Common Path Fiber Tip Interferometer for Sub-nm Wavefront Measurement
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Solution Overview
Problem
Current optical interferometers face challenges in achieving sub-nm RMS wavefront measurement accuracy due to systematic errors from optical elements and sensors, particularly in EUV lithography and mask inspection, where previous designs using metal-coated SMF tips introduce distortion and thermal issues, and non-common path modes fail to cancel out errors.
Innovation Solution
A common path mode fiber tip diffraction interferometer design utilizing a wedged, super-polished, and uncoated SMF tip to generate a near-perfect spherical reference wave, with a second SMF tip or pinhole generating a test wave, ensuring both waves travel the same path and cancel out systematic errors, and optionally using a high-quality thin film as a beam combiner to further reduce errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a metal film coated SMF tip is used to increase reflectivity, then the reflectivity of the SMF tip surface is improved, but the sphericity of the diffraction wavefront is deteriorated due to coating roughness, curvature, and multiple reflections
Solution Approach 1:
The patent removes the metal film coating from the SMF tip surface, extracting the harmful element that caused wavefront distortion. The uncoated silica surface eliminates coating roughness, curvature, and multiple reflection issues while maintaining sufficient reflectivity for interferometry through the natural silica reflection
Solution Approach 2:
The patent uses the inherent reflective property of the uncoated silica fiber tip, eliminating the need for expensive and complex metal film coating processes. The simple uncoated surface provides adequate reflectivity without introducing wavefront errors
2Illumination intensity
If a metal film coating is applied to the fiber tip, then reflectivity is increased, but thermal effects and light power absorption are introduced
Solution Approach 1:
The metal film coating is completely removed from the fiber tip, eliminating the source of thermal absorption. The uncoated silica surface reflects light without converting significant energy to heat, avoiding thermal lensing and alignment instability
Solution Approach 2:
The patent changes the material parameter from metal coating to uncoated silica, fundamentally altering the optical and thermal properties. This parameter change reduces light absorption and eliminates thermal effects while maintaining the necessary reflective functionality
3Measurement precision
If two different SMF tips are used to produce test and reference waves, then wavefront measurement capability is achieved, but systematic errors from extra beam combiners do not cancel out
Solution Approach 1:
The patent merges the reference wave and test wave into a common optical path, allowing both waves to traverse the same optical components. This merging enables systematic errors from beam combiners and other elements to cancel out during interference, improving measurement reliability
Solution Approach 2:
The single SMF tip serves multiple functions: generating the reference wave, providing a beam combining surface through its end face, and eliminating the need for separate beam combiner components. This multi-functionality simplifies the optical path and reduces systematic errors
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 design achieves accurate wavefront measurement with reduced systematic errors, improving spatial resolution and avoiding complex calibration processes, thereby meeting the sub-nm RMS accuracy target while minimizing thermal and distortion issues.
Implementation Method 1
A near-perfect spherical reference wave can be generated for interferometry by using the far field diffraction wave of a single mode optical fiber tip
Implementation Method 2
The test wave, which carries the wavefront aberration information of the measured optics or imaging system, interferes with the reference wave on a 2D sensor, generating interference patterns called interferograms
Data Source
AI summary
Reference and test waves are directed in a common path mode in a fiber tip diffraction interferometer. A first fiber can be used to generate the reference wave and a second fiber can be used to generate the test wave. Each fiber can include a single mode fiber tip that defines a wedge at an end without a coating on end surface or a tapered fiber tip. The fiber tip diffraction interferometer can include an aplanatic pupil imaging lens or system disposed to receive both the test wave and the reference wave and a sensor configured to receive both the test wave and the reference wave.