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1. (WO2016050453) FOCUS MONITORING ARRANGEMENT AND INSPECTION APPARATUS INCLUDING SUCH AN ARRAGNEMENT

Pub. No.:    WO/2016/050453    International Application No.:    PCT/EP2015/070410
Publication Date: Fri Apr 08 01:59:59 CEST 2016 International Filing Date: Tue Sep 08 01:59:59 CEST 2015
IPC: G03F 7/20
G03F 9/00
Applicants: ASML NETHERLANDS B.V.
Inventors: SINGH, Amandev
PELLEMANS, Henricus, Petrus, Maria
Title: FOCUS MONITORING ARRANGEMENT AND INSPECTION APPARATUS INCLUDING SUCH AN ARRAGNEMENT
Abstract:
An inspection apparatus (300) includes a focus monitoring arrangement (500, 500'). Focusing radiation (505) comprises radiation having a first wavelength and radiation having a second wavelength. Reference radiation and focusing radiation at each wavelength are provided with at least one relative frequency shift so that the interfering radiation detected in the detection system includes a time-varying component having a characteristic frequency. A focus detection system (520) comprises one or more lock-in detectors (520b, 520c, 900). Operating the lock-in detectors with reference to both the first and second characteristic frequencies allows the arrangement to select which of the first and second focusing radiation is used to determine whether the optical system is in focus. Good quality signals can be obtained from targets of different structure.