In recent years, non-destructive sensing technology using terahertz radiation (hereinafter also referred to as THz wave) has been researched and developed. In this specification, THz wave means radiation containing a component in a frequency band between 30 GHz and 30 THz. Application of an examining apparatus using THz wave to internal examining means for performing non-destructive quality check of an object has been proposed. In other words, applications to inspection of foreign substance in powdery matter, defect examination of plastics molds, examination of carrier concentration of semiconductor wafers, and the like have been proposed.
Japanese Patent No. 3387721 (PTL 1) discloses technology of examination of characteristic of an object that can be executed by spectral analysis achieved by the Fourier transform of a time-domain waveform of THz wave pulse transmitted through the object. The time-domain waveform can be obtained by the THz time-domain spectroscopy (THz-TDS). In this technology, composition characteristic at each point of the object is recognized by comparison between signals acquired and information of data base of substances acquired beforehand, and the point of the object is moved to obtain an image of the composition characteristic of the object.
Further, Japanese Patent Laid-open No. 2002-98634 (PTL 2) discloses technology that composition characteristic of a semiconductor wafer, such as mobility, electric conductivity and carrier concentration, can be calculated from its complex refractive index acquired by the THz-TDS using Drude model. This technology has been developed as non-destructive means for examining a wafer of Si, GaAs or the like.