Interferometer Wavelength Disperser for Surface Metrology
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Solution Overview
Problem
Existing interferometric techniques face limitations such as time-consuming measurements, phase ambiguity issues, and instability due to fiber path length drift, particularly in micro-scale and nano-scale surface characterization.
Innovation Solution
A short coherence length interferometer apparatus with a wavelength disperser, such as grating or prism, is used to cause chromatic dispersion in the reference path, allowing for unambiguous measurement of surface height without phase-wrapping issues and fiber path length drift, using a broadband light source and a detector to determine the balance point of the measurement and reference paths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If white light scanning interferometry is used to measure surface characteristics, then measurement range is improved, but measurement time increases and sample must remain stationary
Solution Approach 1:
The patent employs periodic phase shifting of the reference beam to generate a series of interferograms. By capturing multiple interferograms with known phase differences and processing them through phase extraction algorithms, the system achieves both extended measurement range and reduced measurement time compared to traditional white light scanning interferometry.
2Productivity
If long coherence length interferometric techniques are used, then measurement speed is improved, but phase ambiguity issues arise limiting measurement range
Solution Approach 1:
The patent introduces a wavelength disperser as an intermediary element in the reference arm that creates wavelength-dependent optical path differences. This disperser acts as a mediator that maps absolute path difference to spectral distribution, allowing phase unwrapping and eliminating the 2π ambiguity inherent in traditional long coherence length interferometry while maintaining high measurement speed.
Solution Approach 2:
The patent changes the optical path length parameter in the reference arm as a function of wavelength by introducing a wavelength disperser. This creates a wavelength-dependent optical path difference that encodes absolute distance information in the spectral domain, enabling unambiguous phase measurement over extended ranges while maintaining the speed advantages of long coherence length techniques.
3Measurement precision
If fiber-optic Michelson interferometer with wavelength dispersion is used, then phase ambiguity is eliminated, but fiber path length drift causes instability
Solution Approach 1:
The patent merges the measurement and reference optical paths into a common fiber-optic path before the wavelength disperser. This common-path configuration ensures that both beams experience identical environmental disturbances and fiber path length variations, eliminating differential drift and improving measurement stability while retaining the phase unwrapping capability provided by wavelength dispersion.
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
Enables stable and accurate surface metrology and displacement measurements without phase-wrapping ambiguities, allowing for high-resolution surface topography analysis and film thickness measurement, while being less vulnerable to fiber path length drift.
Implementation Method 1
a wavelength disperser to cause wavelength dispersion of light along the measurement or the reference path
Implementation Method 2
a combiner to cause light from the sample surface and light from the reference surface to produce an interference pattern or interferogram
Data Source
AI summary
An interferometer apparatus comprising: a short coherence length or broadband light source; a light director to direct light from the light source along a measurement path to a surface of a sample and also along a reference path to a reference surface; a wavelength disperser to cause wavelength dispersion of light along one of the measurement and the reference paths; a combiner to cause light from the sample surface and light from the reference surface to produce an interference pattern or interferogram; a detect—or to detect intensity values of the interference pattern as a function of wavelength; and a determiner to determine from the detected intensity values the wavelength at which the measurement and reference paths are balanced, wherein the wavelength disperser is at least one of: a grating wavelength disperser, a prism wavelength disperser, and an optical dispersive medium.