Autofocus Device Using Virtual Image for Precision
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Solution Overview
Problem
Existing autofocus devices for imaging devices face limitations in precision due to unwanted modulation of intensity variations caused by object structures, which affects the accuracy of defocus determination and focusing precision, particularly in applications like lithography-mask and wafer inspection in the semiconductor industry.
Innovation Solution
An autofocus device and method that utilize an imaging optic with a first and second focal plane, where the focusing image intersects the second focal plane, allowing for precise determination of object deviation from the target position by minimizing the influence of object structures, using an illumination module with a periodically intensity-modulated grating and a control module to adjust the object position based on captured image analysis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an intensity-modulated grating image is projected onto the object to determine defocus, then the autofocusing function is achieved, but object structures cause unwanted intensity modulation that limits measurement precision
Solution Approach 1:
The patent introduces a virtual image as an intermediary element. Instead of directly analyzing the grating image reflected from the object surface (which is contaminated by object structures), the system creates a virtual image of the grating that is formed at a different location. This virtual image serves as a mediator that carries defocus information without being directly influenced by object structure variations, thereby resolving the contradiction between achieving autofocusing and avoiding unwanted intensity modulation.
Solution Approach 2:
The patent segments the optical path into distinct functional zones: the object region, the virtual image formation region, and the detection region. By separating the grating projection path from the object reflection path and forming a virtual image at an intermediate location, the system isolates the defocus measurement function from the object structure interference, allowing precise measurement without contamination from object-generated intensity variations.
2Measurement precision
If the object is positioned close to the focal plane for imaging, then the object detail is captured sharply, but the object structures interfere with defocus determination
Solution Approach 1:
The patent transitions the problem from a two-dimensional object plane to a three-dimensional virtual image space. By forming a virtual image at a different spatial location (at infinity or at a different plane), the system evaluates focus quality in another dimension that is not contaminated by object structures. This dimensional transformation allows simultaneous sharp object imaging and clean defocus measurement without mutual interference.
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 autofocusing by ensuring the focusing image is captured sharply while the object is blurry, effectively suppressing the interference of object structures and enhancing the accuracy of defocus determination, thereby improving focusing precision in semiconductor applications.
Implementation Method 1
the illuminating beam path is folded by reflection from the object
Implementation Method 2
an image capture module having a second focal plane whose position relative to said first focal plane is known
Data Source
AI summary
An autofocus device for an imaging device is provided, which has an imaging optic having a first focal plane and an object table for moving an object to be imaged relative to the first focal plane, wherein said autofocus device comprises a) an image recording module having a second focal plane, the location thereof relative to the first focal plane being known, b) a lighting module (BM) for imaging a focusing image along a lighting beam path in a focusing image plane such that, if the object is positioned in a target position at a predetermined distance to the second focal plane, the lighting beam path is folded because of reflection on the object and the focusing image, which lies in the focusing image plane, intersects the second focal plane or lies therein, and c) a control module, which activates the object table to focus the imaging device so that the object is positioned in the target position, from a signal of the image recording module, which the image recording module generates on the basis of the recording thereof of the focusing image when the object is positioned in the target position, derives the deviation of the object position from the target position, and, based on the derived deviation, the predetermined distance, and the relative location of the first and second focal planes, activates the object table so that the object is positioned in the first focal plane.