Sinusoidal wavelength-scanning interferometer for measurement of thickness and surface profile of thin films

ICO20: optical devices and instruments : 21-26 August 2005 : Changchun, China. : Aug 2005, Changchun, ChinaWe propose a sinusoidal wavelength scanning interferometer for measuring thickness and surfaces profiles with a thin film. An acousto-optic tunable filter (AOTF) is used to produce sinusoidally wavelength-scanned light from a superluminescent laser diode (SLD) with a wide spectral bandwidth of 46nm. The interference signal contains an amplitude Zb of a time-varying phase and a constant phase α. Two measured values of OPD, which are denoted by Lz and Lα, are obtained from Zb and α. By combining Lz and Lα, an OPD longer than a wavelength is measured with an error less than a few nanometers. When the object has two reflective surfaces, the detected interference signal contains two interference signals which are caused by the front and rear surfaces. In this case we must determine the values of Zb1, Zb2, α1, and α2, where suffixes of 1 and 2 are corresponding to the front and rear surfaces, respectively. We define an error function that is the difference between the detected signal and the theoretical signal, and reduce the value of the error function with the multidimensional nonlinear least-squares algorithm to search the values of Zb1, Zb2, α1, and α2. Experimental results show that the thickness and surfaces profiles of a silica glass plate of 20μm-thickness are measured with error less than 1.5nm