Lensless Metrology for Lithographic Overlay
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing inspection apparatuses for lithographic metrology face challenges in achieving high-NA broadband optics with low aberrations, leading to internal scattering and difficulty in detecting weak targets, while also being complex to adjust properly for metrology applications.
Innovation Solution
The implementation of a 'lensless' imaging system using spatially coherent radiation and a processor to calculate a synthetic radiometric image from interference patterns formed by coherent scattered and reference radiation, allowing for the measurement of target properties without relying on the quality of optical components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-NA broadband optics with low aberrations are used, then measurement precision is improved, but device complexity increases and internal scattering occurs limiting detectability of weak targets
Solution Approach 1:
The patent extracts the image formation function from the optical domain to the computational domain. Instead of using complex high-NA optics to form images, the system captures diffraction patterns and computationally reconstructs images, removing the need for complex broadband optics while maintaining measurement precision
Solution Approach 2:
The patent replaces the mechanical/optical image formation system with a computational system. The physical optical components that would normally form images are replaced by algorithms that process diffraction patterns to generate synthetic images, eliminating internal scattering issues while preserving detectability
2Measurement precision
If high-NA broadband optics with low aberrations are used, then measurement precision is improved, but internal scattering increases limiting detectability of weak targets
Solution Approach 1:
The patent removes the source of internal scattering by extracting the image formation process from the optical path. By capturing diffraction patterns directly and forming images computationally, the system eliminates multiple lens-element interfaces that generate internal scattering, thereby improving detectability of weak targets
Solution Approach 2:
The patent creates a computational copy of the image formation process. Instead of physically forming images through optics that introduce scattering, the system captures the diffraction pattern and computationally reconstructs the image, achieving the same measurement precision without the harmful scattering effects
3Adaptability or versatility
If multiple lens elements composed of different materials are used, then broadband performance is improved, but adjustment difficulty increases for metrology applications
Solution Approach 1:
The patent extracts the broadband imaging capability from complex multi-element optics and implements it through computational processing. The system captures diffraction patterns across the broadband spectrum and uses algorithms to synthesize images, achieving broadband performance without the adjustment difficulties of multiple lens elements
Solution Approach 2:
The patent replaces the mechanical adjustment requirements of multiple lens elements with a computational approach. The system uses fixed, simple optics to capture diffraction patterns and employs software-based image formation that adapts to broadband conditions without requiring complex mechanical adjustments
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 enables improved detection of weak targets and reduces the complexity of the optical system, allowing for a larger field of view and increased accuracy across a wide wavelength range without the need for high-NA objective lenses.
Implementation Method 1
forming a beam of spatially coherent illuminating radiation
Implementation Method 2
the reference radiation is coherent with the scattered radiation so as to interfere with the scattered radiation and form an interference pattern at the image detector
Implementation Method 3
calculate from the image data and from knowledge of the reference radiation a complex field of the scattered radiation at the detector
Implementation Method 4
calculate from the complex field a synthetic radiometric image of radiation diffracted by a periodic structure within the target area
Data Source
AI summary
Metrology targets are formed on a substrate (W) by a lithographic process. A target (T) comprising one or more grating structures is illuminated with spatially coherent radiation under different conditions. Radiation (650) diffracted by from said target area interferes with reference radiation (652) interferes with to form an interference pattern at an image detector (623). One or more images of said interference pattern are captured. From the captured image(s) and from knowledge of the reference radiation a complex field of the collected scattered radiation at the detector. A synthetic radiometric image (814) of radiation diffracted by each grating is calculated from the complex field. From the synthetic radiometric images (814, 814′) of opposite portions of a diffractions spectrum of the grating, a measure of asymmetry in the grating is obtained. Using suitable targets, overlay and other performance parameters of the lithographic process can be calculated from the measured asymmetry.