The present invention relates to a method for measuring a thermal diffusivity within a substance having a three-layer laminated structure.  In particular, this invention relates to a method for measuring a thermal diffusivity and an interface thermal resistance, which method is suitable for correctly measuring a thermal diffusivity within a three-layer film structure containing a non-metal substance, such as a semiconductor device and an optical disc type data recording medium. 
With regard to various technical fields, thin film technique is considered to be the most advanced technique and has attracted a considerable public attention.  Particularly, thin film structure plays an important role in a highly integrated semiconductor device, DVD-RAM, MO and the like.  Specifically, a multi-layered thin film structure has been put into practical use, so that analyzing the properties thereof has become an important task in this field.  On the other hand, although there has been a great progress in the measurement of electric, magnetic and optical properties of the above described thin film, the present situation is that there has not been a sufficient progress in the measurement of thermophysical properties of such a thin film structure.  Thus, it is demanded that a rapid development take place in the field of the measurement of the thin film""s thermophysical properties. 
In order to fulfil the above task, the inventors of the present invention have previously developed a picosecond thermoreflectance method which involves a backside heating and a front side temperature measuring, and have been successful in measuring a thermal diffusivity in the thickness direction of a metal film having a thickness of 100 nm, as well as an interface thermal resistance between metal films.  In fact, this method has already been suggested and described in Japanese Unexamined Patent Application Publication No.  2000-83113. 
The basic principle of the above suggested method can be shown in FIG.  5.  Namely, a metal film 10 is deposited on a transparent substrate 11.  Afterwards, an interface 12 formed between the metal film 10 and the transparent substrate 11 is irradiated with a heating pulse light H so as to be heated.  Meanwhile, one surface 13 of the metal film 10 is irradiated with a temperature measuring pulse light P, while a reflected light of the pulse light P is measured, thereby measuring a surface temperature of the metal film 10.  In this way, it becomes possible to directly measure the thermal diffusivity of the metal film, in accordance with the metal film""s thickness d which has been measured in advance, as well as a passed time period lasting from the irradiation using the heating pulse light H to the temperature measurement using the temperature measuring pulse light P.
FIG.  6 is a block diagram showing the aforementioned thermoreflectance system involving the backside heating and the front side temperature measuring, which has already been put into practical use on the base of the above discussed principle.  As shown in the drawing, a light beam emitted from a titanium/sapphire laser 21 capable of generating a picosecond pulse light is splitted by a beam splitter 22, so as to be divided into two light beams, with one being a heating beam H and the other serving as a temperature measuring beam P. The heating beam H is modulated by a frequency which may be for example 1 MHz, using an acoustic optical modulator 23 controlled by an oscillator 29.  The modulated heating beam H is then caused to travel through an optical delay line 25 capable of alterring a light path length by moving a prism 24. 
The heating beam H having passed through the optical delay line 25 is then passed through a lens 26a so as to be focused on to an interface 30 between a metal film 27 and a transparent substrate 28.  On the other hand, the temperature measuring beam P is passed through a xcex/2 plate 33 and a lens 26b, so as to be focused on to the metal film""s another surface 32 just opposite to the heated surface.  The temperature measuring beam reflected from the surface 32 is thus detected by a silicon photo-diode 34.  Meanwhile, AC component of photo-diode signal synchronized by a modulation frequency provided by the above transient thermoreflcetance oscillator 29 is detected by a lock-in amplifier 35.  At this time, a signal is recorded by moving the prism 24 along the delay line 25.  In fact, the above process for measuring a thermal diffusivity has been formed into a practically useful formula, and has been described in detail in a specification of a former patent application of the inventors. 
On the other hand, in order to analyze the heat transfer properties of a multi-layered film structure such as a semiconductor device, DVD-ROM and MO disc, it is absolutely necessary to know not only the values of thermophysical properties of the respective layers forming the multi-layered film structure, but also the value of an interface thermal resistance existing between every two mutually adjacent film layers.  However, in the case where it is required to measure an interface thermal resistance as well as a thermal diffusivity of a multi-layered film structure, the prior art method as described in the above has been proved to be extremely difficult in separately measuring a thermal diffusivity of each film layer and interface thermal resistance. 
Further, with regard to a non-metal film such as a semiconductor film which is different from a metal film, in order for a laser pulse to be absorbed into the film at a depth of about 10 nm from the surface thereof, it is necessary to use a light source capable of emitting a light beam having a short wavelength.  Moreover, since an absorbed light energy is usually accumulated in an excited state due to electron transition between different energy bands, a time period much longer than 1 picosecond is usually needed for arriving at a local thermal equilibrium state in which energy has been relaxed in lattice system.  For this reason, the aforementioned measuring method has been found to be extremely difficult in measuring the thermal diffusivity of a non-metal thin film such as a semiconductor thin film. 
An object of the present invention is to make it possible to correctly measure a thermal diffusivity of a substance even if it is a non-metal substance, also to correctly measure a thermal diffusivity within a three-layer film structure. 
In more detail, the present invention is to make it possible to deal with even a non-metal substance, i.e., to simultaneously measure thermal diffusivity value as well as interface thermal resistance of the non-metal substance, by forming metal layers on both sides of the non-metal substance, using a conventional picosecond thermoreflectance method. 
Further, the present invention is to make it possible to measure a thermal diffusivity within the above three-layer film substance, by preparing a plurality of substances which are different from one another only in their thicknesses and whose thermophysical properties values are unknown, and analyzing measured data about the three-layer sample, thereby making it possible to correctly measure the thermal diffusivity within the above three-layer substance. 
In addition, the present invention is to make it possible to use a response function method to correctly measure the thermal diffusivity within a three-layer film structure, on the base of the above method for measuring the thermal diffusivity within the three-layer substance, in accordance with an area defined by a transient temperature history curve on the backside of the pulse-heated three-layer structure as well as the horizontal axis corresponding to a pulse-heating time. 
In order to achieve the above objects, the present invention provides a method for measuring a thermal diffusivity of a three-layer substance including a middle layer whose thermophysical properties are unknown, and two other layers formed on both sides of the middle layer, said two other layers belonging to the same sort of substance and having known thermophpysical properties, characterized in that one side of the three-layer substance is heated by a pulse light, while at the same time a transient temperature history on the opposite side of the three-layer substance is observed, thereby simultaneously measuring thermophysical properties and interface thermal resistance. 
Further, the above-described thermal diffusivity measuring method of the present invention is characterized in that said method comprises preparing a plurality of substances which are different from one another only in their thicknesses and whose thermophysical properties values are unknown, forming on both sides of each of the substances other substance layers which are in the same identical state and whose thermophysical properties are known, measuring the thermal diffusivity and the interface thermal resistance of each of the three-layer substances. 
Moreover, the present invention is a method for measuring a thermal diffusivity of the above three-layer substance, characterized in that said method comprises calculating an area defined by a transient temperature history curve on the backside of the pulse-heated three-layer substance and a horizontal axis corresponding to a pulse-heating time, thereby simultaneously measuring thermal diffusivity within the three-layer substance as well as the interface thermal resistance.