Optical Apparatus Focus Correction via Displacement Drift Prediction
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Solution Overview
Problem
Defocusing issues in defect inspection apparatuses using EUV light due to thermal expansion cause image blurring and pseudo defects, as the depth of focus is shallow and sensitive to temperature changes.
Innovation Solution
An optical apparatus with a detector, displacement measurement unit, and prediction unit that measures displacement drift in optical elements and predicts focus drift using correlation, allowing for precise control of the distance between the sample and optical system to maintain focus.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If EUV light is used for illumination in defect inspection, then high-resolution imaging is achieved, but focus drift occurs due to thermal expansion causing image blurring
Solution Approach 1:
The system continuously measures the position of optical elements using displacement measurement units (interferometers) and feeds this information back to the control unit. The control unit calculates focus drift based on the measured displacement and correlation data, then adjusts the distance between the optical system and sample in real-time to maintain proper focus, thereby eliminating image blurring while preserving high-resolution imaging capability
Solution Approach 2:
The patent replaces direct mechanical measurement of focus drift with an optical measurement system using interferometers to detect displacement of optical elements. This substitution allows for more precise measurement of thermal expansion effects and enables automated compensation without mechanical contact, improving both measurement precision and focus stability
2Measurement precision
If the depth of focus is made shallow for high-resolution imaging, then imaging precision is improved, but sensitivity to temperature change increases causing focus drift
Solution Approach 1:
The system pre-establishes the correlation between displacement of optical elements and focus drift through measurement and storage in the storage unit. This preliminary action allows the control unit to immediately compensate for focus drift when temperature changes occur, rather than waiting for the drift to manifest as image degradation. The correlation data is prepared in advance to enable rapid response to thermal expansion
Solution Approach 2:
The system implements continuous feedback monitoring of optical element positions using displacement measurement units. When temperature changes cause thermal expansion and displacement of optical elements, the feedback loop detects this displacement, calculates the corresponding focus drift using stored correlation data, and automatically adjusts the optical system-sample distance to maintain focus, thereby reducing sensitivity to temperature changes while preserving shallow depth of focus for high-resolution imaging
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
Enhances focus matching and reduces image blurring by accurately predicting and correcting focus drift caused by thermal expansion, thereby improving defect inspection accuracy.
Implementation Method 1
an interferometer configured to detect a position of the mirror from interference between the laser light emitted to the mirror and the laser light reflected by the mirror
Implementation Method 2
a laser configured to emit laser light
Implementation Method 3
an optical system configured to illuminate the sample with the illumination light and guiding the reflected light reflected by the sample to the detector
Data Source
AI summary
An optical apparatus according to the present embodiment includes a detector for detecting detection light of illumination light reflected by a sample, an optical system for illuminating the sample with the illumination light and guiding the detection light reflected by the sample to the detector, a displacement measurement unit for measuring a displacement drift indicating the amount of drift in the position of an optical element included in the optical system, a storage unit for storing the correlation between the displacement drift and a focus drift indicating the amount of drift in the distance between the sample and the optical system when the detection light detected by the detector is brought into focus, and a prediction unit for predicting a focus drift from the measured displacement drift by using the correlation.