Catadioptric Scatterometer Ghost Reflection Suppression
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Solution Overview
Problem
Catadioptric optical systems in scatterometers experience unwanted detected signals due to ghost reflections at the glass-to-air interface, which are not effectively suppressed by existing anti-reflective coatings across a wide range of illumination wavelengths.
Innovation Solution
The use of a partly filled pupil in the illumination system, where ghost reflections are reflected back into the same point in the pupil plane, excluding this area from the measurement to prevent interference with the signal, and potentially using multiple illumination shots with different areas to cover the full pupil plane.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If anti-reflective coatings are applied to suppress ghost reflections, then measurement precision is improved, but the effectiveness is limited across the wide range of illumination wavelengths
Solution Approach 1:
The pupil plane is divided into multiple segments, with each segment illuminated separately at different wavelengths. This allows ghost reflections from each wavelength to be directed to different regions in the detector plane, enabling wavelength-specific suppression of ghost reflections while maintaining measurement precision across the full spectral range
Solution Approach 2:
The solution moves from a one-dimensional approach (single anti-reflective coating layer) to a two-dimensional approach by utilizing both spatial segmentation of the pupil plane and spectral differentiation. This dimensional expansion allows simultaneous optimization for multiple wavelengths that cannot be achieved with a single coating designed for one wavelength
2Loss of information
If the full pupil plane is illuminated, then measurement completeness is improved, but ghost reflections interfere with the detected signal
Solution Approach 1:
The full pupil plane illumination is segmented into multiple wavelength-specific regions. Each segment is illuminated with a specific wavelength, and ghost reflections from each segment are spatially separated in the detector plane. This allows complete utilization of the pupil plane for comprehensive measurement while preventing ghost reflection interference through spatial separation
Solution Approach 2:
The patent introduces wavelength-specific aperture masks as intermediary elements that control which portions of the pupil plane are illuminated at each wavelength. These masks act as mediators between the full pupil plane illumination requirement and the ghost reflection suppression need, allowing selective illumination that eliminates ghost reflections while maintaining measurement completeness
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 effectively reduces the impact of ghost reflections on angle-resolved spectrum measurements, improving the accuracy and reliability of substrate inspection by excluding the affected areas from the measurement process.
Implementation Method 1
an illumination system configured to provide a beam of electromagnetic radiation by illuminating a first area in an illumination pupil plane of an objective
Implementation Method 2
an objective arranged with the illumination system to illuminate the substrate with the beam of electromagnetic radiation
Implementation Method 3
measuring an angle resolved spectrum arising from the illumination of the substrate
Implementation Method 4
properties of the scattered or reflected beam are measured
Data Source
AI summary
Ghost reflections in a catadioptric scatterometer objective are excluded from an angle-resolved spectrum measurement by using a partial pupil for illumination and for the measurement excluding the area of the pupil plane that has been illuminated. Ghost reflections are reflected back into same point in the pupil plane. The ghost reflections do not interfere with the signal in the non-illuminated area of the pupil plane. An illumination system provides a beam of electromagnetic radiation to illuminate a first area in an illumination pupil plane of the objective. The objective is arranged as to illuminate the substrate with the beam of electromagnetic radiation. The illumination pupil plane is the back projected image of the pupil plane of the objective and is also imaged into the measurement pupil plane at the back focal plane of the objective, via auxiliary optics. A detector is configured to measure an angle resolved spectrum arising from the illumination of the substrate, in a measurement area of the measurement pupil plane of the objective excluding an area corresponding to the first area.