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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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