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
Graduate School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan
b
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
abstract article info
Available online 22 October 2009
Keywords:
Thermal diffusivity
Thermal boundary resistance
Thermoreflectance system
Aluminum oxide
Molybdenum
The thermal diffusivity of aluminum oxide (Al
2
O
3
) films and the thermal boundary resistance between Al
2
O
3
and molybdenum (Mo) films were investigated using ‘rear heating/front detection (RF) type’ picosecond and
nanosecond thermoreflectance systems. Amorphous Al
2
O
3
films sandwiched between Mo films (Mo/Al
2
O
3
/Mo)
were prepared on fused silica substrates by RF magnetron sputtering using Al
2
O
3
and Mo targets. The
thicknesses of the Al
2
O
3
and Mo layers were 0.5–100 nm and 70 nm, respectively. The thermal diffusivity of
the amorphous Al
2
O
3
films was found to be 9.5 × 10
-7
m
2
/s. The thermal boundary resistance between Al
2
O
3
and Mo was 1.5×10
-9
m
2
K/W, corresponding to the thermal resistance of a 4.2 nm thick Al
2
O
3
film or a
77 nm thick Mo film. However, the thermal diffusivity of the amorphous Al
2
O
3
film is approximately one
twelfth that of bulk polycrystalline Al
2
O
3
. This difference was attributed to the smaller mean free path of
phonons in amorphous Al
2
O
3
due to its disordered structure.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Insulating oxide thin films, such as aluminum oxide (Al
2
O
3
), have
been widely used in various semiconductor devices because they
possess high chemical stability and electrical insulating character-
istics. 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 Al
2
O
3
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 Al
2
O
3
films
sandwiched between Molybdenum (Mo) films (Mo/Al
2
O
3
/Mo) were
investigated. Mo films have been also used in semiconductor devices.
The thickness of the Al
2
O
3
was varied from 0.5 nm to 100 nm to
correspond to practical device designs. To characterize the thermal
diffusivity of the Al
2
O
3
layer and the thermal boundary resistance
between the Al
2
O
3
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-
reflectance systems were 780 nm for the picosecond system and
785 nm/1064 nm for the nanosecond system. Although Al
2
O
3
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/Al
2
O
3
/Mo layered structures.
2. Experimental
Mo/Al
2
O
3
/Mo layered films were prepared on unheated fused
silica glass substrates by RF magnetron sputtering with powers of
100 W and 50 W, using Al
2
O
3
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 (Al
2
O
3
)/
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/Al
2
O
3
/Mo
layered structure was fabricated with no exposure to atmosphere
between each deposition. The thicknesses of the Al
2
O
3
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/Al
2
O
3
/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 thermoreflec-
tance signal, is recorded as a function of the delay time relative to the
pump laser pulse. To derive the thermal diffusivity of the Al
2
O
3
film
and the thermal boundary resistance between the Al
2
O
3
and Mo
Thin Solid Films 518 (2010) 3119–3121
⁎ Corresponding author.
E-mail address: yuzo@chem.aoyama.ac.jp (Y. Shigesato).
0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2009.09.180
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Thin Solid Films
journal homepage: www.elsevier.com/locate/tsf