Angular Obscuration Aperture for Ghost Reflection Reduction
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Solution Overview
Problem
Conventional optical measurement systems for microelectronic devices face challenges in reducing parasitic ghost reflections from the bottom interface, particularly in transparent samples like silicon wafers, which interfere with front surface reflection measurements, and existing solutions like reducing field size or increasing numerical aperture are inefficient or complex.
Innovation Solution
An all-optical solution using angular obscuration in the illumination and collection channels to block light propagation paths from regions outside the focal spot, determined by the sample's thickness, refractive index, and detector/source size, employing apertured structures like masks to prevent ghost reflections from reaching the detector.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the field size is reduced to eliminate bottom reflections, then measurement accuracy is improved, but system efficiency is reduced due to lower light flux
Solution Approach 1:
The patent applies local quality by implementing angular obscuration specifically for light rays originating from bottom interfaces, while allowing full collection of light from the top surface. The apertured structure is positioned to block only specific angular ranges corresponding to bottom reflections, preserving top surface signal collection efficiency while eliminating parasitic bottom reflections from the detected signal
2Measurement precision
If the numerical aperture is increased to reduce bottom reflections, then measurement accuracy is improved, but the optical depth of focus is reduced limiting effectiveness
Solution Approach 1:
The patent transitions from controlling bottom reflection through spatial parameters (field size, numerical aperture) to controlling it through angular dimension. By introducing angular obscuration with apertured structures positioned at specific angles in the optical path, the system blocks bottom reflections based on their angular deviation from the optical axis, independent of the depth of focus or numerical aperture settings
3Measurement precision
If a tilt is introduced to displacement bottom reflections, then measurement accuracy is improved, but system complexity increases due to Schiemflug defocus effect and focus sensitivity
Solution Approach 1:
The patent segments the optical path into distinct angular zones using apertured structures. Instead of tilting the entire illumination and collection channels, the system uses discrete aperture masks with blocking regions that selectively obstruct only the angular ranges corresponding to bottom reflections. This segmented approach eliminates bottom reflections without introducing Schiemflug defocus or requiring coordinated tilting of multiple optical components
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
Significantly reduces unwanted bottom reflections, enhancing measurement accuracy by isolating the desired front surface reflection signal and maintaining system efficiency, especially in imaging systems where large fields are required.
Implementation Method 1
angular obscuration of light propagation path blocking angular segments associated with light propagation from regions outside the focal spot
Implementation Method 2
light incident on the front surface of the device is not absorbed within the substrate, but rather continues to propagate inside the substrate, and is then partially reflected back from the rear surface of the substrate
Data Source
AI summary
An optical system and method are presented for use in measurements on an upper surface of a layered sample when located in a measurement plane. The optical system is configured as a normal-incidence system having an illumination channel and a collection channel, and comprises an objective lens unit and a light propagation affecting device. The objective lens unit is accommodated in the illumination and collection channels, thereby defining a common optical path for propagation of illuminating light from the illumination channel toward an illuminating region in the measurement plane and for propagation of light returned from measurement plane to the collection channel. The light propagation affecting device comprises an apertured structure located in at least one of the illumination and collection channels, and configured to provide angular obscuration of light propagation path for blocking angular segments associated with light propagation from regions outside the illuminated region.