Tunable Optical Sensor for Semiconductor Metrology
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Solution Overview
Problem
Current imaging interferometers face challenges in accurately determining the distance between a reference surface and raised surface features on a sample, particularly in semiconductor devices, due to limitations in wavelength selection and coherence length control, which affect axial resolution and measurement precision.
Innovation Solution
The implementation of a tunable light source system that emits multiple light beams with different wavelengths, combined with a processor system that generates images based on coherent reflections from a reference plate, raised surface features, and floor surfaces, using techniques like Fast Fourier Transform to determine distances and improve axial resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a fixed wavelength light source is used in the interferometer, then the device complexity is reduced, but the axial resolution and measurement precision deteriorate
Solution Approach 1:
The patent applies the dynamics principle by implementing a tunable light source that can dynamically adjust its wavelength rather than using a fixed wavelength source. The system sweeps through multiple wavelengths to generate interference patterns at different optical path differences, enabling precise determination of surface feature heights through Fourier transform analysis of the spectral interferograms. This dynamic wavelength adjustment resolves the contradiction by maintaining low device complexity while achieving high axial resolution through temporal modulation of the light source characteristics.
Solution Approach 2:
The patent applies parameter changes by varying the wavelength parameter of the light source across a range of values. By sweeping the wavelength and recording interference patterns at each wavelength point, the system obtains spectral interferograms that contain encoded depth information. The Fourier transform of these spectral patterns yields the axial distance with precision determined by the wavelength sweep range and resolution, thus achieving high axial resolution without increasing overall device complexity.
2Measurement precision
If multiple wavelengths are used to improve axial resolution, then the measurement precision is improved, but the device complexity increases
Solution Approach 1:
The patent applies periodic action by implementing a systematic wavelength sweep through multiple discrete wavelength points. The tunable light source sequentially emits light at different wavelengths, and the detector records interference patterns at each wavelength step. This periodic modulation of the wavelength parameter creates a structured set of spectral interferograms that can be processed through Fourier transform to extract precise axial distance information, achieving enhanced measurement precision through organized temporal sequencing rather than simultaneous multi-wavelength complexity.
Solution Approach 2:
The patent applies self-service by using the interference patterns themselves to provide the depth information needed for measurement. The spectral interferograms generated by the wavelength sweep contain self-encoded axial distance information through their frequency content. By performing Fourier transform on these self-generated patterns, the system extracts height measurements without requiring additional reference measurements or complex external calibration, thus achieving high precision while maintaining relatively simple device architecture.
3Length of stationary object
If the coherence length is increased to measure larger height differences, then the measurement range is improved, but the axial resolution deteriorates
Solution Approach 1:
The patent applies dimensionality change by transitioning from spatial domain measurement to spectral domain measurement. Instead of using spatial coherence length to determine measurement capability, the system uses spectral information from wavelength-swept interferometry. The axial distance is determined by analyzing the frequency content of the spectral interferograms through Fourier transform, where the measurement range is determined by the wavelength sweep extent and the axial resolution is determined by the wavelength sampling density. This spectral dimension approach decouples the trade-off between measurement range and resolution that exists in traditional spatial coherence-based interferometry.
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 enables precise measurement of distances and topological features on semiconductor devices, enhancing axial resolution and measurement accuracy by constructing fringe patterns and performing frequency domain transforms on the images obtained from the interferometer system.
Implementation Method 1
The reflected beam may be a coherent addition of a first reflection of the beam off the surface of the reference plate, a second reflection of the beam off the raised surface feature and a third reflection off the floor of the sample or substrate on which features are located
Implementation Method 2
a first reflection of the beam off the surface of the reference plate, a second reflection of the beam off the raised surface feature and a third reflection off the floor of the sample
Data Source
AI summary
An optical system configured to measure a raised or receded surface feature on a surface of a sample may comprise a broadband light source; a tunable filter configured to filter broadband light emitted from the broadband light source and to generate a first light beam at a selected wavelength; a linewidth control element configured to receive the first light beam and to generate a second light beam having a predefined linewidth and a predetermined coherence length; collimating optics optically coupled to the second light beam and configured to collimate the second light beam; collinearizing optics optically coupled to the collimating optics and configured to align the collimated second light beam onto the raised or receded surface feature of the sample, and a processor system and at least one digital imager configured to measure a height of the raised surface or depth of the receded surface from light reflected at least from those surfaces.