Measurement of dielectric properties of polymers and semiconductor materials using Terahertz Time-domain spectroscopy along with principal component analysis

Optical techniques provide noncontact measurement of thickness and refractive index. These properties have a number of applications in semiconductors, dielectrics, polymers and material science. Simultaneous measurement of thickness and refractive index using various interferometric and other optical techniques have been reported by a number of authors. These techniques utilize visible or IR radiations and therefore cannot be used for many polymers and semiconducting materials which are opaque in optical region. However, terahertz (THz) radiations offer the advantage that many polymers and semiconductors are transparent in this regime. Terahertz Time-domain spectroscopy (THz-TDS) has been used extensively for non-contact measurement of thickness and other optical parameters of materials opaque at other wavelengths. Polymers are among the most commonly used materials in everyday life, hence their distinguishability is important and there is a need to generate a database which can be used as an imaging tool for security applications. Polymers have wide range of applications in aerospace, defense and semiconductor industries. Due to their transparency and lack of any significant peaks in the THz range they are used in manufacturing of THz optics and also as binder during sample preparation for THz studies. They are also used as matrix material is THz-metamaterials. Polymers are difficult to distinguish in THz range as no characteristic feature exist in the spectral range below < 2 THz. In this work, we demonstrate that by combining principal component analysis with THz-TDS one can differentiate such polymers. Additionally, we report experimental measurement of refractive indices and absorption coefficient of SAN and bakellite in the spectral range of 0.2
⁻²
THz for the first time.