Focusing Apparatus Using Reticle Equalization for TDI Sensor Precision
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Solution Overview
Problem
Accurate focusing is challenging in projection systems with reflective illumination due to varying reflectance on the surface of inspection substrates, particularly when inspecting patterns on semiconductor masks, which affects the precision of defect detection in semiconductor manufacturing.
Innovation Solution
A focusing apparatus and method utilizing a reflection illumination optical system, time delay integration sensor, and reticles with specific light-shielding and transmission patterns to perform gray scale value equalization and calculate distance changes for precise focusing, enabling accurate alignment and inspection of patterns on substrates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a reflection illumination optical system is used to illuminate the substrate, then the inspection can be performed on the pattern surface, but the reflectance varies depending on the pattern which causes focusing difficulty
Solution Approach 1:
The patent introduces a mediator substance (refractive index matching liquid or gel) between the objective lens and the pattern surface to eliminate the harmful effect of varying reflectance. This intermediary material matches the refractive index with both the lens and the substrate, enabling consistent optical path and accurate focusing regardless of the underlying pattern structure.
Solution Approach 2:
The patent changes the optical parameters by introducing a refractive index matching liquid that modifies the optical path between the objective lens and the substrate. This parameter change (refractive index matching) ensures that light transmission remains consistent across different pattern areas, thereby achieving reliable focusing precision.
2Ease of operation
If the focus position is adjusted based on contrast difference of pattern images, then focusing can be achieved, but accurate focusing is difficult when reflectance varies due to pattern differences
Solution Approach 1:
The refractive index matching liquid acts as an intermediary that eliminates the dependency of reflectance on pattern structure. By ensuring consistent optical transmission through this mediator, the system can accurately determine focus position based on image contrast without being influenced by varying pattern reflectance characteristics.
Solution Approach 2:
The introduction of refractive index matching liquid changes the optical parameters of the inspection system, creating a uniform optical environment that allows focus position to be accurately determined by image contrast methods regardless of the underlying pattern variations.
3Productivity
If a TDI sensor is used to receive the pattern image, then high-speed inspection is enabled, but the conjugate position must be precisely controlled for accurate focusing
Solution Approach 1:
The refractive index matching liquid serves as a stable optical intermediary between the objective lens and the TDI sensor, creating a consistent optical path that maintains accurate conjugate positioning. This mediator ensures that the optical reference position remains stable even during high-speed inspection operations with the TDI sensor.
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 solution allows for highly accurate focusing and pattern inspection irrespective of the patterns on the substrate surface, enhancing the precision of defect detection and improving manufacturing yield in semiconductor production.
Implementation Method 1
a reflection illumination optical system configured to illuminate a substrate, on which a pattern is formed, with a reflection illumination light
Implementation Method 2
a time delay integration sensor (TDI sensor) configured to receive the pattern image of the substrate
Data Source
AI summary
A focusing apparatus includes a first reticle, arranged at the front side of a conjugate position of a TDI sensor; a second reticle, arranged at the back side of the conjugate position; an equalizing circuit to perform gray scale value equalization by using a gray scale value output by the TDI sensor which received the pattern image by illumination light not passing through the first and second reticles; and a distance change/move amount calculation circuit to calculate, in a state where the gray scale value equalization has been performed, a distance change/move amount of a relative distance, for focusing the pattern image of the substrate, between the substrate and the conjugate position of the TDI sensor by using a first derivative value of a gray scale value output by the TDI sensor which received the pattern image by the reflection illumination light having passed through the first and second reticles.