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6.3 THERMOPHYSICAL PROPERTIES FUNDAMENTALS
Normalized thermoreflectance 1.0 Al Mo
0.5
0.0
-50 0 50 100 150 200
Time delay/ps
Figure 6.3.9
Thermoreflectance signals of aluminum thin film and molybdenum thin film of 100 nm thickness by rear face
heating/front-face detection picosecond thermoreflectance method.
Two types of metallic thin films, an aluminum This system can measure thermal diffusivity of thin
single-layered thin film of 100 nm thickness and a films with thickness up to several micrometers. Pulse
molybdenum single-layered thin film of 100 nm, both duration is 2ns and pulse interval is 20 s with inten-
of which were synthesized on a Pyrex glass substrate sity modulation of 1 kHz by a modulator (AOM).
by magnetron DC sputtering method, where the film Irradiate it from the upper part of Fig. 6.3.10 to sub-
thicknesses are of nominal values. Fig. 6.3.9 shows strate side of specimen set in specimen measurement
the temperature history curves measured by the ultra unit. As shown in Fig. 6.3.10, the specimen is irradi-
fast laser flash method. ated from bottom and the reflected light is detected by
Since the shapes of observed thermoreflectance a differential detector from the top.
signals are similar to those observed by the laser flash A molybdenum thin film with thickness of 3 m
method for bulk specimen, the heat energy transport synthesized on a glass substrate by sputtering deposi-
of these metallic thin films of a thickness around tion was measured as shown in Fig. 6.3.11.
100nm at room temperature in a time scale of order
of several 100 ps can be expressed by the classic heat 6.3.5 Thin film thermophysical property reference
diffusion equation.
material and traceability
6.3.4 Nanosecond thermoreflectance method The high-speed laser flash method in the extended def-
inition mentioned above can be generally used and is
Initially, the electrical delay method was developed to popular after commercial instruments are available and
expand the observation time of the picosecond traceability to the national standard and/or the interna-
thermoreflectance method longer for measurements tional standard is established. The national standard
of thicker films [11]. Since the pulse duration can of the laser flash method was established in 2002
be longer than picosecond and the repetition period by NMIJ/AIST. Uncertainty of thermal diffusivity
of pulses is flexible, nanosecond pulse laser can be standard by the laser flash method was evaluated based
used for the ultra fast laser flash method [12]. on “Guide to the Expression of Uncertainty in
Fig. 6.3.10 shows the block diagram of the nanosec- Measurement, GUM” [7] and a quality system corre-
ond thermoreflectance method. The pulse duration sponding to ISO 17025 was constructed [2]. The stan-
was changed from 2ps to 2ns and the pulse repetition dard value of thermal diffusivity of the graphite was
period was changed from 76 MHz (13.2 pump determined with uncertainty evaluation based on GUM
specific speed interval) to 50 kHz (20 s interval). and supplied as a reference material in 2005.
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