Cavity Ringdown Reflectometer for EUV Optical Surface Contamination
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Solution Overview
Problem
EUV lithography apparatuses face contamination issues with substances like hydrocarbons, which affect the optical properties and service life of reflective optical surfaces, requiring precise monitoring of contamination status to maintain performance.
Innovation Solution
The implementation of a cavity ringdown reflectometer to determine the contamination status of EUV-reflective optical surfaces by measuring reflectivity at specific wavelengths, using a laser and optical resonator to enhance sensitivity and precision, and adjusting temperature to analyze contaminant adsorption and desorption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional reflectivity measurement methods are used, then the measurement process is simple, but the measurement precision and sensitivity are insufficient for detecting contamination
Solution Approach 1:
The patent changes the measurement parameter from simple reflectivity to cavity ringdown time, which is highly sensitive to contamination. By measuring the decay time of light in a resonant cavity, the system achieves high-precision contamination detection while maintaining a relatively simple device structure.
Solution Approach 2:
The patent introduces an optical resonant cavity as an intermediary between the light source and detector. This cavity amplifies the measurement signal by creating multiple reflections, thereby enhancing the sensitivity to contamination without requiring complex direct measurement systems.
2Duration of action of stationary object
If EUV-reflective optical surfaces are operated in vacuum to extend service life, then contamination deposition is reduced, but contamination still occurs and requires monitoring
Solution Approach 1:
The patent implements a feedback mechanism where the cavity ringdown measurement provides real-time information about contamination status. This feedback enables monitoring of optical surface degradation, allowing for timely maintenance or cleaning to preserve both service life and optical performance.
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 method allows for high-precision determination of contamination status, improving the sensitivity and accuracy of reflectivity measurements, enabling effective monitoring and cleaning of EUV-reflective surfaces during exposure processes.
Implementation Method 1
Cavity ringdown spectroscopy involves measuring the decay of laser light in the optical resonator after the laser is rapidly switched off. The decay curve is an exponential curve with a time constant T from which the reflectivity or transmission of the optical element may be deduced.
Implementation Method 2
A cavity ringdown spectrometer typically comprises a laser source, an optical resonator, and a detection unit for detecting the laser radiation reflected or transmitted by the optical element
Implementation Method 3
Optical elements used in EUV lithography apparatuses are typically reflective elements, as no optical materials are known which can provide sufficient transmission for radiation at EUV wavelengths
Data Source
AI summary
The invention relates to an EUV lithography apparatus with at least one EUV-reflective optical surface and a cavity ringdown reflectometer adapted to determine the contamination status of the EUV-reflective optical surface for at least one contaminating substance by determining the reflectivity of the EUV-reflective optical surface for radiation at a measuring wavelength (λm). The invention further relates to a method for determining the contamination status of at least one EUV-reflective optical surface arranged in an EUV lithography apparatus for at least one contaminating substance comprising determining the reflectivity of the EUV-reflective optical surface for radiation at a measuring wavelength (λm) using a cavity ringdown reflectometer.