v was sound velocity in this paper, it was calculated from Young's modulus E and the density ρd. For simplicity, it is assumed that amorphous Al2O3 has the same heat capacity and density as sintered Al2O3. The values of Young's modulus for amorphous and sintered Al2O3 are 142 GPa [16] and 401 GPa [17], respectively. Thus, λph can be calculated using Eqs. (2) and (3). The lph value for the amorphous Al2O3 films is estimated to be 0.47 nm, and for the bulk sintered Al2O3, it is 3.2 nm. The smaller lph value for amorphous films should be largely due to the lack of long-range order in these films. 5. Conclusions The thermophysical properties of amorphous Al2O3 layers were investigated by picoseconds and nanosecond thermoreflectance methods. Three-layer specimens consisting of an amorphous Al2O3 film sandwiched between 70-nm-thick Mo films (Mo/Al2O3/Mo) were fabricated on fused silica substrates by RF magnetron sputtering. The thermal diffusivity of the Al2O3 films was found to be 9.5 × 10− 7 m2/s and the thermal boundary resistance between amorphous Al2O3 and Mo layers was 1.5 × 10− 9 m2K/W. The thermal diffusivity of the amorphous Al2O3 films is approximately one twelfth that of the bulk polycrystalline Al2O3, and this was attributed to the smaller mean free path of phonons in amorphous Al2O3. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Fig. 5. Areal heat diffusion time of Mo/Al2O3/Mo structures and theoretical curve calculated using Eq. (1).

[14] [15] [16] [17]

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