Solid Immersion Lens Gap Control via Broadband Spectroscopy
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Solution Overview
Problem
Existing techniques for controlling the gap between a Solid Immersion Lens (SIL) and a target surface in lithographic processes face challenges, particularly when dealing with varied target structures and materials, as they often require high light source power and are prone to issues like low reflectivity and speckle noise, limiting accuracy and reliability.
Innovation Solution
A method using broadband radiation to illuminate the target surface, measuring the reflected electromagnetic spectrum, and deriving the gap distance from shifts in wavelength features, allowing for continuous servo control of the gap to maintain optimal optical contact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If monochromatic light source is used for generating air gap control signal, then the gap control can be implemented, but process dependent issues appear such as low to almost zero light reflectivity at given wavelength and speckle noise
Solution Approach 1:
The patent changes the spectral parameter of the light source from monochromatic to broadband. This allows the system to integrate reflectivity information across multiple wavelengths, avoiding the problem of low reflectivity at any single wavelength and eliminating speckle noise through incoherent illumination.
2Illumination intensity
If high power light source is used, then sufficient reflected signal can be obtained, but process dependent issues like low reflectivity and speckle noise become more prominent
Solution Approach 1:
The patent replaces the mechanical approach of increasing light source power with an optical approach using broadband illumination. This substitution achieves sufficient reflected signal intensity while simultaneously eliminating speckle noise and process-dependent artifacts through the use of incoherent, multi-wavelength light.
3Measurement precision
If gap is reduced to maintain SIL in effective optical contact with substrate, then measurement accuracy improves, but the gap must be precisely controlled within very small range
Solution Approach 1:
The patent implements a feedback-based gap control system that uses broadband reflected light to continuously monitor and adjust the gap between SIL and substrate. The system measures the spectrum, derives gap information from spectral features, and adjusts positioning to maintain optimal gap, thereby achieving high measurement precision without requiring extremely precise mechanical control.
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 provides a robust and accurate method for controlling the small gap between the SIL and the target surface, improving reliability across a wide range of target materials and structures, reducing process-dependent artifacts, and enhancing sensitivity in metrology applications.
Implementation Method 1
collecting radiation reflected by the target surface back into the optical element
Implementation Method 2
measuring an electromagnetic spectrum of the collected radiation; and deriving information about the distance between the optical arrangement and the target surface based on a shift in wavelength of one or more features in the spectrum
Data Source
AI summary
A broadband spectroscopic analysis is used for controlling a distance (d) between a miniature solid immersion lens (SIL, 60) and a metrology target (30′). An objective lens arrangement (15, 60) including the SIL illuminates the metrology target with a beam of radiation with different wavelengths and collects a radiation (709) reflected or diffracted by the metrology target. A mounting (64) holds the SIL within a distance from the metrology target that is less than the coherence length of the illuminating radiation (703). A detection arrangement (812, 818) produces a spectrum of the radiation reflected or diffracted by the metrology target. The distance between the SIL and the metrology target or other target surface can be inferred from spectral shifts observed in the detected spectrum. Servo control of the distance is implemented based on these shifts, using an actuator (66).