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6.3 THERMOPHYSICAL PROPERTIES                                                FUNDAMENTALS
                  of a thickness less than 1 m. In order to solve this
                  problem, “A picosecond thermoreflectance method” to  1
                  observe change of the temperature of thin film front                       50 nm
                  face by heat diffusion to the inside was developed [8].
                  The optical reflection intensity of the temperature
                  probe pulse is detected by photodiode. Since reflectiv-                    100 nm
                  ity of material surface changes as a function of tem-  Normalized temperature increase
                  perature, a change of specimen front-face temperature
                  is observed by a change of reflected light amplitude.                     500 nm
                  This temperature measurement method with a temper-
                  ature change of such a reflectivity is called as ther-  0
                  moreflectance method. NMIJ/AIST of Japan developed
                  a thin film thermal diffusivity measurement system
                  based on a picosecond thermoreflectance method as        0      20     40      60     80
                  shown in Fig. 6.3.5. The laser beam emitted from a               Delay time (ps)
                  mode-locked titanium sapphire laser is divided into
                  transmitted beam and reflected beam by a quartz plate  Figure 6.3.6
                  and about 90% is used for pulse heating and about  Thermoreflectance signals of three different thicknesses of
                  10% is used for temperature detection by thermore-  aluminum thin films synthesized on glass substrates.
                  flectance method. Area of a diameter of several 10 m
                  of thin film face is heated by the picosecond laser
                  beam and the same position is irradiated by the probe  limited by time duration of the light pulses. On the
                  beam.  Then, the history of surface temperature is  other hand, it is a weak point that sensibility of tem-
                  observed by the thermoreflectance method.      perature detection is low.
                    Since light travels 0.3mm in 1ps. By adjusting dis-  Fig. 6.3.6 shows the results of front-face tempera-
                  tance to specimen after it was divided, the time differ-  ture change by a picosecond thermoreflectance
                  ence that a heating light and a probe light arrive at a  method about three aluminum thin films of different
                  specimen front face can be controlled. Response time  thickness synthesized on glass substrates [9]. For the
                  of thermoreflectance method is incomparably faster  specimen of 500 nm thickness, the heat has not arrived
                  than thermocouples, resistance temperature sensor or  at the substrate within 120 ps after pulse heating and
                  radiation thermometers.  According  to the  pump  the surface temperature changes corresponding to inter-
                  probe method, ultra fast thermometry is possible only  nal heat diffusion of the aluminum thin film. On the
                                                                 other hand, when the film thickness is 100 nm, the
                                                                 temperature decrease after pulse heating is suppressed
                             Pump laser beam                     and deviates from a temperature change of film thick-
                                                                 ness 500nm in 30 ps as shown in Fig. 6.3.6. Because
                                               Probe laser beam  the thermal effusivity of glass substrate is much
                                                                 smaller than thermal effusivity of aluminum thin film,
                                                                 the heat effusion into the substrate is suppressed when
                                                  Probe laser
                   Pump laser                     heating area   the heat arrives at the interface between the thin film
                   heating area                                  and the substrate. For the specimen of 50 nm thick-
                   -50  m                                        ness, since the heat effuses to the substrate just after
                                                                 pulse heating, the temperature change only for the thin
                                                                 film inside cannot be observed as shown in Fig. 6.3.6.

                                                                 6.3.3 Picosecond thermoreflectance method by rear
                         Thin film  50 nm – 500nm
                         T h i n  f i lm  5 0  n m  –  5 00 nm
                                                                 face heating/front-face detection
                                                                 According to the picosecond thermoreflectance
                                                                 method by front-face heating/front-face detection
                            Substrate – 1mm
                            Substrate – 1mm                      stated in the above section, a thermal diffusivity is
                                                                 calculated by a cooling rate after picosecond pulse
                                                                 heating which changes depending on a thermal diffu-
                                                                 sivity of the thin film and the penetration depth of the
                                                                 laser beams. Since the cooling rate of the observed
                  Figure 6.3.5                                   temperature history changes the condition of thin film
                  Specimen and heating/temperature measurement geometry  surface sensitively, it has been difficult to obtain
                  of front-face heating/front-face detection picosecond  reproducible and reliable thermal diffusivity value of
                  thermoreflectance method.                      the thin film by this method [10].

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