Thermophysical properties of thin films and boundary thermal resistances - Measurements by ultra fast laser flash methods and development of their database


T. Baba1, N. Taketoshi1, T. Yagi1 and Y. Yamashita1

1National Metrology Institute of Japan, AIST, Japan

Keywords: laser flash method
property: thermal diffusivity, boundary thermal resistance
material: thin films

Properties of solid materials are not determined only by their composition but change dependent on structure of nano scale, micro scale and macroscopic scale. Since thin films of same composition can be synthesized by completely different processes, such as physical vapor deposition methods, chemical vapor deposition methods, dip-coating, spin-coating etc., from the process for bulk materials, their structures and properties are different from those of the bulk materials of the same condition .

Therefore, reliable measurement of thermophysical property is required for the thin films which are synthesized to the same thickness by the same deposition method as the thin films of interest instead of using thermophysical property value of the corresponding bulk material of the same composition [1].

In addition, it is indispensable to know boundary thermal resistances between the layers as well as thermophysical property of each layer to understand internal heat transfer of multilayered film constituting a semiconductor device or storage media at high integration. However, it is very difficult to separate contribution of thermal resistance between the layers from thermal resistance of each layer if the time resolution of the measurement method is slow compared with the heat diffusion time across the layers [2].

In order to solve these problems, NMIJ, AIST developed "ultra fast laser flash method" which can measure the thermal diffusivity of metallic thin films from several 10 nm to several micrometers thick on transparent substrate in thickness direction by picosecond / nanosecond light pulse heating. The configuration is called as rear face heating / front face detection (RF) type pulsed light heating thermoreflectance method [3]. This configuration is essentially equivalent to the laser flash method which is the well-established standard method to measure the thermal diffusivity of bulk materials [4]. Therefore, thermal diffusivity of thin films can be determined reliably by the observed heat diffusion time and the thickness of the specimen.

In order to determine the thermal diffusivity of a nonmetal layer or a thin film such as a semiconductor by the ultra fast laser flash method, the specimens of three-layered structure including a nonmetal film whose thermal properties are unknown, a first metal film disposed on one side of the nonmetal film, and a second metal film disposed on the other side are prepared. Thermal diffusivity of the metal thin film which is deposited on the same substrate as the single layer can be measured in advance.

When a set of specimens which consist of nonmetal layers of different thickness and metal films of the same thickness were measured by the ultra fast laser flash method, a set of temperature response curves which rises slower for thicker nonmetal layer are observed. The thermal diffusivity of the nonmetal layer and the boundary thermal resistance can be calculated based on the response function method [2]. Thin films of semiconductors, carbon materials, oxides, nitrides, carbides, and polymers can be measured as well as metal thin films have been measured by this approach.

Variety of thin films have been measured, such as transparent conductive films for flat panel display, phase change material for optical media and P-RAM, polymer thin films for organic electro-luminescence, hard coating films, thin films developed for thermoelectric application as well as basic metal thin films. These thermophysical property data are systematically stored to the thermophysical property database developed by NMIJ [5].

References
  1. D. G. Cahill , K. E. Goodson, A. Majumdar, J. Heat Transfer, ASME,124, pp. 223-241 (2002).

  2. T. Baba, Jpn. J. Appl. Phys. 48 (2009) doi: 10.1143/JJAP.48.05EB04

  3. N. Taketoshi, T. Baba, A. Ono, Jpn. J. Appl. Phys.38 pp. L1268-L1271 (1999) doi: 10.1143/JJAP.38.L1268

  4. J. Parker, R. J. Jenkins, C. P. Butler, G. L. Abbott , J. Appl. Phys. 32 pp. 1679-1684 (1961)

  5. http://riodb.ibase.aist.go.jp/TPDB/DBGVsupport/index_en.html

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