Autofocus Overlay Imaging for Faster Wafer Mark Focusing
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Solution Overview
Problem
Existing image acquisition devices require significant time to focus on marks due to varying heights and warpage of semiconductor wafers, affecting the efficiency of overlay measurement.
Innovation Solution
An image acquisition device with an autofocus optical system that detects Z-direction positional information using a light blocking wheel and bi-cell photodiode sensors to adjust the objective lens in the Z-direction based on phase differences, allowing for rapid focusing on marks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional autofocus optical systems are used to focus on each overlay mark, then measurement accuracy is maintained, but the focusing time increases significantly due to height variations and warpage of semiconductor wafers
Solution Approach 1:
The patent applies preliminary action by detecting the Z-direction positional information of the wafer surface in advance using the autofocus optical system while moving from the previous measurement position to the current measurement position. This allows the objective lens to be pre-positioned closer to the correct focus distance before actual image acquisition, significantly reducing the focusing time required at each measurement point while maintaining measurement accuracy.
2Measurement precision
If the objective lens is moved frequently to focus on marks at different heights, then accurate images are obtained, but the MAM (Move, Acquire, Measure) cycle time increases
Solution Approach 1:
The patent implements feedback by continuously detecting the Z-direction positional information of the wafer surface using the autofocus optical system during movement. This real-time feedback allows the system to dynamically adjust the objective lens position to match the actual wafer height at each measurement location, ensuring accurate focus while optimizing the MAM cycle time through intelligent, data-driven positioning rather than frequent brute-force adjustments.
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
Reduces focusing time by anticipating and adjusting for height variations, thereby shortening the MAM (Move, Acquire, Measure) cycle.
Implementation Method 1
an autofocus optical system configured to detect Z-direction positional information of a surface of the sample while changing a position of an observation region of the imaging optical system
Implementation Method 2
forming an image from light reflected from the marks
Implementation Method 3
an imaging optical system configured to form an image from light reflected from the marks
Data Source
AI summary
Proposed is an image acquisition device and method, which includes a stage on which the sample is arranged, an imaging optical system configured to form an image from light reflected from the marks, an autofocus optical system configured to detect Z-direction positional information of a surface of the sample while changing a position of an observation region of the imaging optical system from a previous measurement position of the sample to a current measurement position of the sample by moving at least one of the stage and the imaging optical system in an X-Y plane, and a focus controller configured to move an objective lens of the imaging optical system toward a Z direction on the basis of the Z-direction positional information and to bring a distance between the surface of the sample and the objective lens closer to a reference distance at which the mark is in focus.