Lithography Mask Focus Position Metrology
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Solution Overview
Problem
Current methods for determining the focus position of lithography masks are not reliably reproducible and often depend on the test structure and illumination settings, especially failing with coherent illumination settings having a low pupil filling coefficient.
Innovation Solution
A method that records and evaluates the intensity distributions of a structure-free measurement region to determine the focus position by analyzing speckle contrast, separating imaging aberration contributions, and representing them using Zernike polynomials, allowing for the qualification of the imaging optical unit and reduction of defocus aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional focus determination methods using test structures are used, then focus position can be determined, but the results are not reliably reproducible and depend on test structure and illumination settings
Solution Approach 1:
The invention extracts the focus determination process from dependence on specific test structures and illumination settings by using a structure-free measurement region. The method separates the measurement of wavefront aberrations from the imaging of mask structures, allowing focus determination to be performed independently of the mask pattern and illumination configuration.
Solution Approach 2:
The invention changes the measurement parameter from structure-dependent intensity distributions to structure-independent wavefront aberration measurements. By evaluating the wavefront aberrations in a structure-free region and minimizing defocus contributions, the method achieves parameter independence from illumination settings and mask structures.
2Productivity
If coherent illumination settings with low pupil filling coefficient are used, then illumination efficiency is improved, but conventional focus determination methods fail to function
Solution Approach 1:
The invention replaces structure-based focus determination with wavefront aberration-based measurement. This substitution allows the method to function with coherent illumination settings by measuring the wavefront directly rather than relying on intensity distributions from test structures that require incoherent or partially coherent illumination.
3Measurement precision
If focus stack recording of measurement regions is performed, then focus position information can be obtained, but the evaluation is complex and requires separation of imaging aberration contributions
Solution Approach 1:
The invention segments the focus determination process into two independent parts: recording the focus stack in a structure-free measurement region, and evaluating wavefront aberrations separately from intensity distributions. This segmentation simplifies the evaluation by eliminating the need to separate mask structure contributions from imaging aberrations.
Solution Approach 2:
The invention introduces wavefront aberration measurement as an intermediary between the focus stack recording and focus position determination. By using wavefront aberrations as the intermediate parameter, the method simplifies the evaluation process compared to direct intensity distribution analysis.
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 method enables robust and simple determination of the focus position, independent of mask structures and illumination properties, and is applicable even with coherent illumination, providing reliable and reproducible results.
Implementation Method 1
evaluating the 2D intensity distributions (15zi) includes ascertaining the speckle contrast
Data Source
AI summary
For determining a focus position of a lithography mask (e.g., 5), a focus stack of a measurement region free of structures to be imaged is recorded and the speckle patterns of the recorded images are evaluated.