Linear polarization modulation heterodyne ellipsometer using digital signal processing technique

Abstract

ABSTRACT In this research, a novel linear polarization modulation he terodyne ellipsometer (LPMHE) integrated with a digital signal processor is able to measure ellipsometric parameters of a specimen was developed. In this setup, a pair of orthogonally circularly polarized lights with slightly different frequency of the laser beam is used which behaves like a linear polarization rotator at high speed. By integrating with a digital storage oscilloscope, LPMHE is able to real-time measure ellipsometric parameters precisely. When the incident angles of laser beam are set at 60o and 70o in LPMHE, an accuracy of less than 0.7% on ellipsometric parameters measurement of the SiO 2 thin film deposited on silicon substrate was demonstrated. Keywords: heterodyne interferometry, ellipsometry, dual-frequency laser. 1. INTRODUCTION A Zeeman He-Ne laser which emits a pair of circularly polarized light with slightly different frequency and opposite sense of rotation behaves like a linear polarization rotator (LPR) [1,2]. The rotational frequency of LPR is dependent upon the beat frequency of a Zeeman laser beam. This provides a high speed polarization modulation based on heterodyne interferometry and then can be applied to develop a heterodyne interferometric ellipsometer follows the developed theory [2]. However, the unequal amplitude between two orthorgonally polarized components of a Zeeman He-Ne laser beam reduces the accuracy in measurement [3]. This results in the uncertainty and has been analyzed and discussed in our previous work [4]. We suggest an improved solution by using a single-frequency laser, an electro-optic modulator (EOM) and a quarter wave plate to develop a linear polarization modulation heterodyne ellipsometer (LPMHE) [5]. The accuracy of LPMHE was verified experimentally. Generally, the heterodyne interferometric signal of LPMHE is expressed by I tI I tI t