Metrology Light Source Spectral Control for Overlay Accuracy
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Solution Overview
Problem
Lithographic metrology systems face challenges due to intrinsic stochastic spectral noise in light sources, leading to variations in wavelength and errors in overlay measurements, particularly when short measurement times are desired, as current light sources like supercontinuum sources exhibit unpredictable spectral shifts and fluctuations.
Innovation Solution
A method and apparatus that utilize a spectrometer to measure and control the spectral output of a light source, incorporating a controller to adjust the light source's output based on measured characteristics, thereby stabilizing the wavelength and reducing spectral noise, including the use of band-pass filters and interferometers to compensate for fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a supercontinuum light source is used for metrology measurements, then broadband light is available for illumination, but intrinsic stochastic spectral noise causes wavelength variations that degrade measurement precision
Solution Approach 1:
A spectrometer continuously monitors the instantaneous spectrum of the light source, and a controller adjusts the light source parameters based on measured spectral characteristics to maintain a stable centroid wavelength, thereby compensating for stochastic spectral noise and improving measurement precision
Solution Approach 2:
The light source parameters are dynamically adjusted based on real-time spectral measurements to maintain a stable centroid wavelength, transforming the stochastic spectral output into a controlled and predictable light source for precise metrology measurements
2Productivity
If the measurement time is reduced for faster throughput, then productivity increases, but the stochastic nature of the light source causes greater spectral variations that worsen measurement precision
Solution Approach 1:
Real-time spectral monitoring and control enable precise wavelength stabilization even during short measurement intervals, allowing fast throughput without sacrificing measurement accuracy by compensating for spectral noise on-the-fly
Solution Approach 2:
The system performs preliminary spectral characterization and establishes control parameters before measurements begin, enabling rapid subsequent measurements with maintained precision through pre-configured compensation strategies
3Ease of operation
If the spectral output of the light source is not controlled, then the system is simpler to operate, but wavelength variations cause errors in overlay measurements
Solution Approach 1:
The system automatically monitors and adjusts its own spectral output without requiring manual intervention, measuring the instantaneous spectrum and controlling the light source parameters autonomously to maintain measurement precision while keeping operation simple
Solution Approach 2:
The system dynamically adapts to changing spectral conditions by continuously monitoring and adjusting light source parameters in real-time, maintaining measurement accuracy under varying operational conditions without requiring complex manual 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 significantly reduces spectral noise and measurement errors, improving the accuracy and consistency of metrology measurements by stabilizing the central wavelength and compensating for variations, allowing for faster and more reliable data acquisition without compromising system throughput.
Implementation Method 1
a spectrometer arranged to measure a characteristic of the spectrum of light emitted from said light source
Implementation Method 2
One possible light source for use in metrology applications is a supercontinuum light source, for example based on modulation instability or similar non-linear effects
Implementation Method 3
This broadband source is then band-pass filtered to generate a narrowband source for use in the metrology process
Data Source
AI summary
Provided are light sources and methods of controlling them, and devices and methods for use in measurement applications, particularly in metrology, for example in a lithographic apparatus. The methods and devices provide mechanisms for detection and/or correction of variations in the light source, in particular stochastic variations. Feedback or feedforward approaches can be used for the correction of the source and/or the metrology outputs. An exemplary method of controlling the spectral output of a light source which emits a time-varying spectrum of light includes the steps of: determining at least one characteristic of the spectrum of light emitted from the light source; and using said determined characteristic to control the spectral output.