Geometric Reference Structure for High-Precision Optical Focusing
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Solution Overview
Problem
Existing focusing methods often require multiple optical elements and different light wavelengths, leading to compromised imaging precision and increased complexity, especially when dealing with structured surfaces like those in photolithography masks.
Innovation Solution
A method involving a geometric reference structure illuminated at a non-zero angle, imaged onto the object plane, and evaluated for position relative to a reference position, allowing for precise focusing by adjusting the objective's distance along the optical axis until the image is within a defined area of the reference position.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple optical elements and different light wavelengths are used for focusing, then focusing capability is achieved, but imaging precision is compromised and device complexity increases
Solution Approach 1:
The patent combines the focusing reference structure and the object of interest into the same optical path, eliminating the need for separate focusing optics. The geometric reference structure is imaged through the same objective lens used for the object, merging two functions into one optical path and reducing overall system complexity while maintaining high imaging precision.
Solution Approach 2:
The objective lens serves dual purposes: it images both the geometric reference structure (for focusing determination) and the actual object of interest. This multi-functionality eliminates the need for separate focusing optics and multiple wavelengths, simplifying the optical system while maintaining precise focusing capability.
2Measurement precision
If multiple optical elements are used for focusing, then focusing capability is achieved, but the optical system configuration becomes more complex
Solution Approach 1:
The patent extracts the focusing determination function from the main imaging path by using a geometric reference structure that can be imaged through the same objective. This separation of functions within the same optical path allows precise focusing determination without adding complex dedicated focusing optics to the system.
Solution Approach 2:
The focusing reference structure and object imaging share the same optical path and objective lens, merging the focusing determination system with the imaging system. This integration reduces the number of separate optical elements and simplifies system configuration while maintaining high focusing precision.
3Measurement precision
If prior art focusing methods are used on structured surfaces, then focusing is achieved, but the results show perturbing dependence on structures and edges
Solution Approach 1:
The patent introduces a geometric reference structure as an intermediary object for focusing determination. This reference structure has known, simple geometry that is imaged through the same objective as the actual object. By measuring the position and sharpness of this intermediary reference structure's image, the system determines focus without being affected by the complex structures and edges of the actual object being studied.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables high-precision focusing with minimal effort and cost, specifically for photolithography masks, by using a single wavelength and optimizing the objective for the illumination wavelength, thus simplifying the optical system and improving focusing accuracy.
Implementation Method 1
imaging the geometric reference structure onto the object plane with the objective
Implementation Method 2
evaluating an image of the geometric reference structure in the object plane with respect to its position
Data Source
AI summary
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).