Variable Pitch Diffraction Grating for Autofocus Sensitivity
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Solution Overview
Problem
Fringe projection autofocus systems in lithography face challenges due to unequal sensitivity to wafer height changes across different wavelength bands, exacerbated by wavelength-dependent optical path differences caused by air turbulence, making focus correction difficult.
Innovation Solution
The system adjusts the angle of incidence and/or selects grating pitches for different wavelength bands to ensure equal optical axis and optical path difference error sensitivity, using a diffraction grating with variable grating periods and angles of incidence to project fringes such that diffraction orders for multiple wavelengths propagate along a common path, allowing for equal sensitivity and error correction across bands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the same grating and illumination angle are used for all wavelength bands, then the system structure is simple, but the sensitivity to wafer height changes and OPD errors is unequal across wavelength bands
Solution Approach 1:
The patent applies local quality by making the grating period wavelength-dependent. Different regions of the grating have different periods specifically tailored to compensate for wavelength-dependent OPD errors. This allows each wavelength band to have equal sensitivity to wafer height changes while maintaining a relatively simple overall system structure.
Solution Approach 2:
The patent changes the grating period parameter as a function of wavelength to equalize sensitivity across wavelength bands. By adjusting this physical parameter locally across the grating structure, the system achieves equal sensitivity for focus measurement while avoiding the need for completely separate optical paths for each wavelength.
2Measurement precision
If wavelength-dependent grating periods and angles of incidence are used, then equal sensitivity and error correction across wavelength bands is achieved, but the device complexity increases
Solution Approach 1:
The patent segments the grating into multiple zones with different periods, each optimized for specific wavelength bands. This segmentation allows the complex wavelength-dependent compensation to be implemented in a structured, modular way that reduces overall system complexity compared to using entirely separate optical systems for each wavelength.
3Manufacturing precision
If equal OPD error sensitivity is achieved across wavelength bands, then focus correction accuracy is improved, but the system requires more complex grating configurations
Solution Approach 1:
The patent implements local quality by assigning different grating periods to different spatial regions of the grating, where each region is optimized for specific wavelength bands. This local optimization achieves equal OPD error sensitivity across all wavelengths while maintaining a single integrated grating structure rather than multiple separate systems.
Solution Approach 2:
The patent effectively creates a composite grating structure with spatially varying properties. By combining multiple grating periods in a single grating element, it achieves the functionality of multiple specialized gratings while reducing overall system complexity and improving focus correction accuracy.
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 mitigates the effects of air turbulence and mechanical drifts by ensuring equal sensitivity and error correction across wavelength bands, improving the accuracy of focus adjustments in lithography systems.
Implementation Method 1
a diffraction grating configured to project fringes such that angles of incidence of a received optical radiation flux vary with wavelengths of the optical radiation flux
Implementation Method 2
A detection system is configured to receive projected fringes and produce an indication of a displacement of a target surface
Data Source
AI summary
Fringe projection autofocus systems are provided with variable pitch diffraction gratings or multiple diffraction gratings so that a reference beam and a measurement beam propagate along a common path. Alternatively, an input beam can be directed to a diffraction grating so that the selected diffraction orders propagate along a common path. In some examples, distinct spectral bands are used for reference and measurement beams.