Microlithography Autofocusing Device Using Gap Caustics to Eliminate Moiré Effects
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Solution Overview
Problem
Autofocusing methods in microlithography often suffer from Moiré effects when dealing with periodic and non-periodic structures on lithography masks, leading to measurement difficulties or corruption.
Innovation Solution
A device and method utilizing an imaging device with an autofocusing system that projects a focusing structure in the form of a gap onto the mask, which is imaged as a focus caustic, allowing for the avoidance of Moiré effects and enabling precise determination of the mask's position relative to the focal plane through movement and evaluation of the caustic centers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a periodically structured focus image or non-periodic focus image is projected into the focusing image plane, then autofocusing can be performed, but Moiré effects occur that corrupt the measurement
Solution Approach 1:
The invention extracts the harmful periodic structure from the focusing image and replaces it with a gap structure. By removing the periodic pattern that causes Moiré effects while retaining the ability to generate focus-dependent imaging characteristics, the solution eliminates the measurement corruption while preserving the autofocusing capability.
Solution Approach 2:
The invention changes the structural parameter of the focusing image from periodic or non-periodic patterns to a gap structure. This parameter change fundamentally alters the interaction between the focusing image and the mask structures, eliminating Moiré effect generation while maintaining focus sensitivity.
2Measurement precision
If multiple gaps are imaged on the focus structure, then measurement inaccuracies can be reduced through averaging, but device complexity increases
Solution Approach 1:
The focusing structure is segmented into multiple gaps arranged in a specific pattern. This segmentation allows the system to capture multiple independent measurements simultaneously, which can be averaged to reduce the impact of local inhomogeneities and improve overall measurement precision without requiring complex post-processing.
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 effectively reduces measurement inaccuracies caused by inhomogeneities and aberrations, providing a stable and accurate focus determination by averaging over multiple gaps and compensating for mask inhomogeneities, thus improving the focus setting process.
Implementation Method 1
an imaging optical unit with a focal plane for imaging the mask
Implementation Method 2
The gap is imaged on the structures of the mask as a so-called focus caustic. In this case, a focus caustic is understood to mean an at least partly unsharp imaging of the gap
Implementation Method 3
During focusing by means of the autofocusing methods described in the cited documents, if the lithography masks determined have periodic and/or non-periodic structures, disturbing Moiré effects can occur
Data Source
AI summary
The invention relates to a device for measuring a mask for microlithography, the device including an imaging device and an autofocusing device. The imaging device comprises an imaging optical unit with a focal plane for imaging the mask, an object stage for mounting the mask, and a movement module for producing a relative movement between object stage and imaging optical unit. The autofocusing device is configured to generate a focusing image by way of the imaging of a focusing structure in a focusing image plane intersecting the focal plane, in which the focusing structure is embodied as a gap. Furthermore, the invention relates to an autofocusing method for a device for measuring a mask for microlithography.