Development of Ultrafast Laser Flash Methods for Measuring Thermophysical Properties of Thin Films and Boundary Thermal Resistances Tetsuya Baba  , Naoyuki Taketoshi 1 , and Takashi Yagi 2 National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8563, Japan 1 Standards Planning Office, International Standards Promotion Division, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan 2 Thermophysical Properties Section, Material Metrology Division, National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8563, Japan Received July 23, 2011; accepted August 31, 2011; published online November 21, 2011 Reliable thermophysical property values of thin films are important to develop advanced industrial technologies such as highly integrated electronic devices, phase-change memories, magneto-optical disks, light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), semiconductor lasers (LDs), flat-panel displays, and power electronic devices. In order to meet these requirements, the National Metrology Institute of Japan of the National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) has developed ultrafast laser flash methods heated by picosecond pulse or nanosecond pulse with the same geometrical configuration as the laser flash method, which is the standard method to measure the thermal diffusivity of bulk materials. Since these pulsed light heating methods induce one-dimensional heat diffusion across a well-defined length of the specimen thickness, the absolute value of thermal diffusivity across thin films can be measured reliably. Using these ultrafast laser flash methods, the thermal diffusivity of each layer of multilayered thin films and the boundary thermal resistance between the layers can be determined from the observed transient temperature curves based on the response function method. The thermophysical properties of various thin films important for modern industries such as the transparent conductive films used for flat-panel displays, hard coating films, and multilayered films of next-generation phase-change optical disks have been measured by these methods. # 2011 The Japan Society of Applied Physics 1. Introduction Electrical devices such as central processing units (CPUs), dynamic random access memories (DRAMs), phase-change memories, hard disks, light emitting diode (LEDs), organic LEDs, laser diodes (LDs), flat-panel displays, and power electronics consist of thin films with a thickness of several nanometers to several hundred nanometers. In order to know how heat flows and the distribution of temperature that is induced inside them under operation, the thermophysical properties of thin films and the boundary thermal resistance between thin films are required. 1–8) The thermal properties of such thin films are generally different from those of bulk materials of the same composition. Consequently, it is desirable to measure thin films of the same thickness synthesized under the same deposition conditions by the same deposition method as those of the thin films in devices. 9,10) In addition, it is necessary to know the boundary thermal resistances between the layers as well as the thermophysical properties of each layer to understand the internal heat transfer of multilayered films. 2–6,11,12) However, when multilayered films are measured with conventional measure- ment methods, it is difficult to separate the contribution of thermal resistance between the layers from the thermal conduction of each layer. The quantitative measurement of heat transport properties across thin films thinner than 100 nm is not easy because of the fundamental difficulty of temperature detection for layers as thin as 10 nm. It is extremely difficult to measure the temperature difference between both sides of the same thin film, which is much smaller than 1 mK, if steady-state measurements with a similar geometrical configuration to that of the guarded hot plate method used for measuring the heat transport of insulation materials, are applied to measure the thermal conductivity of thin films. 2. Development of Technology for Thermal Diffusivity Measurements The laser flash method is a well-established and standard method for measuring the thermal diffusivity  of bulk materials such as metals, ceramics, graphite, and semicon- ductors. 13–16) The ultrafast laser flash method is a natural extension of the laser flash method for application to thin films. 5,8,17–24) Thermal conductivity ! is calculated by the equation ! ¼ c& from specific heat capacity c and density & after thermal diffusivity  is measured by the ultrafast laser flash method or conventional laser flash method. 2.1 Laser flash method When thermal diffusivity is measured with the laser flash method, the front face of a planar specimen, kept at constant temperature, is heated uniformly with an impulse of light, as shown in Fig. 1. 15) Heat diffuses one-dimensionally from the heated face to the opposite face, and finally the temperature throughout the specimen becomes uniform. Because the normalized temperature rise at the rear face of the specimen changes proportionally to the thermal diffusivity and inversely proportionally to the square of the specimen’s thickness, the thermal diffusivity is calculated from the thickness of the specimen and heat diffusion time. The following conditions are assumed as ideal: 13,15) 1) The duration of the laser pulse is negligibly short compared with the heat diffusion time. 2) The specimen is adiabatic to the environment. 3) The specimen’s front face is heated uniformly. 4) The temperature change of the specimen’s rear face is measured precisely. 5) The specimen is dense, uniform, and opaque. 6) The change in thermal diffusivity due to the speci- men’s temperature increase after the pulse heating is negligibly small. Under the assumptions mentioned above, when the front face of a plate of thermal diffusivity , specific heat capacity  E-mail address: t.baba@aist.go.jp Japanese Journal of Applied Physics 50 (2011) 11RA01 11RA01-1 # 2011 The Japan Society of Applied Physics REVIEW PAPER DOI: 10.1143/JJAP.50.11RA01