Overlay Measurement Focus Control for Uneven Surfaces
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Solution Overview
Problem
Conventional autofocus systems in semiconductor manufacturing face challenges in accurately finding the focus position due to an uneven laser-reflecting surface, which affects the precision of overlay measurement and circuit formation.
Innovation Solution
An overlay measurement device and method that control focus movement by using a lighting part to orient light towards an overlay measurement target, a collection part with an objective lens and detector to obtain images at various focuses, and a processor to adjust the objective lens position based on focus graphs and contrast indices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional autofocus systems are used on uneven laser-reflecting surfaces, then the system can operate with simple hardware, but the focus position cannot be accurately found
Solution Approach 1:
The patent replaces conventional mechanical autofocus systems with a computational approach. Instead of relying on physical focus search mechanisms that struggle with uneven surfaces, the system uses a processor to generate focus graphs from captured images, calculate contrast indices, and computationally determine the optimal focus position. This substitution of mechanical search with computational analysis enables accurate focus finding on uneven laser-reflecting surfaces while maintaining relatively simple hardware.
2Manufacturing precision
If the objective lens position is not accurately adjusted, then the system operation is simple, but the overlay measurement precision deteriorates
Solution Approach 1:
The patent implements a feedback mechanism where the processor continuously monitors the contrast index from captured images and adjusts the objective lens position accordingly. The system captures images at different focus positions, calculates contrast indices, identifies the maximum contrast point as the optimal focus, and uses this feedback to precisely position the lens. This closed-loop feedback control ensures accurate overlay measurement while using relatively simple actuation mechanisms.
Solution Approach 2:
The patent performs preliminary focus search by capturing multiple images at different focus positions before making the final lens positioning decision. The processor generates focus graphs from these preliminary images, calculates contrast indices for each position, and uses this pre-computed information to determine the optimal focus position. This preliminary action approach enables precise lens positioning without requiring complex real-time control mechanisms.
3Measurement precision
If focus position search is performed on uneven surfaces, then complete surface coverage is achieved, but the time required to find focus increases
Solution Approach 1:
The patent uses a scanline-based approach where the processor analyzes focus information along specific scanlines rather than requiring complete analysis of the entire measurement field. By identifying the optimal focus position along representative scanlines and extrapolating this information, the system achieves accurate focus finding without the time penalty of exhaustive full-surface analysis. This partial action approach maintains measurement precision while significantly reducing focus search time.
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 enables accurate identification of the reference focus and adjustment of the objective lens to achieve the highest contrast focus position, thereby improving the precision of overlay measurement and ensuring reliable circuit formation.
Implementation Method 1
a lighting part being configured to orient lighting toward an overlay measurement target
Implementation Method 2
a collection part comprising an objective lens and a detector for obtaining one or more of images of each focus in the target
Implementation Method 3
a lens focus actuator moving the objective lens to adjust a distance between the objective lens and a wafer
Data Source
AI summary
There is provided an overlay measurement device controlling a focus movement, which includes: a lighting part being configured to orient lighting toward an overlay measurement target; a collection part comprising an objective lens and a detector for obtaining one or more of images of each focus in the overlay measurement target; a lens focus actuator moving the objective lens to adjust a distance between the objective lens and a wafer; and a processor controlling an operation of the lens focus actuator. The processor obtains a focus graph respectively in relation to two layers from one or more of the images of each focus, and the processor identifies a reference focus in relation to the obtained focus graphs to move the objective lens.