| 摘要 |
PURPOSE:To enable an accurate measurement to be made by eliminating influence of vibration or fluctuation of air. CONSTITUTION:Two coherent lights with a different frequency which are emitted from a light source 20 impinge on a beam splitter 21 and a reference light beat signal according to interference is obtained by a Gland Thompson prism 22 which is placed in its reflection direction and a light detector 24 and is input to a signal processing control part 25. In a transmission direction of the beam splitter 21, a prism 27 with a polarization beam splitter part 27a and reflection parts 27b and 27c is provided and a flux of light is separated into two polarization lights according to a polarization direction and is applied to diffraction gratings 31 and 33 which are placed on a mask 28 and wafer 29, respectively. A diffraction light beat signal due to interference of diffraction light is obtained by a Gland Thompson prism 35 and a photo detector 37 for a reflection light and is input to the signal processing control part 25. In the signal processing control part 25, a phase difference between the reference light beat signal and the diffraction light beat signal is obtained, thus enabling a position deviation of the diffraction gratings 31 and 33 to be detected. |
