Self referencing heterodyne reflectometer and method for implementing

Abstract

The present invention is directed to a self referencing heterodyne reflectometer system and method for obtaining highly accurate phase shift information from heterodyned optical signals, without the availability of a reference wafer for calibrations. The self referencing heterodyne reflectometer rapidly alternates between a heterodyne reflectometry (HR) mode, in which an HR beam comprised of s- and p-polarized beam components at split angular frequencies of ω and ω+Δω is employed, and a self referencing (SR) mode, in which an SR beam comprised of p-polarized beam components at split angular frequencies of ω and ω+Δω is employed. When the two measurements are made in rapid succession, temperature induced noise in the detector is be assumed to be the same as for both measurements. A measured phase shift δis generated from the HR beam and a reference phase shift δis generated from the SR beam. The measured phase shift δgenerated from the beat signals of the HR beam is used for film thickness measurements. The SR beam is p-polarized and no significant reflection will result from a film surface and will not carry any phase information pertaining to the film. The reference phase shift δgenerated from the beat signals of the SR beam is equivalent to that obtained using a reference sample. Film phase shift information is then derived from the measured phase shift δand the reference phase shift δwhich is independent of phase drift due to temperature. Film thickness is calculated from the film phase shift information.