Annular Apodizer Layout for Small-Target Overlay Metrology
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Solution Overview
Problem
Current semiconductor metrology tools face challenges in measuring small targets with high precision, particularly in the red spectrum, due to limitations in spectral range and spot size, leading to interference and target noise issues with off-axis illumination, and large spot sizes with on-axis illumination.
Innovation Solution
The use of an annular apodizer with a numerical aperture of approximately 0.2 to 0.25 enables the generation of a beam of light with wavelengths from 400 nm to 800 nm, forming a spot diameter of 2 nm to 5 nm on the semiconductor wafer, allowing for precise metrology in both on-axis and off-axis illumination, and enabling measurement across a broader spectral range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If on-axis illumination is used, then the available spectral range is limited, but the spot size becomes large causing target noise
Solution Approach 1:
The patent applies local quality by using an annular apodizer to create a non-uniform illumination profile with specific radial intensity distribution. This localized modification of the illumination pattern enables precise control over the spot size and intensity distribution on the target, resolving the contradiction between spectral range and target noise by optimizing the local illumination characteristics rather than using uniform illumination across the entire beam.
2Illumination intensity
If off-axis illumination is used, then the wavelength range can be extended, but interference occurs affecting the difference between pupil and field
Solution Approach 1:
The patent employs asymmetry through off-axis illumination geometry combined with an annular apodizer. The asymmetric illumination path allows extended wavelength range while the apodizer's specific annular shape and positioning control the interference patterns, enabling the system to maintain reliability by managing the interference effects that inherently arise from off-axis geometry.
3Measurement precision
If small pitch is used for small targets, then adequate bars can be included in the target, but the open pupil becomes small generating a large spot
Solution Approach 1:
The patent applies parameter changes by systematically optimizing multiple parameters including the annular apodizer inner and outer radii, numerical aperture, and illumination geometry. These parameter adjustments enable the system to achieve small spot diameters (2-5 µm) on small pitch targets while maintaining adequate bar inclusion, effectively resolving the contradiction between target resolution and spot size through coordinated optimization of optical parameters.
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 allows for accurate and noise-reduced measurements of small targets across a wide range of wavelengths, minimizing the risk of spot overlap with target edges and enhancing the precision of overlay calculations in semiconductor manufacturing.
Implementation Method 1
The large spot causes target noise because the incoming light diffracts when it hits the target edge.
Implementation Method 2
The beam of light forms a spot on the semiconductor wafer having a diameter from 2 nm to 5 nm using the apodizer
Data Source
AI summary
Metrology is performed on a semiconductor wafer using a system with an apodizer. A spot is formed on the semiconductor wafer with a diameter from 2 nm to 5 nm. The associated beam of light has a wavelength from 400 nm to 800 nm. Small target measurement can be performed at a range of optical wavelengths.