Method for focusing an object plane and optical assembly

US9091525B2 · kind B2 · utility

Assignee

• KLA-Tencor MIE GmbH · DE

Inventors

• Wolfgang Sulik · Aßlar, DE
• Lambert Danner · Wetzlar, DE
• Alexander Buettner · Weilburg, DE

Key dates

• Filing date: Feb 14, 2011
• Grant date: Jul 28, 2015
• Expiry date: May 30, 2034

Classification

• Technology area (CPC G): Physics
• CPC primary: G02B7/38
• WIPO field: Measurement
• WIPO sector: Instruments

Abstract

A method for focusing an object plane (42) through an objective (30) and an optical assembly (10), with which the method can be carried out, are disclosed. A geometric reference structure (21) is positioned in a plane (36) conjugate to a field plane (34) of the objective (30) and is imaged onto the object plane (42). The geometric reference structure (21) is illuminated with a light beam (24), which encloses a non-zero angle (φ) with a normal direction (38) of the conjugate plane (36). Therefore a position (Y) of an image (22) of the geometric reference structure (21) in the object plane (42) depends on the signed distance (37) between the object plane (42) and the field plane (34), and correspondingly is evaluated for the determination of the focus position. The optical assembly (10) preferentially may be a metrology tool (100) for measuring structures (120) on masks (100), wherein the objective (30) is the measurement objective of the metrology tool (100).