Focus Adjustment for Bending Photomasks
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Solution Overview
Problem
In photomask inspection apparatuses, the bending of samples supported at multiple points can lead to errors in focus position measurement due to membrane stress, causing slow and inaccurate focus adjustment, even with autofocusing mechanisms.
Innovation Solution
An image pickup apparatus and focus adjustment method that acquire bending data, calculate correction data using a biharmonic equation, and adjust focus based on estimation data to account for sample bending, enabling speedy and accurate focus adjustment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a stage supporting the sample at multiple points is used, then the sample can be stably supported, but the sample bends due to its own weight and membrane stress, causing errors in focus position measurement
Solution Approach 1:
The system performs preliminary actions by acquiring bending data through numerical analysis before focus adjustment, calculating the expected bending deformation in advance, and using this information to correct focus position measurements. This preliminary calculation of bending effects allows the system to compensate for the deformation caused by multi-point support, thereby maintaining both stable support and measurement accuracy.
2Extent of automation
If autofocusing mechanism is equipped, then focus adjustment can be automated, but the focus position may widely deviate due to sample bending, causing slow focus adjustment
Solution Approach 1:
The system introduces an intermediary correction mechanism that acts between the autofocusing mechanism and the final focus position. By calculating bending data and using it to correct focus position measurements, this intermediary step provides accurate reference information to the autofocusing system, preventing wide deviations and enabling faster convergence to the correct focus position.
3Measurement precision
If bending data is acquired through numerical analysis and correction data is calculated, then focus position accuracy is improved, but the processing complexity increases
Solution Approach 1:
The system replaces complex physical measurement mechanisms with numerical analysis and computational methods. Instead of using additional mechanical sensors or complex optical systems to measure bending, the invention uses numerical analysis to calculate bending data based on the sample's physical properties and support conditions. This substitution of computational methods for mechanical measurement systems achieves high accuracy while controlling processing complexity.
Data Source
AI summary
An image pickup apparatus includes a stage configured to support a sample at a plurality of support points, a bending data acquisition unit configured to acquire bending data corresponding to a bending of the sample supported on the stage, a height information detection unit configured to detect a height of the sample supported on the stage, a difference value calculation unit configured to calculate a difference value between a height indicated by height information and a height indicated by the bending data at each of a plurality of points on the sample, a correction data calculation unit configured to calculate correction data based on the difference value, and an estimation unit configured to calculate estimation data for estimating the height of the sample by correcting the bending data using the correction data.